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TRANSFORMERS

Power Transformers-Indian Standards

Insulating Oils-Indian & IECstandards

Internet site: MENU-DRIVENGRAPHICAL USERINTERFACE (GUI)PROGRAMS-Performancefrom Transformer equivalentcircuit

OBJECTIVE TYPEQUESTIONS

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 thetransformer 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 slipcurve 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 aturns ratio

a. N1/N2

b. N1/Ö 3/N2

c. N1/N2/ Ö 3

Ans: (b)

2. The use of higher flux density in transformer design a) reduces the transformer weight per kVAb) increases the weight/kVAc) increases the weight/kW

Ans. a

3. The conservator is used in a transformer because

a) it supplies oil to the transformer whenever neededb) protects the transformer from damage when oil expands due to rise in temperaturec) it provides fresh air and cools down the oil.

Ans. b

4. Which of the following has the highest efficiency?

a) dc shunt motorb) transformerc) induction motord) synchronous motor

Ans. b Power Transformers-Indian Standards

TOPPower Transformers [ISD: 2026 (Part –1)-1977] TYPE TESTS

S. No. Type test Purpose1 Measurement of winding resistance To know resistance of winding utilized2 Measurement of voltage ratio and check of

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

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S. No. Special test Purpose1 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 transformersTo check zero sequence impedance of 3-phasetransformers

3 Short circuit test To ensure mechanical strength under short circuitcondition

4 Measurement of acoustic noise level To measure sound level produced by transformerduring rated excitation under no-load condition

5 Measurement of harmonics of the no-loadcurrent

To measure harmonics of the no-load current

6 Measurement of power taken by fans & oilpumps

To know power consumption of fans & oil pumps

3 Measurement of impedance voltage/ short-circuit impedance (principal tapping) and loadloss

To know copper (load) loss and impedance voltage

4 Measurement of no-load loss and current To know core loss & excitation current5 Measurement of insulation resistance To check insulation strength6 Dielectric tests (Lightning impulse on line

terminalsTo verify impulse voltage withstand capacity ofinsulation (Line terminal to earth & other winding)

7 Temperature rise test To check temperature rise of transformer underloading conditions

8 Tests on on-load tap changer (whereappropriate)

To ensure proper operation of OLTC

SPECIAL TESTS

ROUTINE TESTS

S. No. Routine test PurposeType Tests 1,2,3,4,5,6, and 9 will also be the Routine Tests. In addition, the following Dielectric tests will beperformed as a Routine test

Dielectric TestsSeparate AC sourceInduced over voltage

To verify power frequency withstand capacityof winding (winding to other winding and toearth)To verify power frequency withstand capacityof 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

TOPInsulating 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 followingcharacteristics by carrying out relevant tests on the insulating oil.

S. No. Property As per IS: 335(Un-inhibited)

As per IEC: 296Un-inhibited Inhibited -IA

1 Appearance Clear & transparentand free fromsuspended matter orsediments

Clear & transparent andfree from suspended matteror sediments

Clear & transparent andfree from suspended matteror sediments

2 Density

g/cm30.89 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 0C4 Interfacial tension Min. 0.04 N/m at 270C 0.04 N/m at 250C 0.04 N/m at 250C5 Flash point

Pansky-Marten (closed)1400C 1400C 1400C

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Min6 Pour point

Max.-90 C -300 C -300 C

7 Neutralization value

· Total acidityMax.

· Inorganicacidity/alkalinity

0.03mgKOH/gNil

0.03mgKOH/gNil

0.03mgKOH/gNil

8 Corrosive sulpher Non-corrosive Non-corrosive Non-corrosive9 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

· Neutralizationvalue afteroxidation Max.

· Total sludgeafter oxidationMax.

0.40 mgKOH/g 0.10% by weight

0.40 mgKOH/g 0.10% by weight

No limit

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

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