Equivalent Aggregated Modeling of Multi-VSC System for Transient Synchronization Stability Analysis

In a multi-voltage-source-converter (multi-VSC) system, there may be a scenario where some VSCs lose synchronization stability and others keep stable under grid faults. To describe and analyze the transient behavior of the system under such a scenario, accurate equivalent models are indispensable. In this article, a coherency-based equivalent aggregated modeling method of multi-VSC systems is developed for transient stability analysis. By investigating the factors that reflect the dynamic characteristics of VSCs, the pre-fault and post-fault q-axis voltage drops on collector impedance of each VSC are selected as the coherency identification indicators, and a coherency criterion based on these indicators is proposed. Then, by assessing the transient synchronization stability of multiple VSCs, a criterion based on electrical distances is provided to ensure that VSCs with different stability will not be divided into a coherent group. Combining these two criteria, VSCs can be divided into different coherent groups and aggregated as several equivalent VSCs. Thus, the equivalent model of the multi-VSC system can be obtained. The proposed model is verified by simulations of a multi-VSC system connected to the IEEE 39-bus power grid. Compared with the existing equivalent model, the proposed model shows advantages in precisely reflecting the power dynamics of the multi-VSC system where VSCs present different transient stability under faults. It provides an effective tool for behavior prediction and transient synchronization stability analysis of multiple VSCs.