A.M. Rahimi

Bio: A.M. Rahimi is an academic researcher from Illinois Institute of Technology. The author has contributed to research in topics: Converters & Power electronics. The author has an hindex of 9, co-authored 12 publications receiving 1000 citations.

Active Damping in DC/DC Power Electronic Converters: A Novel Method to Overcome the Problems of Constant Power Loads

Abstract: Multi-converter power electronic systems exist in land, sea, air, and space vehicles. In these systems, load converters exhibit constant power load (CPL) behavior for the feeder converters and tend to destabilize the system. In this paper, the implementation of novel active-damping techniques on dc/dc converters has been shown. Moreover, the proposed active-damping method is used to overcome the negative impedance instability problem caused by the CPLs. The effectiveness of the new proposed approach has been verified by PSpice simulations and experimental results.

Loop-Cancellation Technique: A Novel Nonlinear Feedback to Overcome the Destabilizing Effect of Constant-Power Loads

Abstract: Tightly regulated closed-loop converters are problematic when used as a load since they tend to draw constant power and exhibit negative incremental resistance. This negative resistance causes stability problems for the feeder system, whether it is an input filter or another converter. In multiconverter power electronic systems, which exist in different land, sea, air, and space vehicles, including electric, hybrid, plug-in hybrid, and fuel-cell vehicles, there are many converters loaded by other converters. Therefore, the destabilizing effect of the load converters, which are called constant-power loads, is a major issue. In this paper, a novel nonlinear feedback, which is called loop cancellation, is introduced. This technique is used to cancel the destabilizing effect of the constant-power loads. Theoretically, any amount of constant-power load can be compensated by this technique, and it can identically be implemented on different types of converters. The effectiveness of the proposed technique has been verified by PSpice simulations and experimental results.

An Analytical Investigation of DC/DC Power Electronic Converters With Constant Power Loads in Vehicular Power Systems

Abstract: Stability of multi-converter power systems, which exist in advanced more electric vehicles, is of great importance. The stability issue is investigated in this paper, and design considerations and limitations of the methods that stabilize the open-loop converters are presented. By stabilizing the open-loop converter, it is shown that the feedback loop design is usually translated into a conventional feedback design task. The behavior of the unstable converter is also discussed, and a method for decreasing the amplitude of the output voltage oscillations is proposed. The model of a tightly regulated practical converter is presented. This model is used to decide how much damping should be added to make the feeder converter stable. Furthermore, because we have this information about the load converter, the feeder converter can be redesigned so that it does not see the load converter as a constant power load.

Modified Pulse-Adjustment Technique to Control DC/DC Converters Driving Variable Constant-Power Loads

Abstract: Multiconverter-distributed DC architectures have been utilized for power distribution in many applications such as telecommunication systems, sea and undersea vehicles, an international space station, aircraft, electric vehicles, hybrid-electric vehicles, and fuel-cell vehicles, where reliability is of prime concern. The number of power-electronic converters (AC/DC, DC/DC, DC/AC, and AC/AC) in these multiconverter electrical power systems varies from a few converters in a conventional land vehicle, to tens of converters in an advanced aircraft, and to hundreds of converters in the international space station. In these advanced applications, power-electronic converters might need to have a tight output-voltage regulation. From the output perspective, this property is highly desirable. However, since power-electronic converters are efficient, tight regulation of the output makes the converter appear as a constant-power load (CPL) at its input side. Dynamic behavior of CPLs is equivalent to negative impedance and, therefore, can result in instability of the interconnected power system. In order to mitigate the instability of the power converters loaded by CPLs, this paper presents the pulse-adjustment digital control technique. It is simple and easy to implement in application-specific integrated circuits, digital-signal processors, or field-programmable gate arrays. Moreover, its dynamic response is fast and robust. Line and load regulations are simply achievable using this technique. Analytical, as well as simulation and experimental results of applying the proposed method to a DC/DC buck-boost converter confirm the validity of the presented technique.

Digital Control of an Isolated Active Hybrid Fuel Cell/Li-Ion Battery Power Supply

Abstract: One of the important issues in modern electronic equipment is providing higher peak power while preserving high energy density. Hybrid power sources composed of fuel cells and batteries combine the high-energy capabilities of fuel cells with the high-power capabilities of batteries. DC/DC power converters can appropriately control the power flow shared between the fuel cell and battery system. In this paper, we propose a new implementation of hybrid fuel cell/battery systems. A 50-W isolated power electronic interface system for a hybrid solid-oxide fuel cell (SOFC)/Li-ion battery portable power supply is designed and implemented. The control strategy presented in this paper is able to regulate the output current of the fuel cell, the charging current of the battery, and the output voltage of the power supply. The control strategy is implemented in a DSP and tested by simulation and experiments. Experimental results present the flexibility and generality of the control strategy.

Battery, Ultracapacitor, Fuel Cell, and Hybrid Energy Storage Systems for Electric, Hybrid Electric, Fuel Cell, and Plug-In Hybrid Electric Vehicles: State of the Art

Abstract: The fuel economy and all-electric range (AER) of hybrid electric vehicles (HEVs) are highly dependent on the onboard energy-storage system (ESS) of the vehicle. Energy-storage devices charge during low power demands and discharge during high power demands, acting as catalysts to provide energy boost. Batteries are the primary energy-storage devices in ground vehicles. Increasing the AER of vehicles by 15% almost doubles the incremental cost of the ESS. This is due to the fact that the ESS of HEVs requires higher peak power while preserving high energy density. Ultracapacitors (UCs) are the options with higher power densities in comparison with batteries. A hybrid ESS composed of batteries, UCs, and/or fuel cells (FCs) could be a more appropriate option for advanced hybrid vehicular ESSs. This paper presents state-of-the-art energy-storage topologies for HEVs and plug-in HEVs (PHEVs). Battery, UC, and FC technologies are discussed and compared in this paper. In addition, various hybrid ESSs that combine two or more storage devices are addressed.

DC Microgrids—Part I: A Review of Control Strategies and Stabilization Techniques

Abstract: This paper presents a review of control strategies, stability analysis, and stabilization techniques for dc microgrids (MGs). Overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level. As opposed to local control, which relies only on local measurements, some line of communication between units needs to be made available in order to achieve the coordinated control. Depending on the communication method, three basic coordinated control strategies can be distinguished, i.e., decentralized, centralized, and distributed control. Decentralized control can be regarded as an extension of the local control since it is also based exclusively on local measurements. In contrast, centralized and distributed control strategies rely on digital communication technologies. A number of approaches using these three coordinated control strategies to achieve various control objectives are reviewed in this paper. Moreover, properties of dc MG dynamics and stability are discussed. This paper illustrates that tightly regulated point-of-load converters tend to reduce the stability margins of the system since they introduce negative impedances, which can potentially oscillate with lightly damped power supply input filters. It is also demonstrated that how the stability of the whole system is defined by the relationship of the source and load impedances, referred to as the minor loop gain. Several prominent specifications for the minor loop gain are reviewed. Finally, a number of active stabilization techniques are presented.

DC Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues

Abstract: DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.

Overview of Power Management Strategies of Hybrid AC/DC Microgrid

Abstract: Today, conventional power systems are evolving to modern smart grids, where interconnected microgrids may dominate the distribution system with high penetration of renewable energy and energy storage systems. The hybrid ac/dc systems with dc and ac sources/loads are considered to be the most possible future distribution or even transmission structures. For such hybrid ac/dc microgrids, power management strategies are one of the most critical operation aspects. This paper presents an overview of power management strategies for a hybrid ac/dc microgrid system, which includes different system structures (ac-coupled, dc-coupled, and ac–dc-coupled hybrid microgrids), different operation modes, a thorough study of various power management and control schemes in both steady state and transient conditions, and examples of power management and control strategies. Finally, discussion and recommendations of power management strategies for the further research are presented.

Supervisory Control of an Adaptive-Droop Regulated DC Microgrid With Battery Management Capability

Abstract: DC power systems are gaining an increasing interest in renewable energy applications because of the good matching with dc output type sources such as photovoltaic (PV) systems and secondary batteries. In this paper, several distributed generators (DGs) have been merged together with a pair of batteries and loads to form an autonomous dc microgrid (MG). To overcome the control challenge associated with coordination of multiple batteries within one stand-alone MG, a double-layer hierarchical control strategy was proposed. 1) The unit-level primary control layer was established by an adaptive voltage-droop method aimed to regulate the common bus voltage and to sustain the states of charge (SOCs) of batteries close to each other during moderate replenishment. The control of every unit was expanded with unit-specific algorithm, i.e., finish-of-charging for batteries and maximum power-point tracking (MPPT) for renewable energy sources, with which a smooth online overlap was designed and 2) the supervisory control layer was designed to use the low-bandwidth communication interface between the central controller and sources in order to collect data needed for adaptive calculation of virtual resistances (VRs) as well as transit criteria for changing unit-level operating modes. A small-signal stability for the whole range of VRs. The performance of developed control was assessed through experimental results.