Partial Discharge Detector

Abstract

To remind, in an early stage, a user of the necessity of repairing a device before a more serious defect occurs, by checking the device for a partial discharge (PD).SOLUTION: A PD sensor 411 of a PD detection circuit 440 is one of a capacity coupling sensor, a transient ground voltage sensor, and a high-frequency coupling capacitor. A converter 410 includes a light-emitting device (LED) 412 in series with the PD sensor 411, and may include a current limitation resistor in series with or in parallel with the LED 412. The LED 412 may be optically connected to an optical fiber 420 arranged to carry an analog light signal 413 to a remote position for further processing.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Partial discharges (PDs) are small electrical sparks that occur within insulation or near the electrodes of an electrical device. Partial discharges differ from arc discharges because the discharge path does not reach from one conductor to another or to ground. Partial discharge monitoring can be an important tool for detecting the degradation of electrical devices. The presence of PDs can indicate insulation degradation, poor connections, moisture intrusion, high voltage in unintended locations, or other problems.

[0002] Some embodiments include a partial discharge (PD) detection system. The system includes a PD sensor configured to sense a PD event in an electrical system and to generate a sensor signal in response to the PD event. An envelope generator is coupled to receive the sensor signal from the PD sensor. The envelope generator is configured to extract an envelope signal of the sensor signal. A digitizer is configured to convert the envelope signal into a digital representation of the PD event.

[0003] Some embodiments are directed to a method for detecting a PD event, the method including sensing a PD event in an electrical system and generating an electrical sensor signal in response to the PD event, wherein an envelope signal is extracted from the sensor signal and converted into a digital representation of the envelope signal. [Brief explanation of the drawings]

[0004] [Figure 1] 1 is a schematic diagram of a PD detection circuit including an electrical-to-optical converter, according to various embodiments. [Figure 2] 1 is a schematic diagram of a PD detection circuit including an electrical-to-optical converter, according to various embodiments. [Figure 3] 1 is a schematic diagram of a PD detection circuit including an electrical-to-optical converter, according to various embodiments. [Figure 4] 1 is a schematic diagram of a PD detection circuit including an electrical-to-optical converter, according to various embodiments. [Figure 5]1 illustrates a schematic diagram of a PD monitoring system including N PD sensors and N electrical-to-optical converters, according to some embodiments. [Figure 6] FIG. 2 is a block diagram illustrating a representative embodiment of a portion of a central processing circuit, according to some embodiments. [Figure 7] 1 is a graph of the voltage across a coupling capacitor during a PD event. [Figure 8A] FIG. 1 illustrates a block diagram of a partial discharge system, according to some embodiments. [Figure 8B] 1 provides a more detailed block diagram of a PD detection system, according to some embodiments. [Figure 9] The PD sensor signal and the envelope of the PD signal are shown. [Figure 10] FIG. 1 is a simplified schematic diagram of a PD detection system comprising a one-sided direct electrical envelope generator, according to some embodiments. [Figure 11] FIG. 1 is a simplified schematic diagram of a PD detection system comprising a single-sided direct amplification electrical envelope generator, according to some embodiments. [Figure 12] FIG. 1 is a simplified schematic diagram of a PD detection system comprising a double-sided direct electrical envelope generator, according to some embodiments. [Figure 13] FIG. 1 is a simplified schematic diagram of a PD detection system including a single-sided amplified optical envelope generator, according to some embodiments. [Figure 14] FIG. 1 is a simplified schematic diagram of a PD detection system including a double-sided optical envelope generator, according to some embodiments. [Figure 15] FIG. 18 is a simplified schematic diagram of a PD detection system 1800 including a double-sided optical envelope generator 1810, according to some embodiments. [Figure 16A] 1 is a graph of a voltage signal output of a transimpedance amplifier representing an optical signal from a first LED of an electrical-to-optical converter according to some embodiments. [Figure 16B]10 is a graph of a voltage signal output of a transimpedance amplifier representing an optical signal from a second LED of an electrical-to-optical converter according to some embodiments. [Figure 17A] 10 shows a comparison of a voltage signal representing the light signal from the first LED to the maximum signal from the capacitive PD sensor. [Figure 17B] 10 shows a comparison of a voltage signal representing the optical signal from the second LED to the maximum signal from the capacitive PD sensor. [Figure 18] 10 illustrates a polynomial surface used to fit a model that predicts partial discharge charge amplitude based on voltage signals from two LEDs, according to some embodiments. [Figure 19] 10 illustrates a comparison of PD charge measured using a capacitive PD sensor versus predicted PD charge using a model, according to some embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0005] Partial discharge (PD) is more likely to occur in high-voltage components, e.g., components with operating voltages above about 1000 volts, but PD can also affect lower-voltage components. Partial discharge sites may correspond to locations where defects exist in the insulation of an electrical device. Monitoring a device for PD can provide an early warning that the device needs to be repaired before a more serious failure occurs.

[0006] Partial discharge (PD) events are localized discharges that only partially bridge the insulation between conductors or between a conductor and ground. Each PD event generates a high-frequency electrical signal that can be detected.

[0007] Some embodiments discussed herein are directed to monitoring an electrical device for the occurrence of a partial discharge event. An electrical device is an electrically connected component of an electrical system. In one example, the electrical system includes an electrical device, such as a transformer, and a connector that connects the transformer to a power grid or to another electrical device.

[0008] Some embodiments discussed herein are directed to a PD detection circuit including an electrical-to-optical converter. FIG. 1 is a schematic diagram of a PD detection circuit 440 including an electrical-to-optical converter 410 configured to convert an electrical sensor signal from a PD sensor 411 into an optical signal. In general, the PD sensor 411 can be any type of PD sensor. For example, the PD sensor 411 can be or include a capacitively coupled sensor. The PD sensor 411 can be or include a transient ground voltage sensor or a high-frequency coupling capacitor. The converter 410 includes a light-emitting device (LED) 412 in series with the PD sensor 411. In some embodiments, a current-limiting resistor can also be included, either in series or in parallel with the LED 412.

[0009] The LED 412 may include, for example, a light emitting diode or a laser. The PD sensor 411 generates an electrical signal that causes an electrical current to flow in a loop, indicated by arrow 480. The electrical signal drives the LED 412, which generates an analog optical signal 413 in response to the electrical signal, indicative of a PD event. The LED 412 may be optically coupled to an optical fiber 420 arranged to carry the analog optical signal 413 to a remote location for further processing. In some embodiments, the converter 410 and an input end 425 of the optical fiber 420 are disposed at the location of the monitored electrical component 401, and further processing detects the occurrence of a PD event and / or extracts information regarding the occurrence of a PD event at a remote location.

[0010] In some embodiments, information about the PD event may be carried by analog optical signal 413 over optical fiber 420, as in FIG. 1 . In other embodiments, the optical signal is converted back to an electrical signal, and the electrical signal carries information indicative of the PD event to a remote location. For example, analog optical signal 413 may be converted to an electrical signal by a photodetector, and the electrical signal may be processed, for example, by filtering, amplification, and / or analog-to-digital (A / D) conversion. Circuitry implementing the filtering, amplification, and A / D conversion may be located at the monitored component 401. In some embodiments, the digitized electrical signal may be converted back to a digital optical signal before the digital optical signal is coupled to the optical fiber. In such embodiments, the digital optical signal carries information about the PD event from the monitored electrical component to a remote location.

[0011] 2 is a schematic diagram of another version of an electrical-to-optical converter 510, according to some embodiments. In this example, converter 510 optionally includes a high-pass filter 530. In the embodiment shown in FIG. 2, high-pass filter 530 includes a capacitor 531 and a resistor 532 connected in series. High-pass filter 530 attenuates low frequencies in the electrical sensor signal. Resistor 532 is selected to limit the current to LEDs 512, 514.

[0012] The first and second LEDs 512, 514 are arranged parallel to each other with opposite polarity. The first and second LEDs 512, 514 convert the electrical signal generated by the PD sensor 411 into optical signals 513, 515. The first LED 512 generates a first analog optical signal 513 in response to the positive-going portion of the sensor signal, and the second LED 514 generates a second analog optical signal 515 in response to the negative-going portion of the sensor signal.

[0013] The voltage fluctuations caused by partial discharge events are typically very fast (oscillating over tens of nanoseconds). Recording such fast signals requires expensive data acquisition cards. Adding low-pass filtering to the LED circuit results in slower generated light pulses (0.5 microseconds to several microseconds) that can be recorded with less expensive components. In some embodiments, the LEDs 512, 514 can be selected so that the response time of the LEDs 512, 514 provides low-pass filtering of the sensor signal. For example, in some implementations, acceptable low-pass filtering of the sensor signal can be achieved when the LEDs 512, 514 have rise and fall times of approximately 5 nanoseconds. Alternatively, a low-pass filter can be added to the circuit.

[0014] The LEDs 512, 514 may be optically coupled to corresponding first and second optical fibers 521, 522, respectively. The LED 512 is optically coupled to the optical fiber 521, and the LED 514 is optically coupled to the optical fiber 522. In some embodiments, the converter 510 and input ends 525, 526 of the optical fibers 521, 522 are located at the monitored electrical device 401. The optical fibers 521, 522 extend to carry the first and second analog optical signals 513, 515 to a remote location for further processing. The optical interconnection including the LEDs and optical fibers provides a signal path with good electrical isolation and low electromagnetic interference between the monitored component and the remote location where the optical signals are received and processed.

[0015] As mentioned above, in some embodiments, information regarding the PD event may be carried by analog optical signals 513, 515 over optical fibers 521, 522. In other embodiments, the optical signals 513, 515 are converted to electrical signals at the location of the monitored electrical device 401, and the electrical signals carry information indicative of PD to a remote location, as previously described.

[0016] In the case of the direct electrical-to-optical converter shown in Figures 1, 2, and 3, in which an LED is connected to a PD sensor without an active device such as an operational amplifier between the LED and the sensor, a PD event that generates a voltage smaller than the LED's turn-on voltage does not generate an optical signal. However, this can be adjusted by changing the high-pass filter characteristics to couple in some of the 60 Hz (base) frequency. This 60 Hz base frequency can be used to bias the LED so that even small events are converted into an optical signal. It is also possible to use an LED with a lower turn-on voltage to increase the device's sensitivity.

[0017] FIG. 3 illustrates an embodiment of an electrical-to-optical converter 610 for PD detection. Converter 610 has many of the same components as those described above in conjunction with converter 510. Converter 610 additionally includes a rectifier / regulator 641 connected between LEDs 512, 514 and ground, making the effective turn-on voltage of LEDs 512, 514 approximately 0 V. In the configuration shown in FIG. 3, the inclusion of rectifier / regulator 641 allows for measuring charges less than approximately 1 nC, enabling even small PDs to generate light output from LEDs 512, 514. Rectifier / regulator 641 may have a connection to voltage input 411 so that the voltage across the LEDs is related only to the high-frequency components of input signal 411.

[0018] FIG. 4 shows another implementation of an electrical-to-optical converter 710 for detecting PD events in a monitored electrical component 401. The converter 710 includes an operational amplifier 742 coupled between a PD sensor 411 and LEDs 712, 714 arranged in parallel with opposite polarity. A resistor 743 is selected to limit the current to the LEDs 712, 714. Using the amplifier 742 between the PD sensor 411 and the LEDs 712, 714 (compared to a direct LED approach) provides output optical power that is linear with the PD charge. Using this technique, even small PD events can generate a measurable optical signal.

[0019] According to some implementations, a PD monitoring system may include multiple PD converters, such as those described above, positioned on a single component to monitor multiple locations of the component, or positioned on multiple components. Figure 5 schematically illustrates a PD monitoring system including N PD sensors 811-1, 811-2, ∼811-N and N electrical-to-optical converters 810-1, 810-2 ∼810-N. Each PD converter 810-1, 810-2 ∼810-N is positioned proximate to a corresponding electrical component 801-1, 801-2 ∼801-N. The PD sensors 811-1, 811-2, 811-N are coupled to the components 801-1, 801-2, 801-N, respectively, to sense PD events in the components 801-1, 801-2, 801-N. The sensor outputs of the PD sensors 811-1, 811-2, and 811-N are coupled to the PD converters 810-1, 810-2, and 810-N, respectively.

[0020] Each PD converter 810-1, 810-2, . . . 810-N illustrated in this embodiment includes two LEDs, as described above with reference to Figure 3 or 4. Each LED is optically coupled to an optical fiber 821-1, 822-1, 821-2, 822-2, . . . 821-N, 822-N. Input ends 825-1, 826-1, 825-2, 826-2, . . . 825-N, 826-N of the PD converters 810-1, 810-2, . . . 810-N and the optical fibers 821-1, 822-1, 821-2, 822-2, . . . 821-N, 822-N are positioned in close proximity to the monitored components 801-1, 801-2, . . . 801-N. Optical fibers 821-1, 822-1, 821-2, 822-2 to 821-N, 822-N carry optical signals containing information about PD events in monitored components 801-1, 801-2, to 801-N to central processing circuit 860 where the optical signals are converted to electrical signals and processed to extract the PD information.

[0021] FIG. 6 is a block diagram illustrating a representative embodiment of a portion of the central processing circuit 860. The portion of the central processing circuit illustrated in FIG. 6 includes first and second photodetectors 921 and 922 optically coupled to the output ends of optical fibers 821 and 822, respectively. The optical fibers 821 and 822 carry optical signals, e.g., analog optical signals from first and second LEDs in a PD converter located in the component being monitored for PD. The photodetectors 921 and 922 receive the optical signals and convert them to electrical signals. For example, each photodetector may include a silicon pin diode, a silicon photomultiplier, or other type of photodetector. After optical-to-electrical conversion, the electrical signals from each photodetector 921 and 922 are amplified by transimpedance amplifiers 931 and 932, implemented, for example, using operational amplifiers. The amplified electrical signals are then digitized by A / D converters 941 and 942, and the digitized signals are provided to circuitry 950 configured to extract PD information from the digitized signals. In some embodiments, the circuitry includes a processor that executes stored program instructions to extract PD information from the digitized signal. Additional signal processing can be implemented anywhere along the communication link between the PD converter and central processing circuitry 950 and / or by processing circuitry 950.

[0022] Partial discharge events vary in terms of total charge, and it can be useful to determine the total charge (measured in coulombs) transferred in a PD event. PD events of greater charge typically indicate greater damage or voltage stress on the electrical device. For AC systems, the phase angle at which PD occurs can also be used to identify problems in medium- and high-voltage components.

[0023] The magnitude of a PD event can be characterized by the amount of charge on the PD sensor. The amount of charge of a PD event is related to the magnitude of the PD sensor signal. The conversion from sensor signal magnitude to PD event charge can be obtained through a calibration technique implemented by the partial discharge detection processor in which a known charge is injected into the electrical device when the electrical device is turned off.

[0024] If the sensor signal is used to drive an LED directly (e.g., as in the configurations of Figures 1, 2, and 3, with no amplifier between the PD sensor and the LED), the PD charge will not be linearly related to the measured voltage. However, with known voltage magnitude / shape of the event on both LEDs, the PD charge can be calculated. The optical signal, converted back to an electrical signal using a photodiode, can be combined using a two-dimensional fit or formula to determine the charge.

[0025] In some embodiments, the PD event processor may be configured to detect degradation of the electrical system based on the PD event signal. For example, the processor may store information obtained from PD event signals taken at different times. The processor may compare information from successive PD events to determine whether the electrical system is changing, e.g., degrading, over time. In one scenario, the processor may obtain the magnitude and / or total amount of charge transferred of the PD charge signal for successive PD events. If the magnitude of the PD charge signal and / or the rate at which it increases over time, the processor may trigger an alert or notification that enables an operator to take action before a catastrophic failure occurs. In another scenario, the processor may obtain a first signature (e.g., a snapshot) of the PD sensor signal (or other signal) corresponding to a PD event at a first time point and compare this first signature to a second signature obtained from a PD event occurring at a second time point. The processor may compare the morphology, magnitude, timing, envelope rise time, envelope fall time, time, and / or other parameters of the first and second signatures to detect changes in the electrical system. If the change in the signal signature indicates degradation, the processor may trigger an alert or notification in response to the change in the signal signature to enable an operator to take appropriate action. According to some embodiments, the processor may be configured to predict the time of failure of the monitored electrical system based on recent usage trends and / or load pattern trends.

[0026] One problem with partial discharge detection is that high-speed data acquisition devices (faster than 100 million samples per second) are typically required to accurately record PD signals. These high-speed data acquisition devices are expensive. The embodiments discussed herein are directed to detectors and methods for recording high-frequency PD signals (e.g., >20 MHz) on lower-frequency (e.g., 2 MHz) digitizers. Some approaches described herein involve extracting the signal envelope of a PD event. The PD signal envelope can be digitized using a cheaper, lower-frequency digitizer than the PD signal itself.

[0027] To pick up the electrical signal of a PD event, signals from partial discharge sensors, such as capacitive sensors, transient ground voltage probes, high-frequency current transformers, or high-frequency antennas, may be used. Some PD sensors, such as capacitive sensors and / or current transformers, may also be able to detect the base operating frequency of the monitored electrical system if the power is AC. In some scenarios, such as high-voltage DC transmission, there is no intended "base" frequency, but it may be desirable to record the low-frequency components of such signals to check for vibrations and / or other signal anomalies.

[0028] FIG. 8A shows a block diagram of a partial discharge system 1100 according to some embodiments. The system includes a PD sensor 1101 configured to sense a partial discharge (PD) event in an electrical system and generate a sensor signal in response to the PD event. The PD sensor 1101 may include one or more of a coupling capacitor, a transient ground voltage sensor, a current transformer, and an antenna. As shown in FIG. 9 , the PD sensor signal 1200 is a relatively high-frequency signal having frequency components, for example, on the order of tens of megahertz. The envelope 1201 of the PD signal 1200 is a curve having a positive-going portion 1201a connecting the positive-going peaks of the PD signal and a negative-going portion 1201b connecting the negative-going peaks. The highest frequency component of the envelope signal 1201 may be, for example, less than 1 MHz or less than 100 KHz.

[0029] The envelope generator 1110 is coupled to receive the sensor signal from the PD sensor 1101. The envelope generator 1110 extracts an envelope signal from the sensor signal. The envelope generator 1110 may extract the envelope signal from one or both of the positive-going portion of the sensor signal and the negative-going portion of the sensor signal.

[0030] The digitizer 1150 is coupled to the envelope generator 1110 and configured to convert the envelope signal into a digital representation of the envelope signal. In some embodiments, the bandwidth of the digitizer 1150 may be less than about 1 / 5, or even less than about 1 / 10, of the desired frequency component being measured. The output of the digitizer may be provided to a processor 1175 configured to analyze the envelope signal to determine characteristics of the PD event. For example, the processor 1175 may determine the total amount of charge transferred during the PD event. In some embodiments, the processor 1175 may store information obtained from the envelope signal taken at different times. The processor 1175 may compare information from the envelope signals obtained from successive PD events to determine that the electrical system is degrading or otherwise changing over time. For example, the processor may obtain the magnitude of the envelope signal and / or the total amount of charge transferred for successive PD events. If the envelope signal magnitude, phase-resolved PD pattern, and / or total transferred charge increase over time, the processor 1175 may trigger an alert or notification corresponding to the increasing envelope signal magnitude and / or total transferred charge, allowing an operator to take action before a catastrophic failure occurs. In another scenario, the processor may acquire a first signature (e.g., a snapshot) of the envelope signal during a first PD event at a first time point and compare this first signature to a second signature during a second PD event acquired at a second time point. The processor 1175 may compare the morphology, magnitude, envelope timing, rise time, envelope fall time, and / or other parameters of the first and second signatures to detect changes in the electrical system. If the change in the signal signature is consistent with degradation, the processor 1175 may trigger an alert or notification corresponding to the change in the signal signature, allowing an operator to take appropriate action.

[0031] Higher utilization and / or higher load of the electrical system correlates to PD events that occur more frequently, have higher magnitudes, result in a larger total amount of transferred charge, and / or have other characteristics indicative of the utilization and / or load of the system. According to some embodiments, the processor 1175 may keep track of these PD event characteristics and determine the utilization and / or load of the electrical system over time. In some implementations, the processor may predict time to failure of the electrical system based on system usage and / or load trends and / or based on the characteristics of the PD events.

[0032] FIG. 8B provides a more detailed block diagram of a PD detection system 1100B according to some embodiments. A PD sensor 1101 provides an electrical signal in response to a PD event in a monitored electrical system. As shown in FIG. 8B, an envelope generator 1110 includes a high-pass filter coupled to the PD sensor. The high-pass filter 1111 may have a cutoff frequency such that frequencies below approximately 50 kHz in the electrical PD sensor signal are substantially attenuated by the filter 1111. For example, when monitoring an AC electrical system for PD events, the high-pass filter 1111 may typically block line frequencies. The envelope generator 1110 may include a rectifier circuit 1112 that 1) blocks the negative-going portion of the signal and passes the positive-going portion at the output of the high-pass filter 1111; 2) blocks the positive-going portion of the signal and passes the negative-going portion at the output of the high-pass filter 1111; and / or 3) provides absolute values ​​of the positive-going and negative-going portions of the signal at the output of the high-pass filter 1111. The output of rectifier circuit 1112 is coupled to low pass filter 1113, which attenuates high frequency components from the rectified signal, providing an envelope signal at the output of low pass filter 1113. Envelope generator 1110 optionally includes amplifier 1114, which amplifies the envelope signal. The envelope signal is converted from an analog signal to a digital signal by digitizer element 1151 of digitizer 1150. Optionally, the electrical signal at the output of rectifier 1112 may be converted to an optical signal, and the optical signal may be converted back to an electrical signal before amplification by amplifier 1114. Optical signals may be desirable in some implementations for electrical isolation or electromagnetic interference reduction.

[0033] In some embodiments, the cutoff frequency of the high-pass filter 1111 and / or the low-pass filter 1113 may be adjustable. The adjustable filter allows a spectrum of PD signal amplitudes at different frequencies to be generated. The adjustable filter may also be used to select a specific measurement band, thus avoiding picking up environmental background noise, such as radio frequency signals, PD from other sources, etc., and increasing the sensitivity of the PD detection system.

[0034] In some embodiments, PD detection system 1100B can optionally include channel 1160, which detects the operating frequency of the monitored AC electrical system. Channel 1160 includes low-pass filter 1115, which substantially attenuates frequencies above, e.g., 10x, 100x, or 1000x, the operating frequency of the AC system being monitored for PD events, while passing frequencies below the operating frequency of the monitored AC system. The filtered signal can be amplified by amplifier components 1116 and / or 1117. In some embodiments, the signal between amplifiers 1116 and 1117 can be an electrical signal. In other embodiments, the output of amplifier 1116 can be converted to an optical signal and then converted back to an electrical signal before the input of amplifier 1117. The output of amplifier 1117 is converted from an analog signal to a digital signal by digitizer component 1152. By detecting the base frequency, the phase angle of the detected PD event can be easily recorded using the same digitizer.

[0035] The outputs of the digitizer components 1151, 1152 may be coupled to a processor (not shown in FIG. 8B), which may be configured to analyze the envelope signal, for example, as further described in connection with FIG. 8A.

[0036] 10-15 provide simplified schematic diagrams of several PD detection systems according to various embodiments. The schematic configurations shown in Figures 10-15 provide just a few of many circuit implementations for realizing a PD detection system that downconverts the frequency of a PD signal to a lower frequency envelope signal that can be more cost-effectively digitized and analyzed in accordance with the approaches described herein.

[0037] FIG. 10 is a simplified schematic diagram of a PD detection system 1300 including a single-sided direct electrical envelope generator 1310, according to some embodiments. The PD sensor 1101 provides an electrical signal in response to a PD event in a monitored electrical system. As shown in FIG. 10, the envelope generator 1310 includes a high-pass filter 1311a coupled to the PD sensor 1101. In FIG. 10, the high-pass filter 1311 is shown as a passive filter including a capacitor 1321 and a resistor 1322, although it will be understood that other types of high-pass filter circuits may be used. The envelope generator 1310 includes a rectifier circuit 1312, illustrated as a diode arranged to block the negative-going portion of the signal at the output 1311b of the high-pass filter 1311 and pass the positive-going portion. Output 1312b of rectifier circuit 1312 is coupled to low pass filter 1313, which attenuates high frequency components from the rectified signal and provides the envelope signal at output 1313b of low pass filter 1313. In Figure 10, low pass filter 1313 is shown as a passive filter including capacitor 1324 and resistor 1323, although it will be understood that other types of low pass filter circuits may be used. The envelope signal is converted from an analog signal to a digital signal by digitizer component 1351 of digitizer 1350.

[0038] In some embodiments, the cutoff frequency of the high-pass filter 1311 and / or the low-pass filter 1313 may be adjustable. The adjustable filter allows a spectrum of PD signal amplitudes at different frequencies to be generated. The adjustable filter may also be used to avoid picking up environmental background noise, such as radio frequency signals, PD from other sources, etc., and thus may increase the sensitivity of the PD detection system.

[0039] In some embodiments, the PD detection system 1300 can optionally include a second channel 1360 that detects the operating frequency of the monitored AC electrical system. The channel 1360 includes a low-pass filter 1315 that passes frequencies lower than the operating frequency of the electrical system while substantially attenuating frequencies above the operating frequency, e.g., 10, 100, or 1000 times the operating frequency. In FIG. 10 , the low-pass filter 1315 is shown as a passive filter including a capacitor 1325 and a resistor 1326, although it will be understood that other types of low-pass filter circuits may be used. The output 1315b of the low-pass filter 1315 is converted from an analog signal to a digital signal by a digitizer component 1352. The outputs of the digitizer components 1351, 1352 may be coupled to a processor (not shown in FIG. 10 ). The processor may be configured to analyze the envelope signal, for example, as further described in connection with FIG. 8A .

[0040] FIG. 11 is a simplified schematic diagram of a PD detection system 1400 including a single-sided direct amplified electrical envelope generator 1410, according to some embodiments. Many of the components of the PD detection system 1400 are similar to those described above in connection with the single-sided direct electrical envelope generator 1310. The PD detection system 1400 includes an amplifier 1490 and bias resistors 1491, 1492 connected between a high-pass filter 1311 and a low-pass filter 1313. A rectifier 1312 may optionally be connected between the high-pass filter 1311 and the amplifier 1490 and / or between the amplifier 1490 and the low-pass filter 1313. The output of the envelope generator 1410 provides an amplified envelope signal that is provided to a digitizer component 1351. The digitized signal generated by the digitizer component 1351 may be provided to a processor (not shown in FIG. 11 ) that performs further analysis of the PD event, as described above. In this particular embodiment, an optional separate channel may optionally be included, but is not shown, for detecting the operating frequency of the AC system being monitored.

[0041] FIG. 12 is a simplified schematic diagram of a PD detection system 1500 including a two-sided direct electrical envelope generator 1510, according to some embodiments. The PD sensor 1101 provides an electrical signal in response to a PD event in a monitored electrical system. The envelope generator 1510 includes a high-pass filter 1511, depicted here as a passive high-pass filter including a capacitor 1521 and a resistor 1522. As shown in FIG. 12, the envelope generator 1510 includes two electrical channels 1571, 1572, where channel 1571 passes the positive-going signal output of the high-pass filter 1511 and channel 1572 passes the negative-going signal output of the high-pass filter 1511. A rectifier 1512 passes the positive-going, high-pass filtered signal to a low-pass filter 1513. An inverted rectifier 1514 passes the negative-going, high-pass filtered signal to a low-pass filter 1516.

[0042] Low-pass filter 1513 attenuates high-frequency components from the positive-going signal and provides the positive-going envelope portion at output 1513b of low-pass filter 1513. In FIG. 12, low-pass filter 1513 is shown as a passive filter including capacitor 1524 and resistor 1523, but it will be understood that other types of low-pass filter circuits may be used. The low-pass circuit may be built into the digitizer, and in some embodiments, no additional low-pass filter is required. The positive-going portion of the envelope signal is converted from an analog signal to a digital signal by digitizer 1550.

[0043] Low pass filter 1516 attenuates high frequency components from the negative-going signal and provides the negative-going envelope portion at output 1516b of low pass filter 1516. In Figure 12, low pass filter 1516 is shown as a passive filter including capacitor 1528 and resistor 1527, although it will be understood that other types of low pass filter circuits may be used. The negative-going portion of the envelope signal is converted from an analog signal to a digital signal by digitizer 1550.

[0044] The PD detection system 1500 may optionally include a channel 1560 that detects the operating frequency of the monitored AC electrical system. The channel 1560 includes a low-pass filter 1515 that passes frequencies lower than the operating frequency of the electrical system while substantially attenuating frequencies above, e.g., 10, 100, or 1000 times the operating frequency. In FIG. 12, the low-pass filter 1515 is shown as a passive filter including a capacitor 1526 and a resistor 1525, although it will be understood that other types of low-pass filter circuits may be used. The output 1515b of the low-pass filter 1515 may be converted from an analog signal to a digital signal by a digitizer 1550. The output of the digitizer 1550 may be coupled to a processor (not shown in FIG. 12). The processor may be configured to analyze the envelope signal, for example, as further described in connection with FIG. 8A.

[0045] In some embodiments, the cutoff frequencies of the high pass filter 1511 and / or the low pass filters 1513, 1515, 1516 may be adjustable as described above.

[0046] 13 is a simplified schematic diagram of a PD detection system 1600 including a single-sided amplified optical envelope generator 1610, according to some embodiments. The PD sensor 1101 provides an electrical signal in response to a PD event in a monitored electrical system. The envelope generator 1610 includes a high-pass filter 1611, depicted here as a passive high-pass filter including a capacitor 1621 and a resistor 1622. The high-pass filter 1611 may generally include any type of high-pass filter and may optionally have an adjustable cutoff frequency as discussed herein.

[0047] A rectifier 1612 may optionally be coupled at output 1611b of highpass filter 1611 between highpass filter 1611 and amplifier 1690. Rectifier 1612 passes the positive-going portion of the highpass filtered signal. If the high frequency components of the signal at output 1611b exceed the slew rate of amplifier 1690, the amplified signal at the output of amplifier 1690 may be distorted. In some configurations, the characteristics of rectifier 1612 may be selected such that the slew rate requirements of amplifier 1690 are reduced. For example, the response time (rise time and / or fall time) of rectifier 1612 may be selected to attenuate the high frequency components of the signal at output 1611b.

[0048] The signal at output 1690b of amplifier 1690 drives a light-emitting diode (LED) 1693, with the current through LED 1693 limited by resistor 1691. Light produced by LED 1693 is detected by photodetector 1681 and converted to an electrical signal at photodetector output 1681b. Optionally, amplifier 1682 is included in envelope generator 1610 to provide a second stage of amplification. The amplified signal at output 1682b of amplifier 1682 is digitized by digitizer 1650. The digitized signal produced by digitizer 1650 may be provided to a processor (not shown in FIG. 13 ) that performs further analysis of the PD event, as described above.

[0049] In various embodiments, low-pass filtering of the signal at the output of photodetector 1681b may be achieved by reducing the bandwidth of amplifier 1682. The low-pass filtering function of envelope generator 1610 may be provided by rectifier 1612, LED 1693, photodetector 1681, and / or amplifier 1682. The transient input and / or output response characteristics of each, some, or all of these components may provide the low-pass filtering that produces the envelope signal. In this particular embodiment, an optional separate channel may be included for detecting the operating frequency of the monitored AC system, but is not shown.

[0050] 14 is a simplified schematic diagram of a PD detection system 1700 including a two-sided optical envelope generator 1710, according to some embodiments. The PD sensor 1101 provides an electrical signal in response to a PD event in a monitored electrical system. The envelope generator 1710 includes a high-pass filter 1711, depicted here as a passive high-pass filter including a capacitor 1721 and a resistor 1722. The high-pass filter 1711 may generally include any time high-pass filter and may optionally have an adjustable cutoff frequency as discussed herein.

[0051] The signal at output 1711b of high-pass filter 1711 drives LEDs 1793 and 1794. A first channel 1771 containing LED 1793 converts the positive-going portion of the signal at output 1711b to a first optical signal. A second channel 1772 containing LED 1794 converts the negative-going portion of the signal at high-pass filter output 1711b to a second optical signal. Photodetector 1781 reconverts the first optical signal to an electrical signal at output 1781b of photodetector 1781. Photodetector 1783 reconverts the second optical signal to an electrical signal at output 1783b of photodetector 1783.

[0052] Optionally, amplifiers 1782, 1784 are included in envelope generator 1710. The amplified signals at outputs 1782b, 1784b are digitized by digitizer components 1751, 1753, respectively. The digitized signals produced by digitizer components 1751, 1752 may be provided to a processor (not shown in FIG. 14 ) that performs further analysis of the PD event, as described above.

[0053] In various embodiments, low-pass filtering of envelope generator 1710 within signal channel 1771 may be provided by LED 1793, photodetector 1781, and / or amplifier 1782. The characteristics of each, some, or all of these components, such as bandwidth, transient input response, and / or transient output response, may provide the low-pass filtering that generates the envelope signal. Similarly, in various embodiments, low-pass filtering of envelope generator 1710 within signal channel 1772 may be provided by LED 1794, photodetector 1783, and / or amplifier 1784. The characteristics of each, some, or all of these components, such as bandwidth, transient input response, and / or transient output response, may provide the low-pass filtering that generates the envelope signal. In this particular embodiment, an optional separate channel for detecting the operating frequency of the monitored AC system may be included but is not shown.

[0054] FIG. 15 is a simplified schematic diagram of a PD detection system 1800 including a double-sided optical envelope generator 1810, according to some embodiments. Many of the components of the PD detection system 1800 are similar to those previously described in connection with the double-sided optical envelope generator 1700. The envelope generator 1810 includes an amplifier 1890 connected between a high-pass filter 1811 including a capacitor 1821 and a resistor 1822 and LEDs 1893 and 1894. While shown as a passive analog filter, the high-pass filter 1811 can generally be any type of filter. In some embodiments, the high-pass filter is adjustable, as described above. The resistor 1891 is included to limit the current through the LEDs 1893 and 1894. The LED 1873 in channel 1871 converts the positive-going portion of the signal at the output 1890b of the amplifier 1890 into a first optical signal. An LED 1874 in channel 1772 passes the negative-going portion of the signal at output 1890b of amplifier 1890 to a second optical signal.

[0055] The first optical signal is reconverted to an electrical signal at output 1881b of photodetector 1881. The second optical signal is reconverted to an electrical signal at output 1883b of photodetector 1883.

[0056] Optionally, amplifiers 1882, 1884 are included in envelope generator 1810. The amplified signals at outputs 1882b, 1884b are digitized by digitizer components 1851, 1853, respectively. The digitized signals produced by digitizer components 1851, 1853 may be provided to a processor (not shown in FIG. 15 ) that performs further analysis of the PD event as described above.

[0057] In various embodiments, low-pass filtering of envelope generator 1810 within signal channel 1871 may be provided by LED 1873, photodetector 1881, and / or amplifier 1882. The characteristics of each, some, or all of these components, such as bandwidth, transient input response, and / or transient output response, may provide the low-pass filtering that generates the envelope signal. Similarly, in various embodiments, low-pass filtering of envelope generator 1810 within signal channel 1872 may be provided by LED 1874, photodetector 1883, and / or amplifier 1884. The characteristics of each, some, or all of these components, such as bandwidth, transient input response, and / or transient output response, may provide the low-pass filtering that generates the envelope signal. In this particular embodiment, an optional separate channel for detecting the operating frequency of the monitored AC system may be included but is not shown. [Example]

[0058] Testing using a 110V-7000V potential transformer was conducted to demonstrate the PD converter described herein. A capacitive coupler using the PD converter circuit shown in Figure 2 was tested. An internal partial discharge was generated by applying 90 VAC and operating the transformer in air. Figure 7 shows a graph of the voltage across the coupling capacitor during a PD event. Voltage graphs for multiple PD events are overlaid in Figure 7.

[0059] The optical signals of the first and second converter LEDs (elements 512 and 514 in FIG. 2 ) were converted to electrical signals using first and second silicon photomultiplier detectors. The outputs of the first and second photomultiplier detectors were amplified by first and second transimpedance amplifiers. FIG. 16A is a graph of the voltage signal output of the transimpedance amplifier representing the optical signal from the first LED, with different traces representing different PD events. FIG. 16B is a graph of the voltage signal output of the transimpedance amplifier representing the optical signal from the second LED. The LED used in the PD converter of this example had a response time of 500 μsec; therefore, fast oscillations of the signal from the capacitive coupler were not present in the voltage signals shown in FIGS. 16A and 16B .

[0060] 17A shows a comparison of the voltage signal representing the optical signal from the first LED (component 512 in FIG. 2) to the maximum signal (PD charge) from the capacitive PD sensor (component 411 in FIG. 2). FIG. 17B shows a comparison of the voltage signal representing the optical signal from the second LED (component 514 in FIG. 2) to the maximum signal (PD charge) from the capacitive PD sensor (component 411 in FIG. 2). Note from these comparisons that the PD charge of the capacitive PD sensor correlates to the voltage signals representing the optical signals of the first and second LEDs.

[0061] The polynomial surface shown in FIG. 18 was used to fit a model that predicts partial discharge charge amplitude based on the voltage signals from the two LEDs. FIG. 19 shows a comparison of the PD charge predicted using the model to the PD charge measured using a capacitive PD sensor. The comparison shown in FIG. 19 indicates that an electrical-to-optical PD converter such as that in FIG. 2 can provide accurate measurements of PD with magnitudes greater than 1 nC.

Claims

1. 1. A partial discharge detection system, comprising: a partial discharge (PD) sensor configured to sense a PD event in an electrical system and to generate a high frequency sensor signal in response to the PD event; an envelope generator coupled to receive the sensor signal from the PD sensor, the envelope generator including two light emitting diodes (LEDs) configured to convert both positive-going and negative-going portions of the sensor signal into two optical signals, and photodetectors configured to receive the two optical signals conveyed via two optical fibers, respectively, and to extract an envelope signal of the sensor signal from the two received optical signals; and a digitizer configured to convert the envelope signal into a digital representation of the PD event, the digitizer having a bandwidth less than about 1 / 5 of the frequency content to be measured.

2. The partial discharge detection system of claim 1 , wherein the PD sensor includes one or more of a coupling capacitor, a transient ground voltage sensor, a current transformer, and an antenna.

3. The partial discharge detection system of claim 1 , wherein the envelope generator includes at least one amplifier configured to amplify the envelope signal.

4. The partial discharge detection system of claim 1 , wherein the envelope generator includes a high-pass filter.

5. 5. The partial discharge detection system of claim 4, wherein the cutoff frequency of the high-pass filter is adjustable.

6. 1. A method for detecting partial discharge (PD), comprising: using a PD sensor to sense a partial discharge (PD) event in an electrical system and generate a high frequency electrical sensor signal in response to the PD event; converting both the positive-going and negative-going portions of the electrical sensor signal into two optical signals using two light-emitting diodes (LEDs); receiving the two optical signals carried over two optical fibers using a photodetector; extracting an envelope signal from the two received optical signals using an envelope generator including the two LEDs and the photodetector; converting the envelope signal into a digital representation of the envelope signal using a digitizer with a bandwidth less than about 1 / 5 of the frequency components to be measured; A method comprising:

7. The method of claim 6 , wherein extracting the envelope signal comprises high-pass filtering the sensor signal.

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