Damith B. Wickramasinghe Abeywardana

Single-Phase Grid-Connected LiFePO

AC line integrated energy storage systems are attractive as they increase the system efficiency by reducing the number of required power processing stages. In this paper, operation of a recently proposed battery-supercapacitor hybrid energy storage system (HESS) comprising two DC/AC boost converters, battery, supercapacitors, grid connection, state of charge (SOC) estimation, and associated control systems is experimentally verified and further improved. The improvement is achieved by a phase-shifted interleaved operation of the boost converters. The proposed phase-shifted interleaved operation reduces the switching frequency current ripple component in both the battery and supercapacitor currents. Experimental results show that during the interleaved operation, the HESS operates as expected and allocates all fast current variations to the supercapacitor, while the battery responds to slow varying current demands. At the same time, the control system maintains the supercapacitor voltage at around a predefined value and the batterys SOC, estimated using an extended Kalman filter, is maintained within the specified SOC limits.

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A boost DC/AC converter is popular in AC line-integrated energy storage systems (ESSs) based on low-voltage DC sources such as battery, fuel cell, or supercapacitor. The direct DC/AC power conversion in the boost inverter introduces a second-order harmonic ripple current at the DC side of the boost converter, which leads to internal heating of the energy storage devices and degradation of their lifetime. In this paper, a novel current feedback method is proposed to mitigate the second-order harmonic current component at the DC side. The proposed method is able to significantly reduce the second-order harmonic component in the DC-side current of the boost inverter without increasing other harmonic components. Performance of the proposed method is validated on a single-phase grid-connected DC/AC boost inverter-based battery ESS experimental prototype.

Battery–Supercapacitor Hybrid Energy Storage System With Interleaved Boost Inverter

Filter-based battery-supercapacitor hybrid energy storage systems (HESSs) are popular as a way of extending battery lifetime by diverging the high-frequency power variations to the supercapacitor. However, when a traditional supercapacitor voltage controller (SCVC) is employed in the filter-based HESS, precise sizing of the supercapacitor as well as finding filter parameters for the power allocation are challenging due to nonlinearities. These problems can be circumvented by using a supercapacitor energy controller (SCEC) proposed in this paper. The paper presents a method for selection of the SCEC and filter parameters as well as precise sizing of the supercapacitor for a given application. The proposed method is experimentally verified on a single-phase grid-connected HESS used to smooth the power delivered to the grid at the point of common coupling. It is also shown that the size of the supercapacitor when using the SCEC is significantly lower than the one estimated for the traditional SCVC.

An Input Current Feedback Method to Mitigate the DC-Side Low-Frequency Ripple Current in a Single-Phase Boost Inverter

Single conversion stage DC/AC boost inverters are an attractive solution when integrating energy storage devices, such as a battery, fuel cell, or supercapacitor to a single-phase AC grid. However, a second-order harmonic current ripple appears at the DC side of the inverter increasing the internal heat and losses in the energy storage device and degrading its lifetime. In this paper, a rule-based controller is proposed to reduce such harmonic current ripple component. A key feature and advantage of the proposed controller is its ability to reduce the ripple current amplitude in all four inverter output power operating quadrants without being affected by the capacitor tolerances and the internal resistance of the inductors. Presented experimental results validate the performance of the proposed controller on a single-phase grid-connected DC/AC boost inverter-based battery energy storage system.

Supercapacitor Sizing Method for Energy-Controlled Filter-Based Hybrid Energy Storage Systems

The boost-inverter-based battery-supercapacitor hybrid energy storage systems (HESSs) are a popular choice for the battery lifetime extension and system power enhancement. Various sliding mode (SM) controllers have been used to control the boost inverter topology in the literature. However, the traditional SM controllers for the boost inverter topology operate with a high and variable switching frequency which increases the power losses and system components design complexity. This can be alleviated by a pulse width modulation (PWM)-based fixed-frequency SM controller proposed in this paper. The SM controller is implemented using variable amplitude PWM carrier signals generated using the output capacitor voltage and inductor current measurements, thus eliminating the requirement of the output capacitor currents measurement. The battery-connected inductor reference currents for the SM controller are generated by a supercapacitor energy controller which is responsible for the HESS power allocation. First, the theoretical aspects of the SM controller, the operation and parameter selection of the supercapacitor energy controller, and the supercapacitor sizing for the HESS are discussed in the paper. Then, the proposed control system is experimentally verified, and it is shown that the HESS is able to satisfy the HESS output power requirements, while allocating the ripple current and the fast power fluctuations to the supercapacitor while maintaining operation of the supercapacitor within predefined voltage limits. The main advantage of the proposed SM controller, as compared with the traditional double-loop control method, is in eliminating possible DC current injection into the grid when the equivalent series resistance values of the boost inductors become unequal due to the tolerances and temperature variations.

A Rule-Based Controller to Mitigate DC-Side Second-Order Harmonic Current in a Single-Phase Boost Inverter

In conventional parallel hybrid energy storage systems (HESSs), there are usually two cascaded converters active when transferring energy between its energy storage systems (ESSs) that result in the overall efficiency drop. Moreover, the dc-link capacitor has to be oversized to limit the voltage ripple during simultaneous energy exchange between the ESSs and load feeding. The integrated reconfigurable configuration for HESSs proposed in this paper allows direct energy transfer between the ESSs by using only a single converter and bypassing the dc link. This reduces the stress on the dc link, reduces the dc-link capacitor size, and improves the efficiency. The proposed configuration can be reconfigured to operate in different operating modes: “feeding a load” mode, “regenerative” mode, “energy exchange” mode, “energy exchange and feeding a load” mode, “balancing” mode, “charging” mode, and “backup” mode. To control the proposed HESS, a battery peak power limiting method is used to allocate the power components between the ESSs. The operating principle of the proposed configuration and its different modes are explained and experimentally verified.

A Fixed-Frequency Sliding Mode Controller for a Boost-Inverter-Based Battery-Supercapacitor Hybrid Energy Storage System

A transformerless direct single phase grid-connected battery-supercapacitor hybrid energy storage system based on a modified boost inverter configuration and its associated control strategy are proposed in this paper. The proposed control strategy comprises a double loop output voltage controller, active and reactive power droop controller, battery state of charge (SOC) management system based on the extended Kalman filter (EKF) and supercapacitor voltage controller. Simulation results confirm that the proposed system is able to deliver or absorb the required active power and reactive power to or from the grid while allocating the ripple current to the supercapacitor. Furthermore, the system maintains the supercapacitor voltage at a pre-defined value while operating the battery inside a safe operating SOC range.

Integrated Reconfigurable Configuration for Battery/Ultracapacitor Hybrid Energy Storage Systems

Vehicle to grid (V2G) reactive power compensation using electric vehicle (EV) onboard chargers helps to ensure grid power quality by achieving unity power factor operation. However, the use of EVs for V2G reactive power compensation increases the second-order harmonic ripple current component at the DC-side of the charger. For single-phase, single-stage EV chargers, the ripple current component has to be supplied by the EV battery, unless a ripple compensation method is employed. Additionally, continuous usage of EV chargers for reactive power compensation, when the EV battery is not charging from the grid, exposes the EV battery to these undesirable ripple current components for a longer period and discharges the battery due to power conversion losses. This paper presents a way to provide V2G reactive power compensation through a boost inverter-based single stage EV charger and a DC-side capacitor without adversely affecting the EV battery. The operation of the boost inverter-based EV charger with second-order harmonic and switching frequency ripple current reduction, the dynamic behavior of the system, the transition between different operating modes, the DC-side capacitor voltage control above a minimum allowed voltage, and the DC-side capacitor sizing are extensively analyzed. The performance of the proposed system is verified using an experimental prototype, and presented results demonstrate the ability of the system to provide V2G reactive power compensation both with and without the EV battery.

A single phase grid integration scheme for battery-supercapacitor AC line hybrid storage system

A standalone direct AC line battery-supercapacitor hybrid energy storage system based on a novel configuration and its associated control strategy are proposed in this paper. The proposed double loop control strategy incorporating an outer voltage and an inner current loop is verified by simulation under rated conditions, load short circuit and nonlinear load conditions. The advantages of the proposed system are that it allocates the high frequency power component to the supercapacitor while the battery supplies the DC power, maintains operation of both the battery and the supercapacitor within specified current limits, maintains the supercapacitor voltage at a pre-defined value, and charges the supercapacitor from the battery.

Single-Phase Boost Inverter-Based Electric Vehicle Charger With Integrated Vehicle to Grid Reactive Power Compensation

Battery-supercapacitor hybrid energy storage systems (HESSs) are popular as a way of extending the battery lifetime by reducing the battery current fluctuations. In conventional single phase HESSs, both the second-order harmonic power component and the fast power fluctuations are allocated to the supercapacitor. However, the ripple current component leads to supercapacitor degradation due to its internal heating. In this paper, a current-feedback ripple current reduction method is applied to a single-phase battery-supercapacitor HESS in order to alleviate the second-order harmonic supercapacitor current component. Effectiveness of the ripple current reduction method is theoretically analyzed. Experimental results show that the current-feedback method is able to significantly decrease the second-order harmonic component in the supercapacitor current without introducing additional harmonics.