With the increasing depletion of fossil fuels and the urgent need for renewable energy development, solar power has emerged as a promising solution due to its abundance and environmental friendliness. Photovoltaic (PV) systems, particularly off-grid setups, are crucial for providing electricity in remote areas or as backup power sources. In these systems, inverters play a pivotal role in converting DC power from solar panels or batteries into AC power for loads. Among the various types of solar inverters, such as grid-tied, off-grid, and hybrid inverters, off-grid inverters are essential for standalone applications where reliability and power quality are paramount. The performance of these inverters, especially in terms of output voltage harmonic distortion, directly impacts the efficiency and stability of the entire system. This paper focuses on improving the control strategies for single-phase full-bridge off-grid PV inverters to reduce harmonic distortion and enhance load-handling capabilities.
Off-grid PV systems typically consist of PV arrays, DC/DC converters, battery storage, and inverters. The inverter, being the interface between the DC source and AC loads, must maintain a stable output voltage with low distortion under varying load conditions. Various types of solar inverter control methods have been explored, including PI control, deadbeat control, repetitive control, and intelligent techniques like neural networks and fuzzy logic. However, each has its limitations; for instance, PI control is simple but may not handle nonlinear loads effectively, while advanced methods require complex implementations. In this work, we propose an enhanced multi-loop voltage regulation strategy that builds upon the conventional voltage-current double-loop control, specifically addressing the challenges posed by nonlinear loads.
The single-phase full-bridge inverter topology is widely used in off-grid systems due to its higher power capacity and reduced switch current stress compared to half-bridge configurations. The mathematical model of this inverter is derived from its circuit equations. Let \( U_{dc} \) represent the DC input voltage, \( U_{oc} \) the output voltage, \( i_L \) the inductor current, \( i_o \) the output current, \( L \) the filter inductance, \( C \) the filter capacitance, and \( R \) the load resistance. The state equations can be expressed as:
$$ \frac{di_L}{dt} = \frac{U_{dc}}{L} – \frac{U_{oc}}{L} $$
$$ \frac{dU_{oc}}{dt} = \frac{i_L}{C} – \frac{U_{oc}{RC} $$
In matrix form, the state-space representation is:
$$ \begin{bmatrix} \frac{di_L}{dt} \\ \frac{dU_{oc}}{dt} \end{bmatrix} = \begin{bmatrix} 0 & -\frac{1}{L} \\ \frac{1}{C} & -\frac{1}{RC} \end{bmatrix} \begin{bmatrix} i_L \\ U_{oc} \end{bmatrix} + \begin{bmatrix} \frac{1}{L} \\ 0 \end{bmatrix} U_{dc} $$
And the output equation is:
$$ U_{oc} = \begin{bmatrix} 0 & 1 \end{bmatrix} \begin{bmatrix} i_L \\ U_{oc} \end{bmatrix} $$
This model forms the basis for designing control strategies. The inverter employs unipolar frequency-doubling SPWM modulation, which doubles the output ripple frequency and improves harmonic performance compared to other modulation techniques. This is particularly beneficial for reducing electromagnetic interference and enhancing waveform quality in various types of solar inverter applications.
For control implementation, we first consider the voltage-current double-loop PI control strategy. This approach involves an outer voltage loop and an inner current loop. The reference sinusoidal voltage \( U_{ref} \) is compared with the output voltage feedback \( U_o \), and the error is processed by a PI controller to generate the current reference \( I_{ref} \). The inner loop compares the inductor current \( I_L \) with \( I_{ref} \), and the resulting error is adjusted by another PI controller to produce the PWM signals. The transfer functions and controller parameters are tuned to achieve stable operation. The proportional and integral gains, denoted as \( K_P \) and \( K_I \), are critical for dynamic response and steady-state accuracy. This double-loop control is effective for linear loads, but its performance degrades with nonlinear loads due to limited bandwidth and harmonic distortion.
To address this, we introduce a multi-loop voltage regulation strategy that incorporates output current feedback. This adds a negative feedback loop for the output current \( I_o \), which contains harmonic components from load variations. The enhanced control structure includes the inductor current inner loop, output voltage outer loop, and output current feedback, forming a multi-loop system. The output current feedback provides immediate compensation for load disturbances, particularly low-order harmonics from nonlinear loads. The control law can be summarized as:
$$ I_{ref} = K_{P1} (U_{ref} – U_o) + K_{I1} \int (U_{ref} – U_o) dt $$
$$ U_{PWM} = K_{P2} (I_{ref} – I_L) + K_{I2} \int (I_{ref} – I_L) dt – K_3 I_o $$
where \( K_{P1}, K_{I1} \) are the voltage loop PI gains, \( K_{P2}, K_{I2} \) are the current loop PI gains, and \( K_3 \) is the output current feedback gain. This approach significantly reduces voltage distortion under nonlinear loads, as demonstrated in simulations and experiments.
Simulation studies were conducted using MATLAB to evaluate the control strategies under different load conditions. The system parameters are summarized in Table 1.
| Parameter | Value |
|---|---|
| DC input voltage \( U_{dc} \) (V) | 400 |
| Inductance \( L \) (H) | 4e-3 |
| Capacitance \( C \) (F) | 200e-6 |
| Voltage loop proportional gain \( K_{P1} \) | 10 |
| Voltage loop integral gain \( K_{I1} \) | 0.01 |
| Current loop proportional gain \( K_{P2} \) | 0.05 |
| Current loop integral gain \( K_{I2} \) | 0.01 |
| Output current feedback gain \( K_3 \) | -0.5 |
| Rated output voltage \( U_o \) (V) | 220 |
| Rated output frequency \( f \) (Hz) | 50 |
| Load power \( P \) (W) | 1000 |
For linear loads, the double-loop control achieves a peak output voltage of 310.7 V with a total harmonic distortion (THD) of 1.8%, which meets the standard requirement of less than 5%. The output current waveform is sinusoidal with minimal distortion. When switching linear loads, such as connecting and disconnecting a 1000 W load at 0.1 s and 0.3 s, respectively, the THD increases slightly to 2.7%, but the voltage and frequency remain stable. This demonstrates the robustness of the double-loop control for linear applications, which is common in many types of solar inverter systems.
However, under nonlinear loads, such as a rectifier load, the double-loop control shows limitations. With a nonlinear load switched at 0.1 s and removed at 0.3 s, the output voltage THD rises to 7.1%, exceeding the 5% threshold. The current waveform exhibits significant distortion due to harmonic currents drawn by the nonlinear load. This highlights the need for improved control strategies in off-grid types of solar inverter setups where nonlinear loads are prevalent.
The multi-loop control strategy effectively mitigates this issue. With the same nonlinear load conditions, the output voltage THD is reduced to 2.3%, and the voltage and current waveforms remain nearly sinusoidal. The output current feedback quickly compensates for harmonic disturbances, enhancing the inverter’s load-handling capability. This makes the multi-loop approach suitable for various types of solar inverter applications, especially in environments with mixed linear and nonlinear loads.
Experimental validation was performed using a DC microgrid test bench to verify the simulation results. The setup included a single-phase full-bridge inverter, resistive and nonlinear loads, and measurement equipment. The results correlated well with simulations, showing THD values of 1.7% for linear loads, 3.0% for switched linear loads, 7.4% for nonlinear loads with double-loop control, and 2.6% for nonlinear loads with multi-loop control. Minor discrepancies were attributed to practical factors like measurement delays and component tolerances, but the overall trends confirmed the effectiveness of the proposed control strategy. This reinforces the importance of advanced control in enhancing the performance of off-grid types of solar inverter systems.
In conclusion, the voltage-current double-loop control is adequate for linear loads in off-grid PV inverters, but it falls short under nonlinear load conditions. The multi-loop voltage regulation strategy, incorporating output current feedback, significantly reduces harmonic distortion and improves dynamic response. This approach is applicable to a wide range of types of solar inverter systems, contributing to better power quality and reliability. Future work will focus on optimizing the control parameters further and exploring adaptive techniques to handle a broader spectrum of load variations. As solar energy adoption grows, refining these control strategies will be essential for maximizing the efficiency and lifespan of off-grid PV systems.