Felix Kammerer

Predictive Control of AC–AC Modular Multilevel Converters

Multilevel converters can reach medium-voltage operation increasing the efficiency of high-power applications. Among the existing multilevel converter topologies, the modular multilevel converter (MMC) provides the advantages of high modularity, availability, and high power quality. Moreover, the main advantage compared to cascaded multilevel converters is the lack of an input transformer which results in a reduction of cooling requirements, size, and cost. One of the drawbacks of this topology when used as an ac-ac converter is the input and output frequency components in the control loop, resulting in a more complex controller design. In this paper, a single-phase ac-ac MMC predictive control approach is proposed. The controller minimizes the input, output, and circulating current errors and balances the dc voltages. Experimental results show the performance of the proposed predictive control scheme.

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.

Dimensioning and design of a Modular Multilevel Converter for drive applications

The Modular Multilevel Converter is an upcoming topology for medium voltage drive applications in the megawatt power range. This paper presents the procedure of the converters dimensioning, which is used for the design of a prototype MMC-System. The voltage rating and the required size of the capacitors in the cells are calculated. The inductance of the arm choke, depending on the modulation strategy is determined. The prototype is realised by using self-powered cells, which are pre-charged without any additional power supply during the start-up phase. The design of the complete prototype including the signal processing is shown. This system provides the opportunity to analyse different control schemes, modulation strategies, impacts on the machine etc.

Fully decoupled current control and energy balancing of the Modular Multilevel Matrix Converter

The Modular Multilevel Matrix Converter (M3C) is a Modular Multilevel Converter topology which is suitable for high power low speed drive applications. This paper presents a fully decoupled current control which allows an independent input, output and internal balancing current control. To equalize the energy stored in the nine converter arms, an energy and balancing control is presented which includes average, horizontal, vertical and diagonal balancing control loops. Simulation results are used to verify the function of the M3C together with an induction motor drive system. Additionally, the proper function of the recently constructed arm PCB working as single phase multilevel STATCOM is presented. This PCB will be used for each arm in the laboratory prototype of the M3C in the near future.

Straight forward vector control of the Modular Multilevel Converter for feeding three-phase machines over their complete frequency range

The paper presents a control strategy for the Modular Multilevel Converter (MMC), which allows feeding a three-phase machine over its complete frequency range. The machine is controlled by a standard field oriented control in the outer closed loop. The inner control has to met the challenge of balancing the energy stored in the capacitance of the converter arms. In this approach two operation modes are used: a low frequency mode for start-up and low speed operation plus a high frequency mode for higher speed. A special control scheme for the low frequency mode has to be applied to achieve low energy pulsation in the arm capacitances. It uses the common mode voltage of the three-phase machine together with inner circulating currents to ensure a symmetrical energy distribution in the arms of the MMC and to avoid any AC-currents in the DC-source.

A novel cascaded vector control scheme for the Modular Multilevel Matrix Converter

This paper presents a novel cascaded vector control scheme for the Modular Multilevel Matrix Converter (M3C). The inner current control loops allow an independent control of the input and output converter currents. The outer energy control consists of average, vertical and horizontal balancing control loops to equalize the energy stored in the nine converter arms. A modulation method to balance the energy stored in the cells of one converter arm and to generate the desired arm voltage by selecting the appropriate cells is presented. The proposed vector control scheme allows the qualitative operation of the converter even under unbalanced line conditions. The function of the new converter control is verified by simulations. Additionally, three coupled three-phase z-winding arm inductors L are presented for the use with the M3C.

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.

Cell design of a square-wave powered 1AC-3AC modular multilevel converter low voltage prototype

This paper presents a novel design calculation scheme for low voltage Modular Multilevel Converters. One of the most important design values is to estimate the lifetime of the electrolytic cell capacitor. Therefore, a novel calculation method for the arm capacitor root mean square current is presented. Additionally, a new algorithm to estimate the power losses without a detailed simulation of the switching events is introduced. The results from these calculations are used to construct a low voltage Modular Multilevel Converter with 8 cells per arm.