Monitoring the condition of a generator in real-time is just the beginning. Using the data in modern networking environments allows operators to observe trends and anticipate mechanical problems, as J S Edmonds and T H Richards explain
Chelan County’s Public Utility District No 1 has recently been installing the first of 11 hydrogenerator rotor-mounted HydroScan™ scanners, at its Rocky Reach hydro project. The utility had already introduced computerised maintenance management systems (CMMSs) at its facilities — completed at the Rock Island hydro project on 31 March 1998, and four weeks later at Rocky Reach. It is important for these systems to work in concert: when they do the condition monitoring systems will provide the CMMS with a huge range of data and with analysis, that will give the CMMS a much more powerful preventive maintenance capability.
The original version of HydroScan is operated as a stand-alone, continuous monitoring system, whose alarm functions can be connected to the plant’s local alarm system to alert the operator to impending problems and a Widows-based application to diagnose the cause of the alarm. A utility’s generator specialist uses the scanner to perform periodic inspections while the generator is on line and operating at normal stress levels. This information is accessible either at the local workstation in the plant or remotely via modem. Advances in the HydroScan software and the installation of a networked supervisory control and data acquisition system (SCADA) at Rocky Reach prompted Chelan County to integrate information from the scanner into the SCADA system.
The term ‘networked’ covers many solutions. At Rocky Reach, MCM Enterprise decided to base the system on an Ethernet platform using TCP/IP. Ideally, common messaging and data structures should also be defined, but the industry has not determined a standard (MMS, UCA, etc.) for these functions. This first system will therefore use an off-the-shelf communications standard and the HydroScan-based data structures. Both of these elements have been structured to allow conversion to the industry standard when one is adopted.
The scanner’s entry point onto the network is the scanner sensor interface (SSI) via a fibre optic link. The SSI attributes are:
•Generator interface (almost all the Hydroscan components are inside the generator, on the rotor or stator).
•Standard operating system.
•MCM custom software.
•Debug interfaces via Telnet, Serial Comm and Counsole.
•FTP for upload/download.
The archive database serves as a repository for the HydroScan sensor data. It uses an off-the-shelf embedded database engine with a custom interface. In the first application of this system, archived data will be used by scanner software to present analytical results to the SCADA system or make them available on the network for use by the generator specialist.
The MCM components can be placed anywhere on the utility’s network, and new data-processing functions can be written and placed anywhere on the network as needed.
The scanner will be integrated into the Rocky Reach system using an Intellution 6.15 SCADA system. Because of its diagnostic importance, the scanner’s analytic and diagnostic architecture will retain the features contained in the WinScan workstation software as a separate (but network-compatible) application. The scalar ‘trend’ data values will be provided to the Intellution system. In the short term, this information is most useful for on-line management of the generator; for long-term commitments and scheduling, it is imperative to have the results of long term trending.
The following values are provided to the Intellution system:
•Maximum stator temperature (value, slot, and axial location).
•Average stator temperature.
•Standard deviation of stator temperature.
•Minimum air gap 130mm from the top of the stator (value and tooth).
•Minimum air gap 130mm from the bottom of the stator (value and tooth).
•Alarms (high temperature, minimum air gap, and vibration).
•Generator voltage, stator current, and real and reactive generator load are provided elsewhere from unit transducers.
WinScan contains a number of useful Windows controls, including: generator thermal map; partial discharge analysis, trending and display; and polar air gap. To integrate these displays into the SCADA screens will be possible using Intellution FIX Dynamics (the next generation of Intellution SCADA). A thermal map ActiveX control, providing both visual thermal map and pattern recognition services, will then be available in a FIX screen or other ActiveX container-applications, such as MS Word, Excel, etc. The thermal map ActiveX control would obtain its own data from the HydroScan system using the MCM protocol to communicate with the SSI or MCM Archive Database modules.
Plant operators are not expected to be generator experts: the information is usually presented in trend format. For example, an abnormal condition such as an unexplained, steadily increasing generator hot-spot temperature would alert the operator to intervene and take action, while the generator specialist would access the HydroScan on-line information from the generator sensors and the stored generator data in the archive database.
In the diagram, for example, the information in the trend display is insufficient to determine the generator problem. The generator specialist would access additional HydroScan information through WinScan. The unit history, and earlier analysis of the generator data, confirmed that the stator core structure had been detached from the generator frame. This combined information allowed the utility to judiciously operate the generator for a short time until a sufficiently long outage could be scheduled to effect both a turbine runner replacement and to make long-term maintenance repairs to the stator core and reposition the generator on the foundation. All of this information confirmed the root cause of the problem — concrete migration caused by the use of alkali aggregate when the plant was originally constructed.
|The rotor-mounted scanner|
| HydroScan is a permanently installed diagnostic monitoring system that monitors the generator continuously by scanning the generator stator at each revolution. The system uses six types of scanning sensor mounted on the generator to monitor its electrical, mechanical and thermal characteristics. The sensors are on a support structure which extends the full length of the rotor and is mounted on the rotor rim between two field poles. The sensors are: •Infrared thermal sensors, scanning the full length of the stator in 13cm increments.
•Radio frequency interference noise sensors, mounted on a positive pole and a negative pole centreline to detect RFI sources when voltage stress on each coil is at maximum.
•Hall effect magnetic sensors to detect magnetic anomalies in the winding and core.
•Air gap sensors around 20cm from each core end.
•Acoustic sensors to detect the noise from loose components inside the stator.
•Vibration sensors in case any of the rotor components is loose.
For full details of the HydroScan and its application at Boundary plant in Washington, US, see IWP&DC January 1993, pp37-40 and IWP&DC May 1992, pp41-45.