Karim Younsi

The future of nanodielectrics in the electrical power industry

While specialty applications of nanotechnology in the photonics and electronics areas have seen a tremendous growth in the past several years, the use of nanodielectrics in the electrical industry (high power density and high voltage) has not shown the same level of activity. In addition to a review of nanodielectrics, we discuss in this paper, our perspective on the current status, development needs and future potential to build or engineer nanostructured materials for dielectric applications in the electrical power industry. Short and long-term future research and development needs are considered from the point of view of industrial applications.

An online technique for monitoring the insulation condition of AC machine stator windings

A novel online technique for monitoring the insulation condition of ac machine stator windings is proposed in this paper. The concept is to measure the differential leakage currents of each phase winding from the terminal box in a noninvasive manner to assess the insulation condition during motor operation. The conventional differential CTs used for phase fault protection can be replaced with high performance current sensors to measure the leakage current with higher accuracy. Indicators for insulation condition such as the capacitance and dissipation factor are calculated based on the measurements to provide a low cost solution for online insulation condition assessment. A simplified online insulation system model is derived for analysis and interpretation of the measured data. Experimental results on a 15-hp induction motor under simulated insulation degradation conditions show that the proposed technique is a very sensitive method capable of detecting incipient signs of insulation degradation.

On-line capacitance and dissipation factor monitoring of AC stator insulation

A new on-line technique for monitoring the insulation condition of ac motor and generator stator windings is proposed. The approach uses a newly developed High-Sensitivity Current Transformer (HSCT) to precisely and non-invasively measure the differential current (e.g., the insulation current) of each phase winding from the motor junction box. Conventional differential current transformers (CT) used for fault protection can be replaced with the new HSCT to measure the winding insulation current with higher sensitivity and accuracy. The HSCT can serve both motor health monitoring and motor protection functions. Presently, indicators for insulation condition such as capacitance (C), dissipation factor (DF), or insulation power factor (PF) are only obtainable off-line. The new approach can provide a low-cost solution for on-line motor insulation condition assessment. Validation of the new HSCT technology was carried out during an accelerated life testing of a 460 V, 100 HP, 1200 RPM form wound induction motor. The motor discussed in this paper was aged at high temperature (255 °C) as the load cycled between 0 % and 200 % every 5 minutes. Although this highly accelerated life test does not represent how a motor ages in service under real operating conditions precisely, the principal goal was to prove the capability of the new HSCT to accurately detect the insulation current and quantitatively monitor motor insulation gradual aging and health. On-line data from the HSCT correlated well with off-line data from a commercial capacitance and DF bridge. It is hoped that the benefits of the on-line motor health monitoring are fully realized and the method extended to other electrical assets as well.

An on-line technique for monitoring the insulation condition of AC machine stator windings

A novel on-line technique for monitoring the insulation condition of AC machine stator windings is proposed in this paper. The concept is to measure the differential leakage currents of each phase winding from the terminal box in a non-invasive manner to assess the insulation condition during motor operation. The conventional differential CTs used for phase fault protection can be replaced with high performance current sensors to measure the leakage current with higher accuracy. Indicators for insulation condition such as the capacitance and dissipation factor are calculated based on the measurements to provide a low cost solution for on-line insulation condition assessment. A simplified on-line insulation system model is derived for analysis and interpretation of the measured data. Experimental results on a 15 hp induction motor under simulated insulation degradation conditions show that the proposed technique is a very sensitive method capable of detecting incipient signs of insulation degradation

An online groundwall and phase-to-phase insulation quality assessment technique for AC-machine stator windings

An on-line technique for monitoring the condition of groundwall (GW) and phase-to-phase (PP) insulation for 3 phase AC machine stator windings is proposed in this paper. Insulation condition indicators such as capacitance, dissipation factor (tan/spl delta/), and AC insulation resistance are calculated on-line based on the differential leakage current measurements for each phase. A model for a 3 phase AC machine insulation system is derived and analyzed for both off-line and on-line testing. Guidelines for interpreting the measured indicators for determining the 1) overall condition and 2) significant degradation for the individual GW and PP insulation for each phase of the stator are given based on the analysis. Experimental results on a 15 hp induction motor under intentional GW and PP insulation degradation conditions show that incipient insulation degradation can be detected and classified with high sensitivity.

Stator winding failures: contamination, surface discharge, tracking

Stator winding failures are generally attributed to groundwall or turn insulation failures. The failure mechanism involves the gradual development of a weakness in the insulation. This weakness is usually caused by a trapped void that is continuously enlarging due to ever increasing corona or partial discharge activity. The failure mode is from the inside out. Studies of failures in actual machines, and of coils on voltage endurance testing, indicate that failures can occur from the outside inward. Surface contamination leads to intense surface discharge and tracking. This surface discharge can lead to rapid winding failure following the inward motion of the corona activity. This paper looks at tracking as one failure mechanism and describes a series of tests on standard insulation materials aimed at comparing the anti-tracking capability of the materials. The tests show that combinations of insulation materials can reduce the anti-tracking capacity of a rather robust insulation system and predispose it to failure.

Seasonal changes in partial discharge activity on hydraulic generators

Six hydraulic generators in two separate power stations were equipped with partial discharge cable type couplers for long term PD monitoring. The stator windings used four different insulation systems from three different manufacturers. Partial discharge activity was monitored monthly on each generator for periods of time ranging from 2 to 9 years. PD data was acquired in the frequency range of 40 to 350 MHz. External noise pulses were filtered out using the differential method with two cable couplers per phase. The present paper is a case study showing how PD activity varies dramatically depending on the time of the year the online testing was performed. A ratio higher than 100 was recorded between winter and summer readings. Winter readings, (January-March) were always higher than summer readings (July-September). A strong correlation was found between temperature, atmospheric absolute humidity levels (water partial pressure in mm Hg) and the well-known PD basic quantities such as Q/sub max/ and NQN values. An interpretation of the monthly-acquired data is offered based on both external and internal PD activity.

Electrical signature analysis based online monitoring of drive-trains for doubly-fed wind generators

Drive train failures are one of the common failure modes of wind turbines. Their early detection, including the generator bearing and gearbox defects, is considered difficult using the state-of-the-art monitoring techniques. In this paper, a novel electrical signature analysis-based drivetrain monitoring technique is proposed for wind turbines. A novel electrical signature tool, electrical multi-phase imbalance separation technique (eMIST), is proposed to improve the signal-to-noise ratio in electrical signature analysis. The theoretical basis of drivetrain defect detection is also presented in detail. The proposed approach is validated by experimental results obtained from a 25 HP wind drivetrain simulator, designed to simulate 1.5 MW wind turbines. The experimental results show that the proposed approach is capable of providing accurate detection of drivetrain defects at an early stage. The proposed approach is cost effective with high probability of detection (PoD) of drivetrain defects compared to existing techniques.

Online detection of endwinding contamination in industrial motors

This paper discusses a novel online technique to detect moisture and surface tracking of stator end-winding insulation in medium/high voltage AC motors. A novel high sensitivity differential current transformer is used as a sensor for monitoring insulation capacitance and dissipation factor of motors online. A 100 hp, 4160V, 3-phase form-wound induction motor is tested with different contaminants sprayed on its end winding. Partial discharge couplers are also used to monitor partial discharge activity during insulation contamination tests. A comparison of both techniques to detect pre-cursors of motor failure due to end-winding tracking is presented in this paper.

Motor current signature analysis during accelerated life testing of form wound induction motors

The work presented in this paper describes motor accelerated thermo-mechanical aging and helps set the stage for on-line monitoring/data analysis as an effective means to detect a fault as electrical machine develops it and identify its type in real time. As a first step towards this goal, Motor Current Signature Analysis (MCSA) is applied to the data collected during accelerated thermo-mechanical aging of a formed wound induction motor tested to failure. The characteristic frequencies corresponding to detecting broken bars, stator inter-turn shorts, static/dynamic eccentricities and gradual degradation of the machine insulation are identified and sought from the collected data. To accomplish this accelerated aging, the motor was started/stopped frequently and subjected to overload in a steady state manner to subject it to severe thermo-mechanical stresses. Detailed analysis of experimental data is presented. It is hoped that this effort would pave the way for future work where on-line monitoring and diagnostics become a standard tool for life extension, lower maintenance cost, condition based loading, etc. for electrical assets.