Duc Ngoc Nguyen

The use of time domain spectroscopy as a diagnostic tool for rotating machine windings

In order to assess stator winding insulation condition, a field instrument measuring the charge and the discharge current flowing through rotating machine stator winding groundwall insulation was developed at Hydro-Quebec. This instrument applies a constant DC-voltage of 1 kV during a certain time, usually 2000 s. Immediately following the charging, the stator winding is short-circuited during another period of time. The current is continuously recorded as a function of time during both steps. It is suggested that the magnitude and the shape of both curves may be indicative of the degree of aging of the insulating material. A number of measurements obtained either from generator windings at Hydro-Quebec, or from laboratory aged specimens were examined; spare bars or coils were also measured and used as reference. Measurements were conducted essentially on asphalt and epoxy bonded insulating material which are the two major stator groundwall insulating material for the 350 or so generators at Hydro-Quebec. The method was found particularly useful to detect delamination or leakage current due to moisture absorption or end-winding contamination.

Investigation and diagnosis of a 184-MVA air-cooled generator heavily affected by slot partial discharge activity

For many years, partial discharge (PD) activity has been used in air-cooled generators as one of the important indicators of degradation mechanisms in stator winding insulation. Up to a certain level, these degradation processes are slow. Under normal operation, PD measurement and trending can generally indicate that not only electrical but also mechanical, thermal or ambient (contamination) degradation processes are occurring. Partial discharge activity in air-cooled generators also produces ozone so that the presence of ozone inside and around the generator enclosure is indicative of PD activity. In some cases, the ozone concentration produced by sustained PD activity can reach such levels that it becomes a health hazard for plant personnel. Hazardous levels were recorded in 1991, after 12 years of operation, inside the enclosure of the four 184-MVA hydro generators at BC hydros peace canyon (PCN) generating station, revealing a situation that was probably active for some years. Results from annual PD measurement performed using PDA (partial discharge analysis) techniques showed an asymmetry in favor of positive discharges that can be the result of slot discharges. Phase resolved partial discharge (PRPD) measurements combined with ozone measurements taken all around the rear perimeter of the generator core were used to confirm the nature of the PDs. Since one of the generators was scheduled for a complete stator winding and core replacement in 2006, an exhaustive investigation was performed before and after replacing the stator. Some stator bars were removed from the slots for visual inspection and extensive laboratory testing. This paper presents results of the on-line PD and ozone measurements, together with findings from visual inspection and laboratory investigations of the retrieved bars. These results were analyzed collectively in order to establish the root cause of the insulation problem as well as the deterioration mechanism of the generator stator winding. Finally, a new pre-installation testing procedure is recommended to prevent such a costly insulation problem from occurring again.

Investigation on the low frequency dielectric response of ground-wall insulation of rotating machine windings

In order to assess the quality of stator winding insulation, laboratory and field instruments measuring the charge and discharge currents flowing through rotating machine ground-wall winding insulation were developed. These instruments apply a dc voltage step during a variable charging time at various voltage levels followed by a discharge step during which the insulating dielectric is shorted to ground. The current is continuously monitored during both the charge and the discharge steps. An investigation was conducted for epoxy-mica insulation systems in laboratory as well as on complete windings in the field.

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.

Measurements of polarization/depolarization currents for modern epoxy-mica bars in different conditions

DC polarization and depolarization current (PDC) measurements are commonly used in the field in order to assess the quality of stator winding insulating systems of large synchronous rotating machines. Results from the measurement of polarization/depolarization currents on modern epoxy mica bars in various conditions are presented in this paper. The measurements were conducted for DC voltages ranging from 1 to 20 kV. Unaged epoxy-mica spare bar with either iron oxide or silicon carbide stress grading coatings were compared in laboratory with bars of similar technology after aging and with bars with incomplete curing. The use of a two active electrodes set up with a guarded electrode allowed separating the contribution of the ground wall insulation to the measured current from the contribution of the end winding portions of the bars.

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

Low-frequency dielectric response of asphalt bonded insulation

Low frequency dielectric response measurements were performed on asphalt-mica stator insulation of vertical axis hydro-generators. These measurements, used as a diagnostic test, were conducted using a high voltage dc source and measuring the charge and discharge currents after the application of voltage steps of various magnitudes. The same dielectric tests were also performed on individual coils removed from the winging at different location, i.e. near the neutral end, the middle of the phase and the line end. Both the level of the dielectric loss and the non-linearity factor were found to correlate well with the aging conditions of the coils, which were related to their position in the winding.

Case study - high ozone concentration in hydro generators

Ozone emission in hydro generators is a sign of surface partial discharge (PD) in the stator windings. Surface PD activity can take different forms: slot PD, corona at the junction of the slot semiconductive coating with the voltage grading coating outside the slot, PD between adjacent coils in the end-winding, PD between adjacent connections and surface tracking. An ozone concentration above the daily exposure limit, for which there is a health hazard for workers, was found at several locations in a 6-hydro generator plant. The ozone level in the vicinity of the generators was in the same range for all of them. This case study presents how this problem was identified and addressed.

Innovative web system for condition-based maintenance of generators

A novel integrated diagnostic system has been developed at IREQ, the research institute of Hydro-Québec, over the last eight years. The goal of this project was to build an integrated methodology for generator diagnostics using the results from on-site measurement tools, which would help the utility to make the transition from time-based maintenance to condition-based maintenance. The system makes use of information technology (Internet) to provide a new, modern and efficient way to produce a continuous classification of the condition of all generators of a fleet along with individual diagnostics for any unit at any time. The system computes actual measurements provided by plant personnel who transfer data files to a centralized server. The application calculates simple condition indexes from each of six on-line/off-line diagnostic tools and from visual inspections, and aggregates the results into a comprehensive global diagnostic for the selected generator. Plant and generator selection is done via a user-friendly interface displaying a simple rating of the results for every tool. The algorithm underlying the system generates a global diagnostic for any combination of tools, regardless of their number and selection. However, the level of confidence of the diagnostic will increase with the number of tools used for the diagnostic. In addition to the simplified integrated condition index values of all generators and the individual index for each tool, specialists can access and display the complete data for every measurement series. Each tool, some commercial, others developed at Hydro-Québec, was selected based on its ability to characterize specific complementary aspects of the generator. Since the system was developed with an expandable modular approach, it will be possible in the future to add new diagnostic tools, not yet considered in the current version, without affecting the logic of the system. Hydro-Québec has recently implemented this new system and is already noting improvement in maintenance practices [1, 2]. The ready availability of centralized, simplified information makes it possible for generator specialists and managers alike to assess the condition of any generator in a few minutes. Thanks to this new diagnostic system, technical and management staff can now work together with common information and in real time to optimize maintenance intervention on generators showing degradation. Thus, it is possible to plan any corrective action more effectively or request additional testing when doubts remain about active degradation mechanisms. At the same time, efforts in diagnosis and maintenance could be optimized by reducing the number of measurement campaigns for the vast majority of generators that are in good condition as revealed by their condition indexes.

A case study of condition-based maintenance of a 202-MVA hydro-generator

Over the last few decades, partial discharge (PD) measurement has proven to be one of the most useful tools to identify degradation mechanisms of stator windings in air-cooled generators. Under normal operating conditions, PD levels and, more importantly, PD trending can indicate the presence of not only electrical but also mechanical, thermal or ambient (contamination) degradation processes. Hydro-Quebec uses a 2D PDA (partial discharge analysis) technique as its first line of diagnostics during yearly measurements on more than 120 large generators in its fleet. When a PDA measurement reveals a problem, such as high PD levels or sudden increase in level, the Phase Resolved Partial Discharge (PRPD) technique is used to improve the identification of the nature of the active PD sources. This paper presents results of on-line PD measurements from both the PDA and the PRPD techniques on the case study of a 13.8-kV 202-MVA generator. Identification of slot discharge activity in the PRPD patterns in this generator triggered a more detailed diagnostic. In-situ off-line measurement of the electrical contact between individual bars and stator core was performed on selected bars in order to confirm the presence and extent of a semi-conductive coating problem.