Isolation transformers 4

[Thedroid]

We use isolation transformers on all of the critical drives in our plant. They all have the same voltage on the primary and secondary side, and are used to protect the drives from power bumps.

How does this type of sustem work? We have several 4160/4160, and also several 480/480. I'd like to understand them a little better.

Cement Plant Electrician and
Instrumentation Technician

 

[sibeen]

Thedroid, the answer to this question is very closely related to the question you asked in the motors control section.

I doubt that the isolation transformers you have are to protect the drives from power bumps. There are types of transformers that do this called Constant Voltage Transformers, but I don’t think this is what you would have in your plant.

As you found in your recent thread, even with balanced phase loads, the current in the neutral can be a substantial amount. It can be shown mathematically, that in the worst case, the current in the neutral can reach as high as 1.7 times the current in the phases. Due to this, steps are often taken to mitigate the amount of third harmonic currents flowing in a system.

Some configurations of transformer windings have an inability to pass through any third harmonic currents (0 sequence currents). So that if your load is drawing a substantial amount of third harmonic and the correct isolation transformer is placed into the line, the third harmonic current will not be seen at the input to the primary of the transformer, but it will still be seen on the output (secondary) of the transformer. So, by placing isolation transformers at various points around the plant you are stopping third harmonic currents from these loads getting into your main distribution.

I hope that all makes sense. As always ask for any clarification.

 

[Skogsgurra]

These transformers also serve other purposes. One is to isolate the drive from common-mode transients that often is a problem in areas with high ligtning activity. Lightning is known to destroy VFDs. The isolation you get in a transformer keeps common-mode transients away.

The extra inductance (stray inductance) in the transformer acts like a line reactor (series reactor) and is rather good at reducing normal mode transients as well. That may very well be the "bumps" you mention - although a "bump" usually gets through because of its relatively low frequency characteristic.

Then, there is the problem with PWM VFDs polluting the mains. Putting a transformer between mains and VFD reduces that problem considerably. Especially if there's a static screen/shield between primary and secondary winding.

It is wise to do what you have. It reduces a lot of problems with VFDs. But not all.

Gunnar Englund

http://www.gke.org

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

 

[edison123]

LPS for you sibeen and gunnar.

 

[Thedroid]

Thanks! Those were both excellent answers that cleared up my confusion. I had not even thought of relating this question with my last, but I see know how they were related.

The isolation transformers are pretty good sized, and I figured there has to be some good reasons to install them, not just power "bumps".

The big ones that interest me the most are
4160 delta primary, and the secondary is a Y with 2300v to each of the phases. Rectifief duty. Is this the same as 4160/4160?

They each feed an ABB ACS1000 which drives an ABB 1500hp motor rated for 4000v.

We also have numerous smaller ones that feed DC kiln drives and other critical drives.

 

[Thedroid]

Also the bumps Gunnar mentioned do get through. The drives have to be reset manually after one.

 

[Thedroid]

So they work both ways. Not only do they protect the drive, but they keep the harmonics created by the drive from causing problems throughout the rest of the power system.