Harish S. Krishnamoorthy

Survey on Fault-Tolerant Techniques for Power Electronic Converters

With wide-spread application of power electronic converters in high power systems, there has been a growing interest in system reliability analysis and fault-tolerant capabilities. This paper presents a comprehensive review of conventional fault-tolerant techniques regarding power electronic converters in case of power semiconductor device failures. These techniques can be classified into four categories based on the type of hardware redundancy unit: switch-level, leg-level, module-level, and system-level. Also, various fault-tolerant methods are assessed according to cost, complexity, performance, etc. The intent of this review is to provide a detailed picture regarding the current landscape of research in power electronic fault-handling mechanisms.

Multilevel Medium-Frequency Link Inverter for Utility Scale Photovoltaic Integration

A multilevel topology with medium-frequency ac link for medium-voltage grid integration of utility photovoltaic (PV) plants is discussed in this paper. A megawatt-scale PV plant is divided into many zones, each comprising of two series-connected arrays. Each zone employs a medium-frequency transformer with three secondaries, which interface with the three phases of the medium voltage grid. An insulated-gate bipolar transistor full-bridge inverter feeds the MF transformer. The voltages at the transformer secondaries are then converted to three-phase line frequency ac by three full-bridge ac-ac converters. Second line frequency harmonic power does not appear in the dc bus, thereby reducing the dc capacitor size. Cascading several such cells, a high-quality multilevel medium-voltage output is generated. A new control method is proposed for the cascaded multilevel converter during partial shading while minimizing the switch ratings. The proposed topology eliminates the need for line frequency transformer isolation and reduces the dc bus capacitor size, while improving the power factor and energy yield. This paper presents the analysis, design example, and operation of a 10-MW utility PV system with experimental results on a scaled-down laboratory prototype.

A new multilevel converter for Megawatt scale solar photovoltaic utility integration

This paper presents a new multi-level DC-AC-AC converter topology for medium voltage grid integration of Megawatt (MW) scale utility photovoltaic (PV) plants. It is envisioned that a large PV field is divided into many zones, each comprising of two PV arrays. The number of zones depends on the voltage of the grid with which it is interfaced. In the proposed approach, zonal power balancing is achieved by employing a current-sharing technique. The power conversion architecture consists of an IGBT based full-bridge inverter feeding a medium frequency (MF) transformer with three secondary windings. The voltages at the transformer secondaries are then converted to three phase line frequency AC by three, full-bridge AC-AC converters. This also eliminates the 2nd harmonic power from the DC bus, thereby reducing the capacitor size. By stacking several such modules in series, a high quality multilevel medium voltage output is generated. Further, the bulky line frequency utility interface transformer is eliminated. A new control method is proposed for the series connected modules during partial shading while minimizing the switch ratings. This paper presents the analysis, design example and simulation of a 10 MW PV system with preliminary experimental results on a laboratory prototype.

A Fault-Tolerant Three-Phase Adjustable Speed Drive Topology With Active Common-Mode Voltage Suppression

A fault-tolerant adjustable speed drive (ASD) topology is introduced in this paper. A conventional ASD topology is modified to address: 1) drive vulnerability to semiconductor device faults; 2) input voltage sags; 3) motor vulnerability to effects of long leads; and 4) minimization of common-mode (CM) voltage applied to the motor terminals. These objectives are attained by inclusion of an auxiliary IGBT inverter leg, three auxiliary diodes, and isolation-reconfiguration circuit. The design and operation of the proposed topology modifications are described for different modes: 1) fault mode, 2) active CM suppression mode, and 3) auxiliary sag compensation (ASC) mode. In case of fault and sag, the isolation and hardware reconfiguration are performed in a controlled manner using triacs/antiparallel thyristors. In normal operation, the auxiliary leg is controlled to actively suppress CM voltage. For inverter IGBT failures (short circuit and open circuit), the auxiliary leg is used as a redundant leg. During voltage sags, the auxiliary leg, along with auxiliary diodes, is operated as a boost converter. A current-shaping control strategy is proposed for the ASC mode. A detailed analysis of CM performance of the proposed topology is provided, and a new figure of merit, CM distortion ratio (CMDR), is introduced to compare the attenuation of CM voltage with that of a conventional ASD topology. The output filter design procedure is outlined. A design example is presented for an 80 kW ASD system, and simulation results validate the proposed auxiliary leg based fault-tolerant scheme. Experimental results from a scaled prototype rated at 1 hp are discussed in this paper.

A matrix converter-based topology for high power electric vehicle battery charging and V2G application

In this paper, a new three-phase converter topology based on a 3x1 matrix converter (MC) is proposed for Plug-in Hybrid or Battery (PHEV/BEV) electric transit buses. In the proposed approach, the MC directly converts the low frequency (50/60 Hz, three-phase) input to a high frequency (6 kHz, one-phase) AC output without a dc-link. The output of the matrix converter (MC) is then processed by a PWM rectifier via a high frequency (HF) isolation transformer to interface with the EV battery system. The MC-PWM rectifier system is made to operate like a dual active bridge (DAB), facilitating bi-directional power flow suitable for charging and Vehicle-to-Grid (V2G) application. The digital control of the system ensures that the input currents are of high quality under both charging and discharging operations. Due to the absence of dc-link electrolytic capacitors, power density of the proposed rectifier is expected to be higher. Analysis, design example and extended simulation results are presented for a three-phase 208 VLL, 50kW charger.

Wind Turbine Generator–Battery Energy Storage Utility Interface Converter Topology With Medium-Frequency Transformer Link

A medium-voltage (MV) wind turbine generator (WTG)-battery energy storage (BESS) grid interface converter topology with medium-frequency (MF) transformer isolation is introduced in this paper. The system forms a three-port network in which several series stacked ac-ac converters transform the low-frequency (50/60 Hz) utility MV into MF (0.4 to 2 kHz) ac voltage by modulating it with MF square wave. This voltage is then fed to the MF transformer primary windings. The secondary and tertiary windings interface with the WTG side and the BESS side, respectively, after power conversion. The power generated by WTG is transferred to the MF transformer secondary windings through a three-phase pulse width modulation (PWM) rectifier and a three-phase PWM inverter, whereas the power transfer between the BESS and the tertiary winding occurs through a three-phase PWM inverter. It is shown that the utility grid sinusoidal currents, the battery current, and the WTG output currents can be controlled to be of good quality using PI and DQ control strategies. Thus, the proposed MF transformer-based three-port topology results in smaller converter weight/volume. Moreover, the control can handle voltage sags/swells and provide low voltage ride-through capability. Simulation waveforms along with experimental results are shown as proof of concept.

Medium voltage power distribution architecture with medium frequency isolation transformer for data centers

In this paper, a medium-voltage (MV) data center power distribution system (DC-PDS) architecture using medium-frequency (MF) link transformer isolation is introduced. The proposed approach significantly improves power density while maintaining high efficiency compared to conventional line-frequency based solutions. The approach also contributes to a reduction in PVC or copper used in conventional DC-PDS. First the MV transformed from the utility, is interfaced with a MV switch gear system and a diesel power generator (DPG) system. Then this MV is converted to the low voltages (LVs) required by the loads via MF transformer. The MF transformer primary side windings are connected to stacked AC-AC converters. The LV secondary windings are interfaced with several load systems and battery energy storage system (BESS) using different topologies like boost power factor corrector (PFC), PWM inverter, etc. The presence of MV switch gear and MF transformer in the architecture results in higher efficiency and power density.

A Family of New Multiport Power-Sharing Converter Topologies for Large Grid-Connected Fuel Cells

This paper presents a family of new multiport power-sharing converter (PSC) topologies for utility-scale fuel cell (FC) power generation. The proposed system connects multiple FC sources to a medium-voltage grid via a single multilevel neutral point clamp (NPC) inverter interface with high-frequency isolation. High-voltage series-connected inputs are achieved, despite safely referencing each source to ground. Also, unique power-sharing technology decouples series-connected source currents and enables control of each FCs individual power level. Multidimensional modeling and analysis of the proposed systems PI- and PV-behavior is presented. Then, system topology and design considerations are discussed. Case study simulations show how this topology can operate four 250-kW FCs at separate power levels between 20% and 100%, while connecting to a single three-level NPC utility-interface. Correspondingly, a reduced-scale hardware prototype provides further proof of concept. Altogether, the proposed family of multiport PSC topologies realizes independent power control per source as well as increased output power, operational flexibility, thermal balancing, source availability, and cost effectiveness for utility FC power generation.

New medium-voltage Adjustable Speed Drive (ASD) topologies with medium-frequency transformer isolation

In this paper, two new medium-voltage Adjustable Speed Drive (ASD) topologies with medium-frequency-link transformer isolation are introduced. A medium-voltage (MV) medium frequency (MF) transformer is realized along with several series connected AC-AC converters. Operating the multi-winding transformer at MF contributes to weight and volume reduction without sacrificing efficiency. For this application, it is shown that a unidirectional AC-AC converters with diode front end are sufficient to power the MF transformer. The AC-AC converter gating signals are generated by a convolution of a square wave and a PWM sine wave. This modulation is adjusted in such a way that the input current harmonic frequencies are of higher order, thus introducing power factor correction (PFC). The proposed modulation strategy is flexible, allows for high switching frequency operation while ensuring that the transformer core is operated at constant MF over the entire range. Further, the rectified DC-link voltage is of high quality. Two MV-ASD configurations are explored for generating variable voltage variable frequency (VVVF) - (i) a three-phase NPC inverter with medium-voltage IGBTs and (ii) Multi-level single-phase inverters. Overall, the proposed topologies make the ASD size smaller in weight/volume and at the same time, provide unity power factor sinusoidal currents at the utility input.

A new wind turbine generator / battery energy storage utility interface converter topology with medium-frequency transformer

In this paper, a new medium-voltage (MV) wind turbine generator (WTG) - energy storage grid interface converter topology with medium-frequency (MF) link transformer isolation is introduced. The system forms a 3-port network in which several series stacked AC-AC converters transform the low-frequency (50/60 Hz) utility MV into MF (0.4 to 2 kHz) AC voltage by modulating it with MF square wave. This voltage is then fed to the MF transformer primary windings. The secondary and tertiary windings interface with the WTG side and the battery energy storage side respectively after power conversion. The power generated by WTG is transferred to the MF transformer secondary windings through a 3-phase PWM rectifier and a 3-phase PWM inverter, whereas the power transfer between the energy storage and the tertiary winding occurs through a 3-phase PWM inverter. It is shown that the utility grid sinusoidal currents, the battery current and the WTG output currents can be controlled to be of good quality using PI and DQ control strategies. Thus, the proposed MF transformer based 3-port topology results in smaller converter weight/volume. Moreover, the control can effectively handle voltage sags/swells and provide low voltage ride through (LVRT) capability without significant change in the topology. Simulation waveforms along with preliminary experimental results are discussed in this paper.