# New Approaches to Circulating Current Controllers for Modular Multilevel Converters

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Modular Multilevel Converter

_{MMC}. The submodule is a half H-bridge that contains two insulated-gate bipolar transistors (IGBTs), two reversing diodes and a DC storage capacitor. The two switches ($Sw$ and $\overline{Sw}$) in each SM are controlled with complementary signals, resulting in two active switching states that can connect or bypass the respective capacitors to the converter leg. Consequently, the output voltage V

_{SM}(Figure 1) can be determined based on the switching states. When $Sw$ is switched ON, $\overline{Sw}$ is switched OFF. Here, the output voltage is V

_{c}. In contrast, when $Sw$ is switched OFF, $\overline{Sw}$ is switched ON, and the output voltage is zero.

- Capacitor voltage ripple: the circulating current increases the voltage ripple since it depends on the amplitude of the arm currents.
- Power losses: The circulating current increases the arm currents and thus, the power dissipated by R
_{MMC}is higher. Moreover, the power losses in the switching devices are also increased. - Inductor saturation: As in the previous case, the amplitude of the arm current is affected by the circulating current. Therefore, if the circulating current is high, the inductor L
_{MMC}can be saturated in the case of ferromagnetic core inductors. - Power quality: As demonstrated above, the circulating current is composed of several harmonics which reduce the quality of the power exchanged with the grid. Moreover, the reactive power exchanged greatly increases its value. These energy consumptions, reduce the performance of the controllers as well as the capability of the MMC. This fact is critical if the MMC is intended to be used in FACTS applications where the voltage support or reactive power compensation is intended to be used.
- The circulating current can be minimized by increasing the SM capacitors or increasing the inductors. Nevertheless, the cost of the capacitors has to be taken into account. In addition, the size of the inductors has an impact in the MMC performance increasing the losses. Thus, a circulating current controller is the best option to reduce the amplitude of this current.

## 3. dq-Frame Controller

## 4. αβ-Frame Controller

#### 4.1. Resonant Controllers

#### 4.2. Repetitive Controllers

#### 4.3. Stationary Reference Saturator

## 5. Experimental Setup

## 6. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 23.**Circulating current comparison with resonant controllers. (

**a**) Before controller activation; (

**b**) After controller activation.

**Figure 26.**Circulating current comparison with repetitive controllers. (

**a**) Before controller activation; (

**b**) After controller activation.

Controller | Timer Ticks | Time Consumed |
---|---|---|

Resonant controllers | 125 | 2.5 μs |

Repetitive Controller | 1310 | 26.2 μs |

Parameter | Value |
---|---|

Nominal Power | 50 kVA |

Nominal Voltage | 400 V |

N° submodules per phase | 10 |

Submodule Capacitor | 2200 µF |

IGBT | Semikron SKM145GB066D |

IGBT Driver | Semikron Skyper 32 R UL |

DC-bus voltage | 1200 V |

MMC inductor | 0.5 mH |

Grid inductor | 5 mH |

Controller | THD |
---|---|

Without controller | 1.23% |

Resonant controllers | 1.058% |

Repetitive Controller | 1.048% |

© 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Moranchel, M.; Bueno, E.; Sanz, I.; Rodríguez, F.J.
New Approaches to Circulating Current Controllers for Modular Multilevel Converters. *Energies* **2017**, *10*, 86.
https://doi.org/10.3390/en10010086

**AMA Style**

Moranchel M, Bueno E, Sanz I, Rodríguez FJ.
New Approaches to Circulating Current Controllers for Modular Multilevel Converters. *Energies*. 2017; 10(1):86.
https://doi.org/10.3390/en10010086

**Chicago/Turabian Style**

Moranchel, Miguel, Emilio Bueno, Inés Sanz, and Francisco J. Rodríguez.
2017. "New Approaches to Circulating Current Controllers for Modular Multilevel Converters" *Energies* 10, no. 1: 86.
https://doi.org/10.3390/en10010086