Generator Sizing for VFD Driven Motor up to 250HP 1

[Sherwin Cruz]

Hey everyone, I am currently sizing Generators for our Testing Facility and Factory.

The Plan is to have two units, one dedicated to the offices and factory assembly operations, the other for the sole testing facility of Pump up to 250HP Motor. This Motor will be driven by a VFD (PFA for the Specifications).

My colleague which is an Electrical Engineer suggested that we can have 312.5KVA(ESP) for the application, however I am in doubt since the Generator is not rated higher than the VFD. Also, I am not quite sure if we can program it to have a longer ramping time during off-grid/gen-set to compensate the effects of THD. As for now, I am checking possible options to make the design safe without incurring more expenses. I would like to know if installing input line reactors or harmonic cancelling transformers can be cheaper means.

Regards,
Sherwin Cruz
Excel Fire Systems
Doha, Qatar

 

[waross]

250 Hp x 746 Watts per Hp/1000 = 186.5 KVA
186.5 KVA / 0.9 PF = 207.2 KVA
207.2 KVA / .9 Efficiency = 230.2 KVA.
Our code requires motor supply circuits to be oversized by 25%
230.2 KVA x 125% = 287.8 KVA
Looking at it another way, Schneider uses a factor of about 0.85 to convert HP to KVA.
250 x 0.85 = 212 KVA
The generator should have sufficient capacity to drive the motor.

But two caveats:
1. Some AVRs do not like VFDs. Catserveng may provide more information on this possible issue.
2. You may have a regulatory issue.
The code that I work under requires supply equipment to be rated based on the input Amperes.
By that criteria I would need a 400 KVA generator.
However I would consult with the Authority Having Jurisdiction (AHJ) and apply for an exception to allow a generator smaller than 400+ KVA.


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

 

[itsmoked]

Interesting question is; are you loading the 250HP motors to test them and if you are are you using a regenerative drive to return the load energy back to the source?

Does that alter the generator size?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. Altivar list this drive as suitable for pumps and ventilation.
I understand that the OP will be testing pumps.
In this case I can't see regeneration being an issue.
In the broader sense, returning energy to a generator is generally not done intentionally.
It results in motoring and overspeed.
Generation driven systems where regeneration is possible will have a resistor bank to dissipate the returned energy

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

 

[jraef]

That drive is not line regenerative anyway.

That drive has a DC bus choke as standard, so you can assume the THD-I is going to be around 30-35% and when operating from a generator, that current must come from the generator. You don't state your line voltage or motor FLA, but factor that as additional kVA into the formulae that Bill provided.

Because of that as an unknown, I have always used a rule-of-thumb to size a generator kVA as 1.5x the motor HP, which in this case would be 375kVA, closer to Bill's 400kVA value than their 312.5kVA... COULD a 312.5kVA generator work? Probably, but I would do what I could to keep the THD-I under control, i.e. 12 or 18 pulse, or even just a passive 5th harmonic filter.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[catserveng]

Smaller generators tend to have single phase sensing static excitation systems, not very resistant to harmonic issues. The likely results, depending on the actual load harmonics are the generator voltage going low or high or getting unstable. Sometimes filtering helps, but sometimes if the filter is not properly sized or installed can make the problems worse.

My rule of thumb tends be similar but with a condition,

I used to use 1.5 X Motor HP for PMG (or AREP) powered 3 phase sensing AVR equipped units, I used 2 X Motor HP for SE powered single phase sensing AVR equipped units.

You also appear to be in a place in the world where ambient conditions can be extreme, and you may need to apply derate factors to the generator set during certain ambient conditions, like high temperatures.

So to properly size a unit for your application, quite a bit more information is needed. Most of the major generator manufacturers have sizing calculators that take a lot of those issues into consideration, and yes they tend to be conservative, but you may be better with a slightly larger unit than you may need most of the time than one that for some or most of the year doesn't do what you need.

Hope that helps, MikeL.

 

[Sherwin Cruz]

Hey Bill, thanks for the input. I think old design generators can tolerate the parameters in VFD Operations but those features are pretty rare nowadays. Honestly, it is my first time working on an electrical scope, I already checked AHJ regulations but still I was not able to find the code for compliance with this matter. Perhaps, I will consult it to my colleague thank you.

Hi Keith, we will be for testing pumps of different capacities up to a <250BHP, these will be motor driven with VFD Starting. Interesting input, I haven't encountered any regenerative drive yet but it gives me a good idea for future projects thanks!

Hello jraef, great input thanks! I have a doubtful feeling with the 312.5KVA but I do not want to suggest an up sizing without further information.

Hi Mike! Thanks for the information, it does help a lot. I really need to be careful with the AVR-Alternator to VFD Specification details. Yes, ambient parameters peaks up to 50°C at 95%RH mentioned in the code. May I ask if you are pertaining to derate conditions for the Alternator? Prior to purchasing I will be reviewing the assembly of generator, mostly Diesel Engines are sized relatively higher than Alternators. Also the turbocharging feature does help, hence there will be some good considerations in the derate factor for Engine Side, however I am not familiar with the Alternator Side. Thank you

 

[itsmoked]

Bill, Cruz; I totally glazed over on 1) the actall VFD model.

And 2) that the loads were actually pumps not just motors.

²

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hey Bill, thanks for the input. I think old design generators can tolerate the parameters in VFD Operations but those features are pretty rare nowadays

The issue is with the AVRs, it is not with the generators.
The ratio between grid source impedance and a typical generator impedance is so great that if a new design reduced the typical generator impedance by 50% there would probably still be some issues.

I already checked AHJ regulations but still I was not able to find the code for compliance with this matter.

Sizing and demand factors for conductors and breakers is based on the nameplate ratings, not on the actual current draw.
The day will come when you have to test a 300 HP pump and motor. With the generator sized at 150% or 200% of 250 HP it may work.
BUT, the wiring to the VFD will be overloaded.
Most AHJs that I have worked with would accept a generator sized to the expected load but would demand that the wiring be sized for the nameplate current of the VFD.
I did an installation for a remote retreat and meeting facility. When the only occupancy was the caretaker a small generator was used.
When the facility was in use the large generator was used. Even the large generator was not capable of supplying the full 200 Amps to the 200 Amps service but the AHJ demanded the 200 Amp service, the same as if the facility had been supplied by a grid.

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

 

[genhead]

I realise that you are only intending to run the drive and no other electrical load from this generator but that leads me to some other questions which maybe jraef or others could answer.

As well as the extra current, the 30-35% THD-I is going to result in significant THD-V. For a typical generator of that size (312kva) you could expect somewhere around 20-25% THD-V.
Increasing the size (decreasing the impedance) of the generator will reduce the THD-V. It is not uncommon to oversize the generator by a factor of 3 to bring the voltage distortion down to a reasonable level so that other loads are not adversely affected. This can make expensive-looking filters, low-distortion-drives etc look relatively inexpensive.

Anyway, here are my questions:-
Are drives themselves affected by the level of THD-V at their supply input?
Is there a maximum acceptable level of THD-V for drives in general?
Are some more sensitive than others?

 

[waross]

A question for Mike, Gunnar and/or Jeff:
Given that the issues are caused by the effect of the harmonic distortion on the AVR, would a filter on the AVR sensing be effective?
Filtering the sense input would be a lot cheaper than filtering the entire load.
A more sophisticated sense input filter would still be much cheaper than a basic whole load filter.

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

 

[genhead]

Waross, There are two separate issues.

One is the effect that the distorted waveform has on the AVR, sometimes causing poor voltage regulation. Does anybody have any suggestions for suitable AVR sensing filters?

The other is the THD-V which results from the THD-I created by the drive. The level of THD-V is independent of the AVR and is determined by the level of THD-I and the impedance of the generator. Reducing either of those will reduce the voltage distortion.

 

[Sherwin Cruz]

My bad, I was not specific, yup Bill you're right. There is this site that directs me to PMG/AUX/EBS AVR for non-linear loads. In addition, there are also articles that provides VFD with off-grid/genset mode, unfortunately I cannot apply this since my Manager already purchased the drive prior to this assignment. Noted on that thanks!

I used to see the Motor Data in VFD Interface especially the Amperage back then when I was in CUP Operations, I am quite sure that this is the reference for the VFD protection of Overcurrent Limit. Correct me if I am wrong for this statement, I am not an expert in this matter, if my manager really insists that we will use 250HP as reference then we will size up wirings and components based on the Drive Specs, considering we have another unit 250KVA for another load, is it possible to connect it in parallel with the generator unit intended for this to have more capacity? My manager does not see any reason to use 300HP since most of the orders that we received are up to 250HP only, I am just trying to see the possibility.

Anyone have input in line with the relationship of ramp speed to the operations of the setup? It seems that the main issue is the THD and the compatibility of AVR and VFD.