Important Short Questions and Answers: High Voltage Testing of Electrical Power Apparatus

HIGH VOLTAGE TESTING OF ELECTRICAL APPARATUS AND INSULATION CO ORDINATION

1   What are the necessities of High voltage testing?

·    To check whether they are as per the design and as per specifications and standards.

 ·    To ensure that the HV equipment is able to withstand over voltages produced naturally or within the system.

2   What is the specialty of HV Testing?

1.     The H.V. lab requires higher space.

2.     Special equipments are required.

3.     Special Techniques are required.

3   Name how standards for HV Testing

1.     B I S - Bureau of Indian Standards.

2.     I E C - International Electro Tech. Commission.

3.     B S I - British Standard Institution.

4.     I E E E - Instituting Electrical & Electronics Engineering.

5.     I S O P - International Standards Organization.

6.     A N S I - American Standards Institute

7.     C I G R E - International council on large electrical system.

8.     ISS - Indian Standard Specifications

Definitions

What is disruptive discharge voltage?

The Voltage that produces loss of dielectric strength of equipment is called disruptive discharge voltage. In solid-it is called puncture. In liquid or air-it is called Flashover.

 What is Flashover?

When a loss of dielectric strength occurs inside a liquid or gaseous insulation or along the surface of a solid Insulation, it is called flashover.

What is Puncture?

When a loss of dielectric strength occurs inside a solid it is called puncture.

4. What are self restoring and Non self restoring insulation?

Insulation which completely regains its dielectric strength after a disruptive Discharge is called a self restoring insulation. Insulation which does not regain its insulating property after a disruptive discharge is a Non self restoring insulation.

5. What is withstand voltage.

Withstand Test is a Test in which the specified voltage is applied to the test object under specified conditions to check whether the equipment withstands W/o. any discharge/ flash over . The test voltage which is applied to a Test object in a withstand Test is called withstand voltage. It is the voltage that the equipment is capable of withstanding under specified conditions.

6. What is withstand voltage 50% Flashover voltage

The Test voltage which has 50% probability for flashover is called 50% flashover voltage.

7. What is withstand voltage 100% Flashover voltage

The test voltage which causes flashover of the test object at each of its application.

8. Define Creepage Distance:

It is the shortest distance on the contour of the external surface of the insulator that is between the two metal fittings on the insulator.

9. Define AC Test Voltage.

Alternating current voltage of frequency 40 to 60 Hz, approximately sinusoidal (7% deviation is permitted) is called AC Test voltage.

10. Define Impulse voltage.

It is a fast rising slow decaying voltage, characterized by its peak value, time to front and time to half value.

Standard Impulse Voltage

1.     Peak : Tolerance ±3%

2.     Time to Front : T f 1.2sec ±30%

3.     Time to half value : Tt 50sec ±20%

Standard Switching Voltage

1.     Peak : Tolerance ±3%

2.     Time to Front : 250 s ± 20%

3.     Time to hal value

11. How are the Testing of insulators classified

1. Type Test

Done whenever a new brand is introduced and a new design is adopted.

2. Routine Test

Whenever the quality of the individual equipment is to be established say at the time of purchase.

12.                              What are the various High voltage Tests done on insulators

·        Power frequency Flashover Test Dry

·        Power frequency Flashover Test Wet

·        Power frequency Withstand Test (One Minute) Dry

·        Power frequency Withstand Test (One Minute) Wet

·        Impulse withstand Test Dry

·        Impulse Flashover TestDry

·        Pollution Testing (Power Frequency)

·        Partial discharge Test

·        Radio Interference Test.

13.                              What is meant by atmospheric correction with reference to High Voltage Testing?

Normally HV Tests are done under Normal Temperature, pressure & humidity conditions and then the values are corrected to the following conditions.

Temp : 27oC

Pressure : 1013 Millibar 760 torr

Absolute humidity : 17gram/m₃

This is done by applying the following correction factors.

h = humidity correction factor

d = air density correction factor

If

Va = Voltage under Test conditions

& Vs = Voltage under reference atmospheric candidate

Then

Vs = Va x h/d

d = 0.289 b/ (273+t)

Where

b = atmospheric Pressure in millibar t = atmospheric temp in degree C.

h = Can be obtained form graph. (Humidity / Dry bulb thermometer reading)

14.                              What are the various HV Test done on Bushings?

1. Power frequency Tests

·        Power factor Test

·        Partial Discharge Test

·        1 Minute W.S. Test

·        Visible discharge Test

2. Impulse Voltage Test

·        Impulse with stand Test – Full wave (Positive & Negative Polarity)

·        Impulse with stand Test – Chopped wave (Positive & Negative polarity)

·        Switching surge Flashover Test

·        Impulse Flash over Test under oil.

15.                              What are the steps for Impulse withstand Test on Power Transformer

1.     Apply one full Impulse of 75% BIL of Power Transformer

2.     Apply one full Impulse of 100% BIL of Power Transformer

3.     Apply Two chopped wave of 100% BIL

4.     Apply one full wave of 100% BIL

5.     Apply one full wave of 75% BIL

The Power Tr should stand.

Then, it passes the Test.

16.                              What are the various HV Tests done one circuit Breakers?

1.     Power frequency WS Test, 1 minute, dry.

2.     Power frequency WS Test, wet

3.     Impulse voltage WS Test dry.

4.     Switching impulse WS Test

17.                              What are the various Tests (HV Tests) done on surge diverters.

1.     Insulation withstand Test, power frequency both dry and wet.

2.     Power frequency voltage spark over Test.

3.     Standard Impulse voltage spark over Test.

4.     Front of wave voltage spark over Test.

5.     Switching Impulse voltage spark over Test.

6.     Residual voltage Test.

7.     Current Impulse withstand Test.

o High current

o Long duration.

8.     Pressure relief Test (When fitted)

9.     Pollution Tests

18.                              What is the necessity for measurement of RIV?

Sometimes electrical equipment like power Transformer, conductors, rotating machines etc. produce unwanted electrical signals in the radio frequency range of 150k Hz to 30 M Hz, where as the power frequency being 50 Hz.

These signals affect the communication systems & should be prevented. Hence RIV measurement is necessary.

19. What is meant by insulation co-ordination in EHV power system?

Insulation co-ordination is the grading of the insulation level of

(1) Various equipments in a power system

(2) Various parts of the equipments

(3) Protection devices in such a way that, in the event of a serious over voltage, less vital, less important, less costlier, easy to repair equipment/part of equipment breaks down first and thereby avoiding major breakdown & interruption to consumers, cost of replacement etc.

For e.g.

1.     In the event of an over voltage, a string insulator on Transmission line should breakdown before the bushing of a power Transformer.

2.     The bushing of the power Tr. should breakdown first before the Breakdown of the winding of the Power Transformer.

20.                              What is system protection level and its selection depend on what factors?

In the power system, system protection level is established considering the,

1.     Location of the station

2.     Protection level of arrester

3.     Line shielding

21.                              What is BIL?

The basic insulation levels are reference levels fixed by standards for each voltage levels. Basic impulse levels are reference levels expressed in terms of impulse crest voltage(Vp) with a standard lightning impuse voltage(1.2/50 micro seconds wave) for any apparatus the insulation level as demonstated by suitable tests should be greater than or equal to the BIL.

22. While selecting an equipment for a power system what should he its BIL.when compared to the system protection level.

For any equipment insulation level should be more than the BIL

For proper insulation coordination its insulation level should be greater than the system protection level over the margin determined by the following factor

1.     Atmospheric Condition

2.     Station Location

3.     Protection level of arresters.

4.     Importance of the equipments etc.

Hence the system protection level will be less than BIL

GLOSSARY

1.Routine test

They are intended to ensure the reliability of an individual test object and to check the quality of an individual test piece.

2. Withstand Test Voltage

The voltage which has to be applied to a test object under specified conditions in a withstand test is called withstand voltage.

3. Flashover Voltage

The voltage that causes a flashover at each of its applications under specified conditions when applied to test objects as specified is called flashover voltage.

4. Porosity Test

The insulator is broken and immersed in a 0.5 % alcohol solution under a pressure of 13800 KN/sq.m for 24 hours.

5. Surge Diverter

It is a non-linear resistor in series with a spark gap kept at line terminals in the substations.

6. Creeping Distance

It is the shortest distance on the contour of the external surface of the insulator unit.

7. Treeing

The spreading of spark channels during tracking in the form of branches of a tree is called treeing.

8.Radio Interfaerence

The power apparatus produces unwanted signals in the radio and high frequency ranges.

9.Isolator

It is a disconnect or or a mechanical switching device which provides in the open position an isolating distance in accordance with special requirements.

10.Circuit Breakers

It is a switch which automatically interrupts the circuit when a critical current or voltage rating is exceeded.

11. Type test

They are intended to prove or check the design features and the quality. They are done on samples when new designs or design changes are introduced