C. Hudon

Testing of low-voltage motor turn insulation intended for pulse-width modulated applications

The problem of premature failure of low voltage rotating machines fed by inverters could have serious economic impact if it is not addressed. Indeed, even though much already has been written on the different topics related to the use of such power supplies, there is a great need for a better understanding of the physical mechanisms involved, particularly with regards to aging mechanisms of insulating materials. The aim of this paper is to highlight the effect of adjustable speed drives (ASD) on the stator finding insulating materials. Pulse-like electrical stresses have been applied to three types of samples (thin film, twisted pair, and layer to layer). Physical characterization and/or lifetime measurements have been performed. The results are presented and discussed according to the nature of the interaction existing between the electrical stress main characteristics (magnitude, frequency, dV/dt) and the material properties. According to these results, the development of a standardized insulation quality control test, based on a physical understanding of involved stresses and material behavior, appears now to be necessary.

Study of slot partial discharges in air-cooled generators

Two laboratory experiments were set up in order to investigate the influence of different parameters on slot partial discharge (PD) activity. The goal of the first experiment was to establish the relative importance of temperature and gap size on slot partial discharges (PDs). The second experiment was used to determine how different stresses (e.g. electrical, thermal and mechanical) influence the discharge mechanisms of stator bars affected by slot PDs. Finally, the results obtained in the laboratory were confirmed when compared with actual field measurements on a generator.

CFD Analysis of Ventilation Flow for a Scale Model Hydro-Generator

In 2006, the first Computational Fluid Dynamics (CFD) simulations of the ventilation of specific hydro-generator components were performed at the Hydro-Quebec Research Institute (IREQ) and lately the entire ventilation circuit is being investigated. Due to the complexity of flow calculations, a validation process is necessary and for this reason a 1:4 scale model of a hydro-generator has been built at IREQ to get experimental data by means of particle image velocimetry (PIV). This paper presents 2D and 3D simulation results for the scale model obtained with a commercial CFD code and addresses the challenges associated with the application of CFD to hydro-generators. In particular, the effect of rotor-stator interface (RSI) types and configuration is analyzed to determine the approach that best suits this application. Two-dimensional calculations show that the steady state multiple frames of reference (MFR) solution is highly sensitive to the type (frozen rotor (FR) vs. mixing plane (MP)) and location of the RSI. A parametric study is performed where each interface configuration is compared to the transient case results. The MFR-FR interface model produces results that may vary significantly depending on the relative rotor position and the radial location of the RSI in the air gap. The MFR-MP interface model appears to be more coherent with reference values obtained from a transient case, since the radial velocity profiles in the stator are similar. Furthermore with an appropriate radial positioning of the interface, the windage losses are within 20%. Simulations of the complete 3D ventilation circuit revealed a maximum variation of 10% in both total ventilation flow rate and total windage losses, between the RSI configurations studied. However, the relative flow distributions, normalized with respect to the total flow rate, are unaffected by changes in RSI configuration. This paper focuses mainly on sensitivity studies to numerical settings, but this comparison still requires experimental validation before any final conclusions can be made.Copyright

Effect of surface degradation on slot partial discharge activity

To improve our understanding of slot partial discharge (PD) mechanisms, an accelerated aging test was initiated two years ago. This long-term experiment is being performed on six stator bars subjected to slot PDs under electrical, thermal and mechanical stresses. It is well known that slot PD activity in air-cooled generators is harmful to the stator winding ground insulation. The degradation induced by slot PDs will modify the physical properties of the surfaces of the cavity where PDs take place and this in turn will influence the slot PD process. To understand the evolution of slot PD activity and the changes in its PRPD (Phased Resolved Partial Discharge) pattern, it is therefore essential to recognize and understand the interdependence between surface modifications and PD signals. This paper presents results of visual observations of stator bar surface degradation, changes in surface conductivity and the effect on the evolution of PD signals in the presence of slot PD activity under conditions of electrical, thermal and mechanical stresses.

Long term behavior of corona resistant insulation compared to standard insulation of magnet wire

Low voltage motor turn insulation has been aged under stresses similar to those caused by PWM voltage pulses. Commercial standard turn insulation was tested in parallel with a new inverter grade corona resistant (CR) insulation. Both types of insulation were subjected to fast bipolar pulses at 80/spl deg/C and at 130/spl deg/C. When the maximum peak voltage was set to 620 V, below the discharge inception voltage (DIV), no failure was recorded on either insulation system, after aging times of 280 days. When the pulse peak voltage was increased to 830 V, failures rapidly occurred on all specimens with standard insulation and later for all specimens with CR insulation. A third aging test was carried out at 750 V. Here, at 80/spl deg/C, all specimens with CR insulation failed before the specimens with standard insulation. The mean time to failure for CR material was of 133 hours, while it was larger than 1070 hours for the standard type insulation at the same temperature. This behavior was not observed at 130/spl deg/C and 750 V.

Contribution of humidity to the evolution of slot partial discharges

The impact of humidity on the slot partial discharge (PD) process was evaluated in a laboratory experiment conducted in a climatic chamber under constant humidity and temperature. Two stator bars, subjected to slot PDs at two temperatures (28°C and 85°C), were aged at twice their nominal phase-to-ground voltage (16 kV) under three absolute humidity levels (5 g/m3, 9 g/3 and 13 g/3). The phase resolved partial discharge (PRPD) patterns were monitored during short aging sequences and visual observations of the surfaces were made at the beginning and end of each aging sequence. Observation confirms that humidity has a significant impact on the discharge phenomenon but other parameters, such as surface condition and temperature, affected the discharge process just as much. This paper presents the way slot PD activity evolves and affects the related PRPD pattern under different conditions of humidity, temperature and surface degradation.

Effect of temperature and thermal expansion on slot partial discharge activity

The influence of generator load changes on partial discharge (PD) activity can be significant especially when the semi-conductive coating of stator bars is eroded. It is common knowledge that thermal expansion (copper, core and insulation) will decrease the gap size in the slot between the bar and the core and this could lead to a reduction of slot PD activity. In the present study, a surface defect was made on a stator bar and a variable gap between the bare insulation surface and a ground plane was adjusted from close contact to fixed gaps of 0.25, 0.5 and 1.0 mm. Partial discharge measurements were recorded at different temperatures for each of these four gap sizes. The stator bar was both electrically stressed at 8.0 kV and thermally stressed by a circulating current. For each gap dimension, PD tests were carried out at room temperature (22degC), and with circulating current at 53,70,90 and 132degC. By comparing results from different gaps at a fixed temperature and results from different temperatures at a fixed gap, it was possible to determine which one of these two parameters had the greatest influence on the PD signals. The effect of these changes with respect to gap size was compared with an evaluation of the thermal expansion of a bar in its slot under normal operating conditions. This study leads to a better understanding of the change expected in slot PD activity with load variations when measurements will be performed on actual generators.

Statistical analysis of partial discharge data

Recently, considerable efforts were deployed at Hydro-Quebec to retrieve all partial discharge (PD) files recorded with the PDA-H instrument over the last 25 years and which were scattered all over in plant computers. These initial raw data files have been centralized on a corporate computer, where all new measurements are now systematically saved. The statistical analysis of the first 10000 files has served as a framework for establishing five levels of discharges severity. This paper presents the results of this statistical analysis which proposes a two dimensional classification of the discharge severity. It also attempts to compare the statistical distribution of the maximum discharge apparent amplitudes with what can be found in the industry. Finally, a description is given to show how the evolution of partial discharges over the years is used in a more general diagnostic process

Root cause analysis of generator failures

The failure rate in one of Hydro-Quebecs power plants is about three times higher than in the rest of the fleet. Over the years a total of 10 failures occurred for the six units of this plant. Spare bars were put in after every failure and the units returned to service. Apart from a few incidents, the failures were almost exclusively on bottom bars. Failed bars were retrieved and cross-section slices were obtained from them for inspection. The same was done with one un-failed bar removed from service and with a spare bar. Microscopic observations revealed differences between top and bottom bars with regard to curvature of the copper strand radii, the quality of the conductor stacking, the thickness of the groundwall insulation, the type of transposition filler material and the void content. The paper analyses the differences and their impact on the electrical field calculation in the groundwall insulation and the relative contribution of each factor to the high failure rate.

Electromagnetic modelling of existing large hydro generator

The electromagnetic modelling of existing large hydro generator is the main focus of this paper. Electromagnetic commercial software was used where all geometrical parameters of the generator are taking into account such as: pole face geometry, air gap size, the slots geometry and pattern winding sequence. The approach used to model the machine and some simulation results are presented in this paper. The analysis of a 122.6 MVA, 60 poles, 13.8 kV generator is presented as a case study on how advanced electromagnetic numerical field simulations are employed to evaluate generator parameters and performances.