Abstract:
A circuit and method for measuring the switch opening time, in cycles, of a medium voltage air circuit breaker. The circuit senses DC current feeding the trip coil in the circuit breaker. The timer starts a predetermined programmable time interval after the DC current starts flowing in the breaker trip coil circuit. The timer stops when the DC current stops flowing in the breaker trip coil circuit. The programmable time interval is a delay between the opening of the breaker&#39;s primary contacts and the opening of the trip coil circuit switch, and is used to determine the circuit breaker opening time from the starting and stopping of the timer. The timing device does not require connection to the circuit breaker&#39;s main power contacts.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to circuit breakers. More particularly, the present invention is directed to a circuit and method for measuring the switch opening time of a medium voltage air circuit breaker. 
     BACKGROUND OF THE INVENTION 
     Electrical power distribution systems and their components need protection from numerous types of malfunctions, including overcurrent conditions, overvoltage conditions, undervoltage conditions, reverse current flow, and unbalanced phase voltages. Accordingly, electrical distribution systems are provided with circuit breakers, which open to break the circuit a prescribed time after detection of a malfunction condition. The integrity of operability of any power distribution system ultimately depends on the proper functioning of the circuit breakers. 
     Circuit breakers are mechanical switching devices capable of making, carrying, and breaking currents under normal circuit conditions and also making, carrying for a specified time, and breaking currents under specified abnormal conditions such as those described above. The medium in which circuit interruption is performed may be designated by a suitable prefix, for example, air-blast circuit breaker, gas circuit breaker, oil circuit breaker, or vacuum circuit breaker. 
     Circuit breakers are rated by voltage, insulation level, current interrupting capabilities, transient recovery voltage, interruption time, and trip delay. 
     Regardless of the medium of arc quenching and insulation, each circuit breaker unit consists of the following construction elements: (1) main contact at system voltage, (2) insulation between main contact and ground potential (porcelain, oil, gas), (3) operating and supervisory devices as well as accessories out of reach of the system voltage-life zone, and (4) an insulated link between operating device and main contact. 
     In operation, a circuit breaker is opened by energizing a trip coil associated with the circuit breaker. Once the trip coil is energized, the main contacts are opened and then auxiliary contacts are opened to prevent damage to the trip coil. 
     A typical medium voltage (e.g., 15,000 volts) air circuit breaker, operating properly, opens in approximately 3-½ cycles, measured from the energizing of the circuit breaker trip coil until distinguishing of the arc between the main contacts and the system. The same breaker, if in need of repair, can take as long as 12 cycles to achieve circuit interruption. Typical circuit breakers can remain closed and in continuous service for many years. Due to the number of mechanical parts associated with a typical circuit breaker, circuit breakers often “stick” if the circuit breaker remains closed for an extended period of time. Lubricants do not adequately remedy this problem. Therefore, it is desirable to measure the circuit breaker&#39;s opening time the first time it is opened, and to remove the circuit breaker if it is defective (i.e., if the breaker does not open within a prescribed period of time). 
     Conventionally, circuit breakers are serviced periodically (e.g., once a year) by shutting down some portion of the power distribution system relatively long periods of time (e.g., hours). For complex power distribution system, it can be difficult to implement such a service program. 
     Therefore, it would be desirable to economically implement a circuit and method for measuring the opening time of a circuit breaker which minimizes disruption of the operation of the power distribution system. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described problems, and provides additional advantages, by providing for a method and circuit for measuring the opening time of a circuit breaker which minimizes down time of the power distribution system and which is not connected to the main power contacts of the circuit breaker. The circuit is powered by the existing substation battery system. A sensing circuit senses a DC current energizing the trip coil in the circuit breaker and starts a timer a programmable time period after the current is sensed. The timer stops when the DC current stops flowing in the breaker trip coil circuit to measure the interruption time of the circuit breaker. The sensing means of the timing circuit includes a current transformer-type sensor, through which a trip coil circuit conductor passes. 
     The circuit according to the present invention advantageously does not require connection to the main power contacts of the circuit breaker, and therefore minimizes shut down time of the power distribution system to measure the switch time of the circuit breakers. According to the disclosed embodiment, the opening time can be measured within approximately 30 seconds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention can be obtained upon reading the following Detailed Description of the Preferred Embodiments, in conjunction with the accompanying drawings, in which like reference indicia are used to designate like elements, and in which: 
     FIG. 1 a block diagram showing a conventional circuit breaker incorporating a circuit according to a first embodiment of the present invention. 
     FIG. 2 is a schematic diagram of the power supply circuit of FIG. 1; 
     FIG. 3 is a schematic diagram of the detection circuit of FIG. 1; and 
     FIG. 4 is a schematic diagram of the oscillator circuit of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a schematic diagram of a conventional circuit breaker incorporating a measuring circuit according to the present invention is shown. As is well known, the circuit breaker includes a set of primary contacts “a” which connect the circuit breaker to an associated power system (not shown) at system voltage, and a set of secondary contacts “b” on either side of a trip coil TC. To open the circuit breaker (e.g., in response to a fault), a DC current is provided to the trip coil TC. In response to the energizing of the trip coil TC, the primary contacts “a” open, followed by the opening of auxiliary contacts “b” to prevent damage to the trip coil TC. 
     A circuit breaker measuring circuit according to the present invention can be connected to each circuit breaker to be monitored in a power distribution system. The circuit according to the present invention includes a detection means such as a current sensing transformer  11  associated with the trip release coil TC of the circuit breaker. The current sensing transformer  11  is preferably embodied by a window-type current transformer in which a primary coil, in the form of a conductor connected to the trip coil, passes through a toroidal magnetic core having a secondary winding. It will be appreciated that other suitable current sensing means will be readily identifiable by those skilled in the art. The primary coil senses DC current flowing to the trip coil TC, and the secondary coil of the transformer  11  outputs an AC voltage proportional to the changing DC current through the primary coil. Since the primary coil current is a DC current, the AC secondary voltage is present only during a change of the primary current. The circuit is powered by a power supply  10 , which accepts an input DC voltage within a range of approximately +43 to +138 volts, and provides outputs of approximately ±4.5 volts. The input voltage range allows the circuit to be powered from conventional substation battery systems, which are typically 48 VDC or 125 VDC. The circuit further includes a detection circuit  12  which senses DC current flowing through the trip coil TC of a circuit breaker. The detection circuit  12  outputs a START or STOP signal depending upon the current flow through the trip release coil, in a manner to be described below. The circuit further includes an oscillator circuit  14  for providing a reliable clock signal of, e.g., 30 kHz. The circuit also includes a counter/display driver  16  which displays the time it takes for the circuit breaker to open based on the START and STOP signals generated by the detection circuit  12  (i.e., the time duration of the energizing of the trip coil TC). The circuit further includes a programmable delay circuit  18 , which is programmed according to the time, in cycles, between the opening of the main circuit breaker contacts “a” and the opening of the auxiliary contacts “b” that open circuit the trip coil. This time delay varies according to circuit breaker manufacturer and/or system type, and this delay can be subtracted from the START-STOP time to determine the opening time of the circuit breaker. 
     Referring now to FIG. 2, a detailed schematic diagram of an exemplary power supply for use with the present invention is shown. The power supply  10  is implemented in this embodiment by a V + supply which includes a fuse  100 , a rectifier diode  102 , a voltage-reduction resistance  104 , a zener diode  106 , an NPN power transistor  108 , and a capacitance  110 , which are connected as shown. The power supply  10  receives an input voltage V in  of, e.g., from +43 to +138 volts. The fuse  100  protects the substation battery subsystems in the event of a short circuit caused by internal electrical component failure. The rectifier diode  102  protects the power supply  10  from incorrect DC voltage polarity. The capacitance  110  filters the V + voltage source during intermittent substation battery high current demands from other equipment. According to a preferred embodiment, the fuse  100  is a 0.25 amp fuse, the voltage reduction resistance  104  is a 27 kΩ resistor, and capacitance  110  is a 1000 uF capacitor. 
     The voltage supply includes a CMOS voltage converter  112 , and capacitances  114  and  116 . The converter  112  converts the V + voltage into a corresponding negative voltage, and the capacitances  114  and  116  operate as charge pumps to provide energy transfer and storage. The V + and V − voltage supplies are preferably approximately +4.5V and −4.5V, respectively, as is necessary to provide power to CMOS circuitry. 
     Referring now to FIG. 3, the detection circuit  12  is shown to include diodes  120  and  122 , which are connected in parallel with the sensing transformer  11 , and in reverse directions. The diodes  120  and  122  operate as a “clipping” circuit to limit the voltage in the secondary winding of the transformer  11  to a range of approximately −0.6V to +0.6V. The output of the clipping circuit is a square wave. As DC current flows through the trip coil, magnetic flux in the transformer  11  increases, thus causing the secondary voltage across diode  120  to decrease to approximately −0.6V. Once the DC current through the trip coil reaches a steady state value, the magnetic flux stops increasing and the secondary voltage returns to substantially 0V. When the trip release coil circuit opens (as a result of the circuit breaker opening), the stored magnetic flux in the transformer  11  decreases, causing the secondary voltage to increase to approximately +0.6V. Once the magnetic flux is exhausted, the secondary voltage returns to substantially 0V. The square wave secondary voltage is supplied to analog amplifiers  124  to separate each non-zero pulse. The complete signal is inverted and amplified by a first operational amplifier  124 A. The output of  124 A is fed to unity gain inverter amplifier  124 B, and to positive output inverting half-wave rectifier amplifier  124 C. The output of amplifier  124 B is supplied to positive output inverting half-wave rectifier amplifier  124 D. 
     A V + output pulse is generated by amplifier  124 C when DC current starts flowing to the trip release coil of the circuit breaker. This V + output pulse generated by the amplifier  124 C is the START signal. A V + output pulse is generated by amplifier  124 D when DC current stops flowing to the trip release coil of the circuit breaker. This V + output pulse generated by the amplifier  124 D is the STOP signal. The START and STOP signals are supplied, via a latch or logic network (not shown) to the counter  16 , such that the counter starts timing the circuit breaker in response to the START signal, and stops timing the circuit breaker in response to the STOP signal. 
     Referring now to FIG. 4, the oscillator circuit  14  is shown to include a quartz crystal  131  and an amplifier  133 . In this embodiment, the crystal  131  is a resonant circuit which oscillates at approximately 30 kHz, and outputs a square wave having a 50% duty cycle which switches between V + (a “high” state) and ground (a “low” state). The crystal  131  is preferably a 30.00 kHz cylinder C- 2  type quartz crystal  131 . The oscillator circuit  14  preferably also includes means for detecting a malfunction. Such a means is implemented, in this embodiment, by an RC network including resistance  130  and capacitance  132 , analog voltage comparators  134 A and  134 B, a voltage dividing network  136 A,B,C which outputs ⅓ V + and ⅔ V + , and an OR gate  138 . In this embodiment, the oscillator crystal output is supplied to resistance  130 , and the output of the RC network is approximately 0.5V + if the oscillator is operating properly; that is, switching between V + and ground. If the oscillator has stopped, the RC network will output the same voltage level as the oscillator (i.e., V + or ground). Voltage comparator  134 A outputs a low state unless the output of the RC network is greater than ⅔ V + , and voltage comparator  134 B outputs a low state unless the output of the RC network is less than ⅓ V + . The oscillator circuit  14  preferably also includes a divide by 50 counter  139  for generating a 600 Hz clock signal for the counter  16 . Such a counter can be implemented by a 12-bit binary counter or other suitable component. 
     The counter/display driver  16  calculates and displays the time, preferably to a tenth of a cycle (where a cycle, in this example, is {fraction (1/60)} second), that it takes for the circuit breaker to open. This time is calculated as the difference between the START and STOP signals, as adjusted by the programmable start delay time. Preferably, the counter/display driver  16  is implemented using a 4553 single chip three-digit BCD counter and multiplexing the BCD outputs to three LCD digit drivers implemented by conventional 4543 BCD-to-seven segment latch/decoder/drivers for liquid crystal displays. The counter receives an input from the oscillator circuit  14  and the detection circuit  12  and calculates the opening time from the START and STOP signals provided by detection circuit  12 . 
     The delay circuit  18  can be implemented by a 74HC688 8-bit magnitude comparator in combination with a 4040 12-bit binary counter, or other suitable circuitry, to provide a programmable time delay to offset the calculated time between the START and STOP signals by the delay time which typically varies between circuit breaker type and manufacturer. The programmable time delay can be set by an electrician or installer of the circuit breaker time circuit of the present invention. Further, it will be appreciated that a mechanical contact “bounce” will occur at the opening of the circuit breaker, that is, the contacts will typically open and close multiple times before stabilizing in the open position. 
     Accordingly, the delay circuit  18  can also include a stop delay function to program a stop delay time, in cycles, to compensate for the mechanical contract bounce of the relay contacts. An exemplary time delay is 0.2 cycles. The stop delay function can be implemented by a 4585 4-bit magnitude comparator or other suitable circuitry. 
     A circuit breaker timing device according to the disclosed embodiments and principles allows the circuit breaker to be closed substantially immediately after being opened to measure the opening time. Conventional circuits and methods require placing the circuit breaker out of service for several hours. In contrast, the timing device of the present invention can measure the opening time in approximately thirty seconds. 
     While the foregoing description includes numerous details and specificities, it is to be understood that these are for purposes of explanation only. Many modifications will be readily apparent to those of ordinary skill in the art which are clearly within the spirit and scope of the invention, as defined by the following claims and their legal equivalents.