UFES
MV SWGR BUS
OIL or DT
XFMR
LV EQUIPMENT
MV BREAKER
REA
BONDING JUMPER
ARC FLASH
50/51
HV BUS
OIL XFMR
TRIP
14
A RCLI M ITE R
TM
A R C FL A SH M I T I G AT I O N S O LU T I O N FO R LV US I N G U FE S
The MV system may have the configuration shown
in Figure 9. The MV system grounding does not en-
ter into this application discussion because the
feeder breaker will be shunt tripped at the same
time as the UFES. Once the breaker is open, all
phase and ground current flow stops. The ground
relays have a chance to time out or pick up but do
not as the ground fault is cleared by the breaker
prior to this happening.
UFES type equipment (high speed ground
switches) have been applied for many years on LV
and MV systems to reduce arc flash incident energy
levels. They are effective but there is a concern
among some liquid-filled transformer manufactur-
ers that solving an arc flash problem with a UFES
may actually be creating a problem at the trans-
former. If the upstream HV-MV transformer is older
and/or has aged insulating paper, the high cur-
rent-induced magnetic vibrations could damage
the insulating paper causing a turn-turn trans-
former fault.
Figure 10, from IEEE C57.109, “IEEE Guide for Liquid
Immersed Transformer Through-Fault Current Du-
ration,” is a visual representation of an assumed
sustained bolted fault on the transformer’s second-
ary reflected to the primary windings. The “X” axis
is in units of times transformer’s full load amps
(base, not top). The “Y” axis, in seconds, is the time
duration capability of the transformer to thermally
and mechanically withstand a sustained secondary
bolted fault. The lower portion of the curve is the
mechanical damage area, considered the area of
most concern from a protection viewpoint.
The graph’s purpose is to assist in the coordination
of the transformer’s primary protective devices.
Even though the transformer is designed to with-
stand this long-term bolted fault level, the fault
time duration is dependent upon the device 51 pro-
tective relay settings and breaker interrupting time
frame or primary fuse interruption. The protective
device time-current-curve must be positioned left
of the transformer’s damage curve.
In order to minimize the through-fault duration, the
UFES’s shunt trip contacts should trip direct
(by-passing the 50/51 protection relay) to the up-
stream MV breaker as in Figure 9. This direct trip
action also minimizes the fault produced voltage
dip duration; a benefit to operating processes.
Upon UFES operation, all three PSEs should be re-
placed. The UFES to ground bonding jumper can be
small thermally, approximately #2 AWG; the zero se-
quence current will be zero.
—
MV-LV transformers fed by MV
breaker
—
09 MV system is not
solidly grounded,
but low resistance
neutral grounded
—
10 Path for fault current
is through the phase C
fuse, to UFES phase C
PSE, to the transformer’s
neutral, but is now lim-
ited by the source neutral
return path’s resistance
—
09
—
10
