Device for overload protection of electric apparatus

A device for overload protection of electric apparatus having a winding immersed in a dielectric liquid, comprising:

• a heat sensing element immersed in a top portion of said dielectric liquid and producing signals corresponding to the temperature of the liquid;
• a load current sensor electrically connected to said winding of the apparatus;
• a function generator electrically connected to said load current sensor and producing signals indicating overheating of said winding of apparatus above the temperature of said dielectric liquid, said signals being corrected with due account to changes in temperature of the hottest spot of said winding caused by the load current;
• and
• a switching circuit comprising:
• low-overload threshold elements receiving signals from said heat sensing element and said function generator and selectively operating at successively rising temperature values of the hottest spot of said winding;
• a high-overload threshold element receiving signals from said heat sensing element and said function generator and producing a signal corresponding to the critical temperature of the hottest spot of said winding;
• alarm circuits controlled by signals of said low-overload threshold elements, containing a time relay with an independent setting, and being electrically connected with respective threshold elements;
• and
• a tripping circuit controlled by signals of said high-overload threshold element and said low-overload threshold elements, said tripping circuit containing a first time relay with an independent setting connected with said high-overload threshold element, and an interlocking circuit for retaining false switching off signals having
• a single-shot multivibrator controlled by one of said low- and high-overload threshold elements respectively,
• an AND circuit receiving signals from said single-shot multivibrator and from said high-overload threshold element,
• a second time relay, with a setting exceeding that of said first time relay, connected with said high-overload threshold element and
• a NOT circuit placed between said high-overload threshold element and said first time relay and controlled by an output signal of said AND circuit.

A device for overload protection of an electric apparatus having a winding immersed in a dielectric liquid, comprises:

• a heat sensing element immersed in a top portion of said dielectric liquid and producing electric signals corresponding to the temperature of the liquid;
• a measuring current transformer connected to the winding of the apparatus;
• a linear current-to-voltage converter electrically connected with said measuring current transformer for generating a voltage proportional to the load current;
• a rectifier electrically connected to said linear converter;
• a diode converter which, in accordance with the exponential law, converts the rectified voltage into a voltage corresponding to changes in the temperature of the winding caused by the load current;
• a switching circuit comprising low-overload threshold elements and a high-overload threshold element connected with said heat sensing element and said diode converter, said threshold elements selectively operating at successively increasing temperature values of the hottest spot of said winding;
• alarm circuits controlled by signals of said low-overload threshold elements;
• a tripping circuit controlled by signals of said high-overload threshold element and of said low-overload threshold elements and comprising
• a time relay with an independent setting receiving a signal from said high-overload threshold element for tripping;
• an interlocking circuit, for retaining false switching off signals, to pass output signals of high- and low-overload threshold elements respectively when they coincide in time;
• a converter converting signals corresponding to the temperature of the hottest spot of said winding into the number of pulses corresponding to the exponential law of said aging of insulation of said winding, said converter being connected with said low- and high-overload threshold elements respectively;
• a counter integrating in time the output signals of said converter for fault signalling about the wear and indicating the end of the safe life of the insulation of the winding of the apparatus;
• and
• an alarm-and-tripping device operatively connected with said time relay for unloading the apparatus.