When single-phase grounding occurs in the power grid, the faulty phase-to-ground voltage will drop significantly or even reach zero voltage. When a three-phase short circuit fault occurs, the line voltage of the faulty phase. will also decrease. One of the phenomena that occurs when the power grid is disrupted is that the grid voltage fluctuates significantly. Therefore, the above two faults will cause the grid voltage to drop.
Answers: B, D
This question tests the classification and performance of electrical equipment. Direct Drive Wind Turbine: Direct drive wind turbine does not have a gearbox and the turbine directly drives the rotation of the generator rotor. Compared to dual-powered units, there is no gearbox, which reduces the mechanical failure rate of the wind turbine. Direct drive machines feature the aadvantages of simple structure, strong reliability, high efficiency and low maintenance costs. In addition, the direct drive wind turbine has good performance when operating at low wind speeds and is particularly suitable for installation and use in Category III domestic wind turbine areas. Since the permanent magnet material of the direct drive generator is prone to magnetism loss under the conditions of vibration, impact and high temperature at the same time, since the permanent magnet material has strong permanent magnetism , it cannot be repaired on site; conditions, so if a problem occurs, it can only be returned to the factory for repair.