PhD Student Grant McKechnie with Advisor A. Domínguez -García

With the rapid shift toward distributed inverter-based resource (IBR) power generation, ensuring the reliability and stability of the electrical grid is becoming increasingly critical. Historically, the grid relied on the rotational inertia of centralized synchronous generators for stability. However, modern grid-following (GFL) inverters offer little inertial support and often go offline if the grid itself fails. To address this, research is focusing on grid-forming (GFM) inverters, which can emulate synchronous generator dynamics and support the grid even during islanded operation.

A major challenge in integrating GFM inverters is their compatibility with legacy grid protection schemes. Traditional devices like circuit breakers are designed for the high fault currents produced by synchronous generators, whereas IBRs have sensitive electronics that strictly limit fault currents. Because IBR fault currents are so low, they often fail to trigger protective devices, risking equipment damage and delayed fault isolation. To overcome this, new control strategies are required that can alter inverter behavior specifically for fault detection and injection.

One promising approach is the use of delta-connected single-phase GFM inverters. While three-phase inverters are common, arranging single-phase units in a delta configuration provides unique opportunities for flexible control. However, maintaining balanced operation and precisely controlling phase differences in such a configuration is complex and requires advanced modeling and control frameworks.

This work investigates a novel control framework that explicitly manages the phase differences between single-phase units in a delta-connected system through analysis of phase balancing dynamics. Using a high-fidelity model (Typhoon HIL), we develop and compare a Linear Quadratic Integral (LQI) controller and a Proportional-Integral (PI) controller for phase angle reference tracking. We evaluate the system’s ability to operate as a balanced three-phase source under nominal conditions while allowing for intentional unbalancing to inject auxiliary signals for fault detection. This flexibility is essential for adapting modern inverter-based resources to meet strict grid protection requirements.

Figure 31. Equivalent-circuit model of three delta-connected single-phase inverters in phasor domain serving a generic constant impedance three-phase load.