Patent Publication Number: US-2012026630-A1

Title: Method and Apparatus For Use In Monitoring Operation of Electrical Switchgear

Description:
BACKGROUND OF THE INVENTION 
     The present application relates generally to electrical switchgear, and more particularly, to a method and apparatus for use in monitoring operation of electrical switchgear. 
     In an industrial power distribution system, power generated by a power generation company may be supplied to an industrial or commercial facility for distribution within the industrial or commercial facility. At least some known power distribution systems use switchgear to divide the power into circuit branches which supply power to various portions of the industrial facility. Generally, circuit breakers are provided in each circuit branch to facilitate protecting equipment within the circuit branch. Additionally, circuit breakers in each circuit branch may facilitate minimizing equipment failures since specific loads may be energized or deenergized without affecting other loads. 
     At least some known circuit breakers use electronics to monitor the current flowing through the circuit branches, and to trip the breaker if the monitored current exceeds a predefined value. Known electronic circuit breakers are adjustable, and may include a protection module that is coupled to one or more current sensors. Such modules continuously monitor digitized current values and compare such values to curves that define permissible time frames in which both low-level and high-level overcurrent conditions may exist. For example, if an overcurrent condition has been maintained for longer than a predefined permissible time frame, the breaker is tripped. However, in some instances, the breakers may be inaccurately tripped, as accurate current readings may be affected by the measuring instruments themselves. For example, current transformer saturation may create errors, even when low-burden static relays are used. 
     At least some known circuit breakers use protection modules to monitor and control other types of faults, such as over- or under-voltage conditions and/or phase loss or imbalances. Such protection modules often require that raw electrical signals, such as those transmitted by instrument sensors, be translated into conditioned signals that are usable by the breaker protection modules. Accuracy in the measurement of the electrical parameters facilitates ensuring that power design limits are not exceeded, while equipment is maintained in service during transient conditions. To facilitate improved measurement accuracy, high quality and stable components may be used in the construction of protective instrumentation. However, such components increase production costs. Another technique used in improving accuracy requires compensating for known or estimated errors in the measurement ability of an instrument system. Once errors are quantified, a countervailing circuit may then be introduced to balance the errors out of the system. However, this technique is often difficult to implement and may lead to increased errors, or less predictable errors, being introduced into the system. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, an instrument transformer is provided that includes a current transformer configured to be coupled to a load, and to generate an analog signal that is proportional to a current flowing through the load. The instrument transformer also includes a protection module and a digitizer module that is coupled to the current transformer. The digitizer module is configured to receive an input signal that is proportional to the analog signal and to convert the input signal to a digital signal. The digitizer module includes a synchronization module for generating at least one timing signal and a fiber optic interface module coupled to the protection module and to the synchronization module for transmitting the digital signal to the protection module. 
     In another embodiment, a digitizer module is provided that is configured to be coupled to a current transformer and to a protection module. The digitizer module includes a current-to-voltage converter module that is configured to receive an input signal that is proportional to a signal generated by the current transformer and to convert the input signal to a voltage signal. The digitizer module also includes a synchronization module for generating at least one timing signal and a fiber optic interface module coupled to the protection module and to the synchronization module for transmitting the voltage signal to the protection module. 
     In yet another embodiment, a method of assembling an instrument transformer is provided that includes coupling a current transformer to a load, wherein the current transformer is configured to generate an analog signal that is proportional to a current flowing through the load. A digitizer module is coupled to the current transformer, wherein the digitizer module is configured to receive an input signal that is proportional to the analog signal and to convert the input signal to a digital signal. The digitizer module includes a synchronization module for generating at least one timing signal and a fiber optic interface module coupled to the synchronization module. A protection module is coupled to the digitizer module through the fiber optic interface module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a known instrument transformer assembly. 
         FIG. 2  is a schematic diagram of an exemplary instrument transformer assembly that may be used in monitoring a switchgear. 
         FIG. 3  is a schematic diagram of an alternative embodiment of an instrument transformer assembly that may be used in monitoring a switchgear. 
         FIG. 4  is a schematic diagram of an exemplary instrument transformer that may be used with the instrument transformer assembly shown in  FIG. 2  or  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic diagram of a known instrument transformer assembly  10 . A primary conductor, or busbar  12  is positioned within an alternating current (AC) switchgear panel (not shown). Busbar  12  extends through a toroidal core  13  of a current transformer  14  such that busbar  12  and a plurality of electrical windings  15  within current transformer  14  are magnetically coupled together. Electrical terminations  16  and  18  couple current transformer  14  to conductors  17  and  19  respectively to transmit a signal generated by current transformer  14  to a measuring device (not shown). 
     During operation, an alternating current transmitted through busbar  12  induces a proportional current signal in windings  15  of current transformer  14 . The current signal is transmitted via terminations  16  and  18  to conductors  17  and  19  which then transmit the current signal to a measuring device. 
       FIG. 2  is a schematic diagram of an exemplary instrument transformer assembly  40  that may be used in monitoring the operation of a switchgear (not shown). In the exemplary embodiment, a switchgear busbar  42  is a conductor that extends through, and that is magnetically coupled to, a current transformer  44 . Electrical terminations  46  and  48  couple current transformer  44  to a digitizer module  50 . In another embodiment, an auxiliary attachment (not shown) is used to couple digitizer module  50  to current transformer  44 . Digitizer module  50 , described in more detail below, produces an output that is transmitted through conduit  52  to a relay protection module (not shown in  FIG. 2 ). In an alternative embodiment, digitizer module  50  may be coupled to other switchgear components, such as, for example, a limit switch, a timer, a transfer switch, a panel control and/or a display (none shown). 
       FIG. 3  is a schematic diagram of an alternative instrument transformer assembly  70  that may be used in monitoring a switchgear (not shown). Instrument transformer assembly  70  is similar to instrument transformer assembly  40  (shown in  FIG. 2 ) and components in instrument transformer assembly  70  that are identical to components of instrument transformer assembly  40  are identified in  FIG. 3  using the same reference numerals used in  FIG. 2 . Accordingly, in the exemplary embodiment, instrument transformer assembly  70  includes switchgear busbar  42  that extends through, and that is magnetically coupled to current transformer  44 . Moreover, in the exemplary embodiment, and in contrast to  FIG. 2 , digitizer module  50  is formed integrally with current transformer  44 . An output of digitizer module  50  is transmitted through conduit  52  to a relay protection module (not shown in  FIG. 3 ). In an alternative embodiment, digitizer module  50  may be formed integrally with and/or coupled to other switchgear components, such as, for example, a limit switch, timer, transfer switch, panel control or display. 
       FIG. 4  is a schematic diagram of an instrument transformer  100  that may be used with instrument transformer assembly  40  or  70  (shown in  FIGS. 2 and 3 , respectively). In the exemplary embodiment, instrument transformer  100  includes current transformer  44 , digitizer module  50 , a protection module  106 , a user interface module  108 , and a fiber optic power supply module  110 . In one embodiment, current transformer  44  is a known current transformer that has been retrofitted to receive digitizer module  50 . In the exemplary embodiment, terminals  112  and  114  couple digitizer module  50  to current transformer  44 . A primary conductor  116  extends through, and is magnetically coupled to, a toroidal-shaped winding  118  of current transformer  44 . In the exemplary embodiment, primary conductor  116  is a switchgear busbar or cable. A raw current signal from current transformer  44  is conducted to a step-down transformer  120  that reduces an amplitude of the raw current signal to an instrument signal that is suitable for processing within digitizer module  50 . 
     In the exemplary embodiment, the instrument signal is transmitted via a conduit  122  to a current-to-voltage converter module  124  that converts the instrument signal to a voltage signal that is proportional to a current of the instrument signal. The voltage signal is transmitted via a conduit  126  to an analog-to-digital converter module  128  that digitizes the signal. The digitized signal is transmitted via a conduit  130  to a filter module  132  that band-pass filters the digitized signal to remove undesired low frequency and high frequency signal components from the digitized signal. Alternatively, filter module  132  may use any other filter, such as a low pass filter and/or a high pass filter, to remove undesired signal components from the digitized signal. 
     In the exemplary embodiment, filter module  132  transmits the filtered signal via a conduit  134  to fiber optic interface module  136 . Fiber optic interface module  136  communicates with protection module  106  via fiber optic bus  138 . The communication between fiber optic interface module  136  and protection module  106  is bi-directional, such that signal data is transmitted to protection module  106 , and such that limit value and/or setup data are transmitted to fiber optic interface module  136 . Because digitizer module  50  communicates with protection module  106  via fiber optic bus  138 , digitizer module  50  facilitates electrically isolating current transformer  44  from protection module  106 . 
     Moreover, in the exemplary embodiment, digitizer module  50  includes a synchronization module  140  that is coupled to fiber optic interface module  136 . Additionally or alternatively, synchronization module  140  may be coupled to any other component within digitizer module  50  that enables instrument transformer  100  to function as described herein. In the exemplary embodiment, synchronization module  140  includes a clock unit and/or a global positioning satellite (GPS) unit (neither shown). The clock unit and/or the GPS unit provides timing signals for use in synchronizing voltage and/or current measurements. In the exemplary embodiment, fiber optic interface module  136  receives the timing signals and associates, or “timestamps” the timing signals with voltage measurement signals received. In one embodiment, fiber optic interface module  136  generates a plurality of synchrophasors based on the voltage measurement signals and/or the timing signals received. In the exemplary embodiment, the synchrophasors and/or the timestamped voltage signals are transmitted from fiber optic interface module  136  to protection module  106  and/or to user interface module  108 . 
     Protection module  106 , in the exemplary embodiment, receives the timestamped voltage signals and/or the synchrophasors via fiber optic bus  138  and selectively opens and closes at least one relay (not shown) based on the timestamped voltage signals and/or the synchrophasors received. Because the voltage signals and/or the synchrophasors include timing information, in addition to voltage information, the decision process of protection module  106  is more efficient and reliable regarding the opening and closing of the relays, in contrast to a protection module  106  that receives only voltage data, for example. 
     In the exemplary embodiment, user interface module  108  receives the timestamped voltage signals and/or the synchrophasors from protection module  106  via a conduit  142  and displays the signals and/or the synchrophasors to a user. In the exemplary embodiment, conduit  142  is a fiber optic conduit and/or bus. More specifically, in the exemplary embodiment, user interface module  108  displays the voltage signals and/or the synchrophasors in a time-based reference frame and/or such that the signals are synchronized with the timing signals. In one embodiment, user interface module  108  includes a thin screen module mounted to a user-accessible portion of a switchgear panel exterior (not shown). In another embodiment, user interface module  108  is remote, for example, in a central control room for use in remote monitoring of instrument transformer  100 . Moreover, in the exemplary embodiment, user interface module  108  includes a user input portion (not shown), for example, a key pad, a touch screen, and/or a communications port. The communications port may be coupled to a personal computer or to another data processing device (not shown). User interface module  108  also includes a display (not shown) for displaying a status of instrument transformer  100 , wherein the status may be, but is not limited to only being, an operational status, a fault status, self diagnostic results, and/or any other programmable status indication that enables instrument transformer  100  to function as described herein. In one embodiment, fiber optic bus  138  facilitates communication with a plurality of protection modules  106  and at least one user interface module  108 . 
     Moreover, in the exemplary embodiment, fiber optic power supply module  110  supplies power to at least one component within digitizer module  50 . More specifically, in the exemplary embodiment, fiber optic power supply module  110  supplies power to a fiber optic power supply  144  via a conduit  146 . Fiber optic power supply  144  supplies power to current-to-voltage converter module  124 , to analog-to-digital converter module  128 , to filter module  132 , and to fiber optic interface module  136  via a power bus  148 . In one embodiment, fiber optic power supply module  110  is positioned within a switchgear panel (not shown) that is located proximate to user interface module  108 . Alternatively, fiber optic power supply module  110  may be located in any location that enables instrument transformer  100  to function as described herein. 
     The flexibility of a fiber optic data communication path enables many systems to be monitored and controlled from a central location or from any number of remote locations. Fiber optic bus  138  and conduit  142  are not limited to a fiber optic architecture, but may be any of a wide array of communication bus architectures that include, but are not limited to only including, Ethernet, RS-485, and/or any other bus architecture that enables instrument transformer  100  to function as described herein. Moreover, fiber optic bus  138  and conduit  142  may use any communication protocol that includes, but is not limited to only including, profibus, profibus DP, TCP/IP, and/or any other communications protocol that enables instrument transformer  100  to operate as described herein. 
     The exemplary embodiment is described as an instrument transformer that includes a current transformer. However, instrument transformers based on other sensors may be incorporated in the same manner as described above. For example, voltage, frequency, temperature, infrared spectrum energy, vibration, and/or flow sensors, as well as interlocks and/or safety devices may be incorporated into the instrument transformer as described herein. In operation, coupling digitizer module  50  directly to current transformer  44  reduces an amount of copper wire required to manufacture switchgear. As such, the conduit material that is used by the exemplary instrument transformer to carry signals is manufactured from a fiber optic material. Accordingly, the exemplary instrument transformer may be manufactured in a more cost-effective manner as compared to known instrument transformers. 
     The current transformer and digitizer module described herein provide a robust and reliable apparatus for monitoring electrical switchgear. The fiber optic interface between the digitizer module and the protection module provides a lower impedance as compared to known switchgear monitoring equipment that uses copper wiring. Moreover, the synchronization module enables timing information to be correlated with the voltage signals produced by the digitizer module. The timing information facilitates synchronizing the voltage signals such that the protection module and/or a user may more efficiently and reliably protect switchgear components as compared to prior art systems without such synchronization. As such, the exemplary embodiment enables reliable and robust communication between the digitizer module and the protection module while facilitating reducing a power loss between the modules as compared to prior art systems. 
     A technical effect of the systems and method described herein includes at least one of: (a) coupling a current transformer to a load, wherein the current transformer is configured to generate an analog signal that is proportional to a current flowing through the load; (b) coupling a digitizer module to a current transformer, wherein the digitizer module is configured to receive an input signal that is proportional to an analog signal and to convert the input signal to a digital signal, and wherein the digitizer modules a synchronization module for generating at least one timing signal and a fiber optic interface module coupled to the synchronization module; and (c) coupling a protection module to a digitizer module through a fiber optic interface module. 
     The above-described instrument transformer is cost-effective and reliable. The instrument transformer includes a current transformer that is coupled to a digitizer module. The digitizer module converts, filters, and synchronizes a signal received from the current transformer with one or more timing signals. The digitizer module transmits and receives signal data to and from a protection module over a fiber optic conduit. As such, the digitizer module is electrically isolated from the protection module. Moreover, coupling the digitizer module directly to the current transformer and using a fiber optic bus rather than copper panel wire facilitates reducing a cost of manufacturing and/or of assembling the instrument transformer and/or the switchgear. 
     Exemplary embodiments of a method and apparatus for use in monitoring operation of electrical switchgear are described above in detail. The method and apparatus are not limited to the specific embodiments described herein, but rather, components of the apparatus and/or steps of the method may be utilized independently and separately from other components and/or steps described herein. For example, the instrument transformer may also be used in combination with other monitoring systems and methods, and is not limited to practice with only the switchgear as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other power system applications. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.