GE Digital Energy L60, Инструкция по эксплуатации

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CHAPTER 9: THEORY OF OPERATION OVERVIEW
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-13
9
• There is a time lag from the instant that a transmitter is keyed until the output reflects a change. This build up is
generally a very short time and is usually insignificant.
• There is the propagation time from the instant the transmitter sends until this signal arrives at the remote location,
approximately 1 ms for every 290 km (180 miles) of distance. The same applies from the instant the transmitter stops
until the remote signal is gone.
• There is the build up time in the receiver from the instant the signal appears at its input until the output reflects the
change of state. This time plus the build up time in the transmitter is called the channel operating time.
• There is the tail off time in the transmitter from the instant the keying is removed until the output signal changes or
disappears. This is generally very short and is usually insignificant.
• There is the tail off time in the receiver from the instant the input changes until the output changes accordingly. This
time plus the tail off time of the transmitter is called the channel release time.
• In ON-OFF channels, the operating and release times are not generally the same. They can vary with frequency and
attenuation.
• In frequency-shift channels, the discriminator employed in the receiver can be balanced so that build up and tail off
times are equal, or it can be unbalanced (biased) to the MARK or SPACE side. For example, if it is biased toward MARK
and the input signal is symmetrical (half-cycle MARK and half-cycle SPACE), the output is more than a half-cycle MARK
and less than a half-cycle SPACE.
• In general, wide-band channels tend to operate and release faster than narrow-band channels. That is, faster
channels use more spectrum than slower channels.
In short, the received signal at any given terminal is not an exact analog of the remote current. There are techniques used
in phase comparison schemes to compensate for this, as outlined subsequently. Until then, assume that the received
signal provides a true representation of the phase position of the remote current.
9.1.5.3 Types of communications media
The communication medium over which the transmitted signal is propagated to the remote receiver can take the
following forms:
•
Directly over the power line (Power Line Carrier)
• Multiplexed over the power line (Single Side Band Carrier)
• Multiplexed over microwave (Microwave)
• Pair of wires (Pilot Wire)
• Leased Facilities:
–
Metallic pilot wire
– Microwave
– Cable
A distinction is made between leased facilities and the other facilities owned by power companies because in many cases
the telephone company defines the characteristics of the channel without defining the type of link.
The ON-OFF type of communication equipment is used exclusively over power line carrier links. The transmitted signal is
propagated along the power line between the transmitter and the remote receiver. This equipment usually operates in the
frequency range of 30 to 200 kHz.
Frequency-shift equipment is available in several frequency ranges. First, there are those in the audio range. These are
generally employed over single side-band, microwave, pilot wires, and leased facilities. There are also frequency-shift
channels in the power-line carrier frequency range. These are employed directly over the power line as are the ON-OFF
types of equipment. Finally there is the frequency-shift equipment that operates in and occasionally outside the power line
carrier spectrum. These are employed over microwave and leased facilities.