Power Line Communication Integrated in a Wireless Power Transfer ...

2014 18th IEEE International Symposium on Power Line Communications and Its Applications

Power Line Communication Integrated in a Wireless Power Transfer System: a Feasibility Study Sami Barmada, Marco Raugi, Mauro Tucci Department of Energy, Systems, Territory, and Construction Engineering University of Pisa Pisa, Italy The in-vehicles applications of PLC systems have been investigated in recent years, and the many results [1] – [18] reveal the interest in the topic. The authors have recently developed new methods and tools for the analysis and modeling of PLC systems [19], pointing attention also to the use of PLC for naval applications [20]. A first analysis of the PLC channel onboard a full Electric Vehicle (EV) has been presented in [21] by the authors while in [22] a feasibility study on the PLC for communication between the vehicle and the power grid (V2G) has been performed. The rationale behind the study reported in [22] is to investigate the impact that EV would have on a Smart Grid environment, in which data might be transmitted through this channel.

Abstract—In this work a characterization of a Wireless Power Transfer (WPT) system as a possible channel for data communication is presented. The frequency response of a real WPT system based on the principle of magnetic resonance coupling is simulated accurately using a lumped equivalent circuit model. The theoretical channel capacity of the channel is calculated as a function of the coupling coefficient of the WPT system, in the presence of additive white Gaussian noise. Furthermore, a general design architecture is presented for embedding a broadband powerline communication, BPLC, system on a WPT system. Keywords—Wireless Power Transfer; Communication; Electric Vehicles; Smart Grid

I.

Power

Line

On the other hand, Wireless Power Transfer (WPT) technology for recharging devices is nowadays attracting research attention as an alternative to a wired connection [23]– [25]. The use of WPT for the recharging of electric vehicles seems a promising potential application of such technology, since the power cable is often subjected to deterioration, mainly caused by outdoor environments and, not rarely, misuse by the operators. The research community demonstrates a growing interest of applying WPT to electric vehicles, and [26] – [29] are just a few examples. The achieved power transfer for such applications is of approximately some hundreds of W at a distance of a few tenth of centimeters.

INTRODUCTION

In the last years, power line communication (PLC) has gained widespread interest as a viable option for broadband communications, from both industry and the scientific community. Commercial modems are, at the present time, capable of reaching speeds of 500 Mb/sec, by using advanced communication techniques. Consequently, an increasing number of applications take advantage from the potential of the PLC technology. In particular, a significant field of application and research is the use of PLC to enable in-vehicle communications and networking, seeing that modern vehicles are provided with a large number of devices and features that require data communication. Some examples include data transfer between sensors and actuators, traction control, and, above all, infotainment. In the automotive field, the most widely spread solution for providing communication links of guaranteed performance quality, is based on employing a dedicated data bus for each link (Controller Area Network, Local Interconnect Network, Media Oriented Systems Transport, FlexRay).

In most of the proposed implementations the frequency operating point is in the MHz range, which is located inside the frequency range used by the Homeplug 2.0 standard for wide band PLC modems. This consideration would theoretically allow the implementation of a V2G communication channel allowing the continuity of the PLC communication between the vehicle and the grid even in the case that battery charging is performed by WPT. This paper is a feasibility study assessing the possibility of implementing such setup, and proposes a set of guidelines for the proper design of the WPT system in order to guarantee an efficient communication channel.

It is commonly recognized, at this time, that PLC is not able to substitute all the types of communication links. This is due to robustness, safety, and standardization issues that have not been completely assessed and resolved so far. For example it is not recommended, presently, to use PLC for providing ABS functionality. On the other hand, PLC may indeed be employed for proving auxiliary features such as the entertainment, air conditioning control, diagnostic signals etc.

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misalignment etc. which can be present in such particular application. The magnetic field develops in air, for this reason all the coupling coefficients are in general low (if compared, for instance, to the typical coupling coefficients for standard low power transformers), allowing small power transfer, but with the correct choice of the frequency of operation (which has to be coincident with the resonant frequency of all the 4 circuits) and the coils quality factors, the global efficiency can be high.

Fig. 1. WPT system for an Electric Vehicle

II.

A. Simulation of the WPT channel frequency response One of the main factors to assess the feasibility of the transmission of a PLC signal in air, through the WPT resonator system, is the attenuation given by the frequency response of the WPT system.

WIRELESS POWER TRANSFER FOR ELECTRIC VEHICLES

The basic scheme of a WPT system for EV charging is reported in Fig. 1. The whole system is based on the transmitting and receiving coils, which is a magnetically coupled resonator system that enables the wireless power transfer. The drive loop and load loop are impedance matching circuits designed for optimal power transfer. Their task is to adapt the impedance of the coils to the impedance of devices, i.e. the radio-frequency (RF) amplifier in the drive loop, and the rectifier in the load loop.

TABLE I.

One of the most implemented circuital solutions is represented in Fig. 2, where the magnetically coupled resonator system is represented by a lumped circuit element system which is composed by four resonant circuits, magnetically linked by coupling coefficients.

TABLE I. CIRCUIT PARAMETERS.

Parameter RSource, RLoad

Value 50Ω

L1, L4

1.0μH

C1, C4

235pF

Rp1, Rp4

0.25Ω

k12, k34

0.10

L2, L3

20.0μH

C2, C3

12.6 pF

Rp2, Rp3

1.0Ω

K23

0.0001 to 0.3

f0

10mHz

In order to evaluate the frequency response of the WPT channel, the authors have considered the real system setup proposed in [31], in which the parameters for the components of the equivalent circuit shown in Fig.2 are given, as in Table I. Therefore the frequency response of the circuit in Fig. 2 has been calculated for different values of the coupling coefficient K 23 and the results are reported in Fig. 3. Among the different curves, the four evidenced ones are relative to an under– coupled case ( K 23  0.005 ), the critically coupled case ( K 23  0.01) and two over coupled cases ( K 23  0.04 and K 23  0.2 ).

Fig. 2. Lumped equivalent circuit of a WPT system

The high frequency generator (with its output impedance Rsource) excites the drive loop which can be modeled as an inductor L1 with parasitic resistance R p1 and a capacitor C1 which makes the drive loop resonant at the frequency of interest. The drive loop is coupled to the transmitter coil, which is realized as a multi turn inductor, modeled by the inductance L2 and parasitic resistance R p 2 , while C2 plays the same role as the capacitance C1 for the drive loop. The coupling coefficient k12 connects the two inductors and is in general a fixed parameter since the drive loop and the transmitter coil are often built in the same device. The same description holds for the receiver coil and the load loop, for this reason the only, and fundamental, parameter subject to variation is the coupling coefficient K 23 between the transmitter and received coil. The coupling coefficient K 23 is of course strictly dependent on the distance between the coils and suffers of distance uncertainty,

Fig. 3. Frequency responses for different coupling coefficient.

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In general, the family of frequency responses shown in Fig. 3, represents the typical responses of a WPT system, and not only the response of the particular circuit considered in the simulation. In fact WPT systems are designed so that the attenuation at the resonance frequency is -6db. In the case of over coupled systems, the resonance occurs at two frequencies, and this represents the design of choice for the WPT systems. The power is transmitted as a high power sinusoid at one of those peak frequencies. By a qualitative analysis we can say that any under–coupled case (not suitable for WPT optimal operation) would not be the best choice as a PLC channel due to the highly selective frequency response. On the other hand over coupled systems tend to be a better channel for PLC. III.

Evaluating the channel capacity in the frequency band 1MHz - 30MHz, by using the Shannon-Hartley expression (1), the data reported in Fig. 4 is obtained. In particular, in Fig. 4 we show the capacity, calculated as in (1), as a function of the coupling coefficient K 23 , and of the injected signal to noise

power ratio db  S I N  , which is not frequency dependent.

Thus, despite the relatively low bandwidth of the channel, if the achievable S/N ratio is high, a high channel capacity can be obtained, under a reasonable noise scenario, allowing for an efficient data transfer between the power grid and the vehicle when it is charged by a WPT system. An interesting result that can be observed from the capacity calculation in Fig. 4, is that the ideal capacity, of the over coupled cases, do not increases significantly when K23 changes from 0.04 to 0.2. Hence the behavior of the PLC system is expected to be stable in any over coupled situation.

EVALUATION OF THE CHANNEL CAPACITY

One of the most important characteristics to be determined when evaluating the possibility of using a channel for PLC is the theoretical channel capacity. The signal sent from the transmitter is filtered by the channel transfer’s function, which is in general selective, and reaches the receiver together with the noise which, in this case, we might think it is mainly produced by the signal generator and RF amplifier. In order to find the ideal channel capacity the Shannon– Hartley’s law can be used B  S f   C   log 2 1   df  N  f   0 





IV.

in which C is the channel capacity in bits per second, B is the bandwidth of the channel, S(f) is the signal power spectrum, N(f) is the noise power spectrum and f is frequency. The signal power spectrum can be expressed as a function of the injected power spectrum and the transfer function, according to 2

S ( f )  H ( f ) S I ( f ) 



There is no global standard on the maximum allowed power for PLC system. However some general guidelines in which the electric field constraints lead to power spectral density limits; as a rule of thumb, the injected PSD should be lower than -50 dBm/Hz in the band commonly used for PLC (up to 30 MHz). Consequently, in order to evaluate the S/N ratio, the knowledge on the noise generated by the RF source is of basic importance. At the time being the authors do not have an accurate indication of such noise level hence the applicability of the PLC technology for WPT devices is evaluated based on the results of Fig. 4. CONSIDERATIONS ON THE PROTOCOL FOR PLC AND ON THE IMPLEMENTATION OF THE SYSTEM



In this work we consider that both the injected power spectrum S I ( f )  S I , and the noise power spectrum N ( f )  N , are frequency independent, and the noise is considered as additive white Gaussian noise, AWGN, at the receiver.

Fig. 4. Channel capacity as a function of S/N ratio

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In order to achieve a correct PLC signal transmission and reception through the WPT coils system, the scheme shown in Fig. 5 is proposed, where we prevent the PLC signal from going through the converters. In the scheme we consider that the power source is represented by the mains, and that a superimposed PLC signal is present at the outlet. The WPT system has two tasks: transmitting the power to the load and enabling the communication to the PLC Device. The high power signal at low frequency is separated from the high frequency, low power PLC signal, by using a low pass and a high pass filter respectively. The power signal is then transformed to DC and then converted to a high power sinusoid at the resonance frequency f 0 of the WPT system (where a peak of the frequency response occurs in Fig.3) by using a DC to radio frequency amplifier. The output of the RF amplifier is filtered by a band pass filter centered at f 0 in order to remove the out of band noise generated by the amplifier. On the other hand the PLC signal is coupled at the exit stage of the amplifier, and a band stop filter with a narrow stop-band centered at f 0 prevents the propagation of the high power signal generated by the DC/RF amplifier through the PLC signal path. At this point, the voltage source Vsource at the input of the drive loop in Fig. 2 is represented by the sum of the high power radio-frequency signal centered in f 0 and the low power PLC signal.

Fig. 5. Scheme of the implementation of the PLC-WPT system

This signal propagates to the load loop through the WPT resonators, according to the frequency response of the system in Fig. 3. At the output of the load loop, a band pass filter centered at f 0 extracts the power signal and feeds a RF/DC rectifier, that provides the power to the DC load. On the other hand a band stop filter centered at f 0 is used to remove the power signal and to obtain the PLC signal, which is coupled to a PLC device.

All the other considerations about PLC on board vehicles (channel attenuation, battery impedance etc) remain valid, since the PLC signal in the proposed technique enters the vehicle basically at the same electrical point as V2G systems. CONCLUSION The analysis carried out in this work demonstrates the feasibility of the use of a WPT system for PLC, achieving theoretical capacities from tens of Kbit/s to few Mbit/s. In this work we calculate the theoretic capacity of the channel by considering its frequency response and additive white Gaussian noise (AWGN). An architecture is proposed for integrating the PLC in a WPT system.

It is worth to note that, while the power transmission is unidirectional, the proposed scheme allows a bidirectional communication. In fact the PLC device connected to the load loop is enabled to transmit data, that will propagate in the same fashion from the RX coil to the TX coil as a low power broadband signal. Frequencies around f 0 should be masked in the PLC protocols, at the physical layer, and this can be easily done in the HomePlug AV devices, that use the OFDM protocol. If we consider that the PLC devices automatically mask the frequencies where the signal to noise ratio is low, then the frequencies around f 0 will be automatically masked, because of the presence of the band stop filters.

As future work, the system performance can be characterized in more detail with a study of the harmonic noise generated by the AC/DC converter, by the DC/RF amplifier and the RF/DC rectifier. REFERENCES [1]

As a final remark, it is important to note that the WPT systems are mainly designed to have two resonance frequencies, and one of the two peaks is not used for power transfer, so the frequencies around that peak are available for the PLC communication. At least a single carrier PLC system could be used that exploit the second resonance peak, which is not used by the WPT system, as a very good channel with -6db attenuation.

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It is noteworthy to say that the solution studied in this paper should not be considered in competition with a hybrid WLAN/PLC approach since wireless data transfer is anyway an alternative solution in any application and the proposed solution retains all the quantities and protocols characterizing PLC systems.

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