4.54
SEL-351A Relay Instruction Manual Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings (Not in SEL-351A-1)
Equation 4.24
However, if the desired voltage pickup for the Wattmetric element is known in
terms of VS channel volts (secondary), then the setting value must be scaled by
PTRS/PTR prior to entry. This pre-scaling makes the 59RES setting match the
scaling the relay does when it internally converts the VS channel value to the
VA, VB, VC voltage base.
For our example system, the desired 3V
0
pickup in terms of the voltage
applied to channel VS is:
Equation 4.25
The example 3V
0
pickup value in terms of the voltage applied to channel VS
is:
Equation 4.26
The 59RES setting is determined as follows:
Equation 4.27
As expected, this is the same value as before.
Similarly, if the desired Wattmetric pickup for the Wattmetric element is
known in terms of VS channel volts (secondary) and IN channel current
(secondary), then the setting value must be scaled by PTRS/PTR prior to
entry. This pre-scaling makes the 32WFP and 32WRP settings match the
scaling the relay does when it converts the VS value into the VA, VB, VC
voltage base.
For our example system, the desired Wattmetric pickup in terms of the voltage
applied to channel VS and the current applied to channel IN is:
Equation 4.28
Forward = 24000 W / (96 • 10) = 25 W secondary.
Reverse = 10000 W / (96 • 10) = 10.417 W secondary.
The 32WFP and 32WRP settings are determined as follows:
32WRP
W primary
PTR CTRN•
-----------------------------------=
10000 W primary
60 10•
------------------------------------------- 16.667 W secondary=
Voltage Value (VS channel base)
V primary
PTRS
------------------------=
Voltage Value (VS channel base)
400 V primary
96
----------------------------------- 4.167 V secondary=
59RES V secondary (VS base)
PTRS
PTR
---------------
• =
4.167
96
60
------
• 6.67 V secondary==
Wattmetric value (VS and IN base)
W primary
PTRS CTRN•
---------------------------------------=
Courtesy of NationalSwitchgear.com
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