Simulation study of single-phase photovoltaic grid-connected system and its anti-islanding strategy

Energy shortage and environmental degradation are increasingly serious global problems. As a green and renewable energy source, solar energy is changing from supplementary energy to alternative energy. Photovoltaic utilization has become a hot spot for countries around the world to develop. Photovoltaic grid-connected power generation, as a development trend of solar photovoltaic utilization, has developed rapidly. However, with the increase in the number of grid-connected inverters put into use, the pollution of grid voltage by the grid-connected current harmonics output by them cannot be ignored. In view of the shortcomings of simple PI control, the control of the inverter in the photovoltaic grid-connected system is improved, and a current tracking control strategy combining repetitive control and PI control is adopted. Repetitive control can suppress the periodic disturbance of the grid-connected output current on the grid side and the load side, and reduce the total harmonic distortion coefficient of the grid-connected current; PI control uses the deviation adjustment principle to make the inverter output grid-connected current track the reference sinusoidal given signal in real time. Since the photovoltaic grid-connected power generation system directly feeds the solar energy to the grid after inversion, various perfect protection measures are required. For the fault conditions that may occur during normal system operation, such as power device overcurrent, power device drive signal undervoltage, power device overheating, solar cell array output undervoltage, and grid overvoltage and undervoltage, it is relatively easy to detect through hardware circuits and judge, identify and handle them with software. However, for photovoltaic grid-connected power generation systems, it is also necessary to consider the response plan under a special fault condition, that is, the prevention and countermeasures of the islanding effect.

l Grid-connected system control strategy

In order to make the inverter output a good grid-connected current waveform, the inverter's output grid-connected current must be closed-loop controlled. When using traditional PI control to track the sinusoidal given signal, there are the following limitations:

(1) When the tracking signal is a rapidly changing sine wave, theoretically, the entire system is a differential system and it is impossible to perform zero-error tracking.

(2) Although the steady-state error can be reduced by increasing the proportional coefficient, increasing the proportional coefficient will lead to a decrease in control accuracy and even cause the system to oscillate. In addition, increasing the proportional coefficient may also amplify the noise signal at the same time. Therefore, the proportional coefficient cannot be too large.

1.1 Repeated control strategy

Using simple PI control, new control strategies and control links must be introduced because it cannot effectively improve the influence of nonlinear factors of the inverter. In the 1980s, Inoue et al. proposed the idea of ​​repetitive control based on the internal model principle. An internal model that can generate the same period as the reference input is set in a stable closed-loop system, so that the system can achieve asymptotic tracking of the external periodic reference signal. The repetitive control system is shown in Figure 1. Because it has excellent robustness, it has a significant effect on eliminating waveform distortion caused by nonlinear loads and other periodic interferences.

The repetitive controller consists of a periodic delay link zN, a first-order low-pass filter Q(z) and a compensator S(z). P(z) is the transfer function of the controlled object, d is the disturbance signal, IV is the number of samples per cycle, and S(z) is a compensation link, which makes the system unit gain in the low and medium frequency bands and has no phase lag link. Although repetitive control can achieve tracking control without steady-state error for the periodic reference sinusoidal given signal, there is a characteristic that the tracking control of the error is output after a reference cycle. Taking comprehensive considerations, the two strategies of repetitive control and PI control are combined to make the system have both good steady-state and dynamic characteristics.

1.2 Repeated control system simulation

In this simulation system, the reference sine wave is 50Hz, and the switching frequency of the inverter is 20kHz. Therefore, the frequency fs=20kHz is used, so the number of sampling beats per cycle N=400 can be determined. Obviously, the cycle delay link zN=z-400, and the Q value of the filter is 0.95. Figure 2 shows the simulation model diagram, and Figure 3 is the simulation result diagram. It can be seen that applying repetitive control to the grid-connected inverter system improves the output current waveform of the power generation system and improves the steady-state error.

2. Island Effect

2.1 The emergence of island effect

The so-called island effect, according to a report provided by the Sandia National Laboratories in the United States, is that when the power supply of an electric power company is interrupted due to a fault or power outage for maintenance, the solar grid-connected power generation systems at each user end fail to detect the power outage in time and disconnect themselves from the municipal power network, forming a self-sufficient power supply island formed by the solar grid-connected power generation system and the surrounding loads that the electric power company cannot control.

The photovoltaic grid-connected system is connected to the local load and connected to the distribution network through a gate switch. Its topological structure is shown in Figure 4. When the power grid is out of power, an island is formed.