PLC communication on Egypt s LV grid for AMR

Transcription

1 PLC communication on Egypt s LV grid for AMR by A Helmy, M Abdel Rahman and MM Mansour, Ain Shams University, Electrical energy consumers are demanding better customer service, higher power quality, higher energy measurement accuracy and more timely data. Utility companies all over the world are being forced to find solutions giving greater information on the population s power consumption. The automatic meter reading system (AMR) is one of the ways in which utilities are go-getting to achieve these goals. Power lines are one of the communication mediums used in the AMR system. However, power lines are a hostile communication environment. This paper describes a prototype for AMR system using power lines showing performance and reliability tests. The measurements are carried out on a sample of Egypt s low voltage network. Considerable savings can be achieved by the utility companies by removing the need for a visual inspection of the meter at each billing term. The process is labor intensive, time consuming and prone to human error. A study shows that on average a meter reader achieves an information rate of only around 1 bps [1], which is highly inefficient in terms of modern standards. AMR also reduces the problems and costs associated with reading meters at hard to access locations. Additional significant cost savings can be achieved by identifying the tampering of meters These can be detected because most of these AMR systems offer bi-directional communications, which allows the current meter data to be checked against the historical data for any suspicious divergence. Several communication technologies are currently in use to achieve automated meter reading. Radio, telephone lines and power lines are the most used mediums where each system has its own advantages and disadvantages. This paper focuses on power line communication (PLC) and its performance as a channel for AMR system, statistical analysis and suggestions for reliable communication is presented. AMR system structure The main parts of an AMR system as shown in Fig. 1 are: Meter interface unit (MIU): Each meter at the consumer side is fit with an interface unit which acts as a transceiver for meter data on the low voltage power lines using a single phase power line modem (PLM). Data concentrator unit (DCU): The data sent by the MIU is received by the data concentrator unit located at the distribution transformer. DCU is fit with three phase PLM receiving data from all meters on different phases. Utility central unit (UCU): The data received by the DCU is then transmitted to the utility central unit. The communication link in this case may be PSTN, RF, GSM or Ethernet which is out of the paper scope. As mentioned before the communication links either between the MIUs and their DCU or between the DCUs and the UCU are bi-directional links. PLC benefits, challenges and regulations Benefits Fig. 1: AMR system structure. By using existing cable infrastructure PLC system eliminates the need for installation and maintenance of dedicated communication links. Already every building or household is connected to the electrical power grid and moreover; every room has power line contact points installed. Without doubt the extent of this existing infrastructure cannot be matched by any other telecommunication technologies that are available today. The challenge As the electric power distribution lines were not originally designed for communication purposes as a result they exhibit highly variable and unpredictable levels of channel noise, signal attenuation and distortion [8]. Standards and regulations Fig. 2: Frequency bands for CENELEC EN standard. There are many established standards that provide regulations on the operating specifications of PLC systems. Federal Communications Commission (FCC) and European Committee for Electro-technical Standardization (CENELEC) govern regulatory rules in North America and Europe respectively. For Europe the energize - November Page 13

2 regulations concerning low voltage signaling are described in CENELEC standard EN In part 1 of this EN-standardization paper the allowed frequency band and the output voltage for communication is indicated [2]. The frequency range allowed for communication is subdivided into five sub bands as shown in Fig. 2 ranges from 3 khz to 148,5 khz, with transmitted power not more than 500 mw. Frequency ranges between 9 95 khz, which is allocated for electrical utility use for applications such as AMR and load management; there is no need for access protocol when operating in this band. The rest of the frequency range ,5 khz comprises of the B, C, and D frequency bands and is reserved for end-user applications. Focus on PLC challenges Noise and disturbances Common causes of noise on the high voltage electrical power networks include corona discharge, power factor correction banks and circuit breaker operation. On the low voltage network, much of this noise is filtered by distribution transformers, so the most common interference in low voltage domestic networks can be attributed to the various household devices and office equipment connected to the network. Noise on the low voltage network is classified to [1], [4]: Noise having line components synchronous with power system frequency: The usual source of this noise are Triacs or silicon controlled rectifiers (SCRs), found domestically, for example, in light dimmers or photocopiers. Noise with a smooth spectrum: caused by universal motors that can be found in a lot of household appliances such as blenders and vacuum cleaners. Single event impulse noise: usually caused due to switching events as closing of contacts. Non synchronous noise: Is characterised by periodic components that occur at frequencies other than harmonics of the mains frequency. Major sources of this noise are televisions and computer monitors. Fluctuated channel impedance Unfortunately, a uniform distributed line is not a suitable model for PLC, since the power line has a number of loads of differing impedances connected in parallel to it for variable amounts of time. It can be seen that the channel impedance is a strongly fluctuating variable that is hard to predict. Signal attenuation Reference [3] states that the attenuation between homes in the U K ranges from 15 db to 25 db per 100 m, for the frequency range khz. The corresponding value for homes in Zurich, Switzerland, was in the range 3 13 db. In [3] analysis of signal attenuation is introduced giving some factors affecting signal attenuation as time dependency where there is a strong day and night sensitivity. Communication system Noise, fluctuated channel impedance, signal attenuation and distortion are big challenges facing selecting an efficient communication system to insure reliable data transmission through this hostile environment. However, remote meter reading and load control are examples of PLC applications with rather low demands on the communication system. This type of applications has two important characteristics: firstly, the amount of information that is transferred is small, and therefore the information bit rate (in bit/second or bps) is low. Secondly, they do not have high real time demands therefore a relatively large communication delay is acceptable. These two characteristics make it easier to establish reliable communication over the power line than other PLC high speed and real time demand applications. Many researchers and investigators have been performing important works to select a proper modulation technique, frequency shift keying (FSK) and spread spectrum (SS) are the most commercially applicable by Echelon, Intellon, ST, Itran, Phillips, Archnet the most famous PLM producers. Spread Spectrum uses wide band, noise-like signals. Because Spread Spectrum signals are noise-like, they are hard to detect. Spread Spectrum signals are also hard to Intercept or demodulate. Further, Spread Spectrum signals are harder to jam than narrowband signals. These low probability of intercept anti-jam and high noise immunity features [6] are why the military has used Spread Spectrum for so many years and it s widely used now commercially in PLC systems. Proposed system configuration The proposed system shown in Fig. 3 consists of: Master controller Unit (MCU) Remote controller Units (RCU) Laptop The main aim of this prototype to have a point-to-point system where the MCU (acting as DCU) will communicate with RCU (acting as energy meter) connected to the same phase at different distances from the The overall impedance of a low voltage network as discussed in [1] and [5] results from three main sources, impedance of the distribution transformer, characteristic impedance of cables and impedance of the devices connected to the network. Various measurements and tests were done to evaluate the impedance of the low voltage network, in [7] the CENELEC A-band maximum absolute impedances are reported increasing from 20 Ω at 20 khz to 80 Ω at 100 khz. Mean values increase from 5 Ω to 17 Ω. Clearly, channel impedance is very low. This presents significant challenges when designing a coupling network for PLC communications. Maximum power transfer theory states that the transmitter, receiver and the channel impedance must be matched for maximum power transfer. With strongly varying channel impedance, this is very difficult. Fig. 3: Proposed system for PLC testing. energize - November Page 14

3 MCU. All data transactions on the same phase are recorded with date and time stamp where various performance and reliability tests are carried out to examine the power line channel as a communication medium for the AMR system. Master control unit (MCU) MCU is shown in Fig. 4 where it mainly consists of a single phase PLM from Archnet [10] communicating with a Laptop through RS232 serial port.the Archnet PLM ATL is based on the direct sequence spectrum (DSSS) and it provides bi-directional half duplex data communication over the low voltage grid at baud rate 300 bps and it complies with CENELEC standard EN A band. Thus, the data could be transferred in bidirectionally between the Laptop and the power lines. Fig. 4 shows the MCU prototype. Remote control unit (RCU) RCU is shown in Fig. 5 where it consists as MCU of PLM communicating serially (RS232) with a microcontroller which is responsible for performing all the calculations required for electrical power quantities measurements besides receiving and sending data from and to PLM. Thus, the data could be transferred in bi-directionally between the microcontroller and the power lines. Fig. 5 shows the RCU prototype. The microcontroller is PIC18F258 from microchip [9] fit with 10 bit analog to digital converter (ADC) required for accurate current and voltage signals sampling, serial port required for interfacing with PLM, high internal bus speed up to 10 MHz and other many advanced features. Laptop The laptop computer is responsible for: firstly, sending control signals to all RCUs or one of them. Secondly, receiving and storing all data transactions with date and time stamp for analyses. Communication datagram As the system designed and constructed is still in the prototyping stage, the communication datagram used in the power line communication was designed initially without any concern for data security and improving data reliability the main aim is to find out the performance of the proposed system in the hostile environment. The data request packet is designed for the purpose of collecting meter readings. MCU sends out this packet to ask all RCU within the network for meter readings at a certain interval. All RCUs respond to this by sending back a data send packet to the source, which contains the information requested. Fig. 4: MCU structure diagram and prototype. For simplicity the data request packet sent by MCU to all RCU is one byte which is the ASCII value of letter t (stands for transmission request).while the data send packet sent by each RCU is 16 bytes with the first two bytes representing the RCU ID which is unique ID for each RCU and stored on the microcontroller. The other 14 bytes are the data reading requested of the specified RCU. Performance and reliability site tests Test method The MCU and a single RCU is communicating on the same phase at a distance 50 m apart. Tests are done at a dense residential area in Cairo the capital of Egypt. The tests involves putting the system under hard test continuously for three weeks where the laptop is programmed to send the data request packet every 60 s and waits for the data send packet sent back from the RCU. The laptop stores all the transactions done either transmitted or received this data is then checked for accuracy of response data request packet/day (1 packet/min) were sent and the results were reported for three weeks. Test objectives The site tests focused on three main objectives: Fig. 5: RCU structure diagram and prototype. Report the general performance of PLC for a medium distance (50 m) Report the best day of the week and the best hour of the day for AMR system data collection Report the influence of large number of TV sets loading the power lines where some results are reported during the African cup of nations for football (Egypt ACN 2006) which reported a large number of spectators (especially for the Egyptian team matches). energize - November Page 15

4 Test results The results are based on the accurate packets received by the MCU from the RCU. If an inaccurate packet is received by the MCU, a checksum error will be reported. However, if the MCU does not receive any reply from the RCU after a certain allocated time, a timeout error is reported. Both the checksum and timeout constitute the failure of the proposed system. Test results are collected and subdivided to: Test results based on a day of the week: samples of these results are shown in Table 1. Test results based on the same day of the week: where results from 1 are averaged for the same day of the week for the whole three weeks as shown in Table 2. Test results based on the hour of the day: Fig. 6 shows histogram samples of these results. Observations Fri 17/2/06 Sat 18/2/06 Sun 19/2/06 Mon 20/2/06 Tue 21/2/06 Wed 22/2/06 Thu 23/2/06 trials CS TO Failure ,7 17, ,7 23, ,7 28, ,3 31, ,1 17,9 Table 1: Day of the week reliability results. Based on the results obtained, the observations lie in the following areas: Reliability: As shown in Table 1 successful attempts in any day do not exceed 91 Timeout: High timeout (TO) error is noted while checksum (CS) is so rare compared to the timeout error forming a total failure percentage up to 35 and increase to 49 under heavy loading conditions Correlation with day of the week: As shown in Table 2 the average successful attempts vary from 67 to 83 depending on the day of the week. Best average results are reported on Thursdays and Saturdays. However, worst results are reported on Sundays and Wednesdays. Fig. 6b and Fig. 6c show a histogram samples for PLC performance on Sunday and Thursday respectively Correlation with time of day: The histogram of the first week of the tests reports very high failure percentage up to 100 on the hours where there is a match for Egypt football team in the ACN 2006 due to very high loading effects by TV sets, this is noted on Tuesday 7 February 2006 from 19h00 to 21h00 and Friday 10 February 2006 from 18h00 to 21h00. Fig. 6a: The histogram of the other two weeks reports very high failure rates up to 100 on the hours of the day between 11h00 to 17h00 and high successful rates up to 100 on the hours between 01h00 to 09h00 as shown in Fig. 6b and Fig. 6c. Suggestions All performance and reliability tests show fluctuating results varying with time. Thus, to overcome this we suggest the following: Initiate communication at specific times, Thursdays and Saturdays between 03h00 am to 09h00 are reported the best results. Avoid times which record high TV sets audience density. Using repeaters to reduce the timeout and one of the error correction methods to reduce the checksum. trials CS TO Failure Mon ,2 25,8 Tue ,1 25,9 Wed ,1 30,9 Thu ,5 16,5 Fri Sat ,5 18,5 Sun ,8 32,2 Table 2: Same day of the week reliability results. energize - November Page 16

5 Conclusion The power lines are a hostile environment for communication with channel parameters highly fluctuating with time. Thus, a special care about choosing the right time for data transmission must be considered by the utility central unit to acquire the meters readings through the AMR system. In spite of the satisfactory performance of the prototype system, there is still work to be carried out. It would be useful to examine the reasons for timeout error and also to determine the actual cause of all the. Error correction should be added to the system in future to increase the reliability of the system and to prevent tampering. Acknowledgement This paper was presented at the IEEE PES Power Africa 2007 conference and exhibition in Johannesburg, and in republished with persmission. References [1] H.C Ferreira, H.M Grove, O. Hooijen, Power Line Communications: An Overview IEEE AFRICON 4th, Vol. 2, Sept. 1996, pp [2] J. Newbury, Communication requirements and standards for low voltage mains signaling IEEE Transaction on Power Delivery, Vol. 13, No.1, January 1998, pp [3] D. Cooper, T. Jeans Narrowband, low data rate communications on the low-voltage mains in the CENELEC frequencies. I. Noise and attenuation IEEE Transaction on Power Delivery, Vol. 17, No. 3, July 2002 [4] R.M Vines, H.J Trussel, L.J Gale and J.B O Neal, Noise on residential power distribution circuits IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-26, No.4, Nov 1984, pp [5] R.M Vines, H.J Trussel, K.S Shuey and J.B O Neal, Impedance of the residential power distribution circuit IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-27, No.1, Feb 1985, pp [6] G. Marubayashi, S. Tachikawa: Spread spectrum transmission on residential power line Spread Spectrum Techniques and Applications Proceedings, IEEE 4th International Symposium on Vol. 3, Sept. 1996, pp [7] J. A. Malack and J. R. Engstrom, RF Impedance of United States and European Power Lines, IEEE Transactions on Electromagnetic Compatibility, vol. EMC-18, no. 1, February 1976, pp [8] Echelon Corporation, A Power Line Communication Tutorial Challenges and Technologies [9] Microchip Corporation, PIC18F258 Datasheet Available: microchip.com Fig. 6: PLC performance histogram. [10] Archnet Corporation, Embedded Power Line Modem ATL and ATL Manual v energize - November Page 17