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TRANSFORMERS

 

Power Transformers-Indian Standards

 Insulating Oils- Indian & IEC standards

 Internet site: MENU-DRIVEN GRAPHICAL USER INTERFACE (GUI) PROGRAMS-Performance from Transformer equivalent circuit

OBJECTIVE TYPE QUESTIONS

 

HOME takes you to the start page after you have read these Topics. Start page has links to other topics.

 

MENU-DRIVEN GRAPHICAL USER INTERFACE (GUI) PROGRAMS- by Prof. H. Sadat. -The transformer program obtains the transformer equivalent circuit from open-circuit and short-circuit tests. It also finds the transformer performance characteristics using the transformer parameters. In the induction motor program the parameters of the equivalent circuit can be entered on the circuit diagram. The program draws the Thevenin's equivalent circuit. Performance characteristics are then obtained for the specified slip and the torque slip curve is plotted on the screen. The analysis can be repeated for different slips. A GUI program deals with the design of a first-order controller in the forward path of a closed-loop control system. This GUI program designs the following controllers: Proportional, phase-lag, phase-lead, PD,PI, and PID controllers. To run the controller design program at the MATLABprompt type>> rldesigngui

 

TRANSFORMERS -OBJECTIVE TYPE QUESTIONS

 

  1. For a delta-star 3-phase transformer the primary winding has N1 turns/phase and a secondary has N2 turns /phase. The single-phase equivalent circuit will have a turns ratio
  1. N1/N2
  2. N1/ 3/N2
  3. N1/N2/ 3

 

Ans: (b)

 

2.        The use of higher flux density in transformer design

 

a)       reduces the transformer weight per kVA

b)       increases the weight/kVA

c)       increases the weight/kW

 

Ans. a

 

3.        The conservator is used in a transformer because

 

a)       it supplies oil to the transformer whenever needed

b)       protects the transformer from damage when oil expands due to rise in temperature

c)       it provides fresh air and cools down the oil.

 

Ans. b

 

4.        Which of the following has the highest efficiency?

 

a)       dc shunt motor

b)       transformer

c)       induction motor

d)       synchronous motor

 

Ans. b

 

 

Power Transformers-Indian Standards

TOP

Power Transformers [ISD: 2026 (Part 1)-1977]

 

TYPE TESTS

 

S. No.

Type test

Purpose

1

Measurement of winding resistance

To know resistance of winding utilized

2

Measurement of voltage ratio and check of voltage vector relationship

To know correct voltage ratio at each tap and to check vector symbol (To ascertain phase displacement)

3

Measurement of impedance voltage/ short-circuit impedance (principal tapping) and load loss

To know copper (load) loss and impedance voltage

4

Measurement of no-load loss and current

To know core loss & excitation current

5

Measurement of insulation resistance

To check insulation strength

6

Dielectric tests (Lightning impulse on line terminals

To verify impulse voltage withstand capacity of insulation (Line terminal to earth & other winding)

7

Temperature rise test

To check temperature rise of transformer under loading conditions

8

Tests on on-load tap changer (where appropriate)

To ensure proper operation of OLTC

 

SPECIAL TESTS

 

S. No.

Special test

Purpose

1

Dielectric test (Modified impulse test on Neutral

To verify impulse voltage withstand capacity of insulation (Neutral terminal to earth & other winding)

2

Measurement of zero sequence impedance of 3-phase transformers

To check zero sequence impedance of 3-phase transformers

3

Short circuit test

To ensure mechanical strength under short circuit condition

4

Measurement of acoustic noise level

To measure sound level produced by transformer during rated excitation under no-load condition

5

Measurement of harmonics of the no-load current

To measure harmonics of the no-load current

6

Measurement of power taken by fans & oil pumps

To know power consumption of fans & oil pumps

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ROUTINE TESTS

 

 

S. No.

Routine test

Purpose

Type Tests 1,2,3,4,5,6, and 9 will also be the Routine Tests. In addition, the following Dielectric tests will be performed as a Routine test

 

Dielectric Tests

  • Separate AC source
  • Induced over voltage

 

  • To verify power frequency withstand capacity of winding (winding to other winding and to earth)
  • To verify power frequency withstand capacity of winding (phases to earth and other winding)

 

 

 

ACCEPTANCE TESTS

 

The routine tests on each transformer are to be considered as acceptance tests

 

SAMPLE CRITERIA: Each power transformer to be tested

TOP

Insulating Oils- Indian & IEC standards

 

INSULATING OILS [IS: 335-1983 OR IEC: 296-1982]

 

There are no specific lit of type/routine /acceptance tests to be performed on manufactured insulating oil. However, the manufacturer shall verify the following characteristics by carrying out relevant tests on the insulating oil.

 

 

S. No.

Property

As per IS: 335

(Un-inhibited)

As per IEC: 296

Un-inhibited

Inhibited -IA

1

Appearance

Clear & transparent and free from suspended matter or sediments

Clear & transparent and free from suspended matter or sediments

Clear & transparent and free from suspended matter or sediments

2

Density

g/cm3

0.89 at 270C

< 0.895 at 200 C

< 0.895 at 200 C

3

Kinematic viscosity at 270 Max.

27 cSt

at 270C

16.5 cSt

at 40 0C

16.5 cSt

at 40 0C

4

Interfacial tension Min.

0.04 N/m at 270C

0.04 N/m at 250C

0.04 N/m at 250C

5

Flash point

Pansky-Marten (closed) Min

1400C

1400C

1400C

6

Pour point

Max.

-90 C

-300 C

-300 C

7

Neutralization value

         Total acidity

Max.

         Inorganic acidity/alkalinity

 

0.03mg

KOH/g

Nil

 

0.03mg

KOH/g

Nil

 

0.03mg

KOH/g

Nil

8

Corrosive sulpher

Non-corrosive

Non-corrosive

Non-corrosive

9

Electric Strength (Breakdown voltage) Min.

         New unfiltered oil

         After filtration

 

 

 

30 kV (RMS)

50 kV (RMS)

 

 

 

30 kV (RMS)

50 kV (RMS)

 

 

 

30 kV (RMS)

50 kV (RMS)

10

Dielectric dissipation factor( tand) at 900 Max.

0.005

0.005

0.005

11

Specific resistivity

         At 900 Min.

         At 27 0 Min.

 

30x 1018 ohm-cm

500x 1018 ohm-cm

-

-

-

-

12

Oxidation stability

         Neutralization value after oxidation Max.

         Total sludge after oxidation Max.

 

0.40 mg

KOH/g

 

 

0.10% by weight

 

0.40 mg

KOH/g

 

 

0.10% by weight

 

No limit

 

 

SAMPLING CRITERIA

 

As per IS:6855-1973 OR relevant specification of equipment where oil is required.