 Open Access
 Total Downloads : 43
 Authors : XiuJuan Ma , JiaYi Teng , HuaQiang Zhang , Tong Yao
 Paper ID : IJERTV7IS030028
 Volume & Issue : Volume 07, Issue 03 (March 2018)
 Published (First Online): 16032018
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Circulating Current Suppression Strategy of Modular Multilevel Converter
XiuJuan MA
Dept. Electrical Engineering Harbin Institute of Technology at Weihai
Weihai, China
HuaQiang Zhang
Dept. Electrical Engineering Harbin Institute of Technology at Weihai
Weihai, China
JiaYi Teng
Dept. Electrical Engineering Harbin Institute of Technology at Weihai
Weihai, China
Tong Yao
Dept. Electrical Engineering Harbin Institute of Technology at Weihai
Weihai, China
AbstractOutput voltage waveforms of modular multilevel converter (MMC) with less harmonics and high quality by switching the submodules, it becomes a hot research in the field of voltage source converter DC transmission. The basic topology structure and working principle of MMC are introduced, the mechanism of MMC circulating current is analyzed, and point out the necessity of controlling. To utilize the carrier phaseshift modulation (CPSM), circulating current suppression strategy based on quasiproportional resonant (quasiPR) control is proposed to suppress circulating current and keep voltage stability of submodule(SM) capacitor. The effectiveness and feasibility of the proposed method is proved by Matlab/Simulink, the simulation results show that the circulating current can be confined in a small range.
Keywords Modular Multilevel Converter; Circulating Current Suppression; Capacitor Voltage Balancing; Carrier PhaseShift Modulation
current in coordinate system are analyzed in this paper. In view of the problem of circulating current, circulating current suppression strategy of modular multilevel converter is proposed to suppress circulating current and keep voltage stability of submodule(SM) capacitor. The effectiveness and feasibility of the proposed method is proved by Matlab/Simulink.

MODULAR MULTILEVEL CONVERTER
P
The modular multilevel converter (MMC) topology is shown in Fig. 1, which consists of three strict symmetrical phase units, each phase unit is divided into the upper and lower two arms, each arm is cascaded by N submodule and an inductor. The function of the inductor is to inhibit the phase circulation caused by the fluctuation of submodule capacitor voltage. Each submodule consists of a halfbridge unit and a DC capacitor, the submodule structure is shown in Fig. 2.

INTRODUCTION
In 1990, the Canadian scholar BoonTeck Ooi proposed highvoltage direct current transmission technology based on
SM1
u SM2
SM1
SM2
SM1
SM2
Idc
voltage source converter (VSCHVDC)[1]. VSCHVDC has no short circuit capacity requirements, it can supply power to passive network and dispense with filter, it can quickly and independently adjust active and reactive power. Therefore, VSCHVDC has a wide range of application prospects in renewable energy grid generation, rural power grid
pa
a ia
b
SMN
ipa
SMN
C1
+
–
SMN
U
transformation, urban network expansion and regional power grid interconnection[23].The voltage source converter is the core component of the flexible direct current transmission system.
In 2001, the german scholar Rainer Marquardt proposed modular multilevel converter (MMC). MMC has a common DC bus, which will help to achieve backtoback system. MMC has the same modular structure, it can generate multi level waveforms, the output waveform has less harmonic
c
una
ina
SMN+1
SMN+2
SM2N
SMN+1
SMN+2
SM2N
SMN+1
SMN+2
SM2N
o dc
C
+
2
–
N
content and highquality waveforms. At the same time, MMC has good fault ridethrough and protection capabilities[45]. So MMC become a potential converter technology for high voltage direct current (HVDC) transmission system.
At present, the researches on MMC mainly focus on topological structure, modulation strategy, capacitorvoltage balance control and circulating current suppression etc. The basic topology structure, working principle and circulating
Fig. 1. Topology structure of MMC
By the energy conservation of DC and AC side can get
VT1
VD1
+
UCap
iza
uaia
Udc
2
k U
sin w t I
sin(w t )
Usmj
VT2
VD2
dc 0
a 0
Udc
Fig. 2. Configuration of Submodule
k I
4 a
cos k I
4 a
cos(2w0t )
Taking aphase as an example, o is the midpoint of the DC side potential of the inverter, assuming that the output voltage of the AC side is uao. It is the same potential between the o point and the equivalent midpoint of the threephase voltage, then the upper and lower arm voltage respectively are:
As can be seen from (5), the circulating current iza mainly includes the DC component and the double fundamental frequency component.
By the active power balance of DC and AC side can get
U
u
1 u

L di
pa Ri
3UI cos Udc Idc
pa
u
na
2
1 U
2
dc ao
dc uao
dt pa

L dipa Ri
dt pa
Where, U and I respectively are virtual value of AC output, Udc and Idc respectively are voltage and current of DC side.
Substituting (4) into (6) can get
The upper and lower arm current respectively are:
kIa
cos 4 I
3 dc
ia
The DC component of the circulating current is
ipa iza
2
i
k I cos 1 I
i
na
a i
2 za
4 a 3 dc
Circulating current expressed with formula as follows:
2
1
As can be seen from (8), when the threephase AC system is balanced, the DC component of the circulating current is onethird of the DC current, That is, at this time, the DC side
iza
ipa ina
current of MMC is evenly distributed among the threephases.
So the upper and lower arm current respectively are:
The submodules input number of the upper and lower arm vary according to the sine law, resulting in the step sinusoidal
ia
ia Idc
2 2 3
AC voltage waveform. The more modules, the more the level, the output closer to sine wave.
ipa iza i2 f
i i I
i a i
a dc i



ANALYSIS OF CIRCULATING CURRENT
na 2 za
2 3 2 f
In order to propose a more accurate method of circulating current suppression, the mechanism of MMC circulating current is analyzed. MMC is a symmetrical structure, take a phase as an example, AC output voltage and current respectively are
So circulating current can be expressed as
i 1 (i i ) Idc i
za 2 pa na 3 2 f
u
k
a (t) Udc
2
sin w0t
The circulation path of the circulating current is shown in Fig. 3.
ia (t) Ia sin(w0t )
upa
SM1
SM2
SM1
SM2
SM1
SM2
P
Idc
+ C
– 1
ipb
i
+
nb +
1
2
i
iza
*
+
u
u
i
*
i
z *
QuasiPR
td>
z
Clark
iz
Clark1
u*
zb
u*
zc
z za
SMN
ia
b a
c
ipa
SMN
SMN
Udc
o
pa +
+
ina ipc+
+
inc
izb
izc
*
QuasiPR
u
+ z
i
*
z
Fig. 4. Dynamic block diagram of circulating current suppression strategy
una
ina
SMN+1
SMN+2
SM2N
SMN+1
SMN+2
SM2N
SMN+1 +
C
2
–
SMN+2
SM2N
N
dc + za
Clark
PI
U *
i
i
*
*
2 Cja
1 2 N u z
i
*
j 1
U *
dc + PI zb
1 2 N i*
Fig. 3. Threephase circulating current circulation path
The existence of MMC circulating current increases the unnecessary loss and distorts the arm current, and the fluctuation of submodule capacitor voltage is too large, resulting in output current and voltage waveforms do not achieve the expected results[67]. Therefore, it is necessary to suppress the circulating current.

CIRCULATING CURRENT SUPPRESSION STRATEGY
The unbalanced voltage drop on the arm generated by a
uCjb z
2
i
*
j 1
U *
dc + PI zc
1 2 N
2
uCjc
j 1
Fig. 5. Dynamic block diagram of circulating current reference value
dc
The circulating current reference value of the controller is not 0, but is obtained by PI control, as shown in Fig. 5. Udc track its reference value U * , Udc is obtained by adding sub
phase arm currents can be derived from Kirchhoff's law:
u L diza Ri
za dt za
modules capacitor voltages, which not only suppress circulating current but also keep voltage stability of sub module(SM) capacitor. The actual value of the circulating current is obtained by the upper and lower arm currents. The reference value and the actual value of circulating current are
Thus, the expression of threephase unbalanced voltage
z
z
drop generated by circulating current can be written:
subjected to Clark transformation to obtain the component on the axis, the actual circulating current values iz and iz are
uza iza iza
compared with the reference values i*
and i*
, and then the
u L d i
R i
circulating current is suppressed by quasiPR controller. The
zb
uzc
dt zb zb
izc izc
output is the reference value of the unbalanced voltage drop on the axis, and then the reference value of the unbalanced voltage drop under the threephase is obtained. And it is used
Equation (12) through Clark transformation can get:
as the regulating variable of the circulating current suppression.
z
z
di
u L
dt

Riz
*
u
Cap _ jk
u
z
L diz
dt

Riz
* +
Circulation Current Suppression
u
zk+
*
CPS SPWM
u
jk
Where, iz and iz respectively are AC component of circulating current in axis, uz and uz respectively are AC component of threephase unbalanced voltage drop in axis. In the coordinate system, iz and iz are independent of each other, the decoupling control is not needed, and the design of the circulating current suppressor is convenient.
Fig. 6. Block diagram of MMC control
jk
The MMC control structure is shown in Fig. 6, u*
is the
According to above analysis, this paper designs a circulating current suppressor based on quasiPR control, as shown in Fig. 4.
reference voltage corresponding to each submodule, *
u
is
jk
taken as the reference amplitude of the modulation wave, N
jk
modulated waves with an amplitude of u*
are compared with
N triangular carriers with the same frequency and the same amplitude, but phase difference of 2/N. Generating groups of N PWM trigger signals, respectively controlling the putinto or cutoff of N submodules.


SIMULATION RESULT AND ANALYSIS
In order to verify the effectiveness of the method, a 5level MMC simulation model is built in MATLAB/Simulink, carrier frequency fc=2kHz, submodule capacitance C=1.8mF, arm inductance L=6mH, DC side voltage Udc=600V, submodule capacitor voltage reference value uCap=150V. The simulation results are shown in Fig. 714.
Fig. 7. Threephase ac output voltage of MMC
Fig. 8. Threephase ac output current of MMC
Fig. 9. aphase circulating current
Fig. 10. aphase circulating current(without circulating current suppression)
Fig. 11. Submodule capacitor voltage in upper arm of aphase
Fig. 12. Submodule capacitor voltage in lower arm of aphase
Fig. 13. Submodule capacitor voltage in upper arm of aphase(without circulating current suppression)
Fig. 14. Submodule capacitor voltage in lower arm of aphase(without circulating current suppression)
Fig. 7 is the phase voltage of the MMC threephase AC output, and Fig. 8 is threephase AC output current. From the simulation waveform, it can be seen that the threephase AC output voltage and the output current are close to the sine wave. Fig. 10 is aphase circulating current waveform without the circulating current suppression, the circulating current fluctuates between 4~6A, Fig. 9 is aphase circulating current waveform after the circulating current suppression, the circulating current fluctuates between 0.5~1.3A, only about 8
% of the original. It shows that the circulating current is suppressed. Fig. 11 and Fig. 12 respectively are the capacitor voltage waveform of the four submodule of aphase the upper and lower arm after the circulating current suppression. Fig. 13 and Fig. 14 respectively are the capacitor voltage waveform of the four submodule of aphase the upper and lower arm
without the circulating current suppression. It can be seen from the figure that when there is no suppression strategy, the capacitor voltage of each submodule fluctuates greatly around the reference value of 150V, which is not conducive to the DC side voltage stability. The capacitor voltage conversion trend of the four submodules is consistent after circulating current suppression. The voltage fluctuates around the reference value 150V, and the positive and negative fluctuation range are controlled within 3 , thus ensuring the DC bus voltage stability. The simulation results show that circulating current suppression strategy of modular multilevel converter is beneficial to suppress circulating current and keep voltage stability of submodule(SM) capacitor. It verifies the correctness and effectiveness of the proposed method.

CONCLUSION
MMC is considered as the most potential topology in the field of highvoltage power conversion in the future. At present, it has been applied in several highvoltage highpower projects, especially in the field of flexible DC transmission. Since Siemens AG of Germany took the lead in building the Trans Bay Cable project in 2010, there are dozens of flexible DC transmission projects under construction or planned to be constructed adopting the structure of MMC in domestic and foreign. In China, the flexible DC transmission project of Nanhui, Nanao, Zhoushan, Xiamen and Luxi is VSC – HVDC project based on MMC[8]. MMC as a popular multilevel converter, stable operation is the key technology of its application. I this paper, circulating current suppression strategy of modular multilevel converter is proposed by utilizing the CPSM. A fivelevel MMC simulation model is built on the MATLAB/Simulink platform, the simulation results show that the control method can suppress the circulating current and stabilize the capacitor voltage.
ACKNOWLEDGMENT
The authors thank the support both the Foundation of National Natural Science (51377168) and the Foundation of Shandong Province Natural Science (ZR2017MEE053), which enabled the achievement of the mentioned research results.
REFERENCES

Chengyong Li, Jianzhong Xu, and Tan Li, DC transmission modeling technology of Modular Multilever Converter, Beijing: China Electric Power Press, 2017, pp. 14.

Dandan Xu, Suppression of Circulating Current Harmonics and Capacitor Voltage Fluctuations in Modular Multilevel Converters, Harbin: Harbin Institute of Technology, 2015, pp.1 3.

Xinhang Xie, Research on control strategy of MMCHVDC, Beijing: Beijing Jiaotong University, 2017, pp. 18.

Kaijing Liu, Suppression Strategy on Bridge Arm Circulation of MMCSTATCOM in ThreePhase Four Wire System, Beijing: North China Electric Power University, 2017, pp. 45.

Ao Li, Research on voltage control and circulation suppression strategy of Modular Multilevel Converter, Wuhan: Huazhong University of Science & Technology, 2015, pp. 47.

Decun Niu, Zhongwei Sun, Nan Li, Yanli Li, Wei Tang, and Lei Zhao, Study on MMCHVDC New Type Loop Current Control Strategy Based on QuasiPR Controller, Power Capacitor & Reactive Power Compensation, vol. 37, pp. 8794, August 2016.

Jinyu Wang, Jun Liang, Chengfu Wang, and Xiaoming Dong, Circulating current suppression for MMCHVDC under unbalanced grid conditions, IEEE Transactions on Industry Applications, vol. 53, pp. 32503259, August 2017.

Qiang Song and Hong Rao, Analysis and design of the converters in VSCHVDC system, Beijing: Tsinghua University Press, 2014, pp. 48.