| At present,there are abundant primary energy in Northwest China,but the load of electricity is mainly concentrated in the eastern coastal and central areas.In order to solve the problem of unbalanced distribution of energy and power load in space,many special and super DC transmission projects have been built in China to realize long-distance and large capacity transmission.With the operation of UHV DC transmission project,it has more and more harm to AC power system.When the HVDC project is in debugging,maintenance or failure,the system is transformed from bipolar operation to single polar earth loop operation.The DC current up to thousands of amperes is injected into the earth through the grounding electrode,which causes the surface potential change near the grounding pole,which leads to DC magnetic bias phenomenon of power transformer.DC bias current enters the winding through the neutral point of transformer,which makes the magnetic flux inside the transformer core saturated seriously,and the waveform of excitation current is distorted,which causes the transformer noise increase,vibration aggravation and local overheating.As the core equipment of power system operation,power transformer plays a very important role.Therefore,it is necessary to study the DC bias of transformer.The main contents of this paper are as follows:1.This paper introduces the calculation method of the ground potential caused by the DC grounding current in several common soil models,analyzes the shortcomings of the calculation method of DC bias current in the high voltage DC transmission regulation,and puts forward the calculation method of the total electric field.In theory,the circuit method in the gauge and the total electric field method are used to calculate the DC bias current of the substation in the simple model The results obtained by comparing the two methods show that the method of procedure is not suitable for the area with dense power grid,while the full electric field method is more applicable in the dense area of power grid.Finally,the simulation model is established by combining with the actual local power grid,and the effectiveness and superiority of the full electric field method for calculating the partial magnetic current are verified.2.A simplified model is used to analyze the influence of different factors on the calculation of DC bias current,including: distance between grounding pole and substation,soil resistivity,fault,and environmental temperature.Comparing the difference of the results of the calculation results of the whole electric field method and the whole electric field method,it can be seen that the total electric field method is much smaller than the regulation method in calculating the partial magnetic current affected by the above factors.Taking a local power grid as an example,the author studies the partial magnetic current algebra and the spatial distribution characteristics similar to potential when the single pole earth circuit of HVDC transmission project is running.The fundamental reason is that the ground electrode current causes the change of the current field in the ground,then causes the change of the ground potential,and then causes the partial magnetic DC between the transformers of the power grid.3.The main three methods of DC current control are compared and analyzed,and the advantages and disadvantages of each method are summarized.In view of the complex condition of two grounding poles operating at the same time in the local power grid,the DC bias of the whole substation is calculated based on the simulation model based on the full electric field method.The substation with the largest DC bias is treated by capacitance type and resistance DC bias control device respectively.On this basis,the resistance type control device and capacitance type control device widely used in practical engineering are reasonably allocated to comprehensively manage the partial magnetic problem of local power grid. |
PDF Full Text Request