142 HANDBOOK OF ELECTRICAL ENGINEERING
It can seen that the instantaneous, and short-term root-mean-square, values of the primary
current can be much higher than their rated values, typically by as much as 10 times. Reference 4
Chapter 5 section XIV explains the phenomenon and offers a method of calculating the shape of
the first half-cycle of in-rush current. The reference also points out that if a HV/LV transformer is
energised from the LV secondary terminals, the in-rush current may in some designs be up to twice
the value in per unit than if the energising is carried out at the primary terminals.
These high values of current cause two particular problems. Firstly the designer of the trans-
former must brace the winding to withstand the very high electromagnetic forces that will exist
between the coils of the windings. These forces will be instantaneous and proportional to the square
of the current magnitude. Secondly these asymmetrical large currents will be seen by the protective
relays upstream of the transformer. They will appear as unbalanced currents in the three lines that
are supplying the transformer. This imposes a stability problem for the designer of the overcurrent
relays. A special circuit will be needed within the relay to stabilise its operation when these in-rush
switching currents occur. Care also need to be taken in setting the relay current versus time curves.
An amount of time delay is usually incorporated into the settings to override the transient time of
the in-rush current, which usually lasts for about 5 cycles of fundamental current.
References
1.Protective relays application guide. Alstom T & D Protection & Control Ltd. Stafford UK Third edition,
reprinted March 1995.
- The English Electric Company Ltd,Protective relays application guide. Printed in England by Buckler &
Webb Ltd. Birmingham, UK (approx 1970). - M. G. Say,The performance and design of alternating current machines.Sir Isaac Pitman and Sons Ltd.
(1963).
4.Electrical transmission and distribution reference book. Central station engineers, of the Westinghouse Electric
Corporation. East Pittsburgh, Pennsylvania, USA Eighth printing. (1964) Library of Congress Card No.
86–081053 ISBN 0 471-85393-3