Types of Earth for Ground Return in HVDC Monopolar Operation

摘要

The monopolar operation of a bipolar dc system involves current return though (ⅰ) Ground or (ⅱ) Metallic Return (Using the conductor in the other pole). It is to be noted even that if the insulation is damaged in a pole conductor, it can still be used as a return path as steady-state voltage stress is small. With metallic return, only one terminal is grounded and other terminal is ungrounded. There are advantages and drawbacks associated with each type of return. With ground return, the losses are less (losses are associated only with the ground electrodes). However, there is a danger of corrosion of buried pipe lines. The ground current may also interfere with DC type rail road signals and may cause saturation in transformers that are located in the vicinity, although these problems can be overcome, the solution may be complicated and restrictions are usually placed on extended operation with ground return. While the metallic return does not have the problems associated with ground return, it has the following drawbacks. 1. Increased losses due to voltage drop in the metallic return. 2. The increased voltage drop requires larger tap range in converter transformers. This is the operation of converters at large angles which result in higher value stresses.