Bidirectional Power Control Strategy for Super Capacitor Energy Storage System based on MMC DC-DC Converter

Abstract

In order to equip more high-energy pulse loads and improve power supply reliability, the vessel integrated power system shows an increasing demand for high-voltage and large-capacity energy storage systems. Based on this background, this paper focuses on a super capacitor energy storage system based on a DC-DC converter. This paper analyzes the different topology of Hybrid Energy Storage System (HESS) and Demand Management System using HESS. Taking into account the shortcomings of the traditional bidirectional power control strategy, this paper proposes a control strategy where the HESS module of each branch independently controls the voltage of the sub-module capacitor. The outputs are validated and verified using MATLAB Simulink platform.

Introduction

I. INTRODUCTION

The integrated power system has received extensive attention in the DC power distribution. In the future, it will be one of the inevitable technical routes for renewable energy systems [1][3]. In recent years, with the increasing demand for higher  power supply reliability, and equipment of pulsed loads and new high-energy weapons, energy storage systems have become an indispensable part of the second-generation  [4], [5]. Therefore, the energy storage converter connected to the medium voltage DC (MVDC) grid needs to be characterized by high voltage and large capacity, voltage conversion, electrical isolation and bidirectional conversion. It is widely used in high-voltage and large-power applications because of its modular structure and fault tolerance [6]. For high-voltage rail transit vehicles, the control strategies of super capacitor energy storage system based on Modular Multilevel Converter  (MMC) are studied in [7], [8]. These two papers realize the balanced decoupling control of the power of super capacitors, and put forward the corresponding energy management strategies. In [9], the control strategy of modular multilevel energy storage system under two operating conditions of grid voltage symmetry and asymmetry is studied, which solves the problem of charge state balance of energy storage elements. To realize the electrical isolation and voltage conversion, isolated bidirectional DC-DC converter needs to be used between MVDC grid and LVDC grid. It is a bidirectional DC-DC converter with electrical isolation capability and modular symmetrical structure. It has attracted extensive attention in the fields of electric vehicles, DC micro grids and energy storage systems However, the above literatures study the application scenario of connecting resistive load on the LVDC bus of MMC-DAB. Their control strategy is that MMC controls the voltage of sub-module capacitors and converter controls the voltage of the LVDC bus. If this strategy is extended to the application scenario with energy storage unit connected on the LVDC side, the converter module usually controls the port current of the energy storage unit. To better control the port current of the energy storage unit, the filter inductor needs to be connected between the converter and the energy storage unit. However, this will increase the volume and weight of the device, and the stability margin of converter control system is small. Especially when discharging the super capacitor, the filter inductor and the filter capacitor at the later stage of converter control form an LC filter with large output impedance, which is easy to cause stability problems [10].

The Figure 8,9, and10 shows the PV outputs, Battery Outputs and Super Capacitor Outputs. To sum up, the energy-based strategy had the greatest effects on urban cycles whose driving conditions varied considerably and consecutively. The SOC-based and voltage-based strategies were more effective than the energy-based one when operated under driving conditions with less fluctuation. Because of the significant reduction of the battery rms current and peak current, the battery voltage drop was also minimized. Thus, undesirable effects on the electric drive system were avoided thanks to the semi-active HESS configuration and the EMS. It could consequently be seen that the battery/SC dual-source system and the proposed filtering strategies were more worthy for  electric cars than for other kinds of EVs that work under smoother driving conditions. The impact of the proposed strategies on the SC under several initial conditions such as the SC being fully charged before the vehicle operating or the car not having been used for a long time needs to be illustrated. Hence, simulation  were carried out at four initial voltages of the SC including U, 0.75U, 0.5U, and 0.25U.

Conclusion

This paper proposed three simple, but effective filter-based strategies for the power allocation of a battery/SC EV, which relied on the SC stored energy, SOC, and voltage. We aimed to show the feasibility in a real-world implementation. Furthermore, the system control and EMS can be transferred from the simulation development. These proposed methods were compared to each other before they were analyzed together with a pure battery car and a conventional filtering method under the same working conditions to find the most effective solution. Simulation results showed that the HESS and EMS had significant roles in protecting the battery under fluctuating driving conditions. In the super capacitor of the battery of the HESS with the energy-based strategy was only about one-third of that of the battery-only vehicle. The super capacitor energy storage unit is connected to LVDC bus, which is conducive to enhance the flexibility and reliability of the energy regulation of the DC grid, and also provides power source for pulse loads on board.

References

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Copyright

Copyright © 2023 Swetha M, G. Swarna Latha, D. Swathi, N. Upendra Reddy , N. Varun Kumar Reddy. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.