Do low-voltage grid-connected cabinets have anti-island protection devices?
Anti-island protection device is a kind of protection equipment in low-voltage grid-connected cabinet, which is very important for photovoltaic grid-connected system. So, is it necessary for low-voltage grid-connected cabinets to have anti-island protection devices?
The answer is: the low-voltage grid-connected cabinet normally needs to install anti-island protection devices.
The photovoltaic grid-connected cabinet is a kind of distribution device that connects the photovoltaic power station with the power grid and is used as the boundary between the photovoltaic power station and the power grid. For low-voltage grid-connected photovoltaic power stations, photovoltaic grid-connected cabinets can also be installed with reference metering and some protection functions. The anti-island protection device is used to solve the “island effect”. Due to external or natural reasons, it is likely to cause local power supply system island operation, which will be very harmful to the equipment and system grid. The general inverter and wind power generation have their own anti-island function, and in order to be safe and reliable, a backup protection is prepared. Therefore, the anti-island protection devices are installed in the photovoltaic grid-connected cabinet.
Anti-island protection has many important functions. For example, when the power grid is at high voltage, low voltage, high frequency, and low frequency failure, the photovoltaic grid-connected switch trips in time. When the power supply is restored and the voltage and frequency reach the allowable value, the grid-connected switch should be automatically closed. This ensures that photovoltaic generation is as effective as possible without having too great an impact on the national grid.
When the power grid fails to trip, due to the limited control strategy and detection capability of the inverter, without effective anti-island protection, the distributed power supply may fail to sense the power grid failure in time and continue supplying power to the local load, thus forming an island.
In case there is an earth fault outside the community, for example, that cuts off main power to the grid outside the community, the solar power generation system may not realize the power failure and thus keeps feeding the appliances in the community. For power maintenance personnel, this will be a great safety hazard. When the maintenance personnel work to debug the main power grid outside the community, they usually think that the lines in the community have already been cut off, but because the island formed by the solar power generation system is still supplying power, the lines and equipment in the community are charged, and the maintenance personnel are prone to electric shock if they come into contact with charged equipment.
In the isolated island operation state, if the output power of the solar power generation system is greater than the load power, the voltage rises. This excessive voltage may break down the insulation layer of the electronic components in the electrical equipment, such as refrigerators and TVs, causing equipment damage.
Although the inverter usually connects the solar power generation system to the grid, the inverter itself can also stabilize the frequency output within a certain range; however, there is still a possibility of fluctuation during island operation. If the frequency is higher than the normal working range of electrical equipment, it will influence the normal work of motor equipment such as an air conditioner compressor motor and washing machine motor, which may result in overheating of the motor, abnormal speed, and other problems, thus shortening the service life of the device.
Power quality
Voltage flicker: There might be voltage flicker when operating in the island due to the interaction between the solar power generation system and the load in the cell. Voltage flicker will make indoor light flicker, which influences the normal operation of some voltage stability.
Harmonic improvement: In the working process, the inverter in the solar power generation system may produce harmonics, and the grid can absorb and suppress the harmonics to a certain extent under normal grid-connected operation. However, during island operation, without the buffer effect from the power grid, harmonics could build up in the power grid within the cell and interfere with normal functioning of sensitive communication equipment such as mobile phone signal amplifiers, wireless routers, etc. and other sensitive electronic equipment. This may lead to poor communication quality, malfunctioning of electronic equipment, among others.
If an inrush occurs, the residents may think that the whole community has been cut off, so as to lower their vigilance. Improper operations on electrical equipment conducted by residents in this situation, such as privately checking the distribution box or trying to repair electrical appliances, will also increase the risk of electric shock.
Demand of anti-island protection device for photovoltaic grid-connected system
- Large and medium-sized photovoltaic power stations
Normally, the large and medium-sized photovoltaic power station would install the island protection in the parallel network to guarantee the security of the grid. Large and medium-sized photovoltaic power stations generate a large amount of power. If an islanding phenomenon occurs, the influence on the power grid may be serious, and even lead to a failure in the power grid. Therefore, to ensure stable operation of the power grid, large and medium-sized photovoltaic stations are usually equipped with anti-islanding protection devices.
- Distributed photovoltaic power station
Due to the small installation capacity and low voltage of grid-connection, distributed photovoltaic power stations will also install anti-island protection devices according to requirements to ensure safety. Although the capacity of distributed photovoltaic power stations is small, the number is large and the distribution is wide. If those distributed photovoltaic power plants form islands when the grid goes down, it will also pose a threat against the security of the grid. Besides, distributed photovoltaic generation stations are usually installed at the users’ side and connected directly to the electrical equipment of the users. If there is no anti-island protection device, in case of power grid failure, the distributed photovoltaic station may continue to supply power to the user, which will not only affect the safety of the user’s electricity but also may cause damage to the user’s electrical equipment. Therefore, the distributed photovoltaic station will also install anti-island protection devices according to local requirements.