Control strategy of doubly‐fed induction generator during the grid voltage swell
2018; Institution of Engineering and Technology; Volume: 2019; Issue: 16 Linguagem: Inglês
10.1049/joe.2018.8834
ISSN2051-3305
AutoresZou Le, Wu Xueguang, Longze Kou, Dong Liu, Fangyuan Li, Mingxiao Han,
Tópico(s)Microgrid Control and Optimization
ResumoThe Journal of EngineeringVolume 2019, Issue 16 p. 1807-1811 Session – Poster CDOpen Access Control strategy of doubly-fed induction generator during the grid voltage swell Zou Le, Corresponding Author Zou Le 18270834746@163.com North China Electronic Power University, Beijing, People's Republic of ChinaSearch for more papers by this authorWu Xueguang, Wu Xueguang Global Energy Interconnection Research Institute, Beijing, People's Republic of ChinaSearch for more papers by this authorKou Longze, Kou Longze Beijing Key Laboratory of DC grid Technology & Simulation, Beijing, People's Republic of ChinaSearch for more papers by this authorLiu Dong, Liu Dong State Key Laboratory of Advanced Power Transmission Technology, Beijing, People's Republic of ChinaSearch for more papers by this authorLi Fangyuan, Li Fangyuan Beijing Key Laboratory of DC grid Technology & Simulation, Beijing, People's Republic of ChinaSearch for more papers by this authorHan Mingxiao, Han Mingxiao North China Electronic Power University, Beijing, People's Republic of ChinaSearch for more papers by this author Zou Le, Corresponding Author Zou Le 18270834746@163.com North China Electronic Power University, Beijing, People's Republic of ChinaSearch for more papers by this authorWu Xueguang, Wu Xueguang Global Energy Interconnection Research Institute, Beijing, People's Republic of ChinaSearch for more papers by this authorKou Longze, Kou Longze Beijing Key Laboratory of DC grid Technology & Simulation, Beijing, People's Republic of ChinaSearch for more papers by this authorLiu Dong, Liu Dong State Key Laboratory of Advanced Power Transmission Technology, Beijing, People's Republic of ChinaSearch for more papers by this authorLi Fangyuan, Li Fangyuan Beijing Key Laboratory of DC grid Technology & Simulation, Beijing, People's Republic of ChinaSearch for more papers by this authorHan Mingxiao, Han Mingxiao North China Electronic Power University, Beijing, People's Republic of ChinaSearch for more papers by this author First published: 13 February 2019 https://doi.org/10.1049/joe.2018.8834Citations: 2AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract This study firstly derives the relationship between the grid voltage and the rotor current of doubly-fed induction generator (DFIG) during the grid voltage swell. Then, an effective control strategy aiming to restrain rotor over-current is proposed and the control strategy of the grid-side converter is also improved. The proposed rotor current suppression and grid voltage suppression method, cooperating with the DC chopper, can not only solve the problem of short circuit of the rotor side converter resulting from the frequent switching of crowbar device, but also maximally improve the capability of wind turbine in dynamic reactive power support without adding any other power electronic devices. With the help of this control strategy, the wind turbine could maintain its normal operation even during the fault and the high voltage ride through of wind turbine can be achieved. As a result, the stability of the wind power integration is significantly improved. Finally, the simulations and analysis in the power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) verify the effectiveness and feasibility of the proposed control scheme. 1 Introduction With the rapid development of wind power, the problem of low voltage ride through has been studied to a certain extent and has been effectively controlled. However, in recent years, the accident of wind turbine disconnected with grid caused by the point of common coupling (PCC) voltage swell have also occurred frequently. This issue has caused researchers to pay close attention to the wind power high-voltage ride through (HVRT). So, some countries in the world have also set corresponding standards for HVRT and made wind power integration requirements [1], as shown in Fig. 1. Fig. 1Open in figure viewerPowerPoint High-voltage ride through codes in some countries Doubly-fed induction generator (DFIG) has the dual adjustment ability of reactive power and active power. Moreover, it can realise the advantage of variable speed constant frequency operation, which has become the mainstream model of wind power generation technology. The stator is directly connected to the grid and the rotor is connected to the converter, when the voltage of the grid suddenly rises, it is easy to cause over-current of the rotor and the over-voltage of the DC bus of the converter, which leads to the grid disconnection of the wind turbine and affect the stable operation of the system. At home and abroad, the research of high-voltage crossing has been carried out, mainly by increasing the hardware facilities and improving the internal control strategy of the wind turbine. Fan et al. [2] analysed the principle of wind turbine disconnection with grid, and a STATCOM reactive power compensation device is proposed to absorb excess reactive power and achieve voltage control. Although this method can realise HVRT, but it increases the cost. The paper [3, 4] analysed the reasons of the grid voltage swell and the possible influence on wind turbines. The dynamic voltage restorer is proposed to realise voltage control. The paper [5] proposed a hysteresis controller to improve the response speed of the traditional proportional integral (PI) controller. The PI controller is used when the system is running normally. When the grid voltage swells, the hysteresis controller is switched to improve the response speed. The simulation analysis shows that the scheme has a certain effect on HVRT. The paper [6] proposed a variable damping control strategy. In the rotor side series dynamic resistance, the impact of rotor current and electromagnetic torque on the wind turbine is reduced effectively, but the system loss is increased by the access of dynamic resistance. The paper [7] proposed a series of grid side converter control strategy. When an asymmetric swell occurs in the grid voltage, the constant voltage of the stator is maintained by the control of the series converter. Although the control strategy can realise the fault through effectively when the grid voltage swells. However, due to the additional increase of a grid side converter, the operation cost of the whole wind power system is greatly increased. The paper [8] proposed to add a crowbar device in the rotor-side converter. When the rotor current exceeds the threshold, all the switching devices of the rotor side converter are turned off, and the fault current through the crowbar device, the impact of rotor over-current on the converter is effectively suppressed. This method is the main measure adopted by the commercial DFIG wind turbines at present. However, there are a number of key technologies that are worth investigating and improving. In this paper, the problem of short circuit of the rotor side converter resulting from the frequent switching of crowbar device is improved. The relationship between the grid voltage and the rotor current is analysed. The proposed rotor current suppression (RCS) and grid voltage suppression (GVS) methods, is also maximally improve the capability of wind turbine in dynamic reactive power support. Finally, the HVRT ability of wind turbine is improved. 2 Methodology 2.1 Analysis of transient process of DFIG under the grid voltage symmetrical swell In this paper, the detailed theoretical analysis of the DFIG transient process under the grid voltage symmetrical swell is given. Also deduced the relationship between grid voltage and rotor current. For simplicity, DFIG stator side as reference frame, the rotor side parameters will be referred to the stator side, and assumed magnetic circuit is linear. The equivalent circuit model of the doubly-fed wind generator (DFIG) is shown in Fig. 2. Fig. 2Open in figure viewerPowerPoint Equivalent circuit model of DFIG The DFIG voltage and flux equation can be expressed as (1) (2) (3) (4) where R s and R r represent the stator and rotor resistance, respectively. L s and L r are the stator and rotor self-inductance, respectively. L m is the magnetizing inductance. i represents the current space vector. ψ s and ψ r are the flux space vector of stator and rotor, respectively. ω represents the rotation angular frequency. From (2), (3) and (4), the following expression is obtained: (5) It is assumed that when t = t0, the grid voltage symmetrically swell suddenly from V s to (1 + p)Vs (p is the voltage swell amplitude), then the grid voltage can be expressed as (6) According to the literature [9], when the grid voltage suddenly swells, the rotor voltage equation can be expressed as (7) where the rotor voltage expression can be written under the frame of the rotor: (8) Then, the rotor phase-a voltage expression is (9) The linear differential equation expression of i ra (t) is (10) where , β is the phase-a rotor voltage angle at the instant the fault occurs. Substitute (7) into (10): (11) With time constants defined as (12) According to (11), the maximum amplitude value of phase-a current of rotor is as follows: (13) Through the above analysis, to facilitate the design of the RCS controller, the K value can be (14) 2.2 Rotor current suppression (RCS) control strategy Based on the analysis of the transient process and the influence on rotor current, a method of RCS is proposed. The control structure of DFIG rotor side is shown in Fig. 3, which is the RCS process in the dashed frame. Fig. 3Open in figure viewerPowerPoint DFIG structure of RSC When the rotor side converter adopts stator voltage directional vector control. The decoupling between active power and reactive power of DFIG is realised. Then, the rotor current closed-loop control can be adopted. The output of active power and reactive power of DFIG can be controlled by controlling the d- and q -axis components of the rotor current. In this paper, an RCS controller is added to the traditional rotor current closed-loop control. When the grid is operating normally or the grid voltage V g 1.1 pu. At this time, the RCS controller switches to the RCS mode and outputs a value of K. Then K is limited within a certain range through the limiter, and the rotor d - and q -axis components are controlled within the normal range, which effectively avoids the rotor over-current impact on the converter. 2.3 Grid voltage suppression (GVS) control strategy The control structure of DFIG grid side is shown in Fig. 4, based on the grid voltage directional vector control. Its main function is to maintain constant DC-bus voltage and realise the transmission of active power and reactive power between the converter and grid. Fig. 4Open in figure viewerPowerPoint DFIG structure of GSC The literature [10] pointed out that when the grid voltage swells, the reactive power output capability of the grid side converter is closely related to the voltage swell amplitude. In normal operation, the grid-side converter adopts DC voltage control to ensure the stability of the DC voltage. On the other hand, GSC operates under the unit power factor, which can reduce the converter loss to a certain extent. As shown in the dashed box in Fig. 4, GVS is added to the grid-side converter. When it is detected that the grid voltage swell amplitude exceeds 1.1 pu, the unit power factor control automatically switches to the GVS. During the grid fault, the GSC outputs a certain inductive current to the system. Without adding any hardware devices, provides a certain dynamic reactive power support for the grid. 3 Results Taking 3 MW DFIG parameters as an example, the simulation model of system is built in the PSCAD/EMTDC. Including the rotor side converter and the grid side converter control, the detailed parameters of DFIG are shown in Table 1. Table 1. Simulation parameters of DFIG Parameter Value rated power 3 MW rated stator voltage 690 V rated frequency 50 Hz stator resistance 0.0054 pu rotor resistance 0.0061 pu stator leakage inductance 0.10 pu rotor leakage inductance 0.11 pu mutual inductance 3.0 pu number of pole pairs 2 transformation ratio 0.3 crowbar starting current 1.75 kA Using the above parameters, set up the grid voltage swell amplitude at 1.5 s (step change increased by 20%, fault duration of 0.5 s), adopt the control strategy proposed in this paper, comparing with the traditional control strategy to get the simulation results are shown in Figs. 5 and 6. Fig. 5Open in figure viewerPowerPoint Simulation results of the DFIG for the grid voltage swell to 1.2 pu: (a) The grid voltage (U g), (b) The voltage of stator (U s), (c) The current of rotor (I r) Fig. 6Open in figure viewerPowerPoint Simulation results of the DFIG for the grid voltage swell to 1.2 pu: (a) The crowbar switch situation, (b) The reactive power output waveform of rotor side, (c) The reactive power output waveform of grid side It can be seen from Figs. 5 a and b that the grid fault at 1.5 s, the voltage swell amplitude is 20%, the stator voltage RMS increases from 1.0 to 1.2 pu. Fig. 5 c shows the waveform of rotor current amplitude before and after the control strategy mentioned in this paper. As can be seen from the figure, the rotor current peak is reduced from 2.03 kA before control to 1.62 kA. From Figs. 6 a and b, this control strategy significantly reduces the number of switch times of crowbar devices. The rotor side converter continues to provide the reactive power support for grid. However, the switching process of crowbar does not affect the normal operation of the grid side converter. From Fig. 6 c, the grid side converter provide the reactive power of 2.4 Mvar for the grid and assisted in the recovery of grid voltage during fault due to adopted the GVS control strategy. 4 Conclusion This paper proposed the RCS and GVS method that can not only solve the problem of short-circuit of the rotor side converter resulting from the frequent switching of crowbar device, but also maximally improve the capability of wind turbine in dynamic reactive power support without adding any other power electronic devices. With the help of this control strategy, the wind turbine could maintain its normal operation even during the fault and the HVRT of wind turbine can be achieved. This method is easy to control and can be used in practical engineering. 5 References 1Huizhu D., Yongning C.: 'Comparative research of wind power integration code', Electr. Power, 2012, 45, (10), pp. 1 – 6 (in Chinese) 2Fan Y., Yunting S., Shuqiang Z.: 'Simulation analysis and measures of large scale wind turbine chain off-grid', Electr. Power Sci. Eng., 2013, 29, (1), pp. 21 – 26 (in Chinese) 3Sung-Min W., Dae-Wook K., Woo-Chol L. et al.: 'The distribution STATCOM for reducing the effect of voltage sag and swell [C]'. The 27th Annual Conf. IEEE Industrial Electronics Society, Denver, USA, 2001, vol. 2, no. 10, pp. 1132 – 1137 4Rathi M.R., Mohan N.: 'A novel robust low voltage and fault ride through for wind turbine application operating in weak grids'. 31st Annual Conf. 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