08-08-2012, 04:04 PM
HARMONICS
HARMONICS.ppt (Size: 2.06 MB / Downloads: 44)
INTRODUCTION
The objective of the electric utility is to deliver sinusoidal voltage at fairly constant magnitude throughout their system.
This objective is complicated by the fact that there are loads on the system that produce harmonic currents. These currents result in distorted voltages and currents that can adversely impact the system performance in different ways.
What is HARMONICS ?
Harmonics are electric voltages and currents that appear on the electric power system as a result of certain kinds of electric loads.
Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50 or 60Hz.
Contd..
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 etc.
E.g.. Of Power system harmonic distortion
Power system harmonics distort the shape of the perfect voltage and current sinusoidal waveforms ideal to the power grid, and are multiples of the fundamental grid frequencies of 50 or 60 hertz found throughout the world.
Determining Characteristic Harmonics
The characteristic harmonics are based on the number of rectifiers (pulse number) used in a circuit and can be determined by the following equation:
h = (n x p) ±1
where: n = an integer (1, 2, 3, 4, 5 …)
p = number of pulses or rectifiers
For example, using a 6 pulse rectifier, the
characteristic harmonics will be:
h = (1 x 6) ± 1 Þ 5th & 7th harmonics
h = (2 x 6) ± 1 Þ11th & 13th harmonics
h = (3 x 6) ± 1 Þ17th & 19th harmonics
h = (4 x 6) ± 1 Þ23rd & 25th harmonics
Types of equipment that generate harmonics
Harmonic load currents are generated by all non-linear loads.
Single phase loads, e.g.
Switched mode power supplies (SMPS)
Electronic fluorescent lighting ballasts
Small uninterruptible power supplies (UPS) units
Three phase loads, e.g.
Variable speed drives
Large UPS units
CONTD…
The terms “linear” and “non-linear” define the relationship of current to the voltage waveform. A linear relationship exists between the voltage and current, which is typical of an across-the-line load.
A non-linear load has a discontinuous current relationship that does not correspond to the applied voltage waveform.
Single phase loads
Switched mode power supplies (SMPS)
The majority of modern electronic units use switched mode power supplies (SMPS).
These differ from older units in that the traditional step-down transformer and rectifier is replaced by direct controlled rectification of the supply to charge a reservoir capacitor from which the direct current for the load is derived by a method appropriate to the output voltage and current required.
Single phase UPS units exhibit very similar characteristics to SMPS.
CONTD..
The advantage – to the equipment are
size, cost and weight is significantly reduced and the power unit can be made in almost any required form factor.
The disadvantage –
Drawing continuous current from the supply, the power supply unit draws pulses of current which contain large amounts of third and higher harmonics
Three Phase Loads
Variable speed controllers, UPS units and DC converters in general are usually based on the three-phase bridge, also known as the six-pulse bridge because there are six pulses per cycle on the DC output.
In theory, the magnitude of each harmonic is the reciprocal of the harmonic number
The magnitude of the harmonics is significantly reduced by the use of a twelve-pulse bridge
EFFECTS OF HARMONICS
Equipment heating.
Equipment malfunction.
Equipment failure.
Communications interference.
Fuse and breaker mis-operation.
Process problems.
Conductor heating.
HARMONIC ANALYSIS
Engineers use a tool called Fourier analysis to mathematically demonstrate a distorted yet periodic waveform is actually made up of a fundamental plus a series of sine wave components combined in the right proportions.
Harmonic analyzers are effective instruments for determining the wave shapes of voltage and current and measuring the respective frequency spectrum. Several types of harmonic measuring instruments are available, with each type having a different capability.
Mitigation
It can be performed by two methods: filtering or cancellation.
A harmonic filter consists of a capacitor bank and an induction coil.
The filter is designed or tuned to the predetermined non-linear load and to filter a predetermined harmonic frequency range.
CONTD..
Usually this frequency range only accounts for one harmonic frequency.
This application is mostly used when specified for a UPS or variable frequency drive motor in a manufacturing plant.
CONTD..
It is performed with harmonic canceling transformers.
A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems.
This type of transformer has patented built-in electromagnetic technology designed to remove high neutral current and the most harmful harmonics from the 3rd through 21st
CONCLUSION
Harmonic currents can have a significant impact on electrical distribution systems and the facilities that they feed.
It is important to consider their impact when contemplating additions or changes to a system.
In addition, identifying the size and location of non-linear loads should be an important part of any maintenance.