Frequency regulation of power systems with high penetration of renewable energy resources

Abstract

For safety of generating equipment, satisfactory performance of electrical loads and reliable power delivery in a large-scale power system with multiple control areas, the system frequency and net inter-area power flows should be maintained at their nominal values. To achieve these targets, traditional power systems make generation follow demand by a three-layer frequency controller including droop control, automatic generation control (AGC) and economic dispatch (ED). However, due to the uncertainty and intermittency of renewable power outputs, this generation-side paradigm may be inadequate to regulate the frequency of modern power systems with high penetration of renewable energy resources. To tackle this challenge, the thesis focuses on three concrete frequency control issues in modern power systems, i.e., generation-side control enhancement, load-side controller design, and frequency regulation with energy storage devices (ESDs).

Firstly, to improve the control performance of AGC, a decentralized event-triggered control (DETC) algorithm is proposed to replace the traditional periodic sampling/communication mechanism of AGC for multi-area power systems. For each control area, event-triggering rules that only rely on local measurements are designed to decide the communication instants. A positive dwell time is introduced to exclude Zeno behaviours. Asymptotic stability of the interconnected power network under the developed event-triggered control law in the presence of small constant net load disturbances is analysed based on a linearized model with all generators in each area being simplified as an equivalent generating unit. Further, in view of the fact that the system net demand fluctuates over time due to the changes of renewables and loads, the $\mathcal{L}_\infty$-stability of the system under the DETC-based AGC with time-varying disturbances is also studied.

Secondly, to exploit the frequency support potential of load-side control, a distributed controller which is robust against system parameter uncertainties is designed for each generating unit and flexible load in a multi-area power network with substantial renewables. The targets of the proposed method are twofold. One is to maintain the nominal frequency and scheduled net inter-area power exchanges; and the other is to optimally coordinate the active powers of all controllable units. Asymptotic stability of the system under the distributed controller is analysed by using the nonlinear structure-preserving model.

Thirdly, distributed model predictive control (MPC) is used for frequency regulation and multi-time slot economic dispatch (MTSED) of power systems with ESDs. The frequency regulation task is formulated as an MPC problem where the energy capacity limits of ESDs and bus frequency limits are considered. Then, a novel projection-based algorithm is proposed to solve the MPC problem in a distributed way. The MTSED problem aims to keep the active/reactive power supply-demand balance of systems with ESDs at minimal operation costs. Compared to the traditional ED, the proposed MTSED takes the constraints on bus voltages and reactive power injections into account by using the newly developed decoupled linearized power flow model in the literature.