Sel SEL-751 User guide

Date Code 20200324 SEL Application Guide 2020-05
Setting Up Maintenance Mode in an SEL-751
Swagata Das
INTRODUCTION
Many users prefer to enable maintenance mode in relays as an added safety when working on ener-
gized equipment. The maintenance mode function, when activated, enables a sensitively set instan-
taneous overcurrent element for faster tripping in those relays without arc-flash protection and
prevents closing of the breaker by any means (e.g., automatic reclose, remote SCADA close, or
local pushbutton close). This function is not included in the default settings of the SEL-751 Feeder
Protection Relay, but you can set it up by making a few settings changes. This application guide
shows the settings necessary to enable and disable maintenance mode in an SEL-751 by pressing a
front-panel pushbutton. The same logic can be used when setting up maintenance mode in an
SEL-751A Feeder Protection Relay as well.
ENABLING AND DISABLING MAINTENANCE MODE WITH APUSHBUTTON
You can order the SEL-751 with either four or eight pushbuttons, as shown in Figure 1. This appli-
cation guide assumes that you have an SEL-751 with eight pushbuttons, that the relay is using the
default settings, and that the AUX1 pushbutton is being used to activate maintenance mode. Pressing
the AUX1 pushbutton asserts the PB05 and PB05_PUL Relay Word bits as shown in Figure 1. The
PB05 Relay Word bit remains asserted for as long as the pushbutton is pressed and is considered a
“follow” output. The PB05_PUL Relay Word bit asserts for only one processing interval (1/4 of a
power-system cycle) and then desserts, even if the button is still being pressed. Because we are
interested in capturing the moment the pushbutton is pressed and not in how long the pushbutton is
pressed, we use the PB05_PUL Relay Word bit in our logic. Note that instead of using the AUX1
pushbutton, you can use any other pushbutton to activate maintenance mode in your application.
Refer to Figure 1 and choose the Relay Word bit associated with the pushbutton that you are plan-
ning to use.
Next, we use the AUX1 pushbutton to drive Latch Bit LT01. The output of LT01 enables and disables
maintenance mode. Figure 2 illustrates the settings. Latch Bit LT01 asserts (maintenance mode
off) when 1) the AUX1 pushbutton is pressed (PB05_PUL = 1 for one processing interval), 2) the
front panel is not locked (LT02 = 1 per the default settings), and 3) the latch bit was previously
deasserted (maintenance mode on). Note that the front panel can be locked or unlocked by pressing
and holding the LOCK pushbutton for 3 s (per the default settings). Latch Bit LT01 deasserts (main-
tenance mode on) when 1) the AUX1 pushbutton is pressed again (PB05_PUL = 1 for one processing
interval), 2) the front panel is not locked (LT02 = 1 per the default settings), and 3) the latch bit
was previously asserted (maintenance mode off).
Application Guide Volume III AG2020-05

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SEL Application Guide 2020-05 Date Code 20200324
ENABLE FASTER TRIPPING DURING MAINTENANCE MODE
The arc-flash function in the SEL-751 provides the fastest tripping during a fault. It uses light and
a fast overcurrent detector to detect faults within 2–5 ms [2] and can remain enabled at all times.
This application guide shows an alternative method for faster tripping when the relay has not been
ordered with arc-flash detection. We set up an instantaneous phase overcurrent element for faster
tripping and enable it only during maintenance mode.
The SEL-751 has four levels of instantaneous phase overcurrent elements. Set one such element
(Element 2 in this example) with sensitive pickup settings that are temporarily activated during
maintenance mode conditions. When determining the pickup setting, the element must be set high
enough to ensure that the relay does not nuisance trip under heavy load conditions. A typical set-
ting is 1.5 times the expected maximum load current [1]. In this example, we use a 50P2P pickup
setting of 2 A secondary, as shown in Figure 3. Set the 50P2TC torque-control equation to NOT
LT01 so that the sensitively set 50P2P overcurrent element is enabled only under maintenance
mode conditions. If you want instantaneous tripping on a ground element, you can also set a
ground instantaneous overcurrent element in a similar manner.
Figure 1 Relay Word Bits Associated With Each Operator Control Pushbutton
Figure 2 Logic to Enable and Disable Maintenance Mode
PB2A_LED
PB02, PB02_PUL
PB2B_LED
PB3A_LED
PB03, PB03_PUL
PB3B_LED
PB4A_LED
PB04, PB04_PUL
PB4B_LED
PB1A_LED
PB01, PB01_PUL
PB1B_LED
AUX4
AUX3
AUX1
AUX2
PB6A_LED
PB06, PB06_PUL
PB6B_LED
PB7A_LED
PB07, PB07_PUL
PB7B_LED
PB8A_LED
PB08, PB08_PUL
PB8B_LED
PB5A_LED
PB05, PB05_PUL
PB5B_LED
BLOCK CLOSE
BREAKER CLOSED
BREAKER OPEN
AUX
In reclosing models only
Not available in models with four pushbuttons

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Date Code 20200324 SEL Application Guide 2020-05
Now add the output of the phase instantaneous overcurrent element to the trip equation. To deter-
mine the correct output to use, refer to Figure 4. When directionality is enabled (setting EDIR = Y
or AUTO), the outputs of the phase instantaneous element are 67P2P (pickup) and 67P2T (time-
delayed pickup). When directionality is disabled (EDIR = N), or when you have ordered the
SEL-751 without directionality, the outputs of the phase instantaneous element are 50P2P (pickup)
and 50P2T (time-delayed pickup). Note that the SEL-751A does not have directionality. The out-
put of the phase instantaneous overcurrent logic in that relay is 50P2P and 50P2T. Refer to the
SEL-751 Relay Feeder Protection Relay Instruction Manual for the logic diagram.
Figure 3 Logic to Activate Element 2 During Maintenance Mode
Figure 4 Phase Instantaneous Overcurrent Element Logic in the SEL-751
Regular
Overcurrent
Element
Sensitive Overcurrent
Element Enables Only
During Maintenance Mode
Max Phase
Current
50P2TC
P2DIR
50P2P 50P2D
0
67P2P/50P2P
67P2T/50P2T
Relay Word Bits
Relay Setting
When Use for Trip Use for Pickup Nonfunctional
EDIR = Y
or AUTO
67P2T 67P2P 50P2T
50P2P
EDIR = N
or hiddena
aEDIR is automatically set to N and hidden when the directional control option is not ordered.
50P2T 50P2P 67P2T
67P2P

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SEL Application Guide 2020-05 Date Code 20200324
Because directionality is disabled in the default settings, we add 50P2T in the trip equation as
shown in Figure 5.
DISABLE CLOSE AND RECLOSE DURING MAINTENANCE MODE
Supervise the conditions listed in the close equation with LT01, as shown in Figure 6, so that the
relay does not allow a close when maintenance mode is enabled.
Figure 5 Trip Logic
(...) OR 50P2T
Figure 6 Close Logic
(...) AND LT01

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Date Code 20200324 SEL Application Guide 2020-05
For models ordered with the reclosing option, in addition to modifying the close equation, use the
latch bit to disable reclosing by adding NOT LT01 in the 79DTL drive-to-lockout equation, as
shown in Figure 7.
TURN ONOR FLASH PUSHBUTTON LED DURING MAINTENANCE MODE
Now we are going to turn on or flash a pushbutton LED when maintenance mode is active.
Turn On LED
Every operator control pushbutton has two LEDs. The PBnA_LED setting governs the top LED,
and the PBnB_LED setting governs the bottom LED (nindicates the pushbutton number). To turn
on the top LED during maintenance mode, set the PB5A_LED setting equal to NOT LT01, as
shown in Figure 8. Disable the bottom LED by setting PB5B_LED equal to 0. The color of the top
LED is dictated by the PBnALEDC setting, and the color of the bottom LED is dictated by the
PBnBLEDC setting. In this example, we set PB5ALEDC = AO. This means that when the
PB5A_LED equation evaluates to logical 1, the LED turns on with an amber color (A for amber).
On the other hand, when PB5A_LED evaluates to logical 0, the LED turns off (O = OFF). The
color of the bottom LED does not matter because it is not being used.
Figure 7 Logic to Disable Reclosing During Maintenance Mode
(...) OR NOT LT01
Figure 8 Logic to Turn On Pushbutton LED During Maintenance Mode

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SEL Application Guide 2020-05 Date Code 20200324
Flash LED
To make the pushbutton LED flash during maintenance mode, set up a SELOGIC variable as shown
in Figure 9. Set the pickup and dropout delay to 0.5 s. These timers control how fast the LED
flashes.
Change the PB5A_LED equation to NOT LT01 AND SV01T, as shown in Figure 10. When main-
tenance mode is enabled, the first part of the equation is true (NOT LT01 = 1) while the second part
of the equation (SV01T) oscillates between 0 and 1, causing the LED to flash with an amber color.
Change the PB5ALEDC setting to change the color of the flashing LED.
UPDATE SER
Add LT01 and 50P2T to one of the SER equations. Figure 11 shows them added to SER3.
Figure 9 Logic to Oscillate Timer Output Between 0 and 1
Figure 10 Logic for Flashing Pushbutton LEDs During Maintenance Mode
Figure 11 SER Trigger List Updated With Relay Word Bits Associated With Maintenance Mode

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Date Code 20200324 SEL Application Guide 2020-05
SHOW AFRONT-PANEL DISPLAY MESSAGE
Some users may also want to include a front-panel display message when maintenance mode is
enabled. Refer to [3] for step-by-step guidelines on how to configure a front-panel display message
in an SEL-751.
UPDATE FRONT-PANEL OPERATOR CONTROL LABELS
Once you change the settings, use the configurable label kit provided with the relay to update the
operator control labels on the SEL-751 to avoid confusion. In this example, we changed the label
for the PB05 pushbutton to MAINT, as shown in Figure 12.
CONCLUSION
The safest way to work on electrical equipment is when it is completely de-energized. However,
should live-line work be necessary, this application guide demonstrates how to configure a mainte-
nance mode function in an SEL-751 to improve personnel safety. You can enable or disable this
function when you are physically present in front of the relay (local-mode only) and press a front-
panel pushbutton. When maintenance mode is enabled, a phase instantaneous overcurrent element
provides faster tripping during a fault if the relay does not have arc-flash detection and blocks all
close operations (e.g., automatic reclosing, a remote SCADA close, or a local pushbutton close).
Figure 12 Update Pushbutton Labels
MAINT
PB2A_LED
PB02
PB2B_LED
PB3A_LED
PB03
PB3B_LED
PB4A_LED
PB04
PB4B_LED
PB1A_LED
PB01
PB1B_LED
AUX4
AUX3
AUX2
PB6A_LED
PB06
PB6B_LED
PB7A_LED
PB07
PB7B_LED
PB8A_LED
PB08
PB8B_LED
PB5A_LED
PB05
PB5B_LED
BLOCK CLOSE
BREAKER CLOSED
BREAKER OPEN
AUX
Pushbutton LED
Operator Control Pushbutton
Pushbutton LED

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SEL Application Guide 2020-05 Date Code 20200324
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REFERENCES
[1] R. Kirby, “Reducing Arc-Flash Hazard Energy During Maintenance With an SEL-351S
Front-Panel Pushbutton,” SEL Application Guide (AG2007-08), 2007. Available: selinc.com.
[2] M. Zeller, D. Haas, and A. Hargrave, “Using the SEL-751 and SEL-751A for Arc-Flash
Detection,” SEL Application Guide (AG2011-01), 2011. Available: selinc.com.
[3] S. Dasgupta and R. McDaniel, “Show Power Factor Lead/Lag Information on an SEL-700
Series Relay Front-Panel Display,” SEL Application Guide (AG2019-21), 2019. Available:
selinc.com.
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please contact us at:
Schweitzer Engineering Laboratories, Inc.
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Pullman, WA 99163-5603 U.S.A.
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