Transformer Oil – DGA From sampling to analysis
Presented by Miguel Ceballos
INTRODUCTION Transformer Oil – DGA From sampling to analysis During the last decades Dissolved Gas Analysis (DGA) has become the most important preventive routine for power transformers
Why DGA is important? •Simple, non intrusive oil analysis, that can predict faults via the analysis of the gases generated on the insulating oil. •The types of dissolved gases in the oil, the amount, relative proportions and changes over time give us clues about what is happening in the transformer
DGA – 3 Key steps •Oil sample extraction •Extraction of the gases •Gases separation & measurement (gas chromatography)
Every DGA starts with the sampling Thousands of gallons in a transformer tank, syringe only 30 ml., the sample must be as representative as possible. Appropriate sampling techniques are indicated on ASTM D923, IEC 60567 and in many company specifications.
Key considerations •Atmospheric conditions •Positive pressure on the tank •Appropriate containers •Accessories (Tygon® tubing, valves) •Adequate syringe handling
Syringes, tubing & Accessories
Flushing about 2 liters to ensure there is no free water or visible contaminants
Allow the pressure in the tank to fill the syringe up to the 10 cc mark (don’t pull the piston)
Push the piston gently to rinse the barrel and remove the bubbles
Fill the syringe up to 26 cc mark to ensure an adequate seal along the piston
Push gently to remove any possible bubbles, close the valves and proceed with the labeling and packaging
Adequate packaging
ASTM D3612 / IEC 60567 DGA extraction methods •ASTM D3612-A “vacuum extraction” •ASTM D3612-B “stripping method” •ASTM D3612-C “head space analysis”
Analytical results must be precise and accurate CIGRE and IEC Round Robin tests (RRTs) have shown that the repeatability (precision) of DGA labs is generally good, but that their deviation from true value (accuracy) is often poor*
* http://www.electricity-today.com/et/issue0602/i06_standards.htm
Every Laboratory should be able to demonstrate their precision and accuracy.
Accuracy of IEC/CIGRE laboratories, from roundrobin tests using DGA standards. Medium gas concentrations
Low gas concentrations
Best lab
±3%
±22%
Average
±15%
±30%
Worst Lab
±65%
±64%
ASTM 3612: Standards
Regular use of standards required by ASTM
Diagnostic reliability is affected by the accuracy of the DGA measurement results
CIGRE result for Round Robin Test (RRT) at low concentration levels. Results of individual laboratories (.) and prepares DGA standard value (x)
Improving the reliability of Transformer gas-in-oil diagnosis – M. Duval, J. Dukarm - 2005
How are fault gases produced? Generic Oil Molecule
CH3 CH2 CH3 n
• Thermal & electrical stresses • Exposure to air • Cellulosic insulation starts degrading • Contaminant induced chemical reactions
H2 CH4 C2H6 C2H4
Chain breaks + Molecular rearrangements
Gases
C2H2 CO2 CO
Type of gases 1. HYDROCARBONS AND HYDROGEN METHANE CH4 ETHANE C2H6 ETHYLENE C2H4 ACETYLENE C2H2 HYDROGEN H2
2. CARBON OXIDES CARBON MONOXIDE CO CARBON DIOXIDE CO2
3. ATMOSPHERIC GASES (non fault gases) NITROGEN N2 OXYGEN 02
Type of faults – Gases involved cont. •
In all types of faults, hydrogen is always present.
•
Low energy faults such as corona partial discharges in gas bubbles, or low temperature hot spots, will form mainly H2 and CH4.
•
Faults of higher temperatures are necessary to form large quantities of C2H4.
•
Faults with a very high energy content, such as in electrical arcs, to form large amounts of C2H2.
•
Arcing is the most concerning type of fault condition as it typically escalates to a transformer failure.
DGA results would allow us to identify the type of fault occurring in a transformer in service.
Type of faults – IEC 60599 1. PD- Partial Discharges (corona) 2. D1 - Discharges of low energy 3. D2 - Discharges of high energy 4. T1 - Thermal faults < 300° 5. T2 - Thermal faults > 300°< 700° 6. T3 - Thermal faults > 700°
What do we do with the data? Many results interpretation techniques • IEC 60599 Ratios • IEEE C57.104, Limits, rates and TDCG • Rogers Ratios • Key Gas Method • Duval Triangle • Trend Analysis • “NEW GUIDELINES FOR INTERPRETATION OF DGA” CIGRE Task force 15.01.01,Octr 1999 • Companies guidelines • More..
Results interpretation – cont. Main diagnostic methods: - IEC ratio codes - IEEE methods ( Dornenburg, Rogers and key gases methods ) - Duval Triangle
Gas ratios should be calculated only if at least one gas value is above typical value.
Typical values (CIGRE Brochure # 296, 2006) C2H2 All transformers
H2 50-150
No OLTC
2-20
Communicating OLTC
60-280
CH4
C2H4
30-130
60-280
C2H6 20-90
CO 400-600
CO2 3800-14000
Typical values Typical rates of gas increase for power transformers, in ppm/year
(CIGRE Brochure # 296, 2006) C2H2 All transformers
H2 35-132
No OLTC
0-4
Communicating OLTC
21-37
CH4
C2H4
10-120
32-146
C2H6 5-90
CO 260-1060
CO2 1700-10000
Typical values - notes •Transformers are all different; •The exact value of the DGA reading is not as important as the trend. If the DGA is at a moderate level but holding steady, there is little concern. On-line monitoring systems will ensure that things stay in check. However, low DGA levels which are trending upward are a higher concern; • When the transformer is first energized, the DGA values will trend toward the “typical” value for that transformer and it should then stabilize; • The actions to be taken after analysis depends on how fast the problem is escalating, the criticality of the transformer and the alternatives available; •The risk of taking the transformer out of service too soon may result in an internal inspection which turns up no evidence; •Waiting too long risks a complete transformer failure. Situations where there is N+1 redundancy provide the luxury of taking the transformer off line for internal inspections with no impact to the load. Mobile transformers provide a similar benefit but can often require hours (or days) to coordinate getting them in place and ready
The value of On-Line Monitoring From “Preventive” to “Predictive” 1st Value : Ability to Detect a change in condition In ALL cases = Protection Value
2nd Value : Ability to Monitor the Evolution of the Condition = Monitoring Value.
3rd Value :
Ability to Diagnose the nature of the “bad” condition = Diagnostic Value
On-Line Monitors Single gas or Key gas OLM
1 gas, H2 + moisture
2 gases, H2 + CO + moisture
Detection Monitoring
Multi-Gas OLM
On-Line Diagnostic Only when it’s required
5 gases, Basic on-line diagnostic 9 gases: Full DGA On-Line
As a network element, the OLM is a powerful Intelligent Electronic Device (IED) capable of transmitting information in a variety of ways. Local • •
Via USB cable Via Ethernet cable
Remote
SCADA
• • • •
Analog outputs RS-232 RS-485 Ethernet (Modbus, DNP-3, IEC 61850)
Analog Outputs / Dry Contacts to the SCADA Network
SCADA RTU •
4-20mA outputs Gas, Moisture, Temperature levels
•
NO/NC Relays Gas alarm, Moisture alarm, Temp alarm, Low Carrier gas, Any Alarm, Any error, Always.
Stable H2 concentration at 500 ppm, 6-month period
Increasing H2 concentration, 100 ppm/month, 1 month period
Sudden increase in H2 concentration, up to 30ppm/hour
Sudden change in H2 generation rate
Portable DGA equipment The best complement for the OLM Portable systems allow having a complete DGA analysis on the spot in two minutes, with lab comparable results, using validated gas extraction and separation techniques.
OLM Deployment Strategy Maximize protection of your assets…at a reasonable cost Detect New
Monitor Diagnose Sub-station
Critical 1
Critical 2
or
When an alarm is triggered…the Diagnostic value is required to understand the nature and severity of the fault.
When the condition assessment requires OnLine DGA to maximize protection of a faulty transformer, a Multi-gas Monitor is recommended.
Beyond DGA Description
Oil Quality Tests
ASTM Number
Dielectric Strength
D877/D-1816
Acidity
D-974
Interfacial Tension
D-971
Color
D-1500
Water Content
D-1533
Density
D-1298
Visual Examination
D-1524
Power Factor
D-924
Inhibitor Content
D-4768
Dissolved Gas Analysis
D-3612
Furan Analysis
D-5837
Detect incipient faults
Detect insulation degradation
Density, Color, Visual
Color, ASTM D1500, typical value 0,5 Specific Gravity, ASTM D1298, typical value 0,890
Dielectric Breakdown ASTM D877
Detect free H2O + Particles, Acceptable limit for serviced aged oil: 25 kV Limit for new oil: 30 kV
Neutralization Number (Acidity) ASTM D974 Acceptable limit for serviced aged oil: 0.2 mg KOH/g Limit for new oil: 0.03 mg KOH/g The acidity is caused by oxidation byproducts called polar compounds
Interfacial Tension ASTM D971
Acceptable limit for serviced aged oil: 18 dynes/cm Limit for new oil: 40 dynes/cm Presence of contaminants
Water Content ASTM D1500 Karl Fisher Titration Acceptable limit: 35 ppm Tank breathing or paper degradation
Inhibitor Content Oxidation increase as inhibitor is consumed, controlling the inhibitor content extends the life of the oil.
Furan Analysis ASTM D5837 Furanic compounds are produced as the solid insulation (cellulose) deteriorates; measuring the concentration of those compounds gives indication on the condition of the solid insulation.
Corrosive Sulfur ASTM D1275 The ASTM 1275 method consist basically in a copper strip immersed in oil for several hours at high temperature. Method A: 19 hours at 140 C° Method B: 48 hours at 150 C°
DGA Summary, Conclusions • Sampling Key considerations / best practices • Analytical results Precision, Accuracy • Interpretation Many tools, typical values, trends, DSS (decision support systems) •The value of OLM From Preventive to Predictive •Beyond DGA
Thank you !!!
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