Study On The Transformers Under DC Bias

Direct current (DC) bias phenomenon is that the DC flows into the windings of the transformers through neutral point, which is an abnormal work station. There are two reasons which arose DC bias:One is the monopolar ground circuit operation mode or the bipolar asymmetrically operation mode of the HVDC transmission system. Under this situation, DC will pass by the windings of the transformers through the earthed neutral point. The other is Geomagnetically Induced Current (GIC) which is caused by the interaction of the geomagnetic field and the dynamic movement of the ionic wind. The change of the geomagnetic field produces electric potential gradient, which give rise to low-frequency induction current. It is DC because its frequency is very low.The work point of the transformer under DC bias will shifts, and its iron will be saturated which affect the normal work of the transformer seriously. This gives rise to lots of bad influence. for example, the peak value of the exciting current increases significantly, and the waveform of the current is seriously distorted with a lot of harmonics; the heavy noise; the serious vibration; the increases of transformer’s leakage and losses and temperature build-up; the local heating of metal structures; the malfunction of relay protection devices of electric power system; which endanger the operation safety of the transformers and electric power system. So the DC bias phenomenon of the transformer is concerned.This paper make a thorough study of the physical mechanism of DC bias. On this basis, the transformer under DC bias is studied by experiment and calculated by simulation. The main research contents are as following:Firstly, the feasible experiment scheme of the single-phase transformer and the three-phase three-legged transformer and three-phase five-legged transformer under DC bias is proposed. The exciting currents and B-H curve of the transformers under DC bias are measured.Secondly, this paper proposes a novel transmission-line model (TLM) of the single-phase transformer under DC bias. In the model, the differential terms in transformer circuit model are discretized. The saturation characteristic and hysteresis effect is described on the basis of J-A ferromagnetic hysteresis theory, and the eddy-current losses resulting from the flux paths through legs are replaced by an equivalent resistance. Furthermore, the copper losses and flux leakage of the primary and secondary windings are also included. So the transformer is described sufficiently and completely. At last, the Newton-Raphson technique is chosen to calculate the model. The results compare with the experiment results. That demonstrates the validity of the TLM and effectiveness of analyzing the DC bias phenomenon of the single-phase transformer.The TLM of the three-phase transformer under DC bias is build. And the Newton-Raphson technique is chosen to calculate the model. The simulation results demonstrate that the TLM can exactly analyze the phenomenon of the three-phase transformer under DC bias.A novel coupling model of electric circuit and magnetic circuit of the three-phase three-legged transformer and three-phase five-legged transformer under DC bias is built. In the magnetic circuit model, the eddy-current losses, the core topology and the saturation characteristic of the core material are taken into account. The magnetomotive force due to eddy current is given in magnetic circuit equations. By combining an electrical circuit with nonlinear characteristic curve of the core, nonlinear equations can be obtained. A series of mathematic transformation are performed and numerical nonlinear algebraic equations are solved finally. Meanwhile DC bias problem of the three-phase three-legged transformer and three-phase five-legged transformers are analyzed. The simulation results compare with the experiment results. That demonstrate the validity of the coupling models and effectiveness of analyzing the DC bias phenomenon of the three-phase three-legged transformer and three-phase five-legged transformer.Lastly, this paper introduces several measures of restraining DC bias based on theory analysis.①reverse current method②series-capacitor method③ series-resistanee method. The three measures are studied by simulation.