The frequency of testing and maintenance of transformers varies depending on the condition of service and the atmosphere in which it operates. In general, a thorough energized inspection, including liquid analysis, fault-gas analysis and infra-red inspection should be performed at least annually.
 

Transformer inspection, testing and maintenance can be divided into two general categories, work that can be performed when equipment is

• Energized

• De-energized

Testing while Transformers are Energized

• Visual Inspection
  HVM technicians look for cleanliness, cracked insulators and bushings, condition of paint, oil levels, oil leaks, oil/winding temperature gauges, nitrogen pressures, relay targets, condition of grounds, etc.

• Infrared Inspection
  Using the latest state of the art infrared cameras projecting high-resolution thermal images, HVM technicians are able to locate hot spots, poor connections, defective surge arresters, etc. The inspection is fast, accurate and practical. It is also recommended by ANSI/NFPA 70B.

• Corona Detection
  Excessive corona produces RF interference and may cause an insulator, bushing or surge arrester to fail. Using an ultrasonic corona detector, HVM can identify improperly shaped medium and high voltage connections, defective insulation, bushings, and surge arresters — all likely causes of excessive corona.

• Nitrogen Gas Tests
  On nitrogen gas blanketed transformers, gas samples are extracted and two tests are performed:

• Percent TCG: Most transformer failures are caused by incipient defects such as hot spots or overloading. These defects create combustible gas and their detection calls for more in-depth gas-in-oil analysis.

• Percent Oxygen: High oxygen content in a transformer may indicate "wet" atmosphere has entered the transformer. Oxygen also accelerates the deterioration of oil.

• Oil Analysis
  When transformer oil deteriorates, sludge ultimately forms and coats the windings, resulting in decreased cooling capacity and degradation of the solid insulation system. Some of the oil analysis tests include:

Measuring specific combustible gas levels in transformer oil is a good way to predict transformer problems. HVM technicians and engineers compare the results of historical records to recent test results as precursors of internal problems such as arcing, corona and overheating.

• Dissolved Water in Oil Analysis
  Excessive water in oil may reduce the dielectric strength of oil, and more importantly, could be absorbed by the cellulose insulation.

Testing while Transformers are De-Energized

• Liquid Filled Transformers:
  Insulation tests such as power factor testing and insulation resistance testing should be performed along with diagnostic tests such as turns ratio testing and exciting current testing. These may lead to the following corrective actions:

Mechanical filtration of the liquid insulation

Repairing leaks in valves, tap changers, bushings and gaskets

Servicing load tap changers

Replacing or repairing bushings

Drying out wet transformer windings

Cleaning or painting the transformer

Dry Type Transformers

• There are two main categories of dry-type transformers:

Sealed gas filled units

Open or ventilated units

Sealed gas filled transformers

Testing these transformers is similar to test procedures for liquid-filled transformers in that de-energized inspection and testing is required. Corrective maintenance for severe leaks, bushing replacement or any other task that requires opening up the unit is more complex, because special equipment and procedures are required which involve removing, storing and replacing the gas in the transformer.

Open or ventilated transformers are tested routinely when de-energized by internal inspection. Since the core and coils of these units are subject to contamination by air flowing through the transformer, maintenance includes cleaning the internal parts. Also, since the voltage impulse levels for dry-type transformers are usually lower than for liquid filled transformers, surge arrestors are often installed either inside the transformer case or immediately adjacent to it. Whenever de-energized maintenance is performed on the transformer, the arrestors should be inspected, cleaned and tested.