Enhanced Current Capability for Modular Multilevel Converters by a Combined Sorting Algorithm for Capacitor Voltages and Semiconductor Losses

The modular multilevel converter (MMC) has become very attractive for high- and medium-voltage applications, generating excellent waveforms at very high efficiencies. One of the main challenges is the appropriate selection of inserted submodules (SMs), commonly done by capacitor voltage balancing algorithms. However, the semiconductor stress can only be balanced up to a certain degree by conventional algorithms, since the stress is not directly monitored. An uneven stress distribution between the SMs does not only result in different lifetime expectations, but also in increased maximum temperatures, for which each SM needs to be designed. With the goal of more effective utilization of chip area, a new balancing approach is introduced for monitoring and balancing not only the capacitor voltages but also the average power losses in each SM. In this way, the MMC current capability is significantly increased only by software without deteriorating the system performance and efficiency

Active thermal control of power electronic modules in smart transformer applications

The Smart Transformer (ST) is a possible solution to obtain intelligent nodes in the electrical grid, which can be used for the grid management and increase the capacity for the integration of renewable energy sources. A problem for the application of the ST in the distribution grid is the expected lower reliability in comparison with the traditional transformer. To address this problem, the knowledge of power system, power electronics and reliability is combined in this work. Following the "Physics of failure" approach, the most frequently failing components are identified, their load profile in the electrical distribution grid is analyzed and finally solutions are developed to improve the reliability. The power semiconductors are found to be the most prone to fail components and most of their failure mechanisms are found to be affected by thermal cycling. For this reason, thermal stress analysis is performed for the three-stage ST. As an opportunity to increase the reliability, active thermal control is introduced, which is a software based solution for the reduction of the thermal stress during operation. The existing approaches from literature are reviewed and categorized into control of the power converter losses and the control of the device loading. For increasing the reliability by control of the power converter losses, one algorithm is introduced and validated for hard switching power converters and one algorithm is introduced for soft switching power semiconductors. Controlling the thermal stress of modular building blocks in a modular power converter, referring to power routing, is proposed. The capability of the algorithm is investigated analytically for series connected and parallel connected modular building blocks. For the validation, the influence of the power routing on the loading of the single cell is demonstrated experimentally for series connected, parallel connected and medium frequency transformer coupled cells in modular power converters

Modular Smart Transformer Topology for the Interconnection of Multiple Isolated AC and DC Grids

The Smart Transformer enables the interconnection of multiple AC and DC grids with potentially different voltage levels. In most configurations, each grid needs to be isolated for preventing the propagation of faults. Actual literature mostly considers a single isolated interconnection, which is not opti-mized for the interconnection of multiple AC and DC grids. By utilizing the modularity of the Cascaded H-Bridge (CHB) converter and the corresponding connected Dual Active Bridge (DAB) to each H-bridge, this work proposes a system design for feeding multiple AC and DC grids. Requirements for the DC grid isolation and integration are discussed and the proposed topology is demonstrated to enable reduced losses compared to the commonly adopted configuration. The commonly adopted and the proposed configuration are compared and experimental results validate reduced losses for a large range of operation compared to the conventional solution

Modulation for Cascaded Multilevel Converters in PV Applications with High Input Power Imbalance

Cascaded multilevel inverters, such as the cascaded H-Bridge (CHB) converter, are an attractive solution for multi-string photovoltaic (PV) systems, because they enable direct connection to the medium voltage grid and maximum power point tracking of multiple strings. As a challenge of the topology, the operation with high power imbalance in the strings is constrained by the over-modulation. This limitation is analyzed for sinusoidal modulation and the impact on the maximum power imbalance is demonstrated. For increasing the operating range with maximum power tracking in the strings, a discontinuous modulation with extended maximum power imbalance and reduced losses is proposed. The method is analyzed in terms of maximum power imbalance, efficiency and power quality. In addition, the method is validated on an experimental test bench

Robustness Analysis of Voltage Control Strategies of Smart Transformer

The increasing penetration of Distributed Generators (DG) in the modern electric distribution network poses high priority on the problem of the stability. In this article the Harmonic Stability of a Smart Transformer-fed microgrid is investigated under different control strategies. The considered microgrid is composed by a Smart Transformer and three Distributed Generators, considering the bandwidth of the DGs unknown. The robustness is evaluated analysing the eigenvalues as a consequence of a variation of the DGs bandwidth. The system is modelled as a Multi Input Multi Output System (MIMO); the eigenvalue based analysis is carried out to assess the stability and compare the robustness of the traditional double-loop PI and a state-feedback (SF) integral controller. The results show that the SF controller ensures a higher robustness than the traditional PI controller with respect to increasing bandwidths of the DGs

FS-MPC Algorithm for Optimized Operation of a Hybrid Active Neutral Point Clamped Converter

The design for reliability has gained a lot of attention in power electronic community in the past few years. The aim is to optimize the design in order to achieve desired reliability goals with minimum margins. However, in most applications, there are stressing conditions, which result in high stress and therefore require higher margins. As an opportunity, adapting the control of the power electronic converters to equally redistribute the stress of the devices can reduce the stressing conditions and also reduce the design margins. This is of great importance for multilevel topologies and in particular the Active Neutral Point Clamped (ANPC) topology. This paper introduces a Finite-Set Model Predictive Control algorithm designed for achieving a balanced device junction temperature in a Hybrid SiC ANPC converter. The inner switches of the converter are replaced by SiC MOSFETs and the control algorithm is designed to utilize the low switching losses of the devices. The obtained experimental results are compared to carrier based benchmark algorithm. It is demonstrated that the temperature difference between the devices is within 1°C and the dc-link voltage deviation is within 0.5V.</p