| The power system stability problem always draws a lot attention from electric power research workers. As the scale of modern power system continuously increases and more kinds of new electric power equipment accesses in the power grid, the power system stability problem has become more complex. With large-scale wind power penetrated, the power system which was composed of controllable synchronous generators, is now coupled with large amount of wind power based generators with unknown response characteristics. The doubly fed induction generator (DFIG) is the most widely used wind power based generator type up to now, the uncertainty caused by wind source’s characteristics and the flexibility which belongs to AC excited generators can be both found in its operation characteristics. This leads the modern power system transient stability analysis with large scale DFIGs integration to face new difficulties and challenges.Making an accurate understanding of the integrated DFIG’s transient behavior has important significance on analyzing the influence mechanism of large scale integrated DFIGs on power system transient stability. In this paper, the transient response process of DFIGs is completely analyzed, and based on the study of transient behavior characteristics of DFIG’s equivalent power angle, the electro-mechanical coupling characteristics and transient instability mechanism of DFIGs are then investigated. It suggests that excited by converters, the coupling relationship between the mechanical transient response in DFIG’s rotor speed and the electromagnetic transient response in DFIG’s generator has been mostly relieved; and in the transient process, the instability mechanism of integrated DFIGs is mainly depended on its converters’operating state, but not subjected to the power angle characteristic. The transient behavior characteristics leads the integrated DFIGs not directly involved in the system’s mutual-synchronization, but to influence the synchronous generators’transient behaviors by affecting their output power response. Knowing this helps providing an important theoretical basis for the follow-up studies on the influence mechanism of integrated DFIGs on power system transient stability.Since the influence of integrated DFIGs on system transient behaviors is a complex and time-variant process which is affected by multi factors, the research on this general influence mechanism turns to be more important for the power system transient stability analysis when considering complex DFIG integration conditions. With the application of the system Center of Inertia (COI) transform, the influence of integrated DFIGs on the dynamic motion of the local system COI, and on the rotor speed and the rotor angle dynamics of each synchronous generator with respect to the COI are deeply investigated, and the variations in the key factors which determine the dynamics of the COI and individual generators with DFIG integrated are clearly analyzed. It is demonstrated that the local system COI’s transient dynamics is affected more by the DFIGs’ active power transient response and whether synchronous generators in the system have been replaced. On the other hand, the rotor speed and rotor angle dynamics of each individual synchronous generator in local system with respect to the COI, which are determined by the transient dynamics of both the COI and the generator itself, turns to be influenced significantly by the access locations of the DFIGs. Time-domain simulation studies provide a complement validation for the theoretical analysis, and also present a concrete insight on how the different DFIG integration conditions influence the local system’s transient behaviors.When faced with a complex and large scale power system, it maybe not suitable to still regard the system as a simple single machine to infinite bus system or a dual-generator system in transient stability analysis, and the system’s transient stability problem turns to be the transient stability of the relative rotor speed and rotor angle swings between interconnected areas. In this situation, the area-based COI (ACOI) transform which is extended by simple system COI concept is applied in the analysis of the influence mechanism of integrated DFIGs on the multi-area interconnected power system (MIPS) transient stability. Theoretical analysis and simulation verifications based on both the two-area interconnected power systems (TIPS) and the MIPS are carried out, by which the general influence mechanism of integrated DFIGs on MIPS transient stability is clearly investigated. Study results reveals that the concrete influence effect of integrated DFIGs is influenced by both the DFIG integration conditions and the original transient stability mode of the MIPS; and with a clear understanding of the system transient stability mode, the core of the research on the influence mechanism of integrated DFIGs on a MIPS is still to figure out the influence on the transient behavior of each separate area.According to the findings fetched in the previous research about the influence mechanism of integrated DFIGs on power system transient stability in this paper, a DFIG transient control strategy is proposed to improve the interconnected areas’ transient stability. This control strategy functions through an additional excitation control by regulating the integrated DFIG’s transient active power output during the fault period. When faced with a new system fault, the system transient stability mode is judged according to the typical fault data of the system, and the additional excitation control acts selectively to exert an additional influence on the transient response of synchronization system’s accelerating power. In this way, with accurate and appropriate additional excitation control of DFIGs’ active power output during the fault period, the first swing transient stability after fault can be effectively improved, which are also illustrated by simulation studies based on a two area system considering the main features of wind power integration in China. |
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