Development of Voltage Regulation Plan by ...

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ScienceDirect Physics Procedia 65 (2015) 295 – 298

27th International Symposium on Superconductivity, ISS 2014

Development of voltage regulation plan by composing subsystem with the SFES for DC On-Line Electric Vehicle S. Junga, J. H. Lee a, M. Yoon a, H. Leeb, G. Janga* b

a School of Electrical Engineering, Korea University, Seongbuk, Seoul, 136-713, Korea School of Railway & Electrical Engineering, Kyungil University, Gyeongsan, Gyeongsangbukdo, 712-701, Korea

Abstract The study of the application process of the relatively small size ‘Superconducting Flywheel Energy Storage (SFES)’ system is conducted to regulate voltage fluctuation of the DC On-Line Electric Vehicle (OLEV) system, which is designed by using DC power system network. It is recommended to construct the power conversion system nearby the substation because the charging system is under the low voltage. But as the system is usually built around urban area and it makes hard to construct the subsystems at every station, voltage drop can occur in power supply inverter that is some distance from the substation. As the alternative of this issue, DC distribution system is recently introduced and has possibility to solve the above issue. In this paper, SFES is introduced to solve the voltage drop under the low voltage distribution system by using the concept of the proposed DC OLEV which results in building the longer distance power supply system. The simulation to design the SFES by using DC power flow analysis is carried out and it is verified in this paper. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the ISS 2014 Program Committee. Peer-review under responsibility of the ISS 2014 Program Committee Keywords: Contactless power transfer system; DC distribution network; Electric vehicles; SFES voltage regulation; Wireless charging

1. Introduction Recently, the advanced technology to build up the efficient transportation infrastructure has been in the spotlight. The electric vehicle (EV) is regarded as the promising solution for environmental pollution in many areas. For connecting the EV system to the grid, it is first needed to do research on the reliability and stability that can have an effect in the power system and the entire utility grid operation. In that sense, the technologies of integrating the EV system into the power system should be verified to resolve the above issues. To reduce the capacity and charging interval of the battery, a wide variety of studies on the way to charge the battery during short stop section or low speed driving section by the inductive power transfer system have been recently conducted [1]. But as the EV charging system is usually designed in the low voltage distribution system which can have influence on other systems around, priority consideration has to be given to the reliability issue. This paper proposes the strategy of Superconducting Flywheel Energy Storage (SFES) application in the DC On-Line Electric Vehicle (OLEV) system so as to compensate voltage drop which frequently occurs in the charging system due

* Corresponding author. Tel.:+82-10-3412-2605; fax: +82-2-3290-3692. E-mail address: [email protected]

1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the ISS 2014 Program Committee doi:10.1016/j.phpro.2015.05.155

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S. Jung et al. / Physics Procedia 65 (2015) 295 – 298

to the system characteristics. The process to apply SFES to the designated wireless transfer system is mainly discussed and the simulation results are analyzed in this paper. 2. DC On-Line Electric Vehicle The OLEV which utilizes the wireless charging is presently developed to support energy to the battery located in the vehicle. The OLEV system has different features as it consists of driving charging sections that enable to charge the battery during the operation of the vehicle [2]. The system can generate the high frequency by using the particular power inverter having IGBT devices. However, the entire power transfer system gets the required power through AC utility grid and it can lead to additional conversion losses such as switching loss compared with the other electrical transportation systems. In addition, the power conversion system based on the AC system has disadvantages due to space limitation because the system is usually located in the urban power grid. The bipolar DC distribution system applying in the DC OLEV system is recently studied to improve the above issues. The proposed system shares substations as shown in Fig. 1 and therefore can improve the voltage fluctuation issues [3]. However, the enhanced method how to reduce voltage regulation should be developed because the proposed system still has the possibility of voltage drop owing to the distance between the inverter and substation beyond control.

Fig. 1. DC OLEV system concept and operational advantages

3. Wireless charging system with SFES 3.1. Suitability for SFES The OLEV system adopts the inductive power transfer system capable of generating the high frequency during the system operation. Since it has fast switching characteristic, the integrated power sources require high response ability. Furthermore, the SFES system has the characteristics of large power and energy density which allow it to respond the requirement of instantaneous charging power. 3.2. Circuit Analysis When the vehicle reaches the charging platform, the substations supply the required power and the SFES discharging process is progressed if the charging platform is designated as target section. From the previously designed network, the node equation for power flow analysis can be derived as shown in Eq. (1): gline1 g sub1 gline1 0 0 V1 I 1 (1) g SFES gline 2 gline3 V2 I 2 0 gline3 gline2 g sub 2 gline3 V3 I 3 0 0 gline3 gline1 gline 2 gveh Vcinv 0 0 gline1 gline 2 Where, gveh is the equivalent admittance of the vehicle, gsubi is the admittance of the substation, gSFES is the admittance of the SFES, glinei is the admittance of the line between the charging inverter and substation, Vi is the voltage of the source, Vcinv is the voltage of the charging inverter, Ii is the equivalent current of the source;

gveh uVcinv

I veh

(2)

The Eq. (2) is used in the iteration. By the iteration process with determined value (I 1, I2 and I3), the solution of the voltage at each node can be obtained. Through this power flow analysis, the specific node voltage fluctuation can be determined.

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3.3. System design Based on the previous configuration, the SFES subsystem is combined between two substations which largest voltage fluctuation occurs in during vehicle charging. Fig. 2 gives an overview of the simulation by adopting the previous design system [3]. It shows a part of the entire system for the intensive analysis. Since the distance between two substations is relatively long, the possibility of voltage fluctuation is higher than other sections. The SFES is designed and integrated in the system at the center of the two charging sections. The main role of the SFES is to regulate voltage by supplying the required power promptly when the vehicles reach on the charging platform. In addition, the SFES can be charged during the non-charging period of the vehicles and therefore achieve the efficient power supply during the operation of the entire system. In order to remove DC voltage variation of the power cable, the SFES determines the operation mode according to the system operation. The system operation plan is based on the previous scheduling process that mainly aims to prevent the simultaneous charging. The energy capacity of the SFES can be calculated by the equation (3). ESFES

- -

(1 Dinv / Dsub )tcG Pmax

(3)

Where Dinv is the distance between SFES and charging inverter, Dsub is the distance between SFES and substation, tc is the designated charging time of vehicle, Ƃ is the relative resistance coefficient determined by the difference between SFES and substation, Pmax is the maximum required power of closest charging inverter.

Fig. 2. Overview of the SFES operation in design system (specific section of the entire route)

The system is basically designed by reflecting the previous research and the parameters such as vehicle charging time (tc). The energy capacity can be calculated by considering the distances including D inv and Dsub because the entire power supply processes are running simultaneously. As the scheduling processes firstly prevent the simultaneous charging between close vehicles, the SFES can be designed by focusing a certain charging inverter which can have a strong influence on the system. The charging process of the SFES is progressing during the non-charging period of the electric vehicles. 4. Case studies In this paper, it is assumed that the SFESs are installed in the DC OLEV system which is previously designed with 3 substations and 8 charging stations. The system data is shown in Table 1 needed for the simulation. The numerical calculated data for above equation is represented in the table. The resistance coefficient ‘δ’ is fully depends on line and substation resistance because the SFES retain almost zero resistance value. Table 1. Numerical data of the performed simulation W(veh) 12 [ton]

Line length 4980 [m]

System voltage 440 [V]

Cveh-rate

CSFES-rate

gveh

gline

gline-inv

gsub

11.4

4.759

1.613

0.13

0.13

0.02956

[kWŘh]

[ kWŘh ]

[Ț]

[Ț/km]

[Ț/km]

[Ț]

δ 3.664

Dinv /Dsub

tc

theadway

1/3

30 [sec]

130 [sec]

The power flow calculation is performed based on the given simulation information and the graph is formed to check the power consumption and voltage variation during the charging process. Fig 3 and Fig. 4 show the comparison of the voltage fluctuation for the same time period by the same scheduling process between the previous system and the system integrated with the SFES. Since the target section includes the simultaneous charging nearby the charging stations, it generates the highest peak power and the lowest voltage drop shown in above graph and these problems can be mitigated by the SFES operation as the figures indicate.

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Fig. 3. V fluctuation curve in the designated section (without SFES)

Fig. 4. V fluctuation curve in the designated section (with SFES)

Fig. 5. Power curve in the designated section (without SFES)

Fig. 6 Power curve in the designated section (with SFES)

Table 2. Numerical results of the simulation Substation Substation 1 Substation 2 Substation 3

Lowest voltage (Target section) Non SFES With SFES 423.32 V 432.33 V 431.51 V 432.39 V 430.37 V 429.43 V

Peak power (Target section) Non SFES With SFES 237.45 kW 153.52 kW 123.89 kW 111.38 kW 140.24 kW 112.12 kW

SFES Power capacity 204.98 kW 175.61 kW 246.23 kW

The discharge energy is recharged at vacancy section as shown in the figures and it is suitable for the vehicle scheduling processes. Fig 5 and Fig. 6 show the comparison of the power consumption curve for the same time period by the same scheduling process between the previous systems integrated with SFES. The peak power occurrence by simultaneous charging nearby charging platform was reduced as the support by SFES. In Table 2, voltage regulation and the peak power reduction have been compared between with and without the installation of the SFES. Compared to the previous system, voltage drop is highly mitigated. Furthermore, the peak power is decreasing by 31.3% from 223.45 kW to 153.52 kW. The required power capacity is estimated to 246.23 kW which generated in the SFES recharging section. 5. Conclusions This paper suggests the SFES on the proposed DC OLEV system to regulate voltage fluctuation and energy utilization. Through case studies, it is verified that the SFES contributes to the voltage regulation and the peak power reduction. Voltages at every station is regulated up to 0.02 [p.u.] and therefore the more efficient operation of the charging inverter is expected. Furthermore, it is anticipated that the SFES application can result in the large amount of the peak power reduction and cost saving strategies because the DC OLEV system is suited to effectively utilize ESS due to its unique characteristics. Acknowledgements This work was supported by the NRF grant (No. 2013H1A2A1034289) and by Human Resources Development of KETEP grant (No. 20114010203010) funded by the Korea government. References Reference to a journal publication: [1] van der Pijl F., Castillia M., Bauer P., Control Method for Wireless Inductive Energy Transfer Systems with Relative Large Air Gap, IEEE Trans. Industrial Electronics, 60, 2013, 382-390. [2] Huh J., Wooyoung L., Gyu-Hyeong C., Byunghun L., Chun-Taek R., Characterization of novel inductive power transfer system for On-Line Electric Vehicles, 26th annual Conf., Applied Power Electronics Conference and Exposition , 2011, 1975-1979. [3] Seungmin J., Hansang L., Chong Suk S., Jong-Hoon H., Woon-Ki H., Gilsoo J., Optimal Operation Plan of the On-Line Electric Vehicle System through Establishment of DC Distribution System, IEEE Trans. Power Electronics, 28, 2013, 5878-5889.