直流入侵三相三柱变压器励磁电流及谐波计算

Computation of Exciting Current and Harmonic for Three-phase and Three Limbs Transformer Under DC Inrushing

直流输电系统单极运行时一大地为直流电流的回路,导致大地电位相对于无限远处升高。直流电流通过两个Y0连接的变压器中性点侵入电力系统,故针对三相三柱变压器直流入侵情况建立了直流入侵下的三相三柱变压器的电路模型和相应的方程,结合三柱磁路的麦克斯韦方程和非线性的B—H曲线方程,利用数字迭代法,在MATLAB环境下编程计算了三相三柱变压器直流入侵下的变压器励磁电流和谐波特征。结果表明直流入侵三相三柱变压器下并无磁通的偏移,励磁电流波形亦无畸变,但波形总体沿纵轴方向平移,平移大小为入侵直流电流的大小,励磁电流增加了直流分量,这将增加电路的电阻性热损耗,其它次谐波没有变化,结论为直流入侵三相三柱变压器没有直流偏磁现象。这与文[7]的试验结果完全一致,从计算上验证了试验结果。

DC current will flow into earth in the case of single-pole operation in DC power transmission system, which leads to the potentials rising of grounding spot of nearby transformers of AC transmission system. DC current flows in the windings of transformer through the grounding spot of YO-connected transformer of AC system. From research, It is known that the flux of single-phase transformer would offset and exciting current of single-phase transformer would be distorted under DC current inrushing. In this paper, taking three-phase three limbs transformer （6300kVA capacity and 110 kVA voltage rate） as example, the circuit equation for three-phase and three limbs transformer under DC rushing is built. Using Maxwell equation in quasi-stationary in magnetic circuit and nonlinear single-value curve equation of core, six independent equations is obtained. A series of transforms are made and a matrix equation is formed. The matrix equation is dispersed and solved by iterative method. The waveform and harmonies of flux and exciting current for three-phase and three limbs transformer are gained in MATLAB. The results could be generalized as following. Exciting current of three-phase three limbs transformer is smaller than that of single-phase transformer with same rated power because of the magnetic coupling among three limbs. The exciting current of middle limb which contained larger third order harmonic has two peak values and is distorted most severely. The harmonies of exciting current contained first and low odd order components. DC inrushing will not lead to flux bias because of the counteraction of DC flux among the three legs. The waveform of exciting current keeps unchanged under DC inrushing. DC current inrushing three-phase three limbs could lead to heat loss increasing in power system and could not make the exciting current distort. The computation result is consistent with experimental result of document .