M. Levesque

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.

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.

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.

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.

Effect of surface conditions on the electric field in air cavities

A numerical PD model was developed to obtain more insight into the understanding of the evolution of slot Partial Discharge (PD) activity. This model, in which multiple PD channels were simultaneously active, was used to evaluate the influence of the surface condition on the electric field distribution inside an air cavity typical of the one where slot PDs occur. It was observed that the insulation degradation induced by slot PD activity increases its surface conductivity, which in turn modifies the dynamic of the surface charges deposited by previous PDs and directly affects the electric field and the slot PD behavior itself. The experimental results of surface conductivity measurements and observations of physical changes in surface conditions obtained from previous laboratory experiments were used to feed the PD model. This paper reports the result of the electric field calculation in the presence of PD activity under different surface conditions.

Determination of stator temperature profile using distributed sensing

The aging of generators is intimately associated with the hotspot temperature of the stator windings groundwall insulation. Common resistive thermal detectors (RTD) installed in the machine are well suited to monitor temperature changes and trends during normal operation, as long as temperature distribution is uniform, but are blind to some hotspots. Moreover, the relationship between the RTD temperature and the one of the copper conductor depends on the operating condition, the machine design and its cooling system so a simple rule of ten degree of difference is not always valid. When generators are operated close to their limit, it is essential to determine as precisely as possible the exact hotspot temperature to prevent any life reduction of the equipment. To optimize generation without any life reduction, Hydro-Quebec Research institute IREQ has been developing over the past decade a generator model that includes finite element modeling of electromagnetism, thermal, mechanical and fluid dynamics of the cooling air. This model relies on experimental data from the field to validate calculation. One of the main sensors use to validate thermal simulation is an optic fiber installed in the stator cores vent ducts. This sensor is use for distributed temperature sensing (DTS) measurement. This paper presents the comparison of the thermal model calculations, the DTS measurements and other sensors used to measure temperature in five case studies on actual generators.

Measurements of slot partial discharges with an antenna during accelerated aging

A long-term aging experiment is currently under way on six stator bars subjected to slot partial discharges (PD) activity under electrical, thermal and mechanical stresses. Slot PD were induced on each bar by abrading a portion of their semi-conductive coating. All bars were subjected to accelerated aging at twice their nominal voltage. Two bars were only electrically stressed, two others were electrically and thermally stressed and the last two were electrically, thermally and mechanically stressed. It was noted that the evolution of slot PD was different for each kind of stress. In order to understand those differences, periodic measurements as a function of the aging time were conducted using a directional antenna to detect the electromagnetic radiated signal from each specific slot PD site. The results show the evolution of slot PD activity for the different stress conditions.

On-line rotor temperature measurements

On-line temperature measurements on hydrogenerators are usually limited to those read by resistive thermal detectors (RTD) on stators and global temperature of the cooling air and water at the input and output of the heat exchangers. Data shows that the proportion of generators where the temperature rise on the rotor is larger than on the stator is non negligible. With such units, temperature monitoring of the rotor should be considered. This paper presents the characterization of an infra-red thermal sensor installed on the stator side to measure the rotor. A single sensor can therefore scan every pole passing in front of it. Preliminary measurements on generators owned by Hydro-Québec (HQ) and Électricité de France (EDF) are presented along with details on the characterization at HQs research laboratories (IREQ) in a controlled environment.

Development of a generator prognostic tool

In the past decades, significant improvements in generator diagnostics were made possible by using continuous online measurements and a number of periodic tests. In recent years, the data provided has been converted into more useful information thanks to integrated diagnostic systems. For example, an integrated methodology for generator diagnostics (MIDA) was developed at Hydro-Québecs research institute (IREQ) using a Web-based application. This comprehensive diagnostic system gives the degradation state of generator stator winding insulation by using a portfolio of diagnostic tools. Combining the various results leads to a health index ranging from 1 (good condition) to 5 (worst condition). This system is used by power plant managers as well as technical support and maintenance engineers at Hydro-Québec in the context of condition-based maintenance (CBM). The next step of development is to add new prognostic-related characteristics. This involves automatic identification of active failure mechanisms, root cause analysis and estimation of the stage of advancement of any active mechanism. These characteristics form the basis of predictive maintenance and support the optimization of maintenance strategies. The approach chosen is based on a number of cause- and-effect chains formed by the combination of sequential physical degradation states that ultimately lead to failure. Each combination of physical states is unique and defines a particular failure mechanism. Failure mechanism analysis was followed by identification of all observable symptoms (diagnostics from MIDA) for each physical state. This paper presents a first step toward the development of a prognostic tool, where the modeling of failure mechanisms is combined with automatic analysis of observable symptoms from our diagnostic system. It puts forward probable failure mechanisms for a given generator.

Insulation degradation analysis of stator bars subjected to slot partial discharges

As part of an extensive study on slot Partial Discharge (PD) activity, a long-term laboratory aging test has been conducted on six stator bars subjected to slot PDs. Two bars were electrically stressed, two others were electrically and thermally stressed, and the last two were electrically, thermally and mechanically stressed. The aging experiment lasted 25,000 hours, and all the bars were periodically characterized using PD measurements, dielectric response measurements, surface conductivity measurements and visual inspections. Following the aging test, microscopic observations were performed on cross-section slices taken from the slot PD sites on each of the six bars. Results confirmed that slot PD activity under a combination of electrical, thermal and mechanical stresses was the worst condition since all six slot PD sites of this group failed. This paper presents the evolution of the dielectric response measurements for every bar with respect to the stress condition and the difference in the groundwall insulation degradation.