Why a Harmonic Analyzer is used in Power Quality studies.
Power quality is important to understand and manage, crucial even since our modern technological society needs clean, dependable, reliable, glitch-free power to run our computers, machines, motors, lights etc. With our understanding of electricity we’ve come to realize that electricity can get “messed-up” on its way from the generation stations to us. It travels from generators at dams, coal plants, or nuclear power stations to your home or business via transmission lines at high-voltage. It then passes through one or more substations, and is eventually dropped down to usable voltages by transformers where we as consumers use or consume it. Keep in mind we have to share the distribution network with other users. As the electricity gets consumed by many users and loads it gets – in simple parlance – altered. The way we use power will affect other loads and our interconnected neighbors, and they will affect us. The degree of alteration – the power “quality” – depends on several factors but the amount of alteration can be measured by examining the voltage stability and fluctuations, and the harmonic content. This is done with a power quality analyzer which usually will have a harmonic analyzer function – like the PowerSight power analyzers.
So what are power-line harmonics? These two graphs of voltage waveforms will illustrate:
The vertical axis is volts and the horizontal axis is time. The first one shows a power waveform that is sinusoidal and is an example of a nice, clean-looking, unaltered waveform that power generators produce. The second graph shows a waveform that is choppy and distorted – it doesn’t look so clean does it?
A French mathematician named Fourier invented a method to analyze waveforms – the Fourier analysis. It will show that the waveform on the left is made of one frequency – for power waveforms that would be 60Hz in the US (50Hz in Europe and other parts of the world.). The analysis for the waveform on the right would show that besides 60Hz there is a jumble of other frequencies that mix, add, subtract, interfere, and alter the original waveform. The analysis would show the mix of other frequencies and the amounts of their contributions, both in terms of amplitude (i.e. how big they are) and their phase position (i.e. at what point in time they are inserted into the mix) – as follows:
The way to understand a harmonic table goes like this. The first harmonic is called the fundamental and is declared to be 60Hz and its magnitude is 100%. The next harmonic (3.00) is the frequency of the fundamental multiplied by 3.00, i.e. = 180Hz. It is 13% as large as the fundamental. So if the fundamental was 208Vrms then the third harmonic would be 13% x 208 = 27Vrms. The next harmonic #5 is 300Hz and is 41.6 Vrms. The time duration of a power waveform to go through one oscillation is 16.666 ms. This equates to 60 times per second or 60Hz. Mathematically we can also express the oscillations as revolutions around a circle – which is a 360 degree rotation. The phase number tells us that the third harmonic (180Hz) doesn’t start at exactly the same time as the fundamental but is delayed by 42 degrees.
So why do we care about power-line harmonics? AC power distribution systems, and all the elements in a typical AC electrical infrastructure (conductors, transformers, breakers etc.) are designed to convey power efficiently at 6oHz. They are not so efficient at other frequencies. By less efficient we mean that some of the power will not be transmitted but will be lost as heat. The number one enemy of reliability is heat. Operating life span is reduced, or the “wear and tear” is increased so component reliability is compromised. If left unchecked or untreated, high levels of power line harmonics can cause transformers and conductors to destructively overheat, burn-up and fail, and for breakers and fuses to misoperate.
What creates the distortion of power waveforms? To begin with, motors and incandescent lamps don’t generally create high levels of harmonics but electronic loads do. We generally refer to these latter types of loads as “non-linear” because the current waveform doesn’t follow the voltage waveform with the same shape. Computers, computer-controlled equipment, electronic lighting and a plethora of modern loads are non-linear. Here’s an example of a computer power supply waveform captured by a PowerSight meter showing the current waveform is not sinusoidal like the voltage but a series of gulps of current, or pulses:
A power line harmonics analyzer can show you waveforms of the electricity at various locations inside your building or operation, and calculate the harmonics in both the voltage and current domains. There are guidelines specified by electrical engineers for acceptable levels of harmonic distortion – the one most commonly referred to is IEEE 519. Once you have an analyzer (and someone knowledgeable enough to use it), you can determine the extent of any harmonics. If they fall outside of the recommended limits you can use the harmonic data to find an appropriate solution or strategy to minimize harmonic distortion.
Generally those measures may involve investing in devices that are designed to ‘clean’ or filter the power coming into your facility, or changing or moving the loads, or changing wiring or transformers to cope with levels of high harmonics. The various solutions are situation (and budget) dependent.
A good power monitor or Power Quality Analyzer or Energy Analyzer will also perform as a harmonic power analyzer.
To summarize: Keeping tabs on power-line harmonics is prudent. Use a harmonic analyzer in your operation or facility to check they don’t become excessive. As an analogy it’s like watching blood pressure or cholesterol to avoid health issues over the long term. There will always be some level of harmonic distortion – you just need to keep it to a minimum.
Author: Michael Daish
Tags: harmonic analyzer, ieee 519, power analyzer, power monitor, power quality
Power Harmonics Analyzer, Power Monitor, Power Quality Analyzer | current_logger November 17, 2011