Analysis and reflection on relay protection rejection accident of an offshore wind farm

The relay protection device is an automatic device that can detect the fault in the power system and carry out the operation, which is an important guarantee to ensure the safe and stable operation in the power system. If the protection devices misoperates or rejection, it will cause unnecessary offline and even expand the fault range. The paper discusses the common reasons for misoperates and rejection, and describes two large-scale wind turbine offline accidents caused by zero-sequence over-current protection rejection in an offshore wind farm. The main reason for the accident is that the setting of the over-current protection device is wrong. The actual grounding impedance of the ground transformer is much larger than the design value, resulting in the actual zero-sequence fault current less than the setting value of the protection, so the protection refuses to operate. At last, the paper gives some suggestions to avoid such accident, such as strengthen the maintenance and adjusting the protection setting value.