4160 Isolation Transformer

[spikes2020]

I have a question regarding a 4160 isolation transformer.

The current setup is 13.8kv to 4160 step down transformer feeding a VFD. The VFD is then fed to an isolation transformer, then to a 4160V motor. There is also a bypass for the VFD but i am not sure how it is connected.

The issue is, i looked at the plate of the transformer and it said 4160-2300.... I am 99% sure it is feeding 4160.

Also i believe we are hooked up to BOTH the secondary feeds off this transformer. It has a primary feed delta, then one Delta and one Wye secondary. I believe both are hooked up and then only one feed hits the motor.

Has anyone seen something like this?

I am replacing the VFD and will be removing the isolation transformer as it is now not needed. I am removing the bypass switch and just going 13.8kv to 4160 to VFD to Motor. I am concerned that i plan on putting 4160 on the motor. The motor is rated for it but could be wired differently.


Transformer plate
https://i.imgur.com/ImAw46Z.jpg

 

[spikes2020]

I believe the 2 secondary feeds are going into some starter/bypass that we have never used. So if the vfd took a dump we could bypass it and still start the motor.

I am just looking for if anyone knows kind of what is going on or seen something like this and could explain it better.

 

[Ingenieur]

sometimes xfmrs like that are used for vfd isolation
I've seen one like this:
prim 4160 delta
sec 3 x 1400 deltas (wire to supply 4200)

I guess yours are wired for the sec delta/wye to produce ~ 4 kv
it's done for harmonics

I can't see the nameplate pic, it would help

 

[Besoeker]

sometimes xfmrs like that are used for vfd isolation
I've seen one like this:
prim 4160 delta
sec 3 x 1400 deltas (wire to supply 4200)

I guess yours are wired for the sec delta/wye to produce ~ 4 kv
it's done for harmonics

I can't see the nameplate pic, it would help

The nameplate is there, just not very clear.
But you may well be correct about harmonic mitigation.
It could be a Ddyn11 to eliminate lower order harmonics.

 

[spikes2020]

I found some drawings, this thing is from like 1990 so the drawings are a bit old and were hard to find. It all makes sense now!

It appears that there is a bypass 4160 strait to the motor. The "isolation transformer is to generate 2 feeds at different phase angles from each other that's why one is 'wye' and the other is a 'delta'. Then the VFD uses these 6 feeds to build a stair step wave form to the motor.

This VFD and transformer is specialy made for each other. My new VFD uses pulse witdth modulation, instead of the stair step method. Thus i wont need this transformer.

I guess you learn something new every day~!

Edit

Here is the drawing i found
https://i.imgur.com/hoHYh6i.jpg

 

[Besoeker]

I found some drawings, this thing is from like 1990 so the drawings are a bit old and were hard to find. It all makes sense now!

It appears that there is a bypass 4160 strait to the motor. The "isolation transformer is to generate 2 feeds at different phase angles from each other that's why one is 'wye' and the other is a 'delta'. Then the VFD uses these 6 feeds to build a stair step wave form to the motor.

This VFD and transformer is specialy made for each other. My new VFD uses pulse witdth modulation, instead of the stair step method. Thus i wont need this transformer.

I guess you learn something new every day~!

The wye and delta windings are to reduce input harmonics. Your "new" PWM output won't change that.
You need to understand this important point before any money goes into the project.

 

[spikes2020]

The wye and delta windings are to reduce input harmonics. Your "new" PWM output won't change that.
You need to understand this important point before any money goes into the project.

I see what your saying the wye/delta wingdings are to reduce the harmonics interceded on the input side of the transformer. I would assume that isn't as bad if the drive is designed correctly with internal capacitors and filters. Also we have an 35MW electric arc furnace so it wont be the largest generator.

 

[Besoeker]

I see what your saying the wye/delta wingdings are to reduce the harmonics interceded on the input side of the transformer. I would assume that isn't as bad if the drive is designed correctly with internal capacitors and filters. Also we have an 35MW electric arc furnace so it wont be the largest generator.

I don't know what your drive configuration is. Most VFDs comprise an input rectifier, six pulse for the most part, twelve pulse for larger units, a DC link filter, and a PWM output stage.

 

[Jraef]

What your drawing is showing there is an old Current Source Inverter (CSI). In that technology, the GTO SCRs on the front-end rectifier would have created a lot of line notching and noise, so the isolation transformer was crucial in the design. The 2 offset secondaries was for harmonic mitigation, what's called a "12 pulse" rectifier.

Most modern MV VFDs are now going to be either Active Front End CSI drives, which don't need the input transformer, or Cascaded H Bridge Voltage Source Inverters (VSI), which will have a 24 pulse transformer built-in. There are still a few people using VSI designs with passive rectifiers and 12 or 18 pulse front-ends, but they are falling out of favor now. but even those will often use an autotransformer type design, the isolation is no longer as important as it was in those old designs.

But regardless, if your feed is 13.8kV, you will end up with an isolation transformer no matter what.

 

[spikes2020]

I don't know what your drive configuration is. Most VFDs comprise an input rectifier, six pulse for the most part, twelve pulse for larger units, a DC link filter, and a PWM output stage.

I do understand how it works, and i am saying that 1MW motor harmonics don't compare to a 35MW arc furnace. But also the DC bus on both the old drive and the new drive are designed completely different. Our old drive did not have any capacitors, while the new drive will. This will reduce the harmonics generated by the drive. Also Jraef response about how they work kind of lines up with my thoughts that it is more of an active front end to control harmonics better.

 

[spikes2020]

What your drawing is showing there is an old Current Source Inverter (CSI). In that technology, the GTO SCRs on the front-end rectifier would have created a lot of line notching and noise, so the isolation transformer was crucial in the design. The 2 offset secondaries was for harmonic mitigation, what's called a "12 pulse" rectifier.

Most modern MV VFDs are now going to be either Active Front End CSI drives, which don't need the input transformer, or Cascaded H Bridge Voltage Source Inverters (VSI), which will have a 24 pulse transformer built-in. There are still a few people using VSI designs with passive rectifiers and 12 or 18 pulse front-ends, but they are falling out of favor now. but even those will often use an autotransformer type design, the isolation is no longer as important as it was in those old designs.

But regardless, if your feed is 13.8kV, you will end up with an isolation transformer no matter what.

Ah thanks! yes this is what was thinking after i found the drawing. Yes we have 2 transformers for each motor. 1 goes from 13.8 to 4160 and then the second one, 4160 to 2 feeds of 2300. I am happy to remove 1 of the transformers to just leave the 13.8/4160. This means one less transformer to test, keep a spare and drain its secondary containment.

 

[Besoeker]

I do understand how it works, and i am saying that 1MW motor harmonics don't compare to a 35MW arc furnace. But also the DC bus on both the old drive and the new drive are designed completely different. Our old drive did not have any capacitors, while the new drive will. This will reduce the harmonics generated by the drive. Also Jraef response about how they work kind of lines up with my thoughts that it is more of an active front end to control harmonics better.

Will your new drive have an active front end?

 

[spikes2020]

Will your new drive have an active front end?

I sure hope so . Rockwell is the one supplying it, they understand what is supplying the drive and what the drive is feeding. So they didnt seem to have any concerns.

 

[Besoeker]

I sure hope so . Rockwell is the one supplying it, they understand what is supplying the drive and what the drive is feeding. So they didnt seem to have any concerns.

Are you the project manager?

 

[Rho_HV]

I am a little confused; so there is a 13.8kV to 4160V transformer feeding a VFD and the output of VFD is connected to a phase-shift transformer powering a "motor"? Shouldn't the phase shift transformer be connected to the 12 pulse rectifier and not its inverter? :?

 

[spikes2020]

I am a little confused; so there is a 13.8kV to 4160V transformer feeding a VFD and the output of VFD is connected to a phase-shift transformer powering a "motor"? Shouldn't the phase shift transformer be connected to the 12 pulse rectifier and not its inverter? :?

The VFD has a bypass in it, with a kirk key interlock. So after finding the drawings, it all makes sense now. There is 13.8kv feeding a transformer to get it down to 4160, then into the bypass part of the VFD. Then that feeds the 4160/2300 phase shift transformer that then in turn feeds the VFD drive portion.

I didn't design the system, we never used the bypass switch, and i am trying to remove the extra equipment out of my MCC rooms. My big question was the phase shift transformer and its purpose and to understand what was going on. I now understand the reason for it and why it's no longer needed.

 

[Jraef]

I think some of the confusion stems from your having described the transformers as being between the drive and the motor, they are not. And although they are Delta-Wye transformers where the L-N potential on the secondary is 2400V, they are not used that way.

So here's what I believe you have:
1) One large single transformer that takes the entire load from 13.8kV to 4160V, that 4160V then feeds the VFD input, AND the input to a FVNR Bypass Starter.
2) Within the VFD feed then, there are two phase shifting transformers that are 4160 in and out, but one is Wye the other is Delta, used in a 12 pulse harmonic mitigation scheme.
3) The output of the VFD goes directly to the motor.

If your new VFD is a PowerFlex 7000, it is probably an AFE drive and the phase shifting transformers would no longer be needed. But they COULD have quoted it as a 12 pulse diode front end, because you ALREADY own those transformers. It's something you should get clear. Either way, you will STILL need that 13.8kV to 4160V transformer, UNLESS because you no longer need the Bypass starter, they are quoting you a new AFE or 18 pulse drive with a NEW 13,8kV isolation transformer. That would be wasteful in my opinion, unless there is a problem with your transformer.

 

[drktmplr12]

I sure hope so . Rockwell is the one supplying it, they understand what is supplying the drive and what the drive is feeding. So they didnt seem to have any concerns.

its up to you to tell them if you want AFE or passive. each has their own maintenance implications which need to be considered based on the skill levels of your maintenance staff.

If possible, I would encourage you to consider mfr's other than Rockwell if footprint is a concern (trying to clear up space in MCC room). Their MV voltage source designs are massive compared to comparable Eaton, ABB, Yaskawa, Siemens ROBICON units.

Be careful letting the mfr design anything other than what is inside the drive. I have had reputable mfr's suggest installing drives in a room where you couldn't get them all out after they were installed.

disregard my post if this has already been considered.

 

[spikes2020]

I think some of the confusion stems from your having described the transformers as being between the drive and the motor, they are not. And although they are Delta-Wye transformers where the L-N potential on the secondary is 2400V, they are not used that way.

So here's what I believe you have:
1) One large single transformer that takes the entire load from 13.8kV to 4160V, that 4160V then feeds the VFD input, AND the input to a FVNR Bypass Starter.
2) Within the VFD feed then, there are two phase shifting transformers that are 4160 in and out, but one is Wye the other is Delta, used in a 12 pulse harmonic mitigation scheme.
3) The output of the VFD goes directly to the motor.

If your new VFD is a PowerFlex 7000, it is probably an AFE drive and the phase shifting transformers would no longer be needed. But they COULD have quoted it as a 12 pulse diode front end, because you ALREADY own those transformers. It's something you should get clear. Either way, you will STILL need that 13.8kV to 4160V transformer, UNLESS because you no longer need the Bypass starter, they are quoting you a new AFE or 18 pulse drive with a NEW 13,8kV isolation transformer. That would be wasteful in my opinion, unless there is a problem with your transformer.

1) Yes, something like that. I believe it goes only to the "FVNR Bypass Starter" and then that feeds the VFD or the motor directly.

2) The phase sift transformer is one unit sitting right next to the large 13.8/4160 transformer. It has 1 primary coil and 2 secondary coils as shown on the plate. Its all one unit and sitting outside taking up valuable space. 1 feed from the VFD/bypass feeds in, 2 feeds out back to the VFD.

3) Yup.

4) Rockwell knows what i am feeding them (3 phase 4160) from the 13.8/4160 transformer, and they know what they need to power the 4160 motor. So i wasn't afraid of an issue, but more curious of how the old system operated. They also had specification on harmonics and power factor that they must meet...


Thanks for the insight though and this has been a learning experience.

 

[spikes2020]

its up to you to tell them if you want AFE or passive. each has their own maintenance implications which need to be considered based on the skill levels of your maintenance staff.

If possible, I would encourage you to consider mfr's other than Rockwell if footprint is a concern (trying to clear up space in MCC room). Their MV voltage source designs are massive compared to comparable Eaton, ABB, Yaskawa, Siemens ROBICON units.

Be careful letting the mfr design anything other than what is inside the drive. I have had reputable mfr's suggest installing drives in a room where you couldn't get them all out after they were installed.

disregard my post if this has already been considered.

No, thanks for your insight!

I have specifications regarding harmonics and power factor injection onto our network. So they can supply what ever equipment they want, but if it contributes any additional harmonics or lowers our power factor they will be paying to fix it.

I got a quotes from other manufactures and they were not significantly different in terms of cost or footprint. We also use Rockwell for most of our equipment and have a very good working relationship with them. Also i rather have one type of equipment for stocking spare parts and integrating systems.