Can line reactors on VFDs be dangerous? Why are there no shorting blocks required for these? 3

[bdn2004]

I'm looking at this engineering package calling out line reactors on the incoming line to each of the new VFDs that will drive a pump.

It's obvious they picked the size of the line reactor from a chart. But how was this value actually determined? And if these are so common why doesn't the VFD manufacturer just build them into the units?
Is there anything that can be screwed up by using these? I don't like to be doing something - just because.

Also, I don't see any special protection for these. They will be operating at 480V. They don't have a finger safe design. Shouldn't they be encased? And as like with CT's that require special shorting terminals, why is that not required on the line reactors?

 

[Skogsgurra]

The line reactors are there to reduce the "flat-topping" of the grid's sine. The flat-topping depends, among other things, on how strong your grid is. A 2200 kVA transformer is less sensitive to the effect than a 300 kVA transformer would be. So, that is one reason why you do not get the reactors pre-installed.

Then, there are different limmits as to how much flat-topping (it is about fifth and higher order odd harmonics) the grid and the other loads can tolerate. Again, it is difficult to know for the manufactorer.

It would be possible to have universal reactors with selectable delta-V (how much percent voltage they drop at nominal load), but you would not buy that brand because it would be too expensive in most cases.

Finger-safe? There are such animals. If they are not, you have to build them into a panel or provide a protective hood for them - don't ignore the need for cooling air to circulate!

Shorting straps? No need for them. The reason you need them for CTs is that there is a 1000:1 or 10000:1 or similar winding ratio and the secondary voltage would be very high, sometimes destroying the CT if you run them with nominal primary current without a connected burden or shorted secondary winding. The mains reactor does not have a secondary and there is no winding where a high voltage could be induced. So, no need for shorting straps.

I hope that this is enough information for you, so that you can start using line reactors with more understanding and confidence

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[bdn2004]

Thanks for the thorough answer.

Ok, it's for reducing the flat topping of the incoming power system's sine wave. This is produced by the VFD on it's output. Does it vary with the load or frequency?
And that output is fed back through the power system. But after it goes through the reactors it's returned to a sine wave....? Right?
What's the harm with the flat top? It's ok for the motor.

 

[jraef]

LINE Reactors are for the LINE side of the VFD, if they are between the VFD and the motor, they are LOAD Reactors. The reactor is the same, just sized differently.

Another tangible benefit that I feel a Line Reactor provides is what I call "cheap insurance" for the VFD front-end. The reactor adds impedance to cut down on voltage transients that occur on the supply line and adds an inductive time constant to any current transients they then create, slowing down their rate of rise and reducing the stress on the line side components of the VFD, i.e. the rectifier on through to the DC bus filtering components. In the past, line transients were generally uncommon, but as system capacity needs increase and distribution system infrastructure gets mired in "NIMBY"* delays, power providers must manage varying demand cycles much more than before. That means more switching around of large grid segments and every time a grid switch opens and another one closes, a significant voltage and then current transient event occurs. Those events, depending on the size of a transformer feeding a facility, can move right down to the terminals of a VFD and damage it. The reactor mitigates that to a large degree.

Why not build them in? I once worked with a VFD manufacturer that did in fact do that. They are out of business now because nobody was willing to pay extra for them. People put up with failing VFDs all the time, then lament it, but ask them to use a line reactor and they complain about the added cost... go figure. Besides, our rule-of-thumb is that if your supply transformer is less than 10X the kVA of the VFD, which is often the case, you likely do not need a line reactor for protection (although you may still want it for Gunnar's reason).

*NIMBY: for those who don't know, means Not In My Back Yard

"Will work for (the memory of) salami"

 

[Skogsgurra]

I'm afraid you are confusing a few things, bdn. Jeff gave you a good explanation but you still need to read up on basic VFD operation. There are lots of texts that will tell you how a VFD really works and instead of the bedside version I think you have heard.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[bdn2004]

Skog,

I now understand the line reactor is to protect the VFD itself. 10X rule of thumb is very helpful. Again that helps to answer WHY instead of "well that's just how we've always done it".

What I could use is a basic understanding on how ac electricity flows through a three phase load. The incoming sine wave comes into a vfd, the vfd converts it to dc, the dc is then inverted to form the ac for the output to the motor. And that wave is squared off. Right? Ultimately though the VFD is not the source of the power, it doesn't complete the circuit...the secondary of the transformer does. There is only three wires feeding the VFD...so why wouldn't the leads to the VFD be at least affected by the action of the VFD if the circuit is completed through these leads?

 

[Skogsgurra]

OK.

First. The Flat-topping is a term that I used to describe what odd harmonics do to a sine. It has nothing at all to do with the square pulses that the VFD feeds to the motor. Absolutely nothing.

Second. The square pulses (PWM voltage) fed to the motor is the result of switching the intermediate DC voltage on/off periodically (DC link and IGBT). Switching a DC voltage on/off results in square pulses, but, as I said before, those pulses' square shape has nothing with the squared off (flat topped) sine voltage feeding the VFD.

Third. The flat-topping comes from the mains voltage being peak rectified by the input rectifier. Since the diodes in the rectifier conduct only when the momentary input voltage (the sine wave) is higher than the DC link voltage, the diodes conduct only a short time (the conduction angle) and it is only then that the mains voltage is loaded by the VFD so that there is a voltage drop, hence the flat tops.

Fourth. If you increase the impedance between mains and VFD rectifier (you do that when you add line reactors), the peak current in the charging pulses gets lower and that means that the sine wave isn't loaded as much and the flat-topping is reduced. Hence lower distorsion, which is the same thing as less harmonics.

Fifth. Lower harmonics is a good thing. There are even limits in some of the national electric codes that say how much harmonics are allowed in certain situations.

Got it?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[bdn2004]

Yes got it...I think.

"Third. The flat-topping comes from the mains voltage being peak rectified by the input rectifier...."
This is what I didn't understand in your first response and now I get...that the VFD itself is CAUSING the havoc on the incoming line, therefore it's a good idea to add the line reactors to reduce distortion.
I'm satisfied with that.

 

[Skogsgurra]

Good. Keep on reading about VFDs. There's a lot to learn.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

There's a lot to learn.

30+ years, I'm still not done...

"Will work for (the memory of) salami"

 

[prc]

Gunnar, What are the best documents to learn about VFD? Basic, intermediate and high level.

 

[prc]

Jraef- If transformer rating is less than 10X VFD kVA, line reactor is not required.

Gunnar- With higher rating of transformer flat topping is not serious( I understood it means reactor is not required)
Any contradiction or my understanding problem.

 

[Skogsgurra]

prc
"With higher rating of transformer flat topping is not serious( I understood it means reactor is not required)"

Right, if the reason you put them there is THD at the PCC.

Wrong, if their main purpose is to protect the input rectifier and the DC link. This latter purpose is "optional" while the first purpose is regulated in codes. Different for different regions. Still.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.