Study on Third Harmonic Fault Current Characteristics of Inverter-Based Resources

With the continuous development of new energy and DC transmission technology, more and more inverter-based resources (IBR) are connected to power grid. The fault characteristics of IBR are quite different from traditional power resources. At present, the research on IBR short-circuit current mostly focuses on the fundamental frequency component, and little study on the harmonic characteristics. This paper deduces the mechanism of third harmonic current outputted by IBR during fault. When the system has an asymmetric voltage sag, the negative sequence component appears in the short-circuit current and voltage, which results in a double fundamental frequency oscillation in the inverter DC bus voltage. This oscillation component will flow through the control circuit of inverter step by step, and eventually lead to a third harmonic generation in the short-circuit current output of IBR. Moreover, the third harmonic characteristics under different fault conditions and different control strategies are analyzed by simulation tests, which verify the correctness of theoretical analysis.