Dielectric Losses: MV/MF Converter Insulation

Research Collection Other Conference Item Dielectric Losses: MV/MF Converter Insulation Author(s): Guillod, Thomas; Krismer, Florian; Kolar, Johann W. Publication Date: 2017 Permanent Link: https://doi.org/10.3929/ethz-b-000225431 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library

Dielectric Losses MV/MF Converter Insulation SCCER FURIES Technical Workshop September 20, 2017 T. Guillod, F. Krismer, J. W. Kolar Power Electronic Systems Laboratory, ETH Zurich, Switzerland

2/29 Insulation in MV/MF Converters MV AC-DC Converter 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 [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 Partial Discharges 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 [RMS Energy] 7 kv / 100 khz EMI [ETHZ PES SwiSS SST]

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? Polarization atomic orientational hopping + + - - space charge + + + + - - - - electronic 10-15 10-10 10-5 10 0 10 5 Time Constant [s] Adapted from Kao, K., Dielectric Phenomena in Solids, 2004

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 E [V/m] 1.0 0.5 Electric Field 0 5 10 Time [µs] E D/(ε 0 ε r,0 ) [V/m] 1.0 0.5 Displacement Field D polarization vacuum 0 5 10 Time [µs] J/(ε 0 ε r,0 / τ debye ) [V/m] 0.6-0.6 Current Density J vacuum polarization 0 5 10 Time [µs] w/(ε 0 ε r,0 ) [V 2 /m 2 ] 0.6 0.3 Supplied Energy Density w losses 0 5 10 Time [µs] f s = 10 khz / t r = 500 ns / Debye relaxation / ε r,0 = 3.0 / ε r,inf = 2.0 / τ debye = 320 ns

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 Permittivity / ε Permittivity / ε DC: σ ε, σ β peak α peak β peak ε Frequency / log(f) α peak ε DC: σ T g ε, σ brittle rubber Frequency / log(f) β peak α peak T g @ 1.6 mhz DC: σ Temperature / T Inverse temperature / T -1 Adapted from Menczel, J., Thermal Analysis of Polymers: Fundamentals and Applications, 2008

10/29 Material: Measured Parameters Measured for a typical HV epoxy resin Damisol 3418 unfilled resin T g = 136 C Frequency and temperature dependence T g is a critical parameter (α peak @ 1.6 mhz) How to compute the losses? 160 Measurements / ε 4.0 T g 5.0 160 Measurements / ε α 0.2 T g 0.4 Epoxy Sample Temperature [ C] 125 90 55 3.5 3.4 3.3 3.2 4.5 4.0 3.5 ε [p.u.] Temperature [ C] 125 90 55 2 3 4 6 β 0.3 0.2 0.1 ε [p.u.] 20 100 1k 10k 100k 1M Frequency [Hz] 10M 3.0 20 100 1k 10k 100k 1M Frequency [Hz] 10M Measured with a Novocontrol Alpha-A Analyzer / vonroll Damisol 3418

11/29 Outline Dielectric Material Parameters Dielectric Losses Computation Case Study: Solid-State Transformer Conclusion

12/29 PWM: Spectral Losses 1.0 PWM Voltage 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 V p / V p / V DC DC 0.5 1.0 0.5 D c / f s 1/( 2f ) 1/( f ) s s Time Switching Transition t r ~ 1/f c 10%-90% t r 2t r Time V n,rms / V 1,RMS [db] 0-40 -80-120 -160 Voltage Harmonics (db voltage ) Power Harmonics (db power ) -20 db/dec V n,rms f c / f s -40 db/dec P n / P 1 [db] 0-25 -50-75 -100-10 db/dec P n f c / f s -30 db/dec P n,c / P 1 [p.u.] 5 4 3 2 1 0 P 1 Power Cumulative Sum P f c / f s P n,c 10 0 10 2 10 4 10 6 n = f / f s 10 0 10 2 10 4 10 6 n = f / f s 10 0 10 2 10 4 10 6 n = f / f s f s = 1.0 khz / t r = 100 ns / D c = 0.5

13/29 PWM: Constant Duty Cycle 1.0 PWM Voltage f s [Hz] 100k 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 10k Frequency / Rise Time 10 5.5 10 5.0 10 4.5 10 4.0 10 6.0 10 5.5 10 5.0 10 4.5 10 4.0 1k 10 3.5 10n 100n 1µ t r [s] P [1/s] P [1/s] 50 10 3 40 10 3 30 10 3 20 10 3 10 10 3 0 10 3 Duty Cycle V p / V DC V p / V DC 0.1 0.5 0.9 D c [p.u.] 0.5 1.0 0.5 V DC D c / f s 1/( 2f ) 1/( f ) s s Time Switching Transition t r 10%-90% t r 2t r Time Circuit Topology V p DUT f s = 1 khz / t r = 100 ns / D c = 0.5 / P = P/(ε C 0 V DC2 )

14/29 PWM: Sinusoidal Modulation 1.0 Full Bridge Inverter V 1 V p P 1 [1/s] 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 Modulation Index / Fundamental 150 80 10 3 100 50 0 n c = [1,5] 0.1 0.4 0.7 1.0 M i [p.u.] P harm [1/s] 60 10 3 40 10 3 20 10 3 Modulation Index / Harmonics n c = 2 n c = 1 M i [p.u.] V p / V DC V p / V DC 0 10 3 n c = 3 n c = 4 0.1 0.4 0.7 1.0-1.0 1.0-1.0 V DC / n c 0 5 10 15 20 Time [ms] Multilevel Inverter V1 V p 0 5 10 15 20 Time [ms] Circuit Topology (n c = 3) V p DUT f g = 50 Hz / f s = 1 khz / t r = 100 ns / P = P/(ε C V 2 ) = P 0 DC 1 + P harm

15/29 PWM: Scaling Laws PWM with constant D c f s t r D c V DC ε P ~ f s log(const./f s ) P ~ log(const./t r ) P ~ const. P ~ V DC 2 P ~ ε switching frequency switching speed duty cycle voltage material loss parameter PWM with sinusoidal M i f s f g t r M i n c V DC ε P 1 ~ const. P 1 ~ f g P 1 ~ const. 2 P 1 ~ M i P 1 ~ const. 2 P 1 ~ V DC P 1 ~ ε P harm ~ f s log(const./f ) s P harm ~ const. P harm ~ log(const./t r ) P harm ~ const. 2 P harm ~ 1/n c 2 P harm ~ V DC P harm ~ ε switching frequency grid frequency switching speed modulation index multilevel stages voltage material loss parameter Impact of frequency dependency of ε?

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 ε [p.u.] 3.65 3.60 3.55 3.50 3.45 ε at 120 C 1 - FOM ε ε at 120 C ε meas. f s f c 2 - Kramers-Kronig ε approx. 3 - FOM ε 0 1k 10k 100k 1M 1k 10k 100k 1M Frequency [Hz] Frequency [Hz] f s ε meas. 5 4 3 2 1 ε [p.u.] f s = 1 khz / t r = 800 ns / D c = 0.5/ T = 120 C / Damisol 3418 f c

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 Partner 10 kv SiC MOSFET [Wolfspeed] Considered SST (SwiSS SST) MV Grid Damping EMI filter itcm ZVS Inverter DC-DC insulation / step-down LV Load 6.6 kv AC 6.6 kv AC 7 kv PWM 7 kv PWM 7 kv DC 400 V DC

19/29 Converter: MV/MF DC-DC 10 kv SiC Inverter 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 Transformer Prototype Converter 400 V SiC Test Converter 7 kv L σ L m n:1 400 V

20/29 Converter: Transformer Stress Transformer 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) v AC,MV i AC,MV L σ L m C stray C GND C MV,LV n:1 i AC,LV v AC,LV v [kv] i [A] 3.5-3.5 12 0 v AC,MV Simulated Waveform Voltage v AC,LV n 0 5 10 15 20 i AC,MV Time [µs] Current i AC,LV / n -12 0 5 10 15 20 Time [µs] V [kv] v [kv] 3.5-3.5 10 1 10-1 10-3 10-5 Measured MV Transient Time Domain 15 kv/µs 5.0 5.2 5.4 5.6 Time [µs] Frequency Domain 50k 500k 5M 50M Frequency [Hz] v AC,MV V AC,MV Waveforms shown for SRC DC-DC converter

21/29 Transformer: Prototype Transformer LV GND MV Ratings 25 kw / 31 kva 50 khz ±3.5 kv / ±400 V 15 kv DM/CM insulation 2.8 dm 3 / 170 x 120 x 135 mm 99.55% / 9 kw/dm 3 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? 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 Insulation coordination MV termination shield earthing potting Eddy current in the shield 0.3 0.2 0.1 J RMS [A/mm 2 ] [ma / kv] i frame / V DC 100 50 0 CM currents (EMI) unshielded shielded 5.0 5.2 5.4 5.6 Time [µs]

23/29 Transformer: Computational Workflow Simulation of dielectric losses Voltage/frequency Electric Field Temperature Material Parameters Simulation of insulation losses requires a multiphysics framework Topology Copper and Core Losses FEM Simulations Temperature Dielectric Losses Material Sample Waveforms P [kw/dm 3 ] T [K] Measurements V / I Time MF Transformer Electric Field P [kw/dm 3 ] [Novocontrol Alpha-A] Permittivity [ETHZ PES MEGAlink] E [kv/mm] T [K] f [Hz]

24/29 Transformer: Loss Densities Dielectric losses 110 C hotspot temperature 0.6 kw/dm 3 peak copper/core loss density 2.5 kv/mm RMS electric field 1.0 kw/dm 3 peak dielectric loss density Mostly near the MV winding Dielectric loss density is large and very localized Temperature 110 100 Copper/Core Losses 0.6 0.5 Electric Field 2.5 2.0 Dielectric Insulation Losses zoom 1.0 0.8 90 80 T [ C] 0.4 0.3 0.2 P [kw/dm 3 ] 1.5 1.0 E RMS [kv/mm] 0.6 0.4 P [kw/dm 3 ] 70 0.1 0.5 zoom 0.2 60 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 T g P losess [W] 100 80 60 40 Transformer Losses Transformer Temperature runaway nominal winding cooling T [ C] 20 80 core 0 insulation 60 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 P out [kw] P out [kw] core 160 140 120 100 T g runaway nominal winding Losses simulated for DAB DC-DC converter

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 Epoxy Sample Transformer Prototype

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

29/29 Thank You! Questions? Acknowledgements Raphael Färber Prof. Christian M. Franck Daniel Rothmund Michael Leibl Dr. Jonas Huber Dr. Gabriel Ortiz