Despite being more expensive than oil-filled transformers, open ventilated dry type transformers are not only safer and reliable but also cost-effective, note the authors.

Power and distribution (P&D) transformers can be classified into two main categories: liquid filled and dry type. When referring to P&D transformers, it is the liquid filled transformer that comes to mind. However, advancements in dry type transformer design in more recent years have substantially increased their use. Further, there are many cases in which the application of liquid filled transformers is forbidden due to safety regulations. For instance, in buildings, factories or public assembly places where liquid filled transformers may be employed only under special circumstances.

From the cost viewpoint, the mineral oil filled transformer remains the cheapest. However, dry type transformers can be situated close to the consumer so that use of expensive low voltage supply lines is minimized. Also there is no need for safety installations characteristic of liquid filled transformers. So from TOC point of view, dry type transformer may be less costly than the liquid filled transformer because it evaluates all costs from purchasing, installation, maintenance, operation to disposal.

This paper describes requirements of insulation materials that are used in dry type transformers with a focus on open ventilated dry type transformers (OVDT).

Safety: One of the worst mishaps in New Delhi, India had been the fire at UPHAAR Cinema in 1997. It has been determined that the cause of fire was the explosion of liquid filled transformers. As a result of the accident, the Ministry of Power has mandated that only dry type transformers shall be used for installation inside the residential & commercial buildings [Indian Electricity Rules - 1956, amended with Rule No. 64 (2) (C) (II) in 2001]. In addition to this, IEC60076-11 that is a standard for dry type transformers reinforced safety against fire with a fire behavior test in 2004. This test measures highest temperature and smoke volume emitting from burning coil during a test in the furnace as test criteria. Transformers passed this test may minimize losses on people and properties caused from explosion and fire in transformers.

Composition: Dry type transformers are those which use air or other gases as the primary cooling medium, as opposed to fluids, such as mineral oil. They use solid insulation materials, such as papers, sheet laminates, plastics, varnishes, and resins to protect the electrical circuits from one another and from ground (earth).

It is categorized into ventilated design, solid resin cast design, semi encapsulated design, sealed/gas filled design. Ventilated designs have varnish coatings and rely on cooling ducts for the transfer of heat out of coil, which is normally named by open ventilated dry type transformer (VDT). Most of VDT manufacturers take VPI (vacuum pressure impregnation) technology to produce for power and distribution application.

Relative	Dielectric	Dissipation	Volume
Humidity	Constant	Factor	Resistivity
(%)	(60 Hz)	(60 Hz)	(ohm-cm)
Dry	2.5	0.006	1016
50	2.7	0.006	1016
95	3.2	0.011	1014

The main advantage of open ventilated dry type transformers are the high level of safety when installed in or near buildings; without fire or explosion danger, it is possible to construct the substations in the direct vicinity of the consumers. It makes no difference whether these stations are installed in a cellar, in the center of a floor or under the roof of a factory.

MAIN ADVANTAGES:

1. Can be placed close to the load. It is due to inherent safety of dry type transformers, which enables less energy loss of LV cables.
   
2. Increased emergency overload capability
   
3. Additional advantages of class H VDT are easy to repair (open coils), great flexibility of coil size/shapes (no mold for casting), no cracking (no blocks of solid resin), reduce size and weight per kVA

Common applications of dry type transformers are as follows.

• Power and distribution up to 35KV
• UPS (uninterruptible power supply, class H)
  
• High intensity discharge lighting ballasts
  
• Microwave ovens (220 Deg class)
  
• Welding machine (class H)
  
• Furnace (class H)
  
• Mining, underground (class H)
  
• Nuclear power plants (class H)
  
• General purpose power
  
• Onboard ship / marine power (class H)

WHAT MAKES OVDT SAFER AND RELIABLE?
For the above applications, Nomex® aramid paper is used with its superior performances. It provides a unique combination of properties found in no other insulating materials, having the following properties:

1. Long term stability for continuous exposure to 220ºC
   
2. Will not melt, flow or support combustion below 250 ºC
   
3. Strong resistance to acids and alkalis
   
4. Compatible with all varnishes, resins, adhesives, and fluorocarbons
   
5. Stable properties over a wide range of temperature and moisture conditions
   
6. Inherently flame retardant; self extinguishing

Nomex® is a class C insulation material which performs at 220 Deg C. The insulation used on the conductors for ReliatraN™ cobranded transformers is 2Mil Nomex® paper and all other solid insulation material used for manufacturing of these transformers is class H.

Fire behavior performance: When the fire behavior test is performed on dry type transformers (IEC 60076-11) using Nomex, better has optical transmission factor (OTF) during burning was found. The graph below indicates OTF (visibility) of about 60 to 75 per cent for a time period ranging from 5 minutes to 45 minutes (Max standard temperature range) of burning. Fig 1 illustrates better visibility as the smoke is not dense or black.


In case of fire people do not die of fire but the damage is due to the suffocation in toxic smoke. The injuries to people trapped in the fire are due to stampede because of poor visibility.

The figures alonside compare the flame resistance behavior of Nomex® with other materials used in the dry type transformers, It is found that the Limiting Oxygen Index of Nomex® is very high which means that it requires more oxygen to burn and when this insulation is burnt it does not support combustion so it does not spread fire.

ENVIRONMENTAL AND CLIMATIC CONDITIONS PERFORMANCE
VDT Transformers are well protected from invasion of water high quality vacuum impregnation technology using the most modern equipment (e.g. Epoxylite). Nomex® is not adversely affected by water, even if it does become exposed to 95% RH. It retains-80% of dielectric strength; low dielectric constant and dissipation factor and superior volume resistivity.

The VPI (Vacuum Pressure Impregnated) varnishes used with UL approval for usage at 180°C and higher for continuous operation in the hottest areas. Nomex® insulation is fully coated with varnish makes the encapsulation of the transformer.

TEMPERATURE LIMITS
The insulation system of transformers represent the highest temperature the electrical winding can accept while operating at the highest load condition (Hot Spot) and should have insulating materials which at least match this rating

CONCLUSION
The OVDT aramid based designs safer, reliable and help in reducing the environmental foot print. With the increasing load demand and the load density of individual house hold these transformers with inherent overload capacities are the best option for the builders, utilities and the industry.

The OVDT transformers made out of Nomex® insulation will tend to resist to the fire as the material does not support fire, and an unfortunate case of fire these transformers will help save the human lives and reduce the loss due to fire.

The inherent properties of the insulating material used and the VPI technology make the transformers resistant to environmental effects.

TEMPERATURE LIMITS FOR VARIOUS INSULATION CLASSES
Insulation	Temp	Temp	Ambient	Hot Spot	Hottest
Class	Rise	Class	Temp	Allowance	Temp
Class B	80	130	40	10	130
Class F	100	155	40	15	155
Class H	125	185	40	20	185
Class C	150	220	40	30	220
Highest Temperature = Average Winding Rise (K) + Max. Ambient + Hot Spot Allowance

The higher temperature endurance of the Nomex® helps the OVDT transformers to bear short term overload without damage to the insulation.

(About the authors: Sailesh Purohit, BE (Electrical), ME (Control Systems) is Manager Technical India (ES), DuPont Protection Technologies and Dilip Patel, an Electrical Engineering from M.S. University, Baroda, is Consultant - E.I. DuPont (I) Ltd)

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