In the photovoltaic grid-connected power generation system, to judge the occurrence of the islanding effect, the method of detecting the amplitude change, phase jump and frequency drift of the output terminal voltage is generally used. In practice, there are mainly two kinds of active detection method and passive detection method. . The passive detection method mainly judges the occurrence of islanding by monitoring the operation changes of the voltage frequency and amplitude of the power grid.
When the power grid is disconnected, the voltage at the output end of the inverter system is determined by the load and the grid-connected current, and the phase between the output voltage and current is determined by the load. Therefore, if the grid is not actively interfered with, the operating state of the inverter system after the grid is powered off is completely determined by the load. The passive detection method judges the occurrence of islanding by monitoring different system operating parameters. This method does not need to exert influence on the system operation. There are mainly the following two methods.
(1) Detect the amplitude and frequency of the output voltage
Detecting the amplitude and frequency of the output voltage is a detection method used for over/under voltage and over/under frequency protection. The over/under voltage and high/low frequency detection method is a detection method to stop the grid-connected operation of the inverter when the voltage amplitude and frequency of the common coupling point exceed the normal range. When the inverter is working, the working range of voltage and frequency should be set reasonably to allow normal fluctuation of grid voltage and frequency. Generally, for 220V/50Hz grid, the working ranges of voltage and frequency are 194V≤U≤242V, 49.5Hz≤f respectively. ≤50.5Hz, if the voltage or frequency offset reaches the set threshold for islanding detection, islanding can be detected. However, when the local load of the inverter closely matches its output power, the offset of voltage and frequency will be very small or even zero, so there is a non-detection area in this method. This method is more economical, but due to the large non-detection area, OVR/UVR and OFR/UFR island detection alone are not enough.
The equivalent circuit of the island study is shown in Figure 1. When the grid is normal, the output power of the inverter is P+jQ, the output power of the grid is ΔP+jΔQ, the load power is , and D is the common coupling point. The amplitude and frequency of the voltage at point D are determined by the power grid. When there is no islanding, the over/undervoltage protection and over/underfrequency protection will not operate; when the islanding occurs, when ΔP≠0, the inverter output active power P and If the load active power P1 does not match, the voltage at point D will change; when ΔQ≠0, the inverter output reactive power Q does not match the negative cut reactive power Q1, and the frequency of the voltage at point D will change. If the change of voltage or frequency exceeds the allowable range, over/under voltage protection and over/under frequency protection will be activated to realize islanding state detection.
This method is low-cost and easy to implement, but if the power grid is normal, ΔP=0, ΔQ=0, after the island occurs, the amplitude and frequency of the voltage at point D will not change, and the above method fails to detect. Since the mains voltage and frequency always fluctuate within a certain range, ΔP and ΔQ are not completely required to be equal to zero for this phenomenon to occur. Therefore, the thresholds of over/under voltage and over/under frequency protection cannot be set too small, otherwise it will occur. Malfunction.
(2) Voltage noble detection method
The voltage wave detection method prevents the islanding phenomenon by detecting whether the total Noble wave distortion of the output voltage of the grid-connected inverter exceeds the limit. This method is based on the nonlinear principle of the power transformer of the working branch grid.
When the power generation system is connected to the grid, its output current will flow into the power grid through the common coupling point. Because the network impedance of the power grid is small, the total spectral wave distortion rate of the voltage at the common coupling point is usually low.
Generally, the total THD of the voltage at this time is low. is lower than the threshold value (the THD of the grid-connected inverter is generally required to be less than 5% of the rated current). When the grid is disconnected, since the load impedance is usually much larger than the grid impedance, the common coupling point voltage (spectral wave current The product of the load impedance and the load impedance) will generate large harmonics, and the islanding effect can be effectively detected by detecting changes in the voltage spectrum or spectral wave. However, in practical applications, due to the existence of nonlinear loads and other factors, The harmonics of grid voltage are very large, and the action threshold of harmonic detection is not easy to determine. Therefore, this method has limitations.
Considering that the reference phase of the grid-connected current is provided by the phase of the grid voltage, when the grid is disconnected, the transformer in the system has nonlinear characteristics, which makes the voltage waveform generated by the output transformer current distorted and has a large harmonic content. The voltage waveform is used as the reference signal of the current. By continuously monitoring the output voltage, when the spectral wave is increasing continuously, the island can be effectively detected. In theory, this method can detect the occurrence of islanding in a relatively large range, but this method is still very difficult to select the threshold value, so the application of the voltage nobo detection method is limited to a certain extent
(3) Voltage phase mutation detection method
The voltage phase mutation detection method detects the occurrence of the islanding phenomenon by detecting the phase difference change between the output voltage and current of the photovoltaic grid-connected inverter. When the photovoltaic grid-connected power generation system is connected to the grid, it usually works in the unit power factor mode, that is, the current and voltage (grid voltage) output by the photovoltaic grid-connected power generation system are the same and in phase. The islanding phenomenon of load power supply, at this time, the voltage of the common coupling point is determined by the output current and the load impedance. Due to the action of the phase-locked loop, the voltage of the common coupling point is only synchronized at the zero-crossing point. Between the zero-crossing points, the output current follows the reference current inside the system without sudden change. Therefore, for non-resistive loads, the common coupling point voltage There will be a sudden change in the phase of , so the phase mutation detection method can be used to determine whether the islanding phenomenon occurs.