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Saturday, 28 April 2012

Harmonics


Harmonics
                                             
            
In any alternating current network, flow of current depends upon the voltage applied and the
Impedance (resistance to AC) provided by elements like resistances, reactance of inductive and capacitive nature. As the value of impedance in above devices is constant, they are called linear whereby the voltage and current relation is of linear nature.
However in real life situation, various devices like diodes, silicon controlled rectifiers,
PWM systems, thyristors, voltage & current chopping saturated core reactors, induction & arc furnaces are also deployed for various requirements and due to their varying impedance characteristic, these NON LINEAR devices cause distortion in voltage and current waveforms which is of increasing concern in recent times. Harmonics occurs as spikes at intervals which are multiples of the mains (supply) frequency and these distort the pure sine wave form of the supply voltage & current.

Harmonics are multiples of the fundamental frequency of an electrical power system. If, for
example, the fundamental frequency is 50 Hz, then the 5th harmonic is five times that frequency, or 250 Hz. Likewise, the 7th harmonic is seven times the fundamental or 350 Hz, and so on for higher order harmonics.
Harmonics can be discussed in terms of current or voltage. A 5th harmonic current is simply
a current flowing at 250 Hz on a 50 Hz system. The 5th harmonic current flowing through the
System impedance creates a 5th harmonic voltage. Total Harmonic Distortion (THD) expresses the amount of harmonics. The following is the formula for calculating the THD for current:

When  harmonic currents flow in a power system, they are known as “poor power quality”
or “dirty power”. Other causes of poor power quality include transients such as voltage spikes, surges, sags, and ringing. Because they repeat every cycle, harmonics are regarded as a steady state cause of poor power quality.
When expressed as a percentage of fundamental voltage THD is given by,
THDvoltage =

                                
where V1 is the fundamental frequency voltage and Vn is nth harmonic voltage component.

Major Causes Of Harmonics
Devices that draw non-sinusoidal currents when a sinusoidal voltage is applied create harmonics.
Frequently these are devices that convert AC to DC. Some of these devices are listed below:
1. Electronic Switching Power Converters
Computers, Uninterruptible power supplies (UPS), Solid-state rectifiers
Electronic process control equipment, PLC’s, etc
Electronic lighting ballasts, including light dimmer
Reduced voltage motor controllers
2. Arcing Devices
Discharge lighting, e.g. Fluorescent, Sodium and Mercury vapour
Arc furnaces, Welding equipment, Electrical traction system
3. Ferromagnetic Devices
Transformers operating near saturation level
Magnetic ballasts (Saturated Iron core)
Induction heating equipment, Chokes, Motors
4. Appliances
TV sets, air conditioners, washing machines, microwave ovens
Fax machines, photocopiers, printers
These devices use power electronics like SCRs, diodes, and thyristors, which are a growing
percentage of the load in industrial power systems. The majority use a 6-pulse converter. Most loads which produce harmonics, do so as a steady-state phenomenon. A snapshot reading of an operating load that is suspected to be non-linear can determine if it is producing harmonics.
Normally each load would manifest a specific harmonic spectrum.
Many problems can arise from harmonic currents in a power system. Some problems are
easy to detect; others exist and persist because harmonics are not suspected. Higher RMS current and voltage in the system are caused by harmonic currents, which can result in any of the problems listed below:

Ø  Blinking of Incandescent Lights - Transformer Saturation
Ø  Capacitor Failure - Harmonic Resonance
Ø  Circuit Breakers Tripping - Inductive Heating and Overload
Ø  Conductor Failure - Inductive Heating
Ø  Electronic Equipment Shutting down - Voltage Distortion
Ø  Flickering of Fluorescent Lights - Transformer Saturation
Ø  Fuses Blowing for No Apparent Reason - Inductive Heating and Overload
Ø  Motor Failures (overheating) - Voltage Drop
Ø  Neutral Conductor and Terminal Failures - Additive Triplen Currents
Ø  Electromagnetic Load Failures - Inductive Heating
Ø  Overheating of Metal Enclosures - Inductive Heating
Ø  Power Interference on Voice Communication - Harmonic Noise
Ø  Transformer Failures - Inductive Heating

Overcoming Harmonics
Tuned Harmonic filters consisting of a capacitor bank and reactor in series are designed and
adopted for suppressing harmonics, by providing low impedance path for harmonic component. The Harmonic filters connected suitably near the equipment generating harmonics help to reduce THD to acceptable limits. In present Indian context where no Electro Magnetic Compatibility regulations exist as a application of Harmonic filters is very relevant for industries having diesel power generation sets and co-generation units.

TOTAL HARMONIC DISTRIBUTION FOR DIFFERENT VOLTAGE LEVEL
Bus  voltage at PCC
Individual voltage distribution %
Total voltage distortion %
69 KV and above
3.0
5.0
69.001KV thru 161KV
1.5
2.5
161KV and above
1.0
1.5
PCC=point of common coupling

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