HVDC ground current of the transformer and its suppression measures

　　Title: HVDC ground current of the transformer and its suppression measures
　　Author: Wang Sheng
　　Degree-granting units: Huazhong University of Science and Technology
　　Key words: transformer;; DC transmission;; pole earth loop operation mode;; DC Bias;; neutral blocking capacitor device
　　Abstract:
　　With the high-voltage DC transmission (HVDC) technology network in the country more and more applications, due to its large transmission capacity, transmission distance, adjust quickly, flexible operation, HVDC transmission capacity in long-distance, http://www.999magnet.com/ regional interconnected power system has played a very positive role, but it also brings some new problems. Found in actual operation, when the DC transmission system uses the earth loop operation mode, from the great land of DC current flowing near the AC system will have an impact, especially near the ground electrode will be directly grounded neutral point DC transformer produce more serious Bias, which not only increases the transformer reactive power consumption, affecting the normal operation of the transformer, causing relay malfunction, but also caused a substantial increase of harmonics will also have a greater impact on other electrical equipment, which directly affects the substation , the safe operation of power plants. With HVDC project in China increasing, direct current transmission project put into operation in time and unipolar DC transmission lines each year overhaul of the earth caused by the pole longer total running time, and the DC transmission system operating mode as the earth loop failure the emergency mode is basically inevitable, therefore, caused by the HVDC transformer DC Bias-related research in recent years more and more attention.
　　In this paper, a combined field-circuit approach, the DC loop mode transmission pole earth surface potential (Earth Surface Potential, referred to as ESP) and distributed computing grid is divided into two parts to calculate the ground; deduce the complex image method in the multi-level , vertical multi-layer soil model formula; through the ocean, mountains, lakes, rivers and other terrain alone in the presence of simulation, analysis of terrain factors on the grounding of the grounding performance and very near the earth surface potentials; as potential distribution on the surface of soil structure and the DC current into the transformer neutral point of great influence, through the rational choice of the converter station site for ways to reduce the DC current of the AC power ground effects.
　　This departure from the theoretical model of the transformer, research and analysis of the transformer in the DC bias excitation case, the excitation current and the harmonic changes; established finite element model of the transformer by finite element simulation of the transformer mechanism of magnetic bias occurred; and proposed a practical way to test the tolerance of the DC magnetic bias current transformer capacity.
　　This paper designs a new type of transformer Magnetic lifter neutral blocking capacitor device (Neutral DC Current Blocking Device, referred NCBD), and the blocking device in general compared to the capacitor, the rectifier circuit and no additional turn-off thyristor circuit, the main circuit topology to a large number of simplified, lower unit costs, while the NCBD pure hardware circuit protection and computer protection fully redundant dual technology structure to improve system reliability and intelligence. By analyzing the topology and operating principle, discussed the device DC current the capacitor protection and suppression. Simulation results, and high current test equipment in Guangdong Power Grid, indicating that the NCBD can effectively solve the HVDC pole earth loop mode operation due to DC bias magnetic transformer grounding problem.
　　Degree Year: 2010