Minimum Size Diesel Generator to Start 900 HP Induction Motor 4

[gobigorgohome]

An existing centrifugal water pump requires 3458 lb-ft of torque at 100% speed (1182 rpm)

It's WR^2 is 215 lb-ft^2

It is presently operated by a 900 HP, solid shaft, 1185 rpm (6pole) vertical induction motor. 4000V, 3 phase, SF 1.15

Present starting method, across the line, utility.

Proposed as a an emergency backup, to be operated from a blackout condition, assuming it was possible to use a transfer switch to directly connect the motor to the output of a diesel genset dedicated to this application.

Assume no other base load. The 900 HP motor is the only load.

What would the minimum size of alternator and diesel driver that could bring this load up to speed, assuming no limit of accelleration time, except for motor / alternator thermal damage curves.

Assume worst case would also be that the motor was running prior to blackout and already at operating temperature.

The torque starting characteristics of the pump are; %of full load speed : percent of full load torque.

.00:.08
.10:.02
.20:.04
.30:.09
.40:.17
.50:.25
.60:.37
.70:.48
.80:.65
.90:.83
1.0:1.0

Somewhere between a square and cubic ratio. (Centrifugal Pump).

Any thoughts about normally aspirated vs. turbo diesel?

It would be logical to have this robust enough to exercise the engine once per month, but that could be done with a warm engine, or perhaps syncing to the utility.

Probability of this needing to work from a blackout would be once every 10 years.

 

[davidbeach]

Gen set manufacturers provide free sizing software for just that purpose.

 

[itsmoked]

If it's that dedicated I'd play some games. I'd start the generator and warm it up. Shut it down. Immediately operate the transfer switch. Close the breaker and restart the generator. Least expensive by a great deal I suspect. It would reduce the generator size by a large amount. Nothing would be stressed electrical or mechanical.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If you can connect the motor and start the generator with the motor connected, letting the voltage and frequency ramp up together, you can get by with a generator sized to the motor with a small safety factor.
This may take some creative control modifications. You may have to power the AVR with a UPS and let the Under Frequency Roll Off feature ramp the voltage up with the frequency. It may work fine as it is.
My rule of thumb for a dedicated motor generator combination with DOL starting is a minimum of 250%.
If there are other loads that may be affected by voltage drop, I use 300% of the motor rating for the required capacity.
Turbos are more fuel efficient and often cheaper but there is a slight delay as the turbo spools up when the set is block loaded.
The Permanent Magnet Generator option holds the voltage up better during severe block loading, but check the characteristics at low frequencies before using this option on a direct connected installation.
When you can reduce the generator size from 300% to 110% or 125% and start the motor with much less stress on the motor and generator, it may be worth while to pursue the direct connect option.
And, with the direct connect option the turbo should have no problems spooling up as the load increases, I would recommend the turbo option with this configuration.

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

 

[gobigorgohome]

Thanks waross.

You have added valuable feedback to what I suspect would work. 110 - 125% would make a very efficient system.

Have you seen or know of any examples of this configuration that are documented, any related technical articles or units presently in operation?

I have read a couple of stories on this site, but may now want to find some more tangible reference material.

Thanks.

 

[waross]

I don't have any links. Possibly some other posters may help. I understand that a couple of our regulars have hands on experience with this technique.
Be sure to add the service factor BEFORE adding the safety margin.
Consider the motor to be 900 x 1.15 HP or 1035 HP and add a safety percentage.
For the value of such an installation, the vendors should be able to provide assistance. If not, try another brand.

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

 

[itsmoked]

You will have difficulty finding documentation on this as it represents a substantial loss of revenue to generator suppliers over the 'standard'. They will likely grit their teeth over this.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Good point Keith. But we are still looking at around one megawatt. The supplier would rather sell a 3 MW, but they may rather sell a 1 MW than see the competition sell it. It will be worth the challenge to find an application engineer willing and able to help. Your observation is certainly part of the challenge.

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

 

[catserveng]

I did as suggested by Bill a few times supplying water and sewage pumps during outages with rental units marginally sized.

A few of the challenges, a transfer switch is out, the transfer controls look for good source before transfering, bypassing those has challenges of their own. You may consider a couple of circuit breakers with appropriately designed controls and proper interlocking to prevent safety issues.

Most new AVR's (Automatic Voltage Regulators), mainly the digital ones, see ramping up with load as an "abnormal" condition and can fault, and most current manufacturers I'm familiar with get concerned when you start changing their default settings and protections.

Also some newer engine electronic controls with their built-in protections for generator sets can get confused by adding load below rated speed, and default protections on those may need to be bypassed.

The motor controls themselves may have protections that will need to be bypassed when starting by this method as well, and we ran into some stiff resistance from the motor vendors at one site when using this starting method, simply because it wasn't "normal".

It can be done, we had six 900 HP pumps running off of 6 1 MW rental sets, direct connected and started as described by Bill above, but we had to make a lot of system changes to breakers settings, protective relays and generator protection to make it work with units coming on and off for about three weeks. And we had our manufacturers tech reps, the pump motor tech reps, and the staff at the pump station all resisting the effort because they had not seen it done and it wasn't "right".

For a permanently installed systems there may be some other concerns as well, you may want to check with your AHJ and local engineers to get their take on that.

Also my experience was with rental gen sets that were pretty beefy, 12 cylinder units that would produce 1250kW standby and 1000kW prime, with very robust permanent magnet tail ends and aftermarket Basler AVR's. In my opinion newer model generator packages are not as robust, especially on the tail end excitation systems and AVR's.

Hope that helps.

 

[waross]

I was hoping you would check in, catserveng. Purple star for sure.
How does the cost of a 1250 kW set compare with the cost of a 2500 kW or 3000 kW set?

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

 

[gobigorgohome]

I am really happy to have all this feedback. catserveng has provided pretty much the exact feedback that I was hoping (expecting) to hear. The skiddish reaction of various parties echo the same things I am hearing.

I thought about the transfer system as well, and agree that it will be rather unique and likely custom. I'm okay with that since I am P.eng.

I am thinking about setting up a Multilin 369 with the proper themal model for motor protection. Not at all sure about the generator control end, I suspect that detail conversation with a manufacturer will be in order.

Permanent magnet rotors for generators; I am familiar with large generators and the DC excitors for them, induction machine generators for wind and other, but I was not aware of permanent magnet generators in the 1MW range. To be fair I haven't gone looking, but it sounds like these are "commonplace" or at least readily available? Any statistics about market share of permanent magnet generators vs. induction, slip ring units or self exciting?

 

[waross]

The PMGs we are referring to are part of the excitation system, not the main generator.
An AVR needs a power supply as well as a connection to sense the voltage that is being controlled. A basic AVR uses the generated voltage to supply power. The AVR bootstraps on the small residual voltage of the main generator. Often the machine is at or near full speed before the field and the output come up.

The PMG option is a small generator on the back end of the generator shaft that provides about 220 Volts 3 Ph. to power the AVR.
You may consider a UPS to simplify the transfer switch operation, and if you have issues getting a PMG equipped machine, you may consider powering the AVR with a UPS so as to get a better Volts/Hertz ramp.
Are direct driven diesel powered pumps still available in these sizes? The target for the starting load on the engine will be almost exactly the same as the normal starting load on a direct driven pump.
Ask about engine control packages for direct driven pumps if the normal modern generator controls don't like this service.
What we are doing is assembling a mechanical Variable Frequency Drive and the starting duty is much easier on the engine, generator, motor, and pump than Direct On Line starting.
P.S You can also vary the pump output by varying the speed of the genset, similar to an electronic VFD.

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

 

[catserveng]

PMG equipped tail ends are still pretty common in the 1MW range, although some manufacturers have gone to auxiliary winding in the stator, CAT calls the system AREP, Marathon also does it but can't recall their trade name for it. PM excitation systems are pretty common at 1MW and above, and for prime or industrial service (like rock crushing, materials, or utility service) I've started up units with PM's down to 300 ekW.

KATO and Baldor make Oil Field duty tail ends that I have a great deal of respect for, and would probably be better suited for what you're trying to do rather than a "standard" tail end offered by most genset manufacturers on standby rated machines.

I would also recommend a Basler SSR AVR or similar, since most newer gensets use digital regulators they may have problems in ramping up in load like you're trying to do. They are a bit pricey though, but real workhorses in my opinion.

I have done a fair number of mechanical drive pumps in these sizes and larger with both natural gas and diesel engines. But emissions laws where I live has put a damper on that, although the main sewage lift station for San Diego has two 2500 hp natural gas lean burn engines each driving large sewage lift pumps almost continuously, and a couple of the water districts here also have gas engine driven pumps from 300 to 1000 hp in some areas of the county. The cost of the right angle drives seems to be a big issue these days on the larger units.

Hope that helps, Mike

 

[rbulsara]

You have a couple of choices, do a lot of math and take a chance with bare bone design or do the math and still take a conservative approach.

Refer to my FAQ faq237-766 on the subject.