| Recent years, power system accidents have happened in many countries and areas such as American, Russia, India, which will bring power system transien t stability problems, power system accidents deeply affect people?s normal life and cause huge bad effect. Thus an improved method should be presented to determine whether a power system can remain stable after a failure. Critical clearing time is one of t he most important parameters of transient stablility. In order to improve the accuracy and speed of calculating CCT in different failures, a new method is presented based on critical machine pair model embedded in Power System Analysis Software Package/User Program Interface(PSASP/UPI). Control program and user program(UP) are composed to get real-time data by alternating iterative of PSASP/ST and UP. CCT can be efficiently calculated by using the critical machine pair module to implement critical machine pair selection and power angle curve fitting followed by a control strategy which is called ?first fast, then accurate?.Firstly this paper focus on the principle of PSASP/UPI data interchange. After accurately composing the first line of user program and setting related calling and configuration information, then PSASP/ST can correctly connect with UP. At every single integral moment, ST transfer data to UP through PSASP/UPI, which is the foundation for UP?s further calculation. Using data gotten from every single integral moment and basing critical machine pair module, a main program called control and two user programs UP0, UP1 are composed to accomplish tasks as follows: select several critical machine pairs from power system which is constituted of cr itical machine and non-critical machine; fit angle-curve of selected critical machine pair; calculate single iteration CCT according to Equal Area Criteria; To calculate CCT under different failures without open PSASP window, Control reads and writes related document from the bottom of the files, besides with the help of DOS system, it makes the ST execute program iterate again and again until meeting the constraint, finally it can get the final CCT.When the cut time is very close to CCT, strong time-varying factors will appear, this paper further discuss the problem of critical machine pair angle curve fitting when the angle curve is no longer a standard sine curve due to the strong time-varying factors. Around it, this paper focus on solving those probl ems: A appropriate criteria should be proposed to determine whether there appears the strong time-varying factors; when the sine module can?t be easily used to fit the critical machine pair?s angle curve, the old fitting method need to be changed.To solve the problems spread out above, this paper creates method I and method II to calculate CCT, both method I and method II can remember the interval boundary, meanwhile the length of the interval is shortened. Method I consider the strong time-varying factor exits all the time and controls ST run till it reaches the maximum potential energy point, by this way method I can get accurate resu lt, however it costs more time. To guarantee the efficiency and accuracy, method II consider strong time-varying factor is weak in former iteration, thus ST only need to run for a short time after the failure being cut. When strong time-varying factor becomes strong in the late iteration, method II do as method I for further calculation, method II has the characteristic called “first fast, then accurate”. by calculating the examples, this paper compared merits and demerits of the methods.Finally with the help of China Electric Power Research Institute 8-machine system and the real power system of one province, this paper calculates a three-phase short-circuit failure cluster. Example results show that the method only need few data for further analysis and compared with time domain simulation, it can significantly improve calculation efficiency under the same precision condition. |
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