Your questions answered: Power quality: How to mitigate harmonics

Understand the causes, code requirements for compliance and mitigation techniques for harmonics in these additional responses

By Remi Bolduc May 13, 2020

Remi Bolduc, Competency Center Manager for Digital Power, Schneider Electric, responded to questions left unanswered from the May 5 webcast on Power quality: How to mitigate harmonics.

Can you review definition of harmonics?

Remi Bolduc: Harmonics are multiple of the fundamental frequency that when added together result in a distorted waveform. Basically, in North America our power frequency (fundamental) is 60 Hz, so harmonics will be multiple of 60 Hz. For example, the 3rd harmonic order will be 3 x 60 Hz, so equal to 180 Hz. The 7th harmonic order would be equal to 7 x 60 Hz, so equal to 420 Hz.

The harmonics generated by equipment within a building are often dependent on the specific manufacturer/model used but this isn’t usually known during the design stage. Is there a good way to estimate during the design stage?

Remi Bolduc: Ideally, you would obtain the nonlinear loads data sheets that indicate the THDi level. Some harmonic mitigation solution suppliers have sizing tool that can help estimate the THDi of an installation. Schneider Electric has such a tool free of charge if you are interested. For a rough estimate you could also use a 30% THDi, where you multiply your installation nominal current by 30% (in x 0.3) that will give you the harmonic RMS in current (H RMS), from there you could evaluate your compliance against table 2 of the IEEE 519.

Is it smart to add harmonic mitigation without modelling the full electrical system (like in SKM) and testing response? What is your experience with the how well real-life testing and observations match with software models?

Remi Bolduc: It’s smart to use simulation software and in some cases, it might be mandatory to prove to the utility that you meet their grid code requirement. Based on my experience, a good model with good input data normally gets withing 5% to 10% of real-life results.

How can it be determined if the harmonics are almost strong enough to trip a breaker?

Remi Bolduc: This is tricky because other factors come into play, like the type of breaker (thermal magnetic or equipped with an electronic trip), ambient temperature and how the breaker is integration in the distribution panel. As a guide, we normally oversize circuit breakers when they are supplying harmonic generating loads, 1.25 x in = CB ampere rating.

Just a comment, harmonics can also cause misoperation or nonoperation of protective relays.

Remi Bolduc: Good point, I agree.

When a variable frequency drive is installed with an energy efficient motor it creates a harmonic effect of sort. The correct motor to install is an inverted duty motor. Right?

Remi Bolduc: Good point, I agree. The harmonic effect is caused by the variable frequency drive inverter pulse width modulation output, the effect is reflected on the motor the variable frequency drive is controlling, not on the upstream alternating current supply.

Can harmonics on a system cause vibration in switchgear and therefore noise? If yes, can that noise vary in decibel level as well as not be consistent on location of noise and sometimes no noise at all?

Remi Bolduc: For sure, harmonics in switchgear will create additional heating and stress on switchgear and busways. I would tend to say yes, harmonics could create humming noise in switchgear. However, I don’t have concrete data on the subject.

Is harmonic negative impact only a thermal issue? If so, what is the method of calculating duration of harmonics situation that will lead to a problem?

Remi Bolduc: Many factors would come into play for calculating the duration that will cause a problem, like ambient temperature, loading on the equipment, equipment cooling means and so forth, so it’s difficult to have a precise algorithm. A good example to follow is K-rating calculation on transformers.

Is it more important to measure/consider voltage or current harmonics?

Remi Bolduc: Both are interlinked and both are equally important.

What percentage of a distribution transformers load can be nonlinear without accruing any significant damage or shortening the life of the transformer?

Remi Bolduc: It’s best to follow the recommendations of the IEEE C57.110-2018 when establishing transformers capability when supplying nonlinear loads.

What would you do to reduce capacitors from deteriorating by the harmonics generated from the distribution network?

Remi Bolduc: For electrical systems with a THDi equal or below to 20%, I would equip the capacitor banks with detuning reactors. When the THDi is superior to 20%, I would use an active harmonic filter to correct the power factor and/or reduce the harmonic level.

How do you avoid the resonance when we have several harmonics like 5th, 7th, 13th and 17th for variable frequency drives?

Remi Bolduc: Resonance will occur when you have capacitance in an electrical system. The main capacitance source is normally a capacitor bank. For these cases, use detuned type capacitor banks to control the resonant frequency and force it below the 5th harmonic order. Detuned type capacitor banks are readily available on the market. Alternatively, you can resolve all resonance issue by using an active harmonic filter to correct power factor and to mitigate harmonics.

For variable frequency drives used to modulate fans and pumps up to 100 horsepower in commercial operations, what solutions do you recommend to minimize harmonic distortions?

Remi Bolduc: Use variable frequency drives with built in >3% direct current bus choke or >3% line reactor, then if you have a large quantity of variable frequency drives add an active harmonic filter at the distribution board to bring the TDD as per IEEE 519 table 2.

I have a cap bank with three steps to power factor correction, so I will have three different resonance frequency in my system. Those resonance are 13th, 16th and 22nd. So, is that resonance frequency an issue for my industry if I have only 6-pulse drives? Do I need a filter to correct any harmonic issue with a THDV of 10.5% for worst case?

Remi Bolduc: For your case, I would worry mostly about the 13th resonance frequency because 6-pulse variable frequency drives will produce a fair level of 11th and 13th harmonic order. I can see magnification of the 11th and 13th harmonic order happening. That can be resolved by the use of detuned type capacitor banks or the use of active harmonic filters. At a minimum I would apply 3% direct current bus choke or line reactors on all your variable frequency drives. By the way, a 10.5% THDv level is alarming, consider reducing this level below 8%, a THDv of 5% or less would be my recommendation.

Are direct current bus chokes typically part of variable frequency drives?

Remi Bolduc: It’s more and more common to have variable frequency drives with built-in direct current bus choke, for example, the Schneider Electric Altivar Process range of variable frequency drives all have direct current bus choke. Most good variable frequency drives supplier have options for direct current bus choke or line reactors, I recommend that you ask for it in your specifications. Ask for >3% line reactor or >3% direct current bus choke.

Are filters built in to variable frequency drives?

Remi Bolduc: Sometimes. Line reactors or direct current bus choke sold with some variable frequency drives is a form of filtering, it can reduce the THDi by 50%; it’s the first line of defense. Some other form of variable frequency drives referred to as low harmonic drive have low harmonic emission, these variable frequency drives are also known as active front end variable frequency drives. These variable frequency drives are typically much larger and much more expensive.

Can you review why the 5th harmonic is the predominant harmonic in 3-phase and not the 3rd?

Remi Bolduc: Without going into advanced signal calculations, 6 pulses rectifier (6 diode bridge) generate dominant 5th, 7th, 11th and 13th harmonic order and 4 pulses (4 diodes bridge) generate dominant 3rd, 5th, 7th and 9th harmonic order.

Is dynamic braking the same thing as regenerative braking. What is the benefit?

Remi Bolduc: Correct, dynamic braking and regenerative braking are the same thing. It allows you to reinject power onto the power grid, which reduces your overall kilowatt/hour consumption. It’s a way to save energy. And obviously it also helps brake (stop) the load connected to the variable frequency drive.

Any recommendations to prevent or mitigate power quality issues with LED lighting?

Remi Bolduc: Use LED lights with low harmonic emission or use harmonic mitigation methods at the distribution board, like a 3-phase 4-wire active harmonic filter.

Do harmonics cause an issue at a water pump station, where high-voltage motors are used? Will it cause SCADA data from a programmable logic controller to be distorted, by interfering with the programmable logic controller data?

Remi Bolduc: If you have medium-voltage variable frequency drives, use variable frequency drives with multipulsing rectifier bridges to reduce harmonic emission and/or other form of filtering. There is a possibility that harmonic create communication issues with your SCADA/programmable logic controller (PLC) system. Use shielded cables on your variable frequency drives and communications cables and keep your communication cables away from your power cables as much as possible in order to reduce EMC interference.

What is the typical total cost to implement a 6-pulse versus a 12-pulse versus an 18-pulse rectification for variable frequency drives?

Remi Bolduc: From 6 to 12 pulses, approximately 50% price adder and from 6 to 18 pulses, approximately 65% price adder. The use of multipulsing technology makes sense when you have one or two variable frequency drives that create issues and in medium-voltage application. When you have multiple variable frequency drives, it’s more cost effective to meet a 5% TDD with the use of standard variable frequency drives with line reactor or direct current bus choke and have an active harmonic filter mitigating the remaining harmonics from all the drives.

If you have a large standard (not detuned) capacitor bank at a facility’s electric main, can you install a new separate active harmonic filter? Or will it fight with the old detuned capacitor bank?

Remi Bolduc: It’s possible to have an active harmonic filter operate with a standard capacitor bank, however special considerations must be taken, like adding CT’s at the capacitor bank feeder breaker. It’s best to get your case reviewed by a Schneider Electric specialist.

How much load does an active harmonic filter consume? How much do you have to oversize your distribution to account for active harmonic filter load?

Remi Bolduc: Active harmonic filters are normally ~97% efficient and add very little kilowatt load to your system. For example, a 300 ampere active harmonic filter that can inject 250 kVAR at 480 V has 7.1 kilowatt of losses. Normally active harmonic filters are NOT considered in load calculation because they reduce loading in the electrical distribution by correcting the power factor and reducing the effective (RMS) current due to harmonic mitigation, not to mention the load-balancing benefit.

12- and 18-pulse variable frequency drives reduce common mode voltage and so reduce motor bearing currents and damage. Does harmonic mitigation generally decrease motor bearing current damage?

Remi Bolduc: I am not aware that 12 and 18 pulses can reduce common mode voltage. My understanding is that it’s the variable frequency drive IGBT inverters that can create common mode voltage issue and the best way to eliminate bearing damage due to current flow is the use of microfiber motor shaft grounding ring.

If we know the fundamental RMS neutral current, what would be the worst case (maximum) TrueRMS in theory and in experience?

Remi Bolduc: The maximum current flow in the neutral due to zero sequence harmonic is 1.732 time the phase current. You may have seen some electrical specifications in data centers or computer rooms that ask for 200% rated neutral for that reason.

Can you elaborate on how to add THD (for current and voltage) for different nonlinear devices?

Remi Bolduc: Example for current: load A with 100 A of fundamental with a 15% THDi has 15 A of HRMS, load B with 300 A of fundamental current with a 25% THDi has 75 A H RMS. The total fundamental current is 400 A with 90 A harmonic RMS, so the new THDi is 90/400 A, so a THDi of 22.5%. THDv would be equivalent but with voltage drop.

What is the relationship between harmonics and voltage unbalance?

Remi Bolduc: To my knowledge, there is no real relationship. What creates unbalance voltage is unbalance current that comes from negative and zero sequence current.

Does active harmonic filtering translate into kilowatt-hour savings?

Remi Bolduc: Some small amount because it will reduce losses in the electrical distribution. The major benefit of active harmonic filtering is more related to the reduction of down time, expending electrical equipment life span, reduction of maintenance and to meet the utility grid code requirement.

Should we use active harmonic filter for 480 Vac motor control center?

Remi Bolduc: If the motor control center has multiple variable frequency drives, you should consider active harmonic filter for harmonic mitigation and/or power factor correction.

If you experience harmonic resonance (capacitors are damaged), what are your options to mitigate it?

Remi Bolduc: I would replace the damaged capacitors and I would add de-tuning reactors to the capacitor stages. In the event where the harmonics are still too high, I would also add an active harmonic filter to reduce the harmonic distortion.

What is the typical recommended THD for a service size of 480V/4,000 amps? Is there a threshold?

Remi Bolduc: You would normally target a TDD of 5% or 8% and a THDv of 5% (best practice) or less. The best official reference is the IEEE 519-2014, you can refer to table 1 and 2 of the standard.

You mentioned earlier the effects of harmonics on large capacitors — do they also have an effect on line capacitance?

Remi Bolduc: Line capacitance can also create resonance, which will magnify harmonic in voltage and current at the resonant frequency. I have seen harmonic magnification issues created by LONG runs of buried cables at airport and solar farms.

What kind of commercial applications are most critical for IGBT active harmonic filters? My head goes to hospital equipment.

Remi Bolduc: Active harmonic filters are used in most segments because nonlinear loads are everywhere. Hospital is a good segment for active harmonic filter application.

Are active harmonic filters only for + and – sequence harmonics?

Remi Bolduc: No, they can mitigate ALL the harmonics from the 2nd order to the 51st order.

How does one calculate the appropriate size for the active harmonic filter rating? Is it based on overall system rating or the measured/calculated harmonics in the system?

Remi Bolduc: In existing installations, the active harmonic filter sizing is normally based on measurements. In new installations/projects, the harmonic level is evaluated via sizing tool (software). Schneider Electric has a sizing tool available free of charge if you are interested.

My plant has seen an unexplained increase in utility bill from year to year (20 megawatt load). I noticed some areas have 33% THDi.

Remi Bolduc: Unless the utility applies a penalty for noncompliance to their grid code, the THDi level should not influence your billing. We have a special department in Schneider Electric that assist customer with evaluating customers service provider, you can contact your local Schneider Electric office for support.

What minimum horsepower rating would it be recommended to add an active harmonic filter?

Remi Bolduc: It’s difficult to evaluate the need for active harmonic filter only on horsepower rating. It’s best to follow the IEEE 519-2014 table 2 guide lines and apply them on your low-voltage bus (example 480 V bus).

During electrical planning stage, we don’t have any information about motor characteristics, except its rated voltage and horsepower, how do we size or pick the right harmonic mitigation equipment?

Remi Bolduc: Sizing tools are existing to assist with sizing harmonic mitigation (active harmonic filter) for new projects. Schneider Electric has a tool available free of charge if you are interested.

Regarding facilities who experience an abnormally high number of electronic fluorescent ballast or LED driver failures, is there typically a specific harmonic (3rd, 5th, 7th, etc.) that is suspect as the root cause.

Remi Bolduc: There is no specific harmonic spectrum that could affect the LED driver, but a THDv superior to 5% could affect the life span of the light fixture. There could also be other causes for the LED’s premature failure, like voltage transient or simply poor product quality. For transient issues, it’s recommended to install TVSS on the distribution board.

There was a mention of more impact on generators, rather than from utility supply. Should the harmonic filter be placed only on the GE bus feed?

Remi Bolduc: For the same amount of harmonic current, the voltage drop at harmonic orders will be higher on a generator due to its high impedance. You need harmonic filtering on both type of supply, utility or generator. When on utility supply it allows you to meet their grid code and keep a clean source of power and on genset to off load them from the harmonic stress.

Should we follow IEEE 519 if the generator is the prime source?

Remi Bolduc: I would say yes, alternatively follow the generator manufacturer recommendations for TDD level.

Can you elaborate in how to add THD (for V&I) for different nonlinear loads?

Remi Bolduc: Example for current: load A with 100 A of fundamental with a 15% THDi has 15 A of HRMS, load B with 300 A of fundamental current with a 25% THDi has 75 A H RMS. The total fundamental current is 400 A with 90 A harmonic RMS, so the new THDi is 90/400 A, so a THDi of 22.5%. THDv would be equivalent but with voltage drop.

Do server rooms benefit from active harmonic filter application?

Remi Bolduc: There are two sets of problem with data centers. First, if your blade servers are power with SMPS that yields a high level of THDi (typically older technology or cheaper design) then your power distribution units and/or uninterruptible power supply could benefit from harmonic filtering from an active harmonic filter. Second, if your blade servers are powered by compensated (built-in filtering capacitors) SMPS then a high concentration of these SMPS can cause a leading power factor on the electrical distribution. The main issue with leading power factor is that your backup generator may refuse to start (governor issue) if the leading power factor is outside their operating window. Then an active harmonic filter can be used to correct the power factor and bring the power factor back lagging to allow for proper gen set operation.

Does harmonics have a significant impact on localized equipment (assumed energized), i.e., neighboring variable frequency drives or controls equipment even though overall TDD may be within limits?

Remi Bolduc: Potentially, yes, if the localized THDv is high.

When should consultants recommend adding active harmonic filter, in terms of size in horsepower and quantity of motors connected to a distribution system they have dedicated to mech loads?

Remi Bolduc: When the installation is projected to be outside the set limits recommended by the IEEE 519-2014 table 1 and table 2.

I have a lot of RCCBs installed in a building, on all distribution boards that I’m managing. The primary purpose of the space is for a customer service center so there are a lot of computers. Random RCCBs across the building keep tripping and this is disrupting the operations in the building. A consultant has suggested that the cause might be harmonics and a capacitor bank may be required. Is there any way to confirm that the RCCBs are tripping due to harmonics?

Remi Bolduc: The consultant is correct; it could be caused by harmonics. To be sure, an audit would have to be performed. Schneider Electric could assist with the power monitoring audit if you are interested. The solution is likely to be a 3-phase 4-wire active harmonic filter instead of a capacitor bank though.

Inverters used commonly to convert a direct current source to alternating current power can introduce harmonics into a power system. Will the direct current bus in the inverter have voltage harmonics or can sometimes be called ripple? Can you characterize the order of harmonics that might exist on the direct current bus?

Remi Bolduc: The term that power electronic engineers use normally is direct current ripple and they will look at the ripple frequencies to help with their component design, however I don’t know if they refer to these frequencies as harmonic though.

What size of active harmonic filter is cost effective? What amount of energy can save through installing active harmonic filter?

Remi Bolduc: Active harmonic filters are scalable (60, 120, 200 and 300 A ratings) and are normally cost effective if you are trying to mitigate harmonic for more than three nonlinear loads on a particular low-voltage bus. Active harmonic filters will help reduce losses on an electrical system, however active harmonic filter’s primary purpose is not for energy savings.

Which harmonic mitigation method do you recommend?

Remi Bolduc: I would always recommend the use of direct current bus choke or line reactors on all types of rectifier loads. If you have two variable frequency drives or less that are causing harmonic distortions, then the use of multipulsing technologies or active front end variable frequency drives would be good. In a case where you have three nonlinear loads (example variable frequency drives) or more, then a system approach with an active harmonic Filter is more appropriate and cost-effective.

The harmonic has the same effect at 50 to 60Hz.

Remi Bolduc: Correct, harmonics have the same effects on 50 or 60 Hz networks.

Is it possible to measure high harmonic at a variable frequency drive, but low at PCC? Can harmonics at the variable frequency drive be ignored here?

Remi Bolduc: Yes, it’s possible to have high THDi at the variable frequency drive terminals and a low THDi at the PCC. In cases like this, if you meet the grid code requirements and your THDv is below 5% on your LV bus, then it can be ignored.

How does an energy-efficient motor installed with a variable frequency drive impact motor harmonics?

Remi Bolduc: I would imagine the voltage distortion would be slightly lower since the motor impedance is lower, however I cannot confirm because I don’t have concrete data on the subject.

When do you correct harmonics at individual equipment versus the substation switchgear?

Remi Bolduc: If you have one or two nonlinear loads that is causing most of the harmonic distortion on the network, then it’s a good practice to mitigate the harmonics at the load level. If you have three or more nonlinear loads, then it’s a good practice to compensate at the switchgear level or at the motor control center level. Note: Active harmonic filter is ideal for system level compensation.

Can modern active front end uninterruptible power supply systems significantly affect the system’s power factor and generator systems especially if you have multiple ups modules?

Remi Bolduc: The new generation of uninterruptible power supply equipped with active front end rectifier bridge normally maintain a unity power factor and have a THDi below 5%, so normally they don’t impact the low-voltage network power quality significantly. The active front end technology is normally used on medium and large size uninterruptible power supplies.

What are the pros and cons of using an isolation transformer versus a line reactor?

Remi Bolduc: Both devices will reduce THDi from approximately 80% to 90% to 35% to 40%, they will both protect the variable frequency drives against transient and they will both reduce the short circuit current at the variable frequency drive terminals. The line reactors are normally smaller and cheaper. The isolation transformers are mostly larger and more expensive and they have the benefit of giving a galvanic isolation to the load.

What is the most common/recommended software to run harmonic system analysis?

Remi Bolduc: I would say ETAP or SKM.

Would filtering be best at the main distribution or throughout the system at the subpanels?

Remi Bolduc: Technically speaking, harmonic filtering device would yield better overall performance at the subpanels, but more often than not it will be more cost effective to install one larger piece of filtering device at the main distribution board.

In a hydro power plant, how can harmonics show themselves as distortion in control signals?

Remi Bolduc: I would say that it’s possibly electromagnetic interference, make sure you keep control wiring away from power cables (and don’t run them in parallel to each other), power sources and nonlinear loads. Also use shielded control cables.

What is effect of harmonic to maintain power factor in system? What is true power factor?

Remi Bolduc: Harmonics has the effect of displacing the true power factor, for example, you can have a displacement power of 98% lag but have a true power factor of 88% lag on an electrical system. The calculations are well explained in the presentation, please refer to it for more details.

Author Bio: Remi Bolduc, Competency Center Manager for Digital Power, Schneider Electric