Research on effects of transformer DC Bias on negative sequence protection

Geomagnetically induced currents (GIC) caused by geomagnetic storms, with a main frequency range of 0.0001~0.01Hz, can be regarded as quasi-DC. The GIC, similar to direct currents returned from earth during monopolar operation of HVDC systems, can cause DC bias of transformers. The harmonics due to DC Bias may result in the misopertations of the protection devices. The paper studies waveform distortions of the transformer exciting current and impacts of odd and even order harmonics on negative sequence protection under DC bias of the transformer. Firstly, a simulation model of the saturated transformer can be created in PSCAD/EMTDC by giving the piecewise linear V - I curves. Then exciting current distortions and harmonic characteristics of a 750kV single-phase autotransformer under different levels of direct currents and different loads are simulated. The simulation result indicates that only load change under the same DC mainly alters the fundamental component content of the transformer primary current, not harmonics content. On this basis, finally, the paper analyzes the adverse impacts of the transformer DC bias on the analog and digital negative sequence filters, which are the main elements of negative sequence protection devices, only under different levels of direct currents by simulation calculation. It reveals that compared with digital negative sequence protection, analog negative sequence protection is more susceptible to DC Bias.