Analysis And Suppression Of DC Bias Of UHVDC Receiving-end Power Grid

DC bias current refers to the DC current in the ground wire of the transformer.This DC current will result in distorations in the excitation currents of the transformer,which will cause a lot of damage to the transformer.The distortion of the transformer excitation current will saturate the core of the transformer magnetically,which will cause the transformer to overheat locally and reduce the service life of the transformer.The DC bias current will also cause a large number of harmonics in the AC grid,which will increase the reactive power loss of the transformer and affect the reactive power loss of the AC grid.Also,the noise of the transformer will increase,which will affect the normal operation of the transformer.UHVDC transmission generally operates in a bipolar manner and has return wires.However,in the early stage of system constructions,for economic reasons,sometimes only one line is used.This operation mode is called a unipolar earth return line one.The ground acts as a return wire.As a result,the ground will generate a potential difference,and the DC current will enter the AC grid via the substation directly grounded at the neutral point,causing a DC bias phenomenon.Currently,there are mainly two methods that can be used to calculate the DC bias current of the transformer,one is the direct coupling method of the field-circuit and the other is the indirect coupling method of the field-circuit.The field-circuit direct coupling method comprehensively models the above-ground part and the underground part,and calculates the influence of the DC bias current on the ground surface potential.The calculation result is more accurate but the calculation process is more complicated and takes a long time.The field-circuit indirect coupling model separates the above-ground AC power grid part from the underground soil part and calculates it separately,ignoring the influence of the DC bias magnetic current on the earth’s surface soil potential.The calculation process is however simple and computationally effcient.Based on the parameters of the substations and lines of the AC power grid provided,the disertation investitagtes the DC bias current using the direct and indirect coupling methods of the field-circuit.The results of the two methods are compared with the experimental measurement ones to verify the accuracy of the model.The effects of surface and deep soil resistivity,the distance between the station and the DC ground electrode,and the parallel operation of multiple transformers on the DC bias current distribution were also studied.Based on the DC bias simulation model and multi-valued coding genetic algorithm,an optimal configuration model of the DC bias suppression device is proposed.The minimum number of current limiting devices to be installed is minimized.The DC bias current of each substation that does not exceed the constraint value is selected as the conditions,iteratively calculate the optimal installation plan of the resistance and capacitance suppression device.Finally,on the basis of the above-mentioned DC bias magnetic simulation model and the optimal configuration model of the suppression device,this dissertation explores a case study of a receiving-end AC grid within 150 km of Jinsi electrodes with 99 substations.The current value calculated by the DC bias simulation model is close to the experimental value,which verifies the accuracy of the model.And by optimizing its configuration model,it can effectively reduce the DC blocking device by 41.3%,verifying the effectiveness of the optimization model.Compared with a single optimized configuration of a resistor device or a capacitor device,this optimization methodology is more in line with the actual.