A method for modeling transient stability of doubly-fed wind turbine generators considering double-port grid-connection characteristics

Abstract

This invention discloses a transient stability modeling method for doubly-fed induction generator (DFIG) wind turbines that considers the characteristics of dual-port grid connection. In constructing the mathematical model, this method considers the coupling relationship between the input voltages of the phase-locked loops (PLLs) of the rotor-side converter and the grid-side converter, using the mutual impedance voltage drop as a bridge for the mutual coupling of the two PLLs. The constructed fourth-order transient synchronization model, compared to the traditional second-order transient synchronization model based on a single PLL, can reveal the transient synchronization and stability mechanism of DFIG wind turbines more deeply.

Description

Technical Field

[0001] This invention belongs to the field of transient synchronization analysis of doubly fed grid-connected wind power, specifically involving a method for establishing a reduced-order transient synchronization model that considers the characteristics of dual-port grid connection. Background Technology

[0002] In recent years, the new energy industry has risen rapidly, and the installed capacity of wind power has steadily increased, making wind energy one of the main energy sources in modern power systems. However, power electronic converters, as grid-connected equipment for wind turbines, have synchronization characteristics different from those of traditional power systems, posing new challenges to the stability of the power system. When a large disturbance occurs in the power grid, the wind power grid-connected system may lose synchronization or even lead to large-scale grid disconnection accidents. Especially under weak grid conditions, the voltage at the point of common coupling is easily affected by changes in wind power output, thereby exacerbating system instability.

[0003] To analyze the transient stability of wind power grid-connected systems, most current studies treat the system as a controlled current source oriented by a phase-locked loop (PLL) synchronization link and model it accordingly. However, this method may not be accurate for doubly-fed induction generators (DFIGs) with two-port grid-connection characteristics. In DFIGs, both the grid-side converter and stator windings are connected to the grid, and the current orientation information for the two ports comes from different PLLs, leading to complex dual-PLL coupling synchronization characteristics during grid connection. Therefore, a new transient synchronization model for DFIGs that considers the two-port grid-connection characteristics is needed. Summary of the Invention

[0004] To address the problem that existing second-order transient synchronization models cannot characterize the two-port coupling synchronization characteristics of doubly-fed induction generator (DFIG) wind turbines, a transient stability modeling method for DFIG wind turbines that considers the two-port grid connection characteristics is proposed.

[0005] The technical solution of the present invention is as follows:

[0006] A transient stability modeling method for doubly-fed induction generator (DFIG) wind turbines considering dual-port grid connection characteristics includes the following steps:

[0007] Step 1. Determine the topology and controller structure of the wind power grid-connected system based on a doubly-fed induction generator. Based on the phase-locked loop (PLL) structure, determine the expressions for the output angular frequency and output phase angle of the two PLLs.

[0008]

[0009] Step 2. Establish the q-axis component expression of the common coupling point voltage in the phase-locked loop reference frame of the rotor-side converter and the grid-side converter.

[0010]

[0011]

[0012] Step 3. Determine the expressions for the stator current and the grid-side converter output current in the double phase-locked loop reference system.

[0013]

[0014] Step 4. Combine the above expressions to determine the expressions for the three voltage components that make up the input voltage of the phase-locked loop.

[0015]

[0016]

[0017] Step 5. Combining steps 1 to 4, we can obtain a complete fourth-order transient synchronization model of a doubly-fed wind turbine considering the characteristics of dual-port grid connection.

[0018]

[0019] The beneficial effects of this invention are as follows:

[0020] The fourth-order transient synchronization model constructed in this invention reflects the coupling synchronization characteristics of the two grid-connected ports of the doubly-fed induction generator (DFIG) wind turbine. It can more accurately reflect the complex synchronization characteristics of the DFIG wind turbine during the grid connection process and helps to more comprehensively reveal the transient synchronization stability mechanism of the DFIG wind turbine, thus providing a more reliable basis for the stability analysis and control of the power system. Attached Figure Description

[0021] Figure 1 These are the implementation steps of the technical solution of the present invention.

[0022] Figure 2 This describes the topology and controller structure of a doubly fed wind power grid-connected system.

[0023] Figure 3 This is a simplified version of a doubly fed wind power grid-connected system, consisting of two controlled current sources connected in parallel.

[0024] Figure 4 A fourth-order transient synchronization model for a doubly fed wind turbine considering the characteristics of dual-port grid connection.

[0025] Figure 5 The transient synchronization waveform of the fourth-order mathematical model and simulation system under power grid fault conditions is shown. Detailed Implementation

[0026] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] The technical solution of the present invention adopts the following steps:

[0028] Step 1. Determine the topology and controller structure of the doubly-fed wind power grid-connected system, such as... Figure 2 As shown, a doubly-fed wind turbine consists of a doubly-fed induction generator (DFIG), a rotor-side converter (RSC), and a grid-side converter (GSC). It features dual-port grid connection, meaning the stator windings of the DFIG are directly connected to the grid, and the GSC is connected to the grid via a filter. In this example, a single-inductor filter L is used. f The Thevenin circuit is used as an equivalent for AC low-voltage power grids, U g =U g ∠θ g L is the grid voltage. g and R g These are the mains inductance and mains resistance, respectively. U s This is the stator three-phase voltage, which is also the point of common coupling (PCC) voltage. r I s and I g These are the rotor three-phase current, stator three-phase current, and the three-phase current output by the GSC, respectively. pcc This refers to the total three-phase current injected into the power grid. A phase-locked loop (PLL) is one of the important control structures for synchronizing a doubly-fed induction generator (DFIG) wind turbine with the power grid. Each of the DFIG's RSC and GSC phases has a PLL, and the synchronization angle θ generated by the two PLLs... pll1 and θ pll2 Vector current orientation is used for RSC and GSC respectively. A doubly-fed induction generator (DFIG) for achieving stable current control can be considered as a parallel system of two controlled current sources oriented by different phase-locked loops, such as... Figure 3 As shown, where I s and I g These are the stator current amplitude and the GSC output current amplitude, respectively. and The stator and GSC output current injection angles are respectively, I sqref and I sdref For stator current command, I gqref and I gdref This is a GSC current command. Normally, the GSC reactive current command is zero. The parameters of the doubly fed wind power grid-connected system are shown in Table 1.

[0029] Table 1 Parameters used in the doubly fed wind power grid-connected system

[0030] parameter symbol numerical values parameter symbol numerical values Rated voltage <![CDATA[V nom ]]> 690V Stator leakage self-sensing <![CDATA[L ls ]]> 0.06mH Rated power <![CDATA[P nom ]]> 1.5MW Rotor resistance <![CDATA[R r ]]> 2mΩ DC voltage reference <![CDATA[V dcref ]]> 1150V Rotor leakage self-inductance <![CDATA[L lr ]]> 0.08mH Rated frequency <![CDATA[f nom ]]> 50Hz Grid inductance <![CDATA[L g ]]> 0.7mH Extreme logarithm <![CDATA[n p ]]> 2 Grid resistance <![CDATA[R g ]]> 63.5mΩ Stator-rotor turns ratio <![CDATA[K e ]]> 0.37 RSC PLL proportional gain <![CDATA[k p1 ]]> 0.27 Filter inductor <![CDATA[L f ]]> 0.3mH RSC PLL integral gain <![CDATA[k i1 ]]> 10.2 Magnetizing inductor <![CDATA[L m ]]> 4.4mH GSC PLL proportional gain <![CDATA[k p2 ]]> 0.19 Stator resistance <![CDATA[R s ]]> 2.4mΩ GSC PLL Integral Gain <![CDATA[k i2 ]]> 3.55

[0031] according to Figure 2 The phase-locked loop structures shown are used to obtain the expressions for the output angular frequency and output phase angle of the RSC and GSC phase-locked loops, respectively:

[0032]

[0033] Where ω n The system's rated angular frequency is 100π rad / s.

[0034] Step 2. Establish the three-phase voltage U at the common coupling point s and grid voltage U g The expression for the voltage drop across the grid impedance is as follows:

[0035]

[0036] The two phase-locked loops (PLLs) of a doubly-fed wind turbine correspond to two PLL reference frames. The RSC PLL reference frame is denoted by the superscript "R", and the GSC PLL reference frame is denoted by the superscript "G". The synchronization angle θ of the two PLLs is used respectively. pll1 and θ pll2 Performing a Park transform on the above equation, we obtain the q-axis component expressions of the common coupling point voltage in different reference frames as follows:

[0037]

[0038]

[0039] Step 3. Define the phase angle difference between the GSC phase-locked loop reference frame and the RSC phase-locked loop reference frame as δ. 21 =θ pll2 -θ pll1 The current conversion relationship under different reference frames is as follows:

[0040]

[0041] Ignoring current loop dynamics, the expressions for stator current and GSC output current in the dual phase-locked loop reference system are determined based on the above current conversion relationship.

[0042]

[0043] Step 4. Substitute the above current expression into the expression for the q-axis component of the common coupling point voltage to obtain the specific expression for the q-axis input voltage of the dual phase-locked loop.

[0044]

[0045]

[0046] Where δ1=θpll1 -θ g Let δ2 be the angle between the d-axis of the RSC phase-locked loop reference frame and the grid voltage. pll2 -θ g Let d be the angle between the d-axis of the GSC phase-locked loop reference frame and the mains voltage. Let U be the q-axis input voltage of the RSC phase-locked loop. sq1 It consists of three parts: grid voltage U gq1 The self-impedance voltage drop U generated by the stator current across the grid impedance 11 The mutual impedance voltage drop U generated by the GSC current across the grid impedance 21 Similarly, the q-axis input voltage U of the GSC phase-locked loop... sq2 This also includes grid voltage U gq2 Self-impedance voltage drop U 22 and mutual impedance voltage drop U 12 Three parts.

[0047] Step 5. Combining steps 1 to 4, we can obtain a complete fourth-order transient synchronization model of a doubly-fed wind turbine considering the characteristics of dual-port grid connection.

[0048]

[0049] The corresponding control block diagram model is as follows: Figure 4 As shown, the upper part of the model is the phase-locked loop of RSC, and the lower part is the phase-locked loop of GSC. These two parts are coupled through mutual impedance voltage drop. The fourth-order model proposed in this invention considers the characteristics of dual-port grid connection, making up for the deficiency of traditional second-order transient synchronization models in not being able to reflect the coupling synchronization effect of dual-phase-locked loops, and can more deeply reveal the transient synchronization stability mechanism of doubly-fed wind turbines. The waveforms of angular frequency difference and phase angle difference obtained by solving the fourth-order mathematical model of this invention using the parameters shown in Table 1 are compared with the simulation waveforms. Figure 5 As shown, the initial and steady-state values ​​of the fourth-order model are consistent with the simulation results, and they have the same trend as the transient waveform of the simulated system, which verifies the correctness of the fourth-order transient synchronization model.

Claims

1. A transient stability modeling method for doubly-fed induction generator (DFIG) wind turbines considering dual-port grid connection characteristics, applied to grid-connected DFIG wind power systems, characterized in that... The method includes the following steps: Step 1. Determine the topology and controller structure of the doubly fed wind power grid-connected system. The controller structure includes two independent phase-locked loops, one for the rotor-side converter and the other for the grid-side converter. Based on the two phase-locked loop structures, determine the output angular frequency ω of the rotor-side converter phase-locked loop respectively. pll1 and output phase angle θ pll1 And the output angular frequency ω of the grid-side converter phase-locked loop. pll2 and output phase angle θ pll2 The expression is as follows: In the formula, k p1 and k i1 These represent the proportional gain and integral gain of the PI controller in the rotor-side converter phase-locked loop, U. sq1 ω represents the q-axis component of the common coupling point voltage in the rotor-side converter phase-locked loop reference frame. Variables with the subscript "2" are variables related to the grid-side converter phase-locked loop. n The system's rated angular frequency is 100π rad / s; Step 2. Establish the q-axis component U of the common coupling point voltage in the rotor-side converter phase-locked loop reference frame. sq1 And the q-axis component U of the common coupling point voltage established in the grid-side converter phase-locked loop reference frame. sq2 The expression is as follows: In the formula, the superscript "R" represents the variable in the rotor-side converter phase-locked loop reference system, the superscript "G" represents the variable in the grid-side converter phase-locked loop reference system, and U gq R is the q-axis component of the grid voltage. g and L g These are the mains resistance and mains inductance, respectively. sd and I sq I is the dq-axis component of the stator current. gd and I gq The dq-axis component of the grid-side converter output current; Step 3. Determine the expressions for the stator current and the grid-side converter output current in the dual phase-locked loop reference frame: In the formula, δ 21 =θ pll2 -θ pll1 This represents the phase difference between two reference frames, with the subscript "ref" indicating a given reference value. Step 4. Based on the voltage and current expressions derived in the above steps, determine the three voltage components that make up the input voltage of the two phase-locked loops, namely the grid voltage U. gq1 and U gq2 Self-impedance voltage drop U 11 and U 22 and the voltage drop across the impedance U 21 and U 12 The expression is as follows: In the formula, δ1=θ pll1 -θ g Let δ2 be the angle between the d-axis of the rotor-side converter phase-locked loop reference frame and the grid voltage. pll2 -θ g The angle between the d-axis of the grid-side converter phase-locked loop reference frame and the grid voltage; Step 5. Combining steps 1-4, construct a fourth-order transient synchronization model for a doubly-fed induction generator (DFIG) wind turbine considering the characteristics of dual-port grid connection, as follows:

2. The transient stability modeling method for doubly-fed induction generator (DFIG) wind turbines considering dual-port grid connection characteristics according to claim 1, characterized in that, The topology of the doubly fed wind turbine includes at least one doubly fed induction generator, a rotor-side converter, a grid-side converter, and related electrical components connected to the power grid.

3. The transient stability modeling method for doubly-fed wind turbines considering dual-port grid connection characteristics according to claim 1, characterized in that, The expressions for the output angular frequency and output phase angle of the phase-locked loop are derived based on the dynamic response characteristics of the phase-locked loop and the system electrical parameters.

4. The transient stability modeling method for doubly-fed wind turbines considering dual-port grid connection characteristics according to claim 1, characterized in that, The expression for the q-axis component of the common coupling point voltage takes into account the influence of voltage fluctuations and harmonic components on the synchronization process.

5. The transient stability modeling method for doubly-fed induction generator (DFIG) wind turbines considering dual-port grid connection characteristics according to claim 1, characterized in that, The expressions for the stator current and the grid-side converter output current reflect the dynamic variation characteristics of the current in the double phase-locked loop reference system.

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