Abstract:
An electronic trip unit and a method of protection in an electronic trip unit comprises selecting a limit value, sensing an electrical signal to provide corresponding first and second sensed signals, each indicative of an electrical characteristic of the electrical signal, comparing said first and second sensed signal to determine a rate of rise of said electrical characteristic, comparing said rate of rise to said limit value to detect a spike in said electrical characteristic, and withholding generation of a trip signal when said rate of rise is greater than said limit value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 09/325,605, filed on Jun. 3, 1999, pending, herein incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    The present invention relates generally to circuit breaker trip units. More specifically, the present invention relates to an electronic trip unit with adjustable sensitivity to current spikes.  
           [0003]    The use of electronic trip units in electric circuit breakers is well known. Trip units can be used for, among other purposes, providing short circuit protection to an electrical distribution circuit. In this capacity, the trip unit samples current in the power lines of the distribution system to detect a short circuit. If a short is detected, the trip unit provides a trip signal to an actuating device, such as a trip solenoid, within the circuit breaker. Upon receiving the trip signal, the actuating device separates a pair of contacts within the circuit breaker to open the distribution circuit and protect the distribution circuit from damage caused by the short circuit.  
           [0004]    The construction of an electronic trip unit is also known. Electronic trip units typically comprise voltage and/or current sensors, which provide analog signals indicative of the power line signals. The analog signals are converted by an A/D (analog/digital) converter to digital signals, which are processed by a signal processor. Electronic trip units further include RAM (random access memory), ROM (read only memory) and may also include EEPROM (electronic erasable programmable read only memory) all of which interface with the signal processor.  
           [0005]    To detect short circuits in the distribution circuit, trip units monitor peaks in the current within the power lines. Generally, trip units compare the current in the power lines to some threshold value. For example, this threshold value may be seven times the rated current of the circuit breaker. If the current in the power lines exceeds this threshold value, indicating a short circuit, the trip unit generates the trip signal.  
           [0006]    [0006]FIG. 1 shows a current waveform of fundamental frequency. In the waveform shown, the current peak is higher than the threshold value and, therefore, this waveform is indicative of a short in the circuit. A trip unit would generate a trip signal if the waveform of FIG. 1 were detected. FIG. 2, however, shows a current waveform with current spikes caused by high harmonic content or noise. Such current spikes can cause the circuit breaker to trip, even where no short circuit exists. Trips caused by these current spikes can be a nuisance.  
           [0007]    Attempts have been made to overcome this problem by using processing algorithms to filter out the current spikes. While such is well suited for certain applications, such as drive systems, where current spikes are commonly generated, it is problematic in other applications, such as high-frequency systems (e.g., 400 Hz systems or resistive load circuits), where the user desires the trip unit to trip in response to such current spikes.  
         SUMMARY OF INVENTION  
         [0008]    In an exemplary embodiment of the invention, a method of protection in an electronic trip unit comprises selecting a limit value, sensing an electrical signal to provide corresponding first and second sensed signals, each indicative of an electrical characteristic of the electrical signal, comparing said first and second sensed signal to determine a rate of rise of said electrical characteristic, comparing said rate of rise to said limit value to detect a spike in said electrical characteristic, and withholding generation of a trip signal when said rate of rise is greater than said limit value. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    The present invention will now be described, by way of example only, with reference to the accompanying drawing in which:  
         [0010]    [0010]FIG. 1 is a current waveform of fundamental frequency;  
         [0011]    [0011]FIG. 2 is a current waveform with current spikes;  
         [0012]    [0012]FIG. 3 is a schematic block diagram of a electric power distribution circuit;  
         [0013]    [0013]FIG. 4 is a schematic block diagram of a circuit breaker with an electronic trip unit of the present invention;  
         [0014]    [0014]FIG. 5 is a flow diagram of a short circuit protection program of the present invention;  
         [0015]    [0015]FIG. 6 is a current waveform of fundamental frequency with a plurality of samples for each half cycle;  
         [0016]    [0016]FIG. 7 is a current waveform with current spikes and with a plurality of samples for each half cycle in accordance with the present invention; and  
         [0017]    [0017]FIG. 8 is a flow diagram of an alternate method of short circuit protection of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring to FIG. 3, an electrical power distribution circuit is generally shown at  10 . Distribution circuit  10  comprises a source  12 , an upstream circuit breaker  14 , a downstream circuit breaker  16  and at least one corresponding load  18 . Any number of additional downstream circuit breakers  20  with corresponding loads  22  may be included. It will be appreciated that breakers  14 ,  16 , and  20  may be of similar construction.  
         [0019]    Referring to FIG. 4, a general schematic of a circuit breaker is generally shown at  20 . Circuit breaker  20  comprises a trip unit  22 , actuating device  24 , and contacts  26  all mounted within housing  28 . Contacts  26  form part of distribution circuit  10  and are mechanically connected to actuating device  24 . Actuating device  24  is arranged to receive a trip signal from trip unit  22 , which is electrically connected to distribution circuit  10 . Upon receiving the trip signal, the actuating device  24  separates contacts  26  to stop the flow of current in a portion of the distribution circuit  10 .  
         [0020]    Trip unit  22  comprises a user-adjustable switch  30 , a current sensor  32 , an analog-to-digital (A/D) converter  34 , a microprocessor  36 , and a power supply  37 . Power supply  37  is typically fed from the secondary of current sensor  32 . Current sensor  32  is electrically connected to distribution circuit  10  by a line  33  and provides analog signals indicative of current measurements in distribution circuit  10  to AID converter  34 , via a line  35 . A/D converter  34  converts the analog signal to a digital line signal and presents the digital line signal, via bus  38 , to microprocessor  36 . Power supply  37  is electrically connected to distribution circuit  10  by line  33  for providing operating power to A/D converter  34 , switch  30 , and microprocessor  36 , via a line  41 .  
         [0021]    User-adjustable switch  30  is arranged to provide a signal indicative of a limit value, via bus  40 , to microprocessor  36 . The user-adjustable switch  30 , for example, may be a binary coded decimal (BCD) encoded switch that allows the user of the circuit breaker to alter the limit value provided to the microprocessor  36 . Alternately, the user-adjustable switch  30  may comprise a jumper bit or a user-selectable option in non-volatile memory such as ROM (read only memory)  50 .  
         [0022]    Microprocessor  36  comprises a plurality of registers  42 - 48  and ROM  50  internal thereto. ROM  50  includes trip unit application code, e.g., main functionality firmware, including initializing parameters, boot code, and a short circuit protection algorithm. The plurality of registers  42 - 48  comprises a register  48  for storing the line signal provided by the A/D converter  34 , a register  42  for storing the limit value provided by switch  30 , and registers  44  and  46  for use by the microprocessor  36  in executing the short circuit protection algorithm. It will be appreciated that RAM (random access memory), EEPROM (electronic erasable programmable read only memory) or any combination thereof may be employed by the microprocessor  36  for memory purposes, as is well known. The EEPROM would include, e.g., operational parameters for the application code. It will also be appreciated that ROM  50  may be external to the microprocessor  36 , as is well known. Further, communications within trip unit  22  can be provided through a communications I/O port  51 .  
         [0023]    Referring to FIG. 5, the short circuit protection algorithm (program) is applied to each of the phases of the power lines in distribution circuit  10 . The program is initiated preferably from the boot code at start-up, block  52 , and proceeds immediately to block  54 . At block  54  the program resets a sample count value stored in register  44  to zero. The program continues to block  56  where a peak count value stored in register  46  is reset to zero. At block  58 , the program increments the sample count value in register  44 . The program then waits a predetermined sample period, block  60 , and then proceeds to block  62  where a line signal in register  48  is sampled. The sample period is a parameter stored in ROM  50  and is equal to a fraction of the half-cycle of the current frequency in the distribution circuit  10 . For example, the sample period might be one-eighth of the half-cycle time. Thus, the line signal is sampled eight times per half-cycle (see, e.g., FIGS. 6 and 7).  
         [0024]    At block  64 , the program compares the line signal stored in register  48  to a threshold value (e.g., seven times the rated current) stored in ROM  50 . If the line signal, which is indicative of the current level in the distribution circuit  10 , is less than the threshold value, the program continues to block  66 . At block  66 , the program compares the sample count value in register  44  to a maximum sample value stored in ROM  50 . The maximum sample value is equal to the number of samples per half-cycle of the current frequency in the distribution circuit. Using the example above, the maximum sample value would be eight. If the sample count value in register  44  is less than the maximum sample value, the program loops to block  58  where it increments the value in the sample count register  44  (to continue sampling the same half-cycle). If the sample count is equal to the maximum, the program loops to block  54  where it resets the sample count value in register  44  to zero (to begin a new half-cycle).  
         [0025]    Referring again to block  64 , if the line signal stored in register  48  is greater than the threshold value stored in ROM  50 , the program continues to block  68  where it increments the peak count value in register  46 . At block  70 , the program compares the peak count value in register  46  to the peak limit value in register  42 . If the peak count value is less than the peak limit value, the program continues to block  66  where, as described above, the same half-cycle is sampled again or sampling of a new half-cycle begins. If the peak count value is equal to the peak limit value, the program continues to block  72 , where it initiates a trip signal. The program then ends at block  74 .  
         [0026]    [0026]FIGS. 6 and 7 show examples of a current signal sampled eight times per half-cycle. FIG. 6 represents a half-cycle with five line signals (samples) over the threshold value. In the short circuit detection algorithm of FIG. 5, if the peak limit value stored in register  42 , as set by the user-adjustable switch  30 , is five or less, the half-cycle shown in FIG. 6 would cause the breaker to trip. If set to six or higher, the breaker would not trip. FIG. 7 represents a half-cycle with two line signals (samples) over the threshold value. In this case, if the user set the peak limit to three or greater, the breaker would not trip. As shown in these examples, the user can adjust the sensitivity of the trip unit to current spikes by adjusting the switch  30 .  
         [0027]    Alternately, the short circuit protection algorithm (program) shown in FIG. 8 may be applied to each of the phases of the power lines in distribution circuit  10 . The program is initiated preferably from the boot code at start-up, block  76 , and proceeds immediately to block  78 . At block  78 , the program samples the line signal in register  48 . The program then continues to block  80  where it shifts the line signal stored in register  48  to register  46  and then continues to block  82 . At block  82 , the program waits a predetermined sample period, and then proceeds to block  84  where a new line signal in register  48  is sampled. The sample period is a parameter stored in ROM  50  and is equal to a fraction of the half-cycle of the current frequency in the distribution circuit  10 . For example, the sample period might be one-eighth of the half-cycle time, such that the line signal is sampled eight times per half-cycle.  
         [0028]    At block  86 , the program calculates the quantitative difference between the previous line signal in register  46  and the current line signal in register  48 . The difference is compared to the limit value provided by the user-adjustable switch  30  and stored in register  42 . For example, the limit value may be equal to seven times the rated current. If the difference is greater than the limit value, the program loops back to block  80 . If the difference is less than the limit value, the program continues to block  88  where the line signal in register  48  is compared against a known threshold value (e.g., seven times the rated current) stored in ROM  50 . If the line signal in register  48  is less than the threshold value, the program loops back to block  80 . If the line signal in register  48  is greater than the threshold value, the program continues to block  90 , where it initiates a trip signal. The program then ends at block  92 .  
         [0029]    The short circuit protection algorithm of FIG. 8 uses the rate of rise of two consecutive samples to detect current spikes. If the rate of rise is too steep (i.e., if the quantitative difference between the current and previous line signals is greater than the limit value) this indicates a current spike. The user can adjust the sensitivity of the trip unit to current spikes by adjusting the limit value using switch  30 . If the user desires high sensitivity, the user can adjust switch  30  to increase the limit value. Sensitivity can be reduced by decreasing the limit value.  
         [0030]    The short circuit protection algorithms of FIGS. 5 and 8 may further comprise a power-up feature that sets the trip unit for high sensitivity during power-up and reduces the sensitivity during running state. This feature, for example, can be used on the portions of distribution systems that service electric drive motors. Alternately, switch  30  may include a setting feature that would adjust the trip unit for use in a 400 Hz application, where maximum sensitivity is needed.  
         [0031]    The trip unit of the above-described invention allows the user of the circuit breaker to adjust the trip unit&#39;s sensitivity to current spikes. This feature allows the user to decrease sensitivity for applications such as drive systems, where current spikes are generated, and to increase sensitivity for applications such as high-frequency systems, where maximum sensitivity is needed.  
         [0032]    All of the aforementioned limits, settings or thresholds may be stored in any non-volatile memory or an EEPROM which can be altered by downloading desired settings via communications I/O port  51 . This would include remotely downloading such data when the unit is connected to a system computer (not shown), either directly, over telephone lines, or any other suitable connection. It may also be preferred that such EEPROM comprises a flash memory whereby such data is flashed, as is well known.  
         [0033]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.