Abstract:
A low voltage AC electric power monitoring system includes a processor which samples the current and circuit breaker system includes a processor which samples each of the current components at seventeen samples per cycle to obtain current samples representing a single period of the AC power signal. The processor samples the voltage component at different points in the voltage cycle over several cycles to obtain voltage samples representing a single period of the AC power signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of Ser. No. 07/753,261 filed Aug. 30, 1991, now abandoned. 
    
    
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     The present invention is directed to apparatus for monitoring an electric power distribution system and in particular to a monitoring unit which provides extended monitoring functions and which includes a distributed current and voltage sampling system. 
     In a typical factory power distribution system, high-voltage (i.e. greater than 1,000 volts) power provided by the power company generation station is stepped down to low voltage power using a transformer. The low voltage power is then distributed around the factory to power equipment such as, motors, welding machinery and large computers. 
     Power distribution systems of this type are typically divided into branches, where each branch supplies power to a portion of the factory. The entire power distribution system is protected by installing low voltage fuses or circuit breakers in each branch so that a fault such as a short circuit in a piece of equipment supplied by one branch will not affect the power distributed to equipment coupled to the remaining branches. 
     Typically, these low voltage circuit breakers detect more than just large overcurrent conditions caused by short circuit faults. In addition, they detect lower level long-time overcurrent conditions and excessive ground current. The simplest form of circuit breaker is thermally tripped as a result of heating caused by an overcurrent condition. This type of breaker is best for detecting relatively low level overcurrent conditions since it measures the cumulative heating effect of the low-level overcurrent condition over a period of time. A breaker of this type, however, may respond too slowly to provide effective protection against high-current short circuit conditions. 
     Another type of breaker monitors the level of current being passed through the branch circuit and trips the breaker when the current exceeds a predefined maximum value. Breakers of this type typically include a microcontroller coupled to one or more current sensors. The microcontroller continually monitors the digitized current values using a curve which defines permissible time frames in which both low-level and high-level overcurrent conditions may exist. If an overcurrent condition is maintained for longer than its permissible time frame, the breaker is tripped. 
     Although this type of breaker provides good protection against both long-time and short-time overcurrent conditions, if it does not calculate RMS current values, it may erroneously trip the circuit when a nonlinear load, such as a welding machine, is coupled to the branch that it is protecting. Non-linear loads tend to produce harmonics in the current waveform. These harmonics tend to distort the current waveform, causing it to exhibit peak values which are augmented at the harmonic frequencies. When the microcontroller, which assumes a sinusoidal current waveform, detects these peaks, it may trip the breaker even though the heating effect of the distorted waveform may not require that the circuit be broken. 
     Since circuit breakers of the type described above only monitor overcurrent conditions, other types of faults such as over or under voltage conditions and phase imbalances may be missed unless or until they result in an overcurrent fault. Typically, circuit protection for faults of this type requires special purpose line monitoring and relaying equipment, separate from the overcurrent breakers. 
     Another problem with many existing circuit breakers involves the time required to restore the branch to operation once the breaker has been tripped. For purely transient faults, such as a power surge during an electrical storm, a technician must go onto the factory floor, locate the tripped breakers and reset them. Depending on the experience and knowledge of the technician, this may take a few minutes or a few hours. In this instance, however, the delay may be minimized by using a breaker with an automatic recloser. 
     Faults caused by the equipment that is powered by the branch may be more difficult to locate. Many circuit breakers provide no information on the conditions present at the time the breaker was tripped. Thus, the technician may need to install power monitors on each piece of equipment to determine the magnitude and duration of the current that caused the fault. Due to the limited information provided by currently available breakers, faults of this type may take several days to locate and correct. 
     A final problem with existing low-voltage circuit breaker systems concerns the lack of effective backup protection if the breaker should fail to trip. This problem is more of a concern with microcontroller based trip units than with the older thermal trip units. In general, effective backup protection may include a fuse, in series with the branch line, which blows at a short-circuit current slightly higher than the short-circuit current of the breaker. If the microcontroller or any of its associated circuitry fails, a lower-level overcurrent condition may damage the distribution system and/or the equipment being protected before the backup fuse is blown. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in a power monitoring circuit for low-voltage AC electric power distribution system. The controller circuit monitors the level of current flowing by continually sampling instantaneous current and voltage levels on the branch line. The sampling system produces a number of samples sufficient to characterize the current component during one cycle of the AC power signal and a number of samples sufficient to characterize the voltage component over an interval longer than one cycle of the AC power signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram, partly in block diagram form of a power distribution system which includes a trip unit containing an embodiment of the present invention. 
     FIG. 1a is a block diagram which illustrates the data communications interconnections of selected ones of the trip units shown in FIG. 1. 
     FIG. 2 is a block diagram, partly in schematic diagram form of a trip unit suitable for use in the system shown in FIGS. 1 and 1a. 
     FIG. 2a is an elevation drawing of the front panel of one of the trip units shown in FIGS. 1 and 1a. 
     FIGS. 3a and 3b are graphs of current versus time which are useful for describing the operation of the trip unit shown in FIG. 2. 
     FIGS. 4a through 4g are flow-chart diagrams which are useful for describing the operation of the trip unit shown in FIG. 2. 
     FIGS. 5a through 5c are timing diagrams which are useful for describing the current and voltage sampling scheme used by the trip unit shown in FIG. 2. 
     FIG. 6 is a flow-chart diagram which is useful for describing the operation of the trip unit shown in FIG. 2. 
    
    
     DETAILED DESCRIPTION 
     Overview 
     The present invention is embodied in a dual processor low-voltage circuit breaker and power line monitoring system. In this system, which is shown in greater detail in FIGS. 2 and 2a, the two processors are implemented using respective microcontroller circuits 210 and 250. 
     Referring to FIG. 2, the microcontroller 210 monitors the current flowing through the three-phase power lines 200a, 200b and 200c to detect overcurrent conditions and to trip the circuit breaker 202 immediately if a large overcurrent is detected or if, after a programmable delay time, a relatively small overcurrent is detected. 
     The microcontroller 250 monitors the potential developed across the power lines 200a, 200b and 200c in addition to monitoring the current flowing through the power lines 200a, 200b and 200c. From these values, the controller 250 calculates the power flowing through the lines and the frequency of the power signal. Based on these monitored parameters, the microcontroller 250 can trip the breaker or change the state of an alarm output signal. The alarm signal may be used to actuate an alarm device, such as a light and/or a buzzer, or it may be used, through an interposing relay to open the circuit breaker 202. The microcontroller 250 can also reclose the breaker after receiving a specific command from the host computer 140. 
     In addition to its protection functions, the microcontroller 250 logs minima and maxima for various ones of the monitored variables and logs the occurrence of events such as the detection of overcurrent conditions, also known as pickup events, and trip events. 
     The logged items may be monitored by a remote host computer 140. The computer 140, shown in FIG. 1a, is coupled to multiple trip units to provide, at one location, the continuing status of the electric power distribution system. In addition, many of these logged items may be monitored using a local breaker display unit 117. The host computer 140 can also be used to control the operation of the processor 250. 
     The microcontroller 250 provides backup overcurrent protection by tripping the breaker 202 at overcurrent levels greater than those used by the microcontroller 210. In addition, the microcontroller 250 uses a power supply which is separate and distinct from that used by the microcontroller 210. All input and output signals used by the microcontroller 250, including the operational power signal, are electrically isolated from the outside circuitry to prevent damage to the trip unit circuitry. 
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT OF THE INVENTION 
     FIG. 1 is a diagram of an exemplary electrical power distribution system. The system has been simplified to facilitate the explanation of the invention. In the FIGURE, all of the power lines are three-phase lines even though only one line is shown. 
     As shown in FIG. 1, a high voltage source 110, which may be a power company substation, provides a high voltage electrical power signal to the primary winding of a transformer 112. The secondary winding of the transformer provides three-phase low voltage power to the power distribution system of, for example, a factory. The low voltage power is distributed around the factory floor through respective step-down transformers 124, 126, 128 and 130 to provide power to equipment represented as respective loads 125, 127, 129 and 131. 
     The power distribution system is protected by multiple trip units 114, 116, 118, 120 and 122. In this configuration, the trip units 116, 118, 120 and 122 each protect the system from faults occurring on a respective branch of the power distribution system while the trip unit 114 protects the transformer 112 from faults not handled by one or more of the other trip units and from faults on the main distribution bus 113. 
     FIG. 1a is a block diagram which illustrates how the trip units are connected to the host computer 140 to allow the power distribution system to be monitored from a central location. To simplify the drawing, only three of the trip units 114, 116 and 118 are shown in FIG. 1a. It is contemplated, however, that all of the trip units may be connected to the host computer 140. In this embodiment of the invention, the host computer 140 may be an ACCESS™ electrical distribution communication system, available from Siemens Energy and Automation, Inc. 
     The host computer 140 is coupled to a display device 142 and a keyboard 144. As set forth below, the host computer 140 may periodically poll each of the trip units, via a multi-drop line 141, to monitor the status of the power distribution system at the main bus and at each branch bus. In addition, the host computer 140 can issue commands to the various trip units causing them to open or close their respective breakers or to change the levels at which pickup and trip events occur for certain ones of the monitored parameters. 
     Furthermore, as shown in FIG. 1a, each of the trip units 114, 116 and 118 may be coupled to respective breaker display units (BDUs) 115, 117 and 119 by a separate data communications port. The BDU may be used to monitor the status and history of the power distribution system, at the trip unit. This monitoring function is implemented to be substantially independent of the monitoring functions of the main computer 140. 
     FIG. 2 is a block diagram, partly in schematic diagram form of an exemplary trip unit which controls a breaker 202. For the purpose of this explanation, the trip unit is assumed to be the unit 116 which isolates its branch line from the main bus 113 as shown in FIG. 1. The trip unit includes an overcurrent microcomputer 210 which implements the basic overcurrent protection functions of the trip unit and a communications microcomputer 250 which implements data communications functions and provides auxiliary circuit protection functions. 
     The exemplary overcurrent microcomputer 210 includes an 80C49 microcontroller, available from Intel Corp, which includes an internal memory 211. The memory 211 includes read only memory (ROM) for program and fixed-value data storage as well as a small scratchpad random access memory (RAM). 
     Electrical current flowing through the three-phase lines 200a, 200b and 200c is sensed by three current transformers 204. In the exemplary embodiment of the invention, the current transformers 204 are implemented as respective secondary windings wrapped around each of the bus bars 200a, 200b and 200c. Current induced in the secondary winding is stepped down by three respective current transformers (not shown) internal to the trip unit 116. These stepped-down currents are converted to voltage by three resistors (not shown) which are also internal to the trip unit. These voltages are applied to a multiplexed analog to digital converter (ADC) 212. The ADC 212, under control of the microcomputer 210, sequentially digitizes the voltages generated by the three stepped-down currents. These digitized values are supplied to the overcurrent microcomputer 210 as data values. 
     In addition to the current sensors on the three phase lines 200a, 200b and 200c, the trip unit may also be configured to accept ground current and neutral current at separate current sensor input terminals. As set forth below, the neutral current input signal is used only by the communications microcomputer 250. The ground and neutral current lines are not shown in FIG. 2 to avoid unnecessary complexity in the drawing. 
     Operational power for the overcurrent microcomputer 210 is supplied from the current transformers 204. As shown in FIG. 2, the secondary windings of the transformers 204 are coupled to power supply circuitry 214 which rectifies the provided alternating current power signal to generate direct current (DC) operational power for the overcurrent microcomputer 210. The power supply 214 is also coupled to provide operational power to the ADC 212, watchdog circuitry 218 and front panel display 220. 
     The front panel switches 216 are used to set the pickup and trip levels used for primary overcurrent protection. As set forth above, a pickup level is an overcurrent condition which may cause the unit to trip the circuit breaker, either after a delay dependent on the level of the detected current, or instantaneously, for large overcurrent conditions. The configuration of the front panel switches 216 is described below with reference to FIG. 2a. 
     The watchdog circuit 218 continually monitors the status of the microcomputer 210. The exemplary circuit 218 expects to receive a pulse signal from the microcomputer 210 at regular intervals. If it fails to receive a pulse within an interval centered about an expected time, it causes a liquid crystal device (LCD) array on the front panel display to display the message &#34;DISABLE&#34; and attempts to reset the overcurrent microcomputer 210. Even if it is successful in restarting the microcomputer 210, the watchdog circuit continues to display the DISABLE message once a failure has been detected. 
     If, during its current monitoring, the microcontroller 210 detects a large overcurrent condition indicative of a short circuit condition, or a smaller overcurrent condition which persists for longer than a predefined time interval, the microcontroller 210 activates the trip actuator 230, causing the breaker 202 to break the connection between the branch lines 200a, 200b and 200c and the main bus 113. 
     In addition to tripping the breaker, the microcomputer 210 indicates on the front panel display 220 the type of event which caused the trip. If the event was a long-time overcurrent condition, the word &#34;OVERLOAD&#34; is displayed on the front panel display 220. If the event was a ground overcurrent, sensed from the ground current input terminal (not shown) to the ADC 212, the words &#34;GROUND FAULT&#34; are displayed. If the event was a large overcurrent, causing a short time or instantaneous trip, the words &#34;SHORT CIRCUIT&#34; are displayed. In addition to these event displays, the microcomputer 210 can activate two light emitting diodes (LEDs) on the front panel. One of these LEDs is lighted when a long-time pickup event occurs and the other is activated when a short-time pickup or a ground fault pickup event has occurred. 
     In addition to displaying these events, the overcurrent microcomputer 210 provides signals via a digital message path (DMP) for events which have occurred to the communications microcomputer 250. These signals include all of the signals that activate the front panel display and, in addition, signals that indicate when a short-time or long-time pickup event has occurred or a trip event has occurred. 
     FIG. 2a is an elevation drawing of an exemplary front panel which may be used with this embodiment of the invention. The front panel is described now since it primarily relates to the overcurrent microcomputer 210. 
     The front panel display 220 includes an LCD array 220a the long-time pickup LED 220b and the short-time/ground fault pickup LED 220c. 
     The front panel switches include a switch 216a for setting the current level which will cause an instantaneous trip of the breaker 202. This current is specified a multiple of the rated current of the current sensors 204. In the exemplary embodiment of the invention, this may be set to between twice and fifteen times the rated current of the sensor. Switches 216b and 216c set the pickup level and time delay for a long-time trip. The exemplary pickup level may be set to between one-half of the rated current and the rated current. The delay may be set to between 3.5 seconds and 30 seconds. 
     Switches 216d and 216e determine the short-time trip settings. A short-time pickup may be set to occur for sensed currents between twice and twelve times the long-time pickup setting while the delay from pickup to trip may be set to between 0.08 and 0.4 seconds. 
     The parameters used for a ground-fault trip are controlled by the front panel switches 216f and 216g.  The ground-fault pickup may be set to between 20% and 60% of the rated current for the ground current sensor and the delay can be set to between 0.1 seconds and 0.4 seconds. 
     In addition to the display 220 and control switches 216, the front panel includes a connector 264 which is used by the communications microcomputer 250 to implement data communications with the breaker display unit (BDU) 117. A rear connector (not shown) couples the microcomputer 250 with the host computer 140. 
     Referring again to FIG. 2, the communications microcomputer includes a 68HC11F1 microcontroller available from Motorola, Inc. and a memory. This memory includes an external programmable read-only memory (PROM) 253, which is used to hold the program instructions and an a memory 251 which is internal to the microcontroller. In this embodiment of the invention, the PROM 253 is a 27C256 integrated circuit available from Intel Corp. The program stored in the PROM 253 is included as a software appendix to the present application. 
     The internal memory includes a non-volatile random access memory (NVRAM) portion, which is used to hold certain log entries that may be provided to either the host computer 140 or the BDU 117 and a RAM portion which is used to hold log entries which may change frequently as well as flags and partial values of calculations. 
     Operational power is applied to the microcomputer 250 and to the circuitry to which it is coupled from an external 15 volt direct current instrument supply 260. The operational power for the microcomputer 250 is further shielded by an isolating power supply 258 interposed between the microcomputer 250 and the instrument power supply 260. This isolating power supply 258 may be include, for example, a conventional DC to DC converter. 
     The supply 260 is desirably isolated from the branch lines 200 so that the communications microcomputer 250 can continue to operate even when the breaker 202 is open or tripped. In addition, this alternate operational power source shields the communications processor from power problems which may disable the overcurrent microcomputer 210. 
     In this embodiment of the invention, the communications microcomputer 250 performs both communications and monitoring functions. In addition to monitoring the current flowing through the lines, the microcomputer 250 monitors the voltage between the respective three phase lines and, using this voltage and current data, monitors power, energy and imbalances among the three phases in either voltage or current. 
     Data on the current flowing through the lines 200a, 200b and 200c is collected by an ADC 252 which is coupled, in parallel with the ADC 212, to the current sensors 204. In addition, the ADC 252 is coupled to a potential transformer module 254 which provides a measure of the voltage between each of the three phases. The ADC may also be coupled to receive ground and neutral currents from sensors coupled to the branch lines 200. In an exemplary system, these sensors may include a circuit (not shown) which derives ground current as the vector sum of the three phase currents and a conventional current transformer coupled to the neutral line (not shown) of the branch lines 200. 
     The ADC 252 is a multiplexed ADC which provides instantaneous samples of one of three current signals (five if ground and neutral are used) and three voltage signals. The ADC is controlled by the microcomputer 250 to determine which sample to provide at any given time. 
     The communications microcomputer 250 is coupled to the COMM WATCH LED 256 on the front panel of the trip unit. The function and operation of this LED is described in detail below with reference to FIG. 6. 
     As set forth above, the communications microcomputer 250 provides two substantially independent communications links. One of these links is a dedicated communications port 262 which is coupled to the BDU 117. The other communications link is an RS-485 port 266 through which the communications microprocessor is coupled to the host computer 140. Both of the ports 262 and 266 include conventional opto-isolators to prevent any electrical connection between the communications microcomputer 250 and the BDU 117 or the host computer 140. 
     The communications microcomputer 250 is also configured with an opto-isolated alarm signal output line 268, an opto-isolated breaker position input signal line, an output line to the trip actuator 230 and an opto-isolated breaker-close signal output line 269. These signal lines allow the communications microcomputer 250 to trip, open or close the breaker 202 and, in addition, allow it to log the state of the breaker 202. The alarm line 268 may be coupled to an alarm device so that conditions detected by the communications microcomputer 250 which activate the alarm signal activate the alarm device. 
     Alternatively, as shown in FIG. 2, the alarm line 268 may be coupled to an interposing relay 270 which is coupled to control circuitry (not shown) in the breaker 202. In this configuration, when the alarm signal is activated, the interposing relay closes, causing the breaker to open. The breaker may be closed by activating the breaker-close output line 269. 
     The input signal line 272 is coupled to the breaker control circuitry to determine if the breaker is open or closed. If the breaker 202 has been opened using the interposing relay 270 or tripped using the trip actuator 230, this signal indicates that the breaker is open. 
     The communications microcomputer 250 has three functions in the trip unit. First, it directly monitors line current and interline voltage for each of the three or four wires of the branch line 200 through the ADC 252 and, from these values calculates other values which indicate the status of the line 200. Second, it controls communication between the trip unit 116 and the host computer 140 and BDU 117. Third it performs protective functions which augment the overcurrent protection provided by the overcurrent microcomputer 210. 
     The monitoring function primarily entails collecting voltage and current samples from the branch line 200, calculating and storing functions of these samples which may indicate significant events, and logging these events as they occur. Table I lists the parameters which are monitored by the microcomputer 250. 
     
                       TABLE I______________________________________Measured Parameter Measured Parameter______________________________________Phase A current    Phase A voltage.sup.1Phase B current    Phase B voltage.sup.1Phase C current    Phase C voltage.sup.1Average current (A, B &amp; C)              Average phase voltage.sup.1Neutral current.sup.1              FrequencyGround current.sup.2              Real powerLine voltage A-B   Reactive powerLine voltage B-C   Apparent powerLine voltage C-A   Power factorAverage line-line voltage              Kilowatt hours              Kilowatt hours reverse              Kilovar hours              Kilowatt demand______________________________________ note 1  only for four wire systems (three phases plus neutral) note 2  only when ground sensor is used 
    
     For each monitored parameter, the communications microcomputer 250 records, in a monitored parameter log, the present value as well as the maximum and minimum values that have occurred since the last time the log was cleared. The monitored voltage and current values are RMS values generated from sample values taken over a one-second interval. The frequency parameter is determined by measuring the time interval between successive zero-crossing points of the measured voltage signal for one of the three phases, doubling the measured interval and inverting it to convert it into a frequency. 
     The sampling algorithm and the method used to calculate the RMS current and voltage values are described in detail below. The other values in Table I may be calculated from the current and voltage samples or from the RMS current and voltage values by well known methods. 
     Either the host computer 140 or the BDU 117 may retrieve the monitored parameter values at any time. Either processor may also clear the monitored parameter log. 
     The protective functions implemented on the communications microcomputer 250 allow it to trip the breaker 202 on the occurrence of certain events or to activate the alarm signal to either sound an alarm or open the breaker 202, depending on the system configuration selected by the user. Table II lists the events for which the overcurrent microcomputer 210 (OC) and the communications microcomputer 250 (COM) may trip the breaker 202 and the parameters that are stored in the trip log when the breaker is tripped. 
     
                       TABLE II______________________________________Cause of Trip       Processor Parameters logged______________________________________Overcurrent OC        I.sub.A, I.sub.B &amp; I.sub.CShort Circuit       OC        I.sub.A, I.sub.B &amp; I.sub.CGround Fault       OC        I.sub.A, I.sub.B, I.sub.C &amp; I.sub.GCurrent Imbalance       COM       I.sub.A, I.sub.B &amp; I.sub.COver Voltage       COM       VA-B, VB-C, VC-A &amp; VAVEUnder Voltage       COM       VA-B, VB-C, VC-A &amp; VAVEVoltage Imbalance       COM       VA-B, VB-C, VC-A &amp; VAVEOver Frequency       COM       Freq., VA-B, VB-C, VC-A                 &amp; VAVEUnder Frequency       COM       Freq., VA-B, VB-C, VC-A                 &amp; VAVEReverse Power       COM       KW, KVAR, and KVAShadow      COM       I.sub.A, I.sub.B &amp; I.sub.C______________________________________ 
    
     In Table II, I X  indicates the present current in phase X, VX-Y indicates the present voltage measured from phase X to phase Y, VAVE indicates the average phase-to-phase voltage, KW, KVAR and KVA indicate the present value of real power, reactive power and apparent power, respectively. Table II assumes a three-wire system with no ground current input (except for the ground current trip). Except for the shadow protection, any of the trip events which are implemented using the communications microcomputer 250 must be specifically enabled from the host computer 140 before they can cause the breaker 202 to trip. 
     The alarm/relay functions which may be recognized by the trip unit are listed in Table III. All of these events are recognized by the communications microcomputer 250. 
     
                       TABLE III______________________________________Alarm event        Measured Parameter______________________________________Over phase current I.sub.A, I.sub.B &amp; I.sub.COver ground current              I.sub.G.sup.1Over neutral current              I.sub.N.sup.2Over voltage       VA-B, VB-C &amp; VC-AOver KW            KWOver KVA           KVAOver Frequency     FrequencyOver Reverse KW    Reverse KWOver KW demand     KW demandOver KVAR          KVARUnder Voltage      VA-B, VB-C &amp; VC-ACurrent Imbalance  I.sub.A, I.sub.B &amp; I.sub.CVoltage Imbalance  VA-B, VB-C &amp; VC-APower factor       Power factor______________________________________ note 1  requires ground current sensor note 2  requires four wire system 
    
     The communications microcomputer 250 maintains three logs for use in reporting significant events: the trip log, the event log and the alarm log. The trip log is a nonvolatile memory which holds the last three trip events that have occurred. The host computer 140 can access all three entries in the trip log with time stamps, but the BDU 117 can only access the most recent event without its time stamp. The event log is a volatile memory which holds, on the average, the ten most recent alarm events. More or less may be held, depending on the amount of data logged for each event. This log may only be read by the host computer 140. The alarm log holds the most recent alarm event. This log may only be read by the BDU 117. Either the BDU 117 or the host processor 140 can read the entire monitored parameter log. 
     As shown in Table I, one event which may cause the communications microcomputer 250 to trip the breaker 202 is a shadow event. This event occurs when the communications microcomputer 250 detects an overcurrent condition which should have caused the overcurrent microcomputer 210 to trip the breaker. To understand how the shadow protection function is implemented, it is helpful to understand how the overcurrent microcomputer 210 determines that an overcurrent condition exists and trips the breaker 202. 
     FIG. 3a is a graph of the trip curve 310 and the shadow protection curve 312. FIG. 3b is a similar curve which illustrates how the ground-fault trip function is implemented on a system which provides a ground sensor input signal to the trip unit. 
     In FIG. 3a, the coordinates of point A on the solid-line curve 310 represent the pickup current and delay parameters of the long-time trip setting. The coordinates of point C represent pickup current and delay parameters for the short-time trip setting and the current coordinate of point D represents the instantaneous trip current. 
     Point B on the curve 310 is determined as the intersection of a fixed slope line, originating at the long-time trip coordinates, and a line drawn vertically from the short-time trip coordinate. This line is referred to as an I 2  T curve Similarly, the sloped line between points C and D is a fixed-slope line originating at the short-time trip coordinates and intersecting a line drawn vertically from the instantaneous trip coordinate. The broken line 311 illustrates the trip function without this short-time I 2  T curve. 
     The solid line 310 defines the pickup and trip functions performed by the overcurrent microcomputer 210. A pickup occurs whenever the current sensed on one of the phases can be mapped onto the curve 310. The breaker is not tripped, however, until after the time delay indicated by the time coordinate of the trip curve at the pickup current value. 
     In this embodiment of the invention, the communications microcomputer 250 implements a shadow trip function. The broken-line curve 312 represents this function. It is noted that the shadow trip function can only cause a trip when the main overcurrent function, represented by the curve 310 has failed to do so. In this embodiment of the invention, the long-time (A&#39;), short-time (C&#39;) and instantaneous (D&#39;) trip coordinates are set at values slightly greater than the maximum values which may be set for the overcurrent microcomputer 210 using the front panel switches 216. The trip point values for the shadow overcurrent protection curve are preset and cannot be reset by the user. 
     It is contemplated, however, that the set points for the shadow overcurrent curve may be specified by the operator either through the BDU or through the host microcomputer 140. Alternatively, they may be specified as offsets from the principal overcurrent curve, 310. In this instance, the shadow setpoints would change whenever the operator changed the setpoints for the curve 310 using the front-panel switches. 
     The ground fault curve shown in FIG. 3b consists of two points, a variable trip coordinate E, which may be specified by the operator using the front-panel switches 216, and a short-time trip coordinate F which is automatically set to a current that is 1.5 times the specified ground-fault pickup value and a delay of one-half second. The ramp between the points E and F is a fixed-slope I 2  T curve drawn between the variable trip coordinate and the resulting short-time trip coordinate. 
     All of the functions described above for the communications microcomputer 250 are implemented through a real-time program which controls the operation of the microcomputer. This program consists of a main, or background task and several interrupt handlers or foreground tasks. The main program uses sample values taken in response to a periodic interrupt and performs the calculations needed to generate the various monitoring values. The sampling interrupt routine samples all of the voltage and current waveforms over a one-second interval, squares the sample values and accumulates a sum of squares for use by the foreground task. Other interrupt handlers perform functions such as receiving communications packets from the host processor 140 and BDU 117. 
     In the materials that follow, the main loop of the program for the communications microprocessor 250 is described (FIGS. 4a through 4g). This is followed by a description of the sampling algorithm and the algorithm that controls the comm/watch LED 256 on the front panel of the trip unit (FIG. 6). The object code which implements the functions described with reference to FIGS. 4a through 4g as well as all of the interrupt service routines, including the comm/watch function, is contained in the attached software appendix. This object code is in the form of a program for the PROM 253. 
     In FIG. 4a, the first step 410 in the main loop initializes the hardware registers and various program variables. The next step, 412, logs the initial start-up event in the event log. Step 414 then checks the RS-485 port 266 to determine if the host processor 140 is attempting to communicate with the communications processor 250. If so, any pending packets are processed at this time. The next step, 416, determines if the sum of squares of the voltage and current samples for the last one-second interval are ready for processing. If not, control passes to step 470 via the off-page connector B. 
     If, however, at step 416, the sums of the squares of the five current signals (I A , I B , I C , I G  and I N ) are ready, then, at step 418, the communications microcomputer 250 blocks access to the nonvolatile memory. This step prevents data from the log from being written into the nonvolatile memory by an interrupt service routine while the data is being read from the nonvolatile memory by the main loop. At step 420, the microcomputer 250 calculates the RMS value of each of the five current signals and the average of the three phase currents. In the exemplary embodiment of the invention, the RMS value for one of the signals is derived by taking the square root of the sum of squares value for the signal, provided by the sampling routine, and dividing by the square root of the number of sample values. 
     At step 422, the microcomputer 250 determines if it is operating in a protective mode or in an emulation mode. The emulation mode is used to provide compatibility with other communications systems (not shown). If the system is operating in a protective mode, step 424 is executed which performs current imbalance checks. These checks include determining if an imbalance condition exists, logging the event and starting a timer if the imbalance condition did not previously exist. If the imbalance condition is continuing, this step determines if the timer has expired or if the condition has ceased. If the condition has ceased, an inactive event is logged. After step 424, control is passed to step 426 of FIG. 4b via the off-page connector 2. 
     Step 426 branches around step 428 if the system is in emulation. Step 428 processes the current setpoints based on the RMS values calculated at step 420. This step monitors the signals from the overcurrent microcomputer 210 to determine if a pickup or trip event has occurred. If this is the first occurrence of a pickup or trip event, it is logged. If a previously active pickup condition has been cleared, an inactive event is logged. 
     After step 428, step 430 determines if the breaker counter value held in nonvolatile memory needs to be updated. This counter keeps track of the number of times the breaker 202 has been open/closed or trip/closed in a given trip sequence. Step 432 updates the counter if necessary. 
     Step 434 releases access to the nonvolatile memory and clears a sum-of-squares flag for this set of sum of squares values. The control microcomputer 250 uses two sets of sum of squares values. While one set is being used to collect current and voltage sample data, the other set, generated during the previous one-second interval, is used to generate the next set of RMS current and voltage values. 
     After step 436, the microcomputer 250 performs the digital shadow protection at step 438. As set forth above, the shadow protective function trips the breaker when overcurrent conditions are detected which are beyond the maximum values for a predetermined duration that could cause the overcurrent microcomputer 210 to trip the breaker. If overcurrent conditions at this level are detected, the overcurrent microcomputer 210 must not be operating properly. Consequently, the communications microcomputer 250 trips the breaker directly. 
     Step 440 is executed after step 438. This step determines if the system is able to perform power metering. Power metering functions are not performed, for example, if the system does not have the option enabled. If the system is not ready, control passes to step 470 of FIG. 4c via the off-page connector C. Otherwise, control passes to step 442 which blocks access to the non-volatile memory while the voltages VA-B, VB-C, and VC-A; the line power values, KW, KVAR and KVA; as well as the line frequency values are updated at step 444. After step 444, control passes via the off-page connector 3 to step 446 of FIG. 4c. 
     Step 446 allows communications access to the non-volatile memory and step 448 clears the power metering flag to allow voltage, power and frequency information to be collected for the next one-second interval. Step 450 checks if there are any pending data communications requests from either the host computer 140 or the BDU 117. If so, these requests are processed by this step. 
     At step 452, if the system is in emulation mode, control is transferred via the off-page connector D to step 466 of FIG. 4d. Otherwise, step 454 blocks access to the non-volatile memory and step 458 performs the power protection functions if it is determined, at step 456, that the system can perform these functions. 
     In the exemplary embodiment of the invention, the power protection functions trip the breaker and log an event if any of the following conditions are detected: voltage imbalance, over voltage, under voltage, over frequency, under frequency and over reverse power, assuming that these trip values have been programmed by the operator. After step 458, control passes to step 460 of FIG. 4d via the off-page connector 4. 
     At step 460, if the system is a power metering system, step 462 is executed which processes the power set points. This step may either log the alarm condition and/or activate the alarm signal depending on the options selected by the operator. The alarm functions processed at this step are those described above with reference to Table III. 
     Step 464 releases access to the non-volatile memory so that, at step 466, any pending communications requests may be processed. Step 468 updates the minimum and maximum current voltage, KVAR, KW and KVA values that are stored in the monitored parameter log in volatile memory. 
     Step 470 determines if it is time to update the KW hours, KVA hours and KVAR hours values. These values are updated at four second intervals. If not, control passes, via off-page connector E to step 484 of FIG. 4e. Otherwise, step 472 calculates new values for these parameters. 
     Step 474 determines if it is time to update the values for KW hours, KVA hours and KVAR hours in the non-volatile memory. This operation is performed every eight hours and at any power loss. If not, control is passed to step 484 via the off-page connector E. Otherwise, the non-volatile entries are updated at step 476 and at steps 478 and 480 of FIG. 4e, and the values stored in volatile memory are cleared to allow new values to be calculated. Step 484 processes any pending communications requests that may have occurred while access to the non-volatile memory was blocked. 
     Step 486 of FIG. 4e through step 496 of FIG. 4f perform special processing of the samples being obtained from the sampling interrupt routine when the overcurrent microcomputer 210 has tripped the breaker or has had a long-time or short time pickup. If any of these events has occurred, step 494 calculates RMS current values based on only the 17 current samples that were collected in the last cycle of the current signals I A , I B  and I C  and then logs the calculated values. 
     step 498 permits events which have been flagged in an interrupt service routine to be logged. These events include long-time pickup, short-time pickup, breaker close, breaker open and breaker trip. Step 500 parses any communications requests that were received during the previous processing. 
     Steps 502 to 516 (FIG. 4g) perform various calibration functions on the trip unit under the control of the host computer 120. Finally, steps 518 through 528 of FIG. 4g are invoked periodically to generate a checksum for the data in the non-volatile memory. After step 528, control returns to step 414 of FIG. 4a, described above, via the off-page connector A. 
     FIGS. 5a through 5c are timing diagrams which are useful for describing the operation of the sampling interrupt routine executed by the communications microprocessor 250. FIG. 5a shows successive one-second intervals, T 1 , T 2 , T 3 , . . . T N , T N+1 , each of which is divided into 58 sampling sub-intervals, FS 1  through FS 58 . FIG. 5b shows details of one of the one-second intervals in which each of the sampling sub-intervals FS includes an active sampling interval of 17.07 milliseconds (ms) and an inactive interval of 0.17 ms. FIG. 5c shows details of the active sampling interval which includes 17 0.980 ms sampling intervals and a single 0.408 ms sampling interval. 
     These timing diagrams illustrate the operation of the sampling interrupt routine. This routine periodically interrupts the main program loop, described above with reference to FIGS. 4a through 4g, to collect sample values corresponding to five current signals I A , I B , I C , I G  and I N  (assuming a 4 wire system with a ground current sensor) and three voltage signals VA-B, VB-C, and VC-A (or VA, VB, and VC for a 4 wire system). 
     During each one of the sampling intervals, FS, the routine collects 17 samples of each of the current signals and two samples for each of the voltage signals during each 16.66 ms (1/60 Hz) interval. In addition, in a 0.408 ms offset interval between successive 16.66 ms intervals, the routine collects one more sample for each of the three voltage signals. The combination of the 0.408 ms offset interval and the 0.17 ms inactive interval between successive sampling intervals changes the location in the period of the 60 Hz voltage signal at which samples are taken from period to period. 
     In this embodiment of the invention, the current samples taken during a 16.66 ms interval are sufficient to resolve the 8th harmonic of the 60 Hz current waveform while the voltage samples taken during a one-second interval are sufficient to resolve the 13th harmonic of the 60 Hz voltage waveform, assuming the waveform remains substantially the same over the one-second interval. 
     Starting from time T1, the microcomputer 250 collects three voltage samples for VA-B, VB-C, and VC-A, respectively, and then sets a first interrupt to occur within 20 microseconds and a second interrupt to occur at time T 1A  after a 0.408 ms delay from time T1. The interrupt service routine, POWER --  SAMPLES invoked by the first interrupt squares and accumulates each of the collected voltage values. 
     The interrupt service routine for the second interrupt collects up to five current samples, I A , I B , I C , I G  and I N , as well as three more voltage samples. The current and voltage samples are squared and accumulated by a separate interrupt service routines, SUM --  SQUARES and POWER --  SAMPLES, respectively. As soon as the samples are collected, the microcomputer 250 sets these interrupts to occur before the next sampling interrupt and also sets another sampling interrupt to occur at time T 1B , after a delay of 0.980 ms from time T 1A . 
     At time T 1B , the microcomputer 250 again takes five current samples and three voltage samples, schedules interrupts for SUM --  SQUARES and POWER --  SAMPLES to square and accumulate these sample values and another sampling interrupt, to occur at time T 1C , 0.980 ms after time T 1B . The POWER --  SAMPLES routine continues to schedule itself during the remainder of the 16.6 ms interval in order to calculate the watts and vars measurements. 
     At time T 1C , the microcontroller only collects the five current samples, schedules an interrupt which invokes SUM --  SQUARES to square and accumulate the current samples and schedules the next sampling interrupt. This step is repeated 14 more times until time T 11 , at the end of the 18th sample time of the 17.06 ms sampling interval. During this last sample interval, the microcomputer 250 sets a sampling interrupt to occur at time T 12 , 0.17 ms after time T 11 . The pattern established in the sampling interval FS1 is repeated in the interval FS2 which begins at time T 12 . 
     The multiple interrupt service routines, each consuming only a small amount of processing time, are used to ensure that the samples are collected in a timely manner and that the microcomputer 250 is not kept in an uninterruptable state (i.e. processing an interrupt service routine) when a communications interrupt, circuit breaker trip or other critical event needs to be handled. 
     At the end of each of the sampling intervals FS, five accumulated sum of squares values are available representing the five current signals during the preceding cycle of the three-phase 60 Hz power signal. As set forth above, these sample values are used to calculate and log an RMS value for the current signals after a pickup or trip event, caused by the overcurrent microcomputer 210, is detected by the communications microcomputer 250. The one-cycle RMS values from successive sampling intervals FS are accumulated to produce a set of one-second RMS values which are used by the communications microcomputer 250 in its monitoring and protective functions. The RMS voltage samples are only accumulated over one second intervals. 
     FIG. 6 is a flow-chart diagram which is useful for describing the process by which the communications microcomputer 250 controls the COMM/WATCH LED 256 shown in FIGS. 2 and 2a. The COMM/WATCH LED performs two functions. First, it strobes on for 0.5 seconds and then off for 2.5 seconds of each 3 second interval to indicate that the communications microcomputer 250 is operational. Second, when the communications interrupt is receiving a packet of data from or transmitting a packet of data to the host computer 140, the LED blinks on and off rapidly. These two functions allow an operator to quickly determine that a trip unit is operating properly and is able to receive commands from the host processor. 
     The COMM/WATCH function is implemented across the sampling and communications handling interrupt routines. Thus, this process occurs while the microcomputer 250 is performing the background tasks of the main program and the sampling tasks of the sampling interrupt routine. 
     The COMM/WATCH function is activated at 2.5 ms intervals when, at step 610, the microcomputer 250 strobes its watchdog timer. This strobe is monitored by a watchdog circuit (not shown) which is internal to the microcomputer 250. This circuit resets the microcomputer 250 when an expected strobe does not occur. If, at step 612, a COMM LED flag is not active, control is transferred to step 614 to determine if a communications operation is in progress. If so, then at step 616, the process changes the state of the LED 256 either from on to off or from off to on. At the same time, steps 618 and 620 set the COM LED flag to active and start a COMM LED timer. After step 620, the process is complete until the start of the next 2.5 ms interval. 
     If, at step 612, the COMM LED flag is active, and, as step 622, a communications operation is in progress, step 624 is executed which decrements the COMM LED timer. If the timer has not expired at step 626, the process is complete until the start of the next 2.5 ms interval. If, however, at step 626, the COMM LED timer has expired, step 628 changes the state of the LED 256 and restarts the COMM LED timer. The COMM LED timer is set to cycle the LED 256 on and off at a rate sufficient to be recognized as a blinking light. 
     If, at step 614 or step 622, a communications operation is not in progress, step 632 is executed which resets the COMM LED flag to inactive. Steps 634, 636, 638 and 640 implement the WATCH function. This function continually cycles the LED 256 on for 0.5 seconds and then off for 2.5 seconds. 
     While the present invention has been described in terms of an exemplary embodiment, it is contemplated that it may be practiced as outlined above within the spirit and scope of the appended claims. ##SPC1##