BoP: Electrical Conversion & ConnectionDC/DC and DC/AC converters in grid interfacing

Vesa Väisänen

LUT?

− Lappeenranta University of Technology

− Established in 1969− Located in Lappeenranta,

South Carelia, Finland− Faculty of Technology− Faculty of Technology

Management− School of Business− Number of students ~ 5000− Number of staff ~ 929

Our project

− Project started in 2007− 1 professor and 3 researchers − Partners in co-operation: ABB, Wärtsilä, VTT

Objectives

− Feed the energy from a SOFC stack into electric grid− High efficiency (>95 %)− Reliability− Manufacturability and price− Paying attention to the fuel cell characteristics

DC

Grid converter

DC/DC-converter Fuel Cell

DC-linkLow voltage

DC-linkGrid filter

Current reference

Prototype testing at VTT, results

− 10 kW Power conversion unit successfully integrated to a SOFC system at VTT− Operated over 3000 h− Grid connection is done with ABBs grid converter− Measured losses for power electronics (DC/DC + DC/AC) were 1.1kW,

corresponding to about 43% of total system losses [1].

Requirements

− The requirements for a power conversion unit arise from three major sources:

• Fuel cell (or any other power source)• The supplied load or network• General requirements such as economical constraints,

efficiency requirements, expected operating life, standards, patents…

Power Electronics

FuelCell

Load / Network

LOAD REQUIREMENTS

FUEL CELL REQUIREMENTS

GENERAL REQUIREMENTS

Fuel cell requirements

− Fuel cell voltage drops as a function of current density need for voltage regulation

− Current reference must be accurately followed to avoid stack overloading need for accurate current control

− Low frequency current ripple must be low to avoid process oscillation and overloading ripple mitigation by the controller

− Effects of long term high frequency (> 10 kHz) ripple still unclear? the lower the allowed ripple, the more expensive the filter

Region of activation losses

Region of Ohmic losses

Region of gas transport losses

Ideal voltage

Current density (mA/cm2)

Cell

volta

ge

0

0.5

1.0

Total loss

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

1 % 2 % 3 % 4 % 5 % 6 %

Rel

ativ

e in

duct

or c

ost

HF current ripple %

Effect of high-frequency current ripple on inductor costs (an example)

250 A/cm^2

300 A/cm^2

350 A/cm^2

400 A/cm^2

500 A/cm^2

Fuel cell requirements

− The voltage produced by the fuel cell stack can be low (for example 40-60 V), but the DC/AC converter requires a higher input voltage depending on number of phases and the modulation method:

• One-phase (230 V) Voltage Source Inverter (VSI) VDC-link > 255 V (for preferred linear modulation ≥ 325 V)

• Three-phase (400 V) VSI with Space Vector PWM VDC-link > 628 V (for preferred linear modulation ≥ 693 V)

Need for considerable voltage boost

− Fuel cell has high electrical efficiency, so high efficiency is desired also from the power conversion unit to maintain high overall efficiency converter topology and component selection

How to interface a fuel cell?

DC/DC converter

DC/DCconverter

DC/ACconverter

DC/ACconverter

Fuel cellsUnregulatedDC voltage

RegulatedDC voltage

Low AC-voltage50/60 Hz

3-phase AC-voltage

− Most DC loads require regulated DC voltage. Therefore the DC/DC converter is typically essential.

− Galvanic isolation with a transformer is preferred for safety reasons and for voltage boosting.

− High frequency transformer on DC side is much smaller than a low frequency transformer on AC side.

− For example a 10 kVA, 50 Hz commercial transformer can weigh 72 kg, while a 50 kHz transformer weighs about 2 kg!

Non-isolated DC/DC convertersBoost converter topology

− Simple, non-isolated topology for voltage step-up.

• Inductor L1: stores energy and limits the input current rate of change

• Transistor S1: acts a switching element

• Diode D1: allows inductor current to flow to load while transistor S1 is closed and prevents current flow from load to input.

• Capacitor C1: feeds energy to load while transistor S1 is conducting.

VDC

L1

S1

D1

C1 Vout

Non-isolated DC/DC convertersBoost converter operating principle

Ideal relation between input and output voltage is

where

Dt

tt

V

V

off

offon

DC

out

1

1

offon

on

tt

tD

t

vL

VDC

VDC-Vout

toff

t

iL

ton ton

VDC

L1 D1

C1 Vout

Circuit during toff

iL

vL

VDC

L1

S1C1

vL

iL

Circuit during ton

Vout

Non-isolated DC/DC convertersInterleaving of Boost converters

− The basic boost converter is often scaled to higher power levels by paralleling two or more boost converters.

− The stages are controlled in opposite phases (the transistors do not conduct at the same time), so the total input current ripple is reduced compared to a single converter.

VDC

L1

S1

D1

C1 VoutL2

S2

D2

Isolated DC/DC convertersPhase-shifted full-bridge

− Very common topology capable of zero voltage switching low switching losses in primary transistors.

− Suitable for higher input voltages. S1

S3

S4

S2

Vab

ILout

ILlk

Vcd

t0 t1 t2

0.5T-tsafe

DT

DeffT

tsafe

t3 t4

VDC

S3

S1 S4

S2

Cout Rload

D5Llk

Cin

a

b

D1

D2D3

D4

D6

D7

D8

Lout

c

d

Isolated DC/DC convertersFull-bridge boost

VDC

S1

S4

S3

S2

D1

D2

L1

D3

D4

Cout Rload

Llk

TX1

on off

off on

S1 & S2

S3 & S4

DT (1-D)T

IL1

Isec

Mode1 Mode2 Mode 3 Mode 4t0 t1 t2 t4 t5

Vsec

IS1

ID1

ΔIL1

ΔIL1/n

IL1/2

ΔIL1

T

BTX1

− High voltage conversion ratio− Low input current ripple even without

input capacitors.− Low inrush current.

Isolated DC/DC convertersResonant push-pull boost

on off

onoff

off

offon

on

S1

S2

S3

S4

DT (1-D)T

IL1

Isec

Mode1 Mode2 Mode 3 Mode 4 Mode 5 Mode 6

t0 t1 t2 t3 t4 t5 t6

IS1

IL/2

-IL/2

IS4

VDC

S3

S1

S4

S2

L1

Cout Rload

D2

D1 Cr1

Cc1 Cc2

N1

N1

N2

Cr2

Llk

ipri1

ipri2

DS1 DS2

DS3 DS4

− Twice the voltage conversion ratio compared to full-bridge boost

− Low input current ripple even without input capacitors.

− Low inrush current.− Near sinusoidal current waveforms and

zero current switched secondary.

Isolated DC/DC convertersSources of power losses

− The losses in switching converters can be divided into three categories:• Conduction losses• Switching losses• Core losses in magnetic components

− The dominating loss mechanism depends on the voltage and current as well as converter topology (capability of zero voltage or zero current switching etc.)

− As a rule of thumb:• Low voltage, high current conduction losses dominate• High voltage, low current switching and core losses dominate• High voltage, high current depends strongly on the converter design

Isolated DC/DC convertersConduction losses

− Conduction losses are caused by the conductor resistances and the intrinsic resistances in semiconductor junctions.

− Dissipated power P is the product of resistance R and the current I squared.

2IRP − Example: We have a 10 kW converter and two different stack voltages: 50 V

and 250 V. Let us assume that both converters have 3 mΩ of resistance in the primary circuit.

50 V I = 200 A P = 0.003*2002 = 120 W250 V I = 40 A P = 0.003*402 = 4.8 W

− There is a 96% reduction in conduction losses, when the input voltage changes from 50 V to 250 V!

Isolated DC/DC convertersTransistor switching losses

− Switching losses arise from two major sources:• Overlapping of current and voltage during switching• Charging/discharging of parasitic capacitances in components

− In ZVS the transistor body diode conducts before gate voltage is applied.

− Voltage across the transistor is limited to body diode forward voltage during diode conduction.

− There is no Miller plateau in the gate-source voltage and thus the gate drive losses are also decreased.

t

vGS

iDS

vDS

PSW

t

t

t

vGS(th)

vMiller

Hard switching

t

t

t

t

vGS(th)

iDS

vGS

vDS

PSW

Zero voltage switching (ZVS)

Isolated DC/DC convertersTransistor switching losses

Switching with ideal MOSFET

No EMI, minimal losses

Time

248.7700ms 248.7800ms 248.7900ms248.7607msV(X_SLPS.va)

0V

50V

100V

-20V

120V

0.5 1 1.5 2 2.5 3 3.5

x 104

-20

0

20

40

60

80

100

Cgd

Cgs

Cds

Ld

Ls

Switching with unideal MOSFET Increased EMI and losses!

Isolated DC/DC convertersDiode switching losses

− Switching losses in diodes are caused by forward recovery and reverse recovery phenomena, as a diode requires a finite time to switch from conducting state to non-conducting state and vice versa.

− Forward recovery loss is typically small compared to reverse recovery loss.

− Charge Qrr must be swept away from the junction during the recovery time trr.

− Voltage VR and current IRM behavior during the off-transition defines the switching losses.

− Silicon Carbide (SiC) diodes do not experience reverse recovery effects.

swrjswrrrsw fVCfVQP2

1

− Voltage dependent junction capacitance Cj causes additional switching losses also in SiC diodes.

[2]

Isolated DC/DC convertersMagnetic component core losses

− Magnetic field strength H is related on current I flowing through N turns of conductor surrounded by a magnetic core having a magnetic path length of lm.

− The flux density B in a magnetic material depends on the material permeability µ and the magnetic field strength H.

ml

NIH HB

− Flux density B can be plotted as a function of H to form a hysteresis loop.

− The loop shape depends on the core material.

− The area inside the loop is the energy dissipated in the core material.

Isolated DC/DC convertersMagnetic component core losses

− Core losses depend on the difference between the maximum and minimum flux density (ac flux). The larger the ac flux, the larger the losses.

− The higher the operating frequency, the higher the core loss at certain ac flux.− In transformers there is a trade-off between the number of turns (conduction

losses) and the core losses. An optimal design is found near the point where winding losses and core losses intersect.

Isolated DC/DC convertersExamples of loss distributions

− Example loss distributions are given for a 3 kW full-bridge boost [3] and a 10 kW resonant push-pull converter [4].

− The component stresses are dependent on the input/output parameters, selected topology and component optimization!

MOSFETs65 %

Inductive components

23 %

Diodes12 %

Capacitors0 %

RPP loss distribution exampleTotal loss 663 W, efficiency 93.4%

MOSFETs38 %

Inductive components

18 %

Diodes30 %

Misc14 %

FB boost loss distribution exampleTotal loss 100 W, efficiency 96.7%

Isolated DC/DC convertersExamples of prototype costs

Magnetics30 %

Semiconductors28 %

Cooling13 %

Capacitors12 %

Control17 %

Cost distribution in a 10 kW RPP converterTotal cost € 827

Magnetics21 %

Semiconductors35 %

Cooling16 %

Capacitors7 %

Control21 %

Cost distribution in 2 x 5 kW RPP converterTotal cost € 916

− In modular converters the cost of auxiliary components may be higher in proportion than in single unit converters.

− Magnetic components can be smaller and cheaper in modular systems, but it is easier to achieve higher efficiency with larger components.

− Semiconductor efficiency is typically much better in modular converters due to smaller currents.

DC/DC convertersBidirectional converters

anode

cathode

ELECTROLYTE

Oxidant

Unusedhydrogen

Air, heatand water

Hydrogen

Plantcontroller

BoP

FC stack

UPS

Grid

Resistor bank

DC/DC DC/AC

anode

cathode

ELECTROLYTE

Oxidant

Unusedhydrogen

Air, heatand water

Hydrogen

Plantcontroller

BoP

FC stack

Grid

Resistor bank

Battery pack

DC/DC DC/AC

DC/DC

DC/AC

− Process control backup powering is often implemented with UPS systems connected to the grid side.

− In emergency shutdown the excess stack power is dissipated in resistors.− Bidirectional DC/DC converters can interface the fuel cell to battery packs,

that act as small time constant energy storages.− Some of the stack energy could be recovered also during shutdown.

DC/DC convertersSummary

− A DC/DC converter is an essential component in the power supply chain, unless the voltage levels between the power source and the load are directly compatible.

− It is more efficient to transfer certain power with high voltage and low current than vice versa.

− If galvanic isolation is not needed for safety or voltage step-up, the conversion efficiency is likely to increase and less complex converter topologies can be used.

− If the fuel cell output has a high tolerance for high frequency ripple (> 10 kHz) the DC/DC converter input filter requirements can be less stringent smaller, cheaper and more efficient components.

− Higher efficiency often results in higher initial costs, so the total cost efficiency is dependent on the projected system life time.

DC/AC convertersSingle phase topologies

− Half-bridge inverter− Simple structure and control

− Output peak voltage is ma * Vd/2, where ma is the modulation index (ma ≤ 1 in the linear region) [5]

− Full-bridge inverter− Output peak voltage is ma * Vd,

where ma is the modulation index (ma ≤ 1 in the linear region)

− Bit more complex than the one-leg inverter

There are lots of other variants too especially in wind and solar applications!

Vd

TA+

TA-

C+

C-

oA

VAo

2dV

2dV

N

Vd

TA+

TA-

TB+

TB-

C+

C-

oA

VAo-VBo

2dV

2dV

N

B

DC/AC convertersSingle phase modulation methods

− Bipolar PWM [5]− Half-bridge and full-bridge

inverter− Unipolar PWM [5]− Only full-bridge inverter− Lower harmonic content

DC/AC convertersThree phase topologies

− Able to supply all three phase-loads such as motors or electric grid.− Can be implemented either as voltage source inverter (VSI) or current

source inverter (CSI).− CSI converters are able to boost voltage from input to output.− Input inductor in CSI reduces the ripple current taken from the source.

Vd

TA+

TA-

TB+

TB-

C+

C-

o

2dV

2dV

N

TC+

TC-

A

B

C

VSI CSI

Vd

TA+

TA-

TB+

TB-

N

TC+

TC-

ABC

L1

DC/AC convertersThree-phase modulation methods

− Three-phase PWM for VSI− Triangular wave is compared with

sinusoidal waveforms that are 120° out of phase.

− With linear modulation (ma ≤ 1) the maximum line-to-line rms voltage is

dada VmVm 612.022

3

− The maximum obtainable line-to-line rms voltage with overmodulation is

dd VV 78.06

[5]

DC/AC convertersThree-phase modulation methods

− Space vector PWM for VSI − Eight discrete voltage vectors based

on the logic states of power switches.− Other voltage vectors in a sector can

be produced by using the active vectors and zero vectors for a certain time during the switching period Ts.

− Maximum radius of the red circle (linear region) is

− Theoretical maximum output voltage is

dd V

V577.0

3

dd VV 637.02

[6]

DC/AC convertersMultilevel converters

Vd

S1

S2

S3

S4C+

C-

2dV

2dV

S5

S6

ABC

N

S’1

S’2

S’3

S’4

S’5

S’6

− In two-level inverters the available voltages at output are Vd and –Vd.

− By adding levels to the inverter, more output voltages can be produced (diode-clamp multilevel converter).

− A three-level inverter could provide also the neutral voltage N.

− Additional voltage levels reduce the harmonic distortion, so a filter could be omitted.

− Other types of multilevel converters are flying capacitor converters and cascaded converters with separate DC sources [7].

DC/AC convertersLosses in a VSI inverter

− Loss example of a 10 kW application with Vd = 700 V and fsw = 6 kHz [8].

− IGBTs having larger rated current exhibit smaller conduction losses (smaller junction resistance) but larger switching losses (slower switching).

− Typical VSI power losses range between 1-2% of rated power (depending on the operating point).

− Galvanic isolation or grid filter cause additional losses (typically few percent of rated power).

Conduction23 %

Switching50 %

Diode conduction

3 %

Diode switching24 %

SKiM120GD176D, rated current 120 ATotal losses 168 W --> Efficiency 98.3%

Conduction36 %

Switching47 %

Diode conduction5 %

Diode switching12 %

SK35GD126ET, rated current 35 ATotal losses 128 W --> Efficiency 98.7 %

DC/AC convertersSummary

− DC/AC converter converts DC voltage to grid frequency AC voltage.− The required DC link voltage depends on the converter topology and the

modulation method.− Linear modulation requires higher DC link voltage than overmodulation, but

with linear modulation the output voltage has less harmonics and thus the waveform is closer to pure sine.

− The better the voltage quality, the smaller and more efficient filters can be used.

− DC link voltage and switching frequency can often be adjusted in commercial inverters. The selection is a trade-off between voltage quality and switching losses.

System interconnectionProcess signaling

− Case LUT & VTT− If electrical grid is OK, inverter charges the DC link.− DC/DC initializes and activates PCU OK signal.− If DC/DC is OK current reference is set PCU ON signal is activated.− Inverter active signal is activated inverter running signal is received.

VDC 660-700 V

230/400 V

10 kW

Control unit

Isolated DC/DC

ABB ACSM-204AR-016A Regen Supply

Module

Inverter active2PCU ON, Current reference (from PLC)

30-70 V

Inverter runningPCU OK (to PLC)

Inverter running Inverter active

System interconnectionControl of DC/DC converter

− Reference current is given from the fuel cell plant controller.

− Actual current is measured from the converter input.

− The error between the reference and the measurement is fed to a current controller.

− The current controller increases or decreases the converter duty cycle in order to force the current error to zero.

− Attention is paid to mitigation of the 150 Hz grid harmonic.

[9]

System interconnectionControl of DC/AC converter

− Outer control loop controls the DC link voltage to maintain the power balance of the system.

− Voltage controller gives a d-axis current reference to the current controller.

− Current controller compares the current reference to measured values and forces the error to zero.

− The output of the current controller is a d-q voltage reference.

− The d-q voltage reference is transformed into α-β reference and given to the modulator together with phase angle.

− The modulator produces the switching vectors for the DC/AC power stage. [9]

System interconnectionCoordinate transforms

− Three phase grid voltages and currents are transformed into 2-dimensional rotating coordinates (d-q) through Clarke and Park transforms.

[10]

System interconnectionControl overview

− DC/DC controller controls only the input current with as small low frequency ripple and steady-state error as possible.

− DC/AC converter maintains power balance by keeping the DC link voltage constant.

[9]

References

[1] Halinen, M., et al. (2011). Performance of a 10 kW SOFC demonstration unit. ECS Transactions, 35, pp. 113-120.

[2] Walters, K. (n.d.). Rectifier reverse switching performance. MicroNote Series 302, Tech. Rep. Microsemi.

[3] Nymand, M. and Andersen, M.A.E. (2009). New primary-parallel boost converter for high-power high-gain applications. In: Applied Power Electronics Conference (APEC), 2009, pp. 35-39.

[4] Väisänen, V., Riipinen, T., Hiltunen, J., and Silventoinen, P. (2011). Design of 10 kW resonant push-pull DC-DC converter for solid oxide fuel cell applications. In: Proceedings of the 14th European Conference on Power Electronics and Applications (EPE 2011).

[5] Mohan, N., Robbins, W.P., and Undeland, T.M. (2003). Power Electronics: Converters, Applications and Design, Media Enhanced Third Edition, 3rd ed. John Wiley & Sons.

[6] Sarén, H. (2005). Analysis of the voltage source inverter with small dc-link capacitor. Lappeenranta University of Technology.

[7] Lai, J.-S. and Peng, F.Z. (1996). Multilevel converters – a new breed of power converters. IEEE Transactions on Industry Applications, 32(3), pp. 509-517.

[8] Semikron SemiSel thermal calculator and simulator. url: http://www.semikron.com.

[9] Riipinen, T. (2012). Modeling and control of the power conversion unit in a solid oxide fuel cell environment , D.Sc. thesis. Lappeenranta: Acta Universitatis Lappeenrantaensis. In peer review.

[10] Ross, D., Theys, J., and Bowling, S. (2007). Using the dsPIC30F for vector control of an ACIM. Application note AN908. Microchip Technology Inc. url: http://ww1.microchip.com/downloads/en/AppNotes/00908B.pdf

Thank you! Any questions?