Study On The Voltage Operation Performance Of Large Power Grid With Multi-infeed Direct Current

The unbalanced distribution of energy resources and load demands in our country is caused by the fact that the center of supply-side is concentrated in southeastern coast area while the center of demand-side in southwestern and southeastern area, which is a prominent problem in the high-speed economic development. HVDC is widely used in the power system because of its unique advantages in the power transmission with long distance and great capability. With the construction of power grid and increasing penetration of HVDC transmission project, several regional power grids in China have become bulk MIDC systems. The convertor stations in MIDC systems have closer electrical distance and therefore strong interaction with each other, simultaneously the operations of these stations need to consume a large amount of reactive power while there is usually lack of reactive sources in the AC systems at receiving-end. As the consequence, the voltage operation performance of MIDC systems directly affects the safe and economic operation of whole power grid. Researches on voltage operation performance and measures to improve the voltage stability of bulk MIDC systems have become a popular problem with concern. Therefore, the main topic of this paper is about the voltage operation performance of bulk MIDC systems.First of all, it analyzes the key factors which have influence on the voltage operation of MIDC systems. The impacts of factors including strength of AC system at receiving-end, reactive compensation, load characteristic and excitation system of generator et al., have been discussed in detail. The simulation analyses of possible voltage problems under different system faults have also been performed with the case of a certain large power grid.Secondly, a method based on sensitivity index to determine the allocation of reactive compensations is proposed. The study route starts with the definition of sensitivity method which is simple and clear to implement, and then a specific sensitivity method appropriate for AC/DC hybrid system is deduced from the power flow calculation of the hybrid system. After that the intensity level of each nodes can be reasonable indicated by the sensitivity index and weak points needing reactive compensation in the view of system voltage can be further found out. Simulations and calculations of actual power grid are carried out to obtain alternative nodes needing reactive compensation which have higher sensitivity index. Then same reactive compensation devices are installed at different places and voltage performances after system faults are observed to verify the validity of the proposed allocation method based on sensitivity index. The results indicate that the sensitivity index can effectively illustrate the weak points of AC system in the view of system voltage and provide guidance on configuration of reactive compensation devices.Finally, measures to improve the voltage stability of MIDC systems are studied. It emphatically analyzes the effects on enhancing voltage stability of three measures, i.e. raising the operating voltage level, adjusting regulation coefficient of excitation system and installing reactive compensation. And on this basis, the compensation results of different reactive compensation devices have been compared and reasonable allocation plan of reactive compensation can be put forward combining with actual systems.The increasing MIDC systems have observably increased the complexity of power grid structure and the interaction between AC and DC systems, which synergistically give rise to more problems in system’s voltage operation. The paper analyzes and summarizes the voltage operation performance of MIDC systems and then comes up with the measures to improve the voltage stability, which have the significance on the safe, stable and economic operation of power system.