A message about COVID-19.

Converting Liquid Cooled Spec to Dry-Type

PETE

The basic operation of liquid and dry-type transformers is the same, specifically the transformation of voltage. However, different dielectric medium can produce different specifications. Base dry-type specifications can be found on HPS website at
www.hammondpowersolutions.com.
There are some limitations with dry-type transformers and generally are not considered a replacement for liquid filled transformers when:

Voltage is above 44,000 VAC
BIL is above 250 kV
Size is above 40 MVA
Pole mounted units
Liquid filled transformers mounted in areas accessible to the general public

In addition to these specification points (see table below), there are some liquid filled transformer options that do not have dry-type equivalents. These should be removed from any dry-type specification and would include:

Dead Front
Bushing Wells
Drain Plug
Liquid Level Gauge
Pressure Gage
Valve

  Liquid Filled  Dry-Type
 No-Load Tap Changer  A rotary no-load tap changer is often used in
liquid filled transformers to change tap voltages. 
 Rotary tap changers are rarely used in dry-type
designs.  Coil face taps utilizing jumpers should be
specified instead. 
 Enclosure  By their very nature, a liquid filled transformer is
a sealed unit
 
 Dry-type power transformers will usually be
ventilated Type 1 for indoor or Type 3R for outdoor.
 Efficiency  U.S. DOE 2016 regulations have established
minimum efficiency levels for liquid filled
and dry-type transformers up to 2500 kVA.
Canadian NRCan 2019 efficiency specifications
go up to 10 MVA.
 Liquid filled efficiency levels will be slightly higher in
the national specifications and should be updated to
dry-type levels if converted.  
 Site Preparation  U.S. DOE 2016 regulations have established
minimum efficiency levels for liquid filled
and dry-type transformers up to 2500 kVA.
Canadian NRCan 2019 efficiency specifications go up to 10 MVA.
 Liquid filled efficiency levels will be slightly higher in
the national specifications and should be updated to
dry-type levels if converted.  
 Termination  Liquid filled units have fewer locations for
termination, especially if close coupling to
switchgear is required. 
 Dry-type units can be easily directly terminated to
switchgear, cabling and other methods. 
 Temperature Rise  Liquid filled transformers have lower winding
temperature rise restricted by the insulation
and liquid thermal capabilities. Because of the
thermal properties of the liquids and cooling
fins, liquid filled transformers have lower
operating temperatures.
 Dry-type transformer insulation has higher thermal
capabilities and hence temperature rises are much
higher with more variations to work in higher
ambient temperatures.
 Dead Front  Dead front is standard within the non-ventilated
compartment of liquid filled transformers.  
 Dry-type transformers can be ordered with the
primary and/or secondary points wired to a non-ventilated
air terminal chamber which in turn can have dead front construction added.
 Sound Levels  Standards specify the maximum sound level of
liquid filled transformers lower than same rating
of a dry-type transformer.
 In general, dry-type transformers will be louder than
the equivalent liquid filled unit.   

  Cooling Terms1 
Liquid Filled   Dry-Type
ONAN: Liquid Natural Air Natural ANN:  Air Cooled Natural
ONAF: Liquid Natural Air Forced AFN:  Forced air inside & natural outside
OFAF: Liquid Forced Air Forced  AFF: Forced air inside & outside or (AFN)
Water ancillary cooled
 OFWF: Liquid Forced Water Forced  

1Cooling terms are not an exact cross and are provided for review. Applications need to be reviewed to verify that the cooling methods are suitable.

Dry-Type Temperature Rise Terms
Liquid filled and dry-type transformers use significantly different insulation systems. Liquid filled cool through a larger surface area and run cooler even when losses are similar. Dry-type transformers compensate for higher running temperatures by using advanced insulation systems such as NOMEX. Please consider using the following dry-type temperature rises.

Rise Max Ambient Average 24 hour Ambient Service Factor @ 30o
Average Ambient
 150o  40oC  30oC  1.00
 115o  75oC  60oC  1.15
 80o  110oC  90oC  1.30

Dry-type and liquid filled transformers both have unique advantages. Care must be taken in converting specifications and in many cases the specific technologies of the two designs may provide a significant advantage for one type of transformer.

HPS Millennium E vacuum pressure impregnated (VPI) and HPS EnduraCoil cast resin transformers can provide many advantages over liquid filled transformers. For more information, please contact Hammond Power Solutions.

 

X
Cookies help us improve your website experience. By using our website, you agree to our use of cookies.
Confirm