REPORT

Semester : VII Branch : Electrical & Electronics Engineering Seminar Topic: PLCC

1.INTRODUCTION

Power line communication (PLC) is a technology that employs the infrastructure of electrical power distributed system as communication medium. PLC technology could provide the consumer with a spectrum of services such as internet, home entertainment, home automation, and enable the electricity supply authority to efficiently mange their distribution networks in a competitive manner.

This technique has immediate attraction for meter communication system, since every consumer is connected to the communication network and that network is owned and controlled by the electricity supply authority. In a meter reading communication system high power signals are transmitted through the network, which are then received by all connected meters. This system has been extensively implemented in Europe and especially in France.

PLC systems can also be used to transfer data inside buildings using power lines discounting the cost of insulating communication cables. A recent survey shows that one third of new broadband customers will choose power line communication by 2012(in Europe and France). PLC technology could also let the power distribution companies open lucrative revenue streams by bundling electricity supply with broadband telecommunication access providing high speed and reliable communication traffic including Internet access.

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1.1 POWER LINE COMMUNICATION

Power line communication, also known as PLC, uses existing power distribution wires to communicate data. This, however, is not a new idea. In 1838 the first remote electricity supply metering appeared and in 1897 the first patent on power line signaling was issued in the United Kingdom. In the 1920's two patents were issued to the AmericanTelephone and Telegraph Company in the field of "Carrier Transmission over PowerCircuits". One would think that the long-ago conceived idea of power line communications would be well developed by now. However, this is not the case because the power line is not well suited for data communication.

The greatest advantage of power line as communication media is its reach to customers. Every building is connected to the power grid and moreover every room has power line contact point. The extent of this existing wiring cannot be matched to any other communication medium. In, addition PLCC can provide a multitude of new services to the users which are difficult to implement by other technologies, e.g., remote electricity meter reading, appliance control and maintenance, energy management, home automation etc.

Power line communication technology has been slow to evolve because the lines were designed solely for the purpose of 50 Hz main power distribution. Unfortunately, power lines are a rather hostile medium for data transmission. The medium has varying impedance, considerable noise, and high attenuation all which can change as different types of devices are connected to the electrical supply.

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1.2 COMMUNICATIONS BACKGROUND

In order to better understand PLC, the following section provides an overview of a general communications system. This will include a discussion of the elements of a communications system, the methods for transmitting data, and performance measures.

1.2.1 Communications System Model

Figure 2.1 shows a simplified model of a digital communications system. The overall objective of a communications system is to communicate information from a source to a destination over some channel.

Communication System Model

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1.2.2 Source and Destination:

The source can be any digital source of information. If the source is analog such as speech, then an analog to digital converter must precede the transmitter. At the receiving end, the decoded information is delivered to the destination.

The source may also compress redundant data, which minimizes the number of bits transmitted over the channel, but can also create a loss of source information. The data is unpacked at the destination to either an exact replica of the source information (lossless data compression) or a distorted version (lossy data compression).

1.2.3 Channel Encoder and Channel Decoder:

Channel coding reduces the bit error probability by adding redundancy (extra check bits) to the bit sequence. The check bits are computed over a k-symbol input sequence to create an n-symbol output code sequence. This determines the code rate Rc where Rc = k/n and Rc ≤ 1. This is the ratio of the number of actual data bits to the total number of bits transmitted. The channel decoder uses the extra bits to detect and possibly correct errors which occurred during transmission.

The number of extra bits added depends on how much error detection and correction is needed. Channel coding (also known as error control coding) is a heavily studied area. It is used to improve performance over noisy channels (such as the power line). Two major classes of codes exist: block codes and convolution codes. Block codes are implemented by combinational logic circuits. Reed-Solomon (RS) codes are a popular block code. Convolution codes (also known as tree codes or trellis codes) are implemented by sequential logic circuits.

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1.2.4 Channel Modulator and Channel Demodulator:

The purpose of the modulator is to take the encoded data and produce an analog signal suitable to propagate over the channel. The data is converted from a stream of bits into an analog signal. At the receiver, the demodulator tries to detect which waveform was transmitted, and convert the analog information back to the sequence of bits. Modulation is typically performed by varying the amplitude, the phase, or the frequency of a high-frequency carrier signal.

For example, if the input signal of the modulator is used to vary the amplitude of the carrier signal, the modulation is called Amplitude Shift Keying (ASK). There are severalother modulation techniques including FSK (Frequency Shift Keying), PSK (Phase ShiftKeying) and QAM (Quadrature Amplitude Modulation).

1.2.5 Channel:

The channel can be any physical transmission medium including coaxial cable, twisted pair, optical fiber, air, water, or for this work - the power line. It is important to know the characteristics of the channel, such as the attenuation and noise level because these parameters directly affect the performance of the communication system.

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2.POWER LINE AS A COMMUNICATION MEDIUM

A power line network is a very huge infrastructure covering most of the part of the inhabited area. Power is typically generated at high voltages and transmitted over high voltage network to medium voltage line then to low voltage line by substations. These substations then distribute the power to large number of customer using low voltage distribution network.

Power line communication uses the available power line network as a communication medium. The greatest advantage of using PLCC is its huge and fine grain network. Already every building is connected to the power grid and moreover every room has power line contact points. The extent of this existing wiring cannot be matched by any other communication network. Thus, the emerging PLCC technology opens up new opportunities for the mass provision of cost effective local access.

A communication channel is defined as the physical path, using which signal propagates between the transmitter and the transmitter and the receiver node. The quantity of any channel depends upon the noise present and attenuation of the channel. If noise at the channel depends upon the noise present and attenuation of the channel. If noise at the receiver is high, then it makes detection of the original signal difficult. Also, as the signal attenuation increases, less signal power is available at the receiver. This makes the detection harder in presence of noise.

Through PLCC is a emerging field, it has its own limitation. The main objective of power lines is to deliver power. Their characteristic depends on the load condition and varies as load changes. As there are different loads on power line whish generate very strong noise signals, it is a very noisy communication channel. Also, as power line are designed to carry electrical power energy, and not for communication, it has very strong attenuation for the carrier signal frequencies. Thus, PLCC is strongly location and time dependent.

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2.1 KEY ADVANTAGES OF PLCC

There are various strong points and key advantages to consider the PLCC as a communication medium.

1. The power grid offer last mile connectivity: Through PLCC, the power line offers what is sometimes called the last mile connectivity to the customer. In this regard, it is not the only last-mile technology, but it appears to emerge as a significant competitor to the other communication medium. Depending situation, varying demand and capability, the overall communication infrastructure will have a hybrid character. The power line is certainly a very good alternative, since it could provide a permanent-access, two-way, always-on-line connection to the customer, 24 hours a day.

2. The power grid supports information based services: Although PLCC technology is recent and in need of further advances, both in terms of telecom capabilities and equipment cost reduction, present relatively limited data rate transfer speeds are sufficient for many useful innovative applications.

3. The power grid is already in place, thus enhancing cost-effectiveness: As the power line infrastructure is already in place, it is potentially cheaper than other forms of local telecommunications access, as it will require tremendous investment to achieve comparable scale and grain size.

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2.2 COMPARING PLCC WITH OTHER RELEVANT TECHNOLOGIES

There are several alternative technologies, beside PLCC, for the data communication and services. Each technology has its own strength and weakness. A state of art comparison between PLCC and other relevant technologies is presented in and summarize as below.

1. Public switched telephone network

Merits:Mature and robust, good installed based, wide choice of product, easy to use and install, relatively cost effective.

Demerits: Relatively slow, on demand only

2. Cable Modem

Merits: Excellent Performance, permanently on lineDemerits: Limited geographical coverage, currently only available in limited trial, connection based.

3. Microwave Communication

Merits: Superior Performance, good geographical coverage, flexible of configuration.Demerits: Costly compared to PLCC, coverage not wide as PLCC.

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3.COUPLING THE SIGNAL ONTO THE CHANNEL

Several closed current paths which may be considered:

1. In the case of differential mode coupling, the "line" wire is used as one terminal and the "neutral" wire is used as the second terminal.

2. In the case of common mode coupling, the "line" and “neutral” wires are used together, forming one terminal, and the "ground" wire serves as second terminal. This coupling mode is known to yield up to 30 dB better coupling than differential mode coupling. In some countries, common mode coupling is not allowed on low voltage networks, due to potential dangers for the customers.

Two ways of connecting the PLCC unit to the network are possible:

1. The first is known as capacitive coupling. A capacitor is responsible for the actual coupling and the signal is modulated onto the network's voltage waveform.

2. The second is known as inductive coupling. An inductor is used to couple the signal onto the network's current waveform.

Inductive coupling is known to be rather lossy. However, no physical connection to the network has to be made, which makes it safer to install than capacitive coupling.

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3.1 BASIC COUPLING CIRCUIT

Figure shows the basic coupling circuit concept.

An inductor or transformer is generally put parallel to the capacitor to form a high pass filter. The high pass filter response passes the higher frequency communication signal. The coupling circuit also includes various protection components such as fuses, and

diodes to combat high voltage surge and short circuit current.

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3.2 COMMUNICATION BETWEEN A POWER LINE

The relevant technology in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network in known as Wave Trap.

Wave trap also known as Line trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/tele-protection panel in the substation control room (through coupling capacitor). The signals are primarily tele-protection signals and in addition, voice and data communication signals. It blocks the high frequency carrier waves (24 kHz to 500 kHz) and let power waves (50 Hz - 60 Hz) to pass through. It is basically an inductor of rating in milli henry.

The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation bus bars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible.

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PLCC IMPLEMENTED BUILDING

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4.LIMITATION OF PLCC

The preferred method of data transmission is carrier communication with digitally modulated signal being transmitted over the power network. However, compared to other communication channels power line present unique problem.

Power line characteristics are highly time and location dependent which represent most critical obstacle for data communication. The noise signal level and attenuation depends partly on the load connected to power line, which vary with time. The communication channel, which is time variant, makes the design of communication system very complex. Thus, at some time a strong noise source could completely block the communication.

The line characteristics differ according to line voltage, i.e., high voltage or low voltage line. Normally in conventional communication, impedance matching is attempted. The power line network is not matched. The input and output impedance vary with time, with different load and location it can be as low as 0.5 ohms to as large as few hundreds of ohms. Variation due to cable impedance mismatch leads to poor degree of transmission.

In developing a potential communication channel using low voltage network, different component of the power line need to be taken into consideration. These include low voltage distribution substation with its inherent transformer, switchgear and cable. Different distribution line parameters vary with type of the cable, age, current, and voltage specification. Thus, the design of a proper and reliable power line communication system is very complex.

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4.1 NOISE AND DISTURBANCE IN POWER LINE

Mean noise spectral density as a function of day.

The noise is always present in all communication media. The 3 voltage network i.e, high voltage (HV), low voltage (LV), and medium voltage (MV) have different source of channel disturbance. The distribution line noise is location and time dependent. As the load on the distribution line changes, noise intensity is reflected accordingly. This also varies according to the time of the day. In residential buildings the peak noise power can be observed on morning or night hours compares to other hours of the day.

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4.2 TYPES OF NOISE

There are four types of disturbances that are present in the power line, they are as follows:-

4.2.1 Impulsive noise:-

Histogram of noise impulses as a function of time of the day.

Impulsive noise is defined as those entire disturbances on the power line that last for very small fraction of time. This type of noise is primarily due to all kinds of switching phenomenon. This type of disturbance affects the whole frequency band but its duration is for very short time.

From the given histogram one can clearly see that there is dependency between the time of the day and the number of impulses. The number of impulses in day time is very large comparing to remaining hours.

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4.2.2 Switching period noise:-

Switching period noise

This type of noise is caused by all kinds of switching devices, which commonly work synchronously to the power system frequency like light dimmers. Switching period noise is a major component in measurements. A very common measurements like the striking one in shown in figure, where a persistent, high amplitude switching period noise is present.

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4.2.3 Non-Switching period noise:-

Non-switching periodic noise.

The appearance of this noise is quite different from the switching period noise. This noise has a continuous, long-lasting behavior in the time domain. The disturbance is periodic and non-synchronous in the power system frequency. This must correspond to some harmonics of devices operating at that frequency.

This kind of noise is usually generated by television set and computer monitors. The repetition of pulse depends upon the screen scanning that varies among the television and computer standards.

4.2.4 Background noise:-

If we subtract all different noise, mentioned earlier, from the total noise measured at a certain location, then the remaining part portion of noise is the background noise. Unlike the other types, the background noise is present all the time on any measurement. This produces large transient voltages on the power line, depending on the size of the capacitor. Wideband additive noise is present across the entire frequency range.

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4.3 ATTENUATION IN PLCC

The attenuation of power line is of prime importance in carrier application because it determines the fraction of transmitted energy available at the receiving end to overcome noise and interference. There is different signal attenuation in the distribution line, as a communication channel, which are as follows.

1. To transmit the modulated signal on distribution line an isolation of transmitter from high voltage signal is required. This is done using a coupler both at transmitter and receiver. The signal experiences attenuation due to coupling circuits, both coupling-in and coupling-out. Such attenuation is unavoidable because direct injection of signal is impossible without isolation. But it can be reduced through better coupling circuits.

2. There is attenuation due to the branching of the circuits. Branching causes discontinuities in power line, which results in that part of the signal gets reflected and part of its gets transmitted. Reflection is undesirable as it causes signal attenuation.

3. The signal attenuation that is due to the impedance of the power line is called Line Attenuation. The power line impedance is dependent on the size and length of the line.

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4.4 IMPEDANCE IN PLC

The opposition in a circuit presents to an electrical circuit in called impedance. The power network is a widely spread network, with the medium/low voltage transformer's secondary as the driving force and many loads connected in parallel. The channel impedance is thus a strongly fluctuating variable, depending on specific loads being connected to the net at specific times. Impedance matching may be important since the signal power at the receiver side reaches a maximum when the impedances of transmitter, receiver and channel are matched. The overall impedance of a network results from:

1. Impedance of the distribution transformer: This increases with frequency. (Note that within the transformer, the various phases are coupled, thus it is possible to transmit a signal across phases without the use of coupling circuits.)

2. Characteristic impedance of the cables used. : A wide variety of cables is used, which can be modeled as a serial connection of inductors and resistors. The value of the characteristic impedance is indicated to vary between 70 Ω and 100 Ω.

3. Impedances of the devices connected to the network. : This typically varies between 10 Ω and 1OOO Ω.

Channel model with different types of impedances

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4.5 PLCC ELECTROMAGNETIC COMPATIBILITY

Power lines are leaky radiate high-frequency electromagnetic signals. Sensitive electronic devices need to be hooked up to power lines having severe perturbations, and carrying voltages far outside the input ranges of these electronic devices. Van der Laan gives a treatise on Electromagnetic compatibility (EMC) problems. He considered disturbances such as transients, voltage fluctuations and harmonics. Reference can be made to the CBEMA curve shown in fig. below, developed by theComputer and Business Equipment Manufactures Association, as a guide for specifying acceptable voltage limits. Van der Laan suggests concentrating on the front end in order to keep disturbances away from electronics. As diversion, one or more filters with passive L, C and R components, restricted to the frequency band of the signal, should precede a limiter. Limiters, which also have a finite life, should be used as little as possible.

The CBEMA curve.

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4.6 MULTI-PHASE NETWORK ERROR

One drawback to the designed protocol is that the design has considered transmission on only a single phase. In an office environment with three phase supply, this means that approximately two thirds of power outlets would be unavailable for transmission. Also, since not all phases are present in one room, access to the network would be limited. A solution to this problem would be to form a separate network for each phase and connect them through a network bridge. A network bridge bridges data across two or more networks (in this case the three phases). A bridge does not forward all data that it receives; rather it has built in logic that makes forwarding decisions based on the destination of the data. In Figure if computer A wants to talk to computer C, and the network bridge recognizes that they are on different networks, it forwards the data on computer C’s network. If computer B wants to talk to computer D, the bridge does no forwarding of data because B and D are on the same phase or network. This approach reduces the amount of data flooding the network.

Three phase power line network.

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4.7 SECURITY

PLCC is shared channel (like WiFi). The transmission of data over a network that anybody has access to could also pose a data security problem, however. Tapping the signal could allow somebody to eavesdrop on communications. Only data encryption eliminates that problem. Robust security is serious issue, as security increases complexity also increases.

However, it is necessary to connect the personal computer installed inside the building to the Internet, resulting in low versatility. A wide range of node capabilities is anticipated. Some will be computers with a full user interface and a powerful processor. Others will be cheap electronic devices, with perhaps a single button that may be pressed to signal intent. In between, we will have televisions, personal video recorders, DSL routers and the like with various user interfaces and computing capabilities. The protocols have to support devices over this entire range.

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5.APPLICATION FOCUSED SERVICES

1. Automated Meter Reading: - Remotely obtain meter reading of customer’s meter, load & consumption data, and detect energy theft.

2. Remote Connect/Disconnect: - Can connect or disconnect electrical supply from central location, in demand side management.

3. Appliance Management: - individual appliances can be monitored and managed. Customer can be informed if appliance has abnormal usage or appears inefficient compared to modern alternatives.

4. Comfort & Savings:- provide home comfort and energy/cost savings function, like, reduction in peak current consumption.

5. Multi-Utility Services: - Same channel can be used to transmit power as well as can be used for data communication, audio communication, etc…

6. Network Control: - Operating efficiencies are targeted through automation of the distribution network.

7. Energy Usage Management: - The customer information interface is used to provide energy usage information and advices.

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6. CONCLUSION

Power line communication (PLC) system is a relatively recent and evolving technology aimed at utilization of electrical power lines for transmission of data. The power line communications channel is a notoriously bad channel which has been developed without regard for any communications considerations. Originally designed for the transmission of electrical signals.

However, it is so widely distributed that considerable cost savings can be achieved, if use is made of its cable infrastructure. Because of the wide geographical coverage of reticulation networks, coordinating bodies have formulated specifications to restrict the bandwidth and power levels of communications signals, in order to limit interference with other users of low frequency radio communications.

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7. REFERENCE

1. Niovi Pavlidou, A. J. Han Vinck, Javad Yazdani and Bahram Honary, “Power Line Communications: State of the Art and Future Trends”, IEEE Communications Magazine, Vol.41 No. 4 pp. 34-39, April 2003.

2. Motorola, MC68360 Quad Integrated Communications Controller User’s Manual.

3. Lars Selander, “Power-Line Communications: Channel Properties and Communication Strategies”, M. Sc. , Lund University, 1999.

4. Echelon Corporation, A Power Line Communication Tutorial – Challenges and Technologies.

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