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.