01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 1
Members of JWG A2/D1.41 HVDC transformer insulation: Oil conductivity A. Küchler, Convenor (DE) U. Piovan, Secretary (IT) M. Berglund (SE) G. Chen (UK) A. Denat (FR) J. Fabian (AT) R. Fritsche (DE) T. Grav (NO) S. Gubanski (SE) M. Kadowaki (JP) Ch. Krause (CH) A. Langens (DE) S. Mori (JP) B. Noirhomme (CA) H. Okubo (JP) M. Rösner (DE) F. Scatiggio (IT) J. Schiessling (SE) F. Schober (DE) P. Smith (UK) P. Wedin (SE) Corresponding Members I. Atanasova Höhlein (DE) Ch. Perrier (FR) 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 2
Final Report of JWG A2/D1.41 HVDC transformer insulation: Oil conductivity Contents of the Tutorial 1. Introduction 2. Measurement Techniques and Standards 3. Concept for Oil Conductivity Measurements 4. Oil Conductivity Values at Production and in Service 5. Concept for Pressboard Conductivity Measurements 6. Dielectric Test Effectiveness and Reliability 7. Conclusions and Suggestions Annexes in the TB 646: A B Recommendations for Oil Conductivity Measurements Recommendations for Oil impregnated Pressboard Conductivity Measurements 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 3
HVDC insulation systems HVDC (high voltage direct current) transmission is getting important. Effective way of energy transport by using direct current Insulation systems of HVDC transformers mainly consist of (mineral) oil and oil impregnated pressboard in barrier arrangements. Insulating materials are stressed both with AC and DC voltage. AC field DC field basically determined by permittivities ε electrical conductivities σ 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 4
Background: JWG A2/B4.28 HVDC Converter Transformers Design review is difficult to perform. Test procedures are difficult to define. Aging and reliability in service is difficult to asses for HVDC transformers. From field calculation studies, it was concluded that oil conductivity is the dominant factor. JWG A2/D1.41 HVDC transformer insulation: Oil conductivity was initiated. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 5
JWG A2/D1.41 HVDC Transformer Insulation: Oil Conductivity Term of References (I): 1. Review available literature on conduction mechanisms in organic liquids, in systems with uncovered and covered electrodes. 2. Review techniques and standards for measurement of conductivity of liquids, and how representative they are for conditions in an oil gap in a composite HVDC insulation system. RRT 1 3. Give recommendation of sampling and handling of samples of oil taken from service. 4. Guidance for evaluation and interpretation. 5. Advise on possibilities for a simple and representative test of oil quality to be used by suppliers, OEMs and End Users. This is to be used for acceptance tests and during service, if possible. RRT 2 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 6
JWG A2/D1.41 HVDC Transformer Insulation: Oil Conductivity Term of References (II): 6. Advise a test procedure for measurement of conductivity of oil impregnated pressboard. RRT 4 7. Perform a campaign of measurements to determine oil conductivity values of HVDC transformers at production and in service. RRT 3 8. Analyze the impact of the values found in respect of dielectric test effectiveness and reliability. Simulations 9. Suggestions for new standards, if possible. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 7
Methods for conductivity determination Standard methods: Standards IEC 60247 (oil) IEC 61620 (oil) IEC 60093 (pressboard) Electrical stress 0.25 kv/mm 0.1 kv/mm 0.1, 0.5, 1 kv, sometimes up to 15 kv Time of electrification t = 60 s 0.45 s t 5 s 1, 2, 5, 10, 50 or 100 min For oils: Short times of electrification and low electrical stress Different from HVDC conditions (steady state, high electrical stress) Non standard methods: mainly based on FDC (frequency domain spectroscopy) and PDC (polarization / depolarization current) analysis Comparison of methods by the JWG:RRT 1 on unused mineral oils 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 8 8
Results of RRT 1 on unused mineral oil comparison of methods 6.5 1.9 4.7 1.9 10 1.914.1 1.9 8 8 11.2 2.2 1.9 1.9 DC Rect. Median 14.3 12.4 Fig. 1: Apparent conductivity of mineral oil at 25 C and at 0.1 to <1 kv/mm 1 10 100 1000 10000 100000 t in s RRT 1 results from different laboratories and different methods Huge spread (3 orders of magnitude) at different times of electrification and different electrical stresses using various measuring methods Development of recommendations for conductivity determination is necessary to reduce the spread! 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 9
Development of conductivity measuring concept Measuring method PDC analysis Recommendations for oil sample handling and measurement parameters Verification of spread reduction RRT 2 Recommended characterisation of conduction processes in oil by the so called Three Stress Points Characterisation Concept 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 10
PDC analysis for oil v Test voltage (1) (2) (3) t 0 Conduction and polarisation current i c ( t) + ip ( t ) t c id ( t ) Depolarisation current t PDC analysis is a step response measurement in time domain (voltage applied, current measured). Current is caused by time dependent conduction due to ion drift. Conductivity: idc d DC A: Electrode area d: Sample thickness V A 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 11
Example of test cell and guard electrode arrangement for PDC measurements Glass vessel additional weight, optional for oil, required for PB Measuring cable Temperature sensor Glass plate Guard ring Oil Spacers Measuring electrode High-voltage terminal Oil gap High-voltage electrode Measuring electrode with guard ring arrangement Hermetically enclosed system 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 12
Field calculations at electrode edges Electrode edges need rounding in order to avoid field enhancements 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 13
RRT 2: Oil conductivity of new, unused mineral oil at 30 C and 90 C Conductivity of Oil A at 30 C and at a time of electrification of 3600 s No. 1 No. 2 No. 3 No. 5 No. 6 No. 8 Conductivity of Oil A at 90 C and at a time of electrification of 3600 s No. 1 No. 2 No. 3 No. 5 No. 6 Apparent conductivity of an unused mineral oil at a time of electrification of 3600 s and at different temperatures, RRT results from different laboratories Compared with RRT 1, spread was reduced remarkably in RRT 2 by one order of magnitude (at 90 C) to two orders of magnitude (at 30 C). 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 14
RRT 2: Oil conductivity of used mineral oil at 30 C and 90 C Conductivity of used Oil D at 30 C and at a time of electrification of 3600 s No. 1 No. 2 No. 3 No. 5 No. 8 Conductivity of used Oil D at 90 C and at a time of electrification of 3600 s No. 3 No. 5 Apparent conductivity of a used mineral oil at a time of electrification of 3600 s and at different temperatures, RRT results from different laboratories. Used oil from Canada out of a single phase 345/55/55 kv, 134 MVA transformer manufactured in 1985 (HVDC, back to back installation). Significantly reduced spread. Oil conductivity was higher for the used oil than for the unused oil. Conductivity may change over transformer life and should be monitored. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 15
Conduction behaviour of mineral oil (1): Initial conductivity at 1 s, comparable to AC conductivity and to conductivity according to IEC 61620. (4): Conductivity at 1 min, according to IEC 60247 (2), (3): Long-duration or steady-state DC conductivity at 10000 s, related to HVDC conditions. [S/m] 1E-08 1E-09 1E-10 1E-11 0,1 kv/mm 0,3 kv/mm 1 kv/mm 2 kv/mm 3 kv/mm 6 kv/mm 10 kv/mm (1). at low field strength 1E-12 (3). at high field strength 1E-13 1E-14 (2). at medium field strength 1E-15 1E-16 1E-17 1 10 100 1000 10000 Apparent oil conductivity vs. time of electrification at different field stresses t / s t [s] 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 16
Conduction behaviour of mineral oil (RRT 2) Example for apparent conductivity vs. field stress at different times of electrification: 1 s (unfilled symbols) and 3600 s (filled symbols). New, unused mineral oil. RRT results from different laboratories. Maximum values Initial conductivity at low electrical stress (1) Minimum values Long duration conductivity at medium stress (2) High stress behaviour Long duration conductivity at high stress (3) Three Stress Points Characterisation Concept 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 17
Three Stress Points Characterisation Concept for oils 3 1 2 Stress points E Time of electrification Electrical Stress E (1) Initial conductivity at low electrical stress 1 s 0.1 kv/mm (2) Long duration conductivity at medium electrical stress 3600 s 1 kv/mm (3) Long duration conductivity at high electrical stress 3600 s 6 kv/mm Compared with RRT 1, spread was reduced remarkably in RRT 2 by one order of magnitude (at 90 C) to two (at 30 C) orders of magnitude. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 18
Aim of RRT 3 Evaluation of conductivity values for oils taken from transformers at different stages of production and in service (1) Oils sampled before dielectric tests in factory: Oil E from 500 kv / 300 MVA HVDC transformer A Oil B from 400 kv / 261 MVA HVDC transformer B Oil F from 400 kv / 300 MVA HVDC transformer C (2) Oils sampled before transformer energisation on site: Oil E from 500 kv / 300 MVA HVDC transformer A Oil B from 400 kv / 261 MVA HVDC transformer B (3) Oils sampled after many years in service: Used Oil G from AC transformer (UK, in service since 1980) Used Oil D from HVDC transformer (Canada, in service since 1985) Note: Used Oil D in RRT 2 is the same oil type, but it came from a brother transformer. Used Oil H from HVDC transformer (Norway, in service since 1975) 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 19
Oil conductivity at different stages of transformer life Oil E, 30 C (RRT 3) Measurements according to the Three Stress Points Characterisation Concept Prior to transformer testing in factory Conductivity of Oil E before transformer testing at 30 C No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 1 t 1 s 2 3 Before transformer energisation on site Conductivity of Oil E before energisation on site at 30 C No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 1 t 1 s 2 3 No significant difference was observed between conductivity values prior to testing in factory and before energisation on site. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 20
Oil conductivity at different stages of transformer life Oil E, 90 C (RRT 3) Measurements according to the Three Stress Points Characterisation Concept Prior to transformer testing in factory Conductivity of Oil E before transformer testing at 90 C 1 No. 1 No. 2 No. 3 No. 5 No. 6 t 1 s 3 2 Before transformer energisation on site Conductivity of Oil E before energisation on site at 90 C 1 t 1 s 2 No. 1 No. 2 No. 3 No. 5 No. 6 3 No significant difference was observed between conductivity values prior to testing in factory and before energisation on site. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 21
Oil conductivity at different stages of transformer life Oil B, 30 C (RRT 3) Measurements according to the Three Stress Points Characterisation Concept Prior to transformer testing in factory Conductivity of Oil B before transformer testing at 30 C No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 1 t 1 s 2 3 Before transformer energisation on site Conductivity of Oil B before energisation on site at 30 C No. 1 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 1 t 1 s 2 3 No significant difference was observed between conductivity values prior to testing in factory and before energisation on site. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 22
Oil conductivity at different stages of transformer life Oil B, 90 C (RRT 3) Measurements according to the Three Stress Points Characterisation Concept Prior to transformer testing in factory Conductivity of Oil B before transformer testing at 90 C No. 1 No. 2 No. 3 No. 5 No. 6 1 t 1 s 2 3 Before transformer energisation on site Conductivity of Oil B before energisation on site at 90 C No. 1 No. 3 No. 5 No. 6 No. 8 1 t 1 s 2 3 No significant difference was observed between conductivity values prior to testing in factory and before energisation on site. Reduced spread at 90 C. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 23
Oil conductivity, Oil F prior to transformer testing, 30 C and 90 C (RRT 3) Measurements according to the Three Stress Points Characterisation Concept 30 C prior to transformer testing in factory 90 C Conductivity of OIl F before transformer testing at 30 C No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 1 t 1 s 2 3 Conductivity of Oil F before transformer testing at 90 C No. 1 No. 2 No. 3 No. 5 No. 6 1 t 1 s 2 3 No samples of Oil F have been provided before transformer energisation on site. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 24
Used Oil G from AC transformer in service since 1980 in UK (RRT 3) Measurements according to the Three Stress Points Characterisation Concept 30 C used oil 90 C 1 Conductivity of used Oil G at 30 C t 1 s 2 No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 3 Conductivity of used Oil G at 90 C 3 2 1 t 1 s No. 1 No. 2 No. 3 No. 5 No. 6 0 1 2 3 4 5 6 Used oil from an old AC transformer exhibited a high conductivity compared to new, unused oils of today (One to two orders of magnitude higher). 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 25
Used Oil D from HVDC transformer in service since 1985 in Canada (RRT 3) Measurements according to the Three Stress Points Characterisation Concept 30 C used oil 90 C 1 Conductivity of used Oil D at 30 C t 1 s 2 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 3 t 1 s 1 2 Conductivity of used Oil D at 90 C No. 2 No. 3 No. 5 No. 6 3 Used oil from an HVDC transformer exhibits higher conductivity values than new, unused oils of today. Determination of oil conductivity over transformer life is recommended. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 26
Used Oil H from HVDC transformer in service since 1975 in Norway (RRT 3) Measurements according to the Three Stress Points Characterisation Concept 30 C used oil 90 C 1 Conductivity of used Oil H at 30 C No. 2 No. 3 No. 4 No. 5 No. 8 1 Conductivity of used Oil H at 90 C t 1 s No. 2 No. 3 No. 5 No. 8 t 1 s 2 3 2 3 Original conductivity at commissioning of the transformer is not known. Determination of oil conductivity over transformer life is recommended. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 27
PDC analysis for oil impregnated pressboard v Test voltage (1) (2) (3) t Glass vessel additional weight, optional for oil, required for PB Measuring cable Temperature sensor Conduction and polarisation current Glass plate Guard ring i ( t) c + ip ( t ) t c Oil 0 id ( t ) Depolarisation current Step response measurement in time domain (PDC analysis) Current is mainly caused by polarization processes. DC conductivity can only be calculated if steady state is reached. before that only an apparent conductivity can be given Apparent conductivity: t app ( t) Measuring electrode High-voltage terminal Oil-impregnated pressboard sample High-voltage electrode Example of a test cell for PDC measurements 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 28 i( t) V d A
Conductivity determination of oil impregnated pressboard (RRT 4) No. 2 No. 3 No. 4 No. 5 No. 7 30 40 50 60 70 80 90 T in C Apparent conductivity of oilimpregnated pressboard, measured at the recommended parameters: 30 C, 1 kv/mm and electrification time or 3 h RRT 4 results from different laboratories Apparent conductivity increases exponentially with temperature. Spread of apparent conductivity values is very low compared to spread of oil conductivity values. Spread in oil conductivity measurements is mainly caused by the oil, by the test cell and its treatment and not by the measurement procedure. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 29
Field simulations with measured conductivity values in order to investigate effectiveness of dielectric tests Aim of the simulations: Field stresses as a function of voltage waveshapes Parameters for a six barrier insulation model in S/m. in S/m Voltage waveshape (pos./neg./pos.) 10-14 10-15 600 kv, 90/ 90/ 45 min 10-12 10-15 600 kv, 90/ 90/ 45 min 10-14 10-15 600 kv, 360/360/180 min 10-12 10-15 600 kv, 360/360/180 min 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 30
Field simulation geometry for a six barrier insulation model Note: Also a four barrier model was investigated giving very similar results 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 31
Calculated field stresses (creep) in a six barrier model For low conductive oil, longer polarisation times lead to higher stresses 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 32
Calculated field stresses (maximum) in a six barrier model For low conductive oil, longer polarisation times lead to higher stresses 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 33
Conclusions and suggestions (1) (1) Conductivities of oil and oil impregnated pressboard cannot be measured according to the standard procedures. (2) Oil conductivity shall be measured according to the JWG recommendations based on the Three Stress Points Charaterisation Concept (Annex A of the TB 646). Using the Three Stress Points Characterisation Concept, spread was reduced and material properties were achieved that are relevant for HVDC test and service stresses. (3) Oil impregnated pressboard conductivity shall be measured according to the JWG recommendations (Annex B of the TB 646). A very low spread was achieved. Spread of oil conductivity measurements is not caused by the measurement procedure but by the oil itself or by its treatment in the laboratories. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 34
Conclusions and suggestions (2) (4) Oil conductivity shall be measured after testing, after commissioning and throughout the lifetime of a transformer. Conductivity of the oil from the transformer during factory acceptance test shall be in the same range (ratio less than a factor of 10) as the conductivity of the oil from the transformer right after commissioning in order to achieve relevance of test. This should be reported to IEC. In case of exceeding this limit, agreement shall be found between manufacturer and producer based on information about insulation system robustness discussed at design review stage. (5) The physical behaviour of oil is not sufficiently described just by conductivity. Ion drift and space charge may have a complex influence on electrical stress distribution. Therefore, material models based on the RC model (conductivity/permittivity model) give first approximation results only. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 35
Conclusions and suggestions (3) (6) Transient field calculations based on these RC material models indicate that time constants of insulation systems can be long in comparison with standard test durations, especially for low conductive oils. In oils with a very low conductivity, more physical processes than just conduction alone come into play when trying to determine the effect of an applied electrical stress. Examples of such physical processes are: Ion generation (strongly non linear for high electrical stress) and transport, charge injection/interaction with pressboard interface or Electro Hydro Dynamic (EHD) movement of oil. (7) In order to achieve effective test, electrical stresses can be increased by increasing PR test voltage or duration or a combination of both which has been discussed in a previous working group A2/B4.28. (8) The working group agrees that polarisation times are getting longer for low conductive oils. But unfortunately, the JWG has no clear quantitative indication on how to change PR test. 01.03.2016 JWG A2/D1.41 HVDC transformer insulation: Oil conductivity 36
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