Electronic solid state, i.e., static trip units are being implemented in increasing numbers in industrial circuit breakers in place of the traditional thermal and magnetic trip units to initiate circuit interruption automatically in response to an overcurrent condition. Increasing popularity of static trip circuit breakers is largely due to their inherent versatility. That is, a circuit breaker static trip unit is readily susceptible to convenient trip setting adjustment such that a highly repeatable trip-time curve can be rather precisely tailored to the load so as to provide full overcurrent protection thereof ranging from light overload to heavy short circuit proportions. Another attractive aspect of static trip units is the fact that they can be readily and economically supplemented to afford ground fault protection, as well as overcurrent protection, to load circuits.
Typically, static trip units are self-powered, in that, rather than being powered from a separate electrical source, they receive operating power from the same source feeding the protective load circuit. More specifically, current transformers, one coupled with each phase or line conductor of the protected load circuit, are utilized to develop secondary currents proportional to the load circuit phase currents. These secondary currents are full-wave rectified pursuant to providing an output current of a magnitude proportional to the highest level of phase current experienced by any one of the three current transformers. From this full-wave rectified output current, the static trip unit extracts sufficient energy to electrically power itself while preserving the informational or signal content thereof regarding the highest phase current magnitude.
The signal content of this output current is manifested as a signal voltage developed across a burden resistor. The signal voltage is then processed by the trip unit pursuant to initiating a circuit breaker trip function either instantaneously or after a varying time delay, depending upon the severity of the overcurrent condition manifested by the signal voltage amplitude. An important consideration in the design of static trip units is the prevention of spurious or nuisance tripping of the circuit breaker in response to a momentary high level of overcurrent and overcurrents of rapidly decreasing magnitudes. In the latter case, it is extremely important that the trip unit take into account the decreasing nature of the overcurrent condition so as not to operate on the basis of an overcurrent level which no longer exists. It is equally important that the trip unit accurately process a signal voltage of uniform amplitude manifesting a persistent overcurrent condition in order that the appropriate delay interval can be imposed prior to initiating a trip function. All of these factors contribute to accurately defining the trip-no trip boundary of the circuit breaker trip-time curve, as established through adjustment of the trip unit settings.
It is accordingly an object of the present invention to provide a circuit breaker static trip unit including an improved detector circuit for generating a signal current proportional to the peak signal voltage developed across a current transformer burden resistor.
A further object is to provide a peak detector circuit of the above character which is capable of closely following rapid changes in the signal voltage peaks.
An additional object is to provide a peak detector circuit of the above character which is capable of generating an essentially ripple free DC signal current.
Another object is to provide a peak detector circuit of the above character which is efficient in construction and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.