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liukawa
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Question on How to Connect Relay for Restricted Earth Fault Protection For 4 Wire System Wye Transformer

10/08/2018 8:53 AM

Dear All

I have an application whereby the client is requesting for Restricted Earth Fault protection and standby earth fault protection for transformer.

Client has requested us to wire based on the wiring diagram below.

But i have some doubts on their wiring diagram.

If there is a phase to neutral fault before the phasor CT and neutral CT, then the phasor CT and neutral CT will not be able to detect the fault ? and relay won't trip with REF ? Is this correct?

What will be the correct wiring diagram for 4 wire system REF protection and standby earth fault protection?

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PWSlack
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#1

Re: Question on How to Connect Relay for Restricted Earth Fault Protection For 4 Wire System Wye Transformer

10/08/2018 9:03 AM

None of the equipment indicated is there to protect against a <...phase to neutral fault...>; that is given by overload protection apparatus that has not been detailed.

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raghun
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#2

Re: Question on How to Connect Relay for Restricted Earth Fault Protection For 4 Wire System Wye Transformer

10/08/2018 11:48 AM

The REF protection is for phase to earth faults and what ever happens between phase and neutral, whether it is a single phase load or a short is not the business of REF protection.

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liukawa
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#3
In reply to #2

Re: Question on How to Connect Relay for Restricted Earth Fault Protection For 4 Wire System Wye Transformer

10/08/2018 12:16 PM

I am thinking of connecting the neutral wire after the REF CT, then we do not need to install additional CT at Our switchboard neutral end

If we will to do this,then we need to set our standby earth fault relay to trip on directional Earth fault only when there is current flow from neutral to ground ?

Since standby earth is for protection at the downstream side only ?

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PWSlack
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#4
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Re: Question on How to Connect Relay for Restricted Earth Fault Protection For 4 Wire System Wye Transformer

10/08/2018 2:22 PM

<...thinking of connecting the neutral wire after the REF CT...>

Then think again: the 51G apparatus will operate all the time, as current is never balanced across phases!

Do the right thing, and install the equipment the right way.

If in doubt, consult a qualified local Electrical Engineer.

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liukawa
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#5
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Re: Question on How to Connect Relay for Restricted Earth Fault Protection For 4 Wire System Wye Transformer

10/08/2018 8:35 PM

Yes I know current will flow through neutral of load back to the source.

Which is why I am enquiring if we will to connect neutral after the REF CT, I will want to set my standby earth fault as directional earth fault, so that the 51G will only trigger when there is current flow from the tranaformer neutral point to ground.

Any problems with this solution ?

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PWSlack
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#7
In reply to #5

Re: Question on How to Connect Relay for Restricted Earth Fault Protection For 4 Wire System Wye Transformer

10/09/2018 7:28 AM

An inability to understand is the biggest one.

<...we...>

Consult a local Mentor.

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raghun
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#6
In reply to #3

Re: Question on How to Connect Relay for Restricted Earth Fault Protection For 4 Wire System Wye Transformer

10/09/2018 12:45 AM

Probably you have in mind REF protection scheme that uses only 4-CTs.

Such a scheme exists and is also popular. In that scheme, the 51G CT and REF neutral CT will be two distinct CTs (not two cores of the same CT). The REF neutral CT is located between neutral wire and transformer winding star point whereas, the 51G CT is located between neutral wire and the earth point.

The 5-CT scheme (as shown in original post) is preferred by many vendors, as it is easier to accommodate one single CT (with two cores) in the transformer.

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