The demand of electricity in Pakistan has been increasing rapidly in the last few years. There is a mushroom growth in the high rise buildings, shopping areas, CNG and petrol stations, residential colonies, etc. Due to the scarcity of space in cities, installation of conventional oil type transformers in such congested and populated areas is always a big safety risk. The developed as well as many developing countries are installing dry type transformers as a solution for ensuring safety in such scenarios.
Types of Dry Type Transformers
There are primarily two main manufacturing techniques for dry type transformers; Cast Resin and Vacuum Pressure Impregnated (VPI) with their own typical features. The main difference is their insulation class; Cast Resin is Class F (155°C) and VPI is Class H (180°C). The Vacuum Pressure Impregnated transformers not only offers in-build safety feature but there are many other advantages of using these transformers such as improved reliability, reduced losses, minimal maintenance, enhanced overloading capability etc.
PEL’s Initiative for Technology Transfer
Considering these advantages of dry type transformers, Pak Elektron Limited (PEL) took the initiative and went into a Technology Transfer Agreement in March 2007 with DuPont Pakistan Limited (DPOL) for Designing and Manufacturing of Vacuum Pressure Impregnated dry transformers with Nomex® paper. PEL is now pleased to announce that PEL will be the first local company in the production of dry type transformers in Pakistan.
Advantages of Dry Type Transformers with VPI Technology using NOMEX®
Enhanced Safety With the increasing costs of low voltage cables, it has become a trend to bring high voltage lines close to installed loads. For cost reasons, transformers are located near habitable areas such as buildings where fire risk has to be minimized. Additionally, industries such as chemical, mining, etc. are also a potential risk of fire. Ventilated Dry type transformers (VDT) have no fluids to spill, explode or burn, and the insulation used (NOMEX®) does not support combustion in air. Therefore, these transformers ensure enhanced safety of equipment against fire hazards.
Dry type transformers can be located close to their loads, such as; inside factories, schools, hospitals and apartments and as a result low-voltage lines can be shortened with a consequent reduction of the related losses.
Increased Reserve Capacity
If transformers are to be operated continuously at or near their rated loads, efficiency is of prime importance. In this case, one could select 80°C-rise in VDT units insulated with NOMEX®, allowing these units to operate continuously above (potentially up to 30 percent more than) the rated load (if needed, for example, due to unplanned expansions) at a much lower cost (in increased load loss) than that associated with adding an additional transformer.
Transformers are inherently reliable pieces of equipment. Surveys published by IEEE show failure rates of modern ventilated dry-type transformers equivalent to conventional fluid-filled units in the same power and voltage classes. However, in case of difficulty, dry types can be easily inspected and minor repairs conducted on site and repair time for dry-type units is also considerably shorter.
Dry type transformers require an annual inspection and only cleaning of dust which can block path of the cooling medium i.e. air. This is only where the maintenance is required to be carried out. This considerably reduces the running cost. Whereas periodic inspection for leaks and rust is required for liquid cooled transformers. Liquid coolants need continuous monitoring and have to be reconditioned when necessary.
Reduced Cost of Cabling
VPI transformers installation does not require particular fire safety arrangements; consequently the transformers can be installed near the load. This results in reducing the high costs of LT cabling.
Resistance to Humidity
Vacuum pressure resin impregnation of the insulation paper covering the conductors and the other insulation components make it resistant to moisture and polluted atmospheres. This allows the placement of VDT units in many locations not considered in the past.
Friendly to the environment
In these transformers, no liquids is used, thus no risks for any kind of leakage, moreover, no toxic or corrosive gasses, nor halogens are released from the transformer in case of any accident/fire. At the end of their useful life, VDT units can easily be dismantled, and the copper (or aluminum) conductors recovered, along with the steel core. A further impetus to wider use of dry-type transformers has been the PCB hazard which is present for many oil filled transformers.
Improved Capability for Harmonic Loading
Transformers supplying loads with heavy harmonic content frequently see high hot-spot temperatures. Standard transformers typically reduce the base temperature to compensate, which increases the size of the unit. Dry-type transformers designed with NOMEX® still have a large reserve temperature capability to accommodate these hot spots, with no loss of life.
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