use of RCD`s in thyristor power controller circuits 3

[gww]

I have an application that uses thyristor power controllers to heat up a "melting pot". The supply is 415v 2 phase that is fed through a stepdown transformer giving a secondary voltage of 240v centre tapped. This is fed to thyristor power controller which has a resistive load . If a wire is broken @ the o/p of the thyristor and shorts down to earth i need to detect a current of 30mA or more and "trip" out the supply. I have tried using an RCD but this does not work. Anybody any ideas please?

 

[ScottyUK]

Where did you tried installing the RCD? Any chance of a sketch?


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If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

Should also have said: some RCDs do not play well with any DC offset although there are specific types which are more tolerant. A slight imbalance in the thyristor firing angle symmetry would give you a net DC current which could be upsetting the RCD.



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If we learn from our mistakes I'm getting a great education!

 

[gww]

Hi Scotty UK please find attached a sketch of what i have @ the moment. The reason for 414v supply is that i have 12 of these altogether and so i am trying to balance out the load. PS the load current is approx 25A.

 

[ScottyUK]

What application opens a .emf file?

Sorry if it's a dumb question, I seem to be asking more and more of them...


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If we learn from our mistakes I'm getting a great education!

 

[balms52]

Windows picture or fax viewer!

 

[gww]

double clicking on it with microsoft office installed on your m/c should open it. if not please let me know. Thank you.

 

[ScottyUK]

Oh FFS, that is embarassing!

Does the RCD trip when it is not supposed to, or refuse to trip when it should? On re-reading your original post I suspect that it is not detetcting the fault but presumably is not experiencing a false trip.

I suspect this is a DC offset problem because with a broken wire from one thyristor you will have an assymetric output and there will be a net DC component. RCDs use a toroidal core with a multi-turn sense winding to trip the device. If there is DC present then the toroid will saturate and the current in the sense winding will not be a accurate replica of the primary current, causing mis-operation or non-operation of the RCD. I think the RCDs which are tolerant of DC must use an air-gapped core although I admit I've never looked.


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If we learn from our mistakes I'm getting a great education!

 

[PHovnanian]

gww, does the thyristor module have any harmonic filters built in? Specifically, does it have any capacitors that b ypass h.f. harmonics to ground? If so, the frequencies with multiples of three of the fundamental will not sum to zero (even for a balanced system) and the resulting current can trip an RCD.

 

[gww]

PHovnanian thanks for the reply. No it does not have any filters, it is just an off the shelf type A rcd. Looking into this further i am thinking of using a type B such as the siemens range. Hopefully this will work due to the ability to detect dc fault current component of the thyristor phase angle controlled output feeding the load. What does everybody think? Any other thoughts / suggestions would be appreciated, thank you.

 

[ScottyUK]

You might want to look at some of Bender's products. This one

http://www.bender.org/gfci.aspx

seems like it would be very suitable. Bender don't do 'cheap' but the couple of times I've seen their products they seem well built and reliable. Might be tricky to obtain on this side of the pond.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Another way to detect a ground fault is to connect a resistor into the center tap ground connection and then put a voltage sensing device across it. This will also give the secondary benefit of limiting the ground fault current.

 

[PHovnanian]

No it does not have any filters, it is just an off the shelf type A rcd.

Not asking about the RCD. The thyristor module.

 

[ozmosis]

For our VFD's we will generally advise a Type B RCD:

http://hvacity.danfoss.com/pdf_files/rcd.pdf

The same logic would apply to DC drives.
However, 30mA would be too low for an AC drive due to the inherrent leakage current from filters, cabling, switching frequency etc. Not sure if this is the case with DC drives.

 

[waross]

Try a resistor from the center tap to ground. Select a voltage relay and connect it in parallel with the grounding resistor. You can both limit the ground current and detect it with the same resistor/relay. Note that the relay impedance will form part of the grounding impedance. Also the relay impedance may change as the relay operates. Selecting a relay with an impedance many times the impedance of the grounding resistor will minimize the effect of the relay impedance. You may also consider using a DC relay and full wave rectifying the voltage across the grounding resistor.

Bill
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"Why not the best?"
Jimmy Carter