A method for suppressing oscillation of wind farms connected to the grid via modular multilevel converters

Abstract

The invention discloses an oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter. The invention adopts an MMC controller of a d / q coordinate system, improves and optimizes the MMC control method from three parts, namely, an AC voltage outer loop, a current inner loop with current decoupling and voltage feedforward, and an additional sub- / super-synchronous oscillation suppression link, so as to suppress the sub- / super-synchronous oscillation of wind power sent out and connected to the grid via the MMC. The invention can suppress the sub- / super-synchronous oscillation of the entire wind farm by optimizing and improving the MMC control strategy without increasing the hardware cost. The invention can suppress the sub- / super-synchronous oscillation of the wind farm and improve the transient response performance of the wind farm by independently adjusting the d-axis proportional coefficient Kd and the q-axis proportional coefficient Kq of the current inner loop. The invention is simple and reliable, and the parameter setting is convenient and easy to debug.

Description

Technical Field

[0001] The present invention relates to the technical field of renewable energy power generation, and in particular to an oscillation suppression method for a wind farm connected to the grid via a modular multi-level converter. Background Art

[0002] In the new power system dominated by renewable energy generation, the stable transmission, safe grid connection and absorption capacity of large-scale wind power generation over long distances on land or at sea face many difficulties and challenges. Among them, the flexible direct current transmission system grid connection scheme with wind power via modular multilevel converter (MMC) as the core has significant advantages and has been increasingly studied and widely used. However, wind turbine converters and modular multilevel converters are both power electronic devices, and the strong nonlinearity of power electronic devices will lead to broadband oscillation between interconnected power electronic devices, which seriously threatens the safe and stable operation of the devices and systems, especially the sub / super synchronous oscillation problem is the most common. The document "Mechanism and suppression method of broadband oscillation of direct-drive wind farms connected to weak power grids (II): A broadband oscillation suppression method based on impedance reshaping" proposes an impedance reshaping control strategy based on active damping and virtual admittance to solve the broadband oscillation stability problem of direct-drive wind turbines connected to weak power grids, reshape the impedance characteristics of wind turbines, improve the amplitude margin and phase margin, and thus solve the broadband oscillation problem; the patent "A subsynchronous oscillation based on additional virtual impedance control" (patent number: 201911350960.X) proposes a subsynchronous oscillation suppression method based on additional virtual impedance control for the subsynchronous oscillation problem of double-fed wind turbines connected to the grid via VSC-HVDC. The above schemes are improvements and optimizations made to the wind turbine control strategy. However, in actual applications, large-scale wind farms contain dozens or even hundreds of wind turbines. The above schemes need to adjust the parameters of each wind turbine, which is difficult and the actual effect is difficult to guarantee. Summary of the invention

[0003] In order to overcome the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide an oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter. Without increasing the wind turbine control hardware, the MMC control method is improved and optimized, thereby solving the sub / super synchronous oscillation problem of wind power sent out and connected to the grid via the MMC. The method is simple and reliable, with convenient parameter setting and easy debugging.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is:

[0005] An oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter, characterized in that an MMC controller of a d / q coordinate system is adopted, and the MMC control method is improved and optimized from three parts: an AC voltage outer loop, a current inner loop with current decoupling and voltage feedforward, and an additional sub / super synchronous oscillation suppression link, so as to suppress the sub / super synchronous oscillation of wind power sent out and connected to the grid via the MMC;

[0006] The specific steps include:

[0007] Step 1: The AC voltage outer loop adopts PI1 regulator to realize feedback value usd tracking d-axis voltage amplitude reference value Usd*, feedback value usq tracking q-axis voltage amplitude reference value Usq*, and the three-phase voltage ua / ub / uc of the grid point is transformed into usd and usq via abc-dq coordinate system. The abc-dq coordinate system transformation transforms the three-phase voltage instantaneous value ua / ub / uc measured by the voltage transformer from the abc three-phase coordinate system to usd and usq in the d / q two-phase synchronous coordinate system.

[0008] Step 2: The current inner loop has current decoupling and voltage feedforward, and a PI2 regulator is used to realize that the feedback value id tracks the d-axis current reference value id*, and the feedback value iq tracks the q-axis current reference value iq*;

[0009] The three-phase current ia / ib / ic at the grid connection point is transformed into id and iq through the abc-dq coordinate system. The abc-dq coordinate system is transformed to transform the instantaneous value ia / ib / ic of the three-phase current measured by the current transformer from the abc three-phase coordinate system to id and iq in the d / q two-phase synchronous coordinate system;

[0010] Step 3 adds a sub- / supersynchronous oscillation suppression link, including a d-axis additional component and a q-axis additional component, wherein the d-axis current at the grid-connected point is multiplied by the proportional coefficient Kd to obtain the d-axis additional component, and the d-axis additional component is superimposed on the d-axis output result of the current inner loop; the q-axis current at the grid-connected point is multiplied by the proportional coefficient Kq to obtain the q-axis additional component, and the q-axis additional component is superimposed on the q-axis output result of the current inner loop.

[0011] In step 1, the PI1 regulator and the PI2 regulator include a proportional link and an integral link. The proportional link refers to multiplying the input signal of the regulator by a proportional coefficient, and the integral link refers to performing an integral operation on the input signal of the regulator, and the operation results of the two are superimposed on each other.

[0012] In the step 1, the d-axis voltage amplitude reference value Usd* is 1 pu; and the q-axis voltage amplitude reference value Usq* is 0 pu.

[0013] In step 2, the d-axis current decoupling term of the current inner loop is obtained by multiplying the q-axis current iq by the decoupling coefficient X LThe q-axis current decoupling term of the current inner loop is obtained by multiplying the d-axis current id by the same decoupling coefficient X L get.

[0014] In the step 2, the d-axis current reference value id* of the current inner loop is generated by the d-axis voltage outer loop; and the q-axis current reference value iq* of the current inner loop is generated by the q-axis voltage outer loop.

[0015] In step 2, the voltage feedforward items of the current inner loop correspond to the d-axis voltage usd and the q-axis voltage usq of the grid connection point respectively.

[0016] In step 3, the proportional coefficient Kd of the d-axis additional component and the proportional coefficient Kq of the q-axis additional component are not equal, so that independent control of the d-axis or q-axis is achieved; Kd or Kq takes values ​​in the entire real number domain.

[0017] When the value of Kd or Kq is greater than 0, it indicates that the damping of the MMC converter is reduced; when the value of Kd or Kq is greater than 0, it indicates that the damping of the MMC converter is increased; when the value of Kd or Kq is equal to 0, it indicates that the damping characteristics of the MMC converter remain unchanged.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] 1. Without increasing the cost of hardware transformation, the sub / super synchronous oscillation suppression of the entire wind farm can be achieved only by optimizing and improving the MMC control method.

[0020] 2. By independently adjusting the d-axis proportional coefficient Kd and the q-axis proportional coefficient Kq of the current inner loop, the wind farm / supersynchronous oscillation can be suppressed and the transient response performance of the wind farm can be improved.

[0021] 3. This method is not limited to modular multilevel converters, but is also applicable to wind farms connected to the grid via voltage source converters.

[0022] 4. The solution is simple and reliable, with convenient parameter setting and easy debugging. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a control flow chart of the present invention.

[0024] Figure 2 It is a system structure diagram of the present invention.

[0025] Figure 3 This is a diagram showing the oscillation suppression effect of the present invention, wherein: Figure 3 (a) is the active power oscillation suppression effect diagram, Figure 3 (b) is the diagram of reactive power oscillation suppression effect.

[0026] Figure 4This is a low voltage ride-through waveform diagram of the present invention, wherein: Figure 4 (a) is the active power transient response waveform. Figure 4 (b) Reactive power transient response waveform. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below with reference to the accompanying drawings.

[0028] See also Figure 1 , an oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter, using an MMC controller in a d / q coordinate system, improving and optimizing the MMC control method from three parts: an AC voltage outer loop, a current inner loop with current decoupling and voltage feedforward, and an additional sub / super synchronous oscillation suppression link, to suppress the sub / super synchronous oscillation of wind power sent out and connected to the grid via the MMC, specifically including the following steps:

[0029] Step 1: The AC voltage outer loop adopts PI1 regulator to realize feedback value usd tracking d-axis voltage amplitude reference value Usd*, feedback value usq tracking q-axis voltage amplitude reference value Usq*, and the three-phase voltage ua / ub / uc of the grid point is transformed into usd and usq via abc-dq coordinate system. The abc-dq coordinate system transformation transforms the three-phase voltage instantaneous value ua / ub / uc measured by the voltage transformer from the abc three-phase coordinate system to usd and usq in the d / q two-phase synchronous coordinate system.

[0030] Step 2: The current inner loop has current decoupling and voltage feedforward, and a PI2 regulator is used to realize that the feedback value id tracks the d-axis current reference value id*, and the feedback value iq tracks the q-axis current reference value iq*;

[0031] The three-phase current ia / ib / ic at the grid connection point is transformed into id and iq through the abc-dq coordinate system. The abc-dq coordinate system is transformed to transform the instantaneous value ia / ib / ic of the three-phase current measured by the current transformer from the abc three-phase coordinate system to id and iq in the d / q two-phase synchronous coordinate system;

[0032] Step 3 adds a sub- / supersynchronous oscillation suppression link, including a d-axis additional component and a q-axis additional component, wherein the d-axis current at the grid-connected point is multiplied by the proportional coefficient Kd to obtain the d-axis additional component, and the d-axis additional component is superimposed on the d-axis output result of the current inner loop; the q-axis current at the grid-connected point is multiplied by the proportional coefficient Kq to obtain the q-axis additional component, and the q-axis additional component is superimposed on the q-axis output result of the current inner loop.

[0033] In step 1, the PI1 regulator and the PI2 regulator include a proportional link and an integral link. The proportional link refers to multiplying the input signal of the regulator by a proportional coefficient, and the integral link refers to performing an integral operation on the input signal of the regulator, and the operation results of the two are superimposed on each other.

[0034] In the step 1, the d-axis voltage amplitude reference value Usd* is 1 pu; and the q-axis voltage amplitude reference value Usq* is 0 pu.

[0035] In step 2, the d-axis current decoupling term of the current inner loop is obtained by multiplying the q-axis current iq by the decoupling coefficient X L The q-axis current decoupling term of the current inner loop is obtained by multiplying the d-axis current id by the same decoupling coefficient X L get.

[0036] In the step 2, the d-axis current reference value id* of the current inner loop is generated by the d-axis voltage outer loop; and the q-axis current reference value iq* of the current inner loop is generated by the q-axis voltage outer loop.

[0037] In step 2, the voltage feedforward items of the current inner loop correspond to the d-axis voltage usd and the q-axis voltage usq of the grid connection point respectively.

[0038] In step 3, the proportional coefficient Kd of the d-axis additional component and the proportional coefficient Kq of the q-axis additional component are not equal, so that independent control of the d-axis or q-axis is achieved; Kd or Kq takes values ​​in the entire real number domain.

[0039] When the value of Kd or Kq is greater than 0, it indicates that the damping of the MMC converter is reduced; when the value of Kd or Kq is greater than 0, it indicates that the damping of the MMC converter is increased; when the value of Kd or Kq is equal to 0, it indicates that the damping characteristics of the MMC converter remain unchanged.

[0040] like Figure 2 As shown, a wind farm grid-connected system via MMC transmission is composed of wind turbines G1~GN, converter transformer T, MMC converter on the wind farm side (referred to as WF-MMC), transmission line TL (overhead line or cable), MMC converter on the grid side (referred to as GS-MMC), and AC power supply Vs. Among them, the controller C of WF-MMC collects the three-phase voltage ua / ub / uc and the three-phase current ia / ib / ic between the converter transformer T and the wind turbine, controls the WF-MMC to output the reference voltage Vd / Vq, and realizes the transmission of active power of the wind farm.

[0041] like Figure 3As shown in the figure, in the entire period of t = 0s ~ 2.0s, Kd = 0 is always maintained to examine the inhibitory effect of Kq on the oscillation phenomenon. It can be seen that in the period of t = 0s ~ 0.5s, Kq = 0.3, the active power P and reactive power Q can operate stably without oscillation; in the period of t = 0.5s ~ 1.5s, Kq = 0, both the active power P and reactive power Q oscillate, and the oscillation of reactive power Q is more obvious; in the period of t = 1.5s ~ 2.0s, Kq = 0.3, the active power P and reactive power Q can operate stably without oscillation.

[0042] like Figure 4 As shown in the figure, at t = 0.2s, a metallic grounding fault of the A-phase and B-phase AC busbars occurs at the common connection point of the converter transformer T and the fans G1 ~ GN, and the fault is cleared at t = 1.7s. Set Kq = 0.3 to ensure that the system will not oscillate during steady-state operation. When Kd = 0, during the fault duration of 0.2s to 1.7s, the active power P and reactive power Q are almost zero (as shown by the dotted line in the figure), which means that the power transmission is interrupted, which will seriously threaten the safe operation of the system. At the 1.7s fault exit moment, the active power P and reactive power Q fluctuate greatly, which will also affect the safe operation of the system; when Kd = -0.3, during the fault duration, the active power P and reactive power Q are not zero (as shown by the solid line in the figure), and the size of the transmission power is proportional to the voltage drop, which means that the system can maintain the maximum power transmission capacity under the current fault, and at the fault exit moment, the fluctuation of the active power P and reactive power Q is significantly reduced, which is conducive to the safe and stable operation of the system.

Claims

1. A method for suppressing oscillation of a wind farm connected to the grid via a modular multilevel converter, characterized in that: The MMC controller of d / q coordinate system is adopted to improve and optimize the MMC control method from three parts: AC voltage outer loop, current inner loop with current decoupling and voltage feedforward, and additional sub / super synchronous oscillation suppression link, so as to suppress sub / super synchronous oscillation of wind power sent out through MMC and connected to the grid. The specific steps include: Step 1: The AC voltage outer loop adopts PI1 regulator to realize feedback value usd tracking d-axis voltage amplitude reference value Usd*, feedback value usq tracking q-axis voltage amplitude reference value Usq*, and the three-phase voltage ua / ub / uc of the grid point is transformed into usd and usq via abc-dq coordinate system. The abc-dq coordinate system transformation transforms the three-phase voltage instantaneous value ua / ub / uc measured by the voltage transformer from the abc three-phase coordinate system to usd and usq in the d / q two-phase synchronous coordinate system. Step 2: The current inner loop has current decoupling and voltage feedforward, and a PI2 regulator is used to realize that the feedback value id tracks the d-axis current reference value id*, and the feedback value iq tracks the q-axis current reference value iq*; The three-phase current ia / ib / ic at the grid connection point is transformed into id and iq through the abc-dq coordinate system. The abc-dq coordinate system is transformed to transform the instantaneous value ia / ib / ic of the three-phase current measured by the current transformer from the abc three-phase coordinate system to id and iq in the d / q two-phase synchronous coordinate system; Step 3 adds a sub- / supersynchronous oscillation suppression link, including a d-axis additional component and a q-axis additional component, wherein the d-axis current at the grid-connected point is multiplied by the proportional coefficient Kd to obtain the d-axis additional component, and the d-axis additional component is superimposed on the d-axis output result of the current inner loop; the q-axis current at the grid-connected point is multiplied by the proportional coefficient Kq to obtain the q-axis additional component, and the q-axis additional component is superimposed on the q-axis output result of the current inner loop.

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: In step 1, the PI1 regulator and the PI2 regulator include a proportional link and an integral link. The proportional link refers to multiplying the input signal of the regulator by a proportional coefficient, and the integral link refers to performing an integral operation on the input signal of the regulator, and the operation results of the two are superimposed on each other.

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: In the step 1, the d-axis voltage amplitude reference value Usd* is 1 pu; and the q-axis voltage amplitude reference value Usq* is 0 pu.

4. The oscillation suppression method for a wind farm connected to the grid via a modular multilevel converter according to claim 1, characterized in that: In step 2, the d-axis current decoupling term of the current inner loop is obtained by multiplying the q-axis current iq by the decoupling coefficient X L The q-axis current decoupling term of the current inner loop is obtained by multiplying the d-axis current id by the same decoupling coefficient X L get.

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: In the step 2, the d-axis current reference value id* of the current inner loop is generated by the d-axis voltage outer loop; and the q-axis current reference value iq* of the current inner loop is generated by the q-axis voltage outer loop.

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: In step 2, the voltage feedforward items of the current inner loop correspond to the d-axis voltage usd and the q-axis voltage usq of the grid connection point respectively.

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: In step 3, the proportional coefficient Kd of the d-axis additional component and the proportional coefficient Kq of the q-axis additional component are not equal, so that independent control of the d-axis or q-axis is realized; Kd or Kq takes values ​​in the entire real number domain.

8. The method for suppressing oscillation of a wind farm connected to the grid via a modular multilevel converter according to claim 7, characterized in that: When the value of Kd or Kq is greater than 0, it indicates that the damping of the MMC converter is reduced; when the value of Kd or Kq is greater than 0, it indicates that the damping of the MMC converter is increased; when the value of Kd or Kq is equal to 0, it indicates that the damping characteristics of the MMC converter remain unchanged.

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