capable of continuous operation under contingency system and bank conditions,
provided the following limitations are not exceeded:
1. Capacitor units should be capable of continuous operation including harmonics,
but excluding transients, to 110% of rated IED root-mean-square (RMS) voltage
and a crest voltage not exceeding of rated RMS voltage. The capacitor should also
be able to carry 135% of nominal current. The voltage capability of any series
element of a capacitor unit shall be considered to be its share of the total capacitor
unit voltage capability.
2. Capacitor units should not give less than 100% nor more than 110% of rated
reactive power at rated sinusoidal voltage and frequency, measured at a uniform
case and internal temperature of 25°C.
3. Capacitor units mounted in multiple rows and tiers should be designed for
continuous operation for a 24h average temperature of 40 °C during the hottest
day, or −40 °C during the coldest day expected at the location.
4. Capacitor units should be suitable for continuous operation at up to 135% of rated
reactive power caused by the combined effects of:
• Voltage in excess of the nameplate rating at fundamental frequency, but not
over 110% of rated RMS voltage
• Harmonic voltages superimposed on the fundamental frequency
• Reactive power manufacturing tolerance of up to 115% of rated reactive power
5. Capacitor units rated above 600 V shall have an internal discharge device to
reduce the residual voltage to 50 V or less in 5 or 10 minutes (depending on
national standard).
Note that capacitor units designed for special applications can exceed these ratings.
Thus, as a general rule, the minimum number of capacitor units connected in parallel
within a SCB is such that isolation of one capacitor unit in a group should not cause a
voltage unbalance sufficient to place more than 110% of rated voltage on the
remaining capacitors of that parallel group. Equally, the minimum number of series
connected groups within a SCB is such that complete bypass of one group should not
pause voltage higher than 110% of the rated voltage on the remaining capacitors of that
serial group. The value of 110% is the maximum continuous overvoltage capability of
capacitor units as per IEEE Std 18-1992.
The SCB typically requires the following types of IED protection:
1. Short circuit protection for SCB and connecting leads (can be provided by using
PHPIOC, OC4PTOC, CVGAPC, T2WPDIF/T3WPDIF or HZPDIF functions)
2. Ground-fault protection for SCB and connecting leads (can be provided by using
EFPIOC, EF4PTOC, CVGAPC, T2WPDIF/T3WPDIF or HZPDIF functions)
3. Current or Voltage based unbalance protection for SCB (can be provided by using
EF4PTOC, OC4PTOC, CVGAPC or VDCPTOV functions)
4. Overload protection for SCB
1MRK504116-UUS C Section 3
IED application
497
Application manual
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