基于发电机定子接地燃弧电阻变化特征的主动降压熄弧机理与抑制方法

Mechanism and Suppression Method of Active Arc Extinguishing Based on the Variation Characteristics of Arc Resistance of Stator

定子接地燃弧故障是制约大型发电机安全可靠运行的技术瓶颈。为解决定子绕组接地电弧引发的绝缘击穿与铁心损伤问题,该文提出基于发电机定子接地燃弧电阻变化特征的柔性接地主动降压熄弧方法。该文建立了定子接地燃弧电阻与弧道电压的解析关系,揭示了定子接地故障点电压降低过程中,接地燃弧电阻增大的变化规律;明确了将接地故障点电压抑制在电弧重燃电压以下时,可实现定子绕组击穿间隙回归正常绝缘状态的抑制方法。根据大型发电机定子柔性接地方式降压熄弧策略,在接地故障燃弧阶段,中性点电压源输出故障点工频与3次谐波反向电势,主动降低故障点电压,将故障点电压降低至燃弧电压以下,使定子接地电弧熄灭。利用注入信号感知定子接地燃弧状态,实时监测定子阻尼率矢量角变化趋势,判定降压消弧过程中接地燃弧故障是否消除。所提方法无需将故障点电压降低至零,可有效降低非故障相电压抬升,降低定子非故障相绕组槽内击穿风险。在PSCAD/EMTDC仿真环境模拟定子接地故障燃弧工况,对所提方法进行试验验证。试验结果表明,基于定子接地燃弧电阻变化特征的柔性接地消弧方法可主动降低定子接地故障点电压,助增故障点间隙绝缘电阻回归,抑制接地电弧重燃,降低定子非故障相绝缘击穿风险,实现定子接地燃弧故障的可靠消弧。

The stator grounding arc fault is a technical bottleneck that restricts the safe and reliable operation of large generators.The continuous burning arc is prone to burn or melt the stator winding insulation and iron core.residual currents,active and harmonic currents of grounding faults is not eliminated completely by the existing methods.And it is difficult to limit the residual currents of grounding faults to the allowable value of safe grounding currents and cannot meet the requirements of existing regulations.By analyzing the variation characteristics of stator grounding arc suppression fault voltage and arc resistance,the flexible grounding method for stator grounding arcing fault voltage reduction and arc suppression is proposed.Firstly,the analytical relationship between the stator grounding arc resistance and the arc voltage was established.Then,during the voltage reduction at the stator grounding fault point,the stator arc resistance increase.The voltage at the grounding fault point is suppressed below the arc reignition voltage.So,the suppression method can achieve returning the breakdown gap of the stator winding to the normal insulation state.When the method of generator stator is the flexible grounding,the grounding fault point is output with the power frequency and third harmonic reverse electromotive.The voltage of the fault point is reduced to below the gradient of the arcing electric field.And the stator grounding arc is extinguished.By injection signals to sense the stator grounding fault status,the vector angle change trend of stator winding damping rate is real-time monitored.And the grounding arc fault is monitored during the voltage reduction and arc suppression process.In the proposed method,the fault point voltage is not required reducing to zero.The risk of breakdown in the slot of the stator winding is effectively avoided during the voltage rise of the non-fault phase.The proposed method was experimentally validated by simulating the arcing conditions of stator grounding faults in the PSCAD/EMTDC simulation environment.The experimental results showed that the flexible grounding voltage source was put into operation.The input potential amplitude was changed,and the fault point voltage was regulated.The fault point voltage value was lower than the arc breakdown voltage,and the stator grounding arc extinguished within 100 ms.The stator grounding arc is unable to reignite.After the flexible grounding voltage source is put into use for voltage reduction and arc suppression,the damping vector angle is less than 12°.And the change value is negative.The state of stator grounding arc burning out is effectively distinguished during the voltage reduction and arc suppression process.The following conclusions can be drawn from the analysis:(1)The field strength of the stator grounding fault through the gap is inversely proportional to the arc resistance.By reducing the voltage at the stator ground fault point,the electric field strength of the arc is reduced,and the arc resistance is increasing.So,the current at the fault point is reduced.(2)By utilizing a controllable voltage source in parallel with the stator neutral point,the voltage at the fault point is reduced to achieve reliable suppression of grounding fault arcs.(3)The voltage asymmetry of the stator system is reduced during the voltage reduction and arc suppression process in the method.the degree of voltage rise of the non-fault phase winding and the risk of breakdown can be reduce.The maximum displacement voltage shall not exceed the rated voltage of the system and shall be consistent with the stator voltage offset caused by natural ground faults.