Transformer grounding

[lukedric]

I was asked to look into a grounding issue inside a substation because of circulating ground currents.

The incoming service is 13.8kV 3 phase, 3 wire. There are 4 step down delta-wye transformers with neutrals grounded to station ground.

The problem is with a single phase 13.8kV-208V step down transformer which was replaced recently from an ungrounded secondary to grounded secondary because the inspector said so, following NEC.

Part of the problem is that the transformer was specified without a center tap so they grounded one secondary terminal. Is there a correct terminal to be grounded considering that the other 3 ph transformers' wye secondary terminals are also grounded? If the polarity of the 1 ph tx is subtractive, will the connections be reversed.

If the wrong terminal is grounded, isn't it equivalent to inducing common mode supply on the secondary which can cause leakage currents even if the tx is not loaded.

I think the above best explains why most residential tx have center taps. The transformer is forced to be balanced at the secondary and there's no room for mis-application.

 

[busbar]

A basic North American reference comparing secondary-winding connections related to systems grounding is ANSI Std C84.1 Annex 1. “13.8/208” would be an unusual transformer if the secondary is 1ø 2- or 3-wire. Can you clarify the as-found transformer-secondary voltage and grounding form (and additionally, versus that specified at design stage?)

A crucial consideration are nameplate voltage ratings of secondary-load equipment, and overcurrent-device rating and location. [A default 1ø arrangement may be 120/240V 3-wire.]

 

[lukedric]

Thanks Busbar.

I don't have information on the replaced 40 years old transformer because I wasn't involved with the installation. But based on conditions at the other sites, it looked like a 3-wire 208V, 1ph secondary. The center tap was not grounded nor used as a neutral.

The user load is a critical signalling equipment that requires an ungrounded source. Normally, they are supplied from a 2 pole breaker off a 208V 3 phase supply (grounded at the wye neutral). The loads are referencing grounds elsewhere for their phasors and a grounded source will give them skewed phase diagrams. So based on this, instead of supplying a 3 phase transformer it was assumed that the old setup is grandfathered and the specified single phase customized transformer will do the job.

I did a search here in this forum and bought the "grounding neutral" standard you recommended elsewhere but there is nothing in the document about polarity/phasing problems.

 

[rbulsara]

I a bit unclear what you are saying. I did u actually replace the single phase tx or just grounded one of the leg of 208V secondary?

If it is a 2 winding 13.8kV/208V single phase Tx, it really does not matter which end you ground. Nor it has anything to with how other tx are grounded. You can not mix them up with other delta-wye units' supply so I am not sure what you are asking?

Your second post is even more confusing,what does the 2-pole circuits from a 3 phase wye secondary of a transformer has to do with the single phase unit you are referrring to in the first post? If the 3 phase grounded wye is used to feed so called 'ungrounded' circuits, what does the single phase unit feeds? You are thoroughly confused in what a ungrounded circuit means. Explain loads referencing grounds elsewhere?

 

[lukedric]

rbulsara (Electrical) Jan 21, 2005 wrote:
I a bit unclear what you are saying. I did u actually replace the single phase tx or just grounded one of the leg of 208V secondary?
******
*Replaced with a grounded leg
**********
If it is a 2 winding 13.8kV/208V single phase Tx, it really does not matter which end you ground. Nor it has anything to with how other tx are grounded. You can not mix them up with other delta-wye units' supply so I am not sure what you are asking?
******
* Each transformer feeds its own load.
* When you say it does not matter which end to ground, you are implying that two separately derived systems can be grounded at a common point. It is like interconnecting the grounds of a CG delta secondary to a wye grounded secondary, do you think you will get circulating currents?

Note that 13.8kV is line to line.

**********
Your second post is even more confusing,what does the 2-pole circuits from a 3 phase wye secondary of a transformer has to do with the single phase unit you are referrring to in the first post? If the 3 phase grounded wye is used to feed so called 'ungrounded' circuits, what does the single phase unit feeds? You are thoroughly confused in what a ungrounded circuit means. Explain loads referencing grounds elsewhere?

******
* I am not going to confuse you further with the loads connected.

 

[rbulsara]

Thanks for you clarifications.

Yes , you can ground two separately dervied systems to a common point..its routinely done.

What do you think of the system when there are multiple stepdown transformers in building whose secondaries are grounded to say a building steel? or for that matter your other four delta-wye transformers?
'
In fact vast majority of grounded systems in world are connected to a common point..the earth..

 

[mc5w]

The reason why the inspector said that the transformer secondary had to be grounded is that National Electrical Code requires solid grounding if there is a way to ground the system so that the maximum voltage to ground is 150 volts.

Sounds to me that your existing delta-wye transformers are solidly grounded on the secondary side right at the secondary and there are separate equipment grounds for conduits and boxes and an isolated equipment ground the is insulated from conduits and boxes except at the common grounding point. I am correct that this is your arrangement?

How come the replacement transformer was not ordered with a 4-terminal wye secondary? Is this a dry transformer or a liquid filled transformer? You can ignore an extra terminal if you do not need it for your application but if you do need it you have it.

Mike Cole

 

[lukedric]

Let me rephrase the original post in electromagnetic terms:

Suppose we impress Vab (3 ph source) to both H1-H2 terminals of the above 1ph tx and 3ph tx so that they are in parallel.

The X1-X0 will be in phase with H2-H1 phasors on both transformers so that both X0 terminals can be grounded at a common point. X0-X1 will be 180 degrees off-phase wrt H2-H1, so what happens if we connect the X1 terminal of the 1ph tx to the X0 terminal of the 3ph tx?

We know X1-X2 is 30 degrees off-phase wrt H1-H2 on the 3ph d-y tx.

My electromagnetics is rusty and I am hoping someone out there can enlighten.

 

[rbulsara]

Nothing!

Two separate Tx have nothing to do with each other.