Stator Winding Insulation Systems Research Articles

Considerations regarding the impulse ignition test for level of protection “eb” and ignition test for protection levels “eb” and “ec” stator insulation systems of Ex-proof electric motors

According to the provisions of the 5th edition of the standard IEC 60079 – 7, equipment protection by increased safety “e”, for Level of protection “eb” and “ec” electrical machines, if the rated voltage exceeds 1 kV, type tests in explosive mixtures have to be carried out, in order to certify such an electrical machine. The purpose of this paper is to provide some considerations regarding the impulse ignition test for level of protection “eb” and ignition test for levels of protection “eb” and “ec” stator insulation systems of explosion proof electric motors and also to present a case study regarding such tests conducted in the specialized laboratory of INSEMEX Petrosani, on a stator insulation winding system of a 6 kV electric motor type of protection Ex eb IIC Gb, case study which will assist manufacturers in designing appropriate stator winding insulation systems for increased safety type of protection electric motors.

Stator ITSC Fault Diagnosis for EMU Induction Traction Motor Based on Goertzel Algorithm and Random Forest

The stator winding insulation system is the most critical and weakest part of the EMU’s (electric multiple unit’s) traction motor. The effective diagnosis for stator ITSC (inter-turn short-circuit) faults can prevent a fault from expanding into phase-to-phase or ground short-circuits. The TCU (traction control unit) controls the traction inverter to output SPWM (sine pulse width modulation) excitation voltage when the traction motor is at a standstill. Three ITSC fault diagnostic conditions are based on different IGBTs’ control logics. The Goertzel algorithm is used to calculate the fundamental current amplitude difference Δi and phase angle difference Δθ of equivalent parallel windings under the three diagnostic conditions. The six parameters under the three diagnostic conditions are used as features to establish an ITSC fault diagnostic model based on the random forest. The proposed method was validated using a simulation experimental platform for the ITSC fault diagnosis of EMU traction motors. The experimental results indicate that the current amplitude features Δi and phase angle features Δθ change obviously with an increase in the ITSC fault extent if the ITSC fault occurs at the equivalent parallel windings. The accuracy of the ITSC fault diagnosis model based on the random forest for ITSC fault detection and location, both in train and test samples, is 100%.

Влияние термомеханической нагрузки на старение изоляции тяговых электродвигателей тепловозов

Purpose: To consider the problem of increasing the reliability of traction electric machines. To assess the effect of thermomechanical stresses on the service lifetime of insulating material of traction electric motors. Methods: Finite element method, implemented in SolidWorks 2005 software package, was chosen as the main method for studying and calculating temperature fields. Results: The results show that thermomechanical stresses play a significant role during dynamic thermal aging of stator winding insulation system. With dynamic temperature changes, an insulation system experiences repeated thermomechanical stresses since the coefficients of thermal expansion of insulating materials and copper conductors are different. Practical importance: The need for further study of the actual thermal state of electrical machines has been identified which will allow to a rise temperature definition accuracy, to give a warning for possible overheating in operation by that extending traction motor lifetime and reducing sudden failure likelihood. The developed methods for determining electrical machine thermal state are recommended for practical use.

Surface partial discharge in hydrogenerator stator windings: Causes, symptoms, and remedies

Partial discharges (PD) are small electrical discharges that occur within voids within the stator winding insulation system of a hydrogenerator. PD is harmful to the insulation: the repeated bombardment of electrons and ions on the void's surface gradually ruptures the chemical bonds of the insulation's organic materials and leads to electric treeing or electric tracking. In typical stator insulation systems that use mica tapes bonded by epoxy, deterioration of the insulation due to PD is usually slow (many years or decades). In hydrogenerator stator windings, PD is more a symptom of an aging process than a cause of deterioration [1].

Voltage Stress Modeling and Measurement for Random-Wound Machine Windings Driven by Inverters

This article studies the voltage stress in random-wound machine windings under steep-fronted voltage excitations from inverter drives. The voltage stress in this article includes overshooting voltages between any two conductors in the windings, and overshooting voltages between individual conductors and the stator ground wall. This stress may accelerate insulation degradation in inverter-fed machines compared to traditional AC machines. A high-fidelity finite element (FE) model was used to analyze individual conductors of random-wound windings and predict the voltage stress during switching transients. Experimental validation of the model was conducted using a purpose-made random-wound three-phase winding, designed to allow access to multiple coils and parallel strands. Firstly, the voltage propagation and the voltage stress for both single-phase and three-phase windings were simulated and tested under Pulse Width Modulation (PWM) voltage excitations without a rotor. Both voltage propagations among all the coils and individual turns were measured and compared with simulated results. The voltage stress on each part of the machine stator winding insulation system was tested and studied. Then, similar simulations and tests were conducted with a demagnetized rotor installed to study how the rotor saliency affects the voltage propagation and voltage stress. The measured winding voltage propagations for all scenarios generally agree with the FE simulated results.

Causes of cyclic mechanical aging and its detection in stator winding insulation systems

This contribution presents effects of cyclic mechanical stress on aging of composite insulation systems in rotating machines and means of its detection by innovative diagnostic methods.

Life‐time characteristics of random wound compressed stator windings under thermal stress

Compression of windings is known to give thermal and performance benefits. The aim here is to investigate how the health of stator winding insulation systems are influenced by on-tooth coil pressing. A set of compressed coils have been aged by exposing them to high temperatures. Winding insulation resistance (IR) was monitored during this process to experimentally validate whether compressed coils remained healthy in terms of electrical insulation. In this case, a single stress (i.e. temperature) model is used to predict the reliability of the compressed coils. Accelerated life tests have been conducted to inspect the life expectancy and thermal endurance of the compressed stator coils. The employed statistical model (Arrhenius-Weibull) demonstrates that on-tooth coil pressing reduces the life expectancy dramatically due to reduced turn to ground wall insulation dielectric strength.

Thermal Conductivity Evaluation of Fractional-Slot Concentrated-Winding Machines

The use of fractional-slot concentrated windings (FSCW) in electrical machines allows more compact, efficient, and reliable design with respect to machines equipped with distributed windings. However, an electromagnetic design linked to a thermal analysis of the electrical machine is mandatory to achieve the desired performance, and to fulfill the requirements of efficiency and reliability. One of the most critical issues in thermal design of electrical machines is to assign fair values for the input parameters of the thermal simulation models, particularly those related to the stator winding insulation system. This paper deals with the assessment of the equivalent thermal conductivity of the insulation system of FSCW machines. For this purpose, three FSCW electrical machines for different applications were evaluated via an experimental method based on a dc thermal transient test. Whereas the investigated machines present different characteristics among themselves, different approaches were required to properly estimate the thermal conductivity.

Factory Acceptance Tests: A Comparative Analysis to Establish a Baseline for Testing Data

Factory acceptances tests (FATs) of a new high-voltage (HV) rotating machine include comprehensive diagnostic tests of the finished stator winding insulation system. From these tests, the end user obtains the baseline data for the condition monitoring performed during the machine?s operating life. The design geometry of the stator windings and manufacturing processes used to produce them will affect the dielectric test results. There is a significant challenge in comparing the test results from different stators to obtain a statistically useful sample for an acceptance test database. This article compares the dielectric test results from the 14.4-kV windings of several new 1,800-r/min, 26.1-MW (35,000-hp) centrifugal compressor synchronous motor stators of identical design, built for two U.S. chemical plants. The results are compared with those from accompanying sacrificial coils. The stator diagnostic tests include the dissipation factor (DF) or tan delta, power factor tip up (PFTU) or delta tan delta, offline partial discharge (PD), corona inspection with an ultraviolet (UV) analyzer, and online PD. Sacrificial coils produced and processed alongside each stator receive the specified American Petroleum Institute (API) coil acceptance tests and corona inspection with a UV analyzer. The PD measurements on the sample coils were repeated at an elevated voltage, and the results were compared with the corona inspection observations. This article shares the comparative analysis of the stator and coil test results as an excellent example of the baseline FAT data.

Continuous Remote Online Partial Discharge Monitoring of HV Ex/ATEX Motors in the Oil and Gas Industry

This paper presents results from the continued application of a new technique for the remote online partial discharge (OLPD) testing and monitoring of in-service high-voltage (HV) explosive atmosphere (Ex)/atmosphere-explosive (ATEX) motors operating in hazardous gas zones. The technique employs high-current wideband high-frequency current transformer sensors located remotely from the motor under test, at the switchboard end of the HV feeder cable. Significant cost and operational benefits can be gained from this remote monitoring technique as the stator winding condition of these motors can be monitored without having to enter the hazardous gas (Ex/ATEX) zone. The work described in this paper is a continuation of that carried out by the same authors, as reported at the 2012 IEEE Petroleum and Chemical Industry Technical Conference, and includes the development of proposed condition guidelines for assessing the condition of the stator winding insulation systems of large populations of aged HV motors, based on an OLPD “league-table” database. Two case studies are presented wherein the HV stator insulation condition and reliability of in-service motors, operating in hazardous gas zones, and HV generators were assessed using both online and offline partial discharge (PD) testing and monitoring techniques. The case studies emphasize the importance of carrying out extended continuous OLPD monitoring of in-service rotating HV machines to detect underlying trends in PD activity over time. This paper concludes with how condition monitoring data can support reliability-centered maintenance and condition-based management regimes.

Multiobjective optimization of stator slot insulation of high-voltage generator based on coupled SNOPT-finite element method analysis

The article describes an optimization algorithm based on coupling SNOPT to finite element analysis for optimization of the insulation layers in a model generator. An electromagnetic and thermal study of the core, winding, and insulation of stator slots is performed with the main idea of finding the optimum shape and characteristics of the insulation layers to minimize electric stress with respect to the thermal limitations of the materials. For this purpose a novel method of coupled SNOPT-FEM analysis is used for the optimization of insulation layers in a generator. The optimization algorithm in this article builds on the wellknown SNOPT packag, which is a general purpose system for large-scale, nonlinearly constrained optimization. For optimization involving electric fields and geometric properties, such as shape optimization, SNOPT can be connected to FEM to perform optimization based on a finite element model. According to this method, the geometry of the insulation layers is optimized, and optimized values for the material properties are obtained with respect to electrothermal limitations to investigate the possibility of improving the HV generator characteristics. Following a short review of the stator winding insulation system components, the developed optimization algorithm for SNOPT and FEM analysis will be given along with the results of a model generator.

DEVELOPMENT OF VIBRATION CONDITION MONITORING SYSTEM APPLYING OPTICAL SENSORS FOR GENERATOR WINDING INTEGRITY OF POWER UTILITIES

This paper describes the vibration condition monitoring diagnosis system developed for stator and rotor winding integrity assessment of 100MW class gas turbine generator in combined-cycle thermal power plant. High reliability of windings is one of the most essential prerequisite for generators of power utilities. Assessing the condition of stator winding insulation systems requires objective information from condition monitoring system. In-service monitoring is essential if a power plant is following a condition-based maintenance strategy. Generator damages are caused by the high vibration and the power system instability by secondary impacts of an unannounced plant stop and the life of the generator is decreased. The mechanical vibration in generator is induced by both mechanical and magnetic forces. The vibration condition monitoring system is required for the improved savings of operation and maintenance cost in terms of reliability in power plant.

A Review of Results From Thermal Cycling Tests of Hydrogenerator Stator Windings

Thermal cycling of generators, occurring during intermittent operation of power stations, increases the thermomechanical stress on the stator groundwall insulation and can, therefore, contribute to failure of the generator. Thermal cycling stresses can, to some extent, be replicated and accelerated in laboratory tests. Such tests are performed to ascertain whether a new stator winding insulation system can handle intermittent operation, to evaluate the relative difference between different types of insulations or to investigate degradation processes and failure mechanisms. Results from thermal cycling tests of hydrogenerator stator windings are reviewed. Only stator windings of air-cooled hydrogenerators with thermoset groundwall insulation of mica-epoxy or mica-polyester are considered. The published literature of laboratory experiments performed in order to study effects of thermal cycling on groundwall insulation through different types of diagnostic measurements is described and discussed. Partial discharge (PD) and tanδ measurements gave, in some cases, an indication of a trend in the condition during thermal cycling, while in other cases it was difficult to interpret the test results. Many agree that measurement of breakdown voltage of stator windings can give an indication of the aging and remaining lifetime of the insulation. The ability of other measurements such as tan δ, PD, and voltage endurance to assess the remaining lifetime of the stator winding is less convincing since the results vary greatly.

Reduction of Electrical Stresses on Low-voltage Induction Motor Insulation Systems through Remote-controlled Inverters

The use of adjustable speed drives has introduced new electrical stresses in stator insulation systems. Many applications, in which the inverter and motor are placed at different locations, have led to further enhancement of these stresses and to unexpected failures within these motors. In this article, the main responsible agents in this process will be shown, and the effect of pulse rise times, inverter–motor distance, and selected speed in the creation of high voltages applied to the stator windings dielectric materials will be measured. Furthermore, the benefits of using PC-based INTERBUS protocol for remote-controlled inverters without unnecessarily increasing voltages applied to the stator winding insulation systems will be shown.

Historical development of rotor and stator winding insulation materials and systems

This article presents an overview of some of the significant developments in rotating machine insulation systems that have occurred in the past century. This article describes the chronological development of insulation materials and systems, together with some of the key innovations in manufacturing. For the most part, it was the innovation in materials and manufacturing from wound stator groundwall insulation that paced the development of machine insulation systems.

Using on line insulation testing to implement generator predictive maintenance

Aspects of preventive and predictive maintenance for a hydrogenerator stator winding insulation system are discussed in this article. A specific case study on a 31 MVA hydrogenerator is presented. The new method had to provide the staff with a precise and timely assessment of the state of the insulation without prolonged outages and substantial manpower allocations.