* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download assessment of dielectric properties of epoxy resin
Survey
Document related concepts
Thermal runaway wikipedia , lookup
Printed circuit board wikipedia , lookup
Operational amplifier wikipedia , lookup
Schmitt trigger wikipedia , lookup
Josephson voltage standard wikipedia , lookup
Power electronics wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Current source wikipedia , lookup
Power MOSFET wikipedia , lookup
Voltage regulator wikipedia , lookup
Surge protector wikipedia , lookup
Opto-isolator wikipedia , lookup
Rectiverter wikipedia , lookup
Transcript
CHAPTER 4 ASSESSMENT OF DIELECTRIC PROPERTIES OF EPOXY RESIN COMPONENTS UNDER HUMID CONDITIONS. 4.1 INTRODUCTION s’ The previous chapters established a foundation for the presence of moisture on the electrical Insulation In a humid environment. The moisture ingression Into the material was also seen to be significant, and it was also shown that the absorption of water by epoxy resin composites significantly altered their dielectric properties. Therefore the assessment of the increase in the dielectric loss factor and other attributes like leakage current,partial discharge and volume conductivity would be a valuable Information for designing reliable switchgear components. This assumes more importance in the case of medium voltage switchgear components which were shown In the earlier chapter to be prone to moisture condensation under aggravated service conditions. This chapter reports on the investigations carried out on medium voltage switchgear components, such as instrument transformers and bus support bushings, which were subjected to various humidity levels and the dielectric parameters such as loss factor, leakage current and partial discharges were monitored. Based on this data, a new deterioration Index termed as 'Cumulative Characteristic Factorhas been developed to make an assessment on the rate of Increase in these parameters In the presence of high humidity conditions. Also a three dimensional relief model was developed for pictorially depleting the variations of loss factor with humidity and voltage. -\ 4.2. THEORY OF MOIST DIELECTRICS The quantification of the environmental effect on the switchgear components is based mainly on measurable quantities,l.e loss tangent ( tan 8 ), surface leakage current (li) and partial discharges. The other determinable parameters such as volume conductivity,loss index^ancf ^electric power loss are also used for the assessment of effects of humidity on the dielectric properties of the switchgear components. The understanding of the basic mechanism of moist dielectrics Is necessary to establish the usage of these factors in the 64 quantification of environmental effect on epoxy cast equipments. 4,2,1 Theory of dielectric losses in epoxy composites When a dielectric is subjected to an alternating electric field, the orientation of the dipoles, and therefore also the polarisation, will tend to reverse each time when the field polarity reverses. Their ability to follow the field depends on the inertia of the polarising mechanisms. At very high frequencies (1015Hz) only the lightest of the mechanisms can follow the field and hence the electronic and atomic polarisations dominate. At frequencies less than 107Hz, contributions to the dielectric constant come from the movement of permanent dipoles (Debye relaxation) and interfacial polarisation which is due to electronic charges accumulating at the boundaries between different phases in the material. The result of these polarisation mechanisms is that it is necessary to define a complex relative permittivity for a dielectric.subjected to an alternating electric field and is given by e* = e*-je" (4.D in which £’ is simply known as the dielectric constant, E " is the loss tand j = V - 1. The loss tangent, and therefore the loss angle,are defined by the equation tan 5 = £ " /£* (4.2) at any given frequency .the loss factor will be the sum of several mechanisms, and their relative importance will depend on the frequency of the applied field, so that for example: E " = £ " (Debye) + £ " (conduction) + £ " (interfacial) (4.3) Thinking in the same lines in order to explain the dielectric losses in epoxy composites we have to recognise that any specific epoxy re§ln formulation will be affected by the type of filler and the hardening process employed. In the case of a filled resin the degradation observed is dependent on the filler and the cavities present at the electrode-resin interface. Based on this it has been proposed that the dielectric constant of a composite material can be expressed as that of mixtures so that its complex permittivity can be expressed as. 2 £ = £f Vf + £r Vr + £i Vi (4.4) 65 where the subscripts, f. r and 1 refer respectively the filler, resin and lnerface, and V is the volume fraction of each component. It Is considered that interfaclal region plays an Important role In water absorption, and therefore the dielectric properties of the composite must also depend on it. It follows from Equation (4.4) that the relative permittivity and the loss factor for the moist dielectric can also be given by similar relatlons.so that: E' = E'f Vf + E’r Vr + E’l Vt (4.5) and £" = £”f Vf + E'Y Vr + £”i Vj (4.6) The perusal of the Equation (4.6) shows that the dielectric losses in epoxy are contributed by the resin, filler and the Interfaces. In addition to these,cavities present at the interfaces due to manufacturing processes and water ingress during the synthesis of the resin are all contributory to discharges and high dielectric losses. In presence of moisture. Based on this principle, loss langortwas chosen as one of the parameters for the assessment of the dielectric behaviour of switchgear components under high relative humidity conditions. Further, the experimental results due to H. Egger and G.Praxl (401 also subscribe to the above outlined theoretical reasoning. It was shown by these authors during experimentation on epoxy insulation coll overhang.that loss factor increases to a high value within half an hour after exposure to high relative humidity condition. This is illustrated in Figure 4. la. The field investigations reported by Milne and Blower |4J show an increase in the loss factor of about six percent after exposure to high ambient humidity. Hence the choice of losstangcntfor quantification is Justified theoretically as well as experimentally. 4.2.2 Theory of leakage current due to surface conduction Surface resistivity Is affected by moisture absorbed in the bulk of the material and also by moisture available from the surrounding atmosphere. Surface conduction results from moisture or other contaminants present on the surface of a dielectric. Water has a high conductivity and even, a fine film of moisture on the dielectric surface is enough to cause a conduction of current whose magnitude depends on the thickness of the film. The resistance of the moisture film adhered to the surface is dependent on the nature of the surface, and hence surface conduction is treated as the property of the dielectric itself. The absorption of moisture by the surface of the dielectric Is highly dependent on 66 (a ) FIG. 4.1 (A) LOSS TANGENT VARIATIONS WITH TIME 67 the relative humidity of the surrounding medium. Hence the ambient environmental conditions are the most important factors that determine the surface conductivity of a dielectric. The lower the polarizability of a substance, the cleaner the surface and the better Its finish,the smaller is the surface conductivity. Polar dielectrics have a lower surface resistivity which greatly diminishes In a moist atmosphere. The epoxy resin being a polymer and a polar dielectric and water also being a polar dielectric it can be inferred that adherence of water on the epoxy resin components would significantly affect the surface resistivity. The leakage current ii can be expressed as, U=f{V,A,a) Where V = Voltage (4.7) A = Cross - sectional area of the water film a = Conductivity of the water film The factors A and O are functions of relative humidity (RH) Hence ii = f(V,RH) (4.8) The experimental investigations due to Eagger and Praxl (401 established an increase in surface conductance at a relative humidity of 95 percent,which is shown in Figure 4.1b . Based on the above theoretical reasoning and experimental results, leakage current was chosen as one of the important parameters for quantification of the environmental effect on the epoxy cast resin components. 4.2.3 Theory of Partial Discharges Honda and others (101 have characterised the behaviour of epoxy mould Insulation under sustained AC Voltage. They have attempted to study the deterioration of epoxy moulded insulation even under low partial discharge levels as low as 0.1 pC. The studies indicated that, even on practically void - free moulded insulation the deterioration is developed under the application of high electrical stress. The above said deterioration is closely related to an occurrence of small partial discharge over the boundary between the embedded electrodes and the epoxy resin. The discharge magnitude that actually effects deterioration is approximately 0.1 pC. Among possible causes of small partial discharges, electric field concentrate around micro protrusions over embedded electrode surfaces is dominant In creating the phenomenon. On the other hand, fine epoxy debonding from the electrode surfaces caused by different thermal expansion coefficients of electrode material and resin could serve as another reason for creating the small partial discharges. Earlier to 68 20 RH - 93 V. r 15 CONDUCTANCE ( J1S t 10 5 ___________ I__________________L 0 12 TIME ( Hrs) —► 18 (b) FIG. 4.1(B) SURFACE CONDUCTANCE VARIATION WITH TIME 24 69 this work. Dieter Kind and Konig [23J have established that partial discharges, in voids in epoxy resin insulations subjected to long-term voltage stressing, can lead to breakdown of the insulation. For identical electrode spacing and void dimensions, the resistance to breakdown is dependent upon the epoxy resin system, the condition of the surface of the void and the size of the void. Epoxy resin filled with powdered quartz was found to be more resistant than the corresponding unfilled resin. Small and narrow voids were found to be particularly dangerous to the dielectric even though the magnitudes of the partial discharge quantities were small. Therefore partial discharge levels were also chosen as one of the parameters for observation during characterisation of epoxy cast components under aggravated service condition. 4.3 4.3.1 EXPERIMENTAL SET-UP AND PROCEDURE Specimen Selection Before deciding upon the experimental methodology the choice of specimen is very important. The characterisation of dielectric materials reported in literature usually makes use of models or thin samples of the original insulation system. However it was felt that from the practical view point, it would be better if the component themselves, such as instrument transformers and bus support insulation are used as test specimens. The equipments, namely, instrument transformer and bushings are either impregnated or cast with bisphenol resin composites. Figure 4.2 gives the details of the specimen used for characterisation. The composition of the bisphenol epoxy resin is provided in Table 4.1. The mechanical, physical and electrical properties of the bisphenol resin is tabulated in Table 4.2. 4.3.2 Methodology The characterisation of the dielectric properties of switchgear components under aggravated service conditions requires the following experimental conditions: — Variation of relative humidity from 45 percent to 95 percent — Variation of the temperature from 35 to 45°C — Measurement of loss factor, leakage current and partial discharges. Variation of relative humidity: The experiments were carried out in a thermal humidity chamber shown in Figure 4.3. The humidity chamber had the facility for changing 11 FIG . 4.2 DETAILS kV ENTRANT BUSHING 11 kV PT OF SPECIMEN QVOT t o ID d u jvs /0 0 Z ' A>Ul 3-3 kV CT sc ale ; 70 71 TABLE 4.1 EPOXY RESINS FORMULATION FOR EXPERIMENT Resin formulation Curring Schedule Casting . SPC 20 - 40 gms 2 hrs. at room Silica - 600 gms temperature Hardener -, 40 gms Impregnation Resin CY 205 300 gms Hardener HY 905 Silica 300 gms 750 gms Plastizier 061 .06 gms Accelerator 040 .004 gms 110°C at 6 hours TABLE 4.2. ELECTRICAL PROPERTIES Properties Temperature Electric strength, 2mm plague, 20-s value, 50 Hz. Electrodes: plate-to- plate, 25mm and 75mm, In castor oil 23°C Arc resistance Electrolytic corrosion Loss factor tanS 23°C Standard SI Units Values IEC 243 kV/mm 18-20 ASTM-D 495 s 185-195 DIN 53484 grade LI DIN 53480 grade KA2 IEC 112 V 475 DIN 53489 grade A-l DIN 53483 % 2-3 60°C Dielectric constant 23°C 3-4 DIN 53483 3.9-4.3 DIN 53482 4.3-4.7 1016 60°C Volume relstlvlty 23°C 60°C £2cm 1015 72 MEATER X FIG. 4.3 THERMAL HUMIDITY CHAMBER 73 the relative humidity from 40 percent to 95 percent. The special feature of the chamber was the provision to raise the temperature of the air to a required level for a pre-set relative humidity. This enables the experiments to be conducted at dilferent temperature levels. In Chapter 2 it was shown that the maximum temperature raise on a current transformer was 42°C. Hence during experiments conducted on current transformer the temperature was maintained at 42°C. This condition simulated the temperature raise likely to occur during t normal operations of the equipment. The losses occurring in potential transformer and Bus support insulators are not considerable. Therefore for these components the temperature was maintained at 38°C which was the average ambient temperature of Southern part of India. Pre-conditioning of the epoxy components: The experimental investigations referred in earlier literature show^ that within one hour of moisturing time the values of loss factor and leakage current attain stabilization. Based on this the samples were conditioned in the humidity chamber for three hours, at each level of humidity. During this conditioning the voltages were maintained at nominal rated values. The secondary circuit of the potential transformer was connected to a resistance equivalent to that in the service condition. The secondary windings of the current transformer were short circuited. The samples were initially dried and cleaned before putting into the chamber. All the samples were tested as per IEC specifications before the commencement of the characterisation. The samples not passing the routine test were not used for the experimental studies. No micro-discharges or corona were allowed during bulk property characterisation. Measurement of Dielectric parameters: For the measurement of capacitance and loss factor (tan 8 ) of the test specimens a high voltage Schering Bridge of M/s TETTEX AG INSTRUMENTS make. Type 2801 was used with an electronic null detector and an automatic guard potential regulator. Screened leads were used for connecting the test specimen and the standard capacitor. The Schering Bridge was kept sufficiently away from the high voltage source so as to minimise its influence on the measurement. The circuit adopted for measurement Is shown In Figure 4.4 Partial discharge measurements were carried out using a Discharge Detector.Model 5, Type 500, Bonar Instruments, U.K. The instrument incorporates two particular features that facilitated routine discharge testing, a discharge magnitude meter and a gating unit which allows meaningful discharge magnitude meter reading to be taken in the presence of persistent electrical interferences. Of the various test configurations, the standard test circuit shown in Figure 4.5. was chosen for measurement of partial discharge in this study. The single phase test voltage derived from mains supply is applied through a z FIG. 4.4 (a) SCHEMATIC DIAGRAM LOSS MEASURING CIRCUIT (b) MEASUREMENT CIRCUIT DIAGRAM 74 FIG. 4.5 Cb Cx - BUSHING FREE UNDER DISCHARGE TEST CAPACITOR PARTIAL DISCHARGES DETECTION CIRCUIT 75 76 regulator to the primary of 230V/150kV high voltage transformer. The output of this high voltage transformer Is connected to a discharge free capacitor, the test specimen and a voltmeter resistor. The test voltage Is monitored on the discharge detector via the voltmeter resistor. For a better accuracy !n the measurement the input unit must match the tuning capacitance provided by the test circuit. This Is the total external capacitance appearing across the terminals A and B, and in the circuit chosen it is the series combination of the specimen and blocking capacitances. Precautions were taken to eliminate discharges In the external circuit. Also the bus bars were cleaned and polished to reduce the surface Irregularities to a minimum. The discharge detector was kept In a shielded cubicle to reduce external interference to a minimum. The leakage current was measured using a 3-1/2 digit precision digital multimeter. The characteristic of the current was monitored using oscilloscope. The measurement of current was taken after application of the voltage for 30-60 minutes. 4.4. CASE STUDIES The assessment of dielectric properties of the switchgear components under varying humidity was undertaken in the following manner: — Case study I:Variations of dissipation factor changes with relative humidity. The voltage stress was kept below the Ionization point. — Case study II:Variations of leakage current with relative humidity,at nominal rated voltage. — Case study HkVariations of partial discharge with relative humidity. — Case study IV:Variatlons of dissipation factor with Voltage at various levels of humidity. — Case study V:Variatlon of leakage current with Voltage under various humidity levels. — Case study VkVariatlon of volume conductivity with humidity. The above six case studies Involved totally experimental Investigation of twenty five variations. The characteristics obtained for various equipment under various conditions are listed In Table 4.3. No. C ase Voltage w as held at llk V RMS % Support b u sh in g s ttnn 3 iv) ? a Jj Voltage w as held at llk V RMS % a -U llk V 5) 05 c a -p 5 E n tran t b u sh in g T em perature w as m aintained a t 42°C Hum idity variable 3.3kV 100/5A JJ iii) T em perature w as m aintained a t 42° C Hum idity variable llk V 200/5A E C u rren t T ransform er held constant hum idity variable Single Phase llk V Si Voltage a t P aram eter for O bservation -U ii) llk V held constant hum idity variable Voltage a t C ondition for Experim en t 5 llk V /llO V 33K V /110 V Single Phase Voltage C lass -o c *) CD c a •P Potential T ransform er Potential T ransform er i) E q u ip m en ts TABLE 4 .3 CASE STUDIES 77 a § mU r J. CO CO CO CO 5 3 c t) <T> z 4J CO 3 CO CO CO TABLE i) a cu rren t Voltage was held at 1 lkV RMS B llk V llk V E n tran t B ushings Partial discharges Partial discharges s -3£ 3 cn V) eS At each level of hum idity, Voltatge w as varied At each level of hum idity, Voltage w as varied Hum idity w as varied llk V Potential T ransform er B ushings Hum idity w as varied llk V cu rren t Leakage cu rren t Leakage cn 11) 1) 111) Potential T ransform er 3.3KV Support B ushings iv) 1) llk V o 3 E n tran t B ushings u ill) l Voltage was held a t 3.3 kV RMS Leakage Voltage w as held a t 3.3 kV RMS T em perature held at 42° C uV C urrent 3.3kV Leakage Voltage w as held a t 11 kV RMS llk V Potential T ransform er 4 .3 CASE STUDIES (Contd.) 78 r u £0 % 5 > llk V 6.6kV llk V E n tran t B ushings B us S upport iii) E n tran t B ushing ii) i) llk V Potential T ransform er i) TABLE 4 .3 CASE STUDIES (C ontd.) Relative Humidity w as varied w as varied At each level of hum idity, Voltage w as varied At each level of hum idity, Voltage w as varied At each level of hum idity. Voltage Leakage Volume Conductivity cu rren t Leakage cu rren t Leakage cu rren t 79 £ 80 4.5. ANALYSIS OF THE CHARACTERISTICS CURVES 4.5.1 Variation of loss tangent with relative humidity The characteristics curves of the loss factor variations with humidity for the various equipments under constant voltage stress are shown in Figures 4.6 to 4.9. The loss factor for all the equipment were fairly constant upto 60 percent relative humidity and thereafter they increased exponentially towards higher values. The increase in loss factor was six to ten times the initial values obtained at 50 to 55 percent relative humidity. In absolute terms the loss tAngentof the complete equipment at an average humidity of eighty five percent was about 10 to 13 percent for instrument transformers. While in case of bushings it was ranging from 3.5 percent to 6 percent. The specific values of loss factor as per IEC 243, for filled resins was 3 to 7 percent. But it was found that during high humidity condition the loss factor value rose upto 18 percent. Further it was found that instrument transformers have a higher loss factor value than bushings. The above characterisation leads to a thought, that the micro voids on the surface increase in size and number,due to differential expansion and.contraction brought about due to heating and cooling taking place on the surface,because of the increase and decrease in the values of loss kmqerttduring high and low humidity conditions. Physically this will weaken the matrix structure of the resin composite. Though it may not cause infant mortality of the equipment but it will nevertheless lead to surface degradation with time. Further dielectrics with higher losses will introduce substantial heat in the-insulating material which would result in the progressive degradation of the bulk property of the insulation system. 4.5.2. Variation of loss-fcongenfcwith Voltage The characteristics curves showing the variation of loss factor with voltages at a fixed level of humidity are shown in Figures 4.10, 4.11 and 4.12. At lower percentage of relative humidity loss fcaaqenfc values gradually increase upto 75 percent of the rated voltage and thereafter increases exponentially to higher values. At higher relative humidity the initial values of the lossfangcntare high even for low voltages and steadily increases to higher values more linearly. A perusal of the characteristics shows that at high relative humidity conditions the loss toyngeofc are at elevated levels even for low voltage stresses and they become purely dependent on humidity. This type of characteristics may be due to increase in absorption current because of the Increase in the surface conductivity. Secondly there is a definite point of transition termed as ‘Ionization Point* beyond which there is a steep DISSIPATION t a n g e n t 81 RELATIVE HUMIDITY (V.) ---- ► (a) FIG. 4.6(a) VARIATION OF LOSS TANGENT WITH RELATIVE HUMIDITY FOR 11KV POTENTIAL TRANSFORMER LOSS 82 RELATIVE HUMIDITYC/.) —► (b) FIG. 4.6(b) VARIATION OF LOSS WITH RELATIVE HUMIDITY FOR 33KV POTENTIAL TRANSFORMER LOSS T A N ft EN T. 83 T—J)L_______ IIII1I— 40 50 60 70 80 90 RELATIVE HUMIDITY (•/.)—*► FIG. 4.7 VARIATION OF LOSS TA NftFNT WITH RELATIVE HUMIDITY FOR 3.3KV CURRENT TRANSFORMER. 100 LOSS TA N G E N T — * 84 FIG. 4.8 VARIATION OF LOSS TANGENT WITH RELATIVE HUMIDITY FOR 3.3KV BUS SUPPORT LOSS TANffiEMT**.. » 85 FIG. 4.9 VARIATION OF LOSS TANGENT WITH RELATIVE HUMIDITY FOR 11KV ENTRANT BUSHING A- VOLTAGE STRESS AT 7 kV BELOW IONISATION POINT 86 ( l A> ) 39 V110A < h> -aid 5S01 DNiHsna uawaaa unouio aod aovnoA hum XNasNVj- ssoi do noiiviuva —— fo To -JN'3?9toVl. SS01 FIG. 4.11 3000 7000 VO LTAG E ( kV 9000 ) J_____ L 1100 13000 I 15000 FOR 11KV, 200/5 AMP. CURRENT TRANSFORMER VARIATION OFLOSSTAN(?»ENTWITH VOLTAGE 5000 17000 87 LOSS LOSS 88 FIG. 4.12 VARIATION OF LOSS TAMCP.ENT FOR POTENTIAL TRANSFORMER A - Relative Humidity 80% B - Relative Humidity 50% 89 increase in loss factor. This transition point takes place at lower voltages under high humidity conditions. 4.5.3 Variation of leakage current with relative humidity The variations of leakage current with relative humidity is shown in Figures 4.13, 4.14 and 4.15. The leakage current was found to be fairly constant upto a certain low level of humidity (50% -60%) and thereafter it steadily increased to higher values of about 7 times the initial value for instrument transformer. Similar change was also observed for bushings. It was further found that the leakage current increased linearly with voltage at 50 percent relative humidity but at higher percentages of relative humidity the Increase in the current was more pronounced. This may be attributed to the onset of micro-discharges and more ionisation at the surface due to an increase in humid layer thickness which ranges from 100 to 600 )i m . 4.5.4 Variation of Partial Discharges with relative humidity The initial values of partial discharges of instrument transformer were observed to be very high. As the humidity increased the values rose to 2 to 3 times the initial value. So at 95 percent humidity the values, did not drastically Increase. But the partial discharge inception Voltage decreased sharply with increase In humidity as shown in Figure 4.16. For example, partial discharge inception voltage came down from an initial value of llkV to 7.5KV at 85 percent humidity. 4.6. STATISTICAL ANALYSIS OF THE DATA Explanation outlined in the previous sections were of more qualitative in nature. In order to have a meaningful interpretation of the results, statistical methods were used. Statistical regression theory presumes that a functional relationship exists between the variable to be predicted and the variables which can be observed and that this relationship can be determined from the observed data. There are many types of relationships that might be exhibited by the data points. They can be listed as given below: Y = ko + kih (linear fit) Y = kohki (power law fit) Y = koekih (exponential fit) (4.9) (4.10) (4.11) where Y is dielectric parameter, h is relative humidity, ko and ki are constants. O K> 90 CURRENT LEAKAGE 4O 0 t YF 40 JL JL JL 50 60 70 80 90 RELATIVE HUM I DITY (V.)—► FIG. 4.13 VARIATION OF LEAKAGE CURRENT WITH RELATIVE HUMIDITY FOR 11KV POTENTIAL TRANSFORMER 100 > in cr V) 6O 0 ( JU.A ) O O 40 ( JJ. A ) a o O O CURRENT W Ki -* LEAKAGE 0 > _ _ _ _ _ _ _ _ _ _ ► o ** o 91 0 ss----- >SJ 40 JL _L 50 60 RELATIVE JL JL 70 60 90 HUMIDITY (V.) - FIG. 4.14 VARIATION OF LEAKAGE CURRENT WITH RELATIVE HUMIDITY FOR CIRCUIT BREAKER ENTRANT BUSHING 100 LEAKAGE CURRENT (JUA ) 92 0 40 50 60 70 RELATIVE HUMIDITY 80 90 100 (V.) —► FIG. 4.15 VARIATION OF LEAKAGE CURRENT WITH RELATIVE HUMIDITY FOR 3.3KV CURRENT TRANSFORMER T A N fl& N T LOSS FIG. 4.16 VARIATION OF PARTIAL DISCHARGES WITH RELATIVE HUMIDITY FOR BUSHING A - Partial Discharge Inception Voltage B - Loss tangent C - Partial Discharge (pC) t 94 After fitting the curve the regression co-efficients were determined. 4.6.1 Analysis of Loss tan^enlData The loss factor variations with humidity at constant voltage and time was first analysed. To this data set, all the above listed curve fittings were tried. It was found that exponential fit was the best fit for the data. The regression model for various equipments are listed In Table 4.4. The closeness of the magnitude of the regression to unity is the indication of the correctness of the fit. The establishment of exponential fit is supportive of the physical fact that the values are fairly constant upto 50 to 60 percent humidity and thereafter Increases steeply to higher values with Increase In relative humidity. Hence the variation of lossfcangentcan be expressed as tan8 = koekih 4.6.2 ' (4.12) Variation of Leakage Current with Humidity Having derived an expression for the variation of tan8 with humidity it was proceeded in the same manner to derive an expression for leakage current. The variation of leakage current with humidity when voltage and time were held constant was found to be following a power law. Thus ii = kohki (4.13) The Table 4.5 shows the regression coefficient ‘r’ for the different data sets. This shows that the leakage current is very much dependent upon humidity condition when voltage and time were held constant. 4.6.3 Variation of Partial Discharge with humidity The similar techniques of curve fitting was adopted for the partial discharge variations with relative humidity. The Table 4.6 shows the regression coefficient and the constants for an exponential model.which was found to be the best fit. Further it can be Potential T ransform er CB E n tran t B ushing B us Support Potential T ransform er llk V 33kV 33kV ao llk V M - 4.60566 - 8.70286 - 12.6063 - 6.73088 - 5.94963 COt o H £ o in 05 05 05 T o H X co ■'* (N CO CD 9 o eg CD O m co CO io »—■< eg CO o **H <g CO 05 r—i CO o CO cs t—i 0.0437897 0.0625452 0.187167 0.0615491 0.0366777 «—« C urrent T ransform er TANGENT Average 0.958872 0.912369 0.957318 0.980323 0.950099 0.994251 M 3.3kV Equipm ent TABLE 4.4 REGRESSION MODEL FOR LOSS 95 Potential Transform er CB E ntrant bushing Bus support Potentlial Transform er Transformer (llk V test) Transformer (IkV test) Transformer (6kV test) Transformer (4kV test) Transformer (2kV test) llkV llkV 33kV 33kV llkV llkV llkV llkV llkV I"*-* (N CD CO O) <N r00 in 1—1 CO 5 3o o H H 1 0 o I-t <N in 01—< CO o <N CD 0.300645 T o H £ - 1.64365 - 3.44293 o -2 .1 5 0 8 2 eg CO 1.88941 r—4 - inb - 9.74302 - 22.465 - 7.37446 - 1.20183 <N 05 in 00 c4 10.4648 - •a OV Current Transform er •3 33 kV 0.0319708 0.913273 0.116389 0.537907 0.276072 0.471034 0.508149 0.393029 2.69408 5.91228 3.03401 2.39605 3.39939 •o 1911510 Equipment POWER LAW MODEL FOR LEAKAGE CURRENT 0.993494 0.859393 0.952606 0.990622 0.950325 0.998433 0.99342 0.960378 0.957994 0.964862 u Z01ZV9- TABLE 4.5 96 41.78896956 76.83428278 20.824474751 27.19034214 47.47715433 Potential Transformer Potential Transformer Bus support current Transformer E ntrant Bushing 33kV 33kV 33kV 1 lkV 0.01013336 0.01241795 0.013530219 0.01188156 0.016868036 EXPONENTIAL LAW MODEL FOR PARTIAL DISCHARGES 11 kV Equipment TABLE NO. 4.6 0.864455009 0.84210749 0.8674293 0.854700601 0.751625558 Y = acbx Y = a*bx Y = a*bx 1 Y = acbx Y = aebx Model 97 98 seen from the table the regression coefficients are not as close to unity as in the case of leakage current and loss factor. This shows that increase in partial discharge values with humidity is not so pronounced as In the case of loss factor and leakage current. The values of partial discharges at higher values of humidity are mostly due to surface discharges occurring on the humid layer of the components. 4.7. DEVELOPMENT OF A NEW DETERIORATION FACTOR TERMED AS CUMULATIVE CHARACTERISTIC FACTOR Various parameters measured during experimentation were brought under a single specification by coining a new factor termed as ‘Cumulative Characteristic Factor’ involving loss factor, leakage current and partial discharge levels. Based on the regression analysis each of these parameters were given weightages. It can be seen that the loss factor and leakage current have good dependency on relative humidity conditions. In accordance p ^>\e with the principal^ loss tangent and the leakage current were given higher weights while partial discharge level, capacitances and partial discharges inception voltage were given a lower weightage. The various weights are shown below: Leakage current ... Loss ... Parlial Discharge ... Capacitances ... Partial discharge Inception voltage ... ... ... ... =.40 = .35 =.15 = .05 = .05 The weighted values were normalised to a base of one at the minimum humidity level (50 Percent Relative Humidity Optimal Condition). The optimal condition of 50 percent was chosen on the basis that the average minimum relative humidity for the Southern part of India is 50 percent. Further Headley, Milne and Blower |4] while evaluating the insulation characteristics of medium voltage switchgear found that loss factor values show a good recoveiy characteristics when subjected to an atmosphere of 50 percent relative humidity at 21°C. In a CIGRE paper [41] by Kamer it was demonstrated that at 50 percent humidity the values of dielectric parameters are at lower levels. Muller [42J and others working on the behaviour of organic Indoor insulation under electrical and climatic stresses found that at 50 percent humidity the layer conductivity for hydroscopic surface was as low as 10'5 micro Simens. Based on the above facts reported in literature and observations during current investigation the 50 percent humidity level was decided as optimal operating condition. Thereafter with increase in humidity this base value at 50 percent relative humidity was lowered depending upon the increase in the factors. The sum of all the weighted attribute 99 values was defined as the rate Index ‘R\ The reciprocal of rate Index was termed as ‘Cumulative Characteristic Factor’ (CCF). The expression for this factor can be given as below CCF = 1 / IR where R =ZW x M W = weights of the attributes and M = the parametric values The Cumulative Characteristic Factor was plotted against the relative humidity. Further regression analysis was done to establish the correlation between this factor and relative humidity. Tables 4.7 and 4.8 list the aggregated characteristic factor for various switchgear components. Figures 4.17 and 4.18 give the variation of Cumulative characteristic factor with relative humidity. It can be seen from regression analysis that CCF has good correlation with relative humidity variations. Hence it would be a good Index for the assessment of variation of dielectric properties with relative humidity. 4.7.1 Utility of Cumulative Characteristic Factor This factor will be a valuable tool in the hands of the insulation designer to assess new material and designs for aggravated service conditions. If 70 percent relative humidity is chosen as the base for aggravated service conditions, CCF above 2 indicates steep increases in loss factor, leakage current and partial discharges. Similarly at 95 percent humidity the CCF ranges from 2 to 5. Based on thls.a procedure is prescribed for the assessment of the materials and components under aggravated service conditions. — Take measurements of loss factor, leakage current and partial discharges at 60, 70 and 90 percent humidity for the new system under observation. — Assign the weights as given In the previous paragraph. — Aggregate the values of these parameters according to the procedure outlined earlier. — Determine the cumulative characteristic factor and obtain its variation with humidity. — Intercept the curve at 70 percent and 90 percent relative humidity and obtain CCF for the design and compare with initial values. — In general if the CCF is below 2 for 70 percent relative humidity and below 3 for 90 100 J________ I________ 1________ 1________ I________I________ L_ AO 50 60 70 80 90 100 RELATIVE HUMIDITY (•/.)—► FIG. 4.17 VARIATION OF CUMULATIVE CHARACTERISTIC FACTOR WITH RELATIVE HUMIDITY FOR 11KV POTENTIAL TRANSFORMER CCF - Cumulative Characteristic Factor 101 FIG. 4.18 VARIATION OF CUMULATIVE CHARACTERISTIC FACTOR WITH RELATIVE HUMIDITY FOR 3.3KV CURRENT TRANSFORMER CCF - Cumulative Characteristic Factor humidity Relative TABLE 4.7a. 0s t o rH H >P o in w o o o o to 00 O) 0) 0.658434595 195 (0.56) 200 (0.55) 251 (0.44) k i = 0.012569843 r = 0.976705042 Ko = H CCF = KoeklU d 110(1) 105 (0.95) 'o (pf> 10.49 (1). 10.79 (0.97) 11.31 (0.92) 14.3 (0.73) 17.7 (0.59) 21.2 (0.49) ” 10.4 (0.98) 10.1 (0.95) 9.5 (0.89) 8.9 (0.83) 8.2 (0.77) 10.6(1). 1.07 1.184 1.8868 2.4213 3.125 0.413 0.3205 C.C.F 0.933 tn CO 00 in o' o' TABLE 4.7b Regression Co-efficients 0.045(1) 0.048 (0.93) 0.057 (0.78) 0.095 (0.47) 0.135 (0.33) 0.195 (0.23) s' Partial discharge inception voltage Pi 103.6 (0.36) 128 (0.29) ( )IA) Loss factor Capacitance I 38(1) 42 (0.9) 45.1(0.84) 73.2 (0.51) current Leakage Partial discharges CUMULATIVE CHARACTERISTIC FACTOR FOR POTENTIAL TRANSFORMER. 102 Humidity (%) Relative Loss Factor £7 00 in o o 23.8 38.6 56.2 96.0 in 00 q <N 2.2 (0.78) 2.0 (0.71) 0.478 0.383 0.2835 CCF 1.156 1.538 2.619 3.527 o cs (0.13) 2.8 (1) 2.7 (0.96) 2.6 (0.92) Partial Discharge Inception (kV) Voltage <[ o 00 (0.22) (0.32) (0.52) 52 (0.96) 70 (0.71) 85 (0.58) 80 (0.62) 102 (0.49) 31.8 (1) 36.3 (0.87) 42.23 (0.75) 62.43 (0.509) 68.4 (0.46) 73.13(0.43) tance (PC) 50(1) Capaci- Partial Discharge »—■ 0.062 (0.93) 0.08 (0.72)) 0.102 (0.56) 0.145 (0.4)) 0.198 (0.291) 12.5 (1) 16.3 (0.76) Leakage Current (micro amp) CUMULATIVE CHARACTERISTIC FACTOR FOR CURRENT TRANSFORMER 890 9980 TABLE 4-8 103 o o o 00 o o 05 in 05 in CO 104 TABLE 4.8b REGRESSION ANALYSIS FOR CUMULATIVE CHARACTERISTIC FACTOR Model tan5 li ccf ccf =aebh = ahb =aebh = aebU a b r 1.662423x1O'4 0.6615491 .95009911 2.1922xl0'2 2.39605 .957994 0.132418 0.035104 .936399 0.6162240 0.156045 .972526119 105 percent relative humidity the new designs can be said to be an Improvement over the existing system when exposed to aggravated service conditions.4.8 DEVELOPMENT OF THREE DIMENSIONAL MODEL > Based on the data on the variations of the dielectric properties with voltage and humidity, a three dimensional relief model has been developed. The loss bo^y^eakage Current are funcUons of voltage and humidity at a given time. Using multiple regression technique, polynomial power lit was attempted. The analysis of the data base established a power fit for the variation of the two factors with respect to changes in humidity and voltage. The generalised power model for the variation was estimated as, yi = aix iblX2cl y2 = a2Xib2 iX2c2 (4.14) (4.15) where yi = loss tangent y2 = leakage current xi = relative humidity X2 = voltage ai, a2, bi, b2. ci and C2 are constants. The models for the various equipments are given in the Table 4.9. 4.8.1 Three dimensional surface chart The Multiple regression model was used to create a surface chart. The software programme 'ENER GRAPHICS’ developed by IBM was used to create the surface chart. The surface chart has three dimensions. X axis corresponds to relative humidity. Y axis corresponds to voltage and Z axis represents loss factor. The humidity ranged from 10 percent to 95 percent. The three dimensional relief model obtained for two equipments namely a potential transformer and an entrant bushing Is shown In the Figure 4.19 and Figure 4.20. The three dimensional surface chart clearly shows abnormal increase of dielectric losses with high relative humidity even at low voltage stresses. It also shows a change In behaviour In the case of Impregnated systems (Instrument transformers) and the cast systems (bushings). For example beyond sixty percent humidity, loss factor increases to higher values with Increase In voltage. While In the case of bushings, the surface profile gradually increases upto 70 percent of voltage, thereafter there is a steep increase in the value of loss factor. This finding Is In conformity with the physical fact that In the case of Impregnated component there would be an Increase In the number of cavities at the 106 FIG. 4.19 3-D SURFACE CHART FOR VARIATION OF LOSS TANGENT WITH RELATIVE HUMIDITY FOR POTENTIAL TRANSFORMER 107 FIG. 4.20 3-D SURFACE CHART FOR VARIATION OF LOSS TANGENT WITH RELATIVE HUMIDITY FOR ENTRANT BUSHING 4.945 7.705 18.59 -20.144 11 kV Entrant Bushing 7.0344 11 kV Current Transformer 1.7140210 Log a x> 11 kV Potential Transformer Equipment o I8SS0 TABLE 4.9 MULTIPLE REGRESSION MODEL 8 o °o H 2.175 Model 108 > > > 8 s II ii ii > >< < N 109 resin-filler interface and the windings. These cavities have more affinity to draw moisture and hence the degradation of the dielectric properties are much m6re pronounced in the presence of humidity and voltage stress conditions. It can also be noted here that the physical variation portrayed in these diagrams have similar to that of characteristic factor (CCF) variations with humidity described in the previous section. Hence a relief model developed for the first time in this thesis work gives a good physical picture of increase in loss factor for an equipment working at high humidity conditions. 4.9. DIELECTRICS PARAMETER CHART To have a ready reckoner of the various attributes of the equipment effected by humidity, a dielectric parametric chart was compiled using determinable factors such as' — Loss factor index — Volume conductivity — Dielectric power loss ' The value of these factors for 50 percent and 80 percent (RH) are shown in Table 4.10. 4.9.1 Loss factor Index This is a determinable quantity describing the effect of combination of permittivity and loss factor. Loss factor Index =£. tan 8 (4.16) This helps one to have a better insight into the phenomenon of dielectric losses since one of the basic properties of the material namely permittivity is also taken along with loss factor. The permittivity chosen for this calculation was 5.6 since its rise was found during experimentation to be 5.6 due to moisture ingress. At 50 percent level,permittivity chosen was 3.5. The Table provides information about this factors at 50 percent level and 80 percent level of humidity. 4.9.2 Volume conductivity • The volume conductivity was calculated by using the formula given below: .126 .18 50% RH 80% RH CB 50% RH BUSHING GO BUSHING • vP 80% RH 50% RH 21 .21 0 -0 CN TR \J 00 O CT H .042 .39 it PT c* o o 80% RH Loss Tangent Index .276 Condition DIELECTRIC PARAMETERS CHART H 50% RH Specimen 4 .1 0 .0864 .0102 ? a 7.00 3.5 5.833 .583 1.16 1.32 .11 21.66 10.83 .055 15.2 7.6 10"12m ho/cm Vol. Conductivity ? OS li p 0001 (X, ft o 301' TABLE CO CO H H 11.66 110 r Ill Active volume conductivity = f E tan 8 / 1.8 x 1012 mho / cm (4.17) where f = supply frequency E = relative permittivity of epoxy resin cast material. The variation of volume conductivity with relative humidity is shown In Figures 4.21 and 4.22. The active volume conductivity Is a good measure of the property of dielectric’s responses to high humidity environmental stress. The Increase In the volume conductivity seen In the graph has been attributed to an Increase In a charge carrier with the concentration of the absorbed moisture. The volume conductivity also increased exponentially to higher values after 60 percent relative humidity. Specifically the Increase is about 8 to 10 times the Initial values at 55 percent relative humidity. Such a steep Increase in the value is due to the Increase In conductivity of the moist dielectric because of high activation energy of the charge particles of water, concentration of charge carriers and the diffusion coefficients. 4.9.3 Dielectric Power loss The losses In the dielectric was computed using the formula given below: P = E2 f £ tan 8 / 1.8 x 1012 W / cm3 where E = Electrical field intensity at the Electrode resin interface, f = supply frequency E = relative permittivity (4.18) The value of E, estimated using charge simulation method, was found to be around 15 to 25kV/cm at the electrode-resin interface. The details of the method adopted are outlined in a later chapter. From the perusal of the Table 4.10 it can be seen that the dielectric power losses are more in instrument transformers than In bushings. It can also be pointed out In the case of Instrument transformer that the dielectric power losses are more pronounced even at 50 percent relative humidity level. These losses contributed to the faster surface deterioration of the epoxides under high humidity hazards. 4.10. CONCLUSION The characterisation of the bulk properties of switchgear components under high humidity condition has resulted in the following findings: Hit) It) 3. A T AT 6b0B U if i; 0 AT 113V 2 5 U / U O L . C O N T H JC T I U I T Y I N w h o . 'C N x 1 0 123. 1RU 93 VU. G3. 4U_ 33. ■—i— ■■n" ] MJ I. * f til V«i > i Lit 1 ] U L IIUMiMiy FIG. 4.21 Vv IN X ~r UU UL —#► VARIATION OF VOLUME CONDUCTIVITY WITH RELATIVE HUMIDITY FOR 11KV POTENTIAL TRANSFORMER T UU T u\» 16 .. 14 A AT 6508 U □ AT 10725 U / / 12. t 1A 0 \ o £tU . 04. >6.04 r* /* r-4.84. D fi z0 U2.04. ... —oA- 1— nr mi t.'.> v»» KLi.ritU’i: mifiiunv ~T~ bt) v:» in u:» 1 ‘JO 1 ■J'j mo * FIG. 4.22 VARIATION OF VOLUME CONDUCTIVITY WITH RELATIVE HUMIDITY FOR ENTRANT BUSHING 114 The dielectric parameters were fairly constant upto Sixty percent relative humidity and thereafter they rose exponentially. For example, the Increase In loss factor was six to ten times the initial value obtained at fifty percent relative humidity. — Experiments demonstrate that the humidity hazard is continuously present beyond sixty five percent relative humidity. — Regression models were used to quantify the characteristic changes In the parameters. A multiple regression analysis on the data base established pqwer fit for the two factors namely loss factor and leakage current with respect to humidity and voltage. — This relation was used to plot a three dimensional relief model Involving loss factor, voltage and humidity. The three dimensional surface chart clearly shows abnormal Increase of dielectric losses with high relative humidity. It also shows a change of behaviour In the case of impregnated systems (Instrument transformers) and the cast systems (bushings). For example beyond sixty percent humidity, loss factor Increase towards higher values with Increase in voltage. While in the case of bushings, the surface profile gradually Increases upto 70 percent of voltage, thereafter there is a steep Increase In the value of the loss factor. This finding Is in conformity with the physical fact that In the case of impregnated components there would be an Increase In the number of cavities at the resin filler interface and the windings. These cavities have more affinity to draw moisture and hence the degradation of the dielectric properties are much more pronounced In the presence of humidity and voltage stress conditions. — The various properties measured during experimentation were brought under a single specification by coining a new factor termed as ‘Cumulative Characteristics Factor1 involving loss factor,leakage current and partial discharge levels. Based on this factor new designs for equipment and resin formulations can be assessed for their performance under high humidity conditions. — Further, based on the above principle a ready reckoner of dielectric parametric chart Involving loss index, volume conductivity and dielectric power loss was compiled for usage In the design of equipments under aggravated service conditions.