Analysis On Transient Electric Field Distribution For High Voltage Converter Transformer’s Polarity Reversal

HVDC is an important way to solve high voltage, large capacity, long distance power transmission and grid interconnection. The failure of equipment in DC transmission system is a primary factor to the forced outage of a DC system, and the failure of converter transformer caused the highest number of DC system forced outage.In operation, different from ordinary power transformers, converter transformer withstand not only high magnitude DC and AC voltage and a variety of over-voltage, but also many other dynamic DC voltage, include staring-, stopping and polarity reversal voltage. Due to the fact that static and dynamic electric field distribution are closely associated with the resistance, which is influenced by temperature, impurities, material and field strength, so there is a big uncertainty on the design of DC insulation. Further study on the distribution characteristics of the AC-DC hybrid electric field and PR electric field has great significance to converter transformer’s design, manufacture and maintenance.The polarity reversal electric field of converter transformer belongs to quasi static electric field, a state equation was built based on the finite element method. The boundary conditions were combined into the equation by means of penalty method. Then Several commonly used numerical methods were compared, for the nonlinear medium problem, we need to use the so called Runge-Kutta method.A practical converter transformer’s losses were calculated by using finite element method in combination with Ansoft commercial software. and then using Gambit software to establish a converter transformer model(including the oil-paper insulation structure surrounding) which can be used to do some numerical calculation, then some parameters were sited to the model, and apply the loss data calculated to the core and windings of converter transformer correspondingly, the temperature contours of transformer can be calculated by Fluent software.Finally, the transient electric field was calculated of a practical converter transformer and valve-side outlet bushing under polarity reversal test voltage, which take the electric field strength-conductivity nonlinear character and temperature-conductivity nonlinear character into account, and through contrast the calculation results of linear to nonlinear, the influence of nonlinear character on electric field distributions can be got, also provide the basis for the accurate designing of insulation.