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High-temperature superconducting power transformers with fault current limiting properties
Physica C 372–376 (2002) 1698–1701 www.elsevier.com/locate/physc
High-temperature superconducting power transformers with fault current limiting properties Emmanuel Sissimatos *, Bernd R. Oswald Institute of Electric Power Systems, University of Hanover, Appelstrasse 9A, P.O. Box 6009, 30167 Hanover, Germany
Abstract The superconducting technology presents nowadays an innovation in the area of the electrical power supply. One of the interesting applications is the use of high-temperature superconducting (HTS) windings for power transformers. The advantages of such transformers regarding the losses and the construction volume are far well known. In this paper the influence of superconducting units with fault current limiting coils on the network design and operation is examined from the utility engineers point of view. The specific demands on the transformer design are discussed and the advantages are described. For the computation of the load flow and the short-circuit a simplified model based on the design characteristics is used. The results of the simulations are analysed and compared. With regard also to further superconducting devices, like HTS-cables, a look in the electrical power networks of the near future is attempted. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconducting transformer; SFCL transformer; Fault current limiter; Power system
1. Introduction In the current process of liberalising the electricity market a strong competition in transmission and distribution has been introduced leading to reduced prices for the consumers. The deregulated energy market and the political demand for an increase of alternative power generation forces the utilities to make changes in their power transmission and distribution networks. Simple network constructions and standardised equipment in order to reduce the investment, operational and maintenance costs are in discussion [1]. The optimisation of the networks and the unavoidable lacing of
alternative power generation units, as for example wind parks, are today a challenge for every utility. New technologies as high-temperature superconducting (HTS) equipment could play a key role in this procedure of restructuring. One of these equipment is the HTS transformer. It brings well known benefits such as low losses, smaller weight and volume, etc. However, when fault current limiting property (SFCL transformer) is integrated, the benefits for the entire power system network design and operation are far greater.
2. Application of SFCL transformers in a power system
*
Corresponding author. Fax: +49-511-762-2369/2807. E-mail address: [email protected] (E. Sissimatos).
The impedance of a power system consists of the different impedances of the generators, the
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 1 0 4 - 8
E. Sissimatos, B.R. Oswald / Physica C 372–376 (2002) 1698–1701
transformers and the transmission lines or cables. Until now the transformer impedance could not be reduced to a minimum since it limits the maximal short-circuit current during a fault to the value of Ik ¼ Ir =uk until the circuit breakers react. The term uk represents the voltage impedance of the transformer and it depends only on the transformer design. Typical values of uk for power transformers are between 6% and 20%. The high current density of superconducting wires makes it possible to design HTS transformers with an impedance voltage of <3% [2]. This will reduce the impedance of the system during normal operation considerably, but on the other hand there will be an enormous increase of the short-circuit current in the power grid. Hence, the implementation of a current limiting property to the superconducting windings will be necessary. In order to examine the influence of SFCL transformers we assume the network of Fig. 1. This system consists of a generation unit of 100 MV A which supplies through a transmission line of 110 kV the interconnection network of 380 kV and also a distribution network of 10 kV. This generation unit could be for example an offshore wind farm as on of those that are now in planning in many countries as Germany and Denmark. The electric data of the network are listed in Table 1. The loads are assumed to be 40 MV A (cos / ¼ 0:98) to the 380 kV network and 30 MV A (cos / ¼ 0:98) to the 10 kV network respectively. From the load flow we obtain the following results. The reactive power loss of the network is reduced nearly 80% from 16.75 to 3.40 MV A. Furthermore, a reduction of the active power loss from 1.68 to 1.12 MW is observed mainly due to the low losses in the superconducting windings but also in the transmission lines, since the current through the generator transformer T1 is reduced from 383.6 to
Fig. 1. Power system network.
1699
Table 1 Electric data of the power system network Transformer
uk (conventional, %)
uk (SFCL, %)
T1 (100 MV A) T2 (60 MV A) T3 (40 MV A) Generator Network 380 kV 10 kV
14 15 14 100 MV A/10 kV Sk 10 GV A 700 MV A
3 3 2
377.3 A with the possibility of increasing the transmission capacity. The voltage sag of the 10 kV network decreases from 7.28% to 4.11% which improves the voltage regulation in the network. The stability margin of the generator is increased since the impedance of the power system is reduced, and thus the system has a better steadystate stability. In t ¼ 36 ms, during the zero-passing of the current in phase a of T1, a three-phase fault is occurred in F. The rapid reclosing after 100 ms is not simulated here. The fault currents in all three branches and the resulted current in F calculated with EMTP are shown in Figs. 2 and 3. All three SFCL transformers limit the fault currents before they reach their maximum peak value. After the transient process the three branch currents take approximately the value of the steady state. The maximum fault current in the superconducting
Fig. 2. Currents in phase a of the conventional network.
1700
E. Sissimatos, B.R. Oswald / Physica C 372–376 (2002) 1698–1701
(separated rings) in order to reduce the production cost, but also to make the change of damaged turns cost effectively. The positioning of the different turns of a coil should be made for the fastest reaction during a fault but also for the fastest cool down afterwards. The extreme dependence of the superconducting state of BSCOO 2212 on the applied stray magnetic field of the transformer can be reduced by nested coils and installation of C-cups at the coil ends [4].
4. Advantages of SFCL transformers
Fig. 3. Currents in phase a of the superconducting network.
network would reach in F a peak value of 31.8 kA without the current limiting windings.
3. Design requirements of the SFCL windings The design of a SFCL transformer and the specific parameters of the windings must be studied and fit to the demands of each power system. The limitation of the fault current has to take place before it reaches its peak value. A fast quench of the winding could be achieved by considering also the increase of the stray magnetic field of the transformer. The winding should not limit during the switching on of the no-loaded transformer (inrush current) and the final resistance during a fault should not affect the response of the protecting relays. The insulation has to withstand the overvoltages during the limitation and measures against mechanical stresses have to be considered. Currently, the existing superconducting wires are not appropriate for use in SFCL transformers due to the costs of the silver matrix. But the development of melt cast processed (MCP) BSCCO 2212 bulk material could be a possible alternative for the current limiting windings [3]. Probably, a combination of BSCCO 2223 wire turns and (MCP) BSCCO 2212 bulk turns with FCL characteristics would be the best solution. The bulk turns could be constructed in a modular way
High-temperature superconducting transformers have been designed and prototypes have been constructed in several countries. The high current density of the superconducting wires makes it possible to reduce the volume and the weight of these transformers more than 40% by using less conducting material. By optimising the design further reduction can be achieved [2]. The increase of the efficiency and the reduction of the transformer losses calculated over the lifetime of the transformer are also remarkable. In main power plants there is often a second power transformer installed as a reserve in case of a failure. By reducing the transformer volume and losses it is worth of investigating the use of transformer banks consisting of three 1ph-SFCL transformers with only one 1ph-SFCL transformer as a reserve. The total installed transformer capacity will be reduced dramatically leading to cost savings. The use of the low-cost and environmentally safe liquid nitrogen instead of oil for the insulation and the cooling of the windings is also an important feature. The spill and fire risks associated with dielectric oil are eliminated making it less dangerous to install power transformers near urban areas, in buildings but also in offshore transformer power stations of wind farms. Additionally, the thermal degradation of the insulation during full- and overload having negative influence on transformer lifetime will not occur. More real and reactive power from existing generators is made available by introducing superconducting transformers with a small impedance voltage as it was shown before. The steady-state stability is improved and the in-
E. Sissimatos, B.R. Oswald / Physica C 372–376 (2002) 1698–1701
stallation of capacitor banks for compensation can be reduced, leading to considerable investment cost savings. The capacity and the efficiency of the power network is increased considerably, one of the objectives of the utilities in todays market. It will be possible to connect more consumers to an existing network without upgrading the whole system. By introducing a current limiting functionality to the superconducting transformer the short-circuit current ratings of circuit breakers and in addition the electromagnetic force of other power apparatus can be reduced significantly. The neutral of the SFCL transformer can be grounded directly without use of additional reactors increasing the human safety against step voltage.
1701
protection by introducing SFCL transformers. The installation of HTS cables in the network of Fig. 1 connecting T3 with the 10 kV network increases the consumption of power near the generation unit. This is a main feature of the decentral power supply of todays markets. The SFCL transformer will protect also these cables in case of a fault. The network is becoming more flexible and can match current and future demands efficiently. A new opportunity is opened for the application of power transformers with superconducting windings and current limiting properties (SFCL transformer). Although this transformer is still in the stadium of research and development, it should be implemented into new concepts of electric power networks.
5. Conclusions In spite of the advantages of HTS transformers most of the utilities have not shown any particular interest in the 90s. But today, in the process of liberalising the electricity market, the utilities are interesting in reducing the costs of transmission and improving the stability and capacity of their power system network. These goals can be achieved in a high mass by separating the value of the transformer impedance from the fault current
References [1] R. Hakvoort, in: International Conference on Electric Utility Deregulation and Restructuring and Power Technologies, 2000. [2] E. Sissimatos et al., IEEE Trans. Appl. Supercond. 10 (1) (2001) 1574–1577. [3] M. Noe et al., IEEE Trans. Appl. Supercond. 10 (1) (2001) 1960–1963. [4] A. Godeke et al., IEEE Trans. Appl. Supercond. 10 (1) (2001) 1570–15733.
High-temperature superconducting power transformers with fault current limiting properties Emmanuel Sissimatos *, Bernd R. Oswald Institute of Electric Power Systems, University of Hanover, Appelstrasse 9A, P.O. Box 6009, 30167 Hanover, Germany
Abstract The superconducting technology presents nowadays an innovation in the area of the electrical power supply. One of the interesting applications is the use of high-temperature superconducting (HTS) windings for power transformers. The advantages of such transformers regarding the losses and the construction volume are far well known. In this paper the influence of superconducting units with fault current limiting coils on the network design and operation is examined from the utility engineers point of view. The specific demands on the transformer design are discussed and the advantages are described. For the computation of the load flow and the short-circuit a simplified model based on the design characteristics is used. The results of the simulations are analysed and compared. With regard also to further superconducting devices, like HTS-cables, a look in the electrical power networks of the near future is attempted. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconducting transformer; SFCL transformer; Fault current limiter; Power system
1. Introduction In the current process of liberalising the electricity market a strong competition in transmission and distribution has been introduced leading to reduced prices for the consumers. The deregulated energy market and the political demand for an increase of alternative power generation forces the utilities to make changes in their power transmission and distribution networks. Simple network constructions and standardised equipment in order to reduce the investment, operational and maintenance costs are in discussion [1]. The optimisation of the networks and the unavoidable lacing of
alternative power generation units, as for example wind parks, are today a challenge for every utility. New technologies as high-temperature superconducting (HTS) equipment could play a key role in this procedure of restructuring. One of these equipment is the HTS transformer. It brings well known benefits such as low losses, smaller weight and volume, etc. However, when fault current limiting property (SFCL transformer) is integrated, the benefits for the entire power system network design and operation are far greater.
2. Application of SFCL transformers in a power system
*
Corresponding author. Fax: +49-511-762-2369/2807. E-mail address: [email protected] (E. Sissimatos).
The impedance of a power system consists of the different impedances of the generators, the
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 1 0 4 - 8
E. Sissimatos, B.R. Oswald / Physica C 372–376 (2002) 1698–1701
transformers and the transmission lines or cables. Until now the transformer impedance could not be reduced to a minimum since it limits the maximal short-circuit current during a fault to the value of Ik ¼ Ir =uk until the circuit breakers react. The term uk represents the voltage impedance of the transformer and it depends only on the transformer design. Typical values of uk for power transformers are between 6% and 20%. The high current density of superconducting wires makes it possible to design HTS transformers with an impedance voltage of <3% [2]. This will reduce the impedance of the system during normal operation considerably, but on the other hand there will be an enormous increase of the short-circuit current in the power grid. Hence, the implementation of a current limiting property to the superconducting windings will be necessary. In order to examine the influence of SFCL transformers we assume the network of Fig. 1. This system consists of a generation unit of 100 MV A which supplies through a transmission line of 110 kV the interconnection network of 380 kV and also a distribution network of 10 kV. This generation unit could be for example an offshore wind farm as on of those that are now in planning in many countries as Germany and Denmark. The electric data of the network are listed in Table 1. The loads are assumed to be 40 MV A (cos / ¼ 0:98) to the 380 kV network and 30 MV A (cos / ¼ 0:98) to the 10 kV network respectively. From the load flow we obtain the following results. The reactive power loss of the network is reduced nearly 80% from 16.75 to 3.40 MV A. Furthermore, a reduction of the active power loss from 1.68 to 1.12 MW is observed mainly due to the low losses in the superconducting windings but also in the transmission lines, since the current through the generator transformer T1 is reduced from 383.6 to
Fig. 1. Power system network.
1699
Table 1 Electric data of the power system network Transformer
uk (conventional, %)
uk (SFCL, %)
T1 (100 MV A) T2 (60 MV A) T3 (40 MV A) Generator Network 380 kV 10 kV
14 15 14 100 MV A/10 kV Sk 10 GV A 700 MV A
3 3 2
377.3 A with the possibility of increasing the transmission capacity. The voltage sag of the 10 kV network decreases from 7.28% to 4.11% which improves the voltage regulation in the network. The stability margin of the generator is increased since the impedance of the power system is reduced, and thus the system has a better steadystate stability. In t ¼ 36 ms, during the zero-passing of the current in phase a of T1, a three-phase fault is occurred in F. The rapid reclosing after 100 ms is not simulated here. The fault currents in all three branches and the resulted current in F calculated with EMTP are shown in Figs. 2 and 3. All three SFCL transformers limit the fault currents before they reach their maximum peak value. After the transient process the three branch currents take approximately the value of the steady state. The maximum fault current in the superconducting
Fig. 2. Currents in phase a of the conventional network.
1700
E. Sissimatos, B.R. Oswald / Physica C 372–376 (2002) 1698–1701
(separated rings) in order to reduce the production cost, but also to make the change of damaged turns cost effectively. The positioning of the different turns of a coil should be made for the fastest reaction during a fault but also for the fastest cool down afterwards. The extreme dependence of the superconducting state of BSCOO 2212 on the applied stray magnetic field of the transformer can be reduced by nested coils and installation of C-cups at the coil ends [4].
4. Advantages of SFCL transformers
Fig. 3. Currents in phase a of the superconducting network.
network would reach in F a peak value of 31.8 kA without the current limiting windings.
3. Design requirements of the SFCL windings The design of a SFCL transformer and the specific parameters of the windings must be studied and fit to the demands of each power system. The limitation of the fault current has to take place before it reaches its peak value. A fast quench of the winding could be achieved by considering also the increase of the stray magnetic field of the transformer. The winding should not limit during the switching on of the no-loaded transformer (inrush current) and the final resistance during a fault should not affect the response of the protecting relays. The insulation has to withstand the overvoltages during the limitation and measures against mechanical stresses have to be considered. Currently, the existing superconducting wires are not appropriate for use in SFCL transformers due to the costs of the silver matrix. But the development of melt cast processed (MCP) BSCCO 2212 bulk material could be a possible alternative for the current limiting windings [3]. Probably, a combination of BSCCO 2223 wire turns and (MCP) BSCCO 2212 bulk turns with FCL characteristics would be the best solution. The bulk turns could be constructed in a modular way
High-temperature superconducting transformers have been designed and prototypes have been constructed in several countries. The high current density of the superconducting wires makes it possible to reduce the volume and the weight of these transformers more than 40% by using less conducting material. By optimising the design further reduction can be achieved [2]. The increase of the efficiency and the reduction of the transformer losses calculated over the lifetime of the transformer are also remarkable. In main power plants there is often a second power transformer installed as a reserve in case of a failure. By reducing the transformer volume and losses it is worth of investigating the use of transformer banks consisting of three 1ph-SFCL transformers with only one 1ph-SFCL transformer as a reserve. The total installed transformer capacity will be reduced dramatically leading to cost savings. The use of the low-cost and environmentally safe liquid nitrogen instead of oil for the insulation and the cooling of the windings is also an important feature. The spill and fire risks associated with dielectric oil are eliminated making it less dangerous to install power transformers near urban areas, in buildings but also in offshore transformer power stations of wind farms. Additionally, the thermal degradation of the insulation during full- and overload having negative influence on transformer lifetime will not occur. More real and reactive power from existing generators is made available by introducing superconducting transformers with a small impedance voltage as it was shown before. The steady-state stability is improved and the in-
E. Sissimatos, B.R. Oswald / Physica C 372–376 (2002) 1698–1701
stallation of capacitor banks for compensation can be reduced, leading to considerable investment cost savings. The capacity and the efficiency of the power network is increased considerably, one of the objectives of the utilities in todays market. It will be possible to connect more consumers to an existing network without upgrading the whole system. By introducing a current limiting functionality to the superconducting transformer the short-circuit current ratings of circuit breakers and in addition the electromagnetic force of other power apparatus can be reduced significantly. The neutral of the SFCL transformer can be grounded directly without use of additional reactors increasing the human safety against step voltage.
1701
protection by introducing SFCL transformers. The installation of HTS cables in the network of Fig. 1 connecting T3 with the 10 kV network increases the consumption of power near the generation unit. This is a main feature of the decentral power supply of todays markets. The SFCL transformer will protect also these cables in case of a fault. The network is becoming more flexible and can match current and future demands efficiently. A new opportunity is opened for the application of power transformers with superconducting windings and current limiting properties (SFCL transformer). Although this transformer is still in the stadium of research and development, it should be implemented into new concepts of electric power networks.
5. Conclusions In spite of the advantages of HTS transformers most of the utilities have not shown any particular interest in the 90s. But today, in the process of liberalising the electricity market, the utilities are interesting in reducing the costs of transmission and improving the stability and capacity of their power system network. These goals can be achieved in a high mass by separating the value of the transformer impedance from the fault current
References [1] R. Hakvoort, in: International Conference on Electric Utility Deregulation and Restructuring and Power Technologies, 2000. [2] E. Sissimatos et al., IEEE Trans. Appl. Supercond. 10 (1) (2001) 1574–1577. [3] M. Noe et al., IEEE Trans. Appl. Supercond. 10 (1) (2001) 1960–1963. [4] A. Godeke et al., IEEE Trans. Appl. Supercond. 10 (1) (2001) 1570–15733.