Harmonics

Yaskawa America, Inc. welcomes you to the Harmonics eLearning Module. This eLearning Module provides a basic understanding of electrical harmonics, with an explanation of how they are created and how they can be eliminated from the electrical grid.

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00:00:05 [Music] the Technical Training Department of yasawa America Incorporated presents harmonics a guide to understanding and successful control I'm Steve Kaylor and today we are going to answer these questions what are harmonics why do we care and when we care what we going to do about it now if you're involved with a facility that uses a large amount of

00:00:34 electrical power these are very important questions and there are lots of good reasons that the answers matter it's essential to have an understanding of how harmonics work and how to handle their effects so to begin our understanding let's get down to the fundamentals all electrical power is distributed as a sinusoidal voltage

00:01:01 why a sine wave well it's easier to understand that if you look at the basics of how a generator works at its simplest a generator is a rotating magnet in a stationary coil of wire the rotating Magnet or rotor is called the field winding the stationary coil is called the stator winding the field winding or rotor is excited by direct current by means of carbon brushes and

00:01:28 slipp Rings the field winding is turned at a constant speed by a mechanical power source connected to the shaft as the field winding turns the magnetic flux sweeps by the stationary stator windings which induces voltage in the stator windings and in a two-pole generator the induced voltage makes a complete cycle of values for each complete mechanical Revolution and what

00:01:51 do you have a sine wave in a two-pole generator the frequency of the induced voltage in cycles per second or or Hertz is the same as the speed of the rotor or field winding in revolutions per second the electrical frequency is synchronized with the mechanical speed and for that reason this type of equipment is called a synchronous generator so a two pole synchronous generator turned at 60

00:02:19 revolutions per second or 3600 RPM will produce a 60 Herz voltage sine wave power generation in the world today is a three-phase system so synchronous generators used for power generation produce three phase voltages displaced 120° in time with respect to each other a simple three-phase two-pole synchronous generator with one coil per phase is shown right

00:02:51 here when an electrical load is connected to the voltage source it draws current to perform work if the current follows in the same signus soidal pattern is the voltage then the load is said to linearly follow the voltage and in short is called a linear load however if the connected load does not follow the sinusoidal wave shape of the voltage then the load is called a nonlinear load

00:03:17 nonlinear loads cause stress on the Transformers and generators that make up the power system also known as the power grid this stress has been found to be primarily thermal in nature and in other words stuff gets hot and even though the load that is doing the work can be very efficient the overall energy used to perform the work of a nonlinear load is much higher than a linear load Power

00:03:43 Systems carrying nonlinear loads are less efficient so it's important to find them and take corrective action to reduce their negative impact on the power grid now in the real world the grid is filled with all types of devices that create non linear loads like power supplies for computers and electronics lamp dimmers variable speed

00:04:08 drives a great many things consume current intermittently in blocks or bursts causing the sine waves of the power grid to become distorted now if you're an electrical power utility distorted current waves are a problem and as mentioned earlier distorted waveforms increase line losses they stress generators they overheat Transformers and they can destroy power

00:04:33 factor capacitors the other users down the street who share your electrical grid care as well distorted current drawn by your nonlinear load can result in distorted voltage on the grid distorted voltage can shorten the life of your neighbors's power line equipment and increase the cost of their power it overheats their Transformers stresses their switch gear and increases neutral

00:04:59 currents The Greener World also cares about distorted current because it wastes energy and it increases heat there is an important aspect to keep in mind when it comes to distorted voltage on the grid the distorted current that is drawn by the nonlinear load is local in nature and flows only into the nonlinear load consuming that current however when distorted current is

00:05:25 transported to the nonlinear load it causes distorted voltage drops all all along the power system which leads to distorted voltage on the grid voltage is common to all users and distorting this common grid voltage can create widespread losses and may result in faulty operation of many types of electrical equipment in order to reduce the level of distorted voltage it is

00:05:51 important to limit the flow of distorted current to get a clearer understanding of why this is so let's use one of my favorite simple teacher analogies and imagine that voltage and current in the power system are like pressure and water flow in a household or multif family water system if you are opening and closing the water system of one area well bursts

00:06:18 of water are coming out but bursts of pressure are going in the other direction in the system as well if the bursts are strong enough the water system of the guy next door the one that normally flows so smoothly won't be so smooth anymore at least it won't until you stop surging the system on your end so obviously there needs to be a remedy to distorted waveforms on the power grid

00:06:44 but before we can try to fix power inconsistencies we really need to Define and quantify them as mentioned earlier a distorted current waveform creates heat in the power delivery equipment resolving a nonlinear waveform into its sinusoidal components is called harmonic analysis harmonic analysis will estimate the heating effects due to nonlinear

00:07:12 currents flowing through circuit breakers and Transformers and that's where this gentleman Mr forier enters the picture an 18th century mathematician and physicist Jean Baptist Joseph forier introduced the notion that even the most complex waveforms could be understood and mathematically defined as a composite of a fundamental frequency

00:07:40 sine wave with other sine waves in multiples of that frequency we call these waves harmonics forier also gave us a complex mathematical equation to define the relationship an equation that has struck Terror into the hearts of engineering students and physic teachers ever since right here is a demo of a wave being resolved back and forth between its fundamental wave

00:08:10 and the distorted or composite Wave by adding or subtracting some of its harmonics now we are looking now at how multiple waves can exist in the same system at the same time by looking at this graphic you can see that constructing a composite wave is fairly intuitive for example if the fundamental wave says go up six while the harmonic says go

00:08:36 down two the result will be a composite wave that goes up four now deconstructing a highly distorted composite wave into its fundamental and say the 20 some harmonics that distorted it is much less intuitive and this is the math that makes that possible the ratio of harmonic content to fundamental is defined as total harmonic

00:09:05 Distortion or THD THD is expressed as a percentage of the fundamental the higher the percentage the more distorted the waveform now our primary concern is voltage harmonics since voltage is what is common to all we have to understand that the only way to limit voltage harmonics is to minimize current harmonics if the current harmonics are

00:09:31 under control the voltage harmonics are generally acceptable theoretically there are both even and odd-numbered harmonics typically in an AC power system even harmonics are absent Our concern here are the odd harmonics which are present in an AC system and do contribute to Total harmonic Distortion odd harmonics that are multiples of three in a three-phase

00:09:58 power system called triplin harmonics theoretically are absent in a balanced three-phase system however perfect balance is rarely the case and one must be ready to deal with triplin harmonics that form a noticeable amount of losses in an AC power system now that we have a measure of harmonics called total harmonic Distortion the next question becomes how

00:10:27 much Distortion is too much well the industry standard is i e 519-9966 DD and how is it different from THD well tdd which again stands for total demand Distortion is a moving average of the THD based on the system rated current value where THD is based on the fundamental current on a cycle to cycle basis which changes with load tdd uses a much longer averaging time as

00:11:24 long as 15 to 30 minutes and is based on the rated current of the system the basic definition of tdd is based on the pure and simple truth that distorted current waveforms result in increased temperature of the power delivery equipment hence a Time variable is introduced into the picture the value of THD on a cycle by cycle basis is not meaningful when the main intent is to

00:11:53 reduce power loss and reduce Heating in the power delivery equipment in an optimal way the 519 standard calls for measurement at the point of common coupling with the utility but Engineers specify its use at the input terminal of the variable frequency drives this standard is often too rigorous but it's so commonly used that it is important to address its abuse the 519 standard is

00:12:20 expressed as a table an important variable described in the table is the relationship between the size of the power source and the size of the load this relationship called the short circuit ratio is a measure of load current to available current if you have a very big power source powering a very small load the 519 standard allows you a large amount of tdd up to 20% if you

00:12:49 have a small power source with a relatively large load the standard dictates the need for a lower level of tdd as low as 5% here's where the discussion comes around to the world of variable speed drives also known as variable frequency drives or vfds what kinds of harmonics do variable

00:13:13 speed drives create well the answer is it depends especially based on the input rectifier scheme and the size relationship between the drive and the power source fortunately there are things you can do to reduce current distortion on the power source and these Solutions are commonly much more costeffective than increasing the power source's

00:13:40 size mitigation step one is to add a DC link inductor otherwise known as a reactor or choke in front of the DC bus capacitors of the drive an inductor is a good way to begin making the wave form more continuous or more sinusoidal you can use an inductor to reduce your THD to about 40% in certain situations mitigation step two is to add an input reactor in front of the drive

00:14:11 an input reactor also helps protect the drive from some forms of noise and power disturbances which is a nice bonus this adds impedance that can bring levels of THD down to about 35% many times simply adding a DC link choke in combination with either 1% or 2% impedance input reactor is sufficient to meet tdd levels at the point of common coupling which typically is the

00:14:38 secondary of the main feed Transformer another method of mitigating harmonics is to use an active filter in facilities with large drives or many small drives active filtering techniques can be used to help reduce THD active filtering techniques typically are more efficient and can be economical as well the shunt active filtering technique adds a set of

00:15:02 cancelling harmonics back into the circuit which are meant to counteract the harmonics generated by the nonlinear load shunt active filtering techniques provide the best performance for the lowest cost however shunt active filtering is a systems approach and requires the active filtering equipment provider to work closely with the plant manager specifying the equipment

00:15:28 location and size of the unit for installation another option to reduce THD is to use a variable speed drive with a 12 pulse input rectifier this approach takes a three-phase input and feeds it into a phase shifting Delta Delta y Transformer delta deltay type isolation Transformers are readily available from a variety of Manufacturers the theory behind this

00:15:56 approach is simple the harmonics generated by a three-phase AC to DC rectifier found in many vfds is given by the formula H equal KQ plus or minus one where H is the harmonic number Q is the number of humps per electrical cycle on the DC bus and K is any integer for a six pulse drive that's six humps on the DC bus per cycle harmonic number H yields values of five 7 11 13 17 19 Etc

00:16:35 as you can see converting a six pulse rectifier to a 12 pulse rectifier that's 12 humps on the DC bus per cycle harmonic number H yields values of 11 13 23 25 Etc the fifth 7th 17th and 19th harmonics are absent in the 12 pulse rectifier scheme which results in a low level of current distortion now we're getting input current that is beginning to look very sinusoidal and

00:17:04 brings us to the level of 9 to 14% THD this approach adds a modest amount of extra cost due to the additional components of the 12 pulse rectifier plus the Transformer but in an application where a low voltage vfd is to be connected to a medium voltage power supply A Step Down Transformer will be needed anyway so a 12 pulse Transformer used in a situation like

00:17:29 this may not be a major cost Adder yasawa offers the 12 pulse input rectifier scheme standard on its a1000 drives One Step Above the 12 pulse rectifier scheme is the 18 pulse rectifier scheme 18 pulse rectifier schemes are typically found in larger horsepower vfds and just like the 12 pulse require a phase shifting Transformer using the formula we

00:17:55 discussed earlier H = KQ Plus orus one an 18 pulse rectifier that's 18 humps on the DC bus per cycle harmonic number H yields values of 17 19 35 37 Etc the 5th 7th 11th 13th 23rd and 25th harmonics are absent in the 18 pulse rectifier scheme which results in a very low level of current Distortion an 18 pulse rectifier yields a very s-like wave 5 to 5.5%

00:18:33 THD the downside of the 18 pulse rectifier approach is that it is usually available only in large siiz drives a typical 15 horsepower Industrial Drive isn't available with an 18 pulse rectifier scheme six pulse schemes are most common and 12 pulse schemes are available in some instances large horsepower HVAC Drive applications are often held to a very strict conformance

00:18:59 of i e 519-9966 hybrid can be accomplished by putting a single deltay transformer into the circuit then adding an inphase reactor the Transformer KVA is reduced to 50% of the standard configuration the parallel reactor is impedance matched to the Transformer and sized for 50% of the load an input reactor is optional similar to the hybrid 12 pulse scheme

00:19:54 there exists a hybrid 18 pulse scheme this configuration requires a phase shifting Transformer sized at 2/3 the size of the drive along with a parallel reactor that is impedance matched to the Transformer and sized for onethird of the load as with the 12 pulse hybrid the chief benefit from this approach is that you will save money from harmonic mitigation one of the most complex

00:20:21 options for harmonic mitigation and one of the most interesting is called the active front end or AF so instead of the passive rectifier Circuit of the conventional Drive the AF uses an insulated gate bipolar transistor Bridge or an igbt Bridge schematically the igbt bridge is similar to the output section of the variable speed Drive the igbts are switched on

00:20:53 and off to bring input power to the DC bus in a sequence that virtually eliminat minates the input harmonics the THD can be reduced to 5% or less in most cases an LCL filter is part of the active front-end design some AF manufacturers add an Emi noise filter to reduce the switching noise from propagating onto the AC power line all yasawa active front end equipment uses

00:21:19 state-of-the-art igbts and an input Emi filter is optional yukawa's AF is a very reliable solution it's cheap drawback is its expense which approaches the cost of the basic vfd on the other hand it offers the possibility of regeneration in addition to harmonic mitigation regeneration or the ability to actively feed Excess power back to the grid can provide a significant degree of energy

00:21:48 conservation in some applications in short the AF uses igbts to minimize THD in a vfd OMG let's take a moment to review at this point you have a fairly good overview of the causes and remedies for harmonics to summarize let's take a look at the main points one more time harmonic analysis is a good way to

00:22:19 quantify nonlinear loads nonlinear loads can be problematic they can cause increased heat losses noise injection and and damage to switch gear and other equipment EXs harmonics can also cause issues with the power grid for you and your neighbors i e 519 is often used and sometimes misused to establish acceptable tddd levels it is intended

00:22:47 for the utility customer interface but the same levels are also applied within manufacturing facilities selecting the point of common coupling at the input of the devices makes analysis simpler but it can result in unnecessary costs by doing this we can end up mitigating an issue that isn't actually an issue now there are a variety of mitigation techniques to choose from a

00:23:14 standard six pulse drive plus DC link reactor an input reactor or a combination of DC link reactor and a small 1% input reactor there are 12 pulse drives 18 pulse drives and active front ends now while this program serves as a good start there is still plenty to learn about harmonics and a good place for you to continue your education is in the supplemental info section at yasawa

00:23:44 docomo it's personal thanks for watching

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