1. High Voltage Engineering Insulation: Liquid dieletrics
Course code : EEE-4123
High Voltage Engineering
Lecture-04
Topics
Insulation: Liquid dieletrics
Presented by
Md. Shamim Sarker
Assistant Professor
Dept. of EEE, KUET

2. Time lag component under step voltage
The time that elapses between the application of the voltage to a gap sufficient to cause breakdown and breakdown occurred is called time lag
Vd = DC voltage to cause breakdown after a long time
Vd1= peak value of step voltage
t = Time lag
ts= Statistical time lag
tf= Formative time lag
Time lag component under step voltage

3. Statistical time lag depends on:
(i) The amount of pre ionization present in between the gap
(ii) Size of the gap
(iii) The amount of over voltage (Vd1-vd) applied to the gap
Formative time lag depends on:
(i) mechanism of breakdown
(ii) Transit time of electron from anode to cathode
(iii) Gap length

4. Application of gases in power system
The gases find wide application in power system to provide insulation to various equipment's and substations. The gases are also used in circuit breakers for arc interruption besides providing insulation
The various gases used are:
(i) air
(ii) oxygen
(iii) hydrogen
(iv) nitrogen
(v) CO2
(vi) electronegative gases like sulphur hexafluoride, arcton etc.

5. The various properties required for providing insulation and arc interruption are:
(i) High dielectric strength (ii) Thermal and chemical stability (iii) Non-inflammability (iv) High thermal conductivity. This assists cooling of current carrying conductors immersed in the gas and also assists the arc-extinction process (v) Commercial availability at moderate cost.

6. Breakdown in liquid dielectrics
Function of liquid dielectric in transformer:
Providing insulation between live and grounded parts
Carrying out the heat from transformer to the atmosphere
In circuit breaker it has an additional function of arc extinguishing.
It must be chemically stable and free from moisture and contamination, because:
Even 0.01% water in oil brings down the dielectric strength to 20%

7. Pure liquid: Which are chemically pure, structurally simple and do not contain any impurity even in trace of 1 in 109 is called pure liquids.
Commercial liquids: are chemically impure and contain mixtures of complex organic molecules.
There are different theories of breakdown in liquid dielectrics:
Suspended solid particle mechanism
Elecronic breakdown
Cavity breakdown
Electroconvection breakdown

8. Suspended solid particle mechanism
If then the force is directed towards higher stress If then the force is directed towards lower stress
Suspended solid particle mechanism
Commercial liquids will always contain solid impurities either as fibers or as dispersed solid particles.
Let the permitivitty of solid is and permitivitty of liquid is
The radious of the solid sphere is r. These particles get polarized in an electric fields E and experience a force of

9. Equating the electrical and viscous force, we get
So particle will move towards higher stress and this motion will be opposed by the viscous force, which is
Where v is velocity and ȵ is the viscosity
Equating the electrical and viscous force, we get

10. (From stokes-einstein relation)
Since the electron are being diffuted from lower stress to higher stress region, we will have to count the drift velocity. Which is given by
(From stokes-einstein relation)
At instance particles will have a single velocity. Thus equating them we get
Hence the dielectric strength depends upon the concentration of particles N, radius r and temperature of the liquid.
Larger the radius of the particles lower the dielectric strength.

11. Breakdown of solid dielectrics
The solid insulation not only provides insulation to the live parts of the equipment from the grounded structures, it sometimes provides mechanical support to the equipment
When breakdown occurs the gases regain their dielectric strength very fast, the liquids regain partially and solid dielectrics lose their strength completely.
Roughly speaking, the product of the breakdown voltage and the log of the time required for breakdown is almost a constant

13. If γ is the Young’s modulus, the mechanical compressive strength is
If the initial thickness of the material is d0 and is compressed to a thickness d under the applied voltage V then the compressive stress developed due to electric field is
If γ is the Young’s modulus, the mechanical compressive strength is
Equating the two under equilibrium condition, we have
……………….. (A)
Differentiating this equation “A” w.r to d and equate it to zero for maximum compression we get,

14. For any real value of V the reduction of thickness can not be less than 40% of its original value. If the ratio V/d at this value of V is less than the intrinsic strength of the specimen, a further increase in V shall make the thickness unstable and the specimen collapses.
The highest apparent strength is then obtained by substituting d = 0.6 d0 in the above expressions.

15. Breakdown due to treeing and tracking
Suppose the solid dielectric with an relative permittivity of has some gas pocket trapped inside it. Then,
E field in the gas= * E field in solid
Break down in gas → Higher charge concentration → more non uniform electric field → More charge in void → breakdown step by step

16. Tracking:
Formation of permanent conducting path on an insulation surface due to conduction through moisture or dust