Study On Stator Winding Single-Phase Grounding Faults Protective Scheme For Powerformers With Selectivity

Powerformer is a new type synchronous generator developed by ABB. Due to the theoretical terminal voltage can up to 400kV, it can directly parallel with the grid without the step-up transformer. On the other hand, it offers high efficiency, long life and low environmental impact which makes the good application prospect. The stator single phase-to-ground fault is one of the most common faults that a generator will suffer, it is possible to develop into phase-to-phase fault that usually results in huge fault current and major equipment damage, and as the characteristics of Powerformer, it would endanger the secure and stable operation of power systems if the single phase-to-ground faults and the faulty Powerformer are not detected accurately. At present, the study on stator single phase-to-ground fault protective scheme for Powerformer at home and abroad is not enough, and the protective schemes have been proposed more or less have some disadvantages. Therefore, the stator single phase-to-ground fault protective schemes for Powerformer with selectivity are studied in this paper.There are six chapters in this paper. In the first chapter, the development course of Powerformer was summarized and the generator neutral point earth mode was analysed firstly. Then, the present research status at home and abroad of stator single phase-to-ground fault protective schemes for traditional generator and Powerformer was analysed and summarized, we can get the result that the traditional protection schemes are not suitable for Powerformers, and the disadvantages of the existing protective schemes for Powerformer were pointed out too. In the second chapter, the design characteristics, operation characteristics and internal fault model of Powerformer were described in detail. In the third chapter, the zero sequence current equivalent network of Powerformer set, when stator single-phase ground fault or external single-phase ground fault occurs, was analysed firstly. Then, the extraction schemes about leakage current and zero-sequence current in Powerformer terminal were proposed, and the correctness of theoretical analysis and the effectiveness of extraction schemes were verified by simulation analysis. In the fourth chapter, the hierarchical clustering based stator winding single-phase grounding faults protective scheme for Powerformers with selectivity was proposed. Firstly, the date set was projected in multidimensional space and divided into fault group and non-fault group by hierarchical clustering algorithm, then, the two cluster centers defined as average value of patterns were calculated. Secondly, the space relative distance among detected pattern and two cluster centers were calculated, and the operation state of Powerformer can be identified by comparing the space relative distance among detected pattern and two cluster centers. At last, the effectiveness of protective scheme was verified. In the fifth chapter, the discriminant analysis based stator winding single-phase grounding faults protective scheme for Powerformers with selectivity was proposed. Firstly, the discriminant model can be constructed after learning the training patterns. Secondly, the detected patterns were substituted into discriminant model, and the operation state of Powerformer can be discriminated according to the discriminant criterion. At last, the effectiveness of protective scheme was verified and the superiority of the aforementioned protective scheme can be shown after comparing with other protective schemes.Simulation results show that the protective schemes proposed in this paper can complete the stator winding single-phase grounding faults protection for Powerformers with selectivity effectively, and not influenced by system operation mode. The comparison of different protective schemes shows the superiority on reliability and sensibility of the protective schemes proposed in this paper.