EPE Association: EPE 2019 - LS6b: Storage and Grid

EPE 2019 - LS6b: Storage and Grid
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 2019 ECCE Europe - Conference > EPE 2019 - Topic 05: Renewable Energy Power Systems > EPE 2019 - LS6b: Storage and Grid

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   Aging Effects of Twice Line Frequency Ripple on Lithium Iron Phosphate (LiFePO4) Batteries
By Alireza GHASSEMI [View]
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Abstract: The charge/discharge current profile is one of the most important factors that affects the behavior of lithium-ion batteries (LIBs). Most of previous studies evaluate the behavior of LIBs under pure constant current conditions, when in reality battery packs in arguably the most important applications experience alternating currents (AC), superimposed on DC components. So-called `ripple currents' commonly present at twice line frequency (i.e., 100/120 Hz) and their presence forces designers of single-phase AC-DC converters to incorporate a large DC-link capacitor into these devices. And yet, all this effort presupposes that AC ripple constitutes a serious problem, when it is still unclear to what extent the LIB lifetime may be reduced by the presence of 100 Hz ripple current. While many studies claim that AC harmonics accelerate the degradation of LIBs, there are others that refute this. Different chemistries of LIBs have been included in these studies, however, a systematic study on cells with LiFePO4 (LFP) as the cathode material, considered as one of the most promising chemistries in many different applications, has not been done yet. This paper presents the results of a detailed and systematic experimental study on aging impacts of 100 Hz-ripple currents on 13 fresh LFP cells. The degradation of the cells is characterized based on capacity fade and impedance rise. The results show that the superimposed 100 Hz ripple currents do not cause a significant amount of degradation on LFP cells. This means that in many inverters, the size of the DC-link capacitor is probably excessive, thereby allowing a marked reduction in size of this expensive system component.

   DC Capacitance Reduction in Three-Phase Photovoltaic Inverters by using Virtual Impedance Emulation
By Andoni URTASUN [View]
[Download]

Abstract: DC voltage regulation in grid-connected three-phase PV inverters is a fundamental requirement. In order to reduce the influence of the PV non-linear behavior and ensure stability in the whole operating range, the input capacitance in high-power inverters is currently oversized, thus increasing the converter cost. This paper proposes a control method which emulates a virtual impedance in parallel with the PV generator, making it possible to reduce the capacitance by a factor of 5. Simulation results confirm that the proposed control is stable and fast enough in the whole operating range with such a small capacitor.

   On the requirements of the power converter for second-life lithium-ion batteries
By Alberto BERRUETA [View]
[Download]

Abstract: The use of lithium-ion batteries is increasing year after year, especially in the automotive sector. Given the high requirements of electric vehicles, their energy storage systems are discarded when they still have around 70\% of its initial capacity. These discarded batteries are being studied as a low-price option for stationary systems, mostly related to renewable energy generation, with lower battery requirements. However, the increasing dispersion of cell capacity detailed in this contribution limits the use of second-life cells if regular battery management systems and power converters. We present in this contribution an experimental comparison of the capacity dispersion between fresh and second-life cells, and detail the relationship between the capacity dispersion and the required BMS functionality. Furthermore, we include the ageing phenomena in the analysis by means of experimental ageing results, given that the capacity dispersion is enlarged as the battery ages. After this, we use this data to quantify advantages and disadvantages of a combined BMS and power converter, based on a multilevel topology, compared to a conventional BMS. The most relevant result, when a 55-cell battery is analysed, is a 65\% increase in capacity during its whole second life if the BMS and power converter are combined by means of a multilevel topology. The increased level of complexity required by the combined BMS-power converter architecture is analysed in this contribution, providing a convenient tool for the selection of the most suitable option for each application.

   Voltage and Frequency Support Scheme for Storage Systems in Distribution Grids
By Anastasis CHARALAMBOUS [View]
[Download]

Abstract: Power electronics converters can enable grid support functionalities under abnormal grid conditions. In this paper, a voltage-frequency support strategy based on the reactance to resistance (X/R) ratio of the grid impedance is proposed for a flywheel energy storage system. Unlike conventional support schemes where voltage and frequency support is decoupled, the proposed strategy considers the coupling between voltage and frequency due to the resistive characteristics of the grid impedance in low voltage distribution grids. In addition, the proposed strategy ensures a fair compensation between voltage and frequency support by utilizing two gains that are defined according to the event type. Simulations and experimental tests are carried out using a laboratory setup to validate the proposed strategy. The capabilities of storage systems to provide such tailor-made ancillary services for supporting the distribution grid are demonstrated.

EPE 2019 - LS6b: Storage and Grid
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 2019 ECCE Europe - Conference > EPE 2019 - Topic 05: Renewable Energy Power Systems > EPE 2019 - LS6b: Storage and Grid
	[return to parent folder]

	Aging Effects of Twice Line Frequency Ripple on Lithium Iron Phosphate (LiFePO4) Batteries By Alireza GHASSEMI	[View] [Download]
Abstract: The charge/discharge current profile is one of the most important factors that affects the behavior of lithium-ion batteries (LIBs). Most of previous studies evaluate the behavior of LIBs under pure constant current conditions, when in reality battery packs in arguably the most important applications experience alternating currents (AC), superimposed on DC components. So-called `ripple currents' commonly present at twice line frequency (i.e., 100/120 Hz) and their presence forces designers of single-phase AC-DC converters to incorporate a large DC-link capacitor into these devices. And yet, all this effort presupposes that AC ripple constitutes a serious problem, when it is still unclear to what extent the LIB lifetime may be reduced by the presence of 100 Hz ripple current. While many studies claim that AC harmonics accelerate the degradation of LIBs, there are others that refute this. Different chemistries of LIBs have been included in these studies, however, a systematic study on cells with LiFePO4 (LFP) as the cathode material, considered as one of the most promising chemistries in many different applications, has not been done yet. This paper presents the results of a detailed and systematic experimental study on aging impacts of 100 Hz-ripple currents on 13 fresh LFP cells. The degradation of the cells is characterized based on capacity fade and impedance rise. The results show that the superimposed 100 Hz ripple currents do not cause a significant amount of degradation on LFP cells. This means that in many inverters, the size of the DC-link capacitor is probably excessive, thereby allowing a marked reduction in size of this expensive system component.

	DC Capacitance Reduction in Three-Phase Photovoltaic Inverters by using Virtual Impedance Emulation By Andoni URTASUN	[View] [Download]
Abstract: DC voltage regulation in grid-connected three-phase PV inverters is a fundamental requirement. In order to reduce the influence of the PV non-linear behavior and ensure stability in the whole operating range, the input capacitance in high-power inverters is currently oversized, thus increasing the converter cost. This paper proposes a control method which emulates a virtual impedance in parallel with the PV generator, making it possible to reduce the capacitance by a factor of 5. Simulation results confirm that the proposed control is stable and fast enough in the whole operating range with such a small capacitor.

	On the requirements of the power converter for second-life lithium-ion batteries By Alberto BERRUETA	[View] [Download]
Abstract: The use of lithium-ion batteries is increasing year after year, especially in the automotive sector. Given the high requirements of electric vehicles, their energy storage systems are discarded when they still have around 70\% of its initial capacity. These discarded batteries are being studied as a low-price option for stationary systems, mostly related to renewable energy generation, with lower battery requirements. However, the increasing dispersion of cell capacity detailed in this contribution limits the use of second-life cells if regular battery management systems and power converters. We present in this contribution an experimental comparison of the capacity dispersion between fresh and second-life cells, and detail the relationship between the capacity dispersion and the required BMS functionality. Furthermore, we include the ageing phenomena in the analysis by means of experimental ageing results, given that the capacity dispersion is enlarged as the battery ages. After this, we use this data to quantify advantages and disadvantages of a combined BMS and power converter, based on a multilevel topology, compared to a conventional BMS. The most relevant result, when a 55-cell battery is analysed, is a 65\% increase in capacity during its whole second life if the BMS and power converter are combined by means of a multilevel topology. The increased level of complexity required by the combined BMS-power converter architecture is analysed in this contribution, providing a convenient tool for the selection of the most suitable option for each application.

	Voltage and Frequency Support Scheme for Storage Systems in Distribution Grids By Anastasis CHARALAMBOUS	[View] [Download]
Abstract: Power electronics converters can enable grid support functionalities under abnormal grid conditions. In this paper, a voltage-frequency support strategy based on the reactance to resistance (X/R) ratio of the grid impedance is proposed for a flywheel energy storage system. Unlike conventional support schemes where voltage and frequency support is decoupled, the proposed strategy considers the coupling between voltage and frequency due to the resistive characteristics of the grid impedance in low voltage distribution grids. In addition, the proposed strategy ensures a fair compensation between voltage and frequency support by utilizing two gains that are defined according to the event type. Simulations and experimental tests are carried out using a laboratory setup to validate the proposed strategy. The capabilities of storage systems to provide such tailor-made ancillary services for supporting the distribution grid are demonstrated.