ECT192

Protection of MV/LV substation
transformers
The choices involved in the protection of MV/LV transformers can appear
to be simple since they are often the result of usual practices of electrical
network designers, or even of policy dictated by technical and economic
considerations. In fact, the choices must be made as a function of the
transformer technology, the type of loads that they are supplying, and
above all the external environment that they are subjected to.
This “Cahier Technique” discusses the stresses to which the transformers
are subjected during operation and the consequences of these stresses
and goes on to present the various protection devices that can be used. It
is necessarily simple, due to the large number of criteria and solutions that
exist. Electrical engineers should however find this document provides the
main information needed to make the right choices.
Contents
1. Introduction 1.1 MV/LV transformers and protection policy p. 4
1.2 A review of transformer technology and uses p. 5
2. Operating stress and failure modes 2.1 Energizing and de-energizing p. 7
2.2 External overvoltages p. 7
2.3 Overloads p. 9
2.4 Short-circuits on the LV network p. 10
2.5 Progression of internal faults p. 10
2.6 Faults related to technology types p. 13
3. Overvoltage protection 3.1 General p. 14
3.2 Lightning arrestors and spark gap protection p. 14
4. Overload protection 4.1 Current measurement protection p. 16
4.2 Temperature measurement protection p. 17
5. Protection by MV fuses and 5.1 Characteristics of MV fuses p. 18
fuse switches combinations
5.2 Limits of fuses p. 19
5.3 Using a fuse switch combination p. 21
6. MV circuit-breaker protection, associated 6.1 Trip curve selection criteria p. 22
tripping devices
6.2 Advantages of earthing protection p. 24
6.3 Independent protection devices: TFL and relays p. 24
6.4 Protection devices with auxiliary power supply:
GRPT, temperature sensors and relays p. 25
7. Conclusion p. 27
Appendix 1: Rules governing selection of a fuse to protect a transformer p. 29
Appendix 2: Calculating transfer and take-over currents of a fuse switch combination p. 30
Bibliography p. 32
Logic diagram of situations, criteria and solutions
Cahier Technique Schneider n° 192 / p.3