The active detection method of islanding effect means that during the operation of the inverter, the control makes its output have periodic disturbances. When the grid is normal, the output of the inverter remains consistent with the grid due to the balancing effect of the grid, and the disturbance amount does not play a role; when a fault occurs in the grid, these disturbance amounts gradually accumulate until they exceed the range specified in the grid connection standard, thus detecting a fault in the grid.
There are three main active detection methods: inverter output power perturbation method, inverter output voltage and frequency perturbation method, and sliding mode frequency offset detection method.
① Output power perturbation method
The output power disturbance method is to make the active power output of the PV power generation system change periodically by controlling the inverter output power. When the islanding effect occurs, the voltage at the output of the inverter changes due to the power disturbance, thus reflecting whether the islanding effect occurs or not.
In practice, to minimize the impact of this method on the inverter output power. Usually, the inverter is controlled in N operating cycles so that the output power is lower than normal or zero in one or half of the cycle interval.
With the development of PV power generation, the number of PV grid-connected power generation systems in the local grid will increase, in which case the power disturbance on the inverter output voltage will become weaker when the islanding effect occurs, which in turn will affect the detection results. In addition, if there is a large nonlinear load in the grid, when the grid stops working, the nonlinear load will supply power to the load, which weakens the detection effect of the power disturbance method on the islanding effect.
② Output frequency perturbation method
Compared with the output power perturbation method, perturbing the output frequency of the inverter voltage is a more effective method to detect the islanding effect. Active Frequency Drift (AFD) is a common method for detecting islanding effect with output frequency perturbation.
Active Frequency Drift works by shifting the frequency of the grid voltage sampling signal at the coupling point, causing a disturbance of the voltage frequency at the load side. If the normal situation, the role of the phase-locked loop is to control the frequency error within a small range, and when the grid fault, the phase-locked loop fails, the inverter frequency changes, and the perturbation is added to make the error increase, accumulate to a certain range, there will be a passive method to detect.
The active frequency offset method, because the direction of the disturbance is fixed, may have offsetting effects on the method because of the nature of the load. For example, capacitive loads are lower than resistive ones, while inductive loads are higher than resistive ones, so if the perturbation direction happens to cancel out with the load impedance characteristics, the perturbation may not be able to accumulate.
To prevent this from happening, the active frequency drift method with positive feedback is used. The direction of the disturbance is determined dynamically by comparing the change in frequency before and after, giving a positive one if the frequency is increasing, and a negative one if the frequency is decreasing in the direction.
③ Sliding mode frequency offset detection method
Slip-Mode Frequency Shift (SMS) is an active islanding detection method. It controls the output current of the inverter so that there is a certain phase difference between it and the common point voltage in order to discern the islanding when the frequency of the common point deviates from the normal range after the grid loses voltage.
The inverter phase angle response curve under normal conditions is designed to be within the range around the system frequency, and the inverter phase angle RLC (short for resistance, inductance, and capacitance combination circuit) load increases fast at unit power factor. When the inverter operates in parallel with the distribution grid, the distribution grid stabilizes the inverter operating point at the working frequency by providing a fixed reference phase angle and frequency.
When the islanding is formed, if there is a small fluctuation in the inverter output voltage frequency, the inverter phase response curve will make the phase error increase and reach a new steady state point. The frequency of the new state point will definitely exceed the action threshold of the frequency relay and the inverter will shut down due to the frequency error.
This detection method actually achieves frequency shifting by phase shifting, which has the same advantages as the active frequency shift method AFD in terms of simplicity of implementation, no additional hardware required, and high reliability of islanding detection, but also has similar weaknesses, i.e., as the load quality factor increases, the possibility of islanding detection failure becomes greater.
In summary, the advantages of the active detection method is the detection of small blind areas and fast detection speed. But the disadvantage is just as obvious, that is, there is a certain impact on the power quality.
(3) Other methods
In addition to the above-mentioned passive and active methods, there are also some methods for detecting the islanding effect outside the inverter. Such as "network-side impedance interpolation method", the method refers to the grid fault automatically inserted in the grid load side of a large impedance, so that the impedance of the network side suddenly changed significantly, thus destroying the system power balance, resulting in changes in voltage, frequency and phase. There is also the use of fault signals from the grid system for control. Once a fault occurs on the grid, the grid side's own monitoring system sends a control signal to the PV power system so that it can cut off the parallel operation of the distributed energy system with the grid in time.
