Research On Hybrid Hvdc Technology Based On LCC And VSC

The development of modern society can not be separated from the electric power, in order to sent it to millions of households, we need a efficient and economical way of transmission. In the early stage, the AC transmission technology has the problems of stability and capacity when using in large scale and long distance transmission, so DC transmission technology has been gradually developed and paid attention. The traditional DC transmission is based on the line commutated converter(LCC), although it can operate reliably and has large transmission capacity, it still has some disadvantages such as the risk of commutation failure, large reactive power consumption and cannot be connected to a AC system with weak strength. With the development of power electronic technology, the technology of VSC-HVDC based on voltage source converter(VSC) and pulse width modulation(PWM) is becoming increasingly mature, it not only has no require for the AC system strength, but also can operate flexibly, however, it also has disadvantages such as large switching loss and high cost. In this paper, a Hybrid HVDC technology based on LCC and VSC is studied, this transmission mode has both adventages of the two converters and has a broad prospects for development.In this paper, we first analyze the basic structure of the Hybrid HVDC system--LCC-HVDC and VSC-HVDC, the characteristics of easy commutation failure are verified by the CIGRE HVDC benchmark system, which demonstrate the insufficient of LCC converter; for the VSC-HVDC system, we established mathematical models of the converter in different coordinate system, and design controllers under two different control methods. Then on the basis of the analysis above, we select VSC converter for one end and LCC converter for the other, to constitute the double-ended Hybrid-HVDC system, it’s topological structure, working principle and control strategy are studied in detail, and builted the related model to simulate and verify this system. Finally, this paper has studied a new transmission technology--Hybrid-MTDC, which can be expanded from double-ended Hybrid-HVDC system, it learned the operation principle and control mode from the traditional MTDC system, and took modified according to its own characteristics, we vertified the feasibility and effectiveness of the system and it’s control strategy by simulation of different conditions in the transient simulation software PSCAD/EMTDC. In addition, this paper also applied Hybrid-MTDC system into large-capacity wind power generation, modified the corresponding control method, using an improved double-fed induction machine(DFIG) structure to promote the low voltage ride through capability of the wind field. In summary, Hybrid HVDC technology has the advantages of both traditional DC transmission and flexible HVDC transmission, and has good steady state and transient characteristics, it can expand the scope of application of the conventional DC transmission system effectively.