ABB DPU 1500R Instruction Book page 176

ABB Distribution Protection Unit 1500R/2000R
Application Note AN-23
Introduction
In power systems, protection schemes include series combinations of reclosers on medium voltage overhead distribution
radial feeder lines. The series combination consists of one or several down-line recloser(s) and a backup substation
recloser. This is shown in Figure 13-5. The down-line and backup reclosers would be set for a typical operating
sequence involving two fast or overcurrent trips. The trip times are coordinated so that the substation breaker does
not normally operate for faults beyond the down-line device.
Figure 13-20. Series Combination of Substation and Down-line Reclosers
Application
If the fault F1 in Figure 13-20 were a permanent fault and the backup and down-line reclosers were set as described
above, the down-line device would trip twice instantaneously and reclose and then begin to time out according to its
time overcurrent setting. The backup recloser, however, sensing the same fault current, (series system) but having
not tripped due to coordination, will now trip twice on its instantaneous trips since their times are faster than the
down-line device's time overcurrent trip; times. The sequence of operations is shown in Figure 13-21. These undesirable
operations of the backup recloser not only interrupts power to more customers than required but also adds unnecessary
wear on the mechanisms and contacts. Zone Sequence Coordination (ZSC) is a coordination method that prevents
undesirable trips of a backup recloser for a fault beyond a down-line recloser. The DPU1500R/2000R provides the
ZSC function in its standard software. The backup DPU1500R/2000R senses the down-line device's interruption of
the fault by entering and quickly exiting the 50/50N pickup state without issuing a trip and then advances to the next
trip in the reclosing sequence. With the ZSC function enabled in the backup recloser, the down-line device alone will
trip for fault F1 while the backup advances its recloser steps to remain coordinated. The correct operations are
13-14
Zone Sequence Coordination
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R
BACKUP
DPU1500R/2000R
website:www.abb.com/papd
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F1
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DOWNLINE
DPU1500R/2000R
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Applications Notes