TRANSFORMER



INTRODUCTION

Transformer can change the magnitude of alternating voltage or current from one value to another. They have no moving parts, rugged and durable in construction, requiring little attention and have a very high efficiency of about 99%.

It consists of two windings, the primary and secondary, wound on a laminated magnetic core. The winding connected to the a.c. source is called primary and the one connected to load is called secondary. The alternating voltage V1, whose magnitude is to be changed, is applied to the primary. Depending upon the number of turns of the primary (N1) and secondary (N2), an alternating e.m.f. E2 is induced in the secondary which causes a secondary current I2 when load is connected. Consequently, terminal voltage V2 will appear across the load. If V2 > V1, it is called a step up-transformer and if     V2 < V1, it is called a step-down transformer.  

WORKING



When an alternating voltage V1 is applied to the primary, an alternating flux is set up in the core which links both the windings and induces primary e.m.f. E1 and secondary e.m.f. E2 according to Faraday’s laws of electromagnetic induction. 


If N2 > N1, then E2 > E1 (or V2 > V1) and if N2 < N1, then E2 < E1 (or V2 < V1). 



If load is connected across the secondary winding, the secondary e.m.f. E2 will cause flow of current I2 through the load. The transformer will transfer a.c. power from one circuit to another with a change in voltage level. The following points may be noted: 

(i) The transformer action is based on the laws of electromagnetic induction.

(ii) There is no electrical connection between the primary and secondary and the a.c. power is transferred from primary to secondary through magnetic flux.

(iii) There is no change in frequency i.e., output power has the same frequency as the input power.

(iv) The losses that occur in a transformer are:

(a) core losses—eddy current and hysteresis losses

(b) copper losses—in the resistance of the windings

In practice, these losses are very small so that output power is nearly equal to the input primary power. In other words, a transformer has very high efficiency.

CONSTRUCTION OF A TRANSFORMER

Usually a power transformer is designed such that it approaches the characteristics of an ideal transformer. To achieve this, following design features are incorporated:

i) The core is made of silicon steel which has low hysteresis loss and high permeability. Core is laminated in order to reduce eddy current loss. 

ii) Instead of placing primary on one limb and secondary on the other, one-half of each winding is wound on one limb to ensure tight coupling between the two windings and minimize leakage flux. 

iii) The winding resistances R1 and R2 are minimized to reduce I2R loss and to ensure high efficiency.



Types of Transformers

Depending upon the manner in which the primary and secondary are wound on the core, transformers are of two types.  

i) Core Type Transformers

In a core-type transformer, half of the primary winding and half of the secondary winding are placed round each limb (Fig. 2). This reduces the leakage flux. It is a usual practice to place the low-voltage winding below the high-voltage winding for mechanical considerations.

ii) Shell-type transformer. 

It involves the use of a double magnetic circuit. Both the windings are placed round the central limb (Fig. 3), the other two limbs act as a low-reluctance flux path.

TRANSFORMER ACCESSORY EQUIPMENT 

  • Liquid-Level IndicatorA liquid-level indicator is installed on liquid-filled transformer tanks.
  •  Pressure-Relief DevicesPressure-relief devices are mounted on transformer tanks to relieve excess internal pressures that might build up during operating conditions.
  •  Liquid-Temperature Indicator Liquid-temperature indicators measure the temperature of the internal liquid. 
  • Winding-Temperature Indicator A winding-temperature simulation method is used to measure the hottest spot in the winding. 
  • Sudden pressure Relay  A sudden pressure relay is used to indicate a quick increase in internal pressure when an internal fault occurs. 
  • Desiccant (Dehydrating) Breathers Desiccant breathers use a material such as silica gel to allow air to enter and exit the tank, removing moisture as the air passes through. 
  • Liquid-Preservation SystemLiquid preservation systems isolates the transformer’s internal environment from the external environment . 
  • Conservator system involves the use of a separate auxiliary tank. The auxiliary tank is allowed to “breathe,” usually through a dehydrating breather.

Buchholz Relay

Its purpose is to detect faults that may occur in the transformer resulting in generation of gases.

COOLING OF TRANSFORMERS

 Heat produced in a transformer, by the iron losses in the core and I2R loss in the windings, is removed by cooling to prevent undue temperature rise.

 i) In small transformers (below 50 kVA), natural air cooling is employed. 

 ii) Medium size power or distribution transformers are cooled by housing them in tanks filled with oil. The oil carries the heat from the windings to the surface of the tank and it insulates the windings.

 iii) For large transformers , external radiators are added to increase the cooling surface. Sometimes cooling fans blow air over the radiators to accelerate the cooling process.