Dc Bias Of Transformer Research

GIC (Geomagnetically Induced Current) and the monopolar ground circuit operation mode or the bipolar asymmetrically mode of HVDC transmission system have grate impact to the transformers with earthed neutral point because direct current will pass by the windings of the transformers. The influence gets outstanding with the scale of the HVDC transmission engineering expanding. This paper analyzes the reason of direct current arising and its influence to transformers. Moreover, the paper introduces several measures of restraining neutral DC of transformer and analyzes merits and defects of them.First, this paper simulates and calculates the single-phase transformer using MATLAB and ANSYS software based on J-A hysteresis theory. The simulation results show that the curves of both B-H loop and excitation current shift upwards because of DC voltage. The positive peak is 1.4 times under 150V DC voltage of that without DC voltage and the waveform is no longer symmetrical. The calculation results show that the leakage flux would increase under DC voltage. This is the reason why transformers have more serious local heat under DC biasing condition.Then, this paper builds electric circuit and magnetic circuit coupled model of the DC biased three-phase three-legged transformer. In the model, the magnetic circuit is set up by a practical PSPICE model with accurate performance for the three-phase three-legged saturated transformer. Meanwhile, the electric circuit equation under DC bias is built. Then, two matrix equations are formed. The matrix equations are dispersed and solved by iterative method. The waveform of the flux and exciting current for the three-phase three-legged transformer are gained in MATLAB. The calculation results show:â‘ the exciting current of the three-phase three-legged transformer is smaller than that of the single-phase transformer with the same rated power because of the magnetic coupling among three legs.â‘¡The exciting current of the middle leg is smaller than that of the left and right legs because magnetic coupling of the middle leg with other legs is serious.â‘¢The waveforms of exciting currents basically do not distort, and only the DC current component is added up to the three-phase exciting currents respectively. So this has almost no effect to the power system. But the DC current component can increase the copper losses of transformer.â‘£The three-phase fluxes shift up under DC biasing current, but their waveforms do not distort.At last, this paper introduces several measures of restraining neutral DC based on theory analysis.â‘ reverse current methodâ‘¡series-capacitor methodâ‘¢series-resistance method.