换相失败后提升送端暂态功角稳定性的实时控制方法

Real-Time Control Method for Improving Transient Power Angle Stability of the Sending-End System after Commutation Failure

直流换相失败引起的功率瞬降易增加送端交流系统的功角失稳风险,但当前研究尚缺少应对多直流换相失败的暂态功角稳定实时控制方法。针对这一问题,该文首先分析了换相失败功率冲击下送端系统发电机相对动能和转速的变化规律,指出送端系统的相对动能变化可仅由等值发电机的相对转速表征,不依赖于直流系统功率跌落和恢复的曲线态势;然后根据直流功率恢复至稳态时等值发电机的相对转速计算转子运动剩余加速面积,进而得到维持送端系统暂态功角稳定性所需的减速面积及相应的切机量;最后根据切机量计算结果制定了实时切机控制策略,并基于简单系统和实际电网进行了仿真分析和验证。仿真结果表明,所提方法能够较准确地计算维持送端系统暂态功角稳定性所需的切机量,有效减小换相失败后送端系统的暂态功角失稳风险。

The active power impact caused by commutation failure(CF)at the inverters of line-commutated-converter-based high voltage direct current(LCC-HVDC)can easily increase the risk of power angle instability in the sending-end ac system.However,the real-time control methods to maintain the transient power angle stability after suffering CF are still insufficient in the existing literatures,since the current researches mainly focus on the qualitative analysis,offline decision and dc control strategy,lacking the quantitative methods and real-time control strategies generator tripping.To solve the issue,the principle of generator rotor acceleration and power angle instability caused by the power impact of CF is firstly analyzed.When a dc system suffers CF,the transmitted active power would experience a sudden drop,equivalent to an increase of the equivalent mechanical power in the sending-end system.If dc system suffers continuous CFs,causing multiple power impacts,the equivalent mechanical power would increase again and form an acceleration area due to the power drop before returning to its initial value.Similarly,the relative kinetic energy under continuous power impacts of CFs can be derived.The analysis results indicate the relative kinetic energy change of the sending-end system can be characterized only by the relative speed of the equivalent generator.If the relative speed of the equivalent generator can be obtained in real time,the relative kinetic energy change of the sending-end ac system is independent of the curve trend of the dc power drop and recovery,as well as the duration of CF.Then the remaining acceleration area of the rotor movement is calculated based on the relative speed of the equivalent generator when DC power is restored to steady state,and the deceleration area and cutting amount required to maintain the transient power angle stability of the system can thus be obtained.Based on the theory researches,a real-time generator tripping control strategy is developed,the specific steps are as follows:(1)If CF occurs at the inverter of dc system,start the calculation and control strategy,calculate the equivalent mechanical power and electromagnetic power of the generators.(2)Monitor whether the transmission power of dc system has recovered to the steady-state value.If so,record the equivalent relative speed of generators at this time and calculate the increased relative kinetic energy after CF.At the same time,calculate the maximum possible deceleration area,if the value of which is less than the relative kinetic energy of increased by power impact,implement the generator tripping control strategy.(3)Calculate the required tripping amount of the generators,and implement the strategy after 200 ms time delay.Since dc overload operation is equivalent to reducing equivalent mechanical power,increase the reference value of the transmitted power of the dc system to the 1.1 times of steady state value during the time delay.Simulations are implemented based on the simple system and actual power grid,which shows that the proposed method can accurately calculate the required cutting amount to maintain the transient power angle stability and effectively reduce the risk of power angle instability after commutation failure.Owing to the generator tripping strategy,voltages of the critical nodes can be maintained at a relatively higher level and the trend of continuous increase of the phase angle difference is effectively suppressed,with the phase angle difference maintained within 180°.