Hot Spot-Temperature
In any transformer design, there is a location in the
winding that the designer believes to be the hottest spot within that
transformer (ANSI=IEEE, 1995). The significance of the ‘‘hotspot temperature’’
measured at this location is an assumed relationship between the temperature
level and the rate-of-degradation of the cellulose insulation.
An instantaneous alarm or trip setting is often used, set at
a judicious level above the full load rated hot-spot temperature (1108C for
658C rise transformers). [Note that ‘‘658C rise’’ refers to the full load rated
average winding temperature rise.] Also, a relay or monitoring system can
mathematically integrate the rate-of-degradation, i.e., rate-ofloss- of-life of
the insulation for overload assessment purposes.
Heating Due to Overexcitation
Transformer core flux density (B), induced voltage (V), and
frequency (f) are related. As B rises above about 110% of normal, that is, when
saturation starts, significant heating occurs due to stray flux eddy-currents
in the nonlaminated structural metal parts, including the tank.
Since it is the voltage=hertz quotient, the level of B, a
relay sensing this quotient is sometimes called a ‘‘volts-per-hertz’’ relay.
The expressions ‘‘overexcitation’’ and ‘‘overfluxing’’ refer to this same
condition. Since temperature rise is proportional to the integral of power with
respect to time (neglecting cooling processes) it follows that an inversetime
characteristic is useful, that is, volts per-hertz versus time. Another
approach is to use definite-timedelayed alarm or trip at specific per unit flux
levels.
Heating Due to Current Harmonic Content (ANSI=IEEE, 1993)
One effect of nonsinusoidal currents is to cause current rms
magnitude (IRMS) to be incorrect if the method of measurement is not
‘‘true-rms.’’where n is the harmonic order, N is the highest harmonic of
significant magnitude, and In is the harmonic current rms magnitude.
If an overload relay determines the I2R heating effect using
the fundamental component of the current only, then it will underestimate the
heating effect. Bear in mind that ‘‘true-rms’’ is only as good as the pass-band
of the antialiasing filters and sampling rate, for numerical relays.
A second effect is heating due to high-frequency
eddy-current loss in the copper or aluminum of the windings. The winding
eddy-current loss due to each harmonic is proportional to the square of the
harmonic amplitude and the square of its frequency as well.
Heating Due to Solar Induced Currents
Solar magnetic disturbances cause geomagnetically induced
currents (GIC) in the earth’s surface (EPRI, 1993). These DC currents can be of
the order of tens of amperes for tens of minutes, and flow into the neutrals of
grounded transformers, biasing the core magnetization.
The effect is worst in single-phase units and negligible in
three-phase core-type units. The core saturation causes second-harmonic content
in the current, resulting in increased security in second harmonic-restrained
transformer differential relays, but decreased sensitivity. Sudden gas pressure
relays could provide the necessary alternative internal fault tripping.
Another effect is increased stray heating in the
transformer, protection for which can be accomplished using gas accumulation
relays for transformers with conservator oil systems. Hot spot tripping is not
sufficient because the commonly used hot-spot simulation model does not account
for GIC.
Load Tap-changer Overheating
Damaged current carrying contacts within an underload
tapchanger enclosure can create excessive heating. Using this heating symptom,
a way of detecting excessive wear is to install magnetically mounted
temperature sensors on the tap-changer enclosure and on the main tank.
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