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Analytical Calculation of Transient Current From an Inverter-Interfaced Renewable Energy

Short-circuit current analysis, especially transient current calculation, provides a basis for protection design. Because the control systems of inverter-interfaced renewable energy sources have a high degree of nonlinearity, it is difficult to analyze the transient current. To address this issue, in this work, a nonlinear differential equation of transient current of symmetrical fault is derived while considering the saturation characteristics of the current loop. Aiming at the solution of this nonlinear differential equation, an analysis method based on the phase plane is proposed, which makes use of the linear relationship between the current deviation and its derivative to simplify the direct determination of the nonlinear working state in the time domain. The analytical expression of the transient current can then be obtained by the inverse transform from the phase plane to the time domain. The proposed calculation method is verified by both simulations and field tests.