3. HARMONICS A harmonic of a sinosuidal wave is a component
frequency of the signal that is an integer multiple of the
fundamental frequency, i.e. if the fundamental frequency is f, the
harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics
have the property that they are all periodic at the fundamental
frequency, therefore the sum of harmonics is also periodic at that
frequency. Harmonic frequencies are equally spaced by the width of
the fundamental frequency and can be found by repeatedly adding
that frequency. For example, if the fundamental frequency is 25 Hz,
the frequencies of the harmonics are: 50 Hz, 75 Hz, 100 Hz
4. CAUSES NON-LINEAR COMPONENTS Diodes Transistors Electric
Motors SMPS(switch mode power supply) Etc.
5. EFFECTS One of the major effects of power system harmonics
is to increase the current in the system. This is particularly the
case for the third harmonic, which causes a sharp increase in the
zero sequence current, and therefore increases the current in the
neutral conductor. This effect can require special consideration in
the design of an electric system to serve non-linear loads. In
addition to the increased line current, different pieces of
electrical equipment can suffer effects from harmonics on the power
6. 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.
Third-harmonic distortion is nothing more than a measurement of the
amplitude of the third harmonic of the input frequency and is the
most prominent distortion component in analog magnetic recording
7. 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.
8. 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.
9. DETECTION OF HARMONICS USING DFT In the proposed method a
harmonic meter using a microcontroller is designed. The meter is
built with a microcontroller and the full wave rectifier front-end
circuit. 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 effectiveness of the proposed harmonic
meter is confirmed by experiment.
10. 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,
11. 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 2.
12. We may decompose the periodic waveform, f(t) into the
summation of a number of sinusoids waveform easily using the
Discrete Fourier Transform . A0 is the amplitude of DC components.
For AC voltage waveform, A0 is zero .
13. The amplitude for each harmonic can be computed as
14. FLATTENED TOP AC VOLTAGE CAUSED BY A LOW POWER FACTOR
SWITCH MODE POWER SUPPLY
15. 32-point sample of half ac wave
16. CIRCUIT DISCRIPTION From the above design, first of all
input signal is applied to the circuit which is 220Vac. The bridge
rectifier rectifies the ac input to the pulsating dc, which would
further made constant 5V with the help of filter-capacitor
arrangement and voltage stabilizer so that any change in ac would
not proportionally change dc. The dc output is given to the
variable resistor which is used to select the voltage which in case
of PLL cannot be done because it needs a separate phase detector
for different frequencies (voltages) this is then applied to the
digital oscilloscope which shows the total coverage of half cycle
by capturing 32 samples. The output from this is given to the
microcontroller where the software performs the DFT calculations
and the final results are displayed on the LCD i.e. the fundamental
frequency, the third order harmonic component and the harmonic
distortion in percentage.
17. SOFTWARE USED MPLAB 8.84 MPLAB development tools for the
PIC16F877A Microcontroller Architecture support every level of
software developer from the professional applications engineer to
the student just learning about embedded software development. The
Keil 8051 Development Tools are designed to solve the complex
problems facing embedded software developers.
18. 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
19. DISADVANTAGE It is designed for very less range of voltage
if distortion more than 5% is present then the microcontroller will
get damaged. It can only detect % distortion but cannot filter it
out because as ac is used then this technique of dft is not
possible in this case.
20. CONCLUSION As a matter of fact, the frequency analysis of
discrete time signals is conveniently performed on a digital signal
processor which in our case is designed and programmed in an
embedded microcontroller. The project approach applies a
computational convenient representation of discrete time signals
known as Discrete Fourier transform to estimate the fundamental and
third harmonic frequency of the signal. The computation of this
ratio can identify the distortion in ac voltage while being drawn
through different nonlinear devices so that after detecting the
harmonic component we can improve the quality of an ac
21. CONCLUSION (CONT..) WE WILL TRY TO BUILD A CHEAP INSTRUMENT
FOR MEASURING THE THIRD HARMONIC OF AC POWER LINE WITH PIC 18F255