Calculate generator capacity 1

[Shaaban Emad]

Hello
I need to calculate the 0.4k generator capacity needed to supply 6.6kv 250kw induction motor using 2.5MVA 0.4kv/6.6kv transformer with C.B .considering the in rich current of transformer and then the motor
What is the expected starting currents ?

 

[cranky108]

My standard answer is: look at the LRA number on the nameplate.

That said, does it have a nameplate, and what would be the expected voltage drop from the source to the motor?

 

[ScottyUK]

The transformer is the problem here, not the motor. Which side of the transformer is the circuit breaker? If the CB is on the high side then the generator capacity to handle the motor without severe voltage disturbance will be in the region of 1250kVA or so, IF you can directly couple the transformer to the generator and bring it up gently. The transformer is really too large to close directly onto an LV generator without causing a massive disturbance and potentially tripping the protection.

 

[waross]

Motor starting current times 3 for a standby generator current rating.
Motor starting current time 2.7 for a prime rated gen set current rating.
Or
The generator current rating = other loads plus 3 times the motor FLC.
This is the absolute minimum to start the motor. Other loads may experience noticeable voltage dips but most loads will carry through.
Most customers, when made aware of the cost of a larger gen-set will accept the voltage dip.
Failing that, several of the major gen-set manufacturers have free generator sizing software on their websites.
The software will give a conservative gen-set size.
I have over 12 Standby sets that have been in service for over 15 years.
All of the customers are happy with the performance.
Caterpillar software says that all 12 sets are too small.
Your choice will depend on your situation and customers budget.


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

 

[ScottyUK]

Bill, anyone would think Cat are trying to sell expensive generators where a cheaper one would do...

Actually I think they're just ultra-conservative so there's very little chance of any comeback for an under-sized generator being supplied based on their estimating software.

 

[catserveng]

Of course CAT wants to sell more expensive units. Sizing has always been a problem, and while some smart people can regularly figure it out, a lot of people start from "what is the cheapest unit I can get a way with". When I was doing this for a living, first thing out of my mouth for a question like the OP's is that I need way more information.

Agree with Scotty above, a 400volt unit picking up a 2500kVA transformer will likely be a problem, unless its pretty good sized, and depends on excitation system, an SE will pretty much always drop voltage more than a PMG or AREP type system.

How are you starting the motor?

What is the motor driving?

Is the generator going be driving any other loads?

More info will likely get you a better answer.

MikeL.

 

[Shaaban Emad]

Hello
Thank you all for giving a hand
I will answer your question for better conclusion
The motor is a gas turbine starting motor,
This situation is temporary because the main EDG are broken and we need to make this situation Incase of black out to to supply the GTG starting motor ,because we do not have 6.6kv generator we are trying to use the step down transformer in reverse

 

[Shaaban Emad]

The generator will be connected directly to transformer ,the transformer will be connected via CB to motor

 

[waross]

Hi Scotty. To be fair to Cat; Their software considers a default voltage dip when starting motors.
The default voltage dip is reasonable but conservative.
Many customers, especially for standby sets, are willing to accept the greater voltage dip of a smaller cheaper set.
I haven't looked at Cat software for years now. I think that you may be able to change the default voltage dip.
On aspect of a generator set is what happens when it is hit with an overload.
If the real portion of the overload is greater than the capacity of the prime mover, the set will slow down.
Once the set drops more than 3 Hz, (57 Hz or 47 Hz). the UFRO (Under Frequency Roll Off) feature of the AVR (Automatic Voltage Regulator) becomes active and drops the voltage to maintain the Volts per Hertz ratio.
This may be thought of as the poor mans version of a VFD.
Before UFRO was a feature, it was common for a mechanic to forget to turn off the AVR when doing adjustments with the set at an idle.
UFRO now has two advantages.
1> It avoids AVR burn-out at slow speeds.
2> It acts to shed the load by reducing the voltage in proportion to a frequency reduction to allow the set to better recover from momentary overloads such as motor starting.

With loads such as this, I recommend, when site conditions permit, that the transformer and motor be connected and that then the set be started. This will bring the transformer and the motor up with a good Volts per Hertz ration and avoid almost all of any starting surges.
You may have to tweak the generator controls and you may have to power an AVR with a UPS (Uninterruptible Power Supply) to ensure voltage output at low RPMs.

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

 

[waross]

I was writing as you were posting,Shaaban Emad.
This is a great application for direct connecting the transformer and the motor and then starting the gen-set.

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

 

[catserveng]

I dug thru some old turbine starting package rules of thumb that came from Duke Energy many years ago, the closet I could find to your situation came in the a recommendation at Motor kW X 2.5 for PMG gen set, Motor kW X 3 for SE genset, pretty close to Bills general rule of thumb above.

We did several temp power jobs for Duke using rental gensets to roll the turbines before permanent station power was in (was a requirement of the job). Those jobs were 480 gensets and rental 1MVA transformers supplying 4160V to the turning motor. We did have to change the undervoltage settings on the rental units to complete a turning cycle.

Hope this info helps, MikeL.

 

[Shaaban Emad]

Thank you all for your help ,based on your help this was my final feedback to my supervisor
Dear Electrical SPV
Option 1 discusses separating the grid in two portions Wafa with 3 GTGs and NC41 with 2 STGs, such separation improve Independence and increase diversity in case of one portion fail down apart from other portion .
On the other hand the risk for partial plant blackout rises by affecting correlation
Based on STGs & GTGs performance experimented power plant operation team may confirm how far STGs & GTGs can work independently.

In option 2, it was arguing to supply a GTG starting motor using temporary generator through reversing power in a 6.6/0,4kv transformer ,which could be appropriate due to unavailable of 6.6 Gen.
Considering the motor inrush current and transformer reactance ,the generator capacity kw rating= 2.5 times motor kw for PMG generators and 3 times motor kw for SE generators ,
Hence in our situate a PMG generator in range of 1.25MVA is capable to handle the motor.
Protection system shall implement to accept the voltage dips during motor acceleration if the Generator capability is in minimum.
Also TR may connected directly with Gen to bring up the transformer currents with the generator starting and overcame the TR inrush.
Regards
SR.Tech Emad Shaaban

 

[jraef]

The starting method of the motor is a consideration here as well. If it is Across the Line (DOL), the values posted above are what I’ve always used (I generally just say 3x because I rarely know the type of generator). But for a soft starter, I’ve been lucky at 2x, even though I typically tell people 2.5x. With VFDs it can be 1.5x, but there are other factors involved so it should be discussed in depth, or to be safe, use 2x on standard drives (AFE drives need more thought).


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

 

[waross]

Scotty, Jeff and Mike; How about connecting the transformer and motor before starting the generator and bringing the transformer and motor up similar to a Generator Step-Up Transformer?
That will get the set size down to about 1x plus a safety factor. The 1.5x for a VFD drive sounds good.


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

 

[ScottyUK]

Yeah, that's probably the only way of doing it Bill. A transformer that size will be challenging for an LV machine to handle. We don't see LV sets much above 2MW / 2.5MVA over here - maybe you guys have larger ones.

The other question is how does the motor drive the load. If it's a through hydraulic torque converter then the motor will effectively start unladen so it's a nice friendly load.

Don't forget to allow for the magnetising current of the transformer when sizing the generator.

 

[catserveng]

Directly connecting the transformer to the temp generator is likely the best way. On the motor connection, usually there are a number of safeties and permissives on turbine start systems, not only on the motor itself but usually things like prelubrication for gear box and such. Not sure how all that is setup but using a temp gen on a turbine starting package gets to be a real pain sometimes, so a lot of things usually need to be looked at, at least on the units I have done.

I've attached on older Selection and Sizing Guide CAT did that could be used instead of the online sizing tool. There is a later version I think but still a good resource if you take the time to read it thru.

MikeL.

 

[waross]

Good point Mike.
I suspect that when the prices of the sets for the various options are compared, along with the issues of dealing with pre-lube, permissives etc., it may look good financially to use two sets. A small set to power the auxiliary systems and the pre-lube systems and a large, direct connected set to bring up the turbine start motor.

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

 

[ScottyUK]

I think I'd be looking at how best to leave the turbine controls undisturbed and bring the MV power direct to the existing starter.

With a such a small set there's a good chance the MV contactor could clear the available fault current without the MV backup fuses, and by removing the backup fuses you could create both an isolation between generator and the switchgear main busbar and also provide a convenient tie-in point for the generator at the load side of the MV starter fuses.

 

[waross]

I like that suggestion Scotty.
You could do something similar with the control power transformer to provide independent control power.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

That's a useful link Mike - thank you.