hv lab

H V LAB MANUAL

PREPARED BY 09117001 -09117010.

EXPERIMENT NO. # 03

1) AIM:

Determination of breakdown voltage for rod-rod gap.

2) APPARATUS REQUIRED:

S. NO.

APPARATUS TYPE RANGE

1. Rod – Rod gap arrangement

Iron ----

2. High voltage Testing Transformer

A.C. 220 V/ 100 KV/ 200 V, 20 KVA

3. Control panel A.C. ----- 4. Circuit breaker A.C. -----

3) CURCUIT DIAGRAM:

Schematic Circuit showing the source and Breakdown voltage measuring unit

Using standard Rod gap method.

H V LAB MANUAL

PREPARED BY 09117001 -09117010.

4) THEORY:

Measurement of high voltages and currents are having more complex and these

equipment’s have large stray capacitance and large voltage gradient. High vol tage

equipment’s are protected against over voltages. Rod gap method is the one of the

standard method for measurement of peak values of AC, DC voltages in high voltage

circuits. In short duration of time we can measure the breakdown voltage using this

method. Rod electrodes are made with many materials like aluminium, steel, brass, light

alloys, bronze and copper.

The electric breakdown strength of a gas-insulated gap between two metal

electrodes can be improved considerably when one or both of the electrodes are covered

with a dielectric coating i.e. hybrid insulation. The effect of the coating depends on the

electrode shape, voltage polarity, pre-charging and the duration and form of the

applied voltage. Dielectric barriers in air and oil gaps are well known for bringing

improvement to the electric breakdown strength and widely used in high volt age

engineering.

In this method, air is acting as an insulating medium between the rods. The

atmospheric air is the combination of various types of gas molecules which influence

the breakdown voltage of the system. By increasing the applied voltage between the rod

gaps the breakdown of air takes place at a certain applied voltage which is call as

breakdown voltage. The influence of the grounded objects and the shanks, connecting

the rods to the HV supply and to the ground, on the breakdown voltage a nd the field

between equal rods was reported to be small (a few percentage) and depended on the

dimensions of the rods, the gap separation.

A spark over voltage of 30 KV peak at 1cm spacing in air at 20°C and 750 atm

pressure occurs for a rod gap or any uniform field gap. Rod gap are used for voltage

measurements. The rod gap breakdown is independent of the voltage waveform and

hence is highly suitable for all types of waveforms from D.C. to impulse voltages of short

time rise.

Factors that affect the spark over voltage of a rod gap are:

a) Nearby earthed object.

b) Atmospheric conditions and humidity.

c) Irradiation.

d) Polarity and rise time of voltage waveform.

e) Impact of Dust Particles.

H V LAB MANUAL

PREPARED BY 09117001 -09117010.

Consider the effect of atmospheric conditions, air density correction factor is

calculated as:

Where,

b = pressure of air in mm of mercury

t. = Temperature in °C

Breakdown voltage of air between the rod gap is

(

√ )

√(

)

Where,

S = gap between rod electrodes (in cm)

R = radius of the rod electrode (in cm)

5) PROCEDURE:

a) Set the gap of electrode.

b) Ensure interlocking (mechanical)

c) Switch on input supply. Press HT ready push button.

d) Increase the raise button. The recommended rate for increment should be 20 KV/min.

e) Rise the voltage regularity till gap breakdown.

f) After flash over, press lower to make voltage zero.

g) For next adjustment discharge the bushing an electrode by cutting rod.

h) Repeat next cycle for different gap lengths.

H V LAB MANUAL

PREPARED BY 09117001 -09117010.

6) OBSERVATION TABLE:

S.No Rod-rod gap (in cm) Breakdown voltage ( in KV) 1. 2.

8) RESULT:

Determination of breakdown voltage for rod to rod air gap is done.

Breakdown voltage =

% Error =

9) SOURCES OF ERROR & REMEDIES:

i) Rods used are not well polished and are old.

Remedy: Always clean the surface of the electrodes to avoid any dust particles which

would lead to the non-uniformity of electric field between the gap while in turn lowers

the breakdown voltage.

ii) Breakdown voltage depends upon the pressure and temperature at the time of

experiment. Therefore may give a different reading then actual breakdown voltage of

30KV/cm.

Remedy: Multiply the ideal breakdown voltage (i.e. 30Kv/cm) with air density constant

‘δ’ which will account for the surrounding atmospheric conditions.

iii) There may be a lot of difference between the breakdown voltage due to initial

charge present in the rod gap electrode connected to H.V. side.

Remedy: The rod electrode must always be once connected to ground in order to

remove all the excess charge being present in the electrode connected to H.V. side.

10) PRECAUTIONS:

i) Increase the voltage gradually in steps after a break of 30 seconds after each 10 KV

rise of voltage.

H V LAB MANUAL

PREPARED BY 09117001 -09117010.

Ii) After performing the experiment discharge the electrode in bushings by the earthing

rod.

hv lab

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