Hi All,

Apprecate, can someone enlighten me on the following?

I am working on Induction Heaters / melters and our large machine are supplied with multi phase supply in order to minimise feeding harmonics to the mains supply.

Over 1000 KW Units, the power supply configuration is like this:

There is a dedicated transformer, 11 KV or higher, 3 phase primary, and the secondry 575 V taken from 6 phases- two windings, STAR and DELTA. Then rectified by two sets of SCR rectifiers and comes to common DC. Then the the inverter section.

When it operates at the full power (rated power) or less than 100% power, one side always draws more current than the other. I cannot remember Star or Delta, Why Is That? The Load is a common rectifed DC but Star and Delta currents never seen balanced.

 

Do you have a schematic of the overall circuit configuration. On the rectifier side this appears to be a 12-pulse phase controlled converter.

If everything is well balanced one would expect the two 6-pulse halves to be equally sharing the load.

Where are you seeing the current imbalance - on the DC output of either 6-pulse rectifier?

If the SCR rectifiers are phase controlled (α>0°) there may be an imbalance in firing delay for either of the 6-pulse converter stages.

Is there an inter-phase reactor which couples the DC outputs of the two 6-pulse rectifiers?

 

Many thanks for this post.
Sorry, At this stage I don't have a schematic with me and also realised that I have not given enough information.

First it is not a phase controlled rectifier and we use SCRs for two reasons.
1) for rectification
2) as a SCR switch for fast AC interruption

We have developed our own AC Interruption firing system called ACI and it fires all SCR at a fixed frequency 2.1Khz. But it is a closed loop system operation and it monitors DC feed back constantly. Any sudden current overflow will cause a drop in the Voltage feed back will instantly stop firing the SCRs. in order to break the current flow.

After rectification there is a huge current limiting reactor on both Star and Delta side and then DC coming to a common bus. There are adequate Filter capacitors and on any of our machines at 100% power the allowable AC ripple on the DC bus is between 45 to about 75 V rms.
We measure the line currents & Voltages on each phase for the Power calibration etc. and have noticed Star side phases never balance with Delta side phases. This my question why?
Hope given enough info.

 

If you can't supply at least some details of circuit topology and operation then it will be difficult to make any informed suggestions as to the cause of the imbalance. If the system also includes feedback control then that adds an additional layer of complexity.

It's possible you could develop a simulation model using a power electronic simulation application such as PSIM - that may provide some insights.