Showing papers by "Srdjan Lukic published in 2019"

Extreme Fast Charging of Electric Vehicles: A Technology Overview

Abstract: With the number of electric vehicles (EVs) on the rise, there is a need for an adequate charging infrastructure to serve these vehicles. The emerging extreme fast-charging (XFC) technology has the potential to provide a refueling experience similar to that of gasoline vehicles. In this article, we review the state-of-the-art EV charging infrastructure and focus on the XFC technology, which will be necessary to support the current and future EV refueling needs. We present the design considerations of the XFC stations and review the typical power electronics converter topologies suitable to deliver XFC. We consider the benefits of using the solid-state transformers (SSTs) in the XFC stations to replace the conventional line-frequency transformers and further provide a comprehensive review of the medium-voltage SST designs for the XFC application.

Toward Extreme Fast Charging: Challenges and Opportunities in Directly Connecting to Medium-Voltage Line

Abstract: With an increasing awareness of the detrimental effects of the fossil fuel?based transportation sector, which accounts for 14% of human-made greenhousegas emissions globally, individuals, companies, and government entities have made a concerted push to develop solutions to provide modes of transportation that are less carbon intensive. Importantly, several countries, including Norway, India, France, and Britain, have decided to end the sale of internal combustion engine cars in the near future, further accelerating the shift to electric transportation.

Dynamic Adaptive Protection for Distribution Systems in Grid-Connected and Islanded Modes

Abstract: Reliable protection of distribution systems is becoming increasingly challenging with growing distributed energy resources (DERs) and microgrids. Existing adaptive overcurrent relaying (AOCR) schemes are vulnerable to DER ride-through and generally focus on only grid or islanded modes. This paper presents a dynamic AOCR scheme independent of external controllers to estimate relay pickup that ensures significantly less communication overhead. Existing and emerging challenges for the state-of-the-art are discussed and resolved using the proposed scheme. The reliability is tested using transient system models in Simulink for all combinations of state and fault types/impedances in a 15-bus distribution system.

Distributed Secondary Control Strategy for Microgrid Operation with Dynamic Boundaries

Abstract: Microgrids (MGs) with dynamic boundaries, also known as dynamic MGs, are able to support critical loads without energization from utility and allow system topology variation upon request. Utilization of dynamic MGs can provide more flexible solutions toward distribution system restoration from natural disasters. This paper proposes a distributed secondary control strategy for dynamic MG operation under both static topology and topology variation. The proposed control strategy aims to guarantee seamless transitions during dynamic MG reconfiguration and proper power management among distributed generators (DGs) that are grouped dynamically. Smart switches (SSWs) are utilized to identify, process, and implement the reconfiguration request. The proposed control strategy provides a framework to coordinate the operation of multiple DGs in neighboring autonomous MGs and determine the operation status of SSWs, so that no transient over-shoots are introduced as SSWs operate and DGs are able to support the system power demand proportionally. Detailed controller designs are provided. Sufficient conditions under which the proposed controllers are exponentially stable are derived and the dynamic performance of the proposed controller are validated by comprehensive case studies in MATLAB/Simulink.

Combined Foreign Object Detection and Live Object Protection in Wireless Power Transfer Systems via Real-Time Thermal Camera Analysis

Abstract: The increased penetration of electric vehicles (EV) into the market has driven the research and development of more efficient and viable wireless power transfer (WPT) systems as a means of charging. The safe operation of commercial wireless charging EV stations requires both foreign object detection (FOD) and live object protection (LOP) capabilities. In this study, a real-time thermal imaging based combined FOD-LOP system, utilizing a single sensing device is proposed. Experimental results validating the systems ability in detecting foreign metal objects and representative live objects, under varying conditions, is presented.

A 12.47 kV Medium Voltage Input 350 kW EV Fast Charger using 10 kV SiC MOSFET

Abstract: This paper presents a medium-voltage (MV) (12.47 kV), 350 kW electric vehicle (EV) fast charger using 10 kV SiC MOSFETs. Detailed system design procedure based on the 10 kV SiC MOSFET characterization is presented to provide a guide on the 10 kV SiC MOSFET converter development. Taking the advantage of the 10 kV SiC MOSFET’s high voltage blocking capability and efficient switching performance, a single module high power density system is designed with the DC/DC stage operates at 25 kHz and the simulated system efficiency exceeds 98%, input current THD lower than 2%. With all the passive components selected, the designed system power density is 1.6 kW/L.

Distributed Control Strategy to Achieve Synchronized Operation of an Islanded MG

Abstract: To seamlessly transition a microgrid (MG) from islanded to grid-connected mode, it is necessary to synchronize the magnitude, frequency, and phase of the MG voltage to the voltage of the main grid. In this paper, we propose a distributed control strategy to achieve synchronized operation of an islanded MG supported by multiple controllable distributed generators (DGs). The proposed method utilizes a pinning-based consensus algorithm to ensure explicit coordination between magnitude, frequency, and phase angle regulation while ensuring proportional power sharing. System frequency is regulated by all the DGs in proportion to their capacity, while a selected DG eliminates the phase and magnitude regulation errors. Controller design criteria is based on small-signal stability analysis. The proposed control strategy is implemented in hardware controllers and its effectiveness is demonstrated using a real-time hardware-in-the-loop MG testbed.

Passivity-Oriented Discrete-Time Voltage Controller Design for Grid-Forming Inverters

Abstract: Passivity theory provides a promising approach to guarantee microgrid system stability. If all converters in the system can be made passive, prevention of electrical resonance can be achieved. To that end, a passivity-oriented discrete-time voltage controller for grid-forming inverters is proposed in this –paper. Compared with existing methods, the approach ensures not only –superior reference tracking performance and load disturbance rejection capability, but also provides passive output impedance, and therefore, guaranteed stable inverter operation under weak grid condition. A comparison study is carried out and the proposed controller design method is validated by hardware-in-the-loop (HIL) experiments.

Current Injection Methods for Ripple-Current Suppression in Delta-Configured Split-Battery Energy Storage

Abstract: Cascaded H-bridge (CHB) converters are receiving growing attention in battery energy storage systems (BESS) due to their modularity and flexibility. However, direct generation of ac output in CHB-BESSs incurs large second-order current ripple in the batteries, which causes additional loss and might accelerate battery aging. Existing methods for ripple-current suppression usually require bulky passive components due to the high energy content of the ripple components. This paper presents a class of current injection methods for delta-configured CHB-BESSs. The injected currents flow through the CHB arms and transfer the original second-order oscillating power to the fourth or the sixth order, or even to an arbitrarily high-order frequency. As such, the battery current ripple appears at much higher frequencies with lower oscillating energy and can be easily filtered by small passive components. In the laboratory setup, the proposed methods reduced the battery root-mean-square current ripple to less than 10% of the dc component with negligible distortion in the loads. The proposed methods and the filter implementations show good tolerance to load frequencies and to the control error of the injected currents.

Device Access Abstractions for Resilient Information Architecture Platform for Smart Grid

Abstract: This letter presents an overview of design mechanisms to abstract device access protocols in the resilient information architecture platform for smart grid, a middleware for developing distributed smart grid applications. These mechanisms are required to decouple the application functionality from the specifics of the device mechanisms built by the device vendors.

Auxiliary Power Supply for Medium-Voltage Power Converters: Topology and Control

Abstract: This paper presents an isolated auxiliary power supply for medium-voltage power electronics systems. The proposed converter comprises two stages: a non-isolated ac/dc stage that connects directly to the medium-voltage line, and an isolated dc/dc stage that provides 100-W output power at 24 V, with 10 kV isolation. The proposed modular ac/dc stage uses just one active semiconductor device per module, features an internal capacitor voltage balancing, and achieves power factor correction by employing predictive current control. High switching frequency operation of both converter stages enable a reduction in system size and weight when compared to traditional low-frequency transformer-based approach. The proposed converter is simulated and its operation is validated experimentally on a 100-W prototype.

A Distributed Control Architecture for Cascaded H-Bridge Converter

Abstract: Cascaded H-bridge topology has been used in grid-tied converter for battery energy storage system due to its modular structure. To fully utilize the converter’s modularity, we propose a hierarchical distributed control architecture that consists of primary control and secondary control. Primary control ensures correct current tracking, but may lead to unequal power output among modules. A distributed secondary control based on consensus algorithm is presented to establish equal power sharing among modules. Further, a carrier phase shift control is implemented to achieve multilevel output voltage and harmonic reduction. Finally, the effectiveness of the presented control strategy is verified through experiments.

Observer Based Admittance Shaping for Resonance Damping in Voltage Source Converters with LCL Filter

Abstract: An observer based resonance damping controller is proposed for voltage source converters (VSCs) with LCL input filter connected to any arbitrary AC network. In prior art, design and analysis of observer based current controllers has been performed either in z-domain taking a discrete approximation of the LCL filter or in continuous-time but with the observer discretized for digital implementation; both methods introduce significant modelling error in the high frequency range. We use a continuous-time equivalent of the discrete observer and develop a comprehensive model of the VSC including the effects of discretization, controller delay, and PWM process. Leveraging the model, we propose a dual-loop current controller with an inner admittance shaping loop that achieves internal stability of the VSC by damping the LCL resonance without the need for additional sensors; moreover, passive converter admittance is achieved almost up to the Nyquist frequency which provides sufficient condition for stability for any uncertain network impedance that may lead to harmonic resonance within that frequency range. The resonance damping capability of the proposed controller is validated through hardware experiments.

High-isolation Low-coupling-capacitance Standalone Gate Drive Power Supply for SiC-based Medium-Voltage Power Electronic Systems

Abstract: This paper presents a standalone gate drive power supply for medium-voltage SiC-based power electronic systems. The developed power supply is rated at 10 W and provides regulated 24 V at its output with output short-circuit protection. It features high-isolation working voltage of 10 kV RMS, creepage distance of over 100 mm and coupling capacitance of 1 pF between its input and the output terminals. The power supply has 4 times smaller coupling capacitance and 2.6 times higher power density compared to the commercial product.

A low-inductance Sectional Busbar for Snuberless Operation of SiC-based EV Traction Inverters

Abstract: This paper presents a low-inductance dc busbar for SiC-based electric vehicle traction inverters operating at high ambient temperatures. Unlike the laminated busbars, which use parallel copper sheets to establish low-inductance connection between the dc-link capacitor bank and all the switching devices, the proposed busbar uses capacitor sections which are vertically mounted on top of each power semiconductor module. This significantly reduces the busbar’s parasitic inductance and eliminates the unreliable snubber circuit which is typically used with laminated and PCB busbars to reduce the overvoltage spikes across the fast-switching semiconductor devices during the device turn-off transients.

Optimization of Medium Frequency Transformers with Practical Considerations

Abstract: A method to design and optimize a Medium Frequency Transformer (MFT) based on commercially available components and semiconductor and converter constraints is presented. The optimization algorithm is used to redesign the transformer for a scaled-down electric vehicle (EV) fast charger using a three-level resonant circuit topology. The main consideration of this paper is the uncertainty caused by modeling assumptions in optimization algorithms. To reduce this uncertainty, space mapping is used to create an optimized design point. Finally, a comparison of the designs found using the original optimization algorithm and the space mapping technique are compared and analyzed. For simplicity, only a single objective optimization routine is employed.

Restart Strategy for Scalar ( V/f ) Controlled Synchronous Reluctance Machine Driving a High-Inertia Load

Abstract: Momentary power disruptions commonly occur in many industrial settings, and can trip large electric machines, which must then be brought to zero speed before the machine can be restarted. The resulting frequent interruptions in industrial processes can have negative effects on productivity. A restart algorithm brings the machine back to the original speed as soon as power is restored, without waiting for the machine to reach zero speed. This paper proposes an approach for implementing a restart algorithm for synchronous reluctance machines (SynRMs) that use a simple identification algorithm to determine the speed and position of the machine so that the appropriate voltage vector can be applied, thus minimizing the inrush current during the restart. The proposed method only requires nameplate machine parameters and needs no machine-specific tuning, making the approach suitable for implementation with both high-inertia vector and scalar-controlled SynRM.

Restart Strategy for Scalar Controlled Synchronous Reluctance Machine Driving a High-Inertia Load with a Single DC-link Current Sensor

Abstract: This paper proposes an approach for implementing the restart for Synchronous Reluctance Machines (SynRMs) driving a high inertia load. The proposed method uses a simple identification algorithm to estimate the rotor speed and position of the machine so that the appropriate voltage vector can be applied to the machine, thus minimizing the inrush current at the instant of the restart. The novelty is that its implementation only requires nameplate machine parameters and a single DC-link current sensor. The proposed method is suitable for implementation with both high-inertia vector and scalar-controlled SynRMs.

Optimization of DC-Link Decoupling Snubber Circuit for SiC-based EV Traction Inverters

Abstract: DC-link decoupling snubber circuit is usually placed close to the power switches in power converters to suppress voltage ringing on the devices during the device turn-off transient. In this paper, the snubber circuit performance is analyzed numerically based on a detailed forth-order high-frequency equivalent circuit of a double pulse test circuit. The approach to optimize the snubber circuit, not only for the voltage spike suppression but also for the good thermal performance, is proposed. The thermal analysis and simulation results reveal that an additional resistor in the snubber circuit can release the thermal stress on the capacitors, which makes it more reliable at high switching frequency and high-power applications. Finally, the performance of an optimized snubber circuit is confirmed in a 100-kW traction inverter system operating at 40 kHz switching frequency and 105°C ambient temperature.

Circulating Current Suppression in Multi-cell Series-parallel Converter for Cost-effective Medium-voltage Solid-state transformer

Abstract: This work presents a cost-effective and compact medium-voltage (MV) solid-state transformer (SST). The pro-posed SST is rated at 5kW, and provides a low voltage dc output serving dc loads and sources such as batteries, photovoltaics, and household dc loads. The proposed multi-cell topology features an internal capacitor voltage balancing and requires only one dc-link voltage sensor. A major challenge of the proposed topology is the unbalanced circulating currents when using the conventional interleaved pulse-width modulated (PWM) scheme. The unbalanced circulating currents cause uneven component stress, and may require different current rating among the switch devices. By analyzing non-ideal equivalent circuits of the converter, we propose a new PWM scheme that minimizes and balances the circulating currents, thus increasing system efficiency. The proposed modulation scheme is verified on a full scale prototype.

High Resolution Signal Generation and Management in Fast Switching Power Electronic Systems

Abstract: Advancements in power electronics necessitate new developments in technologies interfacing it correspondingly. Signal generation and measurement are among these peripheral technologies. Higher performance of new power devices have made higher switching frequencies and higher ratings of power converters realistic. These new opportunities require high-speed switching transients, which causes new settings at source and measurement ends. In this study, we have introduced a new figure of merit for high-speed signals, which are generated or measured. This figure of merit improves accuracy in comparisons and rigidizes specifications for signals. In addition, we studied high speed signal generation methods. Most methods cannot generate a signal with high figure of merit without increasing power consumption and area. We verified capability of a leading choice. The demonstrated signal is ultra-narrow with duty cycle less than 2%. Subsequently, high-speed signals need superior considerations for accurate measurement. Effects and solutions in accurate measurement of high horizontal resolution and high speed signals have been presented. Furthermore, considerations for noise in measurements is discussed.

A New Distributed Voltage Controller for Enabling Volt-Var Support of Microgrids in Grid-Connected Operation

Abstract: Voltage support is one of ancillary services that can be provided by grid-connected microgrids. This paper presents a new microgrid-level voltage controller to enable microgrids to contribute to distribution system voltage regulation. Like the Volt-Var droop control of the smart inverters, the proposed controller makes sure that the voltage and reactive power flow at microgrid point of common coupling (PCC) follows a proportional relationship. To increase flexibility and avoid centralized computation, a distributed controller design is developed using the distributed averaging algorithm in order to achieve reactive power sharing among multiple distributed energy resources (DERs) within the microgrid. A large signal analysis is performed to study the stability and dynamic characteristics of the closed-loop system. Simulation results show that the proposed controller is effective in providing voltage regulation for distribution systems with multi-microgrid architecture.

Cascaded modular multilevel converter for medium-voltage power electronics systems

Abstract: A cascaded modular multilevel converter for medium-voltage power electronics systems is disclosed. The converter can be used in applications such as an auxiliary power supply for medium-voltage (MV) power electronics system, or a unidirectional SST which powers a home (where very light load conditions are experienced when the home is unoccupied or during the night). Unlike the traditional solutions which use a grid-frequency, bulky, and heavy power transformers, the disclosed converter can operate at higher switching frequencies, weigh less, and provide a higher power density than other approaches. Additionally, the disclosed converter features an internal capacitor voltage balancing and can achieve power factor correction (PFC) using predictive control.

Wireless Charging System for an Electric Autonomous Micro-transit Transportation Vehicle

Abstract: This paper details the design, testing and implementation of a wireless charging system for an autonomous micro-transit transportation (AMT) vehicle. Specifically, the paper covers the topology, controls, preliminary experimental results and integration approach within the Ecological personal transport (EcoPRT) autonomous vehicle. The SAE J2954 standard was used as a guideline for the systems overall mechanical and electrical design. The described system layout and methodology may be used in other autonomous charging applications in which power usage is high and frequent charging is required. Examples of this include autonomous vehicles used for both transportation and industrial applications. Experimental results of a scaled down lab set up are reported, detailing closed loop control of the constant current charging process and zero voltage switching behavior of the inverter output.