Prepared By:- Nisarg Amin

Topic:- Symmetrical Component Method For 3 Phase Power Representation

INTRODUCTION

The 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.

2

Need 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 regulators

Certain connections of CT and PT develop zero sequence components that are used in protective ground relaying scheme.

3

Mathematically

Va= Va1+ Va2+ Va3+---------- Van

Vb= Vb1+ Vb2+ Vb3+---------- Vbn

Vn= Vn1+ Vn2+ Vn3+---------- Vnn

Va, Vb……. Vn are unbalanced set of phosors

Va1, Vb1…… Vn1 first set of n balanced phasors with an angle 2pi/n between components a,b,…..n.

4

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

5

Equation in matrix form

6

Symmetrical 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.

7

The phasor representation of three phase system

8

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.

9

Relations of voltage components in matrix form

10

Symmetrical components of generator

11

Thank You

12