# Challenges and Design Requirements for Industrial Applications of AC/AC Power Converters without DC-Link

## Abstract

**:**

## 1. Introduction

_{ESR}. The heating up of the electrolyte and its consequent evaporation and deterioration of electrical parameters is the most significant factor in the degradation of the electrolytic capacitor. One of the main reasons for the increase in capacitor temperature is the ripple of the capacitor current. In addition, the second most critical parameter for the failure of the capacitor is its voltage capacity, usually determined by the rated voltage or operating voltage, surge voltage, or allowed short-term maximum voltage. The voltage parameter is so critical that exceeding its nominal values for a few tenths of a second can cause an immediate failure or significantly accelerate the degradation of capacitor nominal parameters. For the presented reasons, in power converters, especially high power ones, particular attention is paid to environmental and operational conditions affecting their lifespan, with special emphasis on DC-link capacitors.

_{L}= const [16], f

_{L}= var [15,16,17,18]). In addition, different structures of AC/AC converters are distinguished, such as direct and indirect (with a fictitious DC-link) topology. An example of an AC/AC converter without an intermediate circuit capacitor and f

_{L}= const is a matrix controller (matrix chopper), whereas f

_{L}= var is a matrix converter.

## 2. Description of Selected Topologies

_{L}= var) without DC-link is the matrix converter (MC) in its direct topology shown in Figure 4a [15,16,17,18,22,23,24]. Generally, in three-phase systems, the MC consists of nine bidirectional switches connected in a matrix—each input is connected to each output. Similar to the matrix controller, there is an input low pass filter in the MC topology that fulfills identical functions. MC output voltages are formed from pieces of input voltages, shown in Figure 4b, as exemplary experimental time waveforms.

## 3. Design, Construction, and Implementation Barriers

#### 3.1. Power Semiconductors

#### 3.1.1. Si Semiconductors

#### 3.1.2. SiC Semiconductors

#### 3.2. Control Units

_{Seq}. Since there are three bidirectional switches connected in one output branch (six transistors) and two or three of them cannot be switched on simultaneously, the switching sequence will be responsible for three switching operations during one period T

_{Seq}(Figure 11a). In the classic bridge voltage inverter, for a single branch, there are two transistors working alternately. A pair of complementary control signals are then generated and are negated in relation to each other (Figure 11b). In addition, additional dead times (dead-band) are used to ensure correct current commutation. Modern DSP processors and microcontrollers have built-in PWM modulators that generate complementary control signals that occur in classic bridge voltage inverters with built-in dead time generation functions [28]. The control implementation for the matrix converter using built-in PWM modulators is not simple and requires either multiple-core systems (at least two) or additional logic circuits. It is also possible to solve this problem using software I/O outputs instead of built-in PWM modulator procedures. These are, of course, more complicated solutions than in the classic branches of the inverter bridge.

#### 3.3. Measurement of Voltages and Currents

#### 3.4. Commercialization of Prototype Solutions

#### 3.5. Modular Construction

#### 3.6. The Development of Modulation Methods

_{d}, MPC examines a model of a controlled system and predicts its condition in the next step. The configuration of converter switches is selected to ensure a minimum value of the cost function [40]. Scientific publications with MPC of an MC show better achievements in the quality of current and voltage waveforms, torque ripple, and internal switching power losses [30,41,42]. An example MPC scheme for the MC is shown in Figure 16, and this example is one that minimizes the cost function related to the accuracy of shaping the output current and the input power factor. Development of predictive control methods in AC systems without DC-link capacitors is a future area of extensive conceptual and implementation research.

## 4. Novel Applications of AC/AC Converters without DC-Link Capacitor

_{S}) parameters, after activation of the additional bypass switch. In the case of the configuration shown in Figure 20c, the reliability can be assessed as low. In the event of damage to the AC/AC converter, the whole circuit must be turned off, because it is unable to continue to operate. As a result, all receivers connected to the load side have to be shut off.

## 5. Conclusions

## Funding

## Conflicts of Interest

## References

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**Figure 2.**Distribution of faults in power electronic converters [14].

**Figure 5.**Bi-directional switches: (

**a**) Common emitter IGBT, (

**b**) common collector IGBT, (

**c**) IGBT with a diode bridge, and (

**d**) RB-IGBT reverse blocking IGBTs.

**Figure 6.**MC construction: (

**a**) Dynex 1.2 kV/200 A IGBT; (

**b**) a built-in MC structure in the control cabinet.

**Figure 8.**A photograph of the IGBTs with dedicated drivers: (

**a**) Dynex IGBT with Concept driver; (

**b**,

**c**) Semicron drivers and IGBTs.

**Figure 9.**Main topologies for SiC bi-directional switches: (

**a**) common drain anti-paralleled JFET, (

**b**) common source anti-paralleled SiC MOSFET, and (

**c**) common emitter anti-paralleled SiC BJT.

**Figure 10.**Illustration of the beneficial properties of the use of SiC transistors, taken from [24], presents the efficiency of a two-phase to single-phase 2.5 kW MC in Si and SiC technology (

**a**) as a function of switching frequency at T

_{c}= 125 °C and (

**b**) as a function of power at f

_{s}= 100 kHz, T

_{c}= 125 °C.

**Figure 11.**Distribution of control pulses in a single period of the control sequence T

_{Seq}: (

**a**) in the phase leg of a matrix converter, (

**b**) in the phase leg of a classical inverter.

**Figure 12.**Four-step commutation process in one output phase of the MC: (

**a**) switching diagram; (

**b**) control signals from FPGA devices.

**Figure 14.**Multilevel, medium voltage matrix converters: (

**a**) a multilevel MC with nine basic power switch blocks; (

**b**) a three-phase to single-phase (3 × 2) MC power block.

**Figure 15.**Common switching structures for power-electronic building blocks (PEBBs) in AC/AC power converters (

**a**) single bidirectional switch structure, (

**b**) single output phase of MC switch configuration, (

**c**) two phase of MC switch configuration, (

**d**) three phase of MC switch configuration.

**Figure 16.**Model predictive control (MPC) of an MC with minimalization of the output current ripples and input power factor (reactive power).

**Figure 17.**Compensator for voltage fluctuations, based on the HT with an MC or matrix chopper, installed at connection terminals of an industrial plant, a building, or selected industrial loads.

**Figure 18.**MPC of an HT voltage compensator with an MC and minimalization of the output voltage tracking error and input power factor.

**Figure 19.**Results of compensations of electric power grid voltage fluctuation using an HT with an MC and MPC control (author’s simulation results); red – voltage phase 1, blue – voltage phase 2, green – voltage phase 3.

**Figure 20.**Single phase equivalent models of AC/AC converters installed in the power grid, (

**a**,

**b**) high reliability configurations, (

**c**) low reliability configuration.

Switch Model | Characteristic | Manufacturer | Number of Bidirectional Switches |
---|---|---|---|

DIM200MBS12-A | 1.2 kV/200 A | DYNEX | 1 |

DIM400PBM17-A | 1.7 kV/400 A | DYNEX | 1 |

DIM600EZM17-E | 1.7 kV/600 A | DYNEX | 1 |

SK 60GM123 | 1.7 kV/60 A | SEMICRON | 1 |

SKM 150GM12T4G | 1.2 kV/60 A | SEMICRON | 1 |

SML150MAT12 | 1.2 kV/150 A | SEMELAB | 3 |

SML300MAT06 | 0.6 kV/300 A | SEMELAB | 3 |

18MBI50W-120A | 1.2 kV/ 50A | FUJI | 9 |

18MBI100W-060A | 0.6 kV/100 A | FUJI | 9 |

18MBI100W-120A | 1.2 kV/100 A | FUJI | 9 |

18MBI200W-060A | 0.6 kV/200 A | FUJI | 9 |

Model | Isolation/Operation | Manufacturer | Type |
---|---|---|---|

HCPL-7860 | Optic | Avago Technologies | Current |

HCPL-786J | Optic | Agilent | Current |

HCPL-7800 | Optic | Avago Technologies | Current |

ISO124 | capacitive | Texas Instruments | Current/Voltage |

ISO120/121 | capacitive | Burr-Brown Corporation | Current/Voltage |

INA270 | difference amplifier | Texas Instruments | Current |

LM358 | difference amplifier | Texas Instruments | Current |

AD8210 | difference amplifier | Analog Devices | Current. |

ACS752SCA-100 | magnetic | Allegro MicroSystems | Current |

ACS756xCB | magnetic | Allegro MicroSystems | Current |

LV-25 | magnetic | LEM | Voltage |

LTS 25-NP | magnetic | LEM | Current |

MP25P1 | magnetic | ABB | Current |

VS500B | magnetic | ABB | Voltage |

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**MDPI and ACS Style**

Szczesniak, P.
Challenges and Design Requirements for Industrial Applications of AC/AC Power Converters without DC-Link. *Energies* **2019**, *12*, 1581.
https://doi.org/10.3390/en12081581

**AMA Style**

Szczesniak P.
Challenges and Design Requirements for Industrial Applications of AC/AC Power Converters without DC-Link. *Energies*. 2019; 12(8):1581.
https://doi.org/10.3390/en12081581

**Chicago/Turabian Style**

Szczesniak, Pawel.
2019. "Challenges and Design Requirements for Industrial Applications of AC/AC Power Converters without DC-Link" *Energies* 12, no. 8: 1581.
https://doi.org/10.3390/en12081581