Power Transformer / Distribution Transformer

Online Partial Discharge Monitoring of Power Transformers / Distribution Transformers
Transformer problems can be characterized as those that arise from manufacturing defects, those derived from deterioration processes, and those induced by operating conditions that exceed the capability of the transformer. These conditions may take many years to develop into a problem or failure. However, in some cases, undesirable consequences may develop rapidly.
Deterioration processes relating to aging are accelerated by mechanical, thermal, and voltage stresses. Elevated temperature, along with oxygen content, moisture content, and other contaminants significantly contributes to accelerated insulation degradation. The rate of deterioration may be compounded by the presence of contaminants and by mechanical or electro-mechanical wear. Characteristics of the deterioration processes include sludge accumulation, the weakened mechanical strength of insulation materials such as paper wrapped on the conductor, and shrinkage of materials that provide mechanical support.

One cause of transformer failures is the dielectric breakdown. Failure of the dielectric insulation inside transformers is often preceded by partial discharge activity. A significant increase either in the partial discharge (PD) level or in the rate of increase of partial discharge level can provide an early indication that changes are evolving inside the transformer. Since partial discharge can deteriorate into the complete breakdown, it is desirable to monitor this parameter online. Partial discharges in oil will produce hydrogen, plus lesser quantities of methane, dissolved in the oil. Intermittent sparking from a poorly grounded part produces acetylene, typical for a low-energy discharge, and although it is not a “partial” discharge, such events are detectable with PD systems. The dissolved hydrogen may or may not be detected, depending on the location of the PD source and the time necessary for the oil to carry or transport the dissolved hydrogen to the location of the sensor. The PD sources most commonly encountered are related to moisture in the insulation, cavities in solid insulation, metallic particles, and gas bubbles generated due to some fault condition.