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| Real-Time Control Method for Improving Transient Power Angle Stability of the Sending-End System after Commutation Failure |
| Zhu Yihua1, Zheng Chenyi2, Chang Dongxu1, Zhu Yukun1 |
1. State Key Laboratory of HVDC Electric Power Research Institute China Southern Power Grid Guangzhou 510663 China;
2. School of Electrical Engineering Southeast University Nanjing 210096 China |
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Abstract 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°.
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Received: 30 October 2023
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2024, Vol. 39 
