  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.
