Neutral Return Current Options for LCC and VSC Stations

Abstract

This chapter describes different options for the neutral return current for LCC and VSC links. In an HVDC transmission system, neutral return current occurs as the result of unbalance in the converter pole currents in a bipolar system or in the case of a monopolar link it is the full converter current. The neutral return current will flow in the provided neutral return path consisting of either electrode lines and ground electrodes or a dedicated metallic return (DMR) conductor. Ground electrodes are used in HVDC transmission systems to provide a low resistance current return path during both monopolar and bipolar operation. Ground electrodes may be applied for Line Commutated Converters (LCC) or Voltage Sourced Converters (VSC) configured in a bipolar configuration. In a monopole HVDC system, the neutral return current is equal to the pole current as shown in Fig. 1. In a bipole HVDC system under normal operation, the neutral return circuit carries only the bipolar imbalance current. A bipolar system can operate temporarily or for long periods as monopolar with ground return during single pole outages due to fault conditions or during pole maintenance. The current to ground is equal to the pole current. In some cases, especially where the interference to other facilities due to the ground current is a concern, bipolar transmission systems may also include a dedicated metallic return conductor or may use the conductor of the faulted pole as the neutral return path to limit the duration of earth return operation.