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Transformer Protection
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Winding hot-spot temperature protection
The transformer winding hot-spot temperature is another quantity
that should be used for protection of transformers. Protection based
on winding hot-spot temperature can potentially prevent short
circuits and catastrophic transformer failure, as excessive winding
hot-spot temperatures cause degradation and eventual failure of
the winding insulation. The ambient temperature, transformer
loading, and transformer design determine the winding temperature.
Temperature based protection functions alarm or trip when certain
temperature conditions are met.
GE Multilin relays use IEEE C57.91 compliant thermal models to
calculate the winding hot-spot temperature and the loss of life of
the winding insulation. The top-oil temperature may be directly
measured, or calculated from the ambient temperature, load
current, and transformer characteristics. In addition, the calculations
may use a monthly model of ambient temperature, eliminating the
need for external connections to the transformer and relay. This
winding hot-spot temperature and transformer loss of life
information is used in thermal overload protection to provide
alarming or tripping when unacceptable degradation of the
transformer winding insulation is occurring.
Available in 745, T60.
Application Capabilities
GE Multilin transformer protection relays are suitable for different
transformer protection applications, including medium voltage and
high voltage transformers of any size, dual secondary transformers,
auto-transformers, three-winding transformers, transformers with
dual-breaker terminals.
In addition, these relays are designed for both new and retrofit
installations. New installations typically use wye-connected CTs,
and internally compensate the measured currents for the phase
shift of the protected transformer. Traditional installations may use
delta-connected or wye-connected CTs that externally compensate
the measured currents for the phase shift of the protected
transformer. GE Multilin accommodates both methods as simple
configuration settings.
Beyond these typical applications, GE Multilin transformer protection
relays can be applied on more advanced applications.
Acquiring transformer measurements
Transformer protection requires the use of currents measured from
each winding, and possibly system voltages and transformer top-oil
temperatures. Current measurements are normally taken from
bushing CTs mounted at the transformer, voltages from nearby VTs,
and top-oil temperatures via RTDs from the transformer cooling
controls. Each measurement must be brought back individually by
copper wiring to the transformer protection relay. Top-oil
temperatures, for example, are rarely brought back to the
transformer protection relay because of the need to run RTD wires
across the switchyard. Each copper wire requires numerous
terminations that are designed, documented, field installed, and
tested one at a time.
The HardFiber IEC 61850 Process Bus Solution simplifies the
acquisition of measurement signals for transformer protection. The
Brick interface device is mounted at the transformer. Winding
currents (for 2 windings), ground current, top-oil temperatures, and
sudden pressure relays / Bucholtz relay trips are wired to the Brick.
One fiber optic cable, transmitting sampled values from all
transformer measurements, and digital status information, is pulled
across the switchyard to connect to the transformer protection
relay. The copper wiring and fiber optic cabling uses simple
connectors to attach to the Brick.