| With the rapid development of economy in China, the transmission capacity has been unable to meet the demands of electrical loads; while construction of new transmission corridor within the limited land resources becomes more and more difficult. Therefore, people start to pay attention to methods for capacity-increase of existing AC power transmission lines, among which Tripole DC transmission technology is a specially-designed new method aiming at this objective.After a discussion of different methods that can improve the transmission capacity of existing AC lines, the method of convering AC lines to DC (AC-DC conversion) is proposed due to its prosperous application prospect. Furthermore, some problems related to it are analyzed, such as the selection of DC terminal, voltage and line configuration, the transmission power, line power loss and system redundancy after conversion. Also, the costs of building new AC line and converting to tripole DC lines are compared. The results show that tripole DC transmission technology is more feasible and preponderant.The structure of the main circuit of tripole DC transmission system is presented. Two connection modes of the third pole, namely anti-parrallel valves and anti-parallel thyristors within same valve, which are the key parts in the implementation of tripole DC transmission are compared, and research results show that the latter mode is more suitable to be applied to practical engineering projects. Meanwhile, the principle of current modulation adopted in tripole DC transmission system and some problems concerned in current-modulation are analyzed. Furthermore, by comparing the transmission capacity, internal redundancy and overload capacity after converting AC transmission lines to bipole and tripole DC transmission lines respectively, the improvement of operation characteristics after conversion is analyzed and the result shows the latter is better than the former. At last, the basic requirements of the main equipment are stated.A simulation model of tripole DC transmission system is built on the platform of PSCAD/EMTDC. On the basis of verifying the usability of the simulation model, the influence of various current modulation parameters (including time variable and current variable) on the system operation performance is analyzed through time-domain simulation. Further study results show that the tripole DC transmission system performs well under the condition of a step input signal, varying short circuit ratio of AC system and reversing power flow. In addition, system responses to DC faults (including short-current fault of lines and internal faults of converter) and AC faults are analyzed by simulation to verify the validity of its control and protection strategy. |
PDF Full Text Request