An-Najah National University 

 

Faculty of Engineering & Information Technology 
 

Electrical Engineering Department 
 

 

 

Improve the Stability and Performance of the Power Grid under 

Changing Power Generation According to Dynamic Load 

 
 
 

 

Students: 

Ghena Farouq 

Tariq Dumaide 

Mohammad Qasem 

Mohammad Tubileh 
 

 

Supervisor Name: 

Eng. Omar Tamimi 

 

A Report Submitted in Partial Fulfillment of the Requirements for the Degree of 

Bachelor of Science in Electrical Engineering. 

 

An-Najah National University (Palestine) 

January 1st Semester 

5202/6202  

 



2 

Table of Contents 
List of Tables .............................................................................................................................. 5 

List of Figures ............................................................................................................................. 6 

Abstract in English ..................................................................................................................... 8 

Abstract in Arabic ...................................................................................................................... 9 

Chapter 1: Introduction ............................................................................................................ 10 

1.1 Background .................................................................................................................................................. 10 

1.2 Problem Description .................................................................................................................................. 10 

1.3 Project Goals ................................................................................................................................................ 11 

1.4 Importance of the Study ........................................................................................................................... 11 

1.5 Project Scope  ............................................................................................................................................... 13 

1.6 Organization of the Report  ..................................................................................................................... 13 

Chapter 2: Literature Review and Theoretical Background ..................................................... 15 

2.1 Overview of Power Grids and Stability Challenges  ........................................................................ 15 

2.2 Impact of Renewable Energy Integration  .......................................................................................... 16 

2.3 An Examination of Solar PV Systems: On-Grid, Off-Grid, and Hybrid ................................. 17 

2.3.1 ON-grid solar power systems  ......................................................................................................... 17 

2.3.2 Off-grid solar power systems  ......................................................................................................... 22 

2.3.3 Hybrid solar power systems  ........................................................................................................... 25 

2.3.4 Comparison: On-Grid Systems vs. Off-Grid Systems vs. Hybrid Systems ..................... 30 

2.3.5 Working Mechanisms of Solar Photovoltaic (PV) Systems .................................................. 33 

2.4 A Comprehensive Review of Inverter Technologies for Photovoltaic Systems  ..................... 35 

2.4.1 Grid-Tie Inverters (GTIs)  ............................................................................................................... 35 

2.4.2 Stand-Alone Inverter (Off-Grid Solar Power Systems)  ........................................................ 38 

Stand-Alone Inverter: Types and Applications  .................................................................................. 41 

2.4.3 Hybrid Inverters (Hybrid Solar Power Systems)  .................................................................... 43 

2.4.4 Comparison: Stand-alone vs. Grid-tie Inverters vs. Hybrid Inverter  .............................. 48 

2.5 Grid-Tied Inverters' Function  ............................................................................................................... 52 

2.6 Regulation and Grid Stability  ................................................................................................................ 52 

2.7 Previous Work and Related Research ................................................................................................. 54 

2.8 Summary ....................................................................................................................................................... 58 

Chapter 3: System Design and Methodology............................................................................. 60 

3.1 System Overview  ........................................................................................................................................ 60 

3.2 Design Specifications  ................................................................................................................................. 60 

3.3 Inverter Component Selection  ............................................................................................................... 61 

3.4 Control Strategy and Design  .................................................................................................................. 61 

3.4.1 Design of a PWM Technique and the Control Strategy  ........................................................ 62 



3 

3.4.2 Closed Loop Control Systems: ....................................................................................................... 66 

3.4.3 PI & PID Controllers in Grid-Tie Inverters ............................................................................. 68 

3.4.4 Solar Inverter Parameters for grid support: P(V), Q(V), and Power Factor 

Control............................................................................................................................................................. 73 

3.4.4.1 P(V) Solar Inverter Parameter (Active Power and Voltage Relation)  .......................... 73 

3.4.4.2 Q(V) Solar Inverter Parameter (Reactive Power and Voltage Relation) ..................... 76 

3.4.4.4 Power Factor Control .................................................................................................................... 82 

3.5 Hardware Implementation of the Control and Power Stages  ..................................................... 82 

3.5.2 Comparative Analysis of Switching Technologies ................................................................... 83 

3.5.3 Supporting Control and Interface Circuitry ............................................................................. 84 

3.6 Simulation, Implementation of Models and the Closed-Loop Process ..................................... 85 

3.6.1 Circuit and Control Simulation using LTspice  ........................................................................ 85 

3.6.2 Power Flow Analysis Using CYME Software  ........................................................................... 85 

3.6.2.1 Interpretation of Load Flow analysis from a Real-World Case Study  ......................... 89 

3.6.2.2 Impact of the Proposed Solution on Grid Stability  ............................................................. 98 

3.6.2.3 Comparison with Findings from Literature Review ........................................................... 99 

3.6.2.4 Practical Implications and Challenges  .................................................................................... 99 

3.7 Safety and Protection Considerations ........................................................................................... 101 

3.7.1 Ways to Protect Grid-Tied Inverters ......................................................................................... 101 

Chapter 4: Electrical Circuit Description and Functional Analysis of the Grid-Tied Inverter 102 

4.1 Overview of the Inverter System .................................................................................................... 102 

4.2 Isolated Auxiliary Power Supply Design ..................................................................................... 105 

4.3 Low-Voltage Regulation Stage (VCC to +5 V and +3.3 V) ...................................................... 109 

4.4 Relay Driver Stage ................................................................................................................................. 113 

4.5 Gate Driver and Full-Bridge Power Stage ................................................................................... 117 

4.6 Output Filter Design and Implementation (LCL Stage)  ........................................................ 130 

4.7 Grid Connection and Safety Mechanisms .................................................................................... 134 

4.8 Signal Acquisition and EMI Suppression ..................................................................................... 135 

4.9 Isolated Line-Voltage Measurement Interface ......................................................................... 136 

4.10 Microcontroller Unit (MCU) Interface and Integration  ..................................................... 138 

Chapter 5: Software Design and Closed-Loop Current Control Implementation .............. 144 

5.1 Introduction ............................................................................................................................................. 144 

5.2 Overall Control Architecture Modeling ....................................................................................... 144 

5.3 Grid Voltage Modeling ......................................................................................................................... 145 

5.4 Current Reference Generation  ........................................................................................................ 146 

5.5 Grid Current Sensing............................................................................................................................ 147 

5.6 Error Signal Generation...................................................................................................................... 148 



4 

5.7 PI Current Compensator..................................................................................................................... 148 

5.8 Output Limiting and Saturation ...................................................................................................... 149 

5.9 Triangular Carrier Generation (𝑽𝒕𝒓𝒊) .......................................................................................... 149 

5.10 SPWM Generation ............................................................................................................................... 150 

5.11 Gate Driver and Full-Bridge Inverter ........................................................................................ 152 

5.12 Output Filter and Grid Injection................................................................................................... 152 

5.13 Closed-Loop Performance Verification ..................................................................................... 153 

5.14 Relation to Volt/Var Control .......................................................................................................... 154 

5.15 Chapter Summary  ............................................................................................................................... 155 

Chapter 6: Hardware Implementation and Component Description  ................................ 156 

6.1 Main Inverter Board Architecture and Layout ......................................................................... 156 

6.2 High-Frequency DC-DC Step-Up (Boost) Converter Board ................................................. 162 

6.3 Full-Wave Bridge Rectifier Board .................................................................................................. 164 

6.4 XL4015 CC/CV Step-Down (Buck) Converter Module  ........................................................... 165 

6.5 Switched-Mode Power Supply (SMPS) ......................................................................................... 165 

Chapter 7: Testing and measurement ................................................................................. 167 

1. Testing Setup.............................................................................................................................................. 167 

7.2 Microcontroller Configuration ........................................................................................................ 168 

7.3 Measurement Results .......................................................................................................................... 168 

7.4 Results Analysis ..................................................................................................................................... 170 

Chapter 8: Conclusion and Future Work  ............................................................................. 171 

8.1 Project Conclusion and Hardware Outcomes  ........................................................................... 171 

8.2 Project Challenges ................................................................................................................................. 171 

8.2.1 Hardware Challenges .................................................................................................................. 171 

8.2.2 Simulation Challenges ................................................................................................................ 171 

8.3 Future Work ............................................................................................................................................. 172 

Appendix A: Firmware Architecture and Control Logic Analysis ...................................... 173 

References............................................................................................................................... 181 

 

 

 
 

 

 

 

 



5 

List of Tables 

Table 1.1: Comparison Between Traditional and Controlled Grid Conditions 12 

Table 2.1: Energy Sources Comparison 16 
Table 2.2: PV Inverters Types 17 

Table 2.3: Comparison: On-Grid Systems vs. Off-Grid Systems vs. Hybrid 

Systems 
30 

Table 2.4: Comparison: Stand-alone vs. Grid-tie Inverters vs. Hybrid Inverter 48 
Table 3.1: Comparison Between PI and PID Controllers in Grid-Tied Inverter 

Applications 
71 

Table 3.2: Application Comparison Between PI and PID Controllers in Grid-Tied 

Inverter Control Loops (Voltage, Current, and PLL) 
72 

Table 3.3: Comparison Table: P(V) vs. Q(V) Solar Inverter Parameters 80 
Table 3.4: Comparative Analysis of Switching Technologies 83 
Table 4.1: Summary of input and output voltage levels in the regulation stage. 109 
Table 4.2: Pin assignment and hardware connections of the STM32 Control Unit.  142 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



6 

List of Figures 
Figure 1.1: voltage profile with and without inverter control 12 
Figure 2.1: Comparison Between Traditional and Modern Power Grids 15 
Figure 2.2: Solar Power Generation Over a Day 16 
Figure 2.3: Solar Cell System On Grid Type 17 

Figure 2.4: Solar Cell System Off Grid Type 22 

Figure 2.5: Solar Cell System Hybrid Type 25 

Figure 2.6: Comparison Of On-Grid, Off-Grid, And Hybrid Photovoltaic (PV) Systems 29 

Figure 3.1: The basic principle of Sine-Triangle PWM 62 

Figure 3.2: Unipolar PWM strategy for a single-phase inverter 63 

Figure 3.3: Pure sine wave, modified or square wave 64 

Figure 3.4: Types of Inverter Output 65 

Figure 3.5: Step Response of P, PI, and PID Controllers  68 

Figure 3.6: Setting of Volt–Watt Function Curve  74 

Figure 3.7:  Volt–VAR Curve 77 

Figure 3.8: Volt–VAR Control Curve 77 

Figure 3.9: Case study 1 86 

Figure 3.10: Case study 2 87 

Figure 3.11: Original condition, natural network, without adding a solar system 88 

Figure 3.12: After adding the solar system 88 

Figure 3.13: This is a state of improvement 89 

Figure 3.14: Line-to-Line Voltage Variations Due to Load Fluctuations (Pre-PV 
Integration) 

90 

Figure 3.15: Voltage Increase After PV Addition 91 

Figure 3.16: Voltage Changes After PV Addition 92 

Figure 3.17: Voltage Profile Before Adding Load at PV Bus 93 

Figure 3.18: Voltage Profile Before Adding Load at PV Bus 94 

Figure 3.19: Voltage Profile After Adding Load at PV Bus 95 

Figure 3.20: PV Bus After Adding Parallel Cable as First Solution 96 

Figure 3.21: Voltage Profile After Implementing a Parallel Cable Solution 96 

Figure 3.22: Effect of P(V) and Q(V) Modes on Voltage Level 98 

Figure 4.1: Complete electrical schematic of the single-phase grid-tied inverter 

system. 
103 

Figure 4.2: Detailed schematic diagram of the isolated auxiliary power supply.  106 

Figure 4.3: Schematic diagram of the Low-Voltage Regulation and Relay Driver 

circuits. 
110 

Figure 4.4: Output Stage: Filtering, Sensing, and Grid Interface 117 
Figure 4.5: Schematic diagram of the isolated gate driver circuit utilizing UCC21520.  117 

Figure 4.6: Example of Switch-Node Undershoot 122 
Figure 4.7: Example of Bootstrap Overcharge vs. Resistance 123 
Figure 4.8: Parasitic Inductance 123 
Figure 4.9: Start-up timing diagram showing UVLO release delay. 125 
Figure 4.10: Estimated start-up delay as a function of bootstrap resistance and 
capacitance. 

125 

Figure 4.11: Schematic implementation of the LCL output filter. 131 
Figure 4.12: Inductor design parameters from Micrometals Analyzer. 132 
Figure 4.13: Inductance saturation curve vs. DC current. 133 
Figure 4.14: TINA simulation model for resonance validation. 133 
Figure 4.15: Schematic of the dual-relay grid interface circuit. 134 



7 

Figure 4.16: Schematic of the grid interface, sensing, and EMI suppression circuit.  135 
Figure 4.17: Schematic diagram of the Control Unit based on STM32L475 MCU. 138 
Figure 4.18: Clock Tree Configuration ensuring a stable system frequency derived 
from the external crystal. 

139 

Figure 4.19: STM32CubeMX Peripheral Configuration interface used to enable device 
resources. 

140 

Figure 4.20: STM32CubeMX Pinout Configuration showing the assignment of PWM, 
Sensing, and GPIO pins. 

141 

Figure 5.1: Conceptual control architecture and signal flow of the Grid-Following VSI. 144 

Figure 5.2: LTspice schematic diagram of the complete closed-loop grid-tied inverter 

system. 
145 

Figure 5.3: Grid voltage waveform 𝑣𝑔(𝑡) showing a pure sinusoidal profile. 146 

Figure 5.4: Grid current reference waveform synchronized with grid voltage for unity power 

factor operation. 
147 

Figure 5.5: Measured grid current waveform after the LC filter in closed-loop operation. 147 

Figure 5.6: Output voltage of the PI current controller in closed-loop operation. 149 

Figure 5.7: High-frequency triangular carrier waveform at 100 kHz. 150 

Figure 5.8: Comparison between the reference (modulating) signal and the triangular carrier 

waveform. 
151 

Figure 5.9: Resulting sinusoidal pulse-width modulation (SPWM) signal. 151 

Figure 5.10: Complementary gate signals for inverter legs A and B. 152 

Figure 5.11: Inverter output current before and after LC filtering. 153 

Figure 5.12: Grid voltage and injected grid current showing phase alignment (unity power 

factor operation). 
154 

Figure 5.13: Harmonic spectrum (FFT) of the injected grid current. 154 

Figure 6.0: Preliminary PCB Layout Design of the Control and Power Stage. 156 

Figure 6.1: Top-side PCB assembly showing the digital control unit, LCL output filter, and 

protection relays. 
157 

Figure 6.2: Bottom-side PCB layout featuring the full-bridge power stage MOSFETs and 

current sensing shunt resistors. 
157 

Figure 6.3: Close-up view of power MOSFETs with thermal pads 158 

Figure 6.4: The ST-LINK/V2 debugger/programmer used for flashing the STM32 

microcontroller. 
160 

Figure 6.5: Macro view of the PCB programming header showing the SWD pinout 

assignments (3.3V, SWCLK, GND, SWDIO). 
161 

Figure 6.6: Physical connection of the ST-LINK/V2 to the Main Inverter Board using the 

Serial Wire Debug (SWD) interface. 
162 

Figure 6.7: Close-up of the custom cable adapter connecting the ST-LINK/V2 to the board.  162 

Figure 6.8: Top-view of the High-Frequency DC-DC Step-Up (Boost) Converter board. 163 

Figure 6.9: Top view of the prototype full-wave bridge rectifier and DC-link capacitor 

board. 
164 

Figure 6.10: Top view of the XL4015 CC/CV Step-Down (Buck) Converter module. 165 

Figure 6.11: Switched-Mode Power Supply (SMPS) unit with active cooling system. 166 

Figure 7.1: The experimental setup showing the HAMEG HM407 Oscilloscope, DC Power 

Supply, and the Inverter PCB (DUT). 
167 

Figure 7.2: STM32L475RETx Pinout configuration showing the designated PWM output 

pins (PA8, PA9, PA7, PB0). 
168 

Figure 7.3: Physical experimental setup showing the connections between the laptop, Main 

Inverter Board, and oscilloscope during the control signal verification process. 
169 

Figure 7.4: Waveforms of the control signals measured using the GW Instek GDS-2104E 

Digital Storage Oscilloscope. 
169 

Figure 8.1: Control Block Diagram of a Grid-Following Inverter with Volt-Var Capability 172 



8 

Abstract 

This project addresses the critical challenge of maintaining power grid stability under the 

pressures of intermittent renewable energy integration and dynamic load demands. The primary 

objective is to design, simulate, and implement a single-phase grid-tied inverter capable of 

controlling active and reactive power to mitigate overvoltage issues. The system features a full-

bridge topology controlled by an STM32 microcontroller, employing Sinusoidal Pulse Width 

Modulation (SPWM) and a closed-loop current control strategy. 

A real-world case study using CYME software identified critical overvoltage nodes, validating 

the necessity of smart inverter functions such as Q(V) control. The control logic was verified 

through LTspice simulations, confirming unity power factor operation and low harmonic 

distortion. Practically, a hardware prototype was assembled, featuring isolated sensing and gate 

drive circuits. Experimental results verified the microcontroller's ability to generate precise, 

synchronized control signals, laying the foundation for a robust smart inverter capable of 

enhancing grid resilience. 

 

Keywords: Grid-Tied Inverter, Power Grid Stability, Active and Reactive Power Control, 

Overvoltage Mitigation, STM32 Microcontroller, CYME Simulation, LTspice. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



9 

 الملخص

الكهربائية في ظل الضغوط الناتجة عن  يعالج هذا المشروع التحدي الجوهري المتمثل في الحفاظ على استقرار شبكة الطاقة 
دمج مصادر الطاقة المتجددة المتقطعة ومتطلبات األحمال الديناميكية المتغيرة. الهدف األساسي للمشروع هو تصميم ومحاكاة  

قادر على التحكم في القدرة الفعالة  (Single-phase Grid-tied Inverter) وتنفيذ عاكس أحادي الطور متصل بالشبكة
 (Full-bridge) وبولوجيا القنطرة الكاملةتيتميز النظام .  (Overvoltage)وغير الفعالة للتخفيف من مشاكل ارتفاع الجهد

يتم التحكم فيها بواسطة متحكم دقيق من نوع النبضة الجيبيSTM32  التي    (SPWM) ، باستخدام تقنية تعديل عرض 
 .(Closed-loop current control) واستراتيجية تحكم في التيار مغلقة الحلقة

لتحديد نقاط ارتفاع الجهد الحرجة في الشبكة، مما أكد ضرورة استخدام   CYME تم إجراء دراسة حالة واقعية باستخدام برنامج
تم التحقق من منطق التحكم من خالل  .Q(V) وظائف العاكس الذكي مثل التحكم في الجهد عن طريق القدرة غير الفعالة

مع انخفاض  (.Unity P.F) ، والتي أكدت تشغيل النظام عند معامل قدرة موحد LTspiceعمليات المحاكاة باستخدام برنامج
لفانياً.  جفي التشوه التوافقي. من الناحية العملية، تم تجميع نموذج أولي لألجهزة يتضمن دوائر استشعار وقيادة بوابة معزولة 

وقد أثبتت النتائج التجريبية قدرة المتحكم الدقيق على توليد إشارات تحكم دقيقة ومتزامنة مع الشبكة، مما يرسخ األساس لبناء 
 .عاكس ذكي قوي قادر على تعزيز مرونة الشبكة الكهربائية

 

العاكس المتصل بالشبكة، استقرار شبكة الطاقة، التحكم في الطاقة الفعالة وغير الفعالة، التخفيف من الكلمات المفتاحية  :
 .LTspice، محاكاة CYME، محاكاة STM32الجهد الزائد، وحدة التحكم الدقيقة 

 

 

 

 

 

 

 

 

 

 

 

 



10 

Chapter 1: Introduction 

1.1 Background 

   In modern societies, the electrical power grid is one of the most important infrastructures that 

are required to provide continuous and stable energy supply to the consumers. The safety and 

efficiency of the electrical system depended on the stability of the power grid under varying 

generation and load conditions. Initially, power generation was centralized and predictable; 

with greater penetration of renewables, mainly solar energy, newer challenges for grid stability 

emerged. 

   Due to the weather conditions and varying availability of sunlight, solar energy systems are 

variable in nature. These variations result in unpredictable power injections into the grid, 

thereby creating imbalances between generation and consumption. One of the major problems 

is the increase in voltage levels across distribution networks during excessive generation that 

can severely damage electrical equipment and undermine the grid's reliability. 

   To address the problems, grid tied inverters have evolved to be crucial in power setups today. 

The main functions of grid-tied inverters are to convert the DC power supplied by the solar 

panels into AC power compatible with the grid and actively interface with the grid by 

controlling actual power, reactive power, and power factor. Using these features, grid-tied 

inverters should manage the voltage, power balance, and support the network's overall stability. 

   The project presents a design and implementation of a grid-tied inverter able to stabilize the 

grid under dynamic load and generation conditions. By the employment of advanced PWM 

techniques, the inverter adjusts the output to control excess generation, keep voltage levels 

within safe limits, and protect consumer loads as well as itself. Thus, the development of the 

grid becomes more robust and adaptive toward the seamless integration of renewable energy 

sources, without compromising its stability. 

 

1.2 Problem Description 

   The stability and dependability of the grid are facing serious challenges as a result of the 

growing integration of renewable energy sources, especially photovoltaic solar systems, into 

the electrical distribution network. When the amount of power injected into the grid surpasses 

the demand for local consumption one of the most serious problems occurs. Unusual voltage 

increases across distribution lines, especially at the end-user side, are caused by this imbalance. 

Overvoltage situations can seriously harm electrical appliances, shorten equipment lifespans, 

and compromise the grid's ability to function. 

 

   Furthermore, the dynamic fluctuations brought about by distributed generation sources are 

frequently too great for traditional protection schemes to handle. By continuously injecting 

power without take into account the current grid conditions, improperly controlled grid-tied 

inverters can make these instabilities worse. The networks voltage profile is impacted, and 

overvoltage or overcurrent events may cause inverter protective shutdowns, disrupting power 

flow and lowering system resilience overall. 



11 

   The development of a grid-tied inverter system that can dynamically control real and reactive 

power based on the current grid state is therefore urgently needed. In order to guarantee 

dependable and continuous operation, such a system would actively participate in voltage 

regulation, keep the power factor within reasonable bounds, and include self-protection 

mechanisms. In order to safely and effectively integrate renewable energy into contemporary 

power systems while safeguarding consumer loads and preserving grid stability overall, these 

issues must be resolved. 

 

1.3 Project Goals 

   Designing, implementing, and assessing a grid-tied inverter system that can improve the 

electrical grid's performance and stability under various load and generation scenarios is the 

main goal of this project. In addition to supplying electricity to the grid, the inverter must 

actively support grid protection, reactive power compensation, and voltage regulation. 

 

The project specifically aims to accomplish the following objectives:  

1. Design and Construction of a Grid-Tied Single-Phase Inverter. 

2. Create a workable inverter system with dynamic control, protection, and 

dependability that can replicate actual grid situations. 

3. Putting the PWM-Based Control Strategy into Practices. 

4. To ensure stable operation, use Pulse Width Modulation (PWM) techniques to 

precisely control the inverter's output in terms of real and reactive power. 

5. Protect consumer appliances and preserve power quality by actively controlling 

the inverter's contribution to the grid to avoiding hazardous voltage increases in the 

distribution loads. 

6. Surplus Energy Management: Create control systems that keep the grid stable 

during periods of, excess power generation, preventing network outages or inverter 

shutdowns. 

7. Improving System Resilience: Put protection circuits in place to guard agains 

inverter damage and guarantee uninterrupted operation even in the event of 

unfavorable grid events. 

 

1.4 Importance of the Study  

   This study is important, because of the challenges faced by modern electrical grids due to a 

faster integration of renewable energy resources.  

cause of increasing penetration of renewables, the traditional top-down generation model stilts 

down to the distributed and dynamic systems. With these new complexities arise in the 

operation, particularly concerning voltage control, load balancing, and protection the system. 

 

 



12 

In the absence of active real and reactive power control in the interconnection points, grids can 

have: 

1. High Voltage Events: causing failures of the devices and disruptions of the power supply. 

2. Reduced Power Factor: Inefficient operation of the grid and presence of greater losses. 

3. Instability and Protection Tripping: prominent disconnections and issues in reliability. 

 

   the project directly addresses those counteracting factors, providing a low-cost, scalable, and 

adaptable method for control via grid tied inverters.  

Stabilizing the voltage magnitude and restoring the reactive power support would protect 

consumer equipment and provide some measure of reactive power support needed by the 

stronger grid to accommodate fluctuating renewable generation. 

 

 
Figure 1.1: voltage profile with and without inverter control 

 

 

Table 1.1: Comparison Between Traditional and Controlled Grid Conditions 

Aspect 
Grid with Controlled 

Inverter 

Traditional Grids 

without control 

Voltage Fluctuations Low and regulated High and Unstable 

Equipment Protection Strong Poor 

Renewable Energy 

Accommodation 

Enhanced Limited 

Power Factor High and optimized Low 



13 

1.5 Project Scope 

    The goal of this project is to design, implement, and test a single-phase grid-tied inverter 

that will stabilize grid performance in the event of surplus generation and dynamic loading. 

During grid disruptions, a main focus is on inverter self-protection, reactive power 

compensation, and voltage regulation. 

 

The following tasks are included in the project's scope: 

▪ Design and Assembly: Creation of a workable single-phase inverter system that can 

communicate with a grid simulator that operates at low voltage. 

▪ Implementation of Control: Using (PWM) techniques, real and reactive power are 

dynamically controlled according to grid conditions. 

▪ Monitoring voltage and power quality: involves measuring and assessing the 

inverter's capacity to control voltage and keep power factor within reasonable bounds 

under various load conditions.  

▪ Hardware protection: use of fundamental safeguards to prevent the inverter from 

disconnecting or failing due to overvoltage or overcurrent situations. 

however, the project is purposefully constrained in a number of ways to align with the 

academic framework and the resources that are available: 

▪ Only One Phase: While commercial implementations frequently use three-phase 

systems, the project is limited to a single-phase system. 

▪ Laboratory Environment: Rather than being deployed in the real world on an 

operational distribution network, testing and evaluation are carried out in a controlled 

laboratory setting using simulated loads and generation profiles. 

▪ Energy Storage Exclusion: While future research recommends using energy storage 

devices (like batteries), the design and integration of storage systems are not included 

in the current project scopes. 

 

Note: This project is not designed to support advanced features like dynamic frequency control, 

grid synchronization under islanding conditions, or large-scale multi-inverter coordination. 

 

1.6 Organization of the Report 

From the initial background and problem identification to the final results and 

recommendations, the report is organized into eight main chapters and a technical appendix, 

detailed as follows: 

Chapter 1: Introduction 

This chapter provides an overview of the project's background, problem identification, 

objectives, significance of the study, project scope, and the report structure. 

 

Chapter 2: Literature Review and Theoretical Background 

This chapter establishes the theoretical foundation by reviewing grid stability challenges 

caused by renewable energy and dynamic loads. It compares solar system types (on-grid, off-

grid, hybrid) and inverter technologies, identifying the research gap regarding the combined 

impact of fluctuating generation and dynamic loads. 



14 

Chapter 3: Methodology and System Design 

This chapter details the system design specifications and control strategies, including PWM 

techniques and smart inverter functions (P(V)/Q(V)). It presents a case study using CYME 

software to simulate real-world grid conditions, identifying overvoltage issues and validating 

the proposed control logic. 

 

Chapter 4: Electrical Circuit Description and Functional Analysis 

This chapter presents the comprehensive hardware design. It details the schematics of critical 

subsystems, including the isolated auxiliary power supply, the STM32-based digital control 

unit, the galvanically isolated gate drivers, the full-bridge power stage, and the LCL output 

filter design. 

 

Chapter 5: Control Loop Design and Simulation 

This chapter details the development and verification of the closed-loop control system using 

a Model-Based Design (MBD) approach in LTspice. It validates the PI compensator design, 

SPWM generation, and grid synchronization logic through time-domain simulations and FFT 

analysis. 

 

Chapter 6: Hardware Implementation and Component Description 

This chapter describes the physical realization of the project. It covers the PCB layout and 

assembly of the main inverter board, along with auxiliary hardware modules such as the high-

frequency DC-DC boost converter, bridge rectifier, and programming interfaces. 

 

Chapter 7: Testing and Measurement 

This chapter presents the experimental validation of the system. It describes the laboratory test 

bench setup and analyzes the control signals generated by the STM32 microcontroller to verify 

frequency, dead-time, and stability. 

 

Chapter 8: Conclusion and Future Work 

This chapter summarizes the project's achievements, discusses the challenges encountered 

during hardware assembly and simulation, and outlines future developments, specifically the 

implementation of the full Volt-Var control loop. 

 

Appendix A: Firmware Architecture and Control Logic 

This appendix provides a detailed analysis of the embedded software. It explains the code 

structure within STM32CubeIDE, peripheral configuration, and the algorithms used for SOGI-

PLL, PR controller, and DFSDM sensing. 

 

 

 

 

 

 

 



15 

Chapter 2: Literature Review and Theoretical Background  

   Literature Review This chapter provides an overview of studies and technologies that have 

been done in the past that are pertinent to improving the power grid's performance and stability 

under changing power generation scenarios influenced by dynamic loads. It discusses the 

difficulties in integrating renewable energy sources (RES), the critical role that grid-tied 

inverters play in this integration, and the different control strategies used for voltage, 

frequency, and power flow regulation to improve grid stability and power quality. This review 

lays the groundwork for the project's methodology. 

 

2.1 Overview of Power Grids and Stability Challenges 

   Modern power grids face increasing complexity due to changing generation and load 

characteristics. Grid stability is the grid's capacity to maintain equilibrium after a disturbance.  

Large, synchronous generators, which offered inherent inertia and stability support, were 

historically a major component of power grids. However, there are a number of difficulties 

associated with the integration of variable renewable energy sources and the trend towards 

distributed generation (DG). Stable voltages, frequency, and power flow across the system are 

the main concerns.network Variable power output is a result of renewable energy sources like 

solar and wind be intermittent and weather-dependent. This variability raises concerns about 

the stability and security of the utilize grid and can result in anomalies and voltage disruptions. 

Voltage instabilities can cause harmful, especially if they come from dispersed sources. 

transmission assets. Furthermore, it is thought that the current smart grid capabilities, such as 

communication and controls, are insufficient to manage the effects of integrating large amounts 

of solar and wind energy into the power system. 

 

 
Figure2.1: Comparison Between Traditional and Modern Power Grids 

 



16 

Table 2.1: Energy Sources Comparison 

Property Conventional (e.g. Gas) Solar/Wind 

Stability  High  Low 

 Availability High Variable 

Environmental Impact High CO2 Low CO2 

Cost Trend Stable  Decreasing 

Inertia High None 

Predictability  High  Low 

 

 

   Another significant issue with smart grid technology is the inconsistent nature of loades. 

Another issue affecting the grid's condition is a lack of safe, dependable communications 

infrastructure. New control strategies are needed for grid-connected systems to operate 

effectively in these circumstances in order to achieve consistency and dependability in the 

energy produced by renewable sources. Because energy flow fluctuations are periodic, it is also 

essential to incorporate monitoring and control. 

 

2.2 Impact of Renewable Energy Integration 

   The increase in the penetration of PV systems has come as a results of increasing interest  in 

solar energy and simultaneous decreases in PV system costs, among other. Distributed 

generators is increasingly utilizing renewable energy sources. As it discussed in the previous 

section, among the other concerns, the power output of these sources is not reliable. Due to the 

intermittent nature of RES, many issues versus power quality and stability arose in the utility 

grid, mainly in the presence of a growing number of grid-connected solar PV inverters. The 

variation in power output from solar and wind power generation can translate directly into 

irregularities and voltage disturbances. The system needs better abilities than those available 

today with smart grid communication and controls to integrate solar and wind energy in very 

high amounts. The future grid objective is the significant increase of RES integration. The rapid 

integration is promoted due to cost improvement and supportive policy together with global 

concern on extinction of fossil fuels and environmental pollution. But it has to tackle their 

variable nature and ensure grid system stability and dependability. 

 
Figure 2.2: Solar Power Generation Over a Day 

 



17 

Table 2.2: PV Inverters Types 

Inverter Type Stages cost size Control flexibility 

Single-Stage 1 Lower Compact Moderate 

Multi-Stage 2 Higher Larger High 

 Transformer-Based N/A Higher Large Low 

Transformerless N/A Lower Compact  High 

 

 

2.3 An Examination of Solar PV Systems: On-Grid, Off-Grid, and Hybrid 

2.3.1 ON-grid solar power systems 

 
Figure 2.3: Solar Cell System On Grid Type 

Grid-connected solar power systems, also known as photovoltaic (PV) systems tied to a grid, 

are a crucial part of modern electrical systems, including transmission systems, power stations, 

and small-scale standalone three-phase inverters for residential applications (Mnati, 2018). 

They represent a growing application in renewable energy due to the increasing demand for 

high-quality and sustainable energy (Kiriakos, 2019). 

 

Key Components and Operation: 

▪ PV Panels: These panels convert sunlight into DC electrical power (Adekola, 2015) & (Lu, 

2015). The output current and voltage of a solar panel are influenced by factors like light-

generated current, reverse saturation current of the diode, irradiance, and temperature 

(Adekola, 2015) & (Lu, 2015). 



18 

▪ Micro-inverters and Inverters: These devices are fundamental for grid connection, 

converting the DC power from PV panels into AC power that can be fed into the 

electrical grid (Mnati, 2018).  

• Micro-inverters are typically composed of two stages: a DC-DC converter and a 

full-bridge inverter (Gulbahce, 2022). They are common in residential solar and 

plug-in electric vehicle applications. 

• Three-phase inverters are often used for connecting PV systems to the low-

voltage distribution grid (Mnati, 2018). The simulation setup for such a system 

often utilizes components like SiC MOSFET (e.g., CF10120D) transistors, 

operating at a switching frequency of 25 kHz, with a capacity of 5 kVA and 400 

VLL (Mnati, 2018). 

▪ Output Filters: An LCL filter is commonly used in grid-connected inverters to 

minimize harmonic content and ensure the quality of the injected current (Gulbahce, 

2022). The design parameters for LCL filters include output voltage and power, input 

voltage, grid frequency, switching frequency, and input current (Gulbahce, 2022). 

Some designs also incorporate an LC filter (Jiao, 2017). 

▪ Grid Connection: The inverter connects to the grid, which typically consists of three-

phase lines that need to be synchronized with the inverter's output (Wang, 2016). 

Control Strategies for Grid-Connected Inverters: Effective control is essential for stable 

and efficient operation of grid-connected inverters. 

▪ PID Current Control: Proportional-Integral-Derivative (PID) current control is widely 

applied, especially in three-phase photovoltaic inverters connected to the grid (Mnati, 

2018). This control is continuously tuned for the 120-degree bus clamp (BC) pulse width 

modulation (PWM) scheme (Mnati, 2018). The simulation setup for PID current control 

often uses specific parameters, such as Kp=3.5, KI=3.5, and KD=0 in MATLAB (Mnati, 

2018). 

 

• PWM Techniques:  

− Pulse Width Modulation (PWM) is fundamental for generating the switching 

pulses that control the inverter output (Murthi, 2007). 

− 120-degree Bus Clamp PWM (BC-PWM) is a specific operating type that 

helps reduce switching losses in three-phase inverters (Mnati, 2018). Simulation 

results show that using 120-degree BC-PWM can achieve acceptable Total 

Harmonic Distortion (THD) levels, often below 2.333% (Mnati, 2018). 

− Sinusoidal PWM (SPWM) is used to minimize harmonic content (Gulbahce, 

2022). It involves comparing a sinusoidal reference signal with a high-

frequency triangular carrier signal (Murthi, 2007). 

− Space-Vector Modulation (SV PWM) is a digital modulation strategy that can 

increase the amplitude of the output voltage fundamental and shift harmonics to 

higher frequencies (Mohan et al., 2003). 



19 

− Modified Bipolar PWM (MBPWM) is another scheme used, with simulation 

results showing its performance with and without zero-sequence voltage 

(Chapter 2). 

• Maximum Power Point Tracking (MPPT): This algorithm optimizes the power 

extracted from the PV panel under varying conditions of temperature and irradiance 

(Adekola, 2015). Methods like the modified Perturb & Observe (P&O) are studied 

for MPPT (Tan & Thang, 2018). 

• Synchronous Reference Frame (SRF) Control (dq-transformation): This 

technique transforms three-phase AC signals into DC signals (dq-signals) in a 

rotating reference frame, simplifying current control (Wang, 2016). PI controllers 

are typically associated with this control structure (Wang, 2016). The d-axis is 

usually aligned with the grid voltage vector (Wang, 2016). 

• Proportional-Resonant (PR) Controllers and Harmonic Compensators (HC): 

PR controllers are effective for grid-connected inverters, especially for controlling 

the current (Teodorescu et al., 2006). They can be combined with Harmonic 

Compensators (HC) to further reduce harmonic distortion (Teodorescu et al., 2006). 

• Voltage and Current Loops: Grid-connected inverter control schemes often 

feature cascaded control loops, including an outer voltage loop and an inner current 

loop (Teodorescu et al., 2006). The current loop controls the power injected into the 

grid, while the voltage loop regulates the DC-link voltage (Mohammad Alsemaan, 

2016). 

 

▪ Applications and Purpose 

• The primary purpose of on-grid solar power systems is to inject power from 

renewable energy sources directly into the utility grid (Sarkar, 2015). 

• They are widely implemented in various settings, including residential, 

commercial, and large-scale utility solar farms (Adekola, 2015). 

• Driven by concerns about climate change and decreasing PV system costs, grid-

connected solar applications have seen substantial growth, accounting for 99% of 

net installed PV capacity in the market (Adekola, 2015) & (Verma, 2019). 

• In these systems, the utility grid inherently acts as a vast energy storage solution 

for any excess electricity generated, eliminating the need for local batteries  

(Adekola, 2015), (Lu, 2015), (Gulbahce, 2022) & (Haider, 2021). 

 

Advantages 

• Cost-effectiveness: Generally entail significantly lower costs compared to off-grid 

systems because they do not require expensive battery storage (Verma, 2019), 

(Gulbahce, 2022) & (Haider, 2021). 

• High Efficiency: Known for their high efficiency, as power generated is directly 

fed into the grid (Mohammad Alsemaan, 2016), (Adekola, 2015), (Lu, 2015),  & 



20 

(Rodrigues, 2019). Three-phase grid-tie inverters are particularly efficient and 

dominate the market (Rodrigues, 2019). 

• Reliability and Flexibility: The grid provides a reliable backup power source, 

effectively mitigating the intermittency of solar energy (Verma, 2019), (Haider, 

2021) & (Gulbahce, 2022). They also support bidirectional power flow, allowing 

users to sell excess electricity to the grid and draw power when their generation is 

insufficient (Lu, 2015) & (Adekola, 2015). 

• Environmental Benefits: Contribute to reducing greenhouse gas emissions (Ojo, 

2022) and have a lower carbon footprint over their operational lifespan (Kiriakos, 

2019) & (Ojo, 2022). 

• Power Quality Improvement: Can actively support the grid by providing 

harmonic current and reactive power to connected loads (Rodrigues, 2019). Smart 

inverters are designed to improve power quality (Adekola, 2015). 

 

Disadvantages and Challenges 

• Grid Dependency and Anti-Islanding: A crucial safety requirement is that the 

inverter must disconnect from the grid during power outages (known as anti-

islanding) (Adekola, 2015), (Alsemaan, 2016) & (Haider, 2021). This means that 

power is not available to the consumer during a grid failure, even if the sun is 

shining. 

• Intermittency of RES: Despite being connected to the grid, the inherent variability 

of renewable energy sources (e.g., due to weather or seasonal changes) can still pose 

challenges for grid stability and control (Ojo, 2022) & (Adekola, 2015). 

• Synchronization Complexity: Achieving and maintaining precise synchronization 

with the grid's voltage and frequency can be a complex technical challenge 

(Mohammad Alsemaan, 2016), (Wang, 2016), (Lu, 2015) & (Haider, 2021). 

• Power Quality Issues: If not properly filtered, power electronic devices in inverters 

can inject undesirable harmonics into the grid, necessitating compliance with strict 

grid standards (Sarkar, 2015), (Hassaine & Bengourina, 2019), (Hassaine & 

Bengourina, 2020), (Adekola, 2015) & (Zong, 2011). 

• System Control Challenges: The distributed nature of solar generation can create 

complex system control issues for grid operators (Adekola, 2015). There is also a 

recognized need for secure and reliable communication infrastructure for effective 

grid management (Adekola, 2015). 

• Inverter Lifespan: Inverters are often considered the weakest components in PV 

systems, with a typical lifespan of 5 to 15 years, considerably shorter than that of 

PV panels (around 25 years). This contributes to higher operation and maintenance 

costs and energy losses due to downtime (Rodrigues, 2019). Power semiconductors 

and DC-link capacitors are common points of failure (Rodrigues, 2019). 

• Standards Compliance: Systems must adhere to numerous international and 

national standards (e.g., IEEE 1547, IEC 61727, German Grid Code) to ensure safe 

and reliable interconnection (Zong, 2011), (Mohammad Alsemaan, 2016), 

(Rodrigues, 2019), (Zammit et al., 2014) & (Kjær, 2005). 



21 

Efficiency and Losses: 

▪ A major objective for inverters is to increase efficiency by reducing power losses (Mnati, 

2018). Losses in power electronic circuits, such as three-phase inverters, primarily include 

switching losses and conduction losses (Gulbahce, 2022). 

▪ Switching losses are proportional to the switching frequency and transistor ratings (Mnati, 

2018). Conduction losses are related to the transistor's current and resistance (Gulbahce, 

2022). 

▪ The use of wide bandgap semiconductors like Silicon Carbide (SiC) MOSFETs and 

Gallium Nitride (GaN) HEMTs can significantly improve efficiency by reducing these 

losses, especially at high switching frequencies (Gulbahce, 2022). For instance, a SiC 

OSFET (CF10120D) transistor is used in one setup (Mnati, 2018). 

Simulation and Experimental Validation: 

▪ MATLAB/Simulink is widely used for building simulation setups and analyzing control 

systems for micro-inverters and three-phase inverters (Gulbahce, 2022). 

▪ Simulation results often include output waveforms like three-phase line current, voltage, 

and voltage to the negative DC link (Mnati, 2018). 

▪ Experimental setups are used to validate simulation results, measuring parameters like 

THD and observing waveforms (Gulbahce, 2022). Digital Signal Processors (DSPs) are 

frequently employed for implementing controllers in experimental systems (Adekola, 

2015). 

Considerations and Challenges: 

▪ Grid Codes: Inverters must comply with grid code requirements, including harmonic 

current limits (Rodrigues, 2019). 

▪ Time Delays: Digital control systems inherently introduce time delays, which must be 

considered in the design to maintain system stability (Wang, 2016). 

▪ Parasitic Inductance: Stray inductance in switching waveforms can affect performance, 

and balanced layouts can reduce parasitic inductance (Rodrigues, 2019). 

Overall, grid-connected solar power systems require sophisticated control strategies, efficient 

power electronics, and careful design validated through simulation and experimentation to 

ensure stable, reliable, and high-quality power injection into the electrical grid. 

 

 

 

 

 



22 

2.3.2 Off-grid solar power systems 

Off-grid solar power systems, also known as stand-alone systems, are designed to operate 

independently without any connection to the main electricity grid. 

 
Figure 2.4: Solar Cell System Off Grid Type 

Key Components and Operation: 

▪ PV Panels: Serve as the primary standalone source of electricity, especially in remote areas 

(Verma, 2019). They convert solar radiation into DC electricity (Verma, 2019). 

▪ Energy Storage System (ESS): Due to the intermittent nature of solar energy, an ESS, 

typically consisting of batteries, is a crucial component in off-grid systems to ensure the 

continuity of energy supply (Gulbahce, 2022). This is a significant distinction from grid-

connected systems, which generally do not require batteries (Haider, 2021). The energy 

generated during the day is stored in these batteries for use at night or during cloudy periods 

(Haider, 2021). 

▪ Charge Controller (MPPT charge controller): This device is essential for managing the 

energy that enters the battery bank from the solar array (Verma, 2019). It prevents the deep 

cycle batteries from being overcharged during the day and stops power from returning to 

the solar panels at night (Gerard, 2020). 

▪ Inverter (DC/AC Converter): The inverter converts the DC power generated by the PV 

panels and stored in batteries into AC power suitable for household and industrial 

appliances (Verma, 2019). In a stand-alone system, the inverter must generate a specific 

voltage with a proper frequency output to meet the load demand (Mohammad Alsemaan, 

2016). This contrasts with grid-tied inverters, where the voltage and frequency are already 

determined by the utility grid (Mohammad Alsemaan, 2016). Voltage control is typically 

adopted for stand-alone inverters to stabilize the microgrid voltage, often incorporating 

current protection and harmonic distortion control (Lu, 2015). 



23 

▪ Optional Backup: Automatic Backup Generators can be integrated to provide 

additional reliability during extended periods of low solar generation or high demand 

(Gerard, 2020). 

Applications and Purpose: Off-grid solar systems are particularly useful in situations where 

a functional electricity grid is unavailable or geographically distant (Verma, 2019). They are 

common in: 

▪ Remote areas: Providing power for isolated industrial operations, households, cabins, and 

recreational vehicles, especially in developing countries (Verma, 2019). 

▪ Specific applications: Such as telecommunication relay stations, water heaters, water 

pumps, emergency phones, and security systems (Verma, 2019) & (Adekola, 2015). 

▪ Microgrids in island mode: Remote off-grid microgrids are designed to operate 

permanently in an islanded mode due to economic or geographical constraints (Ojo, 2022) 

& (Kiriakos, 2019). These microgrids are traditionally energy self-sufficient (Kiriakos, 

2019). 

Advantages: 

▪ Energy Independence: Users are not reliant on the main power grid, meaning they are 

unaffected by power outages or blackouts (Haider, 2021). 

▪ Rural Electrification: They have been effective in providing electricity to rural areas and 

supporting solar home systems (SHSs) where grid installation is not feasible (Adekola, 

2015). 

▪ Cost Reduction in Remote Areas: Studies show that operating off-grid microgrids 

dominated by renewable sources can reduce the levelized cost of electricity production over 

the project's lifespan in remote or island areas (Kiriakos, 2019). 

▪ Improved Performance: Compared to other renewable sources, solar PV systems offer 

improved performance in remote locations due to the infinite availability of solar irradiation 

(Adekola, 2015). 

Disadvantages and Challenges: 

▪ Energy Storage Limitations: A significant challenge is the difficulty in storing 

generated solar electricity, which often translates to substantial running costs for the 

required energy storage systems (Adekola, 2015). 

▪ High Initial Investment: Off-grid systems typically require a high initial investment 

(Verma, 2019). 

▪ Reliance on Sunlight: Prolonged cloudy weather can negatively impact the energy output 

and availability, as the system relies solely on solar irradiation (Haider, 2021). 

▪ Control Complexity: Operating microgrids autonomously (off-grid) presents a complex 

control problem due to the absence of stabilizing inertia, which is naturally present in 

traditional large synchronous generators connected to the main grid (Ojo, 2022). New, 



24 

robust control strategies are required to regulate frequency and voltage, and ensure power 

sharing optimally for stable autonomous operations (Ojo, 2022). 

▪ Market Share: Grid-connected systems have gained more interest in the international 

market for PV technology and account for the majority (99%) of installations, while stand-

alone systems represent a smaller share (around 1%) (Verma, 2019), (Adekola, 2015). 

Efficiency and Losses 

▪ Storage-related Losses: The requirement for energy storage (batteries) introduces losses 

within the system, which contributes to higher overall running costs (Adekola, 2015). 

▪ Intermittency Impact: The system's complete reliance on solar energy means that 

prolonged cloudy weather can significantly reduce energy output (Haider, 2021). 

▪ Cost vs. Grid-tied: Generally, grid-connected electricity is more economical than off-

grid systems due to the substantial costs associated with off-grid components and storage 

solutions (Adekola, 2015). Grid-connected systems are recognized for their significantly 

reduced costs and higher efficiency, in part because they do not require batteries (Gulbahce, 

2022). 

Simulation and Experimental Validation 

▪ Simulation Tools: Software tools such as MATLAB Simulink can be utilized to model 

and simulate off-grid microgrid systems, assessing aspects like power sustainability and 

Total Harmonic Distortion (THD) (Kiriakos, 2019). PVSYST software is also employed 

for designing and simulating off-grid PV systems (Eshita et al., 2010). 

▪ Experimental Testing: The operation of inverters in grid isolation (off-grid) mode can be 

tested experimentally as a potential area for future research, contributing to the broader 

expansion of PV system applications across both operating modes (Verma, 2019). 

Considerations and Challenges 

▪ High Initial and Running Costs: Off-grid systems typically demand a high initial 

investment and incur substantial running costs, primarily due to the expense of energy 

storage solutions (Verma, 2019) & (Adekola, 2015). 

▪ System Complexity and Maintenance: The integration of batteries and, potentially, 

backup generators adds layers of complexity to the system design and increases 

maintenance requirements (Gerard, 2020). 

▪ Limited Capacity: These systems are usually designed for specific DC/AC electrical 

loads and may not be suitable for the large-scale power demands that grid-tied systems can 

support (Verma, 2019) & (Adekola, 2015). 

▪ Dependency on Solar Conditions: Their sole reliance on solar energy makes them 

vulnerable to extended periods of unfavorable weather conditions (Haider, 2021). 

▪ Geographical Suitability: They are most practical and cost-effective in remote areas 

where grid connection is not feasible or economically viable, or where grid 

independence is desired (Lu, 2015). Common applications include powering remote 



25 

houses, schools, hospitals, telecommunication stations, water heaters, and water pumps in 

rural or developing regions (Adekola, 2015). 

▪ Not Always Economical: They are not always the most economical choice when a utility 

grid connection is available, as grid-connected electricity is typically cheaper (Adekola, 

2015). 

2.3.3 Hybrid solar power systems 

Hybrid Solar Power Systems: An Overview of Types and Applications 

 
Figure 2.5: Solar Cell System Hybrid Type 

Introduction 

   The global energy landscape is undergoing a significant transformation, driven by the need 

to decarbonize power generation and enhance grid resilience. Within this transition, solar 

photovoltaic (PV) technology has emerged as a leading source of renewable energy. However, 

the intermittent nature of solar generation—dependent on daily and seasonal weather 

patterns—presents a fundamental challenge to its reliability and integration into the grid. A 

traditional grid-tied solar system ceases to operate during a power outage, leaving the owner 

without electricity despite having a functional PV array. Conversely, a purely off-grid system 

lacks the reliability and economic benefits of a grid connection. 

   Hybrid solar power systems have been developed to address these limitations by integrating 

solar PV generation with a Battery Energy Storage System (BESS) and a connection to the 

utility grid. These systems are managed by an intelligent hybrid inverter, which serves as the 

central controller, optimizing energy flow to achieve a triad of objectives: maximizing the 

consumption of self-generated energy, providing a reliable backup power supply, and 

intelligently interacting with the utility grid for economic benefit (Masters, 2013, p. 541). This 

paper explores the architecture, operational strategies, performance metrics, and diverse 

applications of modern hybrid solar power systems. 

 



26 

System Architecture and Core Components 

   A hybrid solar system’s effectiveness is defined by its architecture and the seamless 

integration of its components. There are two primary architectural configurations: DC-coupled 

and AC-coupled systems. 

▪ DC-Coupled Systems: In this configuration, both the solar PV array and the battery bank 

are connected on the direct current (DC) side of the hybrid inverter. Excess DC power from 

the solar panels can directly charge the battery through a charge controller without being 

converted to alternating current (AC). This architecture is highly efficient for new 

installations as it minimizes conversion losses (Badwawi, Abusara, & Mallick, 2015, p. 

129). 

▪ AC-Coupled Systems: This architecture is commonly used when retrofitting a battery 

system to an existing grid-tied solar installation. It involves a standard grid-tied PV inverter 

and a separate, bidirectional battery inverter. DC power from the solar panels is first 

converted to AC by the PV inverter. To charge the battery, this AC power must be 

converted back to DC by the battery inverter. While slightly less efficient due to multiple 

conversions, this approach offers greater modularity and flexibility. 

The core components of a typical hybrid system include: 

1. Solar PV Array: Converts sunlight into DC electricity. 

1. Hybrid Inverter: The system's brain. It houses the MPPT controller, the bidirectional 

DC/AC inverter, and the battery charge controller. 

2. Battery Energy Storage System (BESS): Stores excess solar energy. Lithium-ion 

batteries are now the dominant technology due to their high energy density, long cycle 

life, and falling costs (National Renewable Energy Laboratory [NREL], 2021, p. 

12). 

3. Energy Management System (EMS): A sophisticated software platform, usually 

integrated within the hybrid inverter, that executes control algorithms based on user 

settings, grid conditions, and electricity tariffs. 

4. Grid Connection and Automatic Transfer Switch (ATS): The physical connection 

to the utility grid, which includes an ATS to safely disconnect the system from the grid 

during an outage (a function known as anti-islanding) and establish a local microgrid. 

Operational Modes and Control Strategies 

The intelligence of a hybrid system lies in its ability to autonomously switch between various 

operational modes based on real-time data and pre-programmed control strategies. 

 

 



27 

Operational Modes: 

▪ Self-Consumption Mode: During daylight hours, solar generation first powers the 

home's loads directly. Any surplus energy is used to charge the battery. 

▪ Battery Charging Mode: If solar generation exceeds the load demand, the excess DC 

power is routed to the BESS until it reaches a full state of charge. The system may also 

charge the battery from the grid during off-peak hours if programmed to do so. 

▪ Battery Discharging Mode: During the evening or on cloudy days, when solar 

generation is insufficient to meet the load, the system draws stored energy from the 

battery. 

▪ Backup (Islanded) Mode: Upon detection of a grid failure, the ATS disconnects the 

system from the grid. The hybrid inverter then forms its own stable, localized AC grid, 

supplying power to critical home circuits from the BESS. 

Advanced Control Strategies: 

▪ Peak Shaving and Load Shifting: In regions with Time-of-Use (ToU) electricity pricing, 

the EMS can be configured to minimize costs. It discharges the battery to power the home 

during expensive "peak" hours and recharges it with low-cost solar energy or off-peak grid 

power. This strategy shifts the energy consumption pattern to a more economically 

favorable profile (Badwawi, Abusara, & Mallick, 2015, p. 130). 

▪ Grid Support and Virtual Power Plants (VPPs): Advanced hybrid systems can be 

aggregated and controlled by a utility or a third-party operator to form a VPP. As a 

collective, these systems can provide ancillary services to the grid, such as frequency 

regulation and voltage support, helping to stabilize the grid during periods of high demand 

or generation volatility. Homeowners are often financially compensated for participating 

in such programs (Navigant Research, 2019, p. 22). 

 

Types of Inverters within Hybrid Systems (Functionally)  While the sources do not 

explicitly categorize "hybrid inverters" as a distinct type in the same way they categorize 

Voltage Source Inverters (VSIs) or Current Source Inverters (CSIs), they describe inverter 

functionalities that enable hybrid operation: 

▪ Voltage Source Inverters (VSIs): These are commonly used in grid-connected PV 

systems and, when paired with appropriate control and storage, can support hybrid 

operation (Adekola, 2015) & (Verma, 2019). Their ability to act as a voltage source is vital 

for maintaining stable output in islanded microgrids. 

▪ Multi-Stage Configurations: Many PV inverter systems, particularly micro-inverters, 

employ a two-stage power processing architecture, typically involving a DC/DC converter 

followed by a DC/AC inverter (Tan & Thang, 2018), (Rodrigues, 2019). The DC/DC 

stage often performs Maximum Power Point Tracking (MPPT) to optimize energy 



28 

harvesting from PV panels (Tan & Thang, 2018), (Adekola, 2015), (Lu, 2015) & 

(Gulbahce, 2022). A battery can act as an energy buffer between these stages. 

Advantages and Challenges 

Advantages: 

▪ Energy Independence and Resilience: Provides an uninterruptible power supply (UPS) 

during grid outages, ensuring that critical loads remain operational. 

▪ Reduced Electricity Costs: Maximizes self-consumption of free solar energy and enables 

strategic avoidance of high-cost peak electricity tariffs. 

▪ Grid Stabilization: When deployed at scale, hybrid systems can reduce strain on the utility 

grid, defer the need for infrastructure upgrades, and enhance overall grid stability. 

Challenges: 

▪ Higher Initial Cost: The inclusion of a BESS and a more complex hybrid inverter makes 

the upfront investment significantly higher than for a standard grid-tied system. However, 

costs for both batteries and inverters continue to decline (NREL, 2021, p. 4). 

▪ Battery Lifespan and Degradation: Batteries have a finite lifespan, measured in cycles 

and years. Performance degrades over time, and replacement is a significant long-term cost. 

▪ System Complexity: Designing, installing, and commissioning a hybrid system requires 

specialized knowledge of power electronics, battery management, and electrical codes. 

▪ Regulatory Frameworks: The policies governing grid interconnection, net 

metering, and compensation for grid services vary widely and can be complex, 

impacting the economic viability of a project. 

Applications 

Hybrid solar systems are versatile and are being deployed across a wide range of applications: 

1. Residential Sector: The most common application, providing homeowners with backup 

power and lower electricity bills. 

2. Commercial & Industrial (C&I) Sector: Used by businesses to reduce peak demand 

charges, which can constitute a significant portion of their electricity costs. 

3. Remote and Off-Grid Communities: Provide reliable, 24/7 power to locations without 

access to a centralized electrical grid, such as rural villages, islands, and remote industrial 

sites. 

4. Critical Infrastructure: Ensure continuous power for essential services like hospitals, 

telecommunication towers, and data centers. 

Here is a comparison of On-Grid, Off-Grid, and Hybrid Photovoltaic (PV) systems, drawing 

on the provided sources: 



29 

 
Figure 2.6: Comparison Of On-Grid, Off-Grid, And Hybrid Photovoltaic (PV) Systems 



2.3.4 Comparison: On-Grid Systems vs. Off-Grid Systems vs. Hybrid Systems 

Table 2.3: Comparison: On-Grid Systems vs. Off-Grid Systems vs. Hybrid Systems 

Feature On-Grid Systems (Grid-Connected) Off-Grid Systems (Stand-

Alone/Autonomous) 

Hybrid Systems 

Definition All energy produced is directly fed to 

the utility grid (Verma, 2019). These 

systems are primarily designed for 

power injection into the grid 

(Gulbahce, 2022) & (Haider, 2021). 

Systems that function independently of 

the AC grid (Verma, 2019). They are not 

connected to the utility grid (Haider, 

2021) & (Gulbahce, 2022) and are 

designed to provide energy to specific 

DC/AC electrical loads (Verma, 2019). 

Remote microgrids operate continuously 

in an islanded mode (Ojo, 2022). 

Systems that can be utilized for both 

islanding (standalone) mode and 

grid-connected mode (Verma, 2019). 

These combine features of both on-

grid and off-grid systems. 

Battery/Storage Typically, no batteries are needed as 

the utility grid acts as the energy 

storage (Gulbahce, 2022) & (Lu, 

2015). While energy storage can be 

integrated, it is not a primary 

operational requirement (Adekola, 

2015). 

Requires an additional battery system 

(Gulbahce, 2022) & (Haider, 2021). The 

system is designed to produce energy 

throughout the day, which is then stored 

in these batteries (Gulbahce, 2022). 

Involves energy storage (batteries) 

to enable standalone operation 

(Verma, 2019). 

Cost 

Implications 

Costs have significantly reduced in 

recent years (Gulbahce, 2022). They 

offer economic benefits by offsetting 

homeowner's electricity use and 

generating profits from selling excess 

electricity back to the grid. 

While specific comparative costs aren't 

detailed, they are often the only solution 

in remote areas where grid access is 

unavailable (Lu, 2015). The high cost of 

energy storage systems can impact their 

overall feasibility if used as the primary 

load supply (Kiriakos, 2019). 

Dual functionality, they may involve 

a balance between grid-tied cost 

efficiencies and the investment in 

energy storage for standalone 

capability. 



31 

Inverter 

Role/Control 

Inverters are essential for DC/AC 

conversion to produce suitable AC 

voltage forms for the grid and daily 

use (Gulbahce, 2022). Their main 

tasks include:Injecting a sinusoidal 

current into the grid (Kjær, 2005). 

Optimizing the PV module's 

operating point (Kjær, 2005) & 

(Verma, 2019). Controlling active 

and reactive power output to reduce 

grid harmonics (Adekola, 2015). 

Employing Pulse Width Modulation 

(PWM) methods (Mnati, 2018). 

Utilizing current control schemes 

(e.g., PI, PR controllers) (Hassaine & 

Bengourina, 2019). Crucial grid 

synchronization (Haider, 2021). 

Voltage Source Inverters (VSI) are 

commonly used (Verma, 2019). Grid-

feeding inverters operate as 

power/current controlled sources, 

providing prespecified active and 

reactive power (Ojo, 2022). 

Inverters are required to directly feed 

power to local consumers when the grid 

is not connected (Lu, 2015). They 

must:Generate a specific voltage with 

appropriate frequency output to meet load 

demand (Mohammad Alsemaan, 2016). 

Be "grid-forming" by operating as ideal 

AC voltage sources with regulated 

voltage amplitude and frequency(Ojo, 

2022). Focus on frequency and voltage 

regulation, and power sharing (Ojo, 

2022). Can also use Proportional-

Resonant (PR) controllers for AC signal 

control (Ojo, 2022). 

Inverters must be capable of 

handling both grid-connected and 

islanded modes (Verma, 2019). 

This implies control strategies that 

can switch between grid-feeding and 

grid-forming functionalities, 

managing power flow and grid 

synchronization when connected, 

and maintaining stable voltage and 

frequency when isolated. 

Complexity Simpler, less components (no 

batteries) 

Moderate, requires battery management 

and charge controller 

Most complex, requires advanced 

control for dual operation and power 

management 

Grid Interaction Characterized by direct connection 

and interaction with the utility grid 

By definition, these systems do not 

interact with the utility grid (Haider, 

Designed to seamlessly connect and 

disconnect from the utility grid 



32 

(Gulbahce, 2022) & (Haider, 2021). 

Inverters synchronize their output 

voltage, frequency, and phase angle 

with the grid (Haider, 2021. Excess 

generated electricity is transferred to 

the network (Verma, 2019) & 

(Gulbahce, 2022). 

2021). They are designed to operate 

completely independently (Ojo, 2022). 

(Verma, 2019). They can supply 

power to the grid or draw from it, 

and also support local loads when 

disconnected from the main grid. 

Operation 

During Outages 

In the event of a grid power failure, 

on-grid systems typically shut off for 

safety reasons (Haider, 2021) & 

(Kjær, 2005). They are required to 

cease energizing the grid to prevent 

"islanding" (Kjær, 2005). 

Are designed to continue supplying 

power to loads from their stored battery 

energy during utility outages (Haider, 

2021) & (Kiriakos, 2019). They ensure 

continuity of energy supply even when 

the main grid is down. 

Can continue to operate in an 

"islanded" mode and supply power 

to connected loads if the grid fails 

(Verma, 2019). This offers energy 

independence during blackouts. 

Backup Power No backup power during grid 

outages (Haider, 2021) 

Provides power when grid is 

unavailable (Verma, 2019) 

Provides backup power during 

grid outages. 

Key Advantage Cost-effective, net metering benefits, 

no battery maintenance (Kiriakos, 

2019) & (Haider, 2021). 

Energy independence, ideal for remote 

areas (Verma, 2019). 

Versatility, reliability, energy 

independence, grid interaction 

[Previous response] 

Typical 

Applications 

Common in residential PV 

installations where excess power can 

be sold back to the grid. Also widely 

used in solar power plants 

(Gulbahce, 2022) and other renewable 

energy applications connected to low 

voltage grids (Mnati, 2018). 

Primarily suited for remote or rural 

areas lacking grid access (Lu, 2015) & 

(Gerard, 2020). Examples include cottage 

installations (Gerard, 2020) or situations 

where self-generated electricity is the sole 

power solution (Lu, 2015). 

Often implemented in microgrids 

(Kiriakos, 2019) where the system 

needs to support local loads, interact 

with the main grid, and also 

maintain operation during grid 

outages. 



2.3.5 Working Mechanisms of Solar Photovoltaic (PV) Systems 

1. On-Grid PV System (Grid-Tied) 

An on-grid PV system is designed to operate in parallel with the utility grid. It does not include 

a battery storage system, relying on the grid to act as a virtual battery for both supplying deficit 

power and absorbing surplus energy. 

Working Mechanism: 

1. Energy Generation: Solar photovoltaic (PV) panels capture sunlight and convert it 

into direct current (DC) electricity. 

2. DC to AC Conversion: The DC power is channeled to a grid-tie inverter. The 

inverter's primary role is to convert this DC power into alternating current (AC) and, 

crucially, to synchronize its output waveform (voltage, frequency, and phase) precisely 

with that of the utility grid (Teodorescu, Liserre, & Rodriguez, 2011, p. 15). 

3. Power Prioritization (Self-Consumption): The generated AC power is first directed 

to the property's main electrical panel to power any active household or commercial 

loads (e.g., lights, appliances). 

4. Exporting Surplus Energy: If the solar system produces more power than the property 

is consuming at that moment, the excess energy is automatically exported to the utility 

grid. A bidirectional net meter measures this outflow of energy, typically resulting in a 

credit on the owner's electricity bill (Masters, 2013, p. 540). 

5. Importing Power from the Grid: During periods of low or no solar generation, such 

as at night or on heavily overcast days, the property seamlessly draws the required 

power from the utility grid, just as a home without a solar system would. 

6. Grid Outage Scenario: In the event of a utility grid failure, the grid-tie inverter is 

mandated by safety standards (e.g., IEEE 1547) to immediately shut down. This safety 

feature, known as anti-islanding, prevents the PV system from energizing a dead grid, 

which would pose a severe electrocution risk to utility workers performing repairs 

(Teodorescu, Liserre, & Rodriguez, 2011, p. 145). Consequently, an on-grid system 

cannot provide backup power during an outage. 

2. Off-Grid PV System (Stand-Alone) 

An off-grid system operates entirely independently from the utility grid and relies on a battery 

bank to store energy for use when the sun is not shining. It is a self-sufficient power solution. 

Working Mechanism: 

1. Energy Generation: Solar panels produce DC electricity. 

2. Battery Charging and Management: The DC power is sent to a solar charge 

controller. The charge controller's critical function is to regulate the voltage and 

current flowing to the battery bank, preventing overcharging and deep discharging, 

thereby protecting the battery and extending its lifespan (Masters, 2013, p. 555). 



34 

3. Energy Storage: The regulated DC power is stored in a battery bank. The capacity of 

this bank determines how much energy is available to power loads during the night and 

on cloudy days. 

4. Power Inversion: A stand-alone inverter draws DC power from the battery bank. 

This inverter's function is to convert the DC power to a stable AC waveform at the 

required voltage and frequency (e.g., 230V, 50Hz or 120V, 60Hz) to power the 

property's loads. Unlike a grid-tie inverter, a stand-alone inverter acts as a "grid-

forming" device, creating its own independent electrical grid (Rashid, 2017, p. 308). 

5. Supplying Loads: The AC power produced by the inverter is distributed to the 

property's loads. 

6. Energy Limitation: The system's ability to supply power is finite and is dictated 

entirely by the current solar generation and the battery's state of charge. The system 

must be carefully sized to meet the expected load demand throughout the year. 

3. Hybrid PV System 

A hybrid system combines the features of both on-grid and off-grid systems. It is connected to 

the utility grid but also includes a battery bank, offering energy resilience, cost savings, and 

flexibility. 

Working Mechanism: 

1. Energy Generation and Management: DC electricity from the solar panels is fed into 

a hybrid inverter. This intelligent inverter acts as the central hub, managing the flow 

of power between the solar panels, the battery bank, the property's loads, and the utility 

grid (Masters, 2013, p. 541). 

2. Intelligent Power Flow Prioritization: The hybrid inverter operates based on a 

sophisticated hierarchy: 

a. Priority 1: Self-Consumption. Solar power is first used to directly power the 

home's loads. 

b. Priority 2: Battery Charging. Any excess solar power is then used to charge 

the battery bank. 

c. Priority 3: Grid Export. Only when the loads are met and the battery is fully 

charged is any remaining surplus power exported to the utility grid for credit. 

3. Operation with Depleted Solar: At night or during low-sun conditions, the system 

prioritizes drawing power from the stored energy in the battery. It will only begin 

importing power from the utility grid after the battery has been discharged to a pre-set 

reserve level (e.g., 20% state of charge). 

4. Grid Outage Scenario (Backup Function): This is the key advantage of a hybrid 

system. When a grid outage is detected, the hybrid inverter’s automatic transfer switch 

disconnects it from the grid (anti-islanding). The inverter then instantly switches to 

backup mode, drawing power from the battery to supply critical loads in the home, 

functioning exactly like an off-grid system (Teodorescu, Liserre, & Rodriguez, 2011, 

p. 27). This provides seamless, uninterruptible power. 



35 

2.4 A Comprehensive Review of Inverter Technologies for Photovoltaic Systems  

2.4.1 Grid-Tie Inverters (GTIs) 

Grid-tie inverters (GTIs), also known as line-tied or utility-interactive inverters, are power 

electronic devices that serve as an indispensable interface between distributed power 

generation systems (DPGS), such as solar photovoltaic (PV) systems, and the main electricity 

grid (Adekola, 2015), (Wang, 2016) & (Haider, 2021). Their primary function is to convert 

direct current (DC) electricity generated by sources like solar panels or batteries into 

alternating current (AC) electricity that is compatible with and can be fed into the utility grid 

(Haider, 2021), (Kjær, 2005) & (Verma, 2019). This allows users to push excess power into 

the mains socket (Masters, 2013, p. 495). 

1. Key Components A typical grid-connected PV inverter system comprises several key 

building blocks: 

▪ Photovoltaic (PV) Panels: These convert sunlight into DC electricity. 

▪ DC-DC Converter: Often employed as a first stage, especially in micro-inverters and 

string inverters, to boost the PV-array voltage to an appropriate level for the inverter 

or to a well-regulated DC link (Adekola, 2015), (Hassaine & Bengourina, 2020), 

(Gulbahce, 2022), (Jiao, 2017) & (Verma, 2019). Boost converters are crucial because 

standard grid-connected inverters often require DC-link voltages higher than typical 

PV voltages (Adekola, 2015) (Rashid, 2017, p. 158). 

▪ Inverter (DC/AC Converter): This is the core component that performs the DC-AC 

conversion (Verma, 2019), (Lu, 2015) & (Jiao, 2017). It uses fast switching devices 

like Insulated Gate Bipolar Transistors (IGBTs), MOSFETs, or thyristors (Jiao, 

2017), (Sarkar, 2015) & (Ojo, 2022). Two-level Pulse Width Modulation (PWM) 

voltage source inverters (VSIs) are a state-of-the-art topology for grid interaction 

(Wang, 2016). The inverter's output depends on the PWM signals on its switching gates 

(Haider, 2021) (Teodorescu, Liserre, & Rodriguez, 2011, p. 45). 

▪ Filters: To ensure the quality of the AC output and prevent high-frequency switching 

harmonics from entering the utility grid, filters are commonly adopted (Ojo, 2022), 

(Adekola, 2015), (Sarkar, 2015), (Gulbahce, 2022) & (Jiao, 2017). Common filter types 

include L-filters, LC-filters, and LCL-filters (Adekola, 2015), (Gulbahce, 2022), 

(Mnati, 2018), (Adekola, 2015), (Lu, 2015), (Jiao, 2017), (Haider, 2021), (Zong, 2011) 

. LCL filters are particularly noted for their better HF noise damping capabilities 

compared to L filters (Haider, 2021). The THD of output voltage and current is 

significantly reduced with an LCL filter (Gulbahce, 2022), (Teodorescu, Liserre,  

Rodriguez, 2011, p. 77). 

▪ DC-link Capacitor: Often an electrolytic capacitor, it acts as an energy buffer or 

power decoupling element between the PV module and the grid, especially crucial in 

single-phase systems to attenuate power ripple (Kjær, 2005) (Kjær, 2005). A film 

capacitor can also be used for the DC link (Mnati, 2018), (Rashid, 2017, p. 362). 



36 

▪ Isolation Transformer: May be placed between the inverter and the AC source for 

safety reasons and to provide isolation (Lu, 2015). However, transformerless PV 

inverters are also popular, designed to minimize leakage current (Verma, 2019). 

▪ Relay: A solid-state relay connects the inverter to the grid, closing when the inverter 

output is synchronized with the grid (Haider, 2021). 

2. Control Strategies and Operation The design and control of GTIs are complex due to the 

dynamic nature of power systems and the need for high-standard system capabilities to 

maintain stability and robustness (Lu, 2015). 

▪ Core Control Objectives: GTIs aim to:  

o Generate a sinusoidal output current with low Total Harmonic Distortion 

(THD) and unity power factor (Verma, 2019) (Haider, 2021) (Lu, 2015) (Hassaine 

& Bengourina, 2020) (Jiao, 2017). Regulations specify how clean an inverter's 

output must be, with a max allowable THD of 8% (IEEE Standard 1547-2018, p. 

45). 

o Synchronize with the grid voltage in both frequency and phase (Haider, 2021) 

(Lu, 2015) (Hassaine & Bengourina, 2020) (Wang, 2016) (Zong, 2011). A Phase-

Locked Loop (PLL) is a widely used technique for grid synchronization, allowing 

accurate detection of the grid phase signal (Jiao, 2017) (Zong, 2011) (Teodorescu 

et al., 2006) (Lu, 2015) (Adekola, 2015) (Wang, 2016) (Teodorescu, Liserre, & 

Rodriguez, 2011, p. 33). 

o Control active and reactive power output and flow into the grid (Lu, 2015) 

(Adekola, 2015) (Wang, 2016) (Sarkar, 2015) (Rodrigues, 2019) (Zong, 2011) 

o Regulate the DC-link voltage (Wang, 2016) (Lu, 2015). 

o Perform Maximum Power Point Tracking (MPPT) to maximize energy capture 

from the PV array under varying atmospheric conditions (Kjær, 2005) (Hassaine & 

Bengourina, 2019) (Alsemaan, 2016) (Verma, 2019) (Masters, 2013, p. 521). 

▪ Inverter Operation Control Types:  

o Current-Controlled Inverters: These are primarily used for injection into the grid 

and are not suitable for standalone applications (Passey et al., 2009) 1. They 

produce a sinusoidal current output by having their output stage switched to follow 

a sinusoidal reference waveform that is phase-locked to the grid (Passey et al., 2009) 

2. They do not degrade the quality of supply at the point of connection but also 

provide no harmonic improvement (Passey et al., 2009) 3. 

o Voltage-Controlled Inverters: These produce a sinusoidal voltage output and are 

capable of standalone operation (Passey et al., 2009). When connected to the grid, 

they must be connected via an inductance to prevent infinite current flow if their 

voltage or phase is not identical to the grid's (Passey et al., 2009) 4 & 5. They can 

be controlled in magnitude and phase to manage VARs and power (Passey et al., 

2009). Voltage-controlled inverters can improve waveform quality at the point of 

connection and absorb some harmonic current from the grid, reducing harmonics 

seen by the grid (Passey et al., 2009) (Masters, 2013, p. 501). 

 



37 

▪ Advanced Control Strategies:  

o Proportional-Integral (PI) and Proportional-Resonant (PR) Controllers: 

Widely used for current control and suitable for two-dimensional AC signals (Jiao, 

2017) (Teodorescu et al., 2006) (Ojo, 2022) & (Verma, 2019).  

o Hysteresis Control: Can be used to adjust inverter frequency to match the grid 

frequency, offering good current output control (Haider, 2021) (Kjær, 2005). 

o Feed-Forward and Disturbance Observer Methods: Used to compensate for 

disturbances from the grid side, such as grid voltage distortion and grid impedance 

uncertainty, improving control performance and harmonic attenuation (Wang, 

2016) (Jiao, 2017)  

o Droop Control: Explored for power sharing in microgrids (Ojo, 2022). 

o Anti-Islanding: A critical safety feature requiring the inverter to detect faults or 

blackouts in the utility grid and disconnect from the system (Adekola, 2015) 

(Haider, 2021) (Rodrigues, 2019). This prevents the inverter from unsynchronized 

operation that could pose safety risks (Adekola, 2015) (IEEE Standard 1547-2018, 

p. 31). 

3. Performance Metrics and Challenges 

▪ Harmonic Distortion: A significant concern is minimizing Total Harmonic Distortion 

(THD) in the injected current and voltage, as harmonics reduce the efficiency of 

connected devices (Adekola, 2015) (Hassaine & Bengourina, 2020), (Haider, 2021). 

Filters play a crucial role in achieving low THD (Gulbahce, 2022). 

▪ Grid Impedance: The grid impedance is an essential parameter that can significantly 

affect the control performance and stability of grid-connected inverters, potentially 

leading to harmonic resonance (Jiao, 2017). "Weak grids," where grid impedance is 

much larger than the inverter's filter inductance, pose particular challenges (Jiao, 2017) 

& (Teodorescu, Liserre, & Rodriguez, 2011, p. 189). 

▪ Grid Voltage Distortion: In reality, grid voltage is not always ideally sinusoidal and 

can be distorted, introducing additional harmonics into the system (Jiao, 2017). 

▪ Reliability and Efficiency: Smart inverters aim to improve the reliability and 

efficiency of the utility grid (Adekola, 2015). Overall efficiency and standby losses are 

important performance indicators for inverters (Kjær, 2005). 

▪ Standards Compliance: GTIs must comply with international standards, such as IEEE 

1547, which sets requirements for interconnection, including tripping and 

disconnection during grid instabilities (Adekola, 2015) (IEEE Standard 1547-2018, 

p. 1). 

4. Types and Applications GTIs come in various configurations tailored for different power 

levels and applications: 

▪ Microinverters (AC Module Inverters): Designed for a single PV module (typically 50-

350 W) and connect directly to the grid. They perform DC-AC conversion, voltage 

amplification, and MPPT at the module level, offering robustness to single module failures 



38 

and better power tracking (Bielskis et al., 2019) (Verma, 2019) (Hayman, 2009) (Alsemaan, 

2016) (Masters, 2013, p. 515). They are considered "plug and play" devices. 

▪ String Inverters: Connect multiple PV panels in series (forming a "string") to increase the 

DC voltage. They are suitable for medium-scale PV power generation systems (up to 5 

kW), offering more precise MPPT and higher efficiency compared to centralized inverters 

(Alsemaan, 2016) (Verma, 2019) (Rashid, 2017, p. 654). 

▪ Centralized Inverters: Used in high-power applications like solar power stations, 

connecting multiple solar strings in parallel (Alsemaan, 2016) (Mohammad Alsemaan, 

2016). 

▪ Applications: GTIs have wide applications in residential, commercial, and utility-scale 

solar installations (Zong, 2011) (Alsemaan, 2016) (Haider, 2021). They are crucial for 

Distributed Generation (DG) and are becoming essential components of the Smart Grid 

by improving stability, reliability, and power quality of the electricity supply (Adekola, 

2015). GTIs allow renewable energy sources to integrate into the grid and provide ancillary 

services like local voltage and frequency regulation (Wang, 2016). 

 

2.4.2 Stand-Alone Inverter (Off-Grid Solar Power Systems) 

A stand-alone inverter, operating within an off-grid solar power system, is designed to supply 

power to local loads, functioning entirely independently from the main utility grid. Unlike on-

grid systems, these systems do not connect to the public electricity network. 

Overview 

A stand-alone inverter is a fundamental part of many power electronic systems, especially in 

scenarios where a connection to the main grid is either unavailable or intentionally 

disconnected (Mnati, 2018) (Lu, 2015). These inverters operate in "off-grid" situations, serving 

as the sole power provider for local loads (Alsemaan, 2016) (Lu, 2015). They are particularly 

useful for generating self-sufficient electricity in remote areas (Lu, 2015) (Masters, 2013, p. 

552). 

In contrast to grid-connected inverters that primarily function as current sources, stand-alone 

inverters behave as a voltage source, tasked with maintaining stable voltage and frequency by 

actively controlling active and reactive power (Wang, 2016) (Passey et al., 2009) (Teodorescu, 

Liserre, & Rodriguez, 2011, p. 25). This means they must generate a specific voltage and 

frequency to match the load demands (Mohammad Alsemaan, 2016). Examples of their 

application include providing AC power from photovoltaic (PV) arrays or batteries, and serving 

as uninterruptible power supplies (UPS) (Adekola, 2015). The performance of stand-alone 

inverters is notably sensitive to factors such as their control schemes and the types of loads 

they serve (Lu, 2015). 

 



39 

Key Components 

The typical structure of a stand-alone inverter system involves several crucial components: 

▪ DC Energy Source: This could be a PV array (solar panels) or a battery, providing the 

initial DC power (Adekola, 2015), (Mohammad Alsemaan, 2016), (Eshita et al., 2010). 

▪ DC to AC Converter (Inverter): This is the core component, often a full-bridge inverter 

consisting of switching devices like IGBTs (Insulated-Gate Bipolar Transistors) or 

MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) (Adekola, 2015), 

(Gulbahce, 2022), (Mohan et al., 2003), (Murthi, 2007) (Rashid, 2017, p. 245). For higher 

power applications, three-phase Voltage Source Inverters (VSIs) are recommended. 

▪ DC Bus: A stable DC bus is essential to store and provide power to the inverter switches 

(Lu, 2015) . In two-stage micro-inverters, a low-powered battery can act as an energy 

bank between the DC/DC and DC/AC stages (Tan & Thang, 2018). 

▪ Filters: An LC (inductor-capacitor) low-pass filter is commonly used at the output to 

ensure a high-quality sinusoidal waveform and minimize harmonic distortion (Lu, 2015), 

(Gulbahce, 2022), (Rashid, 2017, p. 251). While LCL filters are more often associated 

with grid-connected systems for harmonic suppression, they can also be connected to the 

output of a micro-inverter to reduce harmonic content (Gulbahce, 2022), (Wang, 2016). 

▪ Pulse Width Modulator (PWM): This generates the gating signals for the inverter's 

switches, controlling the output voltage and frequency (Adekola, 2015), (Gulbahce, 2022), 

(Sarkar, 2015) . 

▪ Control Unit: This encompasses the electronic circuitry responsible for managing the 

inverter's operation (Sarkar, 2015) (Hassaine & Bengourina, 2020) (Kjær, 2005). 

Control Strategies and Operation 

The control of stand-alone inverters focuses on providing a stable and regulated AC output to 

the local load. 

A. Core Control Objectives 

▪ Voltage and Frequency Regulation: Ensuring a stable and constant output voltage and 

frequency, which is crucial as the inverter is the sole power provider (Lu, 2015) (Rodrigues, 

2019) (Ojo, 2022). For grid-forming inverters, this means operating as an ideal AC voltage 

source (Teodorescu, Liserre, & Rodriguez, 2011, p. 27). 

▪ Frequency Regulation: Maintaining a stable output frequency to meet load 

requirements1819. (Alsemaan, 2016) (Wang, 2016) 

▪ Power Control: Performing active and reactive power control to ensure system stability 

(Wang, 2016). 

▪ Protection: Providing protection for connected equipment and the inverter itself including 

current protection and harmonic distortion control (Lu, 2015). For specific loads like 

induction motors, maintaining a constant voltage-to-frequency ratio is important to prevent 

magnetic circuit saturation. 



40 

B. Inverter Operation Control Types 

For stand-alone applications, Voltage Source Inverter (VSI) (Mohan et al., 2003), (Mnati, 

2018) controls are predominantly used to deliver specific voltage and frequency values to the 

load (Adekola, 2015), (Rashid, 2017, p. 244). Modern inverters heavily rely on digital 

controllers over analog ones. A common technique for internal control of inverters is Pulse 

Width Modulation (PWM) (Adekola, 2015). Different PWM strategies, such as Sinusoidal 

PWM (SPWM), are employed to control output voltage and frequency while minimizing 

lower-order harmonics (Gulbahce, 2022),  (Adekola, 2015), (Krishna & Prasadarao, 2022), 

(Mohan et al., 2003) (Masters, 2013, p. 498). 

Specific control methods for stand-alone inverters often include: 

▪ PI (Proportional-Integral) Control: Widely used due to its feasibility and ease of 

implementation (Gulbahce, 2022) (Lu, 2015) (Ding et al., 2022) (Bielskis et al., 2019) .For 

stand-alone voltage control, it's a common strategy (Lu, 2015). 

▪ Resonant Control: Based on the Internal Model Principle, this method is ideal for tracking 

sinusoidal signals with zero steady-state error (Lu, 2015). 

▪ Dual-Loop Control: A cascaded control structure (e.g., outer voltage loop and inner 

current loop) is also relevant for grid-forming inverters in stand-alone mode (Ojo, 2022), 

(Lu, 2015). 

C. Advanced Control Strategies 

▪ Maximum Power Point Tracking (MPPT): For PV-based stand-alone systems, 

MPPT algorithms are crucial. They are typically implemented in a DC/DC converter 

stage to optimize energy extraction from solar panels (Tan & Thang, 2018)  (Kjær, 

2005) (Verma, 2019) (Hassaine & Bengourina, 2020) (Alsemaan, 2016) (Tan & Thang, 

2018) (Kjær, 2005) (Hassaine & Bengourina, 2020) (Masters, 2013, p. 521). 

▪ Passivity-Based Control: This framework is used to analyze and design control 

policies that guarantee stability and enhance dynamic performance in microgrids, 

including stand-alone operations (Ojo, 2022). 

4. Performance Metrics and Challenges 

▪ Reliability: Inverters are often considered the weakest link in PV systems due to their 

shorter lifespan (5-15 years) compared to PV panels (around 25 years) (Rodrigues, 

2019), (Teodorescu, Liserre, & Rodriguez, 2011, p. 275). 

▪ Efficiency: A paramount goal is to achieve high energy conversion efficiency 

(Alsemaan, 2016) (Tan & Thang, 2018) (Haider, 2021) (Kjær, 2005). 

▪ Harmonic Distortion (THD): Ensuring a high-quality sinusoidal output waveform 

with minimal harmonic content is essential for sensitive loads (Gulbahce, 2022) 

(Hassaine & Bengourina, 2019) (Jiao, 2017) (Haider, 2021) (Verma, 2019) (Kjær, 

2005). 



41 

▪ Stability: Maintaining stable operation is the most critical aspect of any control system 

(Wang, 2016). 

▪ Cost and Size: Efforts are continuously made to develop cost-effective and compact 

inverter solutions (Alsemaan, 2016) (Verma, 2019) (Kjær, 2005) (Sarkar, 2015) (Zong, 

2011) . 

▪ Transient Performance: The inverter must exhibit good dynamic response to changes 

in load or source conditions (Ojo, 2022), (Hassaine & Bengourina, 2020). 

▪ Protection Capabilities: Stand-alone inverters must