Distribution network modeling for Power Line Communication applications

T.Tran-Anh, P.Auriol Centre de Génie Électrique de Lyon (CNRS n° 5005)

École Centrale de Lyon Écully, France

E-mail: Anh-Tuan.Tran@ec-lyon.fr

T.Tran-Quoc IDEA

BP 46, Lab. Electrotechnique de Grenoble Saint Martin d’Hères, France

E-mail: Tuan.Tran-Quoc@leg.ensieg.inpg.fr

Abstract—Power Line Communication (PLC) is a rapidly evolving technology, aiming to use electrical power lines for the transmission of data. The used signal is high frequency and in the range of 1 to 30 MHz. For PLC applications, this paper presents the modeling of the high frequency distribution network that includes overhead or underground cables and the MV/LV transformer by ATP/EMTP software. Impacts of several factors on the PLC attenuation such as frequency and line length are investigated. A method, that permits to evaluate the influence of 50 Hz power signal on the attenuation of PLC transmission, is proposed as well.

Keywords- PLC, Distribution Network, Cable and Line, Transformer, Modeling

I. INTRODUCTION Power Line Communications is known for many years as

Power Line Carrier. It uses the low bandwidth analog and digital information to communicate over the residential, commercial, and high voltage power lines for AMR (Automatic Metering Reading), home automation, and protective relay… The fast development of new communication services (Internet, voice over IP, video...) and the deregulation of the telecommunication market give both electricity and telecom sectors a new significant business potential.

The main idea of PLC is to use the electrical grid for the communication because it is an existing infrastructure and it covers a wider area than any other traditional communication networks (telephone, TV cable…). This permits to avoid building a new network.

In order to be a high rate communication system, PLC system requires a data transfer speed, in general, greater than 1Mbps. Currently, there is no common standard to enforce the frequency band, but some papers suggest that the frequency of signal should be greater than 2-3 MHz [1] or in range of 1 to 30 MHz [2]. In this paper, we consider that the PLC signal is sinusoidal in the frequency range of 1 to 30 MHz.

Actually the applications of PLC system remain essentially in indoor and low voltage (LV) network. There are several methods [3, 4] that permit to evaluate the attenuation of PLC signals but these methods are either complex or supposed to use for the LV and indoor network. The whole MV/LV system is not considered yet. The 50 Hz signals are totally eliminated,

which assumes a complete decoupling between the 50Hz power transmission and the PLC signal transmission.

Therefore, this paper presents a method that permit to evaluate the propagation of PLC signals. The advantage of the proposed method is:

• Its simplicity and its accuracy

• The use of a well-know software (ATP/EMTP)

• The possibility to study the impact of several factors on the PLC attenuation.

• The consideration of possible coupling of the PLC signal with the 50Hz voltage waveform.

II. CABLE AND LINE MODELING The cable and line modeling is carried out with

ATP/EMTP. This is one of the most popular software in the electro-technical world for the power network transient modeling. The software is able to model the network in time domain or frequency domain with accuracy.

The signal with the frequency in range of 1 to 30 MHz corresponds to a wavelength from respectively 300m to 10m, while the length of cable and line in distribution network varies from few meters to few kilometers. Therefore, the length of line and cable becomes considerable in comparison with wavelength. Thus the line and cable model have to be established by the distributed parameters. In addition, in high frequency these parameters depend strongly on the frequency. So the model for the PLC signal simulation in distribution network, must be distributed and frequency dependent.

In this part, some of the most well-know models in this type are investigated. They are the Semlyen, the Noda, and the J.Marti model. All of these models use Carson’s formula to calculate the parameters of lines or cables, but each of them has the different method to integrate in the power network.

The Semlyen model [5] is a 2nd order recursive – convolution line model. It simplifies the approximation of the parameter of line and cable by using only the 2nd order rational function, thus allows the usage of the recursive convolution in time domain modeling.

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The J. Marti model [6] approximates the frequency dependent parameters of lines and cables into the rational functions. Then theses functions are replaced by an equivalent circuit. This circuit can reproduce all the characteristics of lines and cables.

The Noda [7] model represents the lines and the cables in phase domain rather than in modal domain. In this model, the time domain convolutions are replaced by an ARMA (Auto-Regressive Moving Average) model that helps to reduce the computation work.

Among three above models, we decided to choose the J.Marti model to realize the modeling because of its stability and its convenience for our work.

Figure 1. Cross-section of MV underground (a) and MV overhead cable (b)

In France, as in many countries, the distribution network uses different kinds of normalized section cables, which are either overhead or underground. In this paper four types of cable are used for studies:

• Medium voltage (MV) underground cable

• Medium voltage overhead cable

• Low voltage underground cable

• Low voltage overhead cable

TABLE I. PARAMETER OF CABLES

Parameters MV underground MV overhead

Conductor radius 6,9 mm 4,2 mm

Inner radius of gain 15 mm _

Outer radius of gain 16 mm _

Total radius 20 mm 10 mm

Conductor resistivity 3.10-8 Ω.m 3.10-8 Ω.m

Relative permeability of conductor 1 1

Relative permeability of gain 1 1

Relative permittivity of insulator 3 3

Two among these above cable types, MV underground

cable and MV overhead line are used for the study. They are all tri-phase typical cables. The underground cable is insulated, and is reinforced by a grounded sheath. This sheath is located inside the insulator. For the overhead line, it has a conductor enclosed by only an isolator. The cross-section of the cable and the overhead line is shown on Fig. 1. The parameters are detailed on Tab. 1. The modeling is carried out to determine the

profile of attenuation along the cable or as a function of frequency and line length.

III. TRANSFORMER MODELING The modeling of the MV/LV power transformer in high

frequency is very complex due to the frequency dependent and nonlinear parameters. For our study, we can neglect the nonlinear characteristics, because we consider small amplitude signals. A transformer model at high frequency needs taking into account: self and mutual inductance between coils, skin effects and proximity effects in coil, eddy currents losses in core, self- capacity, and capacity between coils.

In fact, the transformer modeling can be divided in two essential trends: detailed internal winding models and terminal models. The first one represents the transformer by a complex R, L, C circuit. The value of R, L, and C is the result of complex field problem resolutions. This kind of model requires the information on physical layout and construction details of the transformer, and it always needs a huge computation work. The second one considers the transformer as a black box. The equivalent circuit is developed from the frequency characteristics seen from the terminals. Thus this kind of model does not need any knowledge about the internal construction of the transformer. For us the second one is better in term of the easy usage, of the computation time and of the interaction with the other elements in power network. This is why some models of second type are investigated here.

The principle of the first model [8] is to represent the transformer by a complex-value admittance matrix which is symmetrical and frequency dependent. This matrix matches the original of the nodal admittance matrix over the frequency range of interest. The elements of the matrix are approximated with the rational function that contains real as well as complex poles and zeros. Then the equivalent circuit is established in order to be integrated in the simulation program.

The authors in [9] proposed a new approximate method that replaces the admittance matrix of transformer by an equivalent circuit. This method called vector fitting shows the efficiency but needs a complicated measurement.

A third high frequency transformer model [10] is also developed from the frequency response measurement through its terminals. This model needs the preliminary knowledge about the phenomena that happen in the transformer at high frequency. The measurements are carried out with the different connections of the terminals in order to represent each phenomenon by an equivalent circuit. And then the circuits are gathered in a unique circuit that represents the whole transformer. This model takes into accounts:

• The self-capacitances of winding.

• The capacitance between windings, between windings and ground.

• The magnetic induction and the frequency dependent series impedance.

• Multiple resonance

a) b)

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Figure 2. Transformer model used for study

The fourth high frequency transformer model [11] uses the same method as the third one in order to determine the equivalent circuit. However, the equivalent circuits which represent the phenomena are different. And these equivalent circuits are added to the conventional transformer model which consists of an ideal transformer, a winding resistance, a leakage inductance, and a magnetizing conductance and inductance. This model takes into account:

• Winding to winding and winding to enclosure capacity

• Skin effect of winding conductor and iron core

• Multiple resonance

In this paper, the equivalent circuit of transformer model is shown in Fig.2. This model is established for a MV/LV core-type transformer.

IV. PROPOSED METHOD FOR PLC ATTENUATION EVALUATION

This part presents a method that permit to evaluate the PLC attenuations in the distribution network including both 50Hz power and PLC signal. The simulation is carried out by two steps:

1) The distribution network is simulated with 50Hz power signal and without PLC signal (SIG1)

2) The distribution network is simulated with 50Hz power signal and with PLC signal injection(SIG2)

The difference between SIG1 and SIG2 enables us to extract PLC signal injected on the distribution network. This method allows superposing the PLC signals on 50 Hz power signal, thus giving the possibility to study the network in the real condition of work.

V. SIMULATION RESULTS

A. Case 1: Simplified network

In this case a LV or MV distribution network is used for the studies of PLC attenuation. The attenuation is calculated by using the following formula:

=

injectedVmV

att lg20.

where: Vm: Modeled voltage along the line Vinjected: Injected PLC voltage.

Figure 3. 50Hz voltage without PLC signal.

Fig. 3 shows the 50Hz voltage without PLC signal. Fig. 4 shows the 50Hz voltage superposed with PLC signal. The difference gives the PLC signal shown in Fig. 5.

Figure 4. Voltage with both 50Hz and PLC signal.

Figure 5. Extracted PLC signal.

Primary side Connected in delta

Secondary side Connected in star

Phas

e B

Ph

ase

A

Phas

e C

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Figure 6. Influence of frequency and line length on the attenuation of PLC

signals for MV underground cable.

Figure 7. Influence of frequency and line length on the attenuation of PLC

signals for MV overhead cable

Fig. 6, 7 show the influence of frequency and line length on the attenuation of PLC signals for MV underground and MV overhead cables. Several comments are carried out:

• In both types of cable when the frequency increases the number of peak is also increased. This is explained by the one quarter of wavelength phenomena

• With the same line length the number of peak is the same but their amplitudes are different. This is due to their different structure.

• It shows that the attenuation of PLC signal depends on its frequency and the length of line.

B. Case 2: MV/LV network In this case, we use a MV/LV distribution network shown

in Fig. 8 to study. This network includes a MV underground cable, a MV/LV transformer and a LV grid.

The feeder connected to the load LV2, LV3, and LV4 is an underground cable. The section of conductor is normalized at 240 mm2. The feeder connected to the load at LV5, LV6, and LV7 has the same type but its conductor has 95 mm2 normalized section. The transformer is the MV/LV core-type with the rated power 100 kVA.

In order to evaluate the PLC signal attenuation, the following factors can be taken into account for the study:

• Network configuration (MV or LV…)

• Cables (underground, overhead, LV, MV, length…)

• Transformers (rated power, type, connection…)

• Loads

• Frequency of PLC signals

• Injection point of PLC signals (LV or MV…)

• Others…

The impact of these factors on PLC signal attenuation is shown in this part.

Figure 8. MV/LV network used for the study

1) Impact of position of the PLC signal injection :

The influence of PLC signal injection position is studied. The study is carried out for 2 cases. In first case the PLC signal is injected on three phases in LV side (at LV1 point in Fig. 8). In the second case the PLC signal is injected on three phases but in the MV side (at point MV1 in Fig. 8).

For the first case, a PLC signal of 10V in amplitude is used. It is about 2.5% of amplitude of 50Hz power voltage signal (0.4kV). In the second case, suppose that we also use a PLC signal which represents 2.5% of 50Hz power voltage signal (10kV), and then a signal at 250V amplitude is selected to inject in the MV side.

Figure 9. Attenuation for different nodes when the PLC signal is injected in LV side (LV1 in Fig. 8)

The following formulas are used to calculate the attenuation on the MV and LV side:

=

250lg20. mV

MVatt ;

=

10lg20. mV

LVatt

where Vm is the measured PLC signal

115m

31m 13m LV

1 LV2 LV3

27m 25m

LV5 LV6

LV4

LV7

MV Side LV Side

32m

50 Hz Signal

MV1 MV2

18m

-20

-15

-10

-5

0

5

LV2 LV3 LV4 LV5 LV6 LV7

Vol

tage

atte

nuat

ion

(dB

)

Phase A Phase B Phase C

364

The attenuation of the PLC signal for different nodes is shown in Fig. 9, 10.

Figure 10. Attenuation for different nodes when the PLC signal is injected in

MV side (MV1 on Fig. 8)

From this result, we can say that:

• The propagation of PLC signal injected in MV side is more attenuated. It is caused by the strong attenuation along the MV cable and through the MV/LV transformer.

• The attenuation of the PLC signals injected in MV side (Fig.10) presents a slight difference between the phases. This can be explained by the unbalance parameters between phases in the MV/LV transformer.

2) Impact of injection phase

In this case, the PLC signal is injected only on one phase at point MV1 in Fig. 8.

Figure 11. Attenuation in case of the PLC signal injected on one phase

The attenuation of the PLC signal for different nodes is shown in Fig. 11. This demonstrates:

• a coupling between the phases, therefore the PLC signal injected on one phase (phase A) can propagate on two other phases (phases B and C)

• the attenuation is the same for phases B, and C at MV2. But beyond the transformer, the signal is unbalanced in three phases

3) Impact of load

To investigate the impact of loads on the PLC signal attenuation, the network in Fig. 8 is still used. The loads at

LV2, LV3, LV5, and LV6 are varied. The PLC signal is injected at MV1 or LV1. In this case, the attenuation remains the same. This can be explained by the inductive characteristic of load. With high frequency signal, its impedance amplitude becomes very high, almost infinite, this is why the influence of load is very small for the PLC signal attenuation. In order to obtain a higher precision for this study, a frequency dependent load model should be used.

VI. CONCLUSION Some methods in the aim to simulate a distribution network

in high frequency range are investigated. A method that permits to evaluate the attenuation of PLC signal has been proposed. The results are reliable and help to estimate the propagation of PLC signal through an area which is wider than usual home network.

Different factors and elements of a real network are taken into account such as the 50Hz power signal, the network configuration, the injection mode, the load, the transformer, the line and the cable… In the future, we will extend this simulation to a more complex and real system with the on-site experience comparison.

REFERENCES [1] Charles J. Kim, Mohamed F. Chouikha “Attenuation characteristics of

high rate home-networking PLC signals”, IEEE Trans. On Power Delivery, Vol. 17, No. 4, October 2002. pp. 945 – 949.

[2] F. Issa, D. Chaffanjon, E.R. de la Bathie, A. Pacaud, “An efficient tool for modal analysis of multi-conductor transmission lines for PLC network development”, Proc. International Symposium on PLC and its applications, Athens ( Greece ), 2002.

[3] T. Calliacoudas, F. Issa, “Multi-conductor transmission lines and cables solver, an efficient simulation tool for PLC networks development”, Proc. International Symposium on PLC and its applications, Athens ( Greece ), 2002.

[4] L.T. Tang, P.L. So, E. Gunawan, S.Chen, and T.T. Lie “Characterization and modeling of in-building power lines for high speed data transmission”, IEEE Trans. On Power Delivery, Vol. 18, No. 1, January 2003.

[5] ATP Rule Book. Available on www.emtp.org [6] José R.Marti, “Implementation at BPA of a new frequency – dependence

model”, EMTP Newsletter, vol. 2, num. 3, February 1982, pp. 33 – 37. [7] T. Noda, N. Nagaoka, A. Ametani, “Phase domain modeling of

frequency - dependent transmission lines by means of an ARMA model”, IEEE Trans. On Power Delivery, Vol. 11, No. 1, January 1996.

[8] A. Morched, L. Marti, J. Ottevangers, “A high frequency transformer model for EMTP”, IEEE Trans. On Power Delivery, Vol. 8, No. 3, July 1993.

[9] B. Gustavsen, A. Semlyen, “Application of vector fitting to state equation representation of transformers for simulation of electromagnetic transients”, IEEE Trans. On Power Delivery, Vol. 13, No. 3, July 1998.

[10] C. Andrieu, E. Dauphant, D. Boss “A frequency dependent model for MV/LV transformer”, International Conference on Power System Transients, June 20-24, 1999, Budapest, Hungary.

[11] T. Noda, H. Nakamoto, S. Yokoyama “Accurate modeling of core-type distribution transformers for electromagnetic transients studies”, IEEE Trans. On Power Delivery, Vol. 17, No. 4, October 2002.

-70-60-50-40-30-20-10

0

MV2 LV1 LV2 LV3 LV4 LV5 LV6 LV7

Vol

tage

atte

nuat

ion

(dB

)

Phase A Phase B Phase C

-70

-60

-50-40

-30

-20

-10

MV2 LV1 LV2 LV3 LV4 LV5 LV6 LV7

Volta

ge a

ttenu

atio

n (d

B)

Phase A Phase B Phase C

365