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

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



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 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 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 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



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



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




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


СПАСИБО

ЗАВНИМАНИЕ!

THANK YOU!


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