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Type HCB-1 Pilot Wire Relay System
(
) T ;
T
=
1.25 I
tap C
L
The taps must be set the same at all stations. Where
the ct ratios are not identical use auxiliary ct's to
match the currents within 5% of each other.
5.2.2. Ground Fault Sensitivity (R
The ground-fault pickup is determined by G, H and T
taps. (T should be determined by the phase setting.)
The minimum fault current I
lowing:
Tap A or B: I
= .20T (Tap G);
g
Tap C:
I
= .25T (Tap G);
g
For cable circuits, where the line charging current
exceeds 5% of nominal positive sequence pickup
current, set tap G; otherwise set tap H.
The taps must be set identically at all stations. Use
auxiliary ct's where the main ct's have a different
ratio. The currents should match within 5%.
5.2.3. Restraint Tap (Max. and Min. Taps)
Set in maximum restraint tap for all two-terminal
lines. Set in minimum restraint tap for all three termi-
nal lines. The use of maximum restraint on two termi-
nal applications allows the relay to be used for all
pilot-wires as indicated in Table 4. The use of mini-
mum restraint on three-terminal applications com-
pensates for the desensitizing effect of a third
terminal.
NOTE: The relay pick-up calibration will change
slightly (within 5%) between minimum
restraint tap and maximum restraint. In
most applications, it is not necessary to
recalibrate the relay when changing to the
minimum restraint tap.
5.2.4. Tapped Loads
Where one transformer bank is tapped to a line pro-
tected with two HCB-1 relays, the critical point is to
set above the fault current flow for a fault on the other
side of the bank. Set the T for not less than:
(
) T
T
=
1.91 I
tap C
;
PL
(
)
Equ. (10)
=
0.62 I
tap B
L
TAP)
0
should exceed the fol-
g
I
= .10T (Tap H)
g
I
= .12T (Tap H)
g
(
)
Equ. (11)
=
1.52 I
tap B
PL
where I
= total secondary fault current feed from
PL
all terminals to a phase-to-phase fault
on the low side of the bank, divided by
the number of terminals (2 or 3).
NOTE: The tapped bank must not act as a
ground source for high-side faults. Ordi-
narily this means that the R
(G, H) need not be changed since no zero-
sequence current flows in the line when
the low side is grounded.
5.3. SETTING EXAMPLE
Assume:
Two-terminal line.
ct ratio = 600/5
Full-load current = IL = 400A
Minimum 3-phase internal fault
current:
Through Station I = 1500A
Through Station II = 2500A
Minimum internal line-to-ground fault
current:
Through Station I = 400A
Through Station II = 285A
5.3.1. Phase Fault Pickup
1500
I
=
I
=
----------------------------- - x
(
)
max
3P
I
=
0.5I
=
8.3 tap B
(
)
max
3P
400
(
-------- -
I
=
I
=
3.3 tap C
(
)
min
L
120
I
L
(
I
=
----
=
1.7 tap B
(
)
min
2
The phase fault current is sufficient to allow the relay
to be set to prevent tripping on an open circuited
pilot-wire. Therefore, the desired taps would be:
(
T
=
1.25 I
=
4.1 tap C
L
(
T
=
0.62 I
=
2.0 tap B
L
Set both relays for T = 4, R1 = C
41-971.3M
tap settings
0
+
2500
5
(
-------- -
=
16.7 tap C
2
600
(
)
)
)
)
)
)
11
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