Analysis of Cable Overvoltages in Symmetrical Monopolar and Rigid Bipolar HVDC Configuration

The symmetrical monopolar configuration is the prevailing scheme configuration for high voltage direct current (HVDC) interconnectors utilizing the modular multilevel converter (MMC) topology. However, the rigid bipolar configuration is gaining significance, as first projects are currently in planning stage. The purpose of this paper is to analyse both HVDC scheme configurations with regard to cable voltage stresses originating from short-circuit faults. The study focuses on HVDC cables with extruded insulation connected to MMCs equipped with half-bridge (HB) submodules. Basic design aspects in rigid bipolar configuration such as the station grounding concept are discussed. Moreover, various station internal as well as dc faults are examined by means of electromagnetic transient (EMT) simulations. Occurring cable voltage stresses are analyzed with respect to voltage polarity, peak value and wave front-parameters and compared in symmetrical monopolar and rigid bipolar configuration. It is demonstrated that cable overvoltages with same polarity as the dc operating voltage are significantly reduced in schemes operating in rigid bipolar configuration. Voltage reversals caused by the discharge process during a cable fault are nearly independent of the selected scheme configuration. Obtained results are relevant with regard to upcoming projects in rigid bipolar configuration and provide insights to further refine insulation co-ordination aspects related to dc cable systems.