T EC H N I C A L A N D A P P L I C AT I O N G U I D E
15
If the customer does not own the MV line con-
nected to their transformer or the local utility does
not want additional products added on their MV
service line to the customer’s site, we can apply
ABB’s ArcLimiter solution to the LV transformer
side with the same arc flash mitigation effective-
ness and possibly lower installed costs. This ap-
proach should also improve MV power quality
during an AF event.
Note that in Figure 11, the CLF is now relocated
from the MV side to the LV side of the transformer.
These LV CLFs are not as long, but much wider, so
changes in mechanical installation must be consid-
ered. Connect the CLF directly to the transformer’s
secondary LV bushings. The CLF’s load terminals
should connect to the LV busduct or main load ca-
bles routed to the LV switchgear. The PSEs should
be connected from the CLF’s load terminals to
ground. The QRU100 controller can be mounted in
the LV switchgear or the transformer’s secondary
air terminal. As in the previous applications, the
REA101 needs to have communications to the
QRU100. This approach will have less negative im-
pact at the MV level.
LV fault magnitude
Once the PSEs close, the transformer and CLF will
see a bolted LV fault collapsing any LV equipment
AF. That transformer LV through-fault of 52kA will
be reflected to the MV primary as (1 / %Z) * FLA.
Example: 80% of the MV-LV transformers are
2500KVA base rated. Per ANSI, energizing the
transformer will produce an inrush of 12*FLA for
0.1s. If the transformer has an impedance of 5.75%
or 9%, the inrush will be (1 / 0.0575)*FLA =
17.4*FLA. Slightly higher than typical inrush. For
the 9%Z version, the inrush will be (1 / 0.09)*FLA =
11.1*FLA. Slightly lower than typical inrush.
Through-fault duration
A 2500KVA transformer has a top rating typically at
3333KVA (3990FLA at 480V). Therefore, a 4000A
(or 5000A) class L CLF needs to be applied. If a
Mersen A4BQ 4000A (or 5000A) is applied, that
through-fault will last 0.029s or 1.7 cycles (0.148s or
8.9 cycles). The reflected through-fault could be
higher than typical inrush but may last for less time
duration. The mix of transformer KVA vs. CLF sizes
vs. CLF response time is not that large. Each appli-
cation will have to be evaluated and the area FAE(s)
can help here.
Grounding
Similar to the MV applications, most LV applica-
tions for industrial plants have high Z (5A limited)
grounded systems. Upon PSE closure, two of the
CLFs will open, the third CLF will be damaged but
stays intact. The ground alarm will activate.
—
ArcLimiter
TM
solution application at
LV system level
—
11 MV system is not
solidly grounded,
but low resistance
neutral grounded
—
11
