A new optical three-dimensional coordinate measuring system can be used to provide real-time monitoring of generator condition, including air gap, stator circularity, rotor rim position, vibration and more.
By Mohammad S. Eslamipour
Vibration monitoring systems have been used for more than three quarters of a century to analyze the operating condition of hydro generators. This type of monitoring allows for the measurement and analysis of air gap, bearing condition, temperature and more. Figure 1, on page 24, shows the components of a hydro generator.
Vibration monitoring can be essential to protect the generator against sudden failure. Faults can develop quickly, making it necessary to have a system in place that continuously checks various operational parameters, indicates the onset of a fault and provides the possibility of automatic shutdown.
In this environment, the application of recent technology advances can help contribute to more effective monitoring of generators. For example, Mapna Generator has developed the 3D Scanner Camera Monitoring System, an optical three-dimensional (3D) coordinate measuring system to help reduce maintenance and repair costs at hydro plants. With this system, developed in 2010, measurements regarding the status of various pieces of plant equipment are taken using a high-resolution digital camera and associated hardware and software. The camera scans objects with 0.01 tolerance, then transfers these images to a computer.
The software automatically compares the submitted image with stored as-built drawings of various pieces of equipment, then automatically generates a report. A precise model of the equipment being monitored, such as a bearing, can be computed using such parameters as ray intersection, camera position, lens distortion and object coordinates. Ultimately, the information gathered can be used to estimate the life cycle of various components and maintenance intervals for certain pieces of equipment.
Understanding the system
The information in this section shows how the system can be used to monitor equipment in a surface powerhouse that contains four 250 MW turbine-generator units. Each of these vertical Francis turbines weighs about 700 tons and rotates at 187.5 rpm. The generators weigh about 1,050 tons each. The powerhouse generates 2,000 GWh of electricity annually, which is transmitted by 13 single phase step-up transformers (three per unit, with one spare) operating at 105 MVA and 50 Hz.
Because of transportation limitations, the stator frame was designed in five sections and assembled on site into one solid continuous frame. The inner diameter of the 60 ton frame is 10 meters and the outer diameter is 11 meters. The frame is composed of five horizontal rings, vertical columns, vertical ribs and a periphery wall. Over the entire length, the vertical columns are reinforced by a rectangular plate to form a rigid body structure. The stator core, with pressing finger plates and windings, weighs about 400 tons and lies on the lowest of the five horizontal rings.
Despite the various forces and deflections acting on the stator frame, the generator must meet specifications for centering and circularity. Lack of circularity in the stator frame can cause damage to the winding insulation, among other issues. One method to detect such problems before serious damage occurs consists of positioning the 3D camera scanner next to where the sole plates sit on the nuts to monitor movement of the stator.
The stator frame and lower annular extension ring rest on the foundation via base plates, fastened by radial pins to sole plates embedded in the secondary concrete. These pins are needed to ensure centering of the frame and to absorb all the tangential forces during normal operation and short-circuit conditions. This position also can be monitored by installing a system.
The entire rotor must safely withstand all stresses resulting from overload or maximum runaway speed. All parts of this rotor were designed to withstand the turbine runaway speed without the maximum stresses in any part of the rotor exceeding 70% of the yield point.
|Generator rotors must be able to withstand all stresses resulting from overload or maximum runaway speed. The rotor can be monitored for a variety of measurements, including air gap and rotor rim speed and position.|
The rotor run out must be monitored using a properly placed measuring system. Air gap is a measure of the distance between the rotor and stator. Both the stator and rotor are quite flexible, and their shape and location are affected by centrifugal, thermal and magnetic forces during operation. Off-center or out-of-round conditions will, at a minimum, reduce operating efficiency. More severe cases can lead to damage from magnetically induced heating or rubbing of the rotor and stator.
Air gap monitoring can be performed using sensors mounted on the inner bore of the stator in one or more planes. These sensors directly monitor air gap dimension and location, along with rotor shape, during unit operation. Stator shape can be calculated using multiple air gap transducers.
The rotor rim is constructed of flat steel plates stacked in an overlapping arrangement. Fixing and clamping of this lamination helps to achieve a rim that floats during runaway speed but retains its original position while returning to its rated speed. A monitor can be installed to determine its speed and position.
The upper and lower bearing brackets support the generator guide and thrust bearings. These brackets are composed of a central hub and steel plate arms assembled to the hub by bolting and welding. These components are located in the upper and lower parts of the stator frame, and their movement from the original position must be monitored.
The final parameters to be measured consist of the temperature of the air inside the generator and oil temperature.
Understanding the 3D optical monitoring method
Improvements in hydro generator monitoring help boost machine performance by achieving higher productivity and less unit downtime. By catching errors before they appear in components, high-accuracy monitoring systems can reduce maintenance time and lower on-going operational costs.
The 3D optical scanning system can be used to perform synchronized measurements of various components and analyze important parameters. Using this system, digital photogrammetric images can be combined to create a 3D digitized picture of the rotor, stator frame, lower bracket and upper bracket deflections. The software processes the acquired images, which can be taken continuously, and automatically calculates the exact position of reference markers applied to the components being monitored. A report is produced to aid in assessment of generator condition and efficiency.
The 3D scan data also can be used to determine when maintenance will be required and the proper timing for a generator overhaul.
Air gap and all the other components mentioned above can be monitored using this 3D optical scanning system.
An optical 3D coordinate measuring machine can be used to monitor various parameters of hydro generators. With this system, movement and disposition of all components can be recorded using a high-resolution digital camera. The images are automatically evaluated using included software. The resulting information allows the hydro project owner to analyze, trend, correlate, and display position and any other variable of interest.
This system has not yet been installed in a hydro generator.
Mohammad Eslamipour is project manager for Mapna Generator Co., which is a general contractor of energy projects in Iran. The company is managing manufacturing of four generators for the Upper Gotvand project.