Transformer blowing semiconductor fuse

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dm9289

Industrial Maintenance Electrician
Location
Pennsylvania
Occupation
Industrial process repair/ maintenance Electrician
We have a scr controlled heating grid

Input single phase 480v ( 2 wires).
Output SCR varied form of the input to a transformer primary

Secondary is step down 480/240. This feeds a heating grid

Primary semi conductor fuse is 450amp very expensive to replace.
During full load primary draws about 300 amp. Secondary 600 amp.

Finally the problem about once a month the semi conductor fuse blows.
We examined wiring and connections to heating grid, no evidence of poor connections or arcs
We opened the transformer and mega ohm metered appears ok.
Visual transformer inspection seemed good
Grid mega ohm was good
Unfortunately the grid beyond the input is not accessible for about another 8 months due to process conditions.

If there is a bad enough condition on the secondary would it be normal to blow the primary fuse? My feeling is yes

Could you offer me a cost effective way to fuse or protect secondary so once every couple months we are not purchasing fuses.

Or any other suggestions for this problem
 
Driving a transformer primary from an SCR or other electronic "chopper" is extremely hard service for the SCRs.
The very first time you pulse the transformer in a control period you will have to deal with an inrush current that is 10 or more times the full load current rather than simply the steady state magnetizing current plus load current.
Especially problematic if the SCR tries to turn on close to zero crossing in the supplied voltage.

There are ways to design around this, but I can see it being very likely that any semiconductor fuse that is sized to properly protect the SCRs will be vulnerable to nuisance trips.

"Transistors: semiconductor devices that blow promptly to protect expensive fuses."
 
dm9289 said:
We have a scr controlled heating grid

Input single phase 480v ( 2 wires).
Output SCR varied form of the input to a transformer primary

Secondary is step down 480/240. This feeds a heating grid
Click to expand...
Just to be clear the transformer primary has the SCR controller?
 
Is it full wave cycle or phase control?
The absolute worst inrush is created by energizing the transformer at zero crossing and that's the way many SSRs are designed to do.
The actual inrush depends on the amount and direction of residual magnetic field and the phase of input at the very moment of energization.

If you were to repeatedly power cycle a small transformer at a power outlet level, you would hear it make an unhappy thunk, but not every single time. If the transformer is big enough and the timing is just right, you would pop the breaker. So this is probably the reason why its only happening once a month. As others suggested, putting the solid state relay on the secondary side is preferable. Depending on your controls, this can be as simple as putting the proper SSR in line with the heater and plugging the instrumentation control output into the SSR. The SSR needs to be the right type. Zero crossing type can not be used with phase control.
 
To answer questions yes scr is on primary

We have 4 other like circuits without any problem.

That's why this is so confusing, this unit was designed for this purpose
However I am not sure of firing method.

There possibly can be metal fragments getting on the grid. Inside a furnace thus only popping once a month.

I was thinking of s more effective way to deal with this till we can get furnace entrance
As I mentioned might be quit a long time.

Thank you for all your input
 
You're not giving enough info. If the fragment isn't causing direct "terminal to terminal" short and it's just shorting to the frame, the secondary side can be grounded through a high resistance path. A short to ground would cause dozens of amps to flow through the resistance grounding element, which then tell the controls to trip the protective relay while still avoiding tripping from several tenths of amps that may be a normal leakage current.

Clear the short, reset.

A SOLID ground would cause a full fault current to flow and put the need to fall back on the protective relay.
 
Thanks for your input. Currently the transformer is not grounded.

Transformer mounted on floor on cement, furnace is a considerable distance away
One floor up and about 100 ft away.

However by virtue of building arrangement a ground path may exist. Furnace is on metal platform, and anchored to cement. Some encased in concrete grid is connected to building girders.
 
The co I do business with uses this setup....PM me if you wish, I may be of help in resolving your issue. We have units that do this. Or use to do this.
 
dm9289 said:
We have a scr controlled heating grid

Input single phase 480v ( 2 wires).
Output SCR varied form of the input to a transformer primary

Secondary is step down 480/240. This feeds a heating grid

Primary semi conductor fuse is 450amp very expensive to replace.
During full load primary draws about 300 amp. Secondary 600 amp.

Finally the problem about once a month the semi conductor fuse blows.
We examined wiring and connections to heating grid, no evidence of poor connections or arcs
We opened the transformer and mega ohm metered appears ok.
Visual transformer inspection seemed good
Grid mega ohm was good
Unfortunately the grid beyond the input is not accessible for about another 8 months due to process conditions.

If there is a bad enough condition on the secondary would it be normal to blow the primary fuse? My feeling is yes

Could you offer me a cost effective way to fuse or protect secondary so once every couple months we are not purchasing fuses.

Or any other suggestions for this problem
Click to expand...
Some thoughts.
Yes, it could be a fault on the load side. If it is and you can't get access to it for 8 months there's not a lot you can do about that.

Another thought.
As it's single phase I guess it will be a pair of SCRs in a back to back arrangement. Firing is likely to be leading edge phase-angle control.
It gives an output something like this:

Leadingedge-1_zpsd13a176d.jpg


Any imbalance between the positive and negative half cycles will result in a DC component in the transformer current. That would cause saturation and potentially cause fuse blowing.
It could be caused by an occaisional missing pulse. Supply voltage distortion is a possibility but if all four units are on the same supply that would seem unlikely.

Have you tried swapping control cards. Systematic substitution as one of my guys put it.....

I feel for you. A fault as intermittent as you have described can be very difficult to track down and test your patience to the limit.
Good luck.
 
We did replace the entire unit old one was 20 years old and damaged.
Old one did occasionally blow fuse till it was damaged.

I greatly appreciate the education you all have been giving me

As you mentioned it is a dual scr unit. "Supposed" to be a direct replacement for our old unit.
We have 4 other in place working properly

I obviously am open to any suggestions
 
Besoeker said:
Some thoughts.
Yes, it could be a fault on the load side. If it is and you can't get access to it for 8 months there's not a lot you can do about that.

Another thought.
As it's single phase I guess it will be a pair of SCRs in a back to back arrangement. Firing is likely to be leading edge phase-angle control.
It gives an output something like this:

Leadingedge-1_zpsd13a176d.jpg


Any imbalance between the positive and negative half cycles will result in a DC component in the transformer current. That would cause saturation and potentially cause fuse blowing.
It could be caused by an occaisional missing pulse. Supply voltage distortion is a possibility but if all four units are on the same supply that would seem unlikely.

Have you tried swapping control cards. Systematic substitution as one of my guys put it.....

I feel for you. A fault as intermittent as you have described can be very difficult to track down and test your patience to the limit.
Good luck.
Click to expand...


Thank you for your input greatly appreciated
 
Ihave been trying to reply to input of individuals sorry for some reason it is only letting me reply by quote which is not showing up in forum
 
dm9289 said:
We did replace the entire unit old one was 20 years old and damaged.
Old one did occasionally blow fuse till it was damaged.

I greatly appreciate the education you all have been giving me

As you mentioned it is a dual scr unit. "Supposed" to be a direct replacement for our old unit.
We have 4 other in place working properly

I obviously am open to any suggestions
Click to expand...
Are the four others all on the same phase of the supply?
 
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