Power Transformers. Dissolved Gas Analysis Siemens Energy, Inc. All Rights Reserved

Transcription

1 Power Transformers Dissolved Gas Analysis 2008 Siemens Energy, Inc. All Rights Reserved

2 Application ü DGA allows to detect the presence and severity of faults: Air leaks Hot metal faults Arcing and partial discharges ü DGA may help to indirectly detect root cause of faults in: Windings (short circuits, insulation failure) Cleats and leads (high contact resistance, loose contacts) Tanks (ground problems, circulating currents) Tap changer (resistive contacts, leaks into main tank) Core (magnetic flux problems) Oxidation of materials (mostly CO, CO 2 ) 2008 Siemens Energy, Inc., 0-2

3 Key Gases The eight key gases dissolved in transformer oil which are associated with faulty conditions are: Ethylene (C 2 H 4 ) Acetylene (C 2 H 2 ) Hydrogen (H 2 ) Ethane (C 2 H 6 ) Methane (CH 4 ) Carbon Monoxide (CO) Carbon Dioxide (CO 2 ) Oxygen (O 2 ) 2008 Siemens Energy, Inc., 0-3

4 Gas Parameters 2008 Siemens Energy, Inc., 0-4

5 Typical Chromatography 2008 Siemens Energy, Inc., 0-5

6 National and International Standards IEEE Std. C IEEE Guide for the Interpretation of Gases Generated in Oil Immersed Transformers [OUT-DATED] IEEE PC Draft xx (Under Revision) - Guide for the Interpretation of Gases in Oil Immersed Transformers IEC Mineral Oil Impregnated Electrical Equipment in Service: Guide to the Interpretation of Dissolved and Free Gas Analysis 2008 Siemens Energy, Inc., 0-6

7 Key Gases 2008 Siemens Energy, Inc., 0-7

8 IEEE Reference Levels 2008 Siemens Energy, Inc., 0-8

9 IEEE Technique 2008 Siemens Energy, Inc., 0-9

10 Key Gas Technique 2008 Siemens Energy, Inc., 0-10

11 Key Gas Example Thermal Oil Fault Low Energy Partial Discharge CO H2 CH4 C2H6 C2H4 C2H2 0 CO H2 CH4 C2H6 C2H4 C2H Thermal Oil and Cellulose Fault CO H2 CH4 C2H6 C2H4 C2H High Energy Arcing CO H2 CH4 C2H6 C2H4 C2H Siemens Energy, Inc., 0-11

12 Gas Ratios ( IEC / Rogers ) 2008 Siemens Energy, Inc., 0-12

13 Comparative Diagnostic Tools Diagnostic Technique Reference Standard IEEE C IEEE PC IEC Individual & TDCG guidelines P P Doernenburg Ratios P Rogers Ratios P P Basic Gas Ratios P Key Gas Procedure P P TCG Procedure P TDCG Procedure P P Duval Triangle P CO 2 /CO Ratio P P O 2 /N 2 Ratio P C 2 H 2 /H 2 Ratio P 2008 Siemens Energy, Inc., 0-13

14 Comparative Table Unresolved, % Wrong diagnosis, % Total, % 2008 Siemens Energy, Inc., 0-14

15 IEC changes to improve accuracy Duval Triangle method (from 1970 s) Uses three gases only (CH4, C2H4 and C2H2) Low energy faults >>>> high energy faults No possibility of unresolved diagnosis High % of correct diagnosis Plots relative % of CH4, C2H4 and C2H2 Graphical, can show fault evolution over time 2008 Siemens Energy, Inc., 0-15

16 IEC & Duval 6 Different Fault types PD: Partial Discharge (gas bubbles, insulation voids, etc.) D1: Low energy discharge (spark, carbon, surface tracking) D2: High energy discharge (arcing w/metal fusion, internal flashovers, oil contamination) T1: Thermal fault < 300 degc (overload, blocked ducts) T2: Thermal fault < degc (minor circulating current, poor contacts) T3: Thermal fault > 700 degc (major circulating current in tank/core, short circuit or through fault) 2008 Siemens Energy, Inc., 0-16

17 Duval Triangle method 2008 Siemens Energy, Inc., 0-17

18 IEC & Duval Issues w-dga analysis Lab inaccuracy Median IEC in accuracy is normal concentrations North American labs appear worse For low gas concentrations, median inaccuracy is 30-50%. Diagnosis accuracy only 50%, if lab ~~ 30% Typical DGA levels 90% threshold, in-service population > focus on weakness 90% rate of DGA evolution > focus on changing conditions 2008 Siemens Energy, Inc., 0-18

19 IEC & Duval Typical DGA levels With normal concentration, attempted diagnosis + monitor *NOT* xfmr in trouble with normal concentration >> trending IEC Typical Gas Levels H2 CH4 C2H2 C2H4 C2H6 CO CO2 ppm K /year K 2008 Siemens Energy, Inc., 0-19

20 On Line DGA 2008 Siemens Energy, Inc., 0-20

21 On line DGA Bring the lab to the transformer Siemens Energy, Inc., 0-21

22 - continued Siemens Gas Guard 2008 Siemens Energy, Inc., 0-22

23 On Line DGA - continued Oil is sampled automatically via closedloop GC: - No risk of human intervention - Repeatable sampling technique - No atmospheric exposure Data is collected up to hourly: - Resulting in faster, more accurate determination of trends All 8 fault gases + moisture are monitored and correlated with oil temperature and load 2008 Siemens Energy, Inc., 0-23

24 Internal inspection revealed severely damaged crimped connection between LV cables connecting an LV winding-exit to a bushing Siemens Energy, Inc., 0-24