Breakdown Characteristics of Air gaps.docx

8/12/2019 Breakdown Characteristics of Air gaps.docx

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EE 3092Laboratory Practice V

BREAKDOWN

CHARACTERISTICS OF AIR

GAPS

Name

Index No.

Field

Group

Date of Performance

Date of Submission

: W.M.C.N.S.Kirinde

: 100255K

: Electrical Engineering

: G6

: 2013/08/21

: 2013/09/17

Instructed by: Mr. M.K.C.D. Chinthaka

Group Members :

A.M.M. Karunarathna 100243X

K.M.C.G. Karunarathne 100244C

I.M.M. Karunathilake 100249V

A.P.M. Kethumalika 100254G

W.M.C.N.S. Kirinde 100255K


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CALCULATIONS

Correction of observed values for standard temperature and pressure

V = Breakdown voltage at a pressure P and temperature T

= Breakdown voltage at standard pressure and temperature

= Density correction factor

Data taken,

Atmospheric pressure, P = 756.81torr

Room temperature, T = 30

Transformer Ratio = 220V / 100kV

( )

( )

( )

( )

Corrected value of breakdown voltage,

(kV)

Specimen calculation for sphere - sphere air gap breakdown,

Air gap distance = 2.5 cm

Sphere-sphere gap voltage, V = 50V

The corrected value for 50V,

Likewise, all the other calculations are tabulated as follows.


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Corrected voltage values for each electrode pair vs. Air gap distance

Breakdown Voltage (kV)

Air gap distance (cm) Sphere-Sphere Plate-Plate Rod-Rod

0.5 16.991 17.464 17.936

1.0 22.656 17.464 17.936

1.5 32.097 25.016 17.936

2.0 41.064 32.568 17.936

2.5 51.921 40.120 17.936

3.0 60.416 46.729 23.6003.5 69.385 53.336 31.624

4.0 77.409 58.529 35.400

4.5 84.962 60.889 36.345


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0

20

40

60

80

100

120

0 1 2 3 4 5 6

BreakdownVo

ltage(kV)

Distance (cm)

Corrected voltage values vs. Air gap distance

Sphere-Sphere Plate-Plate Rod-Rod

Corrected voltage values vs. Air gap distance


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DISCUSSION

HV lab arrangement

High voltage lab is consisted with 2 floors. Ground floor is where the high voltage

equipment are kept in a caged area. On the first floor, there are control panel, protection and

measuring devices. With this floor and equipment planning, the life stock has been kept

away as much as possible from the high voltage test equipment. High voltage area is covered

with a metal net cage which has been grounded. Entrance is protected by a special protective

plug. High voltage equipment can only be operated after the protective plug has been closed.

Comment upon the necessity and usefulness of safety precaution taken in conducting

the practical

Ground rod plays a vital role in the high voltage lab working as a protective plug. Its

placed at the entrance of the high voltage area. Without closing it any high voltage

equipment in the area cannot be operated. With this rod, all the stray static electric charges

are earthed. Hence, the livestock is protected from the electric shock hazards with the usage

of the ground rod.

High voltage test area is covered with a metal net cage of about 2 meters high. The lab

is designed with 2 floors. Ground floor which contains all the high voltage test equipment is

covered with the above mentioned metal mesh and all the control, protective and measuring

devices are at the 1stfloor. All over the high voltage lab there are necessary, safety signs and

instructions as well. With these, any harm for livestock has been reduced immensely.


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At the entrance, there is a safety interlocking system as well. When the test area was

opened hence the plug, the control panel will be turned off. Hence for operation of the

control panel in order to conduct a test, the door should be closed and then this protective

plug should be closed.

High voltage generators used in the high voltage lab are all protected with safety

relays for protection. After a high voltage discharge during an experiment, it automatically

discharges. Also the plug connected to the entrance door to the test arena is connected to a

safety relay which will open circuit the high voltage power supply when the door is opened.


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Methods of HVAC and HVDC generation

So far HVAC generation is done because of the lesser power losses in transmission

and many other reasons. However, HVDC transmission is more economical and it is the new

trend, hence.

In HVAC systems, ability of transformers to step up and down the voltages with ease

is used here. When transmitting over long distances, it is much more efficient to do with the

intensification of voltages and currents. At the power plants AC voltage is generated and

then stepped upto higher voltage values for transmission. Cascade arrangement of

transformers is also used to step the voltage up to a higher value. Ferranti test circuit

arrangement is used with 2 transformers and an isolation transformer is used to isolate the

high voltage side from the primary side for HVAC generation.

For remote transmission, HVDC systems are less expensive and dissipates lower

power losses. For short distances, the high cost of DC conversion equipment compared to an

AC system can be overcome because of the other benefits. For HVDC generation Foster

generator is used.

Need of a 2Mresistor

The voltages related with the gaseous breakdown are comparatively large, typically

about 30V/cm stress. Since the uniform increase in voltage used in the practical, it is more

probable to occur avalanche breakdown which develops over relatively long periods of time

and its typically more than 1s. The discharge current at the breakdown is suddenly

increased due to avalanche effect. The current should be controlled for the safety of the

equipment. So, this 2Mresistor is used to limit the current at the breakdown.

Factors that affect the breakdown voltage of air for a given pair of electrodes

These are very important in power system protection. These relays are time graded

such that the relay nearer to the fault operates quickly before the relays nearer to the source.

These can isolate only the faulty component form the system.

For vacuum gaps less than about 1 mm, the breakdown voltage is approximately

proportional to the length, all other parameters remaining constant. This gives a constant

breakdown strength. For these small gaps, the breakdown stress is relatively high, being of

the order of 1 MV/cm. Field emission of electrons probably plays an important part in the

breakdown process.

V = k.d for d < 1 mm


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The breakdown voltage of a gap increases on successive flashovers, until a constant

value is reached. The electrodes are then said to be conditioned. This increase in voltage is

ascribed to the burning off by sparking of microscopic irregularities or impurities which may

exist on the electrodes. When investigating the effect of various factors on breakdown, the

electrodes must first be conditioned in such a way that reproducible results are obtained.

The electrode surfaces form the physical boundaries between which the breakdown

finally takes place. Thus it is not surprising to find that the breakdown strength of a given

size of gap is strongly dependent on the material of the electrodes. In general, the smoother

the surface finish, the greater the breakdown voltage.

Increasing the area of the electrodes makes it more difficult to maintain a given

breakdown voltage. Thus, breakdown voltage decreases slightly with increase in surface

area. For example, electrodes of 20 cm2 area gives the breakdown voltage across a 1 mm gap

of 40 kV, whereas electrodes of the same material of area 1000 cm2 gives a breakdown

voltage across the same 1 mm gap as 25 kV. Up to 1 mm gap, the more convex electrodes

have higher breakdown voltage than the more nearly plane electrodes even though at the

same voltage they carried a higher electric field at the surface.

The variation of the breakdown voltage with temperature is very small, and for nickel andiron electrodes, the strength remains unchanged for temperatures as high as 5000C. Cooling the

electrodes to liquid Nitrogen temperature increases the breakdown voltage.


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Method of measuring high voltage using sphere gaps

Ionization of gas molecules and the gas density affects the breakdown strength of the

gas. The breakdown voltage also varies with the gap spacing. Because of the high

consistency of breakdown voltage for uniform air gap, the sphere gap can be used as a

measuring device. Very accurate charts have prepared based on the experiments for differentdiameters and distances of air gap.

Two metal spheres separated by a gas gap is used as a measuring device. Voltage

difference between two spheres is increased until a spark passes among the spheres. This

device can be used to measure the peak value of a voltage wave and for checking and

calibrating of voltmeters.

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