1. A DC/DC Modular-Multilevel Converter for DC Grid
各位领导、各位专家，大家好！
我是全球能源互联网研究院贺之渊，现在我代表“±500kV柔性直流换流阀”研究团队汇报换流阀研制情况。
Global Energy Interconnection Research Institute ,China
Warsaw, Poland
Sep. 13th 2017

2. Background
HVDC- flexible provides a flexible, economical and efficient solution for the access of renewable energy sources.
High voltage DC/DC converter can realize the power transmission and isolation of different voltage DC systems, it is the key equipment of large- scale DC network.
The traditional DC/DC converter can not meet the requirements of high voltage, large capacity, bidirectional energy flow and electrical isolation in the DC grid, new DC/DC converter is urgently needed to be studied.

3. New DC/DC Topology
The topology consists two strings of cascaded sub modules and reactors.
Key features:
step-up and step-down operation.
bi-directional power flowing.
bi-directional fault blocking similar to a DC breaker.

4. New DC/DC Topology
Two special SMs termed LV-SM, constitute the low voltage part of the converter, LV-SM includes three electrical terminals.
Due to low voltage end only includes two LV-SMs and the voltage is limited to low level.
LV-SM
Normal-SM

5. Operation Principle DC Voltage Equation Cap. Energy Equation
Energy Balance Equation

6. Modulation and Strategy
Assume that constant DC voltage control method in the low DC side is adopted.
If the low voltage side voltage and the reference value is not same, the phase difference of the two leg can be adjusted to change the inductor current, and achieve power balance.
SMs in each arm can keep capacitor voltage balance by using basic voltage balance strategy.
Control strategy for constant voltage at low voltage side

7. Bidirectional Fault Blocking
High voltage short circuit: DC/DC converter will be locked immediately, DC fault can be effectively isolated.
Low voltage short circuit: DC/DC converter will be locked immediately, the energy in the two capacitor will quickly release through the fault point, but fault current will not go through the IGBTs.

8. HV Rated output voltage
Simulation
Circuit Parameters of Simulation
Parameters of the simulation are listed in table
Parameter
Value
LV Rated input voltage
10kV
HV Rated output voltage
30kV
Rated power
3MW
R of load
300Ω
Rated voltage of SM
5kV
AC frequency
100Hz
Amount of SM (per arm) 5
Amount of LV-SM 2
L1 and L2
10mH
C of LV-SM
10mF
C of rest SM
6mF

9. Simulation
The DC/DC converter is simulated by MATLAB-Simulink to verify the validity and correctness of the proposed topology and the control strategy.
The low voltage DC current is 300 A, the output voltage is 30 kV, the output current is 100 A, the phase difference of two bridge output AC voltage is about 60 degrees.
The DC voltage and current amplitude are stable, and the fluctuation is small.
Capacitors voltage of LVDC
Capacitors voltage of LVDC
Current and voltage of reactor

10. Conclusion The following conclusions are obtained:
Avoid the IGBT series .
Simple and easy to implement.
LV-SM can be designed separately to enhance utilization rate and the overall economy.
Propose a DC/DC converter based on MMC for connecting micro DC grid to medium or high voltage DC grid.

11. Thank you for your attention!