Naga Brahmendra Yadav Gorla

Tap changing transformer based dual active bridge bi-directional DC-DC converter

Voltage fed Dual Active Bridge (VF-DAB) is the simplest isolated bidirectional DC-DC converter topology with least number of high frequency tank elements. This paper addresses the issue of Phase Shift operated VF-DAB when used for energy storage applications due to their variable nature in terminal voltage based on the state of charge. Circulation currents and limitations in the soft-switching range are the main drawbacks of VF-DAB when operated with phase shift control within the two active bridges and widely varying voltage at one port. This affect their overall efficiencies at low and mid loads. In this paper, a modification in VF-DAB is proposed which reduces the drawbacks of the conventional phase shift controlled DAB by having variable transformer turns ratio, thereby improving the efficiency of the overall converter at medium and light loads. The analysis and design guidelines for the proposed converter are briefly explained and validated with experimental results.

A new control scheme to improve load transient response of single phase PWM rectifier with Auxiliary Current Injection Circuit

This paper presents a new control scheme for a single-phase PWM rectifier with Auxiliary Current Injection Circuit (ACIC) to improve the load transient response and DC bus voltage controller bandwidth. The PWM rectifier and ACIC are controlled to mitigate the 2nd harmonic ripple in the DC bus under steady-state as well as the peak undershoot and overshoot during load transients. Analytical equations governing the operation of the single phase PWM rectifier with proposed ACIC controller are derived and presented. The proposed controller is designed and simulated using MATLAB Simulink and implemented on a 100 W test setup. The simulation and experimental results are presented to validate the performance of proposed controller.

A fault tolerant control approach for a three-stage cascaded multilevel solid state transformer

A solid state transformer (SST) is going to be a vital part of the future grid due to its attractive features such as step-up/step-down of grid voltage, provision for an intermediate DC bus to interface renewable energy source and energy storage, controlled bidirectional power flow between two girds etc. Even though the reliability of the SST is questioned due to high switching device count, redundant modules can be integrated into the SST architecture to ensure uninterrupted operation. Unlike the high short circuit current withstanding capability of the passive copper windings in a conventional iron-and-copper based transformer, an SST can fail in the event of any fault due to the limited over current rating of the switching devices. Several fault identification and isolation techniques for the power converters have been researched, but the power converter operation during the time frame between fault isolation and post fault restoration was neglected. In this paper, the grid current and sub-module capacitor voltage deviations are analysed during the time frame between fault isolation and post-fault restoration. A new control approach is proposed to improve the fault tolerance of the SST by identifying a short circuit fault and limit the short circuit current before a healthy module is brought into operation i.e., post-fault restoration. In the proposed method, the control is designed to switch from the conventional control to a fault tolerant control scheme in the event of fault. The proposed control method is validated using the PLECS real time (RT) hardware-in-loop (HIL) platform and results are presented.

Design of a novel APWM half-bridge DC-DC resonant converter with load-independent soft-switching and reduced circulating current

This paper presents a novel Asymmetrical-Pulse-Width-Modulated (APWM) half-bridge resonant converter with one additional active switching device at the secondary side of the transformer. The proposed converter features load-independent Zero-Voltage-Switching (ZVS) of both of the primary side switches with minimal magnetizing current. Empirical formulae are derived to design the resonant network and the high-frequency transformer systematically. The circulating current is minimized not only by operating the converter exactly at the resonant frequency, but also by reducing the magnetizing current. Moreover, the additional secondary-side switching device (which is a low-voltage and high-current rated MOSFET) achieves zero current turn-off at all load conditions. Thus the efficiency of the overall converter is significantly improved for wide load range. Simulation and experimental results are presented to validate the analysis and to prove the significance of the additional switching device at the secondary side of the transformer.

Improved utilization of grid connected voltage source converters in smart grid through local VAR compensation

In Smart Grid there are abundant number of grid connected Voltage Source Converters (VSC) in the form of Electric Vehicle (EV) battery chargers, Renewable Energy Source (RES) fed grid connected inverters etc., that can be used for active and reactive power support to the grid. The VSCs remain idle when the load in case of EV charger or the energy source in case of RES fed grid connected inverters is not available. This work presents a single-phase PQ theory based control strategy to improve the utilization factor of such converters by using them to support the reactive power to the local loads when they are idle. The proposed control scheme is implementable with less computational effort. The system is further analyzed by calculating appropriate minimum DC bus voltage for a given amount of reactive power to be supported. The analysis for the designed system is validated using MATLAB-Simulink based simulation and a scaled down laboratory based experimental setup. The results are in good agreement with the analysis.

Analysis of active power decoupling in single-phase rectifier using six-switch topology

In this paper, active power decoupling using six switch rectifier with a built-in ripple port (inductive or capacitive or combination of both) to reduce the size of the E-cap is studied. The six switch rectifier is analyzed as a three-port converter: (i) AC source at the input port, (ii) a load at the DC output port, and (iii) an inductor or capacitor connected to the third port called ripple port. With appropriate modulation technique, second harmonic ripple power is circulated between the input port and the ripple port without disturbing the active power flow between the input port and the output port. A suitable closed-loop controller is designed to regulate the DC bus voltage and minimize the second harmonic ripple simultaneously. The step response results presented in the paper confirm the stability of the closed loop design. The relation between modulation indices of ripple port and active port is derived to maximize the utilization of DC bus in these converters. Simulation results are presented to validate the analysis and effectiveness of the circuit. The analysis made for the ripple port can be extended to micro-inverters as well.

A new phase shedding and phase adding control scheme for interleaved DAB converter operating in IPOP configuration

This paper focuses on Voltage Fed Dual Active Bridge (VF-DAB) bidirectional DC-DC converter modules connected Input Parallel and Output Parallel (IPOP) configuration. Modularity or parallel operation of the bidirectional DC-DC converters for energy storage application is not just an economic solution for increasing the power transfer capability, but also provides several advantages such as redundancy, reduced design cost and hot swap feature of faulty module. In this paper, an interleaved VF-DAB with phase-shedding operation at light loads are proposed for interfacing energy storage devices to a common DC bus. A modification in the conventional active current sharing technique for parallel operated converters is proposed to implement phase shedding and phase adding operation. A 12 kW interleaved IPOP based DAB converter is proposed and simulated to validate the claims.