8-2 L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALCT REQUIREMENTS CHAPTER 8: APPLICATION OF SETTINGS8X/R is primary system reactance / resistance ratioR CT is CT secondary winding resistanceR L is AC secondary wiring resistance (one-way)R R is relay burden at rated secondary current5. Practically, it is difficult to meet this criteria and avoid saturation completely. The L30 provides several means toensure security during an external fault, including dual-slope restraint characteristics, adaptive restraint, and CTsaturation mechanism, allowing the relay to be secure during severe external faults, while maintaining excellentperformance for internal faults. Therefore, knee point voltage requirements with a default 87L settings can beapproximately estimated by the following equation:VK > Ktf x I S x Z S Eq. 8-3whereKtf is 2 for system X/R > 20 and Ktf is 1.5 for system X/R ≤ 20This is however applicable, where CT ratios are the same, CT burdens at both sides of the line are very close, and when justone CT at each side is used. When CTs are different at both sides (different CT characteristics, CT ratio and burden) or whenmore than one CT at each side is used, these equation may not provide correct estimation of the required knee pointvoltage.For this situation GE provides a CT saturation analysis tool, described in the following section.8.1.2 CT saturation analysis toolThe CT saturation analysis tool is an Excel spreadsheet provided for users to analyze the L30 reliability during CT saturationcaused by an external fault, to investigate the effect of adjusting 87L settings, and to examine the possibility of reducingthe CT requirement.The tool utilizes the CT model and CT saturation calculation algorithm proposed by the IEEE Power System RelayingCommittee (PSRC), and it simulates the analog/digital signal processing and data calculations in the L30. The figure showsthe interface.Figure 8-1: CT saturation analysis tool main pageFor a system where the CTs are already installed, the CT and system parameters are known. Use the procedure describedin 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, anddistribution through all breakers from short circuit studies, and to analyze L30 performance, having an ability to adjustsettings as needed. The tool has all instructions to perform analysis.
