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High Voltage Engineering

S.R.C.O.E, LONIKAND PUNE

PRACTICAL WORK BOOK For Academic Session 20_ _

HIGH VOLTAGE ENGINEERING

(Elective-III) (403149) 2008 pattern

For

B.E. (Electrical Engineering)

Department of Electrical Engineering

(University of Pune)



SHREE RAMCHANDRA COLLEGE OF ENGG. LONIKAND (096)

DEPT. : ELECTRICAL ENGG. HIGH VOLTAGE ENGINEERING (403149) SEM. : II (BE)

TITLE: LIST OF EXPERIMENTS

Minimum eight experiments

1. Measurement of breakdown strength of solid insulating materials.

2. Breakdown of air under uniform and non-uniform field.

3. Measurement of breakdown strength of liquid insulating materials.

4. Effect of gap length on liquid insulating material.

5. Breakdown of composite dielectric material.

6. Study of impulse generator.

7. High voltage withstand test on cables/safety gloves/shoes as per IS.

8. Surface flashover on the surface of polymer insulator materials.

9. Horn gap arrangement as surge diverter.

10. Measurement audible and visible corona inception and extinction voltage.

11. Surface flashover on corrugated porcelain insulator materials.

12. Sphere gap voltmeter.

13. Development of tracks and trees on polymeric insulation.

14. Measurement of breakdown strength of gaseous dielectrics.

15. Study of output voltage waveform of multistage voltage doubler circuit on CRO.





TITLE: SAFETY PRECAUTION IN HV LAB

All electrical test laboratories shall adequately take care of safety aspects on following points

and ensure compliance with suitable display, training, guidance etc.

a. The laboratories are expected to comply with the safety requirements of electrical shocks:

through use of Earth Leakage Circuit Breaker (ELCB) or any other device and damage to

personnel by use of Safety Helmet as per IS 2925, Gloves as per IS: 13774, Safety shoes

as per IS 12254 & IS 2071 Part 1, Insulated tools as per IS 13772, Earthing rods as per IS

3043, Fire hazards as per SP 30 and Rubber mats as per IS: 5424.

b. Back up controller where necessary shall be provided in order to control the

environmental chambers in case of malfunction of original controller for environmental

chambers.

c. A danger notice board with a sign of skull and bones shall be displayed in Electrical

testing area with voltages above 400 Volts. SP 30 charts where required shall be

displayed.

d. Testing staff working in live electric supply environment shall have knowledge of risk

and hazards involved in testing Supply to vehicles and cranes shall have suitable trip

facility and the metal rails shall be electrically continuous and earthed.

e. Instructions for resuscitation of persons suffering from electrical shocks shall be

prominently displayed.

f. Switchboard shall have clear space around as specified by statutory authority.

g. Due care shall be taken where the equipment is likely to have emissions in test which may

endanger the operator/ equipment.

h. Fire buckets filled with clear and dry sand and water ready for immediate use for

extinguishing fires in addition to fire extinguishers for dealing with electric fires shall be

available.

i. Fire extinguishers of suitable type depending upon class of fire:

Class of Fire Suitable type of Extinguishers

Fires in Ordinary combustibles (Wood, rubber, plastic and the like) Gas expelled, Water

type and anti-freeze type extinguisher and water buckets.

Fires in flammable liquids (paints, grease, solvents & the like) and electrical circuits

Chemical extinguishers of carbon-dioxide & dry powder type, sand buckets and foam

type

Fires in gaseous substances under pressure including liquefied gases Chemical

extinguishers of carbon-dioxide and dry powder type

j. Availability of first aid kit.

k. Ensure minimum safe clearance in air between high voltage terminal and earth during

testing at different voltage levels of AC/ DC/ Impulse applications.

l. Ensure proper earthing of equipment before making physical contact.

m. Operation of equipment by only authorised personnel Use of safety interlocks.

n. Restrict entry with prominent display boards, of un-authorised persons during testing.

o. Where necessary, control desks shall have inter-locks and control shall be with the

authorised persons only.

p. In respect of testing Liquid dielectric, provision for body shower and eye-wash shower

with exclusive over head tank for these showers.





EXPERIMENT NO. : SRCOE/ELECT/BE/HVE/__ PAGE:__-__ Date :

EXPERIMENT TITLE: HIGH VOLTAGE LAB LAYOUT

AIM: To Study High Voltage Lab Layout

APPRATUS:

Sr. No Name of the Equipment Specification Quantity

1

2

3

4

THEORY:

A. General Requirements of HV lab:

1. Customs made according to the type of equipment, available space and accessories.

2. Ground level location is preferred and floor loading has to be considered while designing

the lab.

3. Lab should be free from dust, humidity, draught.

4. Windows should be located at ground level and should have provision for black out so

that arcing can be easily observed.

5. Control room should have good view of the lab.

6. Adequate access door should be provided to bring in the equipment and test specimen.

7. Proper safety and warning system must be provided.

8. Lab should have adequate and proper clearance. Proper spacing should be kept within the

equipment.

9. Area around the equipment should be suitably demarked.

B. Classification of HV lab:

1. Small Lab:

• Less than 10 kW/10 kVA / 10 kJ

• 300kV AC single equipment

• 500 to 600kV (cascaded)

• Less than 100 kV (Impulse generator)

• 200 to 400kV DC

2. Medium Size lab:

• Used in the industries for routine test

• 100 to l000kVA

• 20 to 100kJ

• 200 to 600kV



3. Large size Lab:

• Used in the industries for routine test as well as type tests and also for research.

• Test transformer (1.5 to 2 MV)

• Impulse generator (5 to 6 MV)

• HVDC rectifier (1.2 to 1.5 MV)

4. Ultra high voltage lab:

• Used for basic design of experimental transmission.

C. Tests which can be carried out in the HV lab:

1. Withstand test

2. Flashover test

3. Pollution test

4. Partial discharge test

5. Tan measurement

6. Power frequency test

7. Impulse test:

• Switching

• Lightning

8. DC voltage test

9. Radio interference voltage (RIV) measurement

10. High current test

D. Equipment in HV lab:

1. HV generator (Transformer)

2. Oil testing kit

3. Impulse Generator

4. Testing facilities for RIV testing and partial discharge

5. Sphere gap for voltage measurement

6. Corona cage

E. Grounding of HV lab:

There are three types of grounding.

1. Idea) ground:

• Equipotential plane approximated by copper or galvanized iron sheet

• Very expensive.

2. Single Point Grounding

• Ear thing grid is grounded at single point.

3. Bus grounding:

• Grounding is done at several points in a lab.

• Least satisfactory

F. 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.

G. Rating of SRCOE HVE Lab Equipments:

1 AC/DC high voltage generator/transformer:

• Input: AC- single phase 230 V, 50/60 Hz

• Output Voltage:

• AC: 0 to 100 k V, 50 Hz

• DC: 0 to 60kV

• Output current:

• AC: 100 mA, 50Hz

• DC: 20 mA

2. Horizontal sphere gap voltmeter:

• Model:

• Sphere Diameter 250mm

• Cap scale: 0-100mm

3. Corona Cage:

4. Porcelain Insulators: three disc (each of 11kV)

5. Oil testing kit with filler gauge

6. Horn gap apparatus:

H. Safety Precautions to be taken in HVE lab:

1. 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.

2. Capacitors which are not in use should be shorted.

3. Flashover can be noisy and source of electromagnetic interference.

4. Some equipment require permanent ear thing. These equipments must have connection

which requires mechanical assistance to remove from circuit.

5. Electronic devices which cause or are susceptible to electromagnetic interference should

not be used.

6. Any equipment to be energized must have all exposed metal connected to ground unless

that metal is not a part of circuit.

7. Keep the mobile phones switched off in HV lab.





EXPERIMENT NO. : SRCOE/ELECT/BE/HVE/01 PAGE:__-__ Date :

EXPERIMENT TITLE: BREAKDOWN STRENGTH OF LIQUID DIELECTRICS

AIM: To study Breakdown Strength of Liquid Dielectrics.

APPRATUS:

Sr. No Name of the Equipment Specification

1 Auto transformer 5A, 230 V, 1 Phase 50 Hz AC.

2 Step up transformer 230V to 60/80/100 kV (centre tapped earth) ac.

3 Control panel

4 Oil test cup made of high impact transparent

5 Filler gauge 2.5mm

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 10MV/cm, but practically it is of the order of 100kV/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. It’s ideal

breakdown strength is 15kV/mm. The procedure for testing of oil in HV laboratory is given

below:

CONSTRUCTION:

Al the components are housed in compact and rugged devices is mounted on the front slanted

panel.

H.T. transformer which is epoxy resin cast on which H.T. output terminals are provide on the

top plates. These H.T. electrodes are shaped suitable to accommodate the test cup easily. The

test cup capacity is of 500ml approxm. A transparent hood made of clear plastic / Acrylic

material is provide on the top of the test cup, so that when the tests is carried, the operator

will not come in the hood interlocking micro switch so that if by any change the hood lifted

from its position, the H.T. supply will be cut off automatically.



PROCEDURE:

1. Adjust the gap between the electrodes to 2.5mm by the gauge provide with punch mark

for ‘GO’.

2. 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 microswitch, which acts as a

safety precaution for the operator.

3. Switch ‘ON’ the supply from mains, the corresponding lamp will glow.

4. 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.

5. 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.

6. 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.

7. Note down the readings for breakdown voltage, which is available on the digital meter.

8. Take 6 readings and discard the first one. Take average of remaining 5 readings as

breakdown voltage of oil.

9. Calculate the breakdown strength in kV/mm.

OBSERVATION TABLE:

Gap length between electrodes: ____________ mm.

Sr. no. Breakdown voltage Average breakdown voltage Remark

1. To be discarded

2.

3.

4.

5.

6.

Breakdown strength = average breakdown voltage (kV) / gap length (mm);

CONCLUSION:



230V/100kV

AC

230V,

1ph

50Hz,

AC

supply

2.5mm

0 to 60kV

LV HV

Figure No.

230V/100kV

AC

230V,

1ph

50Hz,

AC

supply

2.5mm

0 to 50kVLV

HV

Figure No. Breakdown strength of liquid dielectric






EXPERIMENT TITLE: EFFECT OF ELECTRODE CONFIGURATION

AIM: To Study Effect of Electrode Configuration on the Breakdown of Air Gap.

APPARATUS:

S. N Name of the Equipment Specification


2 Step up transformer 230V - 60/80/100 kV (centre tapped earth) ac.

3 Control panel.

4 Oil test cup, made of high impact transparent

5 Filler gauge 2.5mm

6 Various electrode configuration

Introduction: 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.

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 centimetre 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:

As the distance between the electrode d increases in the above equation:

For values of d<ds;

And if d=ds then

This is called Townsend’s breakdown criterion and is written as:

Normally is very large, therefore:



For non-uniform field above equation becomes:

Where ∫0d αdx 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:

1. Make arrangements to measure breakdown voltage by using various electrode

configurations.

2. Take anyone electrode configuration says sphere-sphere. Adjust the distance between

the certain values.

3. Apply voltage till gap between spheres breaks down.

4. Repeat above procedure for various distances and electrode configuration.

5. Compare the observations.

OBSERVATION TABLE:

1. Sphere-sphere electrode configuration(sphere dia=25cm)

Gap(mm) Breakdown voltage (kV) Remark

1 2 3 Avg.

10

20

30

40

2. Sphere-sphere electrode configuration(sphere dia=12cm)

Gap(mm) Breakdown voltage (kV) Remark

1 2 3 Avg.

10

20

30

40

Graph: plot graphs of breakdown voltage v/s gap distance.

CONCLUSION:






EXPERIMENT TITLE: EFFECT OF GAP LENGTH ON BREAKDOWN STRENGTH

AIM: To Effect of Gap Length on Breakdown Strength of Liquid Dielectric.

APPRATUS:




3 Control panel.


5 filler gauge 2.5mm

THEORY:

Liquid dielectrics are used in high voltage equipment to serve the dual purpose of insulation

and heat dissipation. They have the advantage that a puncture path is self-healing. Temporary

failure due to overvoltage is reinsulated quickly by liquid flow to the affected area. Highly

purified liquids have dielectric strengths as high as 1MV/cm.

Under actual service condition the breakdown strength reduces considerably due to the

presence of impurities.

A liquid dielectric should withstand breakdown voltage without damage of sparking. It is

measured in kV/mm. the breakdown test is carried out with the help of a standard test cell

with a polished spherical / hemispherical separated by oil gap.

The breakdown in liquid dielectric can be explained

1. Suspended particle theory

2. Stressed oil volume mechanism.

3. Cavitations and bubble mechanism.

All these mentioned above do not consider the dependence of breakdown strength on oil gap

length. They all try to account for the maximum obtainable breakdown strength. However the

experimental evidence shows that the breakdown strength of the liquid depends upon the gap

length.

Where,

d = gap length,

A = constant, and

n = constant, (n<1).



PROCEDURE:

Internal connection diagram is as shown in fig. A testing kit is supplied by phase, 230V,

50Hz supply. In this testing kit there is facility of automatic discharge and it automatically

discharges electrodes. To carry out the experiment we follow the procedure as follow:-

1. The test cell (oil cup) is cleaned, dried and oil is poured to a certain level in the cell

avoiding bubble formation.

2. Set the distance between electrode gaps as 1mm.

3. Switch on the power supply, before that check the door of testing kit is closed properly.

4. Set the maximum test voltage at 70kV.

5. Click on new select and select 70kV as maximum test voltage.

6. Observe the test cell. As soon as the oil breaks down, trip signal is initiated and the

voltage at which this breakdown occurs is recorded as a breakdown voltage for that oil

gap length.

7. Now set various gap distances. Eg. 2mm, 3mm, 4mm, 5mm, 6mm and repeat procedure

from step3.

OBSERVATION TABLE:

Sr.No. Gap

length

(mm)

Break

down

voltage

kV

I

Break

down

voltage

kV

II

Break

down

voltage

kV

III

Break

down

voltage

kV

IV

Break

down

voltage

kV

V

Avg.

break

Down

(kV)

VI

Break

down

strength

(kV/mm)

1.

2.

3.

4.

5.

CONCLUSION:



230V/100kV

AC

230V,

1ph

50Hz,

AC

supply

2.5mm

0 to 60kV

LV HV






EXPERIMENT TITLE: BREAKDOWN OF VARIOUS SOLID DIELECTRICS

AIM: To Study Breakdown of Various Solid Dielectrics.

APPARATUS:



2 High Voltage Transformer 230V to 60kV

3 Sheets of different Solid dielectric material

4 HV Testing Equipments

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:

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

high voltage without breakdown. They must have high dielectrics strength.

2. Their density must be low as they are used on volume basis and not on weight basis.

3. High thermal conductivity is essential.

4. Low co-efficient of thermal expansion to avoid stresses and structural damages.

5. 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:

a) Naturally occurring: Carbon, Varnish, rubber, marble, mica, asbestos, etc.

b) Fibrous nature: Wood, paper, cardboard, cloth, etc.

c) Synthetic materials: Plastic, polythene, polystyrene, ceramic, etc.

d) 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

1. Partial discharge.

2. Tracking on the surface.

3. Chemical and Electrochemical deterioration.

i. Oxidation

ii. Hydrolysis in the presence of moisture.



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

degradation of the insulation.

4. 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.

i. Electronic breakdown

ii. Avalanche or Streamer breakdown.

5. 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.

6. 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 were tested in the laboratory for their

breakdown strength, and results are tabulated in the observation table.

OBSERVATION TABLE:

Sr. No. Type of

paper

Thickness

(mm)

Breakdown Voltage rms (KV) Breakdown

strength

(KV/mm)

I ii Iii Avg

1.

2.

3.

Nature of Graph:

CONCLUSION:






EXPERIMENT TITLE: DIELECTRIC STRENGTH OF COMPOSITE DIELECTRICS.

AIM: To Find Dielectric Strength Of Composite Dielectrics.

APPARATUS:



2 High Voltage Transformer 230V to 60kV

3 Sample of Composite dielectrics.

4 HV testing kit

THEORY:

In insulating system more than one insulating material is used. The different materials can be

used in parallel with each other; such insulation systems are called composite dielectric. Such

composite dielectric is used in low and high voltage appliances such as cables, capacitor, and

transformers oil filled switchgear, bushing, etc.

Properties of composite dielectrics:

A composite dielectric consists of large number of layers arranged over one another. This is

called the “layered construction”.

Important properties of composite dielectrics are given below

1. Effect of layers.

It consists of two or more layers of same materials. Thus two sheets have a higher

dielectric strength then a single layer sheet of same material and same total thickness.

2. Effect of layer thickness.

Increase of layer thickness gives increased br3eakdown voltage p to ascertain limit.

A discharge having penetrated one layer cannot enter the next layer until a part of the

interface also obtain the potential which can produce an electric field stress

comparable to that of the discharge.

3. Effect of interface.

The interfaces between the two dielectric surfaces in a composite dielectric system

determine its breakdown and actual breakdown strength. Discharge usually occurs at

the interfaces and the magnitude of the discharge dep3ends o n the associated surface

and capacitance. When surface conductivity increases the discharge magnitude also

increases resulting in damage dielectric.

Breakdown Mechanism in composite dielectric material:

1. Short time breakdown:

If electric field is very high; failure may occur in seconds or even faster without any

substantial damage to insulating surface prior to breakdown. The breakdown results



from one or more discharges, when applied voltage is high. Breakdown was observed

to occur more readily when breakdown particles are electrons rather than positive

ions. Variations in thickness of soled insulation affect breakdown voltage.

2. Long term breakdown (Ageing of insulation)

The principal effects responsible for ageing of insulation which leads to breakdown are

caused form thermal processes and partial discharges.

i. Ageing and breakdown due to partial discharge

During manufacturing of composite insulation, gas filled cavities will be present

within the dielectric. When a voltage is applied to such a system, discharge occurs

within the gas volume. These discharges are called “Partial discharges” and the

involve transfer of electric charge between the two parts in sufficient quant8ity to

cause discharge of local capacitance. The impact of this charge on dielectric surface

produces a deterioration of insulating properties. Ageing depends upon geometry,

discharge inception voltage Vi and discharge magnitude.

ii. Ageing and breakdown due to accumulation of charges on insulator surface

During discharge a certain quantity of charge (electrons or positive ions) gets

deposited on solid insulators surface. The charge deposits for a large duration. Due to

these charges, the surface conductivity thereby increases, thereby increasing the disc

magnitude and cause damage to the dielectric.

PROCEDURE:

Different composite dielectric with layered construction, were tested in the laboratory for

their breakdown strength, and results are tabulated in the observation table.

OBSERVATION TABLE:

Sr. No. Sample Thickness Breakdown Voltages rms (KV) Breakdown

Voltage

(KV)

1.

2.

3.

4.

CALCULATION:

CONCLUSION:




DEPT. : ELECTRICAL

ENGG.

HIGH VOLTAGE ENGINEERING (403149) SEM. : II

(BE) EXPERIMENT NO. : SRCOE/ELECT/BE/HVE/06 PAGE:__-__ Date : EXPERIMENT TITLE: PHENOMENON OF CORONA.

AIM: To Study The Phenomenon Of Corona.

APPARATUS:




3 Control panel.


5 filler gauge 2.5mm

THEORY:

1. INTRODUCTION: Corona is defined as a self-sustained discharge in which the field

intensified ionization is localized over the distance between the electrodes. Corona is

characterized by discharge emitted from the periphery of a conductor when the surface

electric field exceeds the disruptive field of surrounding air. Corona is a partial discharge

between two electrodes and not a complete breakdown.

2. EFFECTS OF CORONA:

a) Continuous loss of power.

b) Distortion of current waveform interference in communication system,

c) Audible noise.

d) Brush discharge visible in darkness.

3. DISCHARGE MECHANISM:

Free electrons are generally present in the air because of the natural radioactivity or

cosmic rays. The electrons near the energized conductor under the influence of electric

field would move either towards or away from the conductor. During its movement along

the path of field gradient, it would thus ionize them. Due to ionization there would be

positive ions, and free electron leading to further collisions and ionization. The ionization

of air causes redistribution of the field gradient. The ionization of air because

redistribution which might be such that field gradient in the region close to the conductor

would be in excess of the breakdown potential gradient required for air. The air layer at a

distance from the conductor retains its original insulating property where the field

gradient is less that required for breakdown.

In practice, the effect of corona is accounted for operating voltage above 100kv between

phases. It is more important for lines operating at still higher voltages.



4. DISRUPTIVE CRITICLE VOLTAGE:

The potential difference between conductors, at which the electric field intensity at the

surface of conductors exceeds the critical value and generates corona is known as

disruptive critical voltages.

(for 3 phase with equal spacing)

Visual critical voltage:

The potential difference at which corona just becomes visible is called as visual critical

voltage.

Where,

r = Radius of conductor in cm.

D = Spacing between the conductors in cm.

Air density factor.

m = Irregularity factor for the surface of the conductor.

5. EFFECTS OF CORONA:

a. Corona is accomplished by power loss.

b. A luminous glow is observed around the conductor (luminous glow).

c. Hissing sound is produced.

d. Ozone gas is produced.

e. Corona produces interference in communication circuit.

Empirical formula for calculating corona power loss under fair weather condition:

Where,

Phase voltages in KV(rms).

Disruptive critical voltages in KV (rms).

r = Radius of conductor in cm.

D = Spacing between the conductors in cm.

f = Supply frequency.

Air density factor.

3.092*b / (273+ t)

b = Barometric pressure in cm of Hg.

t = Temperature in degree centigrade.



6. FACTORS AFFECTING CORONA:

1) Electrical factors: Increase the transmission voltage, voltage gradient, frequency in

any of these factors increases the corona loss.

2) Conductors Factors: The surface conditions, diameter of the conductor and type of

the conductor affect corona. As the roughness of the conductor surface increases the

Corona loss increases. As the diameter of the conductor increases, corona decreases.

For EHV lines bundled conductors are used to reduce the field gradient due to

increase in the virtual diameter of the conductor.

3) Atmospheric Conditions: As the density of the air reduces corona effect occurs at a

lower value of voltage. Increase in altitude, presence of rain and or dust particles and

conductivity of the air affects the corona loss.

7. EXPERIMENT SET UP:

A. Point Plane Configuration:

In order to observe phenomenon of corona, a point-plane configuration is used. Due

to this configuration non-uniform field is setup with all lines of field concentrated at

the point. A 230V/100KV, 50 Hz, single phase step up transformer is used for this

purpose. With the help of a dimmer, the voltage is gradually increased until corona

effect is observed in the form of luminous spark at the point electrode accompanying

hissing sound.

B. Corona Cage:

A corona cage in simplest arrangement consists of large which forms the outer cage

and a single thin conductor wire is string inside it. The metallic cylinder is kept at the

ground potential and the wire is connected to the high voltage terminal.

The corona conductor and cylinder are co-axial. Normally cage consists of three sections.

A log middle section which is principal cage, with two short guard cages at either end

grounded in order to minimize edge effects. An inspection window is provided in the

middle position to the outer cage so as to observe corona. The principal advantage of cage

arrangement is that it requires lower voltage for creating the required surface field

gradient on the conductor than in an overhead line above the ground.

OBSERVATION:

Sr.

No.

Voltage for Audible

Corona (KV)

Average Value

(KV)

Voltage for Visible

Corona (KV)

Average Value

(KV)

1.

2.

3.

4.

CONCLUSION:






EXPERIMENT TITLE: FLASHOVER ALONG THE SURFACE OF A SOLID DIELECTRIC.

AIM: To Study the Flashover along the Surface of a Solid Dielectric.

APPARATUS:




3 Control panel.

4 Insulator disc Porcelain disc each 11kV

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.

a) Material of the dielectric.

b) Condition of the surface of the dielectric along which the discharge develops.

c) 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. This is shown fig.

Flashover Voltage depends upon the following factors:

I. 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.



II. 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.

III. Material: for different material flashover, voltage is different, if the material is

hygroscopic, flashover voltage is less.

IV. 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:

1. Two samples of a corrugated cylinder.

2. Apply High voltage across the cylinder.

3. Increase the voltage until Flashover occurs.

4. Note the flashover voltages in KV.

OBSERVATION:

EFFECT OF CORRUGATIONS :

Sr. No.

CORRUGATED CYLINDER

Audible noise

voltage (KV)

Flashover Voltage

(KV)

1.

2.

3.

4.

Avg.

1. EFFECT OF FLASHOVER LENGTH ON FLASHOVER VOLTAGES:

SR.

No.

Length of cylinder (mm) Flashover Voltage (KV) Average value (KV)

1 2 3 4

1.

2.

3.

4.



GIVEN:

1. Height of corrugated cylinder = _______.

2. Diameter of corrugated cylinder = ______.

A. Inner = _________.

B. Outer = _________.

CONCLUSION:






EXPERIMENT TITLE: HORN GAP ARRESTOR

AIM: To study Horn Gap as a Surge Arrestor.

APPARATUS:



2 Input

Output

0-230kV A.C

10kV-0- 10kV, 20mA

3 Control panel.

4 Air cooled Air cooled

THEORY:

It is often required to provide some protection to equipment against high voltages. Such

protective equipments can be categorized as

a) Surge arrestor

b) Surge modifier

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. Commonly used surge modifiers

are as

Surge absorbers.

Arcing ground suppressors.

Earthing coil (Peterson’s coil).

Water jet earthing resistance.

Construction of horn gap arrester:

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 centre 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 230V 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:

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

voltage systems only.

2. 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.

3. Roughness of horn gap also affects the performance of the arrester and the frequent

settings are required to be made at the gap

4. 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.

Modifications:

This type of arrester when used on low tension lines, the air gap has to be set very close.

Under this condition there is a danger of an accidental power discharge taking place even by

the insects getting in there and thus bridging the gap. An auxiliary electrode is inserted

between the two horns. In case of an insect getting into the air gap, the auxiliary gap sparks

over. The duration of the spark over is made quite short by inserting the high resistance with

auxiliary electrode in fig.

Fig. Shows the modified horn gap which consists of air gaps. The choking coil is made up

of copper strip which is spirally wound. One end of the strip serves the purposes of one of the

electrode or horns. For violent overvoltage, both the gaps break and current passes to earth

resistance. It is claimed that with this form of L.A. larger air gaps can be made without

materially affecting the performance of the arresters.



PROCEDURE:

1. Connections are as shown in fig.

2. High voltages are applied across the horn gap.

3. The arc is initiated at the bottom and extinguishes as it proceeds upward

4. The effect of gap length for different arrangements is observed.

5. For gap length observed average inception voltage is 20kV(rms)






EXPERIMENT TITLE: SPHERE GAP AS VOLTMETER

AIM: To study Sphere Gap as Voltmeter.

APPARATUS:




3 Control panel.

4 Insulator disc

THEORY:

The breakdown voltage at spark between two metal parts may be used as measure of voltage

highest encountered value in HV testing. The sphere gap method of measuring high voltage is

the most reliable and is used as the standard for calibration purposes.

Configuration of two spheres is a classical example in distance between conductor diameters

D. spheres gap happens to be the commonly acknowledge means in international practice for

measuring direct alternating and pulsating/impulse voltage.

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 spark-over voltage for a given gap

setting. Thus the correction for any air density change must be made. The air density

correction factor δ must be used.

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 spark-

over 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 methods of mounting spheres are

1. Horizontal arrangement

2. Vertical arrangement

Horizontal arrangement:

In the horizontal sphere gap assembly both spheres are connected to the source or one sphere

is grounded. Since the voltage to be measured is applied between HV terminal and earth

between high voltage terminal and sphere supports are mounted on arrangement. At same



time, in order to avoid earth effects, the set is mounted on higher level. It again increases size

of insulators.

Vertical arrangement:

In this arrangement two identical spheres are arranged vertically such that lower sphere is

grounded permanently. This arrangement is very suitable for measuring high voltages. We

can increase the gap by simply moving HV sphere. The height of lab is to be very large i.e.

approximately 15m.

Calibration table is to be completed on basis of spheres and different sizes of spheres and

different gap length at specified condition of temperature and pressure (760mm of Hg 20o C).

The spheres may be made of Aluminium, brass; bronze or light alloys and the surface should

be free from burrs. The radius of curvature should be uniform. The radius of sphere measured

with spereo-meter at various points over an area enclosed by a circle 0.3D around the

sparking points. The surface of the sphere should be free from dust, grease, or any other

coating.

The 100mm sphere gap assembly is of horizontal type and mobile. The main frame stand

structure is made using epoxy fibre glass tubes. These tubes are painted using anti-tracking

paints to avoid any high voltage tracking on the tubes due to dust and moisture. On one side

of the apparatus hand wheel is provided for easy operation of the equipment. The moving

system on the hand wheel side has a built in micrometer scale, with the help of which, the

sphere gap can be adjusted within + or – 0.1mm. Other side of the stand contains lead screw

with nuts. This lead screw is used to adjust zero reading between spheres. Wheel end is

firmly grounded to the base. In the present apparatus spheres are made of 2m thick 99.9%

pure copper sheet and they are so polished that they meet the standard specifications. A water

resistor is supplied along with the equipment and it is fixed on the fixed sphere side.

Advantages :

1. It may be conveniently used for calibration of measuring devices for HV testing.

2. This method is simple and can be used in range of 1kV to 2500kV.

3. Sphere gap is also used for voltage measurement in surge test.

4. It provides cheap, simple and reliable method.

Precaution:

1. Ultra high violet insulation in the gap decreases the spark gap over voltage and therefore

ultraviolet or ionizing radiations should be avoided.

2. Clearance as per IS as shown in fig. should be maintained.

3. In order to avoid pitting of sphere a current limiting resistance of 1ohm/volt may be used.

4. After testing spheres should be cleaned and maintained.

5. No body having conducting surfaces should be present near to sparking point of high

voltage at a distance of [0.25V/300]m.

6. Time interval between consecutive flashover should be large enough to avoid heating of

spheres.



Factors affecting the spark voltage of sphere gap:

Various factors affect the spark over voltage of sphere gap is:

a. Atmospheric conditions.

b. Nearby earthing objects.

c. Polarity and rise time.

A) Effects of atmospheric conditions:

1. The spark over voltage for sphere gap depends upon air density which varies in

change in temp. and press.. if spark over is V under test condition of temp To, press P

Torr. If spark over voltage is Vn under std. condition of temp. & press.

T= 20oC, 760 m of Hg

Then

Since k=constant

Function of air density fraction is given by

Relationship between d & k is

D 0.70 0.75 0.8 0.85 0.90 0.95 1 1.05 1.10

K 0.72 0.76 0.81 0.86 0.90 0.95 1.0 1.05 1.09

2. Humidity :

The spark over voltage increases with humidity is within 3% of normal humidity

range & hence no correction is given for humidity range & hence no correction is

given for humidity.

3. Effect of polarity and waveform:

The presence of nearby earthed object reduces the spark over voltage of sphere gap. If

the specifications regarding clearances are closely observed error is within tolerances

& accuracy.

The spark over voltage for positive and negative polarity impulses is different:

Spheres dia. D Min. value of A Max. value of B Min. value of B

Up to 6.25 7D 9D 14S

10 to 25 6D 8D 12S

25 5D 7D 10S

50 4D 6D 8S

75 4D 6D 7S

100 3.5D 5D 7S

150 3D 4D 6S

200 3D 4D 6S



OBSERVATION TABLE

For sphere of diameter

Gap length(mm)

Sparkover voltage kV at 20o

& 760 torr

at 27o & 719.8 torr kVrms

measured

Peak value (kV)

Draw a graph of ‘k’ v/s ‘d’

For sphere diameter of =

Gap length

Measured sparkover

voltage

Actual correction

factor

Actual spark over voltage = correction factor * standard voltage

V= Vn *k

Air density factor d= P/760*(293/273+T)

From the data air density factor and correction factor (k)

D K

By interpolating value of k

For d= , k= .

Actual b/d voltage :

CONCLUSION

Questions:

1. Explain the effect of following factors on spark over voltage of sphere gap.

a. Nearby earthed object. b. Atmospheric condition. c. Irradiation.

d. Polarity and rise time of voltage waveform.

2. Explain how a sphere gap can be used to measure the peak value of voltage? How

correction for atmospheric temperature and pressure is applied?

high voltage engineering - srespune.org

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