When the amplitude, phase and frequency of the system output voltage do not change significantly after the power grid is powered off, the passive detection fails. Therefore, the active detection method is proposed. The active detection method is to disturb the output current of the inverter and make the system in an island. In the state, when the magnitude and frequency of the common coupling point voltage will deviate from the normal value and exceed the set range, a method of actively stopping the inverter and grid connection, the detection efficiency of the internal active method is high, but due to the introduction of disturbance, The output power quality of the inverter has decreased. According to the parameters and methods of adding disturbance, the following methods are briefly introduced.
(1) Active power disturbance method
The active power disturbance detection method is to periodically change the output power of the grid-connected system to break the power balance between the load and the photovoltaic system, and then detect the change of the voltage amplitude at the output terminal to determine the occurrence of system islanding. According to the IEEE standard, the normal voltage range of the system is 0.88~1.1 times. For a photovoltaic grid-connected system, the grid-connected current is changed by disturbing the output current of the inverter, so that the output power of the inverter changes, and then the output voltage is changed.
The method is simple to control, easy to implement, does not require high-precision sensors, and does not require additional hardware costs. For a single grid-connected inverter whose grid impedance is smaller than the local load impedance, the undetectable area is very small. When the grid is disconnected, the active power disturbance method will interfere with the matching condition of the load and the output power of the grid-connected inverter, resulting in overvoltage or undervoltage protection. However, in a system where multiple grid-connected inverters are connected in parallel, even if all grid-connected inverters adopt an active power disturbance scheme, a dilution effect will still occur. With little to no change in the delta, islanded states are difficult to detect unless the changes are synchronized, which is extremely unlikely.
(2) Active frequency offset method
Active frequency offset method is a common active disturbance detection method. The active frequency shift scheme is used to make the output frequency of the grid-connected inverter slightly distort the current to form a trend of continuous frequency change, which eventually causes the output voltage and current to exceed the limit value of frequency protection, thus achieving the purpose of anti-islanding effect. The active frequency offset method causes disturbance to the voltage frequency of the load terminal by offsetting the frequency of the grid voltage sampling signal at the coupling point.
When using the constant frequency offset method, the current control amount is:
i=Imsin[2π(fk-1+△f)(t-t1)]
In the formula, △f is the frequency offset; fk-1 is the frequency value of the previous cycle; t1 is the forward time of the load voltage sine wave crossing the zero point.
During normal operation, due to the existence of the power grid, the frequency offset of the grid-connected current is very small, and the frequency cannot be continuously changed. After the island is formed, the voltage frequency at the common coupling point will increase or decrease with the load characteristics.
(3) Automatic phase shifting method
The automatic phase shifting method 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 voltage of the common point, in order to determine the islanding when the frequency of the common point deviates from the normal range after the power grid loses voltage. Under normal circumstances, the inverter phase angle response curve is designed in the range near the system frequency, and the inverter phase angle increases faster than the RLC load at unity power factor. When the inverter runs in parallel with the grid, the grid provides a fixed reference phase angle and frequency to stabilize the inverter operating point at the power frequency. After the island is formed, if the inverter output voltage frequency fluctuates slightly, the inverter phase response curve will increase the phase error and reach a new stable state point. The frequency of the new state point must exceed the OFR/UFR action threshold, and the inverter is shut down due to the frequency error. This detection method actually achieves frequency shifting through phase shifting. Like the active frequency shifting method AFD, it has the advantages of simple implementation, no additional hardware, and high reliability of island detection. It also has similar weaknesses, that is, with the increase of load quality factor, islanding The possibility of detection failure increases.
The method is to shift the output current starting phase at each voltage. Its output current command is
i=Imsin{2πfk-1(t-t1)+[θAPS(k)/2πfk-1]}
In the above formula we get:
θAPS(k)=1/α[(fk-1-50/50)×2π+θ0(k)]
In the formula, θ0(k) is the additional phase angle change; α is the phase angle adjustment factor.
The automatic phase shift method calculates the initial phase angle θAPS(k) of the inverter output current by calculating the frequency fk-1 of the previous cycle at each forward zero-crossing point of the terminal voltage. As the load voltage frequency changes with the current initial phase angle, the output current phase angle offset will further increase, and the output voltage frequency will also change rapidly, prompting the protector to act.
(4) Positive feedback frequency offset method
The positive feedback frequency offset method introduces positive feedback, which is an extension of the active frequency offset method. The quantity Δf becomes Δfk, then there is the following relationship:
Δfk=F(△ωk)+△fk-1
In the formula, △ωk is the frequency difference between two cycles; F(△ωk) is the positive feedback function of the frequency increment; △fk-1 is the offset factor of the previous cycle.
Normal power supply from the grid will prevent continuous changes in frequency. When the power grid is disconnected, the frequency increases or decreases, the frequency error also increases, and the output frequency of the inverter increases accordingly until the frequency relay operates.
Compared with the active frequency offset method, the positive feedback frequency offset method can not only accelerate the frequency offset, but also reduce Δfk when the frequency changes to a negative value, which means that under the same Δfk, the positive feedback frequency offset The detection blind area of the moving method is smaller.
(5) Reactive output detection method
The principle of the reactive output detection method is to control the grid-connected current to generate a reactive current of a specific size, and to determine the occurrence of islanding by detecting the existence of the current. The generated reactive current is only when the grid-connected system is connected to the main system. The detection method has high reliability, but its action time is delayed, and its action time exceeds the reclosing time of many automatic reclosers, so the reactive output detection method is generally rarely considered.
The above-mentioned active offset islanding detection methods have the advantages of easy implementation and small detection blind area, but the quality of the output power of the grid-connected inverter is degraded, and it is difficult to determine the offset threshold