Schweitzer Engineering Laboratories SEL-251 - Demand Ammeter Timing Tests

7-16 Maintenance and Testing Date Code 20000421
SEL-251, -2, -3 Instruction Manual
Table 7.2: Current Quantities for 51NT Timing Test Example
IA IB IC
3.45 0.00 0.00 Amperes
0.0° 0.0° 0.0° Degrees
5. Apply a multiple of pickup current to one phase. Record the 51NT element
operating time and compare it to the calculated time.
Note: With current applied to only one phase (e.g., I
A
= 3.45 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
345 0 0 0 0 0
115 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
345 0 0 0 240 0 0 120
345 0 0 120 0 120
115 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 3.45 A.
6. Optional: You may want to repeat this test with various multiples of pickup
current (e.g., M = 3, 5, and 7) and various time dial settings (e.g., TD = 1, 5, and
10) for each of the four curve indexes.
7. Repeat these tests for the phase- and negative-sequence time-overcurrent
elements (51T and 51QT).
Demand Ammeter Timing Tests
Purpose: Verify the operating time of the following demand ammeter thresholds.
PDEM Phase demand current threshold exceeded
QDEM Negative-sequence demand current threshold exceeded
NDEM Residual demand current threshold exceeded