55
6.8.3
MCCBs and HV-Side OCR
An overcurrent-relay remote tripping device (OCR) on
the HV side of the circuit must be coordinated with
the MCCBs on the LV side. The OCR setting must
take into consideration the coordination with the OCR
at the power-utility substation and, at the same time,
the following:
1. The setting of an OCR with an instantaneous-trip
element must be at least 10 times the transformer
current rating, to ensure that the excitation surge
of the latter does not trip the OCR.
2. To ensure short-circuit protection, the OCR must
operate within 2 seconds, at 25 times the
transformer rated current.
Figs. 6.26 and 6.27 show the setup, and the coordinat-
ed characteristics converted to the low-voltage side.
The turns ratio of the CT is 150:5, to match the rated
primary current of 87.5A. Considering cooperation of
the OCR with the upper-ranking substation OCR, the
OCR dial is normally set to 0.2 or less, or 1 second
max. if it has an instantaneous trip element. On the
Mitsubishi Type MOC-E general-purpose relay this is
equivalent to dial setting No. 2. Latching-curve over-
lap, shown by the broken lines in Fig. 6.27, must be
allowed for. The instantaneous trip is set to 30A, in
accordance with §1, above.
For setting the Electronic MCCBs (800 and 600A ver-
sions of Type NF800-SEP), the short-delay tripping
currents of both are set to MIN. NF800-SEP have neg-
ligible latching inertia, so that the reset characteris-
tics (except in the instantaneous-trip region) can be
regarded as the same as the tripping characteristics.
Further, there is very little tolerance variation between
units; thus, the tripping characteristics can be shown
as a single line.
If the NF800-SEP short-delay trip current is set at
MAX (where MIN and MAX respectively correspond
to 2 and 10 times rated current), a 600A rating setting
will correspond to 6000A tripping, and an 800A set-
ting will correspond to 8000A tripping. In this case (at
MAX setting), short-delay latching of the NF800-SEP
will overlap the OCR latching (4710A, secondary con-
version). But if the NF800-SEP and the OCR are all
set to MIN, so that the latching values do not exceed
4710A, good coordination will be achieved.
As the OCR has an instantaneous-trip element, set
at 30A (secondary conversion 28.3kA), the region of
selective interruption between the OCR and the
NF800-SEP will extend to this value.
Considering the coordination of the Electronic
MCCBs with the lower-level MCCBs (NF250-CP), it
NF250-CP
NF800-SEP
800A setting
CT ratio
150/5
CT
Tr
OCR
CB or S
Type MOC-E
5A tap
dial
<
2
NF800-SEP
600A setting
6.6kV/210V
3
f
1000 kVA
Electronic MCCB
PF
Tr
Fig. 6.25 Coordinated PF and Electronic MCCB
Characteristics
3 h
2
1 h
40
30
20
10 min
6
4
1 min
40
30
20
10 s
6
4
2
1 s
0.6
0.4
0.2
0.1
0.06
0.02
0.01
0.006
0.004
0.002
Short-delay tripping
current setting range
Electronic MCCB
characteristic curve
Deteriorated
curve of PF
150
200
300
400
600
800
1,000 2,000 4,000 6,000
20,000
150,000
10,000
40,000
Current (A rms)
Time
Fig. 6.24 Protective Coordination of Electorinic MCCBs
and PF
Fig. 6.26 Electronic MCCBs in Coordination with an HV-
Side OCR