ISOLATED MF DC-DC CONVERTER FOR TRACTION BATTERY APPLICATIONTiago Nabais, Bombardier Transportation, Switzerland

Miloš Stojadinović, Jürgen BielaLaboratory for High Power Electronic Systems, ETH Zürich

Agenda

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• Bombardier Transportation• Catenary free operation (CFO)• Energy solutions• MF DC/DC Building block• Transformer Modeling• Optimization and Design• System Control design• Simulation & measurement results• Standardization & modularity• Talent 3 BEMU DC/DC

BOMBARDIER TRANSPORTATION

3

A global player with a European base

4

Revenues 2017(1):$8.5 billion US

Employees(2):39,850

Global HeadquartersProduction Sites

Mobility solutions

5

Vehicle speed

LocomotivesTRAXX

Monorails, people movers

MetrosMOVIA

Light rail vehiclesFLEXITY

Commute / regional trains

(Very) high-speed trains

Intercity trains

Vehi

cle

capa

city

INNOVIA monorailINNOVIA APM

AVENTRATALENT 3,

TWINDEXX, OMNEO,

BiLevel / Multilevel, AVENTRATALENT 3,

TWINDEXX, OMNEO

ZEFIRO

CATENARY FREE OPERATION (CFO)

6

Benefits and Customer value

7

“Beautiful place”

Spaceconstraints

Difficultcatenary setup

Safetyconcerns

Points of touristic interest Unobstructed urban skyline

Low bridges Obstructing road sign bridges Very narrow roads

Crossing intersections of wide roads Difficult access to power feeders

Rationale Examples Commercial Value Added

Touristic valueRaising urban quality & valueWinning arguments to obtain public approvals

Less costly civil worksFootprint width of 2 tracks reduces from 7.6 m to 7.05 m.No or simplified mesh wiring underneath tracks

CFO as cost efficient alternative

Risk of contacting wires (by overloaded trucks) Clearing the road in case of power outage Electromagnetic interference (e.g. near hospitals

affecting medical devices ) Fire dep’t: No obstacles for fire truck ladder use

Hazard and damage avoidance

Modes of Operation

8

LRV

Mass Transit(Metros)

Main Line(Commuter,Regional, …)

Locomotives

Scheduled CatenaryFree Operation

Catenary free route sections (e.g. historic districts)

Static, dynamic and PRI-MOVE charging schemes

Network extensions covering remote stops via unelectrifiedroutes

Emission free one-seat ride

Last mile Diesel and battery to access sidings and unelectrified routes

Battery-only shunting

Rescue Cases(e.g. power outages)

Movement to next trams top or safe place for passengers to exit

Movement to next metro stop for passengers to exit

Movement to next station to clear the tracks and let passengers exit

Clear the tracks Assistance service to re-move

other locos & trains

Other Modes of Operation

Boosting: Accelerated starting w. limited power

EMI reduction with CFO Energy efficiency increase Increased network stability

Boosting: Energy efficiency increase Increased network stability Sleep mode while parking or

standing (See 1)

Sleep mode while parking or standing (See 1)

Catenary free route sections (e.g. historic districts)

Static, dynamic and PRI-MOVE charging schemes

Boosting: Energy efficiency increase Increased network stability

ENERGY SOLUTIONS

9

State of the art

10

Diesel

Fuel Cell

Battery

Supercaps

Characteristics

State of the art VS applications

11

LRV

Mass Transit(Metros)

Main Line(Commuter,Regional, …)

Locomotives

Diesel

Fuel Cell

Battery

Supercaps

Characteristics

1500V-1800V 10’000kWh (1000litres) 600kW

100V-600V 530kWh (1000litres) 200-400kW

500V-900V 24.5kWh-75kWh 100-200kW

500V-900V 2kWh 600kW

Requirements

100kW 750V Line

200kW 750V Line

200kW 1500V-2000V

200kW 3kV Line

State of the art VS applications

12

LRV

Mass Transit(Metros)

Main Line(Commuter,Regional, …)

Locomotives

Diesel

Fuel Cell

Battery

Supercaps

Characteristics

1500V-1800V 10’000kWh (1000litres) 600kW

100V-600V 530kWh (1000litres) 200-400kW

500V-900V 24.5kWh-75kWh 100-200kW

500V-900V 2kWh 600kW

Requirements

100kW 750V Line

200kW 750V Line

200kW 1500V-2000V

200kW 3kV Line

Standardization &Modularity

State of the art VS applications

13

Scalable solution: 1 or more energy storage systems per MITRAC propulsion system Standardized hardware independent interfaces to MITRAC propulsion system Applicable for all vehicle families from LRV’s to locomotives ‘Plug and play’ solution – modular and scalable

TractionConverter

Motors

EnergyStorage 1

EnergyStorage 2

EnergyStorage n

Any viable technologies (supercaps, batteries, etc.) and chemistries. Mixed (= hybrid) solutions are possible.

The protection concept is specific to the energy storage units. High re-use on homologation and safety approvals

ENERGY STORAGE

Standardize power and communication protocol interfaces between traction converter and energy storage.

Standardized application interface towards traction converter Software as ‘ad-on package’ to minimize application specific

adaptations.

WELL DEFINED INTERFACES

MF DC/DC BUILDING BLOCK

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Project requirements

Battery Voltage Range 518..835V

Nominal DC Link Voltage 2800V

Regulated DC Link Voltage (peak) 3000V

Unregulated DC Link Voltage (peak) 4200V

System Power 183kW

System Power (peak) for 300s >234kW

Current per Battery (continuous) 220A

Current per Battery (peak) for 300s 280A

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Working Insulation Voltage 4.2kV

Impulse Withstand Voltage P-S 18kV

Efficiency > 95%

Ambient Temperature 75°C

Cooling Medium Temperature 60°C

Maximum Dimensions 750x360x200 mm

Maximum Weight 25 kg

Chosen topology and optimization procedure

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Modular Dual Active Bridge (DAB) converter Simplified flow chart of theoptimization procedure

• Pre-selected technologies– Transformer with integrated cooling

• Foil windings• Ferrite core• Water cooling

– Semiconductors• SiC MOSFETs

TRANSFORMER MODELING

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Transformer losses

• Winding losses– High frequency effects

• Skin effect losses• Proximity effect losses

– Optimum foil thickness• Analytical model available in literature1

• Core losses– Steinmetz equation

• Steinmetz parameters 𝑘𝑘,𝛼𝛼 and 𝛽𝛽– Extracted from data sheet– Valid only in a limited 𝐵𝐵-, 𝑓𝑓- and Temp. Range

• Can be used for sinusoidal flux waveforms• DC bias/offset is not considered

– Improved Generalized Steinmetz Equation (iGSE)• Can be used for arbitrary flux waveforms• Relaxation losses missing• DC bias/offset is not considered

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[1] W. G. Hurley et. al, Optimizing the AC Resistance ofMultilayer Transformer Windings with Arbitrary CurrentWaveforms, IEEE Trans. on Power Electronics, 2000.

Assumption:1D – H-field

Transformer leakage inductance calculation

• Winding height smaller than window height

𝐿𝐿𝜎𝜎 = 𝜇𝜇0𝑁𝑁𝑃𝑃2𝜋𝜋𝐷𝐷𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑤𝑤𝐿𝐿

ℎ𝑊𝑊𝑘𝑘𝜎𝜎

𝑤𝑤𝐿𝐿 =𝑑𝑑𝑃𝑃 + 𝑑𝑑𝑆𝑆

3 + 𝑑𝑑𝐿𝐿

𝐷𝐷𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = 𝐷𝐷 + 𝑑𝑑𝑃𝑃 + 𝑑𝑑𝑆𝑆 + 𝑑𝑑𝐿𝐿 −𝑑𝑑𝑆𝑆 − 𝑑𝑑𝑃𝑃

2𝑑𝑑𝑃𝑃 + 𝑑𝑑𝑆𝑆 + 4𝑑𝑑𝐿𝐿𝑑𝑑𝑃𝑃 + 𝑑𝑑𝑆𝑆 + 3𝑑𝑑𝐿𝐿

𝐷𝐷 = 2𝑏𝑏𝑐𝑐 + 𝑑𝑑𝑃𝑃 𝑑𝑑𝑐𝑐 + 𝑑𝑑𝑃𝑃

𝜋𝜋

– Rogowski correction factor2

𝑘𝑘𝜎𝜎 ≈ 1 −𝑑𝑑𝑃𝑃 + 𝑑𝑑𝑆𝑆 + 𝑑𝑑𝐿𝐿

ℎ𝑊𝑊– Different primary and secondary winding heights

ℎ𝑊𝑊 = ℎ𝑐𝑐𝑐𝑐𝑃𝑃ℎ𝑐𝑐𝑐𝑐𝑆𝑆

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(width of the reducedleakage channel)

(equivalent inner diameterof the internal winding)

(mean diameter of thereduced leakage channel)

[2] V. V. Kantor, Methods of Calculating LeakageInductance of Transformer Windings, Elektrotehnika, 2009.

Transformer thermal modeling

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• Types of heat transfer– Conduction

• Usually independent of temperature• Important: Precise material thermal data• Difficulty: Interfaces between materials

– Convection• Conduction + fluid flow• Dependent on temperature• Abundance of analytical and empirical

formulas available in literature

– Radiation• Usually negligible compared to

conduction/convection• Difficult to model – nonlinear / line of sight

𝑅𝑅𝑡𝑡𝑡 =Δ𝑇𝑇𝑃𝑃 =

𝑙𝑙𝜆𝜆𝐴𝐴

𝑅𝑅𝑡𝑡𝑡 =Δ𝑇𝑇𝑃𝑃 =

𝑙𝑙ℎ𝐴𝐴

𝑃𝑃 = 𝜀𝜀𝐴𝐴𝜎𝜎 𝑇𝑇𝑏𝑏4 − 𝑇𝑇𝑚𝑚4

𝑞𝑞 =𝑃𝑃𝐴𝐴

Transformer thermal modeling

• Channel thermal model𝑅𝑅𝑡𝑡𝑡,𝑟𝑟 =

𝑎𝑎𝑤𝑤𝑙𝑙𝜆𝜆𝐻𝐻𝑆𝑆

𝑅𝑅𝑡𝑡𝑡,𝑑𝑑 =1

ℎ𝑙𝑙𝜋𝜋𝑑𝑑with

ℎ =𝑁𝑁𝑢𝑢𝜆𝜆𝑓𝑓𝑓𝑓𝑢𝑢𝑓𝑓𝑓𝑓

𝑑𝑑- heat transfer coefficient

𝑁𝑁𝑐𝑐 - Nusselt number𝜆𝜆𝐻𝐻𝑆𝑆 - thermal conductivity of heat sink material

𝑑𝑑 ≈ 2 ⁄𝐴𝐴 𝜋𝜋 𝑚𝑚 – channel hydraulic diameter of cross-sectional area 𝐴𝐴• Nusselt number is a function of

– Average ducted fluid velocity– Duct geometry– Fluid Prandtl number (𝑷𝑷𝒓𝒓)– Analytical models available in literature3

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[3] Y. S. Muzychka, Generalized models for laminardeveloping flows in heat sinks and heat exchangers, HeatTransfer Engineering, 2013.

Transformer thermal modeling

• Channel thermal model𝑅𝑅𝑡𝑡𝑡,𝑟𝑟 =

𝑎𝑎𝑤𝑤𝑙𝑙𝜆𝜆𝐻𝐻𝑆𝑆

𝑅𝑅𝑡𝑡𝑡,𝑑𝑑 =1

ℎ𝑙𝑙𝜋𝜋𝑑𝑑with

ℎ =𝑁𝑁𝑢𝑢𝜆𝜆𝑓𝑓𝑓𝑓𝑢𝑢𝑓𝑓𝑓𝑓

𝑑𝑑- heat transfer coefficient

𝑁𝑁𝑐𝑐 - Nusselt number𝜆𝜆𝐻𝐻𝑆𝑆 - thermal conductivity of heat sink material

𝑑𝑑 ≈ 2 ⁄𝐴𝐴 𝜋𝜋 𝑚𝑚 – channel hydraulic diameter of cross-sectional area 𝐴𝐴• Nusselt number is a function of

– Average ducted fluid velocity– Duct geometry– Fluid Prandtl number (𝑷𝑷𝒓𝒓)– Analytical models available in literature3

22

[3] Y. S. Muzychka, Generalized models for laminardeveloping flows in heat sinks and heat exchangers, HeatTransfer Engineering, 2013.

Transformer thermal modeling

• Water flow in structures with multiple parallel channels∆𝑝𝑝𝑡𝑡𝑡𝑡𝑡𝑡 = ∆𝑝𝑝1= ∆𝑝𝑝2= ⋯ = ∆𝑝𝑝𝑖𝑖 - pressure loss 𝑃𝑃𝑎𝑎�̇�𝑉 = �̇�𝑉1 + �̇�𝑉2 + ⋯+ �̇�𝑉𝑖𝑖 - flow rates ⁄𝑚𝑚3 𝑠𝑠

• Darcy-Weisbach equation for pressure loss in parallel channels

∆𝑝𝑝 = �̇�𝑉2

∑ ⁄𝐾𝐾𝑓𝑓 𝑓𝑓𝑓𝑓2 ; �̇�𝑉 = 𝑉𝑉 � 𝐴𝐴 ; 𝐾𝐾𝑖𝑖 = 𝜋𝜋2�𝑑𝑑𝑓𝑓

5

8�𝜌𝜌�𝑙𝑙𝑓𝑓

𝑉𝑉 ⁄𝑚𝑚 𝑠𝑠 – fluid velocity𝑓𝑓 – channel friction factor𝜌𝜌 ⁄𝑘𝑘𝑘𝑘 𝑚𝑚3 – fluid density

• Channel friction factor– Different formulas for laminar and turbulent flow

• Functions of Reynolds number• In turbulent flow channel roughness plays a role

– Reynolds number is a function of fluid velocity• Iterative solving necessary• Few iterations sufficient

23

Transformer thermal modeling

• Thermal model of the chosen transformer structure

24

OPTIMIZATION AND DESIGN

25

Chosen topology and optimization results

26

Custom cores (Ferrite N97) Standard cores (Ferrite N97)

Modular Dual Active Bridge (DAB) converter Simplified flow chart of theoptimization procedure

System layout and specifications

28

• Single module specifications– Nominal battery side voltage: 700 V– Nominal DC link side voltage: 700 V– Nominal power: 38.5 kW– Peak continuous power: 50 kW– Switching frequency: 35 kHz– Efficiency: 98.5%– Dimensions: 360x195x118 mm– Power density: 6 kW/dm3

– Weight: 18 kg

Mechanical layout of the system

SYSTEM CONTROL DESIGN

29

System control design

30

• Control parameter optimization

System control design

31

• Module control overview

System control design

32

• Generalized PWM generator with voltage second balancing4

𝑝𝑝1,1 𝑛𝑛 = 𝜙𝜙2𝑥𝑥 + 𝛿𝛿𝑥𝑥 𝑛𝑛 − 1

𝑝𝑝1,2 𝑛𝑛 = 𝜙𝜙2𝑥𝑥 + 𝛿𝛿𝑥𝑥 𝑛𝑛 − 1 + 𝑇𝑇𝐼𝐼

𝑝𝑝1,3 𝑛𝑛 = 𝜙𝜙2𝑥𝑥 + 𝛿𝛿𝑥𝑥 𝑛𝑛 +𝑇𝑇𝑆𝑆2

𝑝𝑝1,4 𝑛𝑛 = 𝜙𝜙2𝑥𝑥 + 𝛿𝛿𝑥𝑥 𝑛𝑛 +𝑇𝑇𝑆𝑆2

+ 𝑇𝑇𝐼𝐼

𝑝𝑝3,1 𝑛𝑛 = 𝜙𝜙2𝑥𝑥 − 𝛿𝛿𝑥𝑥 𝑛𝑛 + 𝑇𝑇𝑆𝑆 − 𝑇𝑇𝐼𝐼

𝑝𝑝2,1 𝑛𝑛 = 𝜙𝜙2𝑥𝑥 − 𝛿𝛿𝑥𝑥 𝑛𝑛 − 1 +𝑇𝑇𝑆𝑆2− 𝑇𝑇𝐼𝐼

𝑝𝑝2,2 𝑛𝑛 = 𝜙𝜙2𝑥𝑥 − 𝛿𝛿𝑥𝑥 𝑛𝑛 − 1 +𝑇𝑇𝑆𝑆2

𝑝𝑝3,1 𝑛𝑛 = 𝜙𝜙2𝑥𝑥 − 𝛿𝛿𝑥𝑥 𝑛𝑛 + 𝑇𝑇𝑆𝑆

[4] M. Stojadinović et al, Generalized PWM generator withtransformer flux balancing for Dual Active Bridge converter, EPE ECCEEurope, 2017.

Generalized PWM generator with voltage second balancing

33

• Properties of the proposed PWM generator:– Seamless transition between different modulation schemes– Allows for flux balancing– No overshoot in the transformer current

SIMULATION & MEASUREMENT RESULTS

34

• Temperature distribution in transformer and cooling structure– Same material parameters in

FEM and analytical– Same losses used– Water temperature 𝟐𝟐𝟐𝟐℃– Input flow 𝟖𝟖 ⁄l 𝐦𝐦𝐦𝐦𝐦𝐦

• Velocity profile of the cooling structure

3D-FEM simulation results

35

𝑻𝑻𝑪𝑪𝑪𝑪 𝑻𝑻𝑪𝑪𝟐𝟐 𝑻𝑻𝑪𝑪𝑪𝑪 𝑻𝑻𝑨𝑨𝑨𝑨𝑨𝑨 𝑻𝑻𝑾𝑾𝑾𝑾 𝑻𝑻𝑷𝑷𝑷𝑷𝑷𝑷 𝑻𝑻𝑾𝑾𝑾𝑾 𝑻𝑻𝑻𝑻𝑷𝑷𝑾𝑾3D FEM 70℃ 40℃ 29℃ 71℃ 78℃ 77℃ 81℃ 43℃

Analytical 72℃ 42℃ 28℃ 68℃ 83℃ 80℃ 88℃ 66℃

FEM Analytical

Fluid velocity 0.115 ⁄m s 0.116 ⁄m s

Fluid velocity in transformer channels

3D-FEM simulation results

• Eddy current distribution in the bottom part of the cooling structure

– Aluminium bar has better thermal behaviour(part of the structure)

– Thickness not limited– Can be placed only on lower side

for easy manufacturability• Influence on the field (non-symmetrical)• Can lead to even higher losses in windings

• Calculated induced losses (FEM)– Whole transformer structure taken into

account

36

with Al bar

Peak current density 𝐽𝐽𝑚𝑚 = 1.3 × 108 ⁄𝐴𝐴 𝑚𝑚2

with Al-Ni bar

Peak current density 𝐽𝐽𝑚𝑚 = 4 × 107 ⁄𝐴𝐴 𝑚𝑚2

Al bar AlNi bar

Induced losses 75W 36W

Measurement results

• Transformer leakage inductance test– Transformer in short circuit connection– Power choke test at 400V– Current ramped up to 65A

37

3D FEM Analytical

Leakage inductance 26.2𝜇𝜇𝜇𝜇 26.5𝜇𝜇𝜇𝜇

Measurement setup

Measurement results

• Hydraulic measurements– Simplified pressure drop relation

∆𝑝𝑝 = 𝜁𝜁𝜌𝜌2�̇�𝑉2

𝐴𝐴𝑐𝑐2~𝑘𝑘𝑉𝑉2

𝜁𝜁 – coefficient of fluid resistance𝜌𝜌 – density of the fluid�̇�𝑉 – fluid flow𝐴𝐴𝑐𝑐 – water channel cross section

38

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 220

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

measured points 25°Cinterpolated curve 25°Cmeasured points 60°Cinterpolated curve 60°C

Characteristic curve for 25°C & 60°C

flow [dm3/min]

pres

sure

dro

p [m

bar]

Measurement setup

Measurement results

• Dielectric withstand test:– Partial discharge measurement between

transformer primary and secondary– High voltage 50Hz source in a Faraday cage

39

PD measurement resultsMeasurement block diagram

• Single Active Bridge (start-up) mode of the converter– DC link side switches are blocked

• Dual active bridge mode in open loop

Open loop tests

40

Measurement setup

Voltage second balancing

Closed loop tests

• Current control– Full power reversal 700V / 60A

41

Measurement setup

Dual active bridge thermal test (700V / 60A)

• @ 90mins, water flow changed from 8 l/min to 15 l/min• Internal temperature on the outer surface of the secondary winding:

– 83°C @ 8 l/min– 78°C @ 15 l/min

42

𝑻𝑻𝑪𝑪𝑪𝑪 𝑻𝑻𝑪𝑪𝟐𝟐 𝑻𝑻𝑪𝑪𝑪𝑪 𝑻𝑻𝑨𝑨𝑨𝑨𝑨𝑨 𝑻𝑻𝑾𝑾𝑾𝑾 𝑻𝑻𝑷𝑷𝑷𝑷𝑷𝑷 𝑻𝑻𝑾𝑾𝑾𝑾 𝑻𝑻𝑻𝑻𝑷𝑷𝑾𝑾

3D FEM 70℃ 40℃ 29℃ 71℃ 78℃ 77℃ 81℃ 43℃

Analytical 72℃ 42℃ 28℃ 68℃ 83℃ 80℃ 88℃ 66℃

Measurement - 63℃ 27℃ - - - 83℃ 66℃

Comparison of calculated andmeasured temperaturesInlet flow 8 ⁄l min

STANDARDIZATION & MODULARITY

Building block

43

State of the art VS applications

44

LRV

Mass Transit(Metros)

Main Line(Commuter,Regional, …)

Locomotives

Diesel

Fuel Cell

Battery

Supercaps

Characteristics

1500V-1800V 10’000kWh (1000litres) 600kW

100V-600V 530kWh (1000litres) 200-400kW

500V-900V 24.5kWh-75kWh 100-200kW

500V-900V 2kWh 600kW

Requirements

100kW 750V Line

200kW 750V Line

200kW 1500V-2000V

200kW 3kV Line

Building block

45

Electrical and Performance dataPower Flow Bidirectional

Isolation Galvanic isolation

Power 50kW

Voltage 500-900V

Max. Current 70A

Cont. Current 55A

Efficiency > 98%

Mechanical dataDimensions 360x195x118 mm

Weight 18 kg

Main cooling medium Water/Glycol

Secondary cooling medium Forced air

Max. Temperature of water/glycol cooling 60°C

Max temperature of cooling air 75°C

Vin

Ip

VoutVp Vs

Is

MF Transformer

1U

1V

2U

2V

Building block

Metro Application example

46

TractionConverter

Motors

EnergyStorage 1

EnergyStorage 2

EnergyStorage n

Mass Transit(Metros)

200kW 750V Line

Vin

Ip

VoutVp Vs

Is

MF Transformer

1U

1V

2U

2V

Building block

Main Line Application example

47

TractionConverter

Motors

EnergyStorage 1

EnergyStorage 2

EnergyStorage n

Main Line(Commuter,Regional, …)

200kW 1500V-2000V

Vin

Ip

VoutVp Vs

Is

MF Transformer

1U

1V

2U

2V

Building block

Locomotive Application example

48

TractionConverter

Motors

EnergyStorage 1

EnergyStorage 2

EnergyStorage n

Locomotives 200kW 3kV Line

Vin

Ip

VoutVp Vs

Is

MF Transformer

1U

1V

2U

2V

Building block

TALENT 3 BEMU DC/DC

49

Summary

50

50 % of German railway network is not electrified

53 % shorter than 40 km

Replace diesel trains by BEMUs on non- or partly-electrified regional railway lines

Operating with battery power and recharge under catenary

Market Product & Technology Talent 3 as base product

PRIMOVE battery technology to be adapted to mainline application

MITRAC Solutions

battery operation of up to 40 km

120 km/h (same as DMU) on non-el. lines

EMU BEMU Diesel Diesel Hybrid Fuel Cell

Energy cons. 39 % 45 % 79 % 67 % 100 %

Energy Price 40 % 40 % 32 % 32 % 100 %

Energy costs 15,2 % 17,3 % 39,4 % 33,3 % 100%

CO2 equivalent 8,6 % 9,7 % 100 % 85,2 % 31,4 %

Performance requirements

51

DC/DC VS MF DC/DC

52

QUESTIONS

53