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| Transient Voltage Stability Analysis of Voltage Source Converters Integrated System under DC Voltage Control |
| Liu Xinyu1, Shan Yongpeng1, Xin Huanhai1,2, Zheng Di3 |
1. College of Electrical Engineering Zhejiang University Hangzhou 310027 China;
2. Zhejiang Key Laboratory of Electrical Technology and System on Renewable Energy Hangzhou 310027 China;
3. College of Mechanical and Electrical Engineering China Jiliang University Hangzhou 310018 China |
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Abstract The transient voltage stability issues in traditional power systems mainly focus on the transient stability of induction motors in receiving-end power grids. However, with the large-scale integration of renewable energy sources (RES) into the sending-end grid, the system also faces serious transient voltage instability problems during fault recovery process. At present, the numerical simulation methods are mostly utilized to analyze the influence of RES on the voltage drop during fault, and the transient voltage stability is analyzed by numerically calculating the time-domain integration of the voltage lower than 0.8(pu), which is determined based on the engineering experience criteria. However, the voltage dynamics during fault recovery process is mainly determined by the voltage control mechanism of various equipments, the voltage stability of the system cannot be determined by the period of voltage recovery process. To avoid the misjudgment of the transient voltage stability of the RES integrated system, it is urgent to reveal the voltage instability mechanism considering the control strategies of RES. Therefore, based on the analogy of the transient instability process of synchronous generators, the transient voltage instability mechanism of RES under DC voltage control is revealed, the similarities and differences of transient voltage stability in RES integrated power systems and the traditional power system is sorted out. Afterwards, based on the quantitative analysis of boundaries for transient synchronous stability and transient voltage stability of RES integrated system, the transient stability dominance factor is proposed, and the rationality of the dominance of transient voltage stability in weak grids is demonstrated. Finally, the effectiveness of the proposed instability mechanism and analysis method is validated by electromagnetic transient simulations.
The following conclusions can be drawn from the simulation analysis: (1) With the DC voltage control, the RES integrated system crosses the unstable equilibrium point under the unbalanced active power during fault. Then, the positive-feedback voltage control mechanism of RES is caused, which results in the continuous increase of the active current and continuous decrease of AC voltage, and the transient voltage instability of RES integrated system happens. (2) When the grid is strong and reactive power gain of RES is large enough, the unbalanced active power during fault is relatively small, the increase of the active current is retarded, and the transient voltage stability of the system is improved. Moreover, increasing the proportional gain of DC voltage controller can enhance the system’s ability to respond the unbalanced power and provide additional braking effect. However, considering the lagging of integrator for DC voltage controller, increasing the integration gain will lead to the decrease of deceleration area of the system, and the transient voltage stability of the system is deteriorated. (3) The RES integrated system faces the transient synchronous instability and transient voltage instability risks simultaneously, and the transient voltage instability issue must be considered first when the grid is weak and the reactive power support of RES is insufficient. (4) The active power output by the RES during fault recovery process is increased with the decrease of PLL control bandwidths, and the transient voltage stability of the RES integrated system is improved after considering the PLL dynamics. Therefore, a conservative evaluation result is derived in this paper due to the neglect of PLL dynamics.
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Received: 30 April 2025
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2026, Vol. 41 
