E+C August 2018

TRANSFORMERS + SUBSTATIONS

pockets will promote further partial discharges and can cause progressive deterioration of the oil. The severity of an electrical or thermal fault can be determined through Dissolved Gas in Oil anal- ysis, as the daily production rate of the relevant gases individually and in relation to each other may indicate where the problem is located and its se- verity. Electrical faults like arcing and corona can cause severe breakdown of the oil in the region of the fault, going hand in hand with the formation of carbon particles as a by-product of the reaction. Acid Acids are produced as degradation products of cellulose that gets dissolved in oil and also by oil oxidation. Acid levels higher than 0.6 mg KOH/g of oil are considered high and are detrimental to the operation of the transformer. It is recommended

ble to determine if there is free water at the bot- tom of the transformer, or the true percentage of water locked up in the core. Water may also be absorbed from the atmosphere in free-breathing units, which is why it is critical to ensure that no moisture can get into the unit, either through the breather or through any leaks in the unit, as this will cause the oil to deteriorate more rapidly. Partial discharges Gas may evolve in oil as a result of thermal stress or spurious electric discharges due to high elec- trical stress. These gas pockets contain hydrogen and hydrocarbon gases produced by decomposi- tion of the oil. Because of the difference in permit- tivity of the gas and the liquid insulation surround- ing it, the electrical stress across the gas voids will be very high. Such high electrical stress in the gas

In a power grid that is already showing many signs of advanced age, transformer inspection and monitoring is an increasingly important task.

Electricity + Control

AUGUST 2018

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