Introduction - GE L30 Instruction Manual

9 APPLICATION OF SETTINGS
9 APPLICATION OF SETTINGS 9.1CT REQUIREMENTS
In general, proper CT selection is required to provide both adequate fault sensitivity and prevention of operation on high-
current external faults that can result from CT saturation. The use of high-quality CTs (such as class X) improves relay sta-
bility during transients and CT saturation and can increase relay sensitivity. A current differential scheme is highly depen-
dent on adequate signals from the source CTs. Ideally, CTs selected for line current differential protection are based on the
criteria described as follows, which presents the high-level CT requirements to avoid saturation and to provide undistorted
CT secondary current.
Check the capability of the CTs, and the connected burden, as follows:
1. The CTs should be class TPX or TPY, or IEC class 5P20 or better. Use class TPZ only after discussion with both the
manufacturer of the CT and GE Multilin.
2. The CT primary current rating should be somewhat higher than the maximum continuous load current, but not too high
relative to the minimum fault current because the differential element minimum pickup setting is 0.2 x CT rating for the
phase differential and 0.1 x CT rating for ground differential. Too high CT primary current rating limits sensitivity of the
differential element for high-resistive faults.
3. As per the IEEE C37.110-2007 "Guide for the Application of Current Transformers Used for Protective Relaying Pur-
poses," the CT must be capable of a secondary saturation voltage V
where
I is maximum CT secondary fault current
S
Z is total secondary burden Z
S
4. As per the IEEE C37.110-2007 "Guide for the Application of Current Transformers Used for Protective Relaying Pur-
poses," the CT must be capable of a secondary saturation voltage V
where
X/R is primary system reactance / resistance ratio
R is CT secondary winding resistance
CT
R is AC secondary wiring resistance (one-way)
L
R is relay burden at rated secondary current
R
5. Practically, it is difficult to meet this criteria and avoid saturation completely. The L30 provides several means to ensure
security during an external fault, including dual-slope restraint characteristics, adaptive restraint, and CT saturation
mechanism, allowing the relay to be secure during severe external faults, while maintaining excellent performance for
internal faults. Therefore, knee point voltage requirements with a default 87L settings can be approximately estimated
by the following equation:
where
K is 2 for system X/R > 20 and K
tf
This is however applicable, where CT ratios are the same, CT burdens at both sides of the line are very close, and when
just one CT at each side is used. When CTs are different at both sides (different CT characteristics, CT ratio and burden) or
when more than one CT at each side is used, these equation may not provide correct estimation of the required knee point
voltage.
For this situation GE provides a CT saturation analysis tool, described in the following section.
GE Multilin
V > I x Z
K S S
= R + 2 x R + R
S CT L R
X
 
V > I × -- - + 1
 
K S
R
V > K x I x Z
K tf S S
is 1.5 for system X/R ≤ 20
tf
L30 Line Current Differential System
9.1 CT REQUIREMENTS
to avoid AC saturation:
K
to avoid DC saturation:
K
× Z
S

9.1.1 INTRODUCTION

(EQ 9.1)
(EQ 9.2)
(EQ 9.3)
9
9-1