GE L60 Instruction Manual

Also see for L60: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

9-20 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUALOVERVIEW CHAPTER 9: THEORY OF OPERATION9the selection between a blocking and a tripping scheme generally results from a compromise between security andreliability. In order to make such a selection, consideration of the pilot pair, its protection, and its physical location inrelation to power conductors must be evaluated.In general, a high speed channel requires pilot wires that have a frequency response that is somewhat better than thestandard telephone voice circuits.Possibly because of the uncertainties of channel characteristics, plus the availability of pilot wire relays that are muchlower in overall cost, phase comparison over privately owned pilot wires is not a common application.9.1.5.7 Leased (telephone company) facilitiesThere has been some use of phase comparison relaying over leased facilities including voice grade pilot wire circuits. Ingeneral, if a customer requires or specifies the characteristics of a leased channel, the local telephone or cable companycan provide this link over microwave, cable, even pilot wires, or a combination of these. In such cases, the selectionbetween tripping and blocking schemes depends on the performance of the channel as specified. The same basicschemes of the previous two figures apply.9.1.5.8 Fiber opticsFiber optic links are commonly used for protective relaying schemes. Since they propagate through the fiber rather thanover the power line, they are generally unaffected by faults and noise on the power system. Thus, with a fiber link there isno problem getting a signal through the fault, so tripping type schemes are very acceptable. An exception occurs when thefiber optic is embedded in the ground wire used on the line. In this case, the fault can be a result of a break in the groundwire, which prevents transmission of the signals.In any case, there is nothing about a fiber channel to alter the previous discussion concerning phase comparisonprotection. The same schemes can be used, with the understanding that the fiber optic signal can be lost on occasion.Phase comparison relaying schemes over fiber optic channels mostly are of the tripping types.9.1.5.9 Conclusion of blocking versus tripping schemesImplementations tend to be hybrids of blocking and tripping schemes. Thus, the following simple definitions exclude anyconsiderations of channel status features:• A blocking scheme is one that requires a specific output signal from the associated receiver in order to block tripping.Tripping can only take place during the time that this signal is absent.• A tripping scheme is one that requires a specific output signal from the associated receiver in order to permit tripping.Tripping can only take place during the time that this signal is present.• Where channel status logic is used, these definitions have to be modified to meet the exact logic of the schemeIn general, the selection of a blocking or a tripping scheme is one that needs to be made in conjunction with the chosenchannel and with a knowledge of the channel characteristics in the face of system noise. Many different combinations arepossible, but of these, only a selected few meet any given requirements.9.1.6 Single versus dual phase comparisonIn all the phase comparison schemes described so far, a trip attempt is made only every other half cycle. In the examplesillustrated, this was every positive half-cycle. Such schemes are termed single phase-comparison as against dual phase-comparison where a trip attempt is made every half-cycle, positive and negative.The only advantage of dual-comparison is that its maximum operating time to trip on internal faults is a half-cycle fasterthan the maximum time for the single phase-comparison. The minimum times for both schemes is the same. Thisdifference in maximum time results because a fault can occur at such an instant in time when the current is just goingnegative. Under such conditions, the single phase-comparison has to wait until the next positive half-cycle, while the dualphase-comparison can trip on the upcoming negative half-cycle.