CHAPTER ONE :

INTRODUCTION

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CHAPTER-1

1.1 Introduction to Power Line Carrier Communication

Power line carrier communication, also known as PLCC, refers to the concept of transmitting information using the electrical power distribution network as a communication channel. This technology allows a flow of information through the same cabling that supplies electrical power. This novel idea of communication helps in bridging the gap existing between the electrical and communication network. It offers the prospect of being able to construct intelligent buildings, which contains many devices in a Local Area Network.

Power line carrier communication 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 signalling was issued in the United Kingdom. In the 1920's two patents were issued to the American Telephone and Telegraph Company in the field of "Carrier Transmission over Power Circuits". 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.

There are two main applications for power line communication - one for broadband Internet access to the home and the other for home and office networking. This work focuses on using power lines for home and office networking.

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, it is difficult to communicate data effectively because the medium was not designed for data transmission. Attenuation, variable impedance, and noise are three factors which make this a harsh medium, making it difficult to achieve optimum signal transfer, low distortion, and high signal-to-noise ratios (SNR).

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1.2 Objectives of the Project

The project aims towards thoroughly exploring the theoretical and practical aspects of power line carrier communication (PLCC) techniques. To this end a number of specific goals were proposed at the start of the project.

1. To gain a detailed knowledge of the challenges faced by PLCC techniques and to understand why they are not a widespread communication methods.

2. To research and design a working PLCC system, which could be employed to couple speech signals with AC signals to transfer voice from one end of a building to the other end using home line AC wiring.

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1.3 Advantages : Motivation for Work

The design of a network should consider several factors, of which the two most important are predicted network traffic and installation cost. The nature of traffic generated by applications such as email, streaming audio or video, file transfer, control systems, application or resource sharing, etc. predicts the type of service needed. The various types of traffic can have different throughput, data integrity, latency, and other requirements. A simple control system network that performs functions such as turning lights on and off, opening and closing the garage door, and controlling the air conditioner does not require high speeds. A high speed network would be much better utilized by a multiple computer network where there is a large amount of file and application sharing or video.

The cost factor refers to the installation cost of a network. High speed networks often require more expensive equipment than low speed networks, so for low speed networks it is not economically smart to install high speed equipment. Installation cost is also affected by the actual setup of the network. Wireless equipment is becoming popular because it is simple to set up and provides high speed and high mobility (computers can access the network as long as they are within a certain distance of the access point). However, the wireless equipment may be too costly for low to medium speed applications. Another solution is to use dedicated network cabling but this is also a high cost solution because retrofitting a home with the required cabling becomes a time consuming and expensive job. Also, once network cabling is installed in a home or office, it does not lend itself easily to reconfiguration – resulting in down time when location of network entities changes.

What is missing is a medium speed technology that is low cost and allows for easy and ubiquitous network access. This project addresses a possible solution to the problem of mobility, ease of installation, and cost of networks by using the in-building power distribution system.

Current power line communication technology that can support applications such as control of devices, network gaming, low resolution image sequences from cameras, security applications and several other applications.

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1.4 Disadvantages : Risk Encountered

It is difficult to communicate data effectively because the medium was not designed for data transmission. Attenuation, variable impedance, and noise are three factors which make this a harsh medium, making it difficult to achieve optimum signal transfer, low distortion, and high signal-to-noise ratios (SNR).

The most important technical problem in power line PLC is to device methods and equipment to couple the low frequency and high voltage signal to the high frequency and low voltage signal. Following problems had to be dealt with

Disturbance created in nearby cast receivers. Protection against high voltage. More than one carrier circuit may cause interference. Switching may cause undesirable disturbances.

The modern practices are achieved by connecting a capacitor of proper rating between

the carrier terminal and high voltage line

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CHAPTER TWO

FOCUS ON PLCC

CHAPTER-2

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2.1 Communication Background

In order to better understand PLC, the following section provides an overview of a

general communications system. This includes a discussion of the elements of a

communications system, the methods for transmitting data, and performance measures.

2.1.1 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 (the

transmission of digital information this thesis considers) from a source to a destination

over some channel.

Figure 2.1 Communication system model.

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).

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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

convolutional codes. Block codes are implemented by combinational logic circuits.

Reed-Solomon (RS) codes are a popular block code. Convolutional codes (also known

as tree codes or trellis codes) are implemented by sequential logic circuits.

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. An M-ary

modulator takes a block of Y binary digits from the channel encoder to select and

transmit one of M analog waveforms at its disposal where M = 2Y and Y ≥ 1. 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 several other modulation techniques including FSK (Frequency Shift Keying), PSK

(Phase Shift Keying) and QAM (Quadrature Amplitude Modulation).

Channel: The channel can be any physical transmission medium including coaxial

cable, twisted pair, optical fibre, 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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2.1.2 Description of Important Performance Parameters

Symbol Rate - This is the transmission rate or number of symbols per second from

the modulator. If the signal duration is T seconds then the symbol rate is 1/T symbols

per second. The symbol rate is also known as the baud rate.

Bits per second (bps) – Also known as bit rate, bits per second is directly related to

the symbol rate. If each symbol represents Y bits and the symbol rate is 1/T baud,

then the bit rate is Y*(1/T) bps. On high quality channels it is easier to send more bits

with one symbol, resulting in higher bps.

Bit Error Probability (Pb) – Pb is the probability that a bit is incorrectly received at

the destination. This is an important performance measure for any digital

communication system that is affected by noise and the disturbances in the channel.

Bandwidth (BW) – The range of frequencies used by the communication system. For a

specific communication method, the bandwidth needed is proportional to the symbol

rate. Bandwidth is a limited resource and is often constrained to a certain small range.

Bandwidth Efficiency – This is the ratio between the bit rate and the bandwidth of a

communication system (bps/BW). Today a telephone system can achieve a bit rate

of 56.6 kbps using a bandwidth of 4 kHz, so the bandwidth efficiency is 56.6/4 =

14.15 bps/Hz.

Noise – This is an unwanted signal on the channel that interferes with the desired

signal. Noise on the power line is a sum of many different disturbances originating from

devices such as television receivers, computers, and vacuum cleaners. The amount of

noise can drastically affect the quality of communication.

Attenuation – When the signal is propagating from the transmitter to the receiver the

signal gets attenuated (loses power). If the attenuation is high, the received signal

power can become low and might not be detected. Attenuation is shown to be high on

a power line, and this puts a restriction on the distance from the transmitter to the

receiver.

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Signal to Noise Ratio (SNR) – This is the ratio of received power to noise power. A

higher SNR makes for easier communication because noise has a smaller effect on the

signal. SNR is also affected by attenuation, which reduces signal power and thus SNR.

SNR can be increased by using filters to reduce noise outside of the bandwidth

occupied by the signal.

Diversity – Used to reduce the error probability of harsh channels. Examples of

diversity are time diversity and frequency diversity. In time diversity the same

information is transmitted at different time instants with the idea that if the channel is

bad at some time instance it might not be at another. Frequency diversity transmits the

same information

in different frequency bands. It can be compared to having two antennas transmitting at

different frequencies; if one of them fails the other might work. Several variations of

time and frequency diversity exist. This thesis explores a form of time diversity,

although not exactly as described above.

2.1.3 Modulation

Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.

Like AM, ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc. Both ASK modulation and demodulation processes are relatively inexpensive. The ASK technique is also commonly used to transmit digital data over optical fiber. For LED transmitters, binary 1 is represented by a short pulse of light and binary 0 by the absence of light. Laser transmitters normally have a fixed "bias" current that causes the device to emit a low light level. This low level represents binary 0, while a higher-amplitude light wave represents binary 1.

Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The

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simplest FSK is binary FSK(BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information. With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency.

Audio FSK :Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented by changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero.

AFSK differs from regular frequency-shift keying in performing the modulation at baseband frequencies. In radio applications, the AFSK-modulated signal normally is being used to modulate an RF carrier for transmission.

Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).

Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is

designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent (and referred to as CPSK).

Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed Differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference

signal to determine the exact phase of the received signal (it is a non-coherent scheme). In exchange, it produces more erroneous demodulations. The exact requirements of the particular scenario under consideration determine which scheme is used.

Quadrature phase-shift keying (QPSK) :Sometimes this is known as quaternary PSK or 4-PSK. QPSK uses four points on the constellation diagram, equi-spaced around a circle. With four phases, QPSK can encode two bits per symbol with gray coding to minimize the bit error rate (BER) — sometimes misperceived as twice the BER of BPSK.

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The mathematical analysis shows that QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal, or to maintain the data-rate of BPSK but halving the bandwidth needed. In this latter case, the BER of QPSK is exactly the same as the BER of BPSK - and deciding differently is a common confusion when considering or describing QPSK.

Quadrature amplitude modulation (QAM) is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by modulating the amplitudes of two carrier waves, using the amplitude-shift keying(ASK) in digital modulation scheme or amplitude modulation (AM) in analog modulation scheme. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature components — hence the name of the scheme. The modulated waves are summed, and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying(ASK),. In the digital QAM case, a finite number of at least two phases, and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as a modulation scheme for digital telecommunication systems. Bandwidth speeds of 6 bytes/hz can be achieved with QAM.

Digital QAM : Like all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the case of QAM, the amplitude of two waves, 90 degrees out-of-phase with each other (in quadrature) are changed (modulated or keyed) to represent the data signal. Amplitude modulating two carriers in quadrature can be equivalently viewed as both amplitude modulating and phase modulating a single carrier.

2.1.4 Error control methods

Error control methods for a communication system can be divided into two categories:

Error detection Error correction

Error detection:

Error detection is the process of monitoring received data and determining when a transmission error has occurred. The most common error detection techniques are:

Redundancy :Redundancy involves transmitting each character multiple times. If the same character is not received a fixed number of times in succession, a transmission error is deduced.

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Echoplex :Echoplex is used in data communication systems where human operators enter data manually from the keyboard. Each character received at the receiver is resent to the transmitter for the operator to confirm that character was actually transmitted.

Exact count encoding:With exact count encoding, the number of ones in each character is the same. The number of ones in a character, and if this total does not equal the present value, then an error has occurred.

Parity checks:In parity checking, a single bit(the bit parity) is added to each data unit to force the octal number of binary ‘1’s in a data unit to be either odd(odd parity) or even (even parity). Parity techniques fail when an even number of bits are in error, making it possible to miss a large number of errors. However parity methods are simple and easy to implement.

Checksum:Checksum is defined as the least significant byte of the arithmetic sum of the data set transmitted. Transmitter and receiver both perform summing operation on the data transmitted, with the checksum appended to the end of a data message at transmission. If the receiver checksum does not equal to the transmitter checksum, an error has occurred. Checksum techniques detect 95% of errors, but are more computation intensive than parity methods.

Cyclic redundancy checks:Cyclic redundancy checking uses a division operation on the transmitted sequence, appending the remainder of the division operation to the message transmitted. At the receiver, this same division process is non-zero, an error has occurred. CRC error checking methods detect approximately 99.95% of errors, but are very computation-intensive.

Error correction methods:Error correction is the process of deducing the correct data that is supposed to be received, and modifying the input data set so that the error ceases to exist. The most common methods of error correction are:

Retransmission: Here, an error detected and the receiver automatically requests for the message to be retransmitted. This method is often referred to as ARQ or automatic request for

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retransmission. ARQ techniques are simple, but can be hindered by the overhead involved with acknowledgement and repeat requests.

Forward error correction(FEC): FEC techniques detect and correct errors at the receiving end without calling for retransmission. With FEC, a number of bits are added to the message. These extra bits are coded in a way that allows for a certain number of errors per message to be detected and corrected. FEC techniques increases message overhead by the addition of these bits, and are relatively computation intensive.

2.2 Existing PLCC Standards

Various protocols have been developed for the purpose of carrier communication over power-line. They differ in their modulation Techniques, channel access mechanisms and the frequency bands they use. A brief overview of most popular protocols is presented here.

2.2.1 X10

The X10 specification was designed for low-bandwidth signalling over power lines within the home. The product was developed by a company in Scotland - Pico Electronics -with the first shipped product to market in 1978. The patent on the standard has since expired and prices have fallen sharply. X10 applications include controlling lights and thermostats as well as devices like the stereo amplifier, garage door opener, television receiver and more.

The X10 system is simple and easy to use. It transmits over the electrical wiring using on off keying (OOK). More precisely, it uses 120 kHz signal bursts, each one millisecond long. These signal bursts are synchronized to the zero crossings (both positive and negative) of the ac power line signal. The specification allows for a signal burst to be within a maximum of 200 µs of the zero crossing point. Each bit transmitted occupies two zero crossings; a binary 1 is represented by a burst followed by a no-burst, while a binary 0 is a no-burst followed by a burst. Also, each one millisecond burst is equally transmitted 3 times to coincide with the zero crossing point of all three phases in a three phase distribution system. Figure 2.2 shows the timing relationship of these bursts relative to the zero crossing.

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Figure 2.2 X10 timing on 60 Hz waveform

An X10 packet, shown in Figure 2.3, encompasses eleven cycles of the power line. It begins with a start of packet identifier consisting of the sequence 'burst, burst, burst, no- burst', which occupies the first two cycles (four zero crossings). The next four cycles represent the House Code, and the last five cycles represent a number code (1 through 16) or a Function Code (On, Off, etc.) This complete block is always transmitted twice, with 3 power line cycles between each group of 2 codes. Hence the total number of power line cycles required to complete a transmission is 2*11 + 3 = 25 power line cycles.

Figure 2.3 X10 packet format.

With this simplicity also comes a low bit rate. 25 power line cycles are required to transmit a frame - which consists of 11 bits. The resulting bit rate is then 60*(11/25) = 26.4 bits/second. This bit rate is only useful for trivial applications, and is much too slow for transmission of audio, video, network gaming traffic, and other higher bandwidth network traffic. However, X10 is inexpensive and easy to use; hence it is a popular choice for basic home automation.

2.2.2 LON works

Lon works is a network protocol created by Echelon Corporation and is intended to support communication between control devices or nodes. Each node in the network – a switch, sensor, motor, motion detector, etc. - performs a simple task. The overall network performs a complex control application such as automating a building. Early standards for the Lonworks protocol used spread spectrum modulation. Spread spectrum communication techniques can be used to improve performance in the presence of tonal noise (noise that is present at specific frequencies only). Spread spectrum improves

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performance by using a wider bandwidth for communication than what is required. The amount of improvement depends on the available bandwidth, or in other words, the degree of spreading. It was first used with a bandwidth of 100 kHz - 400 kHz, but this band was found to be too narrow to provide acceptable performance given the type of noise present on the power line. In addition, European regulations prohibit power line signalling above 150 kHz due to potential interference with low frequency licensed radio services.

Instead, Echelon's latest product, the PLT-22 transceiver, operates using a novel Dual Carrier Frequency mode along with Digital Signal Processing (DSP). The purpose of the DSP is to provide adaptive carrier and data correlation, impulse noise cancellation, tone rejection, and low overhead error correction. The PLT-22 communicates using BPSK with frequency ranges 125 kHz - 140 kHz (primary) and 110 kHz - 125 kHz (secondary). The primary frequency range is used unless impairments prevent successful communication in this range. When this occurs, the PLT-22 automatically switches to the secondary frequency range. The PLT-22 communicates at a raw bit rate of 5 kbps. This is much faster than X10, and hence more useful for more complex control of

electrical devices.

2.2.3 CEBus

In 1984, the Electronic Industries Alliance (EIA) Consumer Electronics Group began an effort whose goal was the formulation of a standard for a communication network for consumer products in the home. The standard came to be called the Consumer Electronic Bus (CEBus). The suite of specifications includes communication on many different types of medium including power line, twisted pair cable, coaxial cable, infrared, radio frequency, and fibre optic. The suite of specifications was labelled EIA-600. The full specification was released in 1992.

This work is concerned with the physical layer coding employed by CEBus. CEBus uses non return to zero (NRZ), pulse width encoding. There are four symbols: ‘1’, ‘0’, EOF, EOP. These symbols are encoded using chirp spread spectrum in the bandwidth 100 kHz to 400 kHz. In spread spectrum, the carrier signal frequency is swept over a range of frequencies. CEBus employs a sequence of up and down frequency sweeps of the carrier that in total for one symbol occupies a period of 100 µs. This symbol interval is the shortest symbol time “1”, or unit symbol time. A 0.1 % margin of error is also defined (100 ns for 100 µs). Also, the time to transmit binary “1” is a unit symbol time (100 µs), while to transmit a binary 0, two unit symbol times are used (200 µs). For random binary data, the average symbol time is then 150 µs, for a bit rate of 7.5 kbps. A unit symbol time is shown in 2.4.

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Figure 2.4 CEBus spread spectrum chirp

One other interesting note in the CEBus standard is coupling between power phases within the home. There are two 60 Hz phases, L1 and L2, in a home that are 180 degrees apart in phase. Home 120V electrical devices - appliances, lights, motors, etc. - normally connect to either L1 or L2. Only 240V devices that connect to L1 and L2 simultaneously provide a signal path between these two branches other than the minimal coupling provided by the distribution transformer. Therefore, a CEBus 120V device on L1 may not communicate with a CEBus 120V device on L2 due to inadequate signal coupling between L1 and L2. To help solve this problem, the CEBus standard says that a signal coupler should be placed between L1 and L2 when needed to improve signal propagation within the power line network.

2.2.4 HomePlug

HomePlug is a non-profit consortium founded in March 2000 by thirteen leading IT companies who have a mutual interest in high-speed networking technologies over power lines. Its membership of now more than 80 companies includes companies specializing in semi-conductor manufacturing, hardware/software supply, and service. The goal of the consortium is to create an open specification for high speed power line networking technology and to promote new products to accelerate its adoption. In June 2001, the HomePlug v1.0 specifications were published.

The HomePlug specification is the most complex of all power line technologies. To achieve higher bit rates, higher frequencies and bandwidth must be used than that for X10, Lonworks and CEBus as discussed above (whose frequencies are less than 500 kHz). HomePlug communicates using Orthogonal Frequency Division Multiplexing (OFDM) in the 4.49 to 20.7 MHz frequency band. This method of multiplexing divides up the available bandwidth into sub-bands. These sub-bands are mathematically orthogonal, meaning that for the specific symbol rate they are placed at specific intervals in the frequency domain that minimizes interference between them. In the bandwidth 0 -25 MHz, there are 128 evenly spaced sub-carriers of which HomePlug uses 84, from the band 4.49 to 20.7 MHz (carriers 23-106 inclusive).

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Before forming a symbol to be transmitted, data bits are processed using several error control coding schemes. Data bits are modulated onto the sub-carriers using differential quadrature phase shift keying (DQPSK), or differential binary phase shift keying (DBPSK). The Inverse Fast Fourier Transform (IFFT) is used at the transmitter to create individual channel waveforms. The whole process is reversed at the receiver .

In addition to this, HomePlug adapts to channel conditions. Special frames are sent and analyzed by receivers to determine which of the 84 sub-carriers are available for communication. Tone Maps (TM) are then created and used by sender-receiver pairs to adapt to varying channel conditions. Only good sub-carriers are used for communication. Also, the modulation scheme can be changed (DBPSK or DQPSK), and the error-control coding can be modified. Altogether, 139 distinct physical data rates are available from 1Mbps to 14.1 Mbps .

Several manufacturers have demonstrated HomePlug technology and it looks promising. Field tests with HomePlug V1.0 devices in 500 homes show that 80% of outlet pairs were able to communicate with each other at about 5 Mbps or higher, and 98% could support data rates greater than 1 Mbps [13]. The HomePlug alliance has announced plans for the development of next generation specifications. Named HomePlug AV, the new specification will be designed to support distribution of data and multimedia-streaming entertainment including High Definition television (HDTV) and data rates of 100 Mbps throughout the home.

The ability to adapt is the real strength of HomePlug. Obviously if the power line channel becomes harsh for communication, data rates will be slow, but reliability will be maintained. Note that HomePlug employs a method of frequency diversity. It also uses complex error control coding and modulation techniques that are good for reliability, but are computationally demanding, power consuming and expensive. HomePlug provides high enough data rates for medium speed, but its complexity and cost is more than necessary.

2.2.5 The Need for a Medium Speed Technology

In the coming years, people are likely to use PLC to network anything that is electrically powered such as heating, ventilation, and air conditioning (HVAC). Different devices and applications will have different throughput requirements ranging from 10 bps to 100 Mbps or more. Simple control of devices (turning lights on and off, controlling thermostat, etc.) is achievable with low bit rates (< 5 kbps). High quality video and high- speed computer networking is at the opposite end of the spectrum requiring speeds up to 100 Mbps.

From the different technologies given above one can see that there exists low cost, reliable systems (X-10, Lonworks, and CEBus) and that high speed systems exist using

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the HomePlug protocol. These systems work well for what they were designed, but have several drawbacks. There really isn’t a device designed for “medium speed” (100 kbps to 1 Mbps). Therefore this project focuses on a low complexity, low cost medium speed technology for power line communication.

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2.3 PLCC Implementation Challenges

The ChannelThe characteristics of the channel must first be explored in order to design a timed transmission protocol. Varying impedance, considerable noise, and high attenuation are the main issues. Channel characteristics depend on the location of the transmitter and receiver in the specific power line infrastructure and are both time and frequency dependent. Hence, the channel can be described as random time varying with a frequency-dependent signal-to-noise ratio (SNR) over the communication bandwidth.

2.3.1 Attenuation

Attenuation is the loss of signals strength as the signal travels over distances for a transmission line the input impedance depends on the type of line. Its length and the termination at the far end. The characteristics impedances of a transmission line (Z) is the impedances measured at the input of this line when its length is infinite. Under these conditions the type of termination at the far end has no effect. A standard distributed parameter model can be obtain the characteristics impedances of an unloaded power

cable, and it is given by the equations that follows

At the frequencies of interest for PLC communication (the high frequency range), this approximates to

, where L and C are the line impedance and capacitance per length.

High frequency signals can be injected on the power line by using an approximately designed high pass filter. Maximum signal power line will be received when the impedances of the transmitter, power line and the receiver are matched. Power line networks are usually made of a variety of conductor types and cross sections joined almost at random. Therefore a wide variety of characteristics impedances are encountered in the networks.

Unfortunately, a uniform distributed line is not a suitable model for PLC communications, since the power line has a number of load (appliances) of differing impedances connected to it for variable amounts of time. Channel impedance is a strongly fluctuating variable that is difficult to predict. The overall impedances of the low

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voltage network’s loads, so the small impedances will play a dominant role in determining overall impedances.

Overall network impedances are not easy to predict either. The most typical coaxial cable impedances used are 50 and75 ohm coaxial cables. A twisted pair of guage-22wire with reasonable insulation on the wires measures at about 120ohms. Clearly, channel impedances are low. This presents significant challenges when designing a coupling network for PLC communications. Maximum power transfer theory states that the transmitter and channel impedances must be matched for maximum power transfer. With strongly varying channel impedance, this is tough. We need to design the transmitter and receiver with sufficiently low output/input impedance respectively to approximately match channel impedances in the majority of expected situation.

2.3.2 Impedance

Power line impedance is important because a transmitter must match this impedance over the desired frequency range to avoid frequency dependent distortion of a broadband signal. Nicholson and Malack measured line impedance in the frequency range 20 kHz to 30 MHz at 36 different commercial locations in the United States. They found that the characteristic impedance increased with frequency. The impedance, averaged over all sites, was approximately 1 Ω at 20 kHz increasing to 100 Ω at 30 MHz. Similar results were obtained in European countries and in Japan. Nicholson and Malack explained that any variation from site to site was attributed to variations in load connected to the line. Figure 2.6 is extracted from their work to show the frequency range 50 to 500 kHz.

2.3.3 Noise

The major sources of noise on power line are from electrical appliances, which utilizes the 50Hz electric supplies and generate noise component, which extend well into the high frequency spectrum. Apart from these induced radio frequency signals from broadcast, commercial , military, citizen band and amateur stations severly impair certain frequency bands on power line. The primary sources of noise in residential environments are universal motors, light dimmers and televisions. This noise can be classified as:

50 Hz periodic noise Noise synchronous to the sinusoidal power line carrier can be found on the line. The sources of this noise tend to be silicon-controlled rectifiers (SCRs) that switch when the power crosses a certain value , placing a voltage spike on the line. This category of niose has line spectra at multiples of 50Hz.

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Single-event impulse noiseThis category includes spike placed on the line by single events, such as a lighting strike or a light switch turn on or off. Capacitor banks switched in and out create impulse noise.

Periodic impulsive noise The most common impulse noise sources are triac controlled light dimmers. These devices introduce noise a they connect the lamp to the AC line part way through each AC cycle. These impulse occur at twice the AC line frequency as this process is repeated every ½ AC cycle.

Continuous impulsive noiseThis kind of noise is produced by a variety of series wound Ac motors. This type of motor is found in devices such as found in vacuum cleaner, drillers, electric shavers and many common kitchen appliances. Commutator arcing from these motors produces impulses at repetition rates in the several kilohertz range. Continuous impulsive noise is the most severe of all the noise sources.

Non-synchronous periodic noiseThis type of noise has line spectra uncorrelated with 650Hz sinusoidal carriers. Television sets generates noise synchronous to their 15734 Hz horizontal scanning frequency. Multiples of this frequency must be avoided when designing a communications transceiver. It was found that noise levels in a closed residential environment fluctuate greatly as measured from different locations in the building. Noise levels tend to decreases in power level as the frequency increases in other words, spectrum density of power line noise tends to concentrate at lower frequencies. This implies that a communication carrier frequency would compete with less noise if its frequency were higher.

Background noise This is what every subscriber sees as already present on the line, and not caused by subscriber appliances. Typically, this originates from the distribution transformer, public lighting systems etc.

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CHAPTER THREE

DEVELOPMENT OF PLCC DEVICE:DESIGN ISSUES

3.1 Design issues

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For the design of any communication system, we have to address a number of important design issues. Modulation technique and transmission methods need to be selected to give suitable performance in the communication environment of choice. Our communication environment, i.e., the power line network possesses some unique design issues of its own.

3.1.1Choice of the modulation methods

Transmission of data across a noisy communication channel requires some manner of separating the valid data from the background noise. The most common way to accomplish this is to modulate the data at the data at the transmission end and to demodulate the data on the reception endpoint, to make sure that the data coming from the receiver is the same as the data being presented to the transmitter. The efficiency of the modulation/demodulation process determines the accuracy of the data coming from the receiver. Therefore, careful consideration must be given to the selection of an appropriate modulation-demodulation scheme.

The modulation band selected for power line communications must meet the required data rate while maximizing resistance to noise and interference with the signal because in any power line, there are several sources of noise and interference, each with its own individual characteristics.

ASK is the simplest scheme but is very rarely used, because of its relatively poor noise performance. The amplitude variations in an ASK signal becomes a source of difficulty. Such signals when amplified by nonlinear amplifiers generate spurious out-of-band spectral components, which are filtered out only with difficulty.

Unlike ASK, a carrier is always present with FSK modulation. This affords the designer several benefits. First, the carrier will load the receiver at all times providing greatly increased noise immunity. Secondly, the strength (or amplitude) of the carrier can be used to determine the quality of the incoming signal. FSK is a ‘non-return to zero’ modulation method. This means that the non-modulated condition is between the “off” and “on ” condition. In other words, the carrier should never be at the centre frequency when modulation is present. The benefit here is noise immunity. Since FSK relies on frequency change, and not amplitude change, to indicate data states, an FSK receiver is inherently immune to amplitude noise. This increased noise immunity suggests a potential for higher data rates.

In fact, FSK systems can achieve significantly higher data rates than the ASK counterparts, albeit at the sacrifice of cost and power consumption.

Considering now the PSK techniques, BPSK and QPSK generate discontinuities in the carrier phase, which are further sources of difficulty. When it is necessary to avoid such

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amplitude and phase discontinuities, frequency modulation is the feasible solution. The FSK waveform has constant amplitude and no matter how discontinuities the modulating waveform maybe, its phase is continuous. Phase delay in the PLC channel is expected and is also unpredictable. The reliable performance of FSK with any reasonable amount of phase delay makes it the modulation scheme of choice for PLCC techniques.

3.1.2Choice of carrier frequency

Generally, the power frequencies are very low such as 50 or 60Hz. Hence if we use low carrier frequency over power frequency then it would be very difficult to separate out the carrier signal from the power signal. Again, the power currents have higher harmonics and due to surge currents and corona effects higher frequency currents are generated. These generally lie between 100 to 50,000Hz. So if carrier frequency is chosen within this range then noise introduced in this path will be large and signal to noise ratio would become too low. However, if the carrier frequency is chosen as high as 500 KHz then the open long line behaves as Arial and radiates out such high frequency signal. For this reason frequency between 50 to 500 KHz is chosen to be used as carrier frequency over the power lines.

The type of channel equipment and bandwidth being used will dictate the minimum frequency separation requirements. Table 3.1 has typical values for FSK transmitters to transmitters (uni-directional) and transmitters to receivers (bi-directional). These tables reflect the minimum requirements, assuming a 15 dB isolation is provided with an external device between the equipment.

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Table 3.1 – Frequency Spacing Requirements in kHz for FSK Equipment

The typical frequencies used in Power Line Carrier range from 30 to 500 kHz. In considering which frequency to use for the specific application, several things must first be considered.

1. Application requirements• What are the bandwidth/frequency spacing requirements?• Is there interference from other sources?

2. Surrounding frequencies in use

3. Frequency Planning

4. Coupling Method

5. Line configuration for noise and attenuation considerations

6. Overhead and/or power cable

3.1.3 Selection for error control method in the PLCC environment

In general, the complexity of the PLCC system influences the selection of the error control method. The optimal solution is to use FEC error detection and correction methods to cope with the majority of errors, and then CRC and ARQ detection and correction methods to cope with errors missed by FEC. However, such a system would be complex to implement. A less complex system with moderate performance would be straight ARQ techniques, with cyclic redundancy checking to detect errors or for simpler system. Straight parity checking and ARQ. Accordingly, we implement parity checking

and ARQ.

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3.2 Coupling Strategy

Once the data signal has been generated, it needs to be placed on the power line by some kind of coupling network. The idea is to superimpose the data signal onto the 240V, 50Hz power waveform, and extract it afterwards at the receiving end. There are three possible combinations of lines on which to couple a signal.

Live to ground, Neutral to line Neutral to ground

Differential mode coupling is a scheme where the live wire is used as one terminal and the neutral as the other. In case where a neutral line is not present the ground line acts as the second terminal.

Common mode coupling involves the live and neutral being treated as one of terminal and the ground as the other. Such a kind of coupling is not potentially safe and hence is not used.

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For our coupler implementation we use differential mode coupling. The basic component used for the coupling may be capacitive or inductive. Inductive coupling provides a physical separation between the power network and communication network, making it safer to install. For safety reasons we have decided upon using an inductive coupling method.

At the receiver side the coupling device should posses a band pass characteristics, blocking the 50Hz mains voltage and passing signal at the carrier frequency . At transmitter side, the coupler should posses high pass properties, passing the communication signal un-attenuated. The coupler should also be impedance matched to the power line for maximum power line for maximum power transfer.

3.2.1 Modes of coupling to power line

As with most systems, there is more than one way to couple the carrier to the power line. The deciding factor may be economic, performance or a compromise of the two. That is, the best performance may be expensive to justify for the line being protected so the next best one may be the preference. Most protective relay channels use single-phase-to-ground coupling, requiring only one set of coupling equipment (line tuner, coupling capacitor and line trap). Multi-phase coupling may be used to improve dependability, but requires multi-sets of coupling equipment. As stated before, the coupling schemes with least losses (ranked in order of least losses) are shown below:

• Mode 1 Coupling (Out on two outer phases, in on the center phase) • Center phase to outer phase (push-pull)• Center phase to ground• Outer phase to outer phase with ground return (push-push) • Outer phase to ground (only on short lines)

The major methods of coupling are discussed as below :

Ground to phase coupling

The best single-phase-to-ground scheme uses the centre phase for coupling. The centre phase provides the most mode 1 coupling. Using one of the outside phases will introduce more mode 2 and mode 3 coupling than desired. Figure 3.1 shows an example of phase-to-ground coupling.Here the carrier terminals are connected between one phase conductor and the ground. The wave trap prevents the carrier currents from flowing to the power station side. The ground is used as the return path. Advantage of using this method is that only half of the coupling devices are required and the cost is lowered.

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Figure 3.1. Single Phase-to-Ground (Center Phase) Coupling

Phase To Phase Coupling

Some applications will require more dependability. When the protected line is of significant importance and the type of protection requires receipt of the signal during an internal fault, multiphase coupling improves ependability of the signal being transmitted through the fault. Since the most frequent type of power system fault is a phase to ground, you can improve your chances of receiving the signal through the fault if more that one phase is used. Figure 3.2 shows how to couple using the push, pull type coupling

It provides metallic go and run paths to the carrier currents. In this case the uncoupled conductor does not have any appreciable influence on the transmission. It has such some advantage as It has less attenuation & transmission characteristics are very constant. Hence, radiation loss is smaller & signal/noise ratio is high. Hence it is used although costlier.

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Figure 3.2. Phase-to-Phase Coupling

3.2.2 Coupling Equipment:

Coupling Capacitors:

The coupling capacitor is used as part of the tuning circuit. The coupling capacitor is the device which provides a low impedance path for the carrier energy to the high voltage line, and at the same time blocks the power frequency current by being a high impedance path at those frequencies. It can only perform its function of dropping line voltage across its capacitance if the low voltage end is at ground potential. Since it is desirable to connect the line tuner output to this low voltage point a device must be used to provide a high impedance path to ground for the carrier signal and a low impedance path for the power frequency current. Depending on line voltage and capacitor type, the capacitance values in use range from 0.001 to .05 microfarads.

Line Tuners

In conjunction with the coupling capacitor, the line tuner provides a low loss path to the power line for the carrier signal. There are two basic types of line tuners, resonant and broad-band. The type used depends on the transmission line and the number of carrier channels to be placed on the line.

Line Traps

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To block the energy from going back into the bus and direct it toward the remote line terminal. This device is called a line trap. The general design of a line trap is that of a parallel LC circuit. This type of a circuit presents high impedance to the carrier signal at its resonant frequency. Thus if the parallel LC circuit were placed in series with the transmission line, between the bus and the coupling capacitor, then the carrier signal would propagate toward the remote terminal. The line trap must be capable of providing a very low impedance path to the power frequency current. The inductor in the trap provides this path, and it is designed to carry the large currents required. Another important function of the line trap is to isolate the carrier signal from changes in the bus impedance, thus making the carrier circuit more independent of switching conditions.

Hybrids & Filters

The purpose of the hybrid circuits is to enable the connection of two or more transmitters together on one coaxial cable without causing inter modulation distortion due to the signal from one transmitter affecting the output stages of the other transmitter.

Hybrids may also be required between transmitters and receivers, depending on the application. High/low-pass and band-pass networks may also be used, in some applications, to isolate carrier equipment from each other.

3.3 Power Line Characteristics at RF

Carrier frequencies exceed power frequencies by a factor of 500 or more. As a result, a transmission line’s response to carrier frequencies will be different from its response to power frequencies. At the power frequency, all power lines are electrically short in terms of wavelength. At carrier frequencies, however, most lines are many wavelengths long because of the much shorter wavelength. The (ƒC) frequency to wavelength (λ) relationship is approximated by:

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Remember that c= 3 x108 meters/seconds (speed of light) or 186,000 miles/second. From this relationship it is clear that a 250 kHz signal will have a wavelength of 1,176 meters (0.73 miles). This means that a 100 Kilometer (62 mile) line will be 85 wavelengths long. At 60 Hz, this line will be only 0.02 of a wavelength long.

3.3.1 Transformer Characteristics at RF

There is no up-to-date reference available on the impedances of power transformers at the carrier frequencies. The discussion below is a general discussion based on past experience, and it must be remembered that the results may be entirely different.

Generally power transformers are accepted as being a high shunt impedance at the carrier frequencies. Depending on their location in the carrier channel, their effect may or may not affect carrier channel performance. It is also commonly accepted that a power transformer connecting two transmission lines of different voltages constitutes a broad band high-frequency blocking device, preventing carrier on one line from reaching the other. Thus when a power transformer is at the terminal location of a carrier channel it will probably appear to the carrier signal as a trap.

If the transformer is terminating a load tap, what effect does it have on the carrier signal? That depends largely on the effective RF impedance to ground of the transformer and how far the transformer is from the tap point. Testing has shown that delta connected windings are more capacitive than wye-connected. This high capacitance produces a lower impedance to ground than might be expected. As discussed in the previous section, if the tap is an odd quarter wavelength long, then the impedance presented to the carrier channel is the opposite value of the terminating transformer impedance. That is, if the transformer impedance is low, then the impedance at the tap point will be high and the tap will have little effect. On the other hand, if the transformer impedance is high, then the impedance at the tap point will be low and the tap will have a significant effect on the carrier channel. In the case of taps at even quarter wavelengths, the high terminating impedance will be reflected as a high impedance with little effect on the channel and the low terminating impedance is reflected as a low impedance with a large effect.

3.3.2 Effects of Mismatches

Any time there is a change of impedance along the carrier signal path, there will be some reflection of the signal. This reflection is caused by the mismatching of the impedances. An example of this is when an overhead line is combined with a power cable circuit. This

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reflection results in a loss of the carrier signal in the transmitted direction. This loss can be calculated by the following equation:

where ML is mismatch loss.

3.3.3 Characteristic Impedance

The characteristic impedance of a transmission line is defined as the ratio of the voltage to the current of a traveling wave on a line of infinite length. This ratio of voltage to its corresponding current at any point the line is a constant impedance, Z0. Carrier terminals and line coupling equipment must match the characteristic impedance for best power transfer.

In practice, the jωC and jωL are so large in relationship to R and G, this equation can be reduced to :

By applying appropriate formulas for L and C, this equation can be expressed in terms of the distance between conductors and the radius of the conductor as follows:

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CHAPTER FOUR

DEVELOPMENT OF PLCC DEVICE:DESIGN IMPLEMETATION

4.1 PLC System Overview

Our device consists of two parts TRANSMITTER and RECEIVER. Here both ends are Transceiver modules; however the calling end is referred to as Transmitter while the called end is sighted as the Receiver. Each room may have the equipment connected to each other through power line. Transmitted signals shall reach the intended receivers, and

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the reply must also be obtained to the calling end. We have demonstrated the working by only two transceiver modules.

Figure 4.1 The PLC System Block Diagram

4.1.2 Design Cosideration

Many factors will affect the reliability of a power line carrier (PLC) channel. The goal is to get a signal level to the remote terminal that is above the sensitivity of the receiver, and with a signal-to-noise ratio (SNR) well above the minimum, so that the receiver can make a correct decision based on the information transmitted. If both of these requirements are met then the PLC channel will be reliable. The factors affecting reliability are:

• The amount of power out of the transmitter.

• The type and number of hybrids required to parallel transmitters and receivers.

• The type of line tuner applied.

• The size of the coupling capacitor in terms of capacitance.

• The type and size, in terms of inductance, of the line trap used.

• The power line voltage and the physical configuration of the power line.

• The phase(s) to which the PLC signal is coupled.

• The length of the circuit and transpositions in the circuit.

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• The decoupling equipment at the receiving terminal (usually the same as the transmitting end).

• The type of modulation used to transmit the information, and the type of demodulation circuits in the receiver.

• The received signal-to-noise ratio (SNR).

The above list may not be all inclusive, but these are the major factors involved in the success or failure of a PLC channel. The paper will deal with each one of the above items in detail, and then use this information to design a reliable power-line carrier channel using an example.

Figure 4.2. Basic Power Line Carrier Terminal

4.2 Power Coupling Components and Circuits

4.2.1 Coupling Capacitors

The coupling capacitors will be discussed before the line tuners since they play a large part in the response of the line tuner. In fact the

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coupling capacitor is used as part of the tuning circuit. The coupling capacitor is the device which provides a low impedance path for the carrier energy to the high voltage line, and at the same time blocks the power frequency current by being a high impedance path at those frequencies. It can only perform its function of dropping line voltage across its capacitance if the low voltage end is at ground potential. Since it is desirable to connect the line tuner output to this low voltage point a device must be used to provide a high impedance path to ground for the carrier signal and a low impedance path for the power frequency current. This device is an inductor and is called a drain coil. The coupling capacitor and drain coil circuit are shown in Figure 4.3.

Figure 4.3 Coupling Capacitor & Drain Coil Combination

It is desirable to have the coupling capacitor value as large as possible in order to lower the loss of carrier energy and keep the bandwidth of the coupling system as wide as possible. However, due to the high voltage that must be handled and financial budget limitations, the coupling capacitor values are not as high as one might desire. Technology has enabled suppliers to continually increase the capacitance of the coupling capacitor for the same price thus improving performance. Depending on line voltage and capacitor type, the capacitance values in use range from 0.001 to .05 microfarads.

4.2.2 Line Tuners

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In conjunction with the coupling capacitor, the line tuner provides a low loss path to the power line for the carrier signal. There are two basic types of line tuners, resonant and broad-band. The type used depends on the transmission line and the number of carrier channels to be placed on the line.

The line tuner should be mounted either in the base of the coupling capacitor, if space is available, or on the structure that supports the coupling capacitor. The reason is that the lead between the coupling capacitor and tuner should be as short as possible. Since the coupling capacitor is part of the filter circuit, the point of connection between it and the line tuner is generally a high impedance point. Any capacitance to ground in the connecting cable will cause losses and change the tuning circuit characteristics. This cable is typically a single conductor that is insulated for high voltage and has a very low shunt capacitance to ground. As mentioned before, coaxial cable should not be used for this connection.

All line tuners will have a protector unit which is connected from the output lead to ground. This protector unit must consist of a grounding switch and a protective gap. The gap is present to protect the tuner from failure during large transients on the power line. These transients have large amounts of high frequency energy which is passed by the coupling capacitor and are present at the tuner because the drain coil is high impedance to these frequencies. The grounding switch is for personnel protection during maintenance. Sometimes the line tuners are supplied with a drain coil in addition to the one supplied in the coupling capacitor. This drain coil should not be considered as the primary drainage path. The coupling capacitor must always have a drain coil and it is considered the primary drainage path for power frequency currents.

Resonant-Single Frequency

The single-frequency tuner, shown in Figure 4.4, has a single inductor and a matching transformer. The inductor is arranged so that it and the coupling capacitor form a series resonant circuit. When this circuit is tuned to the carrier frequency it will provide a low impedance path for the carrier signal to the power line. The matching transformer provides the impedance match between the 50 or 75 ohm coaxial cable and the characteristic impedance of the power line (150 to 500 ohms). This tuner will tune at one frequency,thus the name single frequency tuner.

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Figure 4.4. Single Frequency Line Tuner

Resonant-Double Frequency

The double-frequency tuner, on the other hand, has two sets of resonant circuits so it may be tuned to pass two frequencies to the power line. The two-frequency tuner shown in Figure 4.5 not only provides a low loss path for two frequencies, but it also isolates the two sets of carrier equipment from each other. As seen in Figure 4.5 there are two paths, each with its own matching transformer and series inductor, but each path also has a parallel LC circuit used for blocking the carrier signal from the other path. Each path is tuned to series resonance with the coupling capacitor at its given frequency, and the parallel LC circuits are tuned to resonate at the frequency passed by the other path. For the two-frequency tuners, the minimum frequency separation is generally 25 per cent of the lower frequency or 25 kHz, whichever is smaller.

Figure 4.5. Double Frequency Line Tuner

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Broad Band Tuners

If it is desired to place more than two narrow band frequency groups on the line then one must use broad-band coupling. There are two forms of broad-band coupling used: high-pass and band-pass tuners.

High-pass

The high-pass tuner is the simpler of the two and in most cases is the preferred type. It is usually small enough to fit in the base of the coupling capacitor and as a result does not need an extra outdoor cabinet. Another advantage of the high-pass tuner is that the high impedance lead to the coupling capacitor is very short and not exposed to the elements. The high-pass tuner is shown in Figure 4.6. The equivalent circuit for the high-pass tuner is shown in Figure 4.7. Note that the coupling capacitor is used as one of the series branches of the high-pass circuit. The low-frequency cutoff of the circuit is determined by the size of the coupling capacitor and the terminating impedance of the power line. One should not apply any carrier frequencies close to the cutoff frequency of the circuit since it does not have a stable characteristic impedance in that area. The high-pass tuner has one coaxial cable input. Therefore, all of the carrier sets must be paralleled using the principles described in the section on “Paralleling Transmitters & Receivers.”

Figure 4.6. High Pass Tuner

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Figure 4.7. Equivalent Circuit of High Pass Tuner

Band-pass Tuner

This tuner provides a large bandwidth with a constant coupling impedance over a band of carrier frequencies. The band-band tuner is as shown in Figure. The bandwidth of the band-pass tuner depends on coupling capacitance, the terminating impedance, and the square of the geometric mean frequency (GMF) to which the filter is tuned. One should be careful in applying frequencies too close to the band edges of a band-pass tuner since this area can change with varying temperature and changes in standing waves which may be produced on the power line due to changes in line termination.

ner

Figure 4.8. Band-pass Tuner

4.2.3 Line Traps

When the carrier signal is coupled to the power line it can propagate in two directions, either to the remote line terminal or into the station bus and onto other lines. If the signal goes into the station bus much of its energy will be shunted to ground by the bus capacitance. Also some of this energy would propagate out on other lines thus transmitting the signal to a large portion of the system. This is undesirable since the same frequency may be used on another line. Because of these problems, a device is needed to block the energy from going back into the bus and direct it toward the remote line

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terminal. This device is called a line trap. The general design of a line trap is that of a parallel LC circuit. This type of a circuit presents high impedance to the carrier signal at its resonant frequency. Thus if the parallel LC circuit were placed in series with the transmission line, between the bus and the coupling capacitor, then the carrier signal would propagate toward the remote terminal. The line trap must be capable of providing a very low impedance path to the power frequency current. The inductor in the trap provides this path, and it is designed to carry the large currents required. Another important function of the line trap is to isolate the carrier signal from changes in the bus impedance, thus making the carrier circuit more independent of switching conditions.

Figure 4.9. Characteristic of Single Frequency Trap

Figure 4.10. Characteristic of Double Frequency Trap

Note that both the single- and double-frequency traps have a rather sharp resonance peak which provides a 7,000 to 10,000 ohm blocking impedance at one given frequency, as shown in following figures. On the other hand, the wide-band trap will block a large

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bandwidth of frequencies but its blocking impedance is low, on the order of 500 ohms. Therefore, the resonant traps will have less losses than the wide-band type.

Figure 4.11. Characteristics of a Band-pass Trap

Line traps come in several versions just as the tuners do, and these types are single-frequency, double-frequency, and band-pass. Usually the trap used is the same type as the line tuner, that is, if the tuner is a single-frequency type, the trap will also be a single-frequency type. However, it is not absolutely necessary that the line trap be of the same type as the tuner. As an example wide-band traps could be used at all times. The question of economics and blocking impedance will dictate the type of trap to be applied.

Single Frequency and Double Frequency Figure 4.9 shows the typical characteristic for the single-frequency trap and Figure 4.10 shows double-frequency traps. The trap can have both a low-Q and a high-Q setting. The low-Q setting of the trap provides a lower blocking impedance, but has a wider bandwidth. This setting can thus be used to couple two or more very close frequencies to the line. The high-Q setting of the trap provides the normal high blocking impedance, but it has a very narrow bandwidth which may be very susceptible to variations in the bus impedance. The bus is capacitive at carrier frequencies and it can form a series resonant circuit with the inductance of the trap, and this then can create a low impedance path to ground. Power transformers on the line behind the trap have been known to affect the

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trap and change the tuning characteristic. These types of effects can be detected by comparing the received signal at the other end for two conditions. The first level is measured with the disconnect switch between the trap and the bus open. The second level is with the line normal (disconnect closed). If the signal level changes by a large amount between these two conditions and you are certain the trap is tuned properly, then the low-Q setting should be selected since the station impedance will have less effect on the trap tuning. The channel losses will be a little higher, but the channel will be less affected by switching conditions. Note, that not all traps have the low-Q option, and you should check with the manufacturer of the trap.

Wide-band When applying a wide band trap, two things must be decided, that is, bandwidth requirements and how much blocking impedance is needed. Both these factors will greatly affect the cost of the trap. The blocking impedance and bandwidth are directly related to the required inductance which is a large part of the cost. Also it is suggested that frequencies not be used that are near the band edge of the trap because the tuning in that area may change with system conditions. Figure 16 shows the typical characteristic for the wide band trap.

4.2.4 Hybrids

The purpose of the hybrid circuits is to enable the connection of two or more transmitters together on one coaxial cable without causing inter modulation distortion due to the signal from one transmitter affecting the output stages of the other transmitter.

Hybrids may also be required between transmitters and receivers, depending on the application. The hybrid circuits can, of course, cause large losses in the carrier path and must be used appropriately. High/low-pass and band-pass networks may also be used, in some applications, to isolate carrier equipment from each other.

There are many forms of hybrids, such as resistive hybrids, reactance hybrids, and skewed hybrids to name the most popular types. Simply stated a hybrid is a bridge network. The complete bridge is made up of components internal to the hybrid and the external circuits connected to the hybrid.

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Operation of Resistive Hybrid

Figure 4.12. Resistive Hybrid

Refer to Figure 4.12. The hybrid, in this case, is made of a resistor of 25 ohms, and a transformer with a center tap on the primary. The transformer turns ratio is √2/1 with the √2 turns on the center tapped primary. Let’s assume the secondary of the transformer is terminated with a 50 ohm resistor and a voltage (V) is applied to input port #1. The 50 ohm load will be reflected in the primary of the transformer as a 25 ohm quantity from point (a) to the center tap (ct). This is because there is 1 turn on the primary, (a) to (ct), for every √ √2 turns on the secondary. The impedance will be transferred as the square of the turns ratio which in this case is 2 to 1. The voltage V will divide equally between the 25 ohm resistor and the 25 ohm reflected load into the top half of the primary. Thus each voltage has a value of V/2, and in the direction as shown. Since the center tapped primary of the transformer will act as an autotransformer, a voltage V/2 will also appear on the other half of the primary between point (ct) and (b). The voltage appearing across input port #2 due to the voltage V at input port #1 is the sum of the voltages around the loop from (g) to (y). As shown in Figure 4.12, this resultant voltage is 0 volts, and the hybrid isolates the voltage at one input port from the other input port. This isolation expressed in decibels is called trans-hybrid loss and is the same as return loss. Return loss is the ratio in decibels of the power into a discontinuity to the power reflected from the discontinuity. In terms of impedance this would be the ratio of sum of the impedances to the difference of the impedance. The reciprocal of this impedance ratio is call the reflection coefficient.

The mathematical expressions are:

where RT is its terminating resistance and R is the designed impedance.

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A price must be paid for this isolation, and that is attenuation of the carrier signal from either input port to the output port. This loss is the ratio of the input voltage V and the output voltage V/√2, expressed in dB. The result of this calculation will be 3 dB. However, the transformer will have some losses and the loss from input to output will be on the order of 3.5 dB for most hybrids of the type shown in Figure 4.12. The difference in decibels between the input power to a device and the output power of the device is the insertion loss. This can be expressed as follows:

If we analysis of the hybrid shown in Figure 2 using a termination of 45 ohms, the results would be different than discussed above. That is, the voltage will not divide equally between (a) to (ct) and (ct) to (g) and a resultant voltage will appear across input port #2. Thus the hybrid can only provide the best isolation when it is properly terminated, in this case, with a 50 ohm resistor. It is then appropriate to only apply a non-adjustable hybrid in an area of known termination. In cases where there is a termination of 45 ohms, the trans-hybrid loss (return loss) will be:

Adequate return loss is 30 dB or greater.

Reactance Hybrid

Figure 4.13. Reactance Hybrid

When a hybrid is connected to the power line through a line tuner and coupling capacitor the termination impedance may not always be a 50 Ω resistive. Therefore, the hybrid which is connected to the tuner should be an adjustable type and should be designed to handle non-resistive terminations in order to obtain the best performance. This type of hybrid is called a reactance hybrid and is shown in Figure 4.13. Note that the transformer has impedance matching taps to adjust to different magnitudes of termination. The

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balance network is no longer a simple resistor, as in the resistance hybrid, but a resistor, inductor, and capacitor. This is to enable the hybrid to adjust to non-resistive loads. The reactance hybrid will also use an impedance matching transformer similar to the one used by resistive hybrids. Balanced hybrids have equal losses from each input port to the output port. The success of a PLC channel will depend on the received SNR, and this can be obtained by maximizing the amount of transmitter signal that is coupled to the phase wire.

Unbalanced (Skewed)

It is desirable to use balanced hybrids in most applications, but there may be other factors to consider on long lines where losses may be high. Another type of hybrid can be used in an application of this type. It is called a skewed hybrid. Its name comes from the fact that the losses from input port #1 to the output are not the same as the losses from port #2 to the output. The skewed hybrid may be designed with different magnitudes of unbalance, but the most common is 0.5/12 dB. That is, the loss from input port #1 (transmit port) and the output is 0.5 dB and the loss from output port to the input port #2 (receive port) is 12 dB.

The skewed hybrid then allows the transmitter to be isolated from the receiver with only a 0.5 dB loss instead of the 3.5 dB loss of the balanced hybrid. Thus twice as much transmitter power (3 dB) is applied to the line, and the SNR will be improved by 3 dB. The high losses in the receive path do not affect the SNR since the noise is attenuated by the same amount as the signal. The skewed hybrid will generally have an impedance matching network with a fixed balance network and would be considered a resistive type hybrid. When using a skewed hybrid, the receiver port must be terminated in 50 ohms.

A summary of some of the more important application rules are given below:

• All hybrids in a chain should be resistive type hybrids except the last hybrid, that is, the one connected to the line tuner.

• The last hybrid in the chain should be a reactance type hybrid or a skewed type hybrid.

• When applying transmitters to reactance type hybrids the frequency spacing between the widest spaced transmitters is about 4% for frequencies below 50 kHz and 6% for frequencies above 50 kHz.If this rule is not followed then the hybrid cannot be adjusted to provide the best possible isolation between all transmitters.

• When applying transmitters and receivers to a reactance type hybrid the frequency spacing between the transmitter group and receiver group is of no concern; however, all the transmitter frequencies must meet the frequency spacing rule above. This rule is based on receivers with a high input impedance.

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• When the last hybrid is a skewed type then the receiver port should be terminated with a 50 ohm resistor to obtain proper isolation.

Figure 4.14 Hybrid Connections – Dual Bi-Directional Channel

Figure 4.15. Hybrid Connections – Four Transmitters (Unequal Losses)

4.2.5 Filters

L/C Filters: While not providing the isolation of a hybrid, L/C filters may be used to combine two or more transmitters. The bandwidth response of the series resonant L/C filter is a function of the L:C ratio and the frequency to which it is tuned. The insertion loss of the L/C filter is typically around 2 dB, while the return loss is only around 10 to 15 dB, depending on application. Another disadvantage of the L/C filter is the tuning required during installation dictates accurate tuning to maintain the needed isolation.Minimum frequency separation of the transmitters should be 25 kHz or 10% of the highest frequency. These would typically be used where hybrids could not be applied. However, one should calculate the isolation resulting from use of a resistive hybrid as compared to the LC unit. A miss termination of a resistive hybrid of anywhere from 25 to 100 ohms will produce a 10 dB or greater return loss. The advantage here would not have to tune a LC unit.

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4.3 Study of Carrier Performance on Power line: Modal Analysis

Prediction of carrier performance can be accomplished through the use of Modal Analysis. Modal Analysis is a mathematical tool similar to symmetrical components used for analyzing unbalanced faults on three phase power systems. Like symmetrical components, modal analysis is a practical means whose modes can be electrically generated and measured separately. Modal theory is based on the premise that there are as many independent modes of propagation on a multiconductor line as there are conductors involved in the propagation of energy. What follows is a simplified explanation of Modal Analysis. There are five characteristics of natural modes:

1. The phase-conductor currents or voltages can be resolved into three sets of natural-mode components at any point on a lossy, reflection-free three-phase line.

2. At any point on a line, the mode components will add to the actual phase quantities, as well the total power derived be equal to the sum of the mode powers.

3. The mode characteristic impedance, which is the ratio of mode voltage to mode current, is constant on each phase conductor.

4. Each mode propagates with a specific attenuation, wave length and velocity.

5. One set of mode components can not be resolved into other mode components. There is no inter-mode coupling on a uniform line since the modes are independent.

Figure 4.16. Mode Distribution for a Three-Phase Line

Each mode has it own characteristics. Mode 1 is the least attenuated and least frequency dependent of the three and makes carrier channels possible on long EHV lines. The energy is propagated on the two outer phases and returns on the centre phase. Mode 2 is propagated on one outside phase and returns on the other outside phase. It is more frequency dependent and has more attenuation than mode 1. Mode 3 is the highest

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attenuated mode and is propagated on all three phases and returns via the ground. The attenuation is so high that beyond 10 miles, mode 3 is negligible. Figure 4.16 shows the mode propagation characteristics.

This explanation of Modal Analysis applies to a horizontally spaced, single-circuit three-phase EHV line with two overhead static wires, grounded at each tower. The static wires do not generate any transmission modes if grounded at each tower.

Figure 4.17. Mode Components for Various Types of Coupling

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Figure 4.18. Simplified Presentation of Basic Modes

Calculations from modal analysis can become very complex but for explanation purposes a few assumptions can be made to simplify the process. Assume the following:• All phases and modes have the same surge impedances.• Frequency will not be considered.• Instantaneous currents (phase or modal) will be either in phase or 180 out of phase.

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CHAPTER FIVE

DEVELOPMENT OF PLCC DEVICE:HARDWARE IMPLEMETATION

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CHAPTER-5

5.1 Design of PCB for PLC System Implementation

5.1.1 Negative Making Steps

(a) The printout of the design made on IE3D software is taken on a transparent sheet. And its dimension should approximate with the dimension of the design on software.

(b) Transparent sheet should be cut in proper dimension, then along with AgBr sheet, they kept in a negative making unit in which X-rays is passed, and its print is taken on AgBr sheet.

(c) Then AgBr sheet is dipped into a mixture of NaOH and . Then sheet is dipped into the fresh water. After that it is dipped into AgI solution. The sheet is kept for 10-15 minutes and proper shaking is done all the above processes.

Thus the negative has been developed.

5.2 DIELECTRIC SPECIFICATION:-

DIELECTRIC THICKNESS 1.6 mm

DIELECTRIC CONSTANT

VALUE4.4

Table 5.1: Dielectric specification

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