Dissolved gas analysis

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The study of gases from transformers can be used to give an early indication of abnormal behavior of transformer and may indicate appropriate action that may be taken on the equipment before it suffers great damage. The D.G.A. as a maintenance tool must become a part of the routine practice in the industry.

The conventional buchholz relay and gas collector relay are universally used. However, they have limitation that enough gas must be generated, first to saturate the oil full and then to come out of solution and collect in the relay, typically 10 – 15 % of gas by volume of oil, has to be generated for it to come out of the solution, often by the time the buchholz relay was never meant to be a diagnostic device. The D.G.A. is a very sensitive technique and detect gas in p.p.m. of oil. There are four basic types of faults, which can occur in the transformer. They are

• Arcing or high current break down

• Low energy sparking or partial discharges.

• Localized overheating or hot spots and

• General overheating due to inadequate cooling or sustained overloading

Each of the fault result in thermal degradation of the oil ether alone or in combination with paper insulation. This gives rise to the evaluation of various hydrocarbon gases, hydrogen and oxides of carbon, in quantities depending on the type of fault.

• Heavy current arcing is characterized by the evolution of significant quantities of hydrogen and acetylene (C2H2). If the arcing also involves paper insulation, the oxide of carbon will also be present.

• Partial discharge usually results in evolution of hydrogen and lower order hydrocarbons.

• Localized heating or hot spot gives rise to methane and Ethane in appreciable amount.

• Prolonged overloading or impaired heat transfer can cause CO and CO2 to be generated due overheating paper insulation.

To ensure uninterrupted and economical supply the trouble free performance of vital electrical equipments like power transformers during service is a matter of great importance. They are often subjected to complex environmental condition and variable thermal and electrical stresses. Efforts have been made to assess the health of the transformer during service through a series of diagnostic tests. Major Emphasis of these diagnostic tools is to detect the incipient fault prior to their developing into major faults which has obvious advantages. Various diagnostic-testing techniques can be grouped as follows:

• Traditional

• Non-Traditional

Within these categories the testing equipment or systems used can be conventionally grouped into three types

• Portable

• Continuous on-line

• Special support

Details of various diagnostic techniques usually used with transformers in service are listed in Table 1. Among non-traditional tests all of them are under development stage. In this, sufficient data are needed which should be co-related to actual field performance before it can be accepted as a confident tool. On the other hand, traditional testing is well known, among which confident of DGA is proved. DGA is one such powerful diagnostic tool which helps to detect faults at an early stage by detecting abnormal changes in the composition of gasses dissolved in the transformer oil, before the other protective gadgets like buchholz relay and the other respond. Here we will be discussing about the different stages of the Dissolved gas analysis are mentioned in brief and also some cases studies are described.

Mineral insulating oil is used in transformers to act as coolant and as dielectric medium. It deteriorates under the influence of thermal and electrical stresses and as a result evolves gasses, viz.: hydrocarbons, hydrogen, and oxides of carbons. The type of gas formation depends on the nature of stress subjected and rate of gas evolution is a factor of magnitude of stress level. Common protective devices like buchholz relay or a gas-detected relay cannot recognize these changes in gasses due to incipient fault.

Application of D.G.A:

New transformer: Typical hydrocarbon gases concentration for good new oil after vacuum filtration would be within 5 p.p.m. DGA shall be repeated once a month after commissioning and then at increasing intervals as found necessary.

Overhauled and repaired transformer: Transformer, which have been internally inspected and overhauled, shall be subjected to DGA a week after recommision and again after about three months in service.

Routine and special monitoring: DGA shall be carried out on all EHV power and distribution transformers, the periodicity of test being determined by the nature of results obtained. Transformers with concentration approaching or slightly exceeding limiting values are monitored once in three months. The monitoring interval is reduced depending upon the rate of rise of gas concentration.

Testing after an occurrence: D.G.A has proved to be reliable means of establishing the healthiness of a transformer which have tripped by suspected maloperation of differential protection (due to charging inrush or C.T Circuit problem) or Buchholz relay (due to air suction or control suction problem) can be returned to service with more confidence on the basis of D.G.A results. D.G.A shall also be carried out subsequent to oil filtration by vacuum degasification since such treatment degasifies oil and a new base reading is required to permit the further trend of gas evolution to be monitored.

In interpretation of the results obtained for a particular transformer, due regard should be given to the following factors before arriving at a specific conclusion.

1. Date of commissioning of the transformer

2. Loading cycle of the transformer

3. Date on which the oil was last filtered