Real-time Control of Battery Energy Storage Systems to Provide Ancillary Services Considering Voltage-Dependent Capability of DC-AC Converters

TL;DR

This paper presents a real-time control framework for battery energy storage systems that optimally manages ancillary services by considering converter capabilities and security constraints, validated through experiments.

Contribution

It introduces a convex reformulation of a nonconvex optimization problem for BESS control, enabling efficient and globally optimal real-time setpoint computation.

Findings

Achieved 100 ms update time for control setpoints.

Validated control framework on a 720 kVA BESS at EPFL.

Demonstrated improved computational efficiency and feasibility.

Abstract

Frequency response and voltage support are vital ancillary services for power grids. In this paper, we design and experimentally validate a real-time control framework for battery energy storage systems (BESSs) to provide ancillary services to power grids. The objective of the control system is to utilize the full capability of the BESSs to provide ancillary services. We take the voltage-dependent capability curve of the DC-AC converter and the security requirements of BESSs as constraints of the control system. The initial power set-points are obtained based on the droop control approach. To guarantee the feasibility of the power set-points with respect to both the converter capability and BESS security constraints, the final power set-points calculation is formulated as a nonconvex optimization problem. A convex and computationally efficient reformulation of the original control problem is then proposed. We prove that the proposed convex optimization gives the global optimal solution to the original nonconvex problem. We improve the computational performance of this algorithm by discretizing the feasible region of the optimization model. We achieve a 100 ms update time of the controller setpoint computation in the experimental validation of the utility-scale 720 kVA / 560 kWh BESS on the EPFL campus.