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on VFD supply by diesel gen set why to multiply by two the size of gen
4

on VFD supply by diesel gen set why to multiply by two the size of gen

on VFD supply by diesel gen set why to multiply by two the size of gen

(OP)
   I was talking the other day with a friend of me and tell me that when you use a genset for starting in example some motors with VFD, you´ll need to overdimensioning the size of generator, this means in example if you are with 1,7 MW pf 0,8, this means 2,12 MVA, if you use VFD you would need about 4 MVA generator to drive the load, and not to have overheating on the generator, but i´m still don´t understand this point... could somebody explain it to me... cause my friend just tell me this is what the guys that selling the genset says....maybe it´s right we are not experienced on this VFD
thanks and sorry for my special english.. i hope you´ll understand

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

Quote:

...this is what the guys that selling the genset says....
That should be your first clue; if they get you to believe it, they can sell you more genset than you really need.

In theory, you would need LESS genset capacity to start a motor with a VFD than any other method of starting. In fact in some extreme situations where genset capacity is severely limited and a motor must start, sometimes a VFD is the only way to make it work.

What they may be doing is over-compensating because of some previous experience where someone installed a poorly designed system and the harmonics of the VFD caused the alternator windings to overheat. That was more common in the early days of VFDs, most good engineers know how to better handle that now.

My advice is to hire a qualified experienced Electrical Engineer that understands both generators and VFDs, rather than rely on vendors to design systems for you. Vendors will tend to favor the system that maximizes their profits rather than optimize your application.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

(OP)
thanks, but the question is not about the diesel motor, they said that diesel motor could be low power, but you need oversize genset, i wonder why you get so much temperature on windings, and how do you deal with the armonics inside generator core... i mean, that as i see and understand, the VFD is taking a current that is a PWM current maybe 5 khz, and this is the current i got on my windings, then i get a filter on the windings and this is the flux i got in the airgap.... how is this flux? could overheat the generator in the way i have to oversize it, and then my next question, if i would run with the motor on a stand by base, this mean only several hours per day maybe i don´t need to oversize it too much.
 thanks

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

All diesel generator suppliers recommend increasing the size of the Generator when they know the load connected to the generator is none linear. A VFD is a none linear load that draws current from the generator in pulses.
The pulsing current is full of harmonics that can cause the generator windings to run hotter than they would if the load was linear (sinewave).
The other reason the generator suppliers increase the size of the generator for none linear loads is to limit the amount of Voltage distortion that is caused by the none linear loads.

So if you know that the load you plan to run on a 200 KW generator is also rated at 200 KW and it is a none linear load you must double the size of the generator (not the engine) to 400 KW.  

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

2
The switching frequency (aka carrier frequency) of the VFD (seems to be 5 kHz in your case)doesn't reach the generator's windings in any substantial way. There is a rectifier and a DC link between PWM switches and generator, so all the generator windings see are residues of the switching. Bad as such because they cause HF pollution, but they do not heat the windings any extra.

Some VFDs draw current with a very bad P/S (power factor) ratio because the line current is distorted by the rectifier/capacitor in the VFDs front end. That is why you may need to oversize the generator the same way you need to oversize a transformer that is feeding a VFD. There is something called a K factor that is used to describe how much you need to oversize the windings. You can read more about how the K factor here: http://www.xitrontech.com/images/support/app_notes/AN102%20K-Factor%20Defined.pdf

Since this is a rather large piece of equipment, you should get advice from an EE that knows about VFDs. Not only knows about them, but is an expert in their use.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

(OP)
"So if you know that the load you plan to run on a 200 KW generator is also rated at 200 KW and it is a none linear load you must double the size of the generator (not the engine) to 400 KW. " Yes i mean generator.

skogsgurra, so why if this harmonics and frequency never arrives to the generator, the manufacturer of generator says to oversize generator or the diesel manufacturer talks about oversize double the size of generator, are they supported by some standard or just cause they want to sell big units.
  I understand your point that only residual would arrive to the stator windings.
  In reality i think they are going to use a soft start but now i feel curious about this.... cause normally a manufacturer of diesel if he need to multiply for two the size of gen set, will add a big amount of money to its final price, and you know if other can do with a shorter generator will take the project...but seems all of them try to multiply by two.
thanks in advance, cause i know my english is not the best...
 
 

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

uhpo, the input of a VFD is normally made from a three phase diode bridge (Graetz Bridge) that takes the incoming three phase voltage and turns this into the DC link voltage for the inverter. The currents drawn by this input bridge are non-linear and have a reasonably high harmonic content. It is this high THDi (Total Harmonic Distortion Current) that causes problems for the upstream generator.

The basic Graetz bridge is also called a 6 pulse rectifier, as it draws 6 distinct pulses over one period of the fundamental frequency. If no filter is placed in the circuit the THDi of this style of rectifier approaches approximately 30%. It is with this highly distorted input current that generator oversizing is required.

Fortunately, there are other, far cheaper methods of mitigation that can be employed so that a generator (or just the alternator) need not be oversized.

Graetz bridges can be series or paralleled up so the number of pulses drawn by the incoming current is increased which quickly brings down the THDi. Without any additional filtering, by using two Graetz bridges, and therefore building a 12 pulse circuit, the THDi reduces to approximately 14%. An 24 pulse circuit (4 Graetz bridges) will bring it down even further to approximately 7%.

In practice, manufacturers normally don't build units with more than 18 pulses. This  gives a THDi of about 10%. All of this done without adding a filter. It is quite practical to design a unit with a 12 pulse bridge plus an input filter and get the THDi at the input down to about about 5% when the VFD is running full load.

Even if you are stuck with an inexpensive VFD on your site which uses only a 6 pulse bridge, external filtering , either passive or active, can be placed at the input of the VFD to bring down the amount of harmonic current that is seen by the generator.

The 'rule of thumb' of doubling the generator (or just the alternator) for VFD (or UPS) loads is something that is now about 20 (maybe even 30) years out of date. There are far cheaper options that can be employed to lessen any impact that this device may have on your installed infrastructure.




 

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

Using Caterpillar's SpecSizer software to select a base load generator supplying one 220kW battery charger - a vaguely reasonable approximation to a 200kW drive - produced following results:

6-pulse / 220kW / no filter
292kW / 365kVA

6-pulse / 220kW / with filter
292kW / 365kVA

12-pulse / 220kW / no filter
292kW / 365kVA

12-pulse / 220kW / with filter
256kW / 320kVA

The frequency and voltage deviations were both limited to <10%. As you would expect the smallest set has the poorest response if a step load is applied, although a step load with a VFD is not a typical scenario.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

uhpo

Sibeen and Scotty have already given the information you need. But, since you asked me directly, I want to clarify one thing.

You say: "skogsgurra, so why if this harmonics and frequency never arrives to the generator" That is not entirely true. You must differentiate between HF components and harmonics. The HF components do not harm the generator winding but the harmonics do, if there is too much of them so that the thermal effects heat more than expected.  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

Since you mentioned a softstarter, they are an entirely different problem.  A VFD has a capacitor bank that is drawn from to run the motor.  A soft starter has no capacitors.  They directly chop the incoming current up.  This means far more harmonics and a larger issue with the source.   You will always need a larger generator/source with a softstarter than you would with a VFD.

Keith Cress
kcress - http://www.flaminsystems.com

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

(OP)
REally i get really happy with all the information you guys give me, thanks a lot,and i understand perfectly the question.... but, still one more point, with soft start with a by pass contactor, do you still need to oversize the generator, i mean to use it only for starting and then by pass it.... acording my info not, but i would like to see your opinions....
   And about k factor, so i need to ask VFD manufacturer for K factor and then if i need apply to oversize generator or ask for good rectifier.?
thanks again

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

One thing not realy mentioned is that this is a 5kV application. A 5kV VFD will most likely include harmonic filtering as part of it's design.


Quote:

A soft starter has no capacitors.  They directly chop the incoming current up.  This means far more harmonics and a larger issue with the source.

I haven't found this to be true in practice. The soft-starter current will be much closer to a sinewave compared to a 6-pulse VFD rectifier with no harmonic filtering.

The reason for oversizing is because a soft-starter will likely have to draw 300% to 450% of rated current to start your pump. The genset will have a temporary overload capability so it does not have to be continuously rated for this full current. However, the short term overload capacity of a genset is typically around say 1.5 to 2.5 times. So, the genset needs to be oversized a certain amount to stay within the short term overload capabilities. For example, a genset with a 200% short term overload capability which is oversized by 150% will allow you to reach 300% of the motor rated current.

If you can throttle the pump, you will lower both the required soft-start current and the genset size.

 

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

Actually, you will probably not need to oversize the generator when you use a soft starter. The reason is that the SS goes into full wave conduction (or very close to) when the start is finished so there will not be any harmonics.

The harmonics will not cause much extra heat if the start doesn't take more than 30 - 60 seconds. The thermal time constant of the motor windings is usually in the 10 - 30 minutes range. So heat doesn't build up.

And, of course, if you use a by-pass contactor, the same applies.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

Some diesel gen sets have overload capability, many don't. We had a pair of 600 KW sets that would start losing Hertz at about 650 KW. A prime power set will supply a 10% overload by specification. A standby set does not allow overloads.
That said, it depends on the size of the engine and the fuel stop setting. Sometimes the fuel stop may be adjusted to allow the engine to drive an overloaded generator, sometimes the fuel stop may already be at maximum and there is no more to be had. A naturally aspirated set will pick up a block load better than a turbo aspirated set, but sometimes the extra frequency drop of a turbo-ed set may actually be an aid to motor starting.
IT DEPENDS.
If the generator is dedicated to this motor and not supplying critical loads, you may consider direct connecting the motor to the generator and starting the generator with the motor connected. With the proper attention to the control scheme and AVR power you may be able to use the generator as a Variable Frequency drive with the advantage of a good waveform with few if any harmonics and a generator sized very close to the motor size. You want the generator engine over-sized enough to handle any overloads on the motor without slowing down.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

A couple of resources,

http://samplecode.rockwellautomation.com/idc/groups/literature/documents/wp/drives-wp014_-en-p.pdf

http://www.trane.com/commercial/uploads/pdf/552/admapn019en_0106.pdf

http://ecmweb.com/training/electrical_basics/electric_sizing_gensets_large/

In general most generator manufacturers recommend a generator sized twice the size of the VFD, this recommendation was based on older drive technologies and in response to some pretty ugly problems early on.

In a lot of applications using drives we used to recommend upsizing the tail end one or two frame sizes.  As ratings got more complex this didn't always work out so well.

In my experience from the generator supplier side we usually have three things to deal with when the load is a drive,

Power factor of the load as seen by the generator.  If the drive operates at a lower load for most of the time, a poor power factor can result.  This got missed a lot, especially in the rental markets and lots of fried tail ends were the result.  So the actual operating conditions are important.  
And beware of power factor correction capacitors, they can cause a leading power factor, loss of voltage control and instability.

Harmonics can be a problem in many ways.  Depending on the actual harmonic distortion, it can increase heat in the windings, affect the voltage regulator operation, and other engine/generator controls.  Normally we tried to use PM or AREP tail ends with three phase sensing AVR's.  Lower cost single phase AVR's and SE generators had a much harder time dealing with the harmonics.

Voltage and frequency stability, in the early days many VFDs were touchy about voltage and frequency changes, to combat the complaints by customers we would just supply larger units, that would be less affected by the load changes, and result in fewer complaints.  Many VFDs today have a selection for operating on and off grid.

I have installed a number of systems where the VFD was 85-90% of the generator rating, these systems were engineered by experienced engineers familiar with the drives, the site and load, and the power generation equipment.  I have also worked on a large number of systems with VFDs being a much smaller percentage of the rated generator capacity and the systems are problematic, usually done with little or no engineering, using "rules of thumb", and going by a salesman's recommendations.

My two cents, hope it helps,

Mike L.

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

(OP)
so, first i thought i have a complete vision, but what do you think guys about the documents that catserveng show and the experience with drivers in the motors he commisioned...
thanks, this seems to be a dificult point

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

Quote:

Actually, you will probably not need to oversize the generator when you use a soft starter.

You could attempt that, but I would not recommend it. Your motor has very little chance of starting on a 1:1 sized genset. I have been involved with starting a motor on a closely sized genset a few times. The applications were bow thrusters and luckily a bow thruster has a very low load torque while it is being started. In one case, the customer changed the motor so they could get it started.
 

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

That is true. But from a distortion/harmonics point of view, the soft start does not heat the generator much.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

True Gunner, but does the fact that the genset doesn't need to be oversized due to soft-starter harmonics matter when the genset needs to be oversized so you can start the motor?
 

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

No. That circumstance (going for a bigger generator) alone may make it unnecessary to consider harmonics at all.

Then, if you use a VFD, you can start the motor with a 1:1 generator. But harmonics MAY be a problem. Depending on the harmonics contents of the VFD supply current.

If I were the OP (uhpo), I would still be confused. I would probably read Mike's answer a couple of times and heed his advices.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

(OP)
yes. i´m still confuse, i read a lot of times mike post and i think i agree and it´s close to what i read and people tell me , but on the other hand i´m electrical guy and i understand and i like to understand why mike says that when the point is that the harmonics content didn´t affect too much, or maybe real VFD didn´t have such advantages and we still got a high component of harmonics.

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

That's what I posted Gunner. You have to oversize the genset to be able to soft-start the motor.

Posting that you don't have to oversize the genset with a soft-starter since the soft-starter does not produce any full-speed harmonics is doing everyone reading a dis-service. The running harmonics are not the reason to oversize when using a soft-starter.
 

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

My 'No' was an answer to your 'does the fact that the genset doesn't need to be oversized due to soft-starter harmonics matter when the genset needs to be oversized so you can start the motor?'

That means that I agreed. Nothing else.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

I have found, at least in the size units I mostly deal with, this is a VERY hard discussion to have in general terms.  So many factors affect how the system will perform dynamically, what does it really do when it starts and runs. You're getting lots of great general advice, but to do this properly the devil is in the details.

I got involved with my former employer and a customer installing two generators and two large sewage pumps, the system was a near exact copy of a system installed about 5 years ago.  Same pumps, similar motors (same nameplate ratings and type), same VFD's (by model number but with some "improvements"), same physical layout of plant with very similar load conditions, and the same size (ekW and kVA rated) generators.  First system worked flawlessly for years, after a few commissioning issues that were quickly resolved.  The new system fell on it's rear end.  The differences initally appeared to be minor but with fairly major consequences.

The old VFD's had a single fairly wide voltage and frequency tolerence, the new units had an on and off grid selection, that defaulted to on grid.  This was found early and made some improvement but didn't resolve problems.

The main problem was the differences between the "new" and "old" gensets.  Both were rated as standby's for 1000ekW, both rated for a 0.8 lagging power factor.  As Bill pointed out above, standbys normally have no overload rating, but the pre-EPA rated engine had a bit more reserve and response capability than the new EPA tier 2 engine did, a fact of life nowadays in the US.  So there was a difference in the engine response capability that was actually pretty small, but had significant impact.

There was a significant difference in the tail ends, the old 1MW tail ends were bigger, heavier, more iron and copper.  The new and improved tail ends were smaller, lighter, cheaper, and "more efficient".  Looking at the data was hard to see a possible difference, in reality the ability to deal with the real loads imposed by the VFDs and motors was quite different.

The excitation and AVR's were also different, the old dog used big diodes, and older design, but very robust AVR.  The new generator had smaller diodes, a less capable AVR power source, and a digital AVR with factory default settings.

After a lot of data taking and tweaking, adjusting, and holding our collective breaths, we could get the system to work most of the time.  To make it work reliably the generators will have to be retrofitted with larger tail ends and possibly a horsepower uprate, if the air board folks will buy off on that.

The reason for this long winded story (that is still not over with, will probably go to court) is to try and describe many of the factors that can be involved with trying to closely size a generator to a VFD/motor. It can be done, but I don't think even with all the great minds available to you here, that you can the info you need to make this work as you need from a forum.

What you can get from the advice presented by all who contribute is an idea of the skills needed by the engineer and equipment suppliers you select to help make your specific system function as needed.

Hope that helps, Mike L.

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

I have spec'ed and installed quite a few generators. In this instance I would be guided by catserveng's comments.
Mike; Is there any way you can increase the fuel by changing injectors or the injector pump or are you combustion air limited and already blowing black smoke?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

Bill,

In this particular case our limitation is the nameplate rating of the unit and the air pollution permit.  For many of the standby ratings in the market today, especially with EPA approved ratings, it is VERY hard to change an engines horsepower.  This is an issue for CAT and many others.  I'm afraid the days of being able to tweak a bit more out of an engine are near an end.  The emissions folks don't want you to do it, and the folks selling the iron are selling HP, you want more, pay more.

I'm also seeing the same issue with tail ends, we used to have lots of extra capacity in many of the frame sizes, and we'd see full load field amps at about 1/2 the rated maximum excitation system rating.  But the value engineering employed by many suppliers basically gives you exactly what you paid for and nothing more.  Makes it tough when you're used to a bit of extra cushion that may have not been advertised but was available in the iron none the less.

Guess this is evolution, frankly I miss large, slow engines with huge tailends that ran for 100,000 hours with minimal problems. When I started with CAT we supplied lots of 399 and 398 engines with SRSE and SRCR generators for mostly the oil drilling market.  They were beasts, nearly indestructible, ran for years, took lots of abuse.  Electrically very tolerant of harmonics, noise, spikes, surges and abuse.  But they had lots of iron and copper for their size, and the AVR's needed a small crane to handle. If you could get the customer to keep the slip rings in good shape, they performed well.

Of course the new stuff keeps me busy, so I guess I can't complain.

Mike L.

RE: on VFD supply by diesel gen set why to multiply by two the size of gen

Interesting discussion.  For a given combination of motor and generator then, would a soft starter give the best chance of success when the generator is closely sized (i.e. vs Autotransformer or Part Winding methods, for example)?  I would've thought so, because of better control of accelerating current over time, but sounds like there's more to it than that . . .

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