Adaptive reactive power limit control method for doubly fed wind turbine based on voltage sag depth

Abstract

The present invention discloses a method for adaptive reactive power limit support for doubly-fed wind turbines based on voltage drop depth, comprising: Step 1: Upon detecting a system short-circuit fault, obtaining the stator voltage drop depth; Step 2: Detecting the wind turbine's operating status after the fault and determining a reactive current proportionality coefficient using wind turbine parameters; Step 3: Determining a reactive power setpoint based on the reactive current proportionality coefficient and the voltage drop depth. This invention proposes an adaptive reactive power support control method that replaces the fixed control coefficients used in grid-connection standards. This method enables doubly-fed units to provide maximum reactive power support based on operating conditions, tapping the wind turbine's reactive power potential and thus broadening the range of fault ride-through capabilities.

Description

Technical Field

[0001] The present invention relates to the field of double-fed wind turbine fault ride-through, and in particular to a method for self-adaptive reactive power limit control of a double-fed wind turbine based on voltage drop depth. Background Art

[0002] Doubly-fed wind turbines (DFIGs) offer advantages such as variable-speed operation, high wind energy conversion rates, independent decoupling control of active and reactive power, and low converter costs. However, the stator side of the DFIG is directly connected to the grid, and grid faults can cause the wind turbine to disconnect from the grid. Grid connection standards stipulate that during voltage sags, the wind turbine must provide reactive power to the grid to support voltage recovery. Currently, voltage support during fault ride-through utilizes the reactive power regulation capabilities of the rotor-side converter or reactive power compensation equipment to support voltage recovery during faults. However, adding reactive power compensation equipment not only increases equipment cost but also requires coordination with the wind turbine. Traditional reactive power support typically uses a fixed reactive current proportionality factor, making it difficult to adapt to various post-fault operating conditions. Therefore, there is an urgent need for an adaptive reactive power limit control method for DFIGs based on voltage sag depth, which can determine the reactive current proportionality factor based on wind turbine parameters and operating conditions. Summary of the Invention

[0003] In order to solve the above problems, the purpose of the present invention is to provide a doubly fed wind turbine adaptive reactive limit control method based on voltage drop depth. By deriving the reactive current limit and calculating the reactive current proportional coefficient according to this limit, operating conditions and wind turbine parameters, the fixed control coefficient in the grid connection standard is replaced, so that the doubly fed unit can provide reactive support to the maximum extent according to the operating conditions, develop the reactive potential of the unit, and broaden the range of the unit's fault traversal.

[0004] In order to achieve the above technical objectives, the present invention provides a method for adaptive reactive power limit control of a doubly-fed wind turbine based on voltage sag depth, comprising the following steps:

[0005] Based on the doubly-fed wind turbine, a grid-connected system is constructed to control short-circuit faults in the grid-connected system and obtain the stator voltage drop depth;

[0006] Based on the operating status of the doubly-fed wind turbine after a short-circuit fault and the wind turbine parameters of the doubly-fed wind turbine, the reactive current proportional coefficient is determined;

[0007] Based on the drop depth and the reactive current proportional coefficient, the reactive power set value is obtained to control the doubly fed wind turbine.

[0008] Preferably, in the process of obtaining the drop depth, when a short circuit fault occurs in the grid-connected system, the doubly-fed wind turbine is controlled to provide dynamic reactive support to the grid-connected system;

[0009] The drop depth is calculated by obtaining the reactive current, rated current, and reactive current proportional coefficient of the doubly fed wind turbine.

[0010] Preferably, in the process of calculating the drop depth, the reactive current is expressed as:

[0011] I T ≥K p (0.9-U T )I N (0.2≤U T ≤0.9)

[0012] Among them, U T is the voltage drop depth, I N is the rated current, K p is the reactive current proportional coefficient.

[0013] Preferably, in the process of determining the reactive current proportional coefficient, the reactive current proportional coefficient is determined according to the stator-rotor mutual inductance, the stator self-inductance, the stator output active power and active power, and the voltage drop depth.

[0014] Preferably, in the process of obtaining the reactive current proportional coefficient, the reactive current proportional coefficient is expressed as:

[0015]

[0016] Among them, k smax is the maximum coefficient of reactive current ratio, I rmax is the maximum value of the rotor current, P s is the active power, L m is the stator-rotor mutual inductance, L s is the stator winding self-inductance, ω e is the grid angular frequency, and Δu is the voltage drop depth.

[0017] Preferably, in the process of controlling the doubly fed wind turbine to provide dynamic reactive support to the grid-connected system, based on the stator and rotor mutual inductance and the active power and voltage drop amplitudes of the doubly fed wind turbine, and according to the reactive current proportional coefficient, maximum reactive support is achieved without overcurrent.

[0018] Preferably, in the process of achieving maximum reactive support, the maximum value of the stator side reactive current is obtained according to the RSC capacity, which is expressed as:

[0019]

[0020] Preferably, in the process of obtaining the maximum value of the stator side reactive current, a current limit is obtained according to the RSC capacity, which is used as a limiting condition for the maximum value of the stator side reactive current, wherein the limiting condition is:

[0021]

[0022] Preferably, in the process of obtaining the current limit, a flux equation of the doubly fed wind turbine is constructed to obtain the rotor current, and the current limit is generated based on the obtained rotor current, wherein the flux equation is expressed as:

[0023]

[0024] The rotor current is expressed as:

[0025]

[0026] The current limit is expressed as:

[0027]

[0028] Preferably, in the process of obtaining the reactive power set value, the reactive power set value is expressed as:

[0029]

[0030] The present invention discloses the following technical effects:

[0031] The present invention discloses an adaptive reactive power limit control method for a doubly-fed wind turbine. Upon detecting a change in the bus voltage at the wind turbine's grid connection point, the stator voltage drop depth is determined. The wind turbine's post-fault operating status is then detected, and a reactive current proportionality coefficient is determined using a formula derived in the present invention. A reactive power setpoint is determined based on the reactive current proportionality coefficient and the voltage drop depth. This method utilizes the adaptive reactive current proportionality coefficient based on the post-fault operating status of the doubly-fed wind turbine, thereby expanding the range of fault ride-through capabilities of the unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 is a flow chart of the method for adaptive reactive power limit control of a doubly-fed wind turbine according to an embodiment of the present invention;

[0034] Figure 2 is a grid-connected structure diagram of a doubly-fed wind farm according to an embodiment of the present invention;

[0035] Figure 3 This is a control block diagram of the doubly-fed wind turbine adaptive reactive limit control according to the present invention; DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the application for protection, but merely represents the selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application.

[0037] like Figure 1-3 As shown, the present invention provides a method for adaptive reactive power limit control of a doubly-fed wind turbine based on voltage drop depth, comprising the following steps:

[0038] Based on the doubly-fed wind turbine, a grid-connected system is constructed to control short-circuit faults in the grid-connected system and obtain the stator voltage drop depth;

[0039] Based on the operating status of the doubly-fed wind turbine after a short-circuit fault and the wind turbine parameters of the doubly-fed wind turbine, the reactive current proportional coefficient is determined;

[0040] Based on the drop depth and the reactive current proportional coefficient, the reactive power set value is obtained to control the doubly fed wind turbine.

[0041] Further preferably, in the process of obtaining the drop depth, when a short circuit fault occurs in the grid-connected system, the present invention controls the doubly-fed wind turbine to provide dynamic reactive support to the grid-connected system;

[0042] The drop depth is calculated by obtaining the reactive current, rated current, and reactive current proportional coefficient of the doubly fed wind turbine.

[0043] Further preferably, in the process of calculating the drop depth, the reactive current mentioned in the present invention is expressed as:

[0044] I T ≥K p (0.9-U T )I N (0.2≤U T ≤0.9)

[0045] Among them, U T is the voltage drop depth, I N is the rated current, K p is the reactive current proportional coefficient.

[0046] Further preferably, in the process of determining the reactive current proportional coefficient, the present invention determines the reactive current proportional coefficient according to the stator-rotor mutual inductance, the stator self-inductance, the stator output active power and active power, and the voltage drop depth.

[0047] Further preferably, in the process of obtaining the reactive current proportional coefficient, the reactive current proportional coefficient mentioned in the present invention is expressed as:

[0048]

[0049] Among them, k smax is the maximum coefficient of reactive current ratio, I rmax is the maximum value of the rotor current, P s is the active power, L m is the stator-rotor mutual inductance, L s is the stator winding self-inductance, ω e is the grid angular frequency, and Δu is the voltage drop depth.

[0050] Further preferably, in the process of controlling the doubly fed wind turbine to provide dynamic reactive support to the grid-connected system, based on the stator and rotor mutual inductance and the active power and voltage drop amplitudes of the doubly fed wind turbine, the present invention achieves maximum reactive support without overcurrent according to the reactive current proportional coefficient.

[0051] Further preferably, in the process of achieving maximum reactive support, the maximum value of the stator side reactive current is obtained according to the RSC capacity, which is expressed as:

[0052]

[0053] Further preferably, in the process of obtaining the maximum value of the stator side reactive current, the present invention obtains a current limit according to the RSC capacity, which is used as a limiting condition for the maximum value of the stator side reactive current, wherein the limiting condition is:

[0054]

[0055] Further preferably, in the process of obtaining the current limit, the present invention obtains the rotor current by constructing a flux equation of the doubly fed wind turbine, and generates the current limit based on the obtained rotor current, wherein the flux equation is expressed as:

[0056]

[0057] The rotor current is expressed as:

[0058]

[0059] The current limit is expressed as:

[0060]

[0061] Further preferably, in the process of obtaining the reactive power set value, the reactive power set value mentioned in the present invention is expressed as:

[0062]

[0063] The present invention also discloses a doubly-fed wind turbine adaptive reactive limit control system based on voltage drop depth, comprising:

[0064] A data acquisition module is used to collect information about the grid-connected system including the doubly-fed wind turbine and the drop depth of the stator voltage of the doubly-fed wind turbine when a short-circuit fault occurs;

[0065] a data calculation module for determining a reactive current proportional coefficient based on an operating condition of the doubly-fed wind turbine after a short-circuit fault and wind turbine parameters of the doubly-fed wind turbine;

[0066] The wind turbine control module is used to obtain the reactive power set value based on the drop depth and the reactive current proportional coefficient and control the doubly fed wind turbine.

[0067] The present invention also implements the control logic of the doubly fed wind turbine adaptive reactive limit control method through a computer program, and embeds the computer program into a grid-connected system including the doubly fed wind turbine to realize dynamic reactive support of the grid-connected system by controlling the doubly fed wind turbine when the system is short-circuited.

[0068] The present invention also provides a removable storage device for carrying a doubly-fed wind turbine adaptive reactive limit control system and performing data interaction with a grid-connected system having a doubly-fed wind turbine. When a short-circuit fault is detected in the grid-connected system, the doubly-fed wind turbine is controlled to provide dynamic reactive support to the grid-connected system.

[0069] The present invention proposes an adaptive reactive support control method that replaces the fixed control coefficient in the grid-connected standard, enabling the doubly-fed unit to provide reactive support to the maximum extent according to the operating conditions, developing the reactive potential of the wind turbine, and thus broadening the range of the unit's fault ride-through.

[0070] like Figure 2 As shown, an example of the present invention provides a grid-connected system for a doubly-fed wind farm, which includes: a doubly-fed wind farm, an infinite power grid system, and the doubly-fed wind turbines are connected to the infinite power grid via a two-stage transformer.

[0071] The present invention also provides a method for adaptive reactive power limit control of a doubly fed wind turbine under voltage drop, which is applied to the grid-connected system of the doubly fed wind farm, such as Figure 1 As shown, the following steps are included:

[0072] Step 1: When a short circuit fault is detected in the system, obtain the drop depth of the stator voltage;

[0073] Step 2: Detect the fan operating status after the fault and determine the reactive current proportional coefficient using the fan parameters;

[0074] Step 3: Determine a reactive power set value according to the reactive current proportional coefficient and the voltage drop depth.

[0075] In step 1: when a short circuit fault is detected in the system, the drop depth of the stator voltage is obtained; specifically:

[0076] When a power grid fault occurs, the wind farm needs to provide dynamic reactive power support to the system as required. The reactive current should be:

[0077] I T ≥K p (0.9-U T )I N (0.2≤U T ≤0.9)

[0078] Among them, where U T is the voltage drop depth, I N is the rated current of the wind farm, K p is the wind farm's dynamic reactive current proportional coefficient. During a grid fault, the reactive power a wind turbine injects into the system is related to the voltage dip depth and the reactive current proportional coefficient. Detecting the voltage dip depth is a prerequisite for determining reactive power.

[0079] In step 2: detecting the operating status of the fan after the fault, and determining the reactive current proportional coefficient using the fan parameters;

[0080] Specifically:

[0081] Determine the reactive current proportional coefficient based on wind turbine parameters such as stator-rotor mutual inductance, stator self-inductance, stator output active power, and unit operating conditions such as active power and voltage drop depth:

[0082]

[0083] Among them, k smax is the maximum coefficient of reactive current ratio, I rmax is the maximum value of the rotor current, P s is the active power, L m is the stator-rotor mutual inductance, L s is the stator winding self-inductance, ω e is the grid angular frequency, and Δu is the voltage drop depth.

[0084] By setting the reactive current proportional coefficient according to the above formula, the doubly fed wind turbine can achieve maximum reactive power support without overcurrent based on parameters such as the mutual inductance of the stator and rotor and the amplitude of active power and voltage drop.

[0085] The derivation process is as follows:

[0086] In low voltage ride-through, in accordance with the "GB / T 19963.1-2021 Technical Regulations for Wind Farm Integration into Power Systems," the DFIG should initiate reactive power support when the voltage drops below 0.9 per unit. Therefore, during modeling, the initial stator voltage of the DFIG is set to 0.9 pu. The voltage drop depth after the fault is Δu, and the stator voltage should be 0.9-Δu. When the grid voltage is oriented to the d-axis, it is expressed as: u sd =0.9-Δu=-ω e ψ sq ;u sq = 0. At this time, the d-axis and q-axis stator fluxes of the DFIG are ψ sd =0;ψ sq =-(0.9-Δu) / ω e After adopting stator voltage orientation, the flux equation of the doubly fed wind turbine can be expressed as

[0087]

[0088] Among them L s and L r are the stator and rotor winding self-inductances respectively; L m For mutual induction.

[0089] The DFIG active power P s =-3 / 2(0.9-Δu)i sd The rotor current can be obtained by using formula (1):

[0090]

[0091] Among them, ω e is the grid angular frequency.

[0092] The reactive power output of the doubly fed wind turbine depends on the rotor excitation controlled by RSC, so the reactive output limit is closely related to the RSC capacity. We can get:

[0093]

[0094] From formula (3), the maximum value of the stator side reactive current is

[0095]

[0096] In order to achieve the maximum voltage support, setting the reactive power according to formula (4) can maximize the use of the capacity of the RSC and can be adjusted in real time according to the active power and voltage dynamics. According to the operating status of the wind turbine, the reactive current proportional coefficient can be set to

[0097]

[0098] The step 3: determining a reactive power set value according to the reactive current proportional coefficient and the voltage drop depth.

[0099] Specifically:

[0100] The reactive power set value of the wind turbine is determined by the reactive current proportional coefficient and the voltage drop depth in step 2 above.

[0101]

[0102] If K p =k smax , the reactive current proportional coefficient value is related to both the wind turbine's active power output and the magnitude of the stator voltage drop. If a traditional fixed coefficient is used, the reactive power support provided does not take into account the wind turbine's operating conditions. In this case, there is a high probability that there will still be excess RSC capacity and reactive power margin. If an adaptive reactive current proportional coefficient design is adopted, the doubly-fed wind turbine can achieve maximum reactive power support without overcurrent based on parameters such as the stator-rotor mutual inductance and the magnitude of the active power and voltage drop, thereby expanding the fault ride-through range.

[0103] The present invention is built in DigSILENT PowerFactory 15.2 as follows Figure 2 The wind farm grid-connected simulation system shown in Figure 1 has a capacity of 30 DFIG wind turbines (2 MW). The DFIG transformers are connected to the infinite grid. The basic parameters of the simulation model are shown in Table 1.

[0104] Table 1

[0105]

[0106]

[0107] In Example 1 of the present invention, a three-phase ground fault occurred at busbar B2 at 1 second, lasting 0.625 seconds. Using standard grid-connected control, the unit generated approximately 7 Mvar of reactive power, with a voltage drop of 0.81 pu after the voltage dip. Using the adaptive reactive limit support control proposed in the present invention, the unit generated approximately 50 Mvar of reactive power, with a voltage drop of 0.93 pu during the dip. After stabilization, the rotor current approached 1.2 pu. Further voltage increases and reactive power generation would increase Irq, and the rotor current would exceed 1.2 pu, compromising stable operation.

[0108] In Example 2 of the present invention, a three-phase ground short circuit fault occurred at bus B2 at 1 second, and the fault lasted for 0.625 seconds. When the control used in the grid-connected standard was adopted, the reactive power generated by the unit was 20Mvar, and the amplitude after the voltage drop was 0.48pu. When the adaptive voltage control proposed in this article was adopted, during the fault period, the unit generated 38Mvar of reactive power, and the amplitude after the voltage drop was 0.53pu. Compared with the grid-connected standard, the reactive power increased by 18Mvar and the voltage increased by 0.05pu. In summary, the control strategy in this article achieved the goal of generating as much reactive power as possible without overcurrent.

[0109] The simulation results verify the correctness of the proposed adaptive reactive power limit control method for doubly fed wind turbines based on voltage sag depth.

[0110] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowcharts and / or block diagrams, as well as combinations of processes and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowcharts and / or block diagrams. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0111] In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0112] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. An adaptive reactive power limit control method for a doubly-fed wind turbine based on voltage drop depth, characterized in that: The following steps are involved: Based on the doubly-fed wind turbine, a grid-connected system is constructed, a short-circuit fault is controlled in the grid-connected system, and a drop depth of the stator voltage is obtained; Determining a reactive current proportional coefficient based on an operating condition of the doubly-fed wind turbine after a short-circuit fault and according to wind turbine parameters of the doubly-fed wind turbine; Based on the drop depth and the reactive current proportional coefficient, a reactive power set value is obtained to control the doubly fed wind turbine; In the process of obtaining the drop depth, when a short circuit fault occurs in the grid-connected system, controlling the doubly-fed wind turbine to provide dynamic reactive support to the grid-connected system; Calculating the drop depth by obtaining the reactive current, rated current, and reactive current proportional coefficient of the doubly-fed wind turbine; In the process of calculating the drop depth, the reactive current is expressed as: I T ≥K p (0.9-U T )I N (0.2≤U T ≤0.9) Among them, U T is the voltage drop depth, I N is the rated current, K p is the reactive current proportional coefficient; In the process of determining the reactive current proportional coefficient, the reactive current proportional coefficient is determined according to the stator-rotor mutual inductance, the stator self-inductance, the stator output active power and active power, and the voltage drop depth; In the process of obtaining the reactive current proportional coefficient, the reactive current proportional coefficient is expressed as: Among them, k smax is the maximum coefficient of reactive current ratio, I rmax is the maximum value of the rotor current, P s is the active power, L m is the stator-rotor mutual inductance, L s is the stator winding self-inductance, ω e is the grid angular frequency, and Δu is the voltage drop depth.

2. The method for adaptive reactive power limit control of a doubly-fed wind turbine based on voltage sag depth according to claim 1 is characterized in that: In the process of controlling the doubly fed wind turbine to provide dynamic reactive support to the grid-connected system, based on the stator and rotor mutual inductance and the active power and voltage drop amplitudes of the doubly fed wind turbine, and according to the reactive current proportional coefficient, maximum reactive support is achieved without overcurrent.

3. The method for adaptive reactive power limit control of a doubly-fed wind turbine based on voltage sag depth according to claim 2 is characterized in that: In the process of achieving maximum reactive support, the maximum value of the stator side reactive current is obtained according to the RSC capacity, which is expressed as:

4. The method for adaptive reactive power limit control of a doubly-fed wind turbine based on voltage sag depth according to claim 3 is characterized in that: In the process of obtaining the maximum value of the stator-side reactive current, a current limit is obtained according to the RSC capacity, which is used as a limiting condition for the maximum value of the stator-side reactive current, wherein the limiting condition is:

5. The method for adaptive reactive power limit control of a doubly-fed wind turbine based on voltage sag depth according to claim 4 is characterized in that: In the process of obtaining the current limit, a flux equation of the doubly fed wind turbine is constructed to obtain the rotor current, and the current limit is generated based on the obtained rotor current, wherein the flux equation is expressed as: The rotor current is expressed as: The current limit is expressed as:

6. The method for adaptive reactive power limit control of a doubly-fed wind turbine based on voltage sag depth according to claim 5, characterized in that: In the process of obtaining the reactive power set value, the reactive power set value is expressed as:

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