The gated current source converter has active turn-off capability during commutation. This capability gives it better transient active and reactive power characteristics during transients. Currently

there is a lack of quantitative analysis for this converter's voltage and current during electromechanical transients. Mathematical theory is also missing for its operation in large-grid simulation and reactive power support modes. This paper studies the analytical calculation of the external characteristics during electromechanical transient timescales. First

we divided converter operation into three working conditions based on the firing angle range. We derived analytical expressions for DC voltage and AC current for each condition and each stage. Second

the detailed current expression makes solving the forced commutation angle inconvenient. We proposed an approximate linearization method using FFT analysis. This method achieves the goal of simplifying calculations. Third

we provided the expression for fundamental-frequency AC current on the valve side. We analyzed operating characteristics of active power and reactive power in the converter. Finally

we built an electromagnetic transient simulation model for the receiving-end converter. We verified accuracy of the electromechanical transient analytical expression and the supporting capacity of the converter. We also analyzed how key parameters affect system characteristics. The research results provide theoretical support. This support helps large-grid simulation software build electromechanical transient models for the converter. It also helps design fast transient control.