General Electric Company

Canadian General Electric Company Limited

John C Forde

David R Boothman

A thermal model of an electric apparatus such as a transformer or motor, from which signals can be obtained representing temperatures of the conductors and of the adjacent core material, includes storing first and second values, respectively, representing conductor temperature and core material temperature. The magnitude of the current(s) in the apparatus is sensed and a digital signal representative thereof is added to the first value to represent heat generated in the conductors. Values are determined for both heat transferred from the conductors and heat transferred into the apparatus core based on the difference in the first and second values. The value representing heat transferred from the conductor is subtracted from the first value to give a new first value and the value representing heat transferred to the core is added to the second value. Another value is determined from heat transferred from the core material, based on the difference between the new second value and a reference value, and this is subtracted from the new second value. This other value may be modified when the electric apparatus is a motor to provide for variations in heat loss between when the motor is rotating and when it is stationary or, when some other form of forced cooling is employed, whether the forced cooling is active or not. The first and second values thus, at all times, representing conductor temperature and core or average temperature and these temperatures can be used to interrupt the supply of power to the apparatus when predetermined levels are exceeded.

Thermal model for electrical apparatus

A method of and apparatus for protecting three phase electrical equipment where a signal is derived for each phase representing current in each phase, and the three signals combined to form an analog composite signal. The analog composite signal is converted to digital form. One of the analog signals is used to relate the values of the digital signals to respective phases. The largest value is used to determine heating in the equipent and from the heating the temperature of the equipment is derived. The temperature is compared to critical temperature levels and when a critical level is exceeded, the supply of power to the equipment is interrupted. The digital signals representing the current peaks in each phase are used to determine phase loss and phase unbalance, and the power supply is interrupted when there is a phase loss or when the unbalance exceeds a predetermined critical level.

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Peterborough, Canada

John C Forde