Transformers are mainly used to step down a higher distribution level voltage to a lower utilization level. They are vital links in electrical power systems and are among the most reliable components in the system. Transformers can be divided into two general categories according to their insulating medium and construction: liquid-filled and dry-type.
- Core – Made of thin iron laminations. The purpose of the iron core is to channel the magnetic flux 1 generated by the current flowing around the primary coil, so that as much of it as possible also links the secondary coil.
- Coil – The windings are manufactured from copper or aluminum conductors, separated by layers of insulation. Each conductor is formed with precision around a large spool, with each number of turns dependent on the voltage ratio of the transformer 1 being constructed.
- Taps – The windings are manufactured from copper or aluminum conductors, separated by layers of insulation. Each conductor is formed with precision around a large spool, with each number of turns dependent on the voltage ratio of the transformer 1 being constructed.
- Liquid-Filled – core and coils of liquid-filled transformers are, as the name implies, immersed in a liquid. The liquid serves two purposes. First, it is the fundamental element of the insulating system. Second, liquid transfers heat away from the windings to be dissipated by the cooling fins, tank surface, or radiator. Types of oil: Mineral, Silicone, Hydrocarbon, Natural Ester (vegetable oil).
- Dry-Type – operate in air or gas rather than being liquid filled. Just like with liquid transformers, the air or gas serves as an insulating medium and also to dissipate heat from the windings. The two general types of construction are the open or ventilated dry-type transformer and the sealed or closed-tank type.
- Self-cooled (ONAN) — heat is dissipated by the tank surface and cooling fins or tubes
- Forced-air cooled (ONAF) — fans are employed to force air over the cooling surfaces to augment the self-cooled rating
- Forced-air cooled/forced-oil cooled (ONAF/OFAF) — an oil pump circulates oil through a fan-blown oil-to-air heat exchanger
- Water cooled (OFWF) — heat exchange by means of water pumped through a pipe coil installed inside or outside the transformer tank
Basic Transformer Maintenance
Visual and mechanical inspections
- Oil leaks
- Broken parts
- Worn paint
- Defective support structures
- Malfunctioning temperature and level indicators
- Cooling system problems
- Pump and radiator problems
- Bushing and lightning arrester porcelain cracks, etc.
Dissolved Gas Analysis (DGA)
A DGA is the first indicator of a problem and can identify deteriorating insulation and oil, overheating, hot spots, partial discharge, and arcing. This is, by far, the most important tool for determining the health of a transformer.
Infrared Inspection (IR)
A thermographic camera can be used to quickly detect hot spots, localized heating, bad connections, circulating currents, blocked cooling, tap changer problems, bushing and lightning arrester problems.
Doble insulation testing is an important step in determining the condition of the transformer because it can detect loss of winding insulation integrity, loss of bushing insulation integrity, and winding moisture. This will also often include an excitation current test, which measures the current or amperes required to energize the core.
The turns ratio test detects shorted turns, which indicate insulation failure by determining if the correct turns ratio exists. Shorted turns may result from short circuits or dielectric (insulation) failures.
A sweep frequency analyzer is used to place a signal on each of the high voltage windings, and the signal is detected on the low-voltage windings. This provides a picture of the frequency transfer function of the windings. If the windings have been displaced or shifted, test results will differ markedly from prior tests.
Winding resistance in transformers will change due to shorted turns, loose connections, or deteriorating contacts in tap changers. DC voltage drop is measured across a winding to measure its resistance in ohms.
Insulation resistance tests are made to determine insulation resistance from individual windings to ground or between individual windings. The 10-minute reading divided by the 1-minute reading will produce the polarization index, which is a ratio used to determine the general health of electrical insulation.