Ground fault protection system of flexible DC power grid and protection control method thereof

The invention discloses a grounding fault protection system of a flexible DC power grid metal loop and a protection control method thereof. The grounding fault protection system comprises an upper layer control device, and a pole control device, a main protection device and an auxiliary protection device which are in signal connection with the upper layer control device. The auxiliary protection device is used for monitoring a grounding fault of a metal loop layer of the flexible direct-current power grid and sending a grounding fault signal to the upper-layer control device when the metal loop layer has the grounding fault; and the main protection device is used for calculating a differential current according to the current of the station and the opposite-side current of the metal loop layer, and controlling a direct-current change-over switch of the metal loop layer according to the differential current to carry out protection control. Therefore, the protection of the flexible DC power grid is realized, an