1022
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO. 2, FEBRUARY 2015
Modeling and Control of Gate-Controlled Series Capacitor Interfaced With a DFIG-Based Wind Farm Hossein Ali Mohammadpour, Student Member, IEEE, and Enrico Santi, Senior Member, IEEE
Abstract—This paper presents application and control of the gate-controlled series capacitor (GCSC) for series compensation and subsynchronous resonance (SSR) damping in doubly-fed induction generator (DFIG)-based wind farms. The GCSC is a new series FACTS device composed of a fixed capacitor in parallel with a pair of antiparallel gatecommuted switches. The study considers a DFIG-based wind farm, which is connected to a series-compensated transmission line whose parameters are derived from the IEEE first benchmark model for computer simulation of the SSR. The small-signal stability analysis of the system is presented, and the eigenvalues of the system are obtained. Using both modal analysis and time-domain simulation, it is shown that the system is potentially unstable due to the SSR mode. Therefore, the wind farm is equipped with a GCSC to solve the instability of the wind farm resulting from the SSR mode, and an SSR damping controller (SSRDC) is designed for this device using residue-based analysis and root locus diagrams. Using residue-based analysis, the optimal input control signal to the SSRDC is identified, which can damp the SSR mode without destabilizing other modes, and using root-locus analysis, the required gain for the SSRDC is determined. MATLAB/Simulink is used as a tool for modeling, design, and time-domain simulations. Index Terms—Doubly fed induction generator (DFIG), flexible ac transmission systems (FACTS), gate-controlled series capacitor (GCSC), root-locus diagram, subsynchronous resonance (SSR).
I. I NTRODUCTION UE to the recent rapid penetration of wind power into the power systems [1], some countries in central Europe, e.g., Germany, have run out of suitable sites for onshore wind power projects, due to the high population density in these countries. Moreover, it has been found that the offshore wind power resources are much larger than onshore wind power sources [2]. Therefore, offshore wind farms have a great potential as large-
D
Manuscript received September 13, 2013; revised December 29, 2013, May 23, 2014, and July 10, 2014; accepted July 29, 2014. Date of publication August 12, 2014; date of current version January 7, 2015. This work was supported by the National Science Foundation Industry/University Cooperative Research Center for Grid-Connected Advanced Power Electronic Systems under Grant 0934378. The authors are with the Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208 USA (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2014.2347007
scale sustainable electric energy resources [2]. Recently, the doubly fed induction generator (DFIG) has gained significant attention from the electric power industry in offshore wind farms and renewable energy sources [1], [3]. However, in offshore wind farms, the distance between the wind turbines and the shore is much longer [4] than that in onshore wind farms. Therefore, unlike the onshore wind farms—where the voltage level of the wind farm is usually the same as the voltage level of the distribution system—higher voltage levels with reliable and efficient transmission lines are required for the offshore wind farms to minimize the power losses [2]. Currently, there are numerous large offshore wind farms operating throughout the world [2], [5]. Future projects in offshore wind farms will be larger in size and further away from the shore [2]. This requires defining new concepts for the transmission system, including transmission lines from the offshore wind farm to the shore and network integration to the onshore power system. The transmission system options to transmit the wind power to the shore are high-voltage ac (HVAC) [2] or high-voltage dc (HVDC) [5], [6]. The HVAC solutions are viable for distances up to 250 km, and with series compensation, they may be viable for distances longer than 250 km [2]. Reactive power injection, either shunt or series, into power transmission lines has been used for many years to increase the transmittable power of transmission lines [7]. For the purpose of increasing the power transfer capability of a transmission line, series compensation is preferred compared with shunt compensation. One of the main reasons is that, unlike shunt compensation, series compensation is less sensitive to system load characteristics and equipment location along a transmission line. However, it was found at an early date that using series compensation can cause instability in power systems due to a phenomenon known as subsynchronous resonance (SSR) [7]. Properly designed flexible ac transmission systems (FACTS) could be used to take advantage of series compensation benefits without causing the SSR problem in power systems [7]. Nowadays, FACTS devices are required in order to support massive integration of renewable energy resources into the power networks [8]. The advantages of using FACTS devices such as static synchronous compensator (STATCOM) [7], static VAR compensator (SVC) [9], synchronous series compensator (SSSC) [7], unified power flow controller (UPFC) [10], and thyristor-controlled series compensator (TCSC) [9] in power systems are well known.
0278-0046 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
