Study On AC/DC Power System Digital And Physical Hybrid Simulation

With the development of the UHVAC and UHVDC projects, interactions between ac and dc network are becoming more complex. To ensure the security and safety operation of such a large ac/dc hybrid power system, it's necessary to study the system stability, operation and control issues in detail. For economic and technology concerns, a simulation tool has played important role. Compared with the digital simulator, the physical HVDC simulator with actual control and protection system can better reflect the interactions. Therefore, it's necessary to combine the advantages of the electromagnetic transient full digital simulator with physical HVDC simulator, establish the multiple HVDC and UHVDC hybrid transmission system and study the stability issues of the UHV grid. This work is supported by key projects of the State Grid Corporation as well as several National grid projects of science and technology, the relevant theoretical and experimental research about power system digital and analog hybrid simulation technology are presented. The main contents are organized as follows:The parallel calculation efficiency is the paramount issue in large power system real time simulation. This paper analyzes the parallel efficiency of AC\DC grid analog digital hybrid simulation, determines the impact of various factors on the parallel computing efficiency and provides a basis and reference for real-time simulation. To achieve real-time digital simulation, super computer must be used. While factors that affect the simulation efficiency should be considered in order to get the most optimal calculation efficiency.On the basis of extensive research of existing interface method in accordance with the actual network topologies, the shortcomings of the existing transmission line decoupling method are analyzed, then the improved interfacing method under different conditions (long lines, short lines and none line) are given. If the length of the decoupling line is more than 50km, transmission line decoupling method can be used directly. If the length of the decoupling line is less than 50km, a capacitor compensation method is used to overcome the limit of the length of the decoupling line.If there is none lines, parts of the branch inductance is selected and converted to a lkm line. The improved interface method makes the digital and physical network decoupling can be carried out at any node.The closed-loop stability is the paramount issue in power hardware in-the-loop (HIL) simulation in regard to the operational safety and the experiment reliability. In this paper, a stability analysis of the power HIL simulation using TLM is first introduced with a simple example. Several factors (such as time delay, dc offset and resonance of the interface circuit) with respect to the interface stability are analyzed in detail. Then improvements are adopted to ensure interface stability. Through hybrid simulation and full physical simulation of the power hardware-in-the-loop (HIL) of a UH VDC link, it is revealed that the interface exhibit higher stability and accuracy than before under the given conditions.To more accurately study the interaction between ac and dc grid, PHIL simulation was used in a large power grid existing in China power system. PHIL simulation of a large power system with multiple HVDC\UHVDC lines has been achieved for the first time in the world. An interaction study is performed, including the commutation failures, the recovery characteristics after failure, etc. Comparisons of PHIL simulation against off line simulation proved the usefulness and necessity of this paper.The results of this study are the first realization of the digital and analog hybrid real-time simulation of large scale power network, this will provide strong technical support for planning and operation of interconnection power system in China.