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Transformer Protection
ransformer Pr
Inrush Inhibit during Transformer Energization:
The differential current present during transformer energization
resembles the condition of an internal fault. If no inhibiting
mechanism is provided, the differential element will trip. Since the
magnetizing inrush current has significant 2nd harmonic content,
the level of 2nd harmonic current can be used to differentiate
between inrush and a fault condition. The UR T60 and T35 GE
Multilin transformer relays use two different 2nd harmonic modes
to inhibit the differential element for inrush.
Traditional 2nd harmonic blocking The traditional 2
restraint responds to the ratio of the magnitudes of the 2nd
harmonic and the fundamental frequency currents.
Adaptive 2nd harmonic blocking The adaptive 2nd harmonic
blocking responds to both magnitudes and phase angles of the 2nd
harmonic and the fundamental frequency currents. The differential
element correctly distinguishes between faults and transformer
energization, when the 2nd harmonic current is less than the
entered 2nd harmonic setting. While levels of 2nd harmonic during
inrush often do not go below 20%, many transformers are
susceptible of generating lower 2nd harmonic current during
energization. Setting the 2nd harmonic restraint below 20% may
result in incorrect inhibit of the differential element during some
internal fault events. The adaptive 2nd harmonic blocking allows
settings in the traditional 20% range, while maintaining the security
of the differential element against inrush.
Available in the T60, T35.
An alternative method for inrush inhibit is also available, where
current, voltage, or breaker status is used to indicate a de-energized
transformer. The threshold can be lowered during energization of
the transformer as indicated either by breaker contact, current or
voltage sensing, and will last for a settable time delay. This allows
settings of less than 20% for inrush inhibit during transformer
Available in the 745.
Sensitive Ground Fault Protection to limit Transformer
Differential protection and overcurrent protection are not sensitive
enough to detect faults close to the neutral of wye-connected
transformers with grounded neutrals. Such faults produce less fault
current as shown by the current distribution curve. The restricted
ground fault function can be used to provide differential protection
for such ground faults, down to faults at 5% of the transformer
winding. Restricted ground fault protection can be a low impedance
differential function or a high impedance differential function. The
low impedance function can precisely set the sensitivity to meet
the application requirement. This sensitive protection limits the
damage to the transformer for faults close to the neutral. The
restricted ground fault element uses adaptive restraint based on
symmetrical components to provide security during external phase
faults with significant CT error. This permits the function to maximize
sensitivity without any time delay.
Available in the 745, T60.
Overflux Protection
Transformer overfluxing can be a result of system overvoltages, or
low system frequency. A transformer is designed to operate at or
below a maximum magnetic flux density in the transformer core.
Above this design limit the eddy currents in the core and nearby
conductive components cause overheating which within a very
short time may cause severe damage. The magnetic flux in the core
is proportional to the voltage applied to the winding divided by the
impedance of the winding. The flux in the core increases with either
increasing voltage or decreasing frequency. During startup or
shutdown of generator-connected transformers, or following a load
rejection, the transformer may experience an excessive ratio of
volts to hertz, that is, become overexcited. When a transformer core
is overexcited, the core is operating in a non-linear magnetic region,
and creates harmonic components in the exciting current. A
significant amount of current at the 5th harmonic is characteristic
of overexcitation.
Available in the 745, T60, and T35.
Ground fault current for impedance grounded neutral transformer for faults at
different % of the winding.