During a fault inside the transformer, gases are created as the polymeric compounds of the oil and the winding insulation, usually cellulose, are broken down to form simpler gases such as ethylene, methane, carbon monoxide, nitrogen, etc.
The type and the quantity of the gas evolved is dependent on the nature of the fault inside the transformer.
The reasons for the evolution of fault gases can be classified into three types ,
- Pyrolysis or the breakdown of polymers due to high temperature
- Arcing, caused by short-circuits or earth faults in the alternator
- Partial Discharge or Corona
Pyrolysis refers to the splitting of large organic compounds in oil and the insulating cellulose due high temperature.The pyrolysis of oil leads to the generation of gases such as methane, ethane, ethylene and hydrogen. When cellulose is pyrolysed, it results in carbon-di-oxide or carbon monoxide.
Arcing is caused due to electrical faults in the alternator such as short-circuits or earth faults. Hydrogen and Acetylene are the gases which are generally evolved during faults of this nature.
Corona or Partial discharge is caused by the ionization of gas or oil around a high voltage conductor. Corona in transformers causes the creation of gases such as hydrogen, carbon monoxide and carbon di-oxide.
Dissolved Gas Analysis (DGA), as the name suggests, refers to the analysis of the various gases which are created during a fault conditions and are present in a dissolved form in the oil used for cooling inside the transformer. A sample is extracted from the oil of the transformer. The gases dissolved are extracted for analysis. The nature and quantity of each gas is analysed.
By identifying the type of the gas and the quantity, it is possible to identify the nature of the fault which could have led to that particular gas.
A periodic or continual analysis of the evolved gas helps in the monitoring of the health of the transformer. Hence, gas analysis occupies an important place in the maintenance schedule of the transformer.