Transformers

IEEE SF / IAS Seminar
April 2011
Finn Schenck
Schneider Electric

Schneider Electric 2- Projects and Services – PEC – MV Transformers - 2010
Transformer Types
Liquid-Filled Padmount
Liquid-Filled Unit Substation

Transformer Types
VPI Dry Unit Substation
Cast Unit Substation

Dry Vs. Liquid Transformers
DRY
●No fluid to spill or burn
●Cost is higher than oil
●Resistant to moisture
●Minimum Maintenance
●UL Listed
Liquid
●Lowest Purchase Price
●Sealed Tank
●Lowest Losses per $
●UL Listing
●Loads of Options
●Pad Mounted (Tamper-
Resistant)
●Substation – Flexible

Definitions (Buzzwords)
● BIL (Basic Impulse Level)
The level of momentary over voltage that the transformers insulation can withstand
without damage/failure.
● IMPEDANCE
The percentage of rated voltage required to circulate rated current in one winding with
the other winding short circuited.
● EXAMPLE
●A transformer with a 5.75% impedance and a 480 volt low voltage winding would require
(0.0575 x 480 volt) or 27.6 volts to circulate rated current.
●IN SHORT Impedance is simply the opposition to the flow of current in the transformers
windings. (Impedance has a resistive and an inductive component.)
● REGULATION
The drop in secondary voltage that occurs as the load on the transformer is increased
and is expressed as a percentage of rated voltage.
● EFFICIENCY
The ratio of power absorbed by the transformer to the total power drawn from the supply
line.

“Rule of Thumb” Cost Comparison
●Mineral Oil 1.0
●Hi Fire Point Seed Oil 1.2
●VPI 1.25 – 1.75
●VPE (Epoxy) 1.35 – 1.85
●Cast Coil 1.55 – 2.0
●Lower Temerature Rise 15 % to 40 % Add
●Copper 35% - 35% Add
●5 DB Noise Reduction (Dry) 10%
●Hinged Doors on Dry and Cast Enclosures A bargain
●I.R. Viewing Windows on Dry and Cast Encl. An Even Better Bargain
●Insulated Bus on VPI and Cast Encl. $2K – 5K, but a good
idea just the same.

H.V. BIL Comparison
Dry Type Liquid/Cast
Voltage Class Std Opt Std Opt
2.4 KV 20 KV 30 KV 45 KV 60 KV
5 KV 30 KV 45 KV 60 KV 75 KV
8.3 KV 45 KV 60 KV 75 KV 95 KV
15 KV 60 KV 95 KV 95 KV 110 KV
25 KV 110 KV 125 KV 125 KV 150 KV
35 KV 150 KV ----- 150 KV 200 KV
46 KV ----- ------ 250 KV ------

Impedance
●IEEE Standard
● 5.75 Nominal for 750 kVA and Above
● Manufacturer’s standard within the IEEE Standard range for 500 kVA and
below
● Large units and higher voltages have different standards
●Note
● IEEE allows a +/- 7.5% Tolerance
● Example for 5.75% Impedance Range would be 5.32% to 6.18%

Insulation Classes
TRANSFORMER INSULATION WINDING MAXIMUM HOTSPOT HOT SPOT
TYPE CLASS RISE AMBIENT GRADIENT TEMPERATURE
POWER-DRY 220 DEG. C 150 DEG. C + 40 DEG. C + 30 DEG. C = 220 DEG. C
CAST COIL 185 DEG. C 115 DEG. C + 40 DEG. C + 30 DEG. C = 185 DEG. C
LIQUID-FILLED 120 DEG. C 65 DEG. C + 40 DEG. C + 15 DEG. C = 120 DEG. C

Temperature Rise
Dry 150 Degree C Standard
115 Degree C Optional
80 Degree C Optional
Cast 115 Degree C Standard
80 Degree C Option
Liquid Filled 65 Degree C Standard
55/65 Degree C Optional

Altitude
●STANDARD: Designed for operation at a maximum altitude of
3300 ft. (1000m) above sea level
●If a non standard condition exists (i.e.. Altitude above 3300 ft ASL
then:
● Transformer must be de-rated or
● Transformer must be specifically designed for the special altitude
consideration.

OVERLOAD EXAMPLE
3000 kVA Dry Type Xfmr.
AA + 33%
(5400)
Base kVA + 35%
(4050)
80/150
AA + 33%
(4600)
Base kVA + 15%,
(3450)
115/150
AA + 33%
(4000)
Base kVA
(3000)
150
Fan Cool Rating
(FA)
kVA Rating
(AA)
Temp. Rise
(Deg. C)

NEMA Standard Sound Levels
kVA LIQUID-FILLED VPI CAST
51-150 51 55 N/A
225-300 55 55 58
301-500 56 60 60
501-1000 58 64 64
1001-1500 60 65 65
1501-2000 61 66 66
2001-2500 62 68 68
2501-3000 63 68 68
3001-4000 64 70 70
4001-5000 66 71 71
7500 67 75 75
10,000 68 78 78
NOTE: ALL VALUES ARE IN DECIBELS (db) AND ARE FOR BASE kVA RATINGS (FAN PACKAGES WILL INCREASE
db LEVELS)

Energy Efficient (EE) MV Transformers
● The Department of Energy (DOE) has determined that energy conservation
standards for MV Distribution Transformers will result in significant conservation
of energy so they have passed DOE 10 CFR Part 431 Energy Conservation
program for Commercial Equipment.
● Starting January 1, 2010 all medium voltage distribution transformers with
ratings of 2500 kVA and below, 34.5 KV primary and below and 600V class
secondary voltages must meet minimum efficiency requirements.
● Liquid Filled Padmounts, Liquid Filled Substations, Dry Type VPI and Power
Cast products MANUFACTURED after January 1, 2010 are all included.
● EE Low Voltage transformers implemented in 2006, this law is extending
efficiencies to Medium Voltage.

DOE Energy Efficiency Applies to:
●Medium Voltage Liquid
● » 10-833 kVA 1 phase
● » 15-2500 kVA 3 phase
●2. Medium Voltage Dry
● » 15-833 kVA 1 phase
● » 15-2500 kVA 3 phase

Minimum Efficiency Tables
Liquid Filled
Note: All efficiency values are at 50 percent of nameplate-rated load, determined according to the
Appendix A, DOE Test procedure. 10 CFR Part 431, Subpart K
Table I.1
Table I.1 - Standard levels for Liquid Immersed Distribution Transformers,
tabular Form
10………………….. 15……………………
kVA
Three Phase
Efficiency %
98.62
Single Phase
Efficiency % kVA
15…………………..
25…………………..
37.5…………………
50……………………
75……………………
100………………….
167………………….
250………………….
333………………….
500………………….
667………………….
833………………….
98.76
98.97
99.01
99.08
99.17
99.23
99.25
99.32
99.36
99.42
99.46
99.49
30…………………..
45……………………
75…………………….
112.5……………….
150…………………..
225………………….
300…………………..
500…………………..
750…………………..
1000…………………
1500…………………
2000………………….
98.36
98.62
98.76
98.91
99.01
99.08
99.17
99.23
99.25
99.32
99.36
99.42
99.46
2500………………… 99.49

Applies to all Transformers Sold in:
●All 50 US States
●Unincorporated Territories (Possessions): American Samoa, Guam,
Marshall Islands, Puerto Rico, US Virgin Islands
●Freely-associated states: Federal States of Micronesia, Northern
Marianas, Palau

Does not Apply for Transformers
●Exported to a country outside the US
●Armed Forces bases located outside the US unless specified or to a US
territory
●US Embassies unless specified

Transformers Affected:
●Have an input voltage of 34.5 kilovolts or less;
●Have an output voltage of 600 volts or less; and
●Are rated for operation at a frequency of 60 Hertz

Excluded Transformers
●A transformer with multiple voltage taps, the highest of which
equals at least 20 percent more than the lowest;
●A transformer that is designed to be used in a special purpose
application and is unlikely to be used in general purpose
applications, or
●Any transformer not listed in this definition that is excluded by the
Secretary by rule

Excluded Transformers
● Autotransformers
● Drive (isolation) transformers
● Grounding transformers
● Machine-tool (control) transformers
● Nonventilated transformers
● Rectifier transformers
● Regulating transformers
● Sealed transformers
● Special-impedance transformers
● Testing transformers
● Transformer with tap range of 20 percent or more
● Uninterruptible power supply transformers
● Welding transformers
● Step up Transformers

Seismic

Liquid Filled Transformers
Padmount
Unit Substation

Liquid Filled Padmount
●Product Scope:
●75-5000kVA
● 55, 65, 55/65 Temp. Rise
● 2.4 - 34.5kV Class Primary 150kV BIL Max
● 600V - 5kV Class Secondary 75kV BIL Max.
● Self-Cooled Overload Only (12% with 55/65)
● ONAN only - NO ONAN/ONAF (no fans)
●No Fans means, NO FANS!
●Weather Resistant high and low voltage compartments
●Tamper-Resistant Design
(Available up to 20,000 kVA via special quotation)

Fluid Offerings
MINERAL OIL
Type II (Inhibited) Ergon Hyvolt II
Mainly Outdoor (Fire Point = 165 Deg. C)
Least Expensive of all Fluids
BIOTEMP (ABB)
Renewable Natural Agricultural Product
Certified by UL® as “Less-Flammable Fluid”
Fire Point = 360 Deg. C
Factory Mutual Approved
Biodegrades 97% in 21 days
No fusing restrictions
LESS FLAMMABLE SEED OIL
FR3 FLUID ™ (Cooper Industries)
Certified by UL® as “Less-Flammable Fluid”
Fire Point = 360 Deg. C
Factory Mutual Approved
Biodegrades 99% in 21 days
No fusing restrictions
Extends insulation life
LESS FLAMMABLE SEED OIL

HV Bushings
Drain
Valve
Bay-o-net
Fuses
Tap Changer
Pressure
vacuum
Gauge
Upper Fill Plug
Temperature
Gauge
Liquid Level
GaugePressure Relief
valve
Name PlateLV Bushing
HV
Compartment LV Compartment

Radial Feed vs Loop Feed
A. Radial Feed
3 HV Bushings
B. Loop Feed
6 HV Bushings
A transformer with loop feed construction can be used on 2 types of systems:
1. Dual Source Radial Feed
2. Loop Feed System

Live Front Construction
●Contains “exposed current carrying parts” in HV
compartment (IEEE C57.12.34)
●Components: Porcelain Bushings
●Live Front bushings arranged in horizontal pattern,
Radial and Loop Feed

Live Front Transformer
Note: 5/8” eyebolt terminal-optional 2 or 4 hole pad is available by special quote

Dead Front Construction
“No exposed current carrying parts HV compartment” (IEEE
C57.12.34)
Components: Wells, Inserts, One Piece Bushings, Feed-thru inserts
Available In: 15, 25,35kV Classes
Available In: 200 Amp - Loadbreak
600 Amp - Non-Loadbreak
Note: Can not interchange different classes of components.
Radial Feed: Bushings in horizontal pattern.
Loop Feed: “V” Type Configuration

Dead Front-Radial Feed Dead Front-Loop Feed

Dead Front Bushing Well

Bushing Insert

No Load Tap Changer

Switching
All transformers (radial and loop feed construction) are not equipped with
HV switches. Switching can be accomplished remotely.
Padmount can be equipped with:
1. Two Position (ON-OFF) Switch
(Can be used on radial and loop units.)
2. Three Position Switch
Line A Only, Line B Only, Off
(Dual source application.)
3. Four Position Switch - Permits Sectionalizing
(Loop feed transformers only.)

Arresters
●Arresters
● Silicone carbide or MOV (Metal-Oxide Varistor) most common.
●Operation
● MOV is a granular ceramic (semi-conductor)
● Conducts during a “High Amplitude Transient” and diverts excess voltage to
ground
● Provides a transient path to a common point earth ground.

Bayonet Type Fuse

Liquid Filled Unit Substation

●225-10,000kVA
● 55, 65, 55/65 Temp. Rise
● 2.4 - 34.5kV Class Primary - 250 kV BIL Max.
● 600V - 8.7kV Class Secondary - 75kV BIL Max.
● ONAN/ONAF also known as AA/FA
●Available up to 20,000 kVA - 69 kV primary, 5 kV up to 34.5 kV
secondary via special quotation with Square D Name Plate.
●Now available up to 60 MVA base rating, 161 KV primary, 34.5
Secondary with ABB Name Plate

●Standard efficiency (Energy Efficient per DOE2010)
● K-Rated units
● UL Listing
● Factory Mutual
● Evaluated loss designs
● Oil, Silicone, and Less Flammable Seed Oil

Fluid Offerings
MINERAL OIL Type II (Inhibited) Ergon Hyvolt II
• Mainly Outdoor (Fire Point = 165 Deg. C)
• Least Expensive of all Fluids
SILICONE (Dow Corning 561)
• “Less Flammable Liquid” (Fire Point = 343 Deg. C)
• Indoor Use
• Expensive Fluid
BIOTEMP (ABB)
Renewable Natural Agricultural Product
• Certified by UL® as “Less-Flammable Fluid”
• Fire Point = 360 Deg. C
• Factory Mutual Approved
• Biodegrades 97% in 21 days
• No fusing restrictions
• LESS FLAMMABLE SEED OIL
FR3 FLUID ™ (Cooper Industries)
• Certified by UL® as “Less-Flammable Fluid”
• Fire Point = 360 Deg. C
• Factory Mutual Approved
• Biodegrades 99% in 21 days
• No fusing restrictions
• LESS FLAMMABLE SEED OIL

Unit Substation Transformers
●Close coupled to HV or LV switchgear
● Includes throats, flanges, etc.
● Coordination through Unit Substation Coordination Group
●Stand alone with full or partial height ATC’s
●Connection to HV Non-Seg Busway or LV I-Line Busway
● Includes standard 12” Throat
● Busway Plant will provide connection box to throat
● Coordination through MV Transformer Group
●Special retro-fit connections/dimensions available

Cooling Options
●(ONAN) Self-Cooled (Oil Natural Air Natural)
(55/65 Deg. C Self-Cooled = 12% Overload)
●(ONAN / FONFA) * Self-Cooled With Provisions For Future
Addition Of Fans
●(ONAN / ONFA) * Self-Cooled With Fans To Provide Additional
Capacity
● *NOTE: Controlled By: Top Of Oil or Winding Temperature Indicator
(Winding Temp Ind. is expensive option)

Liquid Filled Transformers
Fans Increase Capacity As Follows:
< 2000 kVA
Increase 15%
2500 Through 10000 kVA
Increase 25%

Arrester Classes
1. Distribution Class Heavy Duty Type
2. Intermediate Class Higher Energy Handling Capability
3. Station Class Provide Greatest Degree Of Protection,
Highest Level Of Energy Handling, Max
Capability = 80kA
Note: Intermediate or Station Class Arresters require larger ATC’s

End of Section

Dry Transformers

Dry Transformers
Advantages
●Excellent for Indoor Use
● Schools, Hospitals, Office Buildings
● Locations where a liquid spill can not be tolerated
● No containment pit required, providing for lower installation and
maintenance costs
●Non-Flammable
● 220C Insulation Class
● Will not support combustion
●Approximately the same Price of a Seed Oil Unit
● Most economical when compared to UniCast II or PowerCast II

Dry Transformers
Disadvantages
● Lower BIL Levels than Liquid Filled
● Standard for 15kV Class is 60kV BIL, Liquid Filled is 95kV BIL
● Standard for 600V Class is 10kV BIL, Liquid Filled is 30kV BIL
● Moisture Absorption
● Must pre-dry unit before energizing to eliminate moisture from coils
● More Susceptible to Airborne Contaminants
● Ventilated enclosure, contaminants may accumulate on coils

Picture of VPI Process

Dry Transformer
COOLING OPTIONS
●(AA) Self Cooled
●(AA/FFA)* Self Cooled with provisions for future forced air
●(AA/FA)* Self cooled with forced air
* Capacity will be increased by 33 1/3% with the addition of fans
For example:
● 1000kVA @ 150C = 1000/1333kVA
● 1000kVA @ 115/150C = 1000/1150/1533kVA
● 1000kVA @ 80/150C = 1000/1300/1733kVA

Selectable Product Scope
225 - 5000 kVA
150, 115, and 80 Degree C Rise
2.4 - 34.5 kV Class Primary
150kV Primary BIL
600V - 5kV Class Secondary
(Available up to 13,000 kVA via special quotation)

Cast Coil Transformers
Power Cast II and UniCast II

Cast Resin Transformers
(The Need)
●Replacement for Askarel
(Non-Flammable)
●Environmentally Safe
(By Products of Combustion)
●BIL Equivalent to Liquid Filled
●Primary Voltages Higher than Dry Type
●Mechanical Short Circuit Strength
(Superior)

(The Coil)
HV/LV Coil

LV Coil w/Cooling Ducts

Core/Coil Assembly

Benefits
Cast Coil vs. Liquid
●No Fluids to Leak, Contaminate, or Burn
●Far Less Maintenance, No Yearly Fluid Testing
●No Decommissioning Costs
●Higher Short Circuit Strength
●Non - Flammable
●Longer Design Life when compared to Mineral Oil
●Greater Fan Overload Capability
●Lower Installation Costs

Benefits of Cast Coil vs.
Conventional “VPI”
●Resistant to Moisture and Contaminants
●Higher Standard BIL Levels
●Lower Operating Costs
Lower Standard Temperature Rise - 80 vs 150
More Efficient
Less Maintenance
●Longer Design Life
●More Reliable
●Greater Design Flexibility
Higher kVA Ratings
Higher Primary and Secondary Voltages

IMPEDANCE
● DEFINITION
The percentage of rated voltage required to circulate rated current in one
winding with the other winding short circuited.
● EXAMPLE
A transformer with a 5.75% impedance and a 480 volt low voltage winding
would require (0.0575 x 480 volt) or 27.6 volts to circulate rated current.
● IN SHORT
Impedance is simply the opposition to the flow of current in the transformers
windings.
(Impedance has a resistive and an inductive component.)

IMPEDANCE
● High or Low Impedance does not necessarily mean Low or High Efficiency.
● Impedance consists of two components:
1) A Resistive Component
2) A Reactive Component
%IZ = ( % IR ) 2 + ( % IX ) 2

Efficiency
Definition:
The ratio of power absorbed by the transformer to the total power drawn from the supply
line.
Efficiency can be calculated using the following formula:
EFF = [1 - (Y + (L)2 (X))/(100L + Y + (L)2 x)] 100
Where X = % IR @ 100% Load
Cosine 0
Cosine 0 = Power Factor Cosine 0 = Power Factor
% IR = Load Loss (KW) x 100 % FE = No Load Loss (KW) x 100
Rated kVA Rated kVA
Y = % FE @ 100% Load L = Load Factor = % Load
Cosine 0 (Per Unit Load) 100

Example:
What is the efficiency at 100% load and 1.0 power factor of the 75kVA
transformer with:
No-Load Loss: 341 Watts
Load Loss: 1478 Watts
Impedance: 3.45 %
% IR = (1.478/75) x 100 = 1.97
PF = 1.0
X = % IR = 1.97 = 1.97
PF 1
% FE = .0341 x 100 = 0.454
75kVA
Y = % FE = 0.454 = 0.454
PF 1
L = % Load = 100 = 1
100 100

Heat Dissipation
Some consultants think that a transformer with a lower
temperature rise is better for vault and indoor applications. This
is not correct.
Transformers have losses.
Each watt of total loss = 3.413 BTU/Hr
Multiply the total loss by this value to determine the heat which
must be dissipated by air conditioning, ventilation, etc.

Tests
● WINDING RESISTANCE
● PURPOSE: The fundamental purpose of this test is to supply resistance data which
will be used to calculate I2R conductor losses and winding temperatures. It also
enables a check of coil continuity and hence, is often used in the field as an indicator
of possible damage to any internal coil connections.
● RATIO, POLARITY, AND PHASE RELATION
● PURPOSE: To verify that the transformer has the correct number of turns in each
winding (ratio and that the polarity and phase relationship of the windings are correct.
● INSULATION POWER FACTOR
● PURPOSE: To verify that the transformer has been properly dried and that its
moisture content is not excessive. Insulation power factor is defined as the ratio of
power dissipated in the transformer insulation (in watts) to the product of the effective
voltage and current (in volt-amperes) while being tested with a sine-wave voltage.

Tests
●EXCITATION LOSS
● PURPOSE: To verify that excitation losses, which consist primarily of winding losses
due to exciting current and losses in the transformer’s core, meet guaranteed values
promised to the customer.
●EXCITATION CURRENT
● PURPOSE: To verify core construction quality
●LOAD LOSS
● PURPOSE: To verify that the load losses meet the guarantees given to a customer.
●IMPEDANCE
● PURPOSE: To verify guarantees given to the customer.

Dielectric Tests
● APPLIED POTENTIAL TEST
● PURPOSE: To verify that the dielectric strength of the insulation from
winding to ground and winding to winding is adequate.
● PROCEDURE: A voltage level defined by IEEE is applied to the winding
under test for a period of 60 seconds. (34KV for 15 KV, 95KV BIL Units)
● INDUCED POTENTIAL TEST
● PURPOSE: To verify the insulation between turns and layers of the same
winding, and phase to phase clearance is sufficient.
● PROCEDURE: A special high frequency generator is used to prevent the
core from saturating due to the high voltage that is induced in the winding.
(120, 180, 240 or 400 Hz are commonly used). A voltage up to two times
the rating of the winding is applied for a period of 7200 cycles.

Impulse Test
● PURPOSE
● To verify that the transformer
insulation (BIL) can withstand the
transient over voltages.
● TYPES OF IMPULSE TESTS
● Half Wave
● Full Wave
● Chopped Wave: A more severe
test as it simulates a transient that is
chopped to ground by an arrester
QC impulse is done on all of our
products. This impulse test includes
full and half wave. The Chopped
Wave is included when ANSI
Impulse Test is included in the
specification.

Design Tests
●TEMPERATURE RISE TEST
● PURPOSE: To verify that the top oil and winding temperature rise does not
exceed the guaranteed levels under full load operation.
●SOUND LEVEL TEST
● PURPOSE: To verify that the sound level produced by the transformer is
within guaranteed values. The principle noise associated with transformers,
aside from that produced by any cooling fans, is an audible hum due to a
phenomenon called Magnetostriction. When core steel is excited by a
magnetic field it contracts slightly. This dimensional change which occurs
120 times per second for a 60 cycle system (once every half cycle) is
defined as Magnetostriction.

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