Control, design, and implementation of a low-cost ultracapacitor test system

Turkish Journal of Electrical Engineering & Computer Sciences http://journals.tubitak.gov.tr/elektrik/ Research Article Turk J Elec Eng & Comp Sci (2013) 21: 630 – 648 c TÜBİTAK ⃝ doi:10.3906/elk-1109-18 Control, design, and implementation of a low-cost ultracapacitor test system Hasan Hüseyin EROĞLU,1,∗ Ahmet Masum HAVA2 Turkish Armed Forces Rehabilitation and Care Center, 06100 Bilkent, Ankara, Turkey 2 Electrical and Electronics Engineering Department, Faculty of Engineering, Middle East Technical University, 06800 Çankaya, Ankara, Turkey 1 Received: 09.09.2011 • Accepted: 21.12.2011 • Published Online: 03.05.2013 • Printed: 27.05.2013 Abstract: This paper reviews the ultracapacitor (UC) test procedures, establishes simple and economical power electronic conversion system-based UC test equipment, and experimentally evaluates the performance of a UC module. The power converter hardware structure and control algorithms of the designed system are discussed in detail. The high bandwidth and high accuracy current programming capability of the converter for the purpose of charging and discharging the UC, as required during testing, is illustrated via the experimental results. The UC equivalent circuit parameters are extracted. Successful constant current and constant power charging/discharging operating performances are demonstrated. The results of this study help with the design of simple and economical UC test equipment. Furthermore, the power converter and control algorithm developed and demonstrated can be used for energy management applications involving UCs. Key words: Ultracapacitor, ultracapacitor test procedures, power electronics converters, constant current tests, constant power test, ultracapacitor energy management 1. Introduction Ultracapacitors (UCs) are capacitors with high capacitance, low equivalent series resistance (ESR), and lowrated voltage values [1–4]. Since UCs are relatively new energy storage devices, they are usually compared with conventional energy storage devices, such as lead acid batteries (LABs) and electrolytic capacitors (ECs). In these comparisons, the energy and power density (E d , P d ), charge/discharge time, charge/discharge efficiency, and charge/discharge cycle life appear as basic comparison parameters [1,2,5]. In Table 1, UCs are compared with LABs and ECs [5]. According to Table 1, UCs have a smaller, symmetric (equal) charge/discharge time compared to LABs, which must be charged slowly compared to the discharge time. Thus, UCs can be charged and discharged with high current levels. Moreover, considering the charge/discharge efficiency, it is seen that UCs are more efficient than LABs. Table 1 also shows that UCs are between the LABs and ECs in terms of energy and power density. Furthermore, it can be seen that the charge/discharge cycle life values of UCs are higher than those of LABs. Following the general comparison in Table 1, 3 different commercial energy storage devices are compared in Table 2 by considering the energy density [1,2]. Table 2 shows that UCs are between the LABs and ECs involving an order of magnitude in Ed . With the UC cost falling and Ed improving continually, the application fields for UCs have been experiencing rapid growth, and this growth is expected to increase in the coming years. ∗ Correspondence: 630 EROĞLU and HAVA/Turk J Elec Eng & Comp Sci Table 1. Comparison of different energy storage devices. Parameters/devices Charge time Discharge time Efficiency Ed : Wh/kg Pd : W/kg Life (cycle) LAB 1–9 h 0.3–3 h 0.7–0.9 10–100 < 103 < 103 UC 0.3–30 s 0.3–30 s 0.85–0.98 1–10 < 104 > 5 × 105 EC 10−6 –10−3 s 10−6 –10−3 s > 0.95 < 0.1 < 105 > 5 × 105 Table 2. Comparison of commercial energy storage devices. Ed (J/mL) (J/g) VRLA1 141.7 4.06 11100 349.36 127.54 UC2 5.832 0.24 335 24.1 17.41 EC3 0.135 0.04 325 3.27 0.411 (1: Valve regulated LAB, Haze, UPS140; 2: Nesscap, ESHSR1200C0002R7A5; 3: CDE, 38LX273M100B102V) Device type E (kJ) Volume (L) Mass (g) The main application field of UCs is energy storage, where the basic operation involves the charging and discharging of UCs. Thus, the whole process could be viewed as the energy management of the UCs. Realizing this process effectively requires an understanding of the electrical performance of the UCs. However, the electrical parameters of UCs cannot be extracted by utilizing standard measurement devices, such as LCR meters, which cannot provide sufficient excitation signal to the UCs due to the very large capacitance [1,2]. Applying a high-valued DC current to the UC terminals in the charging and discharging modes and observing the response of the device is a common method for the performance evaluation of UCs [6,7]. In the market, there are state-of-the-art test devices that are capable of performing UC tests including the application of large DC current signals [8]. However, the cost and complexity of these advanced commercial products is usually high. In this sense, relatively simple and low-cost UC test equipment is favorable to an application/design/R&D engineer involved with UCs. Power electronics converters could be utilized for controlling the charging and discharging processes of UCs. Therefore, specialized test equipment capable of adjusting and controlling the charging and discharging current, as well as the state of the charge level of UCs, could be implemented with power electronics converters. By building up a power electronic converter-based UC test system, designers have both the chance of evaluating the electrical performance of UCs and acquiring the necessary knowledge about the know-how of UC energy management mechanisms [1,2]. In this study, UC tests, based on applying large-valued DC current signals, are reviewed, and in order to carry out the mentioned UC tests, a power electronic converter-based UC test system is designed and implemented. In order to demonstrate the performance of the implemented UC test system experimentally, a laboratory-constructed UC module is utilized. The experimental results demonstrating the electrical performance of the UC module are also included. Additionally, the designed and experimentally verified control system for the energy management (charge-discharge, energy transfer from-to the load, etc.) establishes a good example for the control units and control algorithms of UCs utilizing practical energy systems. 2. Test methods for UCs The test of the UC aims to evaluate the performance and determine the equivalent circuit parameters of the device. The UC equivalent circuit shown in Figure 1 is a relatively simple and generally sufficient model to 631 EROĞLU and HAVA/Turk J Elec Eng & Comp Sci evaluate the performance of UCs for most applications [1,2,6,7,9]. More sophisticated models of UCs can be found in [9–11]. i UC + Rs V UC C UC Rp - Figure 1. Electrical equivalent circuit model of a UC. In the model shown in Figure 1, C U C represents the capacitance, R s (...truncated)