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A Super Premium Circuit Filter is used in applications where a Premium Circuit Filter is insufficient. Generally a Super Premium Circuit Filter is used in applications where the branch lighting circuit length is over 200 metres or where there is PLC equipment that is not FCC approved connected to the same power distribution transformer secondary.
Super Premium Circuit Filters have the disadvantages that they cost more than Premium Circuit Filters and they absorb power at their filter resonant frequency (~7 kHz).
A Super Premium Circuit Filter increases the range and reliability of the PLC communications for lighting control, prevents PLC signals on one branch lighting circuit interfering with PLC signals on another branch lighting circuit, prevents the PLC lighting control signals from interfering with other PLC systems, and prevents other PLC systems interfering with the PLC Lighting Control System.
A Super Premium Circuit Filter functions by preventing PLC band signals from passing through it in either direction. It minimizes lighting control PLC signals that are impressed upon the circuit breaker panel electrical bus; it prevents other PLC signals and electrical noise propagating from the circuit breaker panel electrical buses to branch lighting circuits; and it prevents a low circuit breaker panel electrical bus impedance from attenuating the lighting control PLC signals on the branch lighting circuits.
Pictorial diagram for assembly of a 3 phase Super Premium Circuit Filter.
A Super Premium Circuit Filter is similar in design to a Premium Circuit Filter. However, the Super Premium Circuit Filter has larger inductors and smaller capacitors. The purpose of increasing the inductance is to reduce the transfer function at PLC frequencies. The purpose or reducing the capacitance is to keep the filter resonant frequency close to the filter resonant frequency of the Premium Circuit Filter. It is desirable not to let the filter resonant frequency drop because that will cause more problems with power absorption from low frequency inverter harmonics.
In the Super Premium Circuit Filter the size of the components is primarily limited by component cost and availability considerations. The inductor value choice for 120 volt lighting circuits is:
La = Lb = Lc = Ln = 100 uH.
The inductors must be rated for 15A or 20A depending on the circuit breaker size.
The capacitor value choice for 120 volt lighting circuits is:
Ca = Cb = Cc = 5 uF. This capacitance provides a self resonant frequency close to 7 kHz, which is far removed from the PLC receiver bandpass.
The capacitors must be rated for continuous use with AC. A 600 to 1000 volt AC continuous rating is suggested, although a 250 volt AC continuous rating may be sufficient for a 120 volt lighting circuit.
Dry film capacitors are used to avoid the use of oil filled capacitors. The issue is that over time the oil might deteriorate and become a fire hazard. The capacitors must be engineered such that their effective series resistance and distributed inductance are negligibly low below 500 kHz.
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The maximum 127 kHz band noise coupling from the circuit breaker panel electrical bus to the branch lighting circuit occurs when the branch lighting circuit is short and contains only a few ballasts, so that its impedance is relatively high. Let Zb be the impedance of the branch lighting circuit.
At 127 kHz, for L= 100 uH the term 2WL has a value of:
2WL = 2 X 6.28 X 1.27 X 10^5 X 1.0 X 10^-4 = 159.5 ohms
Consider a branch lighting circuit that is relatively short and that consists of three connected two lamp ballasts:
Information provided by Systel indicates that the impedance of one two lamp ballast at 100 kHz is about 380 ohms inductive, so three ballasts in parallel will give an impedance at 100 kHz of about 126.67 ohms inductive. The corresponding inductance is:
126.7 / (6.28 X 10^5) = 200 uH
At 127 kHz this impedance becomes 1.27 X 126.67 = 160.86 ohms and makes:
Zb ~ 2jWL. Hence, the transfer function simplifies to:
T = (R - (j/WC))/[4(R-(j/WC)+jWL)-(R-(j/WC))]
or
T = (R - (j/WC))/[3R-3(j/WC)+4jWL]
At high frequencies T is inversely proportional to frequency. However, this transfer function has a low filter resonant frequency Wr given by:
(3/WrC) = 4WrL
or
Wr^2 = 3/(4LC)
or
Fr = (1 / 6.28)[3 /(4LC)]^0.5
For L = 100 uH, C = 5 uF:
Fr = 6.17 kHz
At the filter resonant frequency the transfer function simplifies to:
T = (R - (j/WrC))/[3R]
In order to prevent the PLC receiver clipping it is necessary to limit the maximum magnitude of the Premium Circuit Filter output voltage. To achieve this objective limit |T| to unity. Then:
1^2 = (R^2 + (1 / WrC)^2) / 9R^2
or
8R^2 = (1/(WrC)^2)
or
R = (1/WrC)(1/8)^0.5
Thus for L = 100 uH and C = 5 uF, R is given by:
R = 1.82 ohms
Choose 1.5 ohms, 10 watts as a reasonably close standard value.
Recall that the transfer function for Zb = 2jWL is:
T = (R - (j/WC))/[3R-3(j/WC)+4jWL]
Evaluate this transfer function at 127 kHz:
At 127 kHz:
1/WC = .251
4WL = 319.02
so
T = (1.5 - .251j)/[4.5-.753j+319.02j]
= (1.5 - .251j)/[4.5 + 318.27j]
|T| ~ 1.5 / 318.27 = .00471
Hence, for a load consisting of 3 two lamp ballasts the Super Premium Circuit Filter reduces the noise voltage coupled from the circuit breaker panel electrical bus to the branch lighting circuit by a factor of .00471. For design purposes it is convenient to impose the requirement that there are always at least three two lamp ballasts connected to a branch lighting circuit.
Note that if there is a 3.5 volt RMS interfering signal on the electrical bus at the low frequency end of the PLC receiver bandpass this interfering signal is reduced to:
3500 mV X .00471 = 16.5 mV.
Assume the worst case which is a branch lighting circuit that is connected to the same phase of a lighting circuit breaker panel as is a high impedance load that is sensitive to the PLC signal. Assume the worst case that all the other connected electrical loads are off. Then Zb = infinity and the transfer function from the lighting circuit to the circuit breaker panel electrical bus is given by:
T = (R - (j/WC))/[2(R-(j/WC)+jWL)-(R-(j/WC))]
At PLC frequencies this transfer function simplifies to:
T = R / 2jWL
= 1.5 /(2 X 6.28 X 1.27 X 10^5 X 10^-4)
= .00940
Assume that the maximum PLC signal is 2.5 V rms. Then the maximum possible noise impressed on the breaker panel bus by the PLC signal becomes:
2500 mV X .0094 = 23.5 mV.
In order to communicate reliably Systel requires a signal voltage Vs to noise voltage Vn ratio of 8 dB. ie 20 log (Vs/Vn) = 8. Thus log (Vs/Vn) = .4. Hence Vs/Vn = 2.51.
If the circuit breaker panel electrical bus has 3500 mV RMS of interference within the PLC receiver's passband, then with 3 connected ballasts about 16.5 mV of this interference is coupled onto the branch lighting circuit. In this case the minimum received signal necessary to communicate via PLC over the branch lighting circuit is given by:
2.51 X 16.5 mV = 41.41 mV rms.
Thus a transmitted PLC signal level of 2.5 volts RMS allows for an attenuation factor of about:
2500/41.41 = 60
or a 20 log(60) = 35.56 dB signal attenuation over the length of the lighting circuit. This extra performance allows either an increse in the level of the interfering signal from 3.5 V rms to 5 V rms or allows the circuit length to be extended by almost 50%.(ie from 200 m to 300 m)
When the circuit is long the value of Zb will fall to a limiting value that determines the rejection of interference from the electrical bus.We need to experimentally determine this limiting value.This limiting value of impedance, in combination with the experimentally measured largest value of electrical bus noise from external sources and the propagation loss per unit length 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 may exceed the coupled noise from the electrical bus. It is necessary to 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 zone of reliable operation.
This web page last updated September 20, 2005
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