2200 HP soft start for fan ? crisis 1

[Tmoose]

We have several 2200 HP direct drive fans. They use a hydraulic coupling to let the motor go right to full speed, then bring the fan up to speed over a 3 minute period at ~ constant torque.

We have hit a major lack of spare parts problem with one of the couplings. The pressure is immense to get the fan back on line quickly in order to run ~ 6 months.

The Fan manufacturer says Direct coupling would require ~ 4000 HP motor.

ALL alternatives are being considered.
Short term generator and engine rental, giant soft start devices, 4000 HP used motors.

Dan T

 

[waross]

I'm a generator kind of a guy. I like the generator option.
There is probably a wider market and better rental prices for a genset than for the other options.
But, you may be able to rent a used motor short term from a used equipment dealer.
Direct connecting the motor to the genset before starting the generator and bringing the genset and motor up together to lessen starting stresses.
You may be able to bring the existing motor up to speed by direct connecting to a genset before starting the genset.
You may have to use manual field control or an external source of power to the AVR (Automatic Voltage Regulator).
You will be using the generator as a defacto VFD. The speed will determine the frequency and the UFRO feature of a modern AVR will maintain the V/H ratio.
I would locate a suitable used motor so that I had backup available but first rent a genset and try to start the existing motor. You may be back online quickly and at the best price.

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

 

[jraef]

A soft starter might work with the 4000HP option, but would not likely work with the existing 2200HP motors if it takes 3 minutes to accelerate,. You would run smack up against the thermal damage curve of that motor long before you got the fan to full speed. However, do you have the power for a 4000HP soft starter? Assume 450% current for 30 to 60 seconds; that's a lot of strain on an electrical system.

I'm a VFD guy myself, they can accelerate that motor in whatever time frame is necessary, so they can fully replace the fluid coupling. If possible, you could even have one VFD and some clever switchgear arrangements to "recycle" the VFD for each motor. But to be honest you will be hard pressed to get a VFD that quickly right now. Most manufacturers of MV VFDs are in full production just to keep up with demand, deliveries are typically longer than 6 months.

So I think waross' idea may be your best bet for success in that short of a time frame. I saw something similar done on a 6000HP 6600V motor once for an LNG pump system. Worked like a charm, although the generator rental technician on-hand for the startup was a little nervous (they hadn't fully disclosed how they were intending on using the gensets when they rented them).

 

[itsmoked]

Nor would I!
lol

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Gianoli]

Give us a bit more info. Might have some ideas.

A. Motor type. Induction-squirrel cage, Induction-wound rotor, Synchronous,???
B. Motor voltage. 4160, 6600, 13,200,???
C. Is there any possibility of getting the motor and fan to turn slowly before energizing the motor?
D. Is this an ID and FD type of fan setup?

 

[roydm]

Can you reduce the starting load i.e. by closing a damper, perhaps that may allow you to get running DOL.
What about an air clutch, similar to that used on a ball mill with Synchronous motor?
We need more info as Gianoli requests.
Good luck
Roy

 

[Tmoose]

The Primary Air fan inertia is about 50 X the motor inertia.
The originally closed damper is just cracked open to reduce very unpleasant starting flow characteristics.

6000 something volts.
not synchronous. Don't know rotor construction.
The motor mfr estimates the fan would have to be spun ~ 1/2 speed (~1200/2=600 rpm) to allow across-the-lines starting.

Fan owner is looking into air actuated/deactuated clutch.

 

[Gianoli]

I wrote this plan up but now I hesitant about posting it because I have some safety concerns. Specifically surrounding the motor contactor or breaker. However, I would like to post it for discussion purposes only. Would it be worth a try? Would it work? Is it safe? 20 years ago we did this kind of stuff. I don’t think I would do it today.






OK, here is a crazy idea, take it for what it is worth. I am sure it will draw some criticism.

· Motor voltage is likely 6600 volts. System voltage is likely 6900 volts.

· Motor is probably a 6-pole squirrel cage induction motor.

· Install solid coupling between motor and pump or lock up current defective coupling.

· Build small temporary hoarding around motor and try to insulate it with some construction blankets or temporary insulation.

· Get some air conditioners and try to get the motor as cold as you can prior to starting. Let it sit for a day when the temperature reaches it coldest point to ensure the stator winding and the rotor are as cold as possible

· Ask electrical folks if it is possible to raise the bus voltage on the segment of the electrical system feeding the 6600-volt motor. If the supply transformer has a tap changer they should be able to do this. If no tap changer is available they may have capacitor banks they can bring on line, which will raise the voltage temporarily. This is critical to the plan. The starting and running torque of an AC induction motor will vary as the square of the voltage. In general, a 10% voltage increase will get you a 21% torque increase and reduce the starting current 10%-12%. Try to get the bus voltage to about 7500 volts.

· If the motor has stator winding RTD’s set up a way in which you can monitor them remotely during start up.

· Modify the protective relay settings to allow for a longer acceleration time.

· Start motor and fan while monitoring stator-winding temperatures. Abort start if stator temperatures become excessive.

I must qualify the above plan by stating that you will need to have a qualified electrical person on board to carry this out. Raising the bus voltage 10% is not a great idea in general however; if it can be raised temporarily and the electrical system is in good condition it should not cause any long-term damage. The motor stator winding can be monitored with RTD’s, which I would think would be installed on most 6.6 kV motors. The motor rotor cannot. A plan like this could reduce the life of the motor. DO NOT monitor the stator winding temperatures from the front of the motor contactor or breaker door if that is the location of the protective relay. If you have to abort the start the motor contactor or breaker will have to interrupt starting current at a higher than normal voltage. No one should be any were near this unit during the starting period.

 

[edison123]

In general, a 10% voltage increase will get you a 21% torque increase and reduce the starting current 10%-12%. Try to get the bus voltage to about 7500 volts.

You are wrong about drop in current with increase in voltage. We use soft starters, vfd's, star/delta starters etc. to reduce the suppy voltage and inrush current. Also, at the voltage levels you mention, the core saturation will drive the current way too high increasing both the core loss heat and the copper loss heat.

I would go by waross' suggestion of a cold start DG along with the motor. About 2 months back, I suggested this to a client of mine, who wanted to start 6.6 KV, 700 KW pump for a desalination plant for an initial start up, where the local utility did not extend the supply in time. With a hired 415 V, 1000 KVA DG and with a 6.6 KV/415 V, 1.5 MVA back-fed trafo, the initial trial run was successfully completed.

 

[Gianoli]

edison123,
You are right. My mistake.

In general, 10% increase in voltage also results in a 10% INCREASE in starting current. However, it does result in a 7% DECREASE in full load current and a 3C to 4C drop in temperature rise. This according the table I am looking at (Standard Handbook For Electrical Engineers, 12 Edition, Table 20-6.)

Thanks.

 

[ruggedscot]

Liked the tranny and DG 1MVA set up sounds kinda cool. Did you run from genny or switch over to utility once upto speed?

 

[edison123]

Gianoli
That decrease in running current with increased voltage would depend on the level of saturation per original design. These MV machines are custom-built and are generally running close to saturation at the nameplate voltage. Also, having a higher voltage during normal running (which will be a very hard thing to do in a network) raises the risk of insulation failure and definitely lowers its useful life.

ruggedscot

The genny-tranny combo was used to complete the entire pre-commissioning run of the pump since the site had only temporary construction power during that phase. As fas I know, that site is still awaiting the utility supply (which is government owned which explains the inefficiency and the lethargy)and the entire desal project is behind time.

Of course, in OP's case, he would have to change over to his regular supply once the motor gets to full speed (unless "the management" is willing to pony up for the continuous hiring of DG.

 

[itsmoked]

Kind of interesting as I bet a full power DG set would be less expensive than a partial power SS for that beast and would leave a large load of the entire power network. Course don't ask about air quality, then again if there is a need for hot water.. But I digress.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Tmoose;
Let us know the final outcome. If you need to use two gen-sets, let us know. We can probably come up with a few tricks to simplify synchronization and control.
I would suggest starting with one set connected to the motor and accelerate as much as possible. Leave the controls set for 60Hz. The fuel limit should limit any over current to an acceptable level for the time of the start. The dead rack adjustment may be set back to limit the overload at full throttle if needed.
Now the second set should be running already. Set the speed down and sync it with the first set. Set the speed to 60 Hz.
When the fan is up to speed, sync with the utility power.
Last, set the frequency down on the gen-sets to shed most of the load and drop out the gen-set breakers.
This should avoid a lot of scampering and surprises trying to start two sets in sync and then trying to advance both throttles.
Any comments or further suggestions, itsmoked, skogs, jraef, edison, rugged?

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

 

[edison123]

waross

I would skip the synchronization-with-the-utility part, which is going to be a b..ch. Take the motor up to full speed (you could even go a little beyond 60 Hz to get a running speed above the rated), trip the gen breaker. After a 2-3 second delay, switch in the regular utility supply on the fly since the motor speed would not have dropped significantly in 2-3 secs.

The 2-3 sec delay is to allow the flux (from gen supply) to die down.

 

[waross]

If you haven't done much synchronizing it is frightening. When you have done a few, it is not that bad. At full load, the sets will stay in sync. You can drop the speed setting on one set a little and then drop the other a little. The load will shift between sets a little, and the droop will take the frequency down on both sets when either set is adjusted.
I would expect big sets that are well loaded to behave very well. It's the small lightly loaded sets that get flighty and can be hard to sync. A synchro-scope would be nice, but voltmeters or light bulbs may be used.

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

 

[waross]

But, hey, an open transition will work if the fan doesn't slow down too fast.

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

 

[itsmoked]

I doubt THAT fan will do anything 'fast'.

I would love to see either method applied. I suspect edison's method as being a little more fumble proof.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

The bigger question is what happens when the running motor stops again due to whatever reason.

 

[waross]

I went in to a small plant with a mix of 350 kW and 600 Kw diesel sets. Due to a wiring error when the plant was installed every synchronization was done 30 degrees out of sync. This had been going on for years. If they were just a little early, they would shear a drive key. Happened once or twice a year.
Have you heard the saying?
"Close only counts in horseshoes and hand grenades!"
I would add synchronization.

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