Large single phase load causing plant voltage unbalance

[hmchi]

A Steel mill added a heat treatment machine 3500KVA single phase to their total system of 15MVA. It is causing voltage unbalance affecting the motors on the rest of the system.

Are there systems that can accommodate large single phase loads and avoid the voltage unbalance problem ? Railways use Scott Transformers but that is not applicable here ...

 

[Skogsgurra]

Does it have to be single-phase? Most unusual. A joke?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[slavag]

Agreed with Gunnar.
3.5MVA single phase load seems very strange ( on 15MVA total).
Please check this data again.
Regards.
Slava

 

[itsmoked]

That is a most ridiculous design. Think of the sinusoidal loading of the generator somewhere.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[alehman]

I don't see any way to significant voltage unbalance with that kind of single-phase load. I think about all you could do is install some sort of three-phase to single-phase converter.

 

[waross]

You could feed it from a transformer with a delta connected secondary.
The transformer would be sized at 3500/2x3= 5250 KVA
This would give you equal currents on each phase. However, the current on one phase would be in phase, on one phase 60 degrees leading and on the third phase 60 degrees lagging before considering a power factor other than unity.
Not that great a solution but possibly better than what you have now.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hmchi]

No, this is not a joke. It is a machine called ESRR and they bought it single phase. It seems that the machine only comes single phase. They connected it to a 6.6kv delta source.

They are going to a company like Elspec who sells thyrister switched capacitor bank reactive compensation to see if that will help.

Thought I ask the experts to see if there are other clever transformer connections, similar to the Scott connection transformer.

 

[Skogsgurra]

The only "good" solution would be to set up either a rotary converter that runs off 6.6 kV three-phase and delivers a single phase voltage or a static MV frequency inverter with single phase output. Both will be very special and very expensive.

I really can't imagine what sort of heater this is. A huge induction coil with compensation running directly off 50 or 60 Hz? Sounds like old Soviet technology to me. Where in the world are you?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricuwe]

In metal industry application there are a traditionally a lot equipment needing single phase power:

induction heating, induction melting, direct resistance heating, resistance welding, spot welding and electroslag remelting (ESRR seems just to be a new variant of that old process)

The basic reason for requireing single pahse power, is that there is only one load which can't be split to three phases.

When symetric connection of such a load to the supply is intended, there are basicly two solutions:

Classic soltuion: Steinmetz circuit
The power factor of the load is compensated and the load is connected between two phases, one additional inductor and one additional capacitor, both properly sized, connected in the other sides of the delta, ensure symmetrical line currents. However, since load paramters tend to change continously during process, you need a lot of switchgear to addapt all the components properly.

Converter solution: A converter with three-phase input and single phase output is used. Such equipment is availble with up to 16 MW using thyristors and up to 4 MW using IGBT

In both cases you have to keep in mind that the key to provide symmetry is to store energy periodically. Therfore all solutions only using transfomrers fail to provide a sound solution for a single load.

The Scott-Connection is fine when you have two single phase loads with of similar power.

 

[Skogsgurra]

Danke Uwe!

I only see the drive's side of things. Sounds like a bad idea to have that sort of load. It deserves a transformer of its own, if you ask me.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

How about using the classic MV VFD front end with PF correction then just create a single sine wave output.

Or three phase to single phase UPS sans batteries, using enough caps instead.

Whatever you do will be custom but finding someone willing to alter an existing product for the least cost would get you the quickest result.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[hmchi]

Thanks, guys, for the very helpful comments --- especially you, electricuwe.

Keep up your good work !!

 

[achilufya]

I agree with Gunnar that such kind of load needs its own single phase transformer. If for arguments sake, the transformer supplying the other loads(motors) is a 66/6.6kV, then this particular transformer needs to be connected to the 66 kV bus so that voltage distribution for the three phases will not be so much distorted.Other interventions could be used in conjuction with this.
Aubrey.

 

[electricuwe]

forget about modifying 3-phase inverters for that purpose. The DC-link will not be capable to carry the ac current with twice the output frequency and you will not find a drives manufacturer willing to modify his control software for that purpose. And only few drives with air cooling will be able to withstand the harsh enviroment in a steel mill for a long time.

We considerd the approach to modify a drive and later ended with doing our own dedicated design (very successful today)

Connecting to 66 kV is not a good idea either, transformer, switchgear and protection will be expensive.

Try to find an established induction melting /heating company willing to sell an IGBT inverter for that purpose to upgrade the installation.

 

[itsmoked]

Thanks for that info electricuwe.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com