What to look for in a good VFD 1

[Zoobie]

For a medium voltage (4160V) VFD in the 5000-10000 hp range (driving a synchronous motor coupled to a centrifugal compressor), what are some of the things to look at when selecting the drive and vendor? Anything to absolutely stay away from? I am having an electrical engineer do some research for me but I get to be the one who decides what we buy.

There appears to be a number of technologies when it comes to the thyristors. Is there a particular type suited for this application? If so, this will definitely narrow down the search and will probably save me some expensive engineering hours.

One last question: Does anyone have experience with the Siemens Sinamics GM150? This appears to be a newer model and I am not sure if Siemens has done anything before in the medium voltage range.

 

[jraef]

It's difficult to speak of technology differences in a forum that prides itself on unbiased opinions and non-commercialism. In the MV VFD industry, nearly every vendor has a different power component topology that they espouse, making it somewhat proprietary, and most will tell you that all of the others are worthless. The best advice I can offer is pretty much general:

a) Make sure you are not the "guinea pig" for someone's newest whiz-bang gizmo system. That's not to say that new technology is bad, because every technology that we have was new at some point. But the first ones to try it are often beset with all the "whoopsies" that were somehow overlooked or unforseen. Responsible manufacturers look for accepting beta sites that are happy with their prior performance and offer to put up with the trials and tribulations of a new technology in gratitude (and/or a discount). If they don't have a decent installed base of that product, either look elsewhere or insist on a deep deep discount because ultimately you may be taking a costly risk. Most reputable manufacturers will correct any problems, but you will bear the cost of downtime.

b) Look at service reputations. Chances are if they had a poor record of taking care of their customers, that will not change with your order.

c) Look at the whole picture, not just the VFD itself. For instance, if it is MV, it is probably large, and therefore harmonics and other power quality issues are going to loom large.

d) When considering a system, look at all aspects of the equipment necessary to feed or use it. Is a phase shift transformer built-in or do you have to buy that on your own? How much of a load will it be on any air conditioning system? How much will it weight (floor loading issues), and how big is it (floor space, access restrictions)? I mention this bbecause there are some out there right now who are emphasizing how small their drive is, but they fail to mention that it doesn't include a very large and costly transformer necessary to make it practical to use. There are others that include everything, but are sometimes too big to get into an existing building.

Now for the opinion, FWIW. I know nothing of the Siemens drive you mentioned, but they just recently bought Robicon as well, and the Robicon Harmony drive is probably the premier MV VFD on the market today. That is strictly an opinion as a competitor (in LV drives). I have never personally specified or purchased one, but I am around them a lot when setting up LV drives and other equipment. The thing I like most about their system is that the power device structure is made up of multiple LV cells that stack up into being rated for MV power. So if a component goes bad, the cell can be removed and replaced by a technician who is not necesssarilly qualified for MV work, making downtime minimal. That IMHO is a feature worth consideration above the component technology, because anything can (and eventually will) fail.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[DickDV]

Number 1 factor is top quality technical support especially when MV is involved. If support has to come from 500 miles away or similar, its not for you even if the brand has a good reputation.

 

[mil3]

I have to agree with the above as far as support. I have seen timeless times where there are extremely limited support for MV Drives in a region. Your very first step should be to investigate which manufacturer has what support, how many people, and how far away. Lots of manufacturers have the product to sell you but very few actually support it at that specific location. Make sure you investigate this well.
Now for technology. I have to agre with jraef on this too. Not on the specific manufacturer specifically but with the technology he is refering to. There are manufacturers out there that have an offering that utilizes low voltage components and by different switching combinations bring the output level to MV. Technology is also referred to as Neutral Point Clamp. What makes NPC technology appealing is that no output filtering is required in most installations which brings cost and overall size down.
Further...make sure you egt a total package...i.e: three wires in...three wires out. Make sure you have a cost for input transformer, etc, etc. and no surprises will get you later.
I have never been a fan of water cooling. Just too close to high power for my comfort plus has some maintenance issues. I would shy away from that if it was me, but some have different opinions. There is MC drives out there that uses a standard AC induction motor for forced air cooling and even provides redundancy fans on a flow switch that will automaticlly switch over should one fan fail. Starter system is built right into the MV Drive. This makes a good package I feel as service can be done by your very own maintenance staff.

 

[Zoobie]

Thank you very much for the comments. I'm glad the cooling was mentioned as that was one of my questions/concerns (water vs. air). MV drives are a bit more common around here now (Alberta) but I will make sure I do my homework about service capabilities.

I already have the 13.8 to 4.16 transformer (from when the synchronous motor used to run). Is there some reason why this won't do the job for the input transformer?

Again, thanks for the helpful comments.

 

[RodVito]

Zoobie:

A few things about the Robicon Drive; because of the topology of the drive, the Robicon Perfect HArmony does not need additional harmonic filtering, or output filters for dV/dT issues. Basically, the drive consits of stacked voltage cells, as in a flashlight. These voltage cells add together to form the output voltage. Lets say the specific drive has 600V cells, and has an output of 4160V. The switching voltage at the DC bus is about 600V, not 4160, as in most drives, so any overvoltages as a result of long line length (dV/dT), are easily handled by the cables themselves (Typical dV/dT overvoltage is appox 2-2.5x Switching V).


In addition to JRaefs comments about the replacement of the cells: IF a cell fails, the controller will sense this, and close a contact, bypassing that particular cell. Next, the controller will 'shift' the neutral point, in order to present the motor with a balanced voltage (rather than three voltages 120 electrical degrees apart, you might have two at 128 degrees apart, and one at 104). Now, a shutdown can be scheduled, according to process needs. The cells are rack out style, so down time is very minimal.

The Perfect Harmony is either 18 or 24 pulse, which places it well within the harmonic limits as specified by IEEE 519.

As far as cooling, as with most manufactureres, Robicon can either do Air or Water cooling, with redundant systems available for each.

Fort MacMurray has purchased many of these drives, and service is found in Edmonton and Ft Mac. The local rep company for Robicon is Altelec Engineering, with offices in Edmonton and Calgary. If you are interested in some further info, let me know and I can put you in contact with the correct people.

 

[jraef]

At the risk of drawing a Red Flag, I'll say that I know the people at Altelec as well. Great folks, very knowledgeable and professional. That does not mean they are the only ones worthy of being given a chance however. Any company that can make and maintain the investment to be in the MV VFD business cannot survive without making a good product and supporting it well. Just keep in mind that recommendations by ANY anonymous forum posters should not be taken as gospel.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Zoobie]

Thanks for the info on the Robicon drive. The ABB MV drive is 36 pulse. Does this mean that harmonics will be further reduced? What about motor life/performance vs pulse count?

 

[jraef]

IMHO, anything above 24 pulse falls into the "marketing dept. numbers game" category, because it falls victim to the laws of diminishing returns. Adding more diodes and phase shifts to the front end does help reduce harmonics, but adds cost, and if you already meet your needs with a lower pulse count, why? It makes no difference to the motor, the pulse count involves only the conversion section of the drive. The only possible reason for a 36 pulse is if it is a true line regenerative front end, with 2 18 pulse sections, one for conversion as a VFD, one for regeneration as a braking device. If you need regen braking (i.e. it can save you on production time), that's a good way to go. If you don't, don't waste money on it.

 

[mil3]

Zoobie,

Be very careful in the selection of front end devices that promise high pulse input front ends. I'll tell you why.
The front end is only as good as the voltages feeding it. The higher pulse front end you select the mnore crucial it becomes to get a transformer + clean solid power to feed it so that all your phases are at the same levels.
If any of the legs are lower than the others(lets say 3 of them are low), then you will most likely not utilize that leg for DC Bus charging and now your (lets say) 12 pulse has become an 9 pulse....and so on. That puts you back o0n the Harmonics issue again and you might start wondering why the harmonics is so high.
(Just used a 12 pulse as an example here)
Hope this makes sense...if not...let me know and I can try to explain further, but I am sure it is clear.
Try to stick to exactly what you require at your plant. No need to buy more and not utilize it.

 

[hec64]

For your information Siemens bought Robicon. So I do not think Siemens has large VFD units.

Now in regard to harmonics, my company bought two 4160 volts, 1000HP Perfect Harmonics units and I went to the factory to see the test. When we were testing the unit the THD were more than 5% and I asked why? The answer I got was that it was because the plant was connected to a line that has several industrial facilities that was creating the problem more than they were injecting into the system. But what I saw was that the VFD can create enough to surpass the 5% THD. Specially if you are running two or more units. Also if you look at the web site they claim that the VFD will only make less than 1% harmonics, but that is not tru. It depends on yuor facility electrical configuration.

 

[Zoobie]

Thanks hec64. I will keep your comments about harmonics in mind.

Also, for your information, Siemens is marketing a MV drive (Sinamics GM150). I'm not sure but it might just come out this year...who knows maybe its just Robicon guts with a Siemens skin.

 

[jraef]

No, the Robicon deal is less than a month old, so anything Siemens already had is not going to just evaporate, and I'm sure Siemens will continue marketing their existing products in other parts of the world where they have a large installed base. However, in North America they couldn't sell many so the long term prospects for their continuing with it are slim once the Robicon deal sinks in and settles.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla