Over-frequency support in large-scale photovoltaic power plants using non-conventional control architectures

Large scale photovoltaic power plants must provide a frequency regulation service, which is defined in the grid codes. This service has commonly required a response time between 15 and 30 s. But some countries are now introducing more strict regulations and requiring response times below 2 s. The typical centralized control architecture of photovoltaic power plants for frequency regulation can present undesired oscillatory responses (or even become unstable) when tuning the controller to achieve these small time response requirements. The present article proposes an alternative solution based on a hierarchical control architecture. In the proposed solution, inverter controllers apply a local frequency regulation action and the central controller corrects active power errors at the point of connection, which can be caused by power losses or lack of irradiance in some inverters. Simulation models are used to study and test the response of this new control approach. The proposed hierarchical control architecture is compared with a fully centralized and a fully decentralized archirectures. Results show that the hierarchical control architecture is not only capable to obtain a fast and accurate response, but also is robust against communication failures. The proposed hierarchical control architecture advantages could be extrapolated to other services. So, further research is proposed to confirm this hypothesis.