Transformer Faults and Detection

In order to maximize the lifetime and efficacy of a transformer, it is important to be aware of possible faults that may occur and to know how to catch them early. Regular monitoring and maintenance can make it possible to detect new flaws before much damage has been done.

The four main types of transformer faults are:

1. Arcing, or high current break down
2. Low energy sparking, or partial discharges
3. Localized overheating, or hot spots
4. General overheating due to inadequate cooling or sustained overloading

These faults can all lead to the thermal degradation of the oil and paper insulation within the transformer. One way to detect them is by evaluating the quantities of hydrocarbon gases, hydrogen and oxides of carbon present in the transformer.  Different gases can serve as markers for different types of faults. For instance,

• Large quantities of hydrogen and acetylene (C2H2) can indicate heavy current arcing. Oxides of carbon may also be found if the arcing involves paper insulation.
• The presence of hydrogen and lower order hydrocarbons can be a sign of partial discharge
• Significant amounts of methane and ethane may mean localized heating or hot spots.
• CO and CO2 may evolve if the paper insulation overheats; which can be a result of prolonged overloading or impaired heat transfer.

Techniques to Detect Faults

Techniques to detect transformer faults include the Buchholz Relay safety device, dissolved gas analysis (DGA) tests and a range of tests for detecting the presence of contaminants in the oil, as well as for measuring indicators of oil quality such as electric strength and resistivity.

• Buchholz Relay

A Buchholz Relay is also called a gas detection relay.  It is a safety device generally mounted at the middle of the pipe connecting the transformer tank to the conservator. A Buchholz Relay may be used to detect both minor and major faults in the transformer.

This device functions by detecting the volume of gas produced in the transformer tank.  Minor faults produce gas that accumulates over time within the relay chamber. Once the volume of gas produced exceeds a certain level, the float will lower and close the contact, setting off an alarm.

Major faults can cause the sudden production of a large quantity of gas.  In this case, the abrupt rise in pressure within the tank will cause oil to flow into the conservator.  Once this is detected the float will lower to close the contact, which causes the circuit breaker to trip or sets off the alarm.

• Dissolved gas analysis (DGA)

Dissolved gas analysis, or DGA, is a test used as a diagnostic and maintenance tool for machinery. Under normal conditions, the dielectric fluid present in a transformer will not decompose at a rapid rate. However, thermal and electrical faults can accelerate the decomposition of dielectric fluid and solid insulation. Gases produced by this process are all of low molecular weight, and include hydrogen, methane, ethane, acetylene, carbon monoxide and carbon dioxide. These gases will dissolve in the dielectric fluid. Analyzing the specific proportions of each gas will help in identifying faults.  Faults detected in such a way may include processes such as corona, sparking, overheating and arcing.

Abnormal functioning within a transformer can be caught early by studying the gases that accumulate within it.  If the right countermeasures are taken early on, damage to equipment can be minimized.