Normal practice for transformer protection application design considers the usage of the transformer
as well as the power level transformed since the economical aspect comes more signi cant when the
transformer size increases. This means that the price of protection applied should be in line of the cost
of the transformer. For example, there is no point to install high level multifunction transformer IED into
few kVA distribution transformer which is feeding few farms in a rural area network as well as it is even
more pointless to leave few hundred MVA transmission transformer feeding entire cities protected only
with fuses.
When designing transformer protection, it should be considered which protection elements are
necessary to apply suf cient and “good enough” protection. Following table gives a rough idea about
protection methods and elements, which should be considered for each type of transformer, e.g.
protection design below these mentioned suggestions increase risk of having costly problems with
transformer.
Transformer
Risks
Protection
Pole mount <
100 kVA
transformer
Distribution
Mostly environmental, highest risk is
lightning hit to overhead line. If broken,
changing to new in hours is possible.
Relatively cheap.
Feeder overcurrent and earth fault protection, no separate
protection devices normally are applied.
< 500 kVA
transformer in
industrial use,
installation
indoors.
Distribution,
applications.
Overloading biggest risk, possibly cooling
if environmental conditions are dif cult. If
broken, changing to new in hours is
possible. Possible fault extension to other
parts of the network or to building should
be reduced. Relatively cheap.
Feeder overcurrent and earth fault protection, fuses to limit the
possible short circuit current.
>500 kVA <2
MVA
Distribution,
applications,
motors, small
generators.
Overload, overvoltage, transients,
cooling. If broken changing to new is a
bigger problem. Not so cheap, xing
could be considered if fault occurs.
Monitoring is important due to most
probably failure causes more costly
problem than the monitor.
Overcurrent and earth fault protection. Fuses could be
considered for short circuit current. Dedicated pressure guard
(Buchholz gas relay), overload protection with winding
temperature monitors. If transformer is oil insulated then oil level
monitor should be applied.
>2MVA <100
MVA
Distribution,
generation,
sub
transmission
<130 kV.
Overload, overvoltage, transients,
cooling, environmental. If broken
changing to new is bigger problem,
normally off-line time long and
replacement dif cult. Relatively
expensive, wide area effect if fails no
matter where installed, in transmission,
distribution or generation. Monitoring very
important as well as fast fault clearing and
limiting the transformer internal fault time.
Differential overcurrent and earth fault protection, back-up
overcurrent and earth fault protection. Tap changer protection,
Dedicated pressure guard (Buchholz gas relay), overload
protection with numerical and winding temperature monitors. If
transformer is oil insulated then oil level monitor should be
applied. Monitoring of loading and oil ageing estimations. If
transformer has forced cooling, monitor and protection for
cooling systems should be applied. Protections and monitoring
to multifunction relay and back-up, overcurrent –earthfault to
dedicated relays.
>100 MVA
Transmission
> 130 kV
Overload, overvoltage, transients,
cooling, environmental. If broken
changing to new is bigger problem,
normally off-line time long and
replacement dif cult. Extremely
expensive, wide area effect if fails no
matter where installed, in transmission,
distribution or generation. Monitoring very
important as well as fast fault clearing and
limiting the transformer internal fault time.
Redundant differential overcurrent and earth fault protection,
redundant back-up overcurrent and earth fault protection. Tap
changer protection, Dedicated pressure guard (Buchholz gas
relay), overload protection with numerical and redundant winding
temperature monitors. Oil level monitor should be applied.
Monitoring of loading and oil ageing estimations. If transformer
has forced cooling, monitor and protection for cooling systems.
Separated control, monitoring and protection relays.
Transformer faults are many, to mention few most likely causes to faults are dirty, watered or old
transformer oil, oil leaking from the tank, prolonged and multiple heavy overloading and faults in cooling
systems. These reasons can cause transformer windings earth faults, interturn faults or even phase to
phase faults.
AQ-T257
Instruction manual
Version: 2.00
143
© Arcteq Relays Ltd