Secondary of transformer wye system center tap.
- Thread starter kjroller
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- New Jersey
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- Journeyman Electrician
Are you referring the the X0 connection where one side of each coil is connected? Generally a center tap is considered the connection to a single winding like in a 3Ø, 4 Wire, Delta system.
I feel like the answer is no it wouldnt since it would be used for the unbalanced load to complet the circuit but im trying to figure out how a corner grounded delta functions i get how to do it just dont understand why it works that way
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
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- Electrician
The only thing system grounding does is reference the system to earth potential. It will not effect how current flows on the various system conductors.
In other words if i put my meter on the center tap of the wye to ground it would read 0? Also in a cornergrounded delta lets say it loses reference to ground and the neutral is open would get 480 volts on that neutral since ita corner tapped?
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
A truly ungrounded system will tend to float around what would be the neutral voltage.
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- Licensed Electrician
We need to get some terms straight so we are all talking about the same thing in the same way-
Ground = earth
Equipment Ground = a low impedance path back to the voltage source
Grounded Conductor = A conductor at the service intentionally connected to the earth
Neutral = a common point between all transformer windings.
The center tap of a three phase wye is a neutral. The center tap of a single phase service is a neutral. The center tap of a three phase delta is called a neutral, but in truth it is not a neutral because the high leg to center tap is not the same voltage as the other two legs relative to the center tap.
In North America, with some rare exceptions, all neutral conductors are grounded, but not all grounded conductors are neutrals. A corner grounded delta is a case where the grounded conductor is not a neutral.
In a solidly grounded system, yes.
There is no neutral in a corner grounded delta, only a grounded conductor. If the system was ungrounded, for any reason, you would read 480V from phase A to B, A to C, and B to C, and would get some whacky voltages from any of them to ground. If you grounded one of the conductors intentionally, typically B phase, you would read 480V A to B, A to C, and B to C and you would read 0V B to ground.
To answer your question, you were correct in post #3, from the opposite end of any winding it would read 277V .
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
You need to understand that 0V is not special. It is just a defined reference. When we say a plane is flying at 20,000 feet, that number is meaningless unless you also specify the reference; is the plane flying 20,000 feet above the ground? 20,000 feet above mean sea level? Or at a level of atmospheric pressure equivalent to 20,000 feet above mean sea level on a standard day? (Answer, all of the above get used in different contexts.)
It is the same with voltage. You either give the voltage difference between two points, or you give the voltage relative to a defined reference.
The loads connected to a transformer don't care about the 'absolute' voltage of any point. All they care about is the voltage difference between the terminals. The load doesn't care about the connection to the earth, or which conductor is the grounded conductor, or even if there is a grounded conductor (with an important exception that I will get to below). All the load cares about is the voltage difference between the terminals that it is connected to.
The exception is that the insulation system of the load usually does care about voltage to ground. In normal operation, current is flowing through the load terminals, but if the insulation to ground fails then you will have a ground fault with current flowing to ground. That is why limiting voltage to ground is important, but it isn't essential to the normal operation of the load or to normal current flow.
We intentionally create a solid connection between one terminal and soil to force the system-ground voltage to a desired value, and to provide an intentional fault current path. But the loads don't care about the system-ground voltage during normal operation. Sometimes systems are intentionally left ungrounded to create a system that can continue operating even with a single ground fault.
Think about it: transformers work just fine on airplanes. If you had a long enough set of meter probes, you might find that the aircraft is at thousands of volts to ground (probably DC); but internally everything works just fine with the 200V 400Hz 3 phase and 28V DC. The shell of the aircraft is used as the local reference (and usually called 'ground', but clearly it has no connection to earth). There are specific other situations where you will find low voltage transformers 'referenced' to extremely high voltage. An example are the tower lights on AM radio stations. The tower might be at thousands of volts relative to ground, but the lights are supplied with 120 or 240V. They use a special high isolation transformer that derives the 120 or 240V even though the 'local ground' is thousands of volts relative to the nearby soil. ( see https://www.engineeringradio.us/blog/2011/09/tower-lighting-transformers-isolation-chokes-etc/ )
If you have a typical 480/277V wye system, and disconnect the earthing connection between X0 and ground, you will create an ungrounded system. The system would probably continue to operate normally, and typically the voltage between X0 and ground would be low. But without the intentional ground bonding the voltage between X0 and ground is set by things like leakage current and capacitive parasitic current, and during fault conditions the X0-ground voltage can be just about anything. If you have a solid phase-ground fault, X0-ground will be forced to 277V, and during some intermittent faults the X0-ground voltage could be above 480V. Ungrounded systems have a history of cascading failures where one fault pumps the system to very voltage relative to ground, and the high voltage causes insulation to fail.
-Jonathan
It is the same with voltage. You either give the voltage difference between two points, or you give the voltage relative to a defined reference.
The loads connected to a transformer don't care about the 'absolute' voltage of any point. All they care about is the voltage difference between the terminals. The load doesn't care about the connection to the earth, or which conductor is the grounded conductor, or even if there is a grounded conductor (with an important exception that I will get to below). All the load cares about is the voltage difference between the terminals that it is connected to.
The exception is that the insulation system of the load usually does care about voltage to ground. In normal operation, current is flowing through the load terminals, but if the insulation to ground fails then you will have a ground fault with current flowing to ground. That is why limiting voltage to ground is important, but it isn't essential to the normal operation of the load or to normal current flow.
We intentionally create a solid connection between one terminal and soil to force the system-ground voltage to a desired value, and to provide an intentional fault current path. But the loads don't care about the system-ground voltage during normal operation. Sometimes systems are intentionally left ungrounded to create a system that can continue operating even with a single ground fault.
Think about it: transformers work just fine on airplanes. If you had a long enough set of meter probes, you might find that the aircraft is at thousands of volts to ground (probably DC); but internally everything works just fine with the 200V 400Hz 3 phase and 28V DC. The shell of the aircraft is used as the local reference (and usually called 'ground', but clearly it has no connection to earth). There are specific other situations where you will find low voltage transformers 'referenced' to extremely high voltage. An example are the tower lights on AM radio stations. The tower might be at thousands of volts relative to ground, but the lights are supplied with 120 or 240V. They use a special high isolation transformer that derives the 120 or 240V even though the 'local ground' is thousands of volts relative to the nearby soil. ( see https://www.engineeringradio.us/blog/2011/09/tower-lighting-transformers-isolation-chokes-etc/ )
If you have a typical 480/277V wye system, and disconnect the earthing connection between X0 and ground, you will create an ungrounded system. The system would probably continue to operate normally, and typically the voltage between X0 and ground would be low. But without the intentional ground bonding the voltage between X0 and ground is set by things like leakage current and capacitive parasitic current, and during fault conditions the X0-ground voltage can be just about anything. If you have a solid phase-ground fault, X0-ground will be forced to 277V, and during some intermittent faults the X0-ground voltage could be above 480V. Ungrounded systems have a history of cascading failures where one fault pumps the system to very voltage relative to ground, and the high voltage causes insulation to fail.
-Jonathan
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
- Occupation
- Electrician
Also just want to reiterate what was mentioned in post #2 that "center tap" is not the correct term. A center tap is the midpoint of an individual transformer winding, used most commonly for 120/240 single phase and high leg Delta.
- Location
- New Jersey
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- Journeyman Electrician
Yes the terminology helps us to better understand the question.
- Location
- Illinois
- Occupation
- retired electrician
For simplicity think of the corner grounded system just like you think of a 120/240 volt system.
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
How so? I don't think of the two as being remotely similar.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
- Occupation
- Electrician
A corner grounded panelboard would look the same as a single phase panelboard. The only difference would be the voltage between the grounded conductor bus and the phase conductors would be 240 instead of 120.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
To ggunn's point I wouldn't consider them similar given the vastly different voltages.
- Location
- Illinois
- Occupation
- retired electrician
You have 3 conductors with voltaged between then and one of them is a grounded conductor. The installation is identical, however sometimes you see 3 phase equipment used for a corner grounded system, but single phase equipment with the correct voltage rating is permitted.