DC injection heating 3

[DmanD]

Hi,
With 2 of our alternators down with IR value of 1M? it is a precarious condition for us. Any one of the working alternators gets broken down then it would be disastor.
The heating with sodium vapour lamps and heat radiators is proving to be very slow.
I need your help with the procedure to follow for the DC INJECTION HEATING.
Ours being a seashore facility the problem of low IR is very rampant. It is surprising that no one here has tried it till now.

Thanks in advance,
D'man

 

[jraef]

There are pre-made winding heaters on the market. They are UL listed and ready to go, makes the process a lot easier.

[url=http://www.ab.com/en/epub/catalogs/12768/229240/229258/229579/]AB Bul. 1410

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

DC heating is very much a thing to test and try. Since it is used at stillstand, there is not much natural cooling and the temperatures can reach unhealthy levels. I have found that most coils can take about 10 percent of rated wattage with no risk. Conduction cooling suffices. Start at that level and be prepared to adjust voltage if becessary.

Have used that on generators, DC motors and little HV transformers in humid environments.

Last Sunday, there was a drive problem in a steel work and I had to go there to help the guys. They had a DC drive and thyristors in it were non-functional. They changed thyristors and fuses and still problem with fuses and thyristors. "Everything" had been tried and as a last measure, they had changed the DC motor.

The spare motor had 9 Mohms in the armature, no problems. But only 180 kohms in the field winding. We checked with AC and found no big problems and after half an hour the IR was up a bit. So we decided to let the motor stay. Worked for 1,5 days and then field winding burned.

Conclusion: Not all bad insulation levels can be cured by heating.

Gunnar Englund

http://www.gke.org

 

[DmanD]

Hi thanks for the input. I had the idea that there must be some rules like the ones followed for transformer heating. OK, I will go by the rule of 10% rated wattage. What to do with the connections of shaft mounted diode rectifier? They have to be isolated isn't it?

The IR should actually go down initially and after a steady phase it should rise and be steady at a value. Like an inverted trapezoid that happens with the transformer.

Thanks
D'man

 

[edison123]

DC injection heating involves lotsa wiring up and then with one kit, you can heat up either stator or rotor.

A quicker way to improve IR and PI would be to put a solid short across the stator terminals and run the machine with incremental increases in excitation (manual control of exciter circuit is a must). I follow thumb rule of 10% increase in stator current every 2 hours with maximum of 80 to 90% of stator current with a winding temperature not exceeding 90 deg C by RTD's. This would dry out the stator & rotor together in a very short time of 10 to 12 hours.

Be sure to vent the machine (if closed air cooled) once in 2-3 hours to vent the moisture to the atmosphere.



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[DmanD]

Thats nice Edison, do you run the alternator at full speed or less than that. What method are you using to control the exciter current? The time period given by you is exactly what we desire.

D'man

 

[edison123]

The speed will be normally full speed since the prime mover (steam turbine, IC engine, water turbine) will dictate it.

Reg exciter manual control, I need more info on excitation system - whether is brushless or static or dc exciter.

Also, the nameplate of your machine would help.

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[aolalde]

Good recommendation Edison.

I have seen that method for drying hydro-generators. The procedure is the same as that for "short-circuit saturation Curve" from IEEE Std 115 part 4. The voltage regulator is replaced by an adjustable DC power supply. The field current (rotor) is lower than the full load excitation but the stator could handle Full load current if the cooling system is operated.

 

[waross]

I have used that system with a battery charger (12 volts)on the field leads.
This gave us about 50% of rated current through the windings of a 350 kw alternator. Just short the load terminals of the stator (Leaving some space for a clamp-on ammeter).
apply the DC source or battery charger to the field leads.
It took us a little longer than 12 hours, maybe 20 or 30 hours, but we didn't have much to work with, and we had a lot of other hurricane damage to repair and didn't have time to build a better DC supply.
respectfully

 

[edison123]

Thx aolalde and hritiman.

The short circuit ratio of the machine determines the field current required to pass the stator short circuit current.

Short Circuit Ratio = Field current to produce rated stator voltage on open circuit / Field current to produce the rated stator current on short circuit.

This ratio is 0.5 - 0.6 for cylindrical rotors (steam and gas turbines) and 1.0 - 1.5 for salient pole rotors (diesels and hydros).


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[DmanD]

Hi,
It was a hectic week for me. I have gathered details of the alternator in question.
Make: Stamford HCI
1500rpm
670kVA
400/231V
732A
50Hz
Exciter Voltage=43V
Exciter Current=2.5A

It has a DC exciter. The alternator is coupled with a Cummins engine.

Thanks
D'man

 

[edison123]

D'man,

Are you sure it is a dc exciter ? Your exciter values indicate an ac exciter with rotating rectifier.

Anyway, disconnect the exciter field (ac or dc exciter doesn't matter) from the AVR circuit. Connect a 48 V, variable dc voltage source - batteries with rheostats in series (wasteful and dependent on amp-hours of the batteries) - or a 3 phase bridge rectifier with a variac controlled ac input.

Bolt a short (50x6 mm copper) across the phase terminals and put a clamp-on ammeter or use the original CT's.

Start the set (is it a diesel ?). Energize the exciter field thru' your variable DC source and adjust the field current till you read 10% of the stator current i.e 73 Amps. Maintain the current till the winding temperatures stabilize (use the winding RTD's). By varying your dc voltage to the exciter field, increase the stator current in steps of 10% after each temp stabilization.

90% stator current or 90 deg C winding temp is the limit for dry-out. Depending on the absorbed moisture, the dry-out may take 6 to 24 hours.

If it is a closed machine, be sure to vent the machine every two hours to let the moisture vapor out.

Good luck.

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[edison123]

BTW, 1 Megohm (your first post) for a 400 V machine ain't that bad.

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[waross]

Good advice edison123.
I would use the same procedure.
The 350 kw set that I mentioned had been badly contaminated with salt water during hurricane Mitch. There was a a white high water mark (from the salt content of the water) inside the junction box.
We dismantled the machine and washed it for several hours with fresh rain water.
We could not get a megger reading but we could read a low resistance insulation value with a multi meter. The value changed depending on the polarity of the multimeter connection. We could also read a very small voltage from the windings to ground. I took this as an indication that the insulation was in such bad shape that the iron and copper were using copper salts in the water/moisture to act as a battery.
Using basically the same method as proposed by edison123 we were able to bring the insulation resistance up to 6 or 7 megohms and put the generator back in service.
Follow edison123's advice.
respectfully

 

[DmanD]

Dear Edison123,
You have explained the process in a very clear way. ACCOLADES for you.
One small doubt is that we have got a single phase supply near the place. What are the disadvantages of using that?

We have two newer alternators of 505kVA rating but the exciter is shaft mounted with rotating rectifier diodes. In near future we might need to undertake same procedure(judging the weather). How to control the excitation voltage?

Thanks
D'man

 

[waross]

Use the single phase supply to energize the DC power supply (Which I hope you have to provide DC current to the exciter field.).
Same answer. Apply DC to the exciter field. Adjust the value of the DC voltage depending on the current in the main windings of the alternator.
respectfully

 

[edison123]

D'man

If you have single phase, then you need single phase bridge rectifier which would require higher ac input current.

My suggestion would be (since you seem to have many generators) to have a 3 phase variac (stepless auto transformer) with a 3 phase bride rectifier. The whole system should be mobile so that you can move from generator to generator. Locating a 3 phase supply in your system shouldn't be a problem.

If you can post the name-plate details of all your generators here, then I could possibly suggest one variable DC system that could probably fit all the generators.

(Excuse me for the lotsa legalese used here - probably / possibly ... blah, blah, blah). I am going thru' the phase of reading so called courtroom thrillers.


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[DmanD]

Two types of alternators are available with us here - 670kVA and 551kVA. The exciter ratings are very close 43V,2.5A and 45V,2.5A. respectively. Further two new ones have arrived with rated output of 505kVA. All the ratings are alike varying 1 or 2 units max.
If you require Edison I will post the complete nameplate details within two days.

Thanks a lot
D'man

 

[edison123]

D'man

Assuming all your generator exciter fields are within 50 V/5 A and you have 230 V single phase supply, the bill of materials will be (starting from 230 V input)

For Exciter Field Cicuit (after disconnecting AVR cables)
230 V, 5 A wall switchable socket or extension cable

0-230 V, 5 A stepless auto transformer (also called as dimmerstat or variac)

50 V, 5 A Single phase rectifier

100 V, 5 A DC Duty MCB

0-5A DC ammeter

5 sq mm copper cables to connect up the above


For Generator Stator
50 x 10 mm bolt-on copper short for stator phase terminals

1000 A CT or Clamp-on ammeter

 

[edison123]

And some method of measuring stator winding temperature

Non-contact IR thermometer

or Embedded winding RTD's with digital temp indicator