Dr. Milan Prodanović

Converter-interfaced generation (CIG) has been used to transfer energy from solar irradiation and wind to electricity networks. Until now, power plants based on CIG have been used solely to inject power from the primary source whenever it is available. As the share of CIG increases, its impact on the power system operation has become more relevant with some aspects clearly affecting the stability and real-time management of the system. Most characteristics of CIG, including its operation and inertial and overcurrent properties are significantly different from traditional synchronous generators. System operators have acknowledged these changes and some of them have already started requesting additional functionality from CIG and changing the Grid Codes. In future CIG-based power plants connected to transmission networks are expected to contribute to damping of low-frequency electro-mechanical oscillations.

To meet these expectations, future CIG control algorithms need to include a power oscillation damping (POD) controller. This controller is typically designed to adjust the CIG’s active and/or reactive powers in order to damp a specific oscillation of the network frequency. However, it is highly likely a CIG in future will connect to a network with several poorly damped frequencies, potentially leading to multiple low-frequency oscillations. Moreover, the network configuration and the transmission system operating point are constantly changing during the operation. These factors impose major challenges for the design of POD controllers. In addition, POD algorithms need to be applicable to converters operating in Grid-Forming mode that is significantly different from the existing Grid-Following CIGs.

Renewable CIG-based power plants assume a number of local CIG units and a centralized controller. Several events of grid oscillations have already been reported and have been linked to internal delays occurring within the plant, namely those between the centralised controller responsible for the coordination and the local CIG units.

In this presentation, different aspects of power oscillation damping using CIGs will be addressed and several methods for design of POD controller proposed. In particular, the impact of the network topology changes and the presence of stochastic communication network delays on POD will be analysed and the solutions for the respective controllers presented. Moreover, the proposed solutions assume only the local frequency measurement and are suitable for the cases when a detailed power system model is unavailable. Both, Grid-Following and Grid-Forming converter control topologies will be examined for the application of POD service.

Keywords: Power Oscillation Damping, Converter-Interfaced-Generation, Renewable Plants