GE L60 Instruction Manual

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GE L60 Instruction Manual Manual pdf 527 page image

GE Multilin L60 Line Phase Comparison System 9-19 APPLICATION OF SETTINGS 9.1 PHASE COMPARISON ELEMENT 87PC99 APPLICATION OF SETTINGS 9.1PHASE COMPARISON ELEMENT 87PC 9.1.1 DESCRIPTIONThe L60 Phase Comparison relay is designed to provide high-speed protection of transmission lines against all phase andground faults when operated in the “mixed-excitation” mode. The term “mixed-excitation”, when applied to phase compari-son, describes a scheme that first mixes different sequence quantities in a given proportion and phase angle, then performsa phase-comparison based on this mix.A complete explanation of the Phase Comparison Element operation principles can be found in Chapter 8.9.1.2 USE OF SETTINGS87PC SIGNAL:A mixed I_2 – K × I_1 signal, mixed I_1 + I_2 / K signal, or 3I_0 signal can be chosen as the operating signal for FDH andFDL excitation. In mixed excitation mode, the relay provides high-speed protection of transmission lines against all phaseand ground faults. However, if a user wants the relay to operate only during ground faults, the 3I_0 mode can be chosen.87PC MIXED SIGNAL K:The K factor must be chosen for the mixed excitation operating signal. As indicated in chapter 8: Theory of operation, bestresults are obtained using a value of 0.2 (the default setting). The selected K value can range from 0.00 to 0.25. SettingK = 0 makes a phase comparison on the basis of negative-sequence excitation only. In such a scheme, the relay protectsagainst all unbalanced faults; a suitable phase-distance relay should be used to protect against three-phase faults.The user must remember that K is an important tool to set FDL and consequently FDH at the lower setting, especially in thecases when the margin between the maximum load current and the minimum fault current is very small. Reducing K to 0.15or 0.10 makes phase comparison protection less sensitive to load current which in turn allows the user to provide enoughsensitivity to the fault current. From the other hand it makes protection less sensitive to the balanced three-phase faultwhich in fact occurs very rarely.87PC FDL PICKUP:The main function of FDL is keying the transmitter. FDL pickup must be set above the K × I_1 output of the mixing networkfor the maximum expected load. The recommended FDL setting is as follows;• FDL = 1.1 × K × I1L where I1L is the maximum line load current and K is a mixed signal factor as described above.Higher margin may be required to definitely avoid FDL pickup during normal load condition.• If the 3I_0 operating signal has been chosen, FDL should be set as FDL=1.1 ⋅ I1L where I1L is a maximum line loadcurrent.It must be noted that in some cases a channel may also perform other functions if it is objectionable to key the transmitterconstantly. In such cases, FDL could be set well below the K × I_1 value of the mixing network resulting from maximumload but not less than 0.05 pu which defines the minimum required current from current transformers.87PC FDH PICKUP:The main function of FDH is to permit tripping as it arms the tripping output. FDH pickup must be set high enough so that itwill not operate on maximum load. Also FDH must be set high enough to reset itself in the presence of heavy loads follow-ing clearing an external fault. The recommended setting for FDH is as follows:FDH = (4/3) × FDL + 0.375 × Ic1where Ic1 is total positive-sequence charging current under normal conditions.A distance relay is recommended as an external fault detector if the minimum internal three-phase fault is less than twicethe maximum load current. It allows coincidence detector to start comparing the local and received signals and to make tripdecision if FDH is not picked up.If 3I_0 operating signal has been chosen, FDH should be set at most 0.66 times but preferably 0.5 times the minimum inter-nal ground fault current to provide reliable sensitivity of the fault.87PC FDL AUX:This setting is provided for cases when sensitivity of the built-in FDL operating on the I_2 – K × I_1 quantity is not sufficient.This can happen on the weak terminal of the heavily loaded line. FDL AUX is connected in parallel with built-in FDL faultdetector and is intended to be assigned with elements such as distance, negative-sequence overvoltage and overcurrent,