TAPS IN AUTOTRANSFORMERS

ByDr.Tomasz Kalicki E-mail: tomasz.kalicki@HydroOne.com and &V. Sankar P.Eng. E-mail: powertransformer@hotmail.com

Tutorial presented at IEEETransformers CommitteeMeetings on October 25,2010 in Toronto, Canada.

OBJECTIVES• To assist the users in procuring autotransformers which meet

their system requirements at economical prices. • To explain the effects of taps on cost and autotransformer

design and reliability.• To help users in preparing functional specifications that

provide scope for development.• To bring to the attention, that it may be dangerous when taps

are used different to their functions stated in the specifications.• To influence IEEE and IEC to conduct workshops on how to

determine tap range, type of taps, taps location etc.,• To highlight the importance of interaction between users and

manufacturers for mutual benefits.

TOPICS COVERED

• Types of taps

• Specifications and standards

• Considerations for taps location

• Types of tap changers

• Tapping windings

• Conclusions

AUTOTRANSFORMER DESIGNATIONS

TYPE of TAPS

Autotransformer taps can be distinguished based on:

• Function• Type of Tap Changer• Connections• Electrical Location

Taps Functions

• Constant flux tapsIf the voltage changes with each tap position in direct proportion to the turns then volts per turn is constant throughout the tap range. Such taps are known as constant flux taps

• Variable flux taps If the volts per turn changes with each tap position, then the taps are known as variable flux taps. With these taps flux density inthe core changes when the taps are changed.

• Mixed regulation tapsA portion of the taps act as constant flux taps and the remaining portion act as variable flux taps. Many autotransformers are purchased as constant flux taps or as variable flux taps but in service they are mostly used as mixed regulation taps.

IEC definitions:

Tap Changer Type

• DTC – De-Energize Tap Charger

Tap changer operation can only be perform while transformer is de-energized

• LTC – Load Tap Changer

Tap changer operation can be perform while transformer is loaded

Type of Connection

• Linear taps• Coarse / Fine taps• Reversing taps

Electrical Connection

• In Series Winging.• In Common winding.• In both Common and Series windings• In LV line end.

Present Status of Standards

• There are no recommendations

• C57.12.10 is being revise

• Many users are specifying same requirements as recommended for two winding transformers

Types of Tap Changer and electrical connection - DTC

Example DTC – bridging type taps in SV winding

Example DTC – Selector type taps in SV winding

Types of Tap Changer and electrical connection - DTC

Example DTC – location separate winding between CV and SV

CVDT SV

C

Types of Tap Changer and electrical connection - DTC

Types of Tap Changer and electrical connection - DTC

Example DTC – location separate winding outside SV

CV

DTC

SVDTC

Very seldom some specification requires also

• DTC TAPS IN TERTIARY.• DTC TAPS IN LV LINE.

In general, from cost and performance point of view, it is not recommended to specify DTC taps if possible

In case both LTC and DTC taps are required, extend LTC tap range and eliminate DTC taps.

Types of Tap Changer and electrical connection - DTC

Example LTC – Linear type, at series end and in LV line

Types of Tap Changer and electrical connection- LTC

Example LTC – linear, at neutral end and in both series and common windings

Types of Tap Changer and electrical connections - LTC

Example LTC - Reversing; in series winding, in LV line and in common winding

Types of Tap Changer and electrical connections - LTC

Example LTC - Coarse/Fine;in series windings, in LV line and in common winding

Types of Tap Changer and electrical connection - LTC

DTC & LTC TAPSA few Typical electrical arrangements

DTC & LTC TAPSA few Typical electrical arrangements

C O TV LTC CV SV DTC R Reversing Linear E Figure 20A C O TV LTC CV DTC SV R Reversing Linear E Figure 20B C TV LTC CV DTC SV O Reversing Reversing R E Figure 20C C O TV CV LTC DTC SV R Reversing Linear E Figure 20D C O TV CV LTC DTC SV R Reversing Reversing E Figure 20E

Calculated percent impedances on 90MVA base DTC +/- 5% & LTC +/-10% (as recommended for two windings):

Figure 20 A Figure 20 B Figure 20 C Figure 20 D Figure 20 EHV to LV

• DTC1 – LTC1 7.08 6.08 5.97 5.36 5.72 • DTC3 – LTC17 5.75 5.75 5.75 5.75 5.75• DTC5 – LTC33 4.89 5.76 5.84 6.46 6.24• DTC5 – LTC1 5.38 6.12 6.21 5.03 4.34• DTC1 – LTC33 6.52 5.72 5.66 7.02 7.81

TV to LV • LTC1 21.38 17.06 15.68 4.87 6.17 • LTC17 23.99 19.11 17.54 4.68 5.95• LTC33 27.63 21.97 20.16 4.72 5.99

TV to HV• DTC1 32.19 25.91 24.11 11.35 13.42• DTC3 31.26 25.83 24.12 11.11 12.61 • DTC5 30.42 25.77 24.16 10.86 11.87

DTC & LTC TAPS

Taping windings –impact on dielectric design

Centre fed design End fed design

Taping windings –impact on dielectric design

Taping windings –impact on dielectric design

Arrangements A Arrangements B Arrangements C

Taping windings –impact on dielectric design

SV Line

LTC

SV

Taping windings –impact on dielectric design

Taping windings –impact on dielectric design

Arrangements A Arrangements B

Taping windings –impact on dielectric design

Taping windings –impact on dielectric design

Arrangements A Arrangements B

EFFECTS OF TAPS ON STEP-DOWN AND STEP-UP OPERATIONS

Tap-Location Operation Constant Varying CoreVoltage Voltage Flux

____________________________________________________________Series Step-down HV LV variable

LV HV constantStep-up HV LV constant

LV HV variable

LV Line Step-down HV LV constantLV HV variable

Step-up HV LV variable LV HV constant

Common Step-down HV LV variableLV HV variable

Step-up HV LV variableLV HV variable

EFFECTS OF TAPS ON STEP-DOWN AND STEP-UP OPERATIONS

EFFECTS OF TAPS ON STEP-DOWN AND STEP-UP OPERATIONS

EFFECTS OF TAPS ON STEP-DOWN AND STEP-UP OPERATIONS

EFFECTS OF TAPS ON STEP-DOWN AND STEP-UP OPERATIONS

EFFECTS OF HV OR LV TAPS ON TERTIARY

1. BURIED TERTIARY

2. TERTIARY BROUGHT-OUT

3. EFFECTS OF PHYSICAL LOCATION OF HV OR LV TAP WINDING ON TV IMPEDANCES.

OVERLOADS AND TAP SPECIFICATION

1. INADEQUATE INFORMATION IN THE SPECIFICATIONS

2. NO CORRELATION AMONG OVERLOADS, SPECIFICATIONS AND THE TAP RANGE

3. SPECIFICATION MUCH DIFFERENT FROM THE REALISTIC CONDITIONS

Common omissions found in the specifications concerning overload:

TAP WINDINGS

• TAPPED HELIX

• MULTI START

• DISC

Types of tapping windings:

CONCLUSIONS

1. TAPS IN AUTOTRANSFORMERS POSE MORE DIFFICULTS IN DESIGN AND MANUFACTURING COMPARED TO TWO WINDING TRANSFORMERS

2. AS IMPEDANCE VARIATION OVER THE TAP RANGE DEPENDS ON MANY FACTORS AND COULD VARY WIDELY, BASED ON THEIR SYSTEM REQUIREMENTS USERS SHOULD SPECIFY THE VALUES/LIMITS.

3. STRONGLY RECOMMEND NOT TO SPECIFY DTC TAPS.

4. AS THE ELECTRICAL LOCATION OF THE TAPS INFUENCES THE COSTS, THIS SHOULD BE CAREFULLY SELECTED.

CONCLUSIONS

5. IEEE TRANSFORMERS COMMITTEE TO CONDUCT WORKSHOP ON HOW TO DETERMINE THE TAP RANGE AND SPECIFICATIONS WRITING.

6. A TENDER REVIEW MEETING IS MORE ESSENTIAL THAN A DESIGN REVIEW MEETING.

7. SPECIFICATIONS SHOULD ENCOURAGE INNOVATIONS AND DEVELOPMENTS.

8. INTERACTIONS BETWEEN USERS AND MANUFACTURERS BENEFIT BOTH.

ACKNOWLEDGEMENTS

1. FRANK DAVID, FD CONSULTING SERVICES.2. PETER FRANZEN, MANITOBA HYDRO.3. BERNHARD KURTZ, REINHAUSEN MFG. CO.4. SHIVANANDA PRABHU, Retired professor ofElectrical Engg, Ryerson University, Canada. 5. HYDRO ONE NETWORKS.6. CG Power Systems Canada Inc.

FOR COMMENTS/CLARIFICATIONS ON THIS TUTORIAL PLEACE CONTACT

Dr Tomasz KalickiE-mail: tomasz.kalicki@HydroOne.com

TEL: 416-345-6111or

Vallamkonda SankarE-mail: powertransformer@hotmail.com

TEL: 905-634-5926

THANK YOU