SCCER FURIES Technical WorkshopSeptember 20, 2017

T. Guillod, F. Krismer, J. W. KolarPower Electronic Systems Laboratory, ETH Zurich, Switzerland

Dielectric LossesMV/MF Converter Insulation

2/29

Insulation in MV/MF Converters

► New SiC MV devices ► Higher voltages: 15 kV ► Higher switching frequency: 100 kHz ► Higher commutation speed: 150 kV/µs

► New topologies ► Single-stage or multi-cell converters ► Voltages at DC/low/medium frequencies ► High power densities ► High operating temperatures

► Proven to be critical ► Eagle Pass HVDC ► Large harmonics content ► Cable termination lifetime: one week ► Losses in the field grading

► Insulation coordination of MV/MF converters is unclear

▼ MV AC-DC Converter

[ETHZ PES MEGAlink]

▼ Eagle Pass HVDC

[ABB]

Paulsson, L., High-Frequency Impacts in a Converter-Based Back-to-Back Tie: the Eagle Pass Installation, 2003

3/29

MV/MF Electric Field

► Insulation coordination ► Dielectric losses ► Thermal breakdown ► Space charge migration ► Partial

/

surface discharges

► Parasitic resonances / shielding ► etc.

► Electromagnetic compatibility

► Dielectric losses ► Efficiency impact ► Thermal runaway ► Thermal breakdown ► Diagnosis (production and aging) ► Computation is possible

► Dielectric loss is an interesting figure of merit

▼ 7 kV / 100 kHz EMI

[ETHZ PES SwiSS SST]

▼ Partial Discharges

[RMS Energy]

4/29

Outline► Dielectric Material Parameters► Dielectric Losses Computation► Case Study: Solid-State Transformer► Conclusion

5/29

Outline► Dielectric Material Parameters► Dielectric Losses Computation► Case Study: Solid-State Transformer► Conclusion

6/29

Material: Polarization Types

► ε depends on many micro-physical processes► Conduction is usually negligible► Polarization is mostly linear

► How to compute the losses?

Adapted from Kao, K., Dielectric Phenomena in Solids, 2004

Pola

riza

tion

Time Constant [s]10-15 10-10 10-5 100 105

atomic

orientational

hopping

space charge

+

-+

-

+-

+

-+

+-

-

electronic

7/29

Dielectric Losses: Time Domain

► PWM voltage applied to the insulation ► Periodic rectangular pulses ► Finite rise time

► Time domain ► Convolution integrals (step response) ► Loss computation is difficult

► Frequency domain modeling

Supplied Energy Density

D/(ε

0εr,

0) [

V/m

]

-0.6

0.0

0.6

0.0

0.3

0.6

J/(ε

0εr,

0 / τ de

bye)

[V/

m]

w/(ε 0ε

r,0)

[V2 /

m2 ]

Displacement Field Current Density

0.0

0.5

1.0

Electric Field

E [V

/m]

0.0

0.5

1.0

fs = 100.0 kHz / t

r = 500 ns / Debye relaxation / ε

r,0’’ = 3.0 / ε

r,inf’’ = 2.0 / τ

debye = 320 ns

Time [µs]0 5 10

Time [µs]0 5 10

Time [µs]0 5 10

Time [µs]0 5 10

vacuum

E

losses

w

Dpolarization

J

vacuum

polarization

8/29

Dielectric Losses: Frequency Domain

► Polarization ► DC conduction is negligible ► Polarization is linear ► Frequency/temperature dependence ► ► Permittivity should be low

► Losses (dielectric between two electrodes)

►

► Geometry/Capacitance ► Material/Temperature ► Voltage/Frequency

► Dielectric losses depend on many parameters

9/29

Material: Frequency / Temperature

► ε’ / ε’’ for typical polymeric dry-type insulation materials► Loss peaks between polarization mechanisms

► Frequency and temperature dependencies are critical

Perm

itti

vity

/ ε

Perm

itti

vity

/ ε

Temperature / T

Frequency / log(f)

Inverse temperature / T-1

Freq

uenc

y /

log(

f)

Adapted from Menczel, J., Thermal Analysis of Polymers: Fundamentals and Applications, 2008

α peak

brittle rubber

DC: σ

Tg @ 1.6 mHz

β peak

DC: σα peak

ε’’, σε’

Tg

α peakβ peak DC: σ

ε’’, σ

ε’

β peak

10/29

Material: Measured Parameters

► Measured for a typical HV epoxy resin ► Damisol 3418 unfilled resin ► Tg = 136°C ► Frequency and temperature dependence ► Tg is a critical parameter (α peak @ 1.6 mHz)

► How to compute the losses?

Measurements / ε’ Measurements / ε’’

Measured with a Novocontrol Alpha-A Analyzer / vonRoll Damisol 3418

▼ Epoxy Sample

4.0

4.5

5.0

3.5

3.0

ε’ [

p.u.

]

0.0

0.3

0.4

0.1

0.2

ε’’ [

p.u.

]

20

55

90

125

160

Tem

pera

ture

[°C

]

1k 10k 100k 1MFrequency [Hz]

10M10020

55

90

125

160

Tem

pera

ture

[°C

]

1k 10k 100k 1MFrequency [Hz]

10M100

α

3.43.4

3.53.5

3.33.3 3.23.2

0.02

0.02

0.03

0.03

0.04

0.04

Tg4.04.0

0.06

0.06

0.20.2Tg

β

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Outline► Dielectric Material Parameters► Dielectric Losses Computation► Case Study: Solid-State Transformer► Conclusion

12/29

PWM: Spectral Losses

► Hypothesis: ε’’( f ) is constant

► PWM signal ► Switching frequency/speed ► Many harmonics ► Fast transitions lead to large losses ► Switching transition model is required ► Fundamental frequency analysis is incorrect

► Simple computation method is required

V p / V

DC

0.00.0

0.5

1.0

1/( 2f

s ) 1/(

fs )

0.0 tr 2t

r

Time

Time

PWM Voltage

Switching Transition

P n,c

/ P

1 [p.

u.]

P n /

P1 [

dB]

100 102 104 106

V n,RM

S /

V1,

RMS [

dB]

n = f /

fs

Power Cumulative Sum

-160

-120

-80

-40

0

-100

-75

-50

-25

0

0

1

2

3

4

5

Voltage Harmonics (dBvoltage) Power Harmonics (dBpower)

100 102 104 106

n = f /

fs

100 102 104 106

n = f /

fs

fs = 1.0 kHz / t

r = 100 ns / D

c = 0.5

V p / V

DC

0.0

0.5

1.0

tr ~ 1/f

c 10%

-90%

-10 dB/dec

-10 dB/dec

-30 dB/dec

-30 dB/dec

Pn

→ P

P1 P

n,c

-20 dB/dec

-20 dB/dec

-40 dB/dec

-40 dB/dec

Vn,RMS

fc /

fs

fc /

fs

fc /

fs

Dc /

fs

13/29

PWM: Constant Duty Cycle

► PWM with constant duty cycle ► Typical for DC-DC converter ► Finite switching speed

► Closed-form solution ► Approximation of partial sum/residual ► 2.5% accuracy ► Formula and derivation in [Gui16]

► Frequency and voltage are critical

V p / V

DC

0.00.0

0.5

1.0

1/( 2f

s ) 1/(

fs )

0.0 tr 2t

r

Time

Time

PWM Voltage

Switching Transition

P’ [

1/s]

f s [H

z]

tr [s]

Circuit TopologyFrequency / Rise Time Duty Cycle

Dc [p.u.]

fs = 10.0 kHz / t

r = 100 ns / D

c = 0.5 / P’ = P/(ε’’

∙ C

0 ∙ V

DC2)

V p / V

DC

0.0

0.5

1.0

V DC

V p

DUT

0.1 0.5 0.9

50·103

1k 103.5

10k

100k

10n 100n 1µ

104.0

104.5

105.0

105.5

106.0

40·103

30·103

20·103

10·103

0·103

P’ [

1/s]

105.5105.5

104.5104.5

104.0104.0

105.0105.0

tr 10

%-9

0%

Dc /

fs

14/29

PWM: Sinusoidal Modulation

► PWM with sinusoidal duty cycle ► Typical for AC-DC converter ► Multilevel inverters

► Closed-form solution ► Local averaging of PWM with constant D

c

► 3.4% accuracy ► Formula and derivation in [Gui16]

► Single-stage inverters are critical

V p / V

DC

-1.0

0.0

1.0

Time [ms]

Time [ms]

Full Bridge Inverter

Multilevel Inverter

P’1 [

1/s]

Mi [p.u.]

Circuit Topology (nc = 3)Modulation Index / Fundamental Modulation Index / Harmonics

Mi [p.u.]

fg = 50 Hz / f

s = 10.0 kHz / t

r = 100 ns / P’ = P/(ε’’

∙ C

0 ∙ V

DC2) = P’

1 + P’

harm

V p / V

DC

-1.0

0.0

1.0

P’ha

rm [

1/s]

0 5 10 15 20

0 5 10 15 20

0.1 0.4 0.7 1.0

0

50

100

150 80·103

60·103

40·103

20·103

0·103

0.1 0.4 0.7 1.0

V p

DUT

V DC /

nc

nc = [1,5]

nc = 1n

c = 2

nc = 3 n

c = 4

VpV

1

VpV

1

15/29

PWM: Scaling Laws

► PWM with constant Dc

► PWM with sinusoidal Mi

► Impact of frequency dependency of ε’’?

fs

Dc

tr

VDC

ε’’

P ~ fs ∙ log(const./fs)P ~ log(const./tr)

P ~ const.P ~ VDC

2

P ~ ε’’

fs

Mi

tr

VDC

ε’’

P1 ~ const.

P1 ~ const.

P1 ~ Mi2

P1 ~ VDC2

P1 ~ ε’’

fg P1 ~ fg

nc P1 ~ const.

Pharm ~ fs ∙ log(const./fs)

Pharm ~ log(const./tr)Pharm ~ const.

Pharm ~ VDC2

Pharm ~ ε’’

Pharm ~ const.

Pharm ~ 1/nc2

switching frequency

switching speed

duty cycle

voltage

material loss parameter

switching frequency

switching speedmodulation index

voltagematerial loss parameter

grid frequency

multilevel stages

16/29

PWM: Frequency-Dependent Material

► PWM signal ► Frequency dependent permittivity ► Constant ε’’ assumption is inaccurate (50% error)

► Closed-form solution ► Approximation of sum with integral & Kramers-Kronig ► 7% accuracy (Damisol 3418) ► Formula and derivation in [Gui16]

► Simple figures of merit for the losses

Frequency [Hz]

ε’ at 120 °C ε’’ at 120 °C

Frequency [Hz]

fs = 10.0 kHz / t

r = 800 ns / D

c = 0.5/ T = 120 °C / Damisol 3418

3.50

3.55

3.60

3.65

0.00

0.01

0.02

0.03

0.04

0.05

1k 10k 100k 1M 1k 10k 100k 1M

3.45

ε’ [

p.u.

]

ε’’ [

p.u.

]

2 - Kramers-Kronig

3 - FOM ε’

1 - FOM ε’’

ε’ meas.

fc

fs

fs f

c

ε’’ meas.

ε’’ approx.

17/29

Outline► Dielectric Material Parameters► Dielectric Losses Computation► Case Study: Solid-State Transformer► Conclusion

18/29

Converter: Solid-State Transformer

► MV AC-DC converter ► 25 kW ► 6.6 kV AC ► 400 V DC ► Full ZVS (AC-DC and DC-DC)

► Applications ► Renewable collecting grid ► Datacenter supply

► Important stresses for the DC-DC stage

▼ 10 kV SiC MOSFET

▼ Partner

▼ Considered SST (SwiSS SST)

EMI filter iTCM ZVS Inverter DC-DC insulation / step-downDampingMV Grid LV Load

6.6 kV AC 6.6 kV AC 7 kV PWM 7 kV DC 400 V DC7 kV PWM

[Wolfspeed]

19/29

Converter: MV/MF DC-DC

► MV DC-DC converter (single stage) ► Dual-active bridge ► Series-resonant converter

► Ratings ► 25 kW / 50 kHz ► 7 kV to 400 V ► 15 kV CM insulation ► 10 kV / 900 V SiC MOSFETs

► Important stresses for the MF transformer

▼ 10 kV SiC Inverter7

kV

400

V

n:1

Lm

Lσ

▼ 400 V SiC Test Converter▼ Converter

▼ Transformer Prototype

20/29

Converter: Transformer Stress

► Switching ► ZVS achieved with magnetizing current ► 15 kV/µs with ZVS (100 kV/µs without ZVS) ► Relevant stress up to 1 MHz

► Combines all the critical factors (f, V, dv/dt)

▼ Transformer

-12

0

12

-3.5

0.0

3.5

5.0 5.2 5.4 5.6

Frequency [Hz]

0 5 10 15 20Time [µs]

v [k

V]i [

A]

V [k

V]

Time [µs]

0 5 10 15 20Time [µs]

50k 500k 5M 50M

-3.5

0.0

3.5

v [k

V]101

10-1

10-5

10-3

▼ Simulated Waveform ▼ Measured MV TransientVoltage

Current

Time Domain

Frequency Domain

v AC,L

V

iAC,LV

v AC,M

V

iAC,MV

CGND

CMV,LV

Cstray

n:1

Lm

Lσ

Waveforms shown for SRC DC-DC converter

vAC,MV15 kV/µs

VAC,MV

vAC,MV

iAC,MV

iAC,LV

/ n

vAC,LV

· n

21/29

Transformer: Prototype

► Ratings ► 25 kW / 31 kVA ► 50 kHz ► ±3.5 kV / ±400 V ► 15 kV DM/CM insulation ► 2.8 dm3 / 170

x

120

x

135 mm

► 99.55% / 9 kW/dm3

► Construction ► Ferrite core (U-cores) ► Two air gaps (for ZVS) ► Litz wire (54:6 turns) ► MV chamber winding ► Dry-type insulation (Damisol 3418) ► Forced convection cooling

► Insulation coordination ?

▼ Transformer

LV GND MV

T [°C]

▼ Cooling System

Final prototype is insulated with silicone elastomer (instead of Damisol 3418)

22/29

Transformer: Insulation Coordination

► Insulation coordination ► Terminations with creepage extenders ► Vacuum potting of the windings ► Earthing of the cores

► Shielding of the windings ► Resistive coating at the surface ► No additional losses ► Complete analysis in [Gui17]

► Electric field is confined inside the windings

▼ CM currents (EMI)

5.0 5.2 5.4 5.6Time [µs]

0

50

100

i fram

e / V

DC [

mA

/ kV

]shielded

unshielded

▼ Eddy current in the shield

0.0

0.3

0.2

0.1

J RMS [

A/m

m2 ]

▼ Insulation coordinationMV termination

earthingshield

potting

23/29

Transformer: Computational Workflow

► Simulation of dielectric losses ► Voltage/frequency ► Electric Field ► Temperature ► Material Parameters

► Simulation of insulation losses requires a multiphysics framework

Material SampleTopology

MF Transformer

V / I

Time

Waveforms

Electric Field

E [k

V/m

m]

Dielectric Losses

P [k

W/d

m3 ]

Copper and Core Losses

P [k

W/d

m3 ]

Temperature

T [K

]

FEM Simulations

Measurements

Permittivity

T [K

]

f [Hz]

[Novocontrol Alpha-A]

[ETHZ PES MEGAlink]

24/29

Transformer: Loss Densities

► Dielectric losses ► 110 °C hotspot temperature ► 0.6 kW/dm3 peak copper/core loss density ► 2.5 kV/mm RMS electric field ► 1.0 kW/dm3 peak dielectric loss density ► Mostly near the MV winding

► Dielectric loss density is large and very localized

Copper/Core LossesTemperature

60

80

90

110

T [°

C]

100

70

0.0

0.2

1.0

0.6

P [k

W/d

m3 ]

0.4

0.8

Dielectric Insulation Losses

E RMS [

kV/m

m]

Electric Field

0.0

0.5

1.0

1.5

2.0

2.5zoomzoom

zoomzoom

0.0

0.4

0.6

P [k

W/d

m3 ]

0.3

0.5

0.2

0.1

Losses simulated for DAB DC-DC converter

25/29

Transformer: Losses Breakdown

► Dielectric losses ► 17% of the transformer losses ► Frequency/temperature dependence are important ► Thermal runaway at the glass transition temperature

► Dielectric losses are not negligible► Alternative materials with higher or much lower Tg

P lo

sess [

W]

Transformer Losses Transformer Temperature

Pout

[kW]

T [°

C]

0

20

40

60

80

100

60

80

100

120

140

160

Pout

[kW]0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35

Losses simulated for DAB DC-DC converter

winding

nominal

corecoreinsulation

coolingwinding

Tg

runaway runaway

nominal

26/29

Outline► Dielectric Material Parameters► Dielectric Losses Computation► Case Study: Solid-State Transformer► Conclusion

27/29

Conclusion

► Dielectric losses with PWM ► Frequency and voltage are critical ► Materials exhibit dielectric loss peaks ► Simple analytical expressions for the losses

► Insulation coordination with MV/MF electric fields ► Insulation material (e.g. losses, breakdown) ► Terminations / creepage ► Shielding / grading ► Electromagnetic compatibility

► MV/MF transformer ► Resistive shielding ► Dielectric losses are not negligible (17%) ► Typical insulation epoxy resins are not adapted ► Other materials are promising (e.g. elastomers)

► MV/MF electric fields are critical

▼ Transformer Prototype

▼ Epoxy Sample

28/29

Detailed Results

[Gui16] Computation and Analysis of Dielectric Losses in MV Power Electronic Converter Insulation T. Guillod, R. Färber, F. Krismer, C.M. Franck, and J.W Kolar IEEE ECCE 2016, Milwaukee, USA https://doi.org/10.1109/ECCE.2016.7854952

[Gui17] Electrical Shielding of MV/MF Transformers Subjected to High dv/dt PWM Voltages T. Guillod, F. Krismer, and J.W Kolar IEEE APEC 2017, Tampa, USA https://doi.org/10.1109/APEC.2017.7931050

L. Heinemann, “An Actively Cooled High Power, High Frequency Transformer with High Insulation Capability”, APEC, 2002K. Kao, “Dielectric Phenomena in Solids”, Elsevier, 2004M. Birle et al., “Breakdown of Polymer Dielectrics at High Direct and Alternating Voltages Superimposed by High Frequency High Voltages”, ICSD, 2013G. Ortiz et al., “Medium Frequency Transformers for Solid-State-Transformer Applications - Design and Experimental Verification”, PEDS, 2013T. Guillod et al., “Characterization of the Voltage and Electric Field Stresses in Multi-Cell Solid-State Transformers”, ECCE, 2014C. Zhao et al., “Power Electronic Traction Transformer - Medium Voltage Prototype”, IEEE Trans. Ind. Electron., 2014T. Guillod et al., “Computation and Analysis of Dielectric Losses in MV Power Electronic Converter Insulation”, ECCE, 2016R. Färber et al., “Modular Arbitrary Waveform Dielectric Spectrometer for Aging Diagnostics of Recessed Specimens”, CEIDP 2016T. Guillod et al., “Electrical Shielding of MV/MF Transformers Subjected to High dv/dt PWM Voltages”, APEC 2017D. Rothmund et al., “10kV SiC-Based Bidirectional Soft-Switching Single-Phase AC/DC Converter Concept for Medium-Voltage SST”, PEDG 2017

References

29/29

Thank You! Questions?

Acknowledgements

Raphael FärberProf. Christian M. Franck

Daniel RothmundMichael LeiblDr. Jonas HuberDr. Gabriel Ortiz