Transformer Oil – DGA From sampling to analysis

Presented by Miguel Ceballos

Transformer Oil – DGA From sampling to analysis

INTRODUCTION

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

Appropriate sampling techniques are indicated on ASTM D923, IEC 60567 and in many company specifications.

Thousands of gallons in a transformer tank, syringe only 30 ml., the sample must be as representative as possible.

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 60567DGA 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 round- robin 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?

• Thermal & electrical stresses• Exposure to air• Cellulosic insulation starts degrading• Contaminant induced chemical reactions

Chain breaks+

Molecular rearrangements

H2

CH4

CC22 HH44

CC22 HH66

C2 H2

CO2

CO

Gases

Generic Oil Molecule CH2CH3 CH3n

1. HYDROCARBONS AND HYDROGENMETHANE CH4ETHANE C2H6ETHYLENE C2H4ACETYLENE C2H2HYDROGEN H2

2. CARBON OXIDESCARBON MONOXIDE COCARBON DIOXIDE CO2

3. ATMOSPHERIC GASES (non fault gases)NITROGEN N2OXYGEN 02

Type of gases

• In all types of faults, hydrogen is always present.

• Faults with a very high energy content, such as in electrical arcs, toform large amounts of C2H2.

• Arcing is the most concerning type of fault condition as it typically escalates to a transformer failure.

• 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.

Type of faults – Gases involved cont.

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 energy3. 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..

Gas ratios should be calculated only if at least one gas value is above typical value.

Results interpretation – cont.

Main diagnostic methods:

- IEC ratio codes- IEEE methods ( Dornenburg, Rogers and key gases methods )- Duval Triangle

Typical values

C2H2 H2 CH4 C2H4 C2H6 CO CO2

All transformers 50-150 30-130 60-280 20-90 400-600 3800-14000

No OLTC 2-20

Communicating OLTC 60-280

(CIGRE Brochure # 296, 2006)

Typical values

C2H2 H2 CH4 C2H4 C2H6 CO CO2

All transformers 35-132 10-120 32-146 5-90 260-1060 1700-10000

No OLTC 0-4

Communicating OLTC 21-37

(CIGRE Brochure # 296, 2006)

Typical rates of gas increase for power transformers, in ppm/year

•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

Typical values - notes

The value of On-Line Monitoring

From “Preventive” to “Predictive”

1st Value : Ability to Detect a change in conditionIn 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

Detection

Monitoring

On-Line Diagnostic Only when it’s required

1 gas, H2 + moisture

2 gases, H2 + CO + moisture

5 gases, Basic on-line diagnostic

9 gases: Full DGA On-Line

Single gas or Key gas OLM

Multi-Gas OLM

As a network element, the OLM is a powerful Intelligent Electronic Device (IED) capable of transmitting information in a variety of ways.

• Via USB cable• Via Ethernet cable

Local

Remote• Analog outputs• RS-232• RS-485• Ethernet (Modbus,

DNP-3, IEC 61850)SCADA

Analog Outputs / Dry Contacts to the SCADA Network

RTU

SCADA

• 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 equipmentThe 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 StrategyOLM Deployment Strategy

Critical 2

Critical 1

New

Maximize protection of your assets…at a reasonable cost

When an alarm is triggered…the Diagnostic value is required to understand the nature and severity of the fault.

Sub-station

Detect

Monitor

When the condition assessment requires On- Line DGA to maximize protection of a faulty transformer, a Multi-gas Monitor is recommended.

Diagnose

or

Beyond DGA

DescriptionDescription ASTM NumberASTM 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

Oil Quality Tests Detect incipient faults Detect insulation degradation

Density, Color, Visual

Color, ASTM D1500, typical value 0,5

Specific Gravity, ASTM D1298, typical value 0,890

Detect free H2O + Particles, Acceptable limit for serviced aged oil: 25 kV

Limit for new oil: 30 kV

Dielectric Breakdown ASTM D877

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/cmLimit for new oil: 40 dynes/cmPresence 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 !!!

mceballos@morganschaffer.com