WO2025237486A1

WO2025237486A1 - Improved testing of electrical insulation

Info

Publication number: WO2025237486A1
Application number: PCT/DK2025/050068
Authority: WO
Inventors: Peter P MONGEAU; Torben Werge MØLLER; Brian Johansen
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2024-05-17
Filing date: 2025-05-14
Publication date: 2025-11-20
Anticipated expiration: 2026-11-17

Abstract

An apparatus and method for partial discharge testing of electrical components. The apparatus includes a partial discharge probe (50) having a housing (52) and a sensor (56) contained by the housing (52). The sensor (56) is sensitive to electromagnetic and/or acoustic energy emitted by partial discharges (60). The housing (52) has an aperture (54) defining a field of view (62) of the probe (50) and is configured to prevent energy from reaching the sensor (56) except through the aperture (54). The probe (50) can be placed in a plurality of positions relative to the electrical component (72), with each position placing a different portion of the electrical component (72) in the probe's field of view (62). Partial discharge data is generated with the probe (50) in each position by applying one or more excitation waveforms (36) to the electrical component and recording both an amount of energy received by, and the position of, the probe (50). A partial discharge map (110) may be defined based on the partial discharge data.

Description

IMPROVED TESTING OF ELECTRICAL INSULATION

Technical Field

This invention relates generally to the testing of electrical insulators, and more particularly to a method of testing the insulation of an electrical component such as the windings of an electric machine.

Background

Prior to deployment, the windings in large electric machines, such as wind turbine generators, are typically subjected to qualification testing that includes tests on the insulation of the windings. There are a number of known insulation testing techniques that may be used for qualification testing.

One such test involves applying high voltage alternating current (AC) or direct current (DC) across the stator windings as a whole, between the stator windings and the stator core, or between different stator winding sets in a multiple coil system. Leakage current can then be measured in conjunction with the application of the high test voltage to establish an effective insulation resistance for the winding This type of test is commonly referred to as a “high potential” or “hipot” test. Hipot tests are a type of steady state test, and typically use a test voltage having an amplitude several times that of the working voltage, where the working voltage is the maximum voltage to which the winding will be exposed during normal operation. Hipot tests are viewed as a “proof’ test that may be used for serial production testing. Hipot tests may also be conducted from time-to-time in situ, but at much reduced levels so as to not risk damaging the winding insulation of a deployed electric machine. Reduced voltages are generally used for hipot tests in the field since an insulation failure during testing can irreversibly damage the insulation system such that the electric machine cannot be put back in service.

Another insulation test is known as “surge testing”. Surge testing of an electric machine involves applying a high voltage pulse to a winding at different voltage levels to establish turn-to-turn dielectric integrity. Like hipot testing, surge testing is typically used as a serial qualification type test at several times the working voltage. However, surge testing is not commonly performed in situ or used for diagnostic testing as part of normally scheduled maintenance because a surge testing failure often causes irreversible damage to the insulation being tested.

Partial discharge testing is a specialized approach to insulation testing that determines a voltage level at which partial discharges occur in the windings. A partial discharge is caused by localized weak areas in the insulation system which do not show up in standard DC hipot testing. Partial discharge testing does not cause a full-blown failure of the insulation system, but rather merely triggers a precursor to eventual failure. Partial discharge testing generally indicates the voltage level at which the system under test may begin to accumulate wear. A partial discharge in and of itself does not result from a failure of the insulation. However, partial discharges are indicative of a voltage level above which the insulation system may accumulate damage over time. Thus, if this level of voltage is sustained over a long enough period of time, it can lead to outright insulation failure. Partial discharge tests are generally done once in standard acceptance testing but are not performed repeatedly at full test voltages.

Fig. 1 depicts an insulator 10 separating two conductors 12 (e.g., the copper conductor of a winding and the iron core of a stator) and a defect 14 (e.g., a contaminate, void, cavity, etc.) embedded in the insulator 10. Applying a test voltage W to the conductors 12 produces an electric field E in the insulator 10. As depicted by Fig. 2, the resulting voltage distribution 16 across the insulator 10 may be relatively linear. For the purpose of analysis, the insulator 10 may be divided into three regions 10a-10c corresponding to a region 10a covering the distance di between the upper conductor 12 and the defect 14, a region 10b covering the distance c occupied by the defect 14, and a region covering the distance ch between the defect 14 and the lower conductor 12. Each of these regions 10a- 10c may be modeled electrically as a series capacitor 18a-18c having a respective capacitor voltage ^- that sum to the test voltage V .

V_T = V₁ + V₂ + v₃

Referring now to Figs. 3 and 4, as the test voltage VT is increased, it may reach a point at which the electric field E has a sufficient gradient to cause localized ionization at the defect 14, resulting a pulse of current 20. Turning now to the electrical model, the rapid drop in the impedance across region 10b of insulator 10 caused by the localized ionization redistributes the charge previously held by the capacitor 18b to capacitor 18a and capacitor 18c. The value of V2 drops rapidly to near zero, and the values of Vi and V3 increase proportionally as the capacitors are recharged by the source of the test voltage VT. However, the insulator 10 holds and remains essentially undamaged.

The phenomena where only a portion of an insulator breaks down is known as a “partial discharge”. Partial discharges occur when a part of the insulation system breaks down due to a localized electric field exceeding the dielectric’s ability to withstand the resulting voltage. However, the remaining portion of the dielectric is able to withstand the applied electric field. Thus, the insulation system as a whole remains functional. A partial discharge is characterized by a time varying voltage pulse that travels through the insulation 10 and produces the redistribution of charge described above. The current pulse has a relatively low energy due to its short (e.g., < 1 second) duration. However, the current pulse may negatively affect the insulation 10 and, if the current pulse occurs repeatedly over time, enough damage may accumulate to allow a full breakdown and catastrophic failure of the insulator 10.

Partial discharges are characterized by a breakdown along some intermediate path that does not extend all the way from one conductor to another, e.g., from the stator core to the windings. In addition to localized ionization at a defect in the insulation (as described above), a partial conductive path may also form across an exposed surface of the insulator (referred to as “flashover”), or through ionization of the surrounding air (“corona”). However, conduction through an internal defect in the insulation is the mechanism most closely associated with partial discharge because it is also the most difficult to observe and to control. In contrast, surface flashover and corona are relatively easy to detect, and can generally be mitigated by some combination of increasing the flashover path distance, increasing the thickness of the insulation in the region of the discharge, or applying a semi-conductive treatment to suppress the development of excess charge external to the primary insulation system.

High voltage electric machines typically have an increased sensitivity to partial discharges. A limited exposure to partial discharge may be tolerated for test and evaluation purposes. However, partial discharge is typically the dominant wear mode in high voltage electric machines, such as wind turbine generators. Partial discharges cause cumulative damage to the insulation system which can, if left unchecked, lead to a complete dielectric failure over time. Typically, electric machines are tested by increasing the test voltage until some level of partial discharge is detected. This voltage level is referred to as the partial discharge inception voltage. The test voltage may then be reduced while continuing to monitor partial discharge activity to determine the voltage at which partial discharge activity ceases. This voltage is known as the partial discharge extinction voltage.

Adequately high partial discharge inception voltage levels are generally a part of acceptance testing and a “critical to quality” criteria in both high voltage electric machines and inverter driven machines. Partial discharge testing and monitoring is also used as a routine part of recurring maintenance inspections. Monitoring degradation in the partial discharge inception voltage is a useful diagnostic tool. Catching degradation in the partial discharge inception voltage before an outright failure in the insulation occurs can prevent a catastrophic failure and irreparable damage to an electric machine. If detected in time, the electric machine can be reconditioned or taken out of service as a preemptive action before a full breakdown in the insulation system. An outright dielectric fault while the electric machine is in use can lead to catastrophic damage of the electric machine with more extreme impacts not only to the machine itself, but also to equipment connected to the machine. Thus, early detection of insulation degradation may decrease both downtime and repair costs.

Conventional partial discharge measurement systems measure the voltages or currents being applied to the electric machine under test. Fig. 5 depicts an exemplary test environment 22 for performing partial discharge testing on a stator 24. The stator 24 includes a plurality of windings 26 (e.g., three windings 26) and a core 28. One of the windings 26 is operatively couped to the output of a high voltage transformer 30 that provides the test voltage. The voltage applied to the stator may be measured by a measurement device 32 (e.g., an oscilloscope) operatively coupled to the input of the stator winding 26 under test by a coupling capacitor 34. Another measurement device (not shown) may be used to measure current flow through the stator 24. Current flow may also be determined based on the measured voltage and a known impedance of the stator 24.

Fig. 6 depicts an exemplary sinusoidal waveform 36 that may be measured by the measurement device 32. The waveform 36 may include negative polarity pulses 38 on the positive charging portion of the waveform 36 and positive polarity pulses 40 on the negative charging portion of the waveform 36. When present, the positive and negative polarity pulses 38, 40 are indicative of a partial discharge occurring somewhere within the insulation system of the stator 24. However, because the measurements are made on the stator winding 26 as a whole, the only information the partial discharge test produces is that a partial discharge is occurring somewhere within the insulation system between the specific winding 26 being energized and the core 28 of stator 24.

In view of the above, there is a need to improve partial discharge testing of electric machines to provide more detailed information regarding where and by what mechanism the partial discharges are occurring.

Summary

In an aspect of the invention, an apparatus for testing an electric component is provided. The apparatus includes a partial discharge probe, a position indexing system, an excitation waveform generator, one or more processors, and a memory. The one or more processors are operatively coupled to the partial discharge probe, the position indexing system, the excitation waveform generator, and the memory. The partial discharge probe includes a housing and a sensor contained by the housing. The sensor is sensitive to at least one of an electromagnetic energy and an acoustic energy. The housing has an aperture that defines a field of view of the partial discharge probe, and is configured to prevent at least one of the electromagnetic energy and the acoustic energy from entering the housing except through the aperture. The position indexing system is configured to determine a position of the partial discharge probe, and the memory includes program code. When executed by the one or more processors, the program code causes the apparatus to use the excitation waveform generator to generate a first excitation waveform that is applied to the electrical component, record a first amount of energy received by the sensor of the partial discharge probe, and record the position of the partial discharge probe received from the position indexing system.

In an embodiment of the invention, the sensor may be a first sensor that is sensitive to one of the electromagnetic energy and the acoustic energy, and the partial discharge probe may further include a second sensor contained by the housing that is sensitive to the other of the electromagnetic energy and the acoustic energy.

In another embodiment of the invention, one of the first and second sensors may include at least one of a radio frequency detector and a photodetector, and the other of the first and second sensors may include an acoustic detector.

In another embodiment of the invention, the partial discharge probe may further include a gasket that encircles the aperture of the housing and is configured to be placed in contact with a surface of the electrical component and to isolate the sensor from signals generated outside the field of view of the partial discharge probe.

In another embodiment of the invention, the housing may include shielding, and the shielding may be operatively coupled to the gasket.

In another embodiment of the invention, the partial discharge probe may further include an isolator, and the isolator may couple the sensor to the housing and isolate the sensor from the shielding.

In another embodiment of the invention, the first excitation waveform may be applied to the electrical component, the first amount of energy may be recorded by the sensor, and the position of the partial discharge probe may be recorded at each of a plurality of positions relative to the electrical component. Each of the plurality of positions may place a different portion of the electrical component in the field of view of the partial discharge probe. In this embodiment, the program code may further cause the apparatus to, for each of the plurality of positions, use the excitation waveform generator to generate a second excitation waveform that is applied to the electrical component and is different from the first excitation waveform, record a second amount of energy received by the partial discharge probe while the second excitation waveform is applied to the electrical component, and determine a difference between the first amount of energy and the second amount of energy.

In another embodiment of the invention, the first excitation waveform may have a first slew rate, and the second excitation waveform may have a second slew rate different from the first slew rate.

In another embodiment of the invention, the program code may further cause the apparatus to define a partial discharge map based at least in part on the first amount of energy recorded and the position of the partial discharge probe recorded at each of the plurality of positions.

In another embodiment of the invention, the apparatus may further include an actuator configured to move the partial discharge probe relative to the electrical component. In this embodiment, the program code may further cause the apparatus to position the partial discharge probe in each of the plurality of positions using the actuator.

In another embodiment of the invention, the electrical component may be a stator of an electric machine that also includes a rotor, and the partial discharge probe may be inserted into an air gap between the stator and the rotor.

In another aspect of the invention, a method for testing the electrical component is provided. The method includes positioning a first partial discharge probe having a field of view in a first plurality of positions relative to the electrical component, with each position placing a different portion of the electrical component in the field of view of the first partial discharge probe. While the first partial discharge probe is in each of the first plurality of positions, the method may apply the first excitation waveform to the electrical component and record the first amount of energy received by the first partial discharge probe, apply the second excitation waveform to the electrical component different from the first excitation waveform and record the second amount of energy received by the first partial discharge probe, and record the position of the first partial discharge probe. The method may further include determining a first difference between the first amount of energy and the second amount of energy recorded at each of the first plurality of positions, and defining a first partial discharge map indicating the first difference between the first amount of energy and the second amount of energy at one or more of the first plurality of positions. In an embodiment of the invention, the first amount of energy received by the first partial discharge probe may include one or more of an amount of electromagnetic energy and an amount of acoustic energy.

In another embodiment of the invention, the first excitation waveform may have the first slew rate, and the second excitation waveform may have the second slew rate different from the first slew rate.

In another embodiment of the invention, the first partial discharge probe may include the housing. In this embodiment, the first sensor may be contained by the housing and sense the first amount of energy received by the first partial discharge probe. The housing may have the aperture and define the field of view of the first partial discharge probe by preventing the first amount of energy received by the first partial discharge probe from entering the housing except through the aperture.

In another embodiment of the invention, the first partial discharge probe may further include a gasket encircling the aperture of the housing. In this embodiment, the method may further include placing the gasket in contact with the surface of the electrical component.

In another embodiment of the invention, the first partial discharge probe may further include the second sensor contained by the housing. In this embodiment, the first sensor may be sensitive to one of the amount of electromagnetic energy and the amount of acoustic energy, and the second sensor may be sensitive to the other of the amount of electromagnetic energy and the amount of acoustic energy.

In another embodiment of the invention, the first and second amounts of energy may be received by the first partial discharge probe at a first time. In this embodiment, the method may further include, at a second time after the first time, positioning one of the first partial discharge probe or a second partial discharge probe in a second plurality of positions relative to the electrical component, each position of the second plurality of positions placing a different portion of the electrical component in the field of view of the first or second partial discharge probe. While the first or second partial discharge probe is in each of the second plurality of positions, the method may apply a third excitation waveform to the electrical component, record a third amount of energy received by the first or second partial discharge probe, and record the position of the first or second partial discharge probe. The method may the determine a second difference between one of the first or second amounts of energy recorded at one or more of the first plurality of positions at the first time and the third amount of energy recorded at one or more of the second plurality of positions at the second time, and define a second partial discharge map indicating the second difference between the first or second amount of energy and the third amount of energy at the one or more of the first and second pluralities of positions.

In another embodiment of the invention in which the first and second amounts of energy are received by the first partial discharge probe at the first time, the method may further include at the second time after the first time, positioning one of the first partial discharge probe or the second partial discharge probe in the second plurality of positions relative to the electrical component each placing the different portion of the electrical component in the field of view of the first or second partial discharge probe. In this embodiment, while the first or second partial discharge probe is in each of the second plurality of positions, the method may apply the third excitation waveform to the electrical component and record the third amount of energy received by the first or second partial discharge probe, apply a fourth excitation waveform to the electrical component and record the fourth amount of energy received by the partial discharge probe, and record the position of the first or second partial discharge probe. The method may determine the second difference as the difference between the third amount of energy and the fourth amount of energy recorded at each of the plurality of positions, determine a third difference between the first difference and the second difference, and define the second partial discharge map as indicating the third difference at the one or more of the first and second pluralities of positions.

In another embodiment of the invention, the first time may be before the electrical component is placed in service, and the second time may be after the electrical component has been placed in service.

In another embodiment of the invention, the electrical component may be the stator of the electric machine that also includes the rotor, the probe used at the second time may be the second partial discharge probe, and the second partial discharge probe may be inserted into the air gap between the stator and the rotor.

In another embodiment of the invention, the first partial discharge probe may be used without the rotor installed in the electric machine at the first time.

In another embodiment of the invention, the electrical component may be the stator of the electric machine that also includes the rotor, and the method may position the first partial discharge probe by inserting the first partial discharge probe into the air gap between the stator and the rotor of the electric machine. In another embodiment of the invention, the electric machine may include an access port. In this embodiment, inserting the first partial discharge probe into the air gap between the stator and the rotor of the electric machine may include accessing the air gap through the access port.

In another embodiment of the invention, the access port may be a first access port of a plurality of access ports of the electric machine. In this embodiment, the method may position the first partial discharge probe in the first plurality of positions relative to the electrical component by inserting the first partial discharge probe into the air gap between the stator and the rotor through both the first access port and a second access port of the plurality of access ports. While the first partial discharge probe is inserted into the first access port, the method may position the first partial discharge probe in each position of a first portion of the first plurality of positions associated with a first portion of the stator. While the first partial discharge probe is inserted into the second access port, the method may position the first partial discharge probe in each position of a second portion of the first plurality of positions associated with a second portion of the stator. The method may then define the first partial discharge map by recording the first and second amounts of energy received by the first partial discharge probe and the position of the first partial discharge probe for each position of the first portion of the first plurality of positions, recording the first and second amounts of energy received by the first partial discharge probe and the position of the first partial discharge probe for each position of the second portion of the first plurality of positions, defining a first portion of the partial discharge map from the first and second amounts of energy recorded for the first portion of the stator, defining a second portion of the partial discharge map from the first and second amounts of energy recorded for the second portion of the stator, and stitching together the first and second portions of the partial discharge map.

The various aspects and embodiments of the invention described above may be used to qualify electric components as having a required partial discharge margin using multiple excitation waveforms that differ from one another in one or more of amplitude, frequency, time rate of change. Mapping this partial discharge data as a multi-dimensional function of variable excitation waveforms, position relative to the electric component, and what type of energy (e.g., radio frequency, optical, or acoustic) is observed provides a detailed characterization or “signature” of the electrical component. The ability to compare signatures obtained at different times and from different electrical components provides significant advantages over known partial discharge monitoring approaches for product development, quality assurance, and longterm performance analysis. Limiting the field of view of the sensors used to receive the energy emitted by partial discharges in the electrical component under test increases both spatial and signal selectivity by excluding noise and partial discharge emissions outside the field of view. Embodiments of the invention can thereby provide partial discharge position and energy data at higher resolutions than known systems.

The combination of a broad band yet selective partial discharge probe and high resolution recording of position and energy of partial discharges across different excitation waveforms provides advantages over known partial discharge monitoring systems. Embodiments of the invention combine data on probe position, detection domain, and excitation domain with partial discharge levels as a function of these variables. This data can then be stored as signatures of electrical components tabulated by serial number (for example) for comparison across time and between components. This provides an ability to observe the long-term performance of electrical devices as they age and for electrical devices having different designs, and provides a new quality metric heretofore unknown in the industry.

Brief Description of the Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the embodiments of the invention.

Fig. 1 is a cross-sectional schematic view of an insulator including a defect.

Fig. 2 is a graphical view showing a voltage distribution across the insulator of Fig. 1.

Fig. 3 is a cross-sectional schematic view of the insulator of Fig. 1 showing a partial discharge in the insulator.

Fig. 4 is a graphical view showing the voltage distribution across the insulator of Fig. 3 during the partial discharge.

Fig. 5 is a schematic view of a partial discharge test environment including a stator.

Fig. 6 is a graphical view of an excitation waveform that may be applied to the windings of the stator of Fig. 5. Figs. 7-9 are schematic views of partial discharge probes being used to detect partial discharges on a device under test.

Fig. 10 is a schematic view of a partial discharge test setup including a partial discharge probe such as depicted in Figs. 7-9 and an excitation waveform generator.

Fig. 11 is a schematic view showing exemplary excitation waveforms that may be generated by the excitation waveform generator of Fig. 10.

Fig. 12 is a schematic view of another partial discharge test setup including an in situ partial discharge probe.

Fig. 13 is a schematic view of an exemplary partial discharge map that may be generated using the test setup of Figs. 10 and 12.

It should be understood that the appended drawings are not necessarily to scale, and may present a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, may be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments may have been enlarged or distorted relative to others to facilitate visualization and a clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration.

Detailed Description

Embodiments of the invention are directed to systems, methods, and computer program products for mapping partial discharge (e.g., the partial discharge inception voltage) across an electrical component (e.g., the stator of an electric machine) for a plurality of test domains. Each test domain may be defined by a combination of detection domain (e.g., optical, radio frequency, or acoustic detection), excitation domain (e.g., characteristics of the waveform applied to the device under test), and other test conditions (e.g., temperature, load, etc.) under which the partial discharge mapping is conducted. The disclosed insulation testing techniques may be particularly applicable to serial testing of production electric machines. The repeatability of partial discharge mapping of the stator may enhance the consistency and quality of delivered electrical machines as compared to conventional partial discharge tests. Although the examples provided below generally involve the testing of stators, it should be understood that the methods and systems disclosed herein may also be used for testing other types of electrical devices with insulation subject to high voltages, such as transformers.

Conventional partial discharge measurements of a stator typically involve measurements of capacitive jumps in the stator windings. This type of inspection only provides a global value for the weakest part of the insulation system, and does not provide any information as to the type of partial discharge or its location within the stator. Embodiments of the present invention combine the use of a new partial discharge probe that is specifically configured to be sensitive to partial discharges in a localized region of the stator.

Embodiments of the partial discharge probe are specifically configured to pick up local emissions from partial discharges within the interior of the stator. The use of this specialized probe enables spatial localization of partial discharge emissions to a physical position, at the exclusion of partial discharges elsewhere in the stator. The partial discharge probe may be physically moved to various regions of the stator to map partial discharge activity. This mapping may be performed with a variety of excitation waveforms to provide a more definitive description of the partial discharge properties of the stator than possible using only a single excitation waveform.

The partial discharge probe may be placed in direct contact with an inner surface of the stator and moved around by a mechanism referred to herein as a positioning mechanism. The positioning mechanism may include an actuator configured to move the partial discharge probe, a position indexing system, or both the actuator and position indexing system. The signals generated by the partial discharge probe may be combined with position information provided by the positioning mechanism to generate a “map” of the stator which indicates not only the specific levels of partial discharge but also to their spatial distribution for the test domain in question. This partial discharge map may provide a pictorial depiction of partial discharge densities in and around the stator based on one or more of the magnitude and number of the partial discharges detected by the partial discharge probe at each position.

Partial discharge mapping of stators may be especially useful as the power generation industry moves towards medium and high voltage electric machines, which are expected to have increased susceptibility to partial discharge. The partial discharge probe enables the locations of partial discharges be determined. To this end, the partial discharge probe may be movable and configured to selectively detect partial discharges within a “field of view” of the probe, while being insensitive to partial discharge activity outside the field of view of the probe. Advantageously, the spatial selectivity of the partial discharge probe may enable partial discharge tests that detect the occurrence of partial discharge at lower levels than is possible with conventional tests. In certain use scenarios, an “in situ” partial discharge probe may be configured to enable the probe to be inserted into the rotor-stator air gap of an electric machine and placed on the surface of the stator. In situ probes may facilitate quality testing at the point of manufacture as well as diagnostic testing after the electric machine has been placed in service.

The partial discharge probe may selectively detect partial discharges using one or more detection domains, e.g., one or more of an optical (e.g., visible, infrared, and ultraviolet light), acoustic, or radio frequency (e.g., microwave and radio wave) detection domain. Because the partial discharge probe is configured to sense partial discharges in a localized area of the stator surface, and ignore partial discharges outside this area, partial discharges detected by the probe can be localized to the area within the probe’s field of view. The partial discharge probe may be used with different types of electrical excitation. When combined with spatial information on the probe’s location, the data provided by the partial discharge probe may be used to define a multidimensional “map” of the stator’s partial discharge characteristics.

The ability to measure and catalog partial discharge data in this way provides a unique perspective on the dielectric integrity of electric machines. Embodiments of the invention also provide a unique diagnostic tool that can be used for initial product development and qualification, quality assurance testing during production, and as a diagnostic predictor of the remaining life of the insulation system after the electric machine is in service. The partial discharge probe may be configured for multi-domain detection (more than one domain of partial discharge sensing) or for a single domain. Embodiments of the invention using single domain partial discharge probes may use multiple probes of different types to obtain a full picture of the partial discharge sensitivity of the stator.

Partial discharge probes may be positioned around the stator surface of an electric machine using appropriate indexing, e.g., manually or via mechatronics. One or more partial discharge voltages (e.g., inception voltage or extinction voltage) may then be measured and mapped to a spatial distribution database. The partial discharge probe may include one or more sensors, with each sensor being sensitive to a particular emission of the partial discharge, e.g., an electromagnetic or acoustic emission in a predetermined frequency range.

During testing, the excitation waveform may be tailored to focus on a particular aspect of the insulation system, partial discharge, and physical location within the stator. Excitation waveforms and their adjustable characteristics may include, but are not limited to, variable frequency and sinus voltage, square wave voltage of adjustable amplitude, and variable trapezoidal voltage waveform of variable ramp rate. Multi-domain partial discharge mapping may be performed at the point of manufacture on a bare stator as well as after assembly and installation of the electric machine. In situ probes used to inspect assembled electric machines may be small enough to fit in the rotor-stator gap. In situ probes may therefore have reduced spatial resolution as compared to physically larger probes that can only be used for inspecting bare stators. However, maps made from data collected by in situ probes may nevertheless be compared to earlier bare stator testing to look for changes that indicate a reduced remaining life or imminent failure.

Fig. 7 depicts an exemplary partial discharge probe 50 including a housing 52 having an aperture 54 and one or more sensors 56. The partial discharge probe 50 is shown in proximity to a device under test 58 (e.g., a stator winding) that is experiencing partial discharges 60. The one or more sensors 56 may include one or more of a radio frequency detector (e.g., an antenna operatively coupled to an amplifier), a photodetector (e.g., a photodiode, phototransistor, charge coupled device, complementary metal-oxide-semiconductor device, etc.), and an acoustic detector (e.g., a piezo-electric or other suitable microphone). The housing 52 and aperture 54 thereof may be configured to define a field of view 62 of the partial discharge probe 50 by reducing the amount of electromagnetic or acoustic energy the one or more sensors 56 receives from partial discharges 60 outside the field of view 62.

The partial discharge probe 50 may be configured to have high sensitivity to partial discharges 60 occurring in a localized region within a line of sight provided by the aperture 54 so that the probe detects partial discharges 60 in a specific region of the device under test 58. This region may be a portion of a stator surface as well as the windings, end turns, connectors, power leads, etc. The partial discharge probe 50 may be further configured so that it is insensitive to a partial discharge 60 occurring outside of the line of sight provided by the aperture 54. The resulting combination of high sensitivity to partial discharges 60 within the field of view 62 and low sensitivity to partial discharges 60 outside the field of view 62 may enable the partial discharge probe 50 to generate output signals having a high signal to noise ratio.

To improve isolation between the sensor 56 and signals generated outside the field of view 62, the housing 52 may include shielding 57 that further prevents energy from entering the housing 52 except through the aperture 54. The shielding 57 may comprise one or more separate layers of the housing 52 (as shown), or may be provided by the housing material itself. For example, the housing 52 may be made out of a suitable shielding material. The type of materials that provide suitable shielding may depend on the detection domain in question. For example, conductive materials (e.g., silver, gold, copper, aluminum, etc) may provide effective shielding for radio frequency and optical detection domains. Likewise, opaque non-conductive materials may provide effective shielding for optical detection domains, and vibration absorbing materials (e.g., foam rubber) may provide effective shielding for acoustic domains.

The sensor 56 may be mounted directly to the housing 52, or the partial discharge probe 50 may include an isolator 59 (shown) that isolates the sensor 56 from the housing 52. The isolator 59 may be configured to attenuate any signals originating from outside the field of view 62 that manage to penetrate the housing 52. By way of example, the isolator 59 may be made from a non-conductive material for electromagnetic detectors and from a sound-decoupling material for acoustic detectors. Further improvements in the partial detection probe 50 may include configuring the housing 52 to have an interior surface that absorbs energy in the detection domain of the sensor 56. The aperture 54 may comprise an opening in the housing 52, or the aperture 54 may include one or more dielectric materials, baffles, lenses, filters, or other suitable devices (not shown) that contribute to defining the field of view 62 by directing energy emitted within the field of view 62 onto the one or more sensors 56 or preventing energy emitted outside the field of view 62 from reaching the one or more sensors 56.

Fig. 8 depicts an exemplary partial discharge probe 50 that further includes a gasket 64. The gasket 64 may encircle the aperture 54 of housing 52, and may be made of a conductive compressible foam or other suitable material to further isolate the sensor 56 from noise or other signals generated outside the probe’s field of view 62. For embodiments of the partial discharge probe 50 that include shielding 57 separate from the housing 52, the shielding 57 and the gasket 64 may be operatively coupled to provide a continuous barrier against signals originating from outside the field of view 62. In operation, the partial discharge probe 50 may be urged into contact with the device under test 58 so that the gasket 64 is compressed sufficiently to seal out external signals. Thus, the gasket 64 may make this embodiment particularly suited to placement in close proximity to (or in contact with) the device under test 58. This feature may improve the ability of the partial discharge probe 50 to sense partial discharges 60 in small, localized regions of the device under test 58 by shielding the sensor 56 from neighboring areas so that the sensor 56 only detects partial discharges 60 within a clear line of sight provided by the aperture 54.

Fig. 9 depicts an exemplary in situ version of the partial discharge probe 50 of Fig 8. The in situ partial discharge probe 50 is shown inserted into an air gap 66 between the stator 24 and rotor 68 of an electric machine. In situ partial discharge testing of an assembled electric machine may involve inserting the partial discharge probe 50 into the air gap 66 and axially indexing the probe’s position. Different access provisions may allow axial partial discharge probe sampling to be done at multiple angular locations, thereby allowing two-dimensional mapping of the stator surface of an assembled electric machine. Partial discharge probes 50 designed for in situ use may be configured to have a larger field of view 62 than those used in more open environments, e.g., by using a relatively wider aperture 54. The width of the field of view 62 may be selected (e.g., by appropriate sizing of the aperture 54) to be commensurate with the number of axial access ports that are provided in the electric machine being tested.

Fig. 10 depicts an exemplary partial discharge test setup 70 that may be used for testing the insulation system of an electrical component 72, such as a stator 24. The depicted test setup 70 includes a computer 74, a partial discharge probe 50 operatively coupled to an actuator 76 by a shaft 78, an excitation waveform generator 80, a switch 82, a transformer 84, and a position indexing system 85. The transformer 84 may be used to step up the voltage output by the excitation waveform generator 80 to a level sufficient to trigger partial discharge in the electrical component 72. The switch 82 may selectively couple a portion of the electrical component (e.g., a winding 26) to the excitation waveform generator 80 via the transformer 84. The computer 74 may be operatively coupled to the partial discharge probe 50, actuator 76, excitation waveform generator 80, and switch 82 to control testing and collect data. The position indexing system 85 may be used to track of the position of the probe 50.

The position information provided by the position indexing system 85 may be used to map both the axial and circumferential position of the partial discharge probe 50. The partial discharges 60 detected may then be mapped to their positions on the cylindrical inner surface of the stator 24 using the position information. The computer 74 may include one or more processors and a memory storing program code that, when executed by the one or more processors, causes the computer 74 to control the partial discharge test setup 70 and to perform partial discharge mapping of the electrical component 72. The partial discharge mapping process may include one or more of moving the partial discharge probe 50, acquiring data from the partial discharge probe 50 and position indexing system 85, and adjusting the excitation waveforms 36 applied to the electrical component 72. In some cases, the partial discharge probe 50 may also be moved by hand between measurements, in which case the primary function of the computer 74 may to collect position and partial discharge data. The acquired data may be stored in a database and used to define one or more partial discharge maps.

In one embodiment, the test setup 70 may be configured to test the stator 24 of an electric machine prior to installation of the rotor 68. The absence of the rotor 68 may facilitate inserting the partial discharge probe 50 into the stator 24. In another embodiment, the test setup 70 may be configured to test the stator 24 of an electric machine after installation of the rotor 68. In this embodiment, the partial discharge probe 50 may be configured to fit in the air gap 66 between the rotor 68 and stator 24. By way of example, the air gap 66 in a large electric machine is typically about 5 to 8 mm. Access for the partial discharge probe 50 may be obtained via access ports in the housing of the electric machine that allow access for visual inspections.

In cases where the partial discharge probe 50 is inserted into an electric machine through an access port, it may only be possible for the partial discharge probe 50 to map a portion of the stator through each port. In this scenario, the entire stator 24 may be mapped by sequentially inserting the partial discharge probe 50 into different access ports to map different potions of the stator 24, then stitching the portions of the partial discharge map together to define a complete partial discharge map. Thus, partial discharge data may be collected from either a bare stator sub-assembly where the entire stator internal surface is accessible (such as in a series manufacturing quality inspection) or in an assembled electric machine having multiple axial access ports into which the partial discharge probe 50 is inserted into the air gap 66.

Each portion of the electrical component 72 being mapped may be selectively coupled to the excitation waveform generator 80 via the switch 82 and transformer 84 so that the computer 74 can control the shape and amplitude of the excitation waveform 36 provided to the electrical component 72. The excitation waveform 36 provided to the electrical component 72 may be varied depending on the size and intended application of the electric machine, as well as the type of mapping being performed.

During the test, the computer 74 may cause the actuator 76 to move the partial discharge probe 50 to obtain partial discharge data over a predetermined portion of the electrical component 72, e.g., the “central windings” of a stator 24. The actuator 76, shaft 78, and position indexing system 85 may collectively form a positioning mechanism that moves the probe 50 in one or more of an axial direction (as indicated by double-headed arrow 86), a circumferential direction (as indicated by double-headed arrow 87), and a radial direction (as indicated by double-headed arrow 88) in response to signals from the computer 74. The positioning mechanism may also support other forms of movement, such as in the x, y, and z-directions of a Cartesian coordinate system. The partial discharge probe 50 may also be moved or positioned to capture partial discharge data of other portions of the electrical component 72, i.e., the “end windings” of a stator 24.

A recently generated partial discharge map (i.e., “later” partial discharge map) may be compared to another partial discharge map generated for the electric machine at a prior time (i.e., “earlier” partial discharge map). This comparison may include generating a partial discharge divergence map that shows a difference in the amount of partial discharge activity in the later partial discharge map as compared to the earlier partial discharge map. Partial discharge divergence maps may also be generated by comparing maps obtained from different electrical components 72 of the same type to identify the level of consistency between electric machines as well as the acceptability of each electric machine for release. A partial discharge map may provide a partial discharge signature for the electric machine which can be compared to partial discharge signatures obtained at different times for the same machine, or from a known good reference machine. Partial discharge maps may be generated for different detection domains, e.g., optically detected, radio frequency detected, and acoustically detected partial discharges, as well as for different frequency ranges within those domains.

The electrical component 72 may be placed in a darkened test chamber for partial discharge tests including optical domain detection, or in an acoustic anechoic chamber or other sound- insulated chamber for partial discharge tests including acoustic domain detection. Radio frequency sensors may be sensitive to a broad range of partial discharges, which typically emit brief electromagnetic bursts having energy concentrated in the 100 kHz band. For partial discharge testing that includes radio frequency domain detection, the electrical component 72 may be placed in a Faraday cage or other type of electromagnetically shielded chamber. Testing may also include tests that use different excitation waveforms 36 to generate multi-dimensional partial discharge maps. Thus, testing and partial discharge mapping of an electrical component 72 may include partial discharge mapping for various combinations of one or more detection domains and excitation domains.

Fig. 11 depicts an exemplary excitation waveform generator 80 as well as some exemplary excitation waveforms 36 that may be generated by the excitation waveform generator 80 and used to excite the electrical component 72. The excitation waveform generator 80 may include one or more of a variable output sinusoidal source 90, and a variable DC source 92 operatively coupled to an inverter 94. The characteristics of the excitation waveforms 36 may be varied to provide a unique and specific diagnostic of the partial discharge characteristics of the electrical component 72. The partial discharges detected may be mapped onto different key regions of the electrical component 72. Exemplary excitation waveforms 36 may include, but are not limited to sinusoid waveforms, chopped sinusoid waveforms, square waveforms, and trapezoidal waveforms. The amplitude (e.g., peak or root mean square voltage), frequency, slew rate/ramp rate of each excitation waveform 36 may be varied to generate different excitation domains. Waveforms may include standard sinusoidal type alternating waveforms of variable voltage and square wave type waveforms corresponding to the typical output of a machine side inverter during pulse-width modulation. Inverter generated waveforms may be connected to the windings via a multi-tapped step-up high-frequency transformer. The net voltage of inverter generated waveforms may also be controlled by adjusting the variable direct current source voltage used to feed the inverter. The combination of different available detection domains, proximity sensitivity, and variable excitation domains may enable partial discharge tests to be designed to focus on specific areas of the insulation system that have a proclivity towards partial discharge, e.g., winding heads, slot exit, in slot, etc.

The use of multiple types of excitation waveforms 36 in conjunction with the spatial mapping abilities enabled by the partial discharge probe 50 introduces an entirely new window for examining, qualifying, and quantifying insulation systems. Different excitation waveforms 36 may produce different voltage distributions within the insulation system of the electrical component 72. For example, a square wave or trapezoidal voltage with high slew rate (i.e., large dV/dt) may produce a completely different electric field distribution in the insulation system than a sinus waveform of having the same amplitude. Different excitation waveforms 36 may produce different voltage gradients in the insulation system due to their interaction with internal impedances and electric field time constants in the electrical component 72. The interaction of these internal impedances with the excitation waveform 36 may cause voltages to be concentrated differently with a square wave versus a sine wave, for example. This feature may be particularly relevant in inverter coupled electric machines that synthesize an equivalent sine wave voltage using high frequency pulse width modulation of the connected inverter. Inverter coupled electric machines are commonly found in modern wind-turbine systems, for example.

Fig. 12 depicts another exemplary partial discharge test setup 70 that may be used for in situ testing the insulation system of a stator 24 including windings 26 and a core 28. The test setup 70 includes an in situ partial discharge probe 50, an excitation waveform generator 80 that is operatively coupled to the stator windings 26 by a transformer 84, a probe positioning mechanism 100, and a probe data acquisition device 102 (e.g., a computer 74).

The data collected from the partial discharge probe 50 may be used to generate a partial discharge map of the electrical component 72. The partial discharge map may be used as a development tool, for serial production quality testing, as a maintenance inspection tool for electric machines that are operating in the field, or for any other suitable purpose. Partial discharge maps may be considered multi-dimensional in that they not only include spatial data matched to geometric features of the electrical component 72 (e.g., specific regions, slots, end turns, etc.), but also data that is a function of the excitation waveform 36 used to generate the partial discharge data. For example, the partial discharge data use to create a partial discharge map may include spatial information, partial discharge information (e.g., inception/extinction voltage level), the detection domain used to collect data (e.g., radio frequency, optical, or acoustical detection), and the excitation domain (e.g., shape, amplitude, frequency, slew/ramp rates of the excitation waveform 36).

The excitation waveform 36 used for a specific test may be tailored to provide additional data, such as the partial discharge inception voltage, which is specific to onset level. Excitation waveforms 36 having rapid changes in one or more of current and voltage may be used to provide characteristics with respect to inverter type waveforms, which may produce different voltage concentration factors as compared to purely sinusoidal waveforms. Various types of testing may also be combined with thermal virtual machine testing or other simulated operational environments that warm up the stator under test to obtain partial discharge map comparisons between room temperature and operational temperature conditions. Partial discharge maps may be generated for different electrical components 72 and compared to show repeatability and quality/consistency between components. Partial discharge maps may also be generated for the same electrical component 72 before and after environment over stress testing or other life type testing to show degradation levels over time. Partial discharge characteristics of deployed electric machines may be mapped and compared to partial discharge maps generated from as-built quality control test components to determine life expectancy.

A partial discharge probe 50 combined with a test setup 70 including a controlled excitation waveform generator 80, a positioning mechanism 100 having a position indexing system 85 that tracks the position of the partial discharge probe 50 (e.g., along the inner surface of the stator 24) may be used to make a two-dimensional partial discharge map of an insulation system. The partial discharge map may be tailored for different types of discharges, depending on the total voltage level (V) and voltage rise time (dV/dt) as well as the specific nature of the partial discharge probe, e.g., optical, radio frequency, or acoustic. Maps may include frequency versus magnitude as a function of the onset waveform conditions.

The magnitude of a detected partial discharge 60 may be determined based on the amplitude of the output signal generated by the partial discharge probe 50. The amplitude of the probe output signal may be related to the amount of energy received by the partial discharge probe 50. Magnitude may be characterized by the peak amplitude of the received output signal, an integrated value of the signal over a period of time, (e.g., the area under the curve of the signal for the duration of a partial discharge 60), or using any other suitable characterization. As described above, one way a partial discharge 60 may be characterized is by its inception and extinction voltages. However, the negative and positive polarity pulses 38, 40 used to detect these voltages may also be generated by energy building up in the insulation system as the voltage applied to the electrical component 72 changes. Thus, pulses 38, 40 may be detected as charge is naturally redistributed within the insulation system in ways that are unrelated to partial discharge. These types of charge redistributions are normal and not considered harmful. However, at some level, the redistributions of energy become large enough and discontinuous enough to be considered as caused by partial discharges 60.

There are various standards that may be used to determine both when a partial discharge 60 has occurred and the magnitude of the partial discharge 60. For example, International Electrotechnical Commission (IEC) Pub. No. 60664-1 considers a transformer to have passed partial discharge testing (i.e., no partial discharges detected) if the charge on the stray capacitances between the primary and secondary windings remains below 10 pC during the measurement period. The amount of charge transferred may be determined, for example, based on the amplitude and duration of any pulses 38, 40 detected, e.g., by calculating the area under the curve of each pulse 38, 40. Setting a predetermined threshold for charge redistributions is one way to provide a quantitative criterion for determining when the partial discharge inception voltage has been reached.

The partial discharge extinction voltage is generally less than the inception voltage because the system voltage must be reduced to stop the partial discharge activity once it has been initiated. Partial discharge inception and extinction voltages as defined in conventional partial discharge testing protocols are only meaningful in terms of global detection. That is, partial discharge inception and extinction voltages are a characteristic of the insulation system as a whole. The conventional practice for partial discharge testing is to apply an excitation voltage and look for the partial discharge inception voltage. The partial discharge inception voltage is thus a global criterion which is set by whatever element in the insulation system is the first to exhibit partial discharge, i.e., the weakest link. This means that to fully map the partial discharge characteristics of an electrical component 72 such as a stator 24, it may be necessary to expose the windings to voltages above the normal partial discharge inception voltage to enable mapping of regions of the stator 24 which do not experience partial discharge at the global partial discharge inception voltage. These higher voltages should be tolerated by the electrical component 72 for limited periods of time. The ability of the partial discharge probe 50 to characterize partial discharge as a local event may require new criteria be defined for detecting when partial discharge is occurring. Because the partial discharge probe 50 looks for evidence of partial discharge in localized regions of the electrical component 72, partial discharge activity may not have to be of a high enough magnitude to be detected above the normal charge redistributions occurring throughout the insulation system of the electrical component 72. Accordingly, the partial discharge probe 50 may enable partial discharge detection at a higher sensitivity than is possible using conventional global threshold based detection. The partial discharge probe 50 may also be tuned to a specific combination of detection and excitation domains in use to further improve its sensitivity. This type of specific tuning may open up whole new sets of in situ characteristics that cannot be observed using conventional testing methods. The higher sensitivity of the partial discharge tests enabled by the partial discharge probe 50 may also allow testing at previously undetectable partial discharge levels, i.e. , at voltages below the globally detected partial discharge inception voltage.

During testing, the occurrence of pulses 38, 40 may be correlated in time with the detection of partial discharges 60 by the partial discharge probe 50. The size of the charge redistributions, as indicated by the amplitude and duration of the pulses 38, 40, may then be correlated with the total amount of energy received by the partial discharge probe 50 over the same period. This correlation may enable partial discharges 60 detected by the partial discharge probe 50 to be characterized in terms of an estimated amount of charge involved. In order to fully map the electrical component 72, the characteristics of the excitation waveform 36 may be varied to induce partial discharge in different parts of the insulation system. This may include varying the shape, slew rate, and amplitude of the excitation waveform 36, as well globally exciting the electrical component 72 at a level above that required to induce partial discharge in order to generate detectable partial discharges 60 in parts of the insulation system that are normally not subject to partial discharge.

A detailed partial discharge map can thus be generated and characterized so as to provide valuable insights. These insights may include design robustness as well as the identification of weak or more sensitive areas of the insulation system that need extra treatments or process development. Partial discharge maps may also be used for quality inspection and production fingerprint and acceptance criteria for series produced machines, and spot checks of electric machines in the field using in situ probes to compare relative partial discharge levels versus when the electric machine left the factory. Fig. 13 depicts an exemplary partial discharge map 110 of an exemplary electrical component 72, e.g., a stator 24. The partial discharge map 110 includes a central winding region 114 corresponding to the central windings of the stator 24, and two end winding regions 116, 118 corresponding to the end-windings of the drive end and the end windings of the non-drive end of stator 24, respectively. The partial discharge map 110 may be divided into cells 120 by a grid 122 including grid lines 124. The location of each cell 120 of partial discharge map 110 may be identified by a row and column (e.g., AA, AB, AC etc.) corresponding to the position of the partial discharge probe 50 when the data used to define the cell was collected. Each cell 120 may be associated with one or more partial discharge detection domain parameters for the surface of the stator 24 in a region corresponding to the portion of the partial discharge map 110 defined by the cell 120.

The size of the cells 120 may be determined by the field of view 62 and proximity of the partial discharge probe 50 to the stator 24 at the position represented by the cell 120. Cells 120 may be categorized into regions that correspond to specific parts of the stator 24. These regions may include the central winding region 114, end winding regions 116, 118, or regions corresponding to specific components of the stator 24 or insulation system thereof. Specific components of the stator 24 may include the teeth, slots between the teeth, specific portions of the windings, etc. These regions may be analyzed to identify individual areas experiencing partial discharge breakdowns.

Parameter values extracted from the partial discharge map 110 may be numerically processed to provide a quantitative basis for comparing the partial discharge parameters of a production machine to those of a reference machine. The partial discharge parameters of the production machine after it has been in operation for a period of time may also be compared to those generated by an earlier test, e.g., when the electric machine was new. To this end, partial discharge parameters of the electric machine may be obtained from a partial discharge map 110 thereof and compared to those of another electric machine or the same electric machine from an earlier time. This comparison may be made using partial discharge parameters obtained with the same grid lines 124, mapping coordinate system, test conditions, etc.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the embodiments of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include both the singular and plural forms, and the terms “and” and “or” are each intended to include both alternative and conjunctive combinations, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” or “comprising,” when used in this specification, specify the presence of stated features, integers, actions, steps, operations, elements, or components, but do not preclude the presence or addition of one or more other features, integers, actions, steps, operations, elements, components, or groups thereof. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, “comprised of”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.

While all the invention has been illustrated by a description of various embodiments, and while these embodiments have been described in considerable detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail.

Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the Applicant’s general inventive concept.

Claims

1 . An apparatus for testing an electrical component (72), comprising: a partial discharge probe (50) including a housing (52) and a sensor (56) contained by the housing (52), wherein the sensor (56) is sensitive to at least one of an electromagnetic energy and an acoustic energy, the housing (52) has an aperture (54) that defines a field of view (62) of the partial discharge probe (50), and the housing (54) is configured to prevent the at least one of the electromagnetic energy and the acoustic energy from entering the housing (54) except through the aperture (54); a position indexing system (85) configured to determine a position of the partial discharge probe (50); an excitation waveform generator (80); one or more processors operatively coupled to the partial discharge probe (50), the position indexing system (85), and the excitation waveform generator (80); and a memory operatively coupled to the one or more processors and including program code that, when executed by the one or more processors, causes the apparatus to: use the excitation waveform generator (80) to generate a first excitation waveform (36) that is applied to the electrical component (72); record a first amount of energy received by the sensor (56) of the partial discharge probe (50); and record the position of the partial discharge probe (50) received from the position indexing system (85).

2. The apparatus of claim 1 , wherein the sensor (56) is a first sensor (56) that is sensitive to one of the electromagnetic energy and the acoustic energy, and the partial discharge probe (50) further includes a second sensor (56) contained by the housing (52) that is sensitive to the other of the electromagnetic energy and the acoustic energy.

3. The apparatus of claim 2, wherein one of the first sensor (56) and the second sensor (56) includes at least one of a radio frequency detector and a photodetector, and the other of the first sensor (56) and the second sensor (56) includes an acoustic detector.

4. The apparatus of any of claims 1 -3, wherein the partial discharge probe (50) further includes a gasket (64) that encircles the aperture (54) of the housing (52) and is configured to be placed in contact with a surface of the electrical component (72) and to isolate the sensor (56) from signals generated outside the field of view (62) of the partial discharge probe (50).

5. The apparatus of claim 4, wherein the housing (52) includes shielding (57), and the shielding (57) is operatively coupled to the gasket (64).

6. The apparatus of claim 5, wherein the partial discharge probe (50) further includes an isolator (59), and the isolator (59) couples the sensor (56) to the housing (52) and isolates the sensor (56) from the shielding (57).

7. The apparatus of any of claims 1 -6, wherein: the first excitation waveform (36) is applied to the electrical component (72), the first amount of energy is recorded by the sensor (56), and the position of the partial discharge probe (50) is recorded at each of a plurality of positions relative to the electrical component (72), each of the plurality of positions places a different portion of the electrical component (72) in the field of view (62) of the partial discharge probe (50), and the program code further causes the apparatus to, for each of the plurality of positions: use the excitation waveform generator (80) to generate a second excitation waveform (36) that is applied to the electrical component (72) and is different from the first excitation waveform (36); record a second amount of energy received by the partial discharge probe (50) while the second excitation waveform (36) is applied to the electrical component (72); and determine a difference between the first amount of energy and the second amount of energy.

8. The apparatus of claim 7, wherein the first excitation waveform (36) has a first slew rate, and the second excitation waveform (36) has a second slew rate different from the first slew rate.

9. The apparatus of claim 7 or 8, wherein the program code further causes the apparatus to: define a partial discharge map (110) based at least in part on the first amount of energy recorded and the position of the partial discharge probe (50) recorded at each of a plurality of positions.

10. The apparatus of any of claims 7-9, further comprising: an actuator (76) configured to move the partial discharge probe (50) relative to the electrical component (72), wherein the program code further causes the apparatus to: position the partial discharge probe (50) in each of the plurality of positions using the actuator (76).

11 . The apparatus of any of claims 1 -10, wherein the electrical component (72) is a stator (24) of an electric machine that also includes a rotor (68), and the partial discharge probe (50) is inserted into an air gap (66) between the stator (24) and the rotor (68).

12. A method of testing an electrical component (72), comprising: positioning a first partial discharge probe (50) having a field of view (62) in a first plurality of positions relative to the electrical component (72), each position placing a different portion of the electrical component (72) in the field of view (62) of the first partial discharge probe (50); while the first partial discharge probe (50) is in each of the first plurality of positions: applying a first excitation waveform (36) to the electrical component (72) and recording a first amount of energy received by the first partial discharge probe (50), applying a second excitation waveform (36) to the electrical component (72) different from the first excitation waveform (36) and recording a second amount of energy received by the first partial discharge probe (50), and recording the position of the first partial discharge probe (50); determining a first difference between the first amount of energy and the second amount of energy recorded at each of the first plurality of positions; and defining a first partial discharge map (110) indicating the first difference between the first amount of energy and the second amount of energy at one or more of the first plurality of positions.

13. The method of claim 12, wherein the first amount of energy received by the first partial discharge probe (50) includes one or more of an amount of electromagnetic energy and an amount of acoustic energy.

14. The method of claim 12 or 13, wherein the first excitation waveform (36) has a first slew rate, and the second excitation waveform (36) has a second slew rate different from the first slew rate.

15. The method of any of claims 12-14, wherein the first partial discharge probe (50) includes a housing (52) and a first sensor (56) contained by the housing (52) that senses the first amount of energy received by the first partial discharge probe (50), and the housing (52) has an aperture (54) and defines the field of view (62) by preventing the first amount of energy received by the first partial discharge probe (50) from entering the housing (54) except through the aperture (54).

16. The method of claim 15, wherein the first partial discharge probe (50) further includes a gasket (64) encircling the aperture (54) of the housing (52), and further comprising: placing the gasket (64) in contact with a surface of the electrical component (72).

17. The method of claim 15 or 16, wherein the first partial discharge probe (50) further includes a second sensor (56) contained by the housing (52), the first sensor (56) is sensitive to one of an amount of electromagnetic energy and an amount of acoustic energy, and the second sensor (56) is sensitive to the other of the amount of electromagnetic energy and the amount of acoustic energy.

18. The method of any of claims 12-17, wherein the first and second amounts of energy are received by the first partial discharge probe (50) at a first time, and further comprising: at a second time after the first time, positioning one of the first partial discharge probe (50) or a second partial discharge probe (50) in a second plurality of positions relative to the electrical component (72), each position of the second plurality of positions placing a different portion of the electrical component (72) in the field of view (62) of the first or second partial discharge probe (50); while the first or second partial discharge probe (50) is in each of the second plurality of positions: applying a third excitation waveform (36) to the electrical component (72) and recording a third amount of energy received by the first or second partial discharge probe (50), and recording the position of the first or second partial discharge probe (50); determining a second difference between one of the first or second amounts of energy recorded at one or more of the first plurality of positions at the first time and the third amount of energy recorded at one or more of the second plurality of positions at the second time; and defining a second partial discharge map (110) indicating the second difference between the first or second amount of energy and the third amount of energy at the one or more of the first and second pluralities of positions.

19. The method of any of claims 12-17, wherein the first and second amounts of energy are received by the first partial discharge probe (50) at a first time, and further comprising: at a second time after the first time, positioning one of the first partial discharge probe (50) or a second partial discharge probe (50) in a second plurality of positions relative to the electrical component (72), each position of the second plurality of positions placing a different portion of the electrical component (72) in the field of view (62) of the first or second partial discharge probe (50); while the first or second partial discharge probe (50) is in each of the second plurality of positions: applying a third excitation waveform (36) to the electrical component (72) and recording a third amount of energy received by the first or second partial discharge probe (50), applying a fourth excitation waveform (36) to the electrical component (72) different from the third excitation waveform (36) and recording a fourth amount of energy received by the partial discharge probe (50), and recording the position of the first or second partial discharge probe (50); determining a second difference between the third amount of energy and the fourth amount of energy recorded at each of the plurality of positions; determining a third difference between the first difference and the second difference; and defining a second partial discharge map (110) indicating the third difference at the one or more of the first and second pluralities of positions.

20. The method of claim 18 or 19, wherein the first time is before the electrical component is placed in service, and the second time is after the electrical component has been placed in service.

21 . The method of any of claims 18-20, wherein the electrical component (72) is a stator (24) of an electric machine that also includes a rotor (68), the second partial discharge probe (50) is used at the second time, and the second partial discharge probe (50) is inserted into an air gap (66) between the stator (24) and the rotor (68).

22. The method of claim 21 , wherein the first partial discharge probe (50) is used without the rotor (68) installed in the electric machine at the first time.

23. The method of any of claims 12-17, wherein the electrical component (72) is a stator (24) of an electric machine that also includes a rotor (68), and positioning the first partial discharge probe (50) includes: inserting the first partial discharge probe (50) into an air gap (66) between the stator (24) and the rotor (68) of the electric machine.

24. The method of claim 23 wherein the electric machine includes an access port, and inserting the first partial discharge probe (50) into the air gap (66) between the stator (24) and the rotor (68) of the electric machine includes accessing the air gap (66) through the access port.

25. The method of claim 24, wherein: the access port is a first access port of a plurality of access ports of the electric machine; positioning the first partial discharge probe (50) in the first plurality of positions relative to the electrical component (72) includes: inserting the first partial discharge probe (50) into the air gap (66) between the stator (24) and the rotor (68) through both the first access port and a second access port of the plurality of access ports, while the first partial discharge probe (50) is inserted into the first access port, positioning the first partial discharge probe (50) in each position of a first portion of the first plurality of positions associated with a first portion of the stator (24), and while the first partial discharge probe (50) is inserted into the second access port, positioning the first partial discharge probe (50) in each position of a second portion of the first plurality of positions associated with a second portion of the stator (24); and defining the first partial discharge map (1 10) includes: recording the first and second amounts of energy received by the first partial discharge probe (50) and the position of the first partial discharge probe (50) for each position of the first portion of the first plurality of positions, recording the first and second amounts of energy received by the first partial discharge probe (50) and the position of the first partial discharge probe (50) for each position of the second portion of the first plurality of positions, defining a first portion of the partial discharge map (110) from the first and second amounts of energy recorded for the first portion of the stator (24), defining a second portion of the partial discharge map (110) from the first and second amounts of energy recorded for the second portion of the stator (24), and stitching together the first portion of the partial discharge map (110) and the second portion of the partial discharge map (110).