On grid power generation systems always face such problems. The on grid power generation system will directly invert the DC power and send it to the grid. This requires relatively complete protection measures, otherwise various problems will occur, even In major accidents, the following situations may occur when the system is working:
①The driving signal of the power device is undervoltage;
② Overcurrent of power device
③The power device is overheated
④The output voltage of the solar cell array is undervoltage;
⑤Some common fault states such as overvoltage and undervoltage of the power grid.
The above faults can be easily detected by the hardware circuit, and can be judged by the designed software, and then processed. In addition, there is another key issue that the photovoltaic on grid power generation system particularly needs to consider, that is, the prevention and countermeasures of the islanding effect, which is a response plan under a special state of barriers.
In a circuit with capacitors in series, there are two plates connected to the external circuit, and these two plates are not the same capacitor. When there is only current flowing between the two plates, the total amount of charge on the other plates is not When changes occur, this phenomenon is called an island. Islanding is an electrical phenomenon. In the grid system, part of the grid is completely powered by the photovoltaic system. The islanding phenomenon usually occurs at the moment when this part of the grid is disconnected from the main grid. This is still the subject of international photovoltaic on grid standardization. A point of contention, because islands will harm the safety of maintenance personnel of the power company and damage the quality of power supply, the equipment may be damaged when the power supply switch is automatically or manually re-closed to re-supply power to the island grid. Therefore, the inverter usually has a protection Device for islanding effect.
The islanding effect means that when the power supply of the power company stops supplying power due to various reasons (such as failure accidents or power outage maintenance, etc.), the power outage status is not detected in time by the solar on grid power generation systems, but continues to the grid as always Power transmission, such a self-sufficient power supply island beyond the control of a power company is composed of a solar on grid power generation system and surrounding loads.
As long as it is a distributed power generation system, such as fuel cell power generation, wind power generation, etc., or power generation equipment generally connected in parallel to the mains, this problem will occur. Generally, the islanding effect may cause the entire power distribution system equipment and user equipment The adverse effects mainly include the following:
① The safety of the maintenance personnel of the power company’s transmission line;
②It will affect the action program of the protection switch on the power distribution system
③The problem of phase out-of-synchronization caused when the power supply of the power company is restored
④The phenomenon of unstable power supply voltage and frequency occurring in the power island area;
⑤Solar power supply system, if it is single-phase power supply, it will cause the problem of under-phase power supply of the system’s three-phase load.
It can be seen that if you want an on grid inverter device to be safe and reliable, those devices must be able to detect the islanding effect in time and avoid the harm it brings. With the development of new energy technology, photovoltaic on grid power generation systems are increasingly connected to the grid in parallel, and the probability of islanding effects is getting higher and higher. Therefore, it is necessary to find a suitable solution in order to solve the increasingly serious problem of islanding. The detection of power outages in the power grid is a key method to prevent the islanding effect.
Usually, when the power distribution switch trips, the voltage and rating on the city grid will change greatly. This may be caused by the large difference between the power supplied by the solar power system and the load demand of the grid. The fault can be detected by using the over (under) voltage protection set point or over (under) frequency protection set point of the grid voltage specified by the system software and hardware, so as to prevent the islanding effect.
Another situation is when the power supply situation of the solar power supply system and the amount required by the network load meet the balance or the difference is very small. At this time, when the power distribution switch is tripped, the protection circuit is not easy to detect the grid connection The amount of voltage and frequency changes on the city grid near the system will cause islanding effects. Although the possibility of such a situation is very low, once this happens, the consequences will be unimaginable, so in In the application environment of large-scale photovoltaic on grid systems, measures to prevent islands must be considered.
According to the special standard IEEE Std.2000-929, on grid inverters must have the function of anti-islanding effect. At the same time, it also provides that after the on grid inverter detects the islanding phenomenon after the power is cut off, it will The time limit for the disconnection of the inverter from the grid. As shown in Table 1, Unom refers to the normal value of the grid voltage amplitude. fnom refers to the normal value of the grid voltage frequency
|state||Voltage amplitude after power failure||Voltage frequency after power failure||Maximum detection time allowed|
When the photovoltaic system is connected to the grid, it should run synchronously with the grid. The rated frequency of the grid is 50Hz. The allowable deviation of the frequency after the photovoltaic system is connected to the grid should meet the requirements of GB/T15945, that is, the deviation value is allowed to be ±0.5Hz. When the frequency range is exceeded, it should be Operate within 0.2s to disconnect the photovoltaic system from the grid. The specific abnormal frequency response time is specified in Table 2.
|Frequency range/Hz||Response time/s|
A series of studies have now been carried out to evaluate the various possibilities of the islanding effect and its associated risks. Studies have shown that for low-density photovoltaic power generation systems, because the load and power generation capacity are far from matching, it is impossible for the islanding effect to occur. However, for the part of the grid with high-density photovoltaic power generation systems, active islanding effect protection methods It is indispensable, and at the same time, voltage and frequency control are used as auxiliary measures to ensure that the risks brought by photovoltaics are minimized. Most photovoltaic inverters have both active and passive islanding protection. Although there are not many examples of photovoltaics breaking into the grid, the standards in this area have not been relaxed.