Study On The AC System Stability Effected By Inductive Filtering Converter Transformer And HVDC Modulation

In the next two decades, the pattern of the electric power development in Chinais:“the power transmission from west to east, north and south mutual supply andnationwide network”. HVDC (High Voltage Direct Current) transmission technologyundertakes the main task in the power transmission from west to east and nationwidenetwork. With increase of the transmission distance and transmission capacity,reliability of power system is higher standard for the DC transmission. The hotresearch field is how to reduce harmonic injection into AC system and how to reducethe influence that the reactive power compensation under various HVDC operatingconditions on the AC system.China Southern Power Grid is the most complex AC/DC hybrid systems.Utilizing the quick response of the DC system to improve the dynamic stability andsuppress the low frequency oscillations of AC grids under AC fault, which is a keyresearch issue for the security and stability operation of the AC/DC hybrid system.The effect of the DC system to AC system stability is mainly determined by theDC system topology and control strategy in the AC-DC parallel system. Based on thefact that the structure of the inductive filtering converter transformer of HVDCsystem is different from the structure of the traditional converter transformer,combined with the truth that the DC modulation can improve the performance of theAC system. Therefore, this thesis studied the DC system improved the stability of theAC system in these two aspects.Based on “the Eleventh Five” important scientific and technological specialFoundation and National Natural Science Foundation of China, this thesis studied thefiltering characteristic of inductive filtering converter transformer and its reactivepower compensation characteristic on improving AC system’s stability in case of ACsystem fault in the DC transmission system, and also carried out a theoretical researchon DC power modulation that improves the frequency and transient stability of ACsystem in the AC system fault. The fundamental research and the related innovativeachievement in this thesis can be summarized as follows:(1) The short-circuit impedance of the converter transformer is the key to filter,the designer using the genetic algorithm design short-circuit impedance, which makesthe algorithm complex and convergence earlier. This thesis takes the converter valve’sbiggest impact short circuit current as a design principle to optimize the short-circuitimpedance of inductive filtering converter transformer, the algorithm is simplified and can be realized easily.(2) The two main reasons that result harmonic instability are analyzed: thepositive feedback is occurred when the larger amplitude low-order harmonic passesthrough the system that owns larger harmonic impedance; and the transformer coresaturation phenomenon is another reason. Although these two reasons have beenstudied based on inductive filtering DC transmission system, the result was based onexperiment. Essence of this term has not been analyzed yet. In this term, the ACpower will be decomposed into positive sequence voltage and negative sequencevoltage when single phase fault occur in the AC system, the negative and positivesequence harmonic pass from the AC side to the DC side in HVDC system iscalculated. Because of the effect of inductive filtering converter transformer, the DCside harmonic voltage amplitude will be lower than the traditional DC transmissionsystem. Based on the transfer characteristic of the positive sequence, and negativesequence harmonics between AC-and DC-side, this thesis analyzed the equivalentharmonic impedance at the AC-and DC-side of the inductive filtering based HVDCsystem. The research result shows that harmonic equivalent impedance of theinductive filtering based HVDC converter station is smaller than the traditionalHVDC system. The above calculation method of the positive sequence, negativesequence harmonic voltage passing in AC to DC side, demonstrated the advantages ofthe DC transmission system based on the inductive filtering in the suppression ofharmonic instability in mechanism.(3) At present, the study of the inductive filter converter transformer coresaturation phenomenon are based on the measured data and the wave recording, basedon traditional method transformer core saturation, taking the special of inductivefiltering transformer winding arrangement into account. The electromagneticrelationship among windings of the inductive filtering converter transformer isestablished by using the circuit-magnetic coupling model. The simulation resultscoincide with the experimental results. The result contribute to the further study of itsbody features.(4) Calculating the voltage stability of inductive filtering based HVDC converterstation, it is found that the instability of AC system in converter station is often due toreactive power over compensation or de-compensation. Reactive power compensationcharacteristics of the inductive filtering converter transformer better stability of thevoltage in the inverter station than that of traditional HVDC. The former researchcompared commutation reactance and the dropping value of the commutation voltagewith the traditional DC transmission. On this basis, there is a comparison of these twofactors’ affection on the assessment of the voltage regulator. It verified that the inductive filtering based on HVDC transmission system can enhance the voltagestability of the converter station.Signal modulation and control strategies play the key role in the DC modulationresearch. For two main regulation-the AC system frequency regulation and stabilityregulation, the controller has some improvements in the existing linear and nonlinearvariable structure DC modulation strategies.(1) Variable structure DC modulation linear controller is proposed by focusingon researching frequency control when load disturbance occurred in the receiversystem in a parallel AC-DC interconnected power system. Using the optimal formswitching surface design in sliding mode, a simple model is built including offrequency deviation, the AC power deviation and DC power deviation. Using theoptimal quadratic form switching surface design in sliding mode. Compared withtraditional methods, this method is easy to achieve by its simplification with less statevariable, suppress external disturbances, the strong robustness. It also contribute toapplication in multi-terminal AC-DC parallel system.(2) In terms of the transient stability problems of the AC-DC parallel system, thenew design method of nonlinear variable structure DC modulation controller isproposed. Steps of the variable structure design are contrary to the conventional ones.First, the switching surface was designed, and then the controller is designed. Thismethod is easy to carry out make nonlinear variable structure controller in nonlinearsystems become simplified.Based on the background of the application requirements of AC-DC transmissionsystem and took the DC transmission’s impact on the stability of the AC system as thestudy object, this thesis makes a deep research on inductive filtering based HVDCtransmission system can improve the stability of the AC system, It provides a usefulpractical reference for the application of the inductive filtering converter transformer.the joint regulation of the DC system and AC system are also studied. The designmethod of variable structure DC modulator provides a useful reference for thecontroller design of AC-DC parallel operation system.