Transformer Rating
Capacity (or rating) of a transformer is limited by the temperature that the insulation can tolerate. Ratings can be increased by reducing core and copper losses, by increasing the rate of heat dissipation (better cooling), or by improving transformer insulation so it will withstand higher temperatures. A physically larger transformer can dissipate more heat, due to the increased area and increased volume of oil. A transformer is only as strong as its weakest link, and the weakest link is the paper insulation, which begins to degrade around 100 °C. This means that a transformer must be operated with the “hottest spot” cooler than this degradation temperature, or service life is greatly reduced. Reclamation typically orders transformers larger than required, which aids in heat removal and increases transformer life.
Ratings of transformers are obtained by simply multiplying the current times the voltage. Small transformers are rated in “VA,” volts times amperes. As size increases, 1 kilovoltampere (kVA) means 1,000 voltamperes, 1 megavoltampere (MVA) means 1 million voltamperes. Large GSUs may be rated in hundreds of MVAs. A GSU transformer can cost well over a million dollars and take 18 months to 2 years or longer to obtain. Each one is designed for a specific application. If one fails, this may mean a unit or whole plant could be down for as long 2 years, costing multiple millions of dollars in lost generation, in addition to the replacement cost of the transformer itself. This is one reason that proper maintenance is critical.
Percent Impedance
The impedance of a transformer is the total opposition offered an alternating current. This may be calculated for each winding. However, a rather simple test provides a practical method of measuring the equivalent impedance of a transformer without separating the impedance of the windings. When referring to impedance of a transformer, it is the equivalent impedance that is meant. In order to determine equivalent impedance, one winding of the transformer is short circuited, and just enough voltage is applied to the other winding to create full load current to flow in the short circuited winding. This voltage is known as the impedance voltage. Either winding may be short-circuited for this test, but it is usually more convenient to short circuit the low-voltage winding. The transformer impedance value is given on the nameplate in percent.
This means that the voltage drop due to the impedance is expressed as a percent of rated voltage. For example, if a 2,400/240-volt transformer has a measured impedance voltage of 72 volts on the highvoltage windings, its impedance (Z), expressed as a percent, is:
percent Z = (72/2400) x 100 = 3 %
This means there would be a 72-volt drop in the high-voltage winding at full load due to losses in the windings and core. Only 1 or 2% of the losses are due to the core; about 98% are due to the winding impedance. If the transformer were not operating at full load, the voltage drop would be less. If an actual impedance value in ohms is needed for the high-voltage side:
Z = V/I
where V is the voltage drop or, in this case, 72 volts; and I is the full load current in the primary winding. If the full load current is 10 amps:
Z = 72 V/ 10 A = 7.2 ohm
Of course, one must remember that impedance is made up of both resistive and reactive components.
Internal Forces
During normal operation, internal structures and windings are subjected to mechanical forces due to the magnetic forces. These forces are illustrated in Picture 1. By designing the internal structure very strong to withstand these forces over a long period of time, service life can be extended. However, in a large transformer during a “through fault” (fault current passing through a transformer), forces can reach millions of pounds, pulling the coils up and down and pulling them apart 60 times per second. Notice in Picture 1 that the internal low-voltage coil is being pulled downward, while the highvoltage winding is pulled up, in the opposite direction. At the same time, the right-hand part of the figure shows that the high- and lowvoltage coils are being forced apart. Keep in mind that these forces are reversing 60 times each second. It is obvious why internal structures of transformers must be built incredibly strong.
Picture 1: Transformer Internal Forces
Many times, if fault currents are high, these forces can rip a transformer apart and cause electrical faults inside the transformer itself. This normally results in arcing inside the transformer that can result in explosive failure of the tank, throwing flaming oil over a wide area. There are protective relaying systems to protect against this possibility, although explosive failures do occur occasionally.
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