PowerWorld -A Software Approach toTransmission Line Fault Analysis

A paper by Prajakta Dhawane under guidance of Prof. S.U.Kulkarni,

,Bharati Vidyapeeth University College of Engineering , Pune

Absuud- The falllt aulysis is doae for the threephase sym••etric2I fa.at •• d die a~ halts..TlIe unsymmetrical faults include siagie _ to grouod.,tiBe to liDeaJIddoable tiae to grouad &lilt. llae metllodemployed is bllS i_pedaatt matrix wIIidt bas certainadvantages over tlleveaUa's equivaleat _dbocL Theadvautage of this approach over coaveatioeal metllod isto make the analysis of three typical un-symmetricalfaults, namely single-line-to-ground fault, line-to-linefault and double-line-to-ground fault more unified. So itis aa~ to camberso_dy coaaed dlrcc seqllCllcenetworks ",hen calculating the fauft voltagesat acb busand fault carn:ats flowiag from otIe bas to itsneigbboriag bas. Tile resalts have beea obbiaed usingthe POWERWORLD Simulator, which usesthe methodof Bus Impedance i\'btriL

Index Terms- - Bus impedance matrix, fault analysis,powerworld, thevenin 's equivalent.

1. INTRODUCT[ON

The steady state operating mode of a powersystem is balanced 3-phase ac. However due to

sudden external or internal changes in the system,this condition is disrupted. When the insulation of thesystem fails at one or more points or a conductingobject comes in contact with a live point, a shortcircuit or fault occurs. A fault involving all the threephases is known as symmetrical (balanced) faultwhile one involving only one or two phases is knownas unsymmetrical fault. Majority of the faults areunsymmetrical. Fault calculations involve finding thevoltage and current distribution throughout thesystem during the fault. It is important to ·determinethe values qf system voltages and currents duringfault conditions so that the protective devices may beset to detect the fault and isolate the faulty portion ofthe system.

II. FAULTS IN A THREE PHASE SYSTEM

1_ Symmetrical three-phase fault2_ Single line-to-ground fanft (SLG)3_ Line-to-line fault (IL)4. Double line-to-ground fault (OLG)The mostcommon type of fauhs by far is the SLG

fault, followed in frequency of occurrence by the LLfault, DLG fault, and three-phase fault.

Out of the above four faults, two are of the line-to-ground faults. Most of these occur as a result ofinsulator f1asbovcr during electrical storms. Thebalanced three-phase fault is the rarest in occurrenceand the least complex in so fur as the fault currentcalculations are concerned. The other threeunsymmetricaJ faults will require the knowledge anduse of symmetrical components. Unsymmetricalfaults cause unbalanced currents to flow in thesystem. The method of symmetrical components is avery powerful tool which makes the calculations ofunsymmetrical faults almost as easy as thecalculations of a three-phase fault.To analyze un-symmetrical faults, one needs to

develop positive-, negative-, and zero-sequencenetworks of the power system under study, based onwhich one further needs to work' out the impedanceof three thevenin equivalent circuits as viewed fromfaulty point. Then the positive-, negative- and zero-sequence components of phase-a faulty-point-to-ground current can be calculated. To calculate three-pbase currents flowing from one bus to itsneighboring bus and three-phase voltages at each bus,one needs to connect three sequence networksuniquely for each type of fault. This may makecircuit drawing very cumbersome. Furthermore byusing the network with three sequence networksconnected, it is impossible to appreciate theimpedance matrix approach to calculate the sequencevoltage at each bus when fault occurs.To overcome these two drawbacks, paper [1]

introduces a new approach to unify the analysis of

We can work out a universal representation of allf----------- three typical un-symmetrical faults. This

representation is valid with the imposition ofdifferent fault conditions for each typical lID-

symmetrical fault, such as for the single-line-to-ground fault, such as for the single-line-to-groundfault, the fault conditions being Vka=Zrlfa,Itb=Ifc=O.

three typical unsymmetrical faults, namely single-line-to-ground fault, line-to-line fault and double-line-to-ground fault. This new method allows theanalysis of three typical un-symmetrical faults toshare all steps except one. The only different step isbow to calculate the posirive-, negative-, and zero-sequence components of pbase-a-to-ground faultcurrent at faulty point. It also makes impedancematrix approach more UIldastmdable when used tocalculate the sequence voltages at each bus.

All the above four fuuJts ( L,2., 3., 4.) are beingsolved using the bus impedance matrix and the sameresults are obtained from the powerwodd simulator.

®a ~t!fa.~.

®'. -. Zf

b ~rlib(k)~.

c

IiIfc

Fig.l. Single line to ground fault

a

b

c

Zf

Fig.2. Line to line fault

<:0a---~iIfa®

.-b

c -------------~ __--~--------------

ill. BUS IMPEDANCE MATRIX MElHOD

ill the following formulation, per-unit systemis adopted Zero-sequence oltage at each buscontributed by equivalent current source isdetermined by

This gives

AV(') Z(I} Z{I) Z(I) Z(I}If II 11 "' In

AV(') Z(I) Z{I) Z(I) Z(I}1C 21 22 2l: 2n

AV(') Z(I) Z(I) Z(I) Z(I)u t.:I U H:: 1.-..

AV(') Z(I) Z(I) Z{l} Z(I).c nl a2 "" DB

0 -Z(I)I(I)It: b

0 -Z(')I(')21: r••

_el) -Z (1)1(1)fa 1;;:1;:b

o

If pre-fault current is ignored, then the pre-faultvoltage at each bus is the same and equal to that atfault bus k before fault occurs, which is assumed tobe V f . So the positive sequence voltage at each buswhen fault occurs can be written as follows.

-v.:1)If

I)1f

9I

I)1

= + = +VP) v.1) AV~'} v, -Z~~tf t tf

I) v.9 AV{l) v. -t2~rL D of

Vr-~~~V _t:r!I)f ·fa

V _Z(I)~)f H::

In a similar way, the negative-sequence voltage ateach bus can be COG1 uted by

V (2)If

V (1)2f

V (1)H

V (2)Dr

.?-Z (2)1(2)

II;: h

-Z (2)1(2)21;: h

~2) zY) ~!l ~}. I 11 ..

'42) t:1 .. z1:1 .. zg}

_I

.zY}= 4~)zt> Z(2) zi!>.. H:: ..

=

Following figure shows the single line diagram of afour bus system. Let the bus number 4 be faulted. Letus perform the fault analysis for faults (1.,2.,3.,4.).using powerworld.

z<2) zY)nl ril.

liL'; U"'luot'l'_.-.• 2

fig. 4. Sino~ lincdiagram from POWERWORLD

TABLE [

IllN11SINor

SfNGLE LfNE TO GROUND FAUL T VOLT AGES

----~ ~- ~.---.••.~---.-- ~-----.I",~;;;:•••••="=----':F'''",,-=:=,- ••~ ~ c::-- :.~~.-~ -~~ .~--".""•••"'Il>;;;"l!;;------ -~· ·:_~~ __ -_d ~_-::.:-~~ -.p.:.=r.::..

TABLE II

UNE TO LLNE FAULT VOLTAGES

IIliIMI 1IiI--'-- ----- - ~------'-- :-...,.,.".-_. -----,----~ ~-----, •••••• -~&. -"'~

-~-~

'-~ .~~--~.-~

~~ ::.::: :::--::!: ~:11"" ~ LlI8(! 'J! -.nS •• '1I'j"tl&..... ~ - •••• -- •..,. ~. ,"II(

TABLE [[[

THREE PHASE BALANCED FAULT VOLTAGES

~----- - s.iQpr--- ••~-------&lY......r---'-~-'IIMe-~~ ;:~ -~ ~:Nr--=--- ••••.~ -~ -~ -~-.--~--.--~-~~-----

-------- FItiIIDIIiI -au.a. ....~ ~

s= .••••.Uiiii .. --.U3 ~

UiIj'--- - roD

<r<I ..,.-y-----p.------i-.'-'-0-----_·0·

- 1-1 1""'.

TABLE IV

DOUBLE UNE TO GROUND FAUL T VOLT AGES

-; 1'" !•••~

TABLE V

IN-LINE FAULT VOLTAGES

....,..••s:!.ZIll (l;~

- --Uiiri:- - -I~.---~ ~.--.-~--.~

- •••• -- UiIiI!II

~ .•0:. -11m2 ~~ -•.•.-u ••Vi&s - .' ..•.. --~ ••0.--' ---ril ". -~ ._ ..u.... ~-----

IV. CONCLUSION

This paper presents a method to tackle typical un-symmetrical faults. It is foundthat the bus impedancematrix method invol es comparatively lesscomputations than the thevenin's equivalent methodThe software approach using powerworld foeanalyzing the fanlts invovlves the IJ5C of the statedmethod This furthec makes e calculations lesstedious, regardless of the co exity of thepowersystcm, The proposed app radvantage over traditional method that it is moreintuitive whenmatrix approach is adopted to tackle afan h problem,

V. REFERENCES

[1] Darning Zhang," An alternative approach toanalyze un-symmetrical faults in powersystem".

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