1. Modular Multilevel Converter (MMC)
Ghazal Falahi

2. Modular Multilevel Converter (MMC) structure
Series connection of sub-modules
Make the arm of the converter to build
the output voltage stepwise
arm
leg
2. Sub-modules are Half-bridge (HB) or
Full-bridge (FB) converters
3. Each sub-module has a capacitor
with average voltage of
4. With n sub-modules in arm of the converter output voltage has n+1 levels

3. MMC sub-module operation
iarm>0
Sub-module inserted
Capacitor in discharge mode
Sub-module bypassed
iarm<0
Sub-module bypassed
Sub-module inserted
Capacitor in charge mode

4. Modular Multilevel Converter operation
MMC equivalent circuit
Each arm of the converter is equivalent
To a controlled voltage source with
magnitude of and a series
inductor
Where nactive is number of inserted
sub-modules
Number of sub-modules in
MMC arm

5. MMC advantages and disadvantages
Low THD
Low on devices and good voltage sharing for semiconductors
Modular structure with identical modules which has redundancy and allows to substitute failed modules
Scalable and no DC link voltage limitation
Simple mechanical construction
No need for bulk filters on ac side
Lower losses
Disadvantages
Extra controller required for balancing of capacitor voltages
Need for monitoring all capacitor voltages
Circulating current consisting double fundamental frequency component and increases device losses if not suppressed

6. MMC mathematical equations
MMC mathematical model
Arm voltage
Output voltage
Circulating current
MMC grid connection dynamic
Output voltage
Vk : Phase voltage
K= a, b, c
Modulation

7. MMC operation and analysis
Device operation in MMC sub-module
Upper arm reference voltage
Arm current
Varm,ref
iarm
D2 conducts
S2 conducts
D1 conducts
S1 conducts
m = 1
φ = 0

8. Device operation principal in MMC sub-modules
Logic of operation
The arm current flowing out of the sub-module is considered as positive (a) and the current flowing into sub-module is considered as negative
(a) iarm>0
(b) iarm<0
iarm>0
S1 or D2 S1 D2
iarm<0
S2 or D1 D1 S2
Direction of arm current determines which devices can operate
The rate reference arm voltage change determines if a sub-module is inserted or bypassed
State S1 S2
Vsm 1 ON
OFF Vc 2

9. Semiconductor rating Device voltage Device current
Device voltage rating is the average capacitor voltage rating of sub-modules
With a margin depending on maximum allowed ripple
Device current
Device current in MMC is found from the following equations
Valve current:
Valve rms current:

10. Modular Multilevel Converter Conventional Control [1]
An individual capacitor voltage controller
The averaging controller
The system controller
Modulation reference generation
Modulation reference generation
[1] Hagiwara, Makoto, and Hirofumi Akagi. "Control and experiment of pulsewidth-modulated modular multilevel converters." Power electronics, IEEE Transactions on 24.7 (2009):

11. MMC modulation methods
Fundamental switching frequency modulation techniques
Suitable for large number of sub-modules
Lower losses
High switching frequency modulation techniques
Suitable for small & large number of sub-modules
Lower harmonics
× High losses
[1] Wang, Jun, Rolando Burgos, and Dushan Boroyevich. "A survey on the modular multilevel converters—Modeling, modulation and controls." Energy Conversion Congress and Exposition (ECCE), 2013 IEEE. IEEE, 2013.

12. Passive components design ( Arm inductor and Sub-module capacitor)
Criteria to select arm inductor [1]:
Limit the circulation current
Limit the fault current rise rate
Sub-module capacitor selection [2]:
Provide the output power for at least one cycle
If DC link is defective
2. Limit sub-module voltage ripple
Ps: three phase apparent power
K: voltage modulation index
Cosφ: power factor
N: number of sub-modules,
ω0 is the fundamental frequency,
VC: mean value of sub-module voltages
ε: sub-module voltage ripple
[1] Tu, Qingrui, et al. "Parameter design principle of the arm inductor in modular multilevel converter based HVDC." Power System Technology (POWERCON), 2010 International Conference on. IEEE, 2010.
[2] Zygmanowski, Marcin, Boguslaw Grzesik, and Radoslaw Nalepa. "Capacitance and inductance selection of the modular multilevel converter." Power Electronics and Applications (EPE), th European Conference on. IEEE, 2013.

13. Three phase line PWM voltages and phase currents in MMC with sub-modules in each arm
Simulation results of a grid connected MMC with 3 sub-modules in each arm in Matlab

14. Arm voltages and sub-module voltages of the simulated MMC

15. References
[1] Falahi, Ghazal. "Design, Modeling and Control of Modular Multilevel Converter based HVDC Systems." PhD Dissertation NCSU (2014). [2] Falahi, Ghazal, and Alex Q. Huang. "Design consideration of an MMC-HVDC system based on 4500V/4000A emitter turn-off (ETO) thyristor." Energy Conversion Congress and Exposition (ECCE), 2015 IEEE. IEEE, [3] Falahi, Ghazal, and Alex Huang. "Control of modular multilevel converter based HVDC systems during asymmetrical grid faults." Industrial Electronics Society, IECON th Annual Conference of the IEEE. IEEE, [4] Falahi, Ghazal, Wensong Yu, and Alex Q. Huang. "THD minimization of modular multilevel converter with unequal DC values." Energy Conversion Congress and Exposition (ECCE), 2014 IEEE. IEEE, [5] Falahi, Ghazal, and Alex Huang. "Low voltage ride through control of modular multilevel converter based HVDC systems." Industrial Electronics Society, IECON th Annual Conference of the IEEE. IEEE, 2014.