| China has entered a large complex system with AC and DC era. Because of its unique advantages, HVDC plays an important role in the long-distance power transmission and large interconnected power system. But new challenges are also raised in large-scale interconnected system operation. In complicated AC and DC systems, the research to coordinate the system safety and economic operation is imminent. Available transfer capability (ATC) is applied as the economic and technical indicator in inter-regional electric power system, it is of great importance in maintaining security and stability of the system, managing transmission congestion, and guiding the conduct of market transactions and other aspects. Therefore, the assessment and decision-making of ATC in regional AC-DC hybrid electricity transmission system has become a very important research issue, which still has many problems to be solved, under electricity market environment. As a result, tightly connected with this central theme, the paper gives detailed analysis and conducts in-depth research for ATC related problems in inter-regional systems under static and dynamic constraints.Firstly, aiming at existing space which still can be proved in nowadays power flow calculation method for hybrid systems, this paper improved the power flow calculation method in AC-DC hybrid systems. Considering the interaction between AC and DC systems under different control strategies, it brings us a power flow calculation method satisfying inverters changing controlling strategies. It is based on alternating iterative method, giving effective correction for the special nodes in AC system Jacobian matrix. Key state variables of the converter is choosen reasonably according to its changing controlling strategies, and deals with the cosine value of the control angle and the transformation ration product as a state variable. It effectively avoids the cosine value and transformation ration form crossing the limitation during the iterative process. The refined alternating iterative method has clear mathematical concepts, and is easy to be implemented and transferred. It improves the robustness of the power flow calculation method in AC-DC hybrid systems.There are some limitations of Voltage stability index. A simplified formula of local voltage stability L-index is proposed in this paper. It is based on the actual grid line with reactance is much greater than resistance, the characteristics of a small bus voltage phase. It is proposed a fast calculation can be adapted to large-scale grid L-index (LSimple indicators) and voltage stability analysis of bus related information among the LSimPle_Q sensitivity. The differential equation of the simplified L-index is developed to obtain the sensitivity of LSimpe respect to parameters, such as LSimple-Q sensitivity. The physical meaning of LSimple-Q sensitivity is comprehensively presented. Combining simplified L index with LSimple-Q sensitivity online monitoring system to provide multi-voltage information. It can help system operators complete control system voltage running condition and take reasonable measures to improve the system voltage stability. It has some practical value.The ATC optimization model based on conventional static security constrained, combined the improved alternating iterative method and LSimple index,fully considered characteristics of the HVDC system, and join the LSimple index in inequality constraints, constructed a new ATC calculation method, which is based on the OPF hybrid system. The proposed model is solved by primal-dual interior point method, at the same time, this paper analysis the change of the LSimple value constraints and its influence on ATC of AC-DC hybrid system and voltage stability. Finally pointing out that for a certain AC-DC hybrid system with the same ATC optimization results, the votlage stability of ATC model with LSimple index constraints is better than the one who doesn't have LSimple index constraints.As the prerequisite for realizing the nonlinear additional power control strategy, an improved coherency-based equivalence method is proposed to make up for the current deficiency of dynamic equivalence method. The generator dynamic response factor is defined by the combination of coherency-based equivalences and model equivalences, the generator dynamic stability index is deduced and obtained by the dynamic response factor, and the generator dynamic stability after oscillation can be quantitatively evaluated by using the index. By means of the generator dynamic stability index to calculate coherency-based equivalences of the regional interconnected grid, the accuracy of the equivalence can be effectively improved and the amount of computation can be reduced. The improved coherency-based equivalence method can effectively trace the system operating mode changes and accurately identify the dominant cluster in the center of inertia, which is more accord with generator polymerization concept in dynamic equivalence of power system.This method offers a strong theoretical support for the nonlinear additional power control.According to the additional power characteristic of HVDC that its modulation can improve the dynamic stability of the AC system, a rational DC nonlinear additional power control strategy is proposed. A new dynamic equivalent method is proposed to enhance the accuracy of equivalent system. According to the characteristics of the equivalent strategy, a rational nonlinear optimal supplementary control strategy is proposed for the DC modulation. The feedback linearization of differential systems is easily extended to differential-algebraic systems by proposed approach. Utilizing the equivalent strategy, a rational nonlinear optimal supplementary control strategy is proposed for the AC-DC system. The control strategy only requires the critical clusters'speeds and angles as modulation signals, which ensures the control effect and reduces the difficulty of control strategy implementation. Compared with the traditional control strategy, the proposed control strategy can effectively improve the dynamic stability of the AC-DC hybrid system.At last, against the problem that inter-area transmission power for the system can significantly affect the dynamic stability of the interconnected system, and may lead to inter-area low-frequency oscillation, thereby limiting the ATC. Taking system dynamic stability constraints into account, ATC evaluation model is constructed for inter-area AC-DC systems, which makes the damping ratio join ATC optimization model as an indicator to measure the system dynamic stability, and uses the continuous power flow method to solve the model. By additional DC power modulation strategies, the model effectively improves the system dynamic characteristics and significantly raises the of inter-area system ATC, which takes the economic needs into account and meets the premise of the stability of the power system. The AC-DC systems ATC calculation model considering the dynamic system stability constraints opens up a new way for the research of ATC in hybrid inter-area system. |
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