Transformer secondary winding problem with 3-phase full wave bridge

[shamooooot]

Hello guys
I really need a hand here please..

I have a fully controlled bridge rectifier board of (TYN812RG) thyristors, and I want to trigger it through some firing board which I am sure it's functioning properly.

For the input I am using a 3-phase transformer in which the primary windings are connected in delta and the secondary windings can be connected as a delta or star. With 90V or 45V center tapped output as delta, 150V or 78V center tapped output as star.

I tried every combination of the secondary windings and I used R load (trying combinations of 3x100ohms 100w resistors) and RL loads (trying different combinations of the the 100ohm resistors and two 50mH inductors)

I am getting loud intermittent humming and the power on the secondary side is cutting intermittently, I can see a signal on the scope but it's flickering too quickly due to the power failure.

[WattsThat]

Words alone are not working here. “Disconnecting” “power failure” “intermittent” These words are being used in ways that only create more questions in order to understand the result you’re describing.

What is the goal of the phase control board? To control what and for what purpose?

You never mention transformer primary voltage. With you country flag showing USA, three phase is typically available in 208-240 or 460-480 volts which require proper fusing and other precautions for safe usage.

I would strongly suggest the use of photographs to help communicate. Start with the transformer data plate and photos of the actual transformer terminals and how and to what you have them wired.

[Jeroen3]

You need to wire it Yy0 or Dd0.
Or with 120deg phase shift that you compensate by rotating the trigger wires around.
https://electrical-engineering-portal.com/understanding-vector-group-transformer-1

And since you've drawn the scr bridge upside down, ensure you're wiring the correct thyristor to the correct output and the trigger board is set for this topology as well.

[shamooooot]

Thank you for the valuable inputs..

I want to experiment with 3-phase fully controlled bridge rectifer.
The voltage on the secondary winding keeps intermittently and quickly goes to zero (apparently but not 100% sure with every triggering cycle).
400V on the primary side.
The transformer supposedly have 1.5A maximum current on the secondary (that's pretty much all I can find).
I am using the firing board in the datasheet, and it worked well for driving (single, two, and 4 thyristors).
https://docdro.id/uMz93Hj

[shamooooot]

I tried the Yy0 and Dd0 but no luck.
For the Yy0 the Neutral is not in use neither by the bridge nor by the firing board.

[jonpaul]

load KVA, resistive, inductive SMPS ?
trigger pulses  driver?
thyristors may misfire with noisy, inadequate trigger

j

[shamooooot]

I tried these loads, and the results were the same:

If misfire happen would that affect the input supply to the bridge, because I believe the drop in power happens with each switching cycle, would it be helpful to add a resistor on each of the secondary lines to limit the inrush current ?

[shamooooot]

Something else I forgot to mention is that I am connecting the thyristors together in the bridge with a tiny gauge solid core wires, I don't know if that could be a problem..

[moffy]

You do not mention what the traces are, especially the blue trace on your scope capture. The asymmetry on the yellow is concerning. The loud buzzing could be due to excessive magnetic fields, i.e. an intermittent short or some form of core saturation. For the load you should use a balanced star or delta load. What is the primary voltage rating and what are you using to drive it?

[shamooooot]

Hey muffy

The blue line is proving there's a trigger pulses from the firing board, and the yellow is just the ac input (I think the noise is due to loose connection)..
I think it's exactly intermittent short or saturation in the transformer.
The output of the bridge is DC.
The primary is 400V and the secondary is 90V/45V center tapped. Or what do you mean (what are you using to drive it)?

[moffy]

The primary is 400V and the secondary is 90V/45V center tapped. Or what do you mean (what are you using to drive it)?

You are correct, I was asking what voltage you were using to drive the primaries. That secondary waveform looks concerning, they need to be symmetrical otherwise there is a chance of DC on the primary.

[jonpaul]

Thyristors, inductive,loads not a great mix.


you are Missing snubbers across thyristors.

Need complet schematic of entire system, and better scope shots to comment further

j

[shamooooot]

Thyristors, inductive,loads not a great mix.


you are Missing snubbers across thyristors.

Need complet schematic of entire system, and better scope shots to comment further

j

Honestly this is pretty much the schematic, I tried it with resistive load as well but same result..
I am not using snubber circuit across the thyristors... would that be an issue?
When I scope the output of bridge (DC) I get a couple of waves of the expected full wave signal but intermittently going to 0.


Sorry for the bad image but that's almost what I get, with longer dead time..

[moffy]

Remove the bridge from the transformer and test the transformer by itself. You need to know if the fault is in your bridge circuit or the transformer, to do that test the transformer with resistive loads to make sure it works properly. If it is OK then the problem is in your bridge circuit.

[shamooooot]

Remove the bridge from the transformer and test the transformer by itself. You need to know if the fault is in your bridge circuit or the transformer, to do that test the transformer with resistive loads to make sure it works properly. If it is OK then the problem is in your bridge circuit.

I am 100% sure the transformer is fine, but I was thinking when triggering the thyristors with pulses at higher frequencies it saturates/or get shorted some how, because I read harmonics introduced by the rectifier, high inrush current, or spikes in the secondary can cause that. If we eliminated the transformer as there's nothing more to it, it would be great..

[shamooooot]

Someone suggested "imbalance causing excessive current via saturation of at least one leg of the core".

I have LED's connected on each secondary line and while they flickering, I can see one is dead longer, or even completely dead with higher firing angles!

[jonpaul]

Suggest that you research the specs of your thyristors, gate drive required, dv/dt AK limit, etc.


For me, impossible to comment further without

Thyristor drive circuit,
 current and voltage waves at all thyristor gates during fault

Thyristors misfire is symptom of gate noise, wrong drive Z, or inadequate gate drive.

All thyristors with inductive source ( transformers) or load, need a dv/dt limiter, à snubber.
Try 100 ohm, 5 W in series with 50..100 nF, 600  VAC plastic cap rated for snubber service

Bon Soirée

Jon

[shamooooot]

Suggest that you research the specs of your thyristors, gate drive required, dv/dt AK limit, etc.


For me, impossible to comment further without

Thyristor drive circuit,
 current and voltage waves at all thyristor gates during fault

Thyristors misfire is symptom of gate noise, wrong drive Z, or inadequate gate drive.

All thyristors with inductive source ( transformers) or load, need a dv/dt limiter, à snubber.
Try 100 ohm, 5 W in series with 50..100 nF, 600  VAC plastic cap rated for snubber service

Bon Soirée

Jon

I really appreciate it Jon, I will add an RCD snubber circuit and test again and hopefully it will fix the problem.

[jonpaul]

Rebonjour Cher Monsieur

Snubbers are normally used regardless of load, but if the gate firing is noise, or marginal you will,still have misfire, either missing or,extra half cycle.


Suggest you probe both gate  V and I at all thyristors during a fault.

Beware a thyristor usually needs peak gate current equal to the anode current to fire.

1..100 use pulse or Pulse train is common

Bon journée

Jon

[shamooooot]

So I added the snubber circuit to the thyristor and everything runs perfectly now..
Thank you guys for your help.

[jonpaul]

BRAVO! HOME RUN!


Jon