Abstract
Energy Storage Systems (ESS) are an attractive solution in environments with a high amount of renewable energy sources, as they can improve the power quality in such places and if required, can extend the integration of more renewable sources of energy. If a large amount of power is needed, then supercapacitors are viable energy storage devices due to their specific power, allowing response times that are in the range of milliseconds to seconds. This paper details the design of an ESS that is based on a modular multilevel converter (MMC) with bidirectional power flow, which reduces the number of cascaded stages and allows the supercapacitors SCs to be connected to the grid to perform high-power transfers. A traditional ESS has four main stages or subsystems: the energy storage device, the balancing system, and the DC/DC and DC/AC converters. The proposed ESS can perform all of those functions in a single circuit by adopting an MMC topology, as each submodule (SM) can self-balance during energy injection or grid absorption. This article analyses the structure in both power flow directions and in the control loops and presents a prototype that is used to validate the design.
Highlights
Supercapacitors (SCs) have the highest power density relative to Li-ion batteries, flywheels, fuel cells, and superconducting magnetic energy storage systems [1,2]
To reduce the simulation time, a SC with a capacitance value of 10F was used if not specified in the figure
The SC-based Energy Storage System (ESS) that is proposed in this article uses a modular multilevel converter (MMC) that is able to reduce the number of stages by performing the self-balancing process and the DC/DC and DC/AC conversions in a single stage
Summary
Supercapacitors (SCs) have the highest power density relative to Li-ion batteries, flywheels, fuel cells, and superconducting magnetic energy storage systems [1,2]. SC technology is still maturing, this high-power density comes with a cost, namely that SCs are usually low voltage devices, usually around 2.5 V to 3.0 V [3]. This problem can be overcome by using series connections with passive or active balancing systems, the latter being more efficient [4,5]. SCs can be used in conjunction with batteries to create a hybrid ESS, increasing the capabilities of the system [11]
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