HV Motors Research Articles

Fazi logički ekspertski sistem za procenu stanja izolacionog sistema statora VN motora

During operation, the electrical insulation system (EIS) of high-voltage motors is exposed to various factors that can accelerate its aging. Timely knowledge of the actual state of the EIS is of great help in allocating funds for service activities. The condition of the EIS of the HV motor can be assessed to some extent by performing available electrical tests. A major shortcoming is the lack of accuracy and the lack of well-defined criteria in the current standards. In that way, a potential error can bias the evaluation of recorded results. The paper proposes the use of a fuzzy logic expert system for assessing the condition of EIS stators of HV motors based on the results of electrical measurements. Also, the proposed fuzzy approach is fully discussed. The created expert system fully relies on the criteria defined in the IS32-Internal Standard of PE EPS.

Contribution of Electrical–Thermal Aging to Slot Partial Discharge Properties of HV Motor Windings

During the operation of HV motor, the abrasion of stator windings because of vibration caused by electromagnetic force will lead to slot partial discharge (PD). Furthermore, the superheat induced by overcurrent or poor heat dissipation will aggravate the slot PD. To understand the developing process of slot PD and comprehend the influences of aging factors on slot PD properties, two discharge bar models were made under electrical–thermal aging. At triple the rated line-to-earth voltage of 6 kV, two stator bars were aged for 37 days under different temperatures. The slot PD was measured during different aging stages, and visual observations of the insulation and stator core surface were made at the beginning and the end of aging. The experiment results indicate that electrical–thermal aging has a significant impact on the slot PD activities. The slot PD was intense under higher thermal stress, and the surfaces of insulation and stator core were oxidized after the long-time aging. This study can provide a judging basis for fault diagnosis of HV motors.

Changes in Insulation Characteristics of High-Voltage Motor Stator Windings with Temperature and Moisture

This research shows that all of the insulation characteristics (insulation resistance, partial discharge, dissipation factor, and alternating leakage current) of HV motor become worse at high temperature. It is also shows that partial discharge and insulation resistance are reduced at moisture absorption. Therefore, to accurately analyze the insulation degradation trend, it is necessary that those tests always be conducted under the same conditions (temperature and moisture content).

Development of Chemical Process for Recovering High-quality Rare-earth Oxides from HV Motor Magnets

Development of Chemical Process for Recovering High-quality Rare-earth Oxides from HV Motor Magnets

Influences of humidity and temperature on oil contamination discharge of HV motor stator windings

The leaking oil, as one typical contamination in stator windings of high voltage motor, will corrode stator winding and be dissolved in the grounding insulation layer. The oil contamination could lead to the stratification of insulation, the reduction of dielectric strength and the occurrence of partial discharge. In order to investigate the influence rules of humidity and temperature on oil contamination discharge of stator windings, two discharge winding models with oil contamination were made. At twice the rated line-to-earth voltage of 6 kV, two windings were aged for 240 hours under different relative humidity and temperatures, respectively. The phase resolved partial discharge (PRPD) patterns were monitored and analyzed during the short aging period. The experiment results confirm that temperature and ambient humidity have a significant impact on the oil contamination discharge of winding. The PD intensity of oil contamination increases with the rise of temperature. The creeping phenomenon will take place at the higher relative humidity. In addition, it will provide a judging basis for the on-line monitoring and fault diagnosis of the 10 kV three-phase asynchronous motors.

A new control strategy with fault ride‐through capability for VSC‐based offshore high power oil pump motor power supply system

In offshore oil platforms, high voltage and power motors (HV motors) are needed when transporting oil to the land. Traditional platform diesel generators cannot support so much power, so the required system must be supplied by the onshore AC grid. In general, when there is a long distance between the offshore platform and the shore, a DC transmission system is more efficient. This paper proposes a power supply approach for these motors using a voltage source converter (VSC) and derives the mathematical model for it. This method combines motor drive theory and studies in DC transmission field. Compared with the traditional motor control, this method, which makes full use of the advantages of VSC, can simplify the control strategy. And by using the Q–U droop characteristic, the strategy improves the system stability and fault ride‐through capability. Simulation is carried out in PSCAD/EMTDC, and the results verify the validity of the control method. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Understanding surface partial discharges in HV coils and the role of semi-conductive protection

An electrical insulating system in high voltage rotating machines is subjected to several stresses that cause changes of material properties during its lifetime. This phenomenon reduces progressively ability of the insulation to withstand the service stresses. Internal partial discharges can be located inside the high voltage insulation and usually they are dangerous for life-cycle duration. Surface discharges are generated at the solid/air interface in the gaps of machine stator slots or at the slot exits. Surface discharges can cause more intensive deterioration of the insulation, accelerate aging processes and definitively take aim toward premature failure of the machine. The application of conductive or semi-conductive protection layer provides certain protection for solid insulation from these discharges. Semi-conductive corona protection tapes are used on the coils at the slot exit region to electrical field control and prevent surface discharges. Laboratory measurements of the surface partial discharge quantities in specimens of high voltage coils with mica insulation were performed. The aim of the paper is to describe theoretically the development of surface discharges in rotating machines and to present surface partial discharge development in HV motors when semi-conductive layer protection is missing. The results can be found useful to understand better the surface discharge development and importance of high-quality semi-conductive protection technology.

Synchronous multi-channel PD measurements

The synchronous, multi-channel PD measurements provide a new potential in location, identification and separation of multiple PD sources and their discrimination from external noise in variety of HV assets. The independent PD observers, like coupling capacitors, bushing taps, capacitive or inductive sensors made the multi-channel PD evaluation feasible on HV motors, generators, three phase transformers or single-line cable systems with cross-bonding links. Three different multi-channel PD evaluation techniques are proposed: 3-phase amplitude relation diagram (3PARD), 3-phase time relation diagram (3PTRD) and 3-channel frequency relation diagram (3CFRD). The successful application of all techniques is presented.

Effect of mechanical vibration on the behavior of partial discharges in generator windings

As mechanical vibration usually exists in HV motors and generators, its influence on the partial discharge (PD) needs to be studied so as to improve the reliability of on-line PD monitoring method. In this report, the mechanical vibration frequency was synchronized to the power frequency and the effects of vibration phase and frequency were investigated. Because the length of the discharge gap changed with the vibration, obvious variation in the slot-PD patterns was observed at different vibration lead phase and frequency. Moreover, when the vibration amplitude was large, it was also found that the PD behavior at low voltage just above the PD inception voltage was different from that at higher voltage. This was explained in terms of the effects of the field variation in the gap due to the vibration and the charges left on the insulating surface due to PD.

Switching overvoltages in motor circuits

The switching of HV motor circuits in the megawatt range can cause large overvoltages on motor terminals. The amplitude of the resulting voltage spikes depends on breaker type and circuit characteristics. If the amplitude of switching overvoltages exceeds the basic insulation level of the motor, overvoltage protection circuitry should be installed to prevent a motor insulation failure. The decision whether overvoltage protection circuitry (surge pack) must be installed requires a prediction of the overvoltage level. This paper deals with switching overvoltages in motor circuits with a unit transformer between switchgear and motor. Overvoltages of motor circuits were simulated using specially developed simulation models. The results of three case studies revealed a large impact of cabling and transformer on motor overvoltages. In most of the cases, a unit transformer connected between switchgear and motor will provide an overvoltage protection to the motor. However, the transformer can also be a main cause of large overvoltages when connected to cables of specific length and capacitance. The paper provides some simple rules to recognize such critical configurations.

Failure analysis of HV motors in thermal station auxiliary systems

By analysing the causes of failures of HV motors from 35 power stations in India, it is possible to construct a schedule for motors, from design to operation and maintenance including diagnostic testing, that will result in better, more cost effective and efficient performance of motors.

Results of an investigation on the overvoltages due to a vacuum circuit-breaker when switching an HV motor

The results of a study of overvoltages arising when an HV motor is switched by a vacuum circuit breaker are presented. A laboratory test circuit was devised that would reproduce the actual network conditions in a generating plant, and special measurement and acquisition systems were used to enable the switching-surge amplitude and waveform to be recorded with accuracy. Statistical processing of the data was followed by estimation of the probability of occurrence of overvoltages and by evaluation of the number of the surges impinging on the motor-winding during its operating life. The results are discussed, and a number of conclusions are drawn. >