Breakdown maintenance
As the name implies the maintenance is carried out when the equipment fails.For costly equipment such like transformer it is not desirable to wait till the breakdown of the equipment as this cost is more and reliability of power is affected.
The revenue loss due to non availability of the system shall be much more than the cost of the failed transformer.Therefore identifying the defect before failure is more appropriate to plan repair/replacement.
Preventive maintenance
In this type of maintenance, transformers are inspected at a pre defined period of shut-down. Maintenance procedure, periodicity of maintenance and formats for maintaining record are pre-defined.
Condition based maintenance
This type of maintenance technique is adopted to assess the condition of the transformer. The condition of the transformer based on different condition monitoring tests. Some of the tests are done on line and some are done off line. It requires sophisticated equipment and skills for analysing the test results.
Reliability centred maintenance
This is the recent technique being adopted in maintenance philosophy.The basic objective is to keep the transformer at desired level of performance. Optimizing/minimizing the maintenance/shutdown period and major/minor overhauls till it is absolutely necessary.
Preventive maintenance procedures
In order to provide long and trouble free service, it is important that a careful and regular supervision and preventive maintenance of the transformer and its components is required to be carried out.
The frequency and extent of such a supervision and maintenance is dependent on the experience, climatic condition, service conditions, loading pattern etc.All work done on transformer should be recorded in transformer maintenance register for future reference.
General supervision of dirt and dust, rust and treatment, check for any sign of mechanical damage, check on all joints for signs of leakage, check for oil level, check on surrounding areas.
Checks on breather, check on silica gel breather, checks for conservator, visual check for conservator oil level, leakage test for air cell.
Check for cubicle and marshaling kiosk and valve operation check.
Check for auxiliaries, cooling system, cooling system fans controls, calibration of OTI/WTI,check for cooler control, alarm and trip settings, gas pressure relay, buchholz relay, bushings.
Operation checks and Inspection/maintenance of tap changer, tap changer hand operation, maintaining circuit, drive motor, out of step relay, tap change in complete alarm, remote indication, inspection and maintenance of OLTC.
Condition Based Maintenance
As transformer failure is a cause of concern for the reliability of the system and also a operation hazard, it is prudent to assess the health of the equipment to take corrective action in advance.
Followings are the condition based maintenance techniques require to take advance corrective action
- Measurement of Insulation Resistance
- Measurement of Polarization Index
- Measurement of Tan delta and Capacitance
- Partial Discharge detection by Acoustic method
- Measurement of Winding Resistance
- Magnetic Balance Test
- Measurement of Magnetizing Current
- Voltage Ratio Test
- Dissolved Gas Analysis of transformer oil
- On line DGA of transformer oil
- Furfuraldehyde Analysis
- On line dielectric dissipation factor monitoring of HV bushings
- On line hot spot monitoring
- Measurement of Recovery Voltage
- Frequency Response Analysis
Periodic Inspection of transformer
Hourly Inpection
Take hourly readings of OTI/ WTI,Check hourly reading of Load in amp and MW and Voltage of HV and LV side of transformer. Check the neutral current of both winding.
Daily Inspection
Check oil level form oil level indicator glass/MOG of main tank and OLTC tank conservator.
Check oil level in bushing from the gauge glass,If found low then attend defect.
Check oil leakage from main tank, inspection covers, pipe work and accessories. If oil leakage observe then attend the oil leakage.
Check whether cooling fans and cooling pumps are working properly.
Check any abnormal noise from the transformer active part or from cooling system.If any abnormal noise is observed, depending upon the severity of the noise, then take appropriate action to solve defect.
Monthly Inspection
Dehydrating silica gel breather: Make sure that the color is of silica gel is deep blue, If found pink then change silica gel.
Make sure that oil seal cup is filled with oil up to desired level, If oil level in the cup is found low then add oil in cup.
Make sure breathing operation takes place by looking movement of oil in oil cup.If oil cup is found with leakage, replace the oil seal cup.
Half Yearly Inspection
Check Main tank oil and OLTC tank oil dielectric strength by measurement of BDV of oil.
Check Cooler fan and pumps and oil flow indicator local and remote manual and auto mode operation.
Check OLTC all moving parts of drive mechanism. Check operation of tap changer by local motorized and hand crank operation Check oil in driving gear mechanism.
Check condition of power cable and its connections.Check Temp sensor pockets oil level.
Yearly Inspection
Carry out all routine test and compare with previous results or factory results.
Take Oil DGA(Dissolve Gas Analysis) testing.
Check cable box for proper scaling of compound filling holes. Check for cracks in the compound.
Check cleaning of MK and cooler control box, Check tightness of termination, Check healthiness of contractors, space heaters, illumination etc.
Check all protections and alarm circuits by actual operation of all transformer trouble relays and equipment and check operation of relays and their sensitivity.
Check oil in the WTI and OTI pockets and check calibration of OTI & WTI.
Check condition of transformer earth pad connection.Check earthing resistance is less than 1 ohm.Clean and tighten earth connection at transformer and earth pit. Take suitable action for reactivation of earth pit resistance if resistance is noted high.
Check IR value of motor winding after rainy season. Check Noise & vibration of fans.
Check the tightness of gasket joint bolts.
Check tightness of all moving and fix contacts and carbon deposition of diverter switch of OLTC.
Pingback: Types of Transformer Losses and their calculation formula