Frequency Control of Virtual Power Plants

The Virtual Power Plant (VPP) concept refers to the aggregation of Distributed Energy Resources (DERs) such as solar and wind power plants, Energy Storage Systems (ESSs), flexible loads, and communication networks, all coordinated to operate as a single generating unit. Using as starting point a comprehensive literature review of the VPP concept and its frequency regulation technologies, the thesis proposes a variety of frequency control and state estimation approaches of VPPs, as follows. First, the thesis studies the impact of coordinated frequency control of VPPs on power system transients, in which ESSs are utilized to provide fast frequency regulation. The thesis also proposes a simple yet effective coordinated control of DERs and ESSs able to integrate the total active power output of the DERs, and, thus, to improve the overall power system dynamic performance. The impact of topology on the primary frequency regulation of VPPs is also investigated. With this regard, two types of VPPs topologies are considered, that is, a topology where the DERs that compose the VPP are scattered all-over the transmission grid; and a topology where the DERs are all connected to the same distribution system that is connected to the rest of the transmission grid through a single bus. Next, the thesis proposes a control scheme to improve the dynamic response of power systems through the automatic regulators of converter-based DERs. In this scheme, both active and reactive power control of DERs are varied to regulate both frequency and voltage, as opposed to current practice where frequency and voltage controllers are decoupled. To properly compare the proposed control with conventional schemes, the thesis also defines a metric that captures the combined effect of frequency/voltage response at any given bus of the network. Finally, the thesis presents an on-line estimation method to track the equivalent, time-varying inertia as well as the fast frequency control droop gain provided by VPPs. The proposed method relies on the estimation of the rate of change of the active and reactive power at the point of connection of the VPP with the rest of the grid. It provides, as a byproduct, an estimation of the VPP’s internal equivalent reactance based on the voltage and reactive power variations at the point of connection. Throughout the thesis, the proposed techniques are duly validated through time domain simulations and Monte Carlo simulations, based on real-world network models that include stochastic processes as well as communication delays.