| Conventional HVDC system consists of line commutated current source converters based on thyristors. It has the advantages of high rated power, reliability, and low cost. However, because the conventional HVDC system connected to the weak AC power system is inclined to cause commutation failure and the voltage instability, it is only suitable for the strong AC power system. Therefore, such kind of HVDC system is difficult to supply power to the weak AC power system or passive power system. The VSC-HVDC system based on forced commutated voltage source converters, such as GTO, IGBT, IGCT, has the advantages to feed AC power systems with low short circuit power or even passive networks with no local power generation, to continuously adjust reactive power and active power independently, improve the voltage stability of the AC power system, and to supply high quality power. At present, the rated power of VSC-HVDC system is much less than that of conventional HVDC system, but its cost is much higher than that of conventional HVDC system. Therefore, it is impossible now to adopt the technology of VSC-HVDC system to transfer large power for long distance. To obtain the best economical, technical, and environmental benefits, a hybrid multi-terminal HVDC system composed of the advantages of both kinds of converter is investigated as well as its corresponding technologie and application problems.Firstly, by use of the parameter of critical commutating voltage reduction, the mechanism of commutation failures of conventional HVDC system connected to a weak AC system is investigated and its operation characteristics are also investigated. Comparing the influences of different reactive power compensating devices, it is found that STATCOM is the best choice to improve the operation characteristics, to reduce the probability of the commutation failures, to enlarge the operation range of the inverter, to enhance the voltage stability of the weak AC system. However, even using the STATCOM to compensate the reactive power, it is still very difficult to transfer power to passive AC power system by the conventional HVDC technology. Therefore, a new hybrid point-to-point HVDC system including a voltage source inverter is designed. It is found that this kind of HVDC system is able to supply the passive system.Secondly, a novel hybrid multi-terminal HVDC (MTDC) system, composed of line commutated thyristor current source converters (CSC) and GTO voltage source converters (VSC) in parallel, is proposed for the extension of the conventional HVDC system. To verify the effectiveness of the proposed system, a simulation model for the hybrid three-terminal HVDC system is developed, which consists of a VSC terminal, a CSC rectifier, and a CSC inverter. Based on the developed MTDC system, two control schemes are proposed. The performance of the HVDC system is studied by simulation under the following conditions: start-up, normal operations, and faulted conditions in the DC links as well as in the AC systems. Simulation results show that the first control scheme is more effective for the hybrid MTDC system, and such system is able to exploit the advantages of both the conventional HVDC system and the VSC-HVDC system. The results also show a new application for the conventional HVDC system.Thirdly, a new method to calculate the instantaneous symmetrical components in the time-domain is proposed in this paper. This provides a new way to detect the positive and negative sequence currents. Traditional symmetrical components defined in the frequency- domain is effective only for steady-state analyses of unbalanced faults and systems. Although the conventional instantaneous symmetrical component definited in time-domain can be use, time delay will be introduced. A new kind of instantaneous symmetrical component calculation methos is developed in the dissertation. The principle of the proposed method is based on the rolling phasors which are calculated by use of the three phase instantaneous variables. As a result, the positive, the negative and the zero sequence components can be obtained. Additional advantages of the proposed method include: without triangular function calculation, good real-time performance. As an example, the proposed method is used for the detection of the positive and the negative sequence current for a unbalanced three phase power system The simulation results verify the effectiveness of the proposed method.Fourthly, a new dual loop and dual current control scheme based on the instantaneous symmetrical components is proposed in this paper. If the VSC-HVDC system is connected to the three-phase unbalanced power system, negative sequence components will be produced in the AC power system and the 2nth orders non-characterized harmonics component in the DC line. The DC ripple will transfer to the other converter of the HVDC system, and will produce (2n+1)th orders uncharacterized harmonics in the AC power system connected to that converter. Therefore, if the unbalanced fault occurs in the AC power system, it will deteriorate the performance of VSC-HVDC system. In the proposed control scheme, sequence components without time delay are obtained from the instantaneous symmetrical components of both the voltage and the current measurements. As a result, the proposed method can be used in the real-time control especially for those cases the time delay caused by the conventional symmetrical components must be considered. Based on the result obtained, a control scheme including a compensator for the unbalanced component is developed to eliminate the harmonics transfer characteristics of VSC-HVDC system under the unbalanced faults.Finally, the application of hybrid multi-terminal HVDC system for large wind farm is studied. The wind power system using double fed induction generator and its control scheme based on the internal model control theory are investigated. The operation characteristics of the wind farm connected to a hybrid parallel multi-terminal HVDC system are analyzed by use of the PSCAD/EMTDC. Satisfactory results are obtained. |
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