Abstract:
The subject matter herein generally relates to electrical generators and motors, and more specifically, to electrical turbo-generators. In an embodiment, an electrical system includes a circuit. The circuit includes one or more rotating power delivery assemblies comprising a plurality of sliding surfaces that deliver power to a rotating load. The circuit also includes one or more radio frequency current transformers (RFCTs) that measure radio frequency (RF) signals corresponding to arcing events in the one or more rotating power delivery assemblies. The electrical system also includes a processor that receives the measurements from the one or more RFCTs and determines a health value of the circuit based, at least in part, on the received measurements.

Description:
BACKGROUND 
     The subject matter disclosed herein relates generally to electrical generators and motors, and more specifically, to electrical turbo-generators. 
     In general, electricity may be generated by inducing a current on a set of armature windings as a result of the relative motion of a nearby magnetic field. In order to produce this field, some electrical generators may inject a large amount of current (e.g., 1-5 kA) into the field windings of a spinning rotor during operation. Such electrical generators may use collector assemblies, also known as rotating slip ring assemblies, positioned about the rotor and electrically coupling the rotor to a stationary exciter via a number of stationary brushes. Accordingly, these brushes provide conductive paths between the stationary exciter and the spinning rotor such that power may be transmitted to the rotor and the rotating field may be produced. However, during operation, if one or more brushes lose contact with the collector ring, arcing events may occur. For example, a brush and collector ring may arc due to physical wear on the brush and/or ring, excessive vibration of the shaft, the presence of contaminants (e.g., particulates or oil) between the brush and ring, or incorrect brush alignment or installation. 
     Due to the large voltages and currents operating within many generators, arcing events may cause substantial damage to the collector ring and brushes over time, eventually resulting in a flashover event. During a flashover event, a short circuit path may form within the rotor between the positive and negative terminals of the exciter, between the positive terminal of the exciter and ground, and/or between the negative terminal of the exciter and ground. Generally speaking, a flashover event results in a catastrophic failure of the generator, leaving the generator inoperable. Furthermore, a flashover event may cause substantial damage to other electrical components coupled to the generator as well as personnel or equipment that may be physically located near the generator. In general, regularly scheduled inspection and maintenance of a generator is often required in order to verify the integrity of the brush/slip ring assembly and minimize the risk of flashover. 
     However, regularly scheduled inspection and maintenance of a generator is costly. In general, much of the inspection and maintenance of a generator may actually be performed while the generator continues to operate, increasing the complexity and safety risks of such maintenance. Furthermore, reliance on such a maintenance schedule only takes the operational time of the equipment into consideration, and fails to prioritize maintenance based on device performance or other indicators. That is, for strictly schedule-based maintenance, a healthy generator may receive largely unneeded maintenance based upon the maintenance schedule, while an unhealthy machine requiring servicing may be delayed in receiving maintenance simply because it was recently serviced. 
     BRIEF DESCRIPTION 
     In an embodiment, an electrical system includes a circuit. The circuit includes one or more rotating power delivery assemblies comprising a plurality of sliding surfaces that deliver power to a rotating load. The circuit also includes one or more radio frequency current transformers (RFCTs) that measure radio frequency (RF) signals corresponding to arcing events in the one or more rotating power delivery assemblies. The electrical system also includes a processor that receives the measurements from the one or more RFCTs and determines a health value of the circuit based, at least in part, on the received measurements. 
     In another embodiment, a method includes measuring a first radio frequency (RF) signal power in a circuit of an electrical device using one or more radio frequency current transformers (RFCTs) disposed within the circuit. The method also includes measuring a second RF signal power in the circuit using the one or more RFCTs. The method also includes comparing one or more features of the first and second RF signal power measurements and recommending actions to be performed on the electrical device based, at least in part, on the comparison. 
     In another embodiment, an electrical generator includes an exciter circuit. The exciter circuit includes a set of rotating field windings and a direct current (DC) exciter that provides power to the exciter circuit. The exciter circuit also includes a collector assembly that delivers power to the set of rotating field windings and comprises a plurality of brushes and collector rings. The exciter circuit also includes a shaft voltage suppressor (SVS) comprising a plurality of radio frequency current transformers (RFCTs) that measure radio frequency (RF) signals corresponding to arcing events in the exciter circuit. The electrical generator includes a processor that receives the measurements from the plurality of RFCTs, processes the measurements to determine power of the RF signals as a function of signal frequency, and assesses the health of the circuit based, at least in part, on the determined on the determined power of the RF signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates an embodiment of the rotor of a typical electrical generator, in accordance with aspects of the present disclosure; 
         FIG. 2  illustrates an embodiment of an exciter circuit of an electrical generator, in accordance with aspects of the present disclosure; 
         FIG. 3  is a graph illustrating the relative RF signal power in the exciter circuit when one or more brushes lose contact with the collector ring and arcing occurs, in accordance with aspects of the present disclosure; 
         FIG. 4  is a graph illustrating plots of RF signal power versus signal frequency for an exciter circuit with and without substantial arcing occurring, in accordance with aspects of the present disclosure; and 
         FIG. 5  is a flow diagram illustrating a process that a processor may use to measure RF signals in a circuit, access the health of the circuit, and recommend or perform actions based on the health of the circuit, in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed embodiments enable the detection of arcing events in electrical systems that involve the transfer of power to a rotating body. For example, the disclosed embodiments enable the detection of arcing events between the brushes and the collector ring (e.g., the sliding surfaces) of a collector assembly in a generator. Since arcing events typically increase in frequency as certain electrical systems begin to malfunction (e.g., due to accumulated wear on the brushes and collector rings), the disclosed embodiments enable the monitoring of the health of the collector assembly based upon the detected arcing events. As such, the disclosed embodiments enable health-based maintenance, rather than strictly schedule-based maintenance, of electrical equipment, which may afford both cost and safety benefits. 
     Additionally, the disclosed embodiments enable the assessment of the health of a live collector without having to take the equipment offline, redirect power away from portions of the equipment, or otherwise disrupt the electrical system, which minimizes maintenance cost and equipment downtime. Furthermore, the disclosed embodiments enable tuning of the operational parameters (e.g., the power output) of the electrical system based upon the detection of arcing events. That is, the disclosed embodiments enable the tuning of the parameters of a generator or motor to provide an acceptable level of performance that does not cause a substantial degree of arcing to occur. It should also be noted that the disclosed embodiments are generally applicable to any electrical system involving the delivery of power to a rotating body (e.g., via a collector or commutator assembly), including many types of AC generators and DC motors. Indeed, even in certain electrical systems lacking collector or commutator assemblies, the disclosed embodiments may be beneficial for detecting deleterious arcing events within certain elements of the system (e.g., circuit breakers, switches, relays) such that predictions regarding the future performance of the system may be made. 
     With the foregoing in mind,  FIG. 1  illustrates an embodiment of a rotor  10  of an AC electrical generator. The illustrated rotor  10  utilizes a rotating field produced by a spinning electromagnet to induce a current in a set of stationary windings (i.e., the stator, not shown) and, thereby, provide electricity. Accordingly, the rotor  10  may include a shaft  12  disposed though the length of the rotor  10  that may be driven by any mechanical power source. For example, the shaft  12  may be rotated  13  by an engine (e.g., a combustion engine) or a turbine (e.g., a gas-powered, wind-powered, or water-powered turbine) at approximately 3600 revolutions per minute (RPM) for a two pole gas turbine driven turbogenerator. Disposed within a portion of the shaft  12  may be a number of field windings that provide the rotating field when powered. Additionally, a housing  14  may be disposed around the shaft that includes one or more retaining rings  15  to provide support for the rotor windings as well as a number of fans  16  to provide cooling to the rotor  10  during operation. 
     In the illustrated embodiment, a pair of collector assemblies,  18 A and  18 B, are also disposed about the shaft  12  and coupled to an exciter  20 . More specifically, the collector assembly  18 A is coupled to the positive terminal of the exciter  20  and the collector assembly  18 B is coupled to the negative terminal of the exciter  20 . The exciter  20  may be any DC power source, such as a DC generator, battery, or rectifier, capable of supplying DC power to the field windings (e.g., disposed within or about a portion of the shaft  12 ) in order to produce the rotating field. Generally speaking, the collector assemblies  18  provide a conductive path between the exciter  20  and the field windings within the rotating shaft  12 . That is, the collectors  18  electrically couple the stationary exciter  20  and the field windings in the rotating shaft  12  such that the field windings may be energized to produce the rotating field. 
       FIG. 2  illustrates a schematic of an embodiment of a circuit  30  configured to supply power to a set of field windings  32  and configured to detect arcing events in the collector assemblies  18  or anywhere else on the rotor  10 . The illustrated circuit  30  includes an exciter  20 , collector assemblies  18 A and  18 B, and field windings  32 . Each collector assembly  18  provides a conductive path via a number of conductive, stationary brushes  34  contacting a conductive, rotating collector ring  36  disposed about the shaft  12 . These brushes  34  may be manufactured from a number of durable, conductive materials such as a metal, an alloy, or certain types of carbon. The brushes  34  may generally be stationary relative to the rotating collector ring  36 . In general, the brushes  34  may remain in contact with the rotating collector ring  36  via an applied force (e.g., a spring force). As the shaft  12  is rotated  13 , the free ends of the brushes  34  are designed to remain in physical and electrical contact with the rotating collector ring  36  disposed about the shaft  12 . The collector ring  36  may typically be a conductive, metallic ring that is coaxial with the shaft  12 , having a smooth (e.g., machined) or grooved surface for contacting the brushes  34 . 
     In the illustrated circuit  30 , the brushes  34 A of collector assembly  18 A are coupled to the positive terminal of the exciter  20 , and the brushes  34 B of collector assembly  18 B are coupled to the negative terminal of the exciter  20 . Similarly, the collector ring  36 A of collector assembly  18 A is coupled to one end of the field windings  32 , while the collector ring  36 B of collector assembly  18 B is coupled to the opposite end of the field windings  32 . That is, the rotating collector rings  36  of the collector assemblies  18  are electrically coupled to the field windings such that a complete conductive path may be formed from the exciter  20 , through the brushes  34 A and collector ring  36 A of collector  18 A, through the field windings  32 , through the brushes  34 B and a collector ring  36 B of collector  18 B, and back to the exciter  20 . Accordingly, the collector assemblies  18 A and  18 B cooperate with the exciter  20  to provide power to the spinning field windings  32  in order to produce the rotating field. 
     It should be noted that while the collector rings  36  are coupled to the field windings  32 , the collector rings  36  and the field windings  32  are both electrically isolated from the remainder of the shaft  12 . However, the shaft  12  may have voltages induced by the field windings  32 . Accordingly, the illustrated circuit  30  also includes an embodiment of a shaft voltage suppressor (SVS)  38 . In general, the SVS  38  may help to alleviate voltages capacitively coupled to the shaft  12  from the energized field windings  32 . The illustrated SVS  38  includes two high-value capacitors  40  (e.g., 10 μF), two high-value resistors  42  (e.g., 440 kΩ), and two other resistors  44  (e.g., 1.2Ω). While a particular SVS  38  is illustrated in  FIG. 2  for the purpose of illustration, it should be noted SVS designs are numerous and any appropriately rated SVS may be utilized in circuit  30 . In certain embodiments, the SVS  38  may be implemented as part of the exciter  20 , and in some embodiments the SVS  38  may not be present. 
     In order to detect RF signals in the circuit  30 , the circuit  30  may be equipped with one or more radio frequency (RF) current transformers (RFCTs)  46 . For example, the illustrated circuit  30  includes the three RFCTs (e.g.,  46 A,  46 B, and  46 C) that may measure the power of RF signals in the circuit  30 . Generally speaking, RF signals in the circuit may be the result of arcing events at the interface between the brushes  34  and the collector rings  36  of the collector assemblies  18 . That is, imperfect contact between the brushes  34  and the collector rings  36  of the collector assemblies  18  may disrupt the conductive path linking the exciter  20  to the field windings  32 . Due to the high current traversing the brushes  34  of the collector assemblies  18  during operation, a disruption in the conductive path may result in a high energy arcing event between the brushes  34  and the collector rings  36 . Such an arcing event results in the generation of an RF signal, which may be detected via the one or more RFCTs  46  within the circuit  30 . 
     In certain embodiments, the SVS  38  of the circuit  30  may be modified to include one or more RFCTs  46 . For example, the three RFCTs  46 A,  46 B, and  46 C of illustrated circuit  30  are disposed within the SVS  38 . However, in other embodiments, one or more RFCTs may be located at any number of different positions within the circuit  30 , including positions outside of the SVS  38 . In addition to having different positions within the circuit, each RFCT (e.g.,  46 A,  46 B, or  46 C) may be configured to measure the RF signal at different frequencies, with different levels of sensitivity, and/or with different dynamic ranges. For example, strategically positioning RFCTs  46  at different portions within the circuit  30  may enable the isolation and subtraction of background RF noise from the RF signal measurements. In general, the output from the RFCTs  46  may be transmitted to a processor  48  (e.g., a scope, a process controller, or a computer) for display and/or further processing. 
       FIG. 3  depicts a graph  50  that illustrates the general trend for RF signal power as the brushes  34  lose contact with the collector rings  36 . In  FIG. 3 , two points,  52  and  54 , are illustrated for different levels of contact between the brushes  34  and the collector rings  36 . The first point  52  reflects the baseline RF signal power when the brushes  34  are in good contact with the collector rings  36  and there is no arcing or only low levels of sparking. That is, the first point  52  indicates that a certain amount of RF signal (i.e., baseline signal) is expected within the circuit  30  (e.g., RF noise) even when the brushes  34  and the collector rings  36  are in good contact (e.g., as a result of normal sparking). Generally speaking, while sparking and arcing may represent a similar underlying phenomenon (e.g., a voltage across an air gap), sparking events are substantially lower in energy and are typical in the normal operation of the rotor  10 . The second point  54  on the graph  50  illustrates increased RF signal power measured as a result of arcing events within the circuit  30  due to one or more brushes  34  losing contact with the collector rings  36 . It should be noted that the higher RF power observed at point  54  may result from arcing due to the lifting of one or several brushes  34  in one or both collector assemblies  18 . In general, there may be a 2- to 10-fold increase  56  in RF signal power once the contact between one or more brushes  34  and the collector rings  36  begins to falter (e.g., between points  52  and  54 ). 
     As such, using the RFCTs  46  included in the circuit  30  (e.g., within the SVS  38 ) the RF signal power in the circuit  30  may be measured. In certain embodiments, the measurements of the RFCTs  46  may be transmitted to the processor  48  (e.g., a computer or controller) configured to further process the raw RFCT signals. For example, the processor  48  may apply a fast Fourier transform (FFT) operation to determine the RF signal power as a function of signal frequency. In certain embodiments, the processor  48  may also apply filters to the raw RFCT signals to remove RF noise prior to performing the FFT operation. 
       FIG. 4  illustrates a graph  60  of RF signal power (μW) versus signal frequency (kHz), such as may be attained after the processing of the raw signals of the RFCTs  46 , as described above. The first curve  62  of plot  60  represents the RF signal power over the signal frequency range between 1 and 250 kHz when the brushes  34  are in good contact with the collector rings  36 . Since this first curve  62  correlates to the point  52  of  FIG. 3 , only a baseline level of RF signal (e.g., RF noise) is observed, meaning that little to no arcing is detected. The second curve  64  of plot  60  represents the RF signal power over the same signal frequency range when the brushes  34  are not in good contact with the collector rings  36 . Since this second curve  64  correlates to the point  54  of  FIG. 3 , an increased RF signal power is observed at a number of frequencies relative to the first curve  62 , signifying a substantial increase in the amount of arcing occurring in the circuit  30 . 
     Accordingly, the processor  48  may perform an embodiment of the process  70  illustrated in  FIG. 5  in measuring the RF signals, accessing the health of the circuit  30 , and recommending and/or performing actions to the rotor  10 . The process  70  begins with the processor  48  measuring (block  72 ) a baseline RF signal in the circuit using the RFCTs  46 . For example, the processor  48  may measure the baseline RF signal when a rotor  10  is first installed or shortly after servicing the rotor  10 , circuit  30 , and/or the collector assemblies  18 . In general, the baseline RF signal measured in block  72  should correlate to point  52  of  FIG. 3  and curve  62  of  FIG. 4  and, accordingly, represent little to no arcing in the circuit  30 . After some amount of usage, the processor  48  may measure (block  74 ) a subsequent RF signal in the circuit  30  using the RFCTs  46 . If the contact of the brushes  34  and the collector rings  36  is significantly poorer than when the baseline measurement was taken, the subsequent RF signal measured may correlate to the point  54  of  FIG. 3  and curve  64  of  FIG. 4  and, accordingly, represent a substantial degree of arcing within the circuit  30 . 
     The processor  48  may subsequently compare (block  76 ) one or more features of the baseline and subsequent RF signal measurements to determine the health of the circuit  30  and/or the collector assembly  18 . That is, turning once more to  FIG. 4 , the first curve  62  and the second curve  64  of the plot  60  may be compared in a number of different ways to assess the amount of arcing occurring within a circuit  30  and, therefore, assess the health of the circuit  30  and/or the collector assemblies  18 . For example, the processor  48  may compare the maximum, median, or mean RF signal power values of the first and second curves (e.g.,  62  and  64 ) over a particular frequency range. That is, the processor  48  may determine that the first curve  62  has a maximum RF signal power of approximately 1 μW between 45 and 55 kHz, and determine that the second curve  64  has a maximum RF signal power of approximately 4 μW in the same frequency range. In certain embodiments, the processor  48  may subtract a value from the second curve  64  from a value of the first curve  62  in order to subtract out the baseline noise. That is, the computer may subtract 1 μW from 4 μW to determine that the RF signal due to arcing is approximately 3 μW between 45 and 55 kHz. In other embodiments, the processor  48  may instead consider the ratio of a value from the second curve  64  to a value from the first curve  62 . That is, the processor  48  may calculate the ratio of 4 μW to 1 μW to determine that the RF signal during arcing is roughly four times greater than the baseline between 45 and 55 kHz. 
     Additionally, in certain embodiments, the processor  48  may determine the integration of a portion of the first and second curves (e.g.,  62  and  64 ) for comparison. Accordingly, the processor  48  may subsequently subtract the area under the second curve  64  over a certain frequency range (e.g., between 45 and 55 kHz) from the area under the first curve  62  over the same frequency range. In certain embodiments, the processor  48  may determine a ratio of the two integrals. Furthermore, in certain embodiments, the processor  48  may employ a combination approach and may compare multiple portions or features of the first curve  62  to the multiple portions or features of the second curve  64 , as individually described above. For example, the processor  48  may determine the ratio of the maximum RF power of the first and second curves (e.g.,  62  and  64 ) between 45 and 55 kHz as well as determine the ratio of the integrals of the first and second curves (e.g.,  62  and  64 ) between 80 and 90 kHz to determine the level of arcing occurring within the circuit. 
     Turning once more to  FIG. 5 , the processor  48  may subsequently recommend or perform (block  78 ) a number of different actions to the rotor  10  and/or the circuit  30  after processing the raw data and/or plots, as described above. For example, the processor  48  may condense the RF signal power data, possibly in addition to historical RF signal power data, into a single collector health value (e.g., a quantitative or qualitative score). That is, based upon the degree of arcing detected in the RF signal power data, the processor  48  may determine a collector health value ranging from 100 (e.g., perfect health) to 0 (e.g., on the verge of flashover) for the circuit  30 . For example, the processor  48  may determine that a particular circuit  30  has a collector health value greater than 95, meaning that little or no arcing is occurring and that the collector assemblies  18  and/or the circuit  30  are healthy. Accordingly, the processor  48  may output the collector health value, along with any other requested data, to an operator such that the operator may have an indication of the health of the collector without having to disrupt the operation of the equipment. 
     In the event that the processor  48  determines that a particular collector assembly  18  and/or circuit  30  is unhealthy (e.g., having a collector health value less than 20), in addition to outputting the value and/or related data to the operator, the processor  48  may sound an alarm, automatically schedule maintenance for the equipment, or recommend operational parameters for the generator, the circuit  30 , and/or rotor  10 . For example, if the collector health value is below 10, the processor  48  may signal an alarm to inform the operator that flashover may occur in the near future if the rotor  10  is not taken off-line and/or serviced. By further example, if the collector health value is below 80, the processor  48  may automatically generate a maintenance request (e.g., via an email, an electronic maintenance request system, etc.) based on the severity of the case. That is, if the collector health value for one generator is 80 and the collector health value for a second generator is 50, the second generator may be prioritized for maintenance over the first. 
     Additionally, the processor  48  may recommend the adjustment of operational parameters for the generator, the circuit  30  and/or rotor  10  based on the collector health value. That is, if the collector health value of a circuit  30  is 50 when the generator is at 100% power output, the processor  48  may recommend operating the generator at 50% power output. In certain embodiments, the processor  48  may determine the RF signal power after the parameters of the generator, the circuit  30 , and/or the rotor  10  have been adjusted to determine if the collector health value has improved from the adjustment. In certain embodiments, the processor  48  may continually monitor the RF signal power of the circuit  30  in real-time and make automatic adjustments to the parameters of the generator, the circuit  30 , and/or the rotor  10  in order to maintain the collector health value within a specified range. As such, the power output of a generator may be gradually dialed down from 100% as the collector health value of the rotor  10  begins to fall, preventing damage to the rotor  10  by avoiding the flashover event from occurring and allowing the generator to gracefully degrade performance until it is serviced. Additionally, in certain embodiments, the processor  48  may inform the operator that the health of the circuit  30  and/or the rotor  10  is sufficient to allow the generator to be pushed beyond normal operating conditions (e.g., 115% power output) for a limited period of time (e.g., to meet a short term power demand). 
     Additionally, by tracking the RF signal power within the circuit  30  over the life of the rotor  10 , the health of the generator, the rotor  10 , the collector assemblies  18 , and/or the circuit  30  may be tracked over time. That is, based upon the trend of collector health values over time, the processor  48  may predict when the circuit  30  and/or rotor  10  will require servicing, and/or when the circuit  30  and/or rotor  10  is likely to fail (e.g., flashover), and inform the operator and/or schedule maintenance for the equipment. Additionally, the disclosed embodiments enable an operator to immediately determine if the maintenance that a collector assembly  18  and/or circuit  30  receives actually improves the health of the circuit  30 , which may help to prevent damage to the circuit  30  due to operator error during the maintenance process (e.g., improper brush installation). Furthermore, the disclosed embodiments may be implemented in tandem with other equipment health monitoring and process control components as well as interface with industrial control systems such that the collector health values may be considered by the control system when managing the equipment. 
     Furthermore, it should be noted that existing electrical equipment (e.g., electrical generators and/or motors) may be modified to incorporate the features of the disclosed embodiments. For example, an SVS of an existing generator may be modified to include one or more RFCTs capable of sensing RF signals in the exciter circuit. In certain embodiments, the RFCTs may even be installed while the electrical equipment remains online, or while the electrical equipment has a portion of the power temporarily rerouted, allowing for minimal equipment downtime. With the aid of a processor, such a modification would enable the continual or intermittent assessment of the health of the circuit and/or generator and provide recommendations based on this health assessment. 
     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 languages of the claims.