High Voltage Engineering (2160904)

B. E. Semester VI

Manual for Laboratory work

Department of Electrical Engineering

Government Engineering College, Valsad

GOVERNMENT ENGINEERING COLLEGE, VALSAD ELECTRICAL ENGINEERING DEPARTMENT

CERTIFICATE

This is to certify that Mr./Miss_____________________________________________________ of

Electrical Branch, Sem-VI, Enrollment No.__________________________, has

satisfactorily completed his/her term work for the subject High Voltage

Engineering (2160904) during odd term-2015.

Date :

Sign of Faculty Head of the Department

GOVERNMENT ENGINEERING COLLEGE, VALSAD ELECTRICAL ENGINEERING DEPARTMENT

INDEX

Subject: High Voltage Engineering (2160904)

PRACTICAL NO.

TITLE GRADE DATE SIGN OF FACULTY

1 Design planning and layout of H.V lab

2 Study of horn gap arrester

3 Measurement of Breakdown strength of liquid dielectric

4 Measurement of breakdown strength of solid dielectric.

5 Effect of electrode configuration on breakdown of an air(uniform gaps)

6 Effect of electrode configuration on breakdown of an air(non uniform gaps)

7 Peak voltage measurement using sphere gap

8 To obtain electric field distribution by using electrolytic tank method

9 Study of impulse generator

10 Flashover and breakdown test on insulator.

Government Engineering College, Valsad Electrical Engineering Department

B.E Semester VI (2019-20) High Voltage Engineering (2160904)

List of Practical

Sr. No. Name of Practical Course Outcome

1 Design planning and layout of H.V lab CO-3

2 Study of horn gap arrester CO-3

3 Measurement of Breakdown strength of liquid dielectric CO-3

4 Measurement of breakdown strength of solid dielectric. CO-3

5 Effect of electrode configuration on breakdown of an air(uniform gaps)

CO-2

6 Effect of electrode configuration on breakdown of an air(non uniform gaps)

CO-2

7 Peak voltage measurement using sphere gap CO-1

8 To obtain electric field distribution by using electrolytic tank method

CO-3

9 Study of impulse generator CO-1

10 Flashover and breakdown test on insulator. CO-3

High Voltage Laboratory

Practical No: 1 Date: Aim Design, Planning and Layout of the High Voltage Laboratory.

Theory Transportation of large amount of power needs extra high volt transmission lines. Elsewhere in the transmission line of 760 kV have come into operation & transmission line of ratings of 1000 kV or more are coming into operation in the USA. Extension studies are being made in different countries on the possible use of complex extra high voltage DC system of +400 kV and above. In India,at present the transmission voltage has reached a level of 1200kV.

General Requirements of HV Lab: Customs made according to the type of equipment, available space and accessories. Ground level location is preferred and floor loading has to be considered while

designing the lab. Lab should be free from dust, humidity, draught. Windows should be located at ground level and should have provision for black out so

that arcing can be easily observed. Control room should have good view of the lab. Adequate access door should be provided to bring in the equipment and test specimen. Proper safety and warning system must be provided. Lab should have adequate and proper clearance. Proper spacing should be kept within

the equipment. Classification of HV Lab: High voltage laboratories, depending on the purpose for which they are intended, can be classified into mainly three types

Small Laboratories: Energy Rating: Less than 10 kW/10 kVA/ 10 kJ 300 kV AC Single equipment and 100 kV Impulse Generator AC cascade units 500 to 600 kV and 200 to 400 kV DC

Medium size Laboratories: Ground transport Handling equipment Providing room for possibility of increasing the maximum voltage rating Used in Industries for routine test The impulse voltage generator required would have rating of 20 to 100 kJ or more. Other test equipments like the impulse current generator for testing surge diverters and dc test facilities for testing cable and capacitor can also be made available.

Large size Laboratories: One or more HV test halls and Corona Outdoor test area for test on large sized equipments, transmission lines, towers Controlled atmospheric test rooms pollution test chamber

Computer facilities, conference halls, library etc. with good office facilities Provision for overnight tests and stay Test Transformer (1.5 to 2 MV)

Impulse Generator (5 to 6 MV) HVDC Rectifier (1.2 to 1.5 MV)

Test can be carried out in HV Lab: Withstand Test Flashover Test Pollution Test Partial Discharge Test Tan Measurement Power Frequency Test Impulse Test Switching Test Lightning Test DC Voltage Test Radio Interference Voltage (RIV) measurement High Current Test

Equipment in HV Lab:

HV Generator (Transformer) Oil Testing Kit Impulse Generator Testing facilities for RIV Testing and Partial Discharge Sphere Gap for Voltage Measurement Corona Cage

Grounding in HV Lab:

Ideal Ground Equipotential plane approximated by copper or galvanized iron sheet Very expensive

Single Point Grounding Ear thing grid is grounded at a single point.

Bus Grounding Grounding is done at several points in a lab Least satisfactory

Design and Specification of Grounding System: Metal sheet is embedded in a concrete floor. Generally copper conductors are used. Grounding grid is a mesh of 1m x 1m and is connected to metal grids of RCC

construction of lab. Chicken mesh of 1cm x 1cm is used to reduce the electromagnetic interference. Rating of VGEC HV Lab Equipments: High Voltage 30 kV AC / 40 kV DC Test Kit:

Input Supply : 1-Phase 230V AC, 50 Hz Output AC Voltage : 0-30 kV AC Output DC Voltage : 0-40 kV DC Polarity : +ve and –ve Leakage Current : 0-50 mA Detail of Transformer : Oil Cooled, Shell Type with Epoxy Fiber glass bushing Rectifier and Filter : Oil Cooled Bleeding Resistor : Oil Cooled for measuring DC

Sphere Gap Assembly:

Type : Vertical Sphere Material : Copper with 150 mm dia.

Measurement : Micrometer Scale (Vertical Type) Water resistor : 100 to 1000 kΩ

High Voltage Horizontal Type stand suitable for Rod Gap Apparatus Type : Horizontal Measurement : Micrometer Scale (Horizontal Type) Set of Electrodes:Plate-Plate, Rod-Rod Water resistor : 100 to 1000 kΩ

Electrolytic Tank: Dielectric Oil test set (0-60 kV, 2 kV/sec) Solid insulator test kit Horn Gap Apparatus: Centre tap earthed HV Transformer with 20 kV and 100 mA

Safety and Precaution for HV Lab:

Exposed metal parts on equipment which are not in use but in proximity of live equipment should be treated as live to electromagnetic induction and discharged before touching.

Capacitors which are not in use should be shorted. Flashover can be noisy and source of electromagnetic interference. Some equipment require permanent ear thing. These equipments must have connection

which requires mechanical assistance to remove from circuit. Electronic devices which cause or are susceptible to electromagnetic interference

should not be used. Any equipment to be energized must have all exposed metal connected to ground

unless that metal is not a part of circuit. Keep the mobile phones switched off in HV lab.

Conclusion Review Question 1. What are the main facilities should be available in High Voltage Laboratory? 2. Give classification of High Voltage Laboratory. 3. What are different grounding systems used for Impulse Testing Laboratory? 4. Why Fencing, Earth equipments and Shielding are necessary in High Voltage Laboratory? 5. Enlist types of HV Laboratory based on their rating. 6. Define various types of test carried out in HV Laboratory. 7. Enlist and explain list of equipments in VGEC HV Lab with detail.

Horn Gap Apparatus

Practical No: 2 Date: Aim To study Horn Gap Apparatus as a Surge Arrestor.

Apparatus Horn Gap Assembly

Step Up Transformer : 230 V AC to 60/80/100 kV (Centre tapped earth) AC

Output Voltage and Current: 20kV~0~20kV, 100 mA Type of Cooling: Air Cooling Filler Gauge- 2.5 mm

Theory It is often required to provide some protection to equipment against high voltages. Such protective equipments can be categorized as

Surge Arrestor

These are connected between line terminal and earth at substation terminal and always act in parallel with the equipment to be protected. They simply divert the surface to the earth. Surge arrestors used in practice as follows Horn gap Arrestor Multiple Gap Arrestor Lightning Arrestor Electrolytic Arrestor Valve Arrestor

Surge Modifier

Surge modifiers are connected in series with the line at substation terminal. They absorb the surge energy and flatten the wave front of incoming wave. Surge absorbers Arcing ground suppressors Earthing coil (Peterson’s coil) Water jet earthing resistance

Construction of Horn Gap Arrestor The equipment is known as horn gap because two high voltage electrode are of the apace of horn. There is specially designed high voltage transformer with center tap grounded. The horns are connected to the high voltage outputs of the transformer. For the safety of the operator the horns, which are at high voltage are covered with transparent cover. A suitable push button is provided in the front panel of the equipment. The equipment starts operating as soon as we press the button. The input supply is 230 V AC.

Structure and Working of Horn Gap Arrester The horn gap arresters are the oldest type among all the arrester and still they are used in low voltage lines because they are cheap and simple to construct. It consists of two horn shaped pieces of metal separated by a small air gap and mounted on a vertical plane. They are

connected in parallel with the transmission lines between two conductors and earth. The gap between the metallic wires is such that under normal condition, it does not allow any flow of current but under over voltage condition the gap breaks down and diverts the surge voltage from the earth. An arc is produced at the bottom of the horn gap during high voltage surge. The

arc is pushed out towards the top of the horns due to heat of the arc, the gap length towards the horn top is more than that at the base. The overvoltage cannot maintain such a long arc and arc get extinguished. The time taken for the completion operation is about 3 to 5 sec. Limitations

As the time taken for the complete operation is quite high, the scope is limited to low voltage systems only.

The breakdown voltage depends upon atmospheric conditions such as temperature and pressure. At higher altitudes, a longer gap is necessary. The gap length is to be determined by taking account the relative air density.

Roughness of horn gap also affects the performance of the arrester and the frequent settings are required to be made at the gap. The greatest disadvantage of the horn gap type of lightning arrester is its sensitivity to the corrosion and pitting of the horns and that it does not maintain the setting.

1. Mains ON Indicating Lamp 5. H.T. OFF Push Button Switch 2. H.T. ON (Test ON) Indicating Lamp 6. Dimmer (Output Voltage Control) 3. Fuse 7. Mains ON/OFF Toggle Switch 4. H.T. ON Push Button Switch 8. Horn Gap

Procedure

High voltages are applied across the horn gap. The arc is initiated at the bottom and extinguishes as it proceeds upward. The effect of gap length for different arrangements is observed. For gap length observed average inception voltage is 20 kV (rms)

Conclusion

Review Question 1.What do you mean by corona formation? 2.Why we have to study this experiment? 3.How corona is generated in this set up? 4.Use of Horn Gap Apparatus in HV Lab. 5.Explain Surge Arrester. 6.Short Note: Surge Modifier

Breakdown Test of Liquid Dielectric

Practical No:3 Date: Aim To study breakdown strength of Liquid Dielectric.

Apparatus

1-Phase Auto Transformer 0-2 Amp, 230 V, 50 Hz. Step Up Transformer 230 V/ 60 kV (Centre tapped earthed) AC. Control Panel Oil Test Cup (made of high impact transparent) Filler Gauge- 2.5 mm

Theory Liquid dielectrics are more useful as insulating materials than solid dielectrics or gases due to some of its inherent properties. They are the mixtures of hydrocarbons and are weakly polarized. They are 103 times denser than gases. The dielectric strength of gases is ideally considered to be 10 MV/cm, but practically it is of the order of 100 kV/cm. A liquid dielectric should withstand breakdown voltage without danger of sparking. It should be free from moisture, products of oxidation, any fibrous impurity and other contaminants. The presence of water in oil affects the electric strength of insulating oil and it decrease very sharply if fibrous impurities are present in addition to water. Liquid dielectrics are used mainly as impregnates in high voltage cables and capacitors, and for filling up of transformers, circuit breakers, etc. It also acts as heat transfer agents in transformers, and as arc-quenching media in circuit breakers. For the proper operation of transformer, transformer oil is tested in HV laboratory using oil-testing kit.

Construction Set up the lathe and machine the MS rod for different set of cutting parameters. The cutting parameters are speed, feed and depth of cut. In the first set up vary the depth of cut, keeping the speed and feed constant. In the second set up vary the speed, keeping the feed and depth of cut constant. In the third setup, vary the feed, keeping the speed and depth of cut constant. Collect the chips and measure the thickness at four different points along the length.

Fig 3.1 60 kV Transformer Oil Test Kit

1. Supply IN Indicating Lamp 6. Fuse 2. Variac ‘0’ Indicating Lamp (Ready to test) 7. H.T. ON Push Button Switch 3. H.T. OFF Indicating Lamp 8. H.T. OFF Push Button Switch 4. H.T. ON Indicating Lamp 9. Variac forward and reverse (Toggle switch) 5. Mains ON/OFF Toggle Switch 10. kV Meter

Safety Interlock: The circuit is provided with the following safety interlock:

Zero Start Interlock Zero start interlock is provided on the auto – variable transformer so that the HV can be switched “ON” only if the auto variable transformer knob is held in zero position.

Door Interlock Door interlock is provided at the top cover so that HV can be switched ‘ON” when the top covers is closed.

HT Limit Signs and Warning Indicators

Procedure

Adjust the gap between the electrodes to 2.5 mm by the gauge provide with punch mark for ‘GO’.

Fill the test vessel/ cup with the dielectric oil sample to be tested and place it on H.T. electrodes. Close the hood properly, to operate the interlock micro switch, which acts as a safety precaution for the operator.

Switch ‘ON’ the supply from mains, the corresponding lamp will glow. Press ‘HT ON’ push button the contactor will operate and H.T. ‘ON’ lamp will glow. If the

contactor does not operate, it means that the Variac brush arm is not at zero position or the hood interlock is not closed. The zero interlocking of the Variac is another safety feature against switching on the unit directly at a higher voltage. This will be indicated by voltmeter reading.

Keep increasing/ lower switch in lower position to bring the brush arm to zero position and then again push the H.T. ‘ON’ push button.

Raise the voltage by putting the control switch on raise position. The voltage will increase gradually in steps till breakdown of oil on the gap occurs i.e. oil sample test fails, the unit will trip and the kV meter which has been provided with a pointer arresting mechanism will read the breakdown voltage in kV. To lower down the voltage, put the control switch on lower position before start of subsequent test.

Note down the readings for breakdown voltage, which is available on the digital meter. Take 6 readings and discard the first one. Take average of remaining 5 readings as

breakdown voltage of oil. Calculate the breakdown strength in kV/mm.

Observation Gap Length between electrodes: mm.

Dielectric Material

Breakdown Voltage (kV)

Breakdown Strength = Breakdown Voltage (kV)/Gap Length (mm)

Conclusion

Review Question 1.What is the breakdown strength of mineral oil? 2.Which oil is used as transformer oil? 3.What are the parameters that affect the breakdown strength of liquids? 4.What are the factor which affect to conduction and breakdown in commercial liquids? 5.What are the electrical properties and chemical properties of liquid dielectric? 6. Explain BDV Test of Transformer.

Breakdown of Solid Dielectric

Practical No: 4 Date: Aim To study the breakdown of Solid Dielectric Materials.

Apparatus

Auto Transformer 1-Phase 230 V, 5 Amp, 50 Hz AC. High Voltage Transformer : 230 V AC to 60 kV (Centre tapped earth) Control Panel Sheets of different Solid Dielectric Material HT Testing Equipment

Theory Solid insulator are insulators forming barriers to the flow if charge between various parts of apparatus when high voltage is applied across them.

Requirement of Good Dielectric are:

They should have high resistivity to reduce leakage current. They must withstand high

voltage without breakdown. They must have high dielectrics strength. Their density must be low as they are used on volume basis and not on weight basis. High thermal conductivity is essential. Low co-efficient of thermal expansion to avoid stresses and structural damages. They must be chemically inactive.

There is a wide range of synthetically produced as well as natural. Insulator choice depends upon thermal, mechanical chemical and electrical properties. Classification of Solid insulating materials is made as follows:

Naturally occurring: Carbon, Varnish, Rubber, Marble, Mica, Asbestos etc. Fibrous Nature: Wood, Paper, Cardboard, Cloth etc. Synthetic Materials: Plastic, Polythene, Polystyrene, Ceramic etc. Solid Dielectrics: Bituminous, Waxes, Resins, Thermoplastics, Thermostats etc.

Breakdown of Solid Dielectrics:

In practice, breakdown of solid insulating material occurs due to prolonged processes. This can be due to

Partial discharge Tracking on the surface Chemical and Electrochemical deterioration

Oxidation Hydrolysis in the presence of moisture Chemical action in the presence of oxygen moisture, ozone etc., resulting into

degradation of the insulation. Severe Chemical action may results in damage of Insulation and convert the

whole process in mechanical damages.

Intrinsic Breakdown When high voltages are applied only for a short duration of the order of sec, the dielectric strength of solid dielectric increases very rapidly to an upper limit called an intrinsic electric strength. Two types of intrinsic breakdowns have been proposed viz.

Electronic breakdown Avalanche or Streamer breakdown

Electromechanical Breakdown

When high electric field is applied to a solid dielectric, failure occurs due to electrostatic compressive forces which can exceed the electrostatic compressive strength.

Thermal Breakdown

When high voltage is applied to a dielectric, conduction current, however small it may be, flows through the material. If the heat generated exceeds the heat dissipated, breakdown occurs. Thermal breakdown sets up an upper limit for increasing the breakdown voltage when the thickness of the insulation is increased.

Procedure

Different Solid dielectrics in the form of thin sheets can be tested in the laboratory for their breakdown strength and results are tabulated in the observation table.

Observation Table

Sr No

Material Used Breakdown Voltage (kV)

1 2 3 4

Conclusion

Review Question 1. Enlist various types of Solid Insulating Materials. 2. Short Note: Tan Delta Measurement. 3. Explain Insulation Coordination. 4. Short Note: Testing of Surge Diverters. 5. Explain PD Detection Technique in detail. 6. Significance of PD Detection.

Effect of Electrode Configuration (Uniform Fields)

Practical No: 5 Date: Aim To study the effect of Electrode Configuration (Uniform Fields) on the breakdown of Air gap.

Apparatus Auto Transformer 1-Phase 230 V, 5 Amp, 50 Hz AC. Step Up Transformer : 230 V AC to 50 kV (Centre tapped earth) AC

Control Panel

Various Electrode Configuration Theory In uniform fields such as sphere-sphere, coaxial cylinders, etc. The applied field varies across the gap. Referring to Townsend’s current growth equation, the average number of ionizing collisions (α) made by an electron per centimeter travel in the direction of the field varies with gap configurations. The average current in the gap before the occurrence of breakdown is given equation:

I = I0 exp(αd)

1 − γ[exp(αd) − 1]

As the distance between the electrode d increases in the above equation: 1 − γ[exp(αd) − 1] = 0

For values of d<ds; I=I0 and if d=ds then I = infinite This is called Townsend’s breakdown criterion and is written as:

γ[exp(αd) − 1] = 1

Normally is exp(ad) very large, therefore: γ exp(αd) = 1

For non-uniform field above equation becomes:

𝑑

𝛾{exp [∫ 𝑎𝑑𝑥] − 1} = 1

0

Where integration shows the variations of Townsend’s first ionization coefficient with gap for given gap spacing and at a given pressure of value of V which gives the value of α and r satisfying the breakdown criterion gives breakdown voltage and corresponding distance is called sparking distance.

Procedure

Make arrangements to measure breakdown voltage by using various electrode

configurations. Take anyone electrode configuration and adjust the distance between the certain values. Apply voltage till gap between electrode breaks down. Repeat above procedure for various distances and electrode configuration.

Observation Table

Electrode Configuration:___________

Sr No Types of Supply

Breakdown Voltage ( )

Gap Spacing ( )

1

AC Supply

2 3 4 1

Positive DC Supply

2 3 4 1

Negative DC Supply

2 3 4

Conclusion

Review Question 1.What do you mean by Uniform Field and Non-Uniform Field? 2.Enlist various Uniform Electrode Configuration. 3.Which electrode configuration is best for measurement of HVDC and HVAC? 4.Your comments regarding Uniform Electrode Configuration. 5.Your observation on Breakdown Voltage based on various types of supply.

Effect of Electrode Configuration (Non-Uniform Fields)

Practical No: 6 Date: Aim To study the effect of Electrode Configuration (Non-Uniform Fields) on the breakdown of Air gap.

Apparatus Auto Transformer 1-Phase 230 V, 5 Amp, 50 Hz AC. Step Up Transformer : 230 V AC to 50 kV (Centre tapped earth) AC

Control Panel

Various Electrode Configuration Theory In non-uniform fields, the distribution of field intensity in space between electrodes is uneven. If the electrodes have similar profile, field intensity has a maximum value on surface of electrodes and minimum at middle space. If the profile of the electrode is different the greatest value of field intensity occurs on surface of electrode having smaller radius of curvature and region of minimum intensity is shifted to the bigger electrode. The degree of non-uniformity greatly affects breakdown voltage in non-uniform fields.

Procedure

Make arrangements to measure breakdown voltage by using various electrode

configurations. Take anyone electrode configuration and adjust the distance between the certain values. Apply voltage till gap between electrode breaks down. Repeat above procedure for various distances and electrode configuration.

Observation Table

Electrode Configuration:

Sr No Types of Supply

Breakdown Voltage ( )

Gap Spacing ( )

1

AC Supply

2 3 4 1

Positive DC Supply

2 3 4 1

Negative DC Supply

2 3 4

Conclusion

Review Question 1. Enlist various Non-Uniform Electrode Configuration. 2. Practical application of Non-Uniform Electrode Configuration. 3. Enlist various Non-Uniform Electrode Configuration. 4. Your views regarding Non-Uniform Electrode Configuration. 5. Comments on Breakdown Voltage based on various Configuration.

Peak Voltage Measurement

Practical No: 7 Date: Aim To measure pick voltage using Sphere Gap.

Apparatus Auto Transformer 1-Phase 230 V, 5 Amp, 50 Hz AC. Step Up Transformer : 230 V AC to 50 kV (Centre tapped earth) AC

Control Panel

150mm diameter Sphere gap

Theory: The sphere gap method of measuring high voltage is the most reliable and is used as the standard for calibration purposes. The breakdown strength of a gas depends on the ionization of the gas molecules, and on the density of the gas. As such, the breakdown voltage varies with the gap spacing; and for a uniform field gap, a high consistency could be obtained, so that the sphere gap is very useful as a measuring device. In the measuring device, two metal spheres are used, separated by a gas-gap. The potential difference between the spheres is raised until a spark passes between them. The breakdown strength of a gas depends on the size of the spheres, their distance apart and a number of other factors. A spark gap may be used for the determination of the peak value of a voltage wave, and for the checking and calibrating of voltmeters and other voltage measuring devices. The density of the gas (generally air) affects the sparkover voltage for a given gap setting. Thus the correction for any air density change must be made. The air density correction factor is:

The spark over voltage for a given gap setting under the standard conditions (760 torr pressure and at 20oC) must be multiplied by the correction factor to obtain the actual sparkover voltage. The breakdown voltage of the sphere gap is almost independent of humidity of the atmosphere, but the presence of dew on the surface lowers the breakdown voltage and hence invalidates the calibrations. The limits of accuracy are dependent on the ratio of the spacing d to the sphere diameter D, as distance=radius of sphere. Procedure

Make arrangements of Sphere gap assembly.

Adjust the scale at zero and make gap between sphere as zero.

Apply the voltage by adjusting gap between spheres till breakdown occurs.

Compare the meter reading with the standard one. Conclusion:

Electrolytic Tank

Practical No: 8 Date: Aim To study about Electrolytic Tank.

Apparatus

Auto Transformer 1-Phase 230 V, 5 Amp, 50 Hz AC. Electrolytic Tank with Pantograph Arrangement Isolation Transformer Multimeter Drawing Sheet, Pencil etc.

Theory Electrolytic tank is useful tool to draw equipotential lines. Equipotential line is the path along which the voltage remains the same. This experiment plays very important role for the analysis of electric field or electric stresses of a die-electric. Geometrically simple models can be taken and equipotential lines can be drawn.

The basic tank is made of high quality mild steel and it is epoxy powder coated to protect form corrosion. On the top of the tank a transparent glass is fixed with the help of frame. The drawing sheet, on which equipotential lines have to be plotted, is kept and fixed on the glass sheet. The tank has the provision to drain the water after the experiment is over.

Pantograph is the most important part of the electrolytic tank. Pantograph is specially design to have two parallel moving arms one over the another. These arms can be moved in X or Y direction. Lower arm has the provision to hold the probe which can move between the electrodes, kept in the tank to locate the equipotential points. Upper arm has a pencil holder. This is spring loaded and by pressing the top know of the holder point can be located on the drawing sheet. The base of the equipment is made out of square tube. Provision is made to level the tank with the help of the leveling screw, and hence, water in the tank.

The electrodes are placed in the tank and water is poured in the tank such that the electrodes dip at least little more than the half in the water. Some Voltage (say 10V) is applied to one electrode and other electrode is grounded. A drawing sheet is fixed on the glass sheet. Now equipotential lines at 10%, 20%……90% can be drawn by moving the probe to certain points, where, the high impedance meter reads the voltage. First set of points is achieved at 10% i.e. 1V, second set at 20% i.e.2V and so on. Finally, join the equipotential points to get the equipotential lines. Different type of electrode can be used to draw the equipotential lines such as spheres, flat parallel and curved plates of both type i.e. concave and convex.

Procedure

Parallel Plate Model

The Parallel Plate Capacitor model is placed inside the Electrolytic Tank.

Clean water is poured (added) into the Electrolytic Tank up to the tips of the parallel plates.

Now drawing sheet is fixed on the glass plate of the Electrolytic Tank.

Connections are made as per the circuit diagram and keep multimeter knob in the AC mode.

Switch ON the main supply.

Keep the pantograph needle on any one of the electrodes, then applying a small voltage of 10 Volts by using auto transformer with the help of multimeter.

First trace both the plates by using pantograph then trace equipotential lines corresponding to voltage of 2 volts, 4 volts, 6volts and 8 volts respectively.

With different configuration of electrodes, the measurement can be repeated. Conclusion

Review Question 1. Why an electrolyte is used in this method of plotting the equipotential curves? 2. Why AC is preferred to DC in this method of plotting equipotential curves? 3. How would the equipotential curves for two infinitely long plate electrodes look? 4. How do above plotted curves differ from those for infinitely long plate electrodes? 5. In the above case what change do you expect in the shapes of equipotential curves when more

electrodes (which are connected to higher potential) are introduced in the electrolytic tank outside the larger cylinder?

6. What other liquids could be used instead of tap water? Give examples.

Impulse Generator

Practical No: 9 Date: Aim To perform the generation of Impulse Wave.

Theory In extra high voltage transmission lines and power systems, switching surge is an important factor that affects the design of insulation. All transmission lines rated for 220kV and above, incorporate switching surge spark over voltage for their insulation levels. A switching surge is a short duration transient voltage produced in the system due to sudden opening or closing of a switch or circuit breaker or due to arcing at a fault in the system. The wave form is not a unique. The transient voltage may be an oscillatory wave or a damped oscillatory wave of frequency ranging from few hundred hertz to few kilohertz. It may also be consider as a slow rising impulse having a front time of 0.1 to 1 ms, and a tail time of one to several ms. Thus, Switching surges contain larger energy than the lightning impulse voltages.

Several circuits have been adopted for producing switching surges. They are grouped as (i) impulse generator circuit modified to give longer duration wave shapes. (ii) Power transformers or testing transformers excited by dc voltages giving oscillatory waves and these include Tesla coils.

Standard switching impulse voltage is defined, both by Indian Standards and the IEC, as 250/2500 μs wave, with the same tolerances for time to front and time to tail as those for the lightning impulse voltage wave, i.e. time to front of(250+ or - 50) μs and time to half value of (2500+ or- 500)μs. Other switching impulse voltage waves commonly used for testing the lightning arresters are 250/1500 μs with a tolerance of + or – 500 μs in time to half value.

Conclusion

Review Question 1.Define Impulse Wave. 2.Requirement of Impulse Testing. 3.Define: Wave Front Time and Wave Tail Time 4.Short Note: Impulse Current Generator 5.Differentiate Switching Surge and Lightning Wave.

Flashover Test on Insulator

Practical No:10 Date: Aim To study Flashover Test along the surface of a Solid Dielectric.

Apparatus

Auto Transformer 1-Phase 230 V, 5 Amp, 50 Hz AC. Step Up Transformer : 230 V AC to 50 kV (Centre tapped earth) AC Control Panel Insulators : Pin Type and Suspension Type

Theory Introduction of dielectric in an air gap, considerably changes its dielectric strength. In such a case following factors exerts considerable influence on breakdown voltage.

Material of the dielectric. Condition of the surface of the dielectric along which the discharge develops. The form of the dielectric field in the gap between the electrodes.

When breakdown occurs in air along the surface of a soled dielectric, the term, flashover I used instead of breakdown voltage for discharge in the surrounding gas volume. In uniform fields, flashover voltage along the surface of a solid dielectric is always considerably less than the breakdown voltage of a gap purely in air.

Presence of air layers between the dielectric and electrodes exert influence on the values of the flashover voltage, since permittivity of solid dielectric increases field intensity a few times because of which ionization in air layer arises much earlier than in the main air gap. The products of ionization go out from the air layer to the surface of the dielectric and promote much earlier initiation of the discharge along the surface. Therefore, in practical, insulation constructions all measures are taken to ensure compact joint between the electrodes and the solid dielectric. The electrodes are usually fixed to porcelain insulators, with the help of cement, which compact joints of electrodes and the dielectric, the flashover voltage along the surface remains much lower than, for pure air gap.

Factor affect the Flashover Voltage:

Humidity

It is observed that flashover that a flashover voltage increases somewhat in the beginning with increases in relative humidity of air. But at a value of relative humidity corresponding to the consideration of moisture on the surface of the dielectric (60% - 70%) a sharp decrease of flashover voltage takes place.

Hygroscopes

On account of hygroscopes of the material there is always a surface layer of absorbed moisture even at low values of relative humidity. Since water possesses ion conductivity, the field intensity along the surface is distorted and becomes non-uniform and the flashover voltages get reduced.

Material For different material flashover, voltage is different, if the material is hygroscopic, flashover voltage is less.

Length of Flashover

Considerable increases in the flashover voltage can be achieved, if the surface of dielectric has corrugation, acts as barrier in the pure gaps.

Procedure

Two samples of a corrugated cylinder. Apply High voltage across the cylinder. Increase the voltage until Flashover occurs. Note the flashover voltages in KV.

Observation Table

Pin Type Insulator Suspension Type Insulator

Sr No Audible Noise Voltage (kV)

Flashover Voltage (kV)

Sr No Audible Noise Voltage (kV)

Flashover Voltage (kV)

1 1 Conclusion

Review Question 1.Enlist the various types of Insulator with their capacity. 2.Important of Flashover Test in Electric Field. 3.Factor affecting the properties of Insulator. 4.List out the materials used for Solid Insulator. 5.Enlist various insulator as per their Class and temperature rating. 6.Which type of insulators are used in transformer and motor? 7.Short Note: Insulator Testing.