Influence Of HVDC Ground Electrode Current On Grounding Transformer And Its Restraining Measure

As the development of High Voltage Direct Current Transmission (HVDC) technology in China, it plays an important role in power transmission and also brings some new problems.During Monopolar operations of the HVDC electrode, significant DC currents can be observed in the transformer neutrals of substations in the vicinity of the electrode of the HVDC converter. There are no doubts on the origin, cause and mechanisms of the DC currents that appear in the neutral of the high voltage transformers. A portion of the DC current that is flowing between the two HVDC electrodes is captured by the grounding systems of the transmission and distribution lines and substations located in zones where the earth potentials are high and is discharged back to soil by the grounding systems of the electric network at the locations where the earth potentials are lower. In other words, Part of the DC current injected into the electrode finds an easier return path through the AC system via the grounded neutrals of the transformers. This DC current through transformer neutrals may make the transformer DC bias and therefore, increases the noise of transformer. Furthermore, it can cause many problems due to excessive core vibration, overheating regarding the integrity and longevity of the transformers and other related instrumentation problems. So the abnormal operation of transformer will cause the unsafe operation of substation and power plant. The voltage total harmonic distortion of 220kV and 500kV AC transmission network will increase sharply and these will impact other equipments operation. Sequentially, it may damage the transformers and the whole network. In a word, the HVDC ground electrode current makes the transformer with earthed neutral point as a harmonic source.With the increasing of HVDC project of China, the total time of monopolar operations caused by new projects puting into operation and annual overhaul is getting longer. And as an emergency maintenance mode, monopolar operations mode is inevitable. Therefore, the research of transformer DC bias caused by the DC transmission in recent years has become a very active and important research topic.By using field-circuit coupled method, the distribution of the earth surface potential (ESP) calculations are divided into two parts, including the ground and power grid. The formulas of complex image method (CIM) in the horizontal multi-layer model and vertical multi-layer soil model are deduced. The simulation results of the seas, mountains, lakes and rivers in the vicinity of HVDC grounding electrode show that soil structures have a great impact on the surface potential distribution and the size of DC current through transformer neutral point. Selecting the converter station site is an effective way to reduce the impact of HVDC grounding electrode current on AC power grid.Proceeding from the theoretical model of the transformer, the excitation current and its harmonic components of transoformer in the DC bias case are analyzed. By using the finite element mothod (FEM), the model of the transformer in the DC bias case is established. The results show that the mechanism of transformer DC bias. A practical method to test the DC bias current withstanding capacity of transformer is proposed.In this paper, a novel transformer neutral DC current blocking device (NCBD) based on inverse parallel thyristors protection is proposed and developed to solve DC bias problem. Based on the analysis of the topology and operating principles of NCBD, the main circuit is simplified by removing the front-end rectifier circuit and the turn-off circuit of thyristor. Thus, the cost of the device can be cut down a lot. Moreover, the reliability and intelligence of NCBD are improved by its dual redundant structure, which is made up of a set of hardware integrated circuit and a set of microcomputer measure-controlling system. Simulation results, high current tests and field operation results confirm that the NCBD can resolve the demand of DC bias restraint effectively, and provide a practical based engineering solution.