Transformer
Risk level
Protection
2MVA...100
MVA
Distribution,
generation,
sub
transmission
<130 kV.
Risks include overloading, overvoltage, transients, cooling, and
environmental issues. Replacing a broken device
is problematic as the process is difficult and normally takes the
network off-line for a long time. The device is relatively
expensive. Its failure affects a wide area regardless of where it
is installed (transmission, distribution, generation). Monitoring,
clearing faults quickly, and limiting the device's internal fault
time are all very important.
Includes the following protections:
differential, overcurrent and earth fault
protection, backup overcurrent and earth
fault protection, tap changer protection, a
dedicated pressure guard (Buchholz gas
relay), overloading protection with
numerical and winding temperature
monitors.
If the transformer is oil-insulated, oil level
monitoring should be applied in addition to
monitoring of loading and oil-ageing
estimations.
If the transformer has forced cooling,
monitoring and protection for cooling
systems should be applied.
Multifunction relays need protections and
monitoring; dedicated relays require
backup overcurrent and earth fault
protections.
>100 MVA
Transmission
> 130 kV
Risks include overloading, overvoltage, transients, cooling, and
environmental issues. Replacing a broken device is
problematic as the process is difficult and normally takes the
network off-line for a long time. The device is extremely
expensive. Its failure affects a wide area regardless where it is
installed (transmission, distribution, generation). Monitoring,
clearing faults quickly, and limiting the device's internal fault
time are all very important.
Includes the following protections:
redundant differential overcurrent and earth
fault protection, redundant backup
overcurrent and earth fault protection, tap
changer protection, a dedicated pressure
guard (Buchholz gas relay), overloading
protection with numerical and redundant
winding temperature monitors.
Oil level monitor should be applied, as well
as monitoring of loading and oil-ageing
estimations.
If the transformer has forced cooling,
monitoring and protection for cooling
systems should be applied.
Separated relays for control, monitoring
and protection.
There are many transformer faults, e.g. dirty, watered or old transformer oil, oil leakage from the tank,
as well as multiple, prolonged heavy overloading and other faults in the cooling systems. These can
cause earth faults, interturn faults or even phase-to-phase faults in the windings of the transformer.
Why is differential protection needed in transformer protection?
The transformer differential function is based on calculating the difference between the ingoing and
outgoing currents. If the operating status is normal, all power that comes in also goes out. If this is not
the case, the transformer has an internal fault and the device should be de-energized as soon as
possible to avoid extensive damage to the transformer. An operating differential function takes a faulty
transformer off-line for a long time. A quick de-energizing of the fault saves money because in most
cases the transformer can still be repaired which is significantly cheaper than replacing the broken
device with a new one. However, there are some exceptions to this. Faults that occur within the
differential protection zone but without the transformer itself (such as in the bus or in the cables
connected to the transformer). Faults of this type are easily repaired and the transformer can be re-
energized soon after the fault has bee cleared.
A
AQ
Q-T256
-T256
Instruction manual
Version: 2.06
© Arcteq Relays Ltd
IM00028
105
Summary of Contents for AQ-T256
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