Rectifier configuration Harmonics 3

[lansford]

This appliction is two 110 KW, 480 volt three phase rectifiers feeding an electrolysis system.
One rectifier consists of an iso transformer with a double star output with a diode in each output. (6 diodes)
It has a delta primary winding but with back to back SCRs in each line (full wave bridge type)
The second one has identical secondary design but with a primary wye and the same SCR configuration on the primary.
The rectifiers will operate mostly at full load.

If there were NO SCR's the units can be operated togehter and the harmonic spectrum would act as a 12 pulse.
If the rectifiers are being operated at a light load, then I would expect that since the input is a bridge SCR type the combination would behave as a 6 pulse?
If both are operted at full load and the SCR's fully turned on, then I would expect the combination to have a harmonic line spectrum somehwere between a 6 and a 12 pulse? Or maybe more like a 12 pulse?
Any comments?
thank JIM

 

[REM76]

I am thinking 12 pulse. You may want to hook up a fluke 43B on the line side and see what kind of harmonic spectrum you would get. If the dominant harmonics are the 5th and 7th, then this would be six pulse action. If 11th and 13th then 12 pulse.

 

[LionelHutz]

Double star secondaries do not sound like a 12-pulse configuration.

The SCR controllers will produce more harmonics as they are phased back or as the voltage is lowered. They will act somewhat like 6-pulse rectifiers when doing this but have no effect when full-on.

 

[lansford]

Dear LionelHutz,

On the last sentence do you mean that "this but have no effect when full-on." when full on and with one tranformer high side wye and the other delta the combination will then act as a 12 pulse?

Thanks JIM

 

[lansford]

Dear LionelHutz


"The SCR controllers will produce more harmonics as they are phased back or as the voltage is lowered. They will act somewhat like 6-pulse rectifiers when doing this but have no effect when full-on."
Do you mean that when full on the two rectifiers (one with primary delta and one with primary wye) toghether will produce a harmonic line spectrum similar to a 12 pulse?

Thank you
JIM

 

[avogel]

Not to hijack your thread, but I was looking at some 6 pulse rectifiers today. 2 of them were connected in parallel with the other, but the primaries were connected as shown in my attachment. This is supposedly a 12 pulse equivelant.

I do not see how you get a 30 degree shift from T1 to T2. Can someone explain, it looks to me like a 90 degree shift

 

[LionelHutz]

Does the negative connection go back to the Y points of the transformer?

I might have misread the question. You have 2 transformers.
#1 - Y primary, 2 x Y secondary
#2 - Delta primary, 2 x Y secondary
correct?

Each transformer is powering it's own rectifier with 6 diodes in the rectifier, correct? If so, this is a 6-pulse design.

The primary winding changes from Y to delta between the 2 rectifiers to give the equivalent of a 12-pulse design on the power system as long as both rectifiers are operating at the same current.

The SCR controller will produce harmonics similar to a 6-pulse rectifier as it is phased back. Since it is driving a 6-pulse rectifier there's likely no big difference phasing back or not.

 

[lansford]

Dear LionelHutz,

Copying excerpts from your note and my answers in CAPS:

"Does the negative connection go back to the Y points of the transformer?" YES SCHEMATIC IS HARD TO READ BUT LOOKS LIKE DC OUTPUT IS FROM DIODES CONNECTION TO THE NEUTRAL

"I might have misread the question. You have 2 transformers.
#1 - Y primary, 2 x Y secondary
#2 - Delta primary, 2 x Y secondary
correct?" YES

"Each transformer is powering it's own rectifier with 6 diodes in the rectifier, correct? If so, this is a 6-pulse design." YES AND YES AGREE

"The primary winding changes from Y to delta between the 2 rectifiers to give the equivalent of a 12-pulse design on the power system as long as both rectifiers are operating at the same current." AND IF THEY ARE OPERATING WITH THE PRIMARY SCR ABOUT FULLY ON BUT AS THEY ARE PHASED BACK (LOWERING OUTPUT) SINCE THERE ARE SCR ON THE PRIMARY, THE COMBINATION OF THE TWO ACT MORE LIKE A 6 PULSE COLLECTIVELY I BELIEVE ?
MOREOVER, AT ABOUT AT AROUND 3/4 LOAD ON BOTH, THE COMBINATION WILL BE PRODUCING A HARMONIC SPECTRUM BETWEEN THE 12 PULSE AND 6 PULSE.?
IF THERE WERE NO SCR ON PRIMARY AND ONLY DIODES ON SECONDARY THEN COMBINATION OF THE TWO WOULD ALWAYS BE AS A 12 PULSE BUT IT WILL BE FULLY ON ANYWAY. I THINK WE AGREE.
"The SCR controller will produce harmonics similar to a 6-pulse rectifier as it is phased back." AGREE
"Since it is driving a 6-pulse rectifier there's likely no big difference phasing back or not." NOT SURE I UNDERSTAND THIS SENTENCE?

THANK YOU SO MUCH FOR YOUR HELPFUL COMMENTS. MUCH APPRECIATED
JIM

 

[LionelHutz]

I don't think it matters if the harmonic producing device is on the primary or secondary of the transformer. Having 2 transformers with the different primary connections should still give the effect of a 12-pulse device.

 

[lansford]

Thanks again LionelHutz for your good insight.
And thanks to this forum, for this valuable service.
JIM

 

[electricuwe]

I'm quite suprised to see a star/double-star configuration being used. A transformer having no delta winding may suffer from flux flowing from yoke to yoke via the tank/the the enclosure.

Are you sure your are buying your equipment from manufacturer experienced in that field ?