Employing unified power flow controller to improve stability of the northern section of Ghana’s electric power transmission system.

Abstract

The demand for electricity in Ghana has been increasing for over a decade, with no corresponding increase in the transmission capacity. This led to the transmission network operating close to its stability limits. In addition, the system has inadequate transfer capability during peak periods resulting in decreased system voltages and increased transmission losses, which lead to voltage instability. This is evident on the major power outages experienced recently in the country. In this thesis, the major instability issue on Ghana’s transmission system is investigated with a computer model of the network developed in MATLAB Simscape Power Systems. Load flow studies conducted on the modelled network revealed that voltage instability was predominant, where some buses experienced voltages outside the stability margin of p.u. Different techniques are used to solve stability related issues and the thesis employed the use of the UPFC, which is a current member of the FACTS family. The device is modelled as two sets of four GTO-based square wave inverters connected back to back with a common DC link capacitor. The controllers of these devices are designed to operate within the power capability of Ghana’s 161 kV network. The performance of these controllers were effective on the system. The Tumu bus was identified as the weakest and a candidate for the UPFC placement using Fast Voltage Stability Index analysis. With the UPFC in circuit, the voltage across the buses improved significantly. The power transfer capability also increased leading to low system losses. The UPFC is therefore recommended to be incorporated into Ghana’s transmission system for a stable and secure operation.