[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

and for CTs is
TCF = RCF – (PA/2600) (2.6.8)
The TCF is mainly applied when the instrument transformer is being used to measure energy usage.
From Table 2.6.5, the limits of TCF are also the same as RCF. A negative RE will yield an RCF > 1, while
a positive RE will yield an RCF < 1. The “adopted” class in Table 2.6.5 is extrapolated from these
relationships and is recognized in industry.
The accuracy-class limits of the CT apply to the errors at 100% of rated current up through the rating
factor of the CT. At 10% of rated current, the error limits permitted are twice that of the 100% class.
There is no defined requirement for the current range between 10% and 100%, nor is there any requirement
below 10%. There are certain instances in which the user is concerned about the errors at 5% and
will rely on the manufacturer’s guidance. Because of the nonlinearity in the core and the ankle region,
the errors at low flux densities are exponential. As the current and flux density increase, the errors become
linear up until the core is driven into saturation, at which point the errors increase at a tremendous rate
(see Figure 2.6.9).
Trends today are driving accuracy classes to 0.15%. Although not yet recognized by IEEE C57.13,
manufacturers and utilities are establishing acceptable guidelines that may soon become part of the
standard. With much cogeneration, the need to meter at extremely low currents with the same CT used
TABLE 2.6.5 Accuracy Classes
Accuracy Class RCF Range Phase Range, min TCF Range
New 0.15
1.0015–0.9985 ± 7.8 1.0015–0.9985
IEEE C57.13 0.3
1.003–0.997 ± 15.6 1.003–0.997
IEEE C57.13 0.6
1.006–0.994 ± 31.2 1.006–0.994
IEEE C57.13 1.2
1.012–0.988 ± 62.4 1.012–0.988
IEEE C57.13 2.4
1.024–0.976 ± 124.8 1.024–0.976
Adopted 4.8
1.048–0.952 ± 249.6 1.048–0.952
for regular loads has forced extended-range performance to be constant from rating factor down to 1%
of rated current. This is quite a deviation from the traditional class.
In the case of the VT, the accuracy-class range is between 90% and 110% of rated voltage for each
designated burden. Unlike the CT, the accuracy class is maintained throughout the entire range. The
manufacturer will provide test data at 100% rated voltage, but it can furnish test data at other levels if
required by the end user. The response is somewhat linear over a long range below 90%. Since the normal
operating flux densities are much higher than in the CT, saturation will occur much sooner at voltages
above 110%, depending on the overvoltage rating.
Protection, or relay class, is based on the instrument transformer’s performance at some defined fault
level. In VTs it may also be associated with an under- and overvoltage condition. In this case, the VT
may have errors as high as 5% at levels as low as 5% of rated voltage and at the VT overvoltage rating.
In CTs, the accuracy is based on a terminal voltage developed at 20 times nominal rated current. The
limits of RCF are 0.90 to 1.10, or 10% RE from nominal through 20 times nominal. This applies to rated
burden or any burden less than rated burden.
2.6.2.6 Insulation Systems
The insulation system is one of the most important features of the instrument transformer, establishing
its construction, the insulation medium, and the unit’s overall physical size. The insulation system is
determined by three major criteria: dielectric requirements, thermal requirements, and environmental
requirements.
Dielectric requirements are based on the source voltage to which the instrument transformer will be
connected. This source will define voltage-withstand levels and basic impulse-insulation levels (BIL). In
some cases, the instrument transformer may have to satisfy higher levels, depending on the equipment
with which they are used. Equipment such as power switchgear and isolated-phase bus, for instance, use
instrument transformers within their assembly, but they have test requirements that differ from the
instrument-transformer standard. It is not uncommon to require a higher BIL class for use in a highly
polluted environment. See Table 2.6.6A and Table 2.6.6B.
TABLE 2.6.6A Low- and Medium-Voltage Dielectric Requirements
Class, kV Instrument Transformers (IEEE C57.13) Other Equipment Standards a
BIL, kV Withstand Voltage, kV BIL, kV Withstand Voltage, kV
0.6 10 4 — 2.2
1.2 30 10 — —
2.4 45 15 — —
5.0 60 19 60 19
8.7 75 26 75/95 26/36
15.0 95/110 34 95/110 36/50
25.0 125/150 40/50 125/150 60
34.5 200 70 150 80
46 250 95 — —
69 350 140 350 160
a
IEEE C37.06, C37.20.1, C37.20.2,C37.20.3, C37.23.
TABLE 2.6.6B High-Voltage Dielectric Requirements
FIGURE 2.6.9 CT RCF characteristic curve.
Class, kV Instrument Transformers (IEEE C57.13) Other Equipment Standards (IEEE C37.06)
BIL, kV Withstand Voltage, kV BIL, kV Withstand Voltage, kV
115 450/550 185/230 550 215/260
138 650 275 650 310
161 750 325 750 365
230 900/1050 395/460 900 425
345 1300 575 1300 555
500 1675/1800 750/800 1800 860
765 2050 920 2050 960
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC