Abstract:
A multifunction apparatus for monitoring and reporting electric signals on electric circuits is disclosed. The apparatus comprises a first system for receiving input data from at least a field transformer or a line post sensor; a digital signal processor (DSP) system coupled to the first system; a microprocessor system coupled to the DSP system. The first system in combination with the DSP system and the microprocessor system perform metering, power quality, digital fault recording (DFR) and supervisory control and data acquisition (SCADA) functions.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a method and apparatus for monitoring electric signals on electric circuits and, more particularly, it relates to a multifunction intelligent electronic device (IED) for performing metering, power quality, digital fault recording (DFR), and supervisory control and data acquisition (SCADA) functions.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electric power must be monitored and controlled to ensure proper operational performance. Power must be monitored and controlled in real-time to permit switching for maintenance, improved utilization and efficiency, and in response to situations that may compromise safety. Electric power must also be metered at various points for revenue purposes and also to ensure that circuits are optimally loaded. With the proliferation of computer equipment and electronic controllers for motor drives, electric circuits are littered with unwanted signals called “harmonics”. These harmonics contribute to reducing power quality and must, therefore, be identified and controlled to prevent equipment disturbances and failures. As these metered and harmonic signals are relatively small in magnitude, they must be reported with high precision within a range that is typically termed as the “nominal” range. Further, power lines are subjected to various disturbances as a result of natural and environmental effects such as lightning, high winds, ice load, rodents, birds, etc. Man made circumstances such as vehicular accidents further add to these disturbances.  
           [0003]    In contrast to harmonic and metered signals, disturbance signals are usually large in magnitude and occur within a range that is typically termed as the “overcurrent” range. The “overcurrent” range spans from “1-42” times the “nominal range”. Signals in this range are typically recorded with a DFR system. Electric operators require equipment that can accurately, safely, and economically monitor and control power lines in real-time (i.e., via SCADA) in addition to recording and reporting both metering and disturbance information.  
           [0004]    With respect to metering DFR and SCADA functions, known methods typically call for separate equipment units to carry out metering, harmonic spectrum and disturbance functions due to the inability of an integrated device to perform multiple functions in the required range of measurement. Prior methods accomplished utility metering and harmonic spectrum functions with metering devices, while disturbance recording functions were accomplished using a different apparatus such as a DFR system. SCADA was accomplished by interfacing the equipment to be monitored and controlled in real-time to remote terminal units (RTUs). These multiple devices interfaced to the power lines via potential transformers (PTs) and current transformers (CTs). Employing multiple devices to perform a plurality of functions requires more installation room and increased engineering, commissioning, and maintenance costs.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    Accordingly, the exemplary apparatus described herein provides a multifunction intelligent electronic device (IED) and method for performing such functions as metering, power quality, digital fault recording (DFR) over an extended current range, while permitting real-time monitoring and control (SCADA) equipment, such as for example, communication devices, for multiple and primary equipment having a plurality of digital inputs, transducerless AC analog inputs to name a few. The IED may be directly connected to voltage potential and current transformers or line post sensors on electric circuits to monitor electric signals.  
           [0006]    Input data from field transformers, such as current transformers (CTs) and potential transformers (PTs), is received by an A.C. input sub-system which is coupled to field transformers through a field interface terminal unit. The A.C. input subsystem is coupled to a digital signal processor (DSP) system to process the data received from the field transformers for executing metering, harmonic content, and DFR functions. The DSP subsystem converts the analog input signals into digital representations using a MUX/ A/D converter/ Signal conditioning unit. The DSP subsystem includes first and second signal processing devices which operate under the control of a primary microprocessor to control various functions of the IED apparatus including the switching functions of the A.C. subsystem in overcurrent and metering ranges. A second microprocessor works with the primary microprocessor that monitors and controls a plurality of input and output signals for SCADA functions.  
           [0007]    Each transformer further includes normal and common mode surge and fast transient protection circuitry, and a crowbar protection circuit for protection against signals that are higher in positive polarity than a supply voltage +V, and signals that have more negative polarity than a supply voltage −V. Switching circuits of the IED perform appropriate auto-ranging functions depending on whether the field current in a primary circuit of a respective transformer is within a metering range or an overcurrent range. A microprocessor controls the functioning of each of the switching units in addition to providing such functions as analog-to-digital conversion, multiplexing functions, et cetera.  
           [0008]    In one aspect, a multifunction apparatus for monitoring and reporting electric signals on electric circuits, comprising a first system for receiving input data from at least a field transformer or line post sensor; a digital signal processor (DSP) system coupled to the first system; a microprocessor system coupled to the DSP system; and the first system in combination with the DSP system and the microprocessor system perform metering, power quality, digital fault recording (DFR) and supervisory control and data acquisition (SCADA) functions. The first system comprises a plurality of transformers, each transformer operating with respect to one phase of an electric circuit; and a plurality of switching circuits, each circuit coupled to a respective transformer and further adapted to switch to multiple positions depending on whether the current flowing through a primary circuit of a respective transformer is in a metering range or an overcurrent range. The multifunction apparatus further comprises a circuit assembly for providing normal mode surge and fast transient protection. The circuit assembly preferably comprises a gas tube arrestor, a metal oxide varistor (MOV), a transient voltage suppressor, or a capacitor. The multifunction apparatus further comprises a circuit assembly for providing common mode surge and fast transient protection. A secondary circuit of each transformer preferably includes a diode mirror circuit for providing crowbar protection against signals that are higher in absolute value than supply voltage.  
           [0009]    In another aspect, a method for monitoring electric signals on electric circuits, the method comprising electrically coupling a monitoring apparatus to a field sensor; feeding data from the field sensor to an A.C. subsystem of the monitoring apparatus, the A.C. sub-system comprising a plurality of transformers; and causing switching circuits to switch to multiple positions depending on whether current flowing through a primary circuit of a respective transformer is in a metering range or an overcurrent range. The method further comprises: providing a digital signal processor (DSP) sub-system to process data received by the A.C. sub-system; and providing one or more microprocessors for (a) controlling communication software applications of the apparatus, (b) performing supervisory control and data acquisition (SCADA) functions. The method further comprises providing normal mode surge and transient protection circuit between the field sensor and a primary circuit of each of the plurality of transformers; and controlling the A.C. sub-system and the DSP sub-system by at least one or more microprocessors. The method also comprises providing common mode surge and transient protection circuit between the field sensor and a primary circuit of each of the plurality of transformers, providing a crowbar protection circuit against signals that are higher in absolute value than supply voltage.  
           [0010]    In another aspect, a multifunction apparatus for monitoring and reporting electric signals, comprising: a first subsystem receiving input data from at least one field sensor, the first subsystem having a plurality of transformers, one or more switching circuits, each switching circuit capable of switching to multiple positions depending on whether the current flowing in a primary circuit of a respective transformer is in a metering range or an overcurrent range; one or more digital signal processors processing data received by the first subsystem; and one or more microprocessors controlling the first subsystem and the one or more digital signal processors.  
           [0011]    In yet another aspect, an apparatus for monitoring electric signals on electric circuits, the apparatus comprising: an A.C. sub-system having a plurality of transformers, and one or more switching circuits; means for electrically coupling the apparatus to a field sensor; means for feeding data from the field sensor to the A.C. subsystem; and means for causing the switching circuits to switch to multiple positions depending on whether the current flowing in a primary circuit of a respective transformer is in a metering range or an overcurrent range. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows an exemplary high level block diagram embodying the device in accordance with an example embodiment of the present invention;  
         [0013]    [0013]FIG. 2 illustrates an exemplary detailed circuit diagram of the multifunction intelligent electronic device for measuring signals of a three-phase power line in accordance with an example embodiment of the present invention;  
         [0014]    [0014]FIG. 3 illustrates an exemplary Digital Signal Processor (DSP) sub-system of the present invention of FIG. 1;  
         [0015]    [0015]FIG. 4 depicts communications between the DSP sub-system, as shown in FIG. 3, and the main processor of the device in an example embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring now to FIG. 1, there is shown a high level block diagram of the IED according to the present invention. IED  10  generally includes an A.C. input subsystem  82 , a digital signal processor (DSP) subsystem  84 , and a microprocessor system  86 . IED  10  is coupled to a field transformer  80  through a field interface terminal block P 2 . The field transformer  80  may be a current transformer (CT) or a potential transformer (PT). Input data from the field transformer  80  is received by an A.C. input sub-system  82  which is responsible for monitoring metering, power quality, and digital fault recording (DFR) functions. The DSP subsystem  84  includes first and second DSP devices  54 ,  56  (FIG. 3) one of the devices being responsible for metering and power quality signal from the A.C. subsystem  82  while the other is responsible for monitoring DFR functions. A microprocessor system  86  controls the overall operation of the IED device  10  including the functioning of the A.C. subsystem  82  and the DSP  84 . The microprocessor system  86  includes microprocessors  68 ,  70  (FIG. 4). Microprocessor  70  operates under the control of microprocessor  68  to perform SCADA functions. It will be appreciated that the number of microprocessor used should not be limiting of the present invention. Greater or fewer number of processors may be used to accomplish the inventive functions.  
         [0017]    Referring now to FIG. 2, there is shown a detailed circuit diagram  82  of the A.C. input sub-system  82  of the multifunction IED  10 . An input signal from a field transformer  80  is received in the terminal block P 2  through the first terminal block position P 2 - 1 . The received signal enters conductor  11  and exits through the second terminal block position P 2 - 2  where conductor  11  terminates. It should be noted that field transformer  80  need not necessarily be terminated at the terminal block P 2 , rather the connections from transformer  80  may pass through to the primary winding of transformer T 1 . Metal oxide varistor (MOV)  13 , and capacitor  14  provide normal mode surge and fast transient protection to IED  10  from interference surges in respective phase of the power line. A circuit formed by MOVs  15 ,  17 , and the capacitors  16 ,  18  provide common mode surge and impulse protection to circuit created by conductor  11 . In the exemplary embodiment of FIG. 2, the A.C. input subsystem  82  is discussed with respect to three phases, generally identified as phase 1, phase 2, and phase 3, to explain the inventive concept without ambiguity. The present invention may actually be capable of operating on up to 12 phases of an electric circuit. It will further be appreciated that the present apparatus may be readily scaled to accommodate additional phases of an electric circuit.  
         [0018]    Conductor  11  couples to the primary winding of transformer T 1 . Current in the primary winding of transformer T 1  induces a magnetic field causing a signal to appear in the secondary winding. Supply voltages +V and −V are provided to diodes  21 ,  22 , respectively. Diodes  21 ,  22  meet the second winding at junction  23  and provide crowbar protection against signals that are higher in positive and negative polarities than supply voltages +V and −V, respectively. A switching integrated circuit (SIC) U 2  is connected to the secondary winding and controlled by DSP  84  (FIG. 3). DSP  84  is coupled to the AC subsystem  82  through an interface connector  50 . DSP  84  is controlled by microprocessor  68  (FIG. 4). DSP  84  converts analog signals  42 ,  44 ,  46  to digital signals using a MUX/ A/D converter/signal conditioning unit (“MAS”)  64  (FIG. 3). The digital signals are then passed to DSP1  54  (FIG. 3) which then writes into a compressor register located in CPLD B  60  resulting in a control signal at device  32  of FIG. 2. The control signal at device  32  is high indicating compression mode when the analog signals  42 ,  44 ,  46  are in the overcurrent range. The control signal at device  32 , however, is low when analog signals  42 ,  44 ,  46  are in the metering range. Other embodiments of the present invention may include switching to more than the described two positions depending on the interface to the DSP sub-system.  
         [0019]    The SIC U 2  having pins U 2 - 10  &amp; U 2 - 11  and pins U 2 - 14  &amp; U 2 - 15  engages resistance  38  into the circuit when the field current flowing through the primary winding of T 1  is in the metering range, thereby developing a voltage at pin U 2 - 10 , the voltage being proportional to the current flowing in the primary winding of T 1 . When the field current flowing through the primary winding of T 1  is in the overcurrent (DFR) range, resistance  39  is engaged by closing contacts U 2 - 2  &amp; U 2 - 3 , U 2 - 6  &amp; U 2 - 7  and opening contacts U 2 - 14  &amp; U 2 - 15 , U 2 - 10  &amp; U 2 - 11 , thereby developing a voltage at pin U 2 - 7 , the voltage being proportional to the current flowing in the primary winding of transformer T 1 . The voltage developed at pin U 2 - 7  or pin U 2 - 10  of SIC U 2  is now presented at pin  3  of the operational amplifier  40  which buffers the voltage to provide a signal identified at  42 . The signal identified at  42  is multiplexed with similar signals identified at  44 ,  46  with respect to other phases of the power line. The multiplexed signals are converted to digital signals by MAS  64  of the DSP  84 . Other embodiments of the present invention may include a single switching circuit for all of the transformers previously described.  
         [0020]    The DSP  84  includes two digital signal processors (DSP1  54  &amp; DSP2  56 ). DSP  84  further includes DSP I/O and DSP DFR, power quality and metering applications in software form. One of DSP1  54  or DSP2  56  may be dedicated to the collection of AC input data, while the other may be responsible for performing DFR functions. The DSP I/O application runs on DSP1  54  and receives sampled data from MAS  64 , processes the received sampled data, and reports the processed data to processor  68  (FIG. 4) through a dual port memory  72  (FIG. 4). An interrupt routine running on DSP  84  receives samples from the MAS  64  and copies them to primary input buffers. Upon collecting data pertaining to a complete cycle, an interrupt routine of processor  68  sets a flag to initiate a main processing routine by correcting the magnitude component of the data using correction factors identified by a user via a user interface to offset manufacturing tolerances in transformer  80 . The corrected data is then transformed into frequency domain applying a Discrete Fourier Transform (DFT). Phase correction is performed on the transformed data to compensate for the phase shift introduced by analog multiplexers of MAS  64 . Total Harmonic Distortion (THD), Root-Mean Square (RMS) values for currents and voltages, active, reactive, and apparent power, power factor, energy values, and symmetrical components are then computed by DSP1  54 , DSP2  56  under the control of processor  68 . If 60/50 Hz filtering is enabled, then these values are computed using the fundamental frequency component only. However, if 60/50 Hz filtering is disabled, then these values are computed using all DFT series coefficients. The computed values are then copied into a dual-port memory  72  and an interrupt to processor  68  is issued. The sampling period is then adjusted and processing ceases until the interrupt routine sets the flag again, thus indicating that another cycle of data is ready for processing.  
         [0021]    The DSP DFR application runs on DSP2  56  and receives sample data from the A/D converter of MAS  64 . The DSP2  56  applies time stamps and reports the timestamps to processor  68  for further processing by the DFR Data Translation Application (DFR-DTA) residing in processor  68 . The DSP-DFR application communicates with the DFR-DTA application through shared memory accessible to both the DSP DFR and processor  68  via the interprocessor communication subsystem (ICOM) software layer (not shown). ICOM includes a component resident in the code for processor  68  and a mirrored component resident in DSP2  56 . The DSP DFR application functionalities includes boot and control of the DSP DFR processor in DSP2  56 , providing communication functions with processor  68  (i.e., provide the DSP side of ICOM subsystem, now referred to as ICOM-DSP), providing communication functions with the DFR DTA application running on processor  68  according to the DFR DTA/DSP Message Protocol, reading data samples on the A.C. analog inputs and providing a time stamp to each of the analog inputs received by the A.C. subsystem  82 , and transferring data samples to the shared memory to be available to the DFR DTA application. IED  10  is further capable of SCADA functions under the control of processor  70  (FIG. 4). Processor  70  communicates with processor  68  to distribute its data and to receive control operation commands. Thus, IED  10  of the present invention accomplishes metering, power quality, and DFR functions while maintaining SCADA functionality and performance.  
         [0022]    The operation of the A.C. sub-system of the IED is explained with respect to a power line consisting of three phases as illustrated in FIG. 2. Circuit operation with respect to phase 1 is explained in detail as set forth above. Operation of phases 2 and 3 are similar to the operation of phase 1. The advantages of IED  10  include its ability to accurately report in the metering as well as overcurrent (DFR) ranges. Further, the IED  10  interfaces with all popular field transformers, has low input impedance, wide frequency response and operating temperature ranges. It is further immune to current overloads, and electromagnetic interference. The IED accomplishes high accuracy metering, power quality, and wide dynamic range DFR functions while maintaining SCADA functionality and performance.  
         [0023]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.