New Relay User Session
SEL-751A Hands-On Testing
General Information
•
Relays are rated at 15 A continuous. Apply greater current only for a short period
of time.
•
The thermal rating is 250 A for 1 second or 62500 amperes-squared-seconds (I2t). If currents
greater than 15 A are applied, allow sufficient cooling time between tests.
•
Phase elements respond to IA or IB or IC, which equals IA when IB and IC are zero.
•
Negative-sequence elements (3I2) respond to IA + α2 • IB + α • IC, which equals IA when IB and
IC are zero.
•
Residual elements (IG) respond to IA + IB + IC, which equals IA when IB and IC are zero.
•
Neutral elements (IN) respond to IN.
•
Appendix J of the SEL-751A Instruction Manual (available at http://www.selinc.com) contains
the complete Relay Word bit table.
Meter Test
Step 1
Connect single-phase test currents to IA, IB, and IC. Connect a single-phase test current to IN.
Step 2
Apply 5 A of ac, separated by 120 degrees, to the phase current inputs.
Step 3
Apply 2 A of ac, at 60 degrees, to the neutral current input.
Step 4
Use the serial port METER command or the front-panel METER menu to verify the following:
•
Phase currents: IA = IB = IC = Isec • CTR = 5 A • 120 = 600 A at 120 degrees apart
•
Neutral current: IN = Isec • CTRN = 2 A • 120 = 240 A at 60 degrees
•
Ground and negative-sequence currents: IG = 3I2 = 0 A
SEL-751A Hands-On Testing
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Overcurrent Element Pickup Test
Step 1
Connect a single-phase test current to IA.
Step 2
Issue the command SHO 50P1P, and record the pickup setting.
50P1P = __________________
Step 3
Use the SEL-751A Instruction Manual to find the Relay Word bit that will assert when the applied current
goes above the setting. Use the TAR command to monitor the state of the Relay Word bit.
Step 4
Apply a test quantity slightly less than the setting of the element under test. The Relay Word bit should be
a 0. Slowly increase the test current until the element asserts.
50P1P pickup = __________________
Inverse-Time Overcurrent Element Timing Tests
Step 1
Connect a single-phase test current to IA.
Step 2
Calculate the expected trip time of the 51P1T and 51G1T elements using the equations in the SEL-751A
Instruction Manual, and record the results below. TD is the time dial setting of the element, and M is an
arbitrary multiple of pickup.
As an example, for a U3 curve, time dial of 3.00, and test current of 5 times pickup, the operating time
should be as follows:

51P1T t p TD •  0.0963 +
=

3.88 
M 2 –1 
3.88 

51P1T t p 3.0 •  0.0963 + 2 
=
5 –1 

51P1T t p = 0.774 seconds
51P1T pickup time calculated = __________________
51G1T pickup time calculated = __________________
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Step 3
Add the 51 phase and ground Relay Word bits, both pickup and time-out, to the Sequential Events
Recorder (SER).
Step 4
Apply test current corresponding to M times the pickup.
Step 5
When the relay trips, inspect the SER report and calculate the operating time of the element. This is the
difference in time from when the pickup asserted and the time-out asserted. Verify that the actual
operating time of the element matches the expected operating time from Step 2.
51P1T pickup time test = __________________
51G1T pickup time test = __________________
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