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www.GEMultilin.com
www.GEMultilin.com
www.GEMultilin.com
143
www.GEMultilin.com
143
Transformer Protection
T
ransformer Pr
ot
ection
Introduction
Transformers are a critical and expensive component of the power
system. Due to the long lead time for repair of and replacement of
transformers, a major goal of transformer protection is limiting the
damage to a faulted transformer. Some protection functions, such
as overexcitation protection and temperature-based protection,
may aid this goal by identifying operating conditions that may
cause transformer failure. The comprehensive transformer
protection provided by multiple function protective relays is
appropriate for critical transformers of all applications.
Transformer Protection Overview
The type of protection for a transformer varies depending on the
application and the importance of the transformer. Transformers
are protected primarily against faults and overloads. The type of
protection used should quickly isolate the transformer for internal
faults to reduce the risk of catastrophic failure, and to simplify
eventual repair. Any extended operation of the transformer under
abnormal condition such as overexcitation or overloads
compromises the life of the transformer, which means adequate
protection should be provided for such conditions.
Transformer Failures
Failures in transformers can be classified into
windingfailuresduetoshortcircuits(turn-turnfaults,phase-
phase faults, phase-ground, open winding)
corefaults(coreinsulationfailure,shortedlaminations)
terminalfailures(openleads,looseconnections,shortcircuits)
on-loadtapchangerfailures(mechanical,electrical,shortcircuit,
overheating)
abnormal operating conditions (overfluxing, overloading,
overvoltage)
externalfaults
Innovative GE Multilin Solutions to
Transformer Protection Applications
Differential Characteristic
The major operating challenge to transformer differential protection
is maintaining security during CT saturation for external faults while
maintaining sensitivity to detect low magnitude internal faults. CT
saturation reduces the secondary output current from the CT, and
causes a false differential current to appear to the relay. GE Multilin
differential relays meet this challenge in the following ways:
therestraintcurrentisbasedonthemaximummeasured
winding current, as opposed to the traditional magnitude sum
of the currents. This ensures ideal restraint for the actual fault
condition, balancing sensitivity and security.
thedifferentialelementusesadualslope-dualbreakpoint
characteristic. The differential element can be set to account for
both DC and AC saturation of the CTs, ensuring security, while
maintaining sensitivity.
Available in the T60, T35.
Transformer Protection
Conditions
Protection Philosophy
Internal
Winding Phase-Phase,
Phase-Ground faults
Differential (87T), overcurrent (51, 51N)
Restricted ground fault protection
(87RGF)
Winding inter-turn
faults
Differential (87T), Buchholz relay
Core insulation failure,
shorted laminations
Differential (87T), Buchholz relay,
sudden pressure relay
Tank faults
Differential (87T), Buchholz relay and
tank-ground protection
Overfluxing
Volts/Hz (24)
External
Overloads
Thermal (49)
Overvoltage
Overvoltage (59)
Overfluxing
Volts/Hz (24)
External system short
circuits
Time overcurrent (51, 51G),
Instantaneous overcurrent (50, 50G)