Test on Power Transformer Before Commissioning

  

There are following test to be performed before commissioning of power transformers as per Indian Standard Code.

Inspection	Junction box and marshalling kiosks Control panel, relay panel etc.
Primary injection test	To check operation and stability of earth fault relay on high voltage side and low voltage side

Main Accessories of Three Phase Transformer

 

The following are main accessories of the three phase transformer in addition to the core and windings.

• Transformer Tank
• Conservator
• Oil level indicator
• Breather

Star - Delta Transformer

 

• It is used at the substation end of the transmission line. 
• The third harmonic current flows in the secondary delta which produces sinusoidal flux.

Primary winding is star connected

• Phase voltage = Line voltage / √ 3

Delta - Star Transformer

• The primary winding of transformer is connected in delta whereas the secondary winding is connected in star. 
• It is used as the beginning of the transmission line to step up the transmission line voltage.

Primary winding is delta connected

• Line voltage = Phase voltage

Delta - Delta Transformer

 

The delta – delta transformer is applicable where large current requires on low voltage supply and continuity of supply is possible in spite of one phase fails.

Advantages

• No phase displacement between primary winding and secondary winding

Star - Star Transformer

The Star – Star connection is applicable for low current, high voltage applications.

Advantages

• As the phase voltage is 1/ √ 3 to that of line voltage, number of turns per phase and amount of insulation is reduced that proportional.
• No phase shift between primary voltage and secondary voltage.

All day Efficiency

 

ALL DAY EFFICIENCY

• The performance of the Distribution Transformer cannot be judged by ordinary efficiency. 
• As the primary winding of the transformer is energized throughout the day ( 24 hours ) , iron loss occurs continuously whether the transformer is loaded or unloaded. 

Direct Loading Test on Single Phase Transformer

• The circuit diagram for the direct loading test is shown in the figure. 
• As the transformer is loaded, this test is known as direct test. The rated voltage is given to transformer primary side with the help of single phase auto-transformer and secondary side is loaded. 
• The load is increased step-by-step till ammeter A2 shows the full load secondary current. Note down the reading of wattmeter, voltmeters and and ammeters.

Back To Back Test Transformer

 

Main objective

• The main objective of this test is to determine efficiency at any load, regulation and temperature rise of the transformer.
•  If the high-capacity transformer is loaded for long time, temperature rise is known but it is very difficult to arrange suitable load for that transformer and also large amount of wastage of energy.

To perform short circuit test on single phase transformer

 

Significance

The short circuit on the single phase transformer is performed in order to find out equivalent circuit parameters of the transformer. This test is performed to find out

• Copper losses at full load as well as any load
• Equivalent impedance as referred to primary (Z₀₁) or secondary side (Z₀₂)

To Perform Open circuit test on Transformer

Significance

• The No test on the single phase transformer is performed in order to find out no load parameters of the transformer. 
• This test is performed to find out

• No load current
• Magnetizing current
• Loss component of no load current

To Separate out Iron losses in the Transformer

Introduction

• There are no moving parts in the transformer therefore the mechanical losses are zero. 
• The open circuit test and short circuit test on transformer is performed in order to find out core losses and copper losses.  
• We will study how to separate hysteresis loss and eddy current loss.

Compare : Three Phase Induction Motor & Transformer

Construction

• The stator and rotor magnetic field rotate at synchronous speed with each other in the case of three phase induction motor. 
• These fields are stationary with each other therefore a three phase induction motor is equivalent to transformer with air gap between stator and rotor winding. 

Technical Specification of Three Phase Transformer

The following technical Specification is provided by the Manufacturer of transformer.

• Name of Manufacturer
• Service

kVA rating

Transformer Testing : Type Tests, Routine Tests

Testing of Transformer

• The following types of type test, routine test and special tests are performed on the power transformer. 
• These tests are to be performed at ambient air temperature below 50 degree centigrade. 

Standard KVA Rating of Three Phase Power Transformer

Rated kVA

• The rated kVA is related to the product of rated voltage, rated current and appropriate phase factor as given below.

Number of Phases	Phase Factor
1	1
3	1.73

Transformer Name Plate As Per IS

The following information is displayed on the transformer name plate.

• Kind of Transformer

Auto transformer, Booster transformer etc.

• Number of Standard ( IS code )

Transformer : Advantages of Stepped Core Over Square Core

Core of Transformer

• The cross section of the transformer core may be rectangular, square or stepped. Generally the circular coils are preferred for distribution transformer whereas the square and stepped cores are preferred for power transformer. 

Types of Dry Type Transformer

There are following types of dry type transformers.

• Single phase transformer rating less than 1 kVA and three phase transformer rating less than 5 kVA
• Instrument transformers ( Current Transformer and Potential transformer )

Compare Distribution Transformer and Power Transformer

Distribution Transformer

• The rating of the distribution transformer should be below 500 kVA. 
• It is used for distribution of power at consumer end. 
• It is designed maximum efficiency at 50% full load.