Q: Can VFDs damage transformers?

VFDs can both harm and help transformers . Harmonic distortion can add heat and stress to a utility transformer, shortening the equipment lifespan. Poor power factor can also damage a utility transformer, and VFDs can improve the power factor of a system where motors are run across the line. The best situation to prolong the life of transformers is to evaluate all power quality issues and make sure the proper filtering is taking place.

Question 1

What is harmonic distortion?

Accepted Answer

Harmonic distortion is often described as high frequency electrical noise, but technically it is a distortion of the normal electrical waveform. It shows as a multiple of the base frequency. For example, a 60Hz system will have a third harmonic at 180Hz, a fifth harmonic at 300Hz, and so on. Harmonics cause many problems, especially for sensitive equipment and the utility.

Question 2

What causes harmonic distortion in a facility?

Accepted Answer

Harmonic distortion is caused by non-linear electronic loads, especially those that convert AC to DC and back. Things like LED lights, rectifiers, and UPS systems contribute to harmonics, but industrial facilities usually see most of their harmonic distortion come from variable frequency drives.

Question 3

What happens if my facility has high harmonic distortion?

Accepted Answer

High harmonic distortion can cause many issues, including excess heat, tripping, and equipment failures. From the utility side it can lead to penalties or shutdowns.

Question 4

Do utilities regulate harmonic distortion?

Accepted Answer

Most utilities have policies about harmonic distortion, but those policies and enforcement vary. Many are based on the IEEE 519 standard. Enforcement can take the form of penalties, shutdowns, or other increased rates. It’s also expected to become stricter as stress on the electrical grid increases. They are also more likely to enforce corrections if your harmonic distortion is affecting other local facilities or you’re on a grid that has a high amount of harmonics from many “bad neighbors.”

Question 5

Can VFDs damage transformers?

Accepted Answer

VFDs can both harm and help transformers. Harmonic distortion can add heat and stress to a utility transformer, shortening the equipment lifespan. Poor power factor can also damage a utility transformer, and VFDs can improve the power factor of a system where motors are run across the line. The best situation to prolong the life of transformers is to evaluate all power quality issues and make sure the proper filtering is taking place.

Question 6

Do VFDs always need harmonic mitigation?

Accepted Answer

We always recommend some form of harmonic filtering for variable frequency drives, but the level of the filter changes based on several factors. A “big fish in a little pond,” or a large VFD in a smaller facility will have a large impact on the facility's power quality than a small VFD in a large facility. Basic filtering like reactors is always good, but evaluating advanced filters like passive or active harmonic filters makes sense when the impact of harmonics is felt more strongly.

Some advanced VFDs, like medium voltage or active front end (AFE) VFDs function in a way that already produce minimal harmonics. In these cases, an additional filter is not needed. Higher pulse VFDs, such as 12-pulse or 18-pulse drives, also won’t need an additional filter, but their topology requires additional equipment like phase-shifting transformers to function.

Question 7

Which harmonic filter is right for me?

Accepted Answer

There are many factors to consider, but a few rules of thumb may help. For standard 6-pulse drives, smaller VFDs (under 25 horsepower) should have an input line reactor. VFDs above that size likely need a passive (or hybrid) harmonic filter. Active harmonic filters (AHFs) are good when you’re working on correcting harmonics on a common bus or at a point of common coupling (PCC).

Question 8

What is power factor?

Accepted Answer

Power factor is a measurement of the percentage of your electricity that is used to do actual work. Many types of loads in your facility may consume and utilize energy that is not considered “working.” The more prevalent those loads, the lower your power factor will be. Power factor is measured as a percentage but is typically communicated as a decimal. For example, 95% working power would be a power factor of 0.95.

Question 9

Is power factor correction a scam?

Accepted Answer

For commercial and industrial facilities, low power factor is a legitimate issue to evaluate and correct. For residential customers, power factor is a common “snake oil” type of scam. Most utilities won’t penalize residential users for poor power factor. Historically there have been scams that sell plug-in filters to residential customers to fix power factor that have little or no impact on the system.

Question 10

What causes low power factor?

Accepted Answer

Low power factor (on the lagging side) caused by inductive loads, such as AC electric motors. If a motor is oversized it has an even worse impact on power factor. Although more rare, it is possible to have a leading power factor. This would be caused by capacitive loads, the most common of these being capacitors.

Question 11

Why does a utility care if I have poor power factor?

Accepted Answer

Low power factor puts extra stress on utility transformers and equipment and requires the utility to supply you more power to perform the same work. This means that not only does your utility bill go up due to higher consumption, but they often add a penalty to incentivize improvements.

Question 12

How do power factor penalties show up on a utility bill?

Accepted Answer

Each utility bills power factor penalties differently. Some break it out as a specific line item, while others include it as an increased rate in on-peak or total charges. Your utility will have their method on their website, or else you can often call in and speak to their power quality engineers who are dedicated to solving issues like harmonic distortion and low power factor.

Question 13

Does power factor affect my facility or just the utility?

Accepted Answer

While power factor does affect the utility, it has many of the same impacts on your facility. Transformers and other equipment see additional load and heat, leading to shortened lifespans. There is also the increased cost on your utility bill to consider.

Question 14

Which power factor correction method is best?

Accepted Answer

Capacitor banks are the most common way to address low power factor. Because inductive loads causing a lagging power factor are the typical issue, you correct it by adding the opposite type of load (capacitive) to bring you closer to a unity, or 1.0, power factor. There are active filters that are becoming more popular and cost effective to address the same issue, including static var generators. If you’re needing VFDs, you may also look at different options with VFDs to improve power factor as well.

Question 15

Are VFDs a good way to fix power factor?

Accepted Answer

If you need to control the speed of motors, variable frequency drives may be a good way to solve power factor by tackling two problems at once. Most standard VFDs have a power factor around 0.96, much better than your typical AC motor. If you need regeneration in your VFD, it might be worth considering an active front end drive with excess capacity that can actively correct power factor throughout the rest of your facility. These are more common in medium voltage applications.

Even with these situations, VFDs are usually a part of a power factor correction plan, not the entire solution. If you don’t need to control the speed of your motor, you’re probably paying for functionality you don’t need and a different method of power factor correction would be better for you.

Question 16

How do VFDs damage motors?

Accepted Answer

Variable frequency drives output a simulated sinewave, known as pulse width modulation (PWM). Over long lead lengths or through submerged cables, the pulses in this waveform have a tendency to stack on top of each other and cause high voltage spikes that can damage motor windings.

VFDs also produce common mode voltage that leads to bearing currents. These currents seek a pathway to ground, often going through the bearings and causing electric discharge machining (EDM), or damage to the bearing.

Question 17

Is there a difference between reflective wave phenomenon and dV/dt?

Accepted Answer

dV/dt, referring to voltage rise, is something that happens with VFD PWM waveforms. Reflective wave phenomenon is the reflected waves that show as high voltage spikes. In short, dV/dt is produced by the VFD and is the cause, where reflected waves are the symptom that causes damage.

Question 18

Is there a difference between common mode voltage, bearing currents, and electric discharge machining?

Accepted Answer

Common mode voltage is what is generated by a VFD. It results in bearing currents in the motor that seek a path to ground. These currents cause damage that is called electric discharge machining. All three of these concepts are related, but one is the cause, the other is what is happening, and the final is the damage that occurs as a result.

Question 19

What output filter should I choose for my VFD?

Accepted Answer

The rule of thumb is that you should consider using an output reactor or dV/dt filter when the cable length between the VFD or motor is over 100 feet and you should use a sinewave filter when the distance is over 1000 feet or the cables are submerged. If the motor is hard to replace, critical to your processes, or you have any other factors that make you think there should be more protection, it is worth going up to a sinewave filter rather than other methods.

Question 20

Is a dV/dt filter better than an output reactor?

Accepted Answer

There are some cases where a dV/dt filter may work better than a reactor, but our internal testing has shown that for most cases a reactor is your best option between the two. In test across multiple brands at several cable lengths, a quality 5% impedance reactor performed as well or better than a dV/dt filter while typically being a lower cost and more available in the market.

Question 21

How should I protect against bearing currents?

Accepted Answer

The most common method to protect against bearing currents is to add a shaft grounding ring (SGR) to the motor. These rings give bearing currents an alternate path to ground, protecting the bearing. In larger and more expensive motors, you may want to add insulated ceramic bearings to prevent currents from being able to pass through. Be careful of adding insulated bearings to both sides with a shaft ground ring or other path to ground – in those cases the electricity can build up on the shaft of the motor but doesn’t have anywhere to go, building dangerous electrical potential.

There are other methods to address bearing currents by trying to eliminate common mode voltage. There are devices you can add to the cables between the VFD and motor to attempt to solve this issue. There are also emerging VFD output filters that are meant to address reflected wave but also combat common mode.

Power Quality FAQ - Harmonics, Power Factor, Filters, and Motor Protection

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