HI GUYS I HAVE JUST RUN INTO AN UN

[NEWFIEBOY]

HI GUYS
I HAVE JUST RUN INTO AN UNUSUAL ELECTRICAL PROBLEM ON SHIP.
THIS SHIP HAS 4 230 KW 50 HZ 380 VOLT GENERATOR SETS. 2 OF THESE ARE DRIVEN BY THIER OWN DIESEL ENGINE, THE OTHER 2 ARE DRIVEN BY THE SHIPS MAIN ENGINES VIA A POWER TAKE OFF FROM THE GEARBOX. THE PROBLEM I ENCOUNTERED WAS THAT EVERY MOTOR STARTER HAD SIMULTANEOUSLY BURNED UP THE CONTROL TRANSFORMER. THE CONFUSING THING IS THAT ONLY THE CONTROL TRANSFORMERS WERE AFFECTED. THE FLORESCENT LIGHTING, HID LIGHTING, APPLIANCES AND ELECTRONIC EQUIPMENT WERE NOT DAMAGED. MY QUESTION IS HAS ANYBODY SEEN THIS TYPE OF PROBLEM BEFORE? WHAT SORT OF EVENT COULD CAUSE THIS SORT OF THING TO OCCUR?

 

[electricpete]

there was a discussion recently on the subject of starter coil behavior.

When the coil drops out due to low voltage, the current draw increases dramatically. Most controllers have an aux A contact in series with the coil to deenergize it when it drops out.

If your controller does not have such a feature and you experienced low enough voltage to drop out the coils, that could cause high very current draw on all the starter coils.

 

[dydt]

hi..
how much current does these motor starters draw?
what is the size of your control transformer?
did you sized up your control transformer fuses correctly?

BTW, i heard New Foundland/Labrador is a very nice place..friendly people..


cheers
dydt

 

[busbar]

Long shot — Is the 380V system run ungrounded? In many cases OEM control-power transformers turn out to be the weak link with primary-winding-to-ground failures, where operated on ungrounded 480V systems.

 

[NEWFIEBOY]

electricpete.
None of the starter coils on this ship had any low voltage protection. A low voltage problem was one of the first causes of the damage we came up with. We took a good control transformer and hooked it up on the work bench. We then subjected it to a variety of voltages from 0 volts all the way up to twice the rated primary voltage. Nothing we did would cause the transformer to burn. If I understand you correctly, you are saying is that a low voltage on the secondary would cause the contactor coil to draw excess current and overload the transformer. I will have to simulate your theory and see what happens.

dydt
The motors on this ship which were affected ranged from 3 hp up to about 30 hp. The control transformers in the starters were mostly 380/120/50va. The largest transformer we replaced was 380/220/1000va. This last unit was not powering a contactor coil, it was being used to supply power to a 500 watt 220 volt halogen floodlight. The fuses for transformer protection seemed high to me, we found 6 amp fuses protecting 50va transformers on the 380 volt primary side.
Your information about Newfoundland is correct it is a very nice place and the people are friendly. How did you hear about Newfoundland?

busbar
Yes the 380 volt system is an ungrounded system. The lights and plugs are operating from a dedicated transformer bank hooked up in a 3 ph 220 volt delta - Original European system. There is a MG set which is suppling 480 volt 60 hz power. This power is being used to operate North American type equipment which has replaced some of the original equipment. We had no problems with anything on the 60 HZ power. As for the grounding, we found that about half of the control transformers had ground wires attached to the secondary side. This did not make a difference since we had burnt transfromers with and without ground wires attached.
The main 380 volt power does have a ground detection system which consists of 3 pilot lights connected in series and the midpoint connected to ground via a NC bush button. When we arrived on the ship we found that the ground detection indicated that there were ground faults. All the ground faults were cleared once we replaced the burnt transformers.

 

[electricpete]

newfieboy - yes you understood my theory correctly. There is no doubt that a dropped-out contactor coil will draw much more current than a pulled-in coil. If you didn't have the starter coil energized during your simulation it would not cause any problem. With your scenario all four transformers each feeding a starter coil burning up, this seems like a very likely cause to me.

 

[busbar]

NB — Secondary control-power-transformer grounding is the US accepted norm, but will not significantly change the susceptibility of primary-to-ground insulation failure.

The problem of grossly oversized [or nonexistent] primary-side fuses in smaller starters is routine mid continent, and yes, lack of same makes for unforgetttably pungent burnt-enamel fragrance and nice starter-cubicle fires.

Higher-wattage lamps in the ground detector(s) may help.

 

[gigahertz]

My bet would be overvoltage at non-fundamental frequencies.

Do you have IGBT VFAC drives?

http://www.mtecorp.com/solving.html

You might also check:

http://www.groupeschneider.com/en/pdf/ect190.pdf

 

[electricpete]

My bet is still on contactor coils remaining energized after dropping out on undervoltage.

It doesn't seem likely that harmonics, spikes, ferroresonance etc would attack all 4 starters simultaneously and not damage anything else.

On the other hand, if we had a simple undervoltage condition below coil dropout voltage, then burning up of the starter coils will be the expected result (no undervoltage protection was provided). And other electronic equipment would not be damaged.

 

[NEWFIEBOY]

gigahertz

No, there are no vfac drives. This ship is about 20 years old and was built before solid state ac control devices became popular.

electricpete
There were more than 20 motor starters affected simultaneously. Only the control transformers were damaged. No contactor coils had to be replaced. Most of the control transformers were 50 va and probably sized at slightly larger than the coil current requirement. In other starters the transformer was larger than it needed to be, we had 2 transformers at 100 va, 2 at 150 va, and 1 at 250 va. In all cases the transformer was powering a single contactor coil. If a coil were to draw sufficient current to damage such a large transformer would it not seem likely that the coil would be damaged also?

 

[electricpete]

You're right, I would have thought that the starter coil would have burned up before the transformer. But it still doesn't rule out that scenario. Perhaps the transformer secondary has less current capacity than coil, then transformer would burn first.

Still ... even after the transformer started to burn, how/when was the current eventually removed from starter coils before damage occured to them? I can think of a few scenarios:
1 - Transformer secondary turns shorted out. This reduced the output voltage and reduced current to the coils.
2 - Transformer open-circuited, removing voltage from the starter coils.
3 - Transformer series impedance is high enough to limit current in this circuit (including dropped out coil impedance) to below the coil capability, but above the transf capability.
4 - some fuses blown you didn't tell us about?

good discussion. It probably wouldn't be wise to rule out any possibilities without a thorough review.

 

[purplepete]

Gents,
I'm not sure if I missed it in the discussion but is it possible the ship's generating system did a little time at low frequency for a prolonged period but perhaps with voltage even at the lower frequency?
Pete

 

[gigahertz]

Radiation from Comms, Radar, etc?

 

[jbartos]

Suggestion: The EMI should not be overlooked as the previous posting suggests. Rather than the radiation, there might be the electromagnetically induced interferece in Gausses which is different from electrical radiation interference in Volt/meter.
Next, the ungrounded systems often experience arcing and associated high voltages. Then, it would depend how good insulation those control transformers had. Have you tried to trace a root cause of the malfunction?