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.

Description

FIELD OF THE INVENTION

The invention relates to electronic active impedances that can be used to decouple a device (designated in the following as the “victim device”) on an electric line, from the impedance that may occur on the line itself, either on the network side or on the load side. The invention is applicable to several devices that cannot tolerate too low impedances on the network in a determined frequency range, for example active filters. A special case of application is that of “smart” electricity meter devices that record consumption of electric energy and communicate the information to the electricity supplier through a powerline modem.

DESCRIPTION OF RELATED ART

Automatic reading of energy metering is known in the art and rely on several data transmission technologies. In electric meters, powerline communication signals are typically injected between phase and neutral, i.e. in differential mode. The amplitude of the signals may be of about 10V peak or less with typical frequency range 35 kHz to 500 kHz.

Power line communication frequency bands suitable for the present invention are assigned as follows:

35 kHz to 91 kHz in Europe (part of CENELEC A band)

98 kHz to 122 kHz in Europe (part of CENELEC B band)

155 kHz to 403 kHz in Japan (part of the ARIB band)

155 kHz to 487 kHz in the USA (part of FCC band).

A problem with these devices is that the signals are attenuated by the load impedance existing on the powerline. When load impedance is too low, for example below 1 ohm, the signal is attenuated to critically low levels and communication can fail.

Furthermore, loads on the network generate disturbances and spurious signals in the signalling band of frequencies, which can result in transmission errors.

It is known to introduce a choke network, or another passive attenuation network, downstream to the smart meter, between this and the load, to generate an inductance that limits the signal attenuation at 35 kHz and above, without blocking the power current at 50/60 Hz. The choke must be large enough to avoid saturation, and necessarily introduces power losses.

The above situation is not limited to smart meters but it occurs whenever it is necessary to isolate a device on an electric network from the variation of impedance thereof, in a determined band of frequency, as it may be the case for active EMI filters, for example, in a smaller package and at lower cost than with known passive solutions.

BRIEF SUMMARY OF THE INVENTION

The present invention presents an improved solution for stabilizing the line impedance downstream of a victim device in a predetermined band of frequency. In a use case, the victim device is a PLC modem or a smart meter transmitting in the aforesaid band of frequency. The invention overcomes the limitations of the known art, by an active decoupling impedance having the features of the first independent claim. The present invention proposes an active impedance with a transformer having a primary connectable on the electrical power line, like a current transformer, and whose secondary is loaded by an active burden circuit. The active burden circuit is arranged to present a low impedance at the power frequency, and a higher impedance in the targeted band. An additional advantage is that the transformer and burden are galvanically isolated from the power line.

Thanks to the inventive features, the voltage drop at power frequency is contained, due to the low burden value, such that magnetizing flux is contained and a small core is enough to prevent saturation. At the same time, the burden impedance in the targeted band is higher, such that the impedance seen from the primary side of the transformer is proportionally larger, and the signals are not attenuated. The device of the invention can be dimensioned to introduce an additional impedance in the targeted band, and in this way prevent excessive attenuation, irrespective of the load. This impedance is preferably resistive in nature, and its magnitude may be higher than 5 Ohm, or better 10 Ohm. In practical realization, values of about 50-100 Ohm are attainable.

The dependent claims relate to optional advantageous features including the frequency of the targeted band that, according to the standards mentioned above, may be from 35 kHz to 500 kHz; an advantageous active structure with an amplifier or an unity-gain buffer cancelling a current flowing in the burden network in the targeted band, responsive to the output of an high-pass filter; a burden network including a series of impedances. The burden network and the high-pass filter are dimensioned such that at power frequency the output current of the amplifier is essentially zero while, in the targeted band, the burden network draws no current from the transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

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

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.

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

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

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

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

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

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

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

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

FIG. 11 shows a closed-loop voltage-controlled embodiment.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

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.

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.

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.

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.

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_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.

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_p=Z_s·(N_p/N_s)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.

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.

Voltage S/N Ratio

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

V_s Communication signal voltageZ_s Communication signal source impedanceV_m Received voltage at the victim deviceZ_m victim device input impedanceZ_i decoupling impedanceV_n Noise voltage from loadZ_n Noise source impedance from load

The received voltage at the victim device v_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_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_i is computed as follows:

Without the Invention

Received signal voltage at victim device: V_m1=V_s·Z_n/(Z_s+Z_n)Received noise voltage at victim device: V_m2=V_n·Z_n/(Z_s+Z_n)SNR: V_m1/V_m2=V_s/V_n·Z_n/Z_s

With the Invention

Signal voltage at victim device: V_m3=V_s·(Z_n+Z_i)/(Z_s+Z_n+Z_i)Noise voltage at victim device: V_m4=V_n·Z_s/(Z_s+Z_n)SNR: V_m3/V_m4=V_s/V_n·(Z_n+Z_i)/Z_s

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

$\frac{{\mathrm{SNR}}_{\mathrm{with}}}{{\mathrm{SNR}}_{w\mathrm{ithout}}}=\frac{Z_n+Z_i}{Z_n}=\frac{Z_i}{Z_n}+1$

Therefore, to improve voltage signal to noise ratio we must have Z_i>>Z_n

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

Principle of Operation

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

v_p Primary voltagei_p Primary currentL_m Magnetizing inductancei_m Core loss resistancei_m Magnetizing currentL_b Burden inductancei_b Burden currentR_r Residual resistancei_r Residual currenti_c Compensation current

The primary current is defined by i_p=i_m+i_b+i_r−i_c

Power Frequency (FIG. 4a)

Assume i_c≈0 and i_r≈0: an impinging current at power frequency gives i_p=i_m+i_b. When L_m>>L_b then i_p≈i_b, therefore the primary voltage will be v_p=i_p·jωL_b and the magnetizing current i_m=v_p/(jωL_m). Then i_m·jωL_m=i_p·jωL_b.

Therefore, i_p/i_m=L_m/L_b and the transformer core has a saturation current of i_m. The addition of the burden allows the phase current to be i_p=(L_m/L_b)·i_m.

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_b at power frequency.

Target Frequency (FIG. 4b)

Make i_c=i_m+i_b, then i_p=i_r: an impinging signal voltage v_p produces a current i_r. The primary impedance is Z_p=v_p/i_r. Moreover, i_r=v_p/R_r, therefore Z_p=R_r.

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.

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

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:

v_p Primary voltagei_p Primary currentZ_mb Magnetizing and burden equivalent impedancei_mb Magnetizing and burden equivalent currentR_r Residual resistancei_r Residual currenti_c Compensation currentA_v Loop voltage gain

We require I_c=I_mb so that i_p=i_r. Then v_p=i_c·Z_mb and v₀=i_c·(Z₀+Z_mb). Therefore, the loop voltage gain is A_v=v₀/v_p=(Z₀+Z_mb)/Z_mb and the resultant impedance on the primary is given by Z_p=v_p/i_r=R_r.

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

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.

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

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:

v_p Primary voltagei_p Primary currentZ_mb Magnetizing and burden equivalent impedancei_mb Magnetizing and burden equivalent currentR_r Residual resistancei_r Residual currenti_c Compensation currentA_i Loop current gain

We require i_c=i_mb so that i_p=i_r. Then v_p=i_c·Z_mb and i_r=v_p/R_r. Current loop gain is A_i=i_c/i_r=R_r/Z_mb, and the resultant primary impedance is given by Z_p=V_p/i_r=R_r as in the previous example.

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_p signal.

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

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_p through a band-pass filter 105 and a network comprising an integrator 103 whose output is weighted by (1/L_m+1/L_b) and a proportional branch with weight 1/R_m, such that, in the band of frequencies selected by the filter 105, the correction current is given by i_c=v_p/R_m+(1/L_m+1/L_b)·∫v_pdt so that i_p=v_p/R_r. Then Z_p=v_p/i_r=R_r. Note that, since the output current i_c does not directly influence the sensed voltage v_p, this is an open-loop control.

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

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

Another possible example of realization includes, as represented, a controlled voltage source 112 of output v_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_p from sensed current i_p is the forward transimpedance Z_x. Impedance Z_x is the same as the impedance measured on the primary. Amplifier 108 with voltage source 112 is arranged to cancel current i_p flowing in the burden network, such that the impedance of the burden network in the desired band of frequency is increased.

v_p Primary voltagei_p Primary currentZ_mb Magnetizing and burden equivalent impedancei_mb Magnetizing and burden equivalent currenti_c Compensation currentZ_x Forward transimpedance

The forward transimpedance is

$Z_x=\frac{v_p}{i_p}$

Currents are given by

$i_{\mathrm{mb}}=i_p+i_c$ $i_{\mathrm{mb}}=\frac{v_p}{Z_{\mathrm{mb}}}$

So, the current drawn from voltage source 112 is

$i_c=v_p·\left(\frac{1}{Z_{\mathrm{mb}}}-\frac{1}{Z_c}\right)$

The resultant impedance measured on the primary is Z_x

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

• 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

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.