| With the general principle of"West to east power transmission, North-south power exchange and nationwide interconnection"in our country, the HVDC will be extensive adopted in long-distance power-transmission. But, after the use of the HVDC technology, a certain harmful effect for the transmission facilities in AC power network will be elicited, such as neutral point DC bias for transformer. This adverse effects includes the increase for the virtual value of magnetizing current,the proportion of high-order harmonic,the corresponding loss and the higher leakage magnetic flux because of the core saturation, and so on. These effects will lead to local overheating and insulation aging and other issues for transformer.The main purpose of this research project is to prevent DC bias through transformer neutral caused by a DC current from earth arising from two kinds way, one is the low-frequency circular current(Compared with the 50Hz, It is Equivalent to DC)caused by Geomagnetic storm mainly in earth-west grid. The second is the HVDC operating in unipolar operation mode. There are three main measures to restrain DC current through transformer neutrals which are connected to ground: putting a resistor in series between transformer neutral and ground, string capacitors between transformer neutral and ground, imposing reverse current to transformer neutral point. The three kinds of methods have their own advantages and disadvantages. For example, because of their own factor, such as the ability to withstand high temperatures and voltages, the first two can only be applied for small and medium-capacity transformers; the third one is best suited to large-capacity transformer.For the need of a power substation, this paper designs and produces a DC offset supply to suppress the transformer neural point DC bias according to the generation mechanism of DC bias and a suppression method- DC-bit Compensation Act. This offset supply can maintain the neutral DC to a extent permitted by transformer, of course the control system of the supply can display the present DC value in transformer neutral point. |
