Research On Regional Backup Protection With Substation As Basic Unit

The relay protection is the first defense line of power grid safe operation. With the complicating grid structure in recent years, due to only local information applied, the traditional backup protection reveals some drawbacks including difficulty to setting coordination, long tripping time, inadaptation to operation modes and frequent mal-operation caused by overload. These drawbacks seriously restrict protection performance and threaten the safety of power system.With the help of phase measurement units (PMU) and digital substation, the wide area backup protection using multipoint and multi-type information to decide faults becomes an important solution to the above problems. This paper,"regional backup protection with substation as basic unit", directs at a mode of wide area protection with extending function. The research focus protection system structures and partiton methods, fault identification algorithms adapting various grid enviroment, and the improvement of tranditional protection utilizing regional information. The work and results are as belows.1) With the function extension of wide area protection, the system structure of substation concentration and region distribution is proposed. This structure owning rationality includes substation area backup protection and regional backup protection, and the latter one is the research focus of this paper. On that basis, the novel multi-coverage partition method regarding station spacing of regional protection is put forward. The protection scope with appropriate station spacing can limit the communication delay, the treatment principle of boundary elements simplifies the cooperation strategy of main stations, and the multi-coverage principle improves the reliability of protection system. Case studies certify the rationality and availability of the partition method.2) On the basis of power grid theory, a set of fault identification algorithms adapting to random PMU placement is presented. These algorithms contain fault correlation region identification, fault point location and fault element identification. The functions of remote backup protection, local backup protection and fault location are all realized and the synchronization demand is sharply decreased.The branch equivalence of nonsynchronous injection nodes makes protection algorithms adapt to the random PMU placement. The criterion established by variation information of grid structures has high sensitivity because of its amplification effect to fault features. The fault location algorithm based on node voltage difference has ideal precision without location dead zone. The whole algorithm is immune to system oscillation, reacting to symmetric and asymmetric faults persistently, and adapting to developing faults and transferring faults. Moreover, comparing with the traditional backup protection, this set of algorithms accelerates the action speed, simplify the cooperation strategy, and is not affected by the variation of operation conditions and overload resulting from power transferring. Theory analysis and simulation confirm the above performances.3) A set of novel fault identification algorithms based on power information is proposed. These algorithms can identify fault correlation regions, fault elements and high-resistance faults. The remote and local backup functions of transmission lines and remote backup functions of buses and transformers are all included. Meanwhile, this set of algorithms is unnecessary time synchronization.The fault correlation region identification criterion is formed by power information which accommodates to multiple voltage classes. Thus the challenge of substation DC disappearance is solved. The algorithm which extracts the features of transition resistance can sensibly detect high-resistance faults. The reactive power criterion has obvious difference in external and internal faults, and three methods all adopt ratio braking characteristics. The whole protection has high sensitivity and reliability, abilities of persistent reaction to symmetric and asymmetric faults, and strong flexibility to developing and transferring faults. Moreover, this set of methods is featuring quick action, simple match tactics, and good immunity to system oscillation, power transferring and change of operation modes. Theory analysis and simulation demonstrate the above advantages.4) A novel anti-false action scheme of zone3impedance relays based on regional two types of fault features is put forward. Another fault identification algorithm on account of fault condition set also is presented. The two methods improving the traditional zone3impedance relays and local backup protection are suiting to the initial development period of regional protection.The zone3relay is opened by the fault features of protection system and power grid. Thus the zone3relay is converted to the new action mode of "open according to need". This scheme greatly reduces the probability of protection false action, fully prevents cascading trip, and don’t impact the backup function of zone3relays. With the help of multi-information fusion, the fault identification algorithm can decide exactly with two messages non-operation and one message mal-operation. Therefore, it can enhance the reliability of backup protection and simply the match tactics. Theory analysis and simulation certify the validity and availability of the proposed methods.