| With the continuous increasing permeability of green renewable energy in modern power system, many advantages have appeared in modern grid, along with challenges to its safe and stable operation. Large-scale renewable energy resources in China, especially wind power, are mostly distributed in inland-northwest area, but main power consumption groups located in southeast and east coastal area. Thus the development strategy of "power transmission from west to east, complementary of north and south, and nationwide connectivity" is objective requirement to the present energy distribution in China and the large-capacity and long-distance power transmission mode will be the main development trend. Meanwhile, continuous expanding scale of power system leads to unprecedented challenges of safe operation and control. Ensuring safe and highly efficient operation is one of the main goals of the development of modern power system. This paper mainly studies key technologies of security and stability control in modern power system, and does overall thorough study by theoretical analysis and simulation verification. The major conclusions and achievements are as follows:1. A markov switching model of AC/DC system and its nonlinear control strategy are proposed. The additional modulation function of AD/DC system can improve its stability, but the remote feedback information and time-delay decrease control performance of controllers and affect security and stability of system seriously. Considering complex dynamic interaction behavior of AC/DC hybrid system and the time-varying and time-delay characteristics of wide area measurement system, and full-state-feedback linearization and back-step control theory are used to establish a markov switching time-delay model of AC/DC hybrid system on real-time operating state. Based on free-weighting matrices, the nonlinear control strategies which are not sensitive to time-delay are derived on the condition that part of probability transfer matrices are not known, and then the nonlinear markov switching controller is designed. Using PSCAD/EMTDC to build control model of AC/DC system, and effectiveness of nonlinear control strategies are verified.2. Model and improved control method of VSC-MVDC integration lagrge-scale wind farm are proposed. Because distance and resistance of transmission line are inconsistent for VSC-MTDC system, which result in the larger copper loss for whole system especially long distance and large capacity DC system. To realize the coordinated control between different stations which achieves the minimum copper loss, calculation method about voltage droop coefficient for VSC-MTDC with local load/without local load is built, and a DC voltage droop control strategy is proposed. By introducing adjustable capacity space of convertor stations, therefore each convertor stations could participate in droop control power adjustment according to their abilities. The paper also establishes control model of four-terminal DC system integration wind farm by PSCAD/EMTDC, then operating characteristics of DC transmission system are analyzed on the condition of normal operation, quit operation of convertor stations and three phase short-circuit fault of convertor stations, to verify its effectiveness of this proposed control strategy.3. A proportional integral-resonant (PI-R) current controller of the HVDC system on static αβ coordinate system is proposed. It can realize regulation and control of positive-sequence and negative-sequence current without phase sequence decomposition. Mathematical model of rectifier-side of VSC-HVDC system on unbalanced grid voltage condition is established, and the influence of current negative sequence component on reactive and active power’s2nd harmonic is discussed. Three control targets and corresponding current values of each sequence on positive rotation coordinate are calculated, and then the unbalance resonance control strategies are designed in the VSC-HVDC system. Finally, the control model of VSC-HVDC is built using PSCAD/EMTDC, and steady and dynamic control performance of the PI-R current control strategies are verified.4. A power resonance compensation control strategy considering the zero sequence channel of VSC-HVDC under unbalanced grid voltage condition is proposed. The mathematical model of VSC-HVDC system is established under unbalanced voltage condition using symmetrical component method, and the relationship between zero sequence current and active power, reactive power of VSC-HVDC are establised after considering zero sequence channel. A main controller and an auxiliary controller are designed in the current inner loop control part, the main controller is mainly used to control the converter under normal circumstances or DC component under fault condition of ac side, the auxiliary controller is used for power resonant compensation control of twice frequency component under unbalanced grid voltage conditions. Finally, the power resonant control model is built under unbalanced condition by PSCAD/EMTDC, and the validity of steady and dynamic performance of the proposed resonance control algorithm are verified respectively. 5. An economic operation model under the combined effects of wind, solar and power load are proposed. On the basis of existing security and economic operation method of power system mainly considered effect of single random factor. By introducing probability distribution function of wind power and photovoltaic generation and load prediction error, considering relevance of different random factors, this paper establishes the joint probability density function of wind power, photovoltaic generation and load, and also builds risk model and dispatch model on condition of loss load in two typical operation modes. The constraint of load-loss quantity is used by the optimization model instead of load-loss probability constraints in general dispatching model, and improved α-superquantile was introduced, and Monte Carlo simulation and analytic method were combined to resolve optimal model with risk constraints. Then depending on the economic dispatching model in two typical operation modes, the paper calculates optimal scheduling results at different confidence level and different correlation of random factors by MATLAB, and analyzes relationship between power supplying reliability and economics. |
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