Synchronization Requirements for Low Voltage Transfer Switches

Abstract

This thesis inspects the requirements and challenges involving the open-transition automatic transfer switches (ATS), when an electric motor runs as the load of the switch. The research problem is to find out, if a certain phase angle value can be set for the automatic transfer switch so, that the transfer can be performed safely, regardless what kind of motor is run on the load side. At the same time all the possible requirements and challenges involving the synchronization are inspected.

The thesis was done as an assignment given by ABB Oy Protection and Connection in Vaasa. All the simulations were done with guidance from the University of Vaasa, and with their programs. The tests were done at the ABB Oy switch testing laboratory at Vaasa.

In the theory part the structural and functional differences between open and closed-transition switches. The requirements given by the IEC and UL standards were reviewed. The functionality and mathematical principles of the synchronization algorithm were also explained. In the practical part an PSCAD simulation was done for the case, where the source of an electrical motor is switched with open-transition. After this the tests were done at ABB Oy’s test laboratory and the results were compared to the simulations.

It was discovered that the fast open-transition method is very challenging to do safely, especially when running a motor with low inertia which therefore slows down very fast. A certain phase angle with which the transfer could be always performed safely could not be determined. For a simple solution the delayed-transition method was suggested, to allow the motor to fully stop before reconnecting. It was however concluded, that further testing with different motors should be done before any final decisions.