1. Modular Multilevel Converter for Wind Energy Storage Systems
Bora Novakovic
Adel Nasiri

2. Introduction Motivation Objective
Problems introduced by new grid codes for Wind Energy Systems
Fault ride through capabilities
Power smoothing
Frequency and reactive power support
Problems introduced by increased size of wind turbines
Paralleling of low voltage systems
Efficiency
Objective
Power converter topology that Is suitable for the high power (multi-MW) wind energy application
Power conversion system topology that solves or diminishes problems related to new grid codes.

3. Approach Make the conversion system medium voltage
Solves paralleling problems of low voltage systems
Solves efficiency problem
Integrate energy storage system into wind turbines
Solves or alleviates issues introduced by new grid codes
Find appropriate conversion topology to fulfill the requirements stated above.

4. Converter topology – MMC
Modify the MMC topology to integrate storage system
MC supports high number of levels
Modules (cells) can operate at low voltage for HV and MV systems
Allows integration of storage.

5. MMC Modifications
MMC is modified to include energy storage into the cell structure
Cell can also be divided in modules for easier analysis
Any module combination is possible and each has its benefits and drawbacks
Energy storage is assumed to be string of ultra-capacitors.

6. Simulation
To test the performance if the converter single leg was simulated
High detail switching model was used
It is assumed that the converter operates in a 5MW off shore wind turbine
The power imbalance between the AC and DC side is simulated
The excess power goes to the integrated energy storage
This kind of condition can be found in cases of:
Low voltage ride through conditions
Power smoothing .
Description
Value
Base Converter Power
6.25MVA
Generator Power
5MW
Generator Power Factor
0.8
Base Voltage
13.8kV
Base Impedance 30
Base Frequency
60Hz
DC Bus Voltage min.
23kV
Cell Voltage max.
800V
Number of levels 32

7. Results Cell DC voltage illustrates charging of the storage system
Zoomed region for the cell DC voltage. Illustrates good balancing
Output voltage
illustrates 32 level switching.
DC side power and AC side current to illustrate the power imbalance and the dynamic system behavior

8. Thank You!
Questions?