Study On The Operation Characte Ristics And Protection Measures Of The Electric Equipment Under The DC Monopole Working Mode

In China, the disproportion of power resources supply and demand promotes the development of the long-distance and high-capacity transmission lines. Therefore, the HVDC transmission plays an increasingly important role in China’s power grid construction. When the HVDC transmission system working at the monopolar ground circuit mode, the DC current generated by the HVDC system will flow into the ground point of transformers and makes the transformer core saturated seriously, the magnetic flux leakage increased, and produces great harmonic current as well. The DC magnetic biasing may be amplified by the reactive power compensator and causes resonant, which will be a great threat to the safe operation of the electric power capacitor and do harm to the power grid. This problem is quite prominent in HVAC power system, because of which is the starting point of several HVDC transmission lines and served as the source of the national network. It makes the HVAC power system much easier to be influenced by the HVDC system operating at the monopolar ground circuit mode. Therefore, studying the mechanism of the transformer DC bias and the variation of the magnetic dc exciting current, realizing the influence degree of the partial magnetic harmonic wave on the reactive power compensation capacitor, proposing the methods of restraining the DC magnetic biasing and protecting the safe and stable operation of the HVAC-DC power grid have important academic value and project significance.This thesis expounds the operating properties of the transformer in the HVDC transmission system working at the monopolar ground circuit mode, concludes the calculating methods of the exciting current when the transformer under the condition of DC bias, sets up the "magnetic circuit-electric circuit" simulation system of the transformer and studies the influencing law of the DC bias on the transformer’s exciting current. According to the Fourier Analysis Method, the change rule of each harmonic amplitude and the total harmonic distortion (THD) rate was obtained. And on this basis, the influence mechanism of the harmonics on the reactive power compensation capacitor was researched, the simulation of the impacts was carried out to evaluate the harm of the harmonics to the reactive power compensation capacitor and the relationship between the variety of the capacitor’s voltage/current harmonics and the current through the transformer’s neutral point was also revealed in this paper.Aim at the D.C. magnetic biasing, three control measures were proposed in the paper, as the method of connecting a resistance or capacitor at the transformer neutral point in series and the method of injecting reverse current through the neutral point. This paper analyzes the principle of the three kinds of controlling measure in detail and the simulations were carried out for the situation when the power system has an accident of single phase short circuit in order to ascertain the optimal scheme and technical parameters for restraining the DC magnetic biasing, also the possibility to use the capacitor to restrain the DC current was demonstrated. It is suggested that the way of connecting capacitor at the transformer’s neutral point in series meanwhile in parallel the current bypass protection device, which could effectively solve the DC magnetic biasing problem at a lower cost.