The Research On Inductive Filtering Theory And Its Application In DC Transmission Systems

Power filtering technology is the key approach for the improvement of power quality (PQ). The general filtering methods mainly include passive power filtering, active power filtering, and hybrid power filtering. Different from these, this thesis presents a new inductive filtering method. Its development is described, and its special filtering mechanism is revealed. The impedance conditions for the implementation of inductive filtering method is also obtained. From the overall research results, it can be seen that the application of inductive filtering method into DC transmission can improve the filtering performance, suppress the harmonic instability of AC/DC systems, reduce the requirement of reactive power, enhance the commutating reliability of the inverter, and so on. Besides these, the inductive filtering method also represents kinds of technical superiorities when applied into HVDC-Light systems. The engineering application in a large-power DC supply system validates the technical superiorities that inductive filtering can achieve.The performed fundamental research and the related innovative achievement in this thesis are summarized as follows:(1) The operating mechanism and the electromagnetic chareacteristics of the inductive filtering is revealed in theory. In this section, the theoretical analysis method, which applies the superconductivity constant-flux-linkage theorem and the core-balance principle to analyze the working mechanism of the inductive filtering, is proposed, and then the superconductivity closed-loop condition under the specific frequency, which should be satisfied for the implement of the inductive filtering, is revealed from the magnetic and the circuit side, respectively. Furthermore, the field-circuit coupled electromagnetic analysis method for the inductive filtering is proposed and the corresponding field-circuit coupled model is established. Based on this, the electromagnetic characteristics of the inductive filtering equipment are calculated and analyzed in detail. From the research results, it can be found that the inductive filtering efficiently utilizes the inherent ampere-turns balance characteristic that transformer has, and through constructing the superconductivity closed-loop winding, it realizes the separation of the specific harmonic currents from the power grid and the grid winding of the converter transformer. Meanwhile, the implement of the inductive filtering changes the magnetic-circuit relationship of the transformer under the specific harmonic frequency, and suppresses the harmonic magnetic potential to a certain extent, which is advantage to reduce the harm of the harmonic on the converter transformer such insulation, harmonic loss, vibration, and so on.(2) A sensitivity analysis method is proposed to analyze the inductive filtering performance under non-ideal operating conditions. According to multi-winding transformer theory and basic circuit principle, the decoupling circuit model and the corresponding mathematical model for the inductive filtering converter transformer with electromagnetic constraint relation are established. Furthermore, based on the system sensitivity theory, the sensitivity characteristics of inductive filtering effect on the disturbances of harmonic source, network parameters, and inter-impedance parameters are studied in detail. From the research results, it can be found that the inductive filtering can avoid the series/parallel harmonic resonance with the system impedance to a certain extent. On the other side, as long as the reasonable configuration on the three kinds of impedances, that is the equivalent impedance of the grid winding, the filtering winding and the tuned branches, it can promote the interaction among such impedances, and achieve fine inductive filtering performance.(3) The mathematical model is established to analyze harmonic transfer chareacteristics of the new inductive filtering based DC transmission system. A unified equivalent-circuit model and the harmonic transfer model for the inductive filtering based HVDC converter station are established. Firstly, the equivalent-circuit decoupling model, which is expressed by the fundamental and the harmonic impedances, is established. Then, through the detail mathematical modeling, the harmonic transfer model, which can be used to analyze the distribution characteristic of harmonic in HVDC system, is established. Finally, the effects of the inductive filtering on the harmonic transfer characteristic of the HVDC system, and the related inter-action mechanism of such effects, are revealed based on the proposed models. The establishment of the unified equivalent-circuit and the related mathematical model provides the theoretical basis for the technical research on the inductive filtering based DC transmission system.(4) The working mechanism on suppressing HVDC harmonic instability by way of inductive filtering is revealed. By using frequency scanning method, the working mechanism is discussed from the concept of the complementary resonance. Firstly, the impedance equivalent network for the HVDC harmonic instability analysis is established. Then, by using PSCAD/EMTDC to scan such network within frequency domain range, the potential advantages that inductive filtering method has for the suppression of the harmonic instability is revealed from the mechanism side of harmonic instability generation. Finally, the working mechanism that use inductive filtering method to bi-suppress the parallel resonance current is theoretically analyzed. From the research results, it can be found that the implement of inductive filtering modifies the parallel resonance state of the equivalent impedance at the AC side of the converter, and makes such resonance frequency shift to the higher frequency direction, which is advantage to avoid the resonance point between the AC-and the DC-side. In this way, it benefits to the reduction of HVDC harmonic instability occurrence, the increase of the strength of the AC side of the HVDC system, and the improvement of the stable operation of HVDC system.(5) The characteristic that inductive filtering based HVDC converter has about reactive power compensation at the valve winding side is theoretically analyzed. Firstly, according to multi-winding transformer theory and Kirchhoff’s current law, such characteristic is studied by ways of mathematical modeling and vector analysis. Furthermore, it obtains the expression on equivalent impedance calculation for the new converter transformer and the inductive filtering system with considering compensation degree of the reactive power, and then its impedance-frequency characteristic is studied using frequency scanning method, and its change characteristic influenced by the compensation degree under different frequency is revealed. Through introducing the harmonic participation factor, this thesis defines the effects of inductive filtering on the equivalent commutating reactance of the converter, and further studies the reactive power characteristic of the inductive filtering based converter operating at the different control modes. Such research reveals a series of advantages that inductive filtering method has for the reactive power balance of the HVDC converter.(6) The ability of enhancing operating reliability of HVDC inverter is discussed in theory. The influence of inductive filtering on the commutating characteristic of the inverter is studied under the various HVDC operating states such as normal operation, symmetry or asymmetry fault operation. By comparative study with CIGRE HVDC benchmark test system, the influence of inductive filtering on the converter control performance is studied. Furthermore, through the simulation research on the transient response of the inductive filtering based HVDC transmission system under different fault operating conditions, it reveals the related advantages of the new HVDC system on the transient characteristics. From the research results, it can be found that the implement of the inductive filtering can increase the turn-off angle margin of the inverter, reduce the commutation failures, and increase the stability of the HVDC inverter.(7) The mathematical model of the inductive filtering converter transformer is established based on the phase component method. To analyze the operating characteristics of the new converter transformer and the related full-tuned branches from the side of the circuit principle, such mathematical model is established. The model is based on the transformer coupling-circuit principle, and contains the basic model, the node expansion model, and the branch expansion model, which is convenient to the flexible selection in the practice application. Based on this, the protection scheme for the inductive filtering converter transformer is proposed based on the model method, and the protective criterion is set initially. The simulation on the various operating states of the inductive filtering converter transformer verifies the correctness of the proposed protection scheme and criterion.(8) Propose a new HVDC-Light transmission model based on inductive filtering method. The feasibility and the technical characteristics on the application of voltage sourced inductive filtering converter in HVDC-Light system are studied in detail. By the electromagnetic transients simulation, it initially reveals the operating characteristics such as filtering characteristic, converter P&Q operating characteristics, bi-direction power flow control characteristic, voltage stability, fault recovery characteristic, and so on. Such kind of research widens the application fields of inductive filtering theory in the modern DC transmission system.(9) The practical application of inductive filtering in large-power industrial DC supply system is investigated. Currently, the inductive filtering method has been firstly applied into a industrial DC supply system (DC voltage is415V, DC current is22kA, AC voltage is35kV, AC capacity is10080kVA). In this thesis, the electric wiring scheme of the new industrial DC supply system based on inductive filtering technology is presented. Then, it further introduces the configuration and the main design parameters of the inductive filtering rectifier transformer and its related full-tuned branches. Finally, the field test on the new DC supply system is performed to verify the correctness of the related theoretical achievements mentioned in the thesis.This thesis performs the systematical research on the inductive filtering theory and its application in DC transmission systems, and constructs a set of relative complete theory and application research system. Combined with the existing problem of the practice HVDC project, this thesis reveals the action mechanism on the influence of inductive filtering on the technical characteristics of DC transmission. The research achievement deepens the technical theory of the inductive filtering, and widens its application fields, which has the scientific significance for the improvement of theoretical innovation and technical development of both power filtering and DC transmission technologies.