Download an efficient conversion technique enabling utlity

AN EFFICIENT CONVERSION TECHNIQUE ENABLING UTLITY METER TO FUNCTION AS DIGITAL
METER NEEDED IN AMR
M.Moghavvemi, S.Y. Tan
ABSTRACT
In this paper, the electric meter interface module for the implementation of a full-scale AMR system will be
discussed. Firstly, comparison is made between an electronic solid-state meter and a converted electromechanical meter for use in an AMR system. After giving reasons why the converted electro-mechanical
meter is preferred, a brief discussion of the electro-mechanical meter follows. Then, the method for
converting the electro-mechanical meter is discussed to make it suitable for use in an AMR system.
Conversion of electro-mechanical meters requires the use of a digital display to replace the register dials,
so, the LCD is introduced. The various functions at the meter side including meter reading require the usage
of various chips and electronic devices and they are discussed next. These functions of the AMR system to
be implemented include detection of meter tampering. Finally, power supply is examined to provide the +5V
level required by many of the chips utilised, especially for the PIC (Peripheral Interface Controller) micro
controller and an alternative and back-up to the power supply in the form of lithium batteries is mentioned.
INTRODUCTION
An electric utility company has the responsibility to provide electrical service to a defined geographic area.
In return, utilities expect to be fairly reimbursed for this service. To do this on an equitable basis, they utilise
the electric meter, a device that measures customer usage in kilowatt-hours. Therefore, the electric meter is
considered to be the cash register for the utility [1].
The AMR system starts at the meter. Some means of translating readings from rotating meter into digital
form is necessary in order to send digital metering data from the customer site to a central point. In most
cases, the meter that is used in an AMR system is the same ordinary meter used for manual reading. This
type of meters is known as the electro-mechanical or Ferraris’ disc meter. The history of electro-mechanical
meters date back to the early 1900s. In 1899, GE introduces its first attempt at a polyphase meter known
simply as the Thomson Polyphase Wattmeter. This meter was massive due to the large disk and widely
spaced stators in an attempt to eliminate interference between the stators. It was not very popular since the
industry preferred a more compact meter. An engineer at Westinghouse (Paul McGahan) came up with a
workable design for polyphase meters. Two single-phase meters were installed in a tall case with a common
shaft and register. This design was adopted by all manufacturers and built in various forms until 1969 [2].
Alternatively, an electronic meter that functions based on solid-state electronic technology can be also
utilised. The internal mechanism used for metering consumption differs for both these cases. The main
difference is the addition of some device to generate pulses relating to the amount of consumption
monitored, that is, the generation of electronic digital codes that translates to the actual reading on the
meter dials for the case of the electro-mechanical meter.
A point worth noting is that the electro-mechanical meters have been in existence since the late 19th Century
and has proven to be reliable up to 20 years and is much cheaper than a new electronic meter. Schwendner
states that the mature Ferrari's disc technology exhibits the highest quality and reliability over an extremely
long lifetime and it is a fact that using these meters for simple, dedicated applications is still unquestionably
the most economical solution [3]. Even after the addition of the converting equipment for the electromechanical meter, namely, a PIC micro controller, an optical encoder and a alphanumeric LCD display, the
total cost of the converted electro-mechanical meter will be around RM 150 per unit, cheaper than a new
solid-state electronic meter.
A recent enquiry at TNB Research department indicated that there are around 3 million new electromechanical meters still in their possession that have not been distributed out to consumers. Each of these
electro-mechanical meters cost around RM 80 and a total sum of 3 000 000 X 80 = RM 240 million will be
totally wasted if a full-scale AMR implementation is implemented today using new solid-state electronic
meters without converting these old electro-mechanical meters.
THEORY
A brief theory regarding present existing electro-mechanical meters is presented [4]. The electro-mechanical
meter must accurately measure voltage, current and power factor continuously over a period of time to
arrive at kilowatt-hours. To achieve that, there are various components in an electro-mechanical meter and
these components are as follows:
i) Potential Coil - To measure voltage or potential, a coil is used which is made up of many turns of fine wire.
This coil is inductive by nature and this, together with its physical placement within the meter, produces a
lag, or delay, of approximately 90° between the potential coil flux and the line voltage and current coil flux.
ii) Current Coil - A current coil must produce a flux field whose strength is proportional to amperes drawn by
the consumer's load.
iii) Retarding Magnet - Retarding magnets are permanent magnets arranged one on top and one on the
bottom of the air gap so that the disc has to move between them. The flux from the magnets acts upon the
moving disc by inducing a voltage within it. Because the disc is a closed loop, eddy currents are created.
These currents act against the magnetic flux of the potential and current coils to create a negative torque or
braking action to the movement of the disc.
iv) Disc - The disc or rotor is the movable portion of the meter. Supported in the air gap by a bearing system
that affords a relatively frictionless suspension, the disc is free to rotate as the potential and current coil
fluxes interact upon it.
v) Register - The register produces the total electric consumption readings in the form of a moving dial.
Register dial ratio or Rg is the ratio needed to make the register represent correct values of kilowatt-hours
(kWh). It is an indication of how many revolutions of the spinning disc is needed to rotate the register dial in
such that it gives the reading of 1 kWh. The formula for register dial ratio, Rg, in a modern single-phase
meter is given by Rg = 10 000 / Kh, where, Kh is the test constant of the modern, three-wire, 240-volt meter.
The electro-mechanical meter that will be used for the AMR implementation project has a value of Rg = 375,
meaning 375 revolutions of the rotating disc are needed for the register dial to record the reading of 1 kWh.
EXPERIMENTAL SET-UP
An optical encoder that allows a conventional electro-mechanical meter to emit digitised pulses as a
representation of the number of revolutions of its spinning disc is used. The optical encoder used here is
the HEDS-9720 OPT P51 Small Optical Encoder Module manufactured by Hewlett Packard. The HEDS9720 optical encoder is a high performance and low-cost optical encoder module. When operated in
conjunction with a codewheel, the module detects rotary or circular position. The module consists of a
lensed LED (Light Emitting Diode) source and a detector IC enclosed in a small C-shaped The module is
extremely tolerant to mounting misalignment due to the highly collimated light source as well as having a
unique photo-detector array. The optical encoder is shown in Figure 1.
H9720
OPT P51
Optical
Encoder
CH B
CH A
5V
Figure 1 : Diagram Of Optical Encoder
Hand in hand with the optical encoder, there must be a codewheel, that is, a thin piece of glass, film or
metal. The standard HEDS-9700 is designed for use with an 11 mm optical radius code wheel. The
codewheel is basically a circular piece of material with a hole at its centre for attachment to the meter shaft
and a small opening at one of its side. The size of the codewheel must be thin enough to fit inside the
optical encoder’s enclosure but its radius has to be big enough to encompass the entire enclosure of the
optical encoder to provide a more accurate depiction of the meter readings. The codewheel is fixed on to
the shaft, where, the rotating disc is also attached. As the disc rotates, so will the codewheel as they are
both connected together by the shaft.
The optical encoder is a C-shaped emitter and detector module. When coupled with a codewheel, it
translates rotary motion into a two-channel digital output. The module contains a single LED as its light
source. The light is collimated into a parallel beam by means of a single lens located directly over the LED.
Opposite the emitter is the integrated detector circuit (IC). This IC consists of multiple sets of photodetectors and the signal processing circuitry necessary to produce the digital waveforms.
The codewheel moves between the emitter and detector, causing the light beam to be interrupted by the
pattern of hole and solid material on the codewheel. The photodiodes which detect these interruptions are
arranged in a pattern that corresponds to the pattern and count of the codewheel. These detectors are also
spaced such that a light period on one pair of detectors corresponds to a dark period on the adjacent pair of
detectors. The photodiode outputs are fed through a signal processing circuitry. Two comparators receive
these signals and produce the final outputs for the channels. In the normal condition, as the codewheel is a
solid material, the emitted light will be reflected by the codewheel and no light is received by the light
detector. When no light is detected, a High pulse is emitted from the channels of the optical encoder When
the codewheel opening passes through the optical encoder’s enclosure, light will be received at the light
detector and a Low pulse is emitted. Thus, a revolution of the rotating Ferraris’ disc is represented by the
detection of a Low pulse with normal condition set as High. The converted electro-mechanical meter used
as part of the AMR system is as shown in Figure 2. Note that in Figure 2, the optical encoder is at the side
of the meter at the end of the white cable across the rotating disc of the electro-mechanical meter.
Figure 2 : Converted Electro-Mechanical Meter
With the optical encoder and codewheel, the register dial will no longer be utilised and is taken out of the
system. In place of it, with the generation of the digital pulses, an LCD (Liquid Crystal Display) display is
used. LCD is chosen because it gives clear read-outs, is easy to implement and controlled using a PIC
microcontroller and most importantly, it reads data in digitalised ASCII code. Thus, the microcontroller can
take the digital pulses emitted from the converted electro-mechanical meter, count them and lastly, convert
them to ASCII code to be transmitted for display on the LCD. The LCD here gives 2 types of reading,
namely, the total electricity consumed as well as the cost of the power usage. This conversion was done
through PIC programming using standard rates of 21.8 cents for the first 200 units, 25.8 cents for the next
800 units and 27.8 cents for each additional unit after that as imposed by TNB [5]. This function was
incorporated to enable the user to monitor and plan his power usage. Figure 3 shows the circuitry for the
LCD in use at the meter.
Figure 3 : LCD Circuitry at the Meter
The converted electro-mechanical meter also has an added function to detect fraud. Detection of tampering
is implemented through the use of a motion or vibration sensor. The motion sensor or motion switch used in
the implementation of this project is the MSv24 produced by ASSEMtech Europe Ltd. The motion sensor is
placed at the covering of the electro-mechanical meter. It will generate a High output if there is some
vibration that causes the MS 24 to move. Therefore, if an attempt to tamper the converted electromechanical meter is made, the covering of the meter has to be opened first. When the covering is opened,
the motion of the meter covering will ensure that the motion sensor is vibrated such that a High output is
emitted from it. The High output is sent to the PIC to be transmitted to the data concentrator side across the
PLC for appropriate measures to be taken.
TESTING
The proposed meter was used as part of an AMR system to send and receive data over power transmission
lines. PLC or power line carrier communication takes place over the same lines that deliver electricity. This
technique involves injecting a high frequency AC carrier onto the power line and modulating this carrier with
data originating from the remote meter or central station [6]. To send data remotely and automatically to the
utility office from the meter side in AMR systems, a timer has to be used. The timer selected is the LM 555
chip produced by National Semiconductor. The LM 555 is a very common chip that can generate accurate
time delays or oscillation. In the time delay mode of operation, one external resistor and capacitor precisely
control the time. Its timing can range from microseconds through to hours. In our experiment, a time delay of
15 minutes is set to trigger the PIC to send the accumulated meter readings over the PLC. The LM 555
timer circuitry is as shown in Figure 4.plastic package.
LM 555 Timer
1
Cext
5V
8
Vcc
7
DISCHARGE
6
R2
GROUND
2
TRIGGER
OUTPUT
3
4
5
RESET
CONTROL
VOLTAGE
R1
THRESHOLD
0.01uF
Figure 4 : The LM 555 Timer Circuitry
A decoupling capacitor with the value of 0.01 F cis onnected to Pin 5 or the Control Voltage Pin. This
capacitor has no significant effect to the operation of the LM 555. For timing operations, the signal obtained
from the Output Pin or Pin 3 is a rectangular wave. The values for both resistors, R1 and R2 as well as the
external capacitor, Cext, are govern by Equations 1 and 2 below:
fr = 1.44/(R1+2R2)Cext. ….
and
…………………
(Equation 1)
Duty Cycle = tH/(tH+tL)
= (R1+R2)/(R1+2R2) X 100 %…………………(Equation 2)
The duty cycle gives an indication of the ratio or percentage of a full period of the output rectangular
waveform where the signal is High and fr is the frequency of oscillation for the rectangular output waveform
[7]. The duty cycle is controlled by the values of the two resistors, R1 and R2. Ideally, we want the duty cycle
to be 50 % (ratio value = 0.5) or thereabouts. However, the resistors, R1 and R2 together with the external
capacitor, Cext, also determines the frequency of oscillation of the rectangular output waveform, which in turn
also determines the timing provided by the LM 555. Thus, proper calculations and determination of these
three variables of R1, R2 and Cext are essential to provide proper timing that enables data to be sent across
the PLC at intervals of 15 minutes.
The LM 555 is used to trigger the PIC to sent data across the PLC every 15 minutes. By using the LM 555
to provide the external clock to the PIC and using the Timer0 Module of the PIC microcontroller to provide
the interrupt , the 15 minutes timing circuitry can be produced. For the PIC, overflow in the Timer0 Module
which causes the PIC to be interrupted and triggered will only be happen after the detection of 256 external
clock pulses [8]. Thus, the period for each external input clock pulse is therefore
Clock Pulse Period, T = (60 X 15)/256
= 3.516 seconds
The required frequency of oscillation, fr, is therefore
fr (Theoretical) = 1/ 3.516
= 0.2844 Hz
For duty cycle to be around 50 %, from Equation 2, it can easily be observed that the value of the resistor R2
has to be very much bigger than the value of R1. By selecting R1 to be equal to 30 k and R2 to be equal to
8.2 M, we obtain a duty cycle of
Duty Cycle (Experimental) = (30+8200)/(30+16400) X 100 %
= 50.09 %
The experimental value of the duty cycle is close enough to the ideal value of 50 %, that is, the experimental
value is 50.09 % with a difference margin of less than 2 %. Thus, the external capacitance value can be
calculated from Equation 2 previously to obtain a value for f r such that fr is as close as possible to its
theoretical value of 0.2844 Hz limited by the availability in the market for the required value of the external
capacitor, Cext. By choosing Cext = 0.3082 F (parallel combination of three 0.1 F capacitors with one 8200
pF capacitor), we have the frequency of oscillation, fr value to be
fr (Experimental) = 1.44/(0.03+16.4)0.3
= 0.2844 Hz
The experimental value for the frequency of oscillation, fr, is 0.2844 Hz which gives a timing triggering for
the PIC to be exactly 15 minutes since the theoretical value for fr is also 0.2844 Hz. With these selected
values of R1, R2 and Cext, the LM 555 acts as a timer to trigger the PIC 16F870 such that data is sent to the
PLC at regular and fixed intervals of 15 minutes.
To test our converted meter in a full scale AMR system, the set-up as shown in Figure 5 is used.
Figure 5 : AMR System Testing with Converted Meter
In our test of the complete AMR system, we observe the meter readings as indicated at the LCD of the
converted meter and compare it with that received at the central computer. The result is as shown in Table 1
:
Time Lapsed
LCD Meter
Reading at
(Minutes)
Reading (kWh)
Central
Computer
(kWh)
0
0
0
15
2.1
2.0
30
4.0
4.0
45
6.1
6.0
60
8.1
8.1
75
10.1
10.1
90
12.2
12.1
Table 1 : Comparison of Meter Readings with Readings at Central Computer
As observed from Table 1, the meter gives accurate readings of power consumption and corresponds with
the values stored at the central computer. This shows that after conversion, the meter still functions as a
normal electricity meter and gives true usage of power used. The slightly higher readings at the meter might
be caused by the slight time delay for the data to reach the central computer, such that, the actual values
stored at the computer is slightly less than that of the time lapsed.
DISCUSSION
The power supply for all the IC chips including those for the motion sensor, LCD and optical encoder is
drawn from the power distribution lines. However, the required voltage for the IC chips are low compared
with the power lines. Most of the devices and IC chips require only a positive supply of +5V, including the
PIC micro controller, motion sensor, 555 Timer, LCD and optical encoder. A method must be utilised to
reduce the high AC voltage from the LV network to the lower desired voltages. The process for the reduction
of the voltage level involves three steps. Firstly, a step-down centre-tapped power transformer with the
winding ratio, r of 20:3 referred to the transformer primary is used. An ac voltage of 240 X (3/20) = 36V is
produced at the secondary side of the power transformer but since the secondary is centre-tapped, the net
output voltage from the secondary side of the power transformer is 36/2 = 18V only. The next step requires
the conversion of the AC voltage at the power transformer secondary to DC voltage. The full-wave bridge
rectifier circuit is chosen here as a good constant DC voltage can be obtained from it and this circuit is
simple to implement compared to other more complex rectification configurations. Finally, the last step
requires the further reduction of the +18V to levels required by the IC chips and meter devices by using a
voltage regulator chip.
As a support system to the entire AMR system and especially to the LCD of the meter, batteries are utilised.
They are especially important in the cases of power failures and power disruptions that can cause the entire
AMR system to break down and the LCD to lose its stored memory and reading. Normally, power for the
AMR system is drawn from the power lines using standard down-transformers but when power disruption
happens, no power will be supplied to the entire AMR system. Lithium-ion batteries are selected because
these batteries tend to last longer than most other batteries, even up to 10 years. Also, lithium batteries are
light and have a high energy density. In addition to that, it contains no mercury, cadmium nor lead and
therefore, is non-poisonous. However, lithium batteries are more expensive than most other batteries.
CONCLUSION
In this paper, we outlined the methods to convert the conventional electro-mechanical meter into a fully
functioning meter as part of an AMR system using simple and cost effective techniques. The conventional
electro-mechanical meter was converted to emit digitised pulses through the usage of an optical encoder.
Automatic sending out or transmission of meter reading data and detection of meter tampering at the meter
was also successfully carried out. In testing of our AMR system, the meter readings as read from the LCD
gives an accurate picture of the total power consumed as recorded at the central computer.
A back-up power supply using lithium batteries was also discussed. Further improvements can still be made
to enhance the functions of the AMR meter module to include load automation and pre-payment amongst
others. The proposed meter can result in a substantial saving for the electric utility company as well as
enabling the utility to take advantage of fully automated meter reading.
REFERENCES
[1] O’Neal, J. B., “Substation Noise At Distribution-Line Communication Frequencies”, IEEE Transactions on
Electromagnetic Compatibility, Vol. 30, No. 1, February 1988, pp. 71-77.
[2] Dahle, D. "A Brief History of Meter Companies and Meter Evolution".
[3] Schwendtmer, M.F., "Technological Developments in Electricity Metering and Associated Fields", Eighth
International Conference on Metering and Tariffs for Energy Supply, 3-5 Jul 1996.
[4] "Pocket Guide to Watthour Meters", Second Edition, Alexander Publications, Newport Beach, California,
2003.
[5] Tenaga Nasional Berhad, “Tariff Rates”
[6] Tamarkin, T. D., “Automatic Meter Reading”, Public Power, Vol. 50, No. 5, September-October 1992.
[7] Floyd, T.L., "Electronic Devices", 6th. Edition, Prentice Hall, Upper Saddle River, NJ, 2002.
[8] Peatman, J.B., “Design With PIC Microcontrollers”, Prentice-Hall, New Jersey, 1998.
Biography
Speaker: Mahmoud Moghavvemi
Position: Chairman of electrical engineering
Company: University of Malaya
Country: Malaysia
Prof Mahmoud Moghavvemi occupies the chair of electrical Engineering, in the Department of Electrical
Engineering, University of Malaya. Besides many years of teaching and research experience in Electrical
Engineering, he has several years of working industrial experience in USA and Malaysia. Prof. Mahmoud is
the chief editor of the International Journal of Engineering science & technology and, member of board of
reviewer of several International Journals in USA, Canada, and Australia. He served as an external
moderator to University of Oxford external program from 1995-1999 and University of Cambridge from
1998-present. He is a member of technical committee of Several International conferences in Europe,
U.SA, and the Middle East. Prof Mahmoud has served as chairman of technical committee for several
national conferences and as reviewer for other local conferences. He has published extensively in
international journals and in conference proceedings. His research interest is in applied electronics.