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 The equation for (VD) becomes:
(kW) (S) (VDF)
VD
=
1000
8.4.5 Coordination Study. The coordination study, commonly referred to as the
sectionalizing study, is an analysis of all or part of a distribution system. This analysis is
performed to determine the adequacy of sectionalizing device placement and selection based on
the fault current calculations. This study can be performed in conjunction with the voltage
drop study to incorporate the major system changes in terms of loads or a change in system
configuration.
The distribution system coordination objectives may be to minimize outages per consumer per
year, to minimize expense for service restoration after outages, to minimize damage to primary
lines and apparatus during faults, or to minimize the probability of hazardous voltage at ground
level or on grounded objects interconnected with the system neutral.
These objectives are accomplished by the use of sectionalizing devices appropriately selected and
located on the distribution system. The outages, equipment damage restoration expenses, and
hazardous voltage (caused by live fault and overload conditions) can never be completely
eliminated. These dangerous and undesirable line operating conditions, however, can be reduced
to an acceptable level by the use of proper sectionalizing devices.
The performance of coordination studies has been greatly simplified since the introduction of
computer modeling techniques for calculation of load and fault currents. These techniques have
been applied in the performance of voltage drop, load flow, and fault current studies.
Sectionalizing electric systems requires application of three-phase OCRs and breakers with
ground fault sensing capability, however, the computer applications addressed will be limited to
analysis of single-phase and three-phase sectionalizing devices not requiring ground fault sensing
capabilities. Single- and three-phase sectionalizing devices are used frequently on rural electric
systems and require less engineering judgement than larger devices equipped with ground fault
sensing. The purpose of using computer applications is to reduce hand calculations and
measurements.
A coordination study requires the performance of the following four procedures:
(a)
Accumulate and process data.
(b)
Check sectionalizing for coordination and correct ratings.
(c)
Change sectionalizing to correct inadequacies.
(d)
Document findings as required.
8-23
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