[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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where cos is again the power factor of the load, therefore kVA · cos is real energy delivered to the<br />
load. NL is the no-load loss, and LL is the load loss of the transformer. Most distribution transformers<br />
serving residential or light industrial loads are not fully loaded all the time. It is assumed that such<br />
transformers are loaded to about 50% of nameplate rating on the average. Thus efficiency is often<br />
calculated at L = 0.5, where the load loss is about 25% of the value at full load. Since a typical transformer<br />
will have no-load loss of around 25% of load loss at 100% load, then at L = 0.5, the no-load loss will<br />
equal the load loss and the efficiency will be at a maximum.<br />
2.2.11 <strong>Transformer</strong> Loading<br />
2.2.11.1 Temperature Limits<br />
According to ANSI standards, modern distribution transformers are to operate at a maximum 65˚C<br />
average winding rise over a 30˚C ambient air temperature at rated kVA. One exception to this is submersible<br />
or vault-type distribution transformers, where a 55˚C rise over a 40˚C ambient is specified. The<br />
bulk oil temperature near the top of the tank is called the “top oil temperature,” which cannot be more<br />
than 65˚C over ambient and will typically be about 55˚C over ambient, 10˚C less than the average winding<br />
rise.<br />
2.2.11.2 Hottest-Spot Rise<br />
The location in the transformer windings that has the highest temperature is called the “hottest spot.”<br />
Standards require that the hottest-spot temperature not exceed 80˚C rise over a 30˚C ambient, or 110˚C.<br />
These are steady-state temperatures at rated kVA. The hottest spot is of great interest because, presumably,<br />
this is where the greatest thermal degradation of the transformer’s insulation system will take place. For<br />
calculation of thermal transients, the top-oil rise over ambient air and the hottest-spot rise over top oil<br />
are the parameters used.<br />
2.2.11.3 Load Cycles<br />
If all distribution loads were constant, then determining the proper loading of transformers would be a<br />
simple task. Loads on transformers, however, vary through the hours of a day, the days of a week, and<br />
through the seasons of the year. Insulation aging is a highly nonlinear function of temperature that<br />
accumulates over time. The best use of a transformer, then, is to balance brief periods of hottest-spot<br />
temperatures slightly above 110C with extended periods at hottest spots well below 110˚C. Methods for<br />
calculating the transformer loss-of-life for a given daily cycle are found in the ANSI Guide for Loading<br />
(IEEE, 1995). Parameters needed to make this calculation are the no-load and load losses, the top-oil<br />
rise, the hottest-spot rise, and the thermal time constant.<br />
2.2.11.4 Thermal Time Constant<br />
Liquid-filled distribution transformers can sustain substantial short-time overloads because the mass of<br />
oil, steel, and conductor takes time to come up to a steady-state operating temperature. Time constant<br />
values can vary from two to six hours, mainly due to the differences in oil volume vs. tank surface for<br />
different products.<br />
2.2.11.5 Loading Distribution <strong>Transformer</strong>s<br />
Utilities often assign loading limits to distribution transformers that are different from the transformer’s<br />
nameplate kVA. This is based on three factors: the actual ambient temperature, the shape of the load<br />
curve, and the available air for cooling. For example, one utility divides its service territory into three<br />
temperature situations for different ambient temperatures: summer interior, summer coastal, and winter.<br />
The transformer installations are divided into three applications for the available air cooling: overhead<br />
or pad-mounted, surface operable, and vault. The load shape is expressed by the peak-day load factor,<br />
which is defined as the season’s peak kVA divided by the average kVA and then expressed as a percentage.<br />
Table 2.2.1 shows the assigned capabilities for a 100-kVA transformer. Thus this utility would assign the<br />
TABLE 2.2.1 Assigned Capabilities for a 100-kVA <strong>Transformer</strong><br />
Location<br />
Overhead or<br />
pad-mounted<br />
Surface<br />
operable<br />
Vault<br />
<strong>Transformer</strong><br />
Peak-Day Load Factor<br />
Temperature<br />
District kVA 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%<br />
Summer 100 205 196 187 177 168 159 149 140 131 122<br />
interior<br />
Summer coastal 100 216 206 196 186 176 166 156 146 136 126<br />
Winter 100 249 236 224 211 198 186 173 160 148 135<br />
Summer 100 147 140 133 127 120 113 107 100 93 87<br />
interior<br />
Summer coastal 100 154 147 140 133 126 119 111 104 97 90<br />
Winter 100 178 169 160 151 142 133 124 115 105 96<br />
Summer 100 173 164 156 147 139 130 122 113 105 96<br />
interior<br />
Summer coastal 100 182 173 164 155 146 137 127 118 109 100<br />
Winter 100 185 176 166 157 147 138 128 119 110 100<br />
same 100-kVA transformer a peak capability of 87 to 249 kVA depending on its location, the season, and<br />
the load-shape.<br />
2.2.12 <strong>Transformer</strong> Testing<br />
2.2.12.1 Design Tests<br />
Tests that manufacturers perform on prototypes or production samples are referred to as “design tests.”<br />
These tests may include sound-level tests, temperature-rise tests, and short-circuit-current withstand<br />
tests. The purpose of a design test is to establish a design limit that can be applied by calculation to every<br />
transformer built. In particular, short-circuit tests are destructive and may result in some invisible damage<br />
to the sample, even if the test is passed successfully. The ANSI standard calls for a transformer to sustain<br />
six tests, four with symmetrical fault currents and two with asymmetrical currents. One of the symmetrical<br />
shots is to be of long duration, up to 2 s, depending on the impedance for lower ratings. The<br />
remaining five shots are to be 0.25 s in duration. The long-shot duration for distribution transformers<br />
750 kVA and above is 1 s. The design passes the short-circuit test if the transformer sustains no internal<br />
or external damage (as determined by visual inspection) and minimal impedance changes. The tested<br />
transformer also has to pass production dielectric tests and experience no more than a 25% change in<br />
exciting current (Bean et al., 1959).<br />
2.2.12.2 Production Tests<br />
Production tests are given to and passed by each transformer made. Tests to determine ratio, polarity or<br />
phase-displacement, iron loss, load loss, and impedance are done to verify that the nameplate information<br />
is correct. Dielectric tests specified by industry standards are intended to prove that the transformer is<br />
capable of sustaining unusual but anticipated electrical stresses that may be encountered in service.<br />
Production dielectric tests may include applied-voltage, induced-voltage, and impulse tests.<br />
2.2.12.2.1 Applied-Voltage Test<br />
Standards require application of a voltage of (very roughly) twice the normal line-to-line voltage to each<br />
entire winding for one minute. This checks the ability of one phase to withstand voltage it may encounter<br />
when another phase is faulted to ground and transients are reflected and doubled.<br />
2.2.12.2.2 Induced-Voltage Test<br />
The original applied-voltage test is now supplemented with an induced-voltage test. Voltage at higher<br />
frequency (usually 400 Hz) is applied at twice the rated value of the winding. This induces the higher<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC