1.0
Introduction
The most common insulating medium used as dielectric in electrical conduction is air. Hence, it is essential that the behaviour of air under dierent conditions of electric stress is understood. The amount of stress upon the dielectric is relies on the type and the magnitude of applied voltage that is subjected to. The type of applied voltage can be either AC, DC or impulse. The distribution of the electric eld in the dielectric can be either uniform or non!uniform, depending on the geometry of the electrode used. This "ill then in#uence the magnitude of el ectric stress being e$erted on the dielectric "hich in this case is air.
The purpose of the e$periment is to conduct the Corona %nception Test Test in air "ith the objective to investigate the brea&do"n voltage of air under AC or DC voltag voltage e in unifor uniform m and non!un non!unifor iform m elds. elds. %f the electri electric c eld eld is unifor uniform, m, a gradual increase in voltage across a gap produces a brea&do"n of the gap in the form of a spar& "ithout any preliminary discharges. 'n the other hand, if the eld is non!uniform, an increase in voltage "ill rst cause a discharge in the gas to appear at points "ith highest electric eld intensity, namely at sharp points or "here the electrodes are curved or on transmission lines "hich gradually leads to a #ash!over, this phenomenon is the Corona.
Corona is a phenomenon that has the capability for degrading insulators, and causing systems to fail. The corona inception voltage is dependent on the type and polarity of the electric eld e$citation of either DC or AC as "ell as the geometry of the electrode. (or instance, the corona inception voltage has a high li&elihood of occurrence "hen the electrode used is a positive point electrode, "hereas, for a negative point electrode it prevents the corona inception voltage. Corona, also &no"n as partial discharge, is a type of locali)ed emission resulting from from transi transient ent gaseou gaseous s ioni)a ioni)atio tion n in an insula insulatio tion n system system "hen "hen the voltag voltage e stress, i.e., voltage gradient, e$ceeds a critical value. The ioni)ation is usually locali locali)ed )ed over only only a portio portion n of the distance distance bet"een bet"een the electrodes electrodes of the system stem.. Cor Corona ona can occ occur "ithi ithin n voids oids in ins insula ulators tors as "ell "ell as at the the conductor*insulator interface.
Corona inception voltage is the lo"est voltage at "hich continuous corona of specied pulse amplitude occurs as the applied voltage is gradually
1
increased. Corona inception voltage decreases as the freuency of the applied voltage increases. Corona can occur in applications as lo" as -/. This form of discharge is called a corona discharge and can be observed as a bluish luminescence. This phenomenon is al"ays accompanied by a hissing noise, and the air surrounding the corona region becomes converted into o)one.
2.0
Objectives
The rst objective of this e$periment is to investigate the corona inception voltage of air under AC or DC voltage in uniform and non!uniform elds. The second objective is to investigate the brea&do"n voltage of air under AC or DC voltage in uniform and non!uniform elds. The third and nal objective is to understand the relationship bet"een the magnitude of the corona inception voltage and brea&do"n voltage of air "ith varied gap spacing of electrodes.
3.0
Methodology
The e$periment "as carried out by follo"ing a specic methodology as stated in the 0ab anual. 2rior to conduction the e$periment, all the relevant safety precautions as mentioned in the 0ab anual "ere observed and understood. These safety measure "ere follo"ed at all times. 3pon revie"ing the safety measure, the hard"are setup and connections of the e$periment "as done as per the schematic diagram as seen in 4ection 5. of this report. The point!to! plane electrode conguration "as used in the e$periment. 3pon completing the setup, the setup "as veried by a group member as "ell as the lab instructor. 'nce the high voltage setup "as veried and approved, the test area in "hich the e$periment "as being conducted "as cleared and the safety loc& "as closed. All necessary safety precautions "ere ta&en.
The light of the test area "as then s"itched!o. The high voltage setup "as then energi)ed via s"itching on the po"er supply and the control des&. The gap spacing of the electrode "as then set to 1mm via the control des&. After adjusting the gap spacing as desired, the applied voltage of the electrode "as gradually increased "hile observing the tip of the point electrode for the
+
occurrence of #ash!over of a purplish!luminescence. 6hen #ash!over occurred, the voltage at "hich the #ash!over occurred "as recorded as the #ash!over indicates the corona inception voltage for that particular gap spacing. The voltage "as then gradually increased further until orange!yello" spar& occurred indicating the occurrence of brea&do"n and the voltage value at the brea&do"n "as recorded indicating the brea&do"n voltage. T"o reading "ere ta&en for each of the corona inception voltage and the brea&do"n voltage for 1mm gap spacing and the average of these values "ere calculated and recorded. The steps "ere repeated for t"o other dierent values of gap spacing "hich "as -mm and 7mm. The corona inception voltage and brea&do"n voltage for the -mm and 7mm gap spacing "ere recorded. The results "ere recorded and tabulated as seen in Table 1 in 4ection 7. of this report.
After completion of the e$periment, the control des& "as s"itched!o follo"ed by the main po"er supply, thus, s"itching!o the high voltage setup. The safety loc& of the test area "as then opened and a grounding rod "as used to discharge the high voltage setup before dismantling the setup. The #oor "as discharged rst prior to entering the test area. This "as follo"ed by discharging of the electrode, capacitors and transformer. 8o part of the high voltage setup "as touched or contacted before discharging. All the necessary safety precaution "ere ta&en. The high voltage setup "as then dismantled and the components "ere placed bac& accordingly to their respective places.
4.0
Schematic Diagram
-
.0
!esults
The follo"ing results as seen in Table 1 "ere obtained from the e$periment that "as conducted. The results obtained are analy)ed and discussed detail in 4ection 9. of this report.
"able 1# $orona ince%tion voltage and brea&do'n voltage values 'ith di(erent ga% s%acing !eadings
$orona Ince%tion )oltage
,rea&do'n )oltage *&)+
-a%
*&)+ 10mm
30mm
0mm
10mm
30mm
0mm
S%acing 1st
15.:
-.7
-7.7
19.:
--.;
59.7
2nd
15.;
-.9
-<.<
19.;
--.<
59.:
verage
15.:7
-.77
-:.:
19.:7
--.;7
59.-;
/.0
Discussion
5
The average values of the results form Table 1 in 4ection 7. "ere used to plot the graph sho"ing the relationship of the corona inception voltage and brea&do"n voltage "ith the three gap spacing used "hich is 1mm, -mm and 7mm. The graph plot is as seen in (igure 1. As can be seen in (igure 1, both the corona inception voltage and brea&do"n voltage increases as gap spacing increases. Ho"ever, the corona inception voltage is slightly lo"er than the brea&do"n voltage. This is because corona discharge happens rst before the voltage brea&do"n, "hich it can be heard by a hissing noise.
6hen subjected to a non!uniform electric eld under AC condition, corona discharge happens rst in the locali)ed areas "hen there is an increase in e$citation voltage. This is because of the eld strength at the electrode "ith strong curvature of air increases and then the air surrounding it e$periences stress high enough to be dissociated into ions ma&ing the atmosphere conducting. This results in electric discharge around the conductors due to the #o" of these ions, giving rise to a faint luminescent glo", along "ith the hissing sound. Due to their high mobility, the electrons rapidly leave the ioni)ing region of the electric eld. The discharges "ill intensify "ith an increase of stress level to the point that eventually a brea&do"n "ill occur.
igure 1# -ra%h o corona ince%tion voltage and brea&do'n voltage versus ga% s%acing under $ condition
7
Ho"ever, in a non!uniform electric eld under DC condition, the slo"er ions build up a positive space charge in front of the point electrode and change the potential distribution. 6hen the point electrode is negative, the electrons move to"ards the plate. The remaining ions cause very high electric eld strengths immediately at the tip of the point electrode, "hereas the rest of the eld region sho"s only slight potential dierences.
.0
$onclusion
As a conclusion, from the e$periment it has been understood that the corona inception voltage occurs before the occurrence of the brea&do"n voltage "ith the corona inception voltage being lo"er than the brea&do"n voltage. The corona inception voltage is identied by the characteristic purplish!luminescence that it produces and it also produces a hissing noise as the voltage is gradually increased this hissing gets louder. oreover, it is also understood that as the gap spacing increases the voltage value at "hich corona and brea&do"n occurs also increases. The objectives of the e$periment have been achieved successfully.
!eerences
=umurthy, =.>. +-. Corona bet"een point plane electrodes in air at atmospheric pressure Annual Report Conference on Electrical Insulation and Dielectric Phenomena. pp. -7 ! -9.
Hogg, .=., Timosh&in, %./., c=regor, 4.?., 6ilson, .2. and =iven, .?. +17. 2olarity @ects on rea&do"n of 4hort =aps in a 2ointplane Topology in Air. Department of Electronic and Electrical Engineering, University of Strathclyde.
?avadi, H., (ar)aneh, . and 2eyda, A. +1. Determination of @lectric (ield at %nception ased upon Current!/oltage Characteristics of AC Corona in >od!2lane =aps. Iranian Journal of Electrical Electronic Engineering.
/B+11
9
Euel,
@.,
Faengl,
6.4. and
Euel,
?.
+.
High
/oltage
@ngineering
(undamentals. !e"nes #$ford.
ee&, ?.. and Craggs, ?.D. 1<7-. @lectrical rea&do"n of =ases. John %iley , 8e" Gor&.
aglaras, A. and Topalis, (./. +<. %n#uence of ground and corona currents on dielectric behavior of small air gaps. IEEE &rans' Dielectr' Electr' Insul.
1/B1-+!51.
6arne, 0.E., ?orgenson, >.@. and Eunhardt, @.@. +15. Criterion for spar&! brea&do"n in non!uniform elds. Journal of Applied Physics. 11B15 15-!--.
adhu, /. and havya, E. @A43>@@8T '( A%> >@AED'68 /'0TA=@ 34%8= 4TA8DAD 42H@>@ =A2 @TH'D. Journal of Electrical Engineering'
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