Direct vs. Indirect Control Schemes for Grid-Forming Inverters-Unveiling a Performance Comparison in a Microgrid

This article provides insights into the dynamic performance of indirect and direct control strategies for grid-forming (GFM) inverters. The indirect method for grid-forming inverters utilizes three cascaded stages to produce the reference signal sent to the PWM generator. However, the direct method has only one stage. The indirect method consists of the outer loop, which controls the power, the middle loop, which regulates voltage, and the inner loop, which controls the output current. The direct method does not require the middle and inner control loops. The article compares the performance and sensitivity of both indirect and direct control methods under various conditions, including static, inductive and dynamic loads, by adjusting controller gain parameters. A virtual reactance inrush current mitigation technique is implemented with the direct method to provide momentary control over inrush caused by the dynamic loads. The direct method incorporates a feedforward technique known as virtual reactance inrush current mitigation to outperform the performance of the direct control method in the presence of dynamic and inductive loads. The performance of both control methods is experimentally compared, and the direct method outperforms the indirect method. Experimental analyses are performed in a laboratory-scaled microgrid supplied by a 208V, one 5kVA SiC inverter, and one 10kVA Si-based inverter to compare the performance of the indirect and direct methods under different test scenarios, and the outcomes are presented in this article.