Transient Stability Improvement of a Multi- Machine Power System by Using Superconducting Fault Current Limiters

Transcription

1 International Journal of Energy and Power, Volume 5, 06 doi: /ijep Transient Stability Improvement of a Multi- Machine Power System by Using Superconducting Fault Current Limiters Jian Fang, a, Yuwei Dai,b, Lei Chen 3,c, Guocheng Li 3,d Fujian Electric Power Survey & Design Institute, Fuzhou, , China Jilin Electric Power Survey & Design Institute, Changchun, 300, China 3 School of Electrical Engineering, Wuhan University, Wuhan, 43007, China a @qq.com; b @qq.com; c stclchen98@63.com; d @qq.com Abstract In regard to the rapid development of high-temperature superconducting materials, electric power applications of superconductivity have attracted more and more attention from home and abroad. In regard to superconducting fault current limiter ( known as one of the best countermeasures to solve the technical problems related to a short-circuit fault, its practical applications have covered power transmission and distribution systems. In this paper, it is devoted to study the application of flux-coupling type s in improving the transient stability of a multi-machine power system. Related theory deduction and simulation calculation analysis are carried out, and the results demonstrate that, using the flux-coupling type s can effectively reduce the imitated multi-machine system s power-angle fluctuations and increase the critical fault clearance time. As a result, the improvement of the transient stability can be well achieved. Keywords Multi-Machine Power System; Transient Stability; Transmission Line Fault; Superconducting Fault Current Limiter; Simulation Calculation Introduction Along with the rapid expanding of electric power transmission and distribution systems, fault current levels increase and power system transient stabilities become more and more significant [, ]. In the case of that a serious short-circuit fault happens at a large-scale power system, the magnitude of electromagnetic force induced by the fault current is proportional to the square of its first peak value, and that is meaningful and valuable to suppress the fault current quickly and effectively. Thus, it is expected to guard the relevant electric machines/devices and strengthen the system s robustness against the short-circuit fault [3]. Superconducting fault current limiters (s can be regarded as one of the best countermeasures to solve the technical problems related to a short-circuit fault, and they are able to suppress fault current levels, improve reliabilities and securities of power systems as much as possible. Currently, different types of s have been proprosed and investigated by research scholars from home and abroad[4-7]. Among them, the resistive type is an application mainstream with many technical advantages, and it is able to respond to the fault current without additional control devices. The inductive type is another application mainstream, and also it can bring a number of distinctive advantages, such as great design flexibility owing to the use of a coupling transformer, and low heat losses because of its very low resistance. In a way, the combination of resistive and inductive can potentially improve its performance behaviors and extend its applicaton ranges. Accordingly, our research group has proposed and studied an advanced flux-coupling type [8, 9], which is a high-efficient hybrid type. From [0], this can theoretically provide performance advantages from multiple aspects, such as reducing the active/reactive power fluctuations, limiting the system oscillations and protecting the relevant electric power devices. In this paper, the suggested flux-coupling type is selected to play the role, and its impacts on a multi- 4

2 International Journal of Energy and Power, Volume 5, 06 machine power system s transient stability are investigated. Related theoretical analysis of the s working principle and action mechanism to power angle characteristic is conducted, and a detailed simulation calculation model of a typical multi-machine power system with the s is created in MATLAB software. Besides, different simulation cases are considered to verify the s performance behaviors. At the end of this paper, conclusions are summarized and next steps are suggested. Theoretical presentation of the The s electrical structure is shown in Fig.. From this figure, this mainly consists of a coupling transformer (CT, a controlled switch Scs and a superconducting coil (SC. The CT has two basic windings, and their winding directions will be reverse. The controlled switch will be connected in series with one of the CT s winding, and herein the metal oxide arrester (MOA will be aslo adopted. In case of the switching operation, the switching overvoltage may be caused, and thus the MOA can play the role in eliminating the overvoltage. Rmoa I r I cs U s Z s Scs M L L I I SC Is FIG. Configuration structure of the flux-coupling type. In normal condition, the switch Scs is closed, and the current flowing through the SC will be lower than its critical current. Due to that the superconducting state of the SC and the non-inductive coupling can be obtained, the will reflect zero impedance for the main circuit. When the fault happens, the controlled switch will be activated to the opening state, and as the electromagnetic relationship between the CT s two basic windings will be changed, the original non-inductive coupling will be damaged. Meanwhile, owing to the increase of the fault current, the SC will loss the superconducting state and then switch to the normal-resistance state. The current-limiting impedance of the can be expressed as: Z = [RSC +jωl+(knωl /(Rmoa+n ωl. In view of Rmoa» n ωl, Z RSC +jωl can be obtained. S load Influence of Using the s on the Transient Stability of a Multi-Machine System The schematic diagram of a typical two-machine power system integrated with the s is shown in Fig., where two s are installed at the inlet and ending terminals of one of the two transmission lines. When a three-phase grounded fault occurs at the line, the fault transient analysis is carried out as follows. G A T B L C T D G L FIG. Schematic diagram of a typical two-machine power system integrated with the s. X G X T X f X f X T X G G X f 3 G FIG. 3 Equivalent analysis circuit of the two-machine power system under fault condition. 43

3 International Journal of Energy and Power, Volume 5, 06 The two-machine power system s equivalent circuit under fault condition is shown in Fig. 3, where XG, XG, XT, XT, and XL are respectively the generator transient reactance (G or G, transformer reactance (T or T, and transmission line reactance (L and L. Herein it is assumed that the two transmission lines have the same line length, and the distance coefficient c is newly used to estimate the fault distance. The equivalent inductances Xf, Xf and Xf3 can be expressed as: X L(Z La X f = X L L + Z X L(Z Lb X f = X L L + Z (Z La (Z Lb X f3 = X L L + Z ( where X L = X L = X L, X La = cx L, X Lb = (- cx L. The active power of the two-machine power system can be expressed as: E EE P = sinβ + sin(δ β Z Z E EE P = sinβ sin(δ + β Z Z ( where P/P is the active power output of generator /; δ is the absolute rotor angle (denoting the system s power-angle characteristic; E/E is marked as the terminal voltage of generator /; β/ β/ β represents the phase-angle of the impedance Z/ Z/ Z, which is related to XG, XG, XT, XT, Xf, Xf and Xf3. In consideration of that the mechanical power outputs (Pm, Pm of the two generators are unchangeable, the swing-angle acceleration may be defined as: d δ Pm P ΔP = = dt Tj Tj d δ Pm P ΔP = = dt Tj Tj (3 where δ/δ is the rotor angle of generator /; Tj/Tj is the inertia time constant of generator /. In the case of analyzing and assessing the two-machine power system s transient stability, it is necessary to know whether the absolute rotor angle can maintain the original state or switch to a new steady state. Without With the Fault clearance Without With the Fault clearance (a With large current-limiting impedance (b With small current-limiting impedance FIG. 4 Theoretical characteristics of the absolute rotor angle δ and its acceleration α. 44

4 International Journal of Energy and Power, Volume 5, 06 The absolute rotor angle acceleration can be calculated as: d δ d(δδ ΔP ΔP α = = = (4 dt dt T T Considering the different current-limiting impedances of the s, the relationship between α and δ can be shown in Fig. 4, where three different of characteristic curves (αi, αii and αiii corresponding to different fault conditions are taken into account. From this figure, introducing the current-limiting impedances can reduce the acceleration area, and the effects will become more obvious along with the increase of impedance values. In sum, using the suggested flux-coupling type s can reduce the power-angle differences, and thus the transient stability improvement of the two-machine power system can be achieved. Simulation Calculation and Analysis To verify the s performance in a multi-machine power system, the simulation model is built in the MATLAB software, and the specific parameters are expressed as: L=00 mh, L=00 mh, Rsc=5 Ω; the rated capacity T/T is 63 MVA / 63 MVA; the transformation ratio is 0.5 kv/ 0 kv; the length of the transmission line is 0 km. j j R SC R / Time (s FIG. 5 SC s transient characteristics under the process of the current-limitation. (a Single-phase fault (b Phase to phase fault (c double-phase ground fault (d Three phase fault FIG. 6 Power-angle curves of the two-machine power system under different fault types. 45

5 International Journal of Energy and Power, Volume 5, 06 Concerning the simulation modeling of the, the authors use an anti-parallel IGCT pair to reflect the controlled switch, and Fig. 5 shows the SC s transient characteristics under the process of the current-limitation []. From the figure, the SC s quenching time is set as 4 ms, and its recovery time is set as 0.5 s, which is able to cooperate with the auto-reclosing device. Figs. 6-7 show the power-angle curves of the two-machine power system under different fault types and different fault clearance times, and herein the conditions with and without the s are both taken into account. It is observed that, no matter of the differences in the fault types, using the s is able to reduce the demonstrated multi-machine system s power-angle fluctuations effectively. Moreover, employing the s is helpful to increase the fault clearance time. (a Clearance time 00 ms (b Clearance time 300 ms (c Clearance time 400 ms (d Clearance time 500 ms FIG. 7 Power-angle curves of the two-machine power system under different fault clearance times. In light of the two-machine power system without the s, the loss of synchronization will be obtained when the fault clearance time is equal to 0.5 s. However, once the s are introduced, the synchronization can be still maintained. Thus, from this perspective, using the s can increase the critical fault clearance time and enhance the power system s stability margin to a certain extent. Conclusions Concerning the utilization of the flux-coupling-type s for improving the transient stability of a multi-machine power system, this paper conducts the theoretical study as well as simulation verification, and draws a few useful conclusions. According to the simulations under different fault types and fault clearance times, the calculation results confirm that, using the proposed s is able to availably reduce the imitated multi-machine system s power-angle fluctuations and increase the critical fault clearance time. As a result, the improvement of the transient stability can be well achieved. In the near future, some follow-up works will be done, such as the optimized design of the s, the transient characteristic analysis of a multi-machine power system with renewable energy sources. It may be believed, the 46

6 International Journal of Energy and Power, Volume 5, 06 combined use of superconducting power technologies and renewable energy sources can bring more technical advantages from different aspects. ACKNOWLEDGMENT This study was supported by the Wuhan Planning Project of Science and Technology (Grant REFERENCES [] G.-H. Moon, J. Lee and S.-K. Joo: IEEE Trans. Appl. Superconduct. Vol. 3 (03, p [] G.-H. Moon, Y.-M. Wi, K. Lee and S.-K. Joo: IEEE Trans. Appl. Superconduct. Vol. (0, p. 57. [3] J.T. Bialasiewicz: IEEE Trans. Ind. Electr. Vol. 55 (008, p. 57. [4] J.-G. Lee, U.A. Khan, J.-S. Hwang, J.-K. Seong, W.-J. Shin, B.-B. Park and B.-W. Lee: Phy. C Vol. 504 (04, p. 63. [5] Chen L., Li Z., Deng C., Liu H., Weng Y., Xu Q., Wu Z. and Tang Y.: Int. J. Electr. Power Energy Syst. Vol. 69 (05, p. 60. [6] J. Bock, A. Hobl, J. Schramm, S. Krämer and C. Jänke: IEEE Trans. Appl. Supercond. Vol. 5 (05, p [7] A. Colmenar-Santos, J.M. Pecharromán-Lázaro, C. Rodríguez and E. Collado-Fernández: Electr. Power Syst. Res. Vol. 36 (06, p. 89. [8] L. Chen, C. Deng, F. Zheng, S. Li, Y. Liu and Y. Liao: IEEE Trans. Appl. Superconduct. Vol. 5 (05, p [9] L. Chen, F. Zheng, C. Deng, S. Li, M. Li, H. Liu, L. Zhu and F. Guo: Phys. C Vol. 58 (05, p. 44. [0] G. Didier, C.H. Bonnard, T. Lubin and J. Lévêque: Elect. Power Syst. Res. Vol. 5 (05, p. 50. [] J.-S. Kim, S.-H. Lim, J.-C. Kim and J.-F. Moon: IEEE Trans. Appl. Superconduct. Vol. 3 (03, p