Emergency Diesel Generator Calculation

i.engrqasim

Member
Location
Saudi Arabia
Hi everyone,

We've purchased a diesel generator rated 1520 KVA, which was supposed to take a load of 626 KVA. the generator was purchased because of specification requirement of 1400 KVA.
Now for some changes in the project, we require to run 3 number of 250 KW pumps instead of one. that will make a total load of 942 KVA.
The contractor claims that the already purchased generator is not enough for the 942 KVA load.
Contractor's sizing calculation using caterpillar software requires 2 x 1250 KVA gensets for this load.

while I've requested a meeting with the manufacturer for their recommendations, I want to know some experts opinion on the sizing calculation. As I don't see any load surges even crossing 1200 KVA.
the motors are run by VFD's so we don't face any problem with their starting. but the contractor claims the VFD's will cause heat in the generator and thus is insisting on a variation order.

load list:
LOAD LIST_1.jpg

contractor's generator sizing and steps:

Sizing calculation_1.jpg
Sizing calculation_2.jpg
Sizing calculation_3.jpg
Sizing calculation_4.jpg
Sizing calculation_5.jpg
Sizing calculation_6.jpg

Sizing calculation_7.jpg
Links to PDF's:
https://drive.google.com/drive/folders/1Um-Cq3Qfm95kQM0OXd_5Wh7KgCxmtJz2?usp=sharing

thank you very much in advance.

qasim
 

i.engrqasim

Member
Location
Saudi Arabia
Just to clarify, my question is:
> isn't our purchased 1500 KVA DG enough for the complete loads in the load list and and load steps shown in sizing analysis.

His answer is, when a load takes lets say 200 KVA in starting and then comes back to 100 KVA running load, we consider it as 200 in sizing.
Thus the remaining capacity will be 1300 out of 1500 after that load starts.


Hi everyone,

We've purchased a diesel generator rated 1520 KVA, which was supposed to take a load of 626 KVA. the generator was purchased because of specification requirement of 1400 KVA.
Now for some changes in the project, we require to run 3 number of 250 KW pumps instead of one. that will make a total load of 942 KVA.
The contractor claims that the already purchased generator is not enough for the 942 KVA load.
Contractor's sizing calculation using caterpillar software requires 2 x 1250 KVA gensets for this load.

while I've requested a meeting with the manufacturer for their recommendations, I want to know some experts opinion on the sizing calculation. As I don't see any load surges even crossing 1200 KVA.
the motors are run by VFD's so we don't face any problem with their starting. but the contractor claims the VFD's will cause heat in the generator and thus is insisting on a variation order.

load list:
View attachment 21194

contractor's generator sizing and steps:

View attachment 21195
View attachment 21196
View attachment 21197
View attachment 21198
View attachment 21199
View attachment 21200

View attachment 21201
Links to PDF's:
https://drive.google.com/drive/folders/1Um-Cq3Qfm95kQM0OXd_5Wh7KgCxmtJz2?usp=sharing

thank you very much in advance.

qasim
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Not an expert, but clearly the design has to change to accommodate the larger load.

IMHO a 1520 KVA generator should be capable of handling a 942 KVA load, but that assumes a bunch of things, including correct power factor and ability to deal with any sort of transients. As you describe things, you originally had a load of 626 KVA and selected a generator of 1400 KVA.

VFDs will reduce starting transients, but will introduce harmonics. These harmonics _will_ increase the heating of the alternator and reduce its KVA capability. These harmonic currents will also cause harmonic voltage distortion which might affect other loads. You _may_ need to consider harmonic current mitigation, or you _may_ need a larger alternator to deal with this. Do you have transient loads other than the pumps on VFDs which would impact the sizing?

Using the caterpillar sizing tool is like asking the baker how much bread to buy. However IMHO your situation is not a 'slam dunk the generator is big enough' and some design work needs to be done.

-Jon
 

topgone

Senior Member
Hi everyone,

We've purchased a diesel generator rated 1520 KVA, which was supposed to take a load of 626 KVA. the generator was purchased because of specification requirement of 1400 KVA.
Now for some changes in the project, we require to run 3 number of 250 KW pumps instead of one. that will make a total load of 942 KVA.
The contractor claims that the already purchased generator is not enough for the 942 KVA load.
Contractor's sizing calculation using caterpillar software requires 2 x 1250 KVA gensets for this load.

while I've requested a meeting with the manufacturer for their recommendations, I want to know some experts opinion on the sizing calculation. As I don't see any load surges even crossing 1200 KVA.
the motors are run by VFD's so we don't face any problem with their starting. but the contractor claims the VFD's will cause heat in the generator and thus is insisting on a variation order.

load list:
View attachment 21194

contractor's generator sizing and steps:

View attachment 21195
View attachment 21196
View attachment 21197
View attachment 21198
View attachment 21199
View attachment 21200

View attachment 21201
Links to PDF's:
https://drive.google.com/drive/folders/1Um-Cq3Qfm95kQM0OXd_5Wh7KgCxmtJz2?usp=sharing

thank you very much in advance.

qasim
The most important thing when sizing a genset is to make sure the voltage dip will not exceed the allowable voltage dip in your facility. May we know what is the allowed voltage dip? Is this an industrial setup or a hospital. What your sizing software tells us is that the voltage limit used was a very stringent 3%. Your 1250 kVA genset is capable of running the 3 x 250kW (staged starting) and the allowable voltage dip of about 26%.
 

i.engrqasim

Member
Location
Saudi Arabia
Not an expert, but clearly the design has to change to accommodate the larger load.

IMHO a 1520 KVA generator should be capable of handling a 942 KVA load, but that assumes a bunch of things, including correct power factor and ability to deal with any sort of transients. As you describe things, you originally had a load of 626 KVA and selected a generator of 1400 KVA.

VFDs will reduce starting transients, but will introduce harmonics. These harmonics _will_ increase the heating of the alternator and reduce its KVA capability. These harmonic currents will also cause harmonic voltage distortion which might affect other loads. You _may_ need to consider harmonic current mitigation, or you _may_ need a larger alternator to deal with this. Do you have transient loads other than the pumps on VFDs which would impact the sizing?

Using the caterpillar sizing tool is like asking the baker how much bread to buy. However IMHO your situation is not a 'slam dunk the generator is big enough' and some design work needs to be done.

-Jon
Yes we have HVAC load, which are already considered in the sizing calculations and loading steps. Please see the caterpillar sizing calculations.
We already have a larger alternator. In fact we have an alternator of 1774 KVA.
DG KVA_1.jpg
 

i.engrqasim

Member
Location
Saudi Arabia
The most important thing when sizing a genset is to make sure the voltage dip will not exceed the allowable voltage dip in your facility. May we know what is the allowed voltage dip? Is this an industrial setup or a hospital. What your sizing software tells us is that the voltage limit used was a very stringent 3%. Your 1250 kVA genset is capable of running the 3 x 250kW (staged starting) and the allowable voltage dip of about 26%.
It is an industrial setup (a waste water (sewage) pump station).
There is no voltage dip specified in the generator specifications, the 3% is specified voltage drop for cables, which the contractor has used.
The contractor has done calculation for a 1250 KVA. However we have a 1520KVA Generator with 1770 KVA alternator.
Voltage dip shall be in a range that may not harm the VFD's.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Yes we have HVAC load, which are already considered in the sizing calculations and loading steps. Please see the caterpillar sizing calculations.
We already have a larger alternator. In fact we have an alternator of 1774 KVA.
And did your calculations include the harmonic currents drawn by the VFDs? I see them listed as 24 pulse in the initial calculations; is that actually the case or are you and the contractor making different assumptions?

-Jon
 

i.engrqasim

Member
Location
Saudi Arabia
And did your calculations include the harmonic currents drawn by the VFDs? I see them listed as 24 pulse in the initial calculations; is that actually the case or are you and the contractor making different assumptions?

-Jon
The 24 pulse is assumption. we are using ABB low harmonic VFDs. ->
VFD.jpg
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
As I stated, I am not an expert on this topic.

I did a quick look at the VFD that you mentioned, and it uses an active front end rather than a simple rectifier. This results in very low harmonics and near unity power factor.

The system also _boosts_ the input voltage to create the DC rail, which causes the VFD to draw more current. I am guessing that this could interact poorly with the generator if the VFD load is too large; a slight disturbance would cause the generator voltage to dip, which causes the VFDs to draw more current which causes the generator voltage to dip further....

Like I said I am not an expert, so this is simply another factor to consider, not a definitive 'won't work'.

-Jon
 

i.engrqasim

Member
Location
Saudi Arabia
As I stated, I am not an expert on this topic.

I did a quick look at the VFD that you mentioned, and it uses an active front end rather than a simple rectifier. This results in very low harmonics and near unity power factor.

The system also _boosts_ the input voltage to create the DC rail, which causes the VFD to draw more current. I am guessing that this could interact poorly with the generator if the VFD load is too large; a slight disturbance would cause the generator voltage to dip, which causes the VFDs to draw more current which causes the generator voltage to dip further....

Like I said I am not an expert, so this is simply another factor to consider, not a definitive 'won't work'.

-Jon
Thank you very much. That's enough knowledge to have a debate. As I mentioned, I've already requested Mitsubishi Engineer for recommendation. Now I know what to look for in his answers.
Thanks again.

qasim
 

topgone

Senior Member
It is an industrial setup (a waste water (sewage) pump station).
There is no voltage dip specified in the generator specifications, the 3% is specified voltage drop for cables, which the contractor has used.
The contractor has done calculation for a 1250 KVA. However we have a 1520KVA Generator with 1770 KVA alternator.
Voltage dip shall be in a range that may not harm the VFD's.
Please see the difference of the "voltage drop" compared to "voltage dip". Those are not the same animal. Voltage dip is very important as some equipment don't play well when the supply voltage suddenly dips during starting of big motor loads.
 

i.engrqasim

Member
Location
Saudi Arabia
Please see the difference of the "voltage drop" compared to "voltage dip". Those are not the same animal. Voltage dip is very important as some equipment don't play well when the supply voltage suddenly dips during starting of big motor loads.
We don't have large direct on line motors. It's an 8 hour emergency operation. our large motors are run by VFD's, so no voltage dip problem there.
I've read an allowed voltage dip of 30% for industrial loads, however with 1520KVA enginer and 1770 KVA alternator, I highly doubt we'll reach even 20%.

The caterpillar analysis is done for 1250 KVA and that is probably the reason for the shown 26% v dip.
 
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