– Auxiliary transformer used to be wye-wye (Yy) autotransformer
Significant application issues for North American market
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Power Transformer 3-phase core form design, using stacked core Padmount Distribution Transformer 3-phase design, using 5-leg wound core
H1 H2 H3
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Power Transformer Connections
Wye-delta (Ynd1)
– Provides grounding to utility grid – Provides no grounding to collector system – Isolates ground current and V0 – -30 (+ n*120) phase shift – Makes grid line voltage appear as phase voltage on collector, grid phase voltage as line voltage (minus V0)
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Typical Transformer BILs
Line Voltage (kV) 345 230 161 138 115 34.5 Basic Insulation Levels 900, 1050, 1175 650, 750, 825, 900 550, 650, 750 450, 550, 650 350, 450, 550 200 (150 and 125 for distribution)
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Power Transformer Connections
Delta-wye (Dyn1)
– Provides no grounding to utility grid – Provides grounding to collector system – Isolates ground current and V0 – -30 (+ n*120) phase shift – Makes grid line voltage appear as phase voltage on collector, grid phase voltage as line voltage (minus V0)
– Isolation from collector system zero sequence – Less phase-ground overvoltage during feeder islanding during ground fault
Not much connects phase-ground on 575 V side
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Transformer Insulation Level
Ability to withstand transient overvoltage is given by BIL – Basic Impulse Level – 1.2 x 50 s double-exponential waveform
Other wave withstands are factors times BIL – BSL – Basic Switching Level, 250 x 2500 s waveform, 0.83*BIL – CWW – Chopped Wave Withstand, 1.2 x 50 s waveform chopped in 3 s, 1.1*BIL
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Transformers: 1-ph, 3-ph
Three-phase transformers typical for windfarms
– Bank of 3 single-phase transformers costs more – Sometimes used for 4/3 spare, or if total MVA is very large
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Transformer Excitation
Transformer exciting current is very nonlinear
1.15 – 1.20 1.0 Slope = 0.3 – 0.5 p.u. for power = 0.08 – 0.15 for padmount
Wind Farm University
Topic No. 6
Power Equipment
Reigh Walling
May 6, 2004
Copyright 2004 General Electric International, Inc.
Transformer Types
Power – Designed and built to specification – Quality equipment – Applied with extensive protection – Maintained Distribution – Semi-custom design – Commodity equipment – Fuse protection at most – Often installed and ignored
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Power Transformers
Rating is given in MVA (at rated secondary voltage) Typically three MVA ratings; example 50/67/83 MVA
Elevated phase-ground 575 V voltage during grid ground faults for Yy (only TVSS affected?)
– High overvoltages for either Dy or Yy when feeder islands
Dy generally worse (saturation of Yy)
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PMT Connections - History
Originally was developer’s choice GEWE has specified delta-wye (Dyn)
– First rating is natural cooling – ONAN – Second rating is with one stage of fans – ONAF – Last rating is with full fans – ONAF
Steady-state voltage limit is +10% (flux voltage) at no load, +5% at full load
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Power Transformer Connections
Wye-wye (Ynyn0)
– Passes through grid grounding to collector system – Provides no ground source – Does not isolate zero sequence – No phase shift – Collector and grid voltages are scaled images of one another
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Power Transformer Connections
Wye-wye-delta (Ynyn0d1)
– Provides ground source to both grid and collector – Zero sequence isolated, not 100% – Tertiary winding may not even have terminals, rated approx. 30% of full MVA – No phase shift – Collector and grid voltages are scaled images of one another, minus V0