Page 484 - Industrial Power Engineering and Applications Handbook

15/458 Industrial Power Engineering and Applications Handbook
resin cast, oil or SF, insulated and oil or SF6 cooled. feed at a time. The preferred values will follow series R-
Epoxy insulation provides better mechanical and construc- 10ofISO-3 (IEC60059) andasnotedinSection 13.4.1 (4).
tional qualities. They are resistant to humid, contaminated
and corrosive atmospheres and are suitable for all HT 9 Short-time rating
systems. They are mechanically strong and can bear shocks This is not material in voltage transformers, as neither
and impacts. the voltage measuring instruments nor the protective relays
will carry any inrush current during a switching operation
5 Creepage distances or a fault. No short-time rating is thus assigned to such
For outdoor installations the recommended minimum transformers.
creep distances for all types of voltage or current The electromagnetic unit, however, as used in a residual
transformers are given in Table 15. I, according to IEC VT (Section 15.4.3) or a capacitor VT (Section 15.4.4)
60044-1 or IEC 60044-2. should be suitable for carrying the heavy discharge or
inrush currents during a capacitor discharge or switching
6 Tappings respectively.
Tappings are generally not necessary, as a transformer is
designed for a particular voltage system. If and when 10 Accuracy class
such a need arises (as in a control transformer (Section The accuracy of a VT depends upon its leakage reactance
15.4.5)) they can be provided on the primary side of the and the winding resistance. It determines the voltage
transformer. and the phase errors of a transformer and varies with the
VA on the secondary side. With the use of better core
7 Rated output material (for permeability) (Section 1.9) and better heat
The standard ratings, at 0.8 p.f. lagging, may be 10, 15, dissipation, one can limit the excitation current and reduce
25, 30, 50, 75, 100, 150, 200, 300, 400 or 500 VAor as the error. A belter core lamination can reduce the core
the auxiliary circuit may demand. The procedure to size and improve heat dissipation.
determine the total VA burden of a circuit is described in
Section 15.4.5. Typical values of VA burden for a few Measuring voltage transformers Standard accuracy
instruments are given in Table 15.2 from data provided class may be one of 0.1, 0.2, 0.5, 1 or 3. The recom-
by the manufacturers. mended class of accuracy will depend upon the type
of metering and generally as noted in Table 15.3.
8 Rated burden Protection voltage transformers Generally, a measu-
This is the maximum burden the transformer may have to ring voltage transformer may also be used for the
purpose of protection. A protection transformer, however,
is assigned an accuracy class of 3 or 6, which defines
Table 15.1 Recommended values of minimum creepage the highest permissible percentage voltage error at any
distances for a VT or a CT voltage between 5% of the rated voltage up to the
voltage obtained by multiplying the rated voltage by
Pollution level Minimum creepage distance between phase the rated voltage factor of 1.2, 1 .5 or 1.9. And when
and ground mm per kV(rm.s.) the secondary has a burden between 25% and 100% of
(phase to phase) the rated burden at a p.f. of0.8 lagging. This accuracy
class is followed by a letter 'P' such as 3P and 6P etc.
Light 16
Medium 20 The voltage and phase displacement errors should not
Heavy 25 exceed the values noted in Table 15.6.
Very Heavy 31
Table 15.3 Recommended class of accuracy for different
types of meters
Table 15.2 Typical values of VA burdens
Application Class of accuracy
Instruments Maximum burden
f W" 1 Precision testing or as a standard 0.1
VT for the testing of other VTs
I Voltmeter 5 2 Meters of precision grade 0.5
2 Voltage coil of a watt-meter or a power 3 Commercial and Industrial meters 1 .o
factor meter 5 4 Precision Industrial meters (Indicating 0.2 or 0.5
3 Voltage coil of a frequency meter 7.5 instruments, recorders and electronic
4 Voltage coil of a kWh or a kVAr meter 7.5 integrating meters)
5 Recording voltmeters 5 5 General industrial measurements 1 or 3
6 Voltage coils of recording watt-meters and (Indicating instruments and recorders)
power factor meters 7.5 6 Purposes where the ratio is of less 3
7 Voltage coil of a synchroscope 15 importance
"These VA burdens are for moving iron instruments. For electronic Note To choose a higher class of accuracy than necessary is not
meters these values would be of the order of 0.1 to 0.5 VA. desirable.