Branch current monitor with calibration

A meter for measuring electric power consumed by a plurality of branch circuits includes interchangeable current transformers including respective transformer memories for storage of transformer characterization data and enables self-discovery of a phase shift induced by respective current transformers and the phase of current conducted by each branch circuit.

BACKGROUND OF THE INVENTION

The present invention relates to a metering system and, more particularly, to a modular meter for measuring electricity consumption by a plurality of loads.

The total power consumption of a building or other facility is monitored by the electric utility with a power meter located between the utility's distribution transformer and the facility's power distribution panel. However, in many instances it is desirable to sub-meter or attribute the facility's power usage and cost to different occupancies, buildings, departments, or cost centers within the facility or to monitor the power consumption of individual loads or groups of loads, such as motors, lighting, heating units, cooling units, machinery, etc. These single phase or multi-phase electrical loads are typically connected to one or more of the branch circuits that extend from the facility's power distribution panel. While a power meter may be installed at any location between a load and the distribution panel, typically a power meter capable of monitoring a plurality of circuits is installed proximate the power distribution panel to provide centralized monitoring of the various loads powered from the panel.

Flexibility has favored adoption of digital power meters incorporating data processing systems that can monitor a plurality of circuits and determine a number of parameters related to electricity consumption. A digital power meter for measuring electricity consumption by respective branch circuits comprises a plurality of voltage and current transducers that are periodically read by a data processing unit which, in a typical digital power meter, comprises one or more microprocessors or digital signal processors (DSP). The data processing unit periodically reads and stores the outputs of the transducers quantifying the magnitudes of current and voltage samples and, using that data, calculates the current, voltage, power, and other electrical parameters, such as active power, apparent power and reactive power, that quantify electricity distribution and consumption. The calculated parameters are typically output to a display for immediate viewing or transmitted from the meter's communications interface to another data processing system, such as a building management computer for remote display or further processing, for example formulating instructions to automated building equipment.

The voltage transducers of digital power meters commonly comprise a voltage divider network that is connected to a conductor in which the voltage will be measured. The power distribution panel provides a convenient location for connecting the voltage transducers because typically each phase of the power is delivered to the power distribution panel on a separate bus bar and the voltage and phase is the same for all loads attached to the respective bus bar. Interconnection of a voltage transducer and the facility's wiring is facilitated by wiring connections in the power distribution panel, however, the voltage transducer(s) can be interconnected anywhere in the wiring that connects the supply and a load, including at the load's terminals.

The current transducers of digital power meters typically comprise current transformers that encircle the respective power cables that connect each branch circuit to the bus bar(s) of the distribution panel. A current transformer typically comprises multiple turns of wire wrapped around the cross-section of a toroidal core. The power cable conducting the load current is passed through the aperture in the center of the toroidal core and constitutes the primary winding of the transformer and the wire wrapped around the cross-section of the core comprises the secondary winding of the transformer. Current flowing in the primary winding (primary current) induces a secondary voltage and current in the secondary winding which is quantitatively related to the current in the primary winding. The secondary winding is typically connected to a resistor network and the magnitude of the primary current can be determined from the amplitude of the voltage at the output of the resistor network. To measure the power consumed by a plurality of loads making up a facility, a current transformer must be installed encircling each conductor in which the current will be measured. Bowman et al., U.S. Pat. No. 6,937,003 B2, discloses a power monitoring system that includes a plurality of current transformers mounted on a common support facilitating installation of a power meter in an electrical distribution panel.

Accurate measurement of electric power also requires compensation for error introduced by the transducers comprising the power meter. For example, the secondary current of a current transformer is ideally equal to the load current in the power cable (the primary winding) divided by the number of turns in the secondary winding. However, magnetization of the core of the transformer produces ratio and phase errors which may vary with the magnitude of the current being measured and the configuration of the particular transformer, including factors such as core material and turns ratio. Typically, error compensation factors are ascertained by experimentation with sample transformers of each production batch and the compensation factors for correcting the calculated output of the meter are stored in a memory in the power meter for use by the data processing unit during calculation of the meter's output.

While initial installation of a power meter at the distribution panel is simplified by integrating a plurality of current transformers into a single assembly, field repairs, modifications and updating of the power meter or the facility's circuitry can be problematic. A power meter is calibrated with a specific set of current and voltage transducers and modification of a meter or replacement of a failed transducer requires recalibration of the meter. A field repairperson typically does not have the equipment necessary to recalibrate the power meter and store new error correction data or a revised transducer configuration in the power meter's memory. As a result, it may be necessary to install a new, calibrated meter or accept inaccurate readings from a meter that has been altered by repair.

What is desired, therefore, is a electricity meter providing flexible construction, simplified installation and improved serviceability.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring in detail to the drawings where similar parts are identified by like reference numerals, and, more particularly toFIG. 1, a digital power meter20arranged to monitor the voltage and current in a plurality of branch circuits comprises, generally, a data processing module22, a current module24and a voltage module26. The data processing module22comprises a data processing unit30which, typically, comprises at least one microprocessor or digital signal processor (DSP). The data processing unit30reads and stores data received periodically from the voltage module and the current module, and uses that data to calculate the current, voltage, power and other electrical parameters that are the meter's output. The calculated values may be output to a display32for viewing at the meter or output to a communications interface34for transmission to another data processing system, such as a building management computer, for remote display or use in automating or managing facility functions. The data processing module may also include a memory36in which the software for the data processing unit and the data manipulated by the data processing unit may be stored. In addition, the data processing module may include a power supply38to provide power to the data processing unit and to the voltage and current modules.

The voltage module26includes one or more voltage transducers42each typically comprising a resistor network, a voltage sampling unit48to sample the output of the voltage transducers and covert the analog measurements to digital data suitable for use by the data processing unit and a multiplexer44that periodically connects the voltage sampling unit to selected ones of the voltage transducers enabling periodic sampling of the magnitude of the voltage. Typically, each phase of the electricity supplied to a distribution panel is connected to a bus bar23to which is connected the circuit breakers16that provide a conductive interconnection to each of the loads. Since the voltage and phase supplied to all commonly connected loads is the same, a meter for measuring three-phase power typically includes three voltage transducers42A,42B,42C each connected to a respective bus bar23A,23B,23C. The voltage module also includes a voltage sensor memory46in which voltage sensor characterization data, including relevant specifications and error correction data for the voltage transducers are stored. If a portion of the voltage module requires replacement, a new voltage module comprising a voltage sensor memory containing sensor characterization data for sensors of the new module can be connected to the data processing unit. The data processing unit reads the data contained in the voltage sensor memory and applies the sensor characterization data when calculating the voltage from the output data of the replacement voltage module.

The current module24typically comprises a current sampling unit50, a multiplexer52and a plurality of current transducers54communicatively connected to respective sensor positions55of current module. The multiplexer52sequentially connects the sampling unit to the respective sensor positions enabling the sampling unit to periodically sample the output of each of the current transducers54. The current sampling unit comprises an analog-to-digital converter to convert the analog sample at the output of a current transducer selected by the multiplexer, to a digital signal for acquisition by the data processing unit. A clock40, which may be included in the data processing unit, provides a periodic timing signal to the data processing unit which outputs a sampling signal to trigger sampling of the transducer output by the current sampling unit. The current module also includes a current sensor memory56in which are stored characterization data for the current transducers comprising the module. The characterization data may include transducer identities; relevant specifications, such as turns ratio; and error correction factors, for example to correct for magnetization induced errors. The characterization data may also include the type of transducers, the number of transducers, the arrangement of transducers and the order of the transducers attachment to the respective sensor positions of the current module. At start up, the data processing unit queries the current sensor memory to obtain characterization data including error correction factors and relevant specifications that are used by the data processing unit in calculating the meter's output.

Monitoring current in a plurality of branch circuits typically requires a plurality of current transducers, each one encircling one of the plurality of branch power cables that connect the distribution panel to the respective branch circuit. Current sensing may be performed by individual current sensors, such as the current transformer54A, that are connected to the current module. Referring toFIGS. 2-4, on the other hand, a power meter may comprise one or more sensor strips80each comprising a plurality of current sensors attached to a common support, such as sensors54A,54B,54C. The sensors54are preferably current transformers but other types of sensors may be used. Each current transformer comprises a coil of wire wound on the cross-section of a toroidal metallic or non-metallic core. The toroidal core is typically enclosed in a plastic housing that includes an aperture82enabling a power cable88to be extended through the central aperture of the core. The openings82defined by the toroidal cores of the transformers are preferably oriented substantially parallel to each other and oriented substantially perpendicular to the longitudinal axis90of the support86. To provide a more compact arrangement of sensors, the sensors54may be arranged in substantially parallel rows on the support and the housings of sensors in adjacent rows may be arranged to partially overlap in the direction of the longitudinal axis of the support. To facilitate routing the power cables of the branch circuits through the cores of the current transformers, the common support maintains the current transformers in a fixed spatial relationship that preferably aligns the apertures of the toroidal coils directly opposite the respective connections of the power cables88and their respective circuit breakers when the strip is installed in a distribution panel100. For protection from electrical shock, a transient voltage suppressor94may be connected in parallel across the output terminals of each sensor to limit the voltage build up at the terminals when the terminals are open circuited.

The transducer strip80may include the current sensor memory56containing characterization data for the current transformers mounted on the support86. The current sensor memory may also include characterization data for the transducer strip enabling the data processing unit to determine whether a transducer strip is compatible with the remainder of the meter and whether the strip is properly connected to the data processing module. Improper connection or installation of an incompatible transducer strip may cause illumination of signaling lights or a warning message on the meter's display. In addition the transducer strip80may comprise a current module of the power meter with one or more current transformers54, the multiplexer52, the current sampling unit50and the current sensor memory all mounted on the support86. A connector98provides a terminus for a communication link102connecting the transducer strip (current module) to the data processing module22.

While strips of spatially fixed current transducers greatly facilitate installation of metering circuitry in power distribution panels, failure of an individual transducer typically requires replacement of the entire sensor strip because the coils of the transformers and the conductive traces that carry the signals from the transformers are encapsulated in insulating material and a damaged transformer can not be removed from the strip for replacement. In addition, current transformers are intended to operate within a specific current range and it is difficult to customize the strips of transducers for a particular application, that is, to provide a mix of transformers having, respectively, different operating ranges at particular locations on the strip to accommodate branch circuits that transmit substantially different magnitudes of current. The inventors concluded that the benefits of mounting current transformers in a strip could be extended if the current transformers making up the strip could be more readily interchanged.

Referring toFIGS. 5 and 6, the power meter may include one or more current modules comprising strips200of removable current transducers202. A common support204maintains the current sensors in a fixed spatial relationship that preferably coincides with the locations of respective circuit breakers in a distribution panel to facilitate insertion of power cables into the apertures208of the current transformers. Preferably the support204is a rigid or semi-rigid dielectric, but a flexible support installed on a rigid or a semi-rigid supporting member(s) may likewise be used. The transformers are typically arranged in substantially parallel rows on the support and the housings of transformers in adjacent rows may be arranged to partially overlap in the direction of the longitudinal axis210of the support204to provide a more compact arrangement of sensors. The exemplary current transformers are attached to the support by screws212that engage threaded tee-nuts214embedded in the support but other types of fastening and latching elements could be used to secure the current transformers to the support.

The electrical interconnection of the current transformer may be accomplished through plugs220and sockets222that are conductively interconnected with the transformer's wiring and conductive traces224embedded in the insulating material of the support. The current sensor memory of the current module may comprise a current transformer memory226that is located within the housing of the individual current transformer and which may replace or, alternatively, supplement a current sensor memory56for the strip of current sensors. The characterization data may include transducer identities; relevant specifications, such as turns ratio; and error correction factors, for example to correct for magnetization induced errors. The characterization data may also include the type of transducers, the number of transducers, the arrangement of transducers and the order of the transducers attachment to the respective sensor positions of the current module. When the operation of the power meter is initiated, the data processing unit reads the transformer characterization data from the current transformer memory and uses the characterization data in calculating current, power and other electrical parameters related to the branch circuit monitored by the respective current transformer. A transducer strip can be customized with transformers having a variety of operating ranges by replacing individual detachable transformers in the strip. Since the transformer characterization data is stored in the transformer, the data processing module is provided with the appropriate characterization data for each of the individual transformers.

A transient voltage suppressor216may be connected in parallel across the output terminals of each transformer to limit the voltage build up when the terminals are open circuited, for example, when the transformer is removed from the support. In the event, the transformer fails or a transformer with different specifications is desired for a particular branch circuit, a new transformer may be installed by removing the appropriate transformer from the support and securing the new transformer in its place.

When a new current transformer is installed in the current module the data processing unit may read the transformer characteristics that are stored in the sensor memory of the current module. On the other hand, the power meter20is arranged to discover the phase shift induced by the inductance of the respective current transformers. Referring toFIG. 6, the support204of the sensor strip includes traces that terminate in sockets228exposed on the upper surface of the support which a technician can connect with a jumper230that is extended through the aperture in one of the current transformers. Referring toFIG. 7, when initiated302, typically by a command on a touch sensitive display32, the data processing unit of the meter energizes the jumper304with a time varying signal, such as a pulse or a periodic waveform. The data processing unit connects to the secondary winding of the first transformer of the current module304and determines if the signal at the jumper has produced a corresponding quantitative change in the signal in the secondary winding308. If a signal is detected, the data processing unit calculates the phase shift310by detecting the temporal difference between the occurrence of a reference point, such as a zero crossing or a pulse, in the secondary winding signal and the occurrence of the same point in the signal that is input to the jumper. The calculated phase shift is stored in an appropriate position in the current sensor memory or current transformer memory312. The data processing unit checks the current sensor memory or the transformer memory for a stored value for a value for the phase shift, either the value that was stored following calculation or a previously stored value, for the respective sensor position314. If no value is stored for the phase shift characteristic, the data processing unit signals an error316typically with a message on the display or an indicator light. If the data processing unit determines that a phase shift characteristic value is stored for the sensor position or if an error is signaled, the data processing unit determines if the sensor position is the last sensor position318of the current module. If the current sensor position is not the last sensor position of the current module, the data processing unit advances to the next position320. If the current sensor position is the last sensor position of the current module, the process ends322with a phase shift characteristic mapped to each transformer position or with an error signaled to the technician who may move the jumper to another transformer position and restart the process302until a phase shift has been determined for all sensor positions.

Power from the utility is conducted to the bus bars of the facility's power distribution panel. Each of the circuit breakers that enable conduction to one or more of the facility's multiple loads is connected to one or more of the bus bars and conducts a single phase, two phases or three phases of power to the respective load. Each of the different loads connected to a bus bar has the same voltage potential and phase relationship and the power factor, which is a phase relationship between the voltage and current provided to a load, may be determined from the current that is conducted to the particular load and the voltage of the bus bar from which that current is drawn. The power consumed by any particular load can be determined from the voltage and its phase relationship sensed from the appropriate bus bar(s) and the current sensed from the respective power cable(s) supplying the load.

While installation of a power meter at the distribution panel has numerous advantages, the installation can be problematic. To properly connect the power meter, the installer is required to identify the phase of each of the bus bars of the distribution panel and the phase or phases supplied to each circuit breaker. However, there is little consistency in the manner of identifying the phase corresponding to the respective bus bars especially when the phase at each bus bar depends upon its interconnection to the power service from the utility. Similarly, the phase(s) carried by each circuit breaker may be difficult to identify because the connection to the bus bar(s) are typically hidden underneath the installed circuit breaker. To properly install the power meter, the installer needs to determine the phase for the respective bus bars, determine the phase for the circuit breaker associated with each branch circuit, identify the wire interconnecting the circuit breaker and the respective load, associate a particular sensor position with the interconnected wire, and interconnect the current sensor with the appropriate location of the power meter. The present inventors considered the likelihood of improper current sensor installation for branch power monitoring and concluded that the likelihood of correct installation would be improved if the power meter could self discover the phase(s) of the power associated with each current sensor interconnected with the power meter.

Referring toFIG. 8, in a first method of phase discovery400, a signal is applied to a first bus bar402, for example bus bar23A. The data processing unit of the power meter is connected to the first current sensor position and, if the signal is detected404, maps “phase A” to the first sensor position406by storing an identifier of phase A in the memory in conjunction with an identifier of the first sensor position. Following mapping to the first sensor position or if the signal was not detected at the first sensor position and if the current sensor position is not the last sensor position of the meter408, the data processing unit connects to the next sensor position410and attempts to detect the signal at this position404. Once the data processing unit has attempted to detect phase A for all of the sensor positions408, a signal is applied to the second bus bar of the panel412and the data processing unit serially attempts to detect the signal for all of the sensor positions414. If the signal is detected for any of the sensor positions, “phase B” is mapped to the memory for the respective sensor position416. If the position being checked by the data processing unit is not the last sensor position418or if the signal is not detected at the position being checked420, the data processing unit connects to the next position and the detection process is repeated for all current sensor positions. When all the sensor positions have been queried for phase B418, the third bus bar (phase C) is energized422. As with the first two bus bars, the data processing unit serially attempts to detect the signal424for each sensor position and maps “phase C” to the respective sensor positions at which the signal is detected. The data processing unit checks each sensor position426until all positions have been tested430and then terminates the phase discovery process432.

Referring toFIG. 9, in a similar phase discovery process500, the data processing unit is conductively connected to the circuit breaker corresponding to the first current sensor position502. A pulse or other signal is applied to the first bus bar504and if it is detected506, phase A is mapped to the first sensor position508. The signal is applied to the second bus bar510and if it is detected at the first sensor position, phase B is mapped to the first sensor position512. Finally, the test signal is communicated to the third bus bar514. If the data processing unit detects the signal at the first sensor position516, phase C is mapped to the first current sensor position518. If the current sensor position is not the last sensor position520, the data processing unit connects to the next sensor position522and the process is repeated until the last sensor position has been checked520and the program ends524.

FIG. 10illustrates still another method of phase discovery600. In the third method of phase discovery, a coded signal is transmitted from each branch circuit connection at the distribution panel602. The coded signal, for example a single pulse for the first sensor position; two pulses for the second sensor position, etc.; identifies the current sensor position corresponding to the branch circuit. If the data processing unit detects the coded signal identifying the first sensor position at the first bus bar604, phase A is mapped to the first sensor position606. The data processing unit serially attempts to detect each of the coded signals608until it has attempted to detect the code corresponding to the last sensor position610. If the data processing unit has attempted to detect the code for the last sensor position610at the first bus bar, the data processing unit attempts to detect the code for the first sensor position at the second bus bar612. When a code is detected at the second bus bar, phase B is mapped to the respective sensor position614and the data processing unit attempts to detect the next code616until it has attempted to detect all of the codes at the second bus bar618. The process is repeated at the third bus bar620and phase C is mapped to any sensor position corresponding to a detected code622. When all position codes have been tested for all bus bars624,626, the process terminates628. The phase discovery methods enable determination of the number and identities of the phases of the power conducted in each branch circuit.

The power meter provides flexibility enabling customization during installation and repair of individual current sensor elements.

The detailed description, above, sets forth numerous specific details to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid obscuring the present invention.

All the references cited herein are incorporated by reference.