International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol. 3, Issue 1, Mar 2013, 265-272 © TJPRC Pvt. Ltd.
CURRENT SCENARIO FOR POWER LINE CARRIER COMMUNICATION TECHNOLOGY DIVYA ASIJA & PALLAVI CHOUDEKAR Department of Electrical and Electronics Engineering, Amity School of Engineering and Technology, Noida, India
ABSTRACT Power Line Communication (PLC) is a communication technology that enables sending data over existing power cables. This means that, with just power cables running to an electronic device (for example) one can both power it up and at the same time control/retrieve data from it in a half-duplex manner. During the last years PLC technologies have been widely developed mainly due to new modulation techniques used for wireless telecommunication systems that can also be applied to PLC systems. The current state of the art of PLCcommunications is presents many possibilities and opportunities for the utilities. Besides the Distributed Energy Resources integration to the distribution grid is done by plc with in smart grid. The aim of this paper is to make a review of the existing PLCtechnologies as well as the analysis of an interesting range of electric applications of PLC.
KEYWORDS: PLC, Coupling Capacitor, LMU, OFDM, Telemetry INTRODUCTION (WHAT IS PLCC) Power line communication (PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. It is also known as power line carrier, power line digital subscriber line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), and broadband over power lines (BPL). While the idea of sending communication signals on the same pair of wires as are used for power distribution is as old as the telegraph itself, the number of communication devices installed on dedicated wiring far exceeds the number installed on AC mains wiring. The reason for this is not, as one might think, the result of having overlooked the possibility of AC mains communication until recent decades. In the 1920’s at least two patents were issued to the American Telephone and Telegraph Company in the field of “Carrier Transmission over Power Circuits”. United States Patents numbers 1,607,668 and 1,672,940, filed in 1924 show systems for transmitting and receiving communication signals over three phase AC power wiring. Others have suggested that what was required for power line communication to move into the main stream was a commercialized version of military spread spectrum technology. It has been suggested that this is what was needed in order to overcome the harsh and unpredictable characteristics of the power line environment. Commercial spread spectrum power line communication has been the focus of research and product development at a number of companies since the early 1980’s. After nearly two decades of development, spread spectrum technology has still not delivered on its promise to provide the products required for the proliferation of power line communication.
PLC NARROWBAND AND BROADBAND Power Line Communications exploit the power delivery infrastructure to deliver data content. Coupling effects can also be exploited (cross phases). Broadband power line carrier is a fairly recent evolution of the power line carrier technology that has been used by utilities since the 1940s for simple telemetering and rudimentary control of electrical
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equipment. Whereas narrowband power line carrier (PLC) operates at frequencies in the 200 – 500 kHz range and has data rates typically less than 128 Kbps, broadband power line carrier (BPL) operates in the 2 – 30 MHz range and is theoretically capable of supporting data transfer rates of tens of Mbps, depending on the signal-to-noise ratio.
Figure 1: Broadband and Narrowband and PLCC Technology Table 1: PLC Specifications S.No. 1 2 3
4
Power Line Communication Specifications Narrowband Data rate Up Over 1Mbps to 200Kbps Frequency Up to 500kHz FSK, S-FSK, BPSK, SS, Modulation OFDM Building Automation, Renewable Energy, Advanced Metering, Applications Street Lighting, Electric Vehicle, Smart Grid
Broadband Up to 200kbps Over 2MHz OFDM Internet, HDTV, Audio, Gaming
The Table given above shows the different specifications for Narrowband and Broadband PLCC and their applications.
WORKING OF PLCC PLCC system uses the same High Voltage transmission line connecting two sub-stations for telecommunication purpose too.PLCC is used in all power utilities as a primary communication service to transmit speech, telemetry
and
protection tripping commands. This is economic and reliable for inter grid message transfer as well as low bit rate RTU signals. The voice/data are mixed with radio frequency carrier (40-500 kHz), amplified to a level of 10-80W RF power and injected in to high voltage power line using a suitable coupling capacitor. The power line
as a rigid long conductor parallel to ground, guides the carrier waves to travel along the
transmission line. Point to point communication takes place between two SSB transceivers at both ends.
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Figure 2: PLCC Systems Brief Architecture
Figure 3: PLCC System Practical Application The power line carrier component library at its current stage of development includes the following components: line traps, line tuning units, coupling capacitors, balanced resistive and reactance hybrids, skewed hybrids, carrier transmitter, and dBm probes. Figure 2 and 3 shows basic architecture and application of power line carrier communication system. PLCC Terminal It translates voice and data into High Frequency Carrier. Output which has Power =10 to 80W LMU Line Matching Unit: The function of this component is to do impedance matching between line and coaxial cable, which includes high voltage protection devices like drainage coil(20mH), lightening arrestor(500V) and an earth switch. The LTU includes an impedance-matching transformer, a series-resonant L-C circuit tuned to the carrier frequency, and also a protective device. Coupling Capacitor This capacitor couples high frequency carrier with Power Line (4000 to10000pF)
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Line Trap This component do not allow the transmitted HF carrier to enter inside the sub-station. (L = 0.5 to 2mH). With out Line Trap HF carrier get by-passed to some other line on the same bus bar and may leak to ground ( a earth switch inside the yard provided for each bay is kept closed during maintenance). Miscellaneous Components Additional library components include a PLC transmitter, which calculates the 50-Ohm output voltage amplitude based on the specified power (typically 10 W for trip and 1 W for guard), PLC frequency and impedance, and the dBm probe, which provides direct measurement of signal power in decibels referred to 1 mW.
PLC MODULATION SCHEMES Most powerline communications technology implements advanced OFDM (Orthogonal Frequency Division Multiplexing) modulation, a scheme that overcomes most channel difficulties to achieve high data rates while keeping power transmission levels low. OFDM was selected as the most suitable modulation scheme for the powerline channel in particular because it is: •
The most immune to interference, being able to cope with severe changes in channel conditions;
•
Provides the highest level of spectral efficiency and performance.
Orthogonal Frequency Division Multiplexing (OFDM) This modulation is based upon the orthogonality of the carriers. The carriers are chosen so that they are orthogonal among themselves. This minimizes the crosstalk and contributes to elevate the spectral efficiency which is close to the Nyquist rate. Thus, it is possible to use almost the whole frequency range. Besides, OFDM has a white spectrum due to the favourable electromagnetic properties. OFDM modulation requires a very precise synchronization between the transmitter and the receiver, because any deviation of the sub-carriers would affect the orthogonality. This modulation technique is used by DS2. - Gaussian Minimum Shift Keying (GMSK): This kind of modulation is an evolution of the FSK modulation. The bandwidth of the signal changes depending in the binary digit that has to be transmitted and the bandwidth can be controlled by a low-pass Gaussian filter. GMSK
is the modulation used by
ASCOM. - Direct Sequence Spread Spectrum (DSSS): This kind of modulation is also known as DSCDMA, Direct Sequence Code Division Multiple Access and it has been defined by the IEEE in the standard 802.11 for local wireless networks. DSSS uses a pseudo-noise code to modulate the carrier, thus increasing the bandwidth of the transmission and decreasing the spectral density power. The resulting signal’s spectrum is similar to the noise frequency spectrum, so every receptor considers it noise except the one the signal is being directed to.
PLC PROTOCOLS The Power Line has been extensively studied as the media for high frequency signal transmission for use as a communications environment. In recent years, development of several power line communications protocols, namely X10, CEBus and LonWorks, renewed the interest on this matter. X-10 Protocol •
Oldest, uses ASK modulation
•
Originally unidirectional -- controller to devices
Analysis of Heavy Metals by Using Atomic Absorption Spectro-Scopy from the Samples taken around Visakhapatnam
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Some bidirectional products
•
Typically, signals over PL to receivers controlling lights & appliances
•
Poor bandwidth utilization
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(60 bps on 60 Hz line) •
Poor reliability in noisy environments
•
Limited application
CE Bus Protocol •
Peer-to-peer communication
•
Avoids collisions via CSMA/CRCD protocol (carrier sense multiple access/collision resolution & collision detection)
•
Physical layer: spread spectrum technology patented by Intellon Corp.
•
Sweeps thru frequencies rather than hopping
•
•
From 100 to 400 kHz
•
Sweep called chirp -- used for synchronization, collision resolution, data transmission
Data rate ~10kb/s Frequency used limits use in North America
Lon Works Protocol •
Peer-to-peer communication
•
Developed by Echelon Corp.
•
Uses CSMA
•
Narrow band spread spectrum modulation , (125 to 140 kHz)
•
Patented noise cancellation technique and preserves data in presence of noise
•
Can be used in N. America and Europe due to narrow band
Home Plug 1.0 Protocol •
Achieves Ethernet class network on-site using existing electrical wiring
•
Has been introduced in American market
•
Mitigates unpredictable noise
•
Splits bandwidth into many small sub channels •
Masks noisy ones & others
•
Maintain 76 for use in U.S. market
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•
Data rate: 1 to 14 Mbps •
Nodes estimate each 5 sec. & adapt to optimal data rate
•
It still use DSL or cable for Internet into home
•
CSMA/CA -- collision avoidance •
•
•
MAC -- medium access control
Avoidance •
PHY layer detects preamble of frame
•
MAC layer maintains virtual timer
•
Each frame is preceded by contention period, if none transmits during, it can
4 Priorities (for QoS) •
Use back off schedules if detect transmission
•
(8-16-32-64) & (8-16-16-32)
•
Also use decremented counter for ties
PLC Security •
PLC is shared channel (like Wi-Fi)
•
Robust security is serious issue
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Encryption necessary: security vs. complexity
•
•
Rivest: 128 bit key
•
Home Plug: DES - 56 bit key
Intrusion & interference from adjacent subnets •
e.g. Apartments - contention, degradation
•
Decoupling filters - isolate circuits at meter
•
Can also "separate" power line with router
APPLICATIONS OF PLC Power Line Carrier equipment is used for the following applications: Carrier Communication The persons in power stations and receiving stations can communicate with each other using this facility. Carrier Telemetering Telemetering is the process of indicating or recording of quantities at a location remote from the point. The quantities telemetered on power systems are electrical quantities like kilowatts, kilovars, voltage, tap changer position,
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circuit breaker position , and many other quantities. In carrier telemetry by impulse-rate system, frequency or rate of pulse varies according to the magnitude of telemetered quantity. In Impulse-duration system the duration impulses is proportional to the magnitude of the telemetered quantity. The pulse telemetering system is telemeterd by suitable carrier channels. Telecommunication Services Current PLC networks are able to reach speeds of 200Mbps. Telephony and Internet services can be delivered at high speed through broadband PLC networks. Traditional telephony uses Plesiochronous Digital Hierarchy, PDH. PDH uses Time Division Multiplexing, TDM. One possibility is to send the TDM frame over IP, and the voice over TDM, VoTDM. However, this service should accomplish the quality and reliability criteria, like Bit Error Rate, timing and latency, and unfortunately the delay in VoTDM transmissions exceeds 25ms. Nevertheless, it is possible to give a good telephony service over IP. Over TCP/IP, VoIP and Internet services can be delivered at a 200Mbps speed, so it can be possible to compete with technologies Industrial Automation In an industrial environment the PLC communication networks can be used to give electric energy related services, such as meter reading, demand management and remote billing but also to give value added services like remote control and security, automation or even, education, information and e-business opportunities. On the other hand it can also offer telecommunication services such as traditional telephony and Internet.
CONCLUSIONS Power line carrier communication technology is the current upcoming technology which is basically used for carrying data on a conductor that is also used for electric power transmission. It is also known as power line carrier, power line digital subscriber line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), and broadband over power lines (BPL).The advantage of power line communication is to avoid new installation of wire (communication channel) where the new installition imposes limitation due to place amd cost. Also the availability of power line plugs extends the flixiblity of power line as a communication channel. When designing or analyzing a PLC network, the structure and components of it, as well as its topology have to be taken into account. Also, there are different providers that use different technologies so compatibility is an issue. There is a narrowband standard and the broadband is very developed. The transmission quality is also an important factor. In order to characterize a PLC network, attenuation, noise, SNR, crosstalk and Delay Spread parameters have to be measured, analyzed and considered. Finally, the application range of PLC is very wide, it can provide telecommunication services and value added services but it can also be very useful for distribution and transmission system operators in order to guarantee the operation and control of the power grid.
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