E+C August 2018

TRANSFORMERS + SUBSTATIONS

Take Note!

Specifications to consider when selecting a dry-type transformer include: Voltage ratings.

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Insulation methods. Classes of insulation.

heated and dipped in varnish at high temperatures. The coils are then baked to cure the varnish. Cooling ducts in the windings provide an effi- cient and economical method of removing heat produced by electrical losses, by allowing air to flow through the duct openings. This dry-type in- sulation system operates satisfactorily in most ambient conditions. A variant of this type is the encapsulated (or sealed) transformer, which is a standard open-wound distribution transformer en- cased in an electrical-grade silica and epoxy, and totally enclosed in a heavy duty style enclosure. VPI transformers are built with high tempera- ture insulation that exceeds the rating of cellulose or 'O' and 'K' class fluids. They contain materials resistant to high temperatures, and the coils are coated with polyester sealant that resists moisture and high temperatures. The varnish coating of polyester resin is applied in interchanging cycles of pressure and vacuum, after which the coils are cured in an oven. The VPI process is better than the standard dry-type insu- lation because it includes pressure in addition to vacuum, allowing better penetration of the varnish in the transformer coil. In a similar but superior process, vacuum pres- sure encapsulated (VPE) transformers undergo several more dip processes during construction, to encapsulate the coil assembly before the coat- ings are cured in the oven. The result is better pro- tection from harsh and wet environments than the VPI type. In cast coil designs , the coils are solidly cast in resin under a vacuum in a mould. To en- sure even distribution of resin and a high level of precision, the winding processes are controlled by advanced electronics. These transformers are particularly reliable and can withstand heavy power surges and frequent overloads. They can also be exposed to extreme conditions in outdoor installations where mois- ture, salt spray, corrosive fumes, dust and metal particles are present in the air. Even in these demanding conditions, they re- quire minimal maintenance and are commonly found in buildings, tunnels, ships, offshore plat- forms, cranes, mines and even nuclear plants. Classes of insulation The level of insulation in a dry-type transformer determines its dielectric strength and capacity to withstand certain thermal limits. There are var-

ious classes of insulation, according to the tem- perature rise rating to be accommodated: Class A (105°C temperature rise), Class B (130°C), Class F (155°C), Class H (180°C) and Class R (220°C). Effi- ciencies are generally better in the lower temper- ature rise transformers, particularly at loadings of 50% and higher. For instance, full load losses for 115°C transformers are about 30% less than those of 150°C transformers, while 80°C transformers have about 15% fewer losses than 115°C trans- formers and 40% less than 150°C transformers. Advantages The growing popularity of dry-type transformers is because of a range of advantages. Safety factors rank high among these benefits, as the technology presents a low fire hazard and is self-extinguishing. Transformer insulation is a mix of epoxy resin and eco-friendly quartz powder, making the wind- ing flame retardant. In addition, it does not produce toxic gases when arcing occurs.These factors elim- inate the need for costly fire extinguishing equip- ment. The absence of oil in the dry-type transformer not only reduces the fire and explosion risk but means that there is no environmental risk of oil leaks, pollution or ground contamination. Several other costs related to the use of oil are also eliminated, such as liquid level checking, oil testing and recycling, and dielectric testing for moisture absorption. Owing to the higher safety levels, there are fewer restrictions on where and how dry-type transformers are installed on site. For instance, no brick or concrete bund wall is required to catch oil spills, and the units can be located closer to the load, reducing the cost of losses and cabling. The reliability of dry-type transformers is high. Cast resin transformers, for example, typically have a service life of over 25 years and an ex- tremely low failure rate. Maintenance requirements are also low, using only air to cool, liquid testing is unnecessary, and the smooth coil surface eliminates heavy dirt build- up even in the most extreme circumstances. Cast resin transformers are designed to meet Class E2 environmental requirements which include resist- ance to consistent condensation and heavy pollu- tion in terms of IEC 60076-011. Conformance to Class F1 ensures that trans- formers meet the requisite fire compliance require-

A multi winding cast resin transformer.

A cast resin transformer with customised terminations.

A freeze chamber used for C2, E2 and F1 tests.

18 Electricity + Control

AUGUST 2018