Partial discharge and loss factor measurements and PD monitoring on generators and large motors offers the assessment of the condition of the stator winding. This helps avoiding unplanned outages as well as scheduling efficient maintenance outages.
The typical epoxy-mica insulation system of rotating machines is a ‘forgiving’, i. e. a comparably stable insulation system. Due to this electrical stability, partial discharge activity acts as an indicator for a variety of defect mechanisms. Besides the mere electrical aging, further problems, such as contamination, vibration, or mechanical aging find their expression in the phase-resolved partial discharge pattern.
Testing Laboratory:Most suppliers of rotating machines and motors apply partial discharge and loss factor measurements in-line with their quality assurance program and final acceptance tests. Measurement results can help to improve the system design or can help researches in finding suitable insulation materials.The quality of the stator winding insulation system should be verified according to the following brief description of the test procedure.The test requires an AC power source capable of supplying PD-free 50 or 60Hz variable voltage up to at least the rated line-to-ground voltage of the insulation system under test. Partial discharge measurements are taken in voltage steps up to the rated line-to-ground voltage of the insulation system to measure the discharge inception voltage (DIV) and the discharge extinction voltage (DEV). The maximum PD magnitude at line-to-ground voltage should be less than 50pC.
“IEEE Recommend Practice for Measurement of Power Factor Tip-Up of Electric Machinery Stator Coil Insulation”
“Test of insulation of bars and coils of high-voltage machines”
“Guide for a test procedure for the measurement of loss tangent of coils and bars for machine windings”
1) Advanced PD measurement system
2) Standard PD test system
3) Combined PD and TD measurement equipment
Power Diagnostix offers various instruments for continuous monitoring, schedule-based routine testing and in-depth analysis of rotating machinery. The instruments and their control software were continuously improved, based on the feedback and requirements of the end users.Permanently installed partial discharge couplers greatly simplify periodical on-line testing using the ICMsystem without any interruption or downtime. Continuous online PD monitoring with a permanently installed ICMmonitor helps optimizing maintenance intervals and reducing costs, while improving the level of equipment dependability.
Complete PD online monitoring system for motors or generators:
the installation of our PD monitoring system requires one outage of the motor or generator. Depending on the size and number of poles of the generator we recommend three or even up to 12 couplers for each unit.
Tan Delta Measurements
The loss factor or tan delta measurement allows an evaluation of the condition of the complete insulation of a stator winding. All resistive losses between HV potential and ground are measured. Typical values are in a range of 10-3 to 10-2.
The TDAcompact is a digital tanδ and power factor analyzer for off-line dielectric testing to assess the overall healthof the insulation system. Its fiber optic signal transmission simplifies connection even on grounded equipment. The unit comes with reference capacitor and can be combined with any external high voltage source.
Partial Discharge Monitoring
Based on the stand-alone ICMmonitor unit connected to an individual machine, larger networks supervising a multitude of generators in combined-cycle thermal power plants or pump storage hydro power plants have been realized. Such networks include full control of the local instrument via global Intranet access as well as visualization of the monitoring data in monitoring centers. The ICMmonitor software offers automated scanning, pattern acquisition, and analysis of the trending data, while the ICMserver software operates in the background to supervise the communication between the individual instruments and data servers.
High voltage motors are the main assets to keep compressors, cooling pumps, extruders and or large fans running in refineries, oil and gas plants, chemical and petrochemical industry. Here, unplanned outages can cause immense losses. Not only for the HV motor in particular, but, to clean pipelines with remaining product or compressor and extruders that have stalled by motor failure. Further, such motors act as auxiliary drives in thermal and nuclear power plants to run the (emergency) cooling system, for example. Here, failure of a motor stator winding can cause critical subsequent damage. Given the typical 5-yearly outages for such setups, permanent installation of capacitive couplers offers periodical online PD measurements, and, hence, the possibility for an accurate and short follow-up. For the most critical trains continuous monitoring can be installed from the early beginning.
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