4.74
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
When VSCONN := VS and the relay is wye connected (DELTA_Y := WYE),
the 3V
0
source is in secondary volts on the VA, VB, VC input terminal base. In
fact, 3V
0
is calculated from the measured V
A
, V
B
, and V
C
voltages. The
59RES, DIRWFP, and DIRWRP settings are set in terms of this same base.
An example system similar to Figure 2.30, with wye-connected PTs (PT ratio
7200:120; setting PTR = 7200/120 = 60) and a core-flux summation CT
(CT ratio 50:5; setting CTRN = 50/5 = 10), is used to demonstrate the
required setting scaling.
If the desired zero-sequence voltage pickup for the Wattmetric element in pri-
mary 3V
0
is 400 V primary, obtain the proper setting for 59RES by dividing
the primary voltage by the PT ratio for voltage inputs VA, VB, and VC:
If the desired forward Wattmetric element threshold is 24 kW primary, and the
desired reverse threshold is 10 kW primary, the correct settings are:
When VSCONN := 3V0, with a broken-delta 3V
0
voltage source connected to
the VS channel (terminals VS-NS), PTRS must be properly set to give the signal
on the VS channel the correct scaling in primary units, as displayed under VS in
the METER command response, available via serial port or front panel.
The example value PTRS := 96, as specified in Potential Transformer Ratios
and PT Nominal Secondary Voltage Settings on page 2.30, is used for subse-
quent examples. The relay internally converts the VS channel signal to the VA,
VB, VC voltage base before using it as the 3V
0
quantity, as shown in Table 4.26.
Thus, when the zero-sequence voltage pickup for the Wattmetric element is
known in terms of the system primary voltage level, the required calculation
for setting 59RES is the same as the calculation for the VSCONN := VS
example shown previously, which converts the primary zero-sequence voltage
value into a secondary value on the VA, VB, VC input terminal base.
Using the example quantities from the VSCONN := VS subsection:
Note that the primary voltage is divided by the PTR setting, not the PTRS set-
ting.
Similarly, the derivation of the DIRWFP and DIRWRP settings, if they are
known in primary Watts, follows the same formula as before:
59RES
V primary
PTR
------------------------
400 V primary
60
----------------------------------- 6.67 V secondary== =
DIRWFP
W primary
PTR CTRN•
----------------------------------
24000 W primary
60 10•
------------------------------------------ 40.000 W secondary== =
DIRWRP
W primary
PTR CT RN•
----------------------------------
10000 W primary
60 10•
------------------------------------------ 16.667 W secondary== =
59RES
V primary
PTR
------------------------
400 V primary
60
----------------------------------- 6.67 V secondary== =
DIRWFP
W primary
PTR CTRN•
----------------------------------
24000 W primary
60 10•
------------------------------------------ 40.000 W secondary== =
DIRWRP
W primary
PTR CT RN•
----------------------------------
10000 W primary
60 10•
------------------------------------------ 16.667 W secondary== =
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