ROTOR SHORTED TURN MONITOR
The rotor windings of large utility generators may develop inter- turn short circuits, which can increase in severity over a period of time. Rotor shorts produce damage to the mainwall and inter-turn insulation at the site of the short due to overheating, and thermal unbalance due to unsymmetrical heating around the rotor, leading to increased vibration levels which may be difficult to distinguish from potentially disastrous mechanical problems such as crack devlopment. Unbalanced magnetic pull may also add to shaft line lateral vibration, and the imbalance in field endwinding currents causes axial shaft flux and a risk of potentially hazardous shaft currents. All these effects increase the risk of a major, unplanned outage and costly repair to the operator.
Generators can be run with shorts, but it is highly desirable to monitor their development in order to provide continued assurance that no rapid deterioration is taking place and and so reduce the possibility of an unscheduled outage. On-line monitoring also provides the data for cost-effective predictive maintenance strategies to be implemented.
The Generator Rotor Shorted-Turn Monitor developed by Siemens Power Generation Ltd. provides a trouble-free automatic method of detecting the effective shorted-turns in each winding coil whilst the machine is on load and can be applied to any normal design of turbo-generator. The results are continuously and clearly displayed and there is no need for expert interpretation. Experience shows that the load conditions which cause rapid increase in shorts can be detected and subsequently avoided by an appropriate restriction to reactive power loading whilst maintaining rated MW output.
THE DETECTION METHOD
The monitor operates from a single stationary search coil located in the generator airgap, using the voltage generated by the rotor flux pattern during one revolution.
Powerful signal processing techniques are used to remove unwanted effects from the waveform before it is analysed mathematically to determine the effective leakage current in every individual slot. The monitor does not simply compare peak waveform values, as some techniques have done in the past. Any change is automatically re-checked before being reported, to eliminate spurious indications or alarms.
The principle of the shorted turn monitor has been the subject of several years intensive development, making it one of the most advanced systems available world-wide.
Using a PC-based system the operation has been validated against over 200 recorded search-coil waveforms taken from over 30 machines. Extensive pre-production field trials have also been conducted in collaboration with the British National Power Company at one of their power stations.
- Monitoring capability: Up to four generators, 2 or 4 pole, 50 or 60Hz.
- Monitoring resolution: Better than 2% of total slot current at full and part loads.
- Display functions:
- Present shorts:
- Archive of shorts:
- Rotor mimic diagram:
- Search coil waveform plot:
- Store a specimen waveform:
- Compare current and specimen waveforms.
- Search coil input: Analog voltage up to + 20V. (Details available on request)
- Shaft marker input: Analog voltage pulse; up to 20V either polarity.
- MW input: Analog current: 4-20mA.
- MVAr input: Analog current: 4-20mA.
- Alarm outputs: One set per generator.
- Alarm acknowledge inputs: One isolated contact per generator.
The shaft marker is often available from the shaft vibration measurement system, but a signal, derived from the generator terminal voltage may be used instead.The alarm o/p and acknowledge signals are optional; alarms may be acknowledged at the PC. After scaling/conversion in the terminal connection box (TCB), signals are passed to the PC cards via ribbon cables.
An industrial grade PC, the software, the TCB and PC cards are supplied, with software data files configured to the target generators and search coil(s).
A detailed Reference Manual provides instructions to allow the equipment to be connected and set to work by the customer.
The Reference Manual also provides all the information needed to allow changes to the software data configuration files, as might be required for a change of the rotor from one design to another.