M. Mohr F.W. FuchsCSI for fuel cells

Nordic Workshop on Power and Industrial Electronics

NORPIE 2004 Trondheim, 14-16 June

Dimensioning of a Current Source Inverter for the Feed-in of Electrical Energy from Fuel Cells to the Mains

Malte Mohr, Friedrich W. Fuchs

Chair for Power Electronics and Electrical Drives

Christian-Albrechts-Universität zu Kiel, Germany

M. Mohr F.W. FuchsCSI for fuel cells

Outline

1. Introduction

2. Fuel cell 2. Fuel cell

3. Demands on the inverter

3.1 Current source inverter (CSI)

3.2 Dimensioning/optimisation

3.3 Laboratory test setup

4. Conclusion

M. Mohr F.W. FuchsCSI for fuel cells

1. Introduction

• Fuel cells convert chemical energy directly into electrical energy.

• Fuel cells have a high electrical efficiency.• Application in decentral power generation.• Fuel cells deliver dc-current.• Power electronics converts it into ac-current.• A current source inverter fulfils the

requirements for operation at fuel cells.

M. Mohr F.W. FuchsCSI for fuel cells

2. Fuel cell

V [V]

J [A/cm2]

M. Mohr F.W. FuchsCSI for fuel cells

2. Fuel cell

V-I characteristic curve of a single fuel cell:

•voltage depends significantly on current

V[V]

J[A/cm2]

M. Mohr F.W. FuchsCSI for fuel cells

2. Fuel cell

characteristic curve at full fuel supply

V-I characteristic curve of a single fuel cell:

•voltage depends significantly on current• maximum current depends on supplied fuel

J[A/cm2]

V[V]

M. Mohr F.W. FuchsCSI for fuel cells

2. Fuel cell

characteristic curve at partial fuel supply

V-I characteristic curve of a single fuel cell:

•voltage depends significantly on current• maximum current depends on supplied fuel

J[A/cm2]

V[V]

M. Mohr F.W. FuchsCSI for fuel cells

2. Fuel cell

V-I characteristic curve of a single fuel cell:

•voltage depends significantly on current• maximum current depends on supplied fuel• operation in the linear part of the characteristic

curve

J[A/cm2]

V[V]

M. Mohr F.W. FuchsCSI for fuel cells

2. Fuel cell

operating point at partial load

operating point at full load

J[A/cm2]

V-I characteristic curve of a single fuel cell:

•voltage depends significantly on current• maximum current depends on supplied fuel• operation in the linear part of the characteristic

curve

V[V]

M. Mohr F.W. FuchsCSI for fuel cells

3. Demands on the inverter

The inverter has to• draw well smoothed dc-current from the fuel cell

to prevent damages of the cell,• have low harmonics in the mains current,• adapt the varying fuel cell voltage to the mains

voltage,• feed in active power,• be efficient,• be economical.

M. Mohr F.W. FuchsCSI for fuel cells

3.1 Current source inverter (CSI)

Current source inverter (CSI):• meets the demands• increases the voltage towards the mains

M. Mohr F.W. FuchsCSI for fuel cells

3.1 Current source inverter (CSI)

higher efficiency of the inverter system at higher dc input voltages

• input voltage is limited: Vdc < Vline , depending on ac-filter values

load range is limited if the system is optimised for high input voltages

• Losses of the current source inverter depend on the current Id

M. Mohr F.W. FuchsCSI for fuel cells

3.2 Dimensioning/optimisation

Efficiency dependend on input dc voltage:stack with higher system voltage stack with lower

system voltage

maximum inverter current

maximum inverter input voltage

M. Mohr F.W. FuchsCSI for fuel cells

3.2 Dimensioning/optimisation

Efficiency dependend on input dc voltage:

transmitted power at high system voltage

transmitted power at moderate system voltage maximum inverter current

maximum inverter input voltage

transmitted power at low system voltage

M. Mohr F.W. FuchsCSI for fuel cells

3.2 Dimensioning/optimisation

Load range dependend on input dc voltage:

full load range

lower input voltage

low efficiency

no load range

high input voltage

best efficiency

moderate load range

moderate input voltage

better efficiency

M. Mohr F.W. FuchsCSI for fuel cells

3.2 Dimensioning/optimisation

• rated inverter power increases proportional to the input dc current

maximumtransmitted powerPmax at inverterinput voltage

installedpower / max.transmittedpower Pmax

power range,based on Pmax

Vd, max 100 % 0 %

0,8 · Vd, max 125 % 45 %

0,66 · Vd, max 150 % 90 %

reducing the power range yields to better efficiency and less rated inverter power

M. Mohr F.W. FuchsCSI for fuel cells

3.2 Dimensioning/optimisation

Simulation of the current source inverter with Simplorer®

Id*, Id,

Vline/5, iline

t

12.5 25 67.5 50

A, V

ms

-100

-50

50

100

0

0

Id* Id Vline

iline

• rise time < ¼ periode of the mains

• inverter much faster than fuel cell

vline

t

Id* Id

iline

I, V

M. Mohr F.W. FuchsCSI for fuel cells

3.3 Laboratory setup

circuit diagram of the laboratory setup:

M. Mohr F.W. FuchsCSI for fuel cells

3.3 Laboratory setup

measuring and control devices

power circuit with low leakage inductance

overvoltage protection

M. Mohr F.W. FuchsCSI for fuel cells

3.3 Laboratory setup

input dc-current, output ac-current and output ac-voltage; inverter at ohmic-inductive load, open-loop controlled

M. Mohr F.W. FuchsCSI for fuel cells

4. Conclusion

The current source inverter• is suitable to convert electrical energy from fuel

cells to feed in the mains,• has poor utilisation and poor efficiency at low

dc-input voltages.

• Limitation of power range enhances efficiency and inverter utilisation.

• In principle the CSI is suited for higher system voltages and therefore higher power ratings.

M. Mohr F.W. FuchsCSI for fuel cells

Nordic Workshop on Power and Industrial Electronics

NORPIE 2004, 14-16 June

Dimensioning of a Current Source Inverter for the Feed-in of Electrical Energy from Fuel Cells to the Mains

Malte Mohr, Friedrich W. Fuchs

Chair for Power Electronics and Electrical Drives

Christian-Albrechts-Universität zu Kiel, Germany