AC/DC Power System Transient Stability Analysis And Control

The energy resources are dispersed far away from the load centers in China. Because of the great advantages in the long distance and bulk power transmission, HVDC technology has been increasingly used in the west-to-east power transmission, the north-to-south power transmission and the nationwide interconnection in China. However, with more and more HVDC systems commissioned, the transient stability of the AC/DC power system is threatened more frequently. This paper is to study the mechanisms, the quantitative indexes and the enhancement methods for the transient stablity of the AC/DC power system, especially the receiving-end power grid with multiple DC infeed. The main work is organized as follows:(1) The physical mechanisms of the typical transient stablity problems in the AC/DC power system are studied. The simplified models for the two typical types of the AC/DC power system are constructed. From the viewpoints of the transient angle, voltage and frequency stablity, the physical mechanisms and the main related facts of the transient stablity of the AC/DC power system after the AC and the DC faults are revealed. Finally, the similation results of the South China Power Grid (SCPG) prove the effectiveness of the proposed mechanisms.(2) The effectiveness of the multi-infeed effective short circuit ratio (MIESCR) on evaluating the power stabiltiy of the multi-infeed HVDC system is analyzed. The quantitative relationships between the power stability of the multi-infeed HVDC system and the MIESCR are analyzed based on the quasi-steady-state model of the two-infeed HVDC system, and the approximated range of the critical multi-infeed effective short circuit ratio (CMIESCR) is identified. Besieds, the standard rules to evaluate the voltage support strength of the multi-infeed HVDC system using the MIESCR are proposed. The similation results of the SCPG prove the effectiveness of the MIESCR.(3) Three macroscopic indexes are summarized and proposed to evaluate the power grid structure quality of the AC/DC power system, which respectively are the corridor strength index, the voltage support strength index and the frequency support strength index. The corridor strength indexes include the ratio of the AC corridor strength to the DC corridor strength and the strength of the AC corridor to resist the DC corridor power flow shift shock, which can describe the relative strength of the AC and DC corridors and the strength of the AC corridor to resist the shock of the DC corridor power flow shift. The MIESCR is adopted as the voltage support strength index to evaluate the voltage support ability of the AC system for the DC converter stations. The frequency bias factor is used as the frequency support strength index to measure the frequency support ability of the AC/DC power system. The similation results of the SCPG prove that the macroscopic indexes can reflect some important power grid structure quality of the AC/DC power system and provide an effective tool for the quantitative assessments of the AC/DC power system.(4) A grid dynamic segmentation technique based on the fault current limiter (FCL) is proposed. The basic idea of the technique is to install the FCLs on some appropriate AC lines according to the network structure and to segment the power grid into a number of sectors connected by these FCLs. When the short-circuit faults occur inside the AC system, the current-limiting impendence of the FCL is inserted to limit the short-circuit current and obstruct the propagation of the faults among the sectors. Thus, the commutation failure duration of the DC systems is shorten, the recovery of the DC systems is sped, the power imbalance and the transferred system flow caused by the concurrent commutation failures of the multi-infeed DC systems are relieved, and the transient stability of the multi-infeed HVDC system is improved significantly. The simulation results of two simplified multi-infeed HVDC systems and the SCPG verify the effectiveness of the proposed technique.(5) A wide-area measurement system (WAMS) based transient excitation boosting control (WTEBC) is proposed. The WTEBC technique is based on the extended equal area criterion (EEAC). First, the critical swings and the critical machines after the serious AC and DC faults are identified using the prior or real-time simulation. Then according to the real-time measured generator rotor angle from the WAMS, the excitation boosting is applied to the critical machines in the critical swings until their rotor angles drop below a certain threshold after the critical swing. The simulation results of the4-mahicne system and the SCPG prove that the WTEBC can overcome the drawbacks of the conventional transient excitation boosting controls and improve the transient stability of the AC/DC power system more significantly.