Power Quality Optimization with Active Harmonic Filter :
It is widely believed that active harmonics filters (AHF) are very costly and, therefore, are the last choice for power quality solutions. The answer is it depends. Every harmonics mitigation and power factor correction device has its place in the market.
Knowing what a solution does for power quality and the advantages and disadvantages of each solution provide optimized solutions with maximum benefits for the user.
Active harmonics filters provide controlled current injection to remove harmonic current from the source side of electrical system s and reactive current to correct for poor displacement power factor (PF). Active harmonics filters can be applied to a single nonlinear load or many. The nonlinear loads can be mixed types or devices, such as variable frequency drives (VFD), DC motor speed controls (aka DC drives), uninterruptible power supplies (UPS), or thyristor (DC) power supplies, to name a few.
When is the best value achieved with active harmonic filters?
- When more than one nonlinear load and/or type of loads are present : DC drives, UPS, VFD, and others can operate on the same electrical system as an active harmonics filter system
- When input line reactors are used to reduce the total harmonics distortion of nonlinear loads.
- When 6-pulse rectifiers are employed in DC drives, UPS, VFD or DC power supplies.
- When thyristor rectifiers are used and power factor and harmonics correction are required : DC drives and DC power supplies require power factor correction and harmonic mitigation.
- When PF correction is required where high harmonics content is present : Poor power factor in an electrical system a high content of nonlinear loads cannot be performed by traditional power factor correction capacitor systems
- Where rapid in-rush or step load changes require support (aka VAR compensation) to stop flicker : The power quality of an electrical system is quite poor due to the massive voltage variations caused by the loads.
- When meeting harmonics specifications within your facility.
The nature of harmonic currents is that they are vector sums of the amplitudes, current and voltage, at each harmonic order. That is they are the root sum square of each harmonic order times the cosine of the phase angle differences between the vectors. Vector math makes 1+1 = Sqrt 2 (1.414) not 2. Active harmonic filter selection uses this to actually reduce the per amp size of the active filter as the quantity of nonlinear loads increases. Individual solutions (that is per rectifier) must be sized for the full amplitude harmonic spectrum making them, collectively, more expensive and physically larger than AHF
6-pulse rectifiers produce the highest level of harmonic current among three phase loads. If no impedance is present as an input line reactor or a DC bus choke (for diode rectifiers only), 6-p rectifiers may approach 100-120% total harmonic current distortion (TDD). A first step to reduce this very high level of TDD is to insert 3-5 % impedance line reactance to reduce the TDD to about 32-40% per rectifier. Then the AHF is selected for this much lower TDD level. Thus the AHF size is reduced due to the inexpensive insertion of inductors.
Thyristor rectifiers are known as phase controlled rectifiers. Thyristors are turned on at varying points in the AC waveform to control the power delivered to the DC side of the rectifier. This process causes displacement PF variations and harmonics. The interesting point is that maximum harmonics occur at best displacement PF and vice versa. The AHF is the best device to provide exactly what is required to achieve best total or true PF – highest lagging PF and lowest TDD – all the time. This achieves high levels of power quality.
It is always difficult to select tuned PF capacitor systems when the harmonics content is high. The tuning tends to be difficult due to the amplitude of harmonic current that flows through the capacitor circuit. The solution is to make the PF capacitor VARs larger so as not to over load the capacitors. This may preserve the capacitors but can cause leading PF at times. AHF will not cause leading PF at any time and don’t care if harmonics are present while performing PF correction.
Loads that cause flicker (light intensity variations due to rapid line voltage variations) are very short duration events – sometimes as short as 2-3 cycles. AHF are fast enough to provide support in about 100 microseconds. Often the AHF can be packaged with PF capacitor stages to create a hybrid VAR compensation (HVC) system for price and physical size optimization.
When applying harmonics standards inside a facility, the solution is sometimes difficult to attain. This is usually due to the low short circuit current capacity of the point of common coupling chosen within the plant. AHF make meeting or exceeding the standard simple, effective, and complete.
Does the above help you understand AHF and power quality?
What can we do to help you further?
Jim Johnson
will kvar effects generation & transmission
Uitlity systems have the ability to provide KW (voltage and current in phase), KVAR (voltage and current out of phase), and harmonic current (current at integer multiples of the base frequency). VAR and harmonic current are conducted in the power cables.
VAR current proportionally reduces the ability of the grid to provide real current (KW).
Harmonic current causes heating disproportionate to its amplitude. Heating effect is much larger than actual current amplitude.
When loads demand either VAR or harmonic current, the grid capacity is reduced. Therefore, the grid conductors must be oversized where this occurs. The solution to prevent over utilization or oversizing of the grid due to VAR and harmonic current is to provide the source of each locally and remove the demand from the grid – PF correction and harmonic filters.
From the generation side, when rotating generators are used to produce the power, the generator has the ability to provide VAR support readily. It is the electrical nature of the generator. Generators do not like to produce harmonic current – extra heating occurs – but can.
However, as we move to more and more alternative methods of power generation, the electronics used to create the power (from PV or wind or etc.) only produce KW. Thus a need of alternate KVAR and harmonic support is required. This can be local by the user or universal by the utility. The cost must be born by the user in all cases.
From an energy efficiency stand point, local support for both KVAR and harmonic current reduces the cost of transmission (size of cables and alternate sources) and utilization of the grid. It permits more KW to be transmitted on the existing cables and permits planning of future grids to include smaller cables = less mineral resources consumed and processed. It helps make the planet greener.
This blog is fantastic by johnson and is too technical. I simply love it and I want to ask one thing to Jim, What is exactly the diff. between tuned and de-tuned harmonic filter. Which one is appropriate for VFDs? what is the exact process of connecting the filters with the load? Is there any good quality makes available in the market? Please confirm
Thanks for the compliment. The subject is not simple – it gets tough to make it simple.
A PF capacitor when placed in an electrical system has a resonant frequency based upon the inductance of the electrical system (from any point) and the capacitance of the PF capacitors. The PF capacitors are installed parallel to the loads. Adding a reactor in the parallel portion of the circuit in series with the capacitors modifies the frequency of the PF capacitor bank and thus the resonant frequency of the PF capacitor bank in the electrical system. This is ‘detuning.’
PF capacitors will interact with all frequencies present in the electrical system. When harmonics are present this is harmfull to the capacitors. The harmonic currents will overheat the PF capacitors to shorten its life or trip its thermal safety devices. Detuning can be designed to ‘block’ most of the harmonic currents or can be designed to permit the current of one harmonic frequency to flow into the capacitors. The ‘blocking’ design is considered ‘detuned’ within the industry. The design that permits a harmonic order to flow into the capacitors is called a ‘tuned’ capacitor or a harmonic filter.
Detuned and tuned capacitor systems are typically of two application philosophies. One is an electrical system solution where one large bank is used for a group of loads or used at the utility connection. Local and worldwide suppliers of these exist. However, a harmonic study is required for these types of solutions. Be sure the company chosen can provide this before equipment is purchased.
The second philosophy is to place a small tuned capacitor system at each nonlinear load to provide harmonic suppression for the electrical system. To be sure that these small tuned banks do not interact with each other (when more than one is installed) or overload due the presence of other sources of harmonics, a series reactor on the source side of the filter section is required. The series reactor is sized for the full load current of the nonlinear load on which applied.
The tuned filter also provides leading reactive current that must be evaluated within the system to determine if problems may occur due to excessive leading PF. The manufacturers of these type of filters tend to be localized operations within individual conutries. They exists in many countries.
The result of properly applying either device will relieve the utility of conducting VARs and specific harmonics and permit green generation technologies to be more easily employed.
Regards
Dear Jim,
Really admire your in depth knowledge and expertise in the Power Quality issues. Though there’s some other school of thought which I wish to streamline to avoid confusions as there are other terminologies being utilize.
Could you give us particular commercial available asources of Harmonics Mitigatos and filters.
Thanks and regards,
Orlando
Hi, Orlando,
Thank you for your comments and interest in the subject.
The commercial market has a different concern than does the industrial market. For the most part the industrial market utilizes three phase loads. That is what the previous discussions address.
The commercial market has a mixture of three and single phase loads. Typically, these loads are divided into environment control (HVAC equipment) and office environment. The HVAC equipment of buildings is typically isolated from the office enviornment (supplied by separate electrical distribution systems). The comfort equipment is almost all variable frequency drive (VFD) controlled. VFD provide best efficiencies and best and repeatable controls, but do requir e harmonic mitigation due the high concnetration of VFD.
The office area can be divided into two segments today – lighting and all other equipment. Lighting has been one area where the manufacturers have taken very good steps to reduce their harmonic levels. Generally, electronic ballasts are limited to about 20% TDD (total harmonic current distortion – based upon full load level). Some offer ‘premium’ systems that limit harmonc current content to about 5% TDD. Many governments have issue laws demanding these levels.
All office machines (copiers, printers, desk top PC’s, computer peripherals, etc.) utilize switched mode power supplies (SMPS). The early SMPS were very high in harmonic content but operated at near unity displacement power factor.
Since the mid 2000’s, codes and standards have been issued that require SMPS to be low harmonic devices. The results are that new information technology (IT) equipment comply with about 5% TDD. However, the design of the SMPS is now causing leading displacement power factor at low load levels. This has caused much trouble for the UPS manufacturers since the UPS was not designed to handle this high leading power factor levels. The result is that active harmonic filters have been used to correct for the leading power factor by injecting lagging power factor to permit use on the installed UPS. (Note that newer designs of UPS have addressed this issue so the UPS does operate properly with leading power factor loads.)
All of the above is a preamble for the discussion that I think you asked about – harmonic mitigation for the office environment.
There are several points that are universal. 1) Single phase nonlinear loads create neutral harmonics. (Neutral harmonics are sometimes called zero sequence harmonics or triplen (multiples of 3) harmonics.) 2) Neutral harmonics can exceed the amplitude of the fundamental current – maximum levels are 1.73 (square root of 3) times the fundamental current. 3) Most codes and standards now require the neutral to be sized to twice the size of the fundamental current cables (where one neutral services all three phases) or each phase gets its own 100% rated neutral.
In the 480 V/60Hz world, the 208/120 VAC level is derived from the 480 V supply utilizing isolation transformers with a delta primary and wye secondary. This configuration prevents the neutral harmonics from being conducted on the 480 VAC system.
In the 400 V/50Hz world, the single phase loads are connected from a 400 V phase to neutral and operate at 220 VAC. The result is that the 400 VAC system carriers the triplen harmonics.
The emphasis in the 480 V world is to prevent overheating of the neutral and the transformer providing the supply voltage. Protecting the transformer is easy. Many types of neutral ‘filtering’ exist. They range from blocking reactors that have a very high impedance for the third harmonic, to zig-zag transformers that shunt the triplen harmonics into the mains, to tuned third order passive filters, and ultimately active harmonic filters that treat the triplen harmonics. The issue in all of these solutions is that the electrical system upstream from the filter lconnection will have reduced harmonic current, but nothing has been done for the load side of the harmonic solution. The neutrals still carry the harmonic currents that the nonlinear loads need to operate.
The 400 V world has a very different issue. Besides the neutral discussion which are the same, the 400 V system (which can feed many users) total harmonics include the triplens. So, the TDD levels are higher and more dangerous. Surprisingly, the same solutions are used.
I hope this is helpful.
Regards, Jim
Jim,
In my Energy management business I find that harmonics is popping up all over for my industrial and big commercial clients.
I see the benefit of AHF and understand it is quite expensive. At present my feeling is that it is best to make each piece of equipment or lighting circuit as clean as possible because then it seems to me a shopping mall owner can expect his tenants to not contribute any harmonics to the reticulation of the mall.
My question is where can I find someone who has the designs available for both kind of harmonics filtering AFH and LCL or whatever other design is feasable?
My 2nd question is, whether it may be possible to divert the harmonics energy into a resonant frequency tuned circuit, and tap it off for other uses such as resistive heating etc?
I am in Johannesburg RSA and is looking for a filter designer.
Hi, Cobus,
The reason harmonics are a topic is because semiconductor technlogy is being used for energy savings in all types of applications. New washng machines employ a variable frequency drive and digital logic. They have eliminated the mechanical gear box for a direct coupled motor. It is more efficient. Also, all electronic products employ AC to DC rectifiers. Switched mode power supplies (SMPS) are everywhere. As time passes more of these nonlinear loads will employ their own harmonic mitigation methods. But design evolution takes time.
In the mean time, solutions are offered in many varities. Depending upon the type of loads and the concentration of those loads on any given network the solutions vary widely. I am not familiar with the South African market, so I don’t know who might be able to help you. Shaun Wilson at shaun.wilson@za.schneider-electric.com is one contact I do know who may be able to assist.
Harmonics is called ‘imaginary’ or reactive current in electrical engineering. It does not do work but permits work to be done. This is the same definition applied to reactive current that is phase shifted from the fundamental current (Cos phi). There is no known way to capture this power and reuse it for real work.
Regards, Jim
Jim,
I have questions for you regarding the selection and design of harmonics filters. If I have a 400/440 Volt 3 phase 40kVar capacitor bank in a Powerfactor system which needs to be protected against 3rd to 13th harmonics of various amplitudes, how do I choose the inductance or Xl for each leg of the 3 phase supply to the capacitor? My concern is resonance at some of the harmonic frequencies obviously, as well as the internal heat in the inductor/filter and the voltage drop accross the inductor at the base frequency of 50Hz, because that will reduce the capacitors effective kVar slightly.
Then I also wish to know if I can use tuned filters for each harmonic in a smaller inductor/filter design inside the various machines where the harmonics are being generated and if it is a good idea, what must I bear in mind, or do I rather block the harmonics in a machine with a local inductor/filter designed for the machine and what about voltage drop and heat?
Regards
Hi Cobus,
You would porpably need to connect your caps in delta. This decuple the caps from the triplens.
As for the reactor , you need to select a tunning order where the cap/reactor will provide lowest impedance at that order. for instance we tune our filter to 3.7 order. This order will not interfer with utility ripple signal and will decuple the caps from anything higher than 5th order. This will only work when voltage distortion is around 7%. Above that you need to mannage the harmonics via a tuned filter .usually around 5-7 order thus removing almost 80% of the harmonics in the system. The issue with this type of filter is you need to make sure the filter can abosrb all harmonic currents without overloading the caps. The cap will only take 130% of its rated current before it starts to e fail,hence you may need to address the amount of harmonic current in the netwrok then size a filter accordingly. The kvar value of the all the filters will contribute to reactive compenstaion . This is very cruical in defining the correct way to control the filter. The easiest option is to install an Active filter at the main LV borad and allow it to run in PF/ harmonic mode. you hit 2 birds in one stone and you donot have to worry about resonance.
Jim
Is there a much better rectifier system for plating plants other than SCR control, which will have zero or very little harmonic distortion contribution to the system?
Cobus
Hi, Corbus,
Sorry for the late reply. Got a little busy and failed to go back to my notice from the blog.
The thyristor rectifier is the most rugged, most dependable, and easiest to control rectifier on the market. It is also the least cost. No one has spent time, that I know of, in developing a different method.
Regards, Jim
i am parmjit from daviet. we are making a major porject. so pleace tell me circuit diagram of Harmonic filter for better power quality.
i am very thank full to you.
i am suyog from bhopal i am preparing a major porject. so please tell me circuit diagram of Harmonic filter for better power quality.
the paper is excellent but i want full information about the topic .can u give me please??????
my email id is anil05.perala@gmail.com
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