Schweitzer Engineering Laboratories SEL-787, Инструкция по эксплуатации, Страница: 100 / 564

4.22
SEL-787 Relay Instruction Manual Date Code 20081022
Protection and Logic Functions
Basic Protection
In a through-current situation, the worst-case theoretical differential current
occurs when all of the input currents are measured with maximum positive CT
error, and all of the output currents are measured with maximum negative CT
error as well as being offset by maximum LTC variation. Therefore, the
maximum differential current expected for through-current conditions is:
where the summation terms are the total input and output power transformer
secondary currents, after tap compensation. Because these summations must
be equal for external faults and load current, we can express the maximum
differential current as a percentage of winding current:
In addition to the error calculated above, we have to consider additional errors
from transformer excitation current (
≈
3 percent) and relay measurement error
(
≤
5 percent). The maximum total error comes to 36 percent. Therefore, if we
use only one slope, a conservative slope setting, SLP1, is about 40 percent.
This represents a fixed percentage differential application and is a good
average setting to cover the entire current range.
A two-slope, or variable-percentage differential application, improves
sensitivity in the region where CT error is small and increases security in the
high-current region where CT error is great. We must define both slopes, as
well as the slope 1 limit or crossover point, IRS1. If we assume CT error to be
only 1 percent, we can set SLP1 at about 25 percent. A good choice for IRS1
is about 3.0 per unit of tap, while the SLP2 setting should probably be in the
50 percent to 70 percent range to avoid problems with CT saturation at high
currents.
Unrestrained Element Current Pickup
The instantaneous unrestrained current element is intended to react quickly to
very heavy current levels that clearly indicate an internal fault. Set the pickup
level (U87P) to about 10 times TAP. The unrestrained differential element
only responds to the fundamental frequency component of the differential
operating current. It is not affected by the SLP1, SLP2, IRS1, PCT2, or PCT5
settings. Thus, it must be set high enough so as not to react to large inrush
currents.
Note that U87P must be set lower than 31 • I
nom
/TAP
max
, where TAP
max
is the
largest of the TAP settings.
Second-Harmonic Blocking
Transformer simulations show that magnetizing inrush current usually yields
more than 30 percent of IF2/IF1 in the first cycle of the inrush. A setting of 15
percent usually provides a margin for security. However, some types of
transformers, or the presence within the differential zone of equipment that
draws a fundamental current of its own, may require setting the threshold as
low as 7 percent. For example, the additional fundamental frequency charging
current of a long cable run on the transformer secondary terminals could
“dilute” the level of second harmonic seen at the primary to less than 15
percent.
Idmax 1 e +
( )
IW
n
″
IN
″
∑
1 e –
1 a +
------------ –
•
IW
n
″
O UT
″
∑
•
=
1 e
1 e –
1 a +
------------ – +
2 e
•
a e a
•
+ +
1 a +
-------------------------------------
100%
•
28.18% = =