Research On Loss And Temperature Rise Characteristics Of UHV Transformer Under DC Bias

The unipolar operation of high voltage direct current transmission or geomagnetic storm disturbance may cause the DC bias of transformers in the power grid.After the transformer is subjected to DC bias,the core saturation degree deepens,the leakage flux increases,resulting in the loss of the structure parts such as the tie plate,clamp and other structural parts increases,which leads to the transformer local overheating.The UHV power grid adopts eight split conductors with small resistance per unit length,which is more likely to produce large DC disturbance under the same conditions.The capacity of the UHV transformer is as high as 1000MVA,and the volume of the transformer limited by transportation and space can not be increased in equal proportion,which leads to the large flux leakage on the structural parts of the UHV transformer,so the tolerance to the DC bias is poor.Accurate calculation of hot spot temperature rise of UHV transformer structural parts under DC bias has always been an issue of academic concern,which is of great significance to the research on the ability of UHV transformer to withstand DC bias.In this thesis,"single phase four column ultra-high voltage main transformer"(referred to as "UHV transformer")is taken as an example.In the high-medium rated operation state,the loss and temperature rise characteristics of the UHV transformer under DC bias are mainly studied.The research work of this thesis is mainly carried out from three aspects:"measurement of loss and temperature rise characteristics","calculation of magnetic field and loss","establishment of high precision heat flow coupling model".The main research contents and achievements of this thesis are as follows:To obtain the loss and temperature rise characteristics of the actual transformer products under DC bias,the experimental measurement and analysis of the no-load loss and core temperature rise characteristicsIn order to obtain the loss and temperature rise characteristics of actual transformer products under DC bias,the no-load loss and core temperature rise characteristics of a single-phase autotransformer with rated voltage of 500kV products(hereinafter referred to as "500kV transformer")in the transformer plant were carried out in cooperation with the transformer manufacturer and electric power company.The 500kV transformer absorbs a lot of reactive power after DC bias,but the capacity of the generator in the factory is limited.The load test under DC bias cannot be carried out,and its characteristics under DC bias cannot be fully tested and measured in the transformer plant.And because the precise size of the 500kV transformer and other structural parameters belong to the confidential information of the manufacturer,it is impossible to establish a model for detailed modeling and calculation analysis.Therefore,according to the core type and winding layout of UHV transformer,this thesis specially customized a "single-phase four-column auto-transformer scaled-down model",referred to as "SD transformer" for short.The error between the current of SD transformer obtained directly by B-H curve of silicon steel sheet and the measured value is large.Based on SD transformer,two equivalent B-H curve models are established.Model 1 is:considering the influence of the equivalent magnetic circuit length of the model,the equivalent B-H curve(DC magnetization curve)reflecting the overall excitation characteristics of the transformer is calculated based on no-load voltage and current data.The error between the no-load current calculated based on Model 1 and the measured value is small,which verifies the validity of using the DC magnetization curve to calculate the transformer current without or with DC bias.Model 2 is:a two-dimensional finite element model based on the air gap of silicon steel sheet joints is established,and the local equivalent B-H curve of "joint domain" is obtained.Compared with the calculation of current only by using B-H curve of the silicon steel sheet,the accuracy of the current calculated by combining Model 2 with the B-H curve of the silicon steel sheet is improved,and the magnetic field distribution obtained is closer to the reality.Because it is difficult to obtain the no-load voltage and current waveform data of UHV transformer,this thesis calculates the equivalent B-H curve of the "joint domain" of UHV transformer based on the method of Model 2,and applies it to the simulation calculation of the magnetic field and loss of UHV transformer.The calculation models of loss and temperature of SD transformer are established respectively,and the validity of the calculation model and method is verified by comparing with the experimental measurement results.The transformer oil temperature will change with the change of total loss,and the loss of structural parts and oil temperature together affect the size of the hot spot temperature of structural parts.A field-circuit coupling model was established to calculate and analyze the winding loss,core loss and structural eddy current loss of UHV transformer under DC bias.The voltage of the leakage reactance increases after DC bias.Although the eddy current loss caused by the harmonic component in the winding current increases.the total loss of the winding decreases slightly.The core loss under DC bias is calculated by half-wave average method.Because of the nonlinear permeability and structural irregularity of steel structure parts such as clamp and oil tank of transformer,the eddy current loss distribution of steel structure parts under DC bias is simulated by using the transient eddy current finite element software.UHV transformer has large volume,internal structure and complex cooling system.The calculation of temperature rise of structural parts of UHV transformer under DC bias is much more difficult than that of SD transformer.A method for calculating the outlet oil temperature of the cooler under DC bias is proposed based on the heat circuit model and the cooler characteristic equation.This method avoids the calculation error caused by unreasonable simplification of the cooler in the heat flow coupling model and reduces the calculation scale.A heat flow coupling model of UHV Transformer is established.Taking the oil temperature and oil flow velocity at the outlet of the cooler as one of the boundary conditions,the convective heat transfer coefficient and thermal radiation coefficient are used to equivalent the heat transfer between tank wall and air,the steady-state temperature distribution of the structure under DC bias is calculated,and the temperature variation at typical positions is analyzed.The top oil temperature and winding hot spot temperature are less affected by DC bias,and the structure hot spot temperature is more affected by DC bias.The thermal circuit model is used to calculate the transient change curve of the top oil temperature and the high voltage winding hot spot temperature when a GIC flows through the high and medium voltage winding of UHV transformer.The research methods and conclusions of this thesis are of great reference value for the research on the ability of UHV transformer to withstand DC bias.