3-phase Y neutral
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Re: 3-phase Y neutral
Simba, the delta config is wired start-finish in a closed ring such that the vector sum of all the phases is zero. Get one winding backward, then blooie!
In the wye config, all the start (or finish) terminals are tied to the neutral so that the vector diagram looks like a wye. Get one backward and the vector diagram looks like a crow's foot. Since the windings do not form a closed loop, there is no danger of the xfrmr frying itself, but major problems surface when loads are applied.
Ed, we need some diagrams.
Simba, the delta config is wired start-finish in a closed ring such that the vector sum of all the phases is zero. Get one winding backward, then blooie!
In the wye config, all the start (or finish) terminals are tied to the neutral so that the vector diagram looks like a wye. Get one backward and the vector diagram looks like a crow's foot. Since the windings do not form a closed loop, there is no danger of the xfrmr frying itself, but major problems surface when loads are applied.
Ed, we need some diagrams.
Re: 3-phase Y neutral
The 208/120 configuration on the secondary side of a 240 delta to 208 wye will not have a neutral unless you ground the secondary side of the transformer. It does not make one on it's own.
The transformer will work without a ground, however you will not read voltage on any leg to Ground because you do not have one. You will only be able to read phase to phase 208.
This is the way I understand it anyways. Id like some input as I make it a point to learn something new everyday.
208/120 applications are usually used in small commercial applications where there is not that many large motor loads- if any.
The 208/120 configuration on the secondary side of a 240 delta to 208 wye will not have a neutral unless you ground the secondary side of the transformer. It does not make one on it's own.
The transformer will work without a ground, however you will not read voltage on any leg to Ground because you do not have one. You will only be able to read phase to phase 208.
This is the way I understand it anyways. Id like some input as I make it a point to learn something new everyday.
208/120 applications are usually used in small commercial applications where there is not that many large motor loads- if any.
Re: 3-phase Y neutral
Rattus, I worked in an old oil refinery (circa 1918) that used an ungrounded system so I have seen ungrounded 3 phase Wye transformers, however they were not 120/208, but 2300/4160. But an ungrounded neutral on such a system would function correctly. The rub is when there is a ground fault, there is nothing to trip the circuit unless you install a ground fault sensing device (see note below) and a trip relay into the service equipment to provide protection. And any arcing fault to ground not large enough to cause and alarm will send huge voltage spikes everywhere on the system wreak havoc on the insulation of all wires and motor windings causing that equipment to fail prematurely.
The engineering logic around such a system design is that with the proper alarm and ground fault protection one phase of the circuit could become grounded and the circuit would still supply power to the load. When an alarm condition exists the fault could be located, the branch circuit containing the fault is then isolated and repairs can be made. All this and there was no suprise trip of the circuit that would shut down critical equipment and could cause an explosion or the release of liquids and vapors that would be harmful to the environment.
Modern engineeing practices have all but abandoned those configurations, favoring instead a grounded system. And to place the necessary emergency relief and containment in the process equipment to prevent releases or explosions that might be created under a fault trip condition.
But just like nob and tube wiring and open buss motor control centers these older systems do work and are still found in use today. I just wouldn't recommend one for a modern design.
Imagine making a hot tap to a 2300V open overhead buss in order to install a new pump motor or transformer circuit, quite a hazardous task and not one to be done with out taking the proper safety precautions.
Our modern engineeing and safe work practices are a good thing.
Note: One type of ground fault sensing device
is a transformer with a grounded open delta secondary. No current will flow in the secondary until the ground circuit is complete, and the magnitude current will be limited to the resistance of the fault connection. Although there will always be a small amount of current (micro amps in most cases) always present in the secondary, as the insulation of wires and windings begin to loose their dielectric strength the current will rise proportionatley.
On a 2300V system a fault current of more thatn one amp is cause for alarm and most relays will trip at less than 6 amps. (14KW is not a toaster I'd want to see cooking)
Using a special clamp on ammeter with a hoop large enoug to open over the conduit the faulted circuit can be located. The ground current will only show on the conduit containing wires feeding the fault.
Rattus, I worked in an old oil refinery (circa 1918) that used an ungrounded system so I have seen ungrounded 3 phase Wye transformers, however they were not 120/208, but 2300/4160. But an ungrounded neutral on such a system would function correctly. The rub is when there is a ground fault, there is nothing to trip the circuit unless you install a ground fault sensing device (see note below) and a trip relay into the service equipment to provide protection. And any arcing fault to ground not large enough to cause and alarm will send huge voltage spikes everywhere on the system wreak havoc on the insulation of all wires and motor windings causing that equipment to fail prematurely.
The engineering logic around such a system design is that with the proper alarm and ground fault protection one phase of the circuit could become grounded and the circuit would still supply power to the load. When an alarm condition exists the fault could be located, the branch circuit containing the fault is then isolated and repairs can be made. All this and there was no suprise trip of the circuit that would shut down critical equipment and could cause an explosion or the release of liquids and vapors that would be harmful to the environment.
Modern engineeing practices have all but abandoned those configurations, favoring instead a grounded system. And to place the necessary emergency relief and containment in the process equipment to prevent releases or explosions that might be created under a fault trip condition.
But just like nob and tube wiring and open buss motor control centers these older systems do work and are still found in use today. I just wouldn't recommend one for a modern design.
Imagine making a hot tap to a 2300V open overhead buss in order to install a new pump motor or transformer circuit, quite a hazardous task and not one to be done with out taking the proper safety precautions.
Our modern engineeing and safe work practices are a good thing.
Note: One type of ground fault sensing device
is a transformer with a grounded open delta secondary. No current will flow in the secondary until the ground circuit is complete, and the magnitude current will be limited to the resistance of the fault connection. Although there will always be a small amount of current (micro amps in most cases) always present in the secondary, as the insulation of wires and windings begin to loose their dielectric strength the current will rise proportionatley.
On a 2300V system a fault current of more thatn one amp is cause for alarm and most relays will trip at less than 6 amps. (14KW is not a toaster I'd want to see cooking)
Using a special clamp on ammeter with a hoop large enoug to open over the conduit the faulted circuit can be located. The ground current will only show on the conduit containing wires feeding the fault.
Re: 3-phase Y neutral
The company I work for is putting in several new installations that are 480 impedance grounded wye except for the office and hotel loads that will be 208 grounded wye. I'm not seeing where modern engineering practices suggest grounded 480 is the way to go.
I've got reservations that the solidly grounded 208 improves safety, but the code says ground that, and I certainly will.
carl
I don't know about that, I'm kind of partial to impedance grounded 480 Wye. Reliability (continuity of power) is high. Burns up a lot less equipment when things do fail. And I don't see any safety compromise.
Got one. Use it to chase the ground faults. The impedance is usually set to pull 5 amps(or so) on a grounded phase. One throws the switch on the pulser that shorts out part of the grounding resistor, and the current pulses from 5 to 10 amps. The pulsing current is a lot easier to see with the big clamp-on than a small steady current.
The company I work for is putting in several new installations that are 480 impedance grounded wye except for the office and hotel loads that will be 208 grounded wye. I'm not seeing where modern engineering practices suggest grounded 480 is the way to go.
I've got reservations that the solidly grounded 208 improves safety, but the code says ground that, and I certainly will.
carl
al hildenbrand
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Re: 3-phase Y neutral
Great discussion, Carl & Ray
Could someone say more about this type of ground fault detector? Is the indication of fault done by meter or lamps, both? Is the indication an aggregate reading for all three phases, or are there indicators for each phase?
Great discussion, Carl & Ray
I am guessing this is a corner grounded delta secondary and that the primary is connected to the ungrounded Wye phase legs?
Could someone say more about this type of ground fault detector? Is the indication of fault done by meter or lamps, both? Is the indication an aggregate reading for all three phases, or are there indicators for each phase?
Re: 3-phase Y neutral
Rattus
Simba, the delta config is wired start-finish in a closed ring such that the vector sum of all the phases is zero. Get one winding backward, then blooie!
Are you sure about that Rattus. Seems to me it would buck against the other winding and try to cancel the voltage out?
Wouldn't you have maybe 1.73 volts because of the wye configure and not a blooie?
Edited for correction
Rattus I missread your sentence I thought you where refering to the wye above.
But I still believe even with a delta you would buck and have uneven voltages?
[ March 05, 2005, 12:53 PM: Message edited by: ronaldrc ]
Rattus
Simba, the delta config is wired start-finish in a closed ring such that the vector sum of all the phases is zero. Get one winding backward, then blooie!
Are you sure about that Rattus. Seems to me it would buck against the other winding and try to cancel the voltage out?
Wouldn't you have maybe 1.73 volts because of the wye configure and not a blooie?
Edited for correction
Rattus I missread your sentence I thought you where refering to the wye above.
But I still believe even with a delta you would buck and have uneven voltages?
[ March 05, 2005, 12:53 PM: Message edited by: ronaldrc ]
Ed MacLaren
Senior Member
Re: 3-phase Y neutral
(Note - The sketch below is for illustrative purposes only. A delta connected transformer supplying 120 volts is not commonly used.)
Ed
Right! In the example below, there would be 240 volts driving high circulating current through the winding impedance, if the S leads of phases A and B were connected. (#7)
(Note - The sketch below is for illustrative purposes only. A delta connected transformer supplying 120 volts is not commonly used.)
Ed
Ed MacLaren
Senior Member
Re: 3-phase Y neutral
If one winding is reversed so that you are connecting the Start of C with the Finish of A, (#4) the vector sum is only 120 volts, and you have actually started a delta connection.
Ed
Compare #2 and #4 in the sketch above. Connecting the two Finish leads (#2) begins a wye connection, giving a vector sum of 208 volts.
If one winding is reversed so that you are connecting the Start of C with the Finish of A, (#4) the vector sum is only 120 volts, and you have actually started a delta connection.
Ed
Ed MacLaren
Senior Member
Re: 3-phase Y neutral
Ed
Start and Finish. I got used to that in my motor re-winding days.
Ed
O
oliver100
Guest
Re: 3-phase Y neutral
Some European countries use 380/220 Y configuration without grounded neutral. Also some of the ships I have seen use 480/277 Y configuration with various ground fault alarms. It is difficult to chase the grounded equipment but as it was mentioned here the damage is minimal.
Some European countries use 380/220 Y configuration without grounded neutral. Also some of the ships I have seen use 480/277 Y configuration with various ground fault alarms. It is difficult to chase the grounded equipment but as it was mentioned here the damage is minimal.
Re: 3-phase Y neutral
Ed, I am going to zing you for implying that the vectors in a delta config are 60 degrees apart.
3-phase voltages are generated 120 degrees apart, and you cannot change that. If you let the "S" terminals of your your delta transformer represent the arrowheads of vectors, then the vectors are at 0, -120, and 120 degrees. The difference occurs because you tie the 'F' terminals together in a wye, but in a delta you tie the "S" to "F" and so on. You must ignore the internal angles of the triangle.
As it turns out, SUBTRACTING one wye voltage from another yields the L-L voltage. But, ADDING the three voltages in a delta yields a sum of zero.
Sounds wrong. That is because we are working with vectors.
Ed, I am going to zing you for implying that the vectors in a delta config are 60 degrees apart.
3-phase voltages are generated 120 degrees apart, and you cannot change that. If you let the "S" terminals of your your delta transformer represent the arrowheads of vectors, then the vectors are at 0, -120, and 120 degrees. The difference occurs because you tie the 'F' terminals together in a wye, but in a delta you tie the "S" to "F" and so on. You must ignore the internal angles of the triangle.
As it turns out, SUBTRACTING one wye voltage from another yields the L-L voltage. But, ADDING the three voltages in a delta yields a sum of zero.
Sounds wrong. That is because we are working with vectors.
Ed MacLaren
Senior Member
Re: 3-phase Y neutral
Ed
Hi Rattus, I don't mind being zinged at all.
Ed
Re: 3-phase Y neutral
Ed,
My real point is that saying there is 60 degrees of separation does not clarify the issue for the mathematically impaired. I would suggest instead that you take a purely graphical approach using arrowheads to indicate the direction of the vectors. The vector diagrams so drawn can be analyzed graphically to determine the magnitude and phase of the resultant vectors. Not very precise, but it gets the point across, and that is what we are trying to do, right?
Ed,
My real point is that saying there is 60 degrees of separation does not clarify the issue for the mathematically impaired. I would suggest instead that you take a purely graphical approach using arrowheads to indicate the direction of the vectors. The vector diagrams so drawn can be analyzed graphically to determine the magnitude and phase of the resultant vectors. Not very precise, but it gets the point across, and that is what we are trying to do, right?
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