The Influence Of Direct-current Magnetic Biasing To High Voltage Equipment And Countermeasures

With the general principle of "West to east power transmission, North-south powerexchange and nationwide interconnection" in our country, the HVDC will be extensiveadopted in long-distance power-transmission. But, after the use of the HVDC technology, acertain harmful effect for the transmission facilities in AC power network will be elicited,such as neutral point DC bias for transformer. This adverse effect includes the increase for thevirtual value of magnetizing current, the proportion of high-order harmonic, thecorresponding loss and the higher leakage magnetic flux because of the core saturation, and soon. These effects will lead to local overheating and insulation aging and other issues fortransformer.The main purpose of this research project is to prevent DC bias through transformerneutral caused by a DC current from earth arising from two kinds way, one is thelow-frequency circular current(Compared with the 50Hz, It is Equivalent to DC)caused byGeomagnetic storm mainly in earth-west grid. The second is the HVDC operating in unipolaroperation mode. There are three main measures to restrain DC current through transformerneutrals which are connected to ground: putting a resistor in series between transformerneutral and ground, string capacitors between transformer neutral and ground, imposingreverse current to transformer neutral point. The three kinds of methods have their ownadvantages and disadvantages. For example, because of their own factor, such as the ability towithstand high temperatures and voltages, the first two can only be applied for small andmedium-capacity transformers; the third one is best suited to large-capacity transformer.