State Key Laboratory of Power Transmission and Distribution Equipment and System Security and New Technology (Chongqing University), Researcher of State Grid Chongqing Electric Power Company Electric Power Research Institute Ouyang Jinxin, Tang Ting, Zheng Di, Ren Wenjun, Xiong Xiaofu, Zhong Jiayong, In the 22nd issue of Journal of Electrotechnical Engineering in 2017, the author pointed out that the large-scale application of DFIG wind turbines has changed the fault characteristics of power systems, which greatly restricted the implementation of power system relay protection.
In view of the existing research, the impact of low voltage ride through (LVRT) measures on DFIG fault characteristics is not considered. The short circuit current of DFIG under low voltage ride through control is studied. The DFIG reactive power output is considered to change the terminal voltage to the unit fault. The impact of the output characteristics.
Focusing on the mutual coupling of DFIG stator winding reaction and converter LVRT control, the DFIG short-circuit current expression before and after LVRT control startup is derived by constructing the DFIG vector model under LVRT control, from reactive power output and LVRT control. The startup delay is used to analyze the influence of LVRT control on DFIG short-circuit current. The DFIG fault equivalent model before and after LVRT control is established. The calculation method of short-circuit current of DFIG grid-connected system considering LVRT control is proposed.
The variable speed constant frequency wind turbine with Doubly Fed Induction Generator (DFIG) is the main equipment of current wind power generation. Variable-speed wind turbines rely on power electronic converters to provide excitation control or grid-connected interfaces. Wind turbines may experience off-grid operation under grid faults, resulting in serious shortage of grid power [1].
With the large-scale development of wind power generation, the grids of all countries in the world have developed wind power grid-connected guidelines, requiring grid-connected wind turbines to have low voltage traversal (LVRT) capability, which not only keeps off-grid continuous during grid faults. After running for a while, it can also inject reactive power into the grid to support rapid recovery of grid voltage [2, 3].
The transient process and output characteristics of variable speed wind turbines during grid faults are quite different from those of synchronous generators [4]. In the grid fault, a large overcurrent is generated in the DFIG rotor winding. In order to avoid damage to the Rotor Side Converter (RSC), the Crowbar circuit is often shorted to provide a path for the rotor overcurrent [5].
Crowbar protection can realize DFIG continuous operation without off-net during faults. However, after Crowbar action, DFIG is essentially a conventional induction generator with large rotor resistance, which can not provide reactive power to the grid and cannot meet the requirements of new wind power LVRT [6] ]. Therefore, keeping the RSC connected to the rotor winding during grid faults, adjusting the stator output reactive power within the allowable current range of the converter becomes an effective choice for DFIG to achieve LVRT [5, 7].
During the grid fault, the wind turbines are kept connected to the grid. A large number of wind turbines inject short-circuit current into the grid and provide reactive power to change the grid voltage, which changes the grid fault characteristics [8, 9]. The characteristics and effects of the DFIG output short-circuit current have received attention.
Related researchers have studied the calculation method of the maximum short-circuit current of DFIG after Crowbar protection action [10], the calculation formula of stator and rotor short-circuit current [11] and the DFIG equivalent model of power system short-circuit calculation [12]. Some literatures have studied the short-circuit current of the DFIG output when the Crowbar is not operating, but assume the control mode when the DFIG remains in normal operation during the fault [13].
Since DFIG needs to adopt a new control strategy during the fault to achieve continuous non-off-network operation, the existing research does not accurately reflect the characteristics of DFIG short-circuit current under LVRT. Reference [14] considers the impact of LVRT control on the DFIG fault process, but ignores the fault transition process. In addition, the existing research assumes that the DFIG terminal voltage after the fault is a constant value, and does not consider the influence of the voltage variation caused by the reactive power output.
In this paper, the characteristics and calculation methods of DFIG short-circuit current under symmetrical short-circuit of power grid are studied. The influence of LVRT control on stator short-circuit current is changed by changing rotor excitation voltage and terminal voltage.
Based on the DFIG vector model under LVRT control, the expression of DFIG short-circuit current is derived, and the influence of LVRT control on DFIG transient process is analyzed. The DFIG fixed-rotor response and RSC LVRT control are coupled by the terminal voltage. The fault process is divided into two stages before and after the LVRT control, and the fault equivalent model is established respectively. The calculation method of DFIG short-circuit current is proposed.
Figure 1 DFIG system power frequency equivalent circuit after LVRT control is started
in conclusion
The fault analysis of the existing doubly-fed wind power generation system does not take into account the influence of the DFIG output reactive power under the new LVRT standard. Therefore, this paper analyzes the influence of LVRT control and its startup delay on the DFIG short-circuit process and output characteristics, and establishes the LVRT control. The DFIG fault equivalent model before and after startup is proposed. The DFIG short-circuit current calculation method considering the influence of LVRT control is proposed, and the following conclusions are obtained:
1) Under the new LVRT control, the DFIG output reactive power supports the grid transient voltage during the fault, which causes the DFIG terminal fault voltage to change, and further affects the rotor winding transient process through the armature reaction and RSC control, so that the DFIG output is short-circuited. The current changes.
2) The start of LVRT control is delayed by the action of the terminal voltage at the fault end, which will cause two changes of the DFIG terminal voltage and short-circuit current during the grid fault. The startup speed of the LVRT control affects the size and variation of the DFIG short-circuit current.
3) Short-circuit current analysis without considering the influence of reactive power output and LVRT control start-up delay is difficult to accurately reflect the fault characteristics of DFIG under the new LVRT standard. The calculation method proposed in this paper can accurately reflect the variation law and numerical characteristics of short-circuit current, satisfying The need for power system failure analysis.
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