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Upon the Improvement of the Winding Design of Wind Turbine Transformers for Safer Performance within

Resonance Overvoltages

10th Deep Sea Offshore Wind R&D Seminar24-01-2013- Trondheim-Norway

Amir Hayati SolootHans Kristian Høidalen

Bjørn Gustavsen

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Contents

1. Challenges for wind farms2. Transient phenomena in Offshore wind farm3. Resonance Overvoltages4. Prototype wind turbine transformer for the

investigation of resonance overvoltages5. Measurement results6. Conclusion7. Future plan

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Challenges for wind farms

• Different challenges – Financial– political – Environmental– technological challenges: can be better

understood by observing the failures in wind farm which has occurred up to now.

Breakdown of component failures for on/offshore wind farms (Nitschke et al., 2006)

SINTEF report, “HSE challenges related to offshore renewable energy”, 15-02-12

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Transient phenomena in Offshore wind farm

1. Switching transients Energization and Deenergization2. Lightning transients3. Earth fault

Back to Back power converter

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Resonance Overvoltages1. It may occur during earth fault

or energization transients if:a. The quarter-wave frequency of

cable is close to one of resonance frequencies of transformer, especially the dominant resonance frequency.

b. The surge impedance of cable is much lower than transformer input impedance and much higher than source impedance

2. It leads to the highest overvoltage amplitudes with high du/dt compared to normal energization overvoltages.

0.1 0.102 0.104 0.106 0.108 0.11 0.112-8

-6

-4

-2

0

2

4

6

8

time(ms)

indu

ced

volta

ge(k

V)

V41V51V61

LV voltages for energization at peak of phase A on HV side of no load 300 kVA 11/0.230 kV transformer

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Prototype wind turbine transformer500kVA 11/0.230 kV

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The winding designsLayer winding

Disc winding Pancake winding

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The Diagnosis of resonance frequencies in 500 kVA transformer to select the less critical winding design for fixed-length cable energization in wind farms

104 105 106 10710-3

10-2

10-1

100

Frequency(Hz)

LV o

ver H

V v

olta

ge ra

tio

Pancake WindingLayer WindingDisc Winding

Measurement on 500 kVA transformer

Agilent E5061B-3L5 Network Analyzer

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Measurement Results for voltage drop

104 105 106 10710-3

10-2

10-1

100

101

Frequency(Hz)

Vol

tage

Dro

p/V

t

HVT-V24V24-V22V23-V21

104 105 106 10710-3

10-2

10-1

100

101

Frequency(Hz)

Vol

tage

Dro

p/V

t

VHt-T24T24-T22T23-T21

104 105 106 10710-3

10-2

10-1

100

101

frequency(Hz)

Vol

tage

Dro

p/V

t

VHt-V24V24-V22V23-V21

Voltage drop in taps near to HV terminal of the layer winding

Voltage drop in taps near to HV terminal of the disc winding Voltage drop in taps near to HV terminal of the pancake winding

The frequency response of thethree windings for voltage dropnear to HV terminal can beobserved and compared in thesefigures.

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Measurement Results for voltage drop

104 105 106 10710-3

10-2

10-1

100

101

frequency(Hz)

Vol

tage

Dro

p/V

t

T21-T19-DiscT21-T19-PancakeT21-T19-Layer

104 105 106 10710-3

10-2

10-1

100

101

frequency(Hz)

Vol

tage

Dro

p/V

t

T03-T01-DiscT03-T01-PancakeT03-T01-layer

Voltage drop between taps 21 and 19 in three windings Voltage drop between taps 3 and 1 in three windings

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Measurement Results for voltage to ground

104 105 106 10710-2

10-1

100

101

Frequency(Hz)

Vol

tage

to g

roun

d/ V

t

T02T10T18

104 105 106 10710-3

10-2

10-1

100

101

Frequency (Hz)

Vol

tage

to g

roun

d/V

t

T02T10T18

104 105 106 10710-3

10-2

10-1

100

101

Frequency (Hz)

Vol

tage

to g

roun

d/V

t

T02T10T18

Voltage to ground for tap 2,10 and 18 in layer winding

Voltage to ground for tap 2,10 and 18 in disc winding Voltage to ground for tap 2,10 and 18 in pancake winding

The frequency response of thevoltage to ground at the specifictaps in the three windings

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Measured voltage-to-ground distribution in layer winding

The dominant resonance frequency in the layer winding is approximately 1 MHz, whichmeans the energization of the transformer with about 50 meter cable is not recommended (seethe equation). The voltage ratio can be maximum mainly in the taps near to HV terminal. But,there is also one voltage ratio peak near to ground. Higher cable length is less critical.

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Measured voltage-to-ground distribution in disc windingThe dominant resonance frequency in the disc winding is approximately 70 kHz and thereare many resonance peaks between 100 and 500 kHz , which means the energization of thetransformer with cables more than 100 meter is not recommended. The reason is that thevoltage ratio peaks appeared in all the taps (see the right figure).

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The frequency response of the pancake winding is combination of layer and disc winding, i.e.resonance peaks in both 10kHz < f < 1MHz and f> 1MHz. According to the frequencyresponse, the energization can be performed with 100-500 meter cables considering theinstallation of the protective devices in the taps near to the HV terminal.

Measured voltage-to-ground distribution in pancake winding

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Conclusions• Resonance overvoltages at LV terminal for 500 kVA:

The dominant resonance frequency for layer winding is 1.6 MHz which the amplitude of transferred voltage is around 80 p.u.. The dominant resonance frequency for disc and pancake is 800 kHz which the amplitude is 6 and 38 p.u., respectively.

• Resonance overvoltages inside windings for 500 kVA:1. The voltage drops for taps near to HV terminal of the three windings, have high amplitudes (25 p.u.) at dominant resonance frequencies. 2. The layer and pancake windings have lower values further down in the middle of winding and near to ground. But, the disc winding keeps the high value of voltage drops at resonance frequencies which means more potential of internal stresses.3. The Voltage to the ground in near to HV terminal has low values at resonance frequencies (2 p.u.). But, taps near to ground show high value of voltage to ground at resonance frequencies (about 10 p.u.) for disc and layer winding.

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Future plan

• Developing analytical model of the 500 kVA transformer: 1-verification with the measurements, 2-study the effect of various design parameters on thefrequency response

• Modifying the analytical model with transformer kVA scaling equations in order to observe resonancefrequency shifts in 8 MVA transformer compared to 500 kVA one.

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Thanks for your attentionAny question?