Effect of moisture on a transformer. 

A significant amount of transformer premature ageing and failures can be contributed to the effects of high moisture content in a transformer and often it is not even recognised as the cause of the failure.  After many years of research, engineers have a much better understanding as to how water enters into, and move around in a transformer and how it can affect its lifespan or how it result in failures.  

The lifespan of a transformer is considered to be that of the cellulose paper inside the transformer and often overlooked when drawing up maintenance and monitoring strategies.  Too often the mistake is made of judging the moisture level in a transformer from a laboratory oil sample test and piper charts or even worst just considering the water in the oil (ppm) from an oil sample.  Today we know that such an assessment is open to significant error. 

Not knowing the moisture content in the transformer is the same as not knowing the dielectric withstand capability of a transformer, one of the most important parameters for the safe operation of a transformer.  The dielectric withstand capability of the oil constantly changes due to the complex movement of moisture from the oil-to-paper.  Judging the dielectric strength purely from an oil sample test in the laboratory, is misleading, as it will be affected by the oil temperature and moisture content.  As mentioned before, level of the latter parameter, as well as the temperature changes with ambient and operating conditions; both these can effect the dielectric strength of the oil.   

The extent of moisture penetration and generation within a transformer is often surprising even for transformers fitted with isolation diaphragms and operating in what could be considered dry dessert environment.  In one such case 100 mm of free water was reportedly found at the bottom of the tank. 

High moisture levels in a transformer can also lead to “rain” inside the transformer, effect cellulose insulation surfaces leading to Partial Discharges, lead to bubble formation, short core insulation and generate large volumes of gas due to electrolysis effect on the core. 

The water in oil can exist in oil either in a dissolved, suspended or free state and each of these will have a different effect on the performance or behaviour of the transformer.   As an example, the dissolved water is the part that will movement between the oil and paper with temperature and the component to be considered when assessing the dielectric strength of the transformer. 

Identifying the transformers that might be ranked at high risk to moisture content and carrying out more detailed assessment of the moisture content is an important step towards preventing transformer premature ageing or failure. 

IGS's Solution - determining moisture Levels

IGS experts use the Altmann method of monitoring the water in oil during operation of the transformer.  Operating temperature is monitored when an algorithm is used to simulate the water transfer from oil-to-paper and back.   The oil sample and Karl Fischer method commonly used in the industry, measures all of the moisture with the variable dissolved content, as taken at the time of sampling.  The method of using the piper charts to derive water in paper is thus incorrect, as the total water is used rather than the dissolved component and is further influenced by the fact that it is most unlikely that the water movement was not at quasi equilibrium the time the sample is taken.

Our instruments are connected to the top and bottom oil outlets while on line.  Readings are obtained within a few hours of operation and only in extreme cases would the test continue for longer than a day. 

A Temperature Load Curve (TLC) for the transformer is drawn showing the withstand capability of the transformer as a function of operating temperature.  From this curve, a safe operating range can be derived.  Where the actual operating temperatures exceed the safe operating conditions, precautionary measures are required.

Taking a transformer into a workshop and drying out by vapour phase and re-clamping the windings is a very effective way to reduce moisture, but is not only costly but often a risky exercise as transporting and handling of a transformer with aged paper, which goes hand in hand with high moisture, may lead to failure when placed back into service.  A far less aggressive method is to do online drying of the oil.  The condition of oil or its dielectric strength is improved in the short term and the overall water in paper, or rest of the transformer, is gradually reduced.  Once dielectric strength is restored, the risk of failure during lightning strikes and switching surges are reduced thereby extending the life of the transformer.  In the longer term, the deduction of water in transformer will reduce the Hydrolysis or ageing effect on the paper.  Extreme caution has to be exercised when applying rapid on site dehydration methods to prevent windings from loosening resulting in failure.