Electric Vehicle Charging Station with an Energy Storage Stage for Split-DC Bus Voltage Balancing
Abstract
This paper proposes a novel balancing approach for an electric vehicle bipolar dc charging station at the megawatt level, enabled by a grid-tied neutral point clamped converter. The work uses the presence of an energy storage stage with access to both of the dc buses to perform the complementary balance. It proposes a generic balancing structure that can achieve balance regardless the kind of ESS employed. This is aiming to reduce the hardware requirements of the system and maximize the usage of the ESS, whose main function is to perform the energy management related tasks. To meet this purpose, a three-level dc-dc interface is employed, allowing to compensate the dc currents with a single ESS. Furthermore, in order to prevent the appearance of even-order harmonics in the input current during asymmetrical operation, an alternative switching sequence for the central converter is proposed. Results indicate that, without altering dramatically the charging process of the ESS, it is possible to cover the whole load scenario without the need of a balancing circuit. This allows the use of off-the-shelf products both for the rectifier and the fast chargers. In this paper, simulation and experimental results are presented to validate the proposed balancing strategy.
EXISTING SYSTEM:
In order to address the impacts of large-scale adoption of these vehicles in the utility systems, several studies have been carried out mostly based on the conventional slow charging process of the batteries. This is mainly because, conventional charging is expected to remain as the preferred charging method and also the fast charging process of the EV batteries is still not a widespread practice a the owners, due to the lack of facilities and misconceptions regarding the impact of this process to the battery pack. However, fast charging methods are still essential for a large-scale adoption of EVs, as it will provide more flexibility to the drivers, occasional longer trips addressing range anxiety. Additionally, in order to reduce power consumption from the utility grid during peak consumption hours, the presence of ESS in these stations is gaining attention. An alternative to enable fast charging is in the form of fast charging stations, which refers to the concept of having high-power fast chargers installed off-board, similar to gas stations located in public places. The structure of these charging stations can either be with an ac-bus, where each charging unit is fed by its independent ac-dc stage, or each unit connected to a common dc bus enabled by a single ac-dc stage with higher power ratings. Currently, fast charging is only enabled by standalone units, each one with its independent rectifier stage using the ac-bus concept.
PROPOSED SYSTEM:
The central converter stage plays a fundamental role in this charging architecture, and is desirable to provide several features as low distortion operation, high power capability, fully adjustable power factor, reduce the size of the input filters, while featuring a reduced number in both active and passive components. A the alternatives, conventional two-level voltage source converter might arise, however it has a limited capacity to fulfill power ratings, power quality and efficiency requirements due to semiconductors voltage/currents limits. Other works propose the use of a 12-pulse diode bridge rectifier, improving its harmonic performance through the use of an active filter stage. However, the lack of power factor control and its unidirectional power flow capability reduces the potential of the charging station. A different dc-bus concept is proposed and validated in. The structure is based in the use of a central neutral point clamped (NPC) converter enabling a bipolar dc-bus. The use of this split dc-bus provides flexibility to the connection of the loads, has higher voltage and power handling capabilities and reduced step-down effort of the dc-dc stages. However, given the adoption of the bipolar structure and the intended application, the balance control becomes essential. In this was done by adding an additional circuit to the system, which enhances the balancing capabilities of the grid-tied converter. The result led to balanced dc voltages even under severe imbalance load conditions. Taking a different approach, this paper proposes a novel balancing method that uses the presence of the existing ESS, regardless of its type, to perform the complementary balancing actions. The idea is to relocate the power consumption of the ESS in order to keep the central converter operating in its balanced zone. This is achieved by the use of a three-level dc-dc interface. As it will be demonstrated, the only requirement of the approach is to met the minimal balancing power. However, it is important to mention that the presence of this stage is for managing the energy consumption of the charging station, and its operation will be used towards the prevention of drifts in the dc voltages. It will be shown, that the presence of this stationary load in the system can be used to complement the balancing capabilities of the central converter.
CONCLUSION
A different complementary balancing approach has been developed and successfully validated, which takes advantages from the optional stages that the distributed dc bus architecture allows. In this case, the presence of an energy storage stage, interfaced with a three-level dc-dc converter, allows the elimination of the balancing leg and provide the supplementary balancing ability required. This leads to a reduction in the overall cost of the charging architecture, as the requirements for the rectifier stage has been reduced, allowing to use offthe-shelf equipment. Is important to highlight that despite the ESS converter is providing the additional balancing ability, this does not alter dramatically its operation, allowing to keep its main function which is the charging and discharging of the energy buffer according to the selected energy management strategy. Furthermore, given the features of the three-level dc-dc converter, the minimal load condition does not impose a heavy restriction on the ESS sizing, which means that its ratings are still set by the selected energy management approach. Experimental results using an ultra capacitor stage have been carried out for the validation of the method, but the concept can be extended to different kinds of ESSs. Similar to the balancing method presented in, the proposed solution allows to keep high quality input signals, even under the presence of severe imbalances at the dc side. In addition, the alternate switching sequence allows to perform the complementary balancing while keeping the current free of even-order harmonics.
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