As you know the trans summary is having oil for cooling as well as for insulating purpose. Whether this oil is flammable. Yes there are cases the transformer can catch fire due to various reasons like insulation failure due to partial discharge internal short circuits and continue to overload. Instead of providing electrical protection and fire protection system. So what precautions we can take for that? In addition to fire protection system for the transformer we provide oil, soak pit to arrest the spread of fire. Also, it has another purpose to contain any leakage of oil from the transformer which may have some adverse effect on environment. As a general practice, the soak pit shall be provided for larger rating transformers and 33 kV levels. Moving a little ahead there. Let us now understand few do's and don'ts that should be taken care of on the received of transformer. Tools used in opening the packing must not damage the transformer, any missing or shortage of items or accessories has to be flagged immediately. Oil level on post received and pre dispatch to be verified in case of transformer is received in oil filled state. When nitrogen filled the transformer is dispatched on received nitrogen pressure is to be verified and any leakages to be checked and rectified. Drums containing oil is to be verified properly and shortage has to be flagged immediately. Damage prone items like bushing etcetera are to be checked for any damage and rectification to be done. Post received, the next major task is to store transformer and other accessories properly at site. Storage is a critical activity since improper storage may lead to contamination of oil malfunction of transformer. Let us understand briefly the important do's related to storage of transformer. After arrival at site, it is desirable to erect and commission the transformer with minimum delay. In case the above is not possible. Transformer should be erected at permanent location with the conservator and breather fitted. The transformer should be vacuum dried. Processed oil should be filled in the tank and oil condition to be monitored during long storage. If this is also not possible, then it is preferable that tank containing core and coil be placed under shed. The tank shall preferably be filled with the processed oil to core and winding level with the dry air or nitrogen filling above the oil. Just to be monitored periodically, transformers should be kept in dry air or nitrogen filled condition for maximum period of six months including storage at work, transportation and storage at site. In case the transformer is to be stored for more than six months, it needs to be stored in oil filled conditions. Processed oil should comply with the specification and ppm should be less than or equal to 10 and BDV should be greater than 60 kV. It is recommended to test transformer oil before filling the same in the tank. In case the oil fails to meet the requirement, it should be processed. Oil is filling the main tank of the transformer by using the filtration valve furnished at the lower level of the transformer with air wind at the top kept open. After completing the feeling by verifying the oil level in the MOG, the filter machine is put into operation for removal of moisture and all other impurities. Filter machine is kept in operation until the desired oil characteristics is achieved. Further oil samples are being taken for BDV test. Break down voltage is defined as the voltage at which the oil breaks down, then subject to an AC electric field with the continuously increasing voltage contained in a specified apparatus. The BDV or breakdown value as given in IS 335 is as per the table. The new unfiltered oil has a BDV Of 30 kVrms while the filtered oil has a BDV of 60kVrms. These are the stipulated values as per highest and the higher the BDV achieved, the better the dielectric strength of the oil. Before erection of the transformer we should thoroughly study the manufacturer's recommendation for any specific points to be noted. The manufacturer's drawings are also to be clearly studied. There are some internationally acknowledged best practices that I'll be sharing with you. One, transformer shall be erected on level the foundation. Two, all components dispatched separately shall be cleaned inside and outside before being fitted. Three, temporary transportation braces provided if any to be removed without disturbing any permanent internal arrangements. Four, all erection drawing should be available at site. All tools, tackles and other equipments required for erection work may be arranged at site before work is started. Five, lifting equipments depending upon the transformer like crane of specific capacity will be required. Six, a vacuum oil filtering machine of sufficient capacity with thermostatically controlled heating and filtering facility is required. Seven, the transformer tank containing core oil assembly should be first place in the position selected for the assembly. When the rollers are already fitted, the rollers are placed on rail track, the rollers are locked so that the equipment does not move. Nine, additional supports if any for radiators are to be provided. Now that the transformer is erected. Can we energize the transformer immediately? No, we have to follow some critical steps before charging. This is important, both from a safety standpoint as well as performance standpoint. First step is pre-commissioning test. Here, we have listed some of the tests generally recommended by state utilities. Checking ratio polarity phase relationship. These are the fundamental measures that have to be ensured. Megger test, this is measures insulation for resistance. Buchholz relay test, alarm and trip to be checked by injecting air through the test, petcock. Magnetic oil level gauge, check the health of the mechanism does not stick at any point which may result in false reading. Temperature indicator, this is a condition monitoring and safety mechanism. The context of WTI that is binding temperature indicator and OTI oil temperature indicators are checked and set at required temperature. Marshalling Box, this is where the wires from various components of the transformer come and terminate. The wiring and various accesses are checked here to ensure correctness. Oil, oil samples from top and bottom of main tank are tested as per IS 1866. In addition to the pre commissioning test, the following checks also need to be performed. All oil valves are in correct position closed are opened as required. All air pockets are cleared, thermometer pockets are filled with oil. Oil is at correct level in the bushings, conservator, diverter switch and tank etcetera. Earthing connections are done the color of silica gel and oil in breather cup. Can we energize the transformer now. Yes, provided if all test checks are found satisfactory. As a first step transformer can be energized at no load after setting the protective related to minimum value possible. Any abnormality during commissioning, such as vibrations of radiator, parts, hum etcetera to be observed. After a few hours of energization of the transformer at no load the transformer shall be switched off. The Buchholz relay should be checked for collection of air or gas. Any abnormalities observed to be corrected. Now, if no abnormalities are found, we can plan the energy station of the transformer with normal load. All protective relays should be reset to appropriate values. Transformer can now be re-energized and loaded gradually. We have installed and commissioned the transformer it is all set up and functional. Very important function starts now and that is to maintain the transformer to function efficiently and maintain the life cycle of the transformer. Let us understand about the maintenance of transformer. Maintenance plays a very important role since it has impact on the life of the transformer. Maintenance consists of regular inspection and testing, followed by reconditioning wherever necessary. I'm giving you here a few maintenance recommendations for transformers of capacities for medium voltage. It is advisable to check and maintain the logbook against rated values of oil temperature, winding temperature and ambient temperature. Some maintenance steps are taken in a periodic fashion. Fortnightly, check that air passages are clear and check the color of active agents, dehydrating breather. Check transformer, oil level tightness, check for cracks or damages. Also check for oil spillages. Yearly check for directory strength of oil and water content. Check for acidity and sludge. Also examine relay and alarm contacts, their operation fuses, etcetera, check really accuracy, etcetera. The maintenance requirements are covered in detail in self learning module on maintenance of MV substation.
