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CI535V30 3BSE022162R1 Control module

The operation of the overtemperature function is based on a thermal image, which models the temperature rise of the protected winding. When the current changes, the temperature rise changes from an initial value to a final value according to one or more exponential functions. For a transformer, these represent, for example, the thermal behaviour of the cooling water, the oil, the copper of the windings etc. In the case of a motor or a generator, they concern the iron, end windings, windings in the slots etc. One of these functions is always more pronounced than the others, e.g. the transformer oil or motor iron. The overtemperature function utilises two models for constructing the transient temperature rise. The first employs an exponential function and the second "universal" model a universally applicable function (see Fig. 3.5.19.1). The first model is suitable for the case where one exponential function is so predominant, that the others may be either neglected, or approximately taken into account by ad-justing the thermal time constant. The second model is to be preferred for more complex transient temperature rise functions. This protection function does not monitor instantaneous values as do most of the others. The temperature rise is determined by integration over a given period, e.g. 1/40 of the thermal time constant. The overtemperature function determines the temperature rise curve from the following:
the final temperature rise given by the current
the rate of temperature rise by virtue of a transient function.

Settings for alarm

The protection is usually initialised when the protected unit is still cold, so that the calculation must start at an initial temperature rise of zero and the setting becomes Theta-Begin = 0 % providing it is permissible in the case of machines to neglect the temperature rise due to iron and frictional losses.Neglecting the cooling system, the temperature rise of a winding is given by the square of the current I2 and the resistance of the winding R. Since the resistance varies with temperature, the effective temperature rise is influenced by the temperature of the winding. The overtemperature function provides facility for taking account of this influence by setting the factor "Temp.-Coeff.". The following general equation applies for this influence on temperature

Transformers have two predominant exponential functions, one determined by the oil, the other by the winding, with the following typical values: oil : oil = 50 K oil = 120 min winding : W - oil = 10 K W = 10 min total temperature rise of the winding: W = 60 K The first model operates with a single exponential function and therefore the composite temperature rise of the winding has to be approximated as closely as 5 possible by adjusting the single curve. This can be seen from Fig. 3.5.19.2. The final temperature rise of the equivalent curve is identical to that of the winding as a whole, i.e. W = 60 K in the example above. The time constant, however, usually lies between 60 and 80 % of the time constant for the temperature rise of the oil (see Fig. 3.5.19.3). Similar relationships prevail with regard to modelling the temperature rise of generators and motors.