Dry Type and Liquid-Filled Transformers: A Quick Comparison

Transformers under load generate heat due to winding (copper) and core losses occurring during operation. There is an 'acceptable' temperature rise for transformers used in power applications, and this can even limit their size. This acceptable temperature rise is directly related to the limitations of the transformer materials; safety regulations; or component parts in close proximity that may have high-temperature reliability problems.
High temperatures can damage the winding insulation; the heat generated from core and winding losses must thus be dissipated. This dissipation can be achieved with a combination of radiation and convection from the exposed surfaces of the transformer. Dry type power transformers up to several hundred kVA can usually be cooled by convection or even by fans. Power transformers can also be immersed in coolant liquids - which can range from mineral oils to silicone-based oils or ester-based vegetable oils.
Based on the type of cooling used, transformers are thus classified into 'dry type' and 'liquid-filled'.
Liquid-Filled Transformers

1. Oil-filled Transformers
   Oil-filled transformers primarily use mineral-based oil and cellulose paper (Kraft or Aramid) in their insulation systems. This proven combination exhibits outstanding thermal and dielectric properties at a relatively low cost. So popular and effective are these units, that all other transformer designs are judged in relation to them. They are still unparalleled in terms of purchase cost, among all the options available. The inherent weakness of a mineral oil-filled transformer, of course, is flammability; which is why oil-filled transformers are usually restricted to outdoor installations, or indoor installations that have elaborate means of fire protection.
   Typical Applications:
   Oil-filled transformers, thanks to their lower purchase costs, find applications in literally every sort of power distribution. Of late, the awareness of the fire risks associated with mineral oil-filled transformers has created a movement towards safer alternatives that use non-flammable, biodegradable liquids, or even dry-type transformers.
2. Non-Flammable Liquid-Filled Transformers
   Polychlorinated biphenyl (PCBs) were produced in large quantities starting as early as the 1930s, in response to the electrical industry's need for a less flammable substitute for mineral oil as a cooling/insulating fluid for transformers. Several industrial incidents, however, brought the toxicity of PCBs to the fore. As confirmed organic pollutants, PCBs were banned by the late 1970s. A number of alternatives have since surfaced - major ones being silicone, perchloroethylene, high temperature hydrocarbons, and mixtures of oil with perchloroethylene. The first high molecular-weight hydrocarbon-based fluid (HMWH),  was introduced in 1975. This fluids possesses similar dielectric properties as mineral oil, provide remarkable levels of fire-resistance, and do not have undesirable environmental fallouts.
   Typical Applications:
   Non-flammable liquid-filled transformers can be installed indoors and outdoors, close to buildings, walkways and rooftops. Usually, no additional infrastructure is required to address issues like fire safety.

1. Biodegradable Fluid Liquid-Filled Transformers
   Animal fats and vegetable oils offer substitutes that are significantly less harmful to the environment than petroleum oils. However, vegetable oils were not used in transformers for a long time; a fluid which could be stable in the transformer environment and available in the required quantities was simply not available.

1. Transformer manufacturers have since worked on transformer designs that utilize vegetable oil-based dielectric fluids. Comparable in size and electrical performance to conventional liquid-filled units, these transformers are filled with the less flammable dielectric fluids mentioned above. The immediate advantages of biodegradable fluids are clear: higher fire and flash points, thus reducing flammability concerns. The other, and perhaps more long-term advantage, is the fluid's biodegradability: it has demonstrated high environmental assimilation (over 95% in less than a month) - and it has the virtue of coming from a toxin-free resource.
   Typical Applications:
   Biodegradable fluid liquid-filled transformers find application in wind turbines, and other indoor and outdoor areas of heightened environmental and safety sensitivity.

Dry Type Transformers

1. Vacuum Pressure Impregnated (VPI) Conventional Dry Types
   Dry type transformer construction uses high-temperature insulation that exceeds the ratings of cellulose or 'O' and 'K' class fluids. Modern conventional dry type transformers feature insulation systems comprising carefully coordinated high-temperature (220'C) materials coated with a high-temperature, moisture-resistant polyester sealant. On better quality premium units, the polyester sealant is typically applied with a vacuum pressure impregnation (VPI) process. Units built in this fashion have exhibited high resistance to most chemical contaminants. Dry type transformers are generally rated up to 30MVA; performance under overload is demonstrably limited, but the addition of cooling fans can usually help augment this.
   Typical Applications:
   Dry type transformers have been used effectively in diverse commercial and industrial environments for decades; at ratings exceeding 15 kVA. Appropriately designed and installed, VPI units can even be an exceptionally economical choice in medium voltage distribution (15 kVA, 10 MVA ratings), even with the inclusion of installation costs. Because of reduced fire risks, these transformers have been used successfully in special applications where the public are in close proximity, such as underground tunnels, residential apartments, oil rigs and more.
2. Gas-Filled Dry Types
   Gas-filled dry type transformers are designed for applications where low flammability is a vital consideration. N2, C2F6, and SF6 gases are the ones used in these designs, providing a dielectric medium that is external to the windings. Apart from being the dielectric medium, these gases also act as the thermal medium to transfer heat from windings to tank walls. Gas-filled transformers are an alternative to dry-type construction with fewer fire and contamination risks. 
   Typical Applications:
   Gas media have somewhat limited thermal capabilities; and gas-filled dry type transformers usually do not exceed 3750 kVA (C2F6) or even 2000 kVA (N2). Their design also makes them considerably larger than oil-filled units - anywhere from 20% to 30% bigger. Properly designed and installed gas-filled transformers can operate in any environment and remain truly non-flammable. They are commonly installed in dry-docks to provide local power for shipbuilding. Dry-docks (including transformers) are eventually flooded to float the ship away from the construction site.
3. Vacuum Pressure Encapsulated (VPE) Dry Types
   Military shipboard use is an exacting requirement, and VPE dry type transformers were developed to meet these needs. VPE transformers are similar to VPI transformers, but employ a resin made of silicone instead of polyester. The VPE method includes several dip processes to encapsulate the coil assembly; the coatings are then cured in an oven. The resin coating in the VPE design is also typically thicker, sometimes by as much as a factor of four; as can be expected, the thermal classification of the insulation system is markedly different for the military than for commercial applications. VPE transformers are more resistant to harsh and wet environments than VPI type counterparts.
   Typical Applications:
   VPE dry type transformers typically find applications in very harsh indoor and outdoor environments. VPE technology allows the transformer to be highly resistant to humid and caustic environments, but carefully designed enclosures are still a must.
4. Epoxy Coated Dry Types
   Epoxy coated, or 'epoxy shielded' dry type units offer the benefits of better environmental protection, minimal noise, high basic impulse levels and better short circuit strength than cast coil types. Epoxy shielded transformers are also usually less expensive, flexible, smaller and lighter. There are typically two variants of epoxy coated transformers - in one, a VPI dry-type is given an overcoat of epoxy varnish; in the other, the epoxy varnish is used as a complete replacement for the polyester sealant.
   Typical Applications:
   Epoxy coated transformers can be ideally suited for environments that may be polluted with acids, alkalis and chlorides. They are also resistant to the effects of salt water and high humidity, thus finding numerous applications in areas which concentrations of these problems.

1. RESIBLOC Epoxy Cast Dry Types
   Unlike conventional VPI dry type designs, RESIBLOC  Epoxy Cast dry type transformers do not use insulation papers in the windings. Instead, pure epoxy resin reinforced with glass fiber rovings are wound directly with the wire. Winding processes controlled by advanced electronics also ensure even distribution and high levels of precision. Aluminum/copper foils are used for the low voltage winding; and circular (or rectangular) copper conductors with glass-fiber reinforced epoxy resin insulation are used for the high voltage windings. The completed winding block is then 'cured' in a specially designed oven under rotation. This process enables cast winding production without molds or vacuum. As in most cast epoxy units, the dielectric material on the interior winding is the epoxy itself. RESIBLOC transformers are reliable, environmentally safe, offer extreme fire resistance, have high short circuit withstands, and can be exposed to extreme conditions with minimal maintenance.
   Typical Applications:
   RESIBLOC? transformers often find applications in energization, ovens, or traction; they are commonly found in the railways, marine propulsion and distribution, in the nuclear energy and windmill power sectors, and in the mining industry.

1. Epoxy Cast Dry Types The primary and secondary coils in epoxy cast dry type transformers are usually wound with copper conductors, pre-heated and placed in a mold which will then be filled with de-gassed and mixed epoxy under vacuum. The molds are then cured in special ovens to allow sound, void-free casting. The result is a winding design that is void-free, hermetically sealed, and with a smooth exterior finish.
   Typical Applications:
   Epoxy cast dry type transformers are used inside buildings and tunnels, on ships, offshore platforms and cranes, food-processing plants, and more. They are often combined with primary and secondary switchgear and distribution boards, to form compact substations.

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