Research On Several Issues In HVDC Transmission Design

The fact that there is a reverse distribution between the energy resources and the demands determines our country has to implement the power optimal configuration, e.g. west-to-east power transmission, power exchange between south and north and nationwide interconnection. The advantages of HVDC in the field of long distance and large capacity transmission determine HVDC is the best choice. Although the HVDC technology starts relatively late in our country, it develops rapidly. In order to make further improvement in the localization of HVDC, and promote independent development and technological innovation comprehensively, developing HVDC design software package with independent intellectual property is an important content. This thesis makes researches on several important issues in basic design of HVDC and the main works are organized as follows:(1) According to the valve's surge current level, in combination with three-phase AC maximal short-circuit capacity, rated DC voltage and DC current, the determination of minimal short-circuit impedance of the converter transformer is carried out through analytical formulas, avoiding the procedure of iteration in practical engineering. Based on the actual impedance, the other parameters of converter transformer are calculated. Considering the system's parameters and measuring tolerances, tap range of converter transformer is calculated in detail.(2) The basic parameters for the calculation of main-circuit steady state's parameters in HVDC transmission are discussed, including operation modes, constraint conditions and original data. Then the tap changer control of converter transformer which has close relation with steady state's parameters is introduced, including its functions, categories, advantages and disadvantages. Based on the basic parameters and tap changer control, as well as the system's control, the flow for calculation of main-circuit steady state's parameters is established in detail.(3) The switch principle of reactive power equipments and the factors influencing the reactive power management for HVDC transmission are discussed. According to the system's parameters, the capacity of reactive power equipments and the results of main-circuit steady state's parameters, as well as the switch principle that keeping the exchange of reactive power between AC and DC system within a certain range, the calculation process for reactive power management is established in detail.(4) An algorithm for calculating the three-pulse harmonic voltages in the DC side is researched. In the algorithm, various asymmetries in firing angle, commutation reactance and transformer winding as well as the imbalance of ac voltage are considered. According to the states of valves, the waveform of the three-pulse voltage is divided into 6 segments in one cycle. In each segment, the voltage of the three-pulse model is calculated by considering the AC fundamental and harmonic voltages respectively. Then, through applying Fourier decomposition to the results of three-pulse voltage and merging the same orders of the harmonics, each harmonic order of the three-pulse voltage is obtained.(5) Based on the three-pulse model, an approach for calculating the DC side harmonic currents in HVDC systems is presented, including the harmonic currents flowing directly in the DC line and the grounding electrode line, as well as the harmonic currents induced in their screening wires. The DC transmission line is treated as a network element, and its coupled phase model is used to form the network nodal admittance matrix. The harmonic currents along the DC line are calculated in positive and zero sequence respectively, and then the whole harmonic currents along the DC line are synthesized by the two sequences. The calculation of equivalent disturbing current is also described.(6) For the purpose of designing DC filter more effectively, a DC filter design method which is based on the equivalent principle and is suitable for practical engineering is presented. By transforming multi-tuned filter to several single tuned filters, first the parameters of single tuned filters are calculated according to the restrictions of the multi-tuned filter's main capacitor value and the tuned frequencies. Then transforming the single tuned filters back, the parameters of multi-tuned filter can be calculated. With this method the parameters of multi-tuned filter can be adjusted freely, meanwhile, the main capacitor value and the tuned frequencies can be kept unchanged.