Application of unit protection schemes for auto transformers
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Application of Unit Protection Schemes for Auto-Transformers. Zoran Gaji ć ABB AB Vasteras, Sweden. Authors: Z. Gaji ć, ABB Sweden S. Holst, ABB Sweden. Auto-Transformer. An auto-transformer is a power transformer in which at least two windings have a common part

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Application of Unit Protection Schemes for Auto-Transformers

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Application of unit protection schemes for auto transformers

Application of Unit Protection Schemes for Auto-Transformers

Zoran Gajić

ABB AB

Vasteras, Sweden

  • Authors:

    • Z. Gajić, ABB Sweden

    • S. Holst, ABB Sweden


Auto transformer

Auto-Transformer

  • An auto-transformer is a power transformer in which at least two windings have a common part

  • Typically auto-transformers are used to interconnect two electrical networks with similar voltage levels (e.g. system intertie transformer)

  • In practice auto-transformer tertiary delta winding is normally included. It serves to limit generation of third harmonic voltages caused by magnetizing currents and to lower the zero sequence impedance for five-limb core constructions or for auto-transformers built from three single phase units


Example auto transformer

Example Auto-Transformer


Auto transformer has dual rating

Auto-Transformer has Dual Rating

  • It can be shown that power is transferred in two different ways through an auto-transformer.

  • One part of the power is transferred by galvanic connection and the other part is transferred via magnetic circuit (i.e. transformer action)

  • Auto-transformer is cheaper that corresponding two/three winding power transformer design

  • Possible problem with short circuit current withstand


Auto transformer construction

One three-phase unit

Typically has five-limb core

Three single-phase units connected to form three-phase group

Auto-transformer Construction


Possible ct locations for auto transformer

Possible CT Locations for Auto-Transformer


Possible differential protection principles

Based on autotransformer ampere-turn balance 87T

Based on the first Kirchhoff’s law between galvanically interconnected parts 87B

Possible Differential Protection Principles


Special unit protections

Restricted Earth-Fault

Zero-sequence current based

Dedicated delta winding unit protections

Special Unit Protections


87t differential protection using ct1 and ct2

87T Differential protection using CT1 and CT2

  • SBase=Throughput Power(400MVA)

  • Tertiary delta winding can not be loaded

  • Mandatory zero sequence current reduction


87t differential protection using ct1 and ct21

87T Differential protection using CT1 and CT2


87t differential protection using ct1 ct2 and ct3

87T Differential protection using CT1, CT2 and CT3

  • SBase=Throughput Power(400MVA)

  • Tertiary delta winding can be loaded

  • Mandatory zero sequence current reduction


87t differential protection using ct1 ct2 and ct31

87T Differential protection using CT1, CT2 and CT3


87t differential protection using ct1 ct2 and ct7

87T Differential protection using CT1, CT2 and CT7

  • SBase=Throughput Power(400MVA)

  • Tertiary delta winding can be loaded

  • CT location within delta winding requires “special attention”

  • Zero sequence current reduction not required if it is a five-limb or single-phase construction


87t differential protection using ct1 ct2 and ct71

87T Differential protection using CT1, CT2 and CT7

* Influenced by CT location within tertiary delta winding


87t differential protection using ct1 ct5 and ct7

87T Differential protection using CT1, CT5 and CT7

  • SBase=Magnetic Power(169MVA)

  • Tertiary delta winding can be loaded

  • CT location at neutral point and within delta winding requires “special attention”

  • Zero sequence current reduction not required if it is a five-limb or single-phase construction

  • MV (i.e. 220kV) bushings not protected!


87t differential protection using ct1 ct5 and ct71

87T Differential protection using CT1, CT5 and CT7

Influenced by CT5 location in neutral point

* Influenced by CT7 location within tertiary delta winding


87t differential protection using ct1 ct2 ct7 and ct8

87T Differential protection using CT1, CT2, CT7 and CT8

  • SBase=Throughput Power(400MVA)

  • Tertiary delta winding can be loaded

  • Double CT location within delta winding requires “special attention”

  • Relay with four restraint inputs required

  • Mandatory zero sequence current reduction


87t differential protection using ct1 ct2 ct7 and ct81

87T Differential protection using CT1, CT2, CT7 and CT8

* Influenced by double CT location within tertiary delta winding


87t differential protection using ct1 ct5 ct7 and ct8

87T Differential protection using CT1, CT5, CT7 and CT8

  • SBase=Magnetic Power(169MVA)

  • Tertiary delta winding can be loaded

  • CT location at neutral point and within delta winding requires “special attention”

  • Relay with four restraint inputs required

  • Zero sequence current reduction not required if it is a five-limb or single-phase construction

  • MV (i.e. 220kV) bushings not protected!


87t differential protection using ct1 ct5 ct7 and ct81

87T Differential protection using CT1, CT5, CT7 and CT8

Influenced by CT5 location in neutral point

* Influenced by CT7, CT8 location within tertiary delta winding


87b differential protection using ct1 ct2 and ct6

87B Differential protection using CT1, CT2 and CT6

  • SBase=Throughput Power(400MVA)

  • Zero sequence current reduction not required

  • Tertiary delta winding can be loaded

  • Not sensitive for winding turn to turn faults


87b differential protection using ct1 ct2 and ct61

87B Differential protection using CT1, CT2 and CT6


Ref protection using ct1 ct2 and ct4

REF protection using CT1, CT2 and CT4

  • Base quantity is current (either CT2 or W2 rating)

  • Tertiary delta winding can be loaded but it is not protected

  • Operates only for phase to ground faults


Ref protection using ct1 ct2 and ct41

REF protection using CT1, CT2 and CT4


Dedicated unit scheme for tertiary winding

Dedicated unit scheme for tertiary winding

  • Use simple I> relay to provide only earth-fault protection for tertiary delta winding


Field recording

Field Recording

CT1

CT5

CT2

CT4 & CT7

  • External L2-Gnd fault which before clearing evolved into a L2-L3-Gnd fault

  • CT1, CT2, CT4 and CT7 currents recorded

  • CT5 current calculated

  • Delta winding not loaded, thus CT7 currents are identical in all three phases


Calculated diff currents from field recording

Calculated Diff Currents from Field Recording


Calculated ref diff current from field recording

Calculated REF Diff Current from Field Recording


Conclusion

Conclusion

  • The following data are crucial for proper application of the selected differential protection scheme for Auto-transformer:

    • Which base quantities (i.e. power, no load voltage and current) shall be used

    • Which vector group shall be entered

    • Whether or not zero sequence current elimination shall be enabled


Conclusion1

Conclusion

  • Selection of unit protection schemes for particular auto-transformer application depends on:

    • Available CTs

    • User preference

    • Previous experience


Application of unit protection schemes for auto transformers

СПАСИБО

ЗАВНИМАНИЕ!

THANK YOU!


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