In the process of solar photovoltaic grid-connected power generation, due to the photovoltaic power generation system and the power system are connected to the grid, when the power system fails due to an abnormality for some reason, if the photovoltaic power generation system cannot stop working or disconnect from the power system, it will continue to supply power to the power transmission line. This operating state is vividly called the “island effect”. Especially when the generated power of the photovoltaic power generation system is balanced with the load power consumption, even if the power system is powered off, the parameters such as voltage and frequency at the output end of the photovoltaic power generation system will not change rapidly, so that the photovoltaic power generation system cannot correctly judge whether the power system fails or the power supply is interrupted, which can easily lead to the “islanding effect” phenomenon.
The “island effect” can have serious consequences. When the power system fails or the power supply is interrupted, the photovoltaic power generation system will continue to supply power to the grid, which will threaten the safety of the power supply line repair and maintenance operators and equipment, resulting in electric shock accidents. It not only hinders the maintenance of the power failure and the restoration of the power grid as soon as possible, but also may cause damage to the power distribution system and some load equipment. Therefore, in order to ensure the safety of maintenance operators and the timely recovery of power supply, when the power system is powered off, the photovoltaic power generation system must be stopped or automatically separated from the power system (at this time, the photovoltaic power generation system is automatically switched to an independent power supply system, and will continue to operate to supply power for some emergency loads and necessary loads). The more photovoltaic grid-connected power generation systems are connected to the power system, the higher the probability of “islanding effect” occurs, so there must be corresponding countermeasures to solve the “islanding effect”. In the inverter circuit, the function of detecting the independent operation state of the photovoltaic system is called independent operation detection. The function of detecting the independent operation state and stopping the operation of the photovoltaic power generation system or automatically separating from the power system is called independent operation stop or “islanding effect” prevention. The independent operation detection function is divided into passive detection and active detection.
①Passive detection method. When the power grid fails and the power goes out, the output voltage, output frequency, voltage phase and harmonics of the inverter will all change. The passive detection method monitors the changes of voltage, frequency, phase and harmonics of the power grid system in real time, detects the voltage fluctuation, phase jump, frequency change, harmonic change and other parameter changes when the inverter transitions to the independent operation due to the power failure of the grid power system, and detects the independent operation state.
Passive detection methods include voltage phase jump detection method, frequency change rate detection method, voltage harmonic detection method, output power change rate detection method, etc. Among them, the voltage phase jump detection method is more commonly used.
The detection principle of the voltage phase jump detection method is shown in Figure 1. The detection process is as follows: Periodically detect the AC voltage cycle of the inverter. If the cycle offset exceeds a certain set value, it can be determined as a separate operation state; At this time, the inverter is stopped or disconnected from the grid. Usually, the inverter connected to the power system operates with a power factor of 1 (that is, the voltage of the power system is in phase with the output current of the inverter), the inverter does not supply reactive power to the load, but the power system supplies reactive power. However, when operating alone, the power system cannot supply reactive power, and the inverter has to supply reactive power to the load, resulting in a sudden change in the voltage phase. The detection circuit detects the change of the voltage phase, and it can be determined that the photovoltaic power generation system is in a separate operation state.
The disadvantage of the passive detection method is that it is difficult to detect the occurrence of the “islanding effect” when the output power of the inverter is exactly balanced with the local load power. Therefore, the passive detection method has limitations and a large non-detection area.
② Active detection method. The active detection method is that the output terminal of the inverter actively sends a disturbance signal such as voltage, frequency or output power to the system, and observes whether the power grid is affected, and detects whether it is in a separate operation state according to the parameter changes. When the power grid is working normally, the power grid has a balancing effect, and these disturbance signals cannot be detected. When the power grid fails, the disturbance signal output by the inverter will be detected.
Active detection methods include frequency offset method, active power variation method, reactive power variation method, load variation method, etc. The most commonly used is the frequency offset method.
According to GB/T19939-2005 “Technical Requirements for Grid-connected Photovoltaic System”, the photovoltaic power generation system should run synchronously with the grid when it is connected to the grid, the rated frequency of the grid is 50Hz, and the allowable deviation of the frequency after the photovoltaic power generation system is connected to the grid is ±0.5Hz. When it exceeds the frequency range, it must act within 0.2s to disconnect the photovoltaic power generation system from the grid.
The working principle of active detection of frequency offset mode is shown in Figure 2. This method is to change the frequency of the AC power output by the photovoltaic power generation system within the allowable range according to the load conditions in individual operation, and according to whether the system follows its changes to determine whether the photovoltaic power generation system is in a separate operation state. For example, if the output frequency of the inverter is fluctuated by ±0.1Hz relative to the system frequency, when it is connected to the grid, this frequency fluctuation will be absorbed by the system, so the frequency of the system will not change. When the system is in the state of independent operation, the fluctuation of this frequency will cause the change of the system frequency, and it can be judged as independent operation according to the detected frequency. Generally, when the frequency fluctuation lasts for more than 0.2s, the inverter will stop running or disconnect from the power grid.
The active detection method has high precision and small non-detection area, but the control is complicated, and the quality of the output power of the inverter is reduced. At present, a more advanced detection method is a combined detection method combining a passive detection method and an active detection method.
Read more: Technical requirements for grid-connected inverters