Seminar guide Seminar coordinator

Ms. Aanchal Baranwal Mr. Alok pandey (Assistant professor) (Assistant professor)

Department of Electrical Department of Electrical

Engineering Engineering

A

Seminar on

High voltage testing of Electrical Transformer

PRESENTED BY

Hari Shankar Singh:-1308220902

CONTENTS

Introduction of transformer and HV transformer

Types of testing of transformer and explain with

detail

Advantages of HV testing of transformer

Disadvantages of HV testing of transformer

Conclusion

References

What is Transformer ?

• Transformer is an electrostatic device which

transfer electrical energy one circuit to another

circuit by magnetic flux without change of

frequency . Transformer has based on principle

of electromagnetic induction(EMI).

• Formula used as measured a value,

Vs / V p = Ns / N p = I p / Is

HIGH VOLTAGE TRANSFORMER

• High voltage transformers convert voltages from

one level or phase configuration to another,

usually from higher to lower.

• They can include features for electrical isolation,

power distribution, and control and

instrumentation applications.

• High voltage transformers usually depend on

the principle of magnetic induction between

coils to convert voltage and current levels .

TYPES OF HV TESTING OF ELECTRICAL TRANSFORMER

1. Partial discharge test

2. Impulse transformer test

3. Transformer turns ratio test

4. Insulation resistance test

5. Routine test

6. Temperature rise of oil testing of transformer

PARTIAL DISCHARGE TEST

• Partial discharge is a small localised dielectric

breakdown of an electrical insulation system

under high voltage stress.

• Each discrete partial discharge is the result of an

electrical breakdown of an air pocket within the

insulation.

• These discharges erode insulation and

eventually result in insulation failure.

• Acoustic techniques are used by manufacturers

to locate discharges that occur during HV testing

factory.

• PD occurs if the electric field strength applied to

a dielectric exceeds the dielectric withstand

strength of an insulation system locally.

• This may be caused by either increased field

strengths , e.g. due to design problems, sharp

edges on electrodes, small clearances , ageing ,

manufacture defect , insulation failure , damage

etc.

PDT TEST CIRCUIT DIAGRAM

Fig.1

CAUSES OF FAILURE IN PDT

Fig.2

FAILURE LOCATION OF PDT

Fig.3

FUTURE APPLIANCES FROM PDT

• Cables and Power transformers and bushings.

• Switchgear.

• Motors and generators

• Conclusion PDT testing prevents failure,

1. Off-line testing is a critical pre-commissioning

check.

2. On- line testing is an extremely powerful non-

instructive condition monitoring tool.

IMPULSE TRANSFORMER TEST

• This test is made to prove that the transformer

insulation will withstand voltage surges which

may be caused by lightning or switching.

• This includes insulation to ground, insulation

between turns and windings, and the flashover

value of the associated bushings.

• A high-voltage wave of standard values, and

approximating a lightning surge, is imposed on

the unit to be tested.

• By a lightning pulse method used as measured

for impulse transformer testing followed as full

wave, half wave and front of wave show in Fig.4.

Fig.4:- Graph between ultra frequency and time

CONNECTION DIAGRAM OF IMPULSE TRANSFORMER

TEST

Fig.5:-Connection of Impulse transformer test

TRANSFORMER TURN’S RATIO TEST

• The Transformer Turns Ratio test is used to

make sure that the Turns Ratio between the

winding of the transformer is correct.

• With this information, you can decide what the

output voltage of the transformer will be. The

ratio is calculated under no-load conditions using

formula given below,

V pri /V sec = Z pri/ Z sec = (N pri /N sec )2

INSULATION RESISTANCE TEST

(IR VALUE TEST)

• Insulation ages and deteriorates because of

moisture, dust and electrostatic stress. Insulation

should be monitored continually to avoid sudden

failure of the equipment.

• An insulation resistance test detects insulation

quality within the transformer.

• The conductive impurities or mechanical flows in

the dielectric can be analysis based on this test.

CONNECTION OF IR VALUE TESTING DIAGRAM

Fig.6:- IR Value testing connection diagram

• Normally, MEGGERS have a test voltage of

500V , 2500V , or 5000V .

• IR between HV and LV as well as windings to

earth.

• Minimum IR value for Tap changer is 1000 ohm

per volt service voltage

FACTORS AFFECTING ON IR VALUE OF TRANSFORMER

• The IR value of transformers are influenced by

Surface condition of the terminal bushing.

• Quality of oil.

• Quality of winding insulation.

• Temperature of oil.

• Duration of application and value of test voltage.

IR VALUE TESTING FORMULAS

• 1 Phase Transformer

IR Value (MΩ) = C X E / (√KVA)

• 3 Phase Transformer (Star)

IR Value (MΩ) = C X E (P-n) / (√KVA)

• 3 Phase Transformer(Delta)

IR Value (MΩ) = C X E (P-P) / (√KVA)

Where C= 1.5 for Oil filled T/C with Oil Tank,

30 for Oil filled T/C without Oil Tank or Dry Type T/C.

ROUTINE TEST FOR TRANSFORMER

A Routine test of transformer is mainly for confirming

operational performance of individual unit in a

production lot. Routine tests are carried out on every

unit manufactured.

All transformers are subjected to the following

Routine tests:

• Insulation resistance Test.

• Winding resistance Test.

• Turns Ratio / Voltage ratio Test

• Polarity / Vector group Test

• Open circuit and short circuit Test.

• Continuity Test.

• High Voltage Test.

• Partial discharge Test.

• Separate source AC voltage.

• Induced overvoltage.

• Lightning impulse Tests.

• On -load tap changers Test.

TEMPERATURE OIL TESTING OF TRANSFORMER

• Temperature rise test of oil of Transformer is

included in type test of transformer.

• In this test we check whether the temperature

rising limit of transformer winding and oil as per

specification or not.

• Faults detects at above 75˚C by the help of

Buchholz relay and work as ALARM and TRIP

circuit.

BUCHHOLZ RELAY, EXPAIL BY THE THIS

VIDEO

ADVANTAGES OF HV TESTING TRANSFORMER

• Pure conductivity by IR Value test.

• Controlling of HV surges by Impulse test.

• Removing of discrete waveform by the help of

Partial Discharge test.

• Terminals has used connected correctly by

Transformer Turn’s Ratio test.

• Units measured a correctly and manufacturing

defect check by Routine test.

•

DISADVANTAGES OF HV TESTING OF TRANSFORMER

• After failure, transformer is removed and

replaced with new/repaired one without removing

the cause of failure which results in immediate or

short time failure.

• Insufficient Oil level.

• Faulty operation of tap changer switch.

• Lack of installation checks.

• Prolonged Over loading.

CONCLUSION

• Knowledge of the construction of high voltage test

transformers, their design and use in high voltage

testing system gives the solution of the most

adjustment transformer in respect of parameters

and prices.

• HV testing are main advantages of safety ,

higher efficiency , lower cost ,savings of energy ,

avoiding of failures ,trouble shooting for failure

detection , accurate detection , quality assurance.

REFERENCES

• R.S. Jha, “High Voltage Engineering”, Dhanpat

Rai & Sons, Second Edition in 1996.

• C. L. Wadhwa, “High Voltage Engineering”, Wiley

Eastern Ltd. Third Edition in 2012.

• E.Kuffel and W.S. Zaengl, “High Voltage

Engineering” , Pergamon Press, First Edition

in1984.

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