Characteristic Analysis and Risk Assessment for Voltage–Frequency Coupled Transient Instability of Large-Scale Grid-Connected Renewable Energy Plants During LVRT

The voltage-frequency coupled transient stability and risk assessment issues of weak grid-connected large-scale renewable energy plants (REPs) during low voltage ride through (LVRT) have received little attention to date. The shape and mechanism of voltage-frequency coupled transient instability during LVRT are investigated in detail by the voltage-vector-triangle graphic (VTG) method utilized in this article, which avoids complex mathematical calculations. In addition, the margin of the transmission line's voltage angle (MTVA) is defined in this article and is then used to quantitatively assess the transient instability risk of the large-scale REPs during LVRT. Furthermore, the expressions of the MTVA are deduced under different conditions and the major influencing factors, which resulted in instability risk, are determined correspondingly. On this basis, the transient instability risk assessment standard of the large-scale REPs is built and the instability risk minimized scheduling strategy is proposed in this article, which does not change the internal control structure and parameters of the REPs during LVRT. Finally, both the correctness of the VTG method and the risk assessment standard, and the effectiveness of the scheduling strategy proposed in this article are validated by experiments and simulations.