Xiaohu Liu

Coordinated Control of Distributed Energy Storage System With Tap Changer Transformers for Voltage Rise Mitigation Under High Photovoltaic Penetration

This paper proposes a coordinated control of distributed energy storage system (ESS) with traditional voltage regulators including the on-load tap changer transformers (OLTC) and step voltage regulators (SVR) to solve the voltage rise problem caused by the high photovoltaic (PV) penetration in the low-voltage distribution network. The main objective of this coordinated control is to relieve the tap changer transformer operation stress, shave the distribution network peak load and decrease the transmission and distribution resistive power losses under high solar power penetration. The proposed control method limits the energy storage depth of discharge in order to meet a more than ten-year cycle life. A benchmark distribution network model was developed in the Real Time Digital Simulator (RTDS) and the simulation results from the studied cases verified the proposed coordinated control strategy. The experimental implementation of proposed control algorithms were developed based on a power hardware-in-the-loop (PHIL) test bed with a 22 kWh ESS, a smart meter, Labview controller, and RTDS. The experimental results were consistent with those obtained from simulation study.

Distributed Digital Control of Modular-Based Solid-State Transformer Using DSP+FPGA

This paper proposes a distributed digital control architecture of a modular-based solid-state transformer (SST) using a digital signal processor (DSP) and a field-programmable gate array (FPGA). In particular, the three-stage SST based on a modular structure is the topology of most interest because of its superior controllability. In order to make the modular-based SST, the digital implementation is inevitable to achieve higher performances, improved reliability, and an easy development. In addition, the modular-based SST requires enough capacity for implementing complex control algorithms, multiple interfaces, and a large number of internal variables. In this paper, a digital control platform for the modular-based SST is built using a floating-point DSP and an FPGA that operate cooperatively. As a result, the main control algorithms are performed by the DSP, and the simple logical processes are implemented in the FPGA to synthesize the suitable gating signals and control external devices, respectively. The proposed implementation method enables high-switching-frequency operation, multitasking, and flexible design for the modular-based SST. Experimental results are presented to verify the practical feasibility of the proposed technique for the modular-based SST.

A Start-Up Scheme for a Three-Stage Solid-State Transformer With Minimized Transformer Current Response

This letter is focused on developing a start-up scheme for a three-stage solid-state transformer that includes a high-frequency transformer. The proposed scheme is aiming at minimizing the high-frequency transformer current during the start-up transient. As a result, the input-inrush current is also eliminated. The scheme is implemented with no extra cost by synchronizing the start-up of rectifier stage and dc-dc converter stage. The scheme is compared with other start-up methods. The theoretical analysis and experimental results are provided to verify the proposed start-up scheme.

An Electrolytic-Capacitor-Free Single-Phase High-Power Fuel Cell Converter With Direct Double-Frequency Ripple Current Control

This paper proposes a direct double-frequency ripple current control in a single-phase high-power fuel cell converter that can achieve low-frequency ripple-free input current without using large electrolytic capacitors. To eliminate the double-frequency ripple current disturbance introduced by the single-phase inverter load, a proportional-resonant controller is developed to achieve an extra high control gain at designed resonant frequency. This high gain can be viewed as the virtual high impedance for blocking the double-frequency ripple energy propagation from inverter load to fuel cell stack. More particularly, the proposed control system can realize the utilization of all capacitive ripple energy sources in the system by regulating all the capacitors to have large voltage swing. In addition, this voltage swing is synchronized to keep real-time balancing of the transformer primary- and secondary-side voltages. As a result, the zero-voltage-switching operation for all switching devices in the dc-dc stage can be guaranteed. The controller design guidelines are derived based on the system small-signal model. The experimental results are presented to validate the theoretical analysis and proposed technology.

A new fuel cell power conditioning system with extended life time and minimized DC-bus capacitor

This paper proposes a new fuel cell power conditioning system based on the current-fed dual-half-bridge (CF-DHB) dc-dc converter with extended life time and minimized dc bus capacitor. The traditional minimizing dc bus capacitor method is to adopt the dc active filter by using the dc chopper with active switches. The unique contribution of this paper is to minimize the dc bus capacitor without adding any components. Moreover, the CF-DHB converter realizes zero-voltage-switching (ZVS) operation for all switching devices without auxiliary circuits, resulting in the high-efficiency operation. A 1kW fuel cell power conditioning system experimental results were presented to validate the theoretical analysis and prove the advanced performance of the proposed fuel cell power conditioning system.

A new three-phase high-power soft-switched DC-DC converter based fuel cell power conditioning system with minimized DC capacitor

This paper proposes a new fuel cell power conditioning system based on the three-phase high-power soft-switched dc-dc converter with minimized dc capacitor. The traditional fuel cell system minimizes the dc capacitor by adopting the dc active filter which consists of the dc chopper with active switches. The unique contribution of this paper is to minimize the dc capacitor by using the proposed pulsation power decoupling control with no extra cost. Moreover, the dc-dc converter adopts three-phase interleaved structure based current-fed dual-active-bridge (CF-DAB) converter, resulting in much smaller input current ripple and passive component. The CF-DAB converter realizes zero-voltage-switching (ZVS) operation for all switching devices without auxiliary circuits, resulting in the high-efficiency operation. Simulation and experimental results were presented to validate the theoretical analysis and advanced performance.

Study on the start-up schemes for the three-stage solid state transformer applications

This paper investigates two stage-by-stage start-up schemes for a three-stage solid state transformer (SST) that includes a high frequency transformer. The DC-DC converter transformer current dynamics analysis presented in this paper shows that the stage-by-stage start-up scheme may lead to a relatively large transformer current. As a result, the DC-DC converter would suffer a relatively large input inrush current. The pre-charge start-up scheme can improve the transformer current profile during the transient. The detailed analysis of those two start-up scheme is presented and the experimental verification on laboratory prototype proved the analysis of stage-by-stage start-up scheme.

Power electronics enabled energy management for energy storage with extended cycle life and improved fuel economy in a PHEV

Active combination of the ultracapacitor (UC) with an energy dense Lithium-ion (Li-ion) battery (BU) is seen as the promising approach in plug-in hybrid electric vehicle (PHEV) application. In this paper, the sizing of Li-ion battery and ultracapacitor, as well as the degree of hybridization (DH) between UC power and battery power are analyzed through a different perspective, i.e., to solve an optimization problem targeting maximized fuel economy. To implement this optimized power sharing in real time, a novel energy management strategy is proposed, which includes battery power reference generation, UC state-of-charge (SOC) regulation, and forecast control according to the driver command. Finally, simulation and experimental results are provided to verify the reduced battery current stress and improved fuel economy by the proposed method, in which flywheel + generator + controlled load is used to emulate the vehicle drivetrain in the laboratory.

A PV Residential Microinverter With Grid-Support Function: Design, Implementation, and Field Testing

Microinverter-based photovoltaic (PV) systems now represent about 8% of the U.S. residential market, and offer many advantages including safety, performance, and simplified installation. The next-generation of PV microinverter will include more ancillary functions to support grid stability and reliability in more distributed generation smart-grid systems. A commercial ready PV microinverter not only focuses on efficiency and cost, but also on reliability, manufacturability, compliance of various grid-code, and electromagnetic interference regulations. This paper presents a detailed design and development process of a microinverter system from concept all the way to final commercial-ready prototype. Various design tradeoffs such as topology, control, filter solutions and power supplies, and mechanical packaging are provided. The required prototype testing and final system field tests are also presented. The presented design and test process intends to accelerate the future microinverter system design and development toward a commercial ready product.