26-06-2012, 04:02 PM
Microcontroller Based Single Phase Digital Prepaid Energy Meter
for Improved Metering and Billing System
Microcontroller Based Single Phase Digital.pdf (Size: 178.83 KB / Downloads: 213)
Introduction
The present system of energy metering as well as billing in Bangladesh which uses electromechanical and
somewhere digital energy meter is error prone and it consumes more time and labor. The conventional
electromechanical meters are being replaced by new electronic meters to improve accuracy in meter reading. Still,
the Indian power sector faces a serious problem of revenue collection for the actual electric energy supplied owing
to energy thefts and network losses. One of the prime reasons is the traditional billing system which is inaccurate
many times, slow, costly, and lack in flexibility as well as reliability [1].
Meters, in the past and today in a few countries, were electromechanical devices with poor accuracy and
lack of configurability. Theft detection was also a challenge.Recent developments in this direction seem to provide
opportunities in implementing energy efficient metering technologies that are more precise, accurate, error free, etc.
[2].
A Prepaid Energy Meter enables power utilities to collect electricity bills from the consumers prior to its
consumption. The prepaid meter is not only limited to Automated Meter Reading but is also attributed with prepaid
recharging ability and information exchange with the utilities pertaining to customer’s consumption details. The use
of electronic token prepayment metering has been widely used in UK for customers with poor record of payment
[3]. A paper suggests a design of a system which can be used for data transmission between the personal computer
and smart card [4]. Another paper suggests making use of state of art technologies like WiMAX in Prepaid Energy
Meter owing to the idea of centralized accounting, monitoring and charging [5]. Polyphase prepaid energy metering
systems have also been proposed and developed based on local prepayment and a card reader [6]. Wireless prepaid
energy metering sytem has been proposed which incorporate RF based system [7]. Digital energy metering system
as an alternative for the clectromechanical system has been proposed and developed with the Peripheral Interface
Controller (PIC) and necessary software [8]. Due to the low cost of microcontrollers, Prepaid Energy Meter has
been developed using a microcontroller from the Microchip Technology Inc. PIC family [9].
Voltage Sensor
In this scheme, energy meter IC (AD7751) is biased around the neutral wire and a resistor divider is used to
provide a voltage signal that is proportional to the line voltage. A voltage divider is made in combination of 1 MΩ
resistor and 1 kΩ resistor. The output voltage across the 1 kΩ resistor is applied to the voltage channel of the energy
meter IC.
Current Sensor
The voltage outputs from a calibrated resistor of 3.335 mΩ connected with the neutral wire is applied to the
current channel of the energy meter IC. Current channel has a programmable gain amplifier with gains of 1, 2, 8, or
16. The maximum peak differential voltage is ±660 mV divided by the gain selection.
Energy Meter IC
Energy meter IC has two ADCs that digitizes the voltage and current signals from the supply main. These
ADCs are 16-bit second order sigma-delta converters with an over sampling rate of 900 kHz. A high-pass filter in
the current channel removes any dc component from the current signal. This eliminates any inaccuracies in the realpower
calculation due to offsets in the voltage or current signals. The real-power calculation is derived from the
instantaneous power signal. The instantaneous power signal is generated by a direct multiplication of the current and
voltage signals. In order to extract the real-power component, the instantaneous power signal is low-pass
filtered.The low frequency output of the energy meter IC is generated by accumulating this real-power information.
The output frequency is therefore proportional to the average real-power. This average real-power information can
in turn be accumulated by a counter to generate real-energy information.
Microcontroller
Microcontroller is a programmable device which contains a microprocessor, memory, input-output ports
etc which can be compared with the microcomputer. Microcontroller is a single chip computer. As microcontroller
is a low cost programmable device. It is used in the automatic control application. Now the pulses produced at the
pin CF is directly applied to the counter pin of the microcontroller. The microcontroller counts the pulses that appear
at pin 1 of Microcontroller (ATmega32) within every 20 seconds. The number of pulses per second appeared at pin
22 of Energy Meter IC is directly proportional to the instantaneous real power information for a particular load.
Information such as power, energy, and maximum demand are stored at the EEPROM of the Microcontroller
(ATmega32).
Smart Card and its Communication with Server Terminal and Energy Metering System
A microcontroller (ATtiny13) is used as a smart card in which the numbers of units to be recharged are
loaded by interfacing with the USB port and the user operated PC of server terminal. The ATtiny13 will send the
information contained on it to the microcontroller (ATmega32), when the DIP switch connected with the two
microcontrollers is switched on. Then the information of microcontroller (ATtiny13) will be transferred to the
microcontroller (ATmega32) and stored in the EEPROM of the microcontroller (ATmega32) while erasing the
content of the ATtiny13. The number of recharged units are contained in the EEPROM of the microcontroller
(ATmega32) and will be gradually decreased with the increment of the energy consumption by the load. The
updated value after the execution of the every step will be stored in the EEPROM of the microcontroller
(ATmega32). Now the consumers have to recharge the smart card from server terminal for further use of energy
after finishing the previously recharged units.