Ct Saturation Analysis Tool - GE L30 Instruction Manual

CT REQUIREMENTS
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.

8.1.2 CT saturation analysis tool

The CT saturation analysis tool is an Excel spreadsheet provided for users to analyze the L30 reliability during CT saturation
caused by an external fault, to investigate the effect of adjusting 87L settings, and to examine the possibility of reducing
the CT requirement.
The tool utilizes the CT model and CT saturation calculation algorithm proposed by the IEEE Power System Relaying
Committee (PSRC), and it simulates the analog/digital signal processing and data calculations in the L30. The figure shows
the interface.
8
For a system where the CTs are already installed, the CT and system parameters are known. Use the procedure described
in the tool to analyze the L30 security during CT saturation and investigate the effect of adjusting 87L settings.
The CT saturation analysis tool allows you to check the application, knowing CT parameters, fault current levels, and
distribution through all breakers from short circuit studies, and to analyze L30 performance, having an ability to adjust
settings as needed. The tool has all instructions to perform analysis.
8-2
V > K x I x Z
K tf S S
is 1.5 for system X/R ≤ 20
tf
Figure 8-1: CT saturation analysis tool main page
L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: APPLICATION OF SETTINGS
Eq. 8-3