Fuse
Power fuses have been used for many years to provide
transformer fault protection. Generally it is recommended that transformers sized
larger than 10 MVA be protected with more sensitive devices such as the
differential relay discussed later in this section.
Fuses provide a low maintenance, economical solution for
protection. Protection and control devices, circuit breakers, and station
batteries are not required.
There are some drawbacks. Fuses provide limited protection
for some internal transformer faults. A fuse is also a single phase device.
Certain system faults may only operate one fuse. This will result in single
phase service to connected three phase customers.
Fuse selection criteria include: adequate interrupting
capability, calculating load currents during peak and emergency conditions,
performing coordination studies that include source and low side protection
equipment, and expected transformer size and winding configuration (ANSI=IEEE,
1985).
Overcurrent Protection
Overcurrent relays generally provide the same level of
protection as power fuses. Higher sensitivity and fault clearing times can be
achieved in some instances by using an overcurrent relay connected to measure
residual current.
This application allows pick up settings to be lower than
expected maximum load current. It is also possible to apply an instantaneous
overcurrent relay set to respond only to faults within the first 75% of the
transformer.
This solution, for which careful fault current calculations
are needed, does not require coordination with low side protective devices.
Overcurrent relays do not have the same maintenance and cost advantages found
with power fuses.
Protection and control devices, circuit breakers, and
station batteries are required. The overcurrent relays are a small part of the
total cost and when this alternative is chosen, differential relays are
generally added to enhance transformer protection. In this instance, the
overcurrent relays will provide backup protection for the differentials.
Differential Protection
The most widely accepted device for transformer protection
is called a restrained differential relay. This relay compares current values
flowing into and out of the transformer windings.
To assure protection under varying conditions, the main
protection element has a multislope restrained characteristic. The initial
slope ensures sensitivity for internal faults while allowing for up to 15%
mismatch when the power transformer is at the limit of its tap range (if
supplied with a load tap changer). At currents above rated transformer
capacity, extra errors may be gradually introduced as a result of CT
saturation.
However, misoperation of the differential element is
possible during transformer energization. High inrush currents may occur,
depending on the point on wave of switching as well as the magnetic state of
the transformer core. Since the inrush current flows only in the energized
winding, differential current results.
The use of traditional second harmonic restraint to block
the relay during inrush conditions may result in a significant slowing of the
relay during heavy internal faults due to the possible presence of second
harmonics as a result of saturation of the line current transformers.
To overcome this, some relays use a waveform recognition
technique to detect the inrush condition. The differential current waveform
associated with magnetizing inrush is characterized by a period of each cycle
where its magnitude is very small.
By measuring the time of this period of low current, an
inrush condition can be identified. The detection of inrush current in the
differential current is used to inhibit that phase of the low set restrained
differential algorithm.
Overexcitation
Overexcitation can also be caused by an increase in system
voltage or a reduction in frequency. It follows, therefore, that transformers
can withstand an increase in voltage with a corresponding increase in frequency
but not an increase in voltage with a decrease in frequency.
Operation cannot be sustained when the ratio of voltage to
frequency exceeds more than a small amount. Protection against overflux
conditions does not require high-speed tripping. In fact, instantaneous
tripping is undesirable, as it would cause tripping for transient system
disturbances, which are not damaging to the transformer.
An alarm is triggered at a lower level than the trip setting
and is used to initiate corrective action. The alarm has a definite time delay,
while the trip characteristic generally has a choice of definite time delay or
inverse time characteristic.
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