A transformer neutral current balancing device to restrain half-cycle saturation induced by HVDC monopolar operations

•The neutral DC balancing device leads a new direction of mitigating half-cycle saturation.•Mechanism, simulation model, hardware architecture, and control strategy have been introduced.•Both laboratory tests and field application demonstrate the effectiveness of the proposed device.•Influential factors and comprehensive comparison of other methods are also included.

DC flow on the power transformer windings can lead to severe half-cycle saturation issues and impose great threats on AC power systems. A transformer neutral direct current balancing device is proposed in this paper to address this issue. By injecting direct current into a grounding grid, the DC flow of power transformer windings can be compensated via proper control strategy and thus the biased working flux of power transformers can be mitigated. The simulation model, hardware architecture, and control strategies of the proposed device have been investigated in this paper. Both laboratory and a 220-kV substation field tests have been implemented. Test results demonstrate that, during monopolar operation of HVDC lines, the proposed device restrains the half-cycle saturation of power transformers effectively and economically. The neutral direct current balancing device opens the door to a new direction of restraining half-cycle saturation induced by HVDC monopolar operations.