Questions & Answers on Power Transformers

   

1-What do you mean by power transformer?

It is a static Electro-magnetic machine which transforms alternating current from one AC voltage to another AC voltage at same frequency & at the same apparent power (KVA).

2-What is the principle of Power transformer operation?

Power transformers work on the principle of electromagnetic induction. Which states that, EMF induced in a closed conducting circuit when the magnetic flux linking with that circuit changes in time.

3-What is the main function of a power transformer?

Generally it is used for stepping up or stepping down of Voltage to desired level

4-What are the various parts of power transformer?

General arrangement of Power Transformer

• Casing
• Core
• Primary & secondary windings
• LV & HV bushings
• Radiators & cooling system
• Conservator
• Breather
• Protection devices like Buchholz relay, relief valves, temperature sensors

5-Why does the oil conservator is placed at higher elevation?

Oil conservator is placed at a slightly higher level than that of the tank. It accommodates the contraction & expansion of oil level during lower & higher loads respectively. At higher load, oil temperature rises and hence level in the conservator rises & at lower load, oil temperature decreases & level in conservator drops down.

The above cushioning in oil level is by cushioning bag present in conservator, the air cushion in the conservator permits expansion & contraction of the oil tank without contact with moist air.

6-What is the function of breather in Transformers?

Breather is installed in a pipe from conservator. One end is connected to air cushion bag in the conservator, other end is connected to external air.

Breather is filled with dry silica jel, generally pink in colour. When oil in the conservator rises, air is let out through the breather. During low load when oil level in the conservator decreases, air is sucked into the cushion bag through breather. Silica jet absorbs the moisture & lets only dry air. Wet silica jells are blue in color.

7-What is the function of Buchholzs relay?

It is fitted in the pipe between conservator tank & main oil tank. It operates by gas during arcing or short circuits

8-What are the various cooling methods employed in Power transformers?

• ONAN: Oil Natural & Air Natural
• ONAF: Oil natural & Air forced
• OFAF: Oil forced & Air forced
• OFWF: Oil forced & water forced
• AN: Air natural

9-What is the function of Pressure relief valve in Transformers?

It is fitted on tank to vent out the gases formed in oil & hence protects the transformer from explosion

10-What do you mean by small, medium & large transformers?

• Small transformers: < 5 KVA
• Medium transformers: 5 to 500 KVA
• Large transformers :> 1 MVA

11-What do you mean by core type transformer?

In this type of transformers, winding surround the limbs of core

12-What do you mean by the Shell type transformer?

In this type of transformers, core surrounds the major portion of the windings

13-What is the relation between voltage, current and number of turns on coils in a transformer?

We have following relation:

Vp/Vs = Np/Ns = Is/Ip

Where, Vp and Vs are Voltage on primary and secondary side.

Np and Ns are No. of turns on primary and secondary coils.

IP and Is are Primary side current and secondary side current of a transformer.

14-A 10 KVA single phase 2200/220 Volts Transformer has 60 turns on secondary side, then calculate Primary current, no. of turns on coil and secondary side current.

Given that,

Vp = 2200 V

Vs = 200 V

Ns = 60

Transformer rating = 10 KVA

We know that Vp/Vs = Np/Ns

Np = 60 X 2200/220 = 600 turns

Further, KVA rating of transformer is (Vp X Ip)/1000 and (Vs X Is)/1000

10 = 2200 X Ip/1000

Therefore, current on primary side Ip = 10 X 1000/2200 = 4.54 Amps

Similarly current on secondary side Is = 10 X 1000/220 = 45.45 Amps

15-A Power transformer’s input voltage is 11 KV & output voltage is 110 KV,then calculate the number of turns on secondary side, if Primary side winding has 25 turns

We have

V1 / V2 = N1 / N2

11 / 110 = 25 / N2

N2 = 250 Nos

16-A power transformer input voltage is 11 KV & output voltage is 220 KV, then calculate the secondary side current if it has 2300 Amps of current on primary side?

We have

V1 / V2 = I2 / I1

11 / 220 = I2 / 2300

I2 = 115 Amps

17-What are the materials of composition of lamination cores?

Laminations are thin 0.2 to 0.3mm thick silicon sheets. These are further coated by varnish or insulation oxide.

18-Why do the laminations are made up of silicon steel sheets?

Higher the silicon content in steel sheets increases the resistivity & reduces the eddy current losses. But silicon percentage is restricted up to 3.25% max.to avoid brittleness of sheet.

19-What are the various tests carried out on Transformers?

Meggering

• IR value measurement
• Magnetic balance test
• Magnetizing current test
• Capacitor measurement
• Ratio test
• Vector group test
• Induced voltage test
• Temperature rise test
• BDV test
• No load current test
• No load loss test

Turbine oil and transformer oil standard testing parameters

20-What is the significance of magnetizing current test?

• This test is performed to locate the defects in the magnetic core structure,
• To detect the shifting of winding, failure
• To detect the problems in tap changers

21-What are factors which affect IR value of transformer?

• Surface condition of the terminal bushings
• Quality of oil
• Quality of winding insulation
• Temperature of oil
• Duration of application & value of test voltage

22-What are the various protections given for power transformers?

• Differential protection for earth faults
• High oil temperature & high winding temperature protection
• Over current protection
• Over fluxing protection
• Protection against fire
• Protection against lightening
• Buchholz relay for gas & arcing protection
• Pressure relief valve

23-On what factors transformer loading depends on?

• Transformer current
• Winding temperature
• Oil temperature

24-What are the effects of Transformer over loading?

Overloading results into

• Cellulose insulation becomes mechanically weak & resulting in winding failure
• Oil gets rapidly oxidized

25-What is the function of tap changer in Power transformers?

Tap changer is for adjusting the secondary voltage

26-What do you mean by No-load current?

The current flowing through the terminal of a winding when rated voltage is applied at rated frequency the other winding being left open circuited

27-What do you mean by no-load losses in Power transformers?

The active power absorbed when rated voltage at rated frequency is applied to the terminals of one winding, with other winding being left open circuited

28-What parameters of Transformer oil are tested yearly?

Transformer oil qualities are recommended as per IS-335-1963

• Density
• Kinematic Viscosity
• Flash Point
• Pour point
• Neutralization Number (Acidity)
• Sludge
• Moisture content
• Dissolved Gas Analysis (DGA)
• Dielectric Dissipation test
• Interfacial tension
• Break down Voltage

29-What is the importance of Transformer BDV test?

BDV test is done to detect moisture, dirt & conductive particles in the oil. The BDV value should be more than 50 KV

30-Briefly explain the BDV test of transformer oil?

This test applies an AC voltage of frequency 40 to 60 Hz through two polished electrodes having diameter 12.5mm to 13 mm with oil gap around 2.5 to 4.0 mm.Rise in voltage between the electrodes is at uniform rate of 2 KV/sec.Thus voltage is increased from zero to till breakdown.

The final value will be arithmetic mean of 6 consecutive tests