Safely Starting a 10,000 HP Fan
Safely Starting a 10,000 HP Fan
(OP)
I have a problem with our 7200V system that I want to fix. Our 7200V system is split into 2 busses. While starting a 10,000 HP Induced Draft (ID) fans, we must shut the tie breaker. If we do not, we will drop our 480V power centers on undervoltage. The problem here is that with the tie breaker shut, the available short circuit current (Isc) is 113kA, and the largest-rated breaker I have been able to find is 77 kA. With the tie breaker open, available Isc is only 56kA.
I'm looking at the solution from 2 perspectives: 1. Find a way to start an ID fan without having to shut the tie breaker, or 2. Find a way to limit the Isc with the tie breaker shut.
For limiting Isc with the tie shut, I have calculated the size of CLR I would need to limit the Isc while starting the fan, and 26MVA is the minimum size. Mainly due to physical size constraints, and also due to the huge cost associated with buying 2 of these, I would rather not go this path.
For starting the fan with the tie open, I have thought that maybe a capacitor bank in parallel with the fan may help, and would be cheaper and smaller than the CLR. However, I am not sure whether shifting the power factor will actually help, and am not sure about being able to switch the bank in and out fast enough to not have a bad effect on the rest of the system (specifically the generator). I know getting a different supply transformer, with a lower impedance, will help, but the cost for this makes me not want to go that path either.
The 7200V busses are supplied from the generator through our Unit Auxiliary Transformer (UAT) which is a 40/20/20MVA, supplying 7200V bus A through X winding at 14.054% impedance, and bus B through Y winding at 14.218% impedance. The 480V power centers are all supplied from the 7200V busses through individual transformers.
Does anyone have any other suggestions to:
a. Minimize voltage drop on the 7200V busses during ID fan starting
OR
b. Limit Isc while the tie breaker is closed
I'm looking at the solution from 2 perspectives: 1. Find a way to start an ID fan without having to shut the tie breaker, or 2. Find a way to limit the Isc with the tie breaker shut.
For limiting Isc with the tie shut, I have calculated the size of CLR I would need to limit the Isc while starting the fan, and 26MVA is the minimum size. Mainly due to physical size constraints, and also due to the huge cost associated with buying 2 of these, I would rather not go this path.
For starting the fan with the tie open, I have thought that maybe a capacitor bank in parallel with the fan may help, and would be cheaper and smaller than the CLR. However, I am not sure whether shifting the power factor will actually help, and am not sure about being able to switch the bank in and out fast enough to not have a bad effect on the rest of the system (specifically the generator). I know getting a different supply transformer, with a lower impedance, will help, but the cost for this makes me not want to go that path either.
The 7200V busses are supplied from the generator through our Unit Auxiliary Transformer (UAT) which is a 40/20/20MVA, supplying 7200V bus A through X winding at 14.054% impedance, and bus B through Y winding at 14.218% impedance. The 480V power centers are all supplied from the 7200V busses through individual transformers.
Does anyone have any other suggestions to:
a. Minimize voltage drop on the 7200V busses during ID fan starting
OR
b. Limit Isc while the tie breaker is closed






RE: Safely Starting a 10,000 HP Fan
As for the motor - is it starting across the line now? A reduced voltage starter can limit the current significantly, provided you can accelerate the fan at reduced torque.
An adjustable frequency drive would definitely solve the problem, but that's going to be an expensive drive.
RE: Safely Starting a 10,000 HP Fan
A possibility is a pony motor. Perhaps you could use a smaller motor to get the fan rolling before you attempt to start it. Of course you'll only be able to get it rolling to a slow speed with the smaller motor. You can combine this with a soft starter and that might solve your problem.
Just my thoughts.
------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Safely Starting a 10,000 HP Fan
Muthu
www.edison.co.in
RE: Safely Starting a 10,000 HP Fan
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Safely Starting a 10,000 HP Fan
I would not assume automatically that the soft starter will work - a study would be needed.
I guess another question is why the 480 V power centers are fitted to trip on undervoltage and what is the setting?
What is the actual voltage drop seen at the 480 V substations? The 480 V system can tolerate voltage down to 85% or so for short durations. Below that, the motor starters start to drop out.
RE: Safely Starting a 10,000 HP Fan
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: Safely Starting a 10,000 HP Fan
What am I missing? Can you post a one-line?
RE: Safely Starting a 10,000 HP Fan
The motor is started across the line. I had thought about a soft starter before, but worried about the physical size. Anyone have an idea how big one might be?
The power center source breakers have 2 undervoltage relays. The first set at 90V with a 0.5 second time delay which will trip the source breaker and close the tie breaker (480V). The second is set at 80V with a 2 second time delay, which control UV lockout. I'm not sure how much voltage is actually seen at the power centers. It's been a long time since we tried to start an ID fan with the tie open.
We actually have 56kA at the 7200V bus, with the tie open. Once the tie goes shut, we have 111kA at the same 7200V bus. I've attached a breaker-open and breaker-closed picture from our Easypower software. The bus short circuit values are at the upper right of the bus, with symmetrical current in kA and asymmetrical current underneath that in parentheses. i.e. 70.715 (111.675)
Picture with tie closed:
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Picture with tie open:
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RE: Safely Starting a 10,000 HP Fan
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Safely Starting a 10,000 HP Fan
I'm guessing you don't have two fully-independent sources.
How are you getting > 100kA?
RE: Safely Starting a 10,000 HP Fan
RE: Safely Starting a 10,000 HP Fan
In addition to looking at the electrical supply part of the problem, could you "unload" the backpressure on the fan (with a damper or vent in the ID fan output so there is much less backpressure against the motor as it starts? Then, as soon as the electrical "trip point" has passed - when the fan has begun to spin up - close the damper/vent and let fan backpressure come up to normal.
RE: Safely Starting a 10,000 HP Fan
RE: Safely Starting a 10,000 HP Fan
If so, your fault current seems almost 3X too high to me as well. My quick calculation says the transformer would supply 24kA from both busses combined or 12kA per buss, which compares well with apowereng' calculation of 11.5kA for one buss.
RE: Safely Starting a 10,000 HP Fan
Alan