Methodological Research On Dampening Electromechanical Oscillations And Enhancing Wind Power Plants Connection Capability In Power Systems

The electric power system as part of an infrastructure in each country plays a particularly important role in the national economy. When the social, political, and technological aspects develop, the demand of electricity power grows rapidly, resulting in the increase of the scale and complexity of power systems. Some characteristics, such as long transmission distances over weak grids, highly variable generation patterns, and heavy loading tend to increase the probability of appearance of sustained wide-area electromechanical oscillations. Such oscillations threaten the secure operation of power systems and if not controlled efficiently can lead to generator outages, line tripping, and even large-scale blackouts. On the other hand, due to the reduction in fossil fuels and the environmental destruction by them, renewable energy sources, such as wind, solar, and photovoltaic are connected to the grid system, in which the wind-electric energy generation is one of the renewable energy sources that is by far the fastest growing. Especially, the doubly fed induction generator based on the wind energy conversion system(DFIG-WECS) is widely used. The control for the grid-connected capability of DFIG wind turbine is a difficult task; especially, the disconnection must be avoided to fulfil the generation grid code requirements. Based on the mentioned background above, the aim of this dissertation is to research methodologies for optimizing the performance and operation of electric power system, especially, focusing on solving two main subjects that are the dampening power system oscillations and enhancing wind turbine connection capability in power system.For methodological research on dampening power system oscillations, the main objective is the prevention of the power system from blackout caused by small signal stability. Some major analyses of power system stability investigate in order to understand the main causes and mechanisms. From these analyses, it can establish that the major reasons for the stability problems relate the angle stability directly. A relevant method proposes to prevent power system blackout. For methodological research on enhancing grid connection capability of wind turbine, the main objective is the assurance of the grid-connected capability of DFIG-WECS system during the low voltage conditions. The technologies and grid connection conditions of wind turbine-generator analyze to understand the main causes and mechanisms. FromConnection Capability in Power Systems these analyses, it can establish the main reasons for the disconnection of wind plants relate the low voltage ride through(LVRT) directly. A relevant method proposes in this dissertation to improve the grid connection capability of wind turbine. The main tenor of the dissertation is summarized as follows:A relevant method for seeking an optimal location placement of thyristor-controlled series capacitor(TCSC) or static var compensator(SVC) is proposed to enhance the rotor angle stability and dampen the power system oscillations. This proposed method is developed from the energy approach based on the controllability and observability Gramian matrices of the linearized system. In order to calculate these matrices, the multi-machine power system with TCSC or SVC controller is expressed in the form of a differential algebraic equation(DAE) model. These Gramian matrices are obtained from the unique positive definite solutions of the Lyapunov equations and it depends on the control input, control output, and state matrices of system. The optimal location of TCSC or SVC is chosen based on the total maximum Gramian energy of contingency outage cases that means the control input energy must insure the smallest. To reduce the number of state variables when dealing with large-scale power systems, the balancing realization technique for order reduction of linear systems is recalled. All equivalent models are established in PSS/E and MATLAB software programs. The effectiveness of the proposed methods is demonstrated on the IEEE 39-bus New England network. In addition, the controllability index method is also re-called to make basis for comparing with the proposed approach.In order to detect the grid-connected capability of DFIG-WECS under the low voltage conditions, the conventional strategy is designed using the proportional-integral(PI) controllers, and these PI controllers are designed based on the Butterworth polynomial method. The performances of DFIG-WECS when connecting to the grid are also analyzed under different operating conditions, such as normal operation, variable wind speed, and voltage sag responses. All equivalent models are established in PSCAD/ EMTDC software program.A relevant control strategy for a DFIG-WECS is proposed to enhance the low voltage ride through(LVRT) capability. Within the proposed control method, the current control loops of the rotor side converter(RSC) are developed based on the passivity theory. The control scheme for the grid side converter(GSC) is designed based on a two-term approach to keep the DC-link voltage close to a given value. The first term based on the maximal voltage of GSC is introduced into the GSC control loops as a reference reactive current. The second one reflecting the instantaneous unbalanced power flow between the RSC and GSC is also introduced into the GSC control loops as a disturbance to compensate the instantaneous rotor power, in which the instantaneous power of the grid filter is considered. The effectiveness of the proposed control strategy is verified via time domain simulation of a 2.0MW-575 V DFIG-WECS using PSCAD/EMTDC. In addition, the conventional control strategy using crowbar and DC chopper is also re-called to make a base for comparing with the proposed control strategy.