Instability Analysis and Stability Control of Grid-Connected Inverter Based on Impedance Circuit Equivalent

The renewable energy power generation systems that utilize grid-connected inverters as the power interface are prone to instability issues when the AC grid is weak. The fundamental reason for the wideband oscillation phenomenon when grid-connected inverters are integrated into AC grids is negative resistance. Firstly, to understand the mechanism of the negative resistance and propose the effective oscillation suppression methods, this paper visualizes the current-inner loop, phase-locked loop, voltage feedforward, and decoupling terms of the grid-connected inverter in to the equivalent circuit model in the dq-domain, modified-sequence domain and positive/negative-sequence domain. By studying the interaction of control dynamics within the sub-/super-synchronous frequency range, the circuit essence of the negative resistance is revealed. Secondly, based on the equivalent circuit model of the grid-connected inverter and the inherent essence of the negative resistance, the design principles of active damping control are proposed. Additionally, the optimal location for implementing active damping control is examined. According to the equivalent circuit among various active damping controls, the investigation is conducted to understand the impact of these controls on the negative resistance characteristics. Finally, the oscillation suppression effects of different active damping control strategies are compared through PSCAD/EMTDC simulations and StarSim controller hardware-in-the-loop experiments. Besides, the dynamic performance of the impedance-high-pass filter type active damping control is validated. It is demonstrated that the system oscillation can be effectively suppressed within 200 ms using the designed control method. Additionally, the grid-connected inverter remains stable under weak grid conditions with SCR=1. The main conclusions of this paper are as follows: (1) The interaction between the current inner loop and the phase-locked loop of the grid-connected inverter is the primary cause for the emergence of the negative resistance in the q-axis equivalent circuit. The negative resistance of the q-axis circuit results in the negative resistance characteristic in the traditional sequential-domain impedance, particularly near the fundamental frequency. Under the weak grid scenarios, the negative resistance characteristic becomes more pronounced, elevating the risk of sub-/super-synchronous oscillations. (2) Based on the circuit model resulting from the interaction between the phase-locked loop and current inner loop, the phase-locked loop determines the existence of negative resistance and its frequency range coverage. The magnitude of the negative resistance is influenced by the integral coefficient of the current inner loop. (3) When designing the active damping control parameters, it is essential to enhance the inductance characteristics of the active damping control to mitigate the impact of negative resistance caused by the integral coefficient of the current inner loop. Thus, the system oscillatory stability can be improved. (4) The band-pass filter and band-rejection filter are beneficial for reducing negative resistance within the specific frequency range. Meanwhile, low-pass filters are advantageous for mitigating the high-frequency oscillations, whereas high-pass filters are useful for suppressing low-frequency oscillations. Under the same parameter conditions, the impedance-high pass filter-based active damping control proposed in this paper demonstrates a better suppression effect on sub-/super-synchronous oscillations, resulting in a notable enhancement of the dynamic performance. © 2024 China Machine Press. All rights reserved.