# Calculation Example 1 - GE UR Series Instruction Manual

Line differential relay.

9 APPLICATION OF SETTINGS
9 APPLICATION OF SETTINGS 9.1 L90 CT REQUIREMENTS
In general, proper selection of CTs is required to provide both adequate fault sensitivity and prevention of operation on
high-current external faults that could result from CT saturation. The use of high quality CTs, such as class X, improves
relay stability during transients and CT saturation, and can increase relay sensitivity. A current differential scheme is highly
dependent on adequate signals from the source CTs. Ideally, CTs used for line current differential should be chosen based
on good application practice as described below. If the available CTs do not meet the described criteria, the L90 will still
provide good security for CT saturation for external faults. Its adaptive restraint characteristics, based on estimates of mea-
surement errors and CT saturation detection, allow the relay to be secure on external faults while maintaining excellent per-
formance for severe internal faults. Where CT characteristics do not meet criteria or where CTs at both ends may have
different characteristics, the differential settings should be adjusted as per Section 9.2.1.
The capability of the CTs, and the connected burden, should be checked as follows:
1.
The CTs should be class TPX or TPY (class TPZ should only be used after discussion with both the manufacturer of
the CT and GE Power Management) or IEC class 5P20 or better.
2.
The CT primary current rating should be somewhat higher than the maximum continuous current, but not extremely
high relative to maximum load because the differential element minimum sensitivity setting is approximately 0.2 × CT
rating (the L90 relay allows for different CT ratings at each of the terminals).
The VA rating of the CTs should be above the Secondary Burden × CT Rated Secondary Current. The maximum sec-
3.
ondary burden for acceptable performance is:
CT Rated VA
<
R
R
+
------------------------------------------------------------ -
b
r
(
CT Secondary I
where: R
= total (two-way) wiring resistance plus any other load
b
R
= relay burden at rated secondary current
r
The CT kneepoint voltage (per the V
4.
voltage during a fault. This can be estimated by:
X
>
×
V
I
--- -
+
1
k
fp
R
X
>
×
V
I
--- -
+
1
k
fg
R
where: I
= maximum secondary phase-phase fault current
fp
I
= maximum secondary phase-ground fault current
fg
X / R = primary system reactance / resistance ratio
R
= CT secondary winding resistance
CT
R
= AC secondary wiring resistance (one-way)
L
To check performance of a class C400 ANSI/IEEE CT, ratios 2000/1800/1600/1500 : 5 A connected at 1500:5, and where:
maximum I
= 14 000 A
fp
maximum I
= 12 000 A
fg
impedance angle of source and line = 78°
CT secondary leads are 75 m of AWG No. 10.
BURDEN CHECK:
ANSI/IEEE class C400 requires that the CT can deliver 1 to 20 times the rated secondary current to a standard B-4 burden
(4 Ω or lower) without exceeding a maximum ratio error of 10%.
GE Power Management
)
2
rated
curves from the manufacturer) should be higher than the maximum secondary
k
×
(
)
R
+
R
+
R
for phase-phase faults
CT
L
r
×
(
)
R
2 R
R
+
+
for phase-ground faults
CT
L
r
L90 Line Differential Relay
9.1 L90 CT REQUIREMENTS
9.1.1 INTRODUCTION

9
9-1