2OUTLINE OF THE PRESENTATION INTRODUCTIONNEED FOR THE SYMMETRICAL COMPONENET METHODTECHINQUES USED TO ANALYSIS SYMMETRICALSYMMETRICAL METHODS USED TO STUDY POWER SYSTEM FAULTSCONCLUSIONREFERENCES
3INTRODUCTIONThe normal operating conditions of an electric power system are occasionally disrupted because of faults.Analysis of power systems usually implies the computation of network voltages and currents under a given set of conditions.Under many circumstances we tend to ignore the unbalanced operation in the system and unbalanced operation is always present.
4An organization of power system analysis problem Source (Reference #1)
5Effects of faults on power system Flow of excessive currentAbnormal voltagesVoltage elevation of system neutralInduce over voltages on neighbouring equipments .Hazards to human, equipment and animals.
6Need for fault analysis Design of protection system requires the 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.
7Normal types of fault are: Fault due to lightningTree limbs falling on the lineWind damageInsulation deteriorationVandalism
8Types of faultSymmetrical fault :Usually three phase to ground faultUnsymmetrical faultThe fault is unbalanced in nature
10Sources of Asymmetrical fault are (cont.) One phase open circuitUnbalanced in load mainly the arc loads
11Sources of Asymmetrical fault are (cont.) One phase open circuit
12Sources of Asymmetrical fault are (cont.) SLG fault
13Need for the symmetrical component analysis Negative sequence relay in generator has helped in protecting the generator from over heating in the event of unbalanced loads.The positive sequence segregating network is used to supply the sensing voltage to generator voltage regulatorsCertain connections of CT and PT develop zero sequence components that are used in protective ground relaying scheme.
14Method of adopted for symmetrical component analysis (cont) Mathematically:Va= Va1+ Va2+ Va VanVb= Vb1+ Vb2+ Vb Vbn;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;Vn= Vn1+ Vn2+ Vn VnnWhere:Va, Vb……. Vn are unbalanced set of phosorsVa1, Vb1…… Vn1 first set of n balanced phasors with an angle 2pi/nbetween components a,b,…..n
15Method of adopted for symmetrical component analysis (cont) Mathematically:Va2, Vb2…… Vn2 second set of n balanced phasors with an angle 4pi/n between components a, b………,…..n……………………………………………………….Va(n-1), Vb(n-1)…… Vn(n-1)are (n-1)th set of n balanced phasors with an angle 2pi(n-1)/n between components a, b………,…..n
16The phasor notation of a-operator Phasor a or a-operatorThe phasor notation of a-operator
20Symmetrical components for three Phase system The n-phase system presented above is of academic interest only and only the practical three phase system will be emphasised.Power is generated, transmitted and consumed mostly in three phase only.
21Symmetrical components for three Phase system (cont) The phasor representation of three phase system (source reference#2)
22Symmetrical components for three Phase system (cont) The positive sequence set consisting of three components of equal magnitude, displaced by 120 & 240o respectively and having the phase sequence of abca.The negative sequence set consisting of three components of equal magnitude displaced by 240 & 120o respectively, having phase sequence of acba.The zero sequence set of the component of which being equal both in magnitude and and phase.
23Symmetrical components for three Phase system (cont) Relations of voltage components in matrix form
27Fault analysis using symmetrical components The most common type of fault is the single line to ground fault:
28Fault analysis using symmetrical components (cont) The sequence component connection for the single line to ground fault:
29Fault analysis using symmetrical components (cont) The sequence component connection for the single line to ground fault:The sequence components are connected in series.The three currents in the case of SLG fault are equal.
30Fault analysis using symmetrical components (cont) LL fault:
31Fault analysis using symmetrical components (cont) LL fault:There is no zero sequence component due to absence of ground return path.The positive and negative sequence components are connected in parallel.
32Fault analysis using symmetrical components (cont) LLG fault:
33Fault analysis using symmetrical components (cont) LLG fault:For the zero sequence component it requires to add an external impedance of Zf+3ZgThe networks are connected in parallel.
34Software available for carrying fault analysis The most commonly used soft ware are:MATLABEDSAETAPCYME
35ConclusionPower system analysis using symmetrical component is very helpful in improving the reliability of the power system.The principle adopted for the analysis of unbalanced fault system is symmetrical component method.By knowing the principle, the results obtained from the computer can be analysed.
36References: 1. Paul M. Anderson, Analysis of Faulted Power Systems. 2. W.D. Stevenson, Elements of Power System Analysis.3. A.P.S Meliopoulos, Power System Grounding and Transients.4. Olle. I. Elgerd, Electric Energy Systems Theory.5. IEEE Transactions