@@对海上风力发电场VSC- HVDC输电系统建模和控制

Modelling and Control of Offshore Wind Farm with VSC-HVDC Transmission System
D.C. Kong*, X.-P. Zhang

*Univerisity of Birmingham, Birmingham B15 2TT, UK, kxd854@ Univerisity of Birmingham, Birmingham B15 2TT, UK, x.p.zhang@
2.4 DFIG model The equivalent circuit of DFIG is represented in Fig.2 [5]. And the full and simplified presentations of DFIG model can be given in [1]
2 WF-DFIG System
2.1 Configuration of WT-DFIG system In the recent years, wind turbines with doubly fed induction generators (WT-DFIG) have been widely installed in the wind farms all in the world. The typical configuration of a WT-
Wind Turbine
Shafts
DFIG
Rotor-side Converter AC Prime Mover DC Grid-side Converter Power Grid DC AC
1 Introduction
In the past few years, the UK¶s offshore wind industry has developed rapidly and it has become one of the leading countries for offshore wind energy generation installation in Europe. Considering the future development of offshore wind farms, on the one hand, the overall capacity of an offshore wind farm increases significantly to reach hundreds MW or even 1 GW, consisting of a large quantity of wind power generators (WPG); on the other hand, with the advantages over AC and conventional thyristor-based HVDC techniques, the voltage-source converter (VSC) based HVDC technique is commonly considered having promising future for offshore wind energy delivery. However, it also brings new challenges for the power system dynamic stability analysis. One challenge is how to present effectively the overall dynamic behaviours of such large quantity of WPGs; the other challenge is how to design the coordinated control strategies for the wind farm and the VSC-HVDC system together to improve the overall stability performances of the offshore wind energy system.
Ids jΒιβλιοθήκη qsable to reflect the accurate dynamic behaviors of the wind farm. On the other hand, if the wind farm is represented using detailed models for all the WPGs, great computational efforts should be needed for simulation. For the study of dynamic impacts of offshore wind farms on the power systems, the main interests are focused on the overall dynamic responses of the entire wind farm rather than on dynamic behaviors of each WPG. So the dynamic models of wind farm can be simplified to some extent using few equivalent machines to reflect the key characteristics of the system. With these principles, the dynamic aggregated modelling method is proposed. In this method, the WPGs with similar dynamic responses are selected into a so-called coherency group and then they are aggregated as an equivalent machine to present their common dynamic characteristics. This dynamic aggregated modelling method can be divided into three steps and they are described in Section 3.3-3.5. 3.2 Assumptions and simplifications Some assumptions are given for simplifying this dynamic aggregated modelling method: 1) The wind speed always keeps constant in the wind farm for the normal operation of all the WT-DFIGs at the maximum power point; 2) The WTDFIGs connected in the same row are configured with identical parameters; 3) The transformers and transmission lines for the interconnection of the WT-DFIGs are configured with identical parameters; 4) The disturbances are incurred outside the wind farm system. 3.3 Identification of coherency groups During the large disturbances in the power system, similar dynamic responses can be reflected from some generators in the system. Such generators can be selected into a coherency group, and then they are represented using an equivalent machine to not only present their common dynamic characteristics but also to simplify the system¶s dynamic models. For this method, if similar rotor-angle swing responses can be reflected from the generators during large disturbances, these generators can be selected into the coherency group. The criterion for identification of coherency groups is given [4]: (8) Max 'G i (t ) 'G j (t ) d H
Rotor-side Controller
Grid-side Controller
Protections
Fig. 1 The configuration of WT-DFIG system 2.2 Wind turbine model The kinetic energy extracted from air flow by wind turbine is given by: 3 (1) Pm 0.5US R2Vw C p (O , E ) where Pm is the mechanical power of the wind turbine; ȡ is the air density; R is the wind turbine blade radius; Vw is the wind speed; Cp is the power coefficient and it is usually presented as a function of blade pitch angle ȕ and the tip speed ratio[5]. 2.3 Two-mass shaft model