大型发电机定子接地故障点降压消弧机理与控制方法研究

Neutral ineffectively grounding is commonly adopted in large generators. The fault features are weak for generator stator single-phase grounding fault. The grounding fault accurate detection and reliable arc extinguishing is hard to realize. Therefore, proceed from the fault arcing mechanism, the fault arc was equivalent to a series connection of “arc breakdown voltage gap” and “arc variable resistance”. The new idea of voltage arc extinguishing was proposed which is controlling fault location voltage below the arc breakdown voltage. In this project, the breakdown voltage characteristics and variable resistance characteristics of stator fault arc will be studied, and the arcing mechanism of stator grounding fault will be revealed; then, the stator voltage to ground flexible regulation basic theory will be established, which is connecting controllable voltage source to neutral point to control generator zero sequence voltage, and the fault location voltage flexible regulation is realized; when stator high resistance grounding fault occurs, by controlling fault location voltage amplitude and phase, the fault features can be increased and the sensitivity of fault detection can be improved; during the short-term operation with stator grounding fault, by actively reducing fault location voltage below arc breakdown voltage, the grounding fault current can be eliminated; finally, a prototype of generator voltage reduced arc extinguishing will be developed. This method take advantage of two characteristics and superiority of large generator: generator step-up transformer is delta connection, the short-term regulation of zero-sequence voltage does not affect generator normal operation; the generator zero-sequence impedance is large, voltage source capacity for zero-sequence voltage regulation is small and can be implemented conveniently. This project will realize the sensitive detection and stator grounding fault elimination without power outage.

国内外大型发电机普遍采用中性点非有效接地方式，定子单相接地故障特征微弱，故障精确检测与可靠消弧困难。为此，项目从故障电弧重燃的机理出发，将电弧弧道等效为“弧道击穿电压间隙”与“弧道可变电阻”的串联，提出控制故障点电压低于弧道击穿电压的电压消弧新思路。将研究定子电弧的击穿电压特性与弧道可变电阻特性，揭示定子接地故障燃弧机理；提出定子对地电压灵活调控原理，在中性点接入可控电压源，控制零序电压，实现故障点电压的灵活调控；定子高阻接地故障时，调控故障点电压幅值和相位，增强故障特征，提升故障检测灵敏度；定子带接地故障短时运行时，主动降低故障点电压至弧道击穿电压以下，消除故障电流；并研发发电机降压消弧原理样机。项目巧妙利用了大型发电机的两大特点与优势：升压变三角形接线，零序电压短时调控不影响正常运行；发电机零序阻抗大，调控零序电压的电源容量小，易于实施；项目最终将实现定子接地故障灵敏检测和不停电消除。

大型发电机组内部电磁结构复杂，长期承受电、化学腐蚀和机械损伤，定子绕组与铁芯间绝缘易发生破损，引发定子接地故障。我国大型发电机组普遍采用中性点非有效接地方式，将单相接地故障电流限制在允许范围内，保护单元发信不跳闸，系统可带故障持续运行一段时间，保障系统的能量平衡。然而，中性点非有效接地系统零序阻抗大，定子单相接地故障时故障信号微弱，故障感知、检测与消弧困难。. 本项目从故障电弧重燃的机理出发，研究定子电弧击穿电压特性与弧道可变电阻特性，探究故障暂态高频能量激荡规律及介质绝缘恢复特性，建立了面向发电机内部复杂环境的接地故障燃弧模型，揭示了定子单相接地故障燃弧机理，提出了控制故障点电压低于弧道击穿电压的电压消弧新思路；通过分析定子对地电压主动调控机理，建立了发电机定子对地电压灵活调控基础理论，提出了具备高阻故障辨识能力的定子接地故障灵敏感知方法；研究中性点电压连调时定子故障点电压的随调特性，提出了基于最优降压值搜索的快速降压消弧方法，实现了对定子单相接地故障主动降压快速消弧。为将定子接地故障电流持续抑制到零，从而抑制故障相恢复电压速度及数值，实现定子接地故障可靠消弧，确保定子接地故障运行期间安全性，分析了可控电压源接入对故障暂态恢复电压特性的影响机理，结合定子绕组介质损耗变化规律，提出了基于介损回归正常值的故障零电流辨识方法；研究故障持续运行电压最优调控范围，提出了定子接地故障安全控制策略，并进行了仿真验证。. 最后，为进一步验证本项目所提理论和方法，研发了定子接地故障灵敏感知与降压消弧原理样机，并对样机进行了动模实验和现场试验。经动模实验和现场试验结果表明，该样机在定子绕组接地故障时，各项功能均运行良好，有效解决了发电机定子单相接地故障灵敏感知与可靠消弧困难的问题，保障了发电机持续安全可靠运行，可广泛应用于非有效接地系统。