Mechanism of Transformer Magnetic Bias Caused by Sub-synchronous Resonance of Wind Farm Cluster

When sub-synchronous resonance (SSR) occurs in wind power clusters, the main transformers of wind farms will produce abnormal noise and increased vibration, which brings potential risks to the security and stability of power system. By analyzing the fault records and the equivalent circuit of the system, it is proposed that the injection of more than a certain amount of SSR current into the system causes transformer core saturation, which leads to the increase of the main transformer excitation current, strange noise and vibration. When SSR occurs in the transmission system of the wind farm cluster through the FSC, the primary transformer voltage contains considerable sub-synchronous frequency component and result in corresponding fluctuation of magnetic flux in transformer core. As a result, the core of the transformer is periodically alternating positive and negative magnetic bias, and repeatedly enters the saturation state, and then periodically appears a large peak excitation current spike. The degree of partial magnetic saturation of the transformer core is determined by the magnitude of the power frequency flux and the sub-synchronous flux. When the system with higher operating voltage resonates, the core of the transformer is more prone to magnetic bias saturation. This result is verified by Real Time Digital Simulator (RTDS) simulation and physical transformer low-voltage model experiments. Appropriate reduction of the system voltage can reduce the saturation level and the harm of the main transformer and damping of SSR is the ultimate solution to solve the above problems.