Schweitzer Engineering Laboratories SEL-251 - Time-Overcurrent Element Timing Tests

Date Code 20000421 Maintenance and Testing 7-15
SEL-251, -2, -3 Instruction Manual
5. Repeat these tests for the phase- and negative-sequence definite-time overcurrent
elements (50LT and 50QT).
Time-Overcurrent Element Timing Tests
Purpose: Verify the operating times of the following time-overcurrent elements.
51T Phase time-overcurrent element
51QT Negative-sequence time-overcurrent element
51NT Ground/Residual time-overcurrent element
Method: 1. Disable all external and internal torque control on the time-overcurrent element
pickups in Relay Word row R1. The above time-overcurrent elements are
derived from their respective pickups in Relay Word row R1. If a time-
overcurrent element pickup in row R1 is torque controlled, the respective time-
overcurrent element it drives in Relay Word row R2 is also torque controlled.
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, respectively (none in this case).
2. Set a programmable output (A1-A4) to follow a time-overcurrent element pickup
(e.g., 51NP). Use the assertion of this output (open to close) to start an external
timer.
3. Set another programmable output to follow time-overcurrent element timeout via
the 51NT bit in the Relay Word (51NP drives 51NT). Use the assertion of this
output to stop the external timer.
4. Calculate the expected operating time of 51NT using the appropriate equation
for the curve number (see Time-Overcurrent Curve-Timing and Time Delay
Reset Equations in Section 2: Specifications). This is dictated by the 51NC
setting. TD is the 51NTD time dial setting. M is the multiple of pickup current
to be applied to the relay. Using example relay settings and a current multiple of
pickup equal to three, the equation for the very inverse curve (51NC = 3) is:
t TD
M
M
= +
-
æ
è
ç
ö
ø
÷
00963
388
1
2
.
.
Where: M = Multiples of Pickup = 3
TD = Time Dial = 4
t
M
= 2.33 seconds
For example, if the relay measures 3.45 amperes of residual current, the 51NT
bit in the Relay Word asserts 2.33 seconds after 51NP assertion (51NP = 1.15 A;
3x1.15 = 3.45). Table 7.2 shows current quantities for the previous example.