  This paper presents a fast power-setpoint tracking algorithm to enable
utility-scale photovoltaic (PV) systems to provide high quality grid services
such as power reserves and fast frequency response. The algorithm unites
maximum power-point estimation (MPPE) with flexible power-point tracking (FPPT)
control to improve the performance of both algorithms, achieving fast and
accurate PV power-setpoint tracking even under rapid solar irradiance changes.
The MPPE is developed using a real-time, nonlinear curve-fitting approach based
on the Levenberg-Marquardt algorithm. A modified adaptive FPPT based on the
Perturb and Observe technique is developed for the power-setpoint tracking. By
using MPPE to decouple the impact of irradiance changes on the measured PV
output power, we develop a fast convergence technique for tracking
power-reference changes within three FPPT iterations. Furthermore, to limit the
maximum output power ripple, a new design is introduced for the steady-state
voltage step size of the adaptive FPPT. The proposed algorithm is implemented
on a testbed consisting of a 500 kVA three-phase, single-stage, utility-scale
PV system on the OPAL-RT eMEGASIM platform. Results show that the proposed
method outperforms the state-of-the-art.
