三相三柱变压器对偏磁电流的耐受特性研究

Research on the Withstand Performance of Three-phase Three-limb Power Transformer Under DC Current Biasing

为了分析三相三柱电力变压器在高压直流输电系统大地回流作用下运行性能的变化情况,采用电路—磁路耦合的数值方法建立了包括变压器内部主要漏磁回路在内的该型式变压器详细模型,并利用该模型通过改变漏磁磁阻值重点讨论了漏磁回路对变压器耐受直流电流偏磁能力的影响。仿真分析结果表明,三相三柱变压器内部漏磁回路特性对变压器受直流电流偏磁作用的影响不可忽视。当漏磁磁阻减小至一定程度时,该种型式变压器仍然会受到直流偏磁电流的影响,因此在三相三柱变压器设计制造环节应对其漏磁回路加以充分考虑。

When HVDC system operates in monopolar mode and uses the earth as a return path, the electrode current with the magnitude of several kiloamperes can cause significant potential difference between different locations. Meanwhile, there may be direct currents flowing through the power transformers with neutral points grounded, which results in a series of harmful consequences. Under heavy saturation due to DC bias, lots of magnetic flux will be ＂pushed out＂ of the iron core and pass through leakage paths consisting of insulation media and metal parts. Taking the example of a simplified power system with a 105 MVA/220 kV three-phase three-limb transformer, some simulation results regarding the estimation of Manapouri transformer performance for no load case when HVDC earth return current is presented. Instead of using either a branch impedance or an admittance matrix to represent the iron core of transformer, the coupled electromagnetic method based on the core topology is adopted and a threephase three-limb transformer model has been established. It can represent transformer characteristics very accurately with properly decided parameters. In this model, the loop equations which relate the loop flux to the externally applied magnetomotive forces with reluctances as coefficients are obtained. The effects of transformer leakage paths are modelled by constant magnetic reluctances and discussed in detail. The results with different magnitudes of these leakage reluctances show that the characteristics of leakage paths would influence the immunity of three-phase threelimb transformer from DC biasing current. Since such type of transformer will also be affected by DC biasing current when the reluctances of leakage paths are not large enough, the leakage paths should be considered carefully during the design and manufacture of power transformer.