What is Harmonics?
Power System Harmonics is an area that is receiving a great deal of attention recently. This is mainly due to non-linear (or harmonic producing) loads are comprising an ever increasing portion of the total load for a typical industrial plant.
The objective of electrical utility is to deliver sinusoidal voltage at fairly constant magnitude throughout the system. But this objective is complicated because of the fact that there are loads on the system that produce harmonic currents. There are loads on the system that produce harmonic currents. These current will result in distorted voltages and currents that can adversely impact the system performance in different ways.
Harmonics are a mathematical way of describing distortion to a voltage or current waveform. The term harmonics refers to a component of a waveform that occurs at an integer multiple of the fundamental frequency. Harmonics should not be confused with spikes, dips, impulses, oscillations or other forms of transients.
A common term used in relation to harmonics is THD or Total Harmonic Distortion. THD can be used to describe voltage or current distortion and is calculated as follows:
THD² (%) = ID1²+ID2²+…. + IDn²,
IDn is the magnitude of nth harmonic as % of fundamental.
Note: Voltage distortion is any deviation from the nominal sine waveform of the AC line voltage.
“What is Harmonic & how do we quantify it?”
Fourier Analysis allows us to breakdown a distorted waveform into a set of sine waves with certain characteristics.
The first characteristics deal with the frequency. The distorted waveform repeats itself with some basic frequency. The sine wave associated with this frequency, which is usually 50Hz is called the fundamental. Each successive sine wave, or harmonic, of this particular set has a frequency that is an integer multiple of fundamental. So 2ndharmonic has a frequency of 100Hz, for 3rd, 150Hz, the 4th, 200Hz & so on.
The next characteristic is magnitude, also called Harmonic Distortion Factor. Each of these sine waves may have a different magnitude from others depending on the actual distorted signal. The magnitude is determined by Harmonic Analyzer. Typically, the magnitude of each harmonic is represented as a percentage of RMS value of the fundamental, not the total RMS of distorted wave form.
For example, let the voltage waveform has an RMS value of 120V, but the fundamental is only 100V. If the magnitude of 3rd harmonic is 25V, then it has a 3rd harmonic distortion factor of 25%.
The aggregate effect of all the harmonics is called Total Harmonic Distortion (THD).
THD= RMS Value of all Harmonics/RMS Value of Fundamental
Linear & Non Linear Loads
A linear element in a power system is a component in which the current is proportional to the voltage. In general, this means that current wave shape will be same as voltage.
Eg: Motors, Heaters, Incandescent Lamps etc.
Voltage & Current Waveform for Linear Loads
For non-linear loads, the current waveform is not the same as voltage.
Eg: Rectifiers, UPS, Adjustable Speed Motor Drives, Ferro-Magnetic Devices, Arcing Equipments etc.
Voltage & Current Waveform for Non-Linear Waveform
Current drawn by non-linear loads is not sinusoidal, but it is periodic, meaning that current waveform looks same from cycle to cycle. Periodic waveforms can be described mathematically as a series of sinusoidal waveforms that have been summed together.
The sinusoidal components are integer multiples of fundamental. The only way to measure voltage or current that contains harmonics is to use a true RMS reading meter.
Waveform with Symmetrical Harmonic Components
Note: A symmetrical wave is one in which the positive portion of wave is identical to the negative portion of wave. They produce only odd harmonics.
Causes of Harmonics
Harmonics are caused by non-linear loads, ie, loads that draw non-sinusoidal current form a sinusoidal voltage source. The rectifier can be thought as a harmonic current source and produces roughly the same amount of harmonic current over a wide range of power system impedances. The characteristic current harmonics that are produced by a rectifier are determined by the pulse number.
where, h- is the harmonic number (integer multiple of fundamental)
When a non-linear load draws current, that current passes through all of the impedance that is between load and system source. As a result of the current flow, harmonic voltages are produced by impedance in the system for each harmonic.
These voltages sum and when added to the nominal voltage produce voltage distortion. The magnitude of voltage distortion depends on the source impedance and harmonic voltages produced.
Potential Effects of Harmonics
Power System problems related to harmonics are rare but it is possible for a number of undesirable effects to occur. High levels of harmonic distortion can cause effects such as,
* Increased transformer, capacitor, motor or generator heating
* Mal operation of electronic equipments
* Incorrect readings on meters
* Mal operation of protective relays
* Interference with telephonic circuits etc.
The likelihood of such ill effects occurring is greatly increased if resonant condition occurs. Resonance occurs when a harmonic frequency produced by a non linear load closely coincides with a power system natural frequency. There are two forms of resonance which can occur.
* Parallel Resonance
* Series Resonance
Parallel Resonance
Parallel resonance occurs when natural frequency of the parallel combination of capacitor banks and the system inductance falls at or near a harmonic frequency. This can cause substantial amplification of harmonic current that flows between capacitor and system inductance and lead to capacitor fuse blowing or failure or transformer over heating.
Series Resonance
Series resonance is a result of series combination of inductor and capacitor and presents a low impedance path for harmonic currents at natural frequency. Effect of a series resonance can be a high voltage distortion level between the inductance and capacitance.
Conclusion Harmonic currents can have a significant impact on electrical distribution systemsand thefacilities 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, troubleshooting and repair program.
This article was intended to provide an introduction to the basic principles of power system harmonics. I hope this helped you to get an idea on Power System Harmonics. Thanks to Google for helping me to write this post! :)
