MV vs LV backup generator for serving MV application

[rockman7892]

When considering a standy backup generator to tie into existing 4.16kV Switchgear lineup (through a breaker in lineup) would there be any reason to provide a 480V generator and step-up voltage through transformer to the 4.16kV utilization voltage at the switchgear? I would think that providing a 4.16kV rated genset would be the best approach but not sure how the economics may play into this application when comparing LV vs MV gensets along with transformer. Genset for this application is 1.5MW.

 

[jraef]

Permanently connected or portable?
A key issue is the level of expertise of available electricians if something goes awry right when you need the generator the most. Are there MV qualified personnel available? If not, I would suggest 480V.


" 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]

The added cost of a 1.5 MW transformer may be a factor.
As for the choice between a 480 Volt generator and a 4160 Volt generator, cost and deliver times may also be factors.

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

 

[RRaghunath]

Do you have a motor at 4.16kV that this generator is expected to start. If yes, it may be appropriate to go in for MV generator.
With LV generator, the LV-to-MV transformer could make the motor start difficult (to be checked out)!

 

[waross]

I have to disagree on this.
In many cases the transformer will act as a series impedance and reduce the starting current, making the motor easier to start.

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

 

[rockman7892]

This will actually be a permanent installation that will start as a temporary installation. Due to permitting requirements the first year will require customer to take generator in and out of location on a temporary basis and only connect when needed. After about a year when permitting is completed the generator will be able to be permanently installed.

The idea here would be to purchase gen-set that can be trailer mounted to move in and out the first year and then set on foundation when it comes time for permanent installation. The first year would utilize some sort of quick connection means for connecting.

Site currently has other MV equipment so site personnel are familiar with MV maintenance practices. This is for a generation station to provide backup power when units are down so this will likely not be starting any large motors and will just be serving auxiliary power.

This auxiliary power is currently fed into and existing MV lineup via a utility fed backup transformer and the customer wishes to add a gen-set as an alternative to this backup utility feed (due to costs). The existing protection relay associated with the breaker in the switchgear currently is used for sensing switchgear normal power and current backup source to control interlocking and switching of backup feed (controlled manually via control room).

I'm also trying to consider how we handle the necessary controls, alarms, etc... from the gen-set in a situation where it is a temporary installation. In all likelihood it would just be a situation where they would have to manually override all controls, alarms, etc... Assuming generator would have onboard breaker and protective functions for LV (For MV application would likely need to have protective gen relay to trip to external existing breaker?)

 

[waross]

Check with Cat. They may have a suitable rental unit already installed in a trailer, ready to go. You may be able to rent for a year until the permanent set is installed and commissioned.

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

 

[JASGripen]

Hi Bill, your posts are great.
You mentioned a very interesting datum earlier on the transformer acting as a series impedance for motor starting.
Would this in any way shorten the life of the transformer if used solely for this function?

 

[waross]

Heat is the transformers enemy.
For every 10 degrees C above the rated temperature, the transformers life is cut in half.
So if you overload a transformer 24/7 to the point where it is above the rated temperature it will have 50% of the rated life.
However, if it is run at that temperature for 10 hours, the life expectancy will be shortened by 10/2 = 5 hours.

Now let's look at an overload suddenly applied to a transformer.
A transformer may have a thermal time constant of between 3 and 5 minutes. (Time constant: The time for a measured value to reach 63% of the value causing the change.)(Memory is failing but I seem to remember the exact factor as one minus the reciprocal of the base of natural logarithms. Just use 63%.)
For more and more accurate, Google "Time constant".
It is accepted that for almost all purposes, terminal temperature will be reached in about 5 time constants.
Old mechanical demand meters were designed to mirror transformer heating and two standard windows were 15 minutes and 30 minutes.

Now if your transformer is fully loaded and at maximum temperature you should not be starting a large motor.
Any motor that take more than about 10 seconds to get up to speed, should be started with a reduced current starting method.

So 10 seconds out of 3 minutes(One time constant, the temperature will have reached 63% of the final temperature.)
Most motor starts will not affect the life of the transformer. Some extremely difficult starts may take a few seconds off of the life expectancy.
How much impedance will the transformer add to the starting circuit?
The impedance and voltage drop will be more than that calculated based on the transformer regulation.
The impedance and voltage drop will be less than that calculated based on the transformer impedance voltage.

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

 

[itsmoked]

Well it wouldn't be solely used this way! And, no not perceptibly because they have a large thermal mass and a starting evolution is quite short - a few seconds.

Helluva a cross post. LOL

Keith Cress
kcress -

http://www.flaminsystems.com