Diagnosis of Electrical Performance and Aging Behavior of Transformer Dielectrics Supatra A. Bhumiwat Independent HV Diagnostics Consultant www.kea-consultant.com CIGRE Thailand 29 th November 2013, Bangkok
Agenda The Nature of Electrical Insulation or Dielectrics Classification of problems in transformer dielectrics Identification of aging in transformer dielectrics by oil analysis Identification of aging in transformer dielectrics by electrical tests 2
The Nature of Electrical Insulation Electrical Insulation is sometimes called Dielectrics due to its 2 basic electrical properties: Ability to persist in electrostatic field for a long time (High Resistance or Low Conduction) Ability to be polarized (Polarization) Conduction and Polarization Phenomena occur in every dielectric material, more or less. 3
Polarization From Walter S. Zaengl; Dielectric spectroscopy in time and frequency domains for HV power equipment, Part I: Theoretical Considerations, IEEE Electrical Insulation Magazine, vol. 19 no. 5, pp. 5-19, September/October 2003. Every kind of insulating materials consists, at an atomic level, of negative and positive charges balancing each other.. As soon as a material is exposed to an electric field, the positive and negative charges become oriented thus forming different kinds of dipoles. 4
Polarization Arrangement of charges during Polarisation 5
Polarization Recovery Voltage happens to the phenomenon of Polarisation But not to the phenomenon of Conduction 6
Equivalent Circuit of Electrical Insulation C R Rpi + Cpi Rpn +Cpn 7
Polarization 1 The charges linked with definite molecules of matter are brought into motion. These charges cannot leave the confines of a given molecule. 2 Polarization takes place in all the molecules of a dielectric and causes chemical change or deterioration 3 When the voltage applied to a dielectric is discontinued, the displaced charges may tend to return to their initial positions 4 The absorption current due to polarization decays to zero slowly under a direct voltage. Conduction 1 The free charges (carriers) can move through the entire thickness of a dielectric from one electrode to the other. 2 Conduction is often determined by the presence of impurities or contaminants. It is not attributed to its basic substance 3 The return (or recovery) voltage never happens to the phenomenon of electrical conduction. 4 The conduction current exists and keeps constant so long as a direct voltage is applied to a dielectric. From B. Tareev Physics of Dielectric Materials, MIR Publishers, Moscow, 1975 8
Each problem in a dielectric is produced by the mechanism of either Conduction or Polarization or both. An ability to diagnose and identify one from the other allows problem in a dielectric to be solved correctly. 9
Classification of problems in Transformer Insulation CLASSIFICATION OF PROBLEMS IN TRANSFORMER INSULATION Polarization Moisture in paper / pressboard Oxidation by-products (acid & non-acid type) Polar molecules in oil (polar aromatics, polar compounds, etc) Sludges at oil-paper interface Thermal aging by-products By products from Partial discharges Arcing by-products Corrosive products in oil Conduction Free water in oil Surface humidity Surface contaminants Tracking Carbon dust Metal particles in oil Debris from fault Copper sulphide 10
Deterioration of Oil by Oxidation O 2 + Unstable Hydrocarbon (in Oil) + Catalyst (.Moisture. Copper. Iron ) +Accelerators (. Heat. Vibration. Surges. Stresses ) = Oxidation by product (.peroxide. alcohols. ketones. acid ) Sludges are the accumulation of Oxidation-by-products Sludges in the main ducts block cooling system or prevent heat transfer. 11
Identification of Aging in Transformer dielectrics by Oil analysis Fault (DGA) Normal Aging of Oil - Contaminants which cause Conduction Moisture, Particles, Metal-in-oil, Dielectric Breakdown Voltage - Deterioration products, acid or non-acid type which cause Polarization Conductivity & DDF by IEC61620 (since previous decade), Acid, IFT, Inhibitors, etc. **** Tan delta & DC Resistivity (IEC 60247) or Power factor test of oil (ASTM-D924 detects the combination of Conduction and Polarization**** Normal Aging of Paper (Furans, Methanol & Ethanol ) 12
Performance of Oil based on IEC61620 13
Oil Test Results showing the Independency of Moisture-in-Oil & Oil-Conductivity (IEC61620) ID Oil Condition Oil temperature ( o C) Moisture-in-oil (ppm) Conductivity at 20 o C (ps/m) Acidity when sample is taken (Karl-Fischer method) (oil test: IEC61620) (mgkoh/g Tx-001 A 37.6 35 7.06 0.086 C 40.0 37 1.30 0.029 B + C 17.0 23 0.47 0.013 Tx-002 A 40.7 36 4.81 0.082 C 39.7 37 1.54 0.019 B + C 17.0 25 0.50 0.012 Tx-003 A 16.0 21 10.06 0.082 B 10.0 10 7.88 0.080 B + C 11.0 5 0.83 0.007 Tx-004 A 14.0 14 18.6 0.129 D 9.0 7 19.5 0.128 E 5 5 0.858 - Note: A: Before treatment, B: after vacuum dehydration, C; after clay treat, D after on-line drying, E: after oil replacement Oil Conductivity / DDF based on the new IEC61620 allows the identification of Polarisation phenomena in the insulation. So we can solve oil problem correctly. 14
Moisture (ppm) Moisture-in-oil VS Conductivity based on IEC61620 (Data from 1,207 oil samples) 1.E+02 Moisture VS Conductivity at 20 o C (Data from 1,207 oil samples) 1.E+01 1.E+00 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 Conductivity (ps/m) 15
DDF (60247) at 90 o C DDF (IEC60247) VS Conductivity based on IEC61620 (Data from 1,066 oil samples) 1.E+00 DDF (60247) at 90 o C VS Conductivity (61620) at 20 o C (Data from 1,066 oil samples 1.E-01 1.E-02 1.E-03 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 s at 20 o C (ps/m) 16
Acidity Acidity (Neutralization Number) VS Conductivity based on IEC61620 (Data from 1,066 oil samples) 1.E+00 Acidity VS Conductivity at 20 o C (Data from 1,066 oil samples) 1.E-01 1.E-02 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 Conductivity (ps/m) 17
Oil analysis results of New and Young transformers Oil from Rating of Age Water content s at 20 o C Acidity IFT at 25 o C DDF transformer transformer in service (ppm) (PS//m) (mg KOH/g) (dynes/cm) at 90 o C no. (MVA) ASTM-D1533 IEC-61620 ASTM-D974 ASTM-D971 IEC60247 [35 max] [0.03 max] [40 min] NT-001 22 5 months 3 5.03 0.004 39.00 - NT-002 13 0 3 3.299 0.003 39.7-10 1 month 9 3.644 0.005 37.6 - NT-003 13 3 months 3 3.914 0.005 37.6 - NT-004 25 2 years 7 5.50 0.01 35.4 0.0177 NT-005 25 4 years 6 16.39 0.01 46.4 0.0483 NT-006 20 0 5 0.03 0.003 39.4 - NT-007 20 1 month 3 0.03 0.001 41.0 - NT-008 20 1 year 7 0.12 0.001 35.7 - NT-009 20 1 year 3 0.10 0.001 39.0-18
Classification of oil according to its Conductivity (Test method of new IEC61620) New unused oil (Laboratory quality) < 0.05 ps/m New unused oil (Industrial quality) 0.05..0.10 ps/m Light used oil in good condition 0.1.1.0 ps/m Middle used oil in acceptable condition 1.0.5.0 ps/m Heavy used oil in bad condition > 5.0 ps/m 19
Identification of Aging in Transformer dielectrics by Electrical Test It is very important that test voltage for in-service transformer (or any in-service power equipment) is non-destructive because insulating materials are service-aged more or less and we do not want any diagnosis to further deteriorate or shorten its life. Since many years problems or aging caused by Polarization and Conduction are diagnosed in combination through Insulation Resistance Measurement (with Polarization Index). Power factor test or Tan delta test also does not identify Conduction and Polarization in Dielectrics. Since the last decade, Dielectric Response Analysis by Polarization Depolarization Current (PDC) analysis identifies Conduction and Polarization in Dielectrics (not only for transformers). 20
Currents during Insulation Resistance Measurement The steady-state Conduction Current is caused by Conduction phenomena. The time-dependant Absorption Current is caused by Polarization phenomena. So Insulation Resistance measurement detects both Conduction and Polarization 21
Principle of Test arrangement for PDC measurement Current measurement: from 1 pa HV equipment can be tested at low voltage (e.g. 50V) 22
Current (A) Insulation between windings a New Power Transformer 1.E-07 Test Voltage 100V 1.E-08 1.E-09 1.E-10 1.E-11 Polarisation Current Depolarisation Current Time (s) 1 10 100 1,000 10,000 Measurement results of Polarisation Depolarisation Currents 23
Current (A). Influence of Oil conductivity 1E-7 s oil = 1.6 ps/m s oil = 0.4 ps/m i pol i depol measured 1E-8 1E-9 s oil = 0.1 ps/m new 3-phase power transformer U c = 1'000 V T c = 5'000 s q ~ 15 C pressboard moisture content = 1.0% 1E+0 1E+1 1E+2 1E+3 Time (s) 24
Current (A). Influence of Moisture in Pressboard 1E-7 i pol i depol 1E-8 1E-9 m. c.: 2.5% m. c.: 1.0% new 3-phase power transformer U c = 1'000 V T c = 5'000 s measured q ~ 15 C m. c.: 0.2% s oil = 0.4 ps/m 1E-10 1E+0 1E+1 1E+2 1E+3 Time (s) 25
Insulation between windings Power Transformers Measurement results of Polarisation Depolarisation Currents 26
Insulation between windings 3f, 20 MVA, 33/11 kv Transformer PDC Test on Transformer without oil (Teat Voltage 100 V) Measurement results of Polarisation Depolarisation Currents 27
Current (A) Current (A) Current (A) Current (A) Current (A) Current (A) Current (A) Current (A) Current (A) Some PDC Measurement results 1.E-07 PDC at 100V, 18 o C 1.E-07 PDC at 500V, 5 o C 1.E-08 PDC at 100V, 25 o C 1.E-08 1.E-09 1.E-08 1.E-09 1.E-10 1.E-11 (5.1) I pol. I depol. 1 10 100 1,000 10,000 Time (s) 1.E-09 (5.2) I pol. I depol. 1 10 100 1,000 10,000 Time (s) 1.E-10 (5.3) I pol. I depol. 1 10 100 1,000 10,000 Time (s) 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 (5.4) PDC at 100V, 15 o C I pol. I depol. 1 10 100 1,000 10,000 Time (s) 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 (5.5) I pol. I depol. PDC at 100V, 23 o C 1 10 100 1,000 10,000 Time (s) 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 (5.6) I pol. I depol. PDC at 100V, 26 o C 1 10 100 1,000 10,000 Time (s) 1.E-08 1.E-09 PDC at 50V, 20 o C (no oil) 1.E-07 1.E-08 PDC at 50V, 24 o C (no oil) 1.E-07 1.E-08 PDC at 50V, 30 o C (no oil) 1.E-10 1.E-11 (5.7) T4 T5 1 10 100 1,000 10,000 Time (s) 1.E-09 1.E-10 1.E-11 (5.8) I pol. I depol. 1 10 100 1,000 10,000 Time (s) 1.E-09 1.E-10 1.E-11 (5.9) I pol. I depol. 1 10 100 1,000 10,000 Time (s) From: S. Bhumiwat, S. Lowe, P. Nething, J. Perera, P. Wickramasuriya, P. Kuansatit, Performance of oil and paper in transformers based on IEC61620 and dielectric response techniques. IEEE EI Magazine, vol. 26 no. 3, May/June 2010, pp. 16-23. (can be downloaded from www.kea-consultant.com) 28
PDC Measurement Results of a Transformer which has Overheating in the Solid Insulation between windings From a case study in Identification of overheating in transformer solid insulation by Polarization Depolarization Current Analysis by Supatra A. Bhumiwat, in 2013 Electrical Insulation Conference, Ottawa, Canada, 2-5 June 2013 (can be downloaded from www.kea-consultant.com) 29
Depolarization Index (dp.i.) from PDC (to determine overheating of solid insulation) From a case study in Identification of overheating in transformer solid insulation by Polarization Depolarization Current Analysis by Supatra A. Bhumiwat, in 2013 Electrical Insulation Conference, Ottawa, Canada, 2-5 June 2013 (can be downloaded from www.kea-consultant.com) 30
Evaluation of PDC Measurement Results Oil Conductivity (only oil-paper transformer) Moisture in paper (any oil-paper equipment) Insulation Resistance and Polarisation Index Frequency scan of Capacitance Frequency scan of DDF (tan d) Recovery Voltage Polarisation Spectrum 31
Resistance (W ) Evaluation of Insulation Resistance between windings and P.I. 1.E+12 1.E+11 1.E+10 Time (s) 1 10 100 1,000 10,000 Polarisation Current 32
Insulation between windings Power Transformers R 1 min. 72.1 GW R 10 min. 99.8 GW PI 1.38 tan d at 1 Hz 0.09% Moisture in pressboard 1.0% Oil conductivity at 20 o C 0.32 ps/m R 1 min. 63.5 GW R 10 min. 78.6 GW PI 1.24 tan d at 1 Hz 0.09% Moisture in pressboard 1.5% Oil conductivity at 20 o C 0.12 ps/m Polarisation Index is not a key to judge the wetness of Transformer insulation. 33
PDC Evaluation of Capacitance and DDF 34
C' (pf).. Capacitance and DDF 400 300 2mm pressboard, q = 20 C unaged (m.c.: 4.0%) unaged (m.c.: 2.5%) C' unaged (m.c.: 1.0%) unaged (m.c.: 0.2%) 1E+0 1E-1 tand 1E-2 unaged (m.c.: 4.0%) unaged (m.c.: 2.5%) tand unaged (m.c.: 1.0%) unaged (m.c.: 0.2%) 200 1E-4 1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 Frequency (Hz) 1E-3 2mm pressboard, q = 20 C 1E-4 1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 Frequency (Hz) C & tan delta is more sensitive at lower frequencies than at 50 Hz for the diagnosis of aging in dielectrics. Tan delta tells you good / bad but does not tell you why. Deterioration in insulating materials increase Capacitance at low frequencies. Since previous decade (at the same time of PDC), there has been a diagnostic tool which determines moisture in paper/pressboard based on frequency domain. 35
C ratio C ratio C ratio DDF DDF DDF PDC Evaluation results of DDF and C ratio 1.E+02 1.E+01 T4 at 15 o C T5 at 23 o C 1.E+01 1.E+00 T6 at 26 o C 1.E+00 T4 (no oil) T5 (no oil) 1.E+00 1.E-01 1.E-02 1.E+02 (6.1) 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 Frequency (Hz) 1.E-01 1.E-02 1.E+02 (6.2) 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 Frequency (Hz) 1.E-01 1.E-02 1.E+01 (6.3) 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 Frequency (Hz) 1.E+01 (6.4) 1.E+00 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 Frequency (Hz) T4 T5 1.E+01 (6.5) T6 1.E+00 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 Frequency (Hz) (6.6) T4 (no oil) T5 (no oil) 1.E+00 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 Frequency (Hz) From: S. Bhumiwat, S. Lowe, P. Nething, J. Perera, P. Wickramasuriya, P. Kuansatit, Performance of oil and paper in transformers based on IEC61620 and dielectric response techniques. IEEE EI Magazine, vol. 26 no. 3, May/June 2010, pp. 16-23. (can be downloaded from www.kea-consultant.com) 36
Conclusion 1. Every problem in a dielectric is produced by mechanism of Conduction or Polarization or both. Aging behavior of transformer dielectric can be classified by its nature. 2. It is important to choose diagnostic tool which can identify problem or identify the aging type of the dielectric, in order to solve the problem correctly so insulation life can be prolonged. 3. The use of non-destructive technique or low voltage methods ensures that the trouble or fault is not modified or worsen by the tests. 37
THANK YOU Publications can be downloaded from: www.kea-consultant.com More questions? Please contact supatra@ieee.org 38