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(or excitation or ratio correction factor curve)
6. One way current transformer lead resistance at
25˚C (RL) (when using excitation curve, include
ct winding resistance)
7. Current transformer connections (Wye or delta)
8. ct secondary winding resistance, R
Definitions of Terms:
I
=
Primary current at (KVA)
P
I
=
Relay input current at (KVA)
R
I
, I
, I
RH
RL
RI
high, low and intermediate voltage sides
respectively
I
=
ct secondary current at (KVA)
S
T
=
Relay tap setting
T
, T
, T
H
L
I
high, low and intermediate voltage wind-
ings, respectively
N
=
Number of current transformer turns that
are in use
N
=
N/NT (Proportion of total turns in use)
P
N
=
Current transformer ratio, full tap
T
V
=
"C" accuracy class voltage
CL
Z
=
Burden impedance of any devices other
A
than the HU or HU-1 relays, with maxi-
mum external fault current flowing
I
=
Maximum symmetrical external fault cur-
ext
rent in secondary RMS amperes
Z
=
Total secondary burden in ohms (exclud-
T
ing current transformer winding resis-
tance)
C
P
ALCULATION
ROCEDURE
1. Select current transformer taps , where multi-ratio
types are used. Select a tap to give approxi-
mately 5 amperes at maximum load. This will
provide good sensitivity and will produce no ther-
mal problem to the ct, the leads, or the relay. Bet-
ter sensitivity can be achieved by selecting a tap
to give more than 5 amperes if a careful check is
made of the ct, the leads, and the relay capabil-
ity. For determining the required continuous rat-
ing of the relay, use the expected two-hour
S
M
M
are same as I
except for
R
M
= are same as T except for
maximum load, since the relay reaches final tem-
perature in this time.
2. Calculate the relay currents, I
for relay tap selection should be based on the
same KVA capacity.
3. Calculate the relay current ratio(s) using the low-
est current as reference.
4. Select the tap ratio as close as possible to relay
current ratio from table 1. Choose the first relay
tap ratio using the largest current ratio from step 3.
The other tap ratios should be determined using
the lower tap from the first tap ratio as reference.
I
should not exceed relay continuous rating as
R
defined in Energy Requirement Table.
5. Check IIT operation . The IIT pickup is ten times
the relay tap value for the HU and HU-1. There-
fore, the maximum symmetrical error current
which is flowing in the differential circuit on exter-
nal fault current due to dissimilar ct saturation
should not exceed 10 times relay tap.
6. Determine Mismatch.
For 2 winding banks:
% mismatch =
100
Where S is the smaller of the two terms, (I
I
) or (T
/T
)
RH
L
H
For 3 winding banks:
Repeat calculation of equation (1) and apply sim-
ilar equations to calculate mismatch from the
intermediate to high and from the intermediate to
low voltage windings.
Where tap changing under load is performed the
relay should be set on the basis of the middle or
neutral tap position. The total mismatch, includ-
ing the automatic tap change should not exceed
15% with a 30% sensitivity relay, and 20% with a
35% sensitivity relay. Note from figure 7 that an
ample safety margin exists at these levels of mis-
match.
41-347.12C
. All relay currents
R
(
⁄
)
(
⁄
)
I
I
T
T
–
RL
RH
L
H
-------------------------------------------------------- -
S
(1)
/
RL
11
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