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Anonymous Poster #1

Trouble with 3 Phase Heater with SCR Control

12/11/2011 4:46 AM

Dear gurus,

During commissioning, we have a differential earth fault trip in our 3 phase heater with SCR control.

690V 100kW and connected in Delta. Differential earth fault setting is 300mA and 0.4second. The SCR is controlled 2 phase only (U, V) and W is direct connected to L3.

1. Is it okay to use SCR in 2 phases only? It can create current unbalance?

2. Delta connection is ok? Because we experience heating also
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Tornado
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#1

Re: Trouble on 3 phase heater with SCR control

12/11/2011 5:28 AM

Wow! That must have been engineered by some of the poster children here on CR4.

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SolarEagle
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#2

Re: Trouble on 3 phase heater with SCR control

12/11/2011 1:13 PM

Check to see if someone configured your heater elements in a Y and then grounded the Neutral on it...

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marcot
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#3
In reply to #2

Re: Trouble on 3 phase heater with SCR control

12/11/2011 10:23 PM

Agree. Correct wiring is the first thing to look at. Then highpot the heaters.

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kvsubramanyam
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#4

Re: Trouble with 3 Phase Heater with SCR Control

12/11/2011 11:05 PM

There is nothing wrong with SCR & delta connected heater load. First we have to find the reason for tripping. Can you just test SCR pack on about 5% dummy known completely isolated load and check whether still the system it trips?

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JRaef
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#5

Re: Trouble with 3 Phase Heater with SCR Control

12/12/2011 12:06 AM

It is perfectly normal to do SCR control on 2 of the 3 phases, it is a way to save money on the SCR controller because you only need 4 SCRs instead of 6. There are however two major caveats to doing that:

1) It can only be done on UNGROUNDED 3 phase heater element gonfigurations. They can be wye or delta, but if wye (star) it cannot be a 4 wire connection of the elements, i.e. where the wye (star) connection point is connected to the grounded neutral of the power distribution system.

2) It DOES create a phase current imbalance. Were the load a motor this would be a problem but it is a heater, so no problem for the load. However one thing you can't do is differential (residual) earth fault protection on it as it is INTENTIONALLY imbalanced. So you have two choices: remove the earth fault protection, or if that is not possible, change to a full 3 phase controller (6 SCRs).

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Wal
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#6

Re: Trouble with 3 Phase Heater with SCR Control

12/13/2011 12:07 AM

What type of elements are these?

Are you heating air or water or something else?

Have you tried increasing the Differential earth fault setting to a higher current to see if the "problem" "goes away"?

Is the diff earth fault device legacy or part of the new installation?

You are commissioning. Now is the time to mess with these things.

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JRaef (1); kvsubramanyam (1); marcot (1); SolarEagle (1); Tornado (1); Wal (1)

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