2. WHAT IS THIRD HARMONIC
DISTORTION
Most of the harmonic problem is caused by the 3rd
component. Since the 3rd harmonic is the 2nd highest
energy from the fundamental component.
3.
4. INTRODUCTION
The 3rd harmonic distortion meter has been designed for
measuring the quality of AC supply. The meter is built with a
PIC18F2550 project board and the full wave rectifier front-end
circuit. The AC power line, 220VAC is measured through the step
down isolation transformer. The input signal to the 10-bit ADC is
full wave rectified. The software performs DFT calculation
finding the amplitude of the fundamental frequency and the 3rd
harmonic. The distortion is computed by the ratio of the
amplitude of the 3rd harmonic to the fundamental frequency.
The meter has been tested with the square wave signal resulting
33% distortion. For low voltage AC utility, 220V, the reading
showed approx. 3%. The meter can be applied for high voltage
application with the appropriate signal conditioning.
6. Nowadays an increasing of the electronic devices having
nonlinear characteristics are many used at home and office.
Such devices mostly are computer based equipment with a
low power factor switch mode power supply. The input
circuit of the power supply uses a diode-capacitor at the
front-end circuit. The current drawn is charging capacitor
only near the peak voltage. Thus for a given feeder having
finite impedance, there will be a lost from voltage dropped
near the peak voltage resulting flattened top distortion of
the AC voltage. To measure how high the distortion of AC
voltage is, we may decompose it into the summation of
sinusoid waves using DFT. The PIC harmonic distortion
meter shows a method for finding the amplitude of the
fundamental frequency and the 3rd harmonic. The reading
shows percentage of the 3rd harmonic distortion.
7.
8. Total Harmonic Distortion, THD
To measure the waveshape distortion, we use the
quantity of the Total Harmonic Distortion, THD
(equation 1). THD is the ratio of the power of
harmonic components to the power of fundamental
frequency. Our concern is the voltage distortion, we
can just find the sum of the rms of the harmonic
components, Vn and the rms of the fundamental
frequency, V1.
9. Most of the harmonic problem is caused by the 3rd
component. Since the 3rd harmonic is the 2nd highest
energy from the fundamental component. So we
interest to find only the 3rd harmonic distortion using
equation.
HD = (V3/V2).100
10. WHAT 3RD HARMONIC DISTORTION
METER DOES
IN OUR PROJECT WE WILL MAKE A
MICROCONTROLLER BASED PROJECT USIND
PIC18F2550 SO WHEN WE WILL GIVE AN INPUT
SIGNAL IN THE FORM OF VOLTAGE , IT WILL TRY
TO DETECT THAT WHETHER THRID HARMONIC
COMPONENT IS PRESENT IN THE SIGNAL OR NOT
, IF IT IS PRESENT THEN IT WILL SHOW THE
VALUE OF VOLTAGE TO BE GIVEN SO AS TO KILL
THE THIRD HARMONIC COMPONENT.
11. TESTING WITH INPUT AC SIGNAL
The testing shows the input signal and 3rd harmonic
detector (shown in THD) for a clean sine wave, square
wave .
14. PREVIOUS FIGURE SHOWS THAT THE 3RD
HARMONIC HAS A SUPPRESING OR DEPPRESSING
CHARACTERISTIC
15. ADVANTAGES
DETECTS 3RD HARMONIC IN A SIGNAL.
SHOWS THE VALUE OF INPUT VOLTAGE TO BE
GIVEN SO THAT NO 3RD HARMONIC IS PRESENT
CAN BE USED IN BIG INDUSTRIES FOR CLEARING
LOSSES DUE TO 3RD HARMONIC
16. CONCLUSION
WE WILL TRY TO BUILD A CHEAP INSTRUMENT
FOR MEASURING THE THIRD HARMONIC OF AC
POWER LINE WITH PIC 18F255
MICRCONTROLLER.
