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Earthing (grounding) transformer – Voltages during a groundfault

Earthing (grounding) transformer – Voltages during a ground fault (On photo: Neutral Grounding Transformers that offer protection from faultcurrent damaging transformer and generator systems; Photo credit: powermagneticsinc.com)

Protection from fault current

Earthing Transformer or grounding transformer is the neutral grounding transformer - star connected on theprimary and has an open delta on secondary. Open Delta has two terminals. A resistor is connected across thesetwo terminals.

Whenever a fault occurs, voltage will be induced in the open delta and there will be a voltage drop in theresistor connected.

This voltage drop will be sensed by the earthing transformer connected across this resistor. At the secondary ofthe earthing transformer we have the neutral displacement relay which will give a trip signal or an alarm wheneverits settings exceed a particular value.

Earthing transformer can also be called a large Potential transformer (Open delta PT).

Representation in SLD

Earthing Transformer Connection Diagram

Earthing transformer Single Line Diagram

Connection diagram

The best way to ground an ungroundeddelta system (existing or new) is toderive a neutral point throughgrounding transformers as shown infigure above. The resistor inserted in the“broken delta” leg is reflected to theprimary underground fault conditions andlimits the current to a nominal value asdictated by its design.

Also, sensing the voltage drop across theresistor (device 59G) can be used tosignal an alarm advising that a groundfault has occurred.

The three lights across each individualtransformer will constitute a version of thenormal ground detection schemecurrently employed on ungroundedsystems.

The voltage across the broken delta is simply the sum of system phase to ground voltages, or3V0.The Y side of the Y-ground/Broken delta VT can either be directly connected to the highvoltage terminals or to the secondary of a main step down VT.

It is common to place a resistor in the broken delta as shown in above figure. One rationale for the resistance isthat the resistance stabilizes the measured voltage.

It does this by:

1. Reducing the risk of ferroresonance

2. Act as a grounding bank/transformer.

Voltages during a ground fault

Referring to phaser diagram:

VAG = VAN + VNG = K<0 + 0 /Equation 1/

VBG = VBN + VNG = K<-120 + 0 /Equation 2/

VCG = VCN + VNG = K<120 + 0 /Equation 3/

For a phase A to ground fault, VAN = 0, and the voltage across the neutral resistor that is VNG is essentially thenegative of the Phase A to neutral voltage.

Mathematically:

VAG = 0 since Phase a is faulted

So: VAN + VNG = 0

VNG = -VAN = – K<0 = 1<180 /Equation 4/

Substituting equation – 4 in equation – 2

VBG = VBN + VNG = K<-120 + K<180 = 1.732K<150 /Equation-5/

Similarly substituting equation – 4 in equation -1

VCG = VCN + VNG = K<120 + K<180 = 1.732K<150 /Equation-6/

Equation 5 and 6 proves that voltage of healthy phase rises by 1.732 times during phase to ground fault inone phase.

Sum of the three phase voltage will be:

= VAG + VBG + VCG = 0 + 1.732K<-150 + 1.732K<150 = 3K<180 /Equation-7/

Since broken/open delta sum up the three phase voltage so voltage of equation 7 will be transformed to opendelta secondary side from the Y grounded primary side.

V Broken Delta = V Primary / Turn ratio = (1/turn ratio) 3K<180 /Equation-8/

Equation 8 can be generalized as:

V Broken Delta =(1/Turn ratio) (3 V L-G System) < (Un faulted phase angle + 180)

Resistance Selection

To obtain the maximum capability of the resistor to dampen system transients and dampen ferroresonant circuits,a typical approach to sizing the resistor is to utilize one that can handle all the power that the transformer cansupply during a full neutral offset.

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About Author //

Asif Eqbal

Asif Eqbal - Bachelor of Engineering in Electrical & Electronics engineering, fromManipal University, (Karnataka), India in 2006. Presently involved in the design of EHVoutdoor substation and coal fired thermal power plants for more than seven years.Motto of joining EEP as a contributor is to share my little engineering experience andhelp the budding engineers in bridging the conspicuous gap between academics andIndustrial practice. “If you have knowledge, let others light their candles with it, so thatpeople who are genuinely interested in helping one another develop new capacities foraction; it is about creating timeless learning processes".

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13 Comments

1. Mahesh Wani

Dec 08, 2014

Dear AsifThank you for this article on earthing using star / open delta earthing transformer with resitor. Could youplease also inform me how to calculate the resistance value across the open delta. e.g. 11kV/415V, 100Afor 10 Sec, 250kVA continuous.Please inform if you need any further information.Thanks in advanceBest regards

(reply)

2. Mahendra Rajput

Nov 14, 2014

advantage & disadvantages of it and its application also

(reply)

3. Mahendra Rajput

Nov 14, 2014

What is the advantages and disadvantages of grounding transformer and its application also?

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4. motheo

Nov 12, 2014

why do we earth the S2 or S1 of the current transformer?

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Peter

Nov 18, 2014

Because the current loop on the secondary circuit for a current transformer is isolated,and is therefore prone to voltage rise due to capacitive coupling. The voltage levels could get tolevels above cable and equipment design rating

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5. Frantz St. Phar

Sep 29, 2014

Nice article. Good fundamentals analysis.

(reply)

6. karri.durga ram prasad

Aug 08, 2014

useful website

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7. Kerry Williams

May 13, 2014

Why would you not use a zig-zag winding on the primary rather than a star connectedwinding? Would this not further limit the level of fault current flowing? What effect would it have on the opendelta?I also would like to understand more about the forces involved here as many earthing transformers fail dueto mechanical strength. Your comments eher would also be of help.

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8. Roberto Martínez

May 10, 2014

There is a mistake:where indicated: 1.732K<150 /Equation-5/has to be: 1.732K<-150 /Equation-5/

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9. Emmanuel Antwi Addo

May 06, 2014

wonderful and helpful site

(reply)

10. guest

May 05, 2014

Where is the reference material listed?What about the source of the photos?Please list does things because then you could have serious troubles with copyright and intellectualproperty.

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Asif Eqbal

May 06, 2014

No specific reference material list for this article. Photos have been picked to best suitthe topic, not any project or schematic generated by specific organisation. Mathematical equationand result is what you learned in university no patent or new discovery over here.

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varunahuja

May 24, 2014

gratitude Mr asif,can u explain If there is a YNd11 , 12 MVA , 45/15 KV power transformer having a NGT whichis 1730KVA, %z=92.7%, YNd1 transformer with no resistance across its secondary terminals, what could be the maximum earth fault current of the power transformer on 15 KV side

(reply)

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