Pawan Garg

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.

A fault-tolerant T-type three-level inverter system

Field experiences have demonstrated that semiconductor devices are vulnerable to failures (open-& short-circuit). In many critical applications, such failures are unacceptable and high system reliability is required. In this paper, a topology modification for the T-type 3-level inverter is explored to achieve the fault-tolerant operation and enhance the system reliability in the case of device open-circuit or short-circuit failures. With the proposed topology modification (via fewer additional switches), the device failure ride-through performance can be dynamically achieved without degradation of output capacity. Furthermore, the transition principles from normal operations to post-fault operations are detailed, and the reliability enhancement is calculated. The simulation results are included to verify the validity of the modified topology.

A new medium-voltage energy storage converter topology with medium-frequency transformer isolation

In this paper, a new medium-voltage energy storage converter topology with medium-frequency-link transformer isolation is introduced. A medium-voltage (MV) medium frequency (MF) transformer is realized along with several series connected AC-AC full-bridge converters with bi-directional switches. The AC-AC converters transform the low-frequency (50/60 Hz) AC utility voltage into MF (0.4 to 2 kHz) AC voltage by modulating the utility input with MF square wave. The series stacked AC-AC converters share equal voltages, enabling the usage of lower voltage rated IGBTs/MOSFETs. The 3-phase voltage at the secondary winding of the MF transformer is fed to a three-phase Pulse Width Modulation (PWM) rectifier to enable bi-directional power flow. The rectifier output is filtered and interfaced with the energy storage devices. The rectifier gating signals can be generated using hysteresis or PI control of 3-phase transformer currents, forcing the currents to be of the same shape as the transformer voltages. It is shown that the system input currents as well as the battery current are of good quality and at the same time, the transformer voltages are of medium frequency. Overall, the proposed energy storage converter topology results in smaller weight/volume and at the same time, provides unity power factor sinusoidal input currents. Simulation results along with preliminary experimental waveforms are shown to validate the concept.

Analysis and design of active inductor as DC-link reactor for lightweight adjustable speed drive systems

Size and weight considerations are very important in certain specialized industries as offshore oil drilling and marine/subsea systems that employ adjustable speed drives (ASD). Typical ASD topology consists of a three phase diode rectifier front-end. It is also common to specify a dc-link inductor and/or ac line reactors to reduce utility line current harmonics to acceptable levels. However, the required dc-link inductor/ac line reactors contribute to additional weight and volume. In this paper an active inductor approach is explored to emulate a large size dc-link inductor. Analysis and design of an active inductor for a 1 MW ASD is proposed and analyzed. The required inductance value is commanded and current control regulates the active inductor current to match inductor behavior. It is shown that the proposed active inductor topology is similar in weight/volume as the smaller inductor while emulating a higher per unit inductance. The proposed active inductor meets the required compact and lightweight specifications of ASDs in retrofit and special applications. Furthermore, the active inductor is tunable to achieve high performance operation during faults. Experimental results from a scaled down prototype are presented. A size, weight and loss analysis is detailed and a soft switching method to reduce losses is also explained.

3-Phase AC-DC converter topologies with higher frequency transformer isolation for utility grid interface

Two AC-DC converter topologies for 3-phase utility grid interface using high frequency (HF) or medium frequency (MF) transformers have been proposed in this paper. The transformer primary side converter structure is similar in both the topologies (say T-I and T-II), but the secondary side converter structures as well as the modulation schemes are different. The line-frequency AC voltage from the utility is first converted to HF or MF AC using three 1-phase AC-AC converters. The AC-AC converters can be unidirectional if power flow is desired in only one direction. The transformer core can be of silicon steel, amorphous or nano-crystalline material. The secondary side of topology T-I consists of a 3-phase PWM rectifier, while topology T-II consists of three 1-phase boost power factor correction (PFC) stages with a common output DC-bus. Both topologies avoid DC-link capacitors on the primary side and allow for high density power conversion. Potential applications include electric vehicle battery charging, telecom power supply, adjustable speed drives, server power supplies, etc. Simulation results and experimental waveforms from laboratory prototype evince the operation of the topologies.