Current meter with on board memory

The accuracy and flexibility of a branch circuit monitor is improved by storing specifications, including error correction factors, for a plurality of current transformers in the monitor's memory and enabling current transformers with stored specifications to be selected for use with the monitor.

BACKGROUND OF THE INVENTION

The present invention relates a current monitoring system and, more particularly, to a user configurable branch circuit monitor with a memory for storing transducer characteristics.

The total power consumption of a building or other facility is typically 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, 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 current and power meters, known as branch circuit monitors, 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 the meter's 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 sensing the magnitudes of the current and voltage and, using this sample 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 and/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.

While the voltage transducer(s) of a digital power meter can be interconnected anywhere in the wiring that connects the supply and a load, including at the load's terminals, interconnection of voltage transducers and the facility's wiring is facilitated by wiring connections in the power distribution panel. The power distribution panel provides a convenient location for connecting the voltage transducers because typically each phase of the power is conducted by a separate bus bar in the power distribution panel and the voltage and phase is the same for all loads attached to the respective bus bar. 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 current transducers of digital power meters typically comprise current transformers that encircle the respective power cables that interconnect each circuit to the bus bar(s) of the distribution panel. To measure the power consumed by the plurality of loads making up a facility, a current transformer must be installed in each branch circuit connecting the load(s) to 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.

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 flowing 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 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, a power meter is configured for use with a particular transducer and error compensation factors, ascertained by experimentation with sample transducers, are built into the meter and applied by the data processing unit during calculation of the meter's output.

However, a user may desire to use a different transducer than that specified for use with a particular meter or several different transducers may be required because of a wide variation in the magnitude of the current in the various branch circuits or it may be necessary to replace one or more transducers during use. Using transducers that are not matched to the meter results in inaccurate readings from the meter. What is desired, therefore, is a branch circuit monitor providing flexible construction, simplified installation and improved serviceability.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Electricity delivered by the local utility to a facility's mains is distributed to the various loads in the facility by branch circuits which are conductively connected to the mains at a distribution panel. Referring in detail to the drawings where similar parts are identified by like reference numerals, and, more particularly toFIG. 1, an exemplary distribution panel20includes two three-phase mains22A,22B which are respectively connected to main circuit breakers24A,24B. Each of the phases of each main is connected to a respective bus bar26A,26B,26C and28A,28B,28C. Three of the bus bars extend behind each of two rows of branch circuit breakers30that respectively conductively connect one of the bus bars to a branch circuit conductor32which is conductively connected to the load(s) of the branch circuit. A single phase load, for example, load34, is conductively connected to single bus bar. A two-phase load is connected, typically, through two adjacent circuit breakers in the same row, to two bus bars conducting different phases of the power. A three-phase load, for example, load36, is connected, typically, through three adjacent circuit breakers in the same row, to three bus bars each conducting a different phase of the electricity.

A digital branch circuit monitor40is often installed in an electrical distribution panel, such as the exemplary distribution panel20, to monitor the current or the current and the voltage in the circuits served by the panel. For example, a digital branch circuit monitor produced by Veris Industries, Inc. can monitor the current or the power (current and voltage) in 84 branch circuit conductors and two three-phase mains of a power distribution panel and can determine, in addition to voltage and current, a number of other parameters related to energy consumption for each circuit served by the distribution panel. Data updates occur approximately every two seconds and an alarm may be triggered by the monitor if the operation of a circuit approaches a user configured threshold. The exemplary branch circuit monitor40comprises a main acquisition board42and four adapter boards44,46,48,50facilitating interconnection of the measuring circuitry and the transducers which sense current and voltage in the various conductors of the distribution panel.

Referring also toFIG. 2, the exemplary digital branch circuit monitor40is arranged to monitor the both voltage and current in a plurality of branch circuits and comprises, generally, a data processing unit52, a current measuring section54and a voltage measuring section56. The data processing unit52typically comprises at least one microprocessor or digital signal processor (DSP). The data processing unit52periodically reads and stores data from the voltage56and the current54measuring sections, 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 display58for viewing at the meter or output to a communications interface60for transmission via a communication network62to another data processing system75, such as a building management computer, for remote display or use in automating or managing the facility's operations. A memory64in which the software for the data processing unit and data manipulated by the data processing unit may be stored is associated with the data processing unit. A power supply66provides power to the data processing unit and to the voltage and current measuring sections. One or more clocks68, which may be incorporated into the data processing unit, time intervals between the occurrences of monitored events.

The voltage measuring section56includes one or more voltage transducers70A,70B,70C each typically comprising a resistor network, a voltage sampling unit76to sample the output of the voltage transducers and convert the analog measurements to digital data suitable for use by the data processing unit and a multiplexer78that periodically connects the voltage sampling unit to a selected one of the voltage transducers enabling periodic sampling of the magnitude of the voltage at each voltage transducer. Typically, each phase of the electricity supplied to a distribution panel is connected to a bus bar, for example, bus bars26A,26B,26C, and each branch circuit breaker30connectively interconnects one of the bus bars to a branch circuit conductor32. One or more of the branch circuit conductors are conductively connected to each of the facility's loads, for examples, loads34,36. Since the voltage and phase supplied to all commonly connected loads is the same, a meter for measuring three-phase power typically includes at least three voltage transducers70A,70B,70C each connected to one of the bus bars26A,26B,26C that is conducting a different phase of the power from one of the mains, for example, main22A.

The current measuring section54comprises a current sampling unit80, a multiplexer84, a signal conditioning unit83and a plurality of current transducers. The current transducers are preferably current transformers but other types of current sensors might be used. Current monitoring with a current transformer requires a current transformer for each of the conductors in which the current is to be monitored. In the exemplary branch circuit monitor40a current transformer89senses the current in each of the panel's mains and a branch current transducer88senses the current in each of the panel's branch circuit conductors32.

The auxiliary current transformers89monitoring the current in the mains are connected to a terminal block91on the main circuit board. Each terminal of the terminal block91is conductively connected to the multiplexer84. For convenience, each of the branch current transducers88may be respectively connected to a sensor position90on one of the adapter boards,44,46,48,50. A multi-conductor cable92communicatively connects each of the sensor positions of each of the adapter boards to the multiplexer84on the main circuit board42. The multiplexer84is connected to a signal conditioning unit83which is, in turn, conductively connected to an analog-to-digital converter94of the current sampling unit80. In response to a timing signal from the clock68, the multiplexer sequentially connects each sensor position and each terminal block connection to the signal conditioning unit and, thereby, the analog-to-digital converter which outputs to the data processing unit52a digital signal representing a sample output of the respective current transducer. The data processing unit calculates the current in the power cable monitored by a current transducer from the characteristics of the transducer and a plurality of sample outputs of the transducer accumulated over a period of time.

Each current transformer comprises a coil of wire wound on the cross-section of a toroidal non-metallic or metallic core which is commonly enclosed in a plastic housing. Referring toFIGS. 3,4, and5, the conductor in which the current is to be measured, for example the conductor154, is passed through the central aperture in the toroidal core. This conductor154is the primary winding of the transformer and current flowing in the conductor induces a secondary current and voltage in the secondary winding, the wire wrapped around the cross-section of the core. Referring toFIG. 3, current transformers may have a solid core152requiring that the conductor be disconnected for insertion into core's aperture. On the other hand referring toFIG. 4, current transformers may have a split core comprising two hinged or separable core portions158,160enabling creation of a gap between the core portions for inserting the conductor154while it is still connected. Referring toFIG. 5, after the conductor154is positioned in the area of the core's central aperture, the core portions158,160are joined, closing the gap, for operation. The sensed output of a current transformer may be the current in the secondary winding, typically, having a range of 4-20 milliamps (mA) or 0-5 Amps. On the other hand, the sensed output may be the voltage measured across a burden resistor156connected in parallel with the secondary winding. Typical voltage ranges for current transistors are 0-0.333 volts (v.), 0-5 v. or 0-10v. However, current transformers with other output current and voltage ranges are available.

Ideally, the secondary current of a current transformer is equal to the current in the primary winding, the conductor of the current that is to be measured, divided by turns ratio, the number of turns in the secondary winding divided by the number of turns in the primary winding which is often one. However, actual transformers are not ideal transformers and the magnetization of the core of a current transformer causes the primary current to be less than the product of the secondary current and the turns ratio. Accurate measurement of electric power requires compensation for error introduced by the transducers.

Current transformer error comprises a phase error and a ratio error. Referring toFIG. 6, the ratio error (re) varies with the magnitude of the primary current (I1) as follows:
re(%)=K3+K4(logI1)  (1)

The effect of the ratio error is to alter the relationship between the magnitudes of the measured secondary current (I2) and the primary current (I1) from the theoretical relationship, that is:
I1=I2(n)  (2)

to the relationship:

where I′2=measured secondary current

The magnitude of the measured secondary current (I2′) is related to the theoretical secondary current (I2), as follows:

Referring toFIG. 7, magnetization of the transformer core and windings also causes a phase shift between the current in the primary winding and the current in the secondary winding. The resulting phase error (P) varies with the magnitude of the primary current (I1) approximately according to the relationship:
P=K1+K2(I1−M)  (5)

In practice M is often approximately equal to ½ and, consequently, a square root approximation can often be conveniently employed as part of the overall correction algorithm.

The values of the constants K1, K2, K3, and K4and, therefore, the magnitudes of the phase and ratio errors depend upon the configuration of the particular current transformer. Factors such as core material and turns ratio affect the values of the constants which are typically ascertained by experiment with samples of a given core configuration. Typically, the values of K1, K2, K3, and K4are determined for a particular transformer configuration or production batch by comparing the actual performance of a sample of the transformer configuration to the performance of a standard device when the secondary winding is connected in parallel to a particular impedance or burden.

Typically, a branch circuit monitor is intended for operation with a particular current transformer and is designed to compensate for errors produced by the transformer. The accuracy of the branch circuit monitor suffers if the user elects to use a different current transformer with the meter. However, a user may desire to use current transformers other than those recommended by the meter's manufacturer to, for example, replace a damaged current transformer after installation or use existing current transformers when replacing another branch circuit monitor. The present inventor concluded that the flexibility and serviceability of a digital branch circuit monitor could be improved if the installer could customize the configuration of monitor for the current transformers chosen for use with the meter. The inventor concluded that if the installer was provided with a configuration tool that enabled access to the branch circuit monitor's memory, the installer could configure the monitor for use with current transformers having a wide range of specifications.

The branch circuit monitor configuration tool may be installed on the branch circuit monitor40or may be installed on another computer75, typically, comprising a data processing unit to execute instructions; a memory for storing instructions and data utilized by the computer's data processing unit; one or more input devices, such as a keyboard and a mouse and/or touch pad; and a display. The computer75is communicatively connected, typically, either directly or via a communication network62, such as the Internet, through the monitor's communication interface60to the data processing unit52of the branch circuit monitor40. When the configuration tool is activated, instructions executed by the data processing unit of the branch circuit monitor or the remote computer75cause a series of templates to be displayed on the monitor's display58and/or the display of the remote computer75enabling an installer of the branch circuit monitor to enter and store in the branch circuit monitor's memory64attributes of the current transformers which the user intends to use with the monitor.

Referring toFIG. 8, an initial template200of the configuration tool comprises an interface enabling specification of general characteristics of the branch circuit monitor and its relationship to a larger data processing network, such as a building management system. The data processing unit52of the exemplary branch circuit monitor40can communicate with another data processing network utilizing the MODBUS® protocol. Each device in a network utilizing the MODBUS protocol must have a unique address. By selecting a scan button202on the template, the configuration tool can identify other devices making up a larger system by the device's product name204, serial number206, location208and MODBUS address210. The initial template includes a plurality of user selectable buttons, including a configure device button222, which, when selected, displays a tabbed file223to facilitate the specification of various attributes of the branch circuit monitor. When the general tab225of the initial template is selected, the installer can enter a general description of the branch circuit monitor that is being configured. Each distribution panel has a location identifier212which is used in reporting the operation of the branch circuit monitor and which can be entered from the template. The template also includes one or more icons214,216representing possible arrangements for connecting the plurality of sets of branch current transformers that can supported by the monitor enabling the installer to easily select and specify the arrangement of the current transducers for a particular monitor.

Referring to alsoFIG. 9, selecting the AUX Breaker Size tab234on the initial display200displays a template300enabling specification of the size of a plurality of auxiliary circuit breakers, typically the mains circuit breakers. A typical mains circuit breaker is rated at 225 amps and the configuration tool suggests this size for the mains circuit breakers of the panel. If all the auxiliary circuit breakers have the same current rating the installer can enter the single rating302or if the auxiliary circuit breakers have different ratings, for example if one is not used, each auxiliary circuit breaker's rating can be specified separately304. Clicking a SET CHANNELS button306saves the selected values. On the other hand, if the ratings of the auxiliary circuit breakers are already stored in the branch circuit monitor, selecting a button308in the template causes the monitor's data processing system to be queried and to load the ratings into the configuration tool.

Referring to alsoFIG. 10, selecting the Branch Breaker Size tab236on the initial display200produces a template320enabling specification of the current ratings of the branch circuit breakers. If the branch circuit breakers have a common rating, the common rating can be entered322or, if various breakers have different ratings or are not used, the rating can be entered for specific circuit breaker324. The template suggests a default value for the circuit breaker current rating that is common for branch circuit breakers. If the ratings of the branch circuit breakers are already stored in the branch circuit monitor's memory, the template enables querying the monitor's data processing system to upload the ratings to the configuration tool by selecting a button308on the template. When the size(s) of the branch circuit breakers have been specified, the sizes can be stored in the branch circuit monitor's memory by activating a SET CHANNELS button326. If the monitor is monitoring additional branch circuits, activation of a NEXT CHANNELS button328will display the additional channels, for example, channels43-82.

Referring to alsoFIG. 11, selecting the branch current transformer (Branch CTs) tab238of the configuration tool produces a template350enabling the user to specify the branch current transformers to be used with the branch circuit monitor. The template includes a representation352of the arrangement of the current transformer channels of the branch circuit monitor. Initially, the template displays the current rating (size)354, manufacturer358, model360, core type362and the transformer output364of the recommended current transformer. However, the installer can select one or more current transformer channels356and change the specifications of the current transformer to be used with that channel or group of channels. When a channel is selected, pull-down menus, for example, pull-down menu366, are enabled which display additional current transformer manufacturers, models, and core types stored in the monitor's memory for the size of current transformer specified for the selected channel. The template enables the user to selectively specify the current transformer to be used with each branch circuit that is to be monitored. Relevant specifications95and error correction factors97for correcting phase and ratio errors are stored in the non-volatile memory of the meter for each of the current transformers displayable with the configuration tool. In addition, the configuration tool enables specifications and error factors for additional current transformers to be downloaded from a remote computer75for storage in the monitor's memory. Intermediate or typical error correction factors for OTHER current transformers357of various sizes may also be stored in the branch circuit monitor's memory for use with current transformers other than those having their particular specifications stored in the memory. When the installer has specified the current transformer(s) to be used with the selected channel(s), a SET CHANNEL button368on the configuration tool template, causes the data processing unit to identify in the memory the appropriate specifications, including the appropriate error correction factors, to be used in calculating and displaying the current and other parameters related to the branch circuit(s) corresponding to the selected current transformer channel(s). A NEXT CHANNELS button370displays additional current transformer channels that may be included in the monitor, for example, channels43-82.

Referring to alsoFIG. 12, selecting the AUX CTs tab232produces a template400enabling the user to specify auxiliary current transformers used with the branch circuit monitor. Auxiliary current transformers are typically used to monitor the current in the mains. In a manner similar to the specification the branch current transformers, the installer can select individual auxiliary current transformer channels406and specify the size404of the auxiliary current transformer(s). The configuration tool causes the template to display the manufacturer408, model410, core type412and current transformer output414for various current transformers of the desired size404which have specifications, including error correction factors, stored in the monitor's memory. When the installer has specified the auxiliary current transformer(s) that are used with the selected channel(s), activating the SET CHANNEL button418causes the data processing unit to identify the appropriate error correction factors stored in the memory64which will be used in determining the current sensed by the respective auxiliary current transformer.

The configuration tool simplifies the configuration of the branch circuit monitor by providing templates for entry of current transformer characteristics and enables customization the monitor for improved accuracy with a variety of available current transformers.

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.