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| Small-Disturbance System Voltage Support Strength Assessment Method for Renewables VSC-HVDC Delivery System |
| Ma Fuyilong1, Xin Huanhai1, Liu Chenxi1, Li Shiyang2, Yuan Hui1, Dai Jiang3 |
1. College of Electrical Engineering Zhejiang University Hangzhou 310027 China;
2. Electric Power Research Institute China Southern Power Grid Guangzhou 510663 China;
3. Guizhou Electric Power Dispatching and Communication Bureau Guiyang 550002 China |
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Abstract System voltage support strength (referred to as system strength) is used to describe the voltage response performance under a disturbance and rapidly quantify stability margin. The inverter-based resources in the system generally apply the grid-following control scheme and need to adapt to the current system strength; otherwise, sub-synchronous oscillations or even voltage collapse accidents are likely to occur. Therefore, it is necessary to accurately measure the system strength to ensure the safety and stability of the renewables system. The system strength indicated by short-circuit ratio (SCR) has provided an effective and intuitive reference for grid operators for a long time. However, the existing SCR-based methods rely on the premise that synchronous generators provide short-circuit capacity or voltage support. Due to this premise, these methods are unsuitable for renewables delivery systems with voltage source converter-based high voltage direct current (VSC-HVDC), where all apparatuses are power-electronic interfaces and no synchronous generators provide short-circuit capacity. It is a challenge to assess the system strength expediently and quantify the stability margin rapidly in the renewables VSC-HVDC delivery system (RVDS). As a result, this paper aims to respond this challenge and propose the system strength evaluation method in terms of small-disturbance analysis.
Firstly, to describe the bus voltage response performance under a small disturbance, the sensitivity transfer function matrix of the bus voltage to the renewables multi-feed current is derived based on linearization analysis, and the closed-loop characteristic equation can be accordingly formed. With the bus voltage as output variable, the relationship between the voltage performance and static voltage stability/small-disturbance synchronous stability of RVDS can be described by the closed-loop characteristic equation. Secondly, the generalized short-circuit ratio (gSCR) is extended into the RVDS based on the voltage-source equivalent analysis of VSC-HVDC.Then, a source-grid separation method is presented to evaluate the system strength of RVDS by combining the apparatus critical SCR and the generalized short-circuit ratio. Finally, the proposed method is verified by simulation in multiple wind plants with VSC-HVDC. Simulation results show that the proposed system strength assessment is consistent with the trend of time-domain eigenvalues and electromagnetic transient analysis results; the stronger system strength indicates the lower instability risks. Furthermore, the voltage-supporting characteristics of VSC-HVDC can be analyzed from the perspective of system strength, and the influence of parameters variation of the voltage-current control loop of VSC-HVDC can be interpreted as the variation of gSCR. Improper parameter sets of VSC-HVDC that weaken the system should be avoided, which may trigger the subsynchronous oscillations.
The following conclusions can be drawn: (1) The gSCR of RVDS essentially represents the sensitivity of power networks, reflecting the comprehensive electrical distance between the busbar and the equivalent voltage source for multiple renewable plants. It is independent of the short-circuit current analysis based on the existence of synchronize generators. Therefore, it can adapt to the renewable energy base without the traditional support from synchronous generators. (2) The gSCR can consider the dynamic voltage support characteristics of VSC-HVDC under different voltage control parameters and be suitable for static voltage stability and small-disturbance synchronous stability analysis. It is the extension of the generalized short-circuit ratio theory in RVDS. (3) The system strength of RVDS can be quantified by the relative difference value between the gSCR and apparatus critical SCR, which is convenient for collaborative implementation from both the resource and the network sides and helps to quickly screen and keep away from the system operations with high instability risks.
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Received: 14 February 2023
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2023, Vol. 38 
