By Irfan Akhtar, MSEE, BSEE, MISA, MIEEE
Part I
For process industries including chemical, petrochemical, fertilizers, oil & gas plants, reliable delivery of electricity to run the various pumps, compressors, fans, & critical machinery is all the very important. A loss of supply is simply not a trip of the plant where you can restart and you lose just for the time plant remained offline. In certain industries, material in the pipelines and vessels is not just lost but its removal and restart of the plant implies more time and cost.
Therefore, availability, reliability and dependability of electric distribution system is vital and maintenance department has to be proactive. Simple dependence on corrective or preventive maintenance would not work. In this blog, i would like to draw your attention towards condition based maintenance techniques for individual electrical assets of the plant.
Generator:
Generally a plant will have a power supply: either through its own power generation or from the grid through a substation. However, generators will also be part of the supply for most medium to large sized process plants as standby power sources.
Generator is the most neglected part of the power supply network. I have deliberately started with generator and included it in power distribution system in a plant. For Low voltage small generators, it is important to keep the filters and hence the windings clean. Alternator winding should be kept clean and dry. If machine remains off for more than 4 or 5 days, perform insulation resistance test at 500 V before starting again. If you have got RTDs installed, then make sure that the winding temperature is around 2 to 3 C more than ambient to avoid condensation of moisture on winding that may cause flashover. When machines is off, winding heaters should be turned on.
If there is excessive dust on the winding along with oil seepage from bearings forming a think slag on the winding, windings should be cleaned. For ordinary contamination, low pressure instrument air can be utilized along with a nylon brush. For removing oil, apply a solvent which should not damage the insulation. Isopropyle alcohol known as propanol is a good choice. If contamination is too much, winding should be washed either with distilled water or dry ice blasting can also be employed. Advantage of dry ice blast cleaning method is that the dry ice will evaporate by itself. Reference d mentioned below is an excellent resource for inspection checklists and forms whereas reference b gives account of different types of winding insulation system being used by different manufacturers.
References:
a. Handbook of Large Turbo-Generator Operation and Maintenance (IEEE Press Series on Power Engineering)
b. Electrical Insulation for Rotating Machines: Design, Evaluation, Aging, Testing, and Repair (IEEE Press Series on Power Engineering)c.Condition Monitoring of Rotating Electrical Machines (IET Power and Energy)
d. Inspection of Large Synchronous Machines - Checklists, Failure Identification, and Troubleshooting (IEEE Power Engineering Series)
There is of course no substitute to monitor parameters of the running equipment. For generators, keep an eye on temperatures of the windings, cooler inlet, current, load, field current, vibrations. The trends over a longer period of time showing seasonal, load changes can give useful information. During major outages, physical inspection should be carried out with utmost care before any kind of cleaning is carried out. Look for signs of dust, fretting, partial discharge, overheating, puffiness of coils, braces, ties, blocked cooling ducts, end wedges, wedge fillers. It is also good to carry out boroscopy analysis of stator core and rotor if rotor is not removed.
For medium voltage rated alternators, a complete testing regime with online monitoring of key parameters is the order of the day. Though at the minimum, manufacturer's recommendations are to be followed, but following condition assessment tests should be performed every 3 to 5 years with one set o reading taken just after installation to form a baseline:
1. Insulation Resistance / Polarization Index - For routine diagnostics, 1 KV insulation resistance test set can be utilized. Depending on voltage 5 KV can also be utilized. PI values greater than 2.0 are considered excellent, 1.5 - 2.0 considered good, 1.25 - 1.5 fair and below 1.25 not acceptable. It is pertinent to mention that performing this test tells you only about cleanliness & dryness of the windings. Many people think that 5 or 10 KV IR test set can be used to find insulation faults but remember IR test set has very low leakage current capability and hence cannot be used a substitute for DC Hipot or DC ramp test. Note: I am not endorsing DC Hipot here. IEEE std 43 gives a detailed account of insulation resistance testing. IEEE std 4, std 1415 and std 95 also cover the subject in depth.
2. Power Factor - Power factor test is also known as C&DF, tan delta or just dissipation factor. An AC voltage is applied across winding and ground or other winding, leakage current is measured. Cosine of the angle between applied voltage and measure leakage current is called Power Factor. It gives useful information on the overall health of the winding. Major defects as against minor can be detected more easily. This test is part of quality assurance during manufacturing as well as important condition assessment test. Test is performed from 20% of rated voltage upto 120% of the rated voltage. Difference of these readings and also successive (at 20% intervals) is known as tip up. Tip up of more than 1% is considered bad for winding insulation. A starting value (at 20% of Uo) of less than 0.5% is good. This value greater than 0.5% shows issues of quality of manufacturing and winding curing process initially. During the application of AC voltage Partial Discharge also take place after PD inception voltage (PDIV) and it is prominent in the graph of power factor plotted against voltage esp when machine has PD. Graph generally rises after the onset of PD. If slope is too steep then it shows problems and definitely will show higher tip up values. Another parameter for which tip up is measured is the capacitance of the winding insulation. More capacitance tipup shows higher voids or cavities in the insulation. A detailed account, procedures and limits of this test can be found in IEC 60034 and IEEE 286 standards.
To be continued...
