Electronic decoupling impedance

Abstract

An active decoupling device for stabilizing the impedance on an electric line in a determined band of frequency. The decoupling device of the invention is inductively coupled to the power line and comprises a transformer with a primary winding connectable to the power line, and a secondary winding closed on a burden network. In an application, the invention, is used to decouple a smart meter from variations of the load impedance.

Claims

1. An active decoupling impedance comprising a transformer having a primary winding connectable on an electrical power line between a victim device and an electrical load, and a burden network, connected to a secondary winding of the transformer, wherein the burden network is arranged to generate a first impedance on the powerline at a power frequency, and a second impedance with an absolute value larger than that of the first impedance in a predetermined band of frequency above the power frequency.

2. The active decoupling impedance of claim 1, wherein the transformer is a current transformer.

3. The active decoupling impedance of claim 2, wherein the predetermined band is comprised in any of the following intervals: 35 kHz to 91 kHz; 98 kHz to 122 kHz; 155 kHz to 403 kHz; 155 kHz to 487 kHz.

4. The active decoupling impedance of claim 1, having an absolute value of the impedance seen on the power line in the predetermined band of at least 5 Ohm.

5. The active decoupling impedance of claim 1, having an absolute value of the impedance seen on the power line in the predetermined band of at least 10 Ohm.

6. The active decoupling impedance of claim 1, comprising a filter selecting the components of the line voltage in the predetermined band of frequency, an amplifier having an input connected to an output of the filter, the amplifier being arranged to cancel a current flowing in the burden network in the predetermined band, such that the impedance of the burden network in the predetermined band of frequency is increased.

7. The active decoupling impedance of claim 1 in combination with an electrical meter connectable on the electrical power line between an energy provider and one or several electrical loads for measuring use of electrical energy by the one or several electrical loads, wherein the electrical meter has a PLC interface for transmitting and/or receiving data through the power line in the predetermined band of frequency, the line impedance stabilizer being connected between the electrical meter and the one or several electrical loads.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:

(2) FIG. 1 shows a smart meter installation including the inventive active impedance, in a simplified representation.

(3) FIGS. 2a and 2b shows a simplified model of the transmission channel for a victim device that needs protection from variations of the load impedance, respectively of the network impedance.

(4) FIG. 3 is an equivalent circuit of the device of the invention including a transformer and a burden load.

(5) FIGS. 4a and 4b are the resulting equivalent circuits of the line impedance of the invention at power frequency, respectively in the targeted band.

(6) FIG. 5 shows schematically an embodiment with a ring-type current transformer.

(7) FIGS. 6a and 6b show an open-loop voltage-controlled, voltage source embodiment of the inventive device.

(8) FIGS. 7a and 7b show a closed-loop current-controlled embodiment.

(9) FIG. 8 shows an open-loop voltage-controlled, current source embodiment.

(10) FIG. 9 shows a circuit realization of the active burden for an open-loop voltage-controlled embodiment.

(11) FIG. 10 is a plot of the resulting impedance in the power and signalling bands of a smart meter.

(12) FIG. 11 shows a closed-loop voltage-controlled embodiment.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

(13) FIG. 1 shows a smart meter installation including the inventive decoupling impedance. The area 30 represents an electrical installation of a customer, with several loads connected to a secondary power line 35 that comes from a distribution transformer (not represented) and may serve also other costumers.

(14) A smart meter 40 placed at the entry records the accumulated electrical consumption and transmits it, via the plc modem 45 and the data concentrator 47 to a server 60 of the electricity provider. The consumption information is used for billing and for controlling the network, for example. Bidirectional communication, from the server to the meter, is also possible.

(15) The electronic decoupling impedance 20, placed between the meter 40 and the loads 32, prevents the line impedance from being pulled down by the load impedance, and avoid the attenuation of the communication signal and mitigate the noise emitter from the load mentioned in the introduction.

(16) It is understood that, insofar as the object of the present invention is to decouple a device on an electric network from the variations of impedance on the network, its usefulness is not limited to smart meters, but extends to many situation in which it necessary to protect a device (denoted “victim device”) from the variation of the impedance on a network. Hence, the use case shown in FIG. 1 is not exhaustive, and the invention could be used to decouple any victim device 40, irrespective of the presence of a plc communication interface. The victim device can be protected from impedance fluctuations on the load side, as in FIG. 2a, as well as on the network side, as in FIG. 2b.

(17) FIG. 5 shows how the decoupling device of the invention is inductively coupled to the power line between the victim device 40 and the load 32. The active decoupling impedance of the invention comprises a transformer with a primary winding connectable to the power line, and a secondary winding closed on a burden network 29. Advantageously, the transformer can be a current transformer with a solid-ring core 25 in which the powerline cable or bus can be inserted, such that the number of spires in the primary is N.sub.p=1. In this way, the active decoupling impedance of the invention can be installed without cutting the powerline, as shown in FIG. 3. Other possible arrangements include split-core transformers, which can be clamped on the power line without opening it and, on the other hand, transformers with a wound primary.

(18) In the following description we consider a transformer model with all values referred to the primary side. The transformer model would normally include an ideal transformer where the turns-ratio is considered. For simplicity the transformer in this description is assumed to have a 1:1 turns-ratio. In most real use cases, the transformer will have a primary winding of 1 turn and multiple turns on the secondary. The power line sees the impedance of the burden network scaled by the square of the turn ratio:Z.sub.p=Z.sub.s.Math.(N.sub.p/N.sub.s).sup.2; spurious components such as stray inductance, winding resistance and stray capacitance are also ignored in this description. For suitable circuit operation, the impedance of these components, within the operating frequency band, should be kept small compared to other circuit impedance parameters.

(19) The transformer coupling coefficient, k, should be sufficiently close to 1. In this case the stray inductance will be small compared to other inductance values.

(20) Voltage S/N Ratio

(21) FIG. 2a is a representation of the power line where the victim device is inserted. the victim device may be a smart meter as in FIG. 1, an active EMC filter, or any device that should be decoupled from the variations of impedance of the load. Consider the following voltages and impedances at the point in the network where the victim device is connected

(22) V.sub.s Communication signal voltage

(23) Z.sub.s Communication signal source impedance

(24) V.sub.m Received voltage at the victim device

(25) Z.sub.m victim device input impedance

(26) Z.sub.i decoupling impedance

(27) V.sub.n Noise voltage from load

(28) Z.sub.n Noise source impedance from load

(29) The received voltage at the victim device v.sub.m is the sum of communication signal from the network and noise voltage from the load. The voltage signal to noise ratio, denoted as SNR in the following, indicates device input impedance Z.sub.m is assumed to be high compared to the other impedances. In this approximation, the equivalent circuit of FIG. 2a can be solved and the SNR before and after insertion of the stabilizer impedance Z.sub.i is computed as follows:

(30) Without the Invention

(31) Received signal voltage at victim device: V.sub.m1=V.sub.s.Math.Z.sub.n/(Z.sub.s+Z.sub.n)

(32) Received noise voltage at victim device: V.sub.m2=V.sub.n.Math.Z.sub.n/(Z.sub.s+Z.sub.n)

(33) SNR: V.sub.m1/V.sub.m2=V.sub.s/V.sub.n.Math.Z.sub.n/Z.sub.s

(34) With the Invention

(35) Signal voltage at victim device: V.sub.m3=V.sub.s.Math.(Z.sub.n+Z.sub.i)/(Z.sub.s+Z.sub.n+Z.sub.i)

(36) Noise voltage at victim device: V.sub.m4=V.sub.n.Math.Z.sub.s/(Z.sub.s+Z.sub.n)

(37) SNR: V.sub.m3/V.sub.m4=V.sub.s/V.sub.n.Math.(Z.sub.n+Z.sub.i)/Z.sub.s

(38) The active decoupling impedance of the invention stabilizes the load impedance seen and yields an improvement of the voltage S/N ratio given by

(39) $\frac{{\mathrm{SNR}}_{\mathrm{with}}}{{\mathrm{SNR}}_{w\mathrm{ithout}}}=\frac{Z_n+Z_i}{Z_n}=\frac{Z_i}{Z_n}+1$
Therefore, to improve voltage signal to noise ratio we must have Z.sub.i>>Z.sub.n

(40) Similar considerations apply to the case in which the decoupling impedance 20 is on the network side, as in FIG. 2b.

(41) Principle of Operation

(42) The principle of operation will now be discussed with reference to FIGS. 3, 4a, 4b, of which the first shows an equivalent circuit to the decoupling impedance with

(43) v.sub.p Primary voltage

(44) i.sub.p Primary current

(45) L.sub.m Magnetizing inductance

(46) i.sub.m Core loss resistance

(47) i.sub.m Magnetizing current

(48) L.sub.b Burden inductance

(49) i.sub.b Burden current

(50) R.sub.r Residual resistance

(51) i.sub.r Residual current

(52) i.sub.c Compensation current

(53) The primary current is defined by i.sub.p=i.sub.m+i.sub.b+i.sub.r−i.sub.c

(54) Power Frequency (FIG. 4a)

(55) Assume i.sub.c≈0 and i.sub.r≈0: an impinging current at power frequency gives i.sub.p=i.sub.m+i.sub.b. When L.sub.m>>L.sub.b then i.sub.p≈i.sub.b, therefore the primary voltage will be v.sub.p=i.sub.p.Math.jωL.sub.b and the magnetizing current i.sub.m=v.sub.p/(jωL.sub.m). Then i.sub.m.Math.jωL.sub.m=i.sub.p.Math.jωL.sub.b.

(56) Therefore, i.sub.p/i.sub.m=L.sub.m/L.sub.b and the transformer core has a saturation current of i.sub.m. The addition of the burden allows the phase current to be i.sub.p=(L.sub.m/L.sub.b).Math.i.sub.m.

(57) Note that the above depends from the assumption that the total impedance of the stray inductance and all the winding resistances are significantly less than the impedance L.sub.b at power frequency.

(58) Target Frequency (FIG. 4b)

(59) Make i.sub.c=i.sub.m+i.sub.b, then i.sub.p=i.sub.r: an impinging signal voltage v.sub.p produces a current i.sub.r. The primary impedance is Z.sub.p=v.sub.p/i.sub.r. Moreover, i.sub.r=v.sub.p/R.sub.r, therefore Z.sub.p=R.sub.r.

(60) Preferably, the transformer should have a significantly high coupling coefficient to realize the above expression. A lower coupling coefficient will result in a higher stray inductance which will diminish the effectiveness.

(61) Open Loop Voltage-Controlled Embodiment (FIGS. 6a-6b)

(62) FIGS. 6a and 6b show a possible example of realization of the inventive decoupling impedance with an active burden network configured as an open-loop controller. The burden is connected on the secondary winding of a transformer, as illustrated in FIG. 6b, and FIG. 6a is an equivalent circuit as seen from the primary side. The filter 105 selects the voltage signal in the band of frequencies where the plc communication takes place and, by the amplifier 108 and controlled voltage source 112, generates a compensating voltage. We introduce the following abbreviations:

(63) v.sub.p Primary voltage

(64) i.sub.p Primary current

(65) Z.sub.mb Magnetizing and burden equivalent impedance

(66) i.sub.mb Magnetizing and burden equivalent current

(67) R.sub.r Residual resistance

(68) i.sub.r Residual current

(69) i.sub.c Compensation current

(70) A.sub.v Loop voltage gain

(71) We require I.sub.c=I.sub.mb so that i.sub.p=i.sub.r. Then v.sub.p=i.sub.c.Math.Z.sub.mb and v.sub.0=i.sub.c.Math.(Z.sub.0+Z.sub.mb). Therefore, the loop voltage gain is A.sub.v=v.sub.0/v.sub.p=(Z.sub.0+Z.sub.mb)/Z.sub.mb and the resultant impedance on the primary is given by Z.sub.p=v.sub.p/i.sub.r=R.sub.r.

(72) FIG. 6b shows the active burden 29 in the variant of FIG. 6a, connected directly to the secondary winding of the transformer 25.

(73) FIG. 9 shows a possible implementation of an active burden circuit 29 that could be used in the embodiment of FIGS. 6a-6b. The amplifier 108 generates a voltage that is injected in the node between the inductances L1 and L2, thereby reducing the current flowing through them, in the target band. Other inductance arrangements are possible where L1 or L2 may be omitted.

(74) Closed-Loop Current Controlled Embodiment (FIGS. 7a-7b)

(75) Another possible example of realization includes, as represented, a controlled current source 114 that is driven by an amplifier 108 whose input is connected to current sensor 24, which may be an auxiliary current transformer. The filter 105 limits the action of the compensation circuit to the target band, such that, at power frequency, the decoupling impedance of the invention is equivalent to the circuit of FIG. 4a. We introduce the following abbreviations:

(76) v.sub.p Primary voltage

(77) i.sub.p Primary current

(78) Z.sub.mb Magnetizing and burden equivalent impedance

(79) i.sub.mb Magnetizing and burden equivalent current

(80) R.sub.r Residual resistance

(81) i.sub.r Residual current

(82) i.sub.c Compensation current

(83) A.sub.i Loop current gain

(84) We require i.sub.c=i.sub.mb so that i.sub.p=i.sub.r. Then v.sub.p=i.sub.c.Math.Z.sub.mb and i.sub.r=v.sub.p/R.sub.r. Current loop gain is A.sub.i=i.sub.c/i.sub.r=R.sub.r/Z.sub.mb, and the resultant primary impedance is given by Z.sub.p=V.sub.p/i.sub.r=R.sub.r as in the previous example.

(85) FIG. 7b shows the active burden 29 in the variant of FIG. 7a, connected directly to the secondary winding of the transformer 25 with an additional current transformer 24 used to extract the i.sub.p signal.

(86) Open-Loop Voltage-Controlled Current Source Embodiment (FIG. 8)

(87) FIG. 8 illustrates another possible structure for the circuit of the invention, based on a voltage-controlled current source 114 driven by the input voltage v.sub.p through a band-pass filter 105 and a network comprising an integrator 103 whose output is weighted by (1/L.sub.m+1/L.sub.b) and a proportional branch with weight 1/R.sub.m, such that, in the band of frequencies selected by the filter 105, the correction current is given by i.sub.c=v.sub.p/R.sub.m+(1/L.sub.m+1/L.sub.b).Math.∫v.sub.pdt so that i.sub.p=v.sub.p/R.sub.r. Then Z.sub.p=v.sub.p/i.sub.r=R.sub.r. Note that, since the output current i.sub.c does not directly influence the sensed voltage v.sub.p, this is an open-loop control.

(88) The active burden circuit of FIG. 8 can be connected at the secondary terminals of the transformer, as in FIG. 6b.

(89) Closed-Loop Voltage-Controlled Embodiment (FIG. 11)

(90) Another possible example of realization includes, as represented, a controlled voltage source 112 of output v.sub.p that is driven by an amplifier 108 whose input is connected to current sensor 24, which may be an auxiliary current transformer as in the embodiment of FIGS. 7a/7b. The filter 105 limits the action of the compensation circuit to the signalling band. The transfer function creating the driven voltage v.sub.p from sensed current i.sub.p is the forward transimpedance Z.sub.x. Impedance Z.sub.x is the same as the impedance measured on the primary. Amplifier 108 with voltage source 112 is arranged to cancel current i.sub.p flowing in the burden network, such that the impedance of the burden network in the desired band of frequency is increased.

(91) v.sub.p Primary voltage

(92) i.sub.p Primary current

(93) Z.sub.mb Magnetizing and burden equivalent impedance

(94) i.sub.mb Magnetizing and burden equivalent current

(95) i.sub.c Compensation current

(96) Z.sub.x Forward transimpedance

(97) The forward transimpedance is

(98) $Z_x=\frac{v_p}{i_p}$
Currents are given by

(99) $i_{\mathrm{mb}}=i_p+i_c$$i_{\mathrm{mb}}=\frac{v_p}{Z_{\mathrm{mb}}}$
So, the current drawn from voltage source 112 is

(100) $i_c=v_p.Math.\left(\frac{1}{Z_{\mathrm{mb}}}-\frac{1}{Z_c}\right)$
The resultant impedance measured on the primary is Z.sub.x

(101) FIG. 10 is a Bode plot showing the action of the inventive decoupling impedance, in the embodiment of FIG. 9. The circuit has been optimized for signalling in the CENELEC A band. The bands of frequency of the power distribution and that used by the plc signalling are indicated by the hashed areas 130 and, respectively 140. It can be appreciated that the magnitude of the primary impedance 120 is very low at 50-60 Hz, which leads to low losses and increases the maximum current that can be circulated without saturating the core and rises to approximately 100Ω n the signalling band. As shown by the phase plot 125, the impedance in the signalling band is essentially resistive in nature.

REFERENCE NUMBERS USED IN THE FIGURES

(102) 20 active decoupling impedance, active impedance stabilizer 24 sense transformer 25 transformer 28 auxiliary power supply 29 active burden network 30 customer 32 load 35 power line 40 victim device/smart meter 42 kWh meter 45 PLC modem 47 Data Concentrator 60 server 103 integrator 104 attenuation 105 filter 108 amplifier 112 controlled voltage source 114 controlled current source 120 magnitude 125 phase 130 power frequency band 140 target band of frequency, signalling band