Transformer & OLTC

> What is an On-Load Tap Changer? What is used to control the Tap Changer?

Under variable load conditions, a fixed winding transformer will experience lower output voltage during high load condition and higher output voltage during low load condition.  An On-Load Tap Changer (OLTC) is a motorized mechanism used to change the tapping connection of the transformer winding while the transformer is energized.  It is an interesting mechanism that will change the transformer ratio under load without the need to have interruption of power supply.  For discerning customers requiring uninterrupted, and regulated power supply under varying load conditions, an OLTC is a critical and important mechanism in a regulating power transformer. 
An automatic voltage regulator, known as TAPCON, is specifically designed and used to control the OLTC which regulates the desired voltage.

> Why do we need to do OLTC maintenance?

Maintenance is a key aspect for all high voltage equipment. This ensures that the equipment life cycle is ensured to the maximum. Safety hazards can also be reduced with appropriate maintenance and checks done on a regular basis.
Poor maintenance regime will attain cost savings initially but result in expensive repairs that may even cause disastrous production downtime.

> How often do we need to conduct maintenance on OLTC?

Depending on the number of years of service or number of operations, whichever comes first.  Often customers are confused about time-based vs conditioned based maintenance. For an Oil Tap OLTC, it is recommended to perform time based if the number of operation (condition based) is not exceeded.  A time-based for OLTC with less operation is necessary to keep the OLTC in-check for unusual voltage stress, unusual fatigue in insulation materials and unusual wear in mechanism.  It is also important to remove carbon inside the OLTC which can form conductive path over time. For a maintenance over a period of 7 years, some components will be exchanged and updated at the same time. 

  1. Inspection of contact wear and measurement on resistors values
  2. Checking of mechanical parts, gears, shafts for abnormal wear
  3. Removal of carbon deposits
  4. Replace with new transformer oil, on-site filtration, on-site oil tests
  5. Testing of new protective relay & testing of TAPCON
  6. Replacement of consumable parts (Eg. Head gaskets, relay gaskets, Motor Drive gaskets where applicable)
  7. Updating with new parts

> How to maintain transformer insulation oil?

Transformer oil is usually clean and dry. However in a humid environment and overtime, moisture ingress into the transformer is possible through the conservator breather or from points of leakage. To dry and clean the transformer oil, it is necessary to deploy an oil treatment machine at site for filtration, degasification and removal of moisture. The oil treatment process will also help to dry the transformer insulation.