Optimisation and Control of Distributed Energy Resources for Power and Voltage Regulation in Distribution Networks

Abstract

Increasing levels of distributed energy resources (DERs) have created significant technical challenges in today's distribution networks. Small-scale DERs such as rooftop solar photovoltaic, battery storage and electric vehicles, when not coordinated, can cause undesirable voltage deviations, peak power demand and power losses on the network. When properly optimised and controlled, the same DERs are the key elements in effectively overcoming such challenges and benefiting all energy users.

In this thesis, original optimisation and control approaches are proposed to control real and reactive power of DERs for grid power and voltage regulation. This thesis proposes local approaches that act autonomously to grid demand and voltage variations based only on local measurements. A two-layer central-local control is also proposed, where fast-acting local controllers receive updated parameters from a system-wide optimisation at a slower timescale. These optimisation and control approaches are presented along four main chapters in this thesis: from the third to sixth chapter.

In the third chapter, a local optimisation-based control is proposed to charge and discharge residential battery storage to prevent excessive grid voltage deviation. The proposed approach, formulated as a quadratic program (QP), is compared with other QP approaches. As DER owners experience financial losses due to over-voltage disconnection of inverters, the benefits for both the grid operation and financial savings for customers are quantified. To reduce complexity in this first part, only real power (charge/discharge) of batteries is controlled.

In the fourth chapter, an original optimisation-based control is proposed to actuate both real and reactive power of battery storage. The approach aims to regulate grid voltages and reduce power losses, while providing economic benefits for DER owners. The network model is also included in the control to capture the sensitivity of real and reactive power for voltage feedback regulation. When providing voltage feedback control with hundreds of DERs across the network, it is critical to design parameters to achieve system stability and prevent voltage oscillations in distribution networks.

In the fifth chapter, the condition for system stability is analytically obtained and assessed. Leveraging on the stability condition, a local volt-var-watt (VVW) proportional control is proposed to effectively regulate voltages while reducing grid power losses. The design of control parameters is based on the stability criterion and an original local optimisation problem to weight real and reactive power actuation considering the grid impedance characteristic. This optimisation problem can be solved locally and analytically.

The sixth chapter proposes a central optimisation with system-wide information to design parameters of local VVW proportional control. In this two-layer central-local approach, autonomous VVW controllers act on a millisecond timescale for voltage regulation and have their gain (slope) coefficients updated regularly by the central optimisation problem at a minute timescale. The optimisation is similar to an optimal power flow (OPF) and formulated as a convex multi-period problem solved in a receding-horizon manner.

Numerical simulations are run with real-world customer data and distribution test feeders. We test and compare the optimisation and control proposed in this thesis with suitable benchmark approaches from the literature. In general, proposed approaches provide significant improvement in grid operation, particularly regarding voltage regulation, grid power losses and peak power demand. With fewer technical challenges, the proposed approaches allow for greater integration of low-carbon DERs and contribute to the sustainable electricity grid of the future.