IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 5, OCTOBER 2014
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Energy Exchange Experiments and Performance Evaluations Using an Equivalent Method for a SMES Prototype Xiao Y. Chen, Jian X. Jin, Ying Xin, You G. Guo, Wei Xu, Liang Wen, and Jian G. Zhu
Abstract—This paper presents an energy exchange system for superconducting magnetic energy storage (SMES) study. The controllable power source and equivalent dc load network are with fully user-customizable circuit parameters to imitate various power fluctuations from the equivalent pseudo external system. A new bridge-type chopper is introduced to handle the bi-directional energy exchanges between the high temperature superconducting (HTS) coil and dc load network. A small-scale prototype is developed to experimentally study the buffering effects of SMES for solving different voltage and power fluctuations. The ohmic loss caused by the flux flow in the HTS coil is discussed by integrating a new coil-current-dependent power-law formula and experimental results. The concept of the energy exchange system has the potential to be used in the design, optimization and evaluation of a SMES device prior to its practical applications. Index Terms—Bridge-type chopper, energy exchange, ohmic loss, superconducting magnetic energy storage (SMES), voltage fluctuation.
I. I NTRODUCTION
S
UPERCONDUCTING magnetic energy storage (SMES) systems can store the electric energy in superconducting coils, currently high temperature superconducting (HTS) coils been focused, and release the stored energy when required. A number of SMES devices and systems have been developed and widely used for various power system applications, e.g., output power stabilization in distributed generators or micro girds [1], power flow control in power transmission systems [2], solutions of power quality problems in power distribution systems [3] and uninterrupted power supplies for power consumers [4], etc. In practice, three main types of thyristor based, current source converter (CSC) based and voltage source converter (VSC) based power conditioning systems (PCSs) are adopted to link the superconducting coils to the AC power system. Since the superconducting coils in a SMES device are operated under DC conditions, various DC-AC components including converters, Manuscript received May 22, 2014; accepted July 14, 2014. Date of publication July 31, 2014; date of current version August 20, 2014. X. Y. Chen, J. X. Jin, and L. Wen are with the Center of Applied Superconductivity and Electrical Engineering, School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China (e-mail:
[email protected]). Y. Xin is with the Center of Applied Superconductivity, School of Electrical Engineering and Automation, Tianjin University, Tianjin 300072, China. Y. G. Guo and J. G. Zhu are with the Faculty of Engineering and Information Technology, University of Technology Sydney, Sydney, NSW 2007, Australia. W. Xu is with the School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2014.2344759
filters and transformers are needed to develop a practical PCS. Therefore, most of the current SMES devices have relatively complex system topologies and control strategies. To apply a specific SMES device in the real power grid, on-sit tests are normally carried out to evaluate the interactive influences between the SMES and its external system [1]–[4]. For the superconducting coils used in a SMES device, the targeted power system applications can be transformed into equivalent energy exchange demands. An energy exchange system with equivalent DC circuit topology is consequently proposed and discussed in this paper. The purpose of the proposed energy exchange system is to carry out the performance evaluations of various SMES devices with more convenient operations and lower costs.
II. BASIC P RINCIPLE AND P ROTOTYPE D ESIGN OF THE E NERGY E XCHANGE S YSTEM A. Basic Principle Fig. 1 shows the circuit topology of the proposed energy exchange system. It mainly consists of a controllable power source U , a DC load network, a bridge-type chopper, a DC-link capacitor C and a HTS coil L. The load network has a power-line resistor R1 and three branched power-load resistors R2 –R4 . Each branch can be connected or disconnected to the power-line resistor by controlling its series-connected MOSFET. The power-line resistor is to imitate the consumed power from the power transmission line and power electronic devices in a practical PCS. The power-load resistors are to imitate the external voltage and power fluctuations through the on-line adjustments of U and R2 –R4 . To maintain the voltage UR (t) across the power-load resistors at their rated voltage Ur , the HTS coils should be carried out the dynamic energy exchanges through the on-line conversions of the operation state of the bridge-type chopper. The bridge-type chopper is formed by four MOSFETs S1 –S4 and two reverse diodes D2 , D4 [5]. To carry out the comparative analyses, two MOSFETs S1 , S3 and two reverse diodes D2 , D4 are also used to form a conventional chopper. Besides the three steady operation states of charge state, storage state and discharge state in the conventional chopper, the bridge-type one has two more temporal operation states to avoid the short circuits of the DC-link capacitor. Define the turn-on or turn-off status of a MOSFET as “1” or “0,” all the operation states of the two choppers can be digitalized, as shown in Table I.
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 5, OCTOBER 2014
Fig. 2.
Fig. 1. Circuit topology of the proposed energy exchange system.
Overall setup of the energy exchange prototype. TABLE II S PECIFICATIONS OF THE 0.2 H B I -2223 S OLENOID C OIL
TABLE I D IGITIZED O PERATION S TATES OF THE T WO C HOPPERS
The practical energy exchange processes are described as follows: i) When UR (t) = Ur , the operation state of the two choppers remains in the storage state. ii) When UR (t) > Ur , the operation state is alternately converted between the charge state and storage state to absorb the surplus power. iii) When UR (t) < Ur , the operation state is alternately converted between the discharge state and storage state to compensate the shortfall power. The above two dynamic processes are defined as “charge-storage mode” and “discharge-storage mode”.
firstly charged gradually to a pre-set initial current value I0 . ii) Open S1 and then close S2 , the two choppers are operated in the storage state. iii) Open S5 , and then close one, two or three branched MOSFETs of S6 –S8 , the corresponding branched resistors are connected to the controllable power source through the power-line resistor. iv) Finally, a MCU MSP430 and CPLD EPM240 joint measurement and control module is to implement the on-line voltage monitoring of UR (t), and thus to change the next operation state of the choppers accordingly. III. E NERGY E XCHANGE E XPERIMENTS W ITH B RIDGE -T YPE AND C ONVENTIONAL C HOPPERS
B. Prototype Design
A. Analyses on Energy Exchange Characteristics
Fig. 2 shows the overall setup of the energy exchange prototype. Four OptiMOSTM MOSFETs are introduced to develop the bridge-type and conventional choppers, with the power lines among the four MOSFETs formed by silvered copper bars. Thirty-two conductive polymer aluminum solid electrolytic capacitors are connected in parallel to serve as the DC-link capacitor. The HTS coil is a 0.2 H Bi-2223 solenoid coil immersed in liquid nitrogen (LN2 ). It consists of three solenoids in series for reducing the internal connections. Its main specifications are shown in Table II. The Bi-2223 tapes are the AMSC Bi-2223 high-strength tapes, whose average width is −4.2 mm, average thickness −0.28 mm, critical current −145.8 A at 77 K and self field. The main operation processes of the prototype are described as follows: i) Close S1 , S3 , and S5 , the coil current IL (t) is
In the experiment, the controllable power source U = 15 V is firstly applied to the power-line resistor R1 = 0.25 Ω and two branched power-load resistors R2 = R3 = 1 Ω. Each branched resistor is operated at its rated voltage Ur = 10 V. Assume that one branch and three branches of R2 = R3 = R4 = 1 Ω are connected with the power-line resistor successively, a voltage swell status and a voltage sag status will be appear accordingly. When the 0.2 H Bi-2223 coil is applied, it should be controlled to absorb a mean surplus power Pswell = 100 W and to compensate a mean shortfall power Pshort = 100 W, respectively. As shown in Fig. 3, a whole 100 W energy exchange cycle can be divided into four different segments. The first one is an incomplete absorption segment when IL (t) < 10 A the two choppers are operated in the charge state until UR (t) drops to 10 V. The second one is a complete
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Fig. 4. Measured and calculated results of ηtotal versus I0 .
Fig. 5. Measured and calculated results of ηtotal versus Pref . Fig. 3. Measured results of UR (t) and IL (t) during a 100 W energy exchange cycle: (a) UR (t) versus t; (b) IL (t) versus t.
absorption segment when 10 A ≤ IL (t) ≤ 60 A, the two choppers are operated in the charge-storage mode to maintain UR (t) around 10 V. Once IL (t) reaches its rated operation current ILr = 60 A, the third process enters into a complete compensation segment. In this segment, the two choppers are operated in the discharge-storage mode to maintain UR (t) around 10 V until IL (t) drops to 10 A. The fourth one is an incomplete compensation segment when IL (t) < 10 A, the two choppers are operated in the discharge state to release a decreasing power. To evaluate the performance of SMES before its practical applicatons, the surplus or shortfall power demands from the external system can be simply transformed into the combinations of U and R1 –R4 . Thus the buffering effects of SMES for the given power fluctuations could be obtained equivalently in the above four segments. Since the consumed power from the used OptiMOSTM MOSFETs is much lower over their reverse diodes [5], the bridge-type chopper has shorter absorption time Tabs and longer compensation time Tcom as compared to the conventional one. This means that the bridge-type chopper has higher charge-discharge efficiency ηtotal , which is defined by ηtotal =
Pshort × Tcom . Pswell × Tabs
(1)
From the measured results of the Fig. 3, the ηtotal values of the bridge-type and conventional choppers are about 0.876 and 0.526, respectively. Therefore, the bridge-type chopper is more suitable to apply in the low-voltage end-user applications for high-efficiency SMES operations. In this paper, the bridge-
type chopper is adopted to carry out the energy exchange experiments for SMES study. Fig. 4 shows the measured and calculated results of ηtotal versus I0 . In the 100 W energy exchange cycle, IL (t) increases from I0 to 60 A, and then decreases from 60 A to I0 . It can be seen that ηtotal decreases along with the increment of I0 . This is because the consumed power from the MOSFETs is in direct proportion to the square of the coil current. Fig. 5 shows the measured and calculated results of ηtotal versus Pref . Pref is assumed to be equal to both the Pswell and Pshort . It can be seen that ηtotal increases with a reduced rising slope as Pref increases. B. Analyses on Ohmic Loss Characteristics For the commercial Bi-2223 tapes, a strong anisotropic magnetic field dependence appears in critical current Ic (B// , B⊥ ) and exponential index n(B// , B⊥ ) [6]. The critical current degradation caused by the perpendicular component B⊥ to the widest surface of a Bi-2223 tape is more serious than the parallel component B// with the same magnitude. According to the non-linear power law, the real-time voltage drop E(t) across the Bi-2223 tapes per meter can be calculated by n(B// ,B⊥ ) IL (t) (2) E(t) = Ec × Ic (B// , B⊥ ) where Ec is the critical electric field, 10−4 V/m. Thus, the ohmic loss Pohm (t) of the whole coil is the product of total voltage drop and coil current Pohm (t) =
N i=1
[2πri Ei (t)] × IL (t)
(3)
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 5, OCTOBER 2014
Fig. 6. Magnetic field (T) distributions of the 0.2 H Bi-2223 coil when IL (t) = 60 A: (a) Parallel magnetic field; (b) Perpendicular magnetic field.
Fig. 7. Critical current (A) and ohmic loss (W/m) distributions of the five upper coil layers: (a) Critical current distributions when IL (t) = 40 A; (b) Critical current distributions when IL (t) = 60 A; (c) Ohmic loss distributions when IL (t) = 40 A; (d) Ohmic loss distributions when IL (t) = 60 A.
where N is the total turns of the coil; ri , Ei (t), the geometric radius and voltage drop of i-th turn. In the 0.2 H Bi-2223 coil, the parallel component accounts for the vast majority of the total magnetic field distributed in the inner cavity area. However, the perpendicular component becomes larger as the location gets closer to two coil ends, as shown in Fig. 6. Due to the anisotropy, the turns located at two coil ends have lower critical current and higher ohmic loss as compared to those located at the middle coil part. Fig. 7 shows the critical current and ohmic loss distributions of the five upper coil layers. To ensure all the coil turns work at superconducting state, the maximum Ic (B|| , B⊥ ) calculated is 40 A. The calculated and fitted relations of Pohm (t) versus IL (t) are shown in Fig. 8. It can be seen that Pohm (t) rises slowly when IL (t) is below about 40 A, while it could increase sharply after 40 A. This trend is very similar to the power-law critical state model [7]. Therefore, this paper proposes a coil-currentdependent power-law formula for ohmic loss calculation b IL (t) (4) Pohm (t) = P1 × a × I1 where a and b are the coil-structure-dependent parameters; I1 , the normalizing constant of 1 A; P1 , the normalizing constant of 1 W. For the 0.2 H Bi-2223 coil, the fitted parameters are as follows: when IL (t) ≤ 40 A, a = 3.048 × 10−19 , b = 11.151; when IL (t) > 40 A, a = 2.304 × 10−14 , b = 8.178. The measured IL (t) values can be directly input into the fitted power functions to calculate the ohmic loss Pohm (t) and energy consumption Qohm (t) during the whole chargedischarge cycle, as shown in Figs. 9 and 10. The calculated
Fig. 8.
Calculated and fitted Pohm (t) versus IL (t).
Fig. 9.
Calculated Pohm (t) versus t.
Fig. 10.
Calculated Qohm (t) versus t.
Pohm (t) values at 40 A and 60 A are about 0.22 W and 8.02 W, respectively. The calculated Qohm (t) values in the chargestorage mode and discharge-storage mode are about 6.49 J and 5.32 J, respectively. From the Figs. 4 and 5, the measured ηtotal values are slightly lower than the calculated values. This is because that the calculated curves only take the MOSFETs’ conduction losses into account. If the corresponding energy consumption Qohm (t) in the Fig. 10 is also added, the measured and calculated ηtotal values will be better matched. The average ohmic power Pavg per cycle can be calculated by Pavg
1 = T
T Pohm (t) dt
(5)
0
where T is the time duration of a charge-discharge cycle. From the Figs. 3 and 9, T is about 7.21 s, and Pavg is about 1.64 W. Ignoring the heat leakages from the cryogenic Dewar and current leads, repeated operations of such a charge-discharge cycle within one day will produce about 141.69 kJ energy consumption, and thus consume about 0.88 L LN2 at one
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Fig. 12. Calculated Pohm (t) versus t.
Fig. 11. Measured results of UR (t) and IL (t) during five 100 W energy exchange cycles: (a) UR (t) versus t; (b) IL (t) versus t. Fig. 13. Calculated Qohm (t) versus t.
atmosphere. If a GM-type cryocooler is applied, the realistic specific power is about 12–20 times of that in 77 K [7]. The realistic specific power is about 19.68–32.80 W.
IV. M ULTI -C YCLE E NERGY E XCHANGE E XPERIMENTS W ITH B RIDGE -T YPE C HOPPER Besides the independent charge and discharge tests, the developed prototype can also be used to carry out multi-cycle energy exchange tests to simulate a variety of application conditions. Fig. 11 shows the measured results of UR (t) and IL (t) during five charge-discharge cycles. The 0.2 H Bi-2223 coil is applied to absorb a mean surplus power Pswell = 100 W during the first half cycle, and then to compensate a mean shortfall power Pshort = 100 W during the residual half cycle. Due to the energy consumptions from the conduction losses of the MOSFETs and ohmic loss of the Bi-2223 coil itself, IL (t) decreases continuously after each charge-discharge cycle. From the Fig. 11(b), IL (t) at the ends of the first cycle to fifth cycle are reduced to 24.78 A, 19.66 A, 13.69 A, 5.74 A, and 0.21 A, respectively. It is noticed that an incomplete compensation segment and an incomplete absorption segment appear successively within the time range from about 19.91 s to 20.09 s. The corresponding lowest and highest offset voltage values are about 9.41 V and 10.72 V, respectively. Figs. 12 and 13 show the corresponding ohmic loss Pohm (t) and energy consumption Qohm (t). The calculated Pavg values from the first cycle to fifth cycle are 1.21 W, 0.78 W, 0.52 W, 0.36 W, and 0.26 W, respectively. The corresponding Qohm (t) values are 6.02 J, 9.92 J, 12.53 J, 14.35 J, and 15.66 J, respectively.
V. C ONCLUSION An energy exchange system with equivalent DC circuit topology has been designed and described for SMES device verification study. As compared to the conventional chopper, the bridge-type chopper with higher charge-discharge efficiency is more suitable to build a SMES test apparatus and to be applied in various low-voltage power applications. A coilcurrent-dependent power-law formula for ohmic loss calculation has been derived by using a finite element model. The measured HTS coil current data can be directly input into the formula to calculate the energy consumption of the coil itself during the charge-discharge operations. R EFERENCES [1] H. Y. Jung et al., “A study on the operating characteristics of SMES for the dispersed power generation system,” IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 2028–2031, Jun. 2009. [2] M. Sander, R. Gehring, and H. Neumann, “LIQHYSMES—A 48 GJ toroidal MgB2-SMES for buffering minute and second fluctuations,” IEEE Trans. Appl. Supercond., vol. 23, no. 3, p. 5700505, Jun. 2013. [3] T. Katagiri et al., “Field test result of 10MVA/20MJ SMES for load fluctuation compensation,” IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 1993–1998, Jun. 2009. [4] R. Kreutz et al., “Design of a 150 kJ high-Tc SMES (HSMES) for a 20 kVA uninterruptible power supply system,” IEEE Trans. Appl. Supercond., vol. 13, no. 2, pp. 1860–1862, Jun. 2003. [5] J. X. Jin et al., “Development of a new bridge-type chopper for low-voltage SMES applications,” in Proc. IEEE Energy Convers. Congr. Expo., Denver, CO, USA, Sep. 15–19, 2013, pp. 5258–5265. [6] B. Dutoit, M. Sjöström, and S. Stavrev, “Bi(2223) Ag sheathed tape Ic and exponent n characterization and modeling under DC applied magnetic field,” IEEE Trans. Appl. Supercond., vol. 9, no. 2, pp. 809–812, Jun. 1999. [7] F. Grilli et al., “Computation of losses in HTS under the action of varying magnetic fields and currents,” IEEE Trans. Appl. Supercond., vol. 24, no. 1, p. 8200433, Feb. 2014.
