Date of Award


Document Type


Degree Name

Master of Engineering


Electrical & Electronic Engineering

First Advisor

Michael O'Donovan

Second Advisor

Dr. Joseph Connell


Switching is a vital task in any power system for ensuring its safe and reliable operation. Switching may be necessary for fault clearance, to ensure wider system stability and to prevent damage to plant. It is essential for isolation, to allow technicians to carry out maintenance tasks safely. Also, switching of reactive loads such as shunt capacitor banks and shunt reactors, is crucial for controlling system voltage.

Switching of some loads however, may produce voltage transients and heavy transient inrush currents which can impact on wider system power quality, impact customers and cause damage or deterioration of the insulation of HV equipment. Therefore, it is important to provide some form of measure to control or mitigate transients caused by switching. The main control measures include: metal oxide surge arrestors, pre-insertion resistors, current limiting reactors and synchronised or controlled switching. Controlled switching is the favoured solution for frequently switched loads such as reactive plant, for economic benefits and as it reduces transients in the first instance.

Controlled switching is defined as the use of electronic equipment to control the making or breaking of high voltage circuit breakers at pre-determined points on the system voltage and current waveforms. It has been implemented in Ireland for over 30 years for the energisation of shunt capacitor banks. Over the last two years, the benefits of controlled switching for different applications has become ever more apparent, with increased use such as switching of transmission shunt reactors and the energisation of large power transformers, particularly in remote areas of the network such as wind farm interfaces.

The aim of this thesis is to provide a complete overview of the stages concerned in implementing controlled switching schemes, from examining the impacts of switching certain loads, to performing systems studies, up to site commissioning stage. The research in this thesis looks at both the theory and practice. It draws together the published work, manufacturers guidelines, international standards and simulation results, to give the total awareness of the issues involved in reactive load switching and commissioning regimes.

The various solutions and strategies associated with controlled switching schemes are examined, to ensure that the best and most economical solution has being implemented. Several recent projects where controlled switching has been implemented for switching of transmission reactors and power transformers are also investigated.