Because that a balanced three-phase system is symmetric one. Negative and zero sequence components appear in three-phase systems' non-symmetric regimes.

In other words a balanced (i.e. symmetric) three phase system contains only positive sequence components by definition.

The theory of sequence components which was proposed by Fortescue is generally used to analyze unbalanced power system networks. The theory enables the transformation of three unbalanced power system phases into a three set of balanced phasors (positive, negative, and zero sequence components). This property presents an extremely powerful analysis tool that enables the formation of three separate and uncoupled equivalent networks called the PPS, NPS and ZPS networks provided that the three-phase power system network in physical phase terms is internally balanced. According to the sequence component theory/mathematical manipulation, in a balanced system the negative and zero sequence components will not be present. However, in practice a perfectly balanced system does not exist.  

For knowing unbalance voltage/current (phase and magnitude of all three phase) only negative sequence would be needed, if not grounded( three phase three wire system). Zero sequence along with negative would be needed if system is grounded (three phase four wire system). refer my papers,"Microprocessor based load balance control and fault type detection for.........using symmetrical components".

According to Fortescue’s methodology,  the calculation of symmetrical components

in a three-phase balanced or symmetrical system results in only positive sequence voltages.

Farhan

When a system is 100% balanced, it does not mean that negative and zero sequence networks do not exist. Surprisingly enough, they do!

You should remember here that zero sequence and negative sequence impedances are parameters and not variables. Parameters, along with the system structure, will determine the outputs (which are variables) given inputs (which are also variables). It is noteworthy to remember that parameters exist whether the system is operational or not.

It is known that any power system contains negative and zero sequence networks. In both balanced and unbalanced conditions, all of the three networks (positive, negative, and zero) exist, but since these networks are connected at the point of unbalance (which is absent as provided by the question), they are not connected together.

Now a new question may arise: even if they are not connected, we still should calculate the currents and voltage in each network, should not we? The answer is no, any generator produces a positive sequence voltage since windings are symmetrically distributed through its circumference. Hence, negative and zero sequence networks will simply have no sources of voltage, and yet they are not connected to the positive sequence one, how can they contain any voltage or current be absent at given node or branch?

I hope this was helpful,

Al-Motasem

Dear, motasem 

the difference between the parameter and variable is not clear to me... can u plz differentiate? 

The simplest example is to think about a resistor. The resistor will have a specific resistance value even if it is not in a circuit. Such resistance is simply determined by resistivity, cross-section area, and length. The resistance here is a parameter that determines the value of an output variable (current through it) for a given input variable (voltage across it).

You may extend the idea to inductors as well. Inductance (parameter that exists even if inductor is not inside a circuit), current (input variable), and flux linkages (output variable).

The method of symmetrical components is used to simplify analysis of unbalanced three-phase power systems under both normal and abnormal conditions.

Positive sequence component is present in a balanced power system by default.If any unbalance is there is the system then only the question of zero or negative sequence arises.

For a balanced system in the positive phase sequence, Ib = a2Ia; Ic = aIa. where a is the 120 degree operator (1