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

[uhpo]

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

 

[jraef]

...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

 

[uhpo]

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

 

[delphini]

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.

 

[Skogsgurra]

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 K-Factor Defined.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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[uhpo]

"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...

 

[sibeen]

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.

 

[ScottyUK]

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.


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

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

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

 

[uhpo]

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

 

[LionelHutz]

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.

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.

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

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

 

[catserveng]

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.

 

[uhpo]

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

 

[LionelHutz]

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.

 

[Skogsgurra]

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

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

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?

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...