# Delta-Wye Tranformer



## McCready00 (Aug 22, 2009)

What should I know about these. I know how it works but is there any specific stuff I should know? Like, where is the current mostly going between the winds and the ground when it comes back from the circuits?

Thanks


----------



## headcrab (Aug 22, 2009)

Just to clarify, current returns from circuits along the neutral, not the ground. The ground is the path for fault current.
The only other thing you might need to know is phase control dimmers (most of those that are in use today) generate insane harmonics which add up on the neutral in a three phase system, requiring the neutral to be overrated 130%, and overheating the transformer if it is not sized with the extra harmonics taken into account.
Transformers designed to work with non-linear loads like phase control dimmers and switching power supplies are said to be K-rated.


----------



## STEVETERRY (Aug 22, 2009)

McCready00 said:


> What should I know about these. I know how it works but is there any specific stuff I should know? Like, where is the current mostly going between the winds and the ground when it comes back from the circuits?
> 
> Thanks



You might be interested in the Glossary entry entitled: Transformer, Harmonic Mitigating.

ST


----------



## icewolf08 (Aug 22, 2009)

Also, Delta and Wye are two different types of transformers. I hate to ask why you need to know, because if you need to use/setup or otherwise interface with a transformer and you have to ask this type of question then you should not be doing whatever it is you may be doing. If that is the case you should consult a qualified electrician.


----------



## epimetheus (Aug 22, 2009)

icewolf08 said:


> Also, Delta and Wye are two different types of transformers. I hate to ask why you need to know, because if you need to use/setup or otherwise interface with a transformer and you have to ask this type of question then you should not be doing whatever it is you may be doing. If that is the case you should consult a qualified electrician.



I completely agree with the fact that if you have to ask, you shouldn't be using/setting up or otherwise doing anything with a transformer like this.

That said, delta and wye are not different types of transformers in themselves. Delta and Wye describe the configuration of the windings in a transformer. You can't have just a Delta or just a Wye transformer. The most common types are Delta-Wye, Wye-Delta, Delta-Delta, and Wye-Wye. These terms apply to 3-phase systems only, by the way.


----------



## icewolf08 (Aug 23, 2009)

epimetheus said:


> That said, delta and wye are not different types of transformers in themselves. Delta and Wye describe the configuration of the windings in a transformer. You can't have just a Delta or just a Wye transformer.



Yeah, it had been a long day and I was only thinking in terms of secondary windings...


----------



## epimetheus (Aug 23, 2009)

I figured as much. We've all had those...
_Posted via Mobile Device_


----------



## McCready00 (Aug 24, 2009)

headcrab said:


> Just to clarify, current returns from circuits along the neutral, not the ground. The ground is the path for fault current.
> The only other thing you might need to know is phase control dimmers (most of those that are in use today) generate insane harmonics which add up on the neutral in a three phase system, requiring the neutral to be overrated 130%, and overheating the transformer if it is not sized with the extra harmonics taken into account.
> Transformers designed to work with non-linear loads like phase control dimmers and switching power supplies are said to be K-rated.




Well I know, but isn't the neutral tapped with the ground inside the transformer? And I might be wrong, but the ground cable is also tapped to the windings!..

Then, could we say, in a technically point of view, that the ground is sort of used as the Neutral continuity?


----------



## headcrab (Aug 24, 2009)

It really depends on how the transformer is connected. With the standard delta connection, the neutral is the center tap on one of the three transformers. The ground is also connected here and then earthed. So yes, they are connected.
What do you mean by the neutral continuity?


----------



## epimetheus (Aug 24, 2009)

headcrab said:


> It really depends on how the transformer is connected. With the standard delta connection, the neutral is the center tap on one of the three transformers. The ground is also connected here and then earthed. So yes, they are connected.
> What do you mean by the neutral continuity?



There are many "standard" delta-connected secondaries. A center-tapped transformer is only one of those possible configurations. Normally, the ground is bonded to the neutral in the first disconnecting mains after the transformer. There are situations where this bonding can occur in the transformer, but that requires careful consideration of local applicable electrical codes.

Point of fact, the ground conductor is not a current carrying conductor. Current supplying a load is supplied via phase conductor and returned via another phase conductor (phase to phase loads) or the neutral conductor (phase to neutral loads). Any load current on the ground conductor means you have a ground fault in your system. This is how GFCI's function - they trip when the current going out the phase does not match the current coming back in the neutral.


----------



## McCready00 (Aug 24, 2009)

well.. if, in the worst-case scenario, you get our of phase as 0% on the first line, 50% in the second and 100% in the third, isn't the neutral supposed to suck most of the current. Well, in this case, the ground in the primary becomes an extention to the neutral.

Am I wrong?


----------



## fredthe (Aug 24, 2009)

McCready00 said:


> Well I know, but isn't the neutral tapped with the ground inside the transformer?


Not necessarily. I once spent a week troubleshooting problems in a bunch of communication equipment, only to find that the neutral on the delta-why transfromer had been left ungrounded :shock:

I questioned the electrician on this, and he told me that it was ungrounded because it was supposed to be an "isolation" transformer. (Yes, he was a licensed, union electrician.) Needless to say, once the neutral was properly bonded to ground, all my problems went away.

-Fred


----------



## epimetheus (Aug 24, 2009)

McCready00 said:


> well.. if, in the worst-case scenario, you get our of phase as 0% on the first line, 50% in the second and 100% in the third, isn't the neutral supposed to suck most of the current. Well, in this case, the ground in the primary becomes an extention to the neutral.
> 
> Am I wrong?



In a 3-phase, 4-wire (or 5-wire) system the neutral will carry a sum of the unbalanced phase current. However, this current does not flow through the ground conductor. The neutral current, if any, is carried back to the transformer where it goes back in to the transformer windings. So, even though the neutral and the ground are bonded together, the ground conductor does not carry current unless there is a fault on the system. Connecting loads between a hot phase and ground is a big no-no. The correct way to connect loads either phase-to-phase or phase-to-neutral.


----------



## JD (Aug 24, 2009)

Generally, a building should have a single earth ground and a single point where this ground bonds to the neutral, usually at the service entrance disconnect or main panel. The ground serves two purposes; Outside the building, it is a way for lightning strikes and stray voltages to find their way to the earth. Inside the building, it serves as an error path should a short occur in part of the wiring. In day-to-day usage, it is not a current carrying conductor, although some micro-leakage may occur in appliances that are plugged in. Any grounded appliance that develops a fault that would have made it's case electrically "hot", returns this power via the ground, hopefully overloading the circuit and tripping the supply breaker. Some breakers, such as GFI, are designed to trip as soon as there is any imbalance. 

There are some examples of multi-point ground. As an extreme example, I live in a house that has a lightning rod system. 14 "air terminals" are on the roof, and 6 grounding rods are in the soil at various points. All are bonded together using a special type of ~3 gauge braided cable. This system also bonds to the incoming neutral at the service entrance head, along with a fairly large MOV unit. As this bond is made outside of the building and prior to the service entrance, it is not considered part of the internal wiring of the house.


----------



## church (Aug 26, 2009)

For some additional explanation of grounding (earthing) you can check the following link.

Untitled

although it is an on-line book that provides explanation for Electricians to assist them in understanding the Institution of Electrical Engineers Wiring Regulations the U.K. equivalent of the U.S. NEC and the canadian Electrical Code it provides a good explanation of the principles.


----------



## STEVETERRY (Aug 26, 2009)

church said:


> For some additional explanation of grounding (earthing) you can check the following link.
> 
> Untitled
> 
> although it is an on-line book that provides explanation for Electricians to assist them in understanding the Institution of Electrical Engineers Wiring Regulations the U.K. equivalent of the U.S. NEC and the canadian Electrical Code it provides a good explanation of the principles.



North American technicians should be very careful with this document, as a great deal of it is not allowed under the NEC. This is particularly true of the wide variety of earthing systems found in the UK and Europe. In addition, there are no RCD devices in North America, only GFCI and GFP devices. RCD's are often used in Europe to create ungrounded or impedance-grounded systems that are still safe. This is not allowed in the US or Canada.

In general, North American practice is to hard-ground the neutral as close to the transformer or service entrance as possible. Any number of delta-wye transformers can be installed in a facility, creating "separately derived" neutrals that are hard-grounded at each transformer.

ST


----------



## Dionysus (Aug 26, 2009)

As a side note only the conductor connecting from the mains bond with the neutral out into the ground is called a "ground", all the others commonly called a ground are in actually called a "bond".

All of those green and bare wires are "bonding conductors".


----------



## epimetheus (Aug 26, 2009)

Dionysus said:


> As a side note only the conductor connecting from the mains bond with the neutral out into the ground is called a "ground", all the others commonly called a ground are in actually called a "bond".
> 
> All of those green and bare wires are "bonding conductors".



US NEC has the Grounding Electrode, Grounding Electrode Conductor, Grounded Service Conductor, Main Bonding Jumper, and Equipment Grounding Conductor. What you are calling the "bond", the NEC calls the Equipment Grounding Conductor. Your "ground" = NEC Grounding Electrode Conductor. The way the NEC is written, "ground" and "bond" are not nearly specific enough without modifiers.


----------



## Dionysus (Aug 26, 2009)

epimetheus said:


> US NEC has the Grounding Electrode, Grounding Electrode Conductor, Grounded Service Conductor, Main Bonding Jumper, and Equipment Grounding Conductor. What you are calling the "bond", the NEC calls the Equipment Grounding Conductor. Your "ground" = NEC Grounding Electrode Conductor. The way the NEC is written, "ground" and "bond" are not nearly specific enough without modifiers.



You have pointed out another specific way in which the NEC and CEC differ.

CEC Section 10 "Grounding And Bonding", see section 0.

Bonding - A low impedance path obtained by permanently joining all con-current-carrying metal parts to ensure electrical continuity and having the capacity to conduct safely any current likely to be imposted on it.

Bonding conductor - A conductor that connects the non-current-carrying parts of electrical equipment, raceways, or enclosures to the service equipment or system grounding conductor

Grounding - A permanent and continuous conductive path to the earth with sufficient ampacity to carry any fault current liable to be imposed on it, and of a sufficiently low impedance to limit the voltage rise above ground and facilitate the operation of the protective devices in the circuit.

Grounding Conductor - The conductor used to connect the service equipment or system to the grounding electrode.

Grounding Electrode - A buried metal water piping system or metal object or device buried in, or driven into, the ground to which a grounding conductor is electrically and mechanically connected.

An important note. Rule 26-258 (1)
The conductors supplying transformers shall have an ampacity rating
(a) not less than 125% of the rated primary current of the transformer for a single transformer; or
(b) not less than the sum of the rated primary currents of all transformers plus 25% for the rated primary current of the largest transformer for a group of transformers operated in parallel or on a common feeder.

The 125% rule also applies to secondaries.


----------



## epimetheus (Aug 26, 2009)

It figured we were talking about differences between countries. Just wanted to make sure anybody else reading this realized that.


----------



## TimMiller (Aug 27, 2009)

And this is why in my commercial electrical class we talked about grounding for a month...... In a nut shell, (talking the LV side of the transformer ie secondary side in 120\208 sec voltage) In delta you have a leg of 120, a "high leg" of 208V, and another leg of 120 metering L-N. Now when you meter line to line you will get a voltage of 240. Delta is what people really get into trouble doing a tie-in and not metering the voltage. They end up sending a leg of 208V to their gear and blowing stuff up. In Wye, you have 3 legs of 120 that meter 208V from line to line. Also when wiring up panel boards its absolutely critical you keep your legs in order. You do not want to offset them, ie install a 3ph breaker, a single breaker, or double breaker, then another 3 phase breaker. That will burn up equipment very quickly. I learned this once from an idiot electrician i told not to and he did it anyways. Delta you commonly run into for transformers feeding motor loads. Motors are a lot happier, you drop your startup (locked rotor) amps. Wye, is your common lighting loads (which includes recepticals and 120V appliances)


----------



## STEVETERRY (Aug 27, 2009)

Dionysus said:


> You have pointed out another specific way in which the NEC and CEC differ.



CEC--that's the Chechnyan Electrical Code, right? 


ST


----------



## STEVETERRY (Aug 27, 2009)

TimMiller said:


> And this is why in my commercial electrical class we talked about grounding for a month...... In a nut shell, (talking the LV side of the transformer ie secondary side in 120\208 sec voltage) In delta you have a leg of 120, a "high leg" of 208V, and another leg of 120 metering L-N. Now when you meter line to line you will get a voltage of 240. Delta is what people really get into trouble doing a tie-in and not metering the voltage. They end up sending a leg of 208V to their gear and blowing stuff up. In Wye, you have 3 legs of 120 that meter 208V from line to line. Also when wiring up panel boards its absolutely critical you keep your legs in order. You do not want to offset them, ie install a 3ph breaker, a single breaker, or double breaker, then another 3 phase breaker. That will burn up equipment very quickly. I learned this once from an idiot electrician i told not to and he did it anyways. Delta you commonly run into for transformers feeding motor loads. Motors are a lot happier, you drop your startup (locked rotor) amps. Wye, is your common lighting loads (which includes recepticals and 120V appliances)



You are describing "high-leg delta" and "delta" interchangeably. They are not the same.

High-leg delta provides a center-tap of one of the delta windings that is the grounded neutral. This is used to provide a small amount of 120/240 split-phase power from a transformer where the majority of the loads are 240V 3-phase delta (no neutral). The "high leg" is the non-center tapped phase, which must not be reference to the center tapped neutral, as its voltage will be 208V to that neutral. that is why it is marked with a distinctive color, usually orange. When faced with a high-leg delta service, the entertainment electrician must regard it as a 120/240V single-phase service, and must ignore the high leg.

Delta is a three-phase, three wire winding. Standard voltages are 240V and 480V in North America. It is used primarily for motor loads. There is no neutral of any kind on a delta service.


----------



## TimMiller (Aug 27, 2009)

steve you are correct, that somehow slipped my mind when i was explaining that.


----------



## JD (Aug 29, 2009)

The floating Delta, or straight Delta as Steve describes can also be a "gotcha!" There is no connection to Neutral or Ground, however equipment in use may incorporate bleeder resistors to drain off stray voltage. This may produce a phantom meter read that looks as if all three legs are ~ 120 volts to ground. (Fake Wye) As soon as a load on any one leg comes into play, that leg goes to 0 volts and the other two go to 240! A simple meter check from hot to hot will weed this one out as it will be 240, not 208.


----------



## STEVETERRY (Sep 4, 2009)

JD said:


> The floating Delta, or straight Delta as Steve describes can also be a "gotcha!" There is no connection to Neutral or Ground, however equipment in use may incorporate bleeder resistors to drain off stray voltage. This may produce a phantom meter read that looks as if all three legs are ~ 120 volts to ground. (Fake Wye) As soon as a load on any one leg comes into play, that leg goes to 0 volts and the other two go to 240! A simple meter check from hot to hot will weed this one out as it will be 240, not 208.



I didn't know that.

Thanks!

ST


----------



## mstaylor (Sep 4, 2009)

I've used what we call wild leg delta but I have never been able to get an explanation of exactly how or why it works. I don't know that I have ever used a straight delta system. I have used Wye systems all the time. Learning new things is a good thing. Also my home building also uses 277 for it's lighting circuits. What is the voltage of the three phase at this level? 440, 460 or 480?


----------



## derekleffew (Sep 4, 2009)

mstaylor said:


> ...Also my home building also uses 277 for its lighting circuits. What is the voltage of the three phase at this level? 440, 460 or 480?


What's 277 x √3 ?


----------



## epimetheus (Sep 5, 2009)

derekleffew said:


> What's 277 x √3 ?



It's 480V for those that don't feel like doing the math. Point of note, though, if you have load connected at 277V, you have a 480Y/277, 3 phase system. If all your loads are connected at 480V, and you have no neutral, that's a true 480V delta system.


----------



## mstaylor (Sep 5, 2009)

I wasn't thinking of it 480 as a true Wye system. Even though I am mostly involved in theatrical power, I do get into our physical plant of my civic center at times, the ever expanding hat I wear. Most recently we have installed a generator to cover all of the building instead of just lighting circuits.


----------



## epimetheus (Sep 5, 2009)

480Y/277 is very common in industrial and medium to large commercial installations. The higher the voltage, the more effeciently you can transmit power (less current, therefore less loss due to wire resistance).


----------

