High-voltage Motor Research Articles

A Development of Online Partial Discharge Monitoring System for High Voltage Motors, considering Insulation Deterioration Trend Analysis

High voltage motors have been applied to various industrial applications. Among the high voltage motors in service today, significant proportions were manufactured in the 1960s and 1970s. Machines of this generations will need to be refreshed sooner or later, due to the insulation deterioration.As to insulation deterioration diagnosis for high voltage motors, automatic and accurate Off-Line diagnosis system, where electric diagnosis, mechanical diagnosis and visual inspections are employed with Expert System for total evaluations has been already developed and is applied to various machine treatment and renewal cases.This time, authors have developed On-Line partial discharge monitoring system for those motors. Features of the system are, (1) Simple system configuration using single Split-Type High-Frequency CT, (2) Micro-computer aided Statistical Noise Reduction System based on Phase Synchronous Algorithm, (3) Micro computer aided automatic Likelihood valuation system, implementing expert evaluation for Partial Discharge Phenomena of the motors.This paper describes the system configurations and features. This paper also describes Insulation diagnosis method using the system, based on more than 30 experiences by actual machines of 300kW-10000kW

Analysis of random PWM switching methods for three-level power inverters

This paper presents a generalized theory which covers both two-level and three-level random pulsewidth modulation (RPWM) schemes. Various three-level RPWM schemes with low switching frequency are presented and compared with two-level schemes. Three-level RPWM schemes have less discrete harmonics and continuous noise than two-level RPWM schemes. They have desirable spectral characteristics and can be employed in high-voltage inverter-fed motor drives. Measurements have confirmed the theory and the attractive features of three-level RPWM schemes.

Failure identification and analysis for high-voltage induction motors in the petrochemical industry

This paper gives a synopsis of condition monitoring methods, both as a diagnostic tool and as a technique for failure identification in high-voltage induction motors in industry. New operating experience data for 483 motor units consisting of 6135 unit years are registered and processed statistically to ascertain the connection between motor data, protection and condition monitoring methods, maintenance philosophy and different types of failures. The different types of failures are further analyzed to determine the failure initiators, contributors and underlying causes. The results have been compared with those of a previous survey, the IEEE Report of Large Motor Reliability Survey of Industrial and Commercial Installations, 1985. In the present survey, the motors range from 100 to 1300 kW; 47% of them are between 100-500 kW.

Multilevel converters for large electric drives

This paper presents transformerless multilevel power converters as an application for high-power and/or high-voltage electric motor drives. Multilevel converters: (1) can generate near-sinusoidal voltages with only fundamental frequency switching; (2) have almost no electromagnetic interference or common-mode voltage; and (3) are suitable for large voltampere-rated motor drives and high voltages. The cascade inverter is a natural fit for large automotive all-electric drives because it uses several levels of DC voltage sources, which would be available from batteries or fuel cells. The back-to-back diode-clamped converter is ideal where a source of AC voltage is available, such as in a hybrid electric vehicle. Simulation and experimental results show the superiority of these two converters over two-level pulsewidth-modulation-based drives.

Methods of condition monitoring and fault diagnosis for induction motors

The study gives a synopsis over condition monitoring methods both as a diagnostic tool and as a technique for failure identification in high-voltage induction motors. Methods offault detection are reviewed and analyzed: mechanical (vibrations, shock pulses, acoustic, speed fluctuations ), electro-mechanical (currents, surges, partial discharges, leakage fluxes) and temperature-, oil particle-, gas analysis, and performance- and visual methods. The study indicates how different types of faults generate different frequency components in vibration spectra, in motor supply current, in torque and in magnetic field, and relationships between fault types and spectra have been developed. Supply current waveform analysis has been applied to demonstrate the detection of broken rotor bars and bearing faults.

A study of the natural vibratory response of stator structures to improve condition monitoring strategies for induction motors

On-line condition monitoring is widely used to detect failure mechanisms in large highvoltage three-phase induction motors. The high-voltage winding is installed in a complex, stator core structure which vibrates due to electromagnetic forces. Insulation degradation can be diagnosed via on-line monitoring of partial discharges but the fundamental cause is often mechanical damage due to stator vibration. This paper presents results of an investigation into the natural vibratory response of the complete stator assembly. Results from finite element models of complex stator assemblies, including the frames, are supported by experimental measurements. These include freely suspended stator cores as in previous studies, and also now introduce the much more realistic condition of the complete stator assembly securely bolted on to a solid baseplate. The knowledge gathered in this study will aid the application of a vibration monitoring strategy to detect stator core vibration which can lead to winding insulation damage.

新設備の４００Ｖ配線化

Niigata Mill, Hokuetsu Paper has increased its production by new investment for these four years (1995 to 1998). As one part of it, the mill changed its low voltage distribution system of 200 volt to that of 400 volt. This conversion produced benefit by reducing numbers of high voltage motors, distribution loss, wiring cost, and frequency of disconnecting motors.

高圧インバータによるポンプの省電力

It is said that the groval warment has been much progressed these days, and the energy saving is taking position for it much greater then ever before.At Nomachi Mill, the energy saving action programs in care of all employees have been annually taken and getting effective achievements. For an instance, in a year of 1997, it has been saved by 945 kW of electricity in a thirty activities, and 165 kl/month of heavy fuel oil in two activities. With new facility installations, such as Oxygen reactor, and coater machine, it seems to be a very energy crisis with higher demand of energy consumption. The energy saving is tool for control of the environmental improvement at the higher demand of energy consumption, and taken in action of several programs in manners of the inverter control, flow improvement, high efficiency machine in use and alternative operation.The inverter control system is most widely approved as the main tool of energy saving. So for, a low voltage converter was applied to control a high voltage motor in manner of low voltage regulated. But, presently the high voltage direct inverter control has been developed and come in handy with reasonable prices, and at Nomachi Mill, two high voltage direct inverters have been installed.Following are for an application explanation.

A three-phase model for surge distribution studies in electrical machines

This paper describes a computer model for surge distribution studies in the windings of electrical machines. The computer model takes the coil as the basis for the analysis and uses multiconductor transmission line theory to obtain the transference matrix for the winding. Several practical formulations based on machine design characteristics are used for calculating electrical parameters. Flux penetration into the iron core and mutual coupling between coils in different phase windings are incorporated into the analysis. The model was validated by a comparison of predicted and calculated results in a high voltage motor.

How good is your motor insulation systems

The insulation systems in high voltage motors have evolved through time. There are many machines providing good operating performance. Some care and maintenance will keep them running for years to come. Application of existing machines to inverter duty is to be addressed with caution and awareness. New machine designs are making use of the past experiences, and the continuing evolution will ensure that we will have better reliability and longer life, along with smaller, lower cost machines in the future. Against this background, the authors discuss a number of factors which, if considered further, will lead to long life for both new and older insulation system designs. These factors are: (1) system suitability for application; (2) system design and materials; (3) implementation; and maintenance practice. The authors' intention is to provide the reader with an understanding of insulation systems as related to insulation life and performance.

The use of partial discharge measurements to assess the condition of rotating machine insulation

This article reviews some of the recent improvements in measurement technology that have enabled on-line PD testing to be applied to a significant percentage of high voltage motors and generators in use in North America today.

Stress grading for high voltage motor and generator coils

Stress grading on high voltage motor and generator stator windings is becoming more important as the electrical stress in the ground insulation is increased. While there are a variety of possible ways to relieve surface stress, non-linear resistive tapes have been almost universally adopted because of their simplicity and efficiency. However, selection and quality control on these materials has been problematic, as the means to characterize these materials has not been clearly set out. Most types of material can be characterized in terms of two parameters, a threshold stress and a gradient index, which can be determined from simple current and voltage measurements. It is hoped, therefore, that manufactures of grading products will adopt such measurements and publish these values as a matter of course. This will enable coil manufacturers to improve the specification and quality control of stress grading coatings.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental investigations of high-voltage motor switching surges

The paper presents the results of experimental investigations of surges appearing on the insulation of high-voltage (HV) motors for normal switching operations and during an interrupt of aborted start. High-voltage motors were switched by oil-minimum circuit breakers, air magnetic and vacuum switching devices. Investigations were performed mostly on 6 kV isolated neutral station service cable networks of thermal power plants, industrial or similar installations. On the basis of the results, certain measures are proposed for limiting transients, thereby increasing the reliability of HV motor operation.

Partial discharges in HV machines; initial considerations for a PD specification

Partial discharge (PD) specifications exist for all types of electrical equipment except motors and generators. Whereas specifications essentially require that the equipment is discharge-free the stator conductor bar insulation of the motors and generators under consideration is micaceous and has excellent PD resistance. Consequently, the intention in a specification would be to permit a certain level of PD. It is therefore particularly important that the problem of overcoming the attenuation between the PD site and the measurement equipment at the winding terminals be solved. Within the general context of examining the feasibility of a PD specification, the problem of PD signal attenuation is addressed. The approach adopted was to seek a region of low attenuation by examining the whole of the possible range of PD measurement frequencies. The investigations described consist of experiments on a full-size 500 MW endwinding model and on two large high-voltage motor windings. Careful screening to reduce radiative pick-up from the calibration generator simulating a PD source and a direct earthing scheme enabled the essential elements of PD pulse transfer through a winding to be determined. The frequencies investigated ranged from kHz to GHz. Pulse transmission and frequency spectrum measurements were made. It is concluded that the PD pulse transmits essentially as a travelling wave, the high-frequency components being highly attenuated. No significant crosscoupling was apparent through the end-winding region. The windings acted as a low-pass filter, the cut-off frequency being dependent on the winding length, and being below 1 MHz. Consequently, it is concluded that it should be possible to measure PD, using equipment with a passband below the low-frequency cut-off, with low attentuation.

Investigation of the effect of repetitive voltage surges on epoxy insulation

The aging of epoxy electrical insulation by unipolar, repetitive voltage surges has been investigated to determine whether such surges can precipitate premature failure of high-voltage motor windings and gas insulated switchgear spacers. A full factorial statistical test was devised and performed on 180 pure epoxy insulation specimens to examine the effect of voltage magnitude, polarity, and surge repetition rate on the life of unfilled epoxy specimens containing a needle-plane electrode geometry. All factors and interactions were determined to have a significant effect on the life, and life prediction equations were estimated. Increasing the voltage magnitude or repetition rate, decreased or increased the life, respectively. Aging was found to occur at as low a stress as 2 MV/cm. Gradual aging of the epoxy does occur under repetitive voltage surges. Based on a realistic number of surges with typical voltage amplitudes which can normally occur from vacuum switchgear during service, gradual deterioration of the epoxy turn insulation in motor stator windings is possible in some situations. Situations where surge aging may occur are identified. >

Determination of transient overvoltages caused by switching of high voltage motors

Results of measurements are presented for switching-induced overvoltages at the terminal of machines installed in power plants. Actual front-time and amplitude data of the voltage transients were measured. The results obtained are examined in a discussion of voltage test levels used on the insulation of motor windings and methods of motor-winding protection.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Application of vacuum circuit-breaker to clear faults with delayed current zeros

In medium-voltage systems short-circuit currents with delayed zero crossings can occur either in the proximity of a concentrated generation or when switching off large high-voltage motors, or both. For two typical system configurations it is shown theoretically that vacuum circuit-breakers are well suited to clear under these conditions, provided they can withstand the high peak currents associated with such faults and their contact configuration can withstand prolonged arcing. The theoretical considerations are backed by tests. However, it will be necessary to investigate the conditions of each application to determine the actual stress upon the circuit-breaker. >

Impulse Strength of High Voltage Motor Coil Turn Insulation

Breakdown of the turn insulation in large ac motors eventually leads to the failure of the stator winding. The turn insulation can fail if the motor coils are subjected to steep-fronted surges. The impulse strength of the turn insulation was determined for fifty-three typical coils procured from different manufacturers and tested singly in air. The coils had six different insulation systems and their specifications (voltage, power, size, turns, strands etc) cover a very wide range. Most coils were found to have an impulse strength exceeding 5 times peak line-to-ground voltage (5 pu).

Impact Potential of New Power Electronic Technologies

New technological advances in the area of power electronics are having an increasing impact on the design of aerospace control systems. These next generation power components promise improved system performance through increased electronic efficiencies. Applying state-of-the-art packaging concepts as an integral part of the system design will allow these devices to be utilized in a space efficient and reliable manner. The first portion of this paper looks at two such next generation components. The first is a High Voltage Integrated Circuit (HVIC) that provides a bridge between the low voltage controller logic and the high voltage motor winding invertor. This device achieves size reduction and an increase in reliability through integration of low and high voltage logic networks on a single integrated circuit. The second is the Insulated Gate Transistor (IGT). This device provides a high voltage switch with MOS-like drive characteristics. The present and future expectations of these power devices are discussed. This paper then looks at new packaging techniques for power devices. The impact of parasitic circuit effects have significant impact on power circuit performance. Finally, this paper looks at an example control application. The design is that of a permanent magnet motor driven actuator. The drive motor uses 270 vdc for supply voltage. Within the intelligent system controller, is the capability to control the current demands of the motor. The new power electronics devices are making the design feasible in both thermal and volume efficiency. This topic includes projected controller sizing into the 1990s.

Impulse Strength of High Voltage Motor Coil Turn Insulation

Breakdown of the turn insulation in large ac motors eventually leads to the failure of the stator winding. The turn insulation can fail if the motor coils are subjected to steep-fronted surges. The impulse strength of the turn insulation was determined for fifty-three typical coils procured from different manufacturers and tested singly in air. The coils had six different insulation systems and their specifications (voltage, power, size, turns, strands etc) cover a very wide range. Most coils were found to have an impulse strength exceeding 5 times peak line-to-ground voltage (5 pu).