Column: Tap-changer know-how

Column: Tap-changer know-how

byRainer FROTSCHER


Insulating liquids – Part I: Mineral oils

1. General

Regulated power transformers are equipped with on-load tap-changers (OLTCs) or de-energized tap-changers (DETCs) to change their ratio and, subsequently, adapt the transformer output voltage to the respective conditions. This enables the power supply network to be kept stable under changing load conditions. In order to avoid overloading and optimally utilise all network components, regulated power transformers are utilised to regulate and control the power flow.

Tap-changers are complex mechanical devices which must also adapt to high-voltage conditions. This combination makes them unique components in energy supply technology. Their tap selectors show numerous different potentials on their terminals, and mechanically operated parts such as contacts are optimized to perform more than a million operations. The combination of mechanical and electrical requirements enforces a compromise which necessarily leads to electrode shapes producing moderate inhomogeneous electrical fields.

While the insulating liquid in a transformer has the sole task of cooling and (in combination with the solid insulation) insulating windings and bushings with regard to high voltage, a suitable liquid for tap-changers must also fulfil other requirements (see Table 1):

a) The switching arc must be cooled and quenched.
b) The spring-driven diverter switch must continue to function properly in all occurring ambient conditions, i.e. within the entire permissible oil temperature range, typically -25°C to +115°C. Within this temperature range, the oil viscosity varies by the factor of hundred.
c) All mechanically moving parts (gears and selector contacts) must be sufficiently lubricated in order to reach a high mechanical life which correlates to the lifespan of the transformer (>30 years).
d) Finally, many different metals and non-metallic materials (glass-reinforced plastics, polyamides, rubber mixtures and others) are used inside the tap-changer to achieve high electrical and mechanical functionality and a long working life, all of which must be compatible with the insulating liquid used.

 

Table 1. OLTCs show the highest demands on the surrounding liquid

 

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