CHAPTER 9: THEORY OF OPERATION OVERVIEWL60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-339Figure 9-24: Permissive dual-comparison scheme logic through fault conditionThe external fault behind one relay with a blocking scheme in a dual-breaker application results in transmittal of thecontinuous blocking signal to the remote terminal.The received pulses can be distorted in a number of ways. Some of those distortions must be filtered out, and some ofthem are left as received (their rectification is neither necessary nor safe).The receive information is delivered from the carrier or other receiver as a DC voltage. In prior generations of relays, theinput for this signal was a binary or status circuit that reported only a debounced or filtered true or false indication to thefollowing circuits or microprocessor. In the newest design, this signal is sampled synchronously with the local AC signalsthrough the same A/D converter controlled from the same S&H signal, and at the same high sampling rate. In this way,both the pieces of information (local AC currents, and remote phase signals) are automatically aligned in time, and theanalog value of the receiver output status signal can be utilized to achieve the closest approach to the core phasecomparison operating principles.The first and obvious distortion in the received signal is a time delay added by the communication channel. This must becorrected by buffering the pulses to be aligned for time differentials with respect to the slowest remote channel. Digitaltechnology means that such delays can be used in a precise and straightforward way by buffering the signal samplevalues in a delay queue. Analog technologies can have difficulties in precisely delaying those signals, particularly if thosesignals are of variable length and have other impairments.The second possible distortion is high frequency noise embedded on the mark or space pulses. These are left unaltered.The receiving relay does not have any reliable information as to the real value of the received information, and therefore isnot to alter it based on any assumptions. The phase comparison algorithm has a well-understood security margin due tothe averaging action of the trip integrators. The integrators deal with this kind of noise, yielding a predictable response thatis transparent and easy to grasp by the user.
