Stability enhancement of HVAC grids using HVDC links.

Abstract

Eskom is facing challenging times where the national power grid is placed under extreme pressure, therefore, the long existing poorly damped low frequency inter area oscillations affects the stability constraints thus reducing the power transfer capacity. Consequently new power stations are being built in remote locations to reduce the short fall of generation capacity and the HVDC technology has become appealing to transport large amount of power over long distance. This research aims to prove that stability enhancement of parallel AC systems can be achieved with the use of HVDC schemes. The HVDC system has the rapid ability to control the transmitted power during transient disturbances and this power system control has a significant effect on the dynamic performance of the system after a disturbance therefore the dynamic performance is related to the small signal stability, where the rotor oscillations are minimised and the system is brought back to steady state after an event or disturbance.The fundamentals of small signal stability in terms of observability, controllability, residues, network sensitivities and mode shape are explained together with a dominant oscillation path definition for HVDC links location selection. The key importance in controlling the power of the HVDC link to affect stability requires that the oscillation is observable and controllable. Simulation results on a simple four-generator, twoarea test system are presented, with a view to benchmark the results and develop a fundamental understanding of how using HVDC links for power transfer can stabilise the grid. The eigenvalue analysis of the system indicates the frequency of oscillations in the system and the generator’s participation factors, together with the controllability and observability of the inter area mode (mode of interest). There are a number of test simulations results from a LCCHVDC system (First Cigrê benchmark model) integrated into a test network where the influence on the small signal stability is analysed. Various literature has been reviewed which supports the basic principles, promoting the benefits of using HVDC systems to enhance stability of a parallel AC system (Hybrid) and then integrating supplementary control. This research investigates the use of the HVDC system to enhance the small signal stability with supplementary control which is termed predictive control. Power Oscillation Damping (POD) control through LCC HVDC links is studied to ensure secure operation of power systems. The Power oscillating damper is expressed as a transfer function whereas the MPC (Model Predictive Controller) is expressed as cost functions of a feedback signal which is a measured quantity. Two feedback signals are selected and their effectiveness with regard to their contribution to the damping of the system is investigated. The controller feedback signals are real power and voltage difference across the AC tie lines. Bode plots, root locus plots and time domain simulation results show the comparison between the different selected controller inputs and supplementary controls. The voltage angle difference is most effective as it is more sensitive to changes in the system and assists the controller in bringing the system to steady state in a shorter period of time when compared to the controller input that uses real power across the AC tie line. The controllers with the HVDC integrated, do improve the damping of the system and it is related to shorter mode decay time, the MPC however has been investigated to reduce the change of loading levels of the AC tie lines following a change in system operating conditions. Simulation responses from the research show that this method is more promising and does not require prior knowledge of the possible contingencies due to its ability to handle complex multi variable systems with constraints, by using cost function algorithms to perform predictions of future plant behaviour and calculating the suitable corrective control actions needed to take the predicted output as close as possible to the target value which is the steady state. This research however demonstrates the fundamental principle which proves that the HVDC together with supplementary control can enhance stability of a parallel AC system.