Schweitzer Engineering Laboratories SEL-251 - Overcurrent Element Pickup Tests

Date Code 20000421 Maintenance and Testing 7-13
SEL-251, -2, -3 Instruction Manual
Overcurrent Element Pickup Tests
Purpose: Verify the pickup thresholds of the following overcurrent elements:
51P Phase time-overcurrent element pickup
50L Phase definite-time overcurrent element pickup
50H Phase instantaneous overcurrent element
51QP Negative-sequence time-overcurrent element pickup
50Q Negative-sequence definite-time overcurrent element pickup
51NP Ground/Residual time-overcurrent element pickup
50NL Ground/Residual definite-time overcurrent element pickup
50NH Ground/Residual instantaneous overcurrent element
Method: 1. Using the SET command, set the desired programmable output (A1-A4) to
follow the appropriate overcurrent element. Select one of the overcurrent
elements from the first row of the Relay Word listed above.
2. Disable all external and internal torque control on the overcurrent elements.
Using the SET command, enter NA for the ETC(1) and ITC(1) settings. These
settings specify the overcurrent elements in Relay Word row R1 you want
externally and internally torque controlled (none in this case).
3. Apply current to one phase and observe the pickup and dropout of each element.
Record the results.
Note: With current applied to only one phase (e.g., I
A
= 6 A, I
B
= I
C
= 0),
the following sequence currents result:
[ ]
[ ]
( ) ( )
[ ]
I I I I
A A A
A
I I a I a I
A B C
A B C
0
1
3
1
3
2
1
3
2
6 0 0 0 0 0
2 0
= + +
= Ð °+ Ð °+ Ð °
= Ð °
= + +
Where: a and aº Ð ° º Ð °1 120 1 240
2
( ) ( )
[ ]
[ ]
I A A A
A A A
A
2
1
3
1
3
6 0 0 0 240 0 0 120
6 0 0 120 0 120
2 0
= Ð °+ Ð °+ ° + Ð °+ °
= Ð °+ Ð- °+ Ð °
= Ð °
The negative-sequence and ground/residual overcurrent elements respond to
3xI
2
and 3xI
0
currents, respectively. Thus, for this test all overcurrent
elements see 6 A.
You can also test the following element in Relay Word row R2 using the above
method (bypass step 2 because 50C cannot be torque controlled).
50C Phase instantaneous overcurrent element (50C overrides internal torque
control element 27)