SCR Phase Angle Waveform

[FrankieAugust]

Hi all,

I am controlling a rectifier (AC to DC) for a sintering process with a SCR phase angle control.
Everything is normal at the beginning. But things go wrong when the SCR sending output command 93% or higher. Noise is coming out from the rectifier and then Eearth Fault Relay tripped the incoming power supply. We connect the power quality analyzer to the SCR output. The result as shown below. It not as what I expected. I sent this picture to the SCR supplier, but the vendor told me that the waveform is normal for a phase angle control. Anyone can tell me is it correct or normal for a SCR performance?

Below are the activities that we have done:
1. From the beginning, we suspect there is some current leakage on the rectifier. We sent the rectifier back to rectifier supplier for verification. They did the insulation and hipot test. After installed it back to the machine, same scenario happened. Power tripped when SCR output is >=93%.
2. We connect the SCR to bulb instead of rectifier. we slowly increase the SCR output from 0% to 100%. The bulbs turn on from dim to bright. But the bulb start flickering when output >=93%. Vendor told me this is due to the bulb is low load/resistance. They tested on higher load/resistance, flickering not happen. Is that correct?

Your advices will be very appreciated.

 

[LionelHutz]

Is this a inverse parallel pair of SCRs per phase which then feed the rectifier, or are the SCR's the rectifier?

Turn it down to say 80% and compare the waveform.

 

[3DDave]

What kind of bulb? Incandescent bulbs increase resistance with temperature and brightness increases.

 

[waross]

First guess based on incomplete information:
You are overloading your equipment.
It is going into current limit, self protection.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FrankieAugust]

LionelHutz,

We monitor the waveform for different output percentage. The waveform is similar.

 

[FrankieAugust]

3DDave,

Is incandescent bulbs. My curious is why it flickering happen when the phase angle control output >=93%

 

[FrankieAugust]

waross,

The machine size for 800A. The operation consumption is less than 200A.

 

[LionelHutz]

OK. I have no clue what configuration of circuit you're measuring, and you don't want to explain it.

It should be obvious, but if the waveform is always similar but it only acts up at certain outputs then it's likely not the waveform shape causing the issue.

 

[waross]

Possibly a bad timing circuit and a slight conduction overlap on opposite polarities.
It looks like some momentary current transients.
Put two phases on a dual trace scope and look for a slight overlap.
Try voltage as well as current.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FrankieAugust]

Hi Waross & LionellHutz,

Thanks for the feedback. I will keep monitoring it.

 

[FacEngrPE]

It is possible that the high frequency noise couples well enough to earth, that there is enough energy at the location where the ground detection relay is located that exceeds it's tripping criteria at output settings >93%.
Try placing the power quality monitor in parallel with the ground detection monitor. The waveform at the earthing connection at several output settings may provide some ideas how to proceed.

Typical SCR bridge.

 

[waross]

Your scope trace appears to have transients at switch-on, and transients at the switch-on points of the other phases.
Check for overlapping trigger timing.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FrankieAugust]

Hi FacEngrPE and Waross,

Thanks for the advices.