In small distributed photovoltaic power plants, the national grid requires the installation of anti-islanding protection devices . As a low-voltage microcomputer protection, it has many differences compared to conventional high-voltage protection. However, it is similar.
Question one: installation problems
Under normal circumstances, the installation of anti-islanding protection devices, no matter whether they are conventional 380V grid-connected or 10KV grid-tied, are all installed on the side of grid-connected switches. The presence of faults can be determined by detecting the grid voltage conditions. However, some situations are different. There are several branches under a transformer, and several branches are summarized by the power grid cabinet and then to the grid. In this case, the grid connection should be at the branch or the total grid-connected switch. In this general case, the main switch is installed, and then the branch switch has a voltage-lost trip. In general, the use of switches is also selective. There are two options for switching. One is a plastic shell switch with an electric operation mechanism. The anti-islanding device controls the integration of the grid connection switch by controlling the electric operation mechanism on the switch. There is also an automatic reclosing switch, which can also be used with anti-islanding protection. All of the above are relatively small-capacity photovoltaic power plants. The switch used is a molded case. When the capacity is too large, frame-type circuit breakers are generally used.
Question two: Do not use UPS
For conventional microcomputer protection, it is installed in the switch cabinet or in the form of a screen cabinet installed in the relay room. The power supply is a DC screen. In this way, when the utility power fails, the microcomputer protection device can operate stably and can complete the protection function of the device for a relatively long period of time. There is no DC screen for distributed photovoltaics, so some projects require the installation of UPS as a power supply. However, the addition of a UPS is undoubtedly an increase in cost, so most of the time the switch is required to have a loss of voltage, which means that when the voltage monitored by the switch side is momentarily depressurized, the switch itself will trip. In other non-de-energized cases, a fault occurs by the device to achieve a trip of the switch.
Question 3: How can the switch fail to reclose?
Nowadays, photovoltaics are widely distributed and it is relatively troublesome to operate and maintain them. If it is simply switched on and off due to a trip, it would not be able to follow manually. More importantly, it affects the amount of electricity generated. Therefore, for some transient faults, the switch should close. If the fault recovers, the switch should close automatically. This will neither affect the grid nor affect users. But if it is a permanent failure, it is another matter, such as current protection. When the switch cannot automatically reclose several considerations:
1, fixed value. Whether it is an automatic reclosing switch or a switch with an electrically operated mechanism, the closing must be constant. It is allowed to close at a fixed value.
2, wiring. Correct wiring, and no false connection.
3. Check whether the closing and tripping points of the switch can work normally.
4. For some grid-connected switches, if the recloser fails to reclose at the same time, the anti-islanding protection device must check the value to see if the number of closings can be set.