Abstract Driven by the large-scale adoption of electronic power converters for widespread renewable generation, power grids gradually present weak grid characteristics, i.e., the grid impedance can’t be ignored and fluctuates frequently. The grid impedance directly affects the control loop gain of inverters, which can decrease the stability margin and lead to poor dynamic performance. To solve these issues jointly, an adaptive control strategy based on loop gain measurement technique is proposed in this work to improve the adaptability of inverters to pre-unknown weak grid conditions. The proposed adaptive control strategy includes two important steps: (1) The monitoring of control performance indexes and (2) Auto-tuning of controller parameters. Firstly, a control performance monitor is proposed for online monitoring of control performance indexes (crossover frequency fc, phase margin φM) of inverter systems. These indexes can be used to characterize, in real time, the dynamic performance and stability, respectively. The proposed control performance monitor is designed based on Middlebrook’s loop gain measurement technique with small signal injection. The corresponding signal extraction structure is realized by a high-pass second order generalized integrator (HPSOGI), which can enhance the attenuation ability of low-frequency harmonics. With the measured performance indexes (fc, φM), an auto-tuner is then designed to adjust controller parameters of inverters accordingly. The sensitivity analysis method is adopted to decouple the controller parameters (Kp, Kr) and control performance indexes (fc, φM). A general adaptive law is therefore designed as following: the crossover frequency fc is designed to adjust the proportional gain Kp until the desired crossover frequency $f_{\text{c}}^{\text{REF}}$ is achieved; the phase margin φM is designed to adjust the resonance gain Kr until the desired phase margin $\varphi _{\text{M}}^{\text{REF}}$ is achieved. In this way, high dynamic performance and sufficient stability margin can be guaranteed simultaneously by the proposed auto-tuner. Additionally, the relevant parameters selection and design of regulators are discussed. The experimental results show that the monitoring process can be completed within 60ms with an accuracy of 96%, which is sufficient for the subsequent auto-tuning process. Both the measurement time and the accuracy can be further improved by refining the auto-tuner coefficients. With the proposed adaptive control strategy, the crossover frequency and phase margin can be maintained at expected value (fc=1 kHz, φM=45°) under grid impedance variations and low-voltage ride through conditions. With the increased loop gain, total harmonic distortion (THD) of the injected grid current is also decreased. The following conclusions are drawn from this paper: (1) The HPSOGI structure adopted in control performance monitor can improve accuracy and speed of the monitoring process. (2) Sensitivity analysis method can be used to design adaptive law to improve the adaptability of adaptive control strategy to various regulators such as PI and PR regulator. (3) The proposed adaptive control strategy can maintain high dynamic performance and sufficient stability margin of inverter system under grid impedance variation and fault conditions such as low-voltage ride through.
Zeng Xiangchen,Liu Qing,Wang Jiachen等. Adaptive Control Strategy of Grid-Connected Inverters with Constant Bandwidth and Stability Margin in Weak Grids[J]. Transactions of China Electrotechnical Society, 2024, 39(9): 2682-2695.
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2024, Vol. 39 
