For some power plants with lower stability margins, any failures that occur may cause some or all of the generators in the plant to lose synchronization. For a given fault, the longest fault duration that guarantees that the system will not lose synchronization is called the critical cut-off time. If the actual cutoff time is greater than this value, the system will lose synchronization. In a power plant, a certain number of generators are removed after a fault to prevent some generators from being out of step with other generators in the system. After the fault is removed, these generators will attempt to re-synchronize with the system, a process that takes at least a few minutes (usually takes longer). In this process, a power supply and power deficit will appear in the power system, which needs to be balanced by a series of frequency and load control. If it is considered that some boilers that have been cooled need to be reheated, the new process is re-established due to the rotor air hopper of the computer group, the gap between the rotor and the stator is reduced, and the rated excitation current of the generator is smaller than the evaluation unit. Therefore, the voltage response time of the computer group should be less than the evaluation unit, and the calculated value of 0.55 s should be too large. It can be inferred that the calculated value should be too large. 4 Conclusions (1) When the amplification factor of the excitation regulator is sufficiently large, the calculated value of this paper is the maximum value of the voltage response time, and the actual response time is less than the calculated value. (2) Under the condition of satisfying the high initial excitation system, the minimum control angle of the thyristor can be reasonably selected, so that the overshoot of the terminal voltage is reduced, and the transient characteristics of the power grid to the unit are met. The time it takes to synchronize will be longer. Coordination of the power plant, so that the power plant does not have to cut the machine after the fault occurs, but still can keep pace with the system. Excitation control and fast-closing valve control are designed with a highly adaptive nonlinear PID. 1 Principle of coordinated control If a fault occurs in the power system, some of the generators in the power plant lose synchronization, and the rotor speed of the generator is higher than the synchronous speed. The steam turbine control system will close the valve to reduce the mechanical power, and the reduction of mechanical power will help to re-establish synchronization. In addition, if the synchronization power is large, that is to say, the excitation level is high, it is also beneficial to re-establish synchronization. In short, lower mechanical power and larger synchronous power are beneficial to re-establish stability. 3. In order to promote stability more quickly and more effectively, it is necessary to utilize the coordination of various controllers, such as excitation control and fast shut-off steam. Door control. It is a schematic diagram of the system losing synchronization. Under normal circumstances, the system runs at point A. The occurrence of short-circuit faults reduces the electrical power and causes the system to operate at point B. At this time, the power angle characteristic is shown by the lower curve, the mechanical power is greater than the electrical power, the rotor is accelerated, and the fault reaches the point C. The rotor has excess operating energy, which is equivalent to the area enclosed by the A-BOD in the figure, that is, the acceleration area. The acceleration area is larger than the maximum deceleration area (the area enclosed by D-EF in the figure) so that the rotor enters the out-of-step operation state. When the rotor passes the point F, the rotor is still in an accelerated state because the mechanical power is greater than the electrical power. Before the next equilibrium point J, the mechanical power is always greater than the electrical power, and the second portion of the acceleration energy obtained by the rotor is used to indicate. After passing J point, the electrical power is greater than the mechanical power and the rotor begins to decelerate. The sum of the deceleration areas (ie, the sum of DEF and J-K'-L) is less than the sum of the acceleration areas (ie, the sum of A-BC-D) means that the rotor will not be able to return to the synchronized state. It shows that the acceleration area exceeds the deceleration area because the mechanical power is always high, so reducing the mechanical power can reduce the acceleration area and acceleration and deceleration area. Indicates that when the out-of-step occurs, the mechanical power is quickly reduced to zero, thereby reducing the acceleration area, but the total acceleration area still exceeds the total stability. The acceleration and deceleration area should be adjusted by changing the amplitude of the power angle characteristic to balance the acceleration area. The magnitude of the power angle characteristic depends on the equivalent reactance of the system and the transient potential of the generator. Therefore, the amplitude of the power angle characteristic can be changed by changing the transient potential of the generator, which is implemented by Bang-Bang excitation control. In the middle, when the generator reaches the G point, the excitation voltage is switched to the negative maximum value to quickly reduce the amplitude of the generator transient potential and the power angle characteristic, so that the acceleration area remains unchanged from the excitation voltage and the valve. F-GH'-/-J() is reduced to F-GH/; when the generator reaches/point, the excitation voltage is switched to a positive maximum value to quickly add the magnitude of the generator transient potential and power angle characteristics. Therefore, the deceleration area is added to /KM by the JK'-L() when the excitation voltage and the valve remain unchanged. The total deceleration area is DEF plus /KM The total acceleration area is AB-CD plus FGH- /, the total deceleration area exceeds the total acceleration area. When the point N is reached, the total deceleration area is equal to the total acceleration area. The rotor swings from the point/point, and the generator rotor does not enter the second swing and re-establishes stability. . 2 Coordinated controller design 21 Mathematical model and excitation control design 141 Single-machine infinity system equivalent circuit as shown. Assume that the generator uses a classic third-order model. The mathematical model of the excitation control of the single-machine infinite system is a metric; the system is synchronous angular velocity; D is the generator damping coefficient; H is the inertia time constant of the genset rotor; Uf is the control voltage of the excitation device; T.' is the excitation of the generator Winding time constant; E/ is the q-axis transient potential of the generator; Pm is the mechanical power of the generator; Pe is the electromagnetic power output of the generator, and the amount is obtained; Xd is the d-axis reactance of the generator; xq is the q-axis reactance of the generator; Xd' is the d-axis transient reactance of the generator. According to the nonlinear PID control, the nonlinear feedback compensation law Uf of the excitation control voltage can be expressed as the requirements of the Bang-Bang control, and the final controller is the design of the 2.2 fast-closing valve controller 141. The steam regulation of the steam turbine generator is assumed. The system has an intermediate reheater. Under normal working conditions, the relay normally open contact r is disconnected. The medium voltage regulating valve quick closing controller has no effect due to the output signal being cut off; after the power system fails, the fault is reflected. The relay is activated, the normally open contact r is closed, and the medium pressure regulating valve is controlled by the intermediate pressure cylinder quick closing controller, which produces a so-called "quick closing control" function. In order to simplify the analysis of the problem, the medium and low pressure cylinder system is equivalent to an inertia link, and its equivalent time constant, equivalent power distribution coefficient and equivalent output mechanical power are respectively represented by Tml and CPml. Cml=Cm-Cl, where Cm and Cl are respectively the power corresponding to the medium and low pressure cylinders=Pm+Pl, where Pm and Pl respectively represent the mechanical power outputted by the medium and low pressure cylinders. At the same time, excluding the limiting link in the valve regulation system, and because the time experienced by the electromechanical transient process of the power system is generally much smaller than the time constant of the intermediate reheater, the output power of the medium and low pressure cylinders is not considered regardless of the reheater pressure change. The effect is that the reheater output is constant, and the total output power Pm of the prime mover is the sum of the output power PH of the high-pressure cylinder and the mechanical power PmL of the output of the medium and low-pressure cylinders, that is, Pm=Ph+Pm:. Since the time constants T%, TMg and steam volume time constants Th and Tml of the high-pressure cylinder and the medium-low pressure cylinder are both small, about 0.4 s, the mathematical model of the valve adjustment system can be further simplified, respectively. An inertia link approximates the high-pressure main valve control system and the medium-low pressure quick-closing valve control system. The inertia time constants are respectively The= In the research of the fast-closing valve control problem, it is assumed that under the action of the excitation controller, the generator Keep the q-axis transient potential Eq' constant during the whole dynamic process. The mathematical model of the turbo control system of the single-machine infinite system is the medium and low pressure cylinders, C=Cml); T* equivalent time constant (only When adjusting the high pressure cylinder, T=ThE; only adjust the medium and low pressure cylinders, T=Tm2); Pmx* adjusts the mechanical power of the system (only adjusts the high pressure cylinder; only adjusts the medium and low pressure cylinders, Pmx=PML). Also according to the nonlinear PID control, the nonlinear feedback control law Um of the fast closing valve can be expressed as 2.3. The design of the coordinated control logic is designed by using the nonlinear PID design to quickly close the valve and the excitation controller, and then coordinate according to the principle described above. Control, adjust the parameters of the nonlinear PID excitation controller to make it similar to the Bang-Bang control. The design principle of the coordinated control logic is that when the fast-acting valve is finished, the sign of the excitation voltage depends on the direction in which the rotor sways and whether the generator tends to synchronize or tend to lose synchronization. When the rotor sways forward and the generator tends to lose synchronization, the excitation control takes a negative maximum; when the generator tends to synchronize, the excitation voltage switches to a positive maximum. When the angular velocity deviation changes sign (<3), and the generator begins to enter the synchronous state, the Bag-Bag excitation control is terminated, and the excitation voltage is controlled by the conventional AVR+PSS. At the same time, in the process of the generator tending to the synchronous state, the coordination control logic gives a signal, and after the closing of about Is, the valve is opened to return to the normal running state. 3 Numerical simulation analysis Three-phase short-circuit fault occurred at point F, and t=0.15s fault elimination. The simulation results are as shown. It can be seen from the above that under the action of the fast closing valve and the coordinated control of the excitation controller, the rotor swaying quickly disappears after the fault, and the generator resumes the synchronous running state, which embodies superior control quality and performance. At the same time, it can be seen that the opening and recovery process of the steam valve is very slow, but under the action of the electro-hydraulic regulator, the quick closing can be realized very reliably, and plays a major role in the process of coordinated control. The excitation voltage is approximately Bang stable. Through the coordination of excitation control and quick-closing valve control, the power plant can be kept in sync with the system after the fault occurs, which is simple and easy to implement. When a power plant encounters a dangerous moment of loss of stability, this coordinated controller can replace the practice of cutting the machine after an accident. Simulation analysis shows its effectiveness, allowing the generator to return to synchronous operation after one or at most two asynchronous periods. Zhu Faguo. Nonlinear PID and its application in generator set control. Ph.D. Thesis of Harbin Institute of Technology. 1999: Qiu Yu (1969-), engineer, Ph.D. student; the main research direction is power system transient stability analysis and control.
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