| With rapid development of distributed generator (DG) technology, the distribution network becomes a multi-source network after a large number of distributed generators connected to it. However the current intelligent level of distribution network with DGs is insufficient, the data information that can be used by protection is limited and breaking capacity of lines are inadequately, which makes it harder to apply protection strategies in distribution network with DGs. Meanwhile the distributed generators have the strong characteristics of diversity, randomness and volatility. The fault synchronous identification technology is put forward, based on which the compound criterion for sufficient differential protection is proposed. The differential protection strategy is under the guidance of the principles of sufficient protection, which solves the protection problem in distribution network with DGs effectively.At first, the definition and superiority of sufficient protection principles are expounded. The sufficient protection strategy breaks through the traditional protection requirements, which focuses on the fault characteristics that can identify the existence of fault. By this way, the protection criterions can be structured based on sufficient fault characteristics with fully priority to ensure the selectivity of protection. Besides, the logical relation between fault characteristics and fault identification is not the sufficient and necessary but the sufficient under sufficient protection principles. In the other hand, sufficient protection scheme has the characteristics of adaptability, advantages, asynchrony, discrete inverse time characteristics and reliability, which makes up the characteristics of sensitivity and quick action in in the course of sufficient fault characteristics selection and criterions forming from the overall system level. Supplemented by DG automatic isolation technique, sufficient protection scheme realizes multi-level and comprehensive protection from the macroscopic angle.Secondly, in order to solve the problem of acquisition of the synchronous data in distribution network, this paper puts forward the fault synchronous identification technology, which overcomes traditional synchronization methods that dates of double sides sampling and using must be synchronous. Before and after the fault moment, the time interval between two adjacent zero crossing points of fundamental current with a same trend is measured, he time interval can be used to calculate the fundamental current phase difference and identify the fault, while effectively overcome the difficulty of high precision synchronous sampling.Last but not least, based on the fault synchronous identification technology, the compound criterion for sufficient differential protection is proposed. Only fundamental current amplitude and time dates obtained by the fault synchronous identification technology are used to structure the compound criterion, which consist the improved scalar product restraining criterion and the sufficient current amplitude comparison criterion. What’s more, two braking coefficients S and K1 can be set according to target parameter, which overcomes the contradiction between improving phase shift restraint capacity and sensitivity. Coordination scheme between sufficient differential protection, DG automatic isolation technique and three-section current protection has been presented. Simulation verifies the accuracy of the fault synchronous identification technology and the action features of the compound criterion. |
