Research On Grounding Fault Protection For Neutral Ineffectively Earthed Power Systems

Neutral Ineffectively Earthed Power Systems (NIEPS) include neutral unearthed system, neutral earthed with high resistance system and neutral resonance earthed system. Single-phase-grounding faults potentially cause cables bombing or phase to phase faults in NIEPS, and the safety and power quality of the distribution system will be affected. Traditional single-phase grounding faults protections have low precision and reliability in NIEPS, because of the small fault current, fault arc, harmonics influence and the protection principle limitation. A new type of high-voltage generator which offers a direct connection to the power network without the need for a step-up transformer is called Powerfonner. Under most conditions, Powerformers are with neutral unearthed, and they belong to NIEPS. Since the Powerfonner was invented in1998and has not been researched well, it is very necessary to study the machanics of internal or external fault in Powerfonner, and the stator single phase-to-ground fault protection. The thesis’ aims are to find the shortages of the traditional grounding faults protection methods, and to develop some novel protections for NIEPS in detecting and isolating grounding faults.This thesis contains seven chapters. The introduction of Neutral Ineffectively Earthed Power Systems is introduced in the first part of this thesis. Chapter two to six are devoted to the main works. The details of this paper are as follows:(1) Earth fault feeder detection is a long-standing problem in Neutral Ineffectively Earthed Power Systems. Hierarchical clustering analysis is an important technique in studying for multi-sources information integrating and processing, and is introduced and applied in feeder grounding fault protection in chapter two. A method that can quantitative discriminate the validity of the hierarchical clustering is proposed. Four steady parameters based protection and three transient signal based protection are selected as the fault characters. The models and methods of hierarchical clustering for information fusion technique in feeder grounding fault protection are presented. The scheme has been verified by EMTP simulation, and results show that the scheme has high sensitivity and robustness.(2) Hierarchical clustering analysis is crisp partition, which has opposite law, meaning A and not-A. However, traditional ground fault protection has fuzzy characteristics because of the influence by variation parameters and disturbing signal. In order to describe the fuzzy characteristics, a novel principle for feeder grounding fault protection based on fuzzy clustering analysis is presented in the third chapter. First, the historical data is divided into two groups by fuzzy clustering analysis. The space relative distance among detected patterns and two cluster centers is then calculated to discriminate the fault feeder. The scheme has been verified by EMTP simulation. The results show that the effect of the disturbing signal can be reduced, inherent limitation of the single protection scheme can be eliminated and the precision and robustness of fault detection can be improved.(3) In order to improve the precision and reliability of traditional earth fault feeder detection schemes for Neutral Ineffectively Earthed Power Systems, on the basis of the Mahal distance discriminant theory and in combination with the project practice, a distance discriminant analysis model of earth fault feeder detection scheme is established in chapter four, in which five parameters are selected as the discrimination factor. The back substitution method is introduced to verify the stability of the distance discriminant analysis model and the ratio of mistake-discrimination was equal to zero after the distance discriminant analysis model was trained. By means of the model, other four groups of measured data are tested as testing sample, and the result are consistent with the measured data. The simulation results show that the distance discriminant analysis model has a higher accuracy and low mis-discrimination ratio.(4) In order to resolve the selective protection for Powerformer with more than one generator directly connecting a bus, the characteristics of the stator winding phase-to-ground zero-sequence current (zero-sequence fundamental component and third-harmonic fault component) are analyzed in chapter five. Through comparing the magnitude of zero-sequence currents and fault point energy dissipation, the stator grounding faults can be detected occurred in generator stator winding or not. Simulation results show that the fundamental component and the third-harmonic fault component of zero-sequence current are combined to realize100%coverage of fault detection for the stator windings of Powerformer. What’s more, the protection can distinguish the single-phase earthed fault occurred in stator winding or outside of generator, and can detect stator ground faults in which generator with high sensitivity and selectivity.(5) When a stator single phase-to-ground fault occurs in a powerformer, every generator that is connected in parallel’with the faulty powerfonner has the same voltage. Traditional protection schemes using zero sequence voltage and third-harmonic voltage signals cannot detect which powerformer is faulted. In order to solve this problem for Powerformer protection, a novel stator single phase-to-ground fault protection based on the direction of zero-sequence power is proposed in chapter six. Simulation results have shown that, under different fault conditions, the new scheme can distinguish reliably internal faults from external faults, and can detect stator single phase-to-ground fault occurred in which powerformer with100%of the winding.