Study on Enhancing Transient Stability of Power System by Superconductive Energy Storage Device Zheng Li, Ma Juxin, Li Lichun (Department of Electrical Engineering and Applied Electronics, Tsinghua University, Beijing 100084, China) Cnki.net leads the mathematical model of single-bridge and dual-bridge systems for energy storage and implements various control strategies for active control and active and reactive integrated control.. Computer simulations on stand-alone infinity and multi-machine systems show that superconducting storage The energy device can effectively improve the transient stability of the power system. The influence of the control mode on the transient stability is studied. Many factors affecting the effect, such as the installation site, are also studied in detail.
712:A With the development of superconducting technology, especially the appearance of high-temperature superconductivity, the prospects of superconducting applications in power systems are currently in view. Currently, devices being studied include superconducting energy storage devices, superconducting devices, and superconductors. Transmission line, superconducting current limiter, etc. Among them, superconducting energy storage device SMES can be stored in a magnetic field in a device which converts alternating current into direct current through a rectifying inverter bridge, charging a huge solenoid coil. Since the coil is made of a superconducting material and maintained in a superconducting state, the energy in the coil can be stored almost without damage, and when necessary, the power is discharged from the magnetic field, converted into alternating current power, and sent to the AC grid.
Superconducting energy storage devices have the following characteristics: 1) High conversion efficiency The charging and discharging efficiency of large-scale low-temperature superconducting energy storage devices is approximately 90% to 9%. The efficiency of high-temperature superconducting energy storage devices can reach as high as 94% or even more. high. 2) Fast response. SMES converts from maximum charging power to maximum discharging power in only tens of milliseconds.) SMES can not only adjust active power, but also adjust reactive power, and can also perform independent control of active and reactive power at the same time. Flexibility Due to these excellent characteristics of superconducting energy storage devices, its application range is very wide. According to the size of the capacity, SMES system can be divided into large, medium and small capacity. Large-capacity SMES (5107~1tfMJ) is mainly used to balance the daily peak load and the night trough load of the power grid. Small and medium-capacity SMES (energy storage less than 1tfMJ) can be used to improve the stability of the power system, damp the synchronous oscillation, and improve the power quality, etc. Because of the technical difficulties in manufacturing large-capacity superconducting coils, most of the current research focuses on medium and small-capacity. Starting from the mechanism of superconducting energy storage system, a set of mathematical models for superconducting energy storage is established. Through computer simulation, the effect of SMES on improving the transient stability of the system is studied, and its control law is studied. 1SMES principle and mathematical model 1.1SMES The basic principle of the SMES system consists of a superconducting coil, a converter circuit, a control protection circuit, and a refrigeration device. The superconducting energy storage single-bridge system is actually a purely inductive load three-phase rectification inverter bridge. The control of the rectification inverter bridge is to connect the superconducting energy storage device and the AC system realized by controlling the conduction state of the switching device. Under the above assumption, the mathematical model of the converter bridge (standard value) can be expressed as 3丌 (4) / s = / s1 + / s2. (5) the amount of electricity in the superconducting coil can be expressed as: jrjd / d According to the basic principle of the converter circuit, you can get the following basic relationship: 丌 Where: Vd is the DC side voltage Average value; Psm is the active energy absorbed by SMES; Q- is the reactive power absorbed by SMES; V is the effective value of the phase voltage on the AC side; T, T are the trigger angles. In this strategy, Ak is used as a control signal to control the system to make the power exchanged with the system proportional. When the generator's Ak at the fault node is positive, SMES absorbs active power from the system; when Ak is negative, SMES The active power is released to the system. As shown, the mathematical expression of the control system it represents is 2.2 Active control strategy 2 (P-Ape control) In practical applications, the measurement of the rotor angular velocity k is difficult and the power measurement is It is easy to realize in the power system. We use the active deviation APe=P*-Pe sent by the generator as a control variable to control the active exchange of the SMES and the system. The PI controller and the inertia link are used for the control law.
2.3 Active and reactive power integrated control to obtain T, T2 through the inverse trigonometric function This article uses the block diagram shown in the active and reactive power integrated control, in which the transfer functions of the active controller and the reactive power controller are: 3 Computer simulation 3.1 The simulation system wiring is simulated on the single-machine infinity system and the three-machine system. The fault is: single-machine system: the high voltage side of the step-up transformer on the generator side, three-phase short-circuit fault occurs in 0.2S, and the fault is removed after a certain time , and then run it in a single line.
Three-machine system: When the fault A0.2s occurs, a three-phase short circuit occurs at point A, and the line between the 7 and 8 nodes is cut after a certain period of time. In the case of fault B0.2s, a three-phase short circuit occurs at point B, and then when the line fault C0.2s between the 6 and 7 nodes is cut off, a three-phase short circuit occurs at node 8 and then the line 3.2 between the 8 and 9 nodes is cut off. The simulation results are under the control of PAk, the dynamic process of SMES and system variables can be seen from the figure, when the generator rotor relative angular velocity 0, the trigger angle T is between 0 and n/2, the active power absorbed by SMES is The direct current on the positive superconducting coil “increases; when the Ak 3.3 multi-machine system simulation result simulation system is as shown, for the three-machine system, the structure of the ring network is more closely related, and after the failure the system becomes open-loop, The structure has undergone great changes; at the same time, there are multiple oscillation modes in a multi-machine system. This article examines the effects of superconducting energy storage systems installed in different locations on the transient stability of the system under different fault conditions.
Fushi 4SMESa and the system quality of the excess state IcWcbookmark1 compared the failure A, multi-machine system does not have SMES in the generator 2 machine, installed in the generator 3 machine SMES under a variety of circumstances, the dynamic process of the system. The introduction of SMES can effectively reduce the relative phase angles WW and WW, improve the transient stability, and no matter what the oscillation is inhibited, at the same time, the SMES installation site close to a generator, the impact of the generator is more obvious Simulating faults B and C can give similar results. Table 2 lists the simulation results in Table 2 for different faults, SMES on different generator nodes, the comparison of the extreme cut-off time Tre with the fault type without SMES at (* node at the G3 node at the same time when G2 and G3 first destabilized generator 0.59 The generator did not destabilize at the first pendulum, but thereafter the amplitude even oscillated.
When Tr* is 0.58, the system can be stabilized after oscillation. *G2 is first unstable. (* It is also easy to lose stability. Therefore, it can be known that: a) No matter which generator terminal is installed on SMES, The transient stability of the system has advantages. The size of its effect is related to the electrical distance from the fault to the SMES, the parameters of the generator, etc. b) The generator end of the SMES is installed to improve the stability of the generator. The greatest impact of sex is c) The installation of SMES on the first unstable generator side can improve the transient stability of the system most effectively. 4 Conclusion This paper briefly introduced the basic working principle of the superconducting energy storage device and established A set of nonlinear differential equations of the superconducting energy storage system mathematical model superconducting energy storage device can simultaneously control its active and reactive power exchanged with the AC system. In this paper, various control strategies for improving transient stability of power systems using active power control and active and reactive power control of superconducting energy storage devices are proposed. This paper conducts computer simulations on stand-alone infinity and multi-machine systems and studies superconductivity. The active control of the energy storage system and the integrated control of active and reactive power improve the transient stability of the power system. The simulation results show that the superconducting energy storage device can effectively improve the transient stability of the system. Among them, the active control directly adjusts the electrical power output by the generator and can significantly improve the transient stability of the system; the active and reactive reactive power control is further improved. Improve the effect. In a multi-machine system, SMES is installed on each generator side to effectively improve transient stability, and at which node the superconducting energy storage is installed, the transient process of the generator on this node is affected. For the entire system, SMES is best installed on the first unstable generator.
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