This article proposes modeling and two controller design techniques for a triple active bridge (TAB) three-port dc-dc converter comprising of fuel cell, battery, and load. The sources are integrated, ...employing boost interleaved full-bridge converters in order to get smoother source current profile. The converter is a nonlinear multi-input multi-output (MIMO) system with a large number of control variables. Moreover, a high degree of coupling among the control variables makes its modeling and control system design quite cumbersome and complex. To overcome the complexity of analysis in a higher order system like TAB, a generalized frequency-domain modeling technique is introduced in this article. A new decoupling matrix-based proportional-integral controller design method is also proposed. It reduces the design complexity and improves the system dynamic performance (lower settling time, overshoot/undershoot in the controlled variables) in comparison to similar three-port converters reported in the literature. Further, the performance of the proposed controller is compared by simulation with another popular MIMO system controller design technique, namely the state feedback control. A 1-kW laboratory prototype is built and tested to verify the system performance during dynamic load changes, source current variation, and battery charging/discharging operation.
Doubly fed induction generator (DFIG) is the most popular variable-speed generator for wind energy application due to its superior performance, lower cost, and control flexibility. Instead of ...back-to-back converters, matrix converter (MC) can be a good alternative for connecting the DFIG rotor to the grid. This paper presents a novel lookup-table-based hysteresis controller for a variable-speed constant-frequency DFIG supplied from a direct MC. For the machine control, a modified direct torque controller (DTC) with reactive power and speed control in the outer loops has been used. The reactive component of the MC input current has been also controlled using a hysteresis controller considering all active switch states based on estimated input current and load power factor. This upgradation of the input current control switching table reduces the distortion in the grid-supplied current waveform. Overall, the performance of the DFIG with MC is experimentally shown to be superior while using the proposed controller compared with a standard DTC-based control or space-vector-pulsewidth-modulation-based vector control of MC-fed DFIG reported in the literature.
Interfacing multiple low-voltage energy storage devices with a high-voltage dc bus efficiently has always been a challenge. In this article, a high gain multiport dc-dc converter is proposed for low ...voltage battery-supercapacitor based hybrid energy storage systems. The proposed topology utilizes a current-fed dual active bridge structure, thus providing galvanic isolation of the battery from the dc bus, wide zero voltage switching (ZVS) range of all the switches, and bidirectional power flow between any two ports. The dc bus side bridge uses voltage multiplier cells to achieve a high voltage conversion ratio between the supercapacitor (SC) and the dc bus. Moreover, as the proposed topology employs only one two-winding transformer to achieve a three-port interface, the number of control variables are reduced, which decreases control complexities. The operation of the proposed converter is analyzed in detail, including the derivation of ZVS conditions for the switches and transformer power flow equations. A decoupled closed-loop control strategy is implemented for the dc bus voltage control and energy management of the storage devices under different operating conditions. A 1-kW laboratory prototype is built to verify the effectiveness of the proposed converter, along with the control scheme.
Nonregenerative pulsewidth-modulated (PWM) rectifiers are increasingly being considered for applications, where the power flow is unidirectional, such as power supplies for telecommunications, X-ray, ...the machine-side converter for wind energy conversion systems, etc. They use fewer active switches, which increase their power density and reduce cost. This paper proposes a novel reduced switch topology for a multilevel (five-level or higher) nonregenerative PWM rectifier. It uses only four controlled switches and eight diodes per phase for a five-level rectifier. Half of the diodes are naturally commutated (zero current switching) at the line frequency, which reduces switching losses. This topology has several other advantages compared to similar topologies reported in the literature, such as minimum voltage stress across the devices, elimination of transient voltage-balancing snubbers, no extra hardware for balancing the flying capacitors, the dc-link mid-point voltage, etc. In this paper, switching cycle average modeling and the carrier-based modulation strategy for this rectifier are also presented to maintain a balanced dc link and to regulate flying capacitor voltages, while achieving unity displacement factor at the rectifier input terminals. The overall performance of the rectifier is verified by experimental results.
The VIENNA rectifier is increasingly being considered as the active front-end converter in many applications that do not require regeneration or controllable input reactive power. In such ...applications, this rectifier is normally operated at unity source displacement factor, which derates its nominal voltampere (VA) rating. Moreover, in the "source voltage oriented" reference frame that is normally used for control, this rectifier has complex operating limits on the rectifier terminal voltage (imposed by the operating conditions), which needs to be computed online to avoid the line currents from going through zero-crossing distortions. Also, the source voltage has to be either measured or estimated, which further increases the cost and/or computational burden. This paper proposes a control method that eliminates the requirement of the input voltage sensors (or estimators) and makes the limits on the rectifier terminal voltage far simpler to compute. The control method is implemented in the input line current oriented reference frame, considering unity displacement factor at the rectifier input terminals, which minimizes the VA rating of the rectifier (for a given rated load power). The proposed sensorless current-oriented control approach is compared with the conventional voltage-oriented control strategy through experimental results.
Isolated DC-DC converters in DC Solid-State Transformers (DCSSTs) must conform to stringent requirements such as high voltage conversion ratio, high efficiency, and reliable operation with grid fault ...handling capabilities. This paper proposes a novel current-fed Dual-Active-Bridge (CFDAB) converter, named H5-CFDAB, for modular DCSST application. Compared to the existing topologies, the proposed converter provides a single-stage power conversion with reduced component count and semiconductor Volt-Ampere (VA) rating. The proposed modulation scheme makes the pulse widths of the transformer winding voltages independent of the duty ratio. This allows further optimization of the high-frequency-link (HFL) characteristics like minimized HFL conduction losses and full range Zero-Voltage-Switching (ZVS) at turn ON for the switches, further increasing the converter efficiency. Several modules of the proposed H5-CFDAB converter connected in Input-Series-Output-Parallel (ISOP) configuration makes the system suitable for Medium-Voltage (MV) high-power applications. The operation of the proposed converter is analyzed in detail, including the HFL characteristics and derivation of the ZVS conditions for the switches. In addition, a closed-loop control scheme is proposed for achieving tight regulation of the converter outputs during transients. A 1.2 kW laboratory prototype is built to verify the effectiveness of the proposed converter, along with the control scheme.
In applications, where bidirectional power flow is not required, unidirectional pulsewidth modulated (PWM) rectifiers are advantageous in terms of power density, hardware simplicity, and reliability ...compared to the conventional PWM rectifiers. For high-power medium-voltage applications, a discontinuous pulsewidth modulation (DPWM) strategy is useful in reducing the switching loss. This article proposes a DPWM strategy for a recently proposed five-level unidirectional rectifier. The rectifier is analyzed in the space vector (SV) domain to determine the switching sequences for DPWM. Two such DPWM sequences (i.e., DPWMA and DPWMB) are identified. Implementation of these DPWM sequences in the SV domain is known to be very computation intensive. Therefore, carrier-based implementation of both the DPWM strategies is proposed, which are derived following the SV approach. The proposed strategies are found to be very simple with very few computations. A new voltage-balancing method for the dc-link capacitors is also proposed in this article. Performances of these DPWM strategies over the entire operating range are predicted analytically and verified experimentally. This article is accompanied by MATLAB files demonstrating various performance indexes of the proposed DPWM sequences.
This paper proposes a three-phase, five-level, non-regenerative pulsewidth modulated rectifier using only two active switches (minimum required) per phase, which drastically reduces gate driver ...requirement and hardware complexity. It draws sinusoidal input current at close to unity power factor. All the semiconductor devices are rated at only one fourth of the dc-link voltage, and none of them requires any transient voltage balancing snubber. A total of 8 out of the 14 diodes per phase undergo soft switching transition under all operating conditions, which increases its efficiency. No extra hardware circuitry for balancing the flying capacitors (FCs) or the dc-link mid-point voltage are required, which further reduces hardware complexity and increases the conversion efficiency. The proposed topology does not need any sophisticated startup procedure for charging the FCs either, which solves the problem of semiconductor overvoltage during starting. A 3-kW laboratory prototype is built to experimentally verify the proposed topology. The maximum efficiency obtained from the prototype is 98.7%, and it is always more than 96% for the load range from 15% to its rated.
The <inline-formula> <tex-math notation="LaTeX">LC </tex-math></inline-formula> filter at the matrix converter (MC) input, excited by the PWM MC input current, introduces unwanted resonance ...oscillations in the input line current. The problem is more severe with a direct torque controlled (DTC) MC due to variable switching frequency operation. Active damping of this oscillation is preferred as it eliminates the power loss in the damping resistance. However, no such active damping strategy for a DTC-controlled MC-fed system has been reported in the literature so far. This article presents an input reactive power controller and a novel active damping strategy to improve the input filter current quality of an MC-fed doubly fed induction generator (DFIG) for variable speed constant frequency wind power generation. For the MC input reactive power control, superior performance of a multi-loop control structure over single-loop is established by experimental results. Stability analysis of the closed-loop input filter system using a small-signal model reveals that the system damping is the lowest in the sub-synchronous generating mode with unity DFIG stator power factor and can be improved by proper selection of the active damping controller parameters. Effects of these parameters on the system damping are predicted from the small-signal model and verified experimentally.
This article proposes a two-stage 48-V voltage regulator module using a hybrid switched-capacitor converter in the first stage. Apart from being compact in nature, this first-stage converter has ...several other advantages such as efficient regulation ability and hence scalability, inherent full/partial soft switching for most of the switches, reduced switch voltage and current stresses, and the possibility of wide range variable switching frequency operation. These combined features together help to obtain a flat high-efficiency curve for the first stage. The second-stage multiphase buck converter also exhibits flat high-efficiency characteristics through phase shedding. Steady-state circuit operation and analysis of the first-stage converter is presented in this article. An accurate discrete-time model of the first-stage converter is also developed. Thereafter, a design guideline for the two-stage converter is discussed. Finally, experimental results from an 80-W laboratory prototype show that the proposed converter possesses a relatively flat high-efficiency profile and exhibits a faster load transient response compared to that of the state-of-the-art solutions.
