(1. Department of Electrical Engineering and Applied Electronics, Tsinghua University, Beijing 100084, China; 2 Xuchang Relay Research Institute, Xuchang 461000). With the analysis and evaluation of the protection principle of the gHouse.ngxhebookmark step in the power system of China's power system, on the basis of this, it is proposed to have multiple starting components, which can automatically adjust the out-of-step protection and predictive start according to the frequency variation. An improved scheme for predicting the trajectory of the power angle by using the least squares method. The improved out-of-step protection scheme can distinguish faults, static and stable faults, and stable and out-of-step conditions, and realize the out-of-step prediction function of the generator, which improves the performance of out-of-step protection. The generator out-of-step prediction test is carried out on the external and internal parts of the generator transformer group. The test results verify the correctness and feasibility of the improved scheme. The importance of out-of-step protection is increasingly prominent due to the inertia coefficient of the unit. The relative reduction of the reactance is relatively large, the relative system's equivalent reactance is reduced, and the fault clearing time is long. The factors such as the loss of the synchronous generator set out of step will cause the generator current to violently oscillate, and the terminal voltage fluctuates. The mechanical force and thermal effects generated during the out-of-step process may damage the rotating shaft and stator coil of the generator, and the out-of-step of one generator may be extended to other generators. If the measures are not taken in time, the large-scale power outage will eventually result. The generator set should be equipped with out-of-step prediction and protection components. At present, the principles of protection out-of-step and prediction are mainly based on the protection principle of the apparent impedance path of the machine, based on the protection principle of indirect reflection of the change of the power angle, based on the protection principle of direct measurement of power angle or angular velocity and the protection principle based on energy balance. At present, domestic power plants and major manufacturers, in the generator-transformer group microcomputer protection, the application is relatively mature and widely used is the three-impedance component out-of-step protection based on the machine-side apparent impedance change trajectory. However, the principle of out-of-step protection of the three-impedance element has its own disadvantages, so it is necessary to analyze and improve it to improve its performance in the field. The evaluation of the out-of-step protection of the three-impedance element is based on the power generation of the single-infinite system. The three-impedance component out-of-step protection action characteristic line is as shown in the figure, and its brief comment is as follows: The three-impedance component out-of-step protection does not have the out-of-step prediction function. When it moves, from the point of avoiding the lost step, it is too time In the evening, the result of the protection action can only be cut or unraveled, and there is no time to take the corresponding suppression of out-of-step measures to keep the system stable. After the inner angle of the right side of the lens characteristic element is set, the funded project on the left side: National Major Science and Technology Project (Study) Project Sub-project Fund Project: Bi Daqiang (1973-), male (Han), Hebei, Ph.D. Communication Contact: Wang Xiangyu , Professor, the angle is also determined accordingly, that is, the inner corners of both sides can only be set to the same fixed value, but this setting is not conducive to the selection of the cutting time. When the generator swings 180* for the first time, the principle of the three-impedance component cannot be distinguished. Whether it is static or unstable or unstable. 2 Improvement of out-of-step protection of three-impedance components 2.1 Calculation of system power angle Assume that the impedance angles of the components of the system are the same, then the power angle W between the generator and the system is measured by the machine end impedance Z and the generator transient reactance zg and the generator The closed impedance triangle formed by the contact reactance Zst of the system determines that each impedance is multiplied by the current to obtain the voltage vectors as shown. In the figure, the generator potential Eg and the power angle are calculated to improve the out-of-step protection. Important role. 2.2 Adding out-of-step protection and prediction start-up components In order to make the out-of-step protection and prediction device start reliably, start out-of-step and prediction when the following conditions are met: to ensure the calculation accuracy of the normal operation of the device, the minimum current must be greater than 0.1/N, otherwise the step-out Protection lockout. The voltage amplitude changes during the out-of-step process, and the voltage drop to less than 0.92Un is an accompanying feature of the trend of out-of-step. The out-of-synchronization process is accompanied by the frequency offset. It is distinguished whether the static stability or the dynamic stability is out of step by detecting whether the lower limit of the generator slip frequency is greater than 0.2 Hz; whether the upper limit of the slip frequency is less than 8 Hz is used to distinguish the short circuit from the oscillation. The logical relationship between the generator power angle W and the rated operating power of the above four starting components is (1AND 4, thus ensuring the phase difference between the out-of-step protection and the improved starting of the predictive device 2.3 the improved system potential Es is the work. Angle W is the rate fs= It should be noted that in the steady state, the power angle should use the synchronous reactance Zg of the generator, instead of the zg lens characteristic. The inner angle of the left half is completely determined according to the power angle that facilitates the disconnection of the circuit breaker. The angle has nothing to do with the starting angle, which is more conducive to circuit breaker tripping and predictive alarm. The recording of the number of slips after the out of step is the same as the original protection scheme. 4 Out-of-step prediction algorithm In the oscillation process, the power angle W becomes unstable with time from 0 to 360*, and the improvement between the generator and the system is compared with the original three-impedance element in the short time of the transient process. The post-action characteristics are as shown, which retains the blocker linear impedance element B and the reactance line impedance element C. The key to the improvement is to set the inner angle of the left side of the lens characteristic impedance element A to the out-of-step cutting angle, and to the right. The side opening angle of the right side is determined by the starting condition 3 or 4, because the power angle corresponding to the starting condition 3 is different under different operating conditions, so that the out-of-step protection and prediction device can be adaptively adjusted according to the magnitude of the slip frequency. Start angle. The power angle can be regarded as a process of continuous change with time. The power angle prediction can adopt the polynomial regression model: the numbers a, a\, ..., a, m are calculated according to the historical observation data of the power angle, and have been calculated before starting the prediction. Observed N+1 power angle values ​​W(), W(At), ..., W(NAt) (At is the sampling interval) According to the least squares method, the calculation formula of the parameter vector in the prediction model is: T =, ( k=N+1,...,N+i) The polynomial parameters are calculated by equation (2), and the predicted power angles at different times thereafter are calculated by equation (1): ...+bNkW(NAt). Since H(N) is a constant matrix, B(N) can be calculated offline beforehand and then the prediction equation coefficient vector corresponding to all different prediction times and =r B(N)= can be calculated offline according to the prediction time kAt. The online calculation of the method is small. In order to improve the accuracy of prediction calculation, rolling prediction is used to continuously collect new data, and the data of the prediction data window is updated with the latest sampling value. 25 The difference between different operating conditions is to improve the reliability of prediction, and it is necessary to correctly distinguish short-circuit faults from oscillations and static stability. Destruction and instability. In normal operation, the impedance of the terminal is large. When a short-circuit fault occurs, the impedance of the machine will become very small, that is, a jump will occur. The power angle calculated according to the path of the impedance change of the machine will also be abrupt, and the power angle change rate will be calculated. The corresponding slip frequency will also mutate according to the maximum slip frequency detected, and set the maximum range of the power angle within the specified time to distinguish between short circuit and out of step. Because the process of static and stable damage is relatively slow, the corresponding slip frequency is very small, and the rate of change of the power angle is less than the rate of change of the corresponding power angle when the power angle change rate is less than the minimum slip. It can be distinguished that the static stability and the dynamic stability are different. By causing the impedance trajectory to stay at a point when it is not moving, after distinguishing between the short circuit and the oscillating condition, the difference between the static acceleration and the loss and the loss of 2.6 When the predicted power angle value is greater than the pre-set value (usually 180*), the predicted generator is about to lose the reliability of the out-of-synchronization prediction, and the method of multiple voting may be adopted, and the out-of-step is predicted several times in succession. , it is judged that the generator is about to lose the step. After the out-of-step is predicted, the out-of-step alarm signal is sent, and the corresponding relay outlet is provided. It can be combined with the safety automatic device to improve the stability of the system by means of quick closing valve and electric braking, and avoid generating electricity as much as possible. The test result of the loss of 3 lost step prediction uses the improved out-of-step protection and prediction device of the three-impedance element, and the prediction function of the out-of-step protection is verified on the dynamic model test unit. The main parameters of the generator are: S 36.GQ; The main parameters of the transformer are 30kVA, Zt=2. We, n=1.1kV/400V; when the oscillation center is outside the generator group, the line parameters are: Zs=16.6,85"; when the oscillation center is in the generator group, the line parameters Yes: Zs=2. Magic, h=85:300ms Calculate the change of power angle to predict whether the generator will use the second-order curve prediction model in the test, take the prediction time to 230ms, and use 12 data observed every 10ms. Forming a historical data window, it is predicted that the alarm signal will be sent out after three consecutive steps. To start the prediction early, when the power angle is greater than a small value, the prediction is started for the oscillation center outside the generator group and the generator group. get on The test, limited to the space, only gives the three-phase voltage and current recording diagram of the three-phase test conditions of the oscillation center outside the generator-changing group, and the action of the out-of-step protection start and out-of-step prediction criteria. It can be judged that the situation 1: increasing the active output causes the static stability of the generator to be destroyed. When the power angle is 102* (see), the generator is predicted to be out of step by 210ms in advance; Short circuit, after the fault is cleared, the protection angle is 84* (see), the out-of-step alarm fault is issued 200ms in advance, and the step is lost after the stable fault. When the oscillation center is in the generator set, the test generator loses quickly and leads to prediction. The amount is small. Because the calculation of the power angle in the test uses the transient reactance of the generator, when the generator is statically out of step, although the trend of the power angle change can be reflected, the calculated value of the power angle is too small. During the test, the improved protection scheme can correctly predict the out-of-step condition of the generator. The time advance of the out-of-step prediction will provide the possibility of suppressing the generator stability. 4 Conclusions The shortcomings of the out-of-step protection principle of the three-impedance element Based on the analysis, the improved scheme is proposed to add multiple starting criteria according to the characteristics of the generator out-of-synchronization process, which can distinguish faults and oscillations, static and stable failures and dynamic stability and out-of-step operation, and improve the reliability of out-of-step protection startup. Through the improvement of the out-of-step protection action characteristics, the protection can adaptively adjust the starting angle of the out-of-step protection according to the slip frequency. The calculation and prediction of the power angle are introduced to make the three-impedance out-of-step protection device increase the out-of-step prediction function, which can improve the operational stability of the generator and the system. The experimental results verify the improved principle of the out-of-step prediction function. Thanks to the help of senior engineer Yu Yuwei of the Power System Dynamic Test Laboratory of Xu Ji Group Co., Ltd.
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