Purpose of resistor bank connected to generator? 3

[rockman7892]

I was in a facility the other day which had two identical 900kW generators at 4.16kV. I noticed above each generator there was what appeard to be a fixed resistor bank hanging from the ceiling of the room with each resisotr bank appearing to be connected to the generator output terminal box.

I was hoping that someone could explain to me the purpose for these resistor banks? I know resistor banks are used for load testing of generators but I find it hard to believe these resistor banks would be permenantly installed for such testing.

I was thinking the resistors could be either:

A) Some means of controlling real/reactive output from generator

or

B) A means of dumping regenerative or reverse power fed to the generator from the system similar to a braking resistor in a VFD?

Can someone please help explain the purpose of such resistor banks in this generator application?

 

[MikeHalloran]

Engines run better and last longer when they're heavily loaded.

When you need to bed in a new set of piston rings, you need to load the generator for a while. Resistor banks can be rented for this purpose.

However, in some installations, the guaranteed minimum house load is so small relative to the installed generator capacity that you need to artificially load the generator most of the time it's running in order to assure decent engine life. ... and in what will probably be a growing trend, to keep the exhaust gas temperature up high enough to light off the aftertreatment system.



Mike Halloran
Pembroke Pines, FL, USA

 

[davidbeach]

Are the resistors (three per generator) connected to the terminals of the generator or is the resistor (one per generator) connected to the neutral end of the generator? I'd kinda guess the latter.

 

[waross]

I would suspect either to absorb regenerated energy if connected phase to phase or a Neutral Grounding Resistor if the resistor is connected to the neutral.

However, in some installations, the guaranteed minimum house load is so small relative to the installed generator capacity that you need to artificially load the generator most of the time it's running in order to assure decent engine life. ... and in what will probably be a growing trend, to keep the exhaust gas temperature up high enough to light off the after treatment system.

Good points Mike. You may be correct. However, given the cost of fuel over the lifetime of a generator compared to the much smaller cost of the generator, artificial loading on a continuous basis is seldom cost effective. That is not to say that it hasn't been done.
As for setting the rings with a load bank, you are absolutely correct. Been there done that several times. I boiled of hundreds of gallons of water to ensure that the rings were seated on a 280 KW set.
Thanks for the information on the after treatment systems. I'll be aware of that in the future.

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

 

[KiwiMace]

In the critical facilities area, I have been in several facilities that had permanent fan-cooled load banks installed for testing of generators so that the critical load need not be transferred.

Renting load banks and installing them inside the data space is used for testing/commissioning the whole system (incl. cooling) prior to the racks being installed.

However, permanent units were always outside or at least ducted to external. Put them on the inside and you also spend money cooling them. I have never seen a load bank suspended from the ceiling/soffit. I agree they would be for something else magic with electricity, rather than dumping any significant load.

 

[LionelHutz]

My guess would be the NGR. It doesn't make sense to be dumping 100's of kW inside the building when they have easily been installed outside.

 

[waross]

rockman7892 has not told us the nature of the loads.
The classic case of a permanently installed resistor bank is a downhill conveyor belt where, once started and loaded, the load is always overhauling the motor. Cranes or drag-lines with relatively large hoisting motors are another example.
Diesel generators do not like regenerating loads. Regeneration may over-speed the gen-set.
I am sure that David Beach was thinking NGR when he asked about the possible neutral connection.
With the information that we have, NGRs may be the most logical guess. A load bank for a regenerative load may be too large to hang from the ceiling, and radiate too much heat for indoor mounting.

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

 

[rockman7892]

Thanks for the responses guys

Davidbeach

Ahh yes I totally overlooked the possibility of it being a NGR. Looking at the One-Line diagram however it shows the generator as solidly grounded.

I am going back to the site tomorrow so I will take a closer look at the resistor bank and how it is connected. I'll let you guys know what I find.

Thanks Again!

 

[rockman7892]

Waross

These generators are in a Water Treatment Plant so the majority of the loads are pumps. No conveyors or cranes that I noticed.

With pumps being the majority of the load would you think there would be a need for such a resistor bank to dump regenerative energy?

I'll let you know what I find.

 

[MiketheEngineer]

As a dumb structural - maybe to change the output voltage??

 

[EEJaime]

rockman,
If the generators operate in parallel, and these resistors were load resistors, one would think they would be connected to the common bus to which the generators parallel. I think you will find that waross is probably right and they are NGR's.
Regards,
EEJaime

 

[waross]

"I think you will find that waross is probably right and they are NGR's."
Credit where credit is due. I got the idea from David's post.
Thanks

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

 

[rockman7892]

I went back to the site for a closer look and sure enough these resistor banks were the NGR's rated 6ohm at 400A for the 4.16kV generators. I guess the fact that they were suspended from the ceiling was what was throwing me off since I'm typically used to seeing NGR's located near the ground in transformer applications.

Thanks Davidbeach for pointing this out!

 

[DTR2011]

Advise the client to update drawings!