Summary of Control Technology for Suppressing Torsional Vibration of Large Steam Turbine Generator Shaft System

Intelligent Control 1 Foreword With the commissioning of high-power units, there have been many major accidents in the home and abroad that caused severe damage to the unit caused by torsional vibration of the shaft. From 1969 to 1988, there were more than 30 shafting accidents caused by torsional vibration at home and abroad. Especially since 1999, there have been many vicious accidents in the shaft section of the unit.

A remarkable feature of the torsional vibration of large steam turbine generator sets is the torsional vibration of electromechanical coupling. Therefore, the torsional vibration problem of the network machine is highly valued in design, manufacturing, scientific research and operation. The research on the prediction and control of torsional vibration of the network machine not only has great theoretical significance, but also has great engineering application value for disaster prevention and economic operation. China has listed such issues as a national key basic research development planning project, and this review is proposed to carry out this research work.

2 Electrical disturbances caused by torsional vibration of shafting under electromechanical disturbance include electrical short circuit fault, automatic reclosing, asynchronous grid connection, load rejection and series capacitor compensation, regulation of high voltage direct current transmission and power system stabilizer. Appropriate configuration and other mechanical disturbances include speed control system shaking, fast control valve and so on. The shafting torsional vibration under electromechanical disturbance is divided into three types, namely, subsynchronous resonance (SSR), supersynchronous resonance and oscillating torque shock torsional vibration. The main factors of 2.1-time synchronous resonance induced SSR are series capacitance compensation, direct current transmission, plus Improper installation of power system stabilizers, generator excitation systems, feedback from thyristor control systems and electro-hydraulic control systems. A large number of studies have been carried out on its mechanism, analytical methods, prevention and inhibition measures at home and abroad, and certain effects have been achieved. The following describes the use of control and regulation to suppress SSR.

In order to improve the transmission capacity, the series capacitor compensation in the power system is the main reason for generating SSR. A lot of research done at home and abroad mainly suppresses the SSR by improving the excitation system. In 1975, for the first time, the excitation system was used to suppress the torsional vibration of the shafting. The proposed negative damping stabilizer (NDS) using the reactive power of the generator as a feedback signal expanded the stable region. The literature uses active power and reactive power as the input signals of NDS to obtain a robust stabilization system. However, such controllers typically only suppress a few torsional modes. The Power System Stabilizer (PSS) proposed in the 1970s was originally used to dampen low-frequency oscillations in the system. The paper analyzes the damping effect of PSS on torsional vibration and the feasibility of multi-parameter feedback linear excitation control. It also proposes to suppress multiple unstable torsional modes by linear optimal excitation control (LOEC). Under large-scale capacitance compensation, It can effectively stabilize systems that are prone to SSR. In [6], it is proposed to use the twist angle difference between adjacent masses as the feedback quantity to obtain the additional sub-optimal excitation controller. The analysis shows that the eigenvalue of the system shifts significantly to the left. However, since the optimal excitation control design is complicated, the feedback amount is not easy to measure and has not been implemented yet. Literature [7] proposed using modal control theory to design PSS to provide damping torque for generators. The literature [8] proposed using artificial neural network (ANN) to adapt controller gain to damper in static var compensator and excitation controller. SSR and time domain simulation have proved their effectiveness.

In theory, the steam turbine's valve adjustment can also suppress the SSR. Various control theory techniques for the excitation system can also be used for valve adjustment, and good suppression effects can be achieved.

Due to the interaction between the turbo generator shaft system and the direct thermal power engineering flow system, the commissioning of DC transmission has also become a cause of SSR. The bandwidth of a constant current or constant power control system in high-voltage direct current (HVDC) transmission is generally 10 to 30 Hz, and the low-order torsional vibration natural frequency of a turbogenerator is usually in this frequency range, so the high-voltage DC converter may The torsional vibration mode is excited by a fixed power, current, voltage, and auxiliary power control loop for improving low frequency oscillation stability. Improving the control of the converter or adding an auxiliary subsynchronous damping control to the current controller eliminates this problem. The literature [9] pointed out that since HVDC is active and fast and controllable, it is possible to use the characteristics of HVDC and adopt appropriate control measures to make it a means of suppressing SSR, and design an additional controller for suppressing SSR for HVDC. .

The expansion of the unit capacity makes the first-order torsional vibration frequency of the rotor below 10 Hz. The high-speed servo system and the fast excitation system are sufficient to react to the torsional vibration components in the rotational speed, resulting in the interaction between the speed regulation system and the excitation system and torsional vibration. Torsional vibration. Therefore, the adverse effects of torsional vibration should be fully considered when designing the control system. The literature [10] analyzes the interaction between the turbine governing system and the torsional vibration of the shafting, points out the factors affecting the torsional vibration in the governing system, and proposes measures to prevent such torsional vibration.

Flexible AC Transmission Technology (FACTS) is a new technology for controlling AC transmission. FACTS devices with good control strategies can suppress shafting torsional vibration. Among them, static reactive power compensation (SVC), subsynchronous resonance damper (SSR Damper) and controllable series compensation (TCSC) have been put into operation and can provide damping for SSR. However, TC SC has higher harmonics, and whether SSR can be stimulated is still under study.

2.2 Super-synchronous resonance grid three-phase load imbalance, various asymmetric short circuits can cause super-synchronous resonance.

The fan blades of steam turbine blades and large generators are extremely sensitive to frequency doubling resonance. The long blades of steam turbines are easy to break and fly off in this resonance state, resulting in a large imbalance force in the shafting system and even causing serious accidents.

At present, countermeasures for preventing and suppressing supersynchronous resonance have limitations on negative sequence current and shafting frequency modulation, and reports on applicable models and related control strategies for analysis are still rare at home and abroad.

2.3. Oscillation torque impact torsional vibration transient symmetry and asymmetric short circuit, automatic reclosing, non-synchronous grid connection, load rejection, short-time fast control valve and line switch switching operation and other sudden disturbances, it may be short Time-impact torque, forming short-term impact shafting torsional vibration. Among them, the automatic reclosing and non-synchronous grid connection have the most serious response to the torsional vibration of the shafting. At present, the measures to suppress such torsional vibration are mainly to limit the operating conditions and optimization criteria.

2.4 Active control of shafting torsional vibration The above control of shafting torsional vibration is passive control. In fact, the control requirements for the normal operation of the turbine generator set control system are not consistent with the control requirements for suppressing the shafting torsional vibration. Literature [11] proposed a technical measure to eliminate the hazard of shafting torsional vibration by active vibration control. Then, several experimental studies on damping vibration reduction and active control vibration reduction were carried out on the rotor-shaft torsional vibration active control simulation test rig of the domestic 200 MW steam turbine generator set. The results show that the active control can effectively suppress the shaft torsional vibration. The active vibration control is active control, the energy is supplemented by external energy, and it has the advantages of strong adaptability, convenient adjustment and modification, etc. However, there are many links in the closed-loop control system, and all links may be invalid, and measures must be taken to ensure reliability.

Analysis method of 3-axis torsional vibration The shafting torsional vibration analysis methods widely used at present include the characteristic root method, the sweep frequency method, the complex torque coefficient method and the time domain simulation method. The literature [12] introduced these several analysis methods and compared their respective advantages and disadvantages. The analysis method of shaft torsional vibration is increasingly rich. The literature [13] extends the analysis of torsional vibration to the nonlinear region, and studies the Hopf bifurcation phenomenon of torsional dynamics. The literature [14] proposed the analysis of torsional vibration with ANN. Both provide favorable conditions for the design of the control system.

4 Mechanical vibration and shafting torsional vibration The long-term study of shafting torsional vibration is mainly based on the disturbance of the power system, and the vibration problem of the turbine itself is rarely considered. Among the factors causing mechanical vibration, the vibration source belonging to forced vibration has insufficient rigidity of the bearing housing, the rotor mass is unbalanced, and the center of the unit is not self-excited, and there is oil film oscillation and whirl movement. The mechanical vibration and torsional vibration of the turbine generator rotor are present at the same time. In the traditional system analysis, the mechanical vibration is usually regarded as a secondary factor and ignored. According to the principle of chaotic dynamics, simple nonlinear rules can repeatedly produce complex dynamic behaviors. Therefore, the analysis of the torsional vibration of turbo-generator shaft shafts needs to consider all aspects of interaction.

5 Analysis and control of shafting torsional vibration based on nonlinear science 5.1 Nonlinear dynamics of turbo-generator system Turbine generator set is regulated by unit shafting, steam turbine and speed control system, synchronous generator and synchronous generator The system and the grid are composed of five parts. The dynamic characteristics of each subsystem are very complicated, especially the synchronous generator, including the electromagnetic and electromechanical transition characteristics of the speed control system and the excitation system is still simplified with a nonlinear link, the fast excitation system thermal power engineering 2000 time The constant is relatively small, so the damping is small. The flexible component on the oscillating rotor of the electromechanical coupling system is dynamically coupled with the rotor to generate an additional torsional vibration mode, and the unit shaft has the characteristics of flexibility, mass, and large inertia. Series capacitor compensation forms an RLC loop, which is prone to subsynchronous electrical oscillations. At the same time, there is an interaction between the subsystems. When the system is disturbed, some unpredictable characteristics may occur in the electromechanical coupling state. These constitute the complex nonlinear characteristics of the torsional vibration of the turbo generator shaft.

Due to the complex nonlinearity of turbogenerators and the variability of their operating conditions, it is difficult to analyze them by linear system theory and suppress torsional vibration. This prompted us to seek new theories and control methods to analyze them deeply. Effectively suppress shafting torsional vibration.

5.2 Nonlinear scientific analysis of shaft torsional vibration and its intelligent control According to the principle of thermodynamics, a system that simultaneously exchanges matter and energy with the outside world is called an open system. In the process of exchanging matter and energy from the outside world, the open system can form and maintain a macroscopic time-space ordered structure through the energy dissipation and internal nonlinear dynamic mechanism, which is called the dissipative structure. The dissipative structure theory proposed by Prigogine studies the mechanism, conditions and laws of an open system from chaos to order. It points out that an open system far from the equilibrium state, when the external conditions or certain parameters of the system change to a certain critical value, through the sudden change of the fluctuation, it is possible to change from the original chaotic disorder state to a time, A new state of space or function order. During the operation of the turbo generator system, energy needs to be exchanged continuously, and some of the energy is dissipated due to poor sealing or system oscillation. Uncertain interference causes the system to deviate from the equilibrium state. Under the interaction of various nonlinear factors, the control system can induce positive feedback, which leads to system oscillation, and then the shaft torsional vibration is formed, so that the system is further away from the equilibrium state and meets the formation of the dissipative structure. conditions of.

Usually chaos refers to a certain nonlinear system. Under certain conditions, its state will exhibit a random phenomenon similar to random. One of its basic characteristics is the extreme sensitivity of the state trajectory of the system to the initial conditions, and the occurrence of chaotic phenomena. It is determined by the nonlinear properties of the system itself. Torsional vibration of the shafting is an evolution from equilibrium to unbalance. When the torque balance is damaged, the nonlinearity of the system is further enhanced, and the bifurcation phenomenon occurs. The interaction of nonlinear factors is further aggravated, and finally chaos may occur.

There are three basic forms of the general development trend of a complex system: one is that the system is in a stable structure, the other is that the system oscillates or collapses, and the third is the evolution of the system from one steady-state structure to another. Almost all stable systems can be transformed into instability and oscillation under certain conditions, but the stability of the system structure can be adjusted by changing the structure and parameters of the control system. The theory of catastrophe is based on the study of structural stability of systems. It is believed that the essence of catastrophe is the transition of a system (or process) from one stable structure (state) to another stable structure (state). The random fluctuation inside the system is the decisive factor to promote the system transformation. The fluctuation has two characteristics. With its positive side, the fluctuation can lead to orderly, that is, the chaotic disorder state is transformed into a new state of time and space or function order. For the torsional vibration of the turbo generator shaft, it is to achieve a new torque balance.

The intelligent control system is essentially a complex nonlinear system. The intelligent controller can not only control the parameters of complex objects, but also adaptively change its control structure according to the needs of dynamic characteristics. Turbine generators consist of multiple subsystems with strong interactions with each other. Synergetics is to discuss the synergy and dominance principle of a system composed of many subsystems in the formation of an ordered structure. According to the synergy, the interaction of each subsystem of the unit can be intelligently coordinated and controlled to achieve the best control effect.

The essential nonlinearity of intelligent control systems provides an effective way to control complex nonlinear systems. In recent years, artificial neural networks and fuzzy control have been widely used in power systems, such as transient, dynamic stability analysis, load forecasting and power system control, and have achieved good results. It has been proved that the three-layer forward neural network can approximate arbitrary nonlinear functions with arbitrary precision, and has parallel processing capability, strong robustness, self-organizing self-learning ability, and prediction ability.

Fuzzy control is suitable for solving the difficulties caused by the uncertainty and inaccuracy of the process itself, and the control form is simple and easy to implement. The fuzzy control and neural network control are used to control the torsional vibration of the shafting, which can avoid the occurrence of torsional vibration while maintaining the stability of the system, and further improve and improve the control performance of the system.

6 Outlook With the development and improvement of control theory, many new control theories and techniques have been applied to the power system and achieved good expected results. Some of these advanced control strategies are also expected to suppress shafting torsional vibration.

Differential geometry control of 6.1 shaft torsional vibration With the development of nonlinear control theory, the modern differential geometry theory is introduced into nonlinear control. Under the unremitting efforts of Chinese scholars, nonlinear control theory has been successfully applied to nonlinear excitation control in power systems, nonlinear control of turbine opening of steam turbine generators, etc., and has achieved important results.

It can be considered to apply the differential geometry control theory to study the suppression of the torsional vibration of the shafting. It should be pointed out, however, that the differential geometry control theory is ill-conditioned when it involves the reversible properties of the system and the structural properties under dynamic feedback, and it lacks robustness to parameter perturbation.

6.2 Intelligent control of shaft torsional vibration In view of the complex nonlinearity of electromechanical coupling system and the effectiveness of intelligent control theory analysis and processing of nonlinear systems, we propose to use intelligent control theory and method to twist the shaft of large steam turbine generator sets. The vibration is predicted and controlled. According to the dissipative structure theory, the mechanism and law of torsional vibration are analyzed. Based on this, the chaotic mechanics theory is applied to study the properties of torsion in the torsional vibration, and then the artificial intelligence theory and technology such as fuzzy logic, neural network, genetic algorithm and rough set are applied. A class of intelligent control laws suppress shaft torsional vibration. Scientifically and accurately predict the occurrence of torsional vibration precursors, and perform real-time intelligent control based on the obtained information of the characteristic variables to avoid the violent oscillation of one vibration, which is unstable, suppress the divergent oscillations that cause instability, and accelerate the convergence oscillation. Convergence rate. In summary, we believe that the application of intelligent control theory based on nonlinear science to predict and control the torsional vibration of the shafting is a good new way.

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