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In the SPI Power Line Carrier (PLC) Lighting Control System PLC signalling is used to program individual fluorescent lamp ballasts and to control the dimming and on/off status of software defined groups of these ballasts. It is essential that this signalling be reliable.
The PLC signalling frequency used in the SPI lighting control system is nominally 127 kHz. The PLC receiver is sensitive to interference in the range 112 kHz to 142 kHz.
The issue of PLC communications reliability reduces to the question: "What measures are required to adequately suppress interference in the frequency range 112 kHz to 142 kHz?"
The sources of interference to PLC communications can be divided into three categories.
Low Amplitude Broad Band Noise There are numerous sources of broad band low amplitude 112 kHz to 142 kHz noise on the power line. Examples of these sources are the switched mode power supplies used in electronic fluorescent ballasts and personal computers. Typically the low amplitude broad band noise on the breaker panel electrical bus is about 25 mV RMS / (30 kHz)^0.5 @127 kHz. The Systel PLC receiver has a 30 kHz bandpass and hence, if the noise is unfiltered, captures about 25 mV RMS of this low amplitude broad band noise.
High Amplitude Broad Band Noise High amplitude broad band noise is produced by equipment involving power inverters. Inverters are used in variable speed fan and pump motor drives, inverter type UPS and standby power systems and microturbine based auxilliary power systems. Broad band noise voltage amplitudes of over 100 mV rms / (30 kHz)^0.5 @ 127 kHz have been routinely observed and a broad band noise voltage as high as 400 mV rms / (30 kHz)^0.5 may occur. One of the complications with power inverters is that the electrical noise power near the Premium Circuit Filter and the Super Premium Circuit Filter resonant frequencies (~7 kHz) is sufficient that heat dissipation within the filters is a performance limiting issue.
Narrow Band Interference Other PLC systems often produce narrow band interfering signals of 1 to 3.6 V RMS within the Systel PLC receiver band pass.
The mathematical development used by Systel and the filter designs proposed by Systel implicitly assume that interference from power inverters and from other PLC systems can be ignored. However, inverter type power systems are becoming increasingly more common and interference between different PLC systems is rapidly becoming a major issue. Hence, successful implementation of PLC systems is dependent upon directly addressing this matters.
POWER INVERTERS
A lighting circuit breaker panel may be directly powered from a power inverter or the circuit breaker panel may be switched from grid power to inverter power after a grid power failure occurs. Sometimes the inverter provides supplementary power so that in effect it is grid connected. If the inverter provides AC power during a grid power failure, generally the voltage harmonic content will be highest when the inverter is not connected to the grid. It is essential to measure the worst case voltage harmonic output in such circumstances to ensure compliance with the SPI equipment specifications.
OTHER PLC SYSTEMS
SPI and Xylene have identified the following major sources of interference from other PLC systems:
1. Time-of-Use Electricity Meters
2. Energy Management, Environmental Control and Security Systems
3. Other Lighting Control PLC Systems
4. Broadband PLC Internet delivery systems
TIME-OF-USE ELECTRICITY METERS
The government of Ontario has legislated requirements to the effect that by 2010 all the end use electricity meters in the Province shall be replaced by time-of-use meters. The time-of-use meters record the kWh consumption in each 15 minute time interval and transmit this data to the Public Utility Corporation (PUC). Generally all the meters on the secondary side of a major power distribution transformer send their data via line carrier to a data collection unit, that in turn transmits the data to the PUC via a telephone line. Each 15 minute interval for each meter requires two bytes to identify the time interval and two bytes to identify the consumption. There are additional overhead bytes related to meter identification, signal acknowledgement, time clock synchronization, error correction, etc. Typically 400 kWh meters report to a single data collection unit. Consequently, in some areas remote meter reading causes a lot of PLC traffic. A major supplier for this market is Quadralogic Technology. Their standard equipment transmits 4 watts in the frequency range 10 to 90 kHz. However, due to channel traffic, sometimes this remote meter reading equipment also transmits in the frequency range above 110 kHz.
ENERGY MANAGEMENT, ENVIRONMENTAL CONTROL AND SECURITY SYSTEMS
PLC signalling is widely used as a medium of communication in low cost retrofit Energy Management, Environmental Control and Security Systems. These systems, which often involve some type of polling for communication security, generate almost continuous PLC traffic. Manufacturers such as Echelon provide modular PLC transceivers that are widely used in this market. These transceivers output 2.5 volt RMS signals and are available for carrier frequencies from 95 kHz to 140 kHz. To meet FCC approval criteria these transceivers are normally operated in the 125 kHz to 140 kHz band. Some Echelon equipment generates up to 3.6 V rms signals with a 2 amp p-p current limit.
OTHER LIGHTING CONTROL PLC SYSTEMS
SPI markets another PLC system manufactured by Cristal that is primarily used for outdoor lighting control. A feature of this system is that it reports defective lamps to a central monitoring station. This system was originally developed for control and monitoring of municipal street lights, but is now being applied to other applications such as parking lot and security lighting. Since the outdoor lights are generally connected to the same distribution transformers as other loads, this system is an ongoing source of PLC interference. The no load transmitted signal amplitude is about 2.7 V rms at a frequency of about 131 kHz. With typical low impedance distribution panels the output voltage falls to about 1.4 V rms. This system must co-exist in the same buildings as Systel equipment without the two systems interfering with one another.
BROAD BAND PLC INTERNET DELIVERY SYSTEMS
A situation has arisen in Ontario that may lead to continuous PLC interference in many buildings. Canadian telephone companies are required to provide telephone service to all customers in their monopoly market areas at set rates. Cable television companies are not bound by the same rules and are permitted to deny service to customers who are in locations such that the cable TV company does not deem them to be profitable to connect.
With the advent of high speed Internet, to keep the telephone companies competitive with the cable companies the telephone companies were permited to deny high speed Internet to customers located in expensive to service locations. The Canadain Radio and Telephone Commission has required that all new fibre optic cable be burried. The Ontario government has legislated a wide "green belt" around Toronto in which no new development is permitted.
The net result of these various rules is that surrounding Toronto is a wide belt of customers who want high speed Internet but cannot get it because it is not economically viable for either the telephone company or the cable television company to provide it. There are a few small companies offering low power microwave Internet connections, but these connections are line of sight and do not work through trees, hills and other obstructions. They also frequently do not work in bad weather.
Hence there is a major demand for high speed Internet in the Greater Toronto Area suburbs by those that are unable to get it via the aforementioned means.
To address this market Toronto Hydro, IBM and others are experimenting with Internet delivery via the electric power grid. The technologies used are not FCC compliant and threaten to generate broad band interference in much of the HF and VHF radio spectrum. However, the political demand for high speed Internet in the Greater Toronto Area suburb communities is huge.
From the point of view of Systel technology the problem is the connection from the utility pole to the subscriber premises. These connections are often via direct burried power cable that will not support HF or VHF frequencies. These connections will likely operate in the 110 to 200 kHz frequency range in order to be compatible with existing and planned remote meter reading equipment.
Most power distribution transformers have isolated primary and secondary windings. Unless a distribution transformer is intentionally bypassed it acts as a low pass filter. A PLC signal on the primary winding does not pass to the secondary winding and vice-versa. Thus, power distribution transformers limit the propagation of PLC signals and interference to PLC communication usually originates on the side of the power distribution transformer to which the PLC system is connected.
A large building is often served by a large dedicated transformer that reduces the utility high voltage down to 120/208 V, 277/480 V, 240/416V or 347/600 volts. In such a building interference to PLC communication usually originates somewhere within the building or its outdoor lighting. Similarly, a large building may contain a multiplicity of small power distribution transformers that reduce 480 or 600 volts down to 120 volts. Interference to communication by PLC equipment that is connected to the secondary side of one of the small power distribution transformers usually originates from another device that is also connected to the secondary side of that same transformer.
Interference to PLC communication is suppressed by the use of Circuit Filters. To achieve communications reliability SPI Circuit Filters must be installed between the circuit breakers and the branch lighting circuits.
Circuit Filters are available in Standard, Premium and Super Premium types. In order to market a PLC Lighting Control System it is essential to have a simple guide for determining the type of Circuit Filters that are required for a particular application.
GUIDE:
Standard Circuit Filters Standard Circuit Filters can be used if all of the following statements are true:
a. The broad band noise level on the circuit breaker panel electrical bus is always less than 100 mV RMS / (30 kHz)^0.5 at 127 kHz;
b. The length of the branch lighting circuit is less than 200 m;
c. All electrical loads connected to the power distribution transformer are under the direct control of the purchaser;
d. There is no other PLC equipment connected to the secondary of the power distribution transformer;
e. The purchaser guarantees in writing that no other equipment that generates PLC frequency signals or interference will be connected to the secondary of the power distribution transformer at any time in the future.
f. There is no present or future possibility of time-of-use metering or Internet via powerline.
g. There is no present or future requirement for a PLC based energy management, environmental control or security system.
Note that a broad band noise voltage amplitude of 100 mV RMS / (30 kHz)^0.5 is about four times larger than the noise voltage amplitude that is normally encountered in commercial buildings and is usually caused by equipment containing a power inverter.
Premium Circuit Filters Premium Circuit Filters can be used if all of the following statements are true:
a. Any other PLC equipment connected to the same power distribution transformer secondary transmits at less than 3.6 V rms. Note that SPI is not presently aware of any FCC approved PLC signal sources that transmit at greater than 3.6 V rms;
b. The length of the branch lighting circuit is less than 200 m;
c. The broad band noise level at 7.12 kHz is less than 3.87 V rms / (4.78 kHz)^0.5.
Super Premium Circuit Filters Super Premium Circuit Filters can be used if all of the following statements are true:
a. Any other PLC equipment connected to the same power distribution transformer secondary transmits at less than 5V rms.
b.The length of the branch lighting circuit is less than 300 m;
c. The broad band noise level at 7.12 kHz is less than 3.87 V rms / (2.39 kHz)^0.5.
SPI does not recommend its equipment for application to branch lighting circuits that are longer than 300 m or where the broad band noise level at 7.12 kHz is greater than 3.87 V rms / (2.39 kHz)^0.5.
A noise level of greater than 3.87 V rms / (2.39 kHz)^0.5 may be indicative of defective or regulatory non-compliant power inverter equipment.
For PLC signalling to work reliably the amplitude of the desired PLC signal at the PLC receiver must be at least 8 dB higher than the amplitude of electrical noise within the PLC receiver bandpass. The sources of electrical noise are those set out above and the electronic ballasts connected to the load side of the Circuit Filter.
A signal voltage Vs to noise voltage Vn ratio of 8 dB implies that:
20 log (Vs/Vn) = 8
Thus log (Vs/Vn) = .4
Hence Vs/Vn = 2.51.
Assume that the broad band electrical noise produced by electronic ballasts that are connected to the branch lighting circuit is << 50 mV rms /(30 kHz)^0.5.
If properly applied, the Circuit Filters reduce the maximum amplitude of the external noise captured by a PLC receiver to less than 50 mV rms.
Then, to realize an 8 dB margin the signal received by a PLC receiver located close to the Circuit Filter must be about:
2.51 X 50 mV rms = 125 mV rms
In simple terms, the Standard Circuit Filter must attenuate an interfering voltage on the breaker Panel bus by a factor of 100 mV/50 mV = 2 and a Premium Circuit Filter must attenuate an interfering voltage on the Breaker Panel bus by a factor of:
3.6 V / .05V = 72
Assume that the PLC signal is transmitted from the far end of the branch lighting circuit at an amplitude of 2.5 V RMS. Then the maximum allowable signal attenuation over the length of the branch lighting circuit is:
20 log (2.5 / .125) = 26 dB.
In simple terms the received signal voltage at one end of the lighting circuit must be at least 1/20 of the transmitted signal voltage originating at the other end of the lighting circuit.
The attenuation by the Circuit Filter and the attenuation by the extended lighting circuit are parameters that are easy to understand and simple to measure. These parameters allow definition of a region in which Systel and other PLC technologies will operate reliably while connected to the same power distribution transformer secondary.
When the branch lighting circuit is long the value of its impedance Zb will fall to a limiting value that determines the ultimate rejection by the Circuit Filter of interference from the electrical bus.We need to experimentally determine this limiting value.This limiting value of impedance, in combination with the largest value of the electrical bus interference from external sources and the propagation loss per unit length of branch lighting circuit determines the ultimate capability of the system.
As the number of ballasts connected to a branch lighting circuit increases the noise voltage generated by these ballasts increases. At some point the ballast produced noise voltage may exceed the coupled noise voltage from the electrical bus. It is necessary to experimentally measure the signal attenuation along the branch lighting circuit and the ballast produced noise in order to quantitatively understand these phenomena, so as to establish the ultimate region of operation.
This web page last updated September 20, 2005
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