CHAPTER 9: THEORY OF OPERATION OVERVIEWL60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-219While, as a general rule, high-speed operation and security are opposite considerations, it is possible to design dual phase-comparison schemes that can provide the added speed with little or no loss in security. However, these schemes aresomewhat more complex than equivalent single phase-comparison schemes. The following illustrates the dual phase-comparison tripping scheme that is the counterpart of the single phase-comparison scheme. The differences are asfollows:• The dual scheme uses two separate comparer integrator combinations, one for the positive half-cycle and the otherfor the negative half-cycle.• A three-frequency, frequency-shift channel is used in dual phase comparison. The high-shift operates in conjunctionwith the positive half-cycle while the low-shift works with the negative half-cycle. When the channel is not keyed toeither high or low, it operates on the center frequency. There is no center frequency output from the receiver into therelay tripping logic.• AND3 is included to make it impossible to key both frequencies simultaneously. It also gives preference to the low-shiftthat is sent continuously when FDL is dropped out. Thus, on single-end feed tripping can take place only on thenegative half-cycle.Figure 9-13: Dual phase-comparison tripping schemeThe center frequency, while not actually used in the relay tripping logic, adds security to the scheme during transientconditions.
