High Voltage Experiments

Lab ReportHigh Voltage Engineering

WAPDA Engineering Academy Faisalabad

Submitted To : Sir Rizwan-ul-Hasan

Submitted By : Muhammad Gulraiz Ahmed

Roll No : 10-EL-15

NFC Institute Of Engineering & Fertilizer

Lab Assignment No : 01 Objective:

“Determination of an ac flash over of a 11kv pin type

insulator under dry conditions.”

Apparatus:

11kv pin type insulator,

connecting cup,

current limiting resisitor,

coaxial cable,

measuring capacitor

Theory:

It is one of the earliest designs used for supporting lineconductor. The pin insulator is supported on the forged steelpins and bolds, whichare skewed to the crosssection of thissupporting structure.The conductor is lied toinsulator on thegrooves. In order toincrease the leakagepath, one, two or three rain shades are so designed when theseinsulators are wet even then sufficient dry space is providedby the inner shades.For higher voltages the thickness of the material required forthe purpose of the insulation. But the practical way is to usemore than one insulator unit instead of increasing thethickness.

Flashover distances when the insulators are dry and wet aredifferent. The flash over voltage when the insulator is wet isless. The rain shades should not disturb the voltagedistribution. They are so designed that their surface is rightangle to electrostatic lines of forces or they must try to liein the equipotential.The pin type insulator is reliable for voltage level upto 50KVmax. and They are never used for voltage beyond 80 KV. Sincethe suspension insulator is more reliable and cheaper. Themodern practice is “not to used” the pin type insulator beyond33KV.

There are three types of loads that may be occur on insulator.1- Mechanical loading2- Thermal loading3- Electrical loading

Mechanical loading contains conducting weight, wind storms,snow fall (ice) and birds etc.Thermal loading is due to atmospheric tensionWhile electrical loading is caused by normal operating voltageI.e. 11 KV, 33KV ------(H.T dist) and 400V, 230V---- ( L.TDist ). It also contains overvoltage for short duration, and transientdue to lightening, switching, steady state overvoltage (due tophase to ground fault).

Connection diagram:

220/100kv

10 m ohm resistance

100 pf capacitor

Procedure:

Connected the circuit as shown in the circuit diagram.

Starting from the 50% of the ac test voltage, required

according to the wapda specification.

The voltage is raised to the test voltage within

approximately 10 seconds.ac test voltage is applied for

one minute, the voltage is raised until the flashover

occurs.

The flash over value shall be determined as the mean

value of 3 individual measurements.

Observations:

Serial # Flashover voltage (v)1 902 903 89

Mean flashover voltage=89.6 v

Precautions :

Insulator under test must be cleaned carefully.

All parts which are grounded under normal service

conditions must be grounded for the test also.

Equivalent linear conductor is to be attached to the

insulator.

Conclusion:

When a disruptive charge take place between two electrodes in

a gas or liquid or over a surface of solid insulation it is

called a flashover. insulation flashover is the discharge

through the air taking the form of an arc or a spark or of

several arcs or sparks between conducting part of an insulator

under the application of high voltage which normally having

the operating voltage between them.

What is the objective of performing ac flashover voltage

test under dry conditions?

In order to check the maximum operating voltage which an

insulator can bear and if that voltage is below or above the

standard voltage which are specified by the wapda.so that when

they operate in dry environment they can bear the voltage for

a specific limit.

Compare your reading with the wapda standard readings?

We uses pin type insulator for this test and standard voltage

set by the wapda for pin type insulator under dry condition is

70kv.our mean value of flashover voltages comes out to be 89.6

so as the value is greater than that set by wapda so insulator

we uses met the standard set by the wapda.if the means value

comes out to be less than 70kv then that insulator does not

met wapda standard specification and thus can not be use in

the field.

______________________________________________________________

_____________

Lab Assignment No : 02

Objective:

“To determine the AC flashover voltage of 11 KV Pin type

insulation under Wet condition.”

Apparatus:

HV control disk

Test Transformer

Current limiting resistor

Flexible conductor cable

Measuring capacitor

Connecting cup

Coaxial cable

11 KV pin type Insulator

Theory: It is one of the earliest designs used for supporting line

conductor. The pin

insulator is

supported on the

forged steel pins and

bolds, which are

skewed to the cross

section of this

supporting structure.

The conductor is lied to insulator on the grooves. In order to

increase the leakage path, one, two or three rain shades are

so designed when these insulators are wet even then sufficient

dry space is provided by the inner shades.

For higher voltages the thickness of the material required for

the purpose of the insulation. But the practical way is to use

more than one insulator unit instead of increasing the

thickness.

Flashover distances when the insulators are dry and wet are

different. The flash over voltage when the insulator is wet is

less. The rain shades should not disturb the voltage

distribution. They are so designed that their surface is right

angle to electrostatic lines of forces or they must try to lie

in the equipotential.

The pin type insulator is reliable for voltage level upto 50KV

max. and They are never used for voltage beyond 80 KV. Since

the suspension insulator is more reliable and cheaper. The

modern practice is “not to used” the pin type insulator beyond

33KV.

There are three types of loads that may be occur on insulator.

4- Mechanical loading

5- Thermal loading

6- Electrical loading

Mechanical loading contains conducting weight, wind storms,

snow fall (ice) and birds etc.

Thermal loading is due to atmospheric tension

While electrical loading is caused by normal operating voltage

I.e. 11 KV, 33KV ------(H.T dist) and 400V, 230V---- ( L.T

Dist ).

It also contains overvoltage for short duration, and transient

due to lightening, switching, steady state overvoltage (due to

phase to ground fault).

Circuit connection:

Procedure: Connect the components as shown in the diagram.

Insulator is objected to artificial raining condition .

It is subjected to a spray of water with the help of

nozzle which are so arranged that the water drops fall

approximately at the inclination of 45 degree with the

vertical. The spray of water on the insulator should

begin atleast one min before the voltage application and

should be continue all throughout the test.

Starting from 50% points of the AC test voltage required

according to WAPDA specification.

The voltage is raised to test voltage within 10 sec approx,

the AC test voltage is applied atleast for 1 minute. The

voltage is quickly raised until a flashover occur. The

flashover value shall be determine as the mean value of three

individual measurement.

Observations:

S.NO FLASHOVER VOLTAGE (wet

condition) PIN type

1 40 KV

2 42 KV

3 46 KV

Mean 42.66 KV

Wapda specification:

S.NO TYPE OF

INSULATOR

FLASHOVER VOLTAGE (WET)

1 PIN 40 KV --- H.T Running

2 DISC 50 KV --- H.T Dead line, angle

position

3 SPOOL (D-SHACKLE 15 KV --- L.T Running

4 STRAIN 18 KV --- L.T Guying wire

Precautions:

The insulator under test must be cleaned carefully.

All the parts which are grounded under normal service

condition must be grounded for test also.

Equivalent line conductor is to be attached to the

insulator.

Conclusions:

Why insulator flashes over at lower voltage under wet

conditions?

Since the flash over occurs when air breakdown take place

and also ionization becomes occur between connecting

parts of an insulator so it totally depends on dielectric

strength . At normal condition the dry air dielectric

strength is 2.1kv/cm but in wet conditions the dielectric

strength becomes low and ionization take place at low

voltage. Also air breakdown take place at low voltage so

insulator flash over at low voltage.

Compare your result with WAPDA’S standard specifications

under wet conditions?

Wapda standard specification:

And results taken during the lab

S.NO FLASHOVER VOLTAGE (wet condition) PINtype

1 40 KV

S.NO Type Wet condition

1. Pin type 40kv

2. Disc type 50kv

3. Spool type 15kv

4. Strain type 18kv

2 42 KV

3 46 KV

Mean 42.66 KV

Pin type insulator flashover at 40 KV under wet conditions while from observations it is clear that flashover occurs at 40 KV,42 KV,46 KV and their mean is 40.66 KV. So it is far good and meet the WAPDA standard specifications.

___________________________________________________________________________


“ One minute power frequency withstand voltage

test.”

Apparatus:


connecting cup,


coaxial cable,

measuring capacitor

Circuit diagram:

Procedure:

Connect the component as shown in the figure.

starting from 50 percent ac test voltage required

according to wapda specification, the voltage is raised

to the test voltage within approximately 10 seconds, the

ac test voltage is applied at least for one minute

Observation: Wapda specifications:

Pin type 65kv

Disc type 75kv

Spool type 23kv

Strain type 32kv

Resulst in the lab:

Pin type Voltage kv

1 64

2 62

3 64

Mean value=63.33kv

Precautions: The insulator under test must be cleaned carefully.




insulator.

Conclusion: Q: What is the object of performing this test?

The main objective of this test is to measure the

dielectric strength of insulator.

Q: What do you under stand by one minute power frequency test?

At power frequency of 50 hertz this test is perform to

check the dielectric strength of the insulating material

under the specific condition.

Q: Define following?

Time lag for break down:

This is the time difference between the application of a

voltage sufficient to cause and occurrence of a breakdown

itself.

Statistical time lag:

The time which elapses between the application of voltage

sufficient to cause breakdown and the appearance of

initiating electron

Formative time lag:

After the appearance of initiating electrons a time is

required for ionization processes to develop fully to

cause the breakdown of the gap and this time is called

formative time lag

Total time lag:

The breakdown of the gap will take place after a time

(ts+tf) from the instant of application of breakdown

voltage.

Crona discharge:

Partially ionized air around the conductor accompanied by

hising noise and bluish glow.

______________________________________________________________

_____________


“One minute power frequency withstand voltage

test of pin type insulator under wet conditions.”

Apparatus:


connecting cup,


coaxial cable,

measuring capacitor

Circuit diagram:

Procedure: Connect the component as shown in the figure.

The insulator shall be exposed for 5 minutes before the

application of voltage throughout test to artificial rain

with following characteristics. rainfall intensity 3mm

per min, tolerance plus minus 10 percent, direction above

45 degree to vertical, water resistivity 10000 ohm

centimeter, water temperature not differing more than 10

percent from that insulator.


Pin type 35kv

Disc type 45kv

Spool type 13kv

Strain type 10kv

Resulst in the lab:

Pin type Voltage kv

1 36

2 37

3 35

Mean value=36kv

Precautions: The insulator under test must be cleaned carefully.




insulator.



dielectric strength of insulator under wet conditions.

Q: Define townsends first and second ionization coefficients?

Average no of ionizing collisions made by an electron per cm

travel in the direction of field is the townsends first

ionization coefficient. Townsends second ionization

coefficient can be define as the net yields of electrons due

to secondary process

Q: Secondary ionization processes?

Are those processes by which secondary electrons are produced,

which sustain a charge after it is established due to

ionization by collision and photo ionization following are the

secondary ionization process,

1) Electron emission due to positive ion impact

2) Electron emission due to photon

3) Electron emission due to metastables and neutral atoms

Q: is crona discharge take place during one minute power

factor with stand voltage test. Differentiate between crona

discharge and flash over (arc discharge)?

Yes crona crona discharge take place during one minute power

factor with stand voltage test.

Crona discharge:


hising noise and bluish glow

Flashover:

Electric discharge between two electrodes separated by

insulator.

______________________________________________________________

_____________


“Voltage measurement through capacitor

divider.”

Apparatus:

Test transformer

Insulator

Resistance

Capacitor

Circuit diagram:

Procedure:

Connect the component as shown in the figure.


Pin type 35kv

Disc type 45kv

Spool type 13kv

Strain type 10kv

Resulst in the lab:

Pin type Voltage kv

1 36

2 37

3 35

Mean value=36kv

Precautions:

The insulator under test must be cleaned carefully.




insulator.



dielectric strength of insulator under wet conditions.

Q: Define townsends first and second ionization coefficients?

Average no of ionizing collisions made by an electron per cm

travel in the direction of field is the townsends first

ionization coefficient. Townsends second ionization

coefficient can be define as the net yields of electrons due

to secondary process

Q: Secondary ionization processes?

Are those processes by which secondary electrons are produced,

which sustain a charge after it is established due to

ionization by collision and photo ionization following are the

secondary ionization process,

4) Electron emission due to positive ion impact

5) Electron emission due to photon

6) Electron emission due to metastables and neutral atoms

Q: is crona discharge take place during one minute power

factor with stand voltage test. Differentiate between crona

discharge and flash over (arc discharge)?

Yes crona crona discharge take place during one minute power

factor with stand voltage test.

Crona discharge:


hising noise and bluish glow

Flashover:

Electric discharge between two electrodes separated by

insulator.

______________________________________________________________

_____________

High Voltage Experiments

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