1. Principle of island detection
Grid power failure detection is the basis of islanding detection. The equivalent circuit of inverter grid-connected operation is shown in Figure 1, and the equivalent circuit of islanded operation is shown in Figure 2. Among them, the load is resistor, inductor and capacitor connected in parallel, the output voltage is 380V, P is the active power output by the inverter; Q is the reactive power; PL* is the active power of the load, QL* is the reactive power; ΔP, ΔQ represent the power difference between the grid and the inverter, respectively. As shown in Figure 1, when the photovoltaic power generation system is connected to the grid, it can be regarded as a current source. Therefore, the voltage of the common coupling point depends on the grid voltage. Once the grid is powered off, that is, islanding occurs, the voltage at the point of common coupling depends on the frequency and phase of the output current of the photovoltaic power generation system and is controlled by phase-locking, which makes the current and voltage only synchronized at the zero-crossing of the voltage, and outside the zero-crossing point, the current The frequency and phase are determined by the internal sine table of the system, and the current waveform is a sine wave.
Formulas (1) and (2) can be obtained from Figure 1:
PL=P+△P=U²R——(1)
QL=Q+△Q=U²(1/ωL)-ωC]——(2)
Equations (3) and (4) can be obtained from Figure 2:
P*L=P=U*²R——(3)
Q*L=Q=U*²(1/ωL)-ωC]——(4)
Formula (5) can be obtained from formula (1) and formula (3):
U*²-U²=R△P——(5)
Formula (6) can be obtained from formula (2) and formula (4):
(ω-ω*)×(1+ωω*LC)=ωω*L△Q/U² ——(6)
From formula (5), it can be known that when ΔP=0, the output voltage does not change; from formula (6), it can be known that when ΔQ=0, the output frequency does not change; the load voltage U affects ΔP and frequency. When energy is required, conventional islanding detection fails. When the value of ΔP or ΔQ is large, if the output voltage or frequency of the inverter changes, the inverter will detect the islanding phenomenon; if the value of ΔP or ΔQ is small, it will hardly cause the microgrid. Undetectable dead zones exist when output voltage or frequency changes
2. Island detection methods
According to a report provided by Sandia National Laboratories, the islanding effect is that when the photovoltaic grid-connected power generation system installed at each user end fails to detect the power outage state in time when it stops working due to a fault accident or power outage maintenance, it cannot quickly cut itself off from the mains. A self-sufficient power supply island phenomenon that is beyond the control of the power company is formed by the photovoltaic grid-connected power generation system to supply power to the surrounding loads. As shown in Figure 3.
There are many island detection methods, and two requirements must be met:
① The detection time is as short as possible;
②To detect the island type.
Microgrid generally adopts local anti-islanding strategy, passive scheme generally detects changes in voltage and frequency; active scheme adds disturbance to the system, and then judges the islanding phenomenon, passive scheme is generally simple, but there is a greater possibility of blind spots; active scheme Although blind spots are avoided, the quality of the power grid will be affected. In addition to the passive and active methods commonly used above, there are also some detection methods outside the inverter, such as the “grid-side impedance interpolation method”. This method means that a large impedance is automatically inserted on the load side of the power grid when the power grid fails, so that the impedance on the grid side changes suddenly and significantly, thereby destroying the power balance of the system and causing changes in voltage, frequency and phase. In addition, the fault signal of the power grid system is used for control. Once the power grid fails, the monitoring system on the grid side will send a control signal to the photovoltaic power generation system, so that the parallel operation of the distributed energy system and the power grid can be cut off in time.
Since active solutions have an impact on grid quality, passive techniques (detecting changes in voltage and frequency of the grid) are not sufficient for islanding prevention under power-on and re-power conditions under balanced load conditions, so active techniques must be combined. Active techniques are based on shifting of sample frequency, impedance monitoring of flowing currents, monitoring of phase jumps and harmonics, positive feedback methods, or controller bases for unstable currents and phases. There are many ways to prevent it. There are many related patents in the world, some have been obtained, and some are still in the process of application. Some of these methods, such as monitoring the current pulses flowing through the grid, prove to be inconvenient, especially when multiple inverters work in parallel, degrade the grid quality, and cause islanding due to the mutual influence of multiple inverters detection has a negative impact.
Therefore, the over/under voltage and over/under frequency detection methods are selected here as the anti-islanding strategy of the microgrid, and the islanding detection process is shown in Figure 4.
In Figure 4, Um is the maximum value of the set grid voltage peak value, Us is the detection of the AC bus of the microgrid, that is, the voltage of the common node between the microgrid and the grid, fmin=49.8Hz and fmax=50.2Hz are the set grid frequency minimum and The maximum value, when it is detected that the voltage and frequency cannot meet the conditions of 0.88 Um < Us < 1.1 Um and fmin < f < fmax, a fault signal is detected, it can be considered that the power grid is faulty, and the switch between the power grid and the microgrid is disconnected.