Page 872 - Industrial Power Engineering and Applications Handbook

25/824 Industrial Power Engineering and Applications Handbook

120 Considering the capacitors to be connected in y, then k = 1,
log,
= 2 x - 71.84 and
90
1
lo
120
x
= 2 x - 4.274 = 11.4 seconds R, = 1 x 30 x lod x fix 6600
90 log,-75
The transformer should be able to meet the switching
requirements in view of the following:
That the line impedances have been ignored, which would
further dampen the capacitor charge on every switch-off.
For a fast changeover, a terminal voltage of more than =69k52
75 V is permissible, which in the above case after 10 seconds
will not exceed (6600)'
69 x lo3
and resistance loss = ~ = 631 W
m= fi x 6600 ,(-A the line This is a significant loss. The resistance therefore should be
(ignoring
'lo)
6 impedance) connected during an interruption only, through a switch (or
the no contact of the interrupting device which can easily
carry this short time burden) and should not be left permanently
in the capacitor circuit.
= 126.7 V which is quite low compared to 10% of 6.6 kV For a normal discharge in 10 minutes a resistance of -
Rd
tZ
and will not damage the equipment. t,
69x103 ~10x60
i.e.
Note 10
Use of additional resistance or inductance or both in series = 4.14 MR (say, 4 MR) would be necessary
with the VT must be avoided, particularly when the VT is also
being used for the purpose of measurement. Any introduction This resistance would normally be fitted across the capacitor
of L or R into the VT circuit would affect its accuracy. terminals by their manufacturers and will cause a permanent
loss of
Alternative 2 ",2
The above discharge is also possible through only an extra -
-- watts
resistance in each capacitor switching circuit. Using equations R
(25.5) and (25.6), or 6600 6600 = 10.89 W
4x lo6
which is reasonably low and the resistance can be left
permanently in the capacitor circuit.
Relevant Standards
IEC Title IS BS
60044-311980 Instrument transformers ~ combined transformers - BS 7628/1993
60056/1987 High voltage alternating current circuit breakers 13118/1991 BS 531 1/1996
60099-4/1998 Surge arresters - Metal oxide surge arresters without 3070-3/1993 BS EN 6009941993
gaps for ax. systems
60 129/1984 A.C. disconnectors and earthing switches 9921 (1 to 5) BS EN 60129/1994
Specification for current transformers 2705
60044- 1 / 1 996 General requirements part- 1/1992
Measuring current transformers part-2/1992 BS 7626/1993
60044-6/1992 Protective current transformer for special purpose applications part-3/1992
part-4/1992
Application guide for voltage transformers 420 1/199 1,
60044-2/1997 Specification for voltage transformers 4146/199 1
General requirements 3 156- VI992 BS 762Y1993
3 1S6-2/1992 BS 7729/1995
601 86/1995 Capacitive voltage transformers. For measurement and 3156-3/1992
protection. General requirements 3 156-4/1992
6025211 993 A.C motor capacitors 2993/1998 BS EN 60252/1994
60269-1/1998 Low voltage fuses ~ General requirements 13703-1/1993 BS EN 60269-111994
60282- 1/1994 High voltage fuses - Current limiting fuses 9385-1 to 5 BS EN 60282-1/1996
60289/1988 Reactors 5553 (1 to 8) BS EN 60289/1995