Symmetrical Faults

7/28/2019 Symmetrical Faults

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Flow of excessive current

Abnormal voltages

Voltage elevation of system neutral Hazards to human, equipment and

animals.

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Design of protection system requiresthe knowledge of fault current.

The information obtained from the fault

studies are used:

to select the sizes of circuit breaker,

fuse and characteristic,

setting of relay.

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Symmetrical fault :

Usually three phase to ground fault

Unsymmetrical fault

The fault is unbalanced in nature

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An unbalanced three-phase system can be resolvedinto three balanced systems in the sinusoidal steadystate. This method of resolving an unbalanced

system into three balanced phasor system has beenproposed by C. L. Fortescue. This method iscalled resolving symmetrical components of theoriginal phasor or simply symmetrical

components.

We Will discuss symmetrical componentstransformation and then will present howunbalanced components like Y- or -connected

loads, transformers, generators and transmissionlines can be resolved into symmetrical components.We can then combine all these components togetherto form what are called sequence networks .

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The phasor representation of threephase system

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The phasor notation of a-operator

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The positive sequence set consisting of three components ofequal magnitude, displaced by 120 & 240o respectively andhaving the phase sequence of abca.

The negative sequence set consisting of three components ofequal magnitude displaced by 240 & 120orespectively, havingphase sequence of acba.

The zero sequence set of the component of which being equalboth in magnitude and phase.

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Relations of voltage components inmatrix form

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The most common type of fault is thesingle line to ground fault:

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The sequence component connectionfor the single line to ground fault:

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The sequence component connection forthe single line to ground fault:

The sequence components are connectedin series.

The three currents in the case of SLG faultare equal.

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LL fault:

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LL fault:There is no zero sequence

component due to absence of groundreturn path.

The positive and negative sequence

components are connected inparallel.

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LLG fault


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LLG fault: For the zero sequence component it

requires to add an externalimpedance of Z

f+3Z

g

The networks are connected in

parallel.

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The most commonly used soft ware are:

MATLAB

EDSA

ETAP

CYME

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Power system analysis using symmetricalcomponent is very helpful in improving thereliability of the power system.

The principle adopted for the analysis ofunbalanced fault system is symmetricalcomponent method.

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Symmetrical Faults

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