Faults in transformers are caused in many ways and the development time may vary from microseconds to tens of years. Very fast mechanisms, often involving mechanical failures, cannot normally be prevented by condition monitoring, although the extent of damage may be restricted by fast tripping mechanisms. Occasionally, detection of precursors to such a fault, such as overheating at a loosened bolt may allow preventative maintenance to arrest its progress. Specialist on-line monitors, such as those for on-load tap-changers are now available.
Similarly, faults developing at a very slow rate are difficult to detect as any measured changes are not significant compared to normal scatter of results.
A thermal or electrical fault normally involves overheating of oil or solid insulation and produces degradation products which are often simple gaseous molecules that are soluble in the oil. The molecules products depend on the highest temperature at the fault. The amount of breakdown products is proportional to the total energy associated with the fault. These, and more complicated molecules from cellulose, can be analysed and the results used to predict the temperature and type of fault.
Oil samples taken from the main and tap-change tanks etc., help to locate the fault location.
A minimal approach is to do nothing and in the current economic climate it is a dangerous method. Simply inspecting the colour of silica gel breathers and replacing when necessary at least gives some protection to small plant. However, the low cost of having a simple analysis on a sample of oil allows so much more confidence in plant costing tens of thousands of pounds. The cost of a power transformer is high, but monitoring the performance of the transformer via the oil is inexpensive compared to the cost of a failure in a transformer and the costs of an interruption in power supplies.
A transformer oil carries information about the condition of the transformer. Analysing the oil in service can therefore give early warnings about paper degradation, hot spots, electrical faults and problems with moving parts such as tap-changers and pumps. It allows optimisation of inspection or repair times and resources and assists in spares management.