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Within the electrical power distribution community there is a general misconception that a liquid filled transformer is a less costly alternative and a practically more advantageous option when compared to a dry type transformer. Prior to accepting this generalization, it should be noted that a correctly designed for safety liquid filled transformer and required peripherals can have an initial cost equal to or greater than that of a dry type transformer.
Typical liquid filled safety devices not required for dry type transformers including dielectric level and pressure monitoring gages, rapid rise relays and pressure relief devices, winding temperature indicators, liquid thermometer devices along with any remote monitoring capability (SCADA) associated with liquid monitoring devices and possibly most significantly containment systems about the transformer can drive the initial cost of a liquid filled unit up appreciably.
Other liquid filled related transformer indirect costs such as insurance premiums and monitoring of liquid related characteristics (i.e. dissolved gases, etc.) are not applicable to a comparably rated dry type transformer and contribute to a higher total owning cost liquid versus dry. A dry type transformer could not only have an equal if not lower initial buy price depending on the application requirements, but a lower total owning cost over the life of the unit.
A frequent misconception is that a liquid filled transformer has a smaller “footprint” than that of its dry type counterpart. This is not the case in all applications. True, in many instances the tank alone containing the core and coil and liquid dielectric medium is physically smaller than that of the dry type enclosure. However when consideration is given to the real estate required to include cooling panel radiators and/or cooling fins necessary for proper heat dissipation from the dielectric running through them as well as the area necessary for a liquid containment system about the liquid filled transformer, square footage values change. Space requirements for personnel access to allow maintenance of the dielectric (sampling, draining, etc.) must also be factored in when considering the footprint of the transformer. In the majority of applications when all liquid filled periphery is considered, the footprint is larger than that of a dry type enclosure.
Liquid filled transformers are designed to be stationary and to maintain a seal and a positive tank pressure. Transportation induced forces the tanks are subjected to during shipment can result in failed welds on the pressurized tank or the cooling panels or fins. Damage to drain valves and samplers being struck during handling and resulting in leaks also occurs. It is not unusual for liquid filled transformers arriving at site to be leaking fluid while still on the truck. Costs associated in time and money to repair a leaking transformer can be significant. Dry type transformers have no dielectric medium to leak and therefore not prone to unexpected and required repairs upon arrival at the application destination..
A liquid filled transformer does need space to access a valve and drain fluid, space to place a containment system, space to facilitate cooling devices such as panels and fins and in some cases must be placed at a significant distance from other equipment or a building because of insurance requirements. The ability of a dry type transformer to, in many cases, be placed closer to a load substantially reduces the cost associated with bus and/or cabling connections that otherwise would need to be employed with a liquid filled transformer.
A difficult to appreciate fact is that dry type transformers handle moisture and water better than liquid filled units. An outdoor enclosure for a dry type transformer is not designed to keep water out but it is made to redirect water to run down its panels instead of splashing onto the core and coil assembly. Regarding condensation, contrary to beliefs or concerns about the formation of condensation on a core and coil assembly, inherent heat generated in a ventilated design of a dry type transformer in service prohibits the formation of condensation. In situations where the transformer may be de-energized for an extended time, strip heaters are utilized to stave off the formation of condensation on the de-energized unit.
Moisture or water reaching any core and coil, liquid filled or dry type assembly poses a risk to the operation of the transformer. Moisture or water entering a compromised sealed tank of a liquid filled unit is undetectable and poses a failure threat unless an insulating medium dielectric analysis is conducted.
Using air for cooling provides some inherent advantages. Since air is far less efficient transmitting heat than oil, the design of a dry type transformer needs to consider lower thermal and dielectric stress levels on its components. Subsequently, the components on a dry type transformer need to handle higher temperatures than those on liquid filled units. This results in the customer receiving a more robust unit electrical design, subsequently a longer life expectancy and a design that is less likely to fail due to overheating and premature aging of its insulation system.
The overall size of a liquid filled transformer depends greatly on the components used or required. The more components required, the bigger the tank need be and subsequently more liquid dielectric increasing the weight and possibly footprint. The design approach for a liquid filled transformer is to accommodate as many components as required into the smallest physical package pushing dielectric distance minimums to a point of compromise. Subsequent cramped quarters does not lend flexibility in terms of the core and coil size and connections themselves. In a dry type transformer, the space inside its enclosure allows for much greater customization possibilities including peripheral components. Should customization requirements force the dry type enclosure to be larger than what would be a standard enclosure, no additional dielectric fluid cost increase would be necessary.
Every component on a liquid filled transformer that protrudes from the pressurized tank requires gaskets to maintain a seal. Over time, these gaskets (including bushing gaskets) need to be monitored and eventually replaced as they deteriorate. A dry type transformer maintenance process is much simpler overall, mainly consisting of visual inspection and proper cleaning. Gasket monitoring, dielectric sampling and dissolved gas analysis associated with liquid insulated transformers are more expensive and complex necessary maintenance procedures.
Should a transformer fail, with few exceptions, the repair associated with a dry type transformer significantly less complicated as compared to a liquid filled transformer. A liquid filled unit may involve a person working in a confirmed space and through a tank manhole which can be inherently dangerous. The inconvenience, time and expense associated with draining a liquid filled unit is significant. In worse case scenarios, unwiring the entire core and coil assembly from internal tank components to facilitate removal of the assembly from the tank is a costly and painstaking process. Conversely, a dry type transformer core and coil assembly and associated accessories are easily accessible by simply removing panels from the enclosure frame to access an item needing repair.
Many special application locations are more geared to the use of a dry type transformer. Some of these applications include rooftops, due to the lower weight with dry types, or indoor applications where the use of less flammable liquids, fire sprinkler systems, fire barriers or special fire insurance requirements need not be considered.
Dry type transformers offer more flexibility in many retrofit applications. Even in the case of liquid filled transformer retrofits, dry type transformers offer the ability to more easily get the transformer to the point of use especially in cases where hallways, doorways, elevator and other dimensional constraints like those found in older buildings, etc. make it impractical to maneuver a replacement liquid filled transformer to that point of use.
Dry Type transformers by standard design requirements are better able to address overload requirements. Specific designed temperature rise systems will provide dry type transformers the ability to continuously operate 30% above nominal rated kVA without the use of forced air cooling fans.
In summation, when considering all aspects and requirements needed in any transformer application, it is very likely that a legitimate and advantageous argument for use of a dry type transformer versus a liquid filled transformer can be made.
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