| The insertion of large-scale wind generator has a great impact on the operation of power system, especially when fault occurs, the fault characteristics of large-scale double-fed wind power system may end in difficulty in the correct operation of relay protection, which is configured in accordance with conventional synchronous machine system. With continuous development of large-scale wind power, the mechanism of the effect of wind power system short-circuit current characteristics on relay protection is needed to study preliminarily, measures also should be taken to ensure the protection element tripping when fault occurs in the action region and malfunction when fault occurs outside, also with enough sensitivity. In this paper, research is divided into two parts, firstly the research on the short current characteristics double-fed wind turbine unit with DC damp circuit (Chopper), secondly exploring the affecting mechanism of fault characteristics on protection algorithms and the principles of various system protections is also taken into account.Double-fed wind turbine unit with DC damp circuit (Chopper) on DC-Link to protect the rotor side converter is commonly used for improving the LVRT capability of DFIG. The insertion of Chopper protection has a greater impact on the short-circuit current characteristics of DFIG, however the present research on DFIG fault current features under symmetrical short-circuit is primarily conducted under the assumption that the Crowbar acts immediately after fault inception, whereas, the short-circuit current characteristics of DFIG under Chopper action is rarely studied. According to the analysis on the short circuit current of DFIG after crowbar operating, i.e. the rotor circuit is in series with a resistor, the rotor current loop after fault under chopper action was analyzed, the rotor sider converter and DC damp circuit was equivalent to a variable resistance, the variation of the equivalent resistor along with the voltage drop and running slip was also analyzed. According to the relationship of the DFIG flux and voltage after symmetrical fault a current analytical expression was obtained for DFIG under Chopper action by mathematical analysis methods. Finally a simulation model of DFIG with Chopper protection was built to verify the validity of the proposed method.According to the double-fed short current short circuit current characteristics obtained, the impact of the current characteristics on distance and differential protection was analyzed respectively. The frequency components of the wind turbines unit short-circuit current is especially different from the traditional synchronous machine —the current of wind farms under the three-phase short circuit contains a larger proportion of damping speed frequency components, which exists as inter-harmonics frequency component will be responsible for resulting errors of discrete fast Fourier algorithm (DFT) that microcomputer-based protection uses to extract fundamental frequency phasor, and may affect the reliability of the traditional protection particularly distance protection and differential protection. Taking into account of the wind power system strong weak feedback characteristic, when non-metallic short circuit occurs, it brings even greater impact on the impedance calculation even action behavior of distance protection. In this paper, wind farm side measurement impedance and differential protection’s complex percentage check characteristic was analyzed based on the protection mechanism analysis, aslo DIgSILENT simulation was done to verify the impact of wind farm side short-circuit current’s speed frequency components and wind farm weak feedback characteristics on distance protection and differential protection. |
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