120 research outputs found
Mechanisms Governing the occurrence of Partial Discharges in Insulation Liquids
No full textBreakdown of electrical equipment is unwanted and the use of reliable methods of insulation are therefore necessary. Liquid insulation is one preferred material for electrical devices used in subsea installations due to outstanding qualities related to insulation, heat transfer, safety and incompressibility properties. To ensure that the selected insulating liquid is suitable for the specific equipment it will be necessary to test its insulating quality. This is verified by stressing the liquid with voltages higher than the partial discharge inception voltage (PDIV). While there are existing tests for these properties, such as use of partial discharge measurements, this thesis examines an experimental approach to testing the properties of different liquids in a point-plane gap through the use of high AC voltage.Charges, like ions, in an electrical field creates a current flow. In a liquid, dissociative ionization under applied voltage is the mechanism that generates the most charges. Field emission originating from the negative polarity contributes in some liquids. Moving charges in the liquids results in measurable currents of the order 10 - 80 nA under applied AC voltage of 20 kV in a 20 mm point-plane gap in different liquids. Space charges are also found to affect the partial discharge (PD) behaviour in liquids. Different liquids have different chemical and physical properties and therefore different PD behaviour. It has been observed that PDs are stochastic, and in some sense correlated to earlier stress due to residual ions from previous half periods and permanently change of the chemical structure of some liquid molecules after a PD. The PD rate increases exponentially with increasing voltage, while the maximum charge per half period tends to increase linearly. This observed behaviour is in agreement with what is found in earlier reports.The presence of free electrons is important. A significant increase in the rate of PD at low magnitudes in both polarities occurred when the test cell was exposed to X-rays. PDs start under the influence of a strong field or an electron avalanche. It is essential to be able to test different liquids in laboratory conditions, for economical reasons, in order to identify the most suitable insulating media. Power electronics utilizing quality insulation is more reliable with a lower probability of downtime. The test setup and methods used in this thesis may easily be adapted for test of electrical components and equipment instead of the simple point-plane gap method.Different liquids have different PD phase patterns. This thesis intends to describe the reasons to explain this difference. It is found to be the result of different properties in the liquids for charge creation and differences in their electrical field threshold for PD initiation. PDs of a certain size are rarely occurring events and are strongly dependent on the prehistory of the stressed liquid gap. We do not know whether PDs in itself is harmful to insulating properties or not. It is therefore uncertain whether the IEC 61294 test method, based on PDs, provides any useful information or not. More research is therefore needed in order to fully understand the PD phenomenon
Mechanisms Governing the occurrence of Partial Discharges in Insulation Liquids
No full textBreakdown of electrical equipment is unwanted and the use of reliable methods of insulation are therefore necessary. Liquid insulation is one preferred material for electrical devices used in subsea installations due to outstanding qualities related to insulation, heat transfer, safety and incompressibility properties. To ensure that the selected insulating liquid is suitable for the specific equipment it will be necessary to test its insulating quality. This is verified by stressing the liquid with voltages higher than the partial discharge inception voltage (PDIV). While there are existing tests for these properties, such as use of partial discharge measurements, this thesis examines an experimental approach to testing the properties of different liquids in a point-plane gap through the use of high AC voltage.Charges, like ions, in an electrical field creates a current flow. In a liquid, dissociative ionization under applied voltage is the mechanism that generates the most charges. Field emission originating from the negative polarity contributes in some liquids. Moving charges in the liquids results in measurable currents of the order 10 - 80 nA under applied AC voltage of 20 kV in a 20 mm point-plane gap in different liquids. Space charges are also found to affect the partial discharge (PD) behaviour in liquids. Different liquids have different chemical and physical properties and therefore different PD behaviour. It has been observed that PDs are stochastic, and in some sense correlated to earlier stress due to residual ions from previous half periods and permanently change of the chemical structure of some liquid molecules after a PD. The PD rate increases exponentially with increasing voltage, while the maximum charge per half period tends to increase linearly. This observed behaviour is in agreement with what is found in earlier reports.The presence of free electrons is important. A significant increase in the rate of PD at low magnitudes in both polarities occurred when the test cell was exposed to X-rays. PDs start under the influence of a strong field or an electron avalanche. It is essential to be able to test different liquids in laboratory conditions, for economical reasons, in order to identify the most suitable insulating media. Power electronics utilizing quality insulation is more reliable with a lower probability of downtime. The test setup and methods used in this thesis may easily be adapted for test of electrical components and equipment instead of the simple point-plane gap method.Different liquids have different PD phase patterns. This thesis intends to describe the reasons to explain this difference. It is found to be the result of different properties in the liquids for charge creation and differences in their electrical field threshold for PD initiation. PDs of a certain size are rarely occurring events and are strongly dependent on the prehistory of the stressed liquid gap. We do not know whether PDs in itself is harmful to insulating properties or not. It is therefore uncertain whether the IEC 61294 test method, based on PDs, provides any useful information or not. More research is therefore needed in order to fully understand the PD phenomenon
Condition Assessment of Hydro Generator Stator Bar Insulation Examination of PD void activity versus AC voltage magnitude and frequency
No full textInnføringen av væravhengige fornybare energikilde i det eksisterende nettet vil påvirke det nåværende driftsmønsteret fra strømproduksjon vi kjenner i dag. Når man kombinerer energikilder som sol- og vindkraft med kraftproduksjon fra vannkraft, blir driften endret hvor maskiner blir utsatt for flere start og stopp, samtidig som det kjøres med en tyngre last. Det er uunngåelig at dette påvirker tilstanden til generatorer, men i hvilken grad denne overgangen i kjøremønsteret er skadelig for vannkraftsgeneratorer over tid er ikke kjent. Dette gjør det nødvendig å ha tilstrekkelig diagnostikk og tilstands analyse tilgjengelig for å forlenge levetiden til generatorene.
Hensikten med denne oppgaven er å adressere problemstillingen rundt PD analyse, og bruke tilgjengelige metoder for påvisning og karakterisering av partielle utladninger i høyspennings isolasjon. Målet er å vurdere effekten av høyspenning AC-testing ved veldig lave frekvenser og opp til 50Hz både teoretisk og eksperimentelt.
En litteraturstudie og teoretisk bakgrunn er presentert som et grunnlag for arbeidet som presenteres i resultat og diskusjon.
Det er to deler til denne oppgaven, en teoretisk analyse av PD-egenskaper og et eksperimentelt forsøk for et sett med generator staver for å undersøke PD-egenskaper med tanke på PDIV/PDEV og utladningsstørrelse. For den teoretiske delen har det blitt simulert et sylindrisk og sfærisk hulrom for å undersøke endringen i det elektriske feltet ved økende hulromslengde og bredde med FEM-simulering. Når lengde/bredde forholdet avtar, er feltet nær homogent uten tilsynelatende kanteffekt for hulrommet med 0.2mm bredde og 2mm lengde fordi det svært høye feltet er påvist gjennom hele hulrommet. Kanteffekten hadde en liten økning av det totale arealet som ble påvirket da hulrommet ble mindre, denne effekten avtok ved hulrom over 10mm.
Det maksimale feltet i hjørnene økte sammen med en økende lengde av hulrommet ved konstant bredde.
Det elektriske feltet til et sfærisk hulrom var lik den teoretiske ligningen for hulrom under 10mm i diameter, da den totale isolasjonstykkelsen økte tilsvarende. Ved måling under konstant isolasjonstykkelse for sfærisk hulrom med endring i lengde, ble det oppdaget avvik fra teorien da hulrommets diameter overskred 1/3 av isolasjonstykkelsen allerede ved 1mm. En frekvensavhengighet ble påvist for de sylindriske hulrommene under svært lave frekvenser. Et avvik på 14% i PDIV for et hulrom på 2.2mm ble observert for en VLF på 10-6 sammenlignet med ved 50Hz.
I de eksperimentelle laboratorieundersøkelsene ble det testet to brukte generatorstaver fra nær HV terminal og to tilsvarende ubrukte back-up staver. En av back-up stavene viste en PDIV på 50% av gjennomsnittet av de tre andre test-objektene. Under maksimal spenning i drift har stavene blitt utsatt for en spenning på 1.9kV, mens PDIV viste 500V under denne grensen.
Gjennomsnittet av de tre stavene med like PDIV/PDEV-nivåer kunne tilnærme seg den teoretiske estimeringen av PDIV, der et hulrom på 1.9mm forårsaket den samme PDIV verdien som i test objektene.
Gjennomsnittlig utladningsstørrelse for stav S2 og R2 var begge avhengig av frekvensen og PDIV-nivået. Gjennomsnittlig utladningsstørrelse var 87% høyere for den brukte staven S2, enn back-up staven R2
Condition Assessment of Hydro Generator Insulation Using Partial Discharge Measurements
No full textMore than half of all hydro generator failures are caused by insulation breakdown. Insulation failures inflict serious damage on the generator and represent significant operating costs for utility companies. One of the main reasons for insulation breakdowns is the irreversible degradation caused by internal partial discharges (PD) resulting from voids in the insulation system. Therefore, it is crucial that methods for condition assessment of generator insulation can identify and analyze the partial discharge activity in these voids.
The main purpose of this thesis is to facilitate condition assessment of generator bar insulation by measuring partial discharges resulting from voids in insulating materials. The aim is to clarify how important PD parameters relate to the void size and the discharge mechanisms in the voids. A part of the work also involves comparing the experimental results with the theoretical model of internal partial discharges: the Abc Model. In addition, possibilities and limitations of partial discharge measurements as a diagnostic tool are examined.
The test samples consist of both real generator bar insulation and specially designed laboratory samples containing disk-shaped voids of different diameters. A thin layer of aluminum was applied by vacuum evaporation to the cavity surfaces of one sample group to investigate the effects of increased surface conductivity. Electrical detection of partial discharges was performed using a conventional measuring circuit, and the PD activity was analyzed in the form of inception voltage, as well as the discharge magnitude and discharge frequency at a voltage frequency of 50 Hz.
The inception voltage is found to decrease with increasing void diameter due to lower field enhancement in larger voids. In the case of the laboratory samples, the discharge magnitude is generally increasing with increasing diameter. The discharge magnitude is larger when the void surfaces are conducting since the discharge area then is equal to the void surface area. However, for samples made of aged generator insulation, the discharge magnitude is constant regardless of void diameter. More importantly, it was not possible to conclusively distinguish the PD activity in the voids from the PD activity inherent in the insulation. The discharge frequency tends to increase with increasing void size. This can be explained by the differences in the electric field strength in the different cavities at the specific voltage level, and the occurrence of parallel discharges. Higher void surface conductivity generally leads to lower discharge frequency.
In conclusion, the PD activity is seen to depend on the void size. The theoretical model is successful in describing the PD activity in voids with conducting surfaces, but fails to describe the PD activity in aged generator insulation. PD measurements performed on the laboratory samples can detect the voids and assess the relative void size. However, the voids in aged generator insulation cannot conclusively be detected using the chosen PD approach. This represents an important limitation of the PD method
Condition Assessment of Hydro Generator Insulation Using Partial Discharge Measurements
No full textMore than half of all hydro generator failures are caused by insulation breakdown. Insulation failures inflict serious damage on the generator and represent significant operating costs for utility companies. One of the main reasons for insulation breakdowns is the irreversible degradation caused by internal partial discharges (PD) resulting from voids in the insulation system. Therefore, it is crucial that methods for condition assessment of generator insulation can identify and analyze the partial discharge activity in these voids.
The main purpose of this thesis is to facilitate condition assessment of generator bar insulation by measuring partial discharges resulting from voids in insulating materials. The aim is to clarify how important PD parameters relate to the void size and the discharge mechanisms in the voids. A part of the work also involves comparing the experimental results with the theoretical model of internal partial discharges: the Abc Model. In addition, possibilities and limitations of partial discharge measurements as a diagnostic tool are examined.
The test samples consist of both real generator bar insulation and specially designed laboratory samples containing disk-shaped voids of different diameters. A thin layer of aluminum was applied by vacuum evaporation to the cavity surfaces of one sample group to investigate the effects of increased surface conductivity. Electrical detection of partial discharges was performed using a conventional measuring circuit, and the PD activity was analyzed in the form of inception voltage, as well as the discharge magnitude and discharge frequency at a voltage frequency of 50 Hz.
The inception voltage is found to decrease with increasing void diameter due to lower field enhancement in larger voids. In the case of the laboratory samples, the discharge magnitude is generally increasing with increasing diameter. The discharge magnitude is larger when the void surfaces are conducting since the discharge area then is equal to the void surface area. However, for samples made of aged generator insulation, the discharge magnitude is constant regardless of void diameter. More importantly, it was not possible to conclusively distinguish the PD activity in the voids from the PD activity inherent in the insulation. The discharge frequency tends to increase with increasing void size. This can be explained by the differences in the electric field strength in the different cavities at the specific voltage level, and the occurrence of parallel discharges. Higher void surface conductivity generally leads to lower discharge frequency.
In conclusion, the PD activity is seen to depend on the void size. The theoretical model is successful in describing the PD activity in voids with conducting surfaces, but fails to describe the PD activity in aged generator insulation. PD measurements performed on the laboratory samples can detect the voids and assess the relative void size. However, the voids in aged generator insulation cannot conclusively be detected using the chosen PD approach. This represents an important limitation of the PD method
Condition Assessment of Hydro Generator Stator Bar Insulation Examination of PD void activity versus AC voltage magnitude and frequency
No full textInnføringen av væravhengige fornybare energikilde i det eksisterende nettet vil påvirke det nåværende driftsmønsteret fra strømproduksjon vi kjenner i dag. Når man kombinerer energikilder som sol- og vindkraft med kraftproduksjon fra vannkraft, blir driften endret hvor maskiner blir utsatt for flere start og stopp, samtidig som det kjøres med en tyngre last. Det er uunngåelig at dette påvirker tilstanden til generatorer, men i hvilken grad denne overgangen i kjøremønsteret er skadelig for vannkraftsgeneratorer over tid er ikke kjent. Dette gjør det nødvendig å ha tilstrekkelig diagnostikk og tilstands analyse tilgjengelig for å forlenge levetiden til generatorene.
Hensikten med denne oppgaven er å adressere problemstillingen rundt PD analyse, og bruke tilgjengelige metoder for påvisning og karakterisering av partielle utladninger i høyspennings isolasjon. Målet er å vurdere effekten av høyspenning AC-testing ved veldig lave frekvenser og opp til 50Hz både teoretisk og eksperimentelt.
En litteraturstudie og teoretisk bakgrunn er presentert som et grunnlag for arbeidet som presenteres i resultat og diskusjon.
Det er to deler til denne oppgaven, en teoretisk analyse av PD-egenskaper og et eksperimentelt forsøk for et sett med generator staver for å undersøke PD-egenskaper med tanke på PDIV/PDEV og utladningsstørrelse. For den teoretiske delen har det blitt simulert et sylindrisk og sfærisk hulrom for å undersøke endringen i det elektriske feltet ved økende hulromslengde og bredde med FEM-simulering. Når lengde/bredde forholdet avtar, er feltet nær homogent uten tilsynelatende kanteffekt for hulrommet med 0.2mm bredde og 2mm lengde fordi det svært høye feltet er påvist gjennom hele hulrommet. Kanteffekten hadde en liten økning av det totale arealet som ble påvirket da hulrommet ble mindre, denne effekten avtok ved hulrom over 10mm.
Det maksimale feltet i hjørnene økte sammen med en økende lengde av hulrommet ved konstant bredde.
Det elektriske feltet til et sfærisk hulrom var lik den teoretiske ligningen for hulrom under 10mm i diameter, da den totale isolasjonstykkelsen økte tilsvarende. Ved måling under konstant isolasjonstykkelse for sfærisk hulrom med endring i lengde, ble det oppdaget avvik fra teorien da hulrommets diameter overskred 1/3 av isolasjonstykkelsen allerede ved 1mm. En frekvensavhengighet ble påvist for de sylindriske hulrommene under svært lave frekvenser. Et avvik på 14% i PDIV for et hulrom på 2.2mm ble observert for en VLF på 10-6 sammenlignet med ved 50Hz.
I de eksperimentelle laboratorieundersøkelsene ble det testet to brukte generatorstaver fra nær HV terminal og to tilsvarende ubrukte back-up staver. En av back-up stavene viste en PDIV på 50% av gjennomsnittet av de tre andre test-objektene. Under maksimal spenning i drift har stavene blitt utsatt for en spenning på 1.9kV, mens PDIV viste 500V under denne grensen.
Gjennomsnittet av de tre stavene med like PDIV/PDEV-nivåer kunne tilnærme seg den teoretiske estimeringen av PDIV, der et hulrom på 1.9mm forårsaket den samme PDIV verdien som i test objektene.
Gjennomsnittlig utladningsstørrelse for stav S2 og R2 var begge avhengig av frekvensen og PDIV-nivået. Gjennomsnittlig utladningsstørrelse var 87% høyere for den brukte staven S2, enn back-up staven R2.The introduction of weather dependent renewable energy to the existing grid is affecting the current operating pattern of electricity production. When combining energy sources like PV and wind power to power production from hydro power, the operation is controlled with more starts and stops, and at a heavier load than known before. It is inevitable that this affects the condition of the generator, however in what degree this transition will be harmful for hydro generators over time is not clear. This makes proper diagnostic testing and condition assessment valuable to increase the lifetime of hydro generators.
The main purpose of this thesis is to address the concerning issues using partial techniques for detection of internal critical voids in high voltage insulation. The aim is considering the effect of high voltage AC testing at very low frequencies to 50Hz both theoretically and experimentally. A literature survey is given together with the theory to provide a basis for the work presented in the results and discussion. There are two parts to this thesis, one theoretical analysis of PD characteristics and one experimental laboratory test for a set of stator rods testing their PD characteristics considering PDIV/PDEV and charge magnitude.
For the theoretical part a cylindrical and spherical void has been simulated for their electric field with an increasing void length and gap distance in a FEM-tool. As the gap-length relation decreases, the field is close to homogeneous with no present edge effect for the void with 0.2mm gap distance and 2mm length due to the very high field throughout the field. The edge effect had a small increase in terms of total area affected as the void grew smaller, and no noticeable difference was proven with void lengths bigger than 10mm. The magnitude of the electric field at the very corners increased along with an increasing length when the gap distance was fixed.
The electric field of a spherical void was equal to the theoretical limitation for voids under 10mm diameter when the total insulation thickness increased accordingly. With a fixed insulation thickness and the void diameter increasing, a deviation from the theory was seen already when the void diameter exceeded 1/3 of the insulation thickness by 1mm. A frequency dependency was proven for the cylindrical voids under very low frequencies. A 14% deviation in PDIV for a 2.2mm void was observed for a VLF of 10-6 compared to at 50Hz.
In the experimental laboratory tests, two service aged near HV and 2 back-up stator rods from a Norwegian run-of-the-river power plant was tested.
One of the back-up rods proved a PDIV of 50% of the average of the 3 other rods tested. The peak operating voltage for the rods were 1.9kV, while the PDIV was 500V under this limit.
The average of the three rods with equal PDIV/PDEV levels could be approximated to the theoretical estimation of PDIV, where a void of 1.9mm could cause the equal PDIV in the rods. Rod, S2, with a low PDIV did not fit the theoretical model. The average charge magnitude for rod S2 and R2 were both dependent on the frequency and the PDIV level. The average charge magnitude was 87% higher for the service rod than the back-up rod at 50Hz
Currents in AC stressed liquid insulated needle plane gap
No full textCurrents and space charge phenomena in 8 different dielectric liquids have been investigated for an ac stressed needle plane gap. Applied frequencies ranged from 0.1 Hz to 100 Hz. A high resolution analogue-digital converter and an active suppression of capacitive currents were used to extract the small conductive currents. Tip radii and voltages were varied. The currents varied non-linearly with voltage. At low frequencies the perfluorpolyeter and mineral oils had almost symmetrical current in both polarities, while cyclohexane, white oil and esters had asymmetrical current with lower peak current for positive polarity. Above a certain instantaneous voltage level - within the power cycle - the currents increased with the voltage squared, fitting a space charge limited current model. Clear indications of heterocharge space charges were revealed by varying tip radii and frequencies. The heterocharge will result in an increase of the electric field. The results show that at higher voltages a purely resistive model for the liquids becomes invalid. It is also evident that the presence of space charges will influence partial discharge behavior.Currents in AC stressed liquid insulated needle plane gapacceptedVersio
Partial Discharges in Voids at Variable Voltage Frequency and Temperature Diagnostic Testing of Stator Mainwall Insulation
No full textThe main purpose of this thesis is to improve diagnostic testing by PD detection at variable voltage frequency by developing and experimentally testing theoretical models proposed for measurement of PDs in voids in a mica/epoxy generator bar insulation. The focus of this thesis is to compare estimated and measured values, and from that determine which physical mechanisms best describe the voltage frequency and temperature dependence of observed PD activity. The main contribution to theoretical modelling was to propose an impedance abc-model and include the dielectric response.
Two types of test object were examined: 1) laboratory made, square-shaped insulation sheets using mica/epoxy tape, made with cylindrical voids with different diameter and gap distance and 2) 50 cm long sections of service-aged generator bars.
A frequency sweep procedure for PD detection was developed, based on the relevant condition assessment standard IEC 60034-27 to study the frequency dependence experimentally. PDs were recorded during applying an increasing voltage of 10 equal steps, at reducing voltage frequency, starting at 300 Hz to 0.1 Hz for laboratory objects, and starting at 50 Hz to 0.1 Hz for service-aged bars. The temperature was varied in the range of 20 °C to 155 °C.
Experimental testing compared to the proposed model showed that the capacitive abcmodel needed to be expanded to include both conductive elements and dielectric response; the impedance abc-model, to explain a frequency and temperature dependence for the partial discharge inception voltage (PDIV) and increased residual charge relaxation at high temperatures. The impedance abc-model was also expanded by a time lag and limited discharge area to describe the apparent charge magnitude. The capacitive abc-model describes PDIV for most cases. The effect of a high conductivity and dielectric response can explain the behaviour at low frequencies according to the impedance abc-model. The maximum apparent charge is frequency-dependent, which can be explained by PD ignition at an increased voltage caused by a time lag. Measured apparent charge magnitude indicates that the void area only partly discharges during each PD. This was observed to cause a distribution of PD magnitudes, instead of a few large PDs. The total apparent charge per period is, however, close to the expected values based on the void geometry, and is frequency independent below 100 °C. The total apparent charge per period is frequency-dependent at 130 °C and 155 °C. This dependency was here modelled by an increased relaxation of residual charges, which enabled more PDs to occur during the same period. The relaxation was modelled by the material dielectric response.
The impedance abc-model fits to the experimental results for both insulation systems containing cylindrical voids and generator bars. This includes the important fit to the lowered PDIV and the increased total apparent charge due to an increased dielectric response at high temperatures. A correlation was found between electrical PD measurements and results from microscopy investigations of cross-sections of the generator bars. A higher PD activity was correlated to a larger void content compared to bars with lower PD activity.
This correlation to void geometry is in line with results from test objects with known void geometry. Measured material properties, estimated time lag, and estimated void conductivity were used to describe the voltage frequency and temperature dependence
Detailed Grav-Kalgoorlie (P197020), gravity point data
No full textMaintenance and Update Frequency: notPlannedStatement: This Detailed Grav-Kalgoorlie (P197020), gravity point data contains ground gravity point data for the Detailed Grav-Kalgoorlie (P197020) survey acquired for None. This dataset contains a total of 2134 point data values. The data is located in WA and were acquired in 1970. The point located data were collected in irregular traverses layout at a station spacing between 400 and 800 metres.
Terrain corrections were calculated using the INTREPID Geophysics software package. The processed data are checked by GA geophysicists using standard methods for assessing quality to ensure that the final data are fit-for-purpose.
All data are provided in EPSG:4283 coordinates, Australian Height Datum (AHD) and gravity datum of AAGD07. The units are degrees, meters, and micrometres per second squared, respectively.
Reference:
Intrepid Geophysics, http://www.intrepid-geophysics.com.Gravity data measures small changes in gravity due to changes in the density of rocks beneath the Earth's surface. The data collected are processed via standard methods to ensure the response recorded is that due only to the rocks in the ground. The results produce datasets that can be interpreted to reveal the geological structure of the sub-surface. The processed data is checked for quality by GA geophysicists to ensure that the final data released by GA are fit-for-purpose.<br/> This Detailed Grav-Kalgoorlie (P197020) contains a total of 2134 point data values acquired at a spacing between 400 and 800 metres. The data is located in WA and were acquired in 1970, under project No. 197020 for None
Partial Discharges in Voids at Variable Voltage Frequency and Temperature Diagnostic Testing of Stator Mainwall Insulation
No full textThe main purpose of this thesis is to improve diagnostic testing by PD detection at variable voltage frequency by developing and experimentally testing theoretical models proposed for measurement of PDs in voids in a mica/epoxy generator bar insulation. The focus of this thesis is to compare estimated and measured values, and from that determine which physical mechanisms best describe the voltage frequency and temperature dependence of observed PD activity. The main contribution to theoretical modelling was to propose an impedance abc-model and include the dielectric response.
Two types of test object were examined: 1) laboratory made, square-shaped insulation sheets using mica/epoxy tape, made with cylindrical voids with different diameter and gap distance and 2) 50 cm long sections of service-aged generator bars.
A frequency sweep procedure for PD detection was developed, based on the relevant condition assessment standard IEC 60034-27 to study the frequency dependence experimentally. PDs were recorded during applying an increasing voltage of 10 equal steps, at reducing voltage frequency, starting at 300 Hz to 0.1 Hz for laboratory objects, and starting at 50 Hz to 0.1 Hz for service-aged bars. The temperature was varied in the range of 20 °C to 155 °C.
Experimental testing compared to the proposed model showed that the capacitive abcmodel needed to be expanded to include both conductive elements and dielectric response; the impedance abc-model, to explain a frequency and temperature dependence for the partial discharge inception voltage (PDIV) and increased residual charge relaxation at high temperatures. The impedance abc-model was also expanded by a time lag and limited discharge area to describe the apparent charge magnitude. The capacitive abc-model describes PDIV for most cases. The effect of a high conductivity and dielectric response can explain the behaviour at low frequencies according to the impedance abc-model. The maximum apparent charge is frequency-dependent, which can be explained by PD ignition at an increased voltage caused by a time lag. Measured apparent charge magnitude indicates that the void area only partly discharges during each PD. This was observed to cause a distribution of PD magnitudes, instead of a few large PDs. The total apparent charge per period is, however, close to the expected values based on the void geometry, and is frequency independent below 100 °C. The total apparent charge per period is frequency-dependent at 130 °C and 155 °C. This dependency was here modelled by an increased relaxation of residual charges, which enabled more PDs to occur during the same period. The relaxation was modelled by the material dielectric response.
The impedance abc-model fits to the experimental results for both insulation systems containing cylindrical voids and generator bars. This includes the important fit to the lowered PDIV and the increased total apparent charge due to an increased dielectric response at high temperatures. A correlation was found between electrical PD measurements and results from microscopy investigations of cross-sections of the generator bars. A higher PD activity was correlated to a larger void content compared to bars with lower PD activity.
This correlation to void geometry is in line with results from test objects with known void geometry. Measured material properties, estimated time lag, and estimated void conductivity were used to describe the voltage frequency and temperature dependence
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