Transformer Switching Studies

What are Transformer Switching Studies?

• Energizing a transformer at the wrong time can produce a high inrush current and RMS voltage drop, temporary overvoltage (TOV) due to parallel resonance. Finally, it can lead to power quality issues, transformer insulation damage, surge arrester failure, and maloperation of relays.

• Transformer energization studies can be carried out to evaluate the transient RMS voltage drop, inrush current, and temporary overvoltage during the energization of the transformer under various operating conditions and different configurations.  

What is the Importance of Transformer Switching Studies?

• Transformer energization is a regular operation in the transmission and distribution networks. The transformer energization studies are required to check whether the transformer is compliant or not with the local electrical grid standards in terms of voltage drop.

• Since the network topology is changing continuously due to the integration of large penetration of renewable energy (offshore wind, etc.), there is a need for determining the potential problems associated with the energization of the transformer for at least in a planning or operational stage.

• This study helps to size the pre-insertion resistors which will reduce the voltage dip, improve the protection system, and avoid possible maloperation due to the energization of the transformer.

How Do We Perform Transformer Switching Studies?

• Step-1: Data collection such as single line diagram (SLD), data sheet of all the equipment such as transformers, losses, and saturation characteristics, shunt reactors, capacitor banks, power grid, loads, overhead lines/cables, generators, network short circuit level, etc.

• Step-2: Modeling the study system/substation using PSCAD/EMTDC software.

• Step-3: Preparation of simulation case studies with the study scenarios accepted by clients.

• Step-4: Preparation of technical report and submission to clients.

What Standards can be Referred for Transformer Switching Studies?

• CIGRE Working Group C4.307 - Transformer Energization in Power Systems: A Study Guide.

• IEC 60071-4 - Insulation co-ordination – Part 4: Computational guide to insulation co-ordination and modelling of electrical networks (It can be referred for temporary overvoltage).