12 pulses AC/DC converter

[marbr2005]

I would like to understand the triggering sequence for two thyristors converters connected in series( double secondary transformer wye/delta). The 30 degrees lag pulses for one thyristor converter is clear for me but I cannot understand how the two converters are triggered at the same time if there is a delay between the pulses for "Y" converter and "D" converter.Sorry my english mistakes.
Thank you.

 

[spargher]

The two converters don't need to be triggered at the same time.
I'm trying to understand your application. Most probably you are talkin' about two converters controlling two dc motors mecanically connected toegether.
One of the two converter will act as a master determining the torque sharing between the two motor, the other converter will act as a slave, doing what the master asks.
They will control the dc current/voltage, but ther is no need of simultaneous triggering of the thyristors.

If this is not your application, please be more specific and explain.
Help us to be useful to you

 

[marbr2005]

Thank you spargher
The application for this technology is to drive big DC motors(4000kW).Here in Brazil it is applied, for example, in rolling mills for heavy beams. This configuration , two converters connected in series driving one motor, virtually eliminates 5th and 7th harmonics orders and get lower ripple level.
There are cases where converters are connected in parallel.
This technology comes from the 70's.A modern control module for thyristors triggering is made in U.S. by Enerpro.At google.com if you type "12 pulses converters" you will find at 1st position

http://www.mathworks.com........

describing the theory.At 4th position you will find

http://www.enerpro-inc.com........

describing the triggering control module.
I would appreciate your kind help.

 

[spargher]

Ok, now I understand the problem.
I'm sorry but I've never worked with this kind of configuration, but alwasy with two converters on two motors mechanically connected so there was no electrical connection between the two converters and ther was no need for triggering.
Sorry but I cannot be useful to you in this case

 

[Skogsgurra]

Hi,

You have two six-pulse rectifiers and a transformer with two secondary windings which are mostly one Y and one D. If you look at the phasor diagram, you will see that there is a 30 degree phase shift between the main voltages from the two secondaries. That's how you get the 5th and 7th harmonics reduction and the resulting 12-pulse ripple (12x30 = 360 degrees).

Now, both rectifiers are connected on the DC side and work in parallel to feed the motor armature. Each rectifier has its own trigger unit and the firing pulses are referred to the respective mains voltage. There's nothing special about that. Just plain six-pulse technology. But twice.

The phase shift between the two systems comes from them being fed from phase shifted voltages. No magic, no synchronisation between the two. Load sharing is automatic since there are impedances in the secondary windings. It is also possible to run one as a master and one as a slave (current or torque slave) if you want to control load sharing between the units.

Gunnar Englund

http://www.gke.org

 

[Warpspeed]

Connecting two six pulse rectifiers in parallel is the normal conventional way to do it, and the centre tapped output choke will allow each rectifier to run independently thirty degrees apart. Fairly easy to understand.

But connecting two six pulse rectifiers in series is a completely different thing. The current through both rectifiers must of course always be exactly the same. I have never seen that configuration described in any text book, and cannot visualise how it might work.

There may be some insurmountable commutation problems, and it may not work in a mode that will provide the harmonic cancellation and output control that is desired.

For instance at very small conduction angles of less than thirty degrees, how can there be a current path through both rectifiers if they are in series ? In parallel, yes, but in series I have difficulty in seeing how it could possibly work.

I have seen two six pulse DIODE rectifiers connected in series, but never with SCRs. If it was possible, I am sure it would appear in the reference books, but I have never seen it anywhere.

 

[marbr2005]

Thanks to spargher, warspeed and skogsgurra
My doubt is exactly how they work in SERIES if there is a time delay between corresponding pulses for each converter.
As I said to sparguer I am sure they really work and this can be checked typing at

http://www.google.com

-12 pulses converters
A theory will be find at

http://www.mathworks.com

and a real product made in U.S. will be find at

http://www.enerpro-inc.com

Anyway, thank you.

 

[CJCPE]

The first 3 of the following have at least a little information about two 6-pulse converters fed from Y and D windings and connected in series. The last 3 are often cited as references in papers about converter.

Mazda, F.F., Thyristor Control, John Wiley & Sons Halsted Press Div., 1973

Oliver, G.; Shankar, N., "A 5-kV 1.5-MW variable DC source," Industry Applications, IEEE Transactions on , vol.26, no.1pp.73-79, Jan/Feb 1990

Tanaka, T.; Koshio, N.; Akagi, H.; Nabae, A., "Reducing supply current harmonics," Industry Applications Magazine, IEEE , vol.4, no.5pp.31-37, Sep/Oct 1998

Pelly, B. R., Thyristor phase-controlled converters and cycloconverters, John Wiley &:Sons, 1971

Dewan S. B., Power Semiconductor Circuits, John Wiley &:Sons, 1976

Paice, D. A., Power ElectronicConverter Harmonics, IEEE Press

 

[Drivesrock]

Hi Marbr2005,
the scheme you describe is not so common but exists. I have worked with this several times - in applications in Brazil also.
You did not mention if the bridges are APSH (Anti-Parallel Split Half, or anti-parallel config) - for regeneration back into the mains or reversing of the motor.
Two points to remember; In a dc drive, power factor of a standard 6 pulse dc drive is proportional to the armature volts - so poor power factor at low motor speed. And, armature current can be controlled independently of the armature volts by controlling the thyristors.
Each bridge has individual firing circuits but the two bridges may have an overall master controller. The firing of each bridge is synchronized with the mains supply for that bridge just as a normal 6 pulse drive. There is nothing complicated in that part. The two voltages will be summed and the current limited in one or both bridges. The 12 pulse rectification will eliminate (nearly) your 5th & 7th current harmonics.
Now, the interesting bit comes in the 'master controller' - if you have one:
As seen in other replies the current in the two bridges is the same and therefore the two bridges will have to supply the full motor current requirements but as they are in series the voltages are summed. This could have been done to maintain two standard-voltage thyristor bridges for a higher voltage motor or for power factor control if you have APSH bridges.

Now, the 'master controller' could simply decrease the firing angle on both bridges and accelerate your motor. Armature current can be controlled in one or other of the bridges.
Alternatively, the controller can control the two APSH bridges independent of each other so, for (extreme) example, at zero armature volts one bridge could be fully positive volts and the other bridge fully negative volts. The sum of the 2 bridges will be 0V so the motor will be stopped. In practice, the master controller won't stop the motor by having both bridges with maximum opposite volts but will do this with some lower value and the speeding up and slowing down of the motor will be done by decreasing the firing angle of one bridge and then the other.
The advantage of this control is that the power factor presented back to the supply will not be so low or can be controlled. This is very advantageous in some low speed, high power applications.

Not many dc drives manufacturers offer this power factor control function for 2 bridges in series although most will have 12 pulse operation (normally for parallel bridge configuration). The 'master controller' has extra firmware and has a more powerfull microprocesor or in the older equipment was made up of more analogue circuits on pcbs in a 19inch rack!

Espero que isto ajuda!

Drivesrock

 

[Skogsgurra]

Sorry, read the OP without noticing the series connection. Makes a lot of difference...

Gunnar Englund

http://www.gke.org

 

[electricuwe]

On the initial post:

The series connection works well with the commonly used timing of the gate pulses as soon as contious DC-current has been established. But for starting up you have to provide synchronous pulses in both bridges. This can be done by adding additional pulses 30° behind the initial ones.

On Drivesrock's answer:
If the series connection is used for improving fundamental power factor you loose the benefits of 12-pulse operation in the major part of the operation range.

 

[marbr2005]

Thank you all.
Now it is clear for me how the series converters work.
Considering all answers, specially from electricuwe,and indicated books it was possible the full understanding, including a diode matrix that remakes the starting triggering condition for the cases where very low current level do not establishes the continuous DC current.
The remark is just the fact that apparently this technology is not well known by our forum colleagues although be widely applied in my country.(Alstom supply from France,GEC supply from England)
It was not commonly applied in US or in Europe?
In case of separated drives for top and bottom work rolls GEC had combined -15/+15/0/+30 degrees phase displacement transformers to get an apparent 24 pulses convertion.
Finally, thank you very much indeed, specially to electricuwe.A Happy Christmas for all of you.