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Protection of MV/LV substation transformers ..........................................................................
Collection Technique
Cahier technique n
o
192
Protection of MV/LV substation
transformers
D. Fulchiron
s
Merlin Gerin
s
Modicon
s
Square D
s
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Foreword
The author disclaims all responsibility further to incorrect use of information
or diagrams reproduced in this document, and cannot be held responsible
for any errors or oversights, or for the consequences of using information
and diagrams contained in this document.
Reproduction of all or part of a Cahier Technique is authorised with the
prior consent of the Scientific and Technical Division. The statement
Extracted from Schneider Cahier Technique no..... (please specify) is
compulsory. Didier FULCHIRON
Graduating as an engineer from the Ecole Supérieure d'Electricité
in 1980, he joined the technical department in Merlin Gerin in 1981 at
the high power testing station. Now in the Medium Voltage Division,
he uses his knowledge of public distribution applications to work on
expert assessments, specifications and standards.
n
°
192
Protection of MV/LV substation
transformers
ECT192 first issued july 1998 Cahier Technique Schneider n
°
192 / p.2
Lexicon
Chopped wave: part of an overvoltage wave,
generally lightning generated, which continues
propagating beyond arcing in an air gap (spark
gap or insulator breakdown). The high gradient
of the downward slope generated by arcing is
very severe for certain equipment.
GRPT: device useable on hermetically sealed
immersed type transformers with integral filling
combining monitoring features for gas release,
pressure and temperature.
Overlaying: technical and/or time-based
differences in the users of a network that
enables a maximum power rating to be used that
is much less than the sum of the individual
maximum powers.
Take-over current: value of current
corresponding to the intersection of the time-
current characteristics of two overcurrent
protection devices (VEI 441-17-16).
Transfer current: value of the symmetrical
three-phase current at which the fuses and the
switch exchange the breaking function (in a
combined fuse-switch) (IEC 420). Cahier Technique Schneider n
°
192 / p.3
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
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
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
fuse switches combinations
tripping devices Cahier Technique Schneider n
°
192 / p.4
1 Introduction
1.1 MV/LV transformers and protection policy
Why transformers exist
Transformers are included in distribution
networks in order to:
c
minimize energy losses due to the Joule effect;
increasing voltage by a factor of 10 reduces
these losses by a factor of 100
(Losses = R (P
consumed
/ U)
2
),
c
minimize voltage drops, both resistive (R) and
reactive (X) at the given transmitted power
(U
I
cos ) (

U R
I
cos + X
I
sin ),
c
and possibly ensure electrical separation
between networks of the same voltage
(boundary limits, changes in the neutral
arrangement, etc.).
Even though it is rare to voluntarily interrupt
power distribution, transformers nevertheless
have to be switched under normal operating
conditions, e.g.:
c
for network reconfiguration,
c
for reasons of maintenance and security,
c
to meet a consumption peak,
c
to start or stop a process.
These operations are carried out on the transfor-
mer either under load or with no load which has
a notable influence on the operating conditions
and the resulting transitory electrical phenomena.
Distribution transformers are very reliable passive
devices with a life expectancy of several dozens
of years. A Norwegian utility has cited an annual
failure rate of 0.09 % (9 for 10,000), all reasons
included, for an equipment base of 5,000 transfor-
mers monitored over four years. For underground
networks, the observed failure rate still remains
less than 0.2 %: It can increase to 0.5 % on cer-
tain overhead networks. It is often obsolescence
- the evolution of the power or voltage levels -
which leads to their replacement. Faults in service
are very rare, but the need to provide safety of
property and people as well as continuity of service
nevertheless leads to the use of protection devices.
Stresses suffered by transformers
Transformers are subjected to many external
electrical stresses from both upstream and
downstream. The consequences of any failure
can be very great in terms of damage as well as
in terms of operating losses. Transformers must
therefore be protected against attacks of external
origin on one hand, and isolated from the
network in case of internal failure on the other
hand. The term transformer protection is very
often associated with the action of disconnecting
from the network, even though the transformer is
already failing, and the amalgam is made between
preventative measures (overvoltages, downstream
faults, overloads, temperature) and corrective
measures to isolate the failed transformer.
Protection policy
It is the electrical network designer's
responsibility to define the measures to be
implemented for each transformer as a function
of criteria such as continuity and quality of
service, cost of investment and operation and
safety of property and people as well as the
acceptable level of risk. The solutions chosen
are always a compromise between the various
criteria and it is important that the strengths and
weaknesses of the chosen compromise are
clearly identified. E.g., an operator and a utility
c