# Current Control Strategies for a Star Connected Cascaded H-Bridge Converter Operating as MV-AC to MV-DC Stage of a Solid State Transformer

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## Abstract

**:**

## 1. Introduction

- an intermediate DC stage allowing integration with DC grids (on both MV and LV sides) without additional AC/DC conversion stages;
- precise control of power flow with high dynamics and four-quadrant operation (active and reactive power in either direction), resulting in low transmission losses;
- ability to independently compensate both reactive power and current higher harmonics on either side of the SST;
- very high volumetric power density (kW/dm
^{3}), resulting in reduced use of materials; - per-phase current control with possibility for the stand-alone mode of operation;
- ride-through operation under various voltage disturbances with the inherent decoupling feature between two sides due to DC stage;
- post-fault operation in case of internal or grid-related failure;
- fast dynamic response and/or reconfiguration in response to events in the distribution system due to operator-level communication layer;
- LV-side voltage control with the ability to compensate non-linear loads.

- ability to isolate disturbances between both sides;
- precise current/power control, especially in case of voltage imbalances or other distortions;
- adjustable power factor;
- post-fault operation capability;
- ability to compensate higher harmonics of current.

## 2. Converter Topologies for the MV-AC to MV-DC Stage of the SST

## 3. Control Algorithm of a Star Connected Cascaded H-Bridge Converter

#### MV-AC to MV-DC Stage Dynamics Improvement

## 4. Power Distribution Strategies among the Phases in MV Grid

#### 4.1. Constant Active Power

#### 4.2. Symmetrical Grid Currents

#### 4.3. Constant Reactive Power Mode—Phase Unloading

## 5. Simulation Results

## 6. Experimental Verification

## 7. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 9.**Steady-state operation at nominal power ${P}_{AC,MV}=450$ kW. From the top: grid line voltages ${u}_{AC,x}$, grid line currents ${i}_{AC,x}$, DC voltages ${U}_{DC,xk}$, grid active power ${P}_{AC,MV}$.

**Figure 10.**Change in the direction of energy flow:without PFF (on the left) and with PFF (on the right). From the top: grid line voltages ${u}_{AC,x}$, grid line currents ${i}_{AC,x}$, DC voltages ${U}_{DC,xk}$, grid active power ${P}_{AC,MV}$.

**Figure 11.**Steady-state operation during 50% phase-to-ground voltage sag: (

**a**) constant active power mode of operation, (

**b**) symmetrical currents mode of operation, (

**c**) faulted phase unloading mode of operation. From the top: grid line voltages ${u}_{AC,x}$, grid line currents ${i}_{AC,x}$, DC voltages ${U}_{DC,xk}$, grid active power ${P}_{AC,MV}$, grid reactive power ${Q}_{AC,MV}$.

**Figure 12.**Experimental setup: (

**a**) view of designed power and driver boards of a single SM of a CHB converter, (

**b**) view of designed single SM of a AC-MV to DC-MV converter with H-bridge of an IPOP DAB.

**Figure 15.**Steady-state operation under: (

**a**,

**b**) symmetrical grid voltages, (

**c**,

**d**) unbalanced grid voltages, (

**a**,

**c**) equal SM loads, (

**b**,

**d**) non equal SM loads. From the top: grid line voltages ${u}_{AC,x}$, grid line currents ${i}_{AC,x}$, DC voltages ${U}_{DC,xk}$.

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

Grid inductance | $L=6$ mH |

Grid resistance | $R=3\mathrm{m}\Omega $ |

Single H-bridge sampling frequency | ${f}_{s}=20\mathrm{kHz}$ |

Carrier signal frequency | ${f}_{sc}=60\mathrm{kHz}$ |

System nominal active power | ${P}_{n}=450\mathrm{kW}$ |

Single H-bridge capacitance | ${C}_{cell}=4\mathrm{mF}$ |

Single H-bridge nominal voltage | ${U}_{dc,cell,n}=8100\mathrm{V}$ |

Number of cells in phase | $n=3$ |

Grid nominal voltage | ${U}_{grid}=15\mathrm{kV}$ line-to-line RMS |

Converter Parameters | |

Parameter | Value |

Grid inductance | $L=2$ mH |

Control sampling frequency | ${f}_{s}=10$ kHz |

Single H-bridge switching frequency | ${f}_{sc}=100$ kHz |

System nominal active power | ${P}_{n}=10$ kW |

Single H-bridge capacitance | ${C}_{cell}=1$ mF |

Single H-bridge nominal voltage | ${U}_{dc,xk}=100/3$ V |

Number of cells in phase | $n=3$ |

Grid nominal voltage | ${U}_{grid}=50\sqrt{3}$ V_{line-to-line}, _{RMS} |

Load Parameters | |

Parameter | Value |

Equal load | ${R}_{xk}=35$ Ω |

Unequal load | ${R}_{A1}=70$ Ω |

${R}_{A2}-{R}_{C3}=35$ Ω |

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**MDPI and ACS Style**

Stynski, S.; Grzegorczyk, M.; Sobol, C.; Kot, R.
Current Control Strategies for a Star Connected Cascaded H-Bridge Converter Operating as MV-AC to MV-DC Stage of a Solid State Transformer. *Energies* **2021**, *14*, 4607.
https://doi.org/10.3390/en14154607

**AMA Style**

Stynski S, Grzegorczyk M, Sobol C, Kot R.
Current Control Strategies for a Star Connected Cascaded H-Bridge Converter Operating as MV-AC to MV-DC Stage of a Solid State Transformer. *Energies*. 2021; 14(15):4607.
https://doi.org/10.3390/en14154607

**Chicago/Turabian Style**

Stynski, Sebastian, Marta Grzegorczyk, Cezary Sobol, and Radek Kot.
2021. "Current Control Strategies for a Star Connected Cascaded H-Bridge Converter Operating as MV-AC to MV-DC Stage of a Solid State Transformer" *Energies* 14, no. 15: 4607.
https://doi.org/10.3390/en14154607