Frequency stability analysis and control methods for wind generation

Abstract

The demands on modern power systems require increased penetration of non-conventional and renewable energy sources such as wind and solar, which increasingly displace traditional and large-scale thermal generation based on coal, oil and gas resources. Higher levels of renewable power penetration require substantive changes in power system operation and network topologies which in turn poses significant impacts on its stability. Therefore, to ensure reliable and secure operation of power systems, a comprehensive investigation of the impacts of greater integration of small-scale, distributed, low-inertia generation is essential.

The thesis investigates methods to assess power system frequency stability in the context of significantly increased distributed generation penetration. The work has focused on applicable control theory, power system models and viable approaches applicable for large- scale problem modelling. Partial replacement of large-scale thermal generation with modern wind plant challenges system frequency regulation and threatens stability, in certain situations rendering the frequency to fall below acceptable limits, especially in isolated power systems. Moreover, large wind penetration reduces total system inertia and consequently affects frequency responses to disturbances. This thesis considers the role and impact of wind generation, with specific investigation of the integration of doubly-fed induction generators and the impacts of large-scale penetration onto power grids.

This thesis investigates the role and impact of the widely-used proportional-integral- derivative (P1D) controller on the inertial response of DFIG wind turbines to support the frequency control of a power system in the event of sudden power changes. The work has determined the effect of P1D parameters and established benchmarks so that an adaptive control strategy can be subsequently developed for frequency regulation. The work has considered the influence of supplementary control loop parameters on the inertial response and power system frequency. The results confirm that the DFIG inertial controller scheme is able to provide appropriate frequency support.

Conventional inertial control algorithms which use the rate of change of frequency (RoCoF) and frequency deviation (droop) loops require a great effort to determine appropriate P1D gains suitable for all power grid and wind speeds. Once an understanding of the effect of controller parameters is gained, an active control strategy is proposed for frequency regulation using variable gains (which are tuned offline) in the frequency deviation loop for the inertial controller. The variable gain control approach presented in this thesis is shown to actively respond to system changes to improve the performance. The proposed controller is compared with the conventional fixed-gain PID method demonstrating enhancement in the frequency nadir whilst guaranteeing steady DF1G operation.

Finally, a fuzzy logic adaptive inertial controller (FLAIC) scheme for online tuning of the PD- type inertial controller parameters is proposed. The scheme aims to enhance the performance of a DFIG and improve the frequency nadir whilst guaranteeing stable operation of a DFIG. The proposed controller adapts the controller parameters of the supplementary inertial control of the DFIG wind turbine so that with any disturbance such as load changes, the active power output can be adaptively controlled to mitigate the frequency deviation. This is done by adapting the controller gains automatically online based on the droop feedback thus helping to release more kinetic energy. The performance of the proposed scheme is investigated under various system conditions and again compared with the conventional PD and active gain methods. Simulation studies carried out in Matlab/Simulink software demonstrates the effectiveness and efficiency of the proposed FLAIC-based methodology. The results indicate that the robustness of the adaptive controller to any load variations is better compared to that of the conventional controller with fixed parameters as well as the active control strategy proposed in this thesis. It is therefore concluded that the active frequency response of the power grid with high wind power penetration can be significantly enhanced.

Date of Award	Jul 2019
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Timothy Littler (Supervisor) & Wasif Naeem (Supervisor)