DC insulation resistance and polarization index testing is very widely used to assess the condition of the stator windings in motors and generators. These tests are good at identifying contaminated windings or windings with severely damaged insulation. However, such issues as thermal deterioration or abrasion of the insulation due to coil vibration, are not detected by this test in modern insulation. The DC polarization/depolarization current test, where the charging current is measured for several minutes and then the test object is grounded and the discharging current is measured for several minutes, may provide more diagnostic information. This paper presents a comparison of PDC data collected at different voltages from a stator winding as well as an individual stator coil. For the winding, there was little or no effect of voltage on the charge or discharge currents. For the coil, which had known silicon carbide coating deterioration, the effect of test voltage was ambiguous.

Recently, the number of electrical machines operated via inverters has significantly increased. Multiple works have pointed out that insulation aging occurs faster for inverter-fed machines due to voltage overshoots and uneven voltage distribution caused by high dV/dt switching at several kHz. This necessitates better insulation monitoring methods so that deteriorating insulation quality can be detected, before machines fail and the industrial process is interrupted. A good monitoring method is one that achieves a balance between the reliability of detection and cost requirements. Monitoring methods requiring the installation of expensive and intrusive sensors cannot be justified for all machines. Therefore, a new method for the monitoring of ground insulation condition for inverter-fed machines applying common-mode voltage at motor standstill is proposed in this article. The proposed method utilizes the commonly used sensors usually employed for differential protection and is simple in its application. It can be implemented whenever the motor is off-line for a short period of time and can screen out severe insulation deterioration that could lead to forced outages. The method has been verified experimentally in the lab on two, 380 V, 18.5 kW, and 5.5 kW induction machines.

There are three stator winding manufacturing processes used for epoxy resin penetration: resin-rich (B-stage) tapes, single coil/bar vacuum pressure impregnation (VPI) and global VPI (GVPI) of complete stators. Over the past several years, the GVPI process has been more widely used since it shortens the manufacturing process. The peak magnitude of partial discharge (PD) activity is a well-established indicator of resin penetration and insulation consolidation. A very large number of PD tests have been assembled into a single database that can be statistically exampled to determine what caused significant differences to occur between data subsets. In this paper we compare the online PD results for non-GVPI and global VPI processes in about 7000 machines. It seems that for stators rated 13–15 kV, there appears to be little difference in the PD activity, indicating both methods can produce well-impregnated windings. In contrast, for stators rated 10–12 kV, the non-GVPI process tends to produce lower PD activity.

Several viable online partial-discharge (PD) tests have been developed for monitoring the condition of the solid dielectric insulation in high voltage equipment. The main requirement for on-line tests is the ability to reduce or eliminate noise. Such noise can create false indications that the insulation is failing, which in turn destroys the test's credibility. Several methods have been developed to reduce or eliminate noise. Methods include pattern recognition, frequency and pulse characterization, and pulse propagation effects. The PD detection and noise elimination methods used take advantage of the characteristics of the PD and noise in each type of equipment being monitored. Thus an on-line measuring system for one type of equipment will probably be suitable for another type of equipment. The greatest success with on-line PD measurements has occurred in generators, where the PDA test for hydrogenerators is now routinely used around the world. There has been less success with substation equipment monitoring

Partial discharge (PD) measurement is widely used for all types of high voltage equipment for both factory quality assurance and insulation system condition assessment. For the latter, the PD may be measured off-line or on-line. For motor and generator stator windings in particular, on-line PD testing has been implemented on tens of thousands of machines using a wide variety of methods. How to perform such on-line tests is described in IEC and IEEE standards [1] , [2] . The usefulness of on-line testing has been described in many case study papers, and in particular, reference [3] summarizes the results of about 200 PD case studies presented by machine owners where stator failures were anticipated by the test, or where the findings of the on-line PD results were confirmed by visual inspections, therefore allowing proactive maintenance.

Electrical stresses in industrial variable frequency drive (VFD) motors are increasing with the increasing voltage and dv/dt levels, and with the advent of wide bandgap power devices. This increases the likelihood of partial discharge (PD) in VFD motors, and is expected to increase the risk of stator insulation failures in low voltage (LV) motors for which the insulation is not resistant to PD. To ensure that PD does not occur during operation, LV VFD motors are qualified for PD-free operation in the design or manufacturing stages. However, the PD inception voltage of qualified LV motors decreases with insulation aging exposing them to the risk of PD-induced failure. In this work, an automated off-line test method for VFD-embedded PD testing is proposed. The main idea is to perform PD testing at motor standstill at a voltage higher than the operating voltage to identify PD activity in the insulation early, before it occurs during operation. A series of impulse voltage tests are proposed for stressing the different components of insulation for identifying PD in terminal-end ground, phase, and turn insulation. Testing on a LV, VFD motor verifies that PD can be identified whenever the motor is stopped to provide early warning of insulation failure.

Recently, inverter-operated electrical machines have increased significantly. According to multiple works, insulation aging occurs faster for inverter-fed machines due to voltage overshoots and uneven voltage distribution caused by high dv/dt switching at several kHz. This necessitates better insulation monitoring methods so deteriorating insulation quality can be detected before machines fail and industrial process is interrupted. Good monitoring methods achieve a balance between detection reliability and cost requirements. Monitoring methods requiring the installation of expensive/intrusive equipment cannot be justified for all machines. Therefore, a new method for monitoring of ground insulation condition for inverter-fed machines is presented here. It utilizes commonly used sensors usually employed for differential protection and is simple in its application. It can be implemented before/after the motor is started/stopped and can screen out severe insulation deterioration preventing machine failure. The method has been verified experimentally on a 380 V, 18.5 kW induction machine.

Partial discharge (PD) testing, both off- and online, is widely used by manufacturers and end users of high voltage generators and motors to assess the insulation condition of the stator winding to aid maintenance planning. The widespread use of this technology lead to the development of standards such as IEEE 1434, IEC 60034-27-1 and IEC 60034-27-2 that provide guidance on uniform methods of measurement and data interpretation.

Off-line partial discharge testing has long been used as a quality control test for the stator winding insulation in motors and generators rated 6 kV and above. Both on-line and off-line PD tests are widely used to assess the condition of the insulation to determine if maintenance is needed. Although there have been many papers published on successful PD tests where stator winding insulation problems were detected, this paper tries to point out some of the limitations of PD testing that we have learned over the years from over 18,000 machines. In particular, the issue of false positive indications and false negative indications is discussed, with several examples presented.

Partial discharge (PD) due to fast risetime, pulse-width modulation (PWM) has been identified as one of the leading causes of insulation failure in low voltage (LV) motors with random wound stator windings fed by variable frequency drives (VFD). With the recent increase in electrical stress with faster risetime wide bandgap power devices and higher voltage for improved efficiency and power density of the system, PD-induced insulation failures are expected to increase. Since LV motor insulation is not resistant to PD, an increasing number of VFD motors are being qualified for PD-free operation. However, the PD inception voltage (PDIV) of qualified motors can decrease with insulation aging and the operating environment making LV motors susceptible to failure. In this paper, a new concept for automated inverter-embedded PD testing under impulse excitation is proposed. The main concept is to perform impulse PD testing using the inverter at increased voltage levels whenever the motor is at standstill for providing advanced warning of PD activity for preventing forced outage of VFD motors. Two options for applying increased impulse test voltage for implementing the proposed concept are presented. Test results on an 800 V VFD driven motor shows that the proposed methods can provide automated PD testing whenever the motor is stopped for improving the reliability of the VFD motor stator insulation.

Partial discharge (PD) detection in stator windings is widely used to assess the condition of the electrical insulation. When the winding is excited by 3-phase AC voltage, PD current pulses can occur between each phase and ground (i.e. within the stator slot and just outside of the same slot). PD can also occur in the endwinding region, driven by the phase to phase voltage. In addition, PD occurring in one phase often induces a “cross-coupled” signal into another phase, presumably due to the capacitance between phases in the endwinding region. These three “sources” of pulses can complicate the phase-resolved PD (PRPD) plots from each phase. Although some methods to separate these three types of pulses from one another have been proposed using post-data acquisition processing - they still require considerable expertise to separate them reliably. In this paper, experiments are conducted on stator winding models to identify the characteristics of the phase to ground, phase to phase and cross-coupled PD signals, in order to separate the sources in real time. Such separation is important, since PD in the endwinding is often much easier to repair than PD occurring with stator slots.

Direct Current (DC) high voltage testing is frequently used to assess the condition of stator windings. The list of the reasons why DC tests may be preferred over alternating 50 Hz or 60 Hz voltage tests is long: smaller size of the test equipment, fewer partial discharges, less risk of damage in the case of very poor insulation quality. Different direct current test methods are possible such as DC HIPOT, polarization index test, ramped voltage test, uniform time and graded time voltage step test, insulation profiling, etc. Since the traditional DC hi-pot test is not a diagnostic test (the result of the test is either PASS or FAIL), an improved method of DC testing using ramped direct high voltage was introduced by Bruce McHenry in 1964 [6] and this paper describes the differences between the two ramped direct-high voltage methods proposed in IEEE 95.

As mentioned in a recent editorial in this magazine [1], distributed generation (renewables), automotive and, in general, transport and industrial utilization are fast changing the structure of electrical assets as well as the typologies and design of electrical apparatus. This is due mostly to the more widespread use of renewable generation and the electrification of transport, including power electronic converters and inverters, which can provide higher voltage, lower losses and make the asset more efficient and power dense. In the long run, medium voltage (MV) transformers will tend to disappear and perhaps high voltage (HV) transformers may be endangered. The future typical industrial, transport and, likely, T&D assets will be made by hybrid networks where power will be delivered in AC or DC, depending on convenience, and most of the loads (rotating machines) will be controlled by power electronics.

The limitations of the thermal, vibration, or electrical monitoring of electric machines such as false indications, low sensitivity, and difficulty of fault interpretation have recently been exposed. This has led to a shift in the direction in research toward applying new techniques for improving the reliability of condition monitoring. With the changing environment, the purpose of this article is to provide an overview of the recent trends in the industrial demand and research activity in condition monitoring technology. The new developments in insulation testing, electrical testing, flux analysis, transient analysis, and fault prognostics are summarized. The challenges and recommendations for future work for the new technologies are also explored to help support target research and development efforts according to industrial needs.

The insulation system in form-wound stator windings is exposed to electrical, mechanical and thermal stresses, in addition to factors such as humidity and contamination. These stresses combine to give rise to at least 15 different aging mechanisms, which are described in some detail in [1, 2]. Some of these aging processes are readily identifiable with a visual inspection of the winding. For example, loose windings in the slot (leading to PD), poorly made PD suppression coatings (either the ‘semicon” in the slot area or the silicon carbide grading coating just outside of the slot), and endwinding electrical tracking due to contamination, are easily visually identified. However, there are a number of aging processes that cannot be seen by this method. These processes include thermal aging of the turn and groundwall insulation due to operation at high temperature, delamination of the groundwall insulation due to load (thermal) cycling and PD due to voids or other manufacturing imperfections within the groundwall. The only definitive method to assess the degree of aging with these latter failure processes is to remove multiturn coils or Roebel bars from the stator core and dissect them.

Partial discharge (PD) has been identified as one of the root causes of stator insulation failure even for low voltage (LV) ac motors. For reliable operation of variable frequency drive (VFD) motors, a standard acceptance test procedure (IEC STD 60034-18-41) to ensure PD-free operation of LV motors was recently developed. Although the test can verify whether PD exists in the stator for a voltage surge with predetermined voltage level and risetime, the information on where PD activity is present in the stator insulation is not clearly provided. In this paper, a test method for applying variable risetime surge voltage is proposed for identifying the component of stator insulation where PD occurs in the LV random wound motor stator. The proposed test allows the user to locate whether PD is in the turn, phase, or ground insulation. This information is critical for lowering the PD inception voltage for improving the motor reliability through insulation system design/manufacturing or motor/drive system reconfiguration. The test is implemented with a 4.5 kV variable risetime surge generator prototype, and 4 LV ac motors are tested to show where PD is likely to occur in commercial LV motors.

Recent cases of broken damper bars and shorted field winding turns in synchronous motors (SM) have shown that rotor faults can degrade motor performance and lead to a forced outage of the motor and driven process. However, detection of SM rotor faults has not received much attention since SMs are not as common as induction motors. Detection of SM rotor faults is difficult, as the damper bar is active only under the starting or load transients, and shorted field winding turns result only in a slight increase in rotating asymmetry. SM rotor fault testing in the field mainly relies on visual inspection or off-line tests, and new test methods that can provide reliable detection of the faults without motor disassembly is highly desirable. In this paper, new test methods for not only detecting, but also classifying the two types of SM rotor faults under motor standstill and starting are proposed. An experimental study on a 30 kW salient pole SM is provided to verify the proposed methods. It is shown that damper and field winding faults can be detected and classified with high sensitivity and reliability compared to conventional tests.

IEC 60034-18-41 has been developed as a standard acceptance test to ensure partial discharge (PD)-free operation for improving the reliability of low voltage (LV) variable frequency drive (VFD) motors. Although the test can verify whether PD exists in the stator for a voltage surge with predetermined magnitude and risetime, the information on where PD activity is occurring in the stator insulation is not clearly given. In this paper, a method based on variable risetime surge voltage testing is proposed for identifying the component of the stator insulation system where PD occurs in LV random wound motor stators (turn, phase, or ground insulation). This information can be used for increasing the PD inception voltage for improving the reliability of the motor through insulation system design/manufacturing. The test is implemented with a 4.5 kV variable risetime surge generator prototype. 4 LV ac motors are tested to show where PD is likely to occur in commercial LV motors.

An interturn insulation failure in the field winding of synchronous motors is a common problem that can lead to degradation in performance and accelerated wear of motor components. Existing offline tests for detecting this fault are known to be unreliable and inconvenient to perform, as they require motor disassembly. Online monitoring based on airgap flux monitoring has been proven to be successful, but could lack sensitivity for cases where the number of turns per pole is large. In addition, field winding turn shorts can be intermittent depending on the centrifugal force and could be unobservable at standstill or under an steady state operation. Therefore, monitoring of the turn faults under standstill and starting conditions with high sensitivity and without motor disassembly is desirable. In this paper, the detection of field winding short circuits for salient pole synchronous motors based on an airgap search coil under the starting transient, and new methods for standstill testing without motor disassembly are proposed. Two-dimensional finite element analysis and experimental testing on a 30-kW motor are given to verify the claims made. It is shown that the proposed set of tests can provide sensitive and reliable detection of field winding short circuits under standstill and starting conditions.

Damper bars are often used in high output salient pole synchronous motor (SM) rotors for motor starting and stabilization of performance. Several cases of broken damper bars in the rotor of SMs that lead to motor starting failure and/or forced outage have recently been reported. However, detection of damper bar failures is difficult, since the damper bars are active only under the starting or load transients. Damper bar testing in the field currently relies on off-line visual inspection, and there is no means available for detecting failures on-line. In this paper, a new method for detecting damper bar failures based on the analysis of airgap search coil measurements under the starting transient is proposed. Since airgap search coils are being increasingly installed in large SMs for detecting field winding short circuits, the proposed method can be implemented without additional hardware. The effectiveness of the proposed method is verified with 2 dimensional finite element analysis and experimental testing on a custom-built 30 kW salient pole SM under controlled fault conditions. It is shown that the proposed method can provide reliable and sensitive detection of broken damper bars whenever the motor is started at low additional cost.