By P. Mongenot and J. Lavallée
Electric utilities are always looking for ways to reduce the number of generating-unit shutdowns and prevent major equipment failure through improved monitoring and maintenance. They are also trying to reduce maintenance costs by extending the interval of time between scheduled power interruptions and reducing the duration of the interruptions. One solution would be to implement continuous monitoring and make diagnostic testing a part of scheduled inspection procedures.
One of the main causes of generating unit shutdown is stator winding failure due to the presence of vibrations in the stator coils. Vibration occurs as a result of wedges being incorrectly mounted or through a loosening of the springs behind the wedges during operation.
One of the inspection methods currently used is the "tap-test," which simply involves hitting the wedge with a hammer and assessing wedge tightness based on the sound that is produced. An alternate method consists in inserting filler gauges behind the wedges through the ventilation holes in order to assess spring deflection. These simple methods lack accuracy, however, and require the removal of several poles, or even the entire rotor in order to access the stator. None of the wedging inspection systems currently available have proven adequate.
An analysis of existing systems was done as a first step toward the development of an automated wedging verification system. Several potentially usable principles were then assessed, such as the spectral analysis of the acoustic response, wedge acceleration or displacement following mechanical excitation and analysis of the strength-displacement curve. The latter was found to be the most promising solution, and a first prototype was designed and successfully tested.
The second step involved making fine adjustments to the prototype and automating the measuring process. The third step consisted in developing an automated positioning system and optimizing the measuring system.
This document presents the automated wedging verification system currently being developed at Hydro-Québec. This diagnostic system (off-line inspection), which does not require that the poles or rotor be removed, aims to increase the reliability of inspections and decrease inspection time, while allowing the stator visual inspection. This document presents some background information on wedging, describes the new system and lists the main objectives of the project.
Background
Spring-based wedging system
The wedging system is designed to maintain the stator coils firmly in their slots while preventing any vibration during normal operation. Vibration often leads to coil insulation wear, which may result in electrical failure.
The springs are thus compressed to a certain extent at the time of installation so that an adequate wedging force is obtained. The initial force includes a safety factor related to the settlement of the bars in their slots and to the aging of the spring. Figure 1 presents a horizontal cross section of the stator bar wedging.
Inspection methods in use
The two methods currently used at Hydro-Québec are the "tap-test" method and the Òfiller gaugeÓ method.
Tap-test Method: This method consists of hitting the wedge with a hammer and assessing wedge tightness based on the sound produced. A dry sound means that the wedge is tight. A vibrating sound, however, indicates poor wedging. This method, while simple, has two disadvantages:
It was noted that there is a tendency with this method to overestimate the number of loose wedges when most of them are tight, and to underestimate the number of loose wedges when most of them are loose to begin with.
Filler-Gauge Method: In this meth-od, a filler gauge is inserted behind the wedge at the spring level to determine the spring compression. However, this method also presents several disadvantages:
New System
Design Objectives The design objectives of the new system are as follows:
The new system thus has the following capabilities:
Description of system
The automated wedging verification system currently under development at the Unite Robotique consists of a thin, flat sensor capable of being inserted into the air-gap of the alternator, a computerized automatic measuring system, an automated positioning system and a micro-camera which allows visual inspection of the stator bar. The computerized measuring system comes with a portable case and includes a computer, screen and various electronic control and acquisition modules.
The positioning system is mounted on the rotor and allows the sensor to be inserted and positioned in the alternator air-gap. Figure 2 shows the various system components and their interaction.
Sensor
The sensor was designed for insertion into the air-gap. It may be inserted into air-gaps as small as 10mm. The air-gap is accessed by removing the cover plates near the slots to be measured. The sensor was designed to allow a force to be applied to a given wedge, measure its displacement and deduce wedging force. Data control, acquisition and processing operations are carried out automatically using software. This method has the advantage of providing an actual wedging force value independent of spring aging.
A miniature camera is used to perform a visual inspection of the stator surface area and manually position the sensor for a given wedge. Visual inspection allows several anomalies to be detected, such as red or white powder deposits and damaged wedges and to check ventilation holes. Figure 3 shows the sensor and portable console.
Control and acquisition software
The software developed for this project is used to automate measurements and control the automated positioning system. It allows the operator to enter the measurement parameters (e.g. alternator no., slot no., positioning parameters) and start up a sequence of automated measurements for a given slot through a user-friendly interface. The interface also includes a graphics display where test curves are plotted in real time, and a screen display for the various data. Software includes LED indicators that warn the operator of situations that present a potential measuring problem and functions to automatically stop testing if needed.
The software also includes different modules used for displaying the camera image on screen, calibrating the measuring systems and displaying data from the laboratory test bench. It also allows several stored curves tables of cumulative results and curve data to be displayed.
Automated positioning system
The positioning system allows the user to automatically position the sensor along a given slot. The software's positioning module is used to control the system. The sensor may be moved in manual or automatic mode. Automatic mode includes several option which allow the user to incorporate various wedging system configurations (e.g. constant or variable wedge lengths) and carry out complete measurement sequences (1 by 1), partial sequences (i.e. every second or third wedge sampled).
The method consists of:
The system executes a series of measurements requested for the given slot by recording the results of each wedge in automatically generated files. The user then moves the system over to the next slot and repeats the steps listed above by merely changing the slot number. Work planned for 1997 includes the development of an industrial prototype ad the transfer of the technology to the users. A color result visualization system is being planned for the industrial version of the system. Data will be presented on a diagram of flattened-out stator with different-coloured wedges corresponding to various wedging forces. The tool is designed to help in the interpretation of the results.
By the end of the year we plan to modify the automated wedging verification system so that it can be perform measurements without the rotor. This will allow wedging measurements to be carried out during generating-unit assembly and startup, as well as during major shutdowns (no rotor). It will thus be possible to keep an eye on the wedging starting from the assembly stage and to perform periodic follow-ups over time.
The project team also intends for 1998 to adapt the system so that it can carry out measurements on turbo-generators. A customized positioning system will be designed and the sensor will be modified as needed.
Conclusion
Given the current economic context, reliable measuring devices are needed to justify maintenance work and assist in decision-making regarding generating-unit maintenance. The costs related to wedging inspections, rewedging and wedging related electrical failure are considerable. The methods currently used to check wedging have many major disadvantages in terms of cost and reliability.
The system currently being developed at Hydro-Québec constitutes a reliable and low-cost means of inspecting generator wedging. Since it is fast to carry out, its potential applications are numerous. For instance, it may be used to inspect wedging during generating unit assembly and startup, full or partial inspections, and spot checks. The system is also a useful tool for assessing and analyzing various spring wedging systems and for long-term forecasting of rewedging needs. A patent has already been filed for the system, which presents considerable commercial potential.
Patrick Mongenot and Jean Lavallée are with the IREQ's Unité Robotique. This paper was originally presented at the Canadian Electricity Association's Electricity '97 held in Vancouver, April 21-24. ET