ABB REB670 Applications Manual page 231

1MRK 505 337-UUS -
Application manual
In addition, to fault conditions SCB can be exposed to different types of abnormal
operating conditions. In accordance with IEC and ANSI standards capacitors shall be
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:
Section 7
Current protection
225