Static Electrification in Power Transformer
Transformer represent a significant asset in any power system network. The power transformer are crucial elements of the network. There are two critical phenomenon which affect reliability of power transformers -Static electrification and geometrically induced currents. There are number of transformer failures associated with these phenomenon. The force cooled power transformers are more susceptible to static electrification. Static electrification is strongly correlated with temperature and the dynamics of moisture exchange between paper dielectric system and the insulating oil.
Concept
- When a liquid is in contact with a solid, the system becomes polarized. (The system was neutral initially.). This happens due to physio chemical reactions. This leads to charge distribution and double layer formation. These double layers are formed at the interface between liquid and solid parts. If liquid flows considerably then it leads to flow electrification.
- In power transformers insulation consists of press board and oil. The oil serves the purpose of insulation and cooling. Due to this liquid-solid interface, there exists charge distribution and double layer formation i.e. when oil is forced through tank and cooling tubes, it has electrostatic charge and it contains equal positive and negative ions. When this oil passes over solid insulation (press board, paper, separators), the negative charge is absorbed by solid insulation thus charge separation takes place at solid-liquid interface.
- The negative ions from oil and impurities are absorbed into press board. Since oil is in motion, the viscous forces induce a continuous removal of charges which exist in the walls of double layers. The charge is transported by oil flow. Since the system tries to obtain equilibrium , the interface again absorbs negative ions from oil. Due to this, current flow is initiated from the interface to earthed tank.
- The press board and oil are highly resistive so accumulation of charge will be more. This space charge may also lead to dielectric breakdown.
- The process can be illustrated in following figure.
