Patent Application
20070007968
1. Field of the Invention
The present invention relates to systems for monitoring an electrical power system, and in particular to power monitoring systems that utilize one or more wirelessly communicating electrical power transducers.
2. Background Information
Systems for monitoring alternating current (ac) power systems are well known and typically include discrete analog components which measure ac voltage and current signals from a power system and convert them into direct current (dc) output signals. Such systems also typically include microcomputers for calculating various electrical parameters such as rms currents and voltages, peak currents and voltages, average powers, power factors, alarm limits, over and under voltages, and the like. In addition to monitoring the various electrical parameters of the power system, such monitoring systems also digitally capture portions of the analog waveforms of the power system for harmonic analysis to determine the harmonic content of the waveforms. Examples of such systems are described in commonly owned U.S. Pat. Nos. 5,587,917 and 5,706,204, the disclosures of which are incorporated herein by reference.
Power monitoring systems such as those described above are widely used in many settings and for many purposes. For example, electrical utilities use power monitoring systems for metering purposes to determine the amount of power consumed by its customers. In addition, power monitoring systems are frequently used by power consumers such as businesses and landlords for sub-metering purposes to determine the amount of power consumed by a particular tenant in a building or other location or the amount of power consumed by a particular department or similar unit within a business. Power monitoring systems are also used in process control applications, for example in situations where a motor is powered by a power system, to measure various parameters for preventative maintenance and predictive diagnostics purposes or for implementing load sharing and load shifting schemes. Finally, power monitoring systems are often used for power quality metering to measure parameters such as harmonics, transients, and voltage fluctuations. Power quality metering is often critical in many industrial operating locations, such as pharmaceutical manufacturing facilities, hospitals, data centers or semiconductor fabrication facilities, where it is extremely important to consistently have power of high quality.
In power monitoring, whether it be for single phase, two-phase (also known as split phase) or three-phase ac systems, it is necessary to simultaneously measure the voltage and current of each phase at multiple points of time in the waveform. In existing monitoring systems, this is done with a piece of equipment commonly referred to as a meter box that includes, for each phase, a current measuring device, such as a current transformer or the like, for measuring the current of the phase, and a voltage measuring device, such as a potential transformer, a divider circuit or the like, for measuring the voltage of the phase. Thus, a power monitoring system for a three-phase ac power system will include a meter box having three current measuring devices and three voltage monitoring devices. The current and voltage measuring devices-are connected to an analog-to-digital converter and a processing unit in the meter box which calculates the various monitoring parameters described above.
Existing power monitoring systems present several problems. First, voltage cannot be reliably measured without being physically connected to the conductor, which typically carries voltages on the order of 120-600 VAC or 1,200-40,000 VAC. As a result, the meter boxes that operators must come into contact with are dangerous as they are directly connected to the voltage conductors. In addition. many meter boxes are directly connected to a communications network, such as, for example, Ethernet. The fact that the meter boxes are connected to the voltage conductors present the potential that the voltage could be passed to the communication network when problems arise, which would likely be very harmful to the network and potentially hazardous to the user. Moreover, installation of a metel-box requires that several cables be connected to particular locations. The most common cause of monitoring system failure is errors made in the cable connection process (e.g., phases incorrectly connected).
Thus, there is a need for a power monitoring system with increased safety features that also limits the number of connections that must be made during installation.
These needs, and others, are addressed by the present invention which provides a system for monitoring an electrical power system having one or more phase conductors. The system includes one or more transducer units, each of which has a current measuring device operatively coupled to a respective one of the phase conductors, a voltage measuring device electrically coupled to a respective one of the phase conductors, and a transducer wireless communications device. The current measuring device of each transducer unit generates a current signal indicative of a current of the respective phase conductor and the voltage measuring device of each transducer unit generates a voltage signal indicative of a voltage of the respective phase conductor. The transducer wireless communications device of each transducer unit receives current data based on the respective current signal and voltage data based on the respective voltage signal and wirelessly transmits the current data and the voltage data to a base unit. The base unit has a base wireless communications device and a processor electrically coupled to the base wireless communications device. The base wireless communications device receives the current data and the voltage data transmitted by the transducer wireless communications device of each transducer unit, and the processor generates one or more electrical parameters relating to the electrical power system using the received current data and voltage data. The current data and the voltage data associated with each transducer unit may be time synchronized to enable the calculation of line-to-line voltage values and the determination of relative phase information. In addition the voltage data and the current data may be compared to predetermined limits to determine whether an alarm condition exists. If an alarm condition exists, an alarm signal may be wirelessly transmitted to the base unit.
The system may be used to monitor a three-phase power system including a first phase conductor, a second phase conductor and a third phase conductor. In this case, the system includes: (i) a first transducer unit having a first current measuring device operatively coupled to the first phase conductor, a first voltage measuring device electrically coupled to the first phase conductor, and a first transducer wireless communications device, (ii) a second transducer unit having a second current measuring device operatively coupled to the second phase conductor, a second voltage measuring device electrically coupled to the second phase conductor, and a second transducer wireless communications device, and (iii) a third transducer unit having a third current measuring device operatively coupled to the third phase conductor, a third voltage measuring device electrically coupled to the third phase conductor, and a third transducer wireless communications device. The first current measuring device generates a first current signal indicative of a current of the first phase conductor and the first voltage measuring device generates a first voltage signal indicative of a voltage of the first phase conductor, the second current measuring device generates a second current signal indicative of a current of the second phase conductor and the second voltage measuring device generates a second voltage signal indicative of a voltage of the second phase conductor, and the third current measuring device generates a third current signal indicative of a current of the third phase conductor and the third voltage measuring device generates a third voltage signal indicative of a voltage of the third phase conductor. The first transducer wireless communications device receives first current data based on the first current signal and first voltage data based on the first voltage signal, and wirelessly transmits the first current data and the first voltage data, the second transducer wireless communications device receives second current data based on the second current signal and second voltage data based on the second voltage signal and wirelessly transmits the second current data and the second voltage data, and the third transducer wireless communications device receives third current data based on the third current signal and third voltage data based on the third voltage signal and wirelessly transmits the third current data and the third voltage data. The base wireless communications device receives the first, second and third current data and the first, second and third voltage data, and the processor generates the one or more electrical parameters using the first, second and third current data and the first, second and third voltage data.
Each transducer unit may have an analog-to-digital converter for converting the current signal indicative of a current of each phase conductor into the respective current data and for converting the voltage signal indicative of a voltage of each phase conductor into the respective voltage data. In addition, the current measuring device of each transducer unit may comprise a current transformer, and the voltage measuring device of each transducer unit may comprise an RC divider network. The transducer wireless communications devices and the base wireless communications device may each comprise an RF wireless communications device, such as a Bluetooth device or an 802.15 or 802.15.4 device, or, alternatively, an infrared wireless communications device. The processor may also perform waveform harmonic analysis using the received current data and the received voltage data.
The present invention also provides a method of monitoring an electrical power system having one or more phase conductors, including generating current data and voltage data for each of the phase conductors at a first location, wirelessly transmitting the current data and the voltage data for each of the phase conductors to a second location, and generating at the second location one or more electrical parameters relating to the electrical power system using the current data and the voltage data for each of the phase conductors. The current data for each respective phase conductor is indicative of a current of the respective phase conductor and the voltage data for each respective phase conductor is indicative of a voltage of the respective phase conductor.
The method may be used to monitor a three-phase power system having a first phase conductor, a second phase conductor and a third phase conductor, wherein the step of generating current data and voltage data comprises generating first current data indicative of a current of the first phase conductor and first voltage data indicative of a voltage of the first phase conductor, second current data indicative of a current of the second phase conductor and second voltage data indicative of the voltage of the second phase conductor, and third current data indicative of a current of the third phase conductor and third voltage data indicative of a voltage of the third phase conductor. In this case, the step of wirelessly transmitting comprises wirelessly transmitting the first, second and third current data and the first, second and third voltage data to the second location, and the step of generating one or more electrical parameters comprises generating the one or more electrical parameters using the first, second and third current data and the first, second and third voltage data.
Moreover, the step of wirelessly transmitting may utilize radio frequency waves or infrared light. In addition, waveform harmonic analysis may be performed at the second location using the current data and voltage data that is transmitted to the second location.
A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
Referring to
As seen in
Processor 40 is electrically connected to wireless communications device 45. Wireless communications device 45 may be any type of device capable of wirelessly transmitting data, such as wireless transmitter or wireless transceiver. In addition, wireless communications device 45 may transmit data wirelessly in a number of different ways, such as using radio frequency waves according to, for example, the Bluetooth protocol, or using infrared light. Wireless communications device 45 receives the digital signals generated by processor 40 from the signals received from current measuring device 30 and voltage measuring device 35 and transmits them wirelessly to base unit 25.
Transducer unit 20 also includes battery 50 for providing power to each of the components thereof Battery 50 is connected to trickle charger 55, which in turn is electrically coupled to phase conductor 15. Trickle charger 55 is a known parasitic power charger that draws power from phase conductor 15 and uses it to charge battery 50. Alternatively, an iron-core current transformer may be used to parasitically draw power from the phase conductor 15. Such a current transformer may be the same device that functions as current measuring device 30, or a different device. Other methods for providing power to each of the components of transducer unit 20, such as an ac supply, may also be used.
According to a further aspect of the present invention, the measurements made by two or more transducer units 20 may be time synchronized to allow the streams of sampled raw current and voltage data to be time stamped to a relatively high degree of accuracy. This, in turn, would allow the base unit 25 to calculate accurate line-to-line voltage values and to determine relative phase information (e.g., rotation and phase angle). Time synchronization may be accomplished in a number of ways. For example, a plurality of transducer units 20 may be placed into a single housing, and a single-wire communication system may be used to transmit time synchronization information among the transducers 20, and in particular, their processors 40. As will be appreciated, in such a configuration, one transducer unit 20 (the master will maintain a base time that is transmitted to the other transducer units 20 for time stamping purposes. Alternatively, each of a plurality of time synchronized transducer units 20 may be provided with a secondary wireless receiver for receiving time synchronization information from the base unit 25 (in this case, the base unit 25 maintains the base time used for time stamping). In yet another alternative, a plurality of time synchronized transducer units 20 (e.g., transducer units 20 that are measuring different phases of the same load) may be connected in a daisy-chain configuration (e.g., in separate housings) by a single wire pair to allow time synchronization information to be shared among the various transducer units 20. This technique would require that the transducer units 20 be selectively configurable as a master or slave such that one can be designated the master (maintains the base time) and the others the slaves. All of the described examples allow for relative time synchronization among the transducer units 20. Only the configuration where the base unit 25 maintains the time, however, may be used for absolute time synchronization, which could be required for certain multi-point power system monitoring applications (e.g., sequence-of-events recording, protection coordination, or billing applications).
Thus, monitoring system 5 avoids the safety problems (presented both to humans and associated networks) presented by prior art monitoring systems by eliminating the direct, wired connection between the voltage measuring components and the processing and/or display elements that operators must come into contact with on a regular basis. In addition, because each transducer unit 20 of monitoring system 5 is physically connected to only a single phase conductor without the need for further, extensive cabling, the installation errors often encountered by prior art monitoring systems are avoided.
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art of various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.
