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Doble's MachineGuard PD Monitoring system is an example of a state-of-the-art monitoring system that can manage large amounts of instrumentation data. This has been one of the primary reasons that some online systems are periodic measurements and not truly continuous. The large volume of data created by continuous systems can be problematic if not properly managed.
The software system was designed to decide if an alarm level has been reached and trend the PD charge level. Based on these results, the decision can be made if a deeper analysis or corrective action is warranted. The MachineGuard PD-Analysis software consists of the 5 parts:
- Measurement acquisition
- Job scheduling
- Data storage
- Settings and display
- Remote data viewer
The PD-Guard instrument is the central hub for the MachineGuard system. For a scaled multi-generator site, each machine has its own PD-Guard. It is automatically started after the PD-Guard is switched on.
The client software establishes a connection to the MySQL database and polls regularly
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The MySQL database is the central part of the MachineGuard software system. Measurement tasks, PD-Guard state information, result data and MachineGuard parameters are stored in the database. The database is divided into several tables and each table contains different rows with data fields, e.g. to store measurement results or status information.
The timer is continuously running on the server and schedules the jobs (measurement tasks) for each connected PDGuard. The user may specify the timer cycle time, which is equal to the time interval between two measurement jobs. After a measurement job is finished, the timer does several calculations on the PD raw data and updates the alarm state of each device and channel.
The Frontend software's primary responsibility is to display measurement results. The Frontend software is a user-friendly visualization tool to set MachineGuard parameters, start and stop the timer and display the various PD diagrams as: PD repetition rate vs. charge, trend chart, pulse view and phase-resolved PD pattern. In the main window, the user gets an overview of the alarm status of all connected PD-Guards and detailed alarm information for the currently selected device. The PD-Analysis Software allows users to get information from normal operation and auto-alarming, to enquiry (trending and PD patterns analysis). The DCOM interface provides standardized functions to the Frontend software to control the PD-Guard. Examples of DCOM functions include reading and writing MachineGuard parameters, start and stop the timer and reading PD measurement results.
Noise suppression
Partial discharge measurements are often influenced by interference caused by disturbances. Despite the higher PD level on the windings of rotating machines, in many cases, noise suppression in on-line measurements is essential in power plants to be able to judge the PD behavior of generators. That is why noise suppression is very important in PD monitoring systems for generators. The dominating noise signals in on-line generator PD monitoring are largely caused by converters in the auxiliary power supply.
The disturbances fall into two categories:
- Disturbance that occurs if the test circuit is not energized. This might be caused by switching operations in other circuits, exciting the circuits of commutating machines, radio transmissions, external corona, etc.
- Disturbances that only occur if the test circuit is energized, but don't occur in the test object. These kinds of disturbances occur with increasing test voltage. For instance, they might be caused by sparking of imperfectly grounded objects in the vicinity, by loose connections in the area of the high voltage, e.g. by spark discharges between screens and other high voltage conductors, connected with the screen only for testing purposes.
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