Nader A. El-Taweel

Voltage Regulation in Islanded Microgrids Using Distributed Constraint Satisfaction

Droop control is a key control method for operating islanded microgrids (IMGs). The settings of the droop parameters for distributed generation (DG) units can considerably affect the ability of an IMG to satisfy the required voltage tolerance boundary prescribed in steady-state voltage regulation standards. This paper analyzes the complexity of voltage regulations in droop-controlled IMGs. A new algorithm is proposed to satisfy the voltage regulation requirements of IMGs. The proposed algorithm obviates the need for a centralized secondary controller, where each DG unit updates its own voltage droop parameters, autonomously, via interaction with other DG units, using a low-bandwidth, peer-to-peer communication network. To that end, a distributed constraint satisfaction approach is adopted to formulate the problem of voltage regulation in a multi-agent environment. An asynchronous weak commitment technique is proposed to solve the formulated problem. Several case studies are simulated to evaluate the performance of the proposed algorithm. The results show that the proposed algorithm can effectively mitigate the challenges of voltage regulation in IMG systems.

Operation challenges of feeder shunt capacitors in islanded microgrids

Active distribution systems are moving towards a new paradigm shift, where they can be clustered into microgrids capable of operating in both grid-tied and islanded modes depending on the penetration levels and types of the distributed generation (DG) units. Previous studies put on view lots of advantages and concerns for microgrids operation in islanded mode, whether it is initiated for emergency, intentionally planned or permanent island system purposes. One of the concerns that have not been addressed yet, is the functionality of the existing feeders shunt capacitors (FSCs) when microgrids operate in islanded mode. Hence, this paper investigates the operation conflicts between DG units and FSCs during the islanded microgrid mode of operation. These operation conflicts have been validated through conducting simulations for different local control schemes of FSCs. The results show that major voltage regulation and reactive power control problems might arise when DG units are droop-controlled and FSCs utilize conventional control schemes. Further, a new local control scheme for FSCs has been proposed to mitigate their operational challenges in islanded microgrids.

Optimal design of charging stations for electrified transit networks

In this paper, an optimization model is developed for the design of charging stations in fully electrified transit networks. The proposed model aims to optimize the charging station infrastructure prerequisites: charger size and number of the chargers. It can be applied for different charging concepts, which will result in addressing the operational feasibility of battery electric buses (BEBs), and the procurement of the BEBs charging profile. To that end, three distinct charging concepts are considered in this work, which are based on current transit industry: flash, opportunity, and overnight. The model is tested for three full transit networks with different fleet sizes in Ontario, Canada. The results show that charging concepts and fleet sizes affect significantly the design of charging stations.

A ZV-ZCS electrolytic capacitor-LessAC/DC isolated LED driver with continous energy regulation

Conventional AC/DC LED drivers require a large energy storage capacitor at the output to provide a constant current to the LEDs. In order to minimize the size and cost of the driver circuit, electrolytic capacitors are conventionally used due to its high energy density and low cost. However, electrolytic capacitors are sensitive to operating temperature and have much shorter lifetime than the LED semiconductor devices, which significantly reduces the overall life time of the LED system. Another drawback with the current LED drivers is that the presence of the switching power losses restricts the use of high frequency operation, which results in using bulky passive circuit components in the drivers and significantly reduces the circuit power efficiency. This paper proposes a single-stage high power factor LED driver with almost zero switching losses and without the electrolytic capacitor. In the proposed circuit, discontinuous conduction mode (DCM) boost converter was utilized as a power factor correction (PFC) circuit, where it was integrated with an asymmetrical pulse width modulated (APWM) series resonant converter to form a single stage power conversion unit to drive the LEDs. The proposed circuit is able to achieve zero turn-on and turn-off switching operation and is able to eliminate the conventionally needed electrolytic capacitors by continuously regulating the DC-link voltage. The proposed LED driver was simulated and tested on a 12W design example to confirm that an almost unity power factor and an efficiency of 95% can be achieved.

Integration of step voltage regulators in islanded microgrids

Step voltage regulators (SVRs) are the workhorses of existing distribution feeders, where they provide appropriate voltage control to meet the customer demands. Given the special features and operation characteristics of islanded microgrids (IMGs), the role of SVRs during the operation of distribution networks in islanded mode has not been well investigated, yet. The first step to conduct such investigation is to incorporate the model of SVRs in the power flow studies of IMGs. As such, this paper presents the incorporation of SVRs in a generic droop-based three-phase power flow algorithm of IMGs. The paper also studies the interferences between the local controllers of SVRs and droop-controlled distributed generation units to regulate the voltage in IMGs at different operating conditions.

An Output-Current-Dependent DC-Link Energy Regulation Scheme for a Family of Soft-Switched AC/DC Offline LED Drivers Without Electrolytic Capacitors

In order to provide a constant current to the Light-emitting diodes (LEDs) and to minimize the size and cost of the driver circuit in ac/dc offline LED drivers, electrolytic capacitors are conventionally used due to their high energy density and low cost. However, electrolytic capacitors are sensitive to operating temperature and have much shorter lifetime than the LED semiconductor devices. This paper proposes an improved control scheme that is capable to control the output LED current and regulate the dc-link capacitors energy simultaneously for a family of single-stage soft-switched high power factor LED driver topologies. Each of these topologies consists of an integrated power factor correction (PFC) circuit. Due to the proposed controllers capability to control the LED current directly via variable frequency control and the dc-link capacitors voltage via duty ratio control, the required energy storage capacitance can be significantly reduced, thereby allowing small size film capacitor to be used as the energy storage capacitor. Simulation results are provided on two different LED driver topologies, one with an integrated boost PFC stage and the other one with an integrated buck–boost PFC stage, to verify the performance of the proposed control scheme. Experimental results are also been provided on a 12-W prototype to highlight the merits of this paper.