IntroductionPower line communications (PLC) is a promising

technology that is rapidly gaining traction in smart

meter reading (AMR) demand response and SCADA

applications. In the U.S., the FCC permits the use of

frequencies less than 500 kHz for narrow band PLC.

This has resulted in a host of different technologies

being developed from single carrier systems such as

LonTalk® to multi-carrier systems such as PRIME,

G3 and IEEE P1901.2 (in the process of being

deployed) for communication purposes.

Distribution transformers can support tens or

hundreds of houses in Europe and China, but in

the U.S. and Japan, particularly in rural areas, each

distribution transformer only supports a few houses.

To minimize costs, the data concentrator should

reside in the medium voltage (MV) side and it is

therefore necessary for signals to cross each

distribution transformer to establish communication

between smart meters on the low voltage (LV) side

and the data concentrator.

In order to properly design smart metering net-

works for the different global regions, it is important

to understand the channel and noise characteristics.

Having a channel model allows system designers

to better understand the network performance

for a given network topology and PLC modem

deployment. Once the specification targets regarding

data rates and coverage are understood, the commu-

nication technology that satisfies these constraints

can be chosen. Texas Instruments (TI) has devel-

oped a flexible PLC development platform based

on its 32-bit C2000™ microcontroller architecture

that allows a designer to choose from the different

narrow-band OFDM communication standards

available, examples include PRIME, G3 and IEEE

P1901.2 [1]. As a first step, a thorough understand-

ing of the network characteristics (i.e., the channel

and noise models) is required. This paper proposes

a channel model for the MV/LV case using the

s-parameters with some of the measurements

done in the field for MV/MV, MV/LV cases and an

attempt to match the simulated channel model

against the measurements.

.

Channel modeling of medium to low voltage links for AMI applications of PLC Medium voltage (MV) to low voltage (LV) crossing is a very important application for power

line communications (PLC) in relation to automated metering infrastructure (AMI) in the

United States, Japan and in some parts of Europe. In this context it is important to have a

channel model for both MV/MV and MV/LV links, which allows engineers to simulate the

attenuation that signals may face under different load conditions. Such an analysis can

then be used to do signal-to-noise ratio (SNR) calculations for a given topology. In this

paper, ABCD or scattering (s-) parameters is used for channel model characterization of

MV/LV link specifically for the band 10kHz - 490kHz where narrowband plc systems are

used. An accurate transmit-to-receive signal can also be obtained if the parameters of the

different components are modeled properly. The measurement of s-parameters for MV/LV

transformers and couplers can be compared to measurements from the lab and used to

predict the observed MV/MV and MV/LV channel in the field.

Modeling using s-parameters/ABCD parameters

Scattering (S) -parameters and ABCD parameters are tools used to characterize two-port

networks (Figure 1). These parameters can also be used to characterize different components

in an MV/LV powerline communication channel, including transformers, couplers and cables,

which allows an end-to-end voltage transfer function (channel) characterization [2]-[5].

Channel characterization is essentially a three step process:

• ObtainABCDparametersforindividualcomponentsintheMV/LVline

• ObtainthenetABCDmatrixbyaconcatenationofindividualABCDmatrices,and

• Obtaintheend-endvoltagetransferfunctionbasedonthenetABCDparameters.

Anand Dabak, FellowIl Han Kim, Member of Technical Staff

Tarkesh Pande, Member of Technical Staff Texas Instruments

W H I T E P A P E R

Channel modeling of medium to low voltage links for AMI applications of PLC June 2012

2 TexasInstruments

Fig. 2 – A generic MV-LV communication channel

Equation 2

Equation 1

V 1 V 2Net ABCD, MV → LV

invABCDcoupler1 *ABCDMV –line1 *ABCDtxfmr –eff *ABCDMV –line2 *ABCDtxfmr 2 *ABCDLV –line2Net ABCD, MV → LV

I 1 – I 2

Fig. 1 – Two-port modeling using S/ABCD parameters

For transformers and couplers, their S-parameters may be readily measured using a network analyzer.

Conversion formulas may then be used to convert the S-parameters to ABCD parameters [6].

Figure 2 illustrates a typical MV/LV (or LV/MV) link found in the U.S.

Inthiscase,thetwo-portnetworkbetweenmodem1andmodem2canbemodeledas:

After noting that I 2 where Z eff Z modem2 // Z home2, plugging in I 2 into Equation 1 allows for a

calculation of the voltage transfer function (V 2 / V 1).

where the net ABCD parameters from the modem at the MV side transmitter to the LV side modem for

parameters,V1,I1,V2,I2aregivenby:

Moduleb1

S-parameters for 2-port network ABCD parameters for a 2-port network

b2

a1b1

a1

a2

S11

S21

S12

S22

b2

a2 ModuleV1

I1

V2V1

I1

- I2

AC

BD

V2

I2

ABCDMV—line2

ABCDLV—line2

V2 V1I2 I2

MV LV

LV MV

ABCDMV—line1

ABCDcoupler1ABCDtxfmr2

ABCDTxfmr

ZeffTxfmr2 Coupler1

Load (home1)Load (home)Load (home2)

Modem2 Modem1

Txfmr

V 2Z eff

Channel modeling of medium to low voltage links for AMI applications of PLC June 2012

3TexasInstruments

Fig. 3 – Comparison of measured versus S-parameters based LV/MV magnitude response for a 25 kVA transformer

Fig. 4 – Comparison of measured versus S-parameters based MV/LV magnitude response for a 25 kVA transformer

The three main components that need to be characterized in an MV/LV link are the transformer, coupler and

MV-cable.

Transformer modeling

S-parameters for distribution transformers used in the U.S. are reported in [7]. These S-parameters are con-

verted to ABCD parameters from which corresponding voltage transfer function and equivalent Thevenin im-

pedancesareobtained.Measurementswerethenmadeonanun-energized25kVAtransformerattheTexas

InstrumentsSystemsandApplicationLabinDallastoexperimentallyobtainthevoltagetransferfunctionand

corresponding Thevenin impedance. Figure 3 to Figure 5 illustrate the match between measurements and

s-parameter based modeling.

Measured Response

Measured Response

S-parameter Based Simulated Response

S-parameter Based Simulated Response

Measured versus Simulated Response

Measured versus Simulated Response

Experimental validation of the different components

4 TexasInstruments

Channel modeling of medium to low voltage links for AMI applications of PLC June 2012

Fig. 5 – Measured versus S-parameter based MV → LV and LV → MV Thevenin Impedance for 25 kVA transformer

Fig. 6 – Comparison of measured versus S-parameters based magnitude response for coupler

Fig. 7 – Model and comparison of measured versus ABCD parameters for an MV/MV line

Measured MV → LV

Measured Response S-Parameter Based Simulated Response

S-parameter based MV → LV Measured MV → LV S-parameter based MV → LV

Coupler modeling

Similar to the transformer, the s-parameter approach was used to match measurements for an MV/LV

coupler. The close agreement between the two different approaches is illustrated in Figure 6.

MV-cable modeling

In[8],acircuitmodelisprovidedfortheMVcableandreadilyallowsonetodirectlydeterminethe

correspondingABCDmatrixgiveninFigure7.Fieldmeasurementswerenextdonetodetermine

the attenuation in an MV/MV line [9]. A comparison of the predicted attenuation using the ABCD approach

and the field measurements is given in Figure 7. The R, L, G, C used for this analysis are given by;

L=1.9e-6H/m,C=8e-12F/m,R200=0.03Ohm/m,G200=1.5e-6S/m.

cosh(gL) Zc sinh(gL)sinh(gL)/ Zc cosh(gL)

Zc = (R + jwL) / (G + jwC)

g = (R + jwL)(G + jwC)

ABCD model:

ABCDcable =

Characteristic impedance of above cable

g

5TexasInstruments

Channel modeling of medium to low voltage links for AMI applications of PLC June 2012

Fig. 9– Time varying nature of energized transformers

Fig. 8 – The MV/LV measurements at 0 mile for a MV/LV transformer, MV line and a coupler.

Un-energized Transformer (Not connected to MV side) Energized Transformer (connected to MV side)

Aninterestingexampleisnotedbelow(Figure9),whereimpedancemeasurementsweretakenonboth

energized and un-energized transformers at 4 different time snap shots – each a quarter of the AC main

cycle.Fortheenergizedtransformer,wefindthatitexhibitsatimevaryingresponsethatislinetothehalf

AC mains cycle.

Thisexampleraisesafewquestionsforfutureareasofresearch:whatistheprincipalreasonforthe

transformer impedance and its response variation with half AC mains cycle? Could this effect be captured by

understandingthephysicsofthechangeoffluxinthetransformersynchedtotheACmainscycle?

Measurements also show that the low voltage side impedance, measured in homes, varies as a function

ofACmainscycle.IsthechangeoftheimpedanceontheLVsideduetothechangeofimpedanceofthe

transformer which is close to the LV site or is it also due to the change of impedance within the home itself?

Furthermore, which is the dominant component?

Most RF simulators using s-parameters currently assume the s-parameters to be stationary with time.

However, since the impedance conditions in a MV/LV grid are changing with the AC mains, a cyclo-stationary

modelforthes-parametersismorerelevant.Inthatcase,howcancurrentRFsimulatorsbeusedtosimulate

theend-to-endresponse?Oneoptionistodiscretizetimeintomultiplebinsandthenhaves-parameters

for each of these bins. The s-parameters for the different bins can then be used to simulate the end to end

response for the individual bins.

An example on concatenating different components

Finally, the MV/LV measurement is compared to the simulations for a transformer, MV line and a coupler

at 0 mile distance.

Energized transformers

6 TexasInstruments

This paper demonstrated that s-parameter/ABCD modeling is a viable method for the study of channel

characterization in PLC networks. The measurement results in most cases match with the predicted results

and further characterization for different topologies is on going. Lastly a few open-ended questions were

posed for the PLC community.

1 www.ti.com/plc2 Willam C. Black, Nader Badr, High frequency characterization and modeling of distribution transformers,

IEEEISPLC,2010,pages18-21.

3 EklasHossain,SherozKhan,AhadAli,ModelingLowVoltagePowerLineasaDataCommunication

Channel,Worldacademyofscience,engineeringandtechnology,45,2008

4 Francis Berrysmith A multipath model for powerline communications,

5 IEEEP1901™_Annex_C&Nmodel_v0.0,March2006.

6 DeanFrickey,ConversionsbetweenS,Z,Y,h,ABCD,andTparameterswhicharevalidforcomplex

sourceandloadimpedances,IEEETrans.onMicrowaveTheoryandTechniquesVol42Feb1994,

pages 205-211

7 Itron,Itron-solution-S-Parameters-Measurements-for-DistributionTransformer-Model,IEEE1901.2

contribution document 2wg-11-0069-00-PHM5-s-parameters-measurements-for-distribution-

transformer-model.docx,2011.

8Antonio Cataliotti,Alfredo Daidone, Giovanni Tinè, Power Line Communication in Medium Voltage

Systems:CharacterizationofMVCables,IEEETrans.onpowerdelivery,vol.23,no.4,October2008.

9 TI,L+G,SummaryofChannelandNoiseMeasurementsInTheFCCBandOnARuralUSGrid,IEEE

1901.2 document 2wg-10-0232-00-PHM5-summary-of-channel-and-noise-measurements-in-the-

fcc-band-on-a-rural-us-grid.ppt, Nov., 2010

Conclusion

References

B011012

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