Powerline Carrier System Reliability in Industrial as well as Residential Applications.

CEPCO's Technology features produce Powerline Carrier systems that are proven to be every bit as reliable as direct wire systems, with the added advantages of being much more flexible, and much less costly.

Powerline Carrier Technology, the technique of transmitting signals over the existing AC Powerlines, has held a lot of promise for Security, Event Monitoring, Remote Control, and Energy Management applications. This technology uses the existing AC Power wiring for the transmission of control, status, or emergency type signals from point-to-point within a building, or from one building to another (as long as they share a common utility transformer), without the expense and inconvenience of adding new wires.

Powerline Carrier systems have been limited in their applications because of the large amounts of electrical noise and transients found on the powerlines, particularly in commercial and industrial applications, where a 3-phase power distribution system is used.

Cepco Products Powerline Carrier offers unique solutions to these problems using Zero Crossing Digital Pulse Modulation and Consecutive Pulse count Coding technologies, that are covered by United States, British, and Canadian Patents, with other patents pending. The key to the success of this technology is in the fact that it allows for the economical inclusion of such features as:

• Noise Detection and Evasion
• Time Division Multiplexing
• Multiple Code Word Commands
• 3-Phase power system distribution system operation
  (Industrial and Commercial Applications)

These features are combined in Cepco Products Powerline Carrier, to produce systems that are every bit as reliable as direct wire systems, with the added advantages of being more flexible, and much less costly.

Zero Crossing Digital Pulse Modulation

In the Zero Crossing Digital Pulse Modulation process, the AC Powerline is modulated by placing a high level pulse at the zero crossing of the AC wave, forming a 120 pulse per second pulse train. The amplitude of these pulses is in the order of 100 volts for 120 vac operation. This modulation process offers the following features that are important to reliable system operation.

First, the signal pulses are, for the most part, isolated from the AC Powerline noise and transients since these are not normally present at the zero crossings.

Second, the large amplitude of the modulating pulses insure that the signal is not masked by the AC Powerline noise and transients.

Third, since the zero crossing is a stable and well defined point on the AC wave, it allows system synchronization to facilitate such features as Noise Detection and Evasion and Automatic Multiplexing for the simultaneous operation of multiple Transmitters without interference.

Consecutive Pulse Count Coding

In the Consecutive Pulse Count Coding technique, the code information is contained in the number of consecutive pulses transmitted before a "missing pulse". By missing pulse we mean a zero crossing on the AC wave that does not contain a pulse. For example, 40 consecutive pulses might be code #1, and 41 consecutive pulses might be code #2. Then to transmit code #1 followed by code #2, a modulation pulse would be placed at 40 consecutive zero crossings (code #1), and then at least one zero crossing would be skipped (missing pulse), and then a modulation pulse would be placed at 41 consecutive zero crossings (code #2).

From this we see that each code is a pulse train with a precise number of consecutive pulses. This means that increasing or decreasing the length of the pulse train is the only way alter the code. To increase the length of the pulse train, pulses must be added to either the beginning or end of the pulse train, and to decrease the number of pulses in the pulse, one or more of the existing pulses would have to be eliminated.

At the Receiver, a zero crossing "gate" and level detector are used to reject all pulses that are not at the zero crossing or of sufficient amplitude. As far as the Receiver is concerned, a pulse is any perturbation of either polarity (plus or minus) that exceeds the threshold. When you consider the amplitude of the signal pulses is on the order of 100 volts, with several peaks caused by the "ringing" on the AC Powerline, the likelihood of a pulse being canceled by noise or transients is nonexistent.

In summary then, the key features of the Consecutive Pulse Count Coding are that each code is of a different length, that is, contains a different number of pulses. The code is altered only by the addition of one or more pulses at the beginning or end of the pulse train. Noise or transients that occur in the middle of the pulse train have no effect on the code.

Transmitter Operation

In order to insure that no pulses are added to the beginning of its code, each transmitter continuously monitors the AC Powerline with the same zero crossing gate circuitry that is contained in the Receivers. When it detects a pulse at the zero crossing, it generates a "busy" signal that inhibits the start of its transmission. This means that the transmitter will delay (in increments of 8.33 milliseconds) its transmission until it detects a zero crossing that contains no pulse, thereby insuring that no extra pulse is added to the start of its code. In this way it is able to evade noise or transients, or pulses from other transmitters, that have the potential of adding pulses to the start of its own code. In addition, this feature allows multiple transmitters to avoid interfering with each other. This leaves only the possibility that the code can be altered by the addition of one or more pulses to the end of the pulse train. In order to eliminate the possibility of having a Receiver interpret a pulse added to the pulse train as a valid command, Multiple Code Word Commands are used.

Multiple Code Word Commands

Multiple Code Word Commands means that the receiver is required to receive at least two (2) identical code words within a one second period before it interprets that code word as a valid command.

To show how extremely effective this requirement is in reducing false commands, consider the following example. Assume that the electrical powerline on which we are operating is so noisy that with a single code word command, we can expect one false command to occur each day (24-hour period). From probability theory, we can the calculate the expected false command rate when we require each command to contain two identical code words within a one-second interval.

There are 86,400 one-second intervals in one day (24 hours). From the theory of probability, the probability of two false code words occurring in a one-second interval is:

This shows that as a result of requiring two identical code words in a one second period, we went from a false command rate of one per day, which could not be tolerated, to a false command rate of one in over 237 years.

Time Division Multiplexing

The coding technique and transmitter operation described above allows multiple transmitters to automatically synchronize themselves and to avoid interference from AC Powerline noise and transients, and from each other. As described previously, each transmitter will delay its transmission until it detects a missing pulse, thereby insuring that no other transmitter is signaling. This amounts to Automatic Time Division Multiplexing, which allows each transmitter to transmit in a separate time slot without interference from other transmitters.

Operation On a 3-Phase Power Distribution System

Commercial facilities (factories, institutions, hospitals, office buildings, hotels/motels, etc.) almost always have 3-phase power distribution systems (120V/208V). In a 3-phase power distribution system the times at which the zero crossing occur is different from phase-to-phase. The figure below shows the voltage waveshapes of a 3-phase power distribution system. Note that the point at which the voltage wave crosses zero occurs 120 degrees later on the 2nd phase than on the 1st phase, and 240 degrees later on the 3rd phase. From our discussions on powerline carrier reliability, one of the keys to reliable operation was the placement of the signal pulses at the zero crossings.

The 3 Phase Coupler/Repeater, Model CPR-3, is used to accomplish this when operating on a 3-phase system. Referring again to the 3-phase waveshapes, when the Transmitter is on the 1st phase, it places its pulses at the zero crossings of that phase, and the 3 Phase Coupler/Repeater will repeater the exact pulse count at the zero crossings of the other two phases (with the appropriate delay).

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