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Western Mining Electric Association San Antonio TX Transformer Loading & Short Circuit Considerations NOVEMBER 16, 2012

© SPX Transformer Solutions, Inc.

Layer vs. Disk Windings Discussion

PRESENTED BY

David L. Harris, PE

Customer Technical Executive

SPX Transformer Solutions, Inc.

Office: 262-521-0166

Cell: 262-617-3039

[email protected]

Dave has a BS Electrical Engineering from Clarkson University, Potsdam, New York, and an MS

Engineering Management from Milwaukee School of Engineering. He has been in the transformer

industry for 43 years in design, development, manufacturing, testing, marketing, sales and

management of transformers and load tap changers. Currently, he holds the position of Customer

Technical Executive for SPX Transformer Solutions. Dave is a Life Member of the IEEE and is

active in the Electric Power Industry as a past chair of several Working Groups and

Subcommittees for the IEEE Substations Committee and IEEE Transformers Committee. Dave is

an individual member of CIGRE.

COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2010

GLOBAL INFRASTRUCTURE X PROCESS EQUIPMENT X DIAGNOSTIC TOOLS

Impedance


IEEE Standard Impedance

IEEE Std C57.12.10-2010

High Voltage BIL Without LTC With LTC

≤ 110 5.5 --

150 6.5 7.0

200 7.0 7.5

250 7.5 8.0

350 8.0 8.5

450 8.5 9.0

550 9.0 9.5

650 9.5 10.0

750 10.0 10.5

Table 3 Percent impedance at self-cooled (ONAN) rating

The percent impedance voltage at the self-cooled rating as measured on the

rated voltage connection shall be as listed in Table 3 if the user does not

specify another value.

For cases not covered in Table 3, the percent impedance voltage value shall

be agreed between user and manufacturer, and the user should perform a

system study to determine the proper value of impedance.


IEEE Std C57.12.10-2010

IEEE Standard Impedance AutoTransformers

For autotransformers, the percent impedance voltage

shall be as specified by the user, or it should be the

lower of the value from Table 3 and the value obtained

according to the following equation:

Autotransformer impedance voltage = (Value from

Table 3) × (Autotransformer co-ratio) × 1.5 where

Autotransformer co-ratio = (High-Voltage – Low-

Voltage)/(High-Voltage)

This impedance voltage is the autotransformer

impedance and not the equivalent autotransformer

impedance.


Impedance: Directly Proportional - Frequency & MVA

Inversely Proportional - Volts/Turn

c a

b h

h

cba*turnsIX% 3

1

3

1

2

cestansisReWindingIR%

22 IR%IX%IZ%

Reactance

Resistance

Impedance

Core

Inner

Wdg

Outer

Wdg

Directly Proportional Frequency & MVA

Inversely Proportional Volts/Turn (Excitation)


Impedance Consequences

Low Impedance High Impedance

Increased Secondary Fault Currents

Reduced Secondary Fault Currents

Improved Voltage Regulation Potential Voltage Regulation Issues

Higher Short Circuit Withstand Forces

Lower Short Circuit Withstand Forces

Higher Interrupting Capacity for Secondary Equipment

Lower Interrupting Capacity for Secondary Equipment

Low Leakage Flux, Stray Losses, Winding Losses, Core Losses

Higher Leakage Flux, Stray Losses, Winding Losses, Core

Losses

Smaller footprint, Lighter, Reduced Cooling

Equipment Requirements

Larger footprint, Heavier, Increased Cooling Equipment

Requirements

NOTE: Both high and low impedance

increase costs


Impedance Effect on Voltage Regulation

18/24/30 MVA Transformer

Load Losses = 60 kW @ 18 MVA; Z = 8.0 @ 18 MVA base

= 166.67 kW @ 30 MVA, Z = 13.33 @ 30 MVA base

18 MVA 30 MVA

Power Factor % Regulation Power Factor % Regulation

1.0 0.64 1.0 1.43

0.9 4.02 0.9 6.95

0.8 5.24 0.8 8.92

18/24/30 MVA Transformer

Load Losses = 65kW @ 18 MVA; Z = 10.0 @ 18 MVA base

= 185.56 kW @ 30 MVA, Z = 16.67 @ 30 MVA base

18 MVA 30 MVA

Power Factor % Regulation Power Factor % Regulation

1.0 0.86 1.0 1.99

0.9 5.05 0.9 8.82

0.8 6.57 0.8 11.25 (> 10% LTC)

9

Nominal Voltage Case Study Example The transmission system is a nominal 138 kV transmission line, and the customer

distribution system is regulated 13.2 kV using LTCs for voltage regulation. Assuming the

HV DETCs are set on C tap and the transformer is rated 15/20/25(28) MVA 55ºC/65ºC rise.

Transformer

HV – 138 kV Nominal

Transformer (Alternate)

HV – 134 kV Nominal

HV DETC TAPS: 144.9, 141.45, 138.0, 135.55,

131.1 kV

HV DETC TAPS: 140.7, 137.35, 134.0, 130.65,

127.3 kV

LV: 13.8/7.967 kV GRDY

Rated current @ 28 MVA = 1171 amps

LV: 13.2/7.621 kV GRDY

Rated current @ 28 MVA = 1225 amps

When HV @ C tap (138 kV) and transmission

line operates @ 138 kV; LV @ Neutral tap = 13.8

kV; @ 7L = 13.2 kV

LTC efectively 13.2kV + 23 (14.4%)/- 9 (5.9%) @

steps = 0.0653%(86.25 kV)/step

When HV @ C tap (134 kV) and transmission line

operates @ 138 kV (1.03 per unit over excitation);

LV @ Neutral tap = 13.6 kV; @ 5L = 13.2 kV

LTC effectively 13.2 kV + 21( 13.1%) / - 12(9.25%)

@ steps = 0.0644%(82.5 kV)/step

Transformer Ratio HV C tap – LV N = 17.32 Transformer Ratio HV C tap – LV N = 17.58

Impedance @ 100 % excitation C-N = 8.0% Impedance @ 103% excitation C-N = 7.54%

Impedances and LTC steps are different are different: paralleling issues.

Ratios are different: paralleling issues

138 kV HV Transformer should be specified with full capacity LTC to 13.2 kV to

avoid capacity reduction

134 kV HV Transformer operates as standard @ 1.03% over excitation,

increases sound level, reduces impedance

Largest Shell Transformer Made in

the U.S. Completed and Shipped

• Efacec Power Transformers Inc. has successfully shipped the largest shell transformer made in the U.S. in more than 20 years. The electrical transformer has the unique feature that it can be delivered in four pieces, overcoming significant transportation restrictions.

• The 700-MVA 230-kV GSU transformer is the first of its type ever made in the U.S. It was completely designed, manufactured and tested at the firm’s Rincon, GA facility. A large electric utility bought the first unit made in the U.S., which was shipped last month.

• The huge shell transformer uses “disassociated phase technology," which allows it to be built in four pieces – three units and a fourth piece that fits on top – and assembled at its destination. The technology, perfected at Efacec’s Portugal plant, makes shipping such large transformers feasible.


Voltage Regulation

IEEE C57.12.80

3.495 voltage regulation of a constant-voltage transformer:

The change in output (secondary) voltage that occurs

when the load (at a specified power factor) is reduced from

rated value to zero, with the primary impressed terminal

voltage maintained constant.

14


Voltage Regulation IEEE C57.12.90 - 2010

15


Agenda

Thermal Design Considerations

Industry Guides

Limiting Parameters for Overloading

Theoretical Life

Functional Life

User’s Practice – Worldwide Survey

16

Let’s make this interactive!

If you have questions during the

presentation, please stop me to ask

them.



To Control Vary the Design Parameter

Average oil Amount of coolers (rads,

temperature over fans, pumps) and unit’s

ambient thermal time constant

Winding Gradient Current densities in winding

conductors and cooling

surface within winding

Hot Spot Gradient Current densities in winding

conductors and stray loss

distribution


Hot-spot temperature calculation – C57.12.00-2010, 5.11.1.1

a) Direct measurement during a thermal test in accordance with IEEE Std C57.12.90 A sufficient

number of direct reading sensors should be used at expected locations of the maximum temperature

rise as indicated by prior testing or loss and heat transfer calculations.

b) Direct measurement on an exact duplicate transformer design per a).

c) Calculations of the temperatures throughout each active winding and all leads. The calculation

method shall be based on fundamental loss and heat transfer principles and substantiated by tests on

production or prototype transformers or windings.

The maximum (hottest-spot) winding temperature rise above ambient temperature shall be included in the

test report with the other temperature rise data. A note shall indicate which of the above methods was used

to determine the value.

Electrical Design Process

COMPANY CONFIDENTIAL © SPX Transformer Solutions, Inc.

TRANSFORMERS | SERVICE | TRAINING | COMPONENTS

TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS

The circulation of oil

through the coils into the

tank and cooling is the

process that removes

heat from the windings

to the surrounding

environment.

Non-directed oil flow is

where the oil is allowed

to flow freely through the ducts and other oil channels in the windings

If the free flow of oil is directed along specific pathways in the

windings, it is known as directed oil flow

Circulation of the Oil

19




coils

radiators

T bot Q c T bot

Q r

T r,bot

Q n

Q s

Q c T

c,top

T top

Q r

T top

T varies linearly here

tank

Assumed oil temperature distribution inside tank. The oil flows, Q , as

well as the flow weighted temperatures are also indicated.

.

Typical Thermal Performance Calculation Variables


20



TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS 21

Natural Circulation of the Oil


22

Winding


Oil is free to

find its own

path from the

bottom of the

winding to the

top of the

winding.

Strategic

washers are

placed in the

winding to

direct the oil

flow.

Non Directed Oil Flow___ ____ Directed Oil Flow___




coils

radiators

T bot Q c T bot

Q r

T r,bot

Q n

Q s

Q c T

c,top

T top

Q r

T top

T varies linearly here

tank

Assumed oil temperature distribution inside tank. The oil flows, Q , as

well as the flow weighted temperatures are also indicated.

.

Typical Thermal Performance Calculation Variables


23




Hot-Spot Temperature Calculation – C57.12.00-2010, 5.11.1.1

a) Direct measurement during a thermal test in accordance with IEEE Std C57.12.90 A sufficient

number of direct reading sensors should be used at expected locations of the maximum temperature

rise as indicated by prior testing or loss and heat transfer calculations.

b) Direct measurement on an exact duplicate transformer design per a).

c) Calculations of the temperatures throughout each active winding and all leads. The calculation

method shall be based on fundamental loss and heat transfer principles and substantiated by tests on

production or prototype transformers or windings.

The maximum (hottest-spot) winding temperature rise above ambient temperature shall be included in the

test report with the other temperature rise data. A note shall indicate which of the above methods was used

to determine the value.

24

Electrical Design Process (cont.)




28 kV, 175.5 kV 28 kV, 195.5 kV 28 kV, 255.5 kV 28 kV, 235.5 kV


25

Hot-Spot Temperature Calculation – Example




Hot-Spot Temperature ( C Rise over ambient)

@ 175.5 kV @ 195.5 kV @ 255.5 kV @ 235.5 kV Hottest

Inner Winding 84.1 83.9 88.4 86.1 88.4

Winding 2 77.0 76.9 79.8 78.4 79.8

Winding 3 80.9 77.0 71.2 72.7 80.9

Winding 4 65.7 65.7 70.0 69.8 70.0

Outer Winding 65.4 65.6 69.5 66.2 69.5

Hot-Spot Temperature Calculation – Results

The inner winding and the Winding 3 had to be redesigned to lower the

hot-spot temperature rise below 80 C.

26





The two transformer oil circulation systems in common use:

Natural circulation, or thermosiphon, which relies on the viscosity change of the oil from temperature variation to produce oil flow (explain thermal head)

Forced circulation, which is the use of pumps as the means to create oil flow

Oil Circulation Systems

27


External Cooling

Radiators are the most common means used to increase the amount

of exposed oil surface area to the surrounding air in order to increase

the efficiency of heat exchange rate.

If dictated by loading or space requirements, higher efficiency heat

exchangers that employ pumps or water coolers can be utilized, with a

significant increase in cost.

Fans are a relatively inexpensive means to increase the rate of heat

dissipation from the radiators by increasing the volume of air moving

over the radiator surface.

Noise generated by the cooling fans varies with the blade design and

the operational speed of the cooling fans, and often becomes a limiting

factor in transformer loading and cooling.




Finite Element Analysis (FEA) is

performed on every new design

to generate electromagnetic field

plots

Analysis of loss distribution is

used to calculate the gradient

between the hottest-spot rise and

oil rise

Hottest-Spot Calculations

29


Oil to Winding Temperature Gradient

The gradient is defined as the difference between the average oil

temperature in the tank and the average conductor temperature

in the windings and is directly related to the amount of conductor

surface exposed to the surrounding oil in the windings.

Resistance and eddy losses per unit area are calculated and

entered into the following formula to determine the gradient.

COMPANY CONFIDENTIAL © Waukesha Electric Systems - 2011 31

Industry Practice on Transformer Loading

Industry Guides

C57.91-1981

Distribution Tr.

C57.92-1981

Power Tr.

ANSI/IEEE C57.91-1995 Guide for Loading

Transformers *

C57.115-1991

Power Tr.>100MVA

* This guide is currently being revised by the IEEE

Transformers Committee

IEC Publication 60076-7 is a 2005 update of IEC Pub 60354-1991



Industry Guides

• Main differences between IEEE and IEC standards and guides

are limits and calculation methods for Hot-spot temperature

• IEC limits are lower (160 °C) than the IEEE limit of 180 °C

• Previous IEC guide utilized the H factor of 1.1(for Distribution

Tr.) to 1.3 (for Power Tr.)

• New IEC revision states this H factor ranges 1.0-2.1

o The factor H should be defined either by direct

measurement or by a calculation procedure based on

fundamental loss and heat transfer principles, and

substantiated by direct measurements on production or

prototype transformers or windings.

o General practice still is to use the default values of H

• IEEE requires more accurate calculation and verification

(C57.12.00)



An example of following two industry guides (IEEE v.s.

IEC)

• Typical Power Transformer will have a “gradient” value of 20 C

• Following the IEEE guide, manufacturers can usually estimate

the hot-spot temperatures with 5 C accuracy

• Following the old IEC practice, HST is 26 C (20 X 1.3)

• CIGRE reported that the Hot-Spot factor ranges from 0.9 to 2.1,

verified by fiber optic sensors

o This translates to HST could be 18 to 42 C over the oil



Limiting Parameters for Overloading

• Under old guides;

o Top oil temperature 110 C

o Winding Hot-spot temperature 180 C

o Maximum short-time loading 2 times Maximum Nameplate rating

• Under 1995 guide; (for distribution transformers)



o Short-time loading (1/2 hours or less) 3 times Maximum Nameplate rating

• Under 1995 guide; (for power transformers)



o Maximum loading 2 times Maximum Nameplate rating


Under IEC 60076-7, 2005;

Types of Loading Distribution

Transformers

Medium Power

Transformers

Large Power

Transformers

Normal cyclic loading

Current (p.u.) 1.5 1.5 1.3

Hot-spot temperature and metallic

parts in contact with insulating

material ( C)

120

120

120

Top-oil temperature ( C) 105 105 105

Long-time emergency cyclic loading

Current (p.u.) 1.8 1.5 1.3



material ( C)

140

140

140

Top-oil temperature ( C) 115 115 115

Short-time emergency cyclic loading

Current (p.u.) 2.0 1.8 1.5



material ( C)

N/A

160

160

Top-oil temperature ( C) N/A 115 115





Transformer Thermal Loading Specification

36




604.27273

15000exp

HSTLife

Where, Life = Life in hours at temperature HST

HST = Hot Spot Temperature in C

Theoretical Life

37




Rule of Thumb: For every 6 to 8 degrees the hot spot

temperature is reduced, the theoretical life of the

transformer insulation doubles.

38

Theoretical Life (cont.)




Basis

Normal Insulation Life

Hours Years

50% retained tensile strength of insulation (former IEEE Std C57.92-1981 criterion)

65,000 7.42

25% retained tensile strength of insulation 135,000 15.41

200 retained degree of polymerization in insulation 150,000 17.12

Interpretation of distribution transformer functional life test data (former IEEE Std C57.91-1981 criterion)

180,000 20.55

"Normal insulation life" of a

well-dried, oxygen-free,

65 C average winding

temperature rise insulation

system at the reference

temperature of 110 C.

Theoretical Life (cont.)

39




Insulation Life Testing Process

40




Aging of insulation materials is

caused by a number of factors • Moisture

• Oxygen

• Temperature

• Time

Proper application of oil

preservation systems and

maintenance can minimize

the moisture and oxygen

content

Proper loading can minimize

the hot-spot temperature

Oil Preservation Systems

41


MPEG Video


Functional Life




Defined as “conditions which the transformer does not function as intended”

Overloading can cause “bubbles” in the oil that cause dielectric failures (hot-spot temperatures)

Overloading can cause tank pressure build-up that cause gasket leaks and PRD operation (average oil temps)

Other loading related issues: • Current carrying components’ ratings

• CT saturation

• Lead heating

• Leakage flux

• Overheating

Functional Life (cont.)

43




Sources of bubbles

• Gasses dissolved in oil

• Gasses generated from

decomposition of insulation

• Water vapor from paper insulation

in windings

Sudden release of gas/vapor

as bubbles is possible under

overloading

Functional Life – Bubbles

44




Next series of slides were

presented at the 2001

IEEE T&D Conference in

Atlanta by T.V. Oommen*

Based on two EPRI

reports:

• EL-6761 (March 1990)

• EL-7291 (March 1992)

Functional Life – Bubbles (cont.)

* Permission to use them granted by the author.

45



Functional Life (Bubbles)

• Revised Loading Guide C57.91 will contain the equation for

Bubble evolution temperature.



Functional Life (Bubbles)

• Summary from these reports on Bubbles

o Bubble Generation from Overload is mostly due to water

vapor released from paper insulation

o Gas blanked units and conservator units show little

difference in bubble evolution at low moisture levels

o Increasing gas saturation in oil lowers bubble evolution

temperature only at high moisture levels

o Accepting 140 °C as Hot-spot temperature limit appears

to be valid for moisture content above 1.5%

• CIGRE did survey in 1995 and found similar practice (140 °C

limit)



User’s Practice – Worldwide Survey

(According to the CIGRE Working Group 12.09)

• Why should the Hot-spot temperature be limited?

o 60% of the users Free Gas Bubbles

o 20% of the users Thermal Decomposition

o 20% of the users Both

• How high this temperature can be?

o Depends on many factors including moisture content, gas

saturation level, static pressure, etc

o Generally below 140 °C



Conclusion and Recommendations

Monitoring, Maintenance, Proper implementation of the Operating strategy, and taking appropriate actions in time protect your investment and provide maximum return on that investment

Specifications, Design, Execution of these at OEM, and Verification testing assures built-in robustness




Conductor Tilting Force – Critical

58




Winding Radial Forces

59

Maximum Radial Stress (psi):

Failure Modes:




Winding Beam Bending Forces

60



TRANSFORMER LOADING AND THERMAL DESIGN CONSIDERATIONS 61

Typically a problem

for “Layer” winding

Can happen to “disk”

or “helical” windings

Extent of damage to

paper insulation will

determine how soon

a total unit failure will

happen

Winding Spiral Tightening Forces


Wild Life Outages www.wildlifeoutages.com

62



63



64


Thank you

January 9, 2013 65

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