DEVELOPMENT STRATEGIES OF THE RELIABLE POWER SYSTEMS

摘要

The power system blackout usually goes through two stages. Stage one is related to voltage collapse/transient instability. The reason why most of the instability had happened in the complex and unreliable system configuration, is because of the power grids interconnections and power transmission are liberalized. They become irregular through arbitrary transmission paths. Their power system configuration is subject to an un-controlled condition in case of system disturbances.rnWhile in stage 2, the instability may spread out to "blackout" when the system integrity could not be maintained. Two different strategies exist on how to handle the system instability. The first strategy is from the American NERC Planning Standards. The standards say that "Generation and transmission protection systems should avoid tripping for stable swings on the interconnected systems" , that means the operations of protection tripping the generation and transmission systems are permitted during instability. The consequence is blackout. The second is from the rule of "Three Lines of Defense" in China, i.e. keeping system integrity during instability. The blackout can be avoided.rnThis rule in China is based on close examinations of blackouts all over the world, especially the experience of the North American Blackout in 1965. It was concluded as "The cause of cascading blackouts - Un-controlled power system configuration and protection." Hence for more than 20 years of building proper AC/DC system configuration and to keep system integrity, system instability may occasionally occur, but it could not cascade to widespread blackout, also any disturbance occurred in one region could not spread out to other regions because of HVDC interconnection. Therefore, both the reliable system configuration with sufficient reactive power support as well as maintaining system integrity should be established as the Defense Plan against extreme contingencies to avoid blackout.rnThe rapid power demand growth has been driving the AC systems to extend, also with the introduction of higher voltage levels, above 1000 kV, however, the feasibility of its application has to be studied.rnHVDC has become a mature and reliable technology by the replacement of mercury arc valves to thyristors with digital control in 1980s. New VSC HVDC technology has been developed since 1999, which has the ability to integrate seamlessly into an AC system with improved stability and highest level of controllability for multi-terminal HVDC.rnDevelopment Strategies - Well-coordinated AC/DC configuration will result with more economical and technical benefits. To day, our particular notice is in the ability of HVDC to control power flow, and prevent propagation of severe disturbances, thus limiting instability extension.rnNew HVDC technology will be further developed for use not only for long distance power transmission and interconnection between AC systems, it is anticipative for use as backbone instead of AC UHV The America [GRID 2030 VISION] is an example; it would be a National Vision for Electricity in Second 100 years. The European [Super grid] has been proposed which will be another example.rnAccording to the United Nation document: "Above 1000 kV, however, the practical difficulty and expense of equipment and insulation that can withstand such high voltages becomes prohibitive." If AC UHV system is developed, it must overlay many existing EHV systems with much wider area, all the complicate problems could not be controlled and would be cascaded to blackout the whole area.rnAccording to technical and economical comparison, 1000kV line is not as good as HVDC line for long distance transmission; it is also not as good as the lines of a 500kV double circuit compact tower for short and middle distance transmission. The security and environmental protection of 1000kV lines is particularly much worse than that of both HVDC and 500kV lines. Therefore, AC UHV has no value in use for transmission.