Research On Transient Stability Control Strategies In Power Systems

The generation capacity and voltage grade have been greatly improved in modern power systems. At the same time, the structure and the model of the systems become more complicated, and many uncertain factors are brought to the system because of the application of HVDC and FACTS, the interconnection of the area power systems, and the open of the power market. These bring many hidden troubles of the system security. Many severe accidents in power systems all over the world in these years arouse again the focus on power system security. Many severe accidents in power systems ascribe the loss of angle stability under large disturbances. Angle stability under large disturbances is always called transient stability and is the foundation of the operation of the power systems. So, transient stability control is an important barrier of the power system security.The topics discussed in this paper are on transient stability control strategies in power systems, and they are:â… . A survey of transient stability control is made, which includes control objectives, control techniques and so on. And a brief introduction of nonlinear system adaptive control theories and their implications in power systems is made.â…¡. A nonlinear robust adaptive excitation controller is designed for the generators in multi-machine power systems. The uncertainties of damping coefficient, synchro reactance and transient reactance on d axis of the generators, and the model errors and outer disturbances are considered. The controller guarantees the convergences of the generator angle and frequency in a small neighborhood of the prime operation point. A characteristic of the designed controller is that the model errors and outer disturbances are requested only to be bounded but the boundnesses are needless. In the designed controller, a dynamic estimator is designed for each uncertain iterm, the globally uniformly ultimately boundedness of all state variables and the convergency of the system output are guaranteed, and each controller is decentralized or local. The digital simulation displays the validity of the designed controller used to improve power system transient stability.â…¢. A nonlinear robust adaptive controller is designed for a MIMO nonlinear system with uncertain parameters and model errors, and is applied to the integrated control of generator excitation and turbine governor. The work in the above chapter which is on the SISO control of nonlinear system is extended in this chapter to MIMO control of nonlinear systems. An increase-order operation is made, first, if the anterior n-1 control variables of an n-order nonlinear system convergence to zero, and the n-1 control variables are regarded as state variables, that brings n-1 new control variables. Then, the whole system becomes to be a 2n-1 order nonlinear uncertain system with n control variables. At the end, using back-stepping technique, control laws are designed for each control variable to guarantee the globally uniformly ultimately boundedness of all state variables and the convergency of system output. The design method is applied to the integrated control of generator excitation and turbine governor to improve transient stability of power systems. The digital simulation displays the effectiveness of the designed controllers.â…£. A "COI-tracking" concept is proposed for the design of transient stability excitation controller in multi-machine power systems. Generally, the COI curves of the power system can reflect the global trend of the changings of all generators in transient time, and the curves are usually smooth. So, if all generators in the power system track the dynamic COI of the system, but not try to stay at the initial operation point, the generators could keep synchronous more easily and the transient stability of the system could be greatly improved. The development of WAMs technique could help the realization of proposed global control concept, and the time delay of WAMs signals within a certain range is acceptable. With the proposed concept, an excitation controller is designed for generators using back-stepping technique and is contrasted with the decentralized/local controller designed by the same technique. Digital simulations show that the proposed excitation controller using COI-tracking concept can improve transient stability of the power system obviously and is much better that the other one.â…¤. Using area COI to design the power modulation controller of HVDC and reactance controller of TCSC, to damp the swing between two-area interconnected power systems is proposed. Direct current power modulation of HVDC and the reactance modulation of TCSC can affect the active power transfered between areas and, further, affect the stability of the power system. The models of HVDC and TCSC modulation are expressed by one order models in this paper. With the objective of damping the swing between the area-COI of the two areas, nonlinear robust controllers are designed for power modulation of HVDC and reactance modulation of TCSC. The globally uniformly ultimately boudedness of all state variables and the convergency of the swings between area-COIs are guaranteed. Digital simulations show that the designed controllers are better than the conventional modulation controllers in improving power system stability. The proposed concept could also be extended to the damping control of multi-area power systems.â…¥. At the end, a conclusion is made for the whole paper. Some open problems in this paper are discussed.