dirk audring de academic village barcelona 12-15 may 2003 1 supplying micro-grids with stationary...

1Dirk Audring DE Academic Village

Barcelona 12-15 May 2003

Supplying micro-grids with stationary fuel cell systems

Prof. Dr.-Ing. G. Balzer, Dipl.-Ing. D. Audring

Darmstadt University of Technology

Department of Electrical Power Systems



Structure

• Examined micro-grid

• Short-circuit performance

• Disconnection from power system

• Conclusions



Examined micro-grid

UBB

FC

3PEN~50Hz 230/400V

5 k

VA

cos

= 0

,95

5 k

VA

cos

= 1

,0

50 kVA

M3~

7 kW

Micro-grid

M1

V1

V2

iM1

SC 2

SC 1

ABCD

• Micro-grid is supplied by one fuel cell

• Fuel cell sets voltage and frequency of micro-grid

• Induction motor and two loads are connected to micro grid

• Network connection is simulated for back-up power supply using a disconnecting switch



Short-circuit performance

-0.1

0

0.1

0.2

-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3-0.1

0

0.1

(t-T )[s]SC

-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3(t-T )[s]SC

i [kA]FC Currents of fuel cell:

i [kA]A Currents of feeder A:• Fuel cell acts as a

voltage source with limited current output

• This leads to low short-circuit current at micro-grid

• Selectivity with over-current protection is difficult to achieve




Inverter

=~~~

Main busbar ofMicro-grid

u

i

Fuel cell stack

iFC,2

iFC,3

iFC,1

Short-circuit

• Short-circuit currents are limited by inverter

• Transformation ratio and vector group of inverter-

transformer influences short-circuits at micro-grid




-0,05 0 0,05 0,1-200-100

0

100

-0,05 0 0,05 0,1

200

-200-100

0100

(t-T ) [s]SC1

-0,05 0 0,05 0,1 -0,05 0 0,05 0,1(t-T ) [s]SC1

i [A]SC

i [A]SC

(t-T ) [s]SC1

(t-T ) [s]SC1

Single-phase short-circuit:Phase-to-phase short-circuit

clear of earth:

Two-phase-to-earthshort-circuit:Three-phase short-circuit:



Disconnection from power system

0 0.05 0.1 0.15 0.2 0.25 0.3

-0.3-0.2-0.10.00.10.20.3

-0.1

0.0

0.1

0.4

0.6

0.8

1.0

-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3

-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3

i [kA]IM

u [kV]BB

(t-T )[s]SC

(t-T )[s]SC

(t-T )[s]SC

Busbar voltage of microgrid:

Currents of induction motor:

[p.u.]Revolution of induction motorApplied torque

• Short-circuit at power system leads to voltage drop at connected micro-grid

• Induction motor slows down

• Induction motor must be re-accelerated after disconnection from network

• Power quality is poor during disconnection and not acceptable for PC etc.



Conclusions

• Main characteristic of micro-grid supplied by fuel cell is low

short-circuit current

• Selectivity with over-current protection is difficult to achieve

• Transformation ratio and vector group of inverter-

transformer is to consider for calculation of short-circuit

currents at micro-grid

• Disconnection from power system due to short-circuit has to

carried out fast to reduce voltage disturbances at micro-grid

dirk audring de academic village barcelona 12-15 may 2003 1 supplying micro-grids with stationary...

Documents