Mario Gommeringer

Cascaded Control System of the Modular Multilevel Converter for Feeding Variable-Speed Drives

The modular multilevel converter (MMC) is an upcoming topology for high-power drive applications especially in the medium voltage range. This paper presents the design process of a holistic control system for a MMC to feed variable-speed drives. First, the design of the current control for the independent adjustment of several current components is derived from the analysis of the equivalent circuits. Second, the current and voltage components for balancing the energies in the arms of the MMC are identified systematically by the investigation of the transformed arm power components. These fundamentals lead to the design of the cascaded control structure, which allows the balancing task in the whole operating range of a three-phase machine. The control system ensures a dynamic balancing of the energies in the cells of the MMC at minimum necessary internal currents over the complete frequency range. Simultaneously, all other circulating current components are avoided to minimize current stress and additional voltage pulsations. The performance of the control system is finally validated by measurements on a low-voltage MMC prototype, which feeds a field-oriented controlled induction machine.

Energy balancing of the Modular Multilevel Matrix Converter based on a new transformed arm power analysis

This paper presents a transformed arm power analysis of the Modular Multilevel Matrix Converter (M3C). It enables the energy balancing in the whole frequency range for high power variable-speed drive applications. Four balancing directions are identified for the active power exchange between the converter arms with minimal internal currents. At critical operating points a zero sequence voltage is used. Additionally, the reactive power components can be used to perform a real time calculation of the energy pulsation in all four balancing directions to improve the control performance. A low voltage prototype with 5 cells in each of the nine arms has been realized to verify the theoretical analysis.

A transformerless single-phase PV inverter circuit for thin-film or back-side contacted solar modules

This paper deals with a new transformerless single-phase photovoltaic inverter circuit. The circuit is well suited for thin-film or back-side contacted solar modules because one pole of the solar module can be directly connected to the neutral conductor. The circuit uses a flying capacitor for one of both half-waves, requires only five transistors and is capable of providing inductive as well as capacitive reactive power.

Current control and energy balancing of a square-wave powered 1AC-3AC modular multilevel converter

This paper presents a new control method for a Modular Multilevel Converter (MMC) fed by a push-pull converter via a medium frequency (MF) transformer. Application of this topology is a universal high-precision 3AC voltage source for a Power Hardware-in-the-Loop Emulator with a frequency range from DC up to almost the medium frequency. Advantages are high efficiency, very low harmonic distortion and high dynamics of the output voltage which can be used e.g. for simulation of electrical machines. Due to the medium frequency input of the MMC the transformer size for the galvanic isolation is much smaller compared to a low frequency input. Additionally, the required cell capacitance for the MMC is reduced which saves cost and space. The delivered medium frequency square-wave voltage requires an alternating input current of the MMC. For the square-wave powered 1AC-3AC MMC an energy and current control is proposed. Due to very high current dynamic requirements a dead-beat controller as subordinated controller is used to achieve a trapezoidal input current which allows zero current switching (ZCS) at the push-pull-converter. The design of the superposed energy and balancing controller is also shown. For the coupling between the superposed and subordinated control loop the arm power is analyzed and a calculation scheme is given.

Dimensioning of a transformerless photovoltaic inverter circuit for thin-film or back-side contacted solar modules

This paper deals with the dimensioning of a single-phase transformerless photovoltaic inverter circuit. The circuit contains a flying capacitor and allows the connection of one pole of the solar generator to the neutral conductor. The intended purpose of the design guidelines is the minimization of size and costs and the increasing of lifetime. A prototype inverter with silicon carbide MOSFETs is built up. It reaches a very high efficiency of up to 98.8 percent and is used to verify mathematical models.

A novel modulation scheme for a modular multiphase multilevel converter in a power hardware-in-the-loop emulation system

This paper presents a novel modulation strategy for a Modular Multiphase Multilevel Converter especially for the usage in Power Hardware-in-the-Loop Emulation Systems. The proposed modulation scheme generates the required voltage time area by means of an integrator in a quasi continuous operation mode. Therefore, the fixed modulation period is disestablished to achieve a minimum response time of the emulation converter. The switching states of the parallel half-bridges are selected by a sorting algorithm, dependent on the instantaneous branch currents, to avoid saturation in the flux compensated chokes of the converter. The performance of the modulation scheme is verified at a three phase, seven level multilevel converter, with a PWM-frequency up to 120 kHz and an output power of more than 100 kW.

An ultra-efficient maximum power point tracking circuit for photovoltaic inverters

This paper deals with a new ultra-efficient circuit for the individual maximum power point (MPP) tracking of an unlimited number of photovoltaic strings. The circuit generates a DC output voltage and can be combined with common DC-AC solar inverter circuits. The necessary blocking voltages of the transistors are far below the maximum string voltages, which yields to low losses and small passive components. An appropriate controller structure is developed and a prototype is built up, that reaches a maximum efficiency of 99.85 percent.