GE L60 Instruction Manual page 503

9 THEORY OF OPERATION 9.1 OVERVIEW
The external fault behind one relay with a blocking scheme in a dual breaker application results in transmittal of the contin-
uous blocking signal to the remote terminal.
The received pulses may be distorted in a number of ways. Some of those distortions must be filtered out, some of them
shall be 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, the
input for this signal was a binary or status circuit that reported only a debounced or filtered true or false indication to the fol-
lowing circuits or microprocessor. In the newest design, this signal is sampled synchronously with the local AC signals
through 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 the analog
value of the receiver output status signal can be utilized to achieve the closest approach to the core phase comparison
operating principles.
The first and obvious distortion in the received signal is a time delay added by the communication channel. This must be
corrected by buffering the pulses to be aligned for time differentials with respect to the slowest remote channel. Digital tech-
nology such delays can be used in a precise and straightforward way by buffering the signal sample values in a delay
queue. Analog technologies may have difficulties in precisely delaying those signals, particularly if those signals are of vari-
able length and have other impairments.
The second possible distortion is high frequency noise embedded on the mark or space pulses. These should be left unal-
tered. The receiving relay does not have any reliable information as to the real value of the received information, and there-
fore shall not alter it based on any assumptions. The phase comparison algorithm has a well-understood security margin
due to the averaging action of the trip integrators. The integrators shall deal with this kind of noise, yielding a predictable
response that is transparent and easy to grasp by the user.
The third type of distortion is pulse asymmetry. Modern carrier sets claim to be free of this problem, but historically it has
been observed that either the mark or the space signals were extended at the receiving end compared with the originally
sent signal. Distinction between the delay and asymmetry is relatively straightforward: if the rising edges and the falling
edges of the transmit and receive signals are spaced by the same period of time, one deals with a straight delay. If the
spacing is different between the rising and falling edges is different, pulse asymmetry takes place on top of the delay. In this
case, one of the numbers is labeled as delay, and the difference with respect to the other number is labeled a pulse asym-
metry. Both need to be entered as settings in order to deal with this distortion.
The following figure presents two cases of this channel distortion. For the extended mark, the falling edge must be shifted
forward in time (accelerated). For extended spaces, the rising edge must be advanced. If not corrected, the pulse asymme-
try renders the system unusable for distortions longer than one quarter of a power cycle. This problem shows the advan-
tage of modern DSP technology. Assuming that the signal may be impaired by short lasting noise, it is very difficult to
perform this correction accurately in the analog world.
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GE Multilin L60 Line Phase Comparison System 9-29