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SMES application for frequency control during islanded microgrid operation
Physica C 484 (2013) 282–286
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
SMES application for frequency control during islanded microgrid operation A-Rong Kim a, Gyeong-Hun Kim a, Serim Heo a, Minwon Park a, In-Keun Yu a,⇑, Hak-Man Kim b a b
Changwon National University, Sarim-dong, Changwon 641-773, Republic of Korea University of Incheon, Songdo-dong, Incheon 406-772, Republic of Korea
a r t i c l e
i n f o
Article history: Accepted 27 March 2012 Available online 13 April 2012 Keywords: Frequency control Islanded operation Microgrid SMES
a b s t r a c t This paper analyzes the operating characteristics of a superconducting magnetic energy storage (SMES) for the frequency control of an islanded microgrid operation. In the grid-connected mode of a microgrid, an imbalance between power supply and demand is solved by a power trade with the upstream power grid. The difference in the islanded mode is a critical problem because the microgrid is isolated from any power grid. For this reason, the frequency control during islanded microgrid operation is a challenging issue. A test microgrid in this paper consisted of a wind power generator, a PV generation system, a diesel generator and a load to test the feasibility of the SMES for controlling frequency during islanded operation as well as the transient state varying from the grid-connected mode to the islanded mode. The results show that the SMES contributes well for frequency control in the islanded operation. In addition, a dual and a single magnet type of SMES have been compared to demonstrate the control performance. The dual magnet has the same energy capacity as the single magnet, but there are two superconducting coils and each coil has half inductance of the single magnet. The effectiveness of the SMES application with the simulation results is discussed in detail. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently interests in the microgrid have been growing as a new eco-friendly energy system because it has renewable energy sources such as wind power and solar power. It is anticipated that many microgrids will penetrate into power grids, especially electric power distribution systems, in the near future [1,2]. The microgrid can be operated in two modes: the grid-connected mode and the islanded mode. In the grid-connected mode, a microgrid is interconnected to an upstream power grid. On the other hand, as an islanded operation, microgrid is disconnected from the upstream power grid by fault occurrence in the upstream power grid in order to protect its systems and devices. A very important requirement of microgrid operation is to maintain a frequency, such as 50 Hz or 60 Hz. The frequency is closely related to the balance between power supply and demand. In the grid-connected mode, a difference of the balance is solved by a power trade with the upstream power grid. However, the difference in the islanded mode causes a serious problem to frequency control [3,4]. With these points as background, a superconducting energy storage system (SMES) is a very effective device to overcome this problem [5–9]. In this paper, an SMES has been applied to the frequency control during islanded microgrid operation. The simulated microgrid con⇑ Corresponding author. Address: 55306, Changwon National University, Changwon 641-773, Republic of Korea. Tel./fax: +82 55 281 3150. E-mail addresses: [email protected] (A.-R. Kim), yuik@changwon. ac.kr (I.-K. Yu). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2012.03.065
sisted of a 30 kW wind power generator, a 20 kW solar power generator, a 50 kW diesel generator, and a 75 kW load to test the feasibility of the 640 kJ SMES for controlling frequency during islanded operation using power system computer aided design/ electromagnetic transient including DC (PSCAD/EMTDC). For frequency stabilization, high speed compensation and high output power are required [10,11]. Therefore, a dual magnet SMES which has two superconducting coils and the same energy capacity as the single magnet was considered in this simulation [12]. The dual magnet SMES operates under higher range of operating current than the single magnet SMES because the dual magnet is less affected than the single magnet by the magnetic field to the superconducting wire. Additionally, the current of dual magnet is twice of single magnet. Therefore, the output power of dual magnet is also twice even the dual magnet has the same energy capacity as the single magnet. The simulation results of the frequency control between the dual and single magnet SMES systems were compared and the effectiveness of the SMES application for microgrid is discussed in detail.
2. Configuration of the microgrid in PSCAD/EMTDC 2.1. Grid-connected operation mode Fig. 1 shows the configuration of the microgrid in this simulation in PSCAD/EMTDC. There are a 30 kW wind power generator,
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283
Fig. 1. System configuration of a microgrid.
Table 1 Specifications of the wind power generator and photovoltaic system. System
Parameters
Values
Wind power generator
Rated power Rotor diameter Generator type Cut in speed Rated speed Cut out speed Rated rotate speed Rated power per panel Max. power voltage Max. power current Open circuit voltage Short circuit current Array Total rated power
30 kW 11 m PMSG 3 m/s 12 m/s 25 m/s 120 rev/min 0.2 kW 25.6 V 7.85 A 33.1 V 8.47 A 10 10 20 kW
PV generation system
a 20 kW PV generation system, a 50 kW diesel generator, and a 60 kW load to test the operational feasibility of the microgrid. Output power of the wind power generator and the PV generation system fluctuated by variable wind velocity and irradiance. A static switch (STS) opened to disconnect the microgrid from the upstream power grid. The type of wind power generator is permanent magnet synchronous generator (PMSG). Rotating speed and maximum power point control are possible by back to back converter. In this simulation, the generator side converter controls maximum power point and the grid side converter controls dc link voltage between
the converters as constant. PV generation system consists of PV array module, DC–DC converter and three-phase DC–AC converter. The output power is controlled by maximum power point control. The specifications of the wind power generator and photovoltaic system are shown in Table 1. Fig. 2 indicates the output power of all generators and load in the microgrid. The load is fixed in and the constant active and reactive powers are absorbed. The PV generation system is connected to the grid with DC–DC converter, three-phase DC–AC converter and transformer. The dc link voltage of the converter is 700 V. 2.2. Islanded operation mode To disconnect the microgrid from the upstream power grid, the STS was opened after 5 s of a three phase short circuit. Fig. 3a shows the frequency fluctuation during the islanded operation mode. To estimate the SMES operation, the islanded mode was started and finished during 5 s. The frequency was fallen when STS was opened, and the frequency was increased and fluctuated after reclosing the STS. At that time, the output powers of the other generators also fluctuated extremely as shown in Fig. 3b. 3. Frequency stabilization by the SMES system To stabilize the frequency fluctuation, an SMES system releases or absorbs energy. The SMES system consists of a transformer, a DC–AC converter to control the DC link voltage as constant, a
Fig. 2. Output powers of the generators and load.
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Fig. 3. Simulation results during islanded operation mode, (a) frequency fluctuation; and (b) active output power of generators.
Fig. 4. Control block diagram of the DC–DC chopper for the SMES system, (a) single magnet; and (b) dual magnet.
A.-R. Kim et al. / Physica C 484 (2013) 282–286
285
Fig. 5. Comparison results between the single and dual magnet SMES, (a) the utility frequency; (b) the operating current; (c) the DC link voltage.
DC–DC chopper to charge and discharge the current and an inductor component for the SMES coil in PSCAD/EMTDC. Fig. 4a represents the control block diagram of the single magnet SMES, D–AC converter, transformer and DC–DC chopper for the SMES system. Fig. 4b shows the circuit diagram of dual magnet. In case of the dual magnet SMES, there are two inductor coils and two DC–DC chopper for each coil but one DC–AC converter and transformer. The control algorithm of the single and dual magnet for frequency stabilization is same. The differences between the single and dual magnet are power ratio and upper limit for current. The power ratio of the dual magnet is specified twice the single magnet, and the current is limited by each power ratio. The dual magnet can output twice power of those of single magnet when the frequency is fluctuated. If utility frequency is over 60.1 Hz, the SMES current
is charged, and the SMES current is discharged when utility frequency is less than 59.9 Hz. The capacity of both the single and the dual magnet SMES systems is 0.64 MJ. One of dual magnet has half the inductance of the single magnet, but the rated current and the DC link voltage of the DC–AC converter, are the same as for the single magnet. Therefore, the dual magnet SMES has twice the output power of the single magnet SMES [9]. The inductance of a single magnet is 8 H, the rated current is 400 A and the DC link voltage is controlled at 10 kV. Even if the microgrid is operated in islanded mode, the frequency is controlled within 60 ± 0.1 Hz by the single and dual magnet SMES as shown in Fig 5. Fig. 5a shows that the frequency is more stabilized by the dual magnet SMES. Fig. 5b indicates the operating current. The current of both types of magnet SMES
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systems was initially charged at 200 A. During stabilization, the dc link voltage of the DC–AC converter was well controlled as shown in Fig. 5c. The dual magnet SMES has guaranteed the requirement even though it has the same capacity as the single magnet due to the higher range of operating current than the single magnet SMES. According to the results, the frequency stabilization performance of the dual magnet SMES is better than the single magnet. In the simulation, there was no severe problem such as large voltage spike across the inductor when the SMES coil was changed from charging state to discharging state. But, the spike may be appeared during the operation. In that case, a capacitor will be needed on the right hand side terminal of the inductor.
wire. Therefore, the authors will carry out the magnetic field analysis of the dual and single magnet to estimate the operating range by current variation in the near future. Acknowledgments This work was supported by the Power Generation & Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20111020400220), and in part by Changwon National University in 2011–2012. References
4. Conclusions In this paper, an SMES system has been applied for frequency control during islanded microgrid operation. In the islanded operation mode, the frequency was extremely fluctuated and it caused critical problem. To control the frequency, the SMES system was applied to the grid and the simulation was conducted using PSCAD/EMTDC. The islanded operation mode was considered during 5 s because the SMES system has fast response time to charge and discharge the energy. As a result, the frequency of the islanded microgrid has been effectively controlled by the SMES system. In addition, the application effects between the dual and the single magnet SMES have been compared and the dual magnet SMES has better performance for the frequency stabilization than that of the single magnet. This software based simulation considered the inductance, operating current and output power of the dual and single magnet SMES. However, the magnetic field variations according to the current of the coils have to be considered because the operating current varies due to the magnetic field, especially the perpendicular magnetic field to the superconducting
[1] N. Hatziargyriou, H. Asano, H.R. Iravani, C. Marnay, IEEE Power and Energy Magazine 5 (2007) 78. [2] M. Barnes, J. Kondoh, H. Asano, J. Oyarzabal, G. Ventakaramanan, R. Lasseter, in: International Conference of System of Systems Engineering (2007) 1. [3] H.M. Kim, T. Kinoshita, Y. Lim, Journal of Electrical Engineering & Technology 6 (2011) 284. [4] H.M. Kim, T. Kinoshita, M.C. Shin, Energies 3 (2010) 246. [5] T. Asao, R. Takahashi, T. Murata, J. Tamura, M. Kubo, Y. Matsumura, A. Kuwayama, T. Matsumoto, Electrical Machines and Systems 8 (2007) 302. [6] A.R. Kim, G.H. Kim, J.H. Kim, M.H. Ali, M. Park, I.K. Yu, H.J. Kim, S. Kim, K.C. Seong, IEEE Transactions on Applied Superconductivity 18 (2008) 705. [7] M.H. Ali, T. Murata, J. Tamura, Power Electronics and Drives System 2 (2005) 1611. [8] A.R. Kim, S.Y. Kim, K.M. Kim, J.G. Kim, M. Park, I.K. Yu, S. Lee, S. Kim, M.H. Sohn, H.J. Kim, J.H. Bae, K.C. Seong, IEEE Transactions on Applied Superconductivity 21 (2011) 1367. [9] A.R. Kim, G.H. Kim, K.M. Kim, D.W. Kim, M. Park, I.K. Yu, S.H. Kim, S. Kim, M.H. Sohn, K.C. Seong, Physica C 470 (2010) 1695. [10] A.R. Kim, J.G. Kim, S. Kim, M. Park, I.K. Yu, K.C. Seong, K. Watanabe, Physica C 471 (2011) 1404. [11] H.Y. Jung, A.R. Kim, J.H. Kim, M. Park, I.K. Yu, S.H. Kim, K. Sim, H.J. Kim, K.C. Seong, T. Asao, J. Tamura, IEEE Transactions on Applied Superconductivity 19 (2009) 2028. [12] A.R. Kim, H.R. Seo, G.H. Kim, M. Park, I.K. Yu, Y. Otsuki, J. Tamura, S. Kim, K. Sim, K.C. Seong, IEEE Transactions on Applied Superconductivity 20 (2010) 1334.
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
SMES application for frequency control during islanded microgrid operation A-Rong Kim a, Gyeong-Hun Kim a, Serim Heo a, Minwon Park a, In-Keun Yu a,⇑, Hak-Man Kim b a b
Changwon National University, Sarim-dong, Changwon 641-773, Republic of Korea University of Incheon, Songdo-dong, Incheon 406-772, Republic of Korea
a r t i c l e
i n f o
Article history: Accepted 27 March 2012 Available online 13 April 2012 Keywords: Frequency control Islanded operation Microgrid SMES
a b s t r a c t This paper analyzes the operating characteristics of a superconducting magnetic energy storage (SMES) for the frequency control of an islanded microgrid operation. In the grid-connected mode of a microgrid, an imbalance between power supply and demand is solved by a power trade with the upstream power grid. The difference in the islanded mode is a critical problem because the microgrid is isolated from any power grid. For this reason, the frequency control during islanded microgrid operation is a challenging issue. A test microgrid in this paper consisted of a wind power generator, a PV generation system, a diesel generator and a load to test the feasibility of the SMES for controlling frequency during islanded operation as well as the transient state varying from the grid-connected mode to the islanded mode. The results show that the SMES contributes well for frequency control in the islanded operation. In addition, a dual and a single magnet type of SMES have been compared to demonstrate the control performance. The dual magnet has the same energy capacity as the single magnet, but there are two superconducting coils and each coil has half inductance of the single magnet. The effectiveness of the SMES application with the simulation results is discussed in detail. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently interests in the microgrid have been growing as a new eco-friendly energy system because it has renewable energy sources such as wind power and solar power. It is anticipated that many microgrids will penetrate into power grids, especially electric power distribution systems, in the near future [1,2]. The microgrid can be operated in two modes: the grid-connected mode and the islanded mode. In the grid-connected mode, a microgrid is interconnected to an upstream power grid. On the other hand, as an islanded operation, microgrid is disconnected from the upstream power grid by fault occurrence in the upstream power grid in order to protect its systems and devices. A very important requirement of microgrid operation is to maintain a frequency, such as 50 Hz or 60 Hz. The frequency is closely related to the balance between power supply and demand. In the grid-connected mode, a difference of the balance is solved by a power trade with the upstream power grid. However, the difference in the islanded mode causes a serious problem to frequency control [3,4]. With these points as background, a superconducting energy storage system (SMES) is a very effective device to overcome this problem [5–9]. In this paper, an SMES has been applied to the frequency control during islanded microgrid operation. The simulated microgrid con⇑ Corresponding author. Address: 55306, Changwon National University, Changwon 641-773, Republic of Korea. Tel./fax: +82 55 281 3150. E-mail addresses: [email protected] (A.-R. Kim), yuik@changwon. ac.kr (I.-K. Yu). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2012.03.065
sisted of a 30 kW wind power generator, a 20 kW solar power generator, a 50 kW diesel generator, and a 75 kW load to test the feasibility of the 640 kJ SMES for controlling frequency during islanded operation using power system computer aided design/ electromagnetic transient including DC (PSCAD/EMTDC). For frequency stabilization, high speed compensation and high output power are required [10,11]. Therefore, a dual magnet SMES which has two superconducting coils and the same energy capacity as the single magnet was considered in this simulation [12]. The dual magnet SMES operates under higher range of operating current than the single magnet SMES because the dual magnet is less affected than the single magnet by the magnetic field to the superconducting wire. Additionally, the current of dual magnet is twice of single magnet. Therefore, the output power of dual magnet is also twice even the dual magnet has the same energy capacity as the single magnet. The simulation results of the frequency control between the dual and single magnet SMES systems were compared and the effectiveness of the SMES application for microgrid is discussed in detail.
2. Configuration of the microgrid in PSCAD/EMTDC 2.1. Grid-connected operation mode Fig. 1 shows the configuration of the microgrid in this simulation in PSCAD/EMTDC. There are a 30 kW wind power generator,
A.-R. Kim et al. / Physica C 484 (2013) 282–286
283
Fig. 1. System configuration of a microgrid.
Table 1 Specifications of the wind power generator and photovoltaic system. System
Parameters
Values
Wind power generator
Rated power Rotor diameter Generator type Cut in speed Rated speed Cut out speed Rated rotate speed Rated power per panel Max. power voltage Max. power current Open circuit voltage Short circuit current Array Total rated power
30 kW 11 m PMSG 3 m/s 12 m/s 25 m/s 120 rev/min 0.2 kW 25.6 V 7.85 A 33.1 V 8.47 A 10 10 20 kW
PV generation system
a 20 kW PV generation system, a 50 kW diesel generator, and a 60 kW load to test the operational feasibility of the microgrid. Output power of the wind power generator and the PV generation system fluctuated by variable wind velocity and irradiance. A static switch (STS) opened to disconnect the microgrid from the upstream power grid. The type of wind power generator is permanent magnet synchronous generator (PMSG). Rotating speed and maximum power point control are possible by back to back converter. In this simulation, the generator side converter controls maximum power point and the grid side converter controls dc link voltage between
the converters as constant. PV generation system consists of PV array module, DC–DC converter and three-phase DC–AC converter. The output power is controlled by maximum power point control. The specifications of the wind power generator and photovoltaic system are shown in Table 1. Fig. 2 indicates the output power of all generators and load in the microgrid. The load is fixed in and the constant active and reactive powers are absorbed. The PV generation system is connected to the grid with DC–DC converter, three-phase DC–AC converter and transformer. The dc link voltage of the converter is 700 V. 2.2. Islanded operation mode To disconnect the microgrid from the upstream power grid, the STS was opened after 5 s of a three phase short circuit. Fig. 3a shows the frequency fluctuation during the islanded operation mode. To estimate the SMES operation, the islanded mode was started and finished during 5 s. The frequency was fallen when STS was opened, and the frequency was increased and fluctuated after reclosing the STS. At that time, the output powers of the other generators also fluctuated extremely as shown in Fig. 3b. 3. Frequency stabilization by the SMES system To stabilize the frequency fluctuation, an SMES system releases or absorbs energy. The SMES system consists of a transformer, a DC–AC converter to control the DC link voltage as constant, a
Fig. 2. Output powers of the generators and load.
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Fig. 3. Simulation results during islanded operation mode, (a) frequency fluctuation; and (b) active output power of generators.
Fig. 4. Control block diagram of the DC–DC chopper for the SMES system, (a) single magnet; and (b) dual magnet.
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285
Fig. 5. Comparison results between the single and dual magnet SMES, (a) the utility frequency; (b) the operating current; (c) the DC link voltage.
DC–DC chopper to charge and discharge the current and an inductor component for the SMES coil in PSCAD/EMTDC. Fig. 4a represents the control block diagram of the single magnet SMES, D–AC converter, transformer and DC–DC chopper for the SMES system. Fig. 4b shows the circuit diagram of dual magnet. In case of the dual magnet SMES, there are two inductor coils and two DC–DC chopper for each coil but one DC–AC converter and transformer. The control algorithm of the single and dual magnet for frequency stabilization is same. The differences between the single and dual magnet are power ratio and upper limit for current. The power ratio of the dual magnet is specified twice the single magnet, and the current is limited by each power ratio. The dual magnet can output twice power of those of single magnet when the frequency is fluctuated. If utility frequency is over 60.1 Hz, the SMES current
is charged, and the SMES current is discharged when utility frequency is less than 59.9 Hz. The capacity of both the single and the dual magnet SMES systems is 0.64 MJ. One of dual magnet has half the inductance of the single magnet, but the rated current and the DC link voltage of the DC–AC converter, are the same as for the single magnet. Therefore, the dual magnet SMES has twice the output power of the single magnet SMES [9]. The inductance of a single magnet is 8 H, the rated current is 400 A and the DC link voltage is controlled at 10 kV. Even if the microgrid is operated in islanded mode, the frequency is controlled within 60 ± 0.1 Hz by the single and dual magnet SMES as shown in Fig 5. Fig. 5a shows that the frequency is more stabilized by the dual magnet SMES. Fig. 5b indicates the operating current. The current of both types of magnet SMES
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A.-R. Kim et al. / Physica C 484 (2013) 282–286
systems was initially charged at 200 A. During stabilization, the dc link voltage of the DC–AC converter was well controlled as shown in Fig. 5c. The dual magnet SMES has guaranteed the requirement even though it has the same capacity as the single magnet due to the higher range of operating current than the single magnet SMES. According to the results, the frequency stabilization performance of the dual magnet SMES is better than the single magnet. In the simulation, there was no severe problem such as large voltage spike across the inductor when the SMES coil was changed from charging state to discharging state. But, the spike may be appeared during the operation. In that case, a capacitor will be needed on the right hand side terminal of the inductor.
wire. Therefore, the authors will carry out the magnetic field analysis of the dual and single magnet to estimate the operating range by current variation in the near future. Acknowledgments This work was supported by the Power Generation & Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20111020400220), and in part by Changwon National University in 2011–2012. References
4. Conclusions In this paper, an SMES system has been applied for frequency control during islanded microgrid operation. In the islanded operation mode, the frequency was extremely fluctuated and it caused critical problem. To control the frequency, the SMES system was applied to the grid and the simulation was conducted using PSCAD/EMTDC. The islanded operation mode was considered during 5 s because the SMES system has fast response time to charge and discharge the energy. As a result, the frequency of the islanded microgrid has been effectively controlled by the SMES system. In addition, the application effects between the dual and the single magnet SMES have been compared and the dual magnet SMES has better performance for the frequency stabilization than that of the single magnet. This software based simulation considered the inductance, operating current and output power of the dual and single magnet SMES. However, the magnetic field variations according to the current of the coils have to be considered because the operating current varies due to the magnetic field, especially the perpendicular magnetic field to the superconducting
[1] N. Hatziargyriou, H. Asano, H.R. Iravani, C. Marnay, IEEE Power and Energy Magazine 5 (2007) 78. [2] M. Barnes, J. Kondoh, H. Asano, J. Oyarzabal, G. Ventakaramanan, R. Lasseter, in: International Conference of System of Systems Engineering (2007) 1. [3] H.M. Kim, T. Kinoshita, Y. Lim, Journal of Electrical Engineering & Technology 6 (2011) 284. [4] H.M. Kim, T. Kinoshita, M.C. Shin, Energies 3 (2010) 246. [5] T. Asao, R. Takahashi, T. Murata, J. Tamura, M. Kubo, Y. Matsumura, A. Kuwayama, T. Matsumoto, Electrical Machines and Systems 8 (2007) 302. [6] A.R. Kim, G.H. Kim, J.H. Kim, M.H. Ali, M. Park, I.K. Yu, H.J. Kim, S. Kim, K.C. Seong, IEEE Transactions on Applied Superconductivity 18 (2008) 705. [7] M.H. Ali, T. Murata, J. Tamura, Power Electronics and Drives System 2 (2005) 1611. [8] A.R. Kim, S.Y. Kim, K.M. Kim, J.G. Kim, M. Park, I.K. Yu, S. Lee, S. Kim, M.H. Sohn, H.J. Kim, J.H. Bae, K.C. Seong, IEEE Transactions on Applied Superconductivity 21 (2011) 1367. [9] A.R. Kim, G.H. Kim, K.M. Kim, D.W. Kim, M. Park, I.K. Yu, S.H. Kim, S. Kim, M.H. Sohn, K.C. Seong, Physica C 470 (2010) 1695. [10] A.R. Kim, J.G. Kim, S. Kim, M. Park, I.K. Yu, K.C. Seong, K. Watanabe, Physica C 471 (2011) 1404. [11] H.Y. Jung, A.R. Kim, J.H. Kim, M. Park, I.K. Yu, S.H. Kim, K. Sim, H.J. Kim, K.C. Seong, T. Asao, J. Tamura, IEEE Transactions on Applied Superconductivity 19 (2009) 2028. [12] A.R. Kim, H.R. Seo, G.H. Kim, M. Park, I.K. Yu, Y. Otsuki, J. Tamura, S. Kim, K. Sim, K.C. Seong, IEEE Transactions on Applied Superconductivity 20 (2010) 1334.