September 22, 2011

Richard Parker,

Facilitator

Attached are two spread sheets with calculations and graphs showing the loss and 

efficiency evaluations for a 50 kVA transformer. I compared three of DOE's designs 

of a 50 kVA transformer (DOE square tank) against a design selected from Memphis 

Light Gas and Water's (MLGW) 2010 bid quotation.

The transformer is a loss evaluated single-phase pad-mounted, liquid-immersed, 

22860 GrdY/13200 volt to 240/120 volt, 125 kV BIL, without H-V taps design. The 

"A" and "B" factors used to select the design for MLGW have a value of

"A" - $4.00 $/watt

"B" - $1.27 $/watt

As a side note, MLGW has been using “A” and “B” factors to evaluate transformer 

designs since 1980.

See attached DOE Design Lines.xls and DOE Design Lines w-amorphous.xls files.

For discussion I have labeled these designs;

T1 from MLGW's 2010 transformer bid,

T2 DOE's D01_C03_F3_A11.00_B3.33 design using M3_CuAl_dgw_sh (silicon steel), 

T3 DOE's D01_C05_G3_A16.00_B3.50 design using M2_CuAl_dgw_sh (silicon steel), 

T4 DOE's D01_C07_G3_A16.00_B4.50 design using ZDMH_CuCu_dgw_sh (high-B scribed 

steel),

T5 DOE’s D01_C08_G3_A16.00_B7.50 design using SA1_AlAl_dgw_sh,

and

DOE’s D01_C09_F3_A16.00_B4.84 design using SA1_CuCu_dgw_sh.

The T1 and T2 appear to be similar designs except T2 has a higher price and lower 

load losses. I am puzzled as to the large difference in the initial cost between 

the quoted price from MLGW's 2010 bid price and the DOE price.

The higher efficiency of the T3 and T4 designs do not have sufficient savings to 

offset the higher initial cost listed in the Department of Energy, Distribution 

Transformers Rulemaking, Liquid-immersed Engineering Analysis Results; August 2011. 

This is shown by the pay back years on line 49 and 50 in the Calculations tab in 

the attachment.

Using DOE's 50% loading and the designs from the Aug. 2011 engineering analysis 

gave the following efficiencies:

 Design: T1 (MLGW) T2 (DOE) T3 (DOE) T4 (DOE)

 ============================================

Efficiency: 99.08% 99.15% 99.21% 99.23%

These designs do however incorporate readily available silicon steel from multiple 

suppliers.
- 2 -

To achieve a higher efficiency and a pay back period of less than 28 years would 

require an amorphous metal core be used in the design. This design was 

investigated in the attached file labeled DOE Design Lines w_amorphous.xls. Again 

using DOE's 50% loading and the designs from the Aug. 2011 engineering analysis 

gave the following efficiencies:

 Design: T1 (MLGW) T2 (DOE) T5(amorphous) T6(amorphous)

 ===================================================

Efficiency: 99.08% 99.15% 99.35% 99.52%

See attached DOE Design Lines w_amorphous.xls.

The designs require the use of a single source amorphous metal core.

In 1993 MLGW purchased three amorphous core 3-phase pad-mounted transformers. The 

transformers had to be removed from service and sent to a repair shop for 

inspection and repair due to gassing. The three amorphous core transformers are 

listed as follows:

MLGW

Company Date Received Class Date Location

Number kVA New Item Removed Address

Z00410 750 Feb. 1993 47-6130 Oct. 1998 Rivermont

CR0105 2000 Sept. 1993 47-6205 June 2000 Methodist N.

CR0106 2000 Sept. 1993 47-6205 July 2000 Methodist N.

The inspection and tear down of the three transformers listed above revealed a 

failure in the core insulation. This failure allowed circulating current in the 

cores, frame and core ground.

The 750 kVA transformer was repaired with press board by an outside repair shop and 

returned to MLGW’s transformer stock.

The tear down of the two 2000 kVA transformers revealed arcing had occurred between 

the core laminations. This was due to core insulation failure that allowed 

circulating currents between the core frame and core. The 2000 kVA transformers 

were not repairable as the damage was too extensive to the core laminations.

See attached Scaling Factor.xls file. In regard to the scaling factor used, I 

compared the scaling factor derived by DOE against trend lines with data points 

taken from Memphis Light Gas and Water's (MLGW) 2010 bid quotation. The scaled 

total losses (TL) from a 25 kVA 1-ph pole type (base) and a 50 kVA single-phase 

pad-mounted (base), both within a 125 kV BIL design line, were compared against the 

DOE 0.75 Scaling Rule: TL1 = TL0 x (S1 / S0)^0.75. The pole type transformer 

designs as shown in the attached spread sheet graph scale to a 

4th order polynomial for losses

and a 

3rd order polynomial for price (I.C.).

The 50 kVA pad-mounted (square tank) transformer design shown in the attached graph 

scaled to a factor of 

0.757 for load losses,

0.7538 total losses,

linear for no-load losses

and a 

3rd order polynomial for price (I.C.).
- 3 -

RECOMMENDATION

My recommendation would be to keep the current efficiency levels as published for a 

minimum. In addition to this minimum efficiency requirement I would prefer to see 

a loss evaluation “A” and “B” factor added to the minimum efficiency requirement. 

These factors could initially be in the range of

"A" - $4.00 $/watt no-load (core) 

and

"B" - $1.00 $/watt load loss (winding) 

to select the designs with the best evaluated total ownership cost. I would also 

recommend that the efficiency be based on the p.u. load of base rating that gives 

the highest efficiency. In the loss evaluated 50 kVA pad-mounted transformer 

selected by MLGW the maximum efficiency is 99.09% at a p.u. load of 0.43 of the 50 

kVA base rating. The highest efficiency is that of the T6 referenced design at 

99.54% at a load of 0.35 p.u. of base.

Best regards,

John Rossetti, P.E.

Systems Engineer

Reliability and Power Quality

Memphis Light Gas and Water Division

City of Memphis

Office: (901)528.4743 (7:00 a.m. - 3:30 p.m. c.s.t.)

cc: 

plim@mlgw.org

droberts@mlgw.org

brian.coffey@navigant.com

srosenstock@eei.org

Referenced spread sheet files: 

DOE Design Lines.xls

DOE Design Lines w-amorphous.xls

Scaling Factor.xls