GE L90 Instruction Manual

Also see for L90: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

8-14 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALDISTANCE BACKUP/SUPERVISION CHAPTER 8: APPLICATION OF SETTINGS8Three terminal line applications generally result in larger reach settings for the distance backup and require a calculationof the apparent impedance for a remote fault. Carry this out for each of the three terminals, as the calculated apparentimpedance differs at each terminal.Distance Supervision essentially offers a solution for the LV fault condition, but the differential setting must still beincreased to avoid operation for an external L-g or L-L-g fault external ground fault. In addition, the distance elementreach setting must still see all faults within the protected line and be less than the impedance for a LV bus fault.The effective source impedance ratio (SIR) for the LV fault generally is not high, so that CVT transients do not contribute tomeasuring errors.If the distance supervision can be set to avoid operation for a transformer LV fault, then generally the filtering associatedwith the distance measuring algorithm ensures no operation under magnetizing inrush conditions. The distance elementcan be safely set up to 2.5 × V nom / I peak , where V nom is the system nominal voltage and Ipeak is the peak value of themagnetizing inrush current.For applications where the tapped station is close to one terminal, then it can be difficult to set the distance supervision toreach the end of the line, and at the same time avoid operation for an LV fault. For this system configuration, use a three-terminal L90; the third terminal is then fed from CT on the high side of the tapped transformer.8.4.2 Phase distance8.4.2.1 Phase current supervision and the fuse failure elementThe phase-to-phase (delta) current is used to supervise the phase distance elements, primarily to ensure that in a de-energized state the distance elements are not picked up due to noise or induced voltages on the line.This supervision feature can also be employed to prevent operation under fuse failure conditions. This requires that thesetting be above maximum load current and less than the minimum fault conditions for which operation is expected. Thispotential problem can be avoided by the use of a separate fuse fail function, which means that the phase currentsupervision can be set much lower, typically two times the capacitance charging current of the line.The usage of the fuse fail function is also important during double-contingency events, such as an external fault duringfuse fail conditions. The current supervision alone does not prevent maloperation in such circumstances.Keep in mind that the fuse failure element provided on the L90 needs some time to detect fuse fail conditions. This cancreate a race between the instantaneous zone 1 and the fuse failure element. Therefore, for maximum security, it isrecommended to both set the current supervision above the maximum load current and use the fuse failure function. Thecurrent supervision prevents maloperation immediately after the fuse fail condition giving some time for the fuse failureelement to take over and block the distance elements permanently. This is of a secondary importance for time-delayedzones 2 and up as the fuse failure element has some extra time for guaranteed operation. The current supervision can beset below the maximum load current for the time delayed zones.Blocking distance elements during fuse fail conditions are not be acceptable in some applications and/or under someprotection philosophies. Applied solutions vary from not using the fuse failure element for blocking at all, to using it andmodifying—through FlexLogic and multiple setting groups mechanisms—other protection functions or other relays toprovide some protection after detecting fuse fail conditions and blocking the distance elements, and to using it andaccepting the fact that the distance protection does not respond to subsequent internal faults until the problem isaddressed.8.4.2.2 Phase distance zone 1As typically used for direct tripping, the zone 1 reach must be chosen so that it does not extend beyond the far end(s) of theprotected line. Zone 1 provides nominally instantaneous protection for any phase fault within a pre-determined distancefrom the relay location. To ensure that no overreach occurs typically requires a setting of 80 to 90% of the line length,which covers CT and VT errors, relay inaccuracy and transient overreach, as well as uncertainty in the line impedance foreach phase, although transposition can minimize this latter concern.To be fully operational, the Fuse Failure element must be enabled, and its output FlexLogic operand must beindicated as the blocking signal for the selected protection elements.For convenience, the current supervision threshold incorporates the factor.