Transformer Expert Needed

[T.M.Haja Sahib]

I am concerned about what happens when there is a fault from the one mentioned phase to the ground that is only going to probably have anywhere between 12 - 72 volts depending on which "H" taps are used.

If there is low impedance ground fault for that portion of tap which is already intentionally grounded, the increase in current in the primary side of the transformer may actuate the primary protection. But if the impedance of the ground fault is such that the ground fault current exceeds the rating of winding portion of that tap but still not sufficient to operate the primary protection, damage to the transformer may occur. The same logic applies even when any other point in the delta or star is unintentionally grounded when there already exists one 'earth' point in the winding.
So additional protection of transformer winding may be needed.

 

[mike_kilroy]

infinity

Originally Posted by kwired
As far as NEC goes:

Is the conductor that is grounded a "common conductor"?

I guess it is the closest thing to being one but certainly is not as common as the center point of a Wye system.

IMO only a wye system would have one point common to all three phases.​

aren't you reading more into that statement than you should? IF they meant the common had to be "equal voltage from all phases" I bet they would have written that in the statement too?

Since a corner grounded phase is acceptable, or CT point, then the voltages don't have to be equal to satisfy this statement I think?

But if the whole object of this was to 'get a voltage reading to ground' for an electrician to read, I assume for safety, why not just corner ground it right and be done with it? Is the end customer concerned their electricians would just check a single ph to ground for juice to see if power is off? If so, then perhaps this is NOT a good place to jury rig for the elevator or you may be sucked into the liability suit!

BTW, since this was for temp use, have they ordered the correct transformer yet? If not that too is telling - they may be planning to use this indefinitely?

 

[kwired]

infinity

Originally Posted by kwired
As far as NEC goes:

Is the conductor that is grounded a "common conductor"?

I guess it is the closest thing to being one but certainly is not as common as the center point of a Wye system.

IMO only a wye system would have one point common to all three phases.​

aren't you reading more into that statement than you should? IF they meant the common had to be "equal voltage from all phases" I bet they would have written that in the statement too?

Since a corner grounded phase is acceptable, or CT point, then the voltages don't have to be equal to satisfy this statement I think?

But if the whole object of this was to 'get a voltage reading to ground' for an electrician to read, I assume for safety, why not just corner ground it right and be done with it? Is the end customer concerned their electricians would just check a single ph to ground for juice to see if power is off? If so, then perhaps this is NOT a good place to jury rig for the elevator or you may be sucked into the liability suit!

BTW, since this was for temp use, have they ordered the correct transformer yet? If not that too is telling - they may be planning to use this indefinitely?

I'm not so certain that the center point of a wye is exactly what was in mind. The point proposed for being used as a ground here is not something usual. It is hard to cover all the unusual things that are possible. The specific instance that is mentioned where voltages are not all equal is the delta with a high leg.

As far as the whole object being "to get a voltage reading to ground", yes you would get a voltage reading to ground, but if it is only 20-25 volts, and your ignorant technician is expecting at least 200 or more - how do you know they will interpret it as live vs not live? I would still rather see conventional corner grounding.

 

[Jraef]

Someone ran this by me today. They're reverse wiring a standard Delta/Wye transformer to get 480 volts with a 208 volt supply. Here are the details as I've been told:
1) Standard everyday 225 KVA transformer w/ 480 Delta primary, 208Y/120 volt secondary
2) Temporarily reverse wired to provide 480 volts to elevator motors3) System to operate as grounded* but not a corner grounded Delta

Temporary? Is this just a test bench? If so, why are they so concerned about measurements to ground for crying out loud? The motors will not care, and if it's a test bench, SURELY there will be test procedures established based on it being a 480V delta system right? Seems like a whole lot of running around to accommodate a noob technician.

At first I thought maybe this was a permanent installation, and their concern would be valid because a lot of elevators are serviced by elevator technicians that have expertise in 1) Mechanics, 2) Hydraulics, 3) Overcoming fear of heights, and 4) electrical, in that order of competency. So if this were permanent, I can see it. But then I thought, wouldn't it just be a whole lot easier to just swap out the motors with 208V versions?

And might it still even if it's a test bench? Most elevator motors I have seen are 12 lead, meaning you can connect them 230 or 460V, so a more simplistic autotransformer to buck the 208V to 230V might be a lot easier.

 

[infinity]

More information:

New building with a temporary 208Y/120 volt service. Elevator machine rooms all wired, 480 volt elevator VFD's wired, GC wants to provide temporary 480 volt elevator power cheapest way possible since the utility network won't be up and running for a few months. EC will reverse wire a new, onsite 225 KVA, 480 Delta-208Y/120 volt transformer temporarily to provide the 480. This transformer will be used later on in a normal step-down fashion for permanent 208Y/120 volt power. I'm guessing that they want to avoid using a corner ground system since there are multiple sets of fuses being used but I don't know that for sure. They say that want to have something in place so if an elevator tech or elevator constructor checks from phase to ground they actually can read a voltage.

When asked for my opinion I said that they should choose either corner grounded or ungrounded Delta and take the appropriate action to do so in a code compliant matter. If that wasn't acceptable then buy the correct transformer.
As of right now it looks like they still will go with the original design.:jawdrop:

 

[don_resqcapt19]

They could add a zig-zag grounding transfomer, but that would probably cost more than they want to spend.

 

[Jraef]

Warning: Most VFDs cannot be connected to 480V delta without being modified, and I guarantee they will not like the arrangement you propose. They have surge protection and EMC/RFI filters on them now to meet CE and FCC requirements which are all referenced to ground. If you do not have a Wye system, their Wye point will attempt to become the Wye point for the entire system and it blows the front-end of the drive. It's not pretty, trust me.

Some can be modified in the field to change that connection but someone will have to remember to re-connect them properly when the real service is installed. Those are usually the ones like A-B or the big EU companies like Siemens, Telemecanique and ABB that are made for the North American market. Many of the low cost Asian and some smaller EU drives do not have the ability to be modified and will recommend that you put a Delta-Wye transformer ahead of the drives.

 

[infinity]

Warning: Most VFDs cannot be connected to 480V delta without being modified, and I guarantee they will not like the arrangement you propose. They have surge protection and EMC/RFI filters on them now to meet CE and FCC requirements which are all referenced to ground. If you do not have a Wye system, their Wye point will attempt to become the Wye point for the entire system and it blows the front-end of the drive. It's not pretty, trust me.

Some can be modified in the field to change that connection but someone will have to remember to re-connect them properly when the real service is installed. Those are usually the ones like A-B or the big EU companies like Siemens, Telemecanique and ABB that are made for the North American market. Many of the low cost Asian and some smaller EU drives do not have the ability to be modified and will recommend that you put a Delta-Wye transformer ahead of the drives.

Interesting, these drives for Schindler elevators and are manufactured in Sweden. I will ask if anyone thought about that. Would their grounding scenario solve that problem?

 

[mike_kilroy]

now that vfd's are brought into the picture, I have to agree with Jraef: if this scheme is used my Hitachi drive will blow their input MOVs on power up.... there is a procedure to disconnect the MoVs but few ever see it in the manual until after the smoke is released, and just blow them up. Maybe an ungrounded system is better or if those electricians REALLY need to be able to read a L-G voltage, then either train em how to use their meters to read L-L, or add a simple 480v wye xfmr winding on the system to get an even 277v L-g reading for them.....

 

[infinity]

Can someone explain (in electricians terms ) why the drives would care if the 3? power is derived from a Delta or Wye system?

 

[T.M.Haja Sahib]

Can someone explain (in electricians terms ) why the drives would care if the 3? power is derived from a Delta or Wye system?

I think the problem to the surge arrester/filter in the drives may arise due to any unbalanced loading on three phase power supply of the delta side of the transformer supplying power to the drives also, which can cause higher voltage applied to them than their rating. But in case of wye connection of transformer, no problem because of presence of neutral conductor.

 

[kwired]

Can someone explain (in electricians terms ) why the drives would care if the 3? power is derived from a Delta or Wye system?

I like the suggestion that if there are MOV's from each line to ground for surge protection that they likely will not take 480 volts to ground very well as they were probably intended to start clamping around 300 volts - maybe up to 400 volts.

 

[steve66]

If they are just trying to ground the delta (can we call it the Transformer 1 delta -T1?), couldn't they connect a transformer (T2) with a wye primary, and ground the center of the T2 primary?

So T1 secondary delta supplies the elevator and the T2 wye, and the T2 wye is grounded. The secondary of T2 could just be left open.

T2 can be as small as you want - how small of a transformer can you find?

Just a thought- feel free to shoot holes in it.

 

[kwired]

If they are just trying to ground the delta (can we call it the Transformer 1 delta -T1?), couldn't they connect a transformer (T2) with a wye primary, and ground the center of the T2 primary?

So T1 secondary delta supplies the elevator and the T2 wye, and the T2 wye is grounded. The secondary of T2 could just be left open.

T2 can be as small as you want - how small of a transformer can you find?

Just a thought- feel free to shoot holes in it.

Not sure what you are trying to talk about, the transformer in this case is a separately derived system - there is no connection at all between any portion of primary and secondary aside from what ever point of the secondary one may choose to ground. This puts a ground reference on both primary and secondary but serves no intentional purpose for supplying any load on either side of the transformer. During abnormal conditions it could end up carrying current.

The secondary of this transformer only has three leads plus additional voltage "taps" that were not really intended to be used at same time as another tap for the same phase conductor. They were only intented to be connect conductors to the correct tap to get the desired output voltage (or in reality since this is a backfed unit, they were intended to match number of turns of primary winding to the input voltage to get desired output voltage. They are not half winding taps like you would see on a typical dual voltage input they are only for compensating a small percent change of voltage.

 

[Jraef]

Not sure what you are trying to talk about, the transformer in this case is a separately derived system - there is no connection at all between any portion of primary and secondary aside from what ever point of the secondary one may choose to ground. This puts a ground reference on both primary and secondary but serves no intentional purpose for supplying any load on either side of the transformer. During abnormal conditions it could end up carrying current.

The secondary of this transformer only has three leads plus additional voltage "taps" that were not really intended to be used at same time as another tap for the same phase conductor. They were only intented to be connect conductors to the correct tap to get the desired output voltage (or in reality since this is a backfed unit, they were intended to match number of turns of primary winding to the input voltage to get desired output voltage. They are not half winding taps like you would see on a typical dual voltage input they are only for compensating a small percent change of voltage.

But he indirectly brings up a good point. If all this is doing is feeding the drives temporarily, why not just use 3 single phase buck-boost autotransformers and connect the secondaries in wye? All problems solved. Sure, an isolation transformer might be a better long term solution, but this is temp.

 

[kwired]

But he indirectly brings up a good point. If all this is doing is feeding the drives temporarily, why not just use 3 single phase buck-boost autotransformers and connect the secondaries in wye? All problems solved. Sure, an isolation transformer might be a better long term solution, but this is temp.

I understand that, how do you get that out of what he posted? He had me totally confused as to what he was talking about.

 

[mike_kilroy]

He is just saying the same thing I said in a couple posts..... make the Xo reference with ANOTHER xfmr. Just hook up any old Wye 460v xfmr winding to the 460V 3ph of concern here. A 250va xfmr, a 250kva xfmr, whatever. Ground the Xo. Now all 3 phases will show the electrician 277v to ground to satisfy the OP question.

 

[pfalcon]

High impedance grounding?

 

[Jraef]

I understand that, how do you get that out of what he posted? He had me totally confused as to what he was talking about.

He had me confused too. But I can't say why my mind triggered on this when reading his post. Maybe the mention of creating an X0 on the secondary?

Or it's the drugs I take...

 

[Jraef]

Can someone explain (in electricians terms ) why the drives would care if the 3? power is derived from a Delta or Wye system?

It's in my post; drives have surge protection (usually MOVs) and filters on the front-end, usually set up in a wye to ground. So when you have an delta system feeding the drive or even a resistance grounded wye system, that connection becomes a path of least resistance to ground for the entire distribution system.