A zigzag transformer is a special-purpose transformer with a zigzag or "interconnected star" winding connection, such that each output is the vector sum of two (2) phases offset by 120°.[1] It is used as a grounding transformer, creating a missing neutral connection from an ungrounded 3-phase system to permit the grounding of that neutral to an earth reference point; to perform harmonic mitigation, as they can suppress triplet (3rd, 9th, 15th, 21st, etc.) harmonic currents;[2] to supply 3-phase power as an autotransformer (serving as the primary and secondary with no isolated circuits);[3] and to supply non-standard, phase-shifted, 3-phase power.[1]
9-winding zigzag transformer
Zigzag transformer
Nine-winding, three-phase transformers typically have three primaries and six identical secondary windings, which can be used in zigzag winding connection as pictured.[1] As with the conventional delta or wye winding configuration three-phase transformer, a standard, stand-alone transformer containing only six windings on three cores can also be used in zigzag winding connection, such transformer sometimes being referred to as a zigzag bank.[1] In all cases, six or nine winding, the first coil on each zigzag winding core is connected contrariwise to the second coil on the next core. The second coils are then all tied together to form the neutral, and the phases are connected to the primary coils. Each phase, therefore, couples with each other phase, and the voltages cancel out. As such, there would be negligible current through the neutral point, which can be tied to ground.[4]
Each of the three "limbs" are split into two sections. The two halves of each limb have an equal number of turns and are wound in opposite directions. With the neutral grounded, during a phase-to-ground short fault, a third of the current returns to the fault current, and the remainder must go through two of the three phases when used to derive a grounding point from a delta source.[5]
If one or more phases fault to earth, the voltage applied to each phase of the transformer is no longer in balance; fluxes in the windings no longer oppose. (Using symmetrical components, this is Ia0 = Ib0 = Ic0.) Zero-sequence (earth fault) current exists between the transformer’s neutral to the faulting phase. The purpose of a zigzag transformer in this application is to provide a return path for earth faults on delta-connected systems. With negligible current in the neutral under normal conditions, an undersized (unable to carry a continuous fault load) transformer may be used only as short-time rating is required, provided the defective load will be automatically disconnected in a fault condition. The transformer's impedance should not be too low for desired maximum fault current. Impedance can be added after the secondaries are summed to limit maximum fault currents (the 3Io path).[6]
A combination of Y (wye or star), delta, and zigzag windings may be used to achieve a vector phase shift. For example, an electrical network may have a transmission network of 110 kV/33 kV star/star transformers, with 33 kV/11 kV delta/star for the high voltage distribution network. If a transformation is required directly between the 110 kV/11 kV network an option is to use a 110 kV/11 kV star/delta transformer. The problem is that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the secondary side of the 110 kV/11 kV transformer provides the required earth reference point.[7]
Applications
Zigzag transformers are often required by utilities when connecting three-phase inverters (usually for renewable energy) to the grid to provide a stable neutral voltage and prevent excessive phase-to-ground voltages. This also protects the switching devices inside the inverters, which are usually insulated-gate bipolar transistors (IGBTs).[8]
References
^ abcdLawhead, Larry; Hamilton, Randy; Horak, John (May 2006). Three phase transformer winding configurations and differential relay compensation(PDF). 60th Annual Georgia Tech Protective Relay Conference. pp. 8–10. Retrieved 27 December 2015.
^Khera, P.P. (October 1990). "Application of zigzag transformers for reducing harmonics in the neutral conductor of low voltage distribution system" (PDF). IEEE Trans. on Industry Applications. doi:10.1109/IAS.1990.152320.
^Sankaran, C. (1 July 2000). "The Basics of Zigzag Transformers". EC&M Magazine. Retrieved 22 February 2012.
zigzag, transformer, zigzag, transformer, special, purpose, transformer, with, zigzag, interconnected, star, winding, connection, such, that, each, output, vector, phases, offset, used, grounding, transformer, creating, missing, neutral, connection, from, ungr. A zigzag transformer is a special purpose transformer with a zigzag or interconnected star winding connection such that each output is the vector sum of two 2 phases offset by 120 1 It is used as a grounding transformer creating a missing neutral connection from an ungrounded 3 phase system to permit the grounding of that neutral to an earth reference point to perform harmonic mitigation as they can suppress triplet 3rd 9th 15th 21st etc harmonic currents 2 to supply 3 phase power as an autotransformer serving as the primary and secondary with no isolated circuits 3 and to supply non standard phase shifted 3 phase power 1 9 winding zigzag transformer Zigzag transformer Nine winding three phase transformers typically have three primaries and six identical secondary windings which can be used in zigzag winding connection as pictured 1 As with the conventional delta or wye winding configuration three phase transformer a standard stand alone transformer containing only six windings on three cores can also be used in zigzag winding connection such transformer sometimes being referred to as a zigzag bank 1 In all cases six or nine winding the first coil on each zigzag winding core is connected contrariwise to the second coil on the next core The second coils are then all tied together to form the neutral and the phases are connected to the primary coils Each phase therefore couples with each other phase and the voltages cancel out As such there would be negligible current through the neutral point which can be tied to ground 4 Each of the three limbs are split into two sections The two halves of each limb have an equal number of turns and are wound in opposite directions With the neutral grounded during a phase to ground short fault a third of the current returns to the fault current and the remainder must go through two of the three phases when used to derive a grounding point from a delta source 5 If one or more phases fault to earth the voltage applied to each phase of the transformer is no longer in balance fluxes in the windings no longer oppose Using symmetrical components this is Ia0 Ib0 Ic0 Zero sequence earth fault current exists between the transformer s neutral to the faulting phase The purpose of a zigzag transformer in this application is to provide a return path for earth faults on delta connected systems With negligible current in the neutral under normal conditions an undersized unable to carry a continuous fault load transformer may be used only as short time rating is required provided the defective load will be automatically disconnected in a fault condition The transformer s impedance should not be too low for desired maximum fault current Impedance can be added after the secondaries are summed to limit maximum fault currents the 3Io path 6 A combination of Y wye or star delta and zigzag windings may be used to achieve a vector phase shift For example an electrical network may have a transmission network of 110 kV 33 kV star star transformers with 33 kV 11 kV delta star for the high voltage distribution network If a transformation is required directly between the 110 kV 11 kV network an option is to use a 110 kV 11 kV star delta transformer The problem is that the 11 kV delta no longer has an earth reference point Installing a zigzag transformer near the secondary side of the 110 kV 11 kV transformer provides the required earth reference point 7 Applications EditZigzag transformers are often required by utilities when connecting three phase inverters usually for renewable energy to the grid to provide a stable neutral voltage and prevent excessive phase to ground voltages This also protects the switching devices inside the inverters which are usually insulated gate bipolar transistors IGBTs 8 References Edit a b c d Lawhead Larry Hamilton Randy Horak John May 2006 Three phase transformer winding configurations and differential relay compensation PDF 60th Annual Georgia Tech Protective Relay Conference pp 8 10 Retrieved 27 December 2015 Khera P P October 1990 Application of zigzag transformers for reducing harmonics in the neutral conductor of low voltage distribution system PDF IEEE Trans on Industry Applications doi 10 1109 IAS 1990 152320 Sankaran C 1 July 2000 The Basics of Zigzag Transformers EC amp M Magazine Retrieved 22 February 2012 Post Glover Zigzag Grounding Transformers Das J C 2002 Short Circuit Load Flow and Harmonics CRC Press pp 25 28 Blackburn J Lewis Protective Relaying Marcel Dekker Inc New York 1998 URJA Techniques india Pvt Ltd Zig Zag Transformers Acutran Retrieved 2021 02 11 Retrieved from https en wikipedia org w index php title Zigzag transformer amp oldid 1100831850, wikipedia, wiki, book, books, library,
