Microprocessor controlled circuit breaker trip unit

A microprocessor based circuit breaker trip unit comprising, in combination:

• a plurality of current transformers connected to corresponding lines of a multiphase circuit which are connected to respective circuit breaker poles;
• a trip mechanism energizable to open said circuit breaker poles;
• respective signal conversion means connected to said plurality of current transformers for converting the current output of said current transformers to respective voltage signals;
• respective sample and hold circuits for repetitively and simultaneously sampling and holding sample analog values of each of said voltage signals for a predetermined number of samples;
• analog-to-digital converter means connected to each of said respective sample and hold circuits for converting the analog value produced in said sample and hold circuit after each of said predetermined number of samples to a corresponding digital value;
• a microcomputer means coupled to each of said analog-to-digital converters for computing a true RMS value of the currents of each of said lines;
• an output circuit means connected between said microcomputer and said trip mechanism for operating said trip mechanism in response to predetermined current conditions;
• and
• autoscaling circuit means connected to said microcomputer for dividing a given current range into a firt small range and a second larger range in values of amperes per bit of said microprocessor.

A microprocessor based circuit breaker trip unit comprising, in combination:

• a plurality of current transformers connected to corresponding lines of a multiphase circuit which are connected to respective poles of said circuit breaker;
• a trip mechanism energizable to open said poles of said circuit breaker;
• respective signal conversion means connected to said plurality of current transformers for converting the current output of said current transformers to respective voltage signals;
• respective sample and hold circuits for repetitively and simultaneously sampling and holding sample analog values of each of said voltage signals for a predetermined number of samples;
• analog-to-digital converter means connected to each of said respective sample and hold circuits for converting the analog value produced in said sample and hold circuit after each of said predetermined number of samples to a corresponding digital value;
• a microcomputer means coupled to each of said analog-to-digital converters for computing a true RMS value of the currents of each of said lines;
• said microcomputer includes means for computing ground fault current from the simultaneous samples of current in said lines;
• an output circuit means connected between said microcomputer and said trip mechanism for operating said trip mechanism in response to predetermined current conditions and;
• autoscaling circuit means connected to said microcomputer for dividing a given current range into a first small range and a second larger range in values of amperes per bit of said microcomputer.

A process for operating the trip unit of a multi-pole a-c circuit breaker which protects a circuit having a plurality of a-c lines;

• said process comprising the steps of producing a current signal for each of said lines which is proportional to the instantaneous current in said line;
• converting each of said current signals to respective voltage signals which are proportional to said current signals;
• simultaneously, and repetitively measuring the amplitudes of each of said voltage signals for relatively short sampling time;
• converting the analog value of the amplitudes of said voltage signal to a digital value;
• and separately calculating the true RMS value of each of said voltage signals by taking the square root of the sum of the squares of each of the digital values measured.

A trip unit for a circuit breaker comprising, in combination:

• a plurality of current transformers connected to corresponding lines of a multiphase circuit which are connected to respective poles of said circuit breaker;
• a trip mechanism energizable to open said poles of said circuit breaker;
• respective signal conversion means ocnnected to said plurality of current transformers for converting the current output of said current transformers to respective voltage signals;
• an output circuit means connected between said current transformers and said trip mechanism for operating said trip mechanism in response to predetermined current conditions;
• said trip mechanism including a magnetic latch and a semiconductor switching device;
• said magnetic latch containing a latch operating winding which is connected in series with said semiconductor switching device;
• said semiconductor switching device having a control electrode connected to said output circuit means to be switched into conduction in response to a given output from said output circuit means;
• said winding consisting of a series connected large turn, high resistance section and relatively low turn, relatively low resistance section;
• and
• bypass switching means connected from the node of said series connected sections to said semiconductor switching device whereby, when the output voltage in series with said winding and said switching device exceeds a given value, said bypass switching device conducts and shortcircuits said large turn, high resistance section.

A method of operating a circuit breaker trip unit with the use of a microprocessor comprising the steps of:

• providing said microprocessor with predetermined current data for said trip unit;
• initiating a sample interval timer in said microprocessor upon the start or the sampling period to sequence the samples taken by a plurality of sample and hold circuits;
• constantly determining the voltage of each sample and hold circuit at the end of each sample interval;
• constantly providing the microprocessor with a digitally converted signal voltage and current samples until the end of the sampling period;
• repetitively storing in the microprocessor the sampled current value for 1100 sample times to calculate the total current during the sampling period;
• repetitively comparing in the microprocessor the sample time calculated with the root-mean-square equation and said elapsed time;
• and
• opening the circuit breaker trip unit when the calculated true RMS value indicates an occurrence of at least a predetermined current condition.