off grid solar hybrid inverter Low voltage crossing
The service life of off-grid solar hybrid inverter has been attention by people, protect off-grid solar hybrid inverter in addition to daily maintenance, its protection function also greatly improved the life of the off-grid solar hybrid inverter. The next step is to introduce the protection function of the off-grid solar hybrid inverter.
The protection function of the off-grid solar hybrid inverter can be divided into the protection of island protection, low voltage crossing and the stationary reactive power generating device in the station.
The low voltage cross function refers to the time when the voltage of the grid voltage falls and the grid inverter can be connected to the grid for a period of time, and the low voltage time (zone) of "crossing" is normal until the power grid returns to normal. Island effect protection means that when the grid power is off, the grid inverter should stop the grid immediately and protect the grid for not more than 0.2 seconds. It can be seen that isolated island effect protection and low voltage crossing are contradictory. The two functions cannot coexist at the same time, which needs to be selected according to the scale and requirements of the power station. The general principles are as follows:
For small photovoltaic power station, the grid inverter in the power grid of small capacity, less effect on the power grid, the grid failure will have substantial influence on the stability of power grid, so should have fast monitoring of the island and the ability to disconnect instantly connected to the grid, the grid inverter should choose at this time the islanding protection function.
For large and medium-sized photovoltaic power station, grid inverter in the power grid of capacity is bigger, bigger influence on the power grid, the grid failure will have substantial influence on the stability of the power grid, so should have certain ability to low voltage across, namely the grid inverter should choose low voltage across functions.
For most pv grid systems, the inverters are configured with low voltage across. Large and medium-sized pv power plant low voltage tolerance capacity requirements in order to achieve the function of grid inverter low voltage across, grid inverter need to adopt new software control algorithm, namely static reactive power generator, software control algorithm to real-time monitoring network, and determine whether the grid voltage dips (balanced or unbalanced falls). When the CPU found power grid voltage drop failure occurs, immediately start the function of low voltage across, control the output current and output power, when the grid voltage within a certain range, entered the stage of low voltage through the inverter; When the power grid enters the stage of voltage recovery, the grid inverter outputs reactive power to rapidly support the power grid voltage. If grid drop falling is unbalanced, inverter can output three-phase equilibrium current as objective function, through software control algorithm in the grid voltage unbalance stage, the current of inverter is balanced; When the power grid returns to normal, the inverter is rapidly transferred to the normal grid state.