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Friday, June 10, 2011

substation maintenance: Dissolved Gas Analysis for Insulating Oils

Oil-filled high voltage power equipment particularly power transformer have useful life. In order to maximize its useful life, preventive maintenance is being conducted regularly to abate equipment failure before its specified life span. One way of determining equipment failure is through dissolved gas analysis (DGA) of high voltage equipment. Let us take for example the power transformer. Big power transformers usually has a useful life of 50 years.
Since substation operators cannot see what is inside the main tank, what we can do is to analyze the insulating oil. By doing this, we can determine what is the condition of the windings inside the power transformer. In the event of failure, the force applied to the structure can distort the container and could cause transformer oil fire. Aside from the cost of reconditioning or replacing the damaged unit, unserved energy to the consumers can also add up to the losses that can be incurred. Therefore, DGA is very important in locating the developing thermal and electrical faults.

As a review, the following are different dissolved gases:
1. Hydrogen (H2)   
2. Methane (CH4)  
3. Ethane (C2H6)     
4. Ethylene (C2H4)  
5. Acetylene (C2H2)
6. Carbon Monoxide (CO)
7. Carbon Dioxide (CO2)


The following are the different interpretation of the DGA results in terms of percentage combustibles:
A. CORONA IN TRANSFORMER OIL
(hydrogen-86.0%, methane-13.0%, carbon monoxide-0.2%, ethane-0.5%, ethylene-0.2%, acetylene-0.1%)

B. OVERHEATED CELLULOSE
(carbon monoxide-92.0%, hydrogen-6.7%, methane-1.2%, ethane-0.01%, ethylene-0.01%, acetylene-0.01%)

C. TRANSFORMER OIL OVERHEATING
(ethylene-63.0%, ethane-17.0%, methane-16.0%, acetylene-trace, misc. gases-trace)

D. ARCING IN TRANSFORMER OIL
(hydrogen-60.0%, acetylene-30.0%, methane- 5.0%, ethane-1.6%, ethylene-3.3%)

Another parameter that is being considered is the high temperature range of thermal fault . This can also be determined through DGA. the following can cause high temperature range of thermal fault:
a. delaminating core of shell-form transformer (>700 degrees Celcius)
b. burnt connections in the bottom chamber (300-700 degrees Celcius)
c. burnt internal connections within windings (>700 degrees Celcius)
d. burning on the resistor (>700 degrees Celcius and high amount of ethylene gas)


Reference: Transformer Chemistry Services (TCS)

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