When load is balanced, three phase four wire system would not be needed, as the neutral current would be zero, and voltage would be balanced, as the load is balanced on all three phases, also with sending end delta third harmonics current, would not be allowed to flow in line, as it circulate in close delta.

Dear Sunita,

Transmission lines are not star or delta connected. Its there ends which are star or delta connected. i.e. at one side,secondary of transformer can be star or delta connected and on the other side of transmission line, load / primary of distribution transformer can be star connected or delta connected.

Why Delta is preferred as substation transformer secondary is because..

Transmission lines are usually kilometers in length. So its better to use 3 wire system(Delta) than four wire systems(star) as it reduces cost and also The delta winding allows third-harmonic currents to circulate within the transformer as it is closed in nature, and prevents third-harmonic currents from flowing in the transmission line".

Superposition theorem is only applicable only to linear circuit i.e. circuit consisting of resistances in which Ohm’s law is valid.

Superposition only works for voltage and current but not power because power is a function of V and I i.e V*I or I^2*R or V^2/R. i.e , its not linearly related and hence not a linear function. In other words the sum of the powers of each source with the other sources turned off will not give you the real consumed power. To calculate power using superposition theorem, we should first use superposition to find both current and voltage of each linear element and then calculate the sum of the multiplied voltages and currents.

Reference: Quora by Rafeequddin, Electrical4u

Transmission lines are not star or delta connected. Their ends are, where usually you have a load or a transformer. Another point that should be considered is that superposition only works for voltage and current but not power. To calculate power we should first use superposition to find both current and voltage of each linear element and then calculate the sum of the multiplied voltages and currents.

thank you all...

yes i agree to your answers..but why it is so?why cant we represent trnsmission line in delta fashion or star fashion...?

Dear Sunita,

We are using transformations to make analysis simpler. Not to make it complex. When Transmission line itself is not star or delta why u want  a transformation there!!

Transmission line itself is a straight conductor. To find net impedance of transmission line its just sum of total net R and total net XL.  

Star Delta Transformations allow us to convert impedances connected together from one type of connection to another. We can now solve simple series, parallel or bridge type resistive networks using Kirchoff´s Circuit Laws, mesh current analysis or nodal voltage analysis techniques but in a balanced 3-phase circuit we can use different mathematical techniques to simplify the analysis of the circuit and thereby reduce the amount of math’s involved which in itself is a good thing.

Ref: (http://www.electronics-tutorials.ws/dccircuits/dcp_10.html)