Jawaban Soal Liquid Liquid Dielectric 1. Explain the phenomena of electrical conduction in liquid. How does it different from that gas? Various Various phenomena occur in gaseous dielectrics when a voltage is applied. -When low voltage is applied, small current flow between the electrodes and the insulation retains its electrical properties. -If the applied voltage is large, the current flowing through the insulation increases very sharply and an electrical breakdown occur. A strongly conducting spark formed during breakdown, practically produces a short circuit between the electrodes. The maimum voltage applied to the insulation at the moment of breakdown is called the breakdown voltage. Breakdown in liquids ! In highly highly purified purified li"ui li"uid d dielectr dielectrics, ics, breakd breakdown own is controll controlled ed by phenome phenomena na similar similar to those for gasses and the electric strength is high #of the order of $ %V&c m'. ! (nfo (nfort rtun unat ately ely,, li"u li"uids ids are easily easily contam contamin inat ated ed,, and and may conta contain in solid solids, s, othe otherr li"ui li"uids ds in suspension and dissolved gasses. ! )ow )ow brea breakd kdow own n stre streng ngth th ! *ecause *ecause,, of the tenden tendency cy to become become contam contamina inated ted,, li"uids li"uids are are not usually usually used used alone alone above above $++ kV&cm in continuously energied e"uipment. ! They are are used used at much much higher higher tresse tressess #up to $ %V&cm %V&cm'' in conun conuncti ction on with with solids, solids, which which can can be made to act as barriers, preventing the line-up of solid impurities and localiing of any bubbles which may form. ! The main main funct function ion of of the li"ui li"uid d in such such arrang arrangeme ements nts is to to fill up up the voids. voids. ! The effect effect of these impurities impurities is relatively relatively small for short duration duration pulses #$+ s'. ! owever, owever, if the voltage voltage is applied applied continu continuously ously,, the the solid impurities impurities line line up at right right angles angles to e"uipotential, and distort the field so that breakdown occurs at relatively low voltage. ! The The lineup lineup of partic particles les is a fair fairly ly slow slow proces process, s, and is unlik unlikely ely to affe affect ct the stren strengt gth h on voltages lasting for less than $ ms.
)i"uid dielectrics are used mainly spreading through in high voltage cables and capacitors and for filling transformers, circuit breakers, etc. In addition to their function as a dielectric, li"uid dielectrics have additional functions functions in certain applications. applications. /or eample, li"uid dielectrics act as heat transfer agents agents in transformers transformers and as arc "uenching media media in circuit breakers. breakers. 0etroleum 0etroleum oils are most commonly used as li"uid dielectrics. /or certain applications 1ynthetic hydrocarbons and halogenated hydrocarbons and for very high temperature applications, silicone oils and fluorinated hydrocarbons are used. In recent times, certain vegetable oils and esters are also being used. )i"uid dielectric dielectricss normally are miture of hydrocarbon hydrocarbonss and are weakly polaried. polaried. When used for electrical insulation purposes they should be free from moisture, products of oidation, and other contaminants. The most important factor that affects the electrical strength of insulating oil is the presence of water in the form of fine droplets suspended in the oil. The presence of even +.+$ 2 water in transformer transformer oil reduces reduces the electrical electrical strength to 3+ 2 of the dry oil value. The dielectric dielectric strength strength of the oil reduces more sharply, if it contains fibrous impurities in addition to water. The three most important properties, properties, of li"uid dielectrics dielectrics are #i' the electrical electrical conductivity conductivity,, #ii' dielectric constant, and #iii' the dielectric strength.
In practice, the choice of a li"uid dielectric for a given application is made mainly on the basis of its chemical stability. In addition, other factors like saving of space, cost, previous usage, and susceptibility to the environmental influences are also considered. In capacitors, replacement of the capacitor oil by askarel spreading through in the overall sie of the capacitor by more than 4+2. In practice, a li"uid found satisfactory over a long period of usage is preferred to a new one. 0etroleum li"uid are widely used because of their low cost.
. !hat are commercial liquis dielectrics? How the" are different from pure liquid dielectrics? 0ure li"uids are those which are chemically pure and do not contain any other impurity even in traces of $ in $+ 5, and are structurally simple. 6amples of such simple pure li"uids are n-heane #7 8$9'. n-heptane #7 :$8' and other paraffin hydrocarbons. *y using simple and pure li"uids, it is easier to separate out the various factors that influence condition and breakdown in them. ;n the other hand, the commercial li"uids which are insulating li"uids like oils, not chemically pure, normally consist of miture of comple organic molecules which cannot be easily specified or reproduced in a series of eperiments.
#. !hat are the factors that influence conduction in pure liquid dielectrics and in commercial liquid dielectrics?
1. Temperature 2. Humidity 3. Pressure 4. Solid impurities 5. Gas bubbles 6. Electrode geometry 7. oltage type! applicatio" duratio" #. $re%ue"cy When low electric fields less than $ kV&cm are applied, conductivities of $+-$< -$+-3+ mho&cm are obtained. These are probably due to the impurities remaining after purification. owever, when the fields are high #=$++kV&cm' the currents not only increase rapidly, but also undergo violent fluctuations which will die down after some time. A typical mean value of the conduction current in heane is shown in /ig. 3.9. This is the condition nearer to breakdown.
Conduction Current (A x 1011)
14 12 10 8 6 Electric feld (MV/cm)
4 2
Fig.2.4 Conduction currentelectric feld c!"r"cteri#tic# in !ex"ne "t !ig! feld# 0
0.4
owever, if this figure is redrawn starting from very small currents, a current-electric field characteristics as shown in /ig. 3.>, can be obtained. This curve will have three distinct regions as shown. At very low fields the current is due to the dissociation of ions. With intermediate fields the current reaches a saturation value, and at high fields the current generated because of the field-aided electron emission from the cathode gets multiplied in the li"uid medium by a Townsend type of mechanism .The current multiplication also occurs from the electrons generated at the interfaces of li"uid and impurities. The increase in current by these processes continues till breakdown occurs. The eact mechanism of current growth is not known? however, it appears that the electrons are generated from the cathode by field emission of electrons.
0.
ionic #"tur"tion Field "ided
Conduction Current (Ar$itr"r% unit#)
0
0.2 0.4 0.6 0.8 1.0 Electric feld (MV/cm) Fig.2.& Conduction currentelectric feld c!"r"cteri#tic# in !%droc"r$on li'uid
The electrons so liberated get multiple by a process similar to Townsend@s primary and secondary ioniation in gases As the breakdown field is approaches, the current increases rapidly due to process similar to the primary ioniation process and also the positive ions reaching the cathode generate secondary electrons, leading breakdown. The breakdown voltage depends on the field, gap separation, cathode work-function, and the temperature of the cathode. In addition, the li"uid viscosity, the li"uid temperature, the density, and the molecular structure of the li"uid also influence the breakdown strength of the li"uid. $. Explain %arious theories of brea&down mechanism of the commercial liquid dielectrics. The breakdown mechanism in commercial li"uids is dependent on several factors, such as, the nature and condition of the electrodes, the physical properties of the li"uid, and the impurities and gases present in the li"uid. 1everal theories have been proposed to eplain the breakdown in li"uids, and they are classified as follows a' 1uspended 0article %echanism
b' 7avitation and *ubble %echanism c' 1tressed ;il Volume %echanism a. Suspended Particle Theory In commercial li"uids, the presence of solid impurities cannot be avoided. These impurities will
( ε 3 ) be present as fibres or as dispersed solid particles. The permittivity of these particles
will be
( ε $ ) different from the permittivity of the li"uid . If we consider these impurities to be spherical particles of radius r, and if the applied field is 6, then the particles eperience of force /, where
/B
$ 3
4
r
( ε 3 − ε $ ) 3ε $+ε 3
grad 6 3 #4.$'
ε 3 〈ε $ This force is directed towards areas of maimum stress, if , for eample, in the case of the presence of solid particles like paper in the li"uid. ;n the other hand, if only gas bubbles are present
ε 3 〈ε $ in the li"uid, i.e. , the force will be in the direction of areas of lower stress. If the voltage is continuously applied #d.c' or the duration of the voltage is long #a.c', then this force drives the particles towards the area of maimum stress. If the number of particles is large, they become aligned due to these forces, and thus form a stable chain bridging the electrode gap causing a breakdown between the electrodes. If there is only a single conducting particle between the electrodes, it will give rise to local field enhancement depending on its shape. If this field eceeds the breakdown strength of the li"uid, local breakdown will occur near the particle, and this will result in the formation of gas bubbles which may lead to be breakdown of the li"uid. The value of the breakdown strength of the li"uids containing solid impurities was found to be much less than the values for pure li"uids. The impurity particles reduce the breakdown strength, and it was also observed that the larger the sie of the particles the lower were the breakdown strengths.
a' b' c' d'
b. 'a%itation and the (ubble )heor" It was eperimentally observed that in many li"uids, the breakdown strength depends strongl y on the applied hydrostatic pressure, suggesting that a change of phase of the medium is involved in the breakdown process, which in other words means that a kind of vapor bubble formed is responsible for breakdown. The following processes have been suggested to be responsible for the formation of the vapor bubbles Cas pockets at the surface of the electrodes? electrostatic repulsive forces between space charges which may be sufficient to overcome the surface tension? gaseous products due to the dissociation of li"uid molecules by electron collisions? and Vaporiation of the li"uid by corona type discharges from sharp points and irregularities on the electrode surfaces. ;nce a bubble is formed it will elongated #long and thin' in the direction of the electric field under the influence of electrostatic forces. The volume of the bubble remains constant during elongation. *reakdown occurs when the voltage drop along the length of the bubble becomes e"ual to the minimum value on the 0aschen@s curve for the gas in the bubble. The breakdown field is given as
3πσ ( 3ε $+ε 3 ) 6+ = ε ε − r ( $ 3 ) $
−$ #3r6 + ' Vh
$ 3
#4.3'
ε $
σ where
is the surface tension of the li"uid,
ε 3 is the permittivity of the li"uid,
is the
V b permittivity of the gas bubble, r is the initial radius of the bubble assumed as a sphere and is the voltage drop in the bubble #corresponding to minimum on the 0aschen@s curve'. /rom this e"uation it can be seen that the breakdown strength depends on the initial sie of the bubble which in turn is influenced by the hydrostatic pressure and te mperature of the li"uid. *ut this theory does not take into account the production of the initial bubble and hence the results given by this theory do not agree well with the eperimental results. )ater this theory was modified, and it was suggested that only incompressible bubbles like water globules can elongate at constant volume, according to the simple gas law pv B nDT. (nder the influence of the applied electric field the shape of the globule is assumed to
β be approimately a spheroid. These incompressible bubbles reach the condition of instability when , the ratio of the longer to the shorter diameter of the spheroid is about $.<>, and the critical field producing the instability will be E+
= 8++
πσ ε $ ε$ & ε$ − ε 3
− G
=
σ
#4.4' where
ε $ =
surface tension, DB initial radius of bubble,
ε $ = permittivity of the li"uid dielectric,
−$ $ β cosh β G= 3 − $ $ β − $ ( β 3 − $) 3
permittivity of the globule,
= 3 β 4 3β − $ − $3÷ β $
3
and
σ
µ m /or a water globule having DB$
6c
= 94dyne&cm and ε $ = 3.+
with
#transformer oil', the
= 338 '( & cm
above e"uation gives a critical field which is approimately the maimum strength obtained for commercial oils. In the case of gas bubbles the e"uation for the critical field is rewritten as
AB
6c
ε
9
β
−$−
$
β 3
*B3ε$β 4 − ε 3 ( $ − β 3 )
$
$
3
$
$ &
where,
σ
C,
and D are as above for li"uid globules, and
3: β > ( ε$ − ε 3 ) 4 $ -$ 0D θ = cosh 4 σ ) 4 3
$
3
where 0 is the hydrostatic pressure. #6"uations 4.3-4.9 are in c.g.s. units'. The epressions are "uite complicated, and the breakdown voltages were obtained using a computer. Desults thus obtained showed good agreement with the eperimental results in n-heane. This theory suggests that
sub-microscopic particles #diameter $++-3>+ A' and bubbles greatly influence the maimum electrical strength attainable in commercial li"uids. The critical condition is reached when cavities are formed due to ero pressure conditions given by
0c = coulombic pressure, 0vp = vapour pressure inside the cavity,
0c = 0vp = 0es + 0s + 0h ,
0es = electrostatic pressure, 0s = pressure due to surface tension, and 0h = hydrostatic pressure.
#4.>' where, /rom this condition, an epression has been obtained for the maimum breakdown strength of pure li"uids which was found to be in good agreement with the eperimental results. In general, the cavitation and bubble theories try to eplain the highest breakdown strengths obtainable, considering the cavities or bubbles formed in the li"uid dielectrics. c. Stressed *il +olume )heor" In commercial li"uids where minute traces of impurities are present, the breakdown strength is determined by the Elargest possible impurityF or Eweak linkF. ;n a statistical basis it was proposed that the electrical breakdown strength of the oil is defined by the weakest region in the oil, namely, the region which is stressed to the maimum and by the volume of oil included in that region. In nonuniform fields, the stressed oil volume is taken as the volume which is contained between the
( 6 ma )
( 6 ma )
maimum stress contour and +.5 contour. According to this theory the breakdown strength is inversely proportional to the stressed oil volume. The breakdown voltage is highly influenced by the gas content in the oil, the viscosity of the oil, and the presence of other impurities. These being uniformly distributed, increase in the stressed oil volume conse"uently results in a reduction in the breakdown voltage. The variation of the breakdown voltage stress with the stressed oil volume is shown in /ig. 4.9.
+re",do-n tre## (,V/cm)
400 00
it! #te"d% *olt"ge ri#e 200
one minute -it!#t"nd *olt"ge 100 0 102 1.0
102
104
106
tre##ed oil *olume (cc) Fig .4 o-er re'uenc% (&0 ) ".c $re",do-n #tre## "# unction o #tre##ed oil *olume
,. !hat is the stressed oil %olume theor" and how does it explain brea&down in large %olume of commercial liquid dielectric? In commercial li"uids where minute traces of impurities are present, the breakdown strength is determined by the Elargest possible impurityF or Eweak linkF. ;n a statistical basis it was proposed that the electrical breakdown strength of the oil is defined by the weakest region in the oil, namely, the region which is stressed to the maimum and by the volume of oil included in that region. In nonuniform fields, the stressed oil volume is taken as the volume which is contained between the
( 6 ma )
( 6 ma )
maimum stress contour and +.5 contour. According to this theory the breakdown strength is inversely proportional to the stressed oil volume.
