Research on improved voltage support method for unbalanced grid condition

CAO Xiaoqing; LI Te; LI Lin; CHEN Di; ZHOU Zhengxu; SHI Xiaojie

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Electric Power Construction ›› 2025

Research on improved voltage support method for unbalanced grid condition

CAO Xiaoqing¹, LI Te², LI Lin¹, CHEN Di¹, ZHOU Zhengxu¹, SHI Xiaojie²

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Abstract

[Objective] With the integration of large-scale distributed photovoltaics, issues like reverse power flow and over-voltage arise, posing challenges to the safe and stable operation of the power system. To address this issue, both domestically and internationally, regulations mandate that PV systems must have grid support capabilities, such as voltage-reactive control, i.e., they actively regulate their reactive power based on voltage deviations. However, such regulations are typically designed for balanced voltage scenarios, and unbalanced voltage conditions are rarely considered. Therefore, taking the voltage-reactive power control(Volt-Var control, VVC) as an example, this paper analyzes the influence of voltage imbalance on their reactive power output, and proposes an improved voltage support control method aiming at a minimal three-phase voltage deviation. [Methods] Based on the instantaneous power theory, the mathematical relationship between voltage imbalance and active/reactive power is derived, demonstrating the voltage regulation effect of reactive power and the existence of the minimum voltage point. Then, with the three-phase voltage deviation as an index, the positive sequence voltage and reactive power corresponding to such point are calculated, and dynamic adjustments to the voltage support control curve are made using this point, voltage limits, and reactive power capacity of PV inverters. [Results] Matlab/Simulink simulation results demonstrate that the proposed improved voltage support control method ensures three-phase voltages remain within limits while achieving minimal deviation from the nominal voltage under the lowest reactive power output, accomplishing economical and efficient regulation of three-phase voltage in unbalanced scenarios. [Conclusions] The proposed control strategy effectively coordinates traditional VVC control with negative-sequence current/power control. It maintains three-phase voltages within grid-connected standard limits despite changes in irradiance, temperature, or other factors causing photovoltaic power fluctuations or voltage-unbalance variations, significantly enhancing the adaptability of conventional VVC control functions.

Key words

Unbalanced grid voltage / Grid integration of PV / Voltage support / Voltage-reactive power control / Instantaneous power theory

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CAO Xiaoqing, LI Te, LI Lin, CHEN Di, ZHOU Zhengxu, SHI Xiaojie. Research on improved voltage support method for unbalanced grid condition[J]. Electric Power Construction. 2025

References

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Funding

National Natural Science Foundation of China (51537009); Key R&D Program of Hubei Province (2020BAA022); Fellowship of China National Postdoctoral Program for Innovative Talents (BX2021224).