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D e sign and control of Modular Multilevel Converter as an Active Front End. Panagiotis Asimakopoulos Technical Student, TE-EPC-MPC Supervisor: Examiner:
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Technical Student, TE-EPC-MPC
KonstantinosPapastergiou Massimo Bongiorno
TE-EPC-MPC, CERN Chalmers University of Technology
Aim of part A:
Aim of part B:
1-phase inverter mode, N=2 modules per arm
2N+1 levels in output voltage
Module’s mode of operation
The arm current consists mainly of 3 components:
Difference between the sum of the upper caps voltages
and the sum of the lower caps-mainly first harmonic
Sum of all capacitors’ voltages in a leg-mainly second harmonic
The main reasons for the utilization of the inductance are summarized in the following bullet points:
The arm inductor is rated for the arm current:
For the 1-phase example:
For the 3-phase converter
Arm current filtering
2nd harmonic suppression
The 2nd harmonic increases the losses in the system. It does not really affect the components ratings.
To avoid a significant increase of the arm inductance, two solutions are proposed:
Filter  or dedicated second harmonic controller 
The main reason for the arm inductance is the fault current limitation in the case of a dc fault. The ac voltage source feeds the fault via the antiparallel diodes in the modules.
The modules capacitors are connected in series during the fault via the antiparallel diodes. No need for a high value of the arm inductance. The capacitors should be dimensioned to handle the voltage fluctuation during the fault.
Calculation of the charge lost during the capacitor’s discharge phase
where the instants when the arm current crosses the x-axis. The instants depend on the fundamental frequency value.
For a specified percentage p of voltage ripple:
To avoid the resonant peak by and in 100 Hz:
The average capacitor voltage with an extra margin depending on the maximum allowed ripple is used.
The current load of the components inside the module is not equally distributed. The arm current for a 3-phase MMC is equal to:
The direction of the power is defined by the sign of the dc component of arm the current.
For AFE operation has a negative sign giving a negative dc-offset to the arm current.
For inverter operation has a positive sign giving a positive dc-offset to the arm current.
Upper switch current Lower diode current
Lower switch current Upper diode current
Upper arm current Upper arm current
The decision of N is based on the criteria:
It is a techno-economical decision…
Trade off among:
General control strategy of an AFE converter
General current controller scheme in dq coordinates
Approach simulated: Load oriented sizing of the total system
The load is finally modeled as a controlled current source.
For a maximum modulation index below unity a margin is provided to the H-bridge input voltage (dc-link capacitor nominal voltage):
The capacitance of the dc-link capacitor is:
The maximum output voltage of the converter for a sinusoidal PWM is
The ac grid phase voltage has the amplitude:
The factor 0.8 is applied in order to take into consideration the maximum modulation index and to provide a margin for the current controller.
The rms ratings are much lower than the peak values of the current. The repetitive peak current of the selected device must be also taken into account.
Converter voltage N=4
Converter voltage N=6
Converter voltage N=10
Planning for the prototype tests: