Being an Electrical Engineer, it is very important to have clear concept of Symmetrical Components used for the analysis of fault or unbalance in a Power System. Basically Symmetrical Components are mathematical modeling used to simplify the calculation while maintain the physical significance of components.

*A system of n vectors or quantities can be resolved into n different symmetrical group or system, one of which consists of equal vectors and remaining (n-1) systems consists of n equi-spaced vectors.*

Let us consider a three phase system. Any three coplanar vectors V_{a}, V_{b} and V_{c }can be expressed in terms of three new vectors V_{1}, V_{2} and V_{3}by three simultaneous linear equation with constant coefficients. Thus,

V_{a} = a_{11}V_{1}+ a_{12}V_{2} + a_{13}V_{3}

V_{b} = a_{21}V_{1}+ a_{22}V_{2} + a_{23}V_{3}

V_{c} = a_{31}V_{1}+ a_{32}V_{2} + a_{33}V_{3}

Thus we observe that each of the original vectors has been replaced by three different vectors and can be found by solving the above equations. This is the origination of concept of Symmetrical Components.

According to the concept, three unbalanced vectors V_{a}, V_{b} and V_{c} can be replaced by a set of three balanced system of vectors. Therefore the solution of above simultaneous equation must be unique. The three symmetrical components vectors replacing V_{a}, V_{b} and V_{c} are Positive Sequence, Negative Sequence and Zero Sequence.

**Positive Sequence:**

Positive Sequence components are those which have three vectors of equal magnitude but displaced in phase from each other by an angle of 120° and have the phase sequence same as the original vectors. Phase sequence means the direction of rotation of vectors.

**Negative Sequence:**

Negative Sequence components are having equal magnitude and displaced from each other by an angle of 120° but the direction of rotation of the vectors are opposite to the direction of rotation of original vectors. Figure below depicts the sequence voltages for three phase system.

**Zero Sequence:**

Zero Sequence components have an awesome property. They are equal in magnitude and phase as shown in figure above.

Notice that Positive Sequence components are designated by subscript 1, Negative Sequence by subscript 2 and Zero Sequence by subscript by 0. Theses subscripts are generally adopted to denote sequence components.

In which condition positive sequence,negative sequence,zero sequence currents are created???

Positive sequence current flows under normal balanced condition. Under unbalanced / fault condition, negative as well as zero sequence (if neutral is grounded) current flows.