Study On The Load Loss Of HVDC Converter Transformer Under Nonsinusoidal Current

The HVDC converter transformers are under the nonsinusoidal servicing conditions,which are consisted of the nonsinusoidal load current for high harmonic components and the DC bias,and load losses of converter transformers would change in those service conditions.The harmonic components of load current are considered during predelivery tests,but more accurate results are still needed.Meanwhile,variation of load losses caused by the DC bias is not considered.Thus,load losses determined in the predelivery tests are not the real value and would lead to a higher temperature rise.Hereby,the characteristics of load losses under the load current with high harmonic components and DC bias are analyzed for determining the load losses of converter transformers.To analysis the winding eddy current losses in load losses,the calculation method for winding eddy current losses is derivated by analyzing leakage flux of transformers based on magnetic quasi-state equations.And the frequency characteristic is proposed,winding eddy current losses follow the frequency with the exponent 2,and then multiply by F(Ka)which is the skin effect factor.Subsequently,the FEM analysis of a converter transformer is conducted in which the complex geometrical dimension and electrical connection of windings are taken into consideration,it is shown that frequency characteristic represented by F(Ka)is the most confirmed to the true value compared to the frequency characteristics proposed by IEC 61378-2 and Emanuel.At last,winding eddy current losses of 4 coils are measured to verify the validity of F(Ka),meanwhile,it is shown that the proposed frequency characteristic is not limited to the apply of converter transformer,but also the common coils.It is recommended that F(Ka)should be used to correct the frequency characteristic proposed by IEC 61378-2.To analysis the stray losses in structural parts,the TEAM Problem 21 family which is the benchmark for stray losses in structural parts of transformers is used to analysis stray losses in structural parts,and the harmonic stray losses in structural parts of 15 sub-models are measured with the tested harmonic frequencies ranged from 50Hz to 2500Hz.According to the measured data,the frequency characteristic that stray losses in structural parts follow the frequency with the exponent 0.8 proposed by IEC 61378-2 would lead to less value.Subsequently,the frequency characteristic is divided into the magnetic part and nonmagnetic part by analyzing the measured data.Frequency characteristic for magnetic structural parts is set as the frequency characteristic for the whole structural parts for losses in magnetic structural parts accounts for the majority of stray losses in structural parts.At last,by analyzing the variation of leakage flux distribution in structural parts,it is proposed that stray losses in structural parts follow the frequency with the exponent 1.5;the validity of the proposed frequency characteristic for structural parts is verified with the measured data.As for the variation of load losses under DC bias,the mechanism is revealed that the DC bias would lead to the increase and distortion of the exciting current,and the responded no-load leakage flux would share the same change.Thus,the eddy current losses windings and structural parts induced by no-load leakage flux would increase and cannot be neglected.Subsequently,the experiment of BK-1000VA under DC bias is conducted to verify the theoretical analysis.It is shown that exciting current would increase with the DC current,and the high harmonic current would increase faster than that of the rated frequency.The stray losses in simulated tank are separated from the total losses,and it is shown that stray losses in simulated tank would increase under DC bias obviously.At last,a simplified method is proposed to determinate the variation of load losses under DC bias,which is based on the data of 1.1 times over excited no-load test.The corrected load losses calculation method is proposed by summarizing the whole paper,and then it is compared to the calculation method proposed by IEC 61378-2 to reveal the difference.Subsequently,the harmonic load losses of 2 converter transformers are measured to verify the corrected calculation method.At last,the temperature rises under service condition are measured and compared to those of predelivery tests.It is shown that the temperature rises under service condition are higher and load losses and temperature rises acquired by IEC 61378-2 are not the real value.