TRANSFORMERS

 Power Transformers-Indian   Standards Internet site: MENU-DRIVEN GRAPHICAL USER INTERFACE (GUI) PROGRAMS-Performance from Transformer equivalent circuit OBJECTIVE TYPE QUESTIONS

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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/Ö 3/N2
3. N1/N2/ Ö 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

## 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)

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

SAMPLE CRITERIA:  Each power transformer to be tested

# 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.