Research On The Theory And Application Of A Dual-set Inductive Filtering Transformer

As all types of power harmonics in the power network make the operation environment of the power system very complicated and non-idealized,the traditional passive filtering method could not meet the requirements of high-performance operation under the actual conditions of the power network,thus directly affecting the power quality of the power network and the system stability of the power network operation.A three-winding transformer is used to connect with the power network of the three different voltage in the high-voltage?HV?substation.When the power networks of 110-kV and 35-kV exist in harmonic loads,these may deliver to the power network of 220-kV through the three-winding transformer,thus directly influence the power quality of the power network of 220-kV.The traditional passive filtering method is that the tuned filtering branches are parallel-connected to the power network of 35-kV of the three-winding transformer.Since it can completely remove the harmonic loads of the power network of 35-kV and slightly reduce the harmonic loads of the power network of 110-kV,but it cannot thoroughly eliminate the harmonic loads of the power network of 110-kV,so that these will be transferred to the power network of 220-kV through the three-winding transformer,thereby affecting the power quality of the power network of 220-kV.The purpose of a HV substation adopting the traditional passive filtering method is to eliminate the harmonics of the power networks of 110-kV and 35-kV,so that it can improve the power quality of the power networks of 220-kV.But,there is unqualified problem consisting of 11th characteristic harmonic in the HV power network of 220-kV via investigation and field test,so this method cannot solve the practical problems of the HV substation.Therefore,on the basis of both"the Key Research and Development Project of Hunan Province Educational Department"and"State Grid Hunan Electric Power Corporation Project",a dual-set inductive filtering transformer?DS-IFT?as the core of key technology is proposed to treat these harmonics in the HV substation from the theory and key technology of level which are discuss and put forward to solve the series of thought.In this thesis,the theory and application of the DS-IFT are deeply studied,and a relatively perfect theory and application system is formed.The inductive filtering scheme with the DS-IFT as the core is applied to solve the problem of harmonic load of MV and LV power network in the HV substation.The research achievement have deepened the theory of inductive filtering technology,which has broad prospects and furtherly scientific significance for actively promoting the application of the DS-IFT and technical development of inductive filtering in the HV substation.The theoretical research and the related innovative achievement can be mainly summarized as the following aspects:?1?An inductive filtering scheme with DS-IFT is proposed to reduce the load harmonic currents of the power network of 220-kV in the HV substation.The definition of the DS-IFT is following as:when two different secondary windings of the three-winding transformer exist in harmonic loads,if it need to eliminate these harmonics,a filtering winding is added between the primary-and secondary-windings,and the tuned filtering branches are connected for filtering.Two-set three-winding transformers composed of among the added winding,the primary winding,and respectively two different secondary windings meet the condition that the equivalent calculated impedance is zero,that is,the condition of inductive filtering is satisfied.This kind of transformer is called dual-set inductive filtering transformer?DS-IFT?.Therefore,based on inductive filtering technology,the inductive filtering scheme with the DS-IFT as the core is proposed to eliminate the influence of the load harmonics of the power networks of 110-kV and 35-kV on the power networks of 220-kV,reduce the total harmonic current distortion,and improve the power supply quality of the power network of 220-kV.According to the equivalent zero impedance principle of transformer,the DS-IFT with the filtering windings is designed by adjusting the structure layout of transformer windings,and the tuned filtering branches are connected for filtering.The DS-IFT consists of four windings,which of each phase are set on the same core column along the core to arrange layout for the low-voltage?LV?winding?the tertiary winding?,medium-voltage?MV?winding?the secondary winding?,filtering winding,and HV winding?the primary winding?,the LV winding on the innermost,the HV winding on the outermost layer,and its connection is YN/yn0/d11/d11.In the harmonic environment,the DS-IFT is connected with the tuned filtering branches,and the reverse inductive current is generated by the electromagnetic induction filter principle of the transformer to offset the harmonic currents of the MV and LV power networks,so as to reduce the harmonic current carried by the transformer and directly reduce the total harmonic current distortion of the HV power network.?2?Both the radiation equivalent circuit and the port equation of each winding physical quantity for the DS-IFT are established.On the basis of both the mathematical model and quadrilateral equivalent circuit of the four-winding transformer,as well as the inductive filtering mechanism,it is deduced that two equivalent calculated impedances related the filtering winding are approximately equal to zero,that is,the DS-IFT satisfies the inductive filtering condition.It means that the DS-IFT is of four independent short-circuit impedances(the short-circuit impedance between the HV winding and filtering windingZ_K14,the short-circuit impedance between the MV and LV windingZ_K23,the short-circuit impedance between the MV winding and filtering windingZ_K24,and the short-circuit impedance between the LV winding and filtering windingZ_K34),so the radiation equivalent decoupling model with four nodes and four branches is deduced.Therefore,based on the equivalent decoupling circuit,the mathematical equations of each inner winding physical quantity for the DS-IFT are established,and the characteristics of the fundamental operation and reactive power compensation for the DS-IFT are verified through simulation,prototype experiment,and engineering.Based on the wiring connection of the DS-IFT,the port equation of each winding's physical quantity under the fundamental and harmonic frequencies is established.It is proved that there are two equations both Z_K12?the short-circuit impedance between the HV and MV winding?andZ_K13?the short-circuit impedance between the HV and LV winding?,and the other four essential short-circuit impedances,and the characteristics that the two sets of equivalent calculated impedance associated with the filter windings are zero are deduced,which indicates that the DS-IFT satisfies the constraint condition of inductive filtering.At the same time,the influence of the fundamental impedance parameters of the tuned filter on short-circuit impedance of the DS-IFT is very small.The correctness of the theory is verified by simulation and short-circuit experiment of prototype.Based on the port equation,the operation characteristics and the inductive filtering performance of the DS-IFT are studied.However,the sensitivity analysis method is introduced to analyze the influence of various parameter perturbations on the filtering performance of the DS-IFT.Then,simulation and experiment verify the effectiveness and feasibility of the proposed equivalent decoupling model and port equation.?3?The power flow calculating model of the DS-IFT is established to study.Based on the wiring connection mode of the DS-IFT,the positive sequence node admittance matrix is established to form the power flow calculating model of three-phase symmetry load.Compared with the port equation,the matrix method is more simple,convenient,and practical to analyze the operation characteristics of the DS-IFT at the fundamental and harmonic frequencies.Based on the symmetric component method,the equivalent networks of positive sequence,negative sequence,and zero sequence of the DS-IFT are established,and the admittance matrix of each sequence network is formed.Then,the three-phase magnetic flux leakage admittance matrix in ABC coordinates is derived by the phase-sequence component transformation method,and its singularity is analyzed.A power flow calculation model of three-phase asymmetry load can be constructed.The power flow calculation results of three-phase symmetry and asymmetry loads are obtained by MALTAB simulation.Simulation results show that the theory and the model are correct.?4?The inductance matrix model of the DS-IFT is established to present the differential protection scheme with the inductance matrix coefficient.Firstly,the inductance matrix characteristics are demonstrated from the magnetic circuit structure of the DS-IFT.Note that the two equations that can reflect the inductance parameters may satisfies the constraint condition of the inductive filtering,and it indirectly has a unique filter winding.The inductance parameters of the experimental prototype are obtained by the finite element software,and the simulation results show that the constraint conditions of the two inductance parameters satisfy the condition that the equivalent calculated impedance parameters are zero,thus which verify the correctness of the theory.Then,based on between the coupling theory of multi-winding transformer and the inductive filtering principle,the inductance matrix model of the DS-IFT is established by mean of the phase component method,and the mathematical equation between internal winding and external port is deduced by the correlation matrix method.Based on inductance matrix coefficient,both the differential protection scheme and the corresponding protective criterion are proposed.This scheme is simple to implement and avoids the internal parameters which are difficult to get in the DS-IFT.The simulation results show that the scheme can effectively remove the internal faults of the DS-IFT.?5?The engineering prototype has been successfully applied in a HV substation.According to the actual operation of a substation,an engineering prototype of the DS-IFT of SSZ11-L-180000/220 was developed and applied in the project.Test on the site shows that the prototype works stably,the filtering effect is obvious,and the power supply quality is guaranteed.