While HVDC transmission runs in monopolar mode, a large direct current will enter into the earth through its earth electrode. This current may cause serious interference on buried pipeline even though it is far away from the earth electrode. In this work, numerical simulation was used to study the mechanism and influencing factors of HVDC interference. The results indicated that while a large direct current entered into the earth through the earth electrode, it could create a strong electric field and induce serious interference on the pipeline. The better the pipeline's coating was, the higher interference the pipeline suffered. Mitigating grounding beds had interaction with each other. The mitigating effectiveness was decided by the location and ground resistance of each mitigation grounding bed. Then, a field experiment was carried out to verify numerical simulation results.
High voltage direct current (HVDC) transmission plays a more and more important role in China, due to the energy resource mainly existing in western China. However, the majority of energy consumption is in eastern China. HVDC transmission is usually performed using a bipolar wire-return system. This system has two running modes: bipolar mode or monopolar mode. In bipolar mode, which is the normal running condition, one wire is positive and the other wire is negative. Current through the earth is unbalanced current between the two wires, which is usually less than 1% of wire current (See Figure 1). In the case of a fault or equipment repair, bipolar mode generally turns to monopolar mode. The earth is used as a conductor, which results in large current entering into the earth and inducing serious interference to buried pipeline near HVDC earth electrode (See Figure 2).
HVDC interference will cause serious hazards to buried pipeline such as electric shock risk, corrosion of pipeline, damage of equipment, hydrogen embrittlement, etc. There were several HVDC interference cases around the world.[2-5] Peter Nicholson reported the HVDC interference caused by Quebec-New England Intertie. Verhiel investigated the interference of B.C. Hydro's HVDC system on a 24-in crude oil pipeline. The results indicated that 1200A DC current entering into the earth from earth electrode might induce 0.214V shift of P/S (pipeline to soil) potential. There were several HVDC interference cases detected during our field test. For example, in Shanghai eastern China, a ±500kV HVDC earth electrode being about 20km far away from pipeline induced 2V shift of P/S on-potential, when the earth electrode ran in monopolar mode. In Guangdong southern China, a ±500kV HVDC earth electrode being about 7km far away from pipeline induced 304V shift of P/S on-potential. Moreover, field test results indicated that the mitigating grounding bed might increase the interference of other segments of the same pipeline.