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Member

Join Date: May 2007
Posts: 7

3ph Wye Fault Conditions

05/05/2007 3:12 PM

I am interetsed in what one would expect to happen on a 3ph source if one of the legs became grounded and then if one of the legs were open.

What happens to the voltage, current and frequency on the other two legs under these conditions?

Thanks

wk

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Anonymous Poster
#1

Re: 3phase Wye Fault Conditions

05/06/2007 4:00 AM

In the UK, three phase configurations are referred to as Star and Delta - I'm not quite suer what Wye refers to.

In either system if it is a 3 wire balanced system then there would normally be 3 pole protective devices or overloads. On a motor for example the three element overload unit will disconnect all three phases if it goes out of balance or a fault on one phase. So in the event of one phase earthing then the supply to the load would be disconnected.

In theory without protection, under the conditions you describe then the healthy phase load would use the faulty phase short circuit point as return and the load would operate on a sinle phase basis to a degree dependent upon the quality of the earth path on the earthed supply system. The load on Star system would be half that of a Delta system as the load would flow through two resistances - one on each phase. If the earthing is poor any earthed metalwork could become live

A return value voltage will appear on the load side of the open phase connection.

On a 4 wire unbalanced system system the short circuited phase will trip out.

The open phase load will not operate at all

The healthy phase will operate on single phase - neutral only with single phase equipment working normally.

A return voltage value will appear on the load side of the open phase. The load side of the disconnected fault phase becomes a "neutral conductor at mid point value.

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Power-User

Join Date: Apr 2007
Posts: 178
Good Answers: 3
#2
In reply to #1

Re: 3phase Wye Fault Conditions

05/06/2007 2:31 PM

In the United States. a WYE connection is equivalent to a Star connection as you think of it in the UK

Note that in a phase schematic the star connection resembles the letter "Y". We call this a WYE connection to identify it. In US systems, often the WYE is a 4 wire system with the center of the WYE grounded.

I think that in England, ungrounded residential systems are common.

A US Delta connection is normally ungrounded.

We do also have what is called a wild cat connection where one of the 240 volt delta phases is center tapped and grounded. This can provide a grounded neutral 120/240 service and a 240 volt (phase to phase) 3 phase service.

A modification is to use only 2 transformers. The voltage to ground of the third phase is not normally usable.

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Anonymous Poster
#4
In reply to #2

Re: 3phase Wye Fault Conditions

05/07/2007 5:36 AM

Thanks for that update.

In The UK the electricity companies mainly supply an earth to all customers except those who are fed by LV overhead lines.

All residential premises must have an earth (ground) - if not supplied by the electricity company the customer has to install their own earth electrodes.

Petrol stations are not allowed to us the suppliers earth - they must provide their own electrodes.

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Member

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Posts: 7
#3
In reply to #1

Re: 3phase Wye Fault Conditions

05/06/2007 3:02 PM

Thanks for the comprehensive reply. It is quite helpful.

As per the comment by "Snakers", yes, "Wye" = "Star"


Thanks

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Member

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Posts: 7
#7
In reply to #1

Re: 3phase Wye Fault Conditions

05/07/2007 10:55 PM

Not sure if I replied to you as yet, but, if I haven't, I just wanted to thank you for your comprehensive reply. cheers

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Commentator

Join Date: Feb 2007
Posts: 60
#5

Re: 3ph Wye Fault Conditions

05/07/2007 7:50 AM

Sir

It depends how the star center is grounded.

More information, please refer to

IEEE Guide for the Application

of Neutral Grounding in Electrical

Utility Systems—Part I: Introduction and the other foregoing standards of the family

C62.92.1.2000(2, 3 4 etc).

It is a matter normally dealt with by using symmetrical components and if you read the standards you will understand why, mainly for two and single phase to groud faults.

On ungrounded systems you will have problems of overvoltages for phase to ground faults but there are means to allow adequate protection even for these systems.

Please refer also to papers from SCHWEITZER9 www.selinc.com) such as "NEW DIRECTIONAL GROUND-FAULT ELEMENTS IMPROVE SENSITIVITY IN UNGROUNDED AND COMPENSATED NETWORKS."

Regards

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Member

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Posts: 7
#6
In reply to #5

Re: 3ph Wye Fault Conditions

05/07/2007 10:52 PM

Great, thank you very much for the reference!

zm

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