Skin Effect

When a conductor carries a steady or direct current, this current is uniformly distributed over the cross-section of the conductor. However , the current distribution is non-uniform is the conductor carries alternating current . The current density is higher at the surface then it's center . It decreases with greater depths in the conductor exponentially. Thus, the current is concentrated near the surface of the conductor , as shown in the figure beside. 

                                                                                                                                                                                                                                Due to this skin effect, the effective cross sectional area offered to the flow of current decreases which increases resistance. The increment of this resistance may be up to 30% , so the effective resistance for conductor is more for AC  than the loss when the value of DC is flowing through the conductor.   

The skin effect is predominant with the increase in the frequency . The skin depth is inversely proportional to the square root of the frequency.The frequency where the skin effect just start to limit the the effective cross sectional area of the conductor is called the crossover frequency.

                                                                                                                                        The skin effect is quite large in solid conductors , even at a frequency of 50 Hz . the skin effect depends on the following factors ----

           i. Nature of material 

          ii. Diameter of wire

         iii. Frequency of supply

        iv. Shape of wire

With the increases in diameter of wire , the skin effect increases. Similarly as frequency increases, the skin  effect increases. If we have stranded conductor rather than solid conductor diameter is less. It can be seen that when supply frequency is less than 50 Hz and conductor diameter is less than 1 cm skin effect is negligible. In large conductors at power frequencies the skin effect is a significant factor.                                                                                                                                                                                                                        

Tertiary Winding

This is a special type of winding used generally in star-star transformer. It it wound on the limb of the                                                                                                                                                                                Ns= 120/P                                       transformer which the primary and secondary are wound. It       Ns- synchronous speed              
      P- no. of Pole                                                is  also called stabilizing winding. It is always delta 

                                                                    connected. Its main function is to arrest the this 

     3rd  Harmonic voltage     
                                                                 
                                                                                   
                                                                                                            Pc ∝ F
                                             Pc - Core Loss     ,                                       F - Frequency      

                 
                               

Parallel Operation Single Phase (1Φ) Transformer

Condition :-

              i) The frequency of both transformers which are in parallel must be same.

     ii) The ratio of voltage of parallel contacts transformer must be equal. Otherwise a circulating current will be flow from higher potential to lower potential. This is only limited by the independence  of those transformers if the ratio is larger different value of this current is also larger which may burnt the winding's of both transformer.

   iii) Phase sequence of parallel connected transformer must be same. Otherwise line voltage will appear across the phase winding's.

   iv) The transformer should be of same phasor group.

    v) The ratio of per phase reactance Xe and Rc  should be same both the transformer otherwise a circulating current will be flow .

Parallel Operation

Advantages :-

                             1. If two and more transformer are connected in parallel the service continuity ensure and the system become reliable.

            2. Any transformer can be switch off or depending on their load demand.

            3. If more transformer works in parallel the chance of over loading of a particular transformer reduce.

             4. The cost of stand by unit transformer is reduce.

Disadvantage :- 

             A. When one or more transformer works on parallel then chance of fault occurrence will increased.

            B. The maintenance cost become more.

            C. The space required for installing the transformer is more.

                     D. The protective device of transformer is more.

Scott Connection / T Connection :-

This is a scott connection which convert 3 phase and 2 phase & vice versa if employs two identical transformers one with centre tap primary known as main transformer and the other with primary√ 3/2 tap known as teaser transformer . the main transformer primary centre tap D is connected with one end of the teaser primary as shown in the figure . Now ends A, B, C, are connected to three phase ac supply and two phase four wire supply is obtained.The two phase side makes use of all the turns on teaser winding as well as all the voltage per turns of the two transformer to be equal.

Application :- 

        The voltages in the primaries of two transformer are 90 (degree) phase displaced . So voltages in there secondary voltages will also 90 (degree) displaced i.e. 3 phase supply has been converted to 2 phase.If we interchange the position the load and supply 2 phase can be converted into 3 phase.