This disclosure relates to fault current limiters, and more particularly to fault current limiters used to improve transient stability of a power system.
A fault current limiter is a device that limits fault currents in a power system. The power system may include transmission and distribution networks to deliver power to differing loads. A fault current is an abnormal current in an electrical circuit due to a fault such as a short circuit resulting in a short circuit current. A fault current may occur due to severe weather damaging power lines and components, e.g., lighting striking the power system. When faults occur, a very small load appears instantaneously. The network, in response, delivers a large amount of current (i.e. fault current) to this load or, in this case, the faults. This surge or fault current condition is undesirable as the condition may damage the network or equipment connected to the network. In particular, the network and the equipment connected thereto may burn or, in some cases, explode.
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The FCL 106 may be a superconducting fault current limiter (SCFCL). In general, a SCFCL has a superconductor that is normally in a superconducting state with essentially a negligible resistance during the steady state time interval. During a fault condition, the superconductor transitions from the superconducting state to a normal conducting state (quench). This extra resistance reduces or limits the fault current during the fault condition time interval.
A conventional FCL 106 is primarily dedicated to the fault current limiting function only. An equivalent impedance of the FCL 106 is given in Cartesian form by ZFCL=RFCL+jXFCL, where the real part of impedance is the resistance (RFCL) and the imaginary part is the reactance (XFCL). The SI unit for both the resistance (RFCL) and reactance (XFCL) is the ohm. As described above, the ratio of reactance to resistance or the XFCL/RFCL ratio is greater than 30 for the conventional FCL 106. In most instances, it is typically as high as 100-300.
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Accordingly, there is a need in the art for a fault current limiter that overcomes the above-described inadequacies and shortcomings.
A fault current limiter that maximizes transient stability by minimizing the power swing experienced by the generator during a fault condition is disclosed. A superconducting fault current limiter (SCFCL) is used, whereby the impedance of the SCFCL changes in the presence of a fault. In parallel with the SCFCL is a shunt impedance, which is the impedance seen by the generator during the fault. By decreasing the ratio of the reactance of the shunt impedance to its resistance, the stability of the power system may be enhanced.
For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which like elements are referenced with like numerals, and in which:
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout.
Since the SCFCL 407 and shunt reactor 408 are in parallel, an equivalent circuit can be created, where the series impedance (ZFCL) is equal to RFCL where ZFCL is the parallel combination of Zsh and Zsc.
As mentioned above, under normal operation, the impedance of the superconducting fault current limiter 407 is roughly equal to 0, and therefore, the series impedance (ZFCL) during normal operation is approximately equal to 0 as well. During a fault, the resistance (Rsc) of the SCFCL 407 may increase to a very large value, such that the series impedance (ZFCL) is roughly equal the impedance of the shunt reactor (Zsh=Rsh+jXsh).
Advantageously, the fault current limiter (FCL) 406 has an XFCL/RFCL ratio less than or equal to 30.
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The plots are with the FCL 406 having an XFCL/RFCL ratio less than or equal to 30. In this case, the value of RFCL was much larger than that used in
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The SCFL 600 may include other components such as an enclosure or tank 602 defining a chamber therein. In one embodiment, the tank 602 may be thermally and/or electrically insulating tank 602 such as those made with fiberglass or other dielectric material. In another embodiment, the tank 602 may be a metallic tank comprising inner and outer layers 602a and 602b, and a thermally and/or electrically insulating medium interposed there between. Within the tank 602, there may be one or more fault current limiting units 620 which, for the purpose of clarity and simplicity, are shown as a block. One or more superconducting circuits may be disposed in the fault current limiting units 620.
The SCFL 600 may also comprise one or more electrical bushings 616. The bushings 616 may comprise an inner conductive material (not shown) and an outer insulator. The distal end of the bushings 616 may be coupled to a respective power line 642 (642a and 642b) via terminals 644 and 646. The power lines 642 may be transmission or distribution lines of a power system. The inner conductive material in the bushings 616 may connect the terminals 644 and 646 of the bushings 616 to the fault current limiting unit 620. Meanwhile, the outer insulator is used to insulate the tank 602 from the inner conductive material, thereby allowing the tank 602 and the terminals 644 and 646 to be at different electrical potentials.
The temperature of one or more fault current limiting units 620 may be maintained at a desired temperature range by coolant 614 contained in the tank 602. In one embodiment, it may be desirable to maintain the fault current limiting units 620 at a low temperature, for example, ˜77° K. To maintain at such a low temperature range, liquid nitrogen or helium gas may be used as coolant 614. In another embodiment, it may be desirable to maintain the temperature of the one or more fault current limiting units 620 at other temperature range, and other types of coolant, in gaseous or liquid form, may also be used. For example, it may be desirable to maintain the temperature of the fault current limiting units 620 at a room temperature. In such a case, air maintained at a room temperature may also be used as the coolant 614. When introduced, the coolants 614 may enter the tank 602 via a feed line (not shown) and a port 615 coupled to the tank 602. In the present disclosure, the feed line and the port 615 may preferably be made from thermally and/or electrically insulating material. If the feed line and the port 615 do not provide grounding of the tank 602 or any component contained therein, they may be made from any type of material.
The tank 602 may be supported from the ground by an optional external support 634. Meanwhile, the fault current limiting units 620 may be supported from the tank 602 by an optional internal support 632. Those of ordinary skill in the art may recognize that both of the internal supports 632 and the external support 634 may be optional as the fault current limiting units 620 may be supported from the tank 602 by some other components. If included, each of the internal support 632 and the external support 634 may preferably be made from thermally and/or electrically insulating material.
In operation, a superconductor of the fault current limiting units 620 is in a superconducting state and the SCFCL 600 provides negligible resistance to the system under normal or steady state operating conditions. During a fault condition, the superconductor transitions from the superconducting state to a normal conducting state to add resistance which limits the fault current during the fault condition. The SCFCL 600 has an XFCL/RFCL ratio of equal to or less than 30 to assist with transient stability of the power system to which it is coupled.
There has thus been provided a fault current limiter with an XFCL/RFCL ratio of equal to or less than 30. Such a fault current limiter can improve power system transient stability significantly by damping the dynamic disturbance to the power system. In particular, such a fault current limiter may reduce power swings as the I2R losses of the fault current limiter provides electrical power during the fault. In other words, the lower XFCL/RFCL ratio of the fault current limiter instantaneously inserts a load that sinks active power. In this way, generators of the power system experience a minimum loss of load which promotes a more stable operation.
Furthermore, such a fault current limiter with an XFCL/RFCL ratio of equal to or less than 30 provides additional benefits. One additional benefit is a reduction in transient overvoltage for a circuit breaker in the power system such as the circuit breaker 108 of
Yet another benefit of a fault current limiter with an XFCL/RFCL ratio of equal to or less than 30, is that is reduces the rate of rise of recovery voltage (RRRV) for the circuit breaker, which is the slope of the transient recovery voltage at the instant of current interruption.
The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes.
This application claims priority of U.S. Provisional Patent Application Ser. No. 61/356,285, filed Jun. 18, 2010, the disclosure of which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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61356285 | Jun 2010 | US |