Transformers-Construction

• Main elements of the transformer as below:

         1) Magnetic circuit: limbs, yokes
         2) Electrical circuit:primary secondary and tertiary windings, formers, 
             insulation and bracing devices.
         3) Terminals, tappings and tapping switches, terminal insulators 
         4) Tank oil, cooling devices, conservators, dryers 

• Special alloy steel of high resistance and low hysteresis loss is used as transformer core, CRGO steel.
• Cut the transformer sheets as far as possible along the grain which is the direction in which the material has a higher permeability
• For large three phase transformer, the core is five-limbed core than , which needs a cross section in the yokes less than that required in usual three limbed core but core losses increase
• 
• on account of the easier insulation facilities, the low voltage winding is placed nearer to the core in case of core type and on the outside positions in the case of shell type transformers. 
• Cylindrical concentric helix windings, commonly employed for core-type transformers.
• Cross over coils are suitable for currents not exceeding about 20A. They are used for HV windings in comparatively small transformers, and comprise wires of small circular section with double cotton covering
• Disc coils are made up of a number of flat sections, generally with rectangular wire
• In Disc coils every turn is contact with oil hence better cooling
• Sandwich windings , commonly employed for shell-type transformers, allow of easy control over reactance.
• Bushings are the porcelain insulators used to provide the insulation between conductor end terminals and whole part of the T/F. 
• Conservators are required to take up the expansion and contraction of the oil with changes of temperature in service without allowing the oil to come in contact with air. 
• Breather extracts the moisture from the air. It consists of calcium chloride or silica gel.
• Transformer oil serves the double purpose of cooling and insulating.
• Sludging is the slow formation of semi-solid hydrocarbons, sometimes of an acidic nature, which are deposited on windings and tank walls. The formation of sludge is due to heat and oxidation. 
• Sludge formation is more in the presence of bright copper surfaces

Objective Questions:

1) Sludge formation in transformer oil is due to which one of the following?

 a. Ingress of dust particles and moisture in the oil.
 b. Appearance of small fragments of paper, varnish, cotton and other organic materials in the oil
 c. Chemical reaction of transformer oil with the insulating materials
 d. Oxidation of transformer oil

Ans: d

Explanation: Refer above theory

2) Cores of large power transformers are made from which one of the following? 
a. Hot-rolled steel
b. Cold-rolled non-grain oriented steel
c. Cold-rolled grain oriented steel
d. Ferrite

Ans: c
Explanation: to increase the relative permeability

3)The function of oil in a transformer is to provide

a. Insulation and cooling
b. Protection against lightning
c. Protection against short circuit
d. Lubrication

Ans: a

Explanation: Refer above theory

4)Consider the following statements relating to the constructional features of a large power transformer:

1. The conservator is used to maintain the level of oil in the transformer tank 2. The bushing is used to protect transformer insulation against lightning over-voltages
3. The Buchholz relay is an over current relay
4. Silica gel is used to absorb moisture.
Which of these statements are correct?
a. 1, 2, 3 and 4
b. 2 and 3
c. 1 and 4
d. 1, 2 and 4 

Ans: c

Explanation: Refer above theory

5) In  large power transformer, a "conservator" drum is provided above the tank and connected to it by a short pipe. The conservator drum is linked to external air through a breather. What is the purpose pf providing the conservator?

a) To store reserve oil to make up oil losses due to leakage.

b) To prevent explosion due to rise in oil pressure inside the tank during a fault.

c) To accommodate change in oil level during the "load cycle" of the transformer load

d) To exert additional pressure by the conservator oil on the inside the main tank to prevent disintegration of oil at high temperature. 

Ans: c

Explanation: Refer above theory

6)Match List I (Type of Coil) with List II (Use of Coil) and select the correct answer using the code given below the lists:

List I
A. Sandwitch coils
B. Disc coils
C. Cross-over coils
D. Spiral type

List II
1. Low voltage coils for currents above 100 A
2. High voltage windings of small transformers
3. Cooling oil is in contact with each turn of the winding
4. Shell-type transformer core

Codes;
   A B C D
a. 2 3 4 1
b. 4 1 2 3
c. 2 1 4 3
d. 4 3 2 1

Ans: b

Explanation: Refer above theory

7) Assertion(A):For obtaning improved magnetic properties, the transformer magnetic core is assembled using cold rolled silicon steel sheets.

   Reason(R):  The laminations for the core could be cut out of the cold rolled

  silicon steel sheets, cutting either in the direction of rolling or transverse

 therof, without affecting the magnetic properties in any way. 

a) Both A and R are individually true and R is the correct explanation of A
b) Both A and R are individually true but  R is not the correct explanation of A  
c) A is true but R is false
d) A is false but R is true

Ans: c

Explanation: Refer above theory

8) Assertion(A):Transformer is not used in a D.C. line

   Reason(R): Losses in the Dc circuit are not negligible  

a) Both A and R are individually true and R is the correct explanation of A
b) Both A and R are individually true but  R is not the correct explanation of A  
c) A is true but R is false
d) A is false but R is true

Ans: b

Explanation: Both are true. R is not reason to A. because when T/F operates on Dc line it burns. 

9) Match List I  with List II  and select the correct answer using the code given below the lists:

List I
A. Silica gel
B. Porcelain
C. Mercury
D. Fins

List II
1. Bushing
2. Buccholz relay
3. tank
4. Breather

Codes;
    A B C D
a. 2 1 4 3 
b. 4 3 2 1
c. 2 3 4 1 
d. 4 1 2 3

Ans: d

Explanation: Refer above theory

10) When are eddy current losses in a transformer reduced?   
a. if laminations are thick
b. if the number of turns in primary winding is reduced
c. if the number of turns in secondary winding is reduced
d. if laminations are thin

Ans: d
Explanation: Pe=ke*f^2*B^2*t^2