PhD student Siddhartha Nigam with advisors A. Domínguez-García and P. Sauer

Any collection of interconnected generators and loads capable of islanded operation is generally considered a microgrid. In a prototypical example, a neighborhood comprising homes with rooftop-mounted photovoltaic (PV) arrays, microgrids are typically envisaged to consist entirely of generators that are interfaced through power electronics. However, without the spinning mass inherent in traditional synchronous generators, this class of microgrid has little or no effective inertia, making frequency regulation a key operational challenge. In this work, supported by D-G Cost Share-Grant AC281, we describe the implementation and testing of a centralized and a distributed frequency-regulation controller for islanded inertia-less ac microgrids. We use a simple six-node network, shown in Figure 7, to illustrate the islanded inertia-less ac microgrid with three inverter-based generators as well as loads.

Siddhartha Higam frequency-regulation controller 6-node network

Figure 7. Six-node ac microgrid

For the centralized frequency regulation architecture, we use an NI cRIO-9068 micro-controller. We model the microgrid using the Typhoon hardware in loop (HIL), which emulates real-time operations. The centralized controller monitors the ac microgrid to detect and compute the average frequency error due to load changes. The controller then fixes the set points of the inverter-based generator sources to remove the average frequency error. This process takes place continuously in a closed loop regulating the microgrid frequency. For the distributed frequency regulation architecture, we use six Arduino-based micro-controllers to run a ratio consensus algorithm to compute the average frequency error in the system due to load changes. Based on the consensus results that provide the average frequency error, the frequency controllers of each inverter-based generation resource fix the set points to match the generation and load and thus regulate the system frequency. We implement each frequency controller on the Arduinos and model the microgrid in the Typhoon HIL, as we did for the centralized architecture. Communication between the Typhoon and the NI cRIO-9068 or the six Arduino-based micro-controllers for centralized and distributed architectures is carried out via the Modbus protocol with the Typhoon acting as the slave device and the controllers acting as the master. We describe results for frequency regulation, in particular, the individual change in the set points of the inverter-based generators with respect to the system load changes, and the associated bus angles as well as the average frequency error and its correction over time.