Oscillation suppression method for grid connection of wind power plant through modular multilevel converter

Abstract

The invention discloses an oscillation suppression method for grid connection of a wind power plant through a modular multilevel converter. The method comprises the following steps: introducing a harmonic component of an output current of a wind field side MMC converter as an additional control quantity into voltage control of the wind field side MMC converter, controlling the output current of the wind field side MMC converter to generate an output voltage, and suppressing oscillation when a wind power plant is connected to a grid through the MMC converter. According to the method, the oscillation of the output current of the wind power plant, namely the input current of the MMC at the wind field side, can be suppressed, and the control complexity is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-voltage direct current transmission stabilization, and in particular relates to an oscillation suppression method for a wind farm connected to a grid via a modular multilevel converter (MMC). Background Art

[0002] With the continuous expansion of wind power grid-connected capacity, the use of flexible DC transmission in wind farms has become a typical grid-connected mode that has developed rapidly in recent years. It allows wind farms to operate asynchronously with the AC system, thereby reducing the requirements for grid-connected bus frequency and phase angle in the grid-connected guidelines. At the same time, due to the isolation effect of the DC line, the voltage drop on the system side will not directly affect the stable operation of the wind farm, thereby improving the fault ride-through capability of the wind farm. In addition, the DC cable does not generate reactive current, so there is no need to compensate the reactive power of the transmission line.

[0003] Many wind farms at home and abroad have experienced oscillation during the commissioning or operation phase of flexible DC transmission projects, with the oscillation frequency band covering low, medium and high frequency bands. If the project has a medium-frequency oscillation of about 290Hz during operation, it will cause the filter capacitor of the high-voltage DC offshore platform to burn out, resulting in system shutdown.

[0004] As a typical highly electronic power grid, the coupling relationship between the wind farm and its flexible DC transmission system is complex, and the dynamic mathematical model is not clearly described, making it difficult to judge the system stability using traditional power system stability analysis methods. In addition, the flexible DC transmission system is dominated by power electronic equipment, which can be regarded as a generator without rotational inertia. Its stability support capacity is naturally defective, especially when the flexible DC converter station is connected to the same wind turbine with small rotational inertia, there is a natural risk of system oscillation. Summary of the invention

[0005] The purpose of the present invention is to solve the shortcomings of the above-mentioned background technology and provide an oscillation suppression method for wind farms connected to the grid via modular multi-level converters. The method is simple to implement, has strong applicability, and has high practical value in engineering design.

[0006] The technical solution adopted by the present invention is: an oscillation suppression method for a wind farm connected to the grid via a modular multi-level converter, wherein the wind farm connected to the grid via a modular multi-level converter comprises a wind farm, a wind farm grid-connected bus, and a wind farm-side MMC converter, wherein the wind farm is connected to the wind farm-side MMC converter via the wind farm grid-connected bus;

[0007] The oscillation suppression method for connecting a wind farm to the grid via a modular multilevel converter comprises:

[0008] The three-phase voltage of the wind farm grid-connected busbar is sampled by the voltage sensor and output, denoted as ua_wf 、u b_wf 、u c_wf The AC output current of the MMC converter on the wind farm side is sampled by the current sensor and output, which is recorded as i a_WM 、i b_WM 、i c_WM ;

[0009] Calculate the wind farm grid-connected bus u through Park transformation a_wf 、u b_wf 、u c_wf The d-axis and q-axis output voltage u in the two-phase synchronous rotating dq coordinate system dwf 、u qwf , and MMC converter on the wind farm side a_WM 、i b_WM 、i c_WM The d-axis and q-axis output current i in the two-phase synchronous rotating dq coordinate system dWM 、i qWM ;

[0010] According to the dq axis output voltage u dwf and u qwf , and dq axis output current i dWM and i qWM , through decoupling feedback control, the AC output voltage reference value u of the MMC converter on the wind farm side is obtained * cdWN and u * cqWM ;

[0011] Based on i a_WM 、i b_WM and i c_WM The harmonic component i of the output current of the MMC converter on the wind farm side is obtained through the trap filter. ～ abc_WM ;

[0012] According to the wind farm side MMC converter output current harmonic component i ～ abc_WM Through virtual resistance control and Park transformation, the additional control value Δu of the AC output voltage reference value of the MMC converter on the wind farm side is obtained * cdWN and Δu * cqWM ;

[0013] The additional control quantity Δu * cdWN and Δu * cqWM Added to the AC output voltage reference value u of the MMC converter on the wind farm side * cdWN and u *cqWM , and obtain the output voltage d-axis modulation signal m of the MMC converter on the wind farm side d and the q-axis modulation signal m q ;

[0014] According to the dq axis modulation signal m d and m q After inverse Park transformation, it is converted into a three-phase modulated signal m in a stationary coordinate system a 、m b and m c ;

[0015] According to the calculated three-phase modulation signal m in the stationary coordinate system a , m b and m c , after the wave generation link, the modulation and wave generation control is performed to control the output current of the MMC on the wind farm side to generate the output voltage, and suppress the oscillation of the wind farm when it is connected to the grid through the MMC converter.

[0016] Furthermore, the AC output voltage reference value u of the MMC converter on the wind farm side * cdWN and u * cqWM , is the dq axis output voltage u dwf and u qwf , and dq axis output current i dWM and i qWM , through decoupling feedback control, the calculation formula is:

[0017]

[0018] Where: u * dwf and u * qwf is the dq axis output voltage u dwf and u qwf The reference value, k pWM is the voltage feedback control PI regulator proportional coefficient of the MMC converter on the wind farm side, T iWM is the integral time constant of the PI regulator of the voltage feedback control of the MMC converter on the wind farm side, L is the inductance of the bridge arm reactor, ω is the rated angular frequency, and s is the Laplace operator.

[0019] Furthermore, the wind farm side MMC converter output current harmonic component i ～ abc_WM The calculation formula is:

[0020]

[0021] Where: Q is the quality factor of the notch filter, which is set to 2.5, ω0 is the cutoff angular frequency, which is set to 50Hz, s is the Laplace operator, i ～ abc_WM Included ～ a_WM 、i ～ b_WM and i ～ c_WM ,i abc_WM Included a_WM 、i b_WM 、i c_WM .

[0022] Furthermore, the additional control value Δu of the AC output voltage reference value of the MMC converter on the wind farm side * cdWN and Δu * cqWM , is the harmonic component i of the output current of the MMC converter on the wind farm side ～ a_WM 、i ～ b_WM and i ～ c_WM , obtained through virtual resistance control and Park transformation, the calculation formula is:

[0023]

[0024] Where: K is the virtual resistance control coefficient, θ MMC It is the output voltage phase angle of the MMC converter on the wind farm side output by the phase-locked loop.

[0025] Furthermore, the output voltage reference value is added with the control amount Δu * cdWN and Δu * cqWM Add to the AC output voltage reference value u * cdWN and u * cqWM , and obtain the output voltage d-axis modulation signal m of the MMC converter on the wind farm side d and the q-axis modulation signal m q , the calculation formula is:

[0026]

[0027] Furthermore, the wind farm grid-connected busbar u a_wf 、u b_wf 、u c_wf The d-axis and q-axis output voltage u in the two-phase synchronous rotating dq coordinate system dwf 、u qwf , and MMC converter on the wind farm sidea_WM 、i b_WM 、i c_WM The d-axis and q-axis output current i in the two-phase synchronous rotating dq coordinate system dWM 、i qWM The transformation equation is:

[0028]

[0029] Where: θ MMC It is the output voltage phase angle of the MMC converter on the wind farm side output by the phase-locked loop.

[0030] Furthermore, the transformation equation of the modulation signal from the two-phase rotating coordinate system to the three-phase stationary coordinate system is:

[0031] m a =m d cosθ MMC -m q sinθ MMC

[0032]

[0033] Where: θ MMC It is the output voltage phase angle of the MMC converter on the wind farm side output by the phase-locked loop.

[0034] Furthermore, the wind farm side MMC converter includes an MMC submodule, a bridge arm reactor and a control system; the control system includes a Park transformation module, a notch filter module, a virtual resistance control module and an anti-Park transformation module.

[0035] The present invention introduces the harmonic component of the converter output current as an additional control variable into the voltage control of the MMC converter at the wind farm side.

[0036] The beneficial effects of the present invention are as follows:

[0037] (1) The present invention is an oscillation suppression method for a wind farm connected to the grid via an MMC converter. Through a virtual resistance controller, the oscillation of the wind farm output current, that is, the input current of the MMC converter on the wind farm side, can be suppressed, thereby reducing the control complexity.

[0038] (2) The present invention realizes oscillation suppression by reshaping the impedance of the MMC converter on the wind farm side, has strong versatility, and is applicable to the operating conditions of wind farms with different capacities and different types of wind turbines connected to the grid via MMC converters. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is the MMC topological structure diagram in the embodiment;

[0040] Figure 2This is a schematic diagram of the MMC oscillation suppression control system based on virtual resistance control proposed by the present invention;

[0041] Figure 3 The Bode plot of the notch filter in the oscillation suppression control system;

[0042] Figure 4 The present invention is not used to suppress the oscillation of new energy connected to the grid via MMC;

[0043] Figure 5 The present invention is used to suppress the oscillation of new energy connected to the grid via MMC. DETAILED DESCRIPTION

[0044] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments to facilitate a clear understanding of the present invention, but they do not constitute a limitation on the present invention.

[0045] like Figure 1 and Figure 2 As shown, this embodiment provides an oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter, wherein the wind farm connected to the grid via a modular multilevel converter includes a wind farm 01, a wind farm grid-connected bus 02, and a wind farm-side MMC converter 03, the wind farm 01 is connected to the wind farm-side MMC converter 03 via the wind farm grid-connected bus 02, the wind farm-side MMC converter 03 includes an MMC submodule 04, a bridge arm reactor 05 and a control system; the control system includes a Park conversion module 06, a notch filter module 07, a virtual resistance control module 08 and an inverse Park conversion module 09;

[0046] The Park transformation module 06 is used to calculate the d-axis and q-axis output voltage of the wind farm grid-connected bus 02 voltage in the two-phase synchronous rotating dq coordinate system, and the d-axis and q-axis output current of the AC side output current of the MMC converter 03 on the wind farm side in the two-phase synchronous rotating dq coordinate system. The notch filter module 07 is used to convert the output current of the MMC converter 03 on the wind farm side into the harmonic component of the output current of the MMC converter 03 on the wind farm side. The virtual resistance control module 08 is used to convert the harmonic component of the output current of the MMC converter 03 on the wind farm side into the additional control quantity of the AC output voltage reference value of the MMC converter 03 on the wind farm side. The inverse Park transformation module 09 is used to convert the two-phase rotating coordinate system of the d-axis modulation signal and the q-axis modulation signal of the output voltage of the MMC converter 03 on the wind farm side to the three-phase stationary coordinate system.

[0047] In this embodiment, a method for suppressing oscillation of a wind farm connected to the grid via a modular multilevel converter includes:

[0048] Step 1: Sample the three-phase voltage of the wind farm grid-connected bus 02 through the voltage sensor and output it, denoted as u a_wf 、u b_wf 、uc_wf The AC output current of the wind farm side MMC converter 03 is sampled by the current sensor and output, recorded as i a_WM 、i b_WM 、i c_WM .

[0049] Step 2: Calculate the wind farm grid-connected bus 02 voltage u through Park transformation a_wf 、u b_wf 、u c_wf The d-axis and q-axis output voltage u in the two-phase synchronous rotating dq coordinate system dwf 、u qwf , and the AC side output current i of the MMC converter 03 on the wind farm side a_WM 、i b_WM 、i c_WM The d-axis and q-axis output current i in the two-phase synchronous rotating dq coordinate system dWM 、i qWM , where the transformation equation from the three-phase stationary coordinate system to the two-phase synchronous rotating coordinate system is:

[0050]

[0051] Where: θ MMC It is the output voltage phase angle of the MMC converter on the wind farm side output by the phase-locked loop.

[0052] Step 3: According to the dq axis output voltage u obtained in step 2 dwf and u qwf , and dq axis output current i dWM and i qWM , through decoupling feedback control, the AC output voltage reference value u of the MMC converter on the wind farm side is obtained * cdWN and u * cqWM , the calculation formula is:

[0053]

[0054] Where: u * dwf and u * qwf is the dq axis output voltage u dwf and u qwf The reference value, k pWM is the voltage feedback control PI regulator proportional coefficient of the MMC converter 03 on the wind farm side, T iWM is the voltage feedback control PI regulator integral time constant of the MMC converter 03 on the wind farm side, L is the inductance value of the bridge arm reactor 05, ω is the rated angular frequency, and s is the Laplace operator.

[0055] Step 4: According to the output current i obtained in step 1 a_WM 、i b_WM and i c_WM , the output current harmonic component i of the MMC converter 03 on the wind farm side is obtained through the trap filter ～ a_WM 、i ～ b_WM and i ～ c_WM , and the whole is recorded as i ～ abc_WM , the calculation formula is:

[0056]

[0057] Among them: Figure 3 As shown, Q is the quality factor of the notch filter 07, which is set to 2.5, ω0 is the cutoff angular frequency, which is set to 50Hz, s is the Laplace operator, i ～ abc_WM Included ～ a_WM 、i ～ b_WM and i ～ c_WM ,i abc_WM Included a_WM 、i b_WM 、i c_WM .

[0058] Step 5: The output current harmonic component i of the wind farm side MMC converter 03 obtained in step 4 ～ abc_WM Through virtual resistance control and Park transformation, the additional control value Δu of the AC output voltage reference value of the MMC converter 03 on the wind farm side is obtained * cdWN and Δu * cqWM , the calculation formula is:

[0059]

[0060] Where: K is the virtual resistance control coefficient, θ MMC It is the output voltage phase angle of the MMC converter on the wind farm side output by the phase-locked loop.

[0061] Step 6: Add the additional control quantity Δu * cdWN and Δu * cqWM Add to the AC output voltage reference value u of the wind farm side MMC converter obtained in step 3 * cdWN and u * cqWM, and obtain the output voltage d-axis modulation signal m of the MMC converter 03 on the wind farm side d and the q-axis modulation signal m q , the calculation formula is:

[0062]

[0063] Step 7: According to the dq axis modulation signal m obtained in step 6 d and m q , which is transformed into a three-phase modulation signal m in a stationary coordinate system through inverse Park transformation a 、m b and m c , the transformation equation of the modulation signal from the two-phase rotating coordinate system to the three-phase stationary coordinate system is as follows:

[0064] m a =m d cosθ MMC -m q sinθ MMC

[0065]

[0066] Where: θ MMC It is the output voltage phase angle of the MMC converter on the wind farm side output by the phase-locked loop.

[0067] Step 8: The three-phase modulation signal m in the stationary coordinate system calculated according to step 7 a , m b and m c , after the wave generation link, the modulation and wave generation control is performed to control the output current of the MMC on the wind farm side to generate the output voltage, and suppress the oscillation of the wind farm when it is connected to the grid through the MMC converter.

[0068] The following is a simulation example and combined with the attached Figure 4-Figure 5 The technical solutions provided by the above embodiments of the present invention are further described. Figure 4 The oscillation condition when the wind farm is connected to the grid via the MMC converter 03 is not suppressed by using the present invention, wherein the main parameters of the wind farm and the MMC converter 03 on the wind farm side are shown in Table 1.

[0069] Table 1 Main parameters of wind farm and MMC converter on wind farm side.

[0070]

[0071]

[0072] It can be seen that in the steady-state operation area from 1.8s to 2s, the output current of the MMC converter on the wind farm side is obviously distorted. Figure 5In order to use the present invention to suppress the oscillation condition of a wind farm when it is connected to the grid via an MMC converter, it can be seen that in the steady-state operation region of 1.8s to 2s, the oscillation phenomenon disappears and the output current tends to be stable.

[0073] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. A method for suppressing oscillation of a wind farm connected to the grid via a modular multilevel converter, wherein the wind farm connected to the grid via a modular multilevel converter comprises a wind farm (01), a wind farm grid-connected busbar (02), and a wind farm-side MMC converter (03), wherein the wind farm (01) is connected to the wind farm-side MMC converter (03) via the wind farm grid-connected busbar (02); characterized in that: The oscillation suppression method for connecting a wind farm to the grid via a modular multilevel converter comprises: The three-phase voltage of the wind farm grid-connected bus (02) is sampled by the voltage sensor and output, denoted as u a_wf 、u b_wf 、u c_wf The AC output current of the MMC converter (03) on the wind farm side is sampled by the current sensor and output, which is recorded as i a_WM 、i b_WM 、i c_WM ; Calculate the wind farm grid-connected busbar (02)u by Park transformation a_wf 、u b_wf 、u c_wf The d-axis and q-axis output voltage u in the two-phase synchronous rotating dq coordinate system dwf 、u qwf , and MMC converter on the wind farm side (03)i a_WM 、i b_WM 、i c_WM The d-axis and q-axis output current i in the two-phase synchronous rotating dq coordinate system dWM 、i qWM ; According to the dq axis output voltage u dwf and u qwf , and dq axis output current i dWM and i qWM , through decoupling feedback control, the AC output voltage reference value u of the MMC converter (03) on the wind farm side is obtained * cdWN and u * cqWM ; Based on i a_WM 、i b_WM and i c_WM Through the trap filter, the output current harmonic component i of the wind farm side MMC converter (03) is obtained ～ abc_WM ; According to the output current harmonic component i of the MMC converter (03) at the wind farm side ～ abc_WM , through virtual resistance control and Park transformation, the additional control value Δu of the AC output voltage reference value of the MMC converter (03) on the wind farm side is obtained * cdWN and Δu * cqWM ; The additional control quantity Δu * cdWN and Δu * cqWM Added to the AC output voltage reference value u of the MMC converter (03) on the wind farm side * cdWN and u * cqWM , the output voltage d-axis modulation signal m of the MMC converter (03) on the wind farm side is obtained d and the q-axis modulation signal m q ; According to the dq axis modulation signal m d and m q After inverse Park transformation, it is converted into a three-phase modulated signal m in a stationary coordinate system a 、m b and m c ; According to the calculated three-phase modulation signal m in the stationary coordinate system a , m b and m c , after the wave generation link, the modulation and wave generation control is performed to control the output current of the MMC on the wind farm side to generate the output voltage, and suppress the oscillation of the wind farm when it is connected to the grid through the MMC converter.

2. The oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter according to claim 1, characterized in that: AC output voltage reference value u of MMC converter (03) on the wind farm side * cdWN and u * cqWM , is the dq axis output voltage u dwf and u qwf , and dq axis output current i dWM and i qWM , through decoupling feedback control, the calculation formula is: Where: u * dwf and u * qwf is the dq axis output voltage u dwf and u qwf The reference value, k pWM is the proportional coefficient of the voltage feedback control PI regulator of the MMC converter (03) on the wind farm side, T iWM is the integral time constant of the voltage feedback control PI regulator of the MMC converter (03) on the wind farm side, L is the inductance value of the bridge arm reactor (05), ω is the rated angular frequency, and s is the Laplace operator.

3. The oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter according to claim 1, characterized in that: Wind farm side MMC converter (03) output current harmonic component i ～ abc_WM The calculation formula is: Where: Q is the quality factor of the notch filter, which is set to 2.5, ω0 is the cutoff angular frequency, which is set to 50Hz, s is the Laplace operator, i ～ abc_WM Included ～ a_WM 、i ～ b_WM and i ～ c_WM ,i abc_WM Included a_WM 、i b_WM 、i c_WM .

4. The oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter according to claim 3, characterized in that: Wind farm side MMC converter (03) AC output voltage reference value additional control quantity Δu * cdWN and Δu * cqWM , is the current harmonic component i output by the MMC converter (03) on the wind farm side ～ a_WM 、i ～ b_WM and i ～ c_WM , obtained through virtual resistance control and Park transformation, the calculation formula is: Where: K is the virtual resistance control coefficient, θ MMC The output voltage phase angle of the wind farm side MMC converter (03) output by the phase-locked loop.

5. The oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter according to claim 1, characterized in that: Add the control amount Δu to the output voltage reference value * cdWN and Δu * cqWM Add to the AC output voltage reference value u * cdWN and u * cqWM , the output voltage d-axis modulation signal m of the MMC converter (03) on the wind farm side is obtained d and the q-axis modulation signal m q , the calculation formula is:

6. The oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter according to claim 1, characterized in that: Wind farm grid-connected busbar (02)u a_wf 、u b_wf 、u c_wf The d-axis and q-axis output voltage u in the two-phase synchronous rotating dq coordinate system dwf 、u qwf , and MMC converter on the wind farm side (03)i a_WM 、i b_WM 、i c_WM The d-axis and q-axis output current i in the two-phase synchronous rotating dq coordinate system dWM 、i qWM The transformation equation is: Where: θ MMC The output voltage phase angle of the wind farm side MMC converter (03) output by the phase-locked loop.

7. The oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter according to claim 1, characterized in that: The transformation equation of the modulation signal from the two-phase rotating coordinate system to the three-phase stationary coordinate system is: m a =m d cosθ MMC -m q sinθ MMC Where: θ MMC The output voltage phase angle of the wind farm side MMC converter (03) output by the phase-locked loop.

8. The method for suppressing oscillation of a wind farm connected to the grid via a modular multilevel converter according to claim 1, characterized in that: The wind farm side MMC converter (03) comprises an MMC submodule (04), a bridge arm reactor (05) and a control system; the control system comprises a Park conversion module (06), a notch filter module (07), a virtual resistance control module (08) and an inverse Park conversion module (09).

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