Majid Mehrasa, Radu Godina, Edris Pouresmaeil, Eduardo M. G. Rodrigues, Joao P. S. Catalao |

- 2020-07-10

- 28 M

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In order to reach better results for pulse width modulation (PWM)-based methods,

the reference waveforms known as control laws have to be achieved with good

accuracy. In this paper, three control laws are created by considering the harmonic

components of modular multilevel converter (MMC) state variables to suppress the

circulating currents under nonlinear load variation. The first control law consists

of only the harmonic components of the MMCs output currents and voltages.

Then, the second-order harmonic of circulating currents is also involved with both

upper and lower arm currents in order to attain the second control law. Since

circulating current suppression is the main aim of this work, the third control law

is formed by measuring all harmonic components of circulating currents which

impact on the arm currents as well. By making a comparison between the

switching signals generated by the three proposed control laws, it is verified that

the second-order harmonic of circulating currents can increase the switching

losses. In addition, the existence of all circulating current harmonics causes

distributed switching patterns, which is not suitable for the switches lifetime.

Each upper and lower arm has changeable capacitors, named equivalent

submodule (SM) capacitors in this paper. To further assess these capacitors,

eliminating the harmonic components of circulating currents provides fluctuations

with smaller magnitudes, as well as a smaller average value for the equivalent

capacitors. Moreover, the second-order harmonic has a dominant role that leads to

values higher than 3 F for equivalent capacitors. In comparison with the first and

second control laws, the use of the third control-law-based method will result in

very small circulating currents, since it is trying to control and eliminate all

harmonic components of the circulating currents. This result leads to very small

magnitudes for both the upper and lower arm currents, noticeably decreasing the

total MMC losses. All simulation results are verified using MATLAB software in the

SIMULINK environment.

1 : SIMULINK ⺻

1.1 SIMULINK 1

5

Ķ 7

ùķ̼ Ķ (Configuration Parameters) 8

ùķ̼ 9

Ķ ǥ 9

ǥ 11

2.2 ùķ̼ 13

̺й 17

º 23

DC ùķ̼ 24

Լ 29

й(difference equation) 34

Subsystem(νý) 37

2 :

1. Introduction 42

2. The Modular Multilevel Converter (MMC) 43

3. Evaluation of the Proposed Control Laws 50

4. Accurate Sizing of the Equivalent SM Capacitors of the MMC

Arms 53

5. Simulation Results 56

6. Conclusions 63

7. References 65