The Limitations of Digital Simulation and the Advantages of PHIL Testing in Studying Distributed Generation Provision of Ancillary Services

There is increasing interest in the evaluation of the capability of power-electronic-interfaced distributed generators (DGs) connected to weak medium-voltage (MV) feeders, to provide ancillary services. Classic simulations using simplified DG models have their limitations and may prove insufficient due to the complexity of adequate modeling of power electronic interfaces. Moreover, conventional testing does not allow the investigation of the real generator with the distribution system interactions. Therefore, a scaled-down physical DG (i.e., inverter and dc source) with exactly the same functionalities can be used to evaluate the network integration of the actual DG, by means of power hardware-in-the-loop (PHIL) testing. In this paper, suitable scaling of the power rating and voltage level of the hardware is performed, and an interfacing approach is proposed that achieves stability of simulations without compromising accuracy. The PHIL tests successfully demonstrate potential problems in the coordination of the on-load tap changer controlling the MV feeder with the voltage controller of the DG, such as recurring tap changes, increased reactive power flows, and opposing actions. Moreover, recurring oscillations of the voltage controller of the hardware model are observed at certain system configurations. These inverter control instabilities, which are not visible in purely digital simulations, demonstrate the added value of employing PHIL testing for current and future power system analysis and testing.