Electrical transformer related questions with answers #4

 1.What is tertiary winding? What are reasons of using it ?

 

           Some three phase transformers may have a third winding called tertiary winding apart from primary and secondary. It is also called auxiliary winding or stabilizing winding.

The tertiary winding is provided in a transformer for any one of the following reasons:

• To supply small additional load at a different voltage
• To give supply to phase compensating devices such as capacitors which work at different voltage
•  To limit short circuit current
•  To indicate voltage in high voltage testing transformer

 

 2. How the tertiary winding is connected? Why?

 

            The tertiary winding is normally connected in delta. When the tertiary is connected in delta, the unbalance in the phase voltage during unsymmetrical faults in primary or secondary is compensated by the circulating currents flowing in the closed delta.

 

 3. What is the range of current densities used in the design of transformer winding?

 

             The choice of current density depends on the allowable temperature rise, copper loss and method of cooling. The range of current density for various types of transformers is given below:

δ = 1.1 to 2.2 A/mm2 - For distribution transformers

δ = 1.1 to 2.2 A/mm2 - For small power transformers with self oil cooling

δ = 2.2 to 3.2 A/mm2 - For large power transformers with self oil cooling

δ = 5.4 to 6.2 A/mm2 - For large power transformers with forced circulation of oil                                       

                                      

 4. In mines applications transformers with oil cooling should not be used, why?

              The oil used for transformer cooling is inflammable. Hence leakage of cooling oil may create fore accidents in mines. Therefore oil cooled transformers are not used in mines. Use of Dry transformer is preferable.

 

 5. What are the merits and demerits of using water for forced cooling of transformers?

           The advantage in forced water cooling is that large amount of heat can be removed quickly from the transformer.

            The disadvantage in forced water cooling is that the water may leak into oil and the oil may be contaminated.

 

6. How the heat dissipates in a transformer?

             The heat dissipation in a transformer occurs by all three modes conduction, convection and Radiation.

 

7. What is iron space factor?

              It is defined as the ratio of gross core area to the area of the circumscribing circle.

        Kis = Agi / Ace < 1

 Agi is the gross core area in m²

Ace is the area of circumscribing circle in m²

 

 8.Mention the factors to be considered for selecting the cooling method of a transformer.

              The choice of cooling method depends on KVA rating of transformer, size, application and the site condition where it has to be installed.

 

 9. Write the expression for temperature rise in plain walled tanks.

The temperature rise = Total loss/Specific heat dissipation*heat dissipating surface of tank

                                 =Pi+P/12.5*St 

 Where,

Pi = iron loss

Pc = copper loss

St = Heat dissipating surface of the tank