A Design of DC/DC Converter of Photovoltaic Generation System for Streetcars

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1 Journal of International Council on Electrical Engineering Vol. 3, No. 2, pp.164~168, A Design of DC/DC Converter of Photovoltaic Generation System for Streetcars Masahide Hojo, Hidekadu Takeda* and Yuji Mishima** Abstract A great deal of effort has been made on installation of photovoltaic generation systems. As a conventional way, the dc power generated by solar panels is converted to ac by a utility interactive inverter just beside the panel. Authors have developed a new installation strategy of them by a dc power line for a streetcar. First, it can reduce the total installation cost because it does not require a lot of the traditional utility interactive inverters. Secondly, it is suitable for collecting and consuming the solar power at the heart of a city because the streetcar carries passengers in day time and its railway is widely spread in the city. In this paper, a power conversion unit which consists of two dc/dc converters bridged by an Electric Double Layer Capacitor is proposed and designed for a sample city. Its performance is also verified by simulation study. Keywords: DC/DC converter, DC power distribution, Electric double layer capacitor, Photovoltaic generation, Streetcar 1. Introduction Nowadays, growing anticipation of renewable energies pushes forward with high penetration of photovoltaic generation systems (PV systems). It has been a worldwide trend and various scales of them can be found all over the world [1]. Although their installation styles depend on their scales, the most popular way is to employ a utility interactive inverter just besides a solar panel, which converts the generated dc power to ac in order to inject the generated power to the grid. In case of a PV system for household use, the inverter is implemented at each house. On the other hand, some Megawatt-scale PV plants can be also found in Japan. In the large PV plants, the generated power is collected by dc and converted by a large-scale inverter unit. In any case, the PV systems are equipped with utility interactive inverters. However, increasing number of utility interactive inverters may result in expensive initial costs. Moreover, it may be accompanied by some additional technical challenges such as to suppress line voltage variations or avoid unintentional islanding of them. Authors have developed another installation strategy of many PV systems using a dc power line for a streetcar [2]. Corresponding Author: Dept. of Electrical and Electronic Engineering, The University of Tokushima, Japan (hojo@ee.tokushima-u.ac.jp) * Dept. of Electrical and Electronic Engineering, The University of Tokushima, Japan (h_takeda@ee.tokushima-u.ac.jp) ** Dept. of Electrical and Electronic Engineering, Hakodate National College of Technology, Japan (mishima@hakodate-ct.ac.jp) Received: December 27, 2012; Accepted: March 14, 2013 The output dc power of the PV system is transferred to the street cars through dc/dc power converter. The streetcar is generally supplied by dc of relatively low voltage such as 600V or 750V, which is not so high as compared to the voltage of the solar panel. Therefore, cascaded conventional boosting converter can provide an interface between the supply voltage for the streetcars and the output voltage of the solar panels. In the proposed system, the solar power is not converted but directly transmitted to the power conversion unit by a low voltage dc line. By this topology, the total installation cost can be reduced because it does not require a lot of utility interactive inverters at the customer sides. In addition, the streetcar carries passengers in day time and its network is widely spread in a city. This means that the solar power can be effectively consumed by the streetcar just around its railway without any additional construction of dc power transmission lines. That is, the proposed system is suitable for collecting and consuming the solar power at the heart of the city. In this research, it is assumed that the power conversion units are placed just around streetcar s stops. This research consists of two issues; one is the optimal placement of the conversion units and the other is a suitable circuit topology for the conversion units. The former was calculated by the co-author in an example where the solar panels are placed on the roof of schools [3]. As a result, the proposed topology is expected to achieve 46% cost reduction at the maximum compared to the traditional method using utility interactive inverters. The latter is of a main topic of this paper. Today, we can refer to some practical advanced 164

2 Masahide Hojo, Hidekadu Takeda and Yuji Mishima power units fortunately, which can compensate voltage drops on a dc power line for a streetcar caused by an inrush current of its drive unit [4]. By referring to these technologies, the solar power is converted by two stages in the proposed converter topology. The solar panels are connected to an Electric Double Layer Capacitor (EDLC) by a dc/dc converter, and then the another dc/dc converter is employed to bridge the power to trolley line. The former converter collects the power directly from the solar panels dispersed just around the converter by low voltage dc lines. Although the single converter cannot accomplish the Maximum Power Point Tracking (MPPT) control of the dispersed solar panels, the converter searches a better voltage to receive higher power from the solar panels. In addition to this, this topology is not suitable for power collection from huge number of small-scale solar panels, because it means that a lot of low voltage dc lines are required. Therefore, it is assumed that some large-scale solar panels which are just around the trolley line are utilized. On the other hand, the latter converter not only bridges the solar power to trolley line but also compensates a voltage drop on the trolley line. By these considerations, a prototype converter was designed for a city model. In addition, it was confirmed that the converter can maintain the dc voltage of the trolley line as well as protect the solar cells from a surge current when the tram stopped or started. Then, the possibility of the strategies is verified by simulation study. 2. Installation Strategies of PV System Fig. 1 indicates the proposed installation strategy with comparison of conventional methods. Fig. 1 (a) shows the conventional style of PV system, where each PV output is converted to ac by the utility interactive inverters. As shown in Fig. 1 (a), this traditional method needs a lot of utility interactive inverters with high penetration of PV systems. Fig. 1 (b) depicts the proposed installation strategy. All PV panels are connected to same dc line. The PV outputs are collected by the low voltage dc network and delivered to the dc/dc converter. It should be noted that the low voltage transmission line cause relatively large loss. Therefore, this topology had better not connect PV panels which are far from the conversion unit. This conclusion is also derived from the other viewpoint of MPPT control. As the PV panels are connected same dc line, the MPPT control must be applied simultaneously. However, when the PV panels are close to each other, the operating voltage of the dc line is not beyond its optimal value for the MPPT. 3. Placement of the Converter In order to consider a practical system scale, a city with streetcar networks in Japan is used as a model for this study. The city is 905km 2 in area and it has a population of about 1.2 million. There are two prospective PV sites around a trolley line. One is a junior high school and the other is a public facility. Their specification is summarized in Table 1. The area of the roof is calculated from a city map and the expected maximum power is derived simply from the area of the roof as summarized in Table 2. Table 1. Possible sites for PV panels. a junior high school (at A in Fig. 2) a public facility (at B in Fig. 2) (a) Conventional style of PV system, (b) Proposed installation style to the streetcar system, Fig. 1. Proposed concept of PV for streetcars. Area of the roof Expected maximum power 4,300 m kw 900 m 2 96 kw Arrangement of the sites and the trolley line are depicted in Fig. 2. For the simplicity, the power conversion unit is assumed to be placed anywhere along the trolley line. In the circumstances, the optimal placement of the conversion unit is considered in order to reduce the transmission losses. Assumed that the transmission losses are evaluated by the percentages against the generated power, the optimal site of 165

3 A Design of DC/DC Converter of Photovoltaic Generation System for Streetcars Table 2. Possible sites for PV panels and the expected maximum generated power Expected maximum power [kw] Distance to the conversion unit [m] a junior high school (A) a public facility (B) Trolley line voltage [V] Line currents [A] Efficiency [%] the conversion unit depends on the distance among them. The points of A and B indicate the possible sites for PV panels. In order to place a conversion unit with minimizing the sum of distance among the PV panels and the conversion unit, it can derived as the crossing point of the railway and a segment between A and B, where the A is the mirrored point of A. After their distance calculations, dc line currents and the efficiency of power transmission. Fig. 4 indicates the total system as a test case of the proposed strategy. The trolley line is simply modified by A current source I t imitates an inrush current of a drive unit in a streetcar. Switching frequency of the dc/dc converter at the trolley line side is set up as 600Hz. Voltage regulation is realized by conventional proportional control. On the other hand, the optimal operating voltage of the PV panel is assumed to be known for the simplicity, and the switching frequency of S 3 is 6kHz, which is faster than the converter at the trolley line side. The other parameters are listed up in Table 3. Fig. 2. Arrangement of PV panels and the trolley line. As discussed above, the site of conversion unit is restricted just around the streetcar stops in practical cases. Co-author has analyzed the optimal placement and favorable number of the conversion units considering with these restrictions [3]. 4. Converter Design and Operating Characteristics Fig. 3 shows the proposed power conversion unit which consists of two dc/dc converters. The PV panels are connected at the input terminal V i and the output terminal V o is for the trolley line. Fig. 3. The proposed power conversion unit. Fig. 4. Simulation model. Table 3. Constants for simulation study. Trolley Line Reference voltage Line resistance Power Conversion Unit Inductances Filter EDLC Capacitance Rated voltage PV panels Opened terminal voltage Short circuit current Optimal operating voltage V T * =750V R T1 = R T2 = 0.55Ω L 1 = 3.0mH, L 2 = 30mH L f = 2.0mH, C f = 35μF C 2 = 15F 300V 240V 5.9Α 186V Without any compensation, a voltage drop was observed at the terminal V o, as shown in Fig. 5. As compared with this case, Fig. 6 shows the waveforms of the proposed converter. From these figures, the voltage drop is compensated by the conversion unit as well as V i is regulated at the optimal operating voltage for PV panels. The voltage across the EDLC is also kept at its reference value. 166

4 Masahide Hojo, Hidekadu Takeda and Yuji Mishima 5. Conclusion Fig. 5. A voltage drop without any compensation. This paper proposes a new installation strategy of PV systems using a trolley line. This strategy can achieve reduction of the installation cost as well as local consumption of the generated power. Fundamental operating characteristics are demonstrated by a model case. Detailed analysis will be a future work, considering a possible volume of the conversion unit because the streetcar network often has little space for additional equipment. Acknowledgements This research was partially supported by TEPCO Research Foundation. The authors also highly appreciate technical advices by the engineers of Meidensha Corporation and Railway Technical Research Institute, Japan. (a) Compensated trolley line voltage. (b) Voltage across the EDLC. References [1] F. Katiraei and J.R. Agüero, "Solar PV Integration Challenges," IEEE Power & Energy magazine, vol.9, no.3, pp.62-71, [2] M. Hojo and Y. Mishima, "An Installation Strategy of Photovoltaic Generation to Tram System," Record of Joint Convention of Transportation and Electric Railway and Intelligent Transport Systems, TER , ITS-10-58, Tokyo, Japan, November 2010 (in Japanese). [3] Y. Mishima and M. Hojo, "Optimal Placement of Power Converters for DC Power Collection from Solar Panels," Tohoku-section Joint Convention Record of Institutes of Electrical and Information Engineers, p.208, Miyagi, Japan, August 2011(in Japanese). [4] "The Trolley Voltage Compensation System for the Kagoshima City Transportation Bureau LRT," Toyo Denki Review, no.116, pp.16-19, 2007 (in Japanese). (c) Voltage across the PV panels. Fig. 6. Operating waveforms when the proposed conversion unit is installed. Masahide Hojo received Ph.D degree from Osaka University, Japan, in He is currently an associate professor of the University of Tokushima, Japan. His research interests are in the area of power system control based on power electronics technology. 167

5 A Design of DC/DC Converter of Photovoltaic Generation System for Streetcars Hidekadu Takeda received B.S degree from the University of Tokushima, Japan, in He is currently a student of the Graduate School of Advanced Technology and Science, the University of Tokushima, Japan. His research interests are in the area of photovoltaic generation system and its intelligent control. Yuji Mishima received Ph.D degree from Hokkaido University, Japan, in After his academic experience at Ibaraki University, Japan, he is currently an associate professor of Hakodate National College of Technology, Japan. His research interests include an optimization of planning and operation of power systems and power delivery systems. 168