1. Reliable Modular Multilevel Converters (MMCs) for High-Voltage Direct-Current (HVDC) Transmission
Fujin Deng
Department of Energy Technology
Aalborg University, Denmark

2. HVDC Transmission Technology
Advantages:
Low losses & Small footprint
Long-distance power transmission
Active and reactive power individual control
No reactive power and frequency regulation issue
Power transmission between different AC grids (different voltage or frequency)

3. Outline Basics of MMC-HVDC Fault Detection Method3
Outline
Basics of MMC-HVDC
Fault Detection Method
Fault Localization Method
Fault Tolerant Control

4. I. Basics of MMC based HVDC
Advantages:
Modularity
Scalability
High reliability
Multi-level
High power quality
Low switching frequency SM
F. Deng and Z. Chen, 2015, “Voltage-balancing method for modular multilevel converters under phase-shifted carrier-based pulse-width modulation,” IEEE Transactions on Industrial Electronics, vol. 62, no. 7, pp

5. SM equivalent model: Operation mode SM state S1 S2 usm
Arm current iarm
Capacitor Csm
Voltage uc I On
Off Uc
Positive
Charge
Increased
Negative
Discharge
Decreased II
Positive or negative
Bypass
Unchanged
F. Deng and Z. Chen, “A control method for voltage balancing in modular multilevel converters,” IEEE Transactions on Power Electronics, vol. 29, no. 1, pp , 2014

6. II. Fault Detection for MMCs
Switch open-circuit fault in SM unit
Kalman Filter Method
with
Method: Detecting fault based on inner difference current idiff_j.
Phase A
Phase B
Phase C
Three-phase MMCs

7. Kalman Filter (KF) for estimating inner difference current idiff_j
Fault detection method
F. Deng, Z. Chen, M. R. Khan, and R. Zhu, “Fault detection and localization method for modular multilevel converters,” IEEE Transactions on Power Electronics, vol. 30, no. 5, pp , 2015.

8. IV. Fault Localization for MMCs
iarm g1 g2
Normal operation
T1 fault
iarm flows through
usm
> 0 on
off
d1 & Csm uc T2
< 0
Csm &T1 d2
(1)
Conclusion:
Normal
Fault
Capacitor voltage uc in faulty SM would be increased.
(2)
iarm g1 g2
Normal operation
T2 fault
iarm flows through
usm
> 0 on
off
d1 &Csm uc T2
< 0
Csm &T1 d2
Normal
Fault
Conclusion:
Capacitor voltage uc in faulty SM would be increased.
F. Deng, Z. Chen, M. R. Khan, and R. Zhu, “Fault detection and localization method for modular multilevel converters,” IEEE Transactions on Power Electronics, vol. 30, no. 5, pp , 2015.

9. Fault localization method
(3)
iarm g1 g2
Normal operation
T1 and T2 fault
iarm flows through
usm
> 0 on
off
d1 & Csm uc T2
< 0
Csm & T1 d2
Conclusion:
Capacitor voltage uc in faulty SM would be increased.
Normal
Fault
Normal
Fault
Fault localization method

10. Experimental Studies Experimental setup: kW prototype 4 cells per arm
Dspace controller

11. Normal Operation
Estimated value = Measured value

12. Top IGBT Fault in Cell21

13. Buttom IGBT Fault in Cell21

14. V. Fault Tolerant Control
Equivalent circuit:
Result in fundamental component in

15. Control for eliminating fundamental component in inner difference current of phase A
Control for eliminating fundamental component in inner difference current of three-phase MMCs

16. Experimental Studies Experimental setup: kW prototype 4 cells per arm
Dspace controller
Phase A
Phase B
Phase C

17. Phase A Phase B Phase C No control with control

18. 18
Thanks !