Influence Of DC Bias On Active And Reactive Loss Of Converter Transformer And Commutation Angle

When DC transmission operates in monopole or geomagnetic storm occurs,DC flows through the neutral point of the transformer.DC bias will cause a series of problems,such as the increase of harmonic component of transformer load current,reactive power loss,local overheating,vibration and noise,and threaten the safe and stable operation of the transformer.In recent years,with the continuous development of DC transmission project,its voltage level is continuously improved,the resistance value of conductor on transmission line is becoming smaller and smaller,and the DC bias problem of converter transformer is becoming more and more serious.Different from AC transformer,both sides of converter transformer are connected with AC and DC systems respectively.Its structure and operating conditions are more complex,and the influence of DC bias is also more complex.Therefore,this dissertation takes the commutation process participated by the converter transformer,nonlinear load current and the difference of star delta connection on the valve side as the starting point to study the commutation angle,active power loss of structural parts and the change of overall reactive power consumption of the converter transformer under DC bias.The main work and achievements of this dissertation are as follows:(1)Influence of DC bias of converter transformer on commutation angleConsidering the excitation branch in the T-circuit model of converter transformer,the mathematical expression of commutation angle under DC bias is deduced;A commutation finite element model considering the change of commutation inductance is established.The variation of commutation angle on rectifier and inverter side with trigger angle of converter,direction and magnitude of DC and the relationship between six commutation angles in a cycle are studied.Using the simulation data of commutation current,the waveforms of DC system voltage and AC system phase current under DC bias are constructed,and the influence of commutation angle on its harmonic content is studied.The results show that the commutation angle will decrease under DC bias.The smaller the trigger angle is,the greater the DC is,and the more the commutation angle decreases;The reduction of commutation angle makes high-frequency non characteristic harmonics appear in the phase current of AC system,and the number of voltage harmonics in DC system changes from a multiple of six to a multiple of three.(2)Study on eddy current loss(structural loss)of tank and clamp of converter transformer under DC biasThrough the magnetic circuit method,the distribution of leakage magnetic field on the surface of tank and clamp of converter transformer under rated and no-load DC bias is analyzed.Using the load current data obtained in(1),the eddy current loss of tank and clamp of 800kV converter transformer is simulated and calculated by finite element method,and the DC bias experiment of 380V scaled transformer is carried out.The variation relationship between eddy current loss of tank and clamp and DC and trigger angle of converter is studied.The results show that the loss of structural parts is positively correlated with DC and negatively correlated with the trigger angle of converter;The loss value under DC bias is equal to the rated loss multiplied by the loss proportion coefficient plus the no-load DC bias loss,in which the loss proportion coefficient is 0 to 1.The larger the DC current and the trigger angle of the converter,the smaller the value.According to the research conclusion,a simplified calculation formula of structure loss of converter transformer under DC bias is constructed.(3)Study on eddy current loss of flitch plate of converter transformer under DC biasA simplified mathematical model of eddy current loss of flitch plate under rated sinusoidal excitation is established.Combined with finite element simulation calculation,the variation law of eddy current loss and current frequency of flitch plate is studied.According to the research idea of eddy current loss of tank and clamp in(2),the relationship between eddy current loss of flitch plate of DC bias and DC and trigger angle of converter is further studied.The results show that the loss of converter transformer flitch plate with two full slots is directly proportional to the 1.47th power of frequency under rated state;The loss of flitch plate of DC bias is approximately equal to the sum of rated loss and no-load DC bias loss,and the relationship between no-load DC bias loss and DC is similar to linear,and the proportion coefficient is 0.038.According to the research conclusion,the frequency index in the rated loss formula of flitch plate is modified,and the direct calculation formula between eddy current loss and DC of flitch plate of DC bias is constructed.(4)Study on the relationship between geomagnetic induced current and reactive power loss of converter transformerFor the converter transformer with star winding on the valve side,the core i-φcurve is expressed linearly in two sections.The static mathematical relationship between reactive power loss and GIC(Geomagnetically Induced Current)is studied.Accordingly,the reactive power loss finite element simulation calculation and experimental measurement are carried out to verify the above research conclusions.Considering the triangular connection mode of valve side winding,the influence of quasi DC characteristics of GIC on reactive power loss is studied.For different converter transformer parameters in the circuit model,the sensitivity of its influence on reactive power is analyzed.The results show that based on the actual GIC level in China,the relationship between static reactive power loss and GIC is linear,and the proportion coefficient is between 0.9-0.95;Considering the influence of delta connection winding,when the flow direction of GIC changes from inflow into converter transformer to outflow,the reactive power loss will increase compared with the static value,and vice versa.Rated flux and delta winding resistance have the greatest influence on reactive power loss,and the sensitivity coefficients are:-3.36 and-0.33 respectively.This dissertation not only considers the structural size and material factors of the converter transformer itself,but also simplifies its working conditions(commutation,nonlinear load current and valve side winding wiring mode),which are included in the analysis model,and analyzes the characteristics of the converter transformer under DC bias.The research results can provide a theoretical basis for quickly evaluating the severity of DC bias(commutation angle,active power loss of structural parts and overall reactive power consumption)of converter transformers under different working conditions in rectifier and inverter stations.