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

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.

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.

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.

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.

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

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

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