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Electrical Power Consumption Monitoring using a Real-time System
1
I. Elamvazuthi, 2M. K. A., Ahamed Khan ,
Syafiq Basri Bin Shaari, 4Rajendran Sinnadurai and 5M.Amudha
3
1,3
Dept. of Electrical and Electronic Engineering
Universiti Teknologi PETRONAS
Bandar SeriIskandar, 31750 Tronoh, Perak, Malaysia
&
2,4
Univesiti Industri Selangor
Kampus Bestari Jaya, Jalan Timur Tambahan
Selangor Darul Ehsan
2
[email protected] (Corresponding author)
5Cosmopoint,
Level 16-2, Wisma Sachdev,
Jln Raja Laut, 50350 Kuala Lumpur, Malaysia
[email protected]
Abstract—
In recent years, research into energy saving has been increasing
in view of dealing with environmental problems and effectively
using energy resources. In a plant, power consumption
monitoring of individual inductive devices like motors would
have significant impact on energy savings in the long run.
However, the current practice of measurement of power
consumption of the whole plant rather than individual devices
results in penalties for energy losses due to variation of demand
charges in a plant. Therefore, electrical power consumption
monitoring on a real-time basis is essential to keep it from
exceeding the critical demand level. Power meters are practical
energy saving devices that can help monitor electricity
consumption in a plant. This paper discusses the development
and implementation of a micro-controller based portable digital
power meter that has the capability to measure three phase
power supply for a single device in order to optimize power usage
in a plant. It could also be used as an educational tool for
undergraduate studies.
Keywords- energy savings; electrical power consumption; power
monitoring; real-time system
I.
INTRODUCTION
In recent years, research into energy saving has been
increasing in view of dealing with environmental problems
and effectively using energy resources. Research about power
meter design has been done by [1] and [2-4]. Three phase
electric power meter is a meter that measures power on a three
phase power system. Research done by [1] implements the
single chip application with digital opto-coupler as the
isolation between the microcontroller circuit and the
measurement unit. The method proposed by [2] solely depends
on the AT89S52 chip to calculate power. However, it has no
data logging feature. Approach taken by [4] is more on data
logging which using GSM network to log the power
measurement data. However it is limited to single phase
application.
This paper proposes the design of 3 phase power meter
which is different from the conventional power meters. This
meter is designed to be plugged-in directly into the power
supply socket meanwhile the load will be plugged-in onto the
power meter socket. This method will remove the dependency
of wires that are conventionally used, and suitable for
temporary or permanent installation. This solution allows the
user to measure power for single device so that power usage
optimization can be done in a facility. In addition, the project
also features data logging into a SD memory card. To achieve
these features, Programmable Interface Controller (PIC) is
interfaced with isolated current and voltage measurement
sensors. With programmed calibration the output linearity can
be manipulated.
II.
PRINCIPLES OF POWER CALCULATION
Basically the total instantaneous real power is the
summation of instantaneous power of each phase as shown
below [5].
𝑝3∅ = 𝑣𝑎𝑛 𝑖𝑎 + 𝑣𝑏𝑛 𝑖𝑏 + 𝑣𝑐𝑛 𝑖𝑐 𝑤ℎ𝑒𝑟𝑒 𝑛 = 0,1,2,3, …
(1)
The simplified version of the three phase power formula
which taken from single phase analysis is:
𝑃3∅ = 3|𝑉𝑝 ||𝐼𝑝 | cos 𝜃
(2)
where the cos θ represents the power factor of the system. The
value is equal to which represents the angle difference
between phase voltage and phase current or the impedance
angle. However the instantaneous measurement in equation 1
is not desirable in metering because the value will keep on
changing over time. In addition, equation 2 cannot be
implemented directly to the PIC. Thus, RMS value needs to be
calculated. The single phase RMS power calculation is [5-6]:
𝑃= √
1
1+𝑁
2
∑𝑁
𝑛=0(𝑣𝑛 𝑖𝑛 )
(3)
Thus, equation 3 is suitable to be used in the power meter. So,
the total RMS three phase real power can be calculated as
follows:
𝑃3∅ = √
√
1
1+𝑁
1
1+𝑁
2
∑𝑁
𝑛=0(𝑣𝑎𝑛 𝑖𝑎𝑛 ) + √
2
∑𝑁
𝑛=0(𝑣𝑐𝑛 𝑖𝑐𝑛 )
1
1+𝑁
2
∑𝑁
𝑛=0(𝑣𝑏𝑛 𝑖𝑏𝑛 ) +
(4)
However, the equation 4 does not provide information about
the system power factor. In order to grab power factor value,
apparent power (S) value needs to be calculated. Power factor
(p.f) can be determined by following equation:
p. f = cos ∅ =
𝑃𝑅𝑀𝑆
(5)
𝑆𝑅𝑀𝑆
Apparent power for a single phase system can be determined
by general equation in equation 6. Meanwhile equations 7, 8
and 9 represent RMS apparent power formula for phase a,
phase b and phase c respectively:
𝑆1∅ = 𝑣𝑎 𝑖𝑎
𝑆𝑎∅ = √
1
1+𝑁
𝑆𝑏∅ = √
𝑆𝑐∅ = √
1
1+𝑁
1
1+𝑁
Fig. 2 shows the developed prototype. It consists of main
switch, backlight switch, power and sensor indicator,
navigation buttons, SD card slot and battery casing.
(6)
1
2
∑𝑁
𝑛=0(𝑣𝑎𝑛 ) × √
1+𝑁
2
∑𝑁
𝑛=0(𝑣𝑏𝑛 ) × √
1
2
∑𝑁
𝑛=0(𝑣𝑐𝑛 ) × √
1
1+𝑁
1+𝑁
2
∑𝑁
𝑛=0(𝑖𝑎𝑛 )
(7)
2
∑𝑁
𝑛=0(𝑖𝑏𝑛 )
(8)
2
∑𝑁
𝑛=0(𝑖𝑐𝑛 )
(9)
So, the total apparent power is the summation of the apparent
powers from all phases. By having power factor information,
the reactive power (Q) can be calculated. Equation 10
represents the general equation for reactive power meanwhile
equation 11 is reactive power equation derived from previous
equations.
𝑄𝑛 = |𝑉𝑛 ||𝐼𝑛 | sin ∅
(10)
𝑃𝑅𝑀𝑆
−1
𝑄𝑛 = |𝑉𝑛 ||𝐼𝑛 | sin (cos
)
(11)
𝑆𝑅𝑀𝑆
III.
Figure 1 System Configuration
MATERIALS AND METHODS
A. Hardware
The system configuration is as shown in Fig. 1. It consists
of microcontroller and other supporting components to enable
the power meters features.
Figure 2 Prototype
Internally, it contains the microcontroller circuitry for each
phase. In actual implementation, the circuit needs to be
duplicated for three set which represent measurement for
phase A, phase B and phase C. The circuit is designed to
measure phase to phase voltage. Thus, the combination of the
duplicates will form delta connected measurement. This
measurement technique can be used only for a balance 3 phase
network.
Software
Software includes main program, SPI module program for
data writes into SD card, interrupt, LCD display control
program, EEPROM read and write program and power
calculation program. The program flow chart is shown in Fig.
3 [7].
The power meter is able to do initialization during each
power up. The initialization includes initialization of certain
variables, read calibration and logging configuration from
EEPROM, set I/Os direction and initialize TIMER0 interrupt.
B.
This power meter is designed with four different modes.
During beginning of each loop, user buttons are scanned to
determine which mode to be run and displayed on the LCD.
Throughout the loop, TIMER0 overflow interrupt will run for
each 1 second to update the EPOCH formatted time variable.
The time is important because it is needed in logging time and
date stamp. In addition, the interrupt also handles the logging
interval variable. The interval will determine the time to
record the measured power data into SD card through SPI
communication protocol.
TestingMethods
Figs 4 to 6 show the prototype test setup, LVDAM-EMS
data acquisition interface and four pole 3 phase synchronous
motor respectively.
C.
Figure 4. Prototype Test Setup
Figure 5. LVDAM-EMS Data Acquisition Interface
Figure 6. Four Pole 3 Phase Synchronous Motor
Figure 3. Flow Chart of Main Program
Tests were done using variable three phase power supply
(0-240V/50Hz) and with a four pole synchronous motor as the
load. The motor is rated to be supplied by 380V and at 0.52A.
Prior to the test, the power meter is calibrated using its internal
calibration program in PIC. The measurement result is
compare with the laboratory data acquisition (DA), LVDAM-
EMS. The test records the input voltage and current into the
motor for all phases. Voltage and current are taken because
both values will be used to calculate other power qualities
component such as apparent power, real power and reactive
power.
IV.
recorded without decimal point. Thus, all readings need to be
multiplies with 10-2.
RESULTS AND DISCUSSION
Table I, Table II and Table III show the phase A, phase B
and phase C measurement result respectively.
TABLE I.
PHASE A MEASUREMENT RESULT
Figure 7. Data Logged in the SD Card
No
1
2
3
Voltage (V)
DA
Error (%)
PM
DA
Error (%)
57.1
73.8
105.34
57.28
73.34
106
0.314246
-0.62722
0.622642
0.12
0.10
0.10
0.13
0.098
0.095
7.692308
-2.04082
-5.26316
TABLE II.
No
1
2
3
PHASE B MEASUREMENT RESULT
Voltage (V)
Current (mA)
PM
DA
Error (%)
PM
DA
Error (%)
57.36
73.16
106.17
57.04
73.68
106.18
-0.56101
0.705755
0.009418
0.12
0.10
0.10
0.129
0.099
0.095
6.976744
-1.0101
-5.26316
TABLE III.
No
1
2
3
Current (mA)
PM
PHASE C MEASUREMENT RESULT
Voltage (V)
Current (mA)
PM
DA
Error (%)
PM
DA
Error (%)
57.3
73.2
105.95
57.36
73.74
106.5
0.104603
0.732303
0.516432
0.12
0.1
0.1
0.129
0.099
0.095
6.976744
-1.0101
-5.26316
By analyzing the results from Table I to III, it can be seen
that the voltage measurements have low error which is less
than 1%. However, current measurements have high error at
certain level. The current measurement can be considered as
unstable measurement. There are many reasons that might
affect the measurement such as the noise, the number of
measurement samples taken and the equipment condition. One
most obvious factor is the data acquisition uses 3 decimal
places for current measurement meanwhile the power meter
uses 2 decimal places. The number of samples taken for one
displayed on the power meter is 50 samples. This number is
considered small thus by having more samples might reduce
the error.
Fig. 7 shows the data logged into the SD card. The data that
logged into SD card is stored into file named “METER.TXT”.
From the Fig. 8, the timestamp is recorded using UNIX time
format. So, the data need to be converted using any UNIX to
normal time format converter. The measurement data is
V.
CONCLUSION
Based on the prototype test result the power meter currently
is capable to measure apparent power, voltage and current
with acceptable error. The raw analog value from the A/D
converter readings can be read by software to calculate RMS
real power, reactive power, and power factor. The power
meter also is expected to be able to accept measurement
values from any other voltage and current measurement
technique due to its programmed calibration feature which is
capable to set new input and output relationship. Therefore,
this meter would be able to be tested in real industrial facility
soon. However, the current power meter circuit configuration
limits the power meter to measure a balance three phase power
system only.
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[1]
[2]
[3]
[4]
[5]
[6]
[7]
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25,
2011,
from
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Cafe:
http://www.rfcafe.com/references/electrical/rms.htm
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