Generic closed loop controller for power regulation in dual active bridge DC-DC converter with current stress minimization

This paper presents a comprehensive and generalized analysis of the bidirectional dual active bridge (DAB) DC/DC converter using triple phase shift (TPS) control to enable closed loop power regulation while minimizing current stress. The key new achievements are: a generic analysis in terms of possible conversion ratios/converter voltage gains (i.e. Buck/Boost/Unity), per unit based equations regardless of DAB ratings, and a new simple closed loop controller implementable in real time to meet desired power transfer regulation at minimum current stress. Per unit based analytical expressions are derived for converter AC RMS current as well as power transferred. An offline particle swarm optimization (PSO) method is used to obtain an extensive set of TPS ratios for minimizing the RMS current in the entire bidirectional power range of - 1 to 1 per unit. The extensive set of results achieved from PSO presents a generic data pool which is carefully analyzed to derive simple useful relations. Such relations enabled a generic closed loop controller design that can be implemented in real time avoiding the extensive computational capacity that iterative optimization techniques require. A detailed Simulink DAB switching model is used to validate precision of the proposed closed loop controller under various operating conditions. An experimental prototype also substantiates the results achieved

Modular multilevel converter with modified half-bridge submodule and arm filter for dc transmission systems with DC fault blocking capability

Although a modular multilevel converter (MMC) is universally accepted as a suitable converter topology for the high voltage dc transmission systems, its dc fault ride performance requires substantial improvement in order to be used in critical infrastructures such as transnational multi-terminal dc (MTDC) networks. Therefore, this paper proposes a modified submodule circuit for modular multilevel converter that offers an improved dc fault ride through performance with reduced semiconductor losses and enhanced control flexibility compared to that achievable with full-bridge submodules. The use of the proposed submodules allows MMC to retain its modularity; with semiconductor loss similar to that of the mixed submodules MMC, but higher than that of the half-bridge submodules. Besides dc fault blocking, the proposed submodule offers the possibility of controlling ac current in-feed during pole-to-pole dc short circuit fault, and this makes such submodule increasingly attractive and useful for continued operation of MTDC networks during dc faults. The aforesaid attributes are validated using simulations performed in MATLAB/SIMULINK, and substantiated experimentally using the proposed submodule topology on a 4-level small-scale MMC prototype

Enhanced performance modified discontinuous PWM technique for three phase Z-source inverter

Various industrial applications require a voltage conversion stage from DC to AC. Among them, commercial renewable energy systems (RES) need a voltage buck and/or boost stage for islanded/grid connected operation. Despite the excellent performance offered by conventional two-stage converter systems (DC-DC followed by dc-ac stages), the need for a single-stage conversion stage is attracting more interest for cost and size reduction reasons. Although voltage source inverters (VSIs) are voltage buck-only converters, single stage current source inverters (CSIs) can offer voltage boost features, although at the penalty of using a large DC-link inductor. Boost inverters are a good candidate with the demerit of complicated control strategies. The impedance source (Z-source) inverter is a high-performance competitor as it offers voltage buck/boost in addition to a reduced passive component size. Several pulse width modulation (PWM) techniques have been presented in the literature for three-phase Z-source inverters. Various common drawbacks are annotated, especially the non-linear behavior at low modulation indices and the famous trade-off between the operating range and the converter switches' voltage stress. In this paper, a modified discontinuous PWM technique is proposed for a three-phase z-source inverter offering: (i) smooth voltage gain variation, (ii) a wide operating range, (iii) reduced voltage stress, and (iv) improved total harmonic distortion (THD). Simulation, in addition to experimental results at various operating conditions, validated the proposed PWM technique's superior performance compared to the conventional PWM techniques

Modified dual active bridge DC/DC converter with improved efficiency and interoperability in hybrid LCC/VSC HVDC transmission grids

DC transmission grids are the promising electrical networks in the near future especially with the high penetration of large scale renewables. This paper proposes a modified version of the dual active bridge (DAB) DC/DC converter with AC link capacitors generating reactive power to compensate for non-active power consumption; hence mitigating current stresses and losses to improve efficiency. The proposed topology also enables the connectivity of current source line-commutated HVDC and voltage source HVDC technologies particularly during power reversal; a feature which conventional DAB is incapable of doing. Analysis and detailed design of the proposed converter are addressed and a comparative performance analysis is carried out with conventional DAB. Converter principle of operation is explained and Matlab/Simulink simulations are carried out to verify converter operation particularly under adverse conditions such as rated power reversal and DC fault conditions. A low scale prototype substantiates the theoretical analysis and simulation results

PV single-phase grid-connected converter : dc-link voltage sensorless prospective

In this paper, a dc-link voltage sensorless control technique is proposed for single-phase two-stage grid-coupled photovoltaic (PV) converters. Matching conventional control techniques, the proposed scheme assigns the function of PV maximum power point tracking to the chopper stage. However, in the inverter stage, conventional techniques employ two control loops: outer dc-link voltage and inner grid current control loops. Diversely, the proposed technique employs only current control loop and mitigates the voltage control loop, thus eliminating the dc-link high-voltage sensor. Hence, system cost and footprint are reduced, and control complexity is minimized. Furthermore, the removal of the dc-link voltage loop proportional-integral controller enhances system stability and improves its dynamic response during sudden environmental changes. The system simulation is carried out, and an experimental rig is implemented to validate the proposed technique effectiveness. In addition, the proposed technique is compared with the conventional one under varying irradiance conditions at different dc-link voltage levels, illustrating the enhanced capabilities of the proposed technique

High performance single-phase single-stage grid-tied PV current source inverter using cascaded harmonic compensators

In this paper, a single-phase single-stage photovoltaic (PV) grid-tied system is investigated. The conventional Pulse Width Modulated (PWM) voltage source inverter (VSI) is replaced by a PWM current source inverter (CSI) for its voltage boosting capabilities, inherent short-circuit proof and higher reliability features. Modeling, design and analysis of the considered CSI are presented altogether with enhanced proposed control loops aided with a modified PWM technique. DC-link even current harmonics are commonly reflected as low-order odd harmonics in the grid resulting in a poor quality grid current. In order to overcome the latter, a high performance Proportional Resonant Controller, applied in the inverter inner grid current loop, is proposed using cascaded resonant control units tuned at low-order frequencies to eliminate injected grid current harmonics. Hence, with a less-bulky smoothing inductor at the CSI DC-side, grid power quality and system efficiency are simultaneously improved. Simulation and experimental results verify the proposed controller effectiveness

Improved performance low-cost incremental conductance PV MPPT technique

Variable-step incremental conductance (Inc.Cond.) technique, for photovoltaic (PV) maximum power point tracking, has merits of good tracking accuracy and fast convergence speed. Yet, it lacks simplicity in its implementation due to the mathematical division computations involved in its algorithm structure. Furthermore, the conventional variable step-size, based on the division of the PV module power change by the PV voltage change, encounters steadystate power oscillations and dynamic problems especially under sudden environmental changes. In this study, an enhancement is introduced to Inc.Cond. algorithm in order to entirely eliminate the division calculations involved in its structure. Hence, algorithm implementation complexity is minimised enabling the utilisation of low-cost microcontrollers to cut down system cost. Moreover, the required real processing time is reduced, thus sampling rate can be improved to fasten system response during sudden changes. Regarding the applied step-size, a modified variable-step size, which depends solely on PV power, is proposed. The latter achieves enhanced transient performance with minimal steady-state power oscillations around the MPP even under partial shading. For proposed technique's validation, simulation work is carried out and an experimental set up is implemented in which ARDUINO Uno board, based on low-cost Atmega328 microcontroller, is employed

A new active power controller in dual active bridge DC-DC converter with a minimum-current-point-tracking technique

This article proposes a new controller for power regulation in dual active bridge (DAB) dc-dc converter based on a new scheme that tracks minimum RMS current to ensure minimum losses. The proposed controller is based on an implementation of perturb and observe (PO) tracking method that enables minimum current point tracking (MCPT) at any desired level of active power transfer and dc voltage ratio. The PO is embedded in a closed-loop control scheme which simultaneously regulates active power in DAB converter. The nonlinear I - V characteristic of DAB presents the basis for this proposed controller and the rationale of using PO algorithm. The proposed controller does not require complex nonlinear converter modeling and is not circuit parameter dependent. Design procedure of the proposed controller is presented, and extensive simulation is carried out using MATLAB/Simulink to validate the effectiveness of the proposed MCPT closed-loop controller. An experimental prototype also substantiates the results achieved

Modified variable step-size incremental conductance MPPT technique for photovoltaic systems

A highly efficient photovoltaic (PV) system requires a maximum power point tracker to extract peak power from PV modules. The conventional variable step-size incremental conductance (INC) maximum power point tracking (MPPT) technique has two main drawbacks. First, it uses a pre-set scaling factor, which requires manual tuning under different irradiance levels. Second, it adapts the slope of the PV characteristics curve to vary the step-size, which means any small changes in PV module voltage will significantly increase the overall step-size. Subsequently, it deviates the operating point away from the actual reference. In this paper, a new modified variable step-size INC algorithm is proposed to address the aforementioned problems. The proposed algorithm consists of two parts, namely autonomous scaling factor and slope angle variation algorithm. The autonomous scaling factor continuously adjusts the step-size without using a pre-set constant to control the trade-off between convergence speed and tracking precision. The slope angle variation algorithm mitigates the impact of PV voltage change, especially during variable irradiance conditions to improve the MPPT efficiency. The theoretical investigations of the new technique are carried out while its practicability is confirmed by simulation and experimental results

Effectiveness of Green Roofs and Green Walls on Energy Consumption and Indoor Comfort in Arid Climates

Increased urbanization have many negative effects on human well-being, city infrastructure, electricity usage and the increase of indoor temperatures. A solution may be to retrofit existing buildings, with implementing a vegetated layer to roofs and walls, this may enhance building performance, reduce consumption and improve indoor comfort. Cities with tall buildings may be more adequate to implement a green-wall as it have more area to make impact. This paper examines the energy reduction advantages of adding greenery on buildings in the hot arid climate of Egypt by considering three typical types of residential buildings in the city of Cairo as a case study. Designbuilder software was selected to stimulate the buildings chosen in this research. The results shows that an extensive soil thickness of 15cm performs better in the arid climates. electricity consumption for the base case is 52 kWh/m2 annually when used a traditional external envelop and dropped to 43 kWh/m2 when a vegetated layer added to the whole building (roof & wall), annual electricity consumption reduced by 17% to 25% per annum when added a vegetated layer. In addition to enhancing the indoor thermal comfort by 3 PMV values and indoor air temperature by 5°C