High-voltage DC (HVDC) transmission lines offer highly efficient transport of electric energy. If applied to comparatively weak grids, self-commutated converters are the proper choice because they allow stabilizing the grid in case of faults by injecting reactive power. At high voltages of e.g. 400 kV, modular multilevel converters (MMC) fulfill all requirements. For testing all relevant aspects of a multiterminal HVDC-transmission system, a scaled-down test bench has been realized. Four MMCs define the multiterminal DC system. With regard to practical demands, no explicit communication between the control systems is allowed for stable operation of the transmission system. This paper gives an overview of the test bench and then concentrates on the bidirectional power flow of the HVDC System in a bipolar configuration.

In the future, the established point-to-point HVDC transmission will be expanded to multi-terminal HVDC grids due to the increasing amount of regenerative energy generation. This change will lead to new challenges and requirements for grid-system operators and vendors e.g. in terms of transmission strategy, project planning, control strategies and multi-vendor intercompatibility. Within this paper, the core consequences are discussed and changes in the design-process model are proposed. Challenges, possible solutions and limitations using linear and nonlinear droop characteristics are outlined by discussing multi-terminal simulation results.

High-voltage DC (HVDC) transmission lines offer highly efficient transport of electric energy. If applied to comparatively weak grids, self-commutated converters are the proper choice, because they allow stabilizing the grid in case of faults by injecting reactive power. At high voltages of e.g. 400 kV modular multilevel converters (MMC) fulfil all requirements. For testing all relevant aspects of a multiterminal HVDC transmission system a scaled-down test bench has been realized. Four modular multilevel converters (MMCs) define the multiterminal DC system. With regard to practical demands, no explicit communication between the control systems is allowed for stable operation of the transmission system. This paper gives a short overview of the test bench and then concentrates on the bidirectional power flow of the HVDC System in a monopolar Configuration.

In the context of the nuclear phase-out in Germany and increasing power generated by wind farms in the north, the need for secure and stable grid conditions in the south triggers HVDC connections from the north to the south of Germany. With regard to grid stability and ancillary services like black-start capability, reactive power supply and future extensions, a multiterminal HVDC link based on self-commutated converters is planned by Amprion and TransnetBW. The use of existing towers in a hybrid AC/DC topology will lead to a fast realization of the HVDC link, but introduce new requirements concerning e.g. weather-inducted faults. A concept for multiterminal operation and fault mitigation of a multiterminal HVDC link without explicit communication is presented and verified by measurements at a test bench.

Connecting renewable energy sources to the grid requires stable and reliable control algorithms. Especially in case of solar-energy systems, single-phase connection is often used at low power levels. This can be achieved by using a four-quadrant converter. For optimal reaction to variations of infeed (e.g. by shading by clouds) and grid disturbances, a fast reaction of the control is advantageous. Reaching such properties under the conditions of single-phase grid connection is challenging: The power pulsation and the inability to exchange energy around zero grid voltage require specialized control schemes for dynamic control. This paper presents the control scheme and measurement results for a four-quadrant converter control basing on the `Pole Restraining' concept which allows fast instantaneous-value-based control even for four-quadrant converters connected to a single-phase grid.

The topology and the resulting properties of power-electronic converters influence mitigation concepts for undesirable power oscillations in the connected assets. Power theory in general provides an answer for the optimal way to operate under aspects of efficient energy transmission. However, practical aspects and design considerations (including size and cost) as well as grid-code specifications also influence the chosen technical solution. With this background, control objectives have to be related to the technical options available and the demands given by e.g. grid codes.

DC grids offer highly efficient distribution of electric energy, eliminating components and optimizing the use of cables. Efficient generation, however, is still based on AC generators. Power-electronic devices link generators and grid and customize energy flow to the loads. Efficient distribution with low losses demands high voltage, e.g. in the range of 10 kV for ship-size grids. Such voltages challenge power electronics as well as protective devices: Short-circuit protection with fast auto-reclosing in a context where fuses are not applicable has to be solved. This paper proposes modular multilevel converters (MMC) with full-bride (4QC) modules and demonstrates fault mitigation in controlled MMC operation by measurement results. The associated control separates asset characteristic defining the steady-state and transient behaviour and converter-near control operating in the subtransient and transient regime. The feasibility of the approach is demonstrated by selected measurement results.

High-voltage DC (HVDC) transmission lines offer highly efficient transport of electric energy. If applied to comparatively weak grids, self-commutated converters are the proper choice, because they allow stabilizing the grid in case of faults by injecting reactive power. At high voltages of e.g. 400 kV modular multilevel converters (MMC) fulfill all requirements. The first task to master is getting the converters and the grid into operation. For this a two-step pre-charging sequence is needed, because the control of the converter modules of the MMC is supplied from its own DC-link voltages. For testing all relevant aspects of a multiterminal HVDC transmission system a scaled-down test bench has been realized. Four modular multilevel-converters (MMCs) define the multiterminal DC system. With regard to practical demands, no explicit communication between the control systems is allowed for stable operation of the transmission system. This paper gives a short overview of the test bench and then concentrates on the pre-charging of the converters based on AC and DC voltage and demonstrates the powering up of the DC system.

Induction machines are affordable and robust, presenting a valid option for electric vehicles. Many control schemes for such machines are known and have been published. With regard to fast torque dynamics and robustness against parameter variation, stator-flux-oriented control is a very favorable option. This paper presents a stator-flux oriented control which bases on a comprehensive model of the induction machine. A sophisticated compensation of non-ideal converter behavior further enhances the results obtained. These results are documented by measurements and verify excellent performance: Fast response without current overshoot uses the rating of the converter optimally. Dynamic field weakening keeps excellent torque response in the field-weakening range without any voltage margin, too.Fast torque dynamics are not required for fulfilling the commands issued by the driver but for efficient disturbance rejection and additional features like slip-slide control and maximum-traction-point detection. For such features, the underlying torque control defines the performance consequently, fast and precise reaction is requested.

High-voltage DC (HVDC) transmission lines enable highly efficient transport of electric energy over long distances. The state-of-the-art topology for HVDC is still thyristor-based, with a tendency to employ self-commutated converters on the basis of IGBT devices. Here, several module topologies are employed or under discussion - with relevant influence on the mitigation of faults on the AC and DC side of the converter stations. In this paper fault mitigation options for HVDC are analyzed, comparing the most relevant converter topologies. Advantages and disadvantages are discussed with regard to the intended use and layout of the transmission line (point to point, multiterminal operation, overhead line, cable) and the number and relevance of faults and interruptions of transmission.

Two main aspects demand for high-voltage long distance energy transfer from the north of Germany to the south: Increasing power generated by wind farms in the north and the need for secure and stable grid conditions in the south also after the nuclear phase out. Against this background Amprion and TransnetBW, two of the four Transmission System Operators in Germany, are planning the HVDC connection Ultranet on existing towers, i.e. an AC/DC hybrid link. Multi-terminal capability, mastering DC faults and AC/DC inter-system faults are essential requirements for the Ultranet. The required dynamic performance can be achieved with MMC converter applying a two-level multi-state control. A laboratory test bench representing the Ultranet and the adjacent AC system at appropriate scale is employed to validate simulation results.

In converter-dominated grid structures with low short-circuit power available, the converter controls and nonlinearities of the converters have a strong influence on the overall system stability. E. g. sub-harmonic stability is a key issue in many fields of application like electric ship applications, railways and offshore wind. In order to achieve a reliable systems stability assessment, a comprehensive scenario-based assessment in time domain is unavoidable.Within this paper, the simulation tool VIAvento is briefly presented which allows for these comprehensive time-domain simulations taking the special characteristics of power-electronic assets into account. The capability of VIAvento is demonstrated with simulation results of a large-scale AC ship grid including more than 40 three-phase two-level converters. This capability makes time-domain grid planning and stability assessment in challenging converter topologies possible.

A novel approach to flux-based control, `assessment-based flux trajectory optimization' (AFO), is presented and analysed, which directly takes the properties of the converter into account. It seamlessly integrates stationary operation, slow and fast dynamics. The optimal flux trajectory is defined, based on the relatively few possibilities resulting from the discrete switching states of the converter and their combination on an instantaneous basis. Within each control cycle, optimal switching states for the next control interval are selected. The criterion for optimality bases on weighted assessment rules in time domain. The paper gives basic assessment rules and resulting flux curves for stationary and dynamic operation.