Low voltage ride through control method considering voltage measurement delay estimation

Abstract

The invention discloses a low-voltage ride-through control method considering voltage measurement delay estimation, and the method comprises the steps: carrying out the modeling of a typical vector control system of a doubly-fed wind power system, obtaining a reactive power output limiting condition, and carrying out the building of a voltage measurement equipment output model, on this basis, a cosine interpolation fitting method is adopted to accurately estimate the time delay of a voltage measurement device for the grid-connected point voltage of the doubly-fed fan, and after accurate time delay estimation is determined, the time delay is substituted into a low-order time delay compensation control link, so that the power instruction of the doubly-fed fan is compensated. According to the method, the actual voltage delay of the grid-connected point is accurately estimated, and a voltage measurement delay compensation method is adopted to optimize a voltage control strategy of the doubly-fed fan, so that the problem of overvoltage of the grid-connected point caused by delay of a voltage measurement link is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of voltage control of a novel power system, and in particular relates to a low voltage ride through control method considering voltage measurement delay estimation. Background Art

[0002] The increasing proportion of wind power in the national energy and the continuous expansion of wind farms will cause fluctuations in voltage, power and frequency when wind farms are connected to the grid. Among them, the voltage fluctuation problem at the wind farm grid connection point has become one of the main problems restricting the development of the wind power industry. In order to maintain the stable operation of the power system, various countries have successively issued wind power operation specifications and require that wind turbines connected to the grid have a certain low voltage ride-through capability, so that they can maintain grid-connected operation and even provide reactive power to the grid to support grid recovery until the grid voltage returns to normal.

[0003] Since the equivalent reactance of the wind farm transmission line is generally much larger than the equivalent resistance, the regulation of the grid connection point voltage mainly depends on the wind farm's regulation of reactive power. The grid connection point voltage needs to be collected by the voltage measurement equipment of the doubly fed wind turbine and the voltage detection algorithm is used to determine whether the grid voltage amplitude is lower or higher than the preset voltage threshold during this period. The reactive power instruction of the doubly fed wind turbine is then changed to adjust the reactive power of the doubly fed wind turbine, thereby increasing reactive power to support grid voltage recovery.

[0004] Regarding the method of detecting the voltage amplitude at the grid connection point, the doubly fed wind turbine wind power system widely adopts the method of half-cycle voltage effective value detection, that is, each phase collects half a cycle of grid voltage and calculates its effective value, and then judges whether the grid connection point voltage has reached the threshold value based on the voltage effective value, thereby judging whether the doubly fed wind turbine has entered the fault ride-through state. The detection algorithm has a delay of about 10ms, that is, half a cycle, and the actual voltage signal also has a delay after passing through the voltage measurement equipment. The actual delay is the sum of the two. The voltage detection delay will cause the doubly fed wind turbine reactive power command to lag behind the actual grid voltage amplitude, and then cause the power emitted by the doubly fed wind turbine to lag behind the actual support power required by the grid, which will increase the overvoltage problem at the wind turbine grid connection point. In existing research, the low voltage ride-through control of the doubly fed wind turbine has not optimized the control strategy for the delay caused by the voltage measurement at the grid connection point, or has only considered the delay of the voltage detection algorithm for compensation without considering the delay of the voltage measurement equipment, which will lead to poor suppression of overvoltage phenomena at the grid connection point or under-compensation of the delay time. Therefore, the present invention proposes a low voltage ride-through control method that considers the voltage measurement delay estimation, and accurately estimates and compensates for the voltage detection delay for the reactive power command lag caused by the grid connection point voltage measurement delay and the transient overvoltage problem at the grid connection point caused by it, so as to optimize the reactive power control command, thereby reducing the purpose of overvoltage suppression at the grid connection point, and further improving the safety margin of stable operation of the power grid. Summary of the invention

[0005] The present invention provides a low voltage ride-through control method considering voltage measurement delay estimation, so as to solve the reactive power increase caused by the reactive power command of a doubly-fed wind power system lagging behind the actual grid-connected point voltage due to the voltage measurement delay. The present invention optimizes the voltage control strategy of the doubly-fed wind turbine by accurately estimating the actual grid-connected point voltage delay and adopting a voltage measurement delay compensation method, thereby reducing the grid-connected point overvoltage problem caused by the delay in the voltage measurement link.

[0006] To achieve the above object, the present invention provides a low voltage ride through control method considering voltage measurement delay estimation, comprising the following steps:

[0007] Step 1: Mathematically model the typical low voltage ride-through control system of the doubly fed wind turbine, establish the active and reactive output models and obtain the restriction conditions of the reactive output of the doubly fed wind turbine;

[0008] Step 2: Use the voltage transformer measuring end and oscilloscope to collect the voltage signal of the grid-connected point of the doubly-fed wind turbine before passing through the voltage measurement device in the case of a low voltage ride-through fault of the doubly-fed wind turbine, simulate the voltage signal after passing through the voltage measurement device, and accurately estimate the voltage measurement delay based on the voltage signals before and after.

[0009] Step 3: Based on the voltage measurement delay estimate obtained in step 2, a low-order delay compensation optimization control method is used to compensate for the phenomenon that the reactive power command lags behind the grid connection point voltage change caused by the voltage measurement device delay, thereby optimizing the reactive power command, preventing reactive power overgeneration, and suppressing grid connection point overvoltage.

[0010] Furthermore, the specific method of step 1 is as follows:

[0011] The typical low voltage ride-through control system of the doubly-fed wind turbine is referred to as the doubly-fed wind power system, which includes a doubly-fed wind turbine, a grid-side filter, a grid-side converter, a machine-side converter, a voltage measurement device and a reactive voltage control system thereof, wherein the voltage measurement device is a voltage measurement device for the voltage controller. The doubly-fed wind power system is mathematically modeled to obtain a low voltage ride-through control simulation model of the doubly-fed wind turbine under a typical vector control based on stator voltage orientation, and the following active and reactive output models are established:

[0012]

[0013] Among them, I rd ,I rq is the dq axis component of the rotor current, L s is the stator inductance, L m is the mutual inductance, w g is the synchronous electrical angular velocity, U sd and U sq is the dq axis component of the stator voltage, U s is the grid voltage.

[0014] At the same time, according to the limitations of the maximum rotor current and the maximum stator current, the limiting conditions of the reactive power output of the doubly fed wind turbine are obtained:

[0015]

[0016] Q≤min(Q max1 ,Q max2 )

[0017] Among them, I smax is the maximum stator current, I rmax is the maximum rotor current.

[0018] Furthermore, the specific method of step 2 is as follows:

[0019] Step (2-1): Construct a voltage signal output model suitable for the voltage measurement device of the doubly fed wind turbine grid connection point, wherein the input signal is the voltage signal of the doubly fed wind turbine grid connection point before the voltage measurement device in the case of low voltage ride-through fault of the doubly fed wind turbine collected by the voltage transformer measuring end and the oscilloscope, and this is used as the input U of the grid connection point voltage reference signal pcc_ref, the output signal is the grid-connected point voltage signal after passing through the voltage measurement equipment. Since the grid-connected point voltage is in the form of a sine wave, a three-parameter sine wave combined with a least squares fitting method is used to simulate the output of the grid-connected point voltage signal after being sampled by an analog-to-digital converter (ADC). The three-parameter fitting function is as follows:

[0020] U pcc_fit (t n )=A0cos(2πf0t n )+B0sin(2πf0t n )+C0

[0021] Among them, U pcc_fit (t n ) is the simulated voltage signal output, A0, B0, C0 are the parameters to be fitted, f0 is the nominal frequency of the power grid, t n is the time of the nth sampling point, and the fitted signal can be converted into the form of amplitude A and phase φ by the following formula:

[0022] A0cos(2πf0t)+B0sin(2πf0t)+C0=Acos(2πf0t+φ)+C0

[0023] in,

[0024]

[0025] φ=arctan(B0,A0)

[0026] Thus, the input and output signals of the grid-connected point voltage of the doubly-fed wind power system before and after passing through the voltage measuring equipment are obtained.

[0027] Step (2-2): Establish a voltage measurement equipment delay estimation strategy, by performing a cross-correlation function analysis on the input and output signals of the grid-connected point voltage of the doubly-fed wind power system obtained in step (2-1) before and after passing through the voltage measurement equipment, and using a cosine interpolation fitting method to further accurately estimate the voltage measurement equipment delay. The cross-correlation function calculation method is as follows:

[0028]

[0029] where x c (τ) is the cross-correlation function, U pcc_ref [t n ] is the grid voltage signal that has not been collected by the voltage measurement equipment, U pcc_fit (t n +τ) is the analog voltage measurement signal output after least square method and three-parameter fitting, that is, the grid-connected point voltage signal after passing through the voltage measurement equipment, and τ is the time interval of each sampling point, that is, the delay time.

[0030] By finding the cross-correlation function x c (τ) Maximum peak value, determine the horizontal coordinate τ at the peak moment max The position of the horizontal coordinate τ max The cross-correlation function value x between two adjacent positions c (τ max -1) and x c (τ max +1) uses cosine interpolation fitting to achieve accurate estimation of delay. The delay after interpolation fitting can be calculated using the following formula:

[0031]

[0032] where t d * is the estimated delay, i.e., the estimated value of the voltage measurement delay. floor(m / 2) is half of the number of sampling samples. m is the number of sampling samples. dT is the sampling rate of the voltage measurement device. α and β are intermediate variables, which are mainly used to facilitate calculation and have no practical significance.

[0033] Furthermore, the specific method of step 3 is as follows:

[0034] If the actual delay caused by voltage measurement is t d , the voltage measurement delay caused by it is If you want to completely eliminate this delay, you should add a time compensation term However, this compensation term can only compensate for microsecond delays, while the voltage measurement time is tens of milliseconds, which includes the delay caused by the voltage measurement equipment and the delay caused by the voltage detection algorithm. For the voltage detection algorithm, the half-cycle effective value detection method is widely used in actual engineering, and its delay is half a voltage cycle, which is 10 milliseconds for a 50Hz power system. Therefore, the low-order Taylor expansion method is used to expand and then compensate instead of adding the time compensation term. The expanded result is:

[0035]

[0036] Among them, y(t d ,s) is the high-order error term of Taylor expansion, s is the Laplace operator, t d is the delay that needs to be compensated.

[0037] Because the actual voltage measurement delay t d It cannot be obtained accurately, so the delay estimate t obtained in step (2-2) is used. d *As the target value for delay compensation. For reactive power command, since the signal collected from the grid connection point is discontinuous, the reactive power command Q(t) needs to be discretized. The discretized power command expression is:

[0038]

[0039] Where Q(t) is the reactive power command, and its subscript represents the reactive power command value under different sampling delays, τ s is the sampling period.

[0040] Then the optimized reactive power command Q after voltage measurement delay compensation is * ref (t) is:

[0041]

[0042] in As power command compensation after voltage measurement delay compensation link.

[0043] Considering the output reactive power restriction of the doubly-fed wind turbine in step 1, the optimized reactive power command needs to meet the following requirements:

[0044] Q * ref (t)≤min(Q max1 ,Q max2 )

[0045] Furthermore, due to the actual test results during the low voltage ride-through process, the second-order and third-order compensation delays have little difference in suppressing overvoltage at the wind turbine grid connection point. Therefore, the present invention takes into account the limited computing resources and processing speed in the actual control system, and adopts a second-order delay compensation method, that is, the third-order term and the subsequent remainder are ignored for the obtained discrete power command, and only the second-order compensation term is retained. The reactive power output command is optimized while considering the voltage measurement delay and the maximum reactive power output limit. In the low voltage ride-through control of the doubly fed wind turbine, this method can effectively suppress the reactive power over-generation and grid connection point overvoltage problems caused by the reactive power command lagging behind the grid connection point voltage change due to the voltage measurement delay.

[0046] The beneficial effects of the present invention are as follows:

[0047] The present invention proposes a low voltage ride-through control method that takes into account voltage measurement delay estimation. By modeling a typical vector control system of a doubly-fed wind power system, reactive power output restriction conditions are obtained, and a model of the voltage measurement device output is built. On this basis, the cosine interpolation fitting method is used to accurately estimate the delay of the voltage measurement device of the doubly-fed wind turbine grid-connected point voltage. After determining the accurate delay estimate, the delay is entered into a low-order delay compensation control link, thereby compensating the power command of the doubly-fed wind turbine. The improvements and advantages of the present invention over the prior art are as follows:

[0048] 1. The present invention is aimed at the low voltage ride-through optimization control of the doubly-fed wind power system, ensuring the effectiveness and applicability of the delay estimation method and the vector control method after power command optimization in low voltage ride-through faults, and has universal applicability.

[0049] 2. In the low voltage ride-through control strategy, the present invention fully considers the impact of the grid connection point voltage measurement delay on the doubly fed wind turbine vector control power command and the grid connection point voltage after fault recovery, and realizes accurate evaluation and compensation of the voltage measurement delay, thereby effectively suppressing the overvoltage phenomenon at the grid connection point and reducing the risk of wind power off-grid operation, thereby improving the safety margin of stable operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 is a flow chart of a low voltage ride through control method considering voltage measurement delay estimation provided by an embodiment of the present invention;

[0051] Figure 2 is a structural diagram of a doubly-fed wind power system provided by an embodiment of the present invention;

[0052] Figure 3 is a control block diagram of a machine-side converter of a doubly-fed wind power system provided by an embodiment of the present invention;

[0053] Figure 4 is a low-order delay compensation control block diagram provided by an embodiment of the present invention;

[0054] Figure 5 It is a schematic diagram of a cross-correlation function of a voltage signal at a grid connection point and an input and output voltage signal of a voltage measurement device in a delay estimation method for a voltage measurement device provided by an embodiment of the present invention;

[0055] Figure 6 It is a schematic diagram of an improved low-order delay compensation control considering voltage measurement delay estimation provided by an example of the present invention;

[0056] Figure 7 This is a diagram showing the effect of the improved low-order delay compensation control provided by an embodiment of the present invention on overvoltage control during low voltage ride-through; DETAILED DESCRIPTION

[0057] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0058] See also Figure 1 , Figure 1 : is a flow chart of a low voltage ride through control method considering voltage measurement delay estimation provided by an embodiment of the present invention, the voltage measurement delay estimation and low voltage ride through control method comprising:

[0059] S1. Establish a low voltage ride-through control model for a doubly-fed wind turbine to obtain its reactive power limitation conditions and collect the voltage signal at the grid connection point.

[0060] S2. Use three parameters and cosine interpolation fitting method to build a voltage measurement model and perform delay estimation.

[0061] S3, substitute the delay estimate obtained in S2 into the low-order delay compensation control link, and output the optimized reactive power command to suppress the overvoltage at the grid connection point.

[0062] The embodiment of the present invention provides a low voltage ride-through control method considering voltage measurement delay estimation. Aiming at the deficiency that the traditional low-order delay compensation link only makes a preliminary judgment on the delay of the voltage detection algorithm (half-cycle voltage effective value detection), the low voltage ride-through control model of the doubly-fed wind power system is first constructed to obtain the reactive power output restriction condition and the simulated grid connection point voltage signal, and then the voltage measurement model is constructed by combining three parameters with the least squares fitting method, and the estimation accuracy of the voltage measurement delay is improved by the cosine interpolation method, so that the low-order delay compensation link provides a more accurate reactive power instruction, thereby achieving a better overvoltage suppression effect. It has a simple structure, is easy to implement, reduces the possibility of the doubly-fed wind turbine being disconnected from the grid, and improves the safety margin and operation efficiency of the stable operation of the power grid.

[0063] The specific method is as follows:

[0064] Step S1:

[0065] For high-power double-fed wind turbines, the following are often used: Figure 2 The dual PWM converter shown in the figure is used for low voltage ride-through control, namely the rotor-side converter and the grid-side converter. This back-to-back converter can realize bidirectional power flow between the generator and the grid. During a low voltage ride-through fault, the doubly fed wind turbine can also provide reactive power to the grid through the converter to maintain the stability of the grid voltage until the grid voltage returns to normal levels.

[0066] In the present invention, the control method of low voltage ride-through adopts a typical vector control strategy based on stator voltage orientation, by orienting the stator voltage vector in the d-axis direction and ignoring the voltage drop of the generator winding, that is, U sd =U s ,U sq =0; combined with the mathematical model of the doubly fed wind turbine in the stator synchronous rotating coordinate system dq, the output active and reactive expressions of the doubly fed wind turbine can be obtained:

[0067]

[0068] Among them, I rd ,I rq is the dq axis component of the rotor current, L s is the stator inductance, L m is the mutual inductance, ω g is the synchronous electrical angular velocity, U sd and U sq is the dq axis component of the stator voltage, U s is the grid voltage.

[0069] Through the method of coordinate transformation and grid voltage orientation, it can be realized by controlling I rd and I rq The purpose of decoupling the active power and reactive power of the doubly fed wind turbine is to control the output power of the doubly fed wind turbine. rd and I rq The specific expression is:

[0070]

[0071] As for the grid-connected point voltage of the wind turbine, it is mainly affected by the output power of the doubly-fed wind turbine. Assuming that the doubly-fed wind turbine is connected to the grid through a line with impedance R+jX, and the capacity of the converter is large enough, combined with the output power P and Q of the above wind turbine, the expression of its grid-connected point voltage is:

[0072]

[0073] Among them, U pcc is the grid connection point voltage, R is the equivalent resistance, X is the equivalent reactance, U pcc0 is the initial grid connection point voltage. pcc As a simulated grid-connected point voltage output, the grid-connected point voltage signal in the actual implementation process can be acquired through the voltage transformer measuring end and the oscilloscope.

[0074] Reactive current command used to support grid voltage recovery. According to the requirements of the Technical Regulations for Wind Farm Access to Power System, when the wind turbine grid connection point Upcc When the voltage drops to 20% and 90% of the rated voltage, the doubly-fed wind turbine generator should inject a certain reactive current I into the grid. Q , the reactive current output by the grid-side converter is not considered here, because it accounts for a small proportion of the total reactive output. Then the reactive current will be provided entirely by the rotor-side converter, and then:

[0075] I Q =1.5(0.9-U pcc )I N =I rq

[0076] Among them, I N is the output current during rated operation. When the grid voltage U pcc When falling, the reference value of the rotor current of the doubly fed wind turbine generator is:

[0077]

[0078] Since the output power of the doubly fed wind turbine is affected by the maximum stator current I smax and the maximum rotor current I rmax The reactive power output of the doubly-fed wind turbine shall not exceed the maximum reactive power Q under the maximum stator current limit. max1 and the maximum reactive power Q under the maximum rotor current limit max2 The smaller value between max1 , Q max2 ), the specific expressions of the two are:

[0079]

[0080] When the voltage at the grid-connected point of the double-fed wind turbine drops to between 20% and 90%, the reactive current output command will be triggered to provide Q to the grid. max1 and Q max2 The limited reactive power is used to support the grid voltage recovery until the fault is cleared. During the voltage recovery phase after the fault is cleared, the voltage measurement delay at the grid connection point will cause the reactive current command I rq_ref Failure to withdraw in time will cause reactive power over-generation, and the absorbed reactive power will be negative. Combined with U pcc From the expression of , it can be seen that injecting reactive power will cause overvoltage problems at the grid connection point.

[0081] Step S2: Obtain the signals of the grid connection point voltage before and after the voltage measurement device under the vector control of the doubly fed wind turbine through step 1, and perform delay estimation on the two. The voltage detection algorithm adopts a half-cycle voltage effective value detection algorithm. The specific delay estimation method is as follows:

[0082] S2-1. Construct an output model for the voltage measurement equipment at the grid-connected point of the doubly-fed wind power system, hereinafter referred to as the voltage measurement model. Since the voltage at the grid-connected point of the doubly-fed wind turbine is a sinusoidal wave signal, the voltage measurement model can use the least squares method plus three-parameter fitting to simulate the voltage measurement signal output.

[0083] The three-parameter fitting formula is as follows:

[0084] U pcc_fit (t n )=A0cos(2πf0t n )+B0sin(2πf0t n )+C0

[0085] Among them U pcc_fit (t n ) is the simulated voltage signal output, A0, B0, C0 are the parameters to be fitted, f0 is the nominal frequency of the power grid, t n is the time of the nth sampling point.

[0086] Assume that the data record contains time t1, t2, t3, ..., t M M samples U pcc_ref [t1],U pcc_ref [t2],U pcc_ref [t3],…U pcc_ref [t M ] sequence, the fitting algorithm finds the values ​​of A0, B0, C0 by minimizing the sum of squares of the following differences:

[0087]

[0088] To calculate the values ​​of A0, B0, C0, first construct the matrix:

[0089]

[0090] Minimizing the sum of squared differences is calculated using the following method:

[0091] (U pcc_ref -D0s0) T (U pcc_ref -D0s0)

[0092] The least squares solution is:

[0093]

[0094] The final U pcc_fit (t n ) can be further processed into the form of grid voltage signal:

[0095] U pcc_fit (tn )=C0+Acos(2πf0t n +φ)

[0096] Where A represents the voltage amplitude and φ represents the voltage phase, which can be calculated as follows φ=arctan(A0,B0).

[0097] S2-2. Obtain the analog output signal of the voltage at the grid connection point of the doubly-fed wind turbine through the voltage measuring device through step S2-1, and convolve the two signals before and after the voltage measuring device to obtain the cross-correlation function of the two signals. The calculation method of the cross-correlation function is as follows:

[0098]

[0099] where x c (τ) is the cross-correlation function, U pcc_ref [t n ] is the grid voltage signal that has not been collected by the voltage measurement equipment, U pcc_fit (t n +τ) is the analog voltage measurement signal output after least square method and three-parameter fitting, that is, the grid-connected point voltage signal after passing through the voltage measurement equipment, and τ is the time interval of each sampling point, that is, the delay time.

[0100] By finding the cross-correlation function x c The maximum peak value of (τ) Figure 5 The peak moment abscissa τ is determined max The position of max The cross-correlation function value x between two adjacent time intervals c (τ max -1) and x c (τ max +1) uses cosine interpolation fitting to achieve accurate estimation of delay. The delay after interpolation fitting can be calculated using the following formula:

[0101]

[0102] where t d * is the estimated delay, floor(m / 2) is half of the number of samples, m is the number of samples, dT is the sampling rate of the voltage measurement device, and x c (τ max -1) and x c (τ max +1) is the cross-correlation function x c (τ) is the cross-correlation function value adjacent to the left and right of the peak value. α and β are intermediate variables and have no practical meaning.

[0103] Step S3: adopting a low-order delay compensation optimization control strategy to perform delay compensation for the overvoltage phenomenon caused by the reactive current command lagging behind the grid connection point voltage change caused by the voltage measurement delay, so as to achieve the purpose of suppressing the overvoltage.

[0104] In the vector control of the doubly-fed wind power system, the current command is closely related to the output power of the system. Specifically, the reactive current command lag caused by the voltage measurement delay at the grid connection point is expressed in the complex frequency domain by the following expression:

[0105]

[0106] Among them, I Qref represents the current command of the doubly-fed wind turbine, k p and k i They represent the proportional coefficient and integral coefficient in the power control loop respectively, ΔQ represents the power command difference of the doubly fed wind turbine due to the voltage detection delay, Q ref represents the power command value of the doubly fed wind turbine, Q represents the actual output power of the doubly fed wind turbine; t d Indicates the actual voltage measurement delay at the doubly-fed wind turbine grid connection point.

[0107] To completely eliminate the voltage measurement delay, a compensation term can be introduced into the control loop This term is equivalent to an advance translation in the time domain. However, for microsecond-level delays, simple phase shifting can be used to compensate. However, when the voltage measurement delay reaches tens of milliseconds, the direct use of It is difficult to achieve compensation. We can use the low-order Taylor expansion method to approximate the compensation function. The expansion result is:

[0108]

[0109] Among them, y(t d ,s) is the high-order error term of Taylor expansion.

[0110] according to Figure 4 The generated structure diagram first obtains the reactive power command Q(t). However, since the signal collected by the actual power grid is discontinuous, a discrete differentiator is first used to collect the reactive power command Q(t). The reactive power command collected once is Q1(t), and the reactive power command sampled twice is Q2(t). Figure 4 The actual delay time cannot be estimated completely accurately, so the estimated value of the voltage measurement delay obtained in step 2 is used. d * Substitute it into the control loop for compensation, and the discretized power command expression is:

[0111]

[0112] Then, the optimized reactive power command after voltage measurement delay compensation is Q * ref (t):

[0113]

[0114] Considering the output reactive power restriction of the doubly-fed wind turbine in step 1, the optimized reactive power command needs to meet the following requirements:

[0115]

[0116] in is the power command after the voltage measurement delay compensation link, Q(t) is the reactive power command, τ s is the sampling period.

[0117] Reactive power command obtained in step S3 As compensation according to Figure 6 In this way, the grid connection point voltage signal obtained in step 1 is first collected and simulated by an oscilloscope and a voltage measuring device respectively, and then the two signals are subjected to the delay time estimation method of step (2-1) to obtain an accurate delay time estimation value t d * , substitute the estimated delay time into the control loop, and finally get the optimized reactive power command Q ref * (t), the optimized reactive power command can better deal with the overvoltage problem at the grid connection point caused by the lag in grid connection point voltage acquisition caused by the voltage measurement delay.

[0118] The present invention proposes a low voltage ride-through control method considering voltage measurement delay estimation, aiming to solve the limitations of the traditional low-order compensation link in voltage detection delay estimation. Usually, its voltage delay only considers the delay caused by the voltage detection algorithm end. For example, the half-cycle voltage effective value (RMS) detection will cause a delay of 10ms. Since it can only obtain a rough delay estimate, this leads to poor low-order delay compensation effect. The present invention constructs a voltage measurement model by using the three-parameter least squares method, and uses cosine interpolation technology to improve the accuracy of delay estimation. This method enables the low-order compensation link to provide reactive power compensation instructions more accurately and optimize the overvoltage suppression of the wind turbine grid connection point. The control method is simple in design and easy to execute. It effectively reduces the risk of double-fed wind turbines being disconnected from the grid, enhances the stability and operation efficiency of the power grid, and provides greater protection for the safe operation of the power grid.

[0119] Example

[0120] A simulation example is used to verify the effectiveness of the low voltage ride-through control method based on voltage detection delay estimation. Table 1 shows the MATLAB / Simulink simulation parameter design.

[0121] Table 1 Simulation parameter design

[0122] parameter Numeric parameter Numeric Rated power / Mw 1.5 Stator leakage inductance / pu 0.18 Rated voltage / V 575 Rotor leakage inductance / pu 0.16 Stator resistance / pu 0.023 Rotor resistance / pu 0.016 Rated frequency / Hz 50 DC bus voltage / V 1150

[0123] According to the simulation parameters of the doubly fed wind turbine in step 1 and Table 1, the following can be obtained: Figure 2 Low voltage ride-through control model of doubly-fed wind power system and reactive power output limitation conditions and Figure 3 The rotor side voltage control model is shown in Figure 2.

[0124] According to step 2, the measurement model of the voltage signal of the doubly-fed wind turbine grid-connected point and the accurate estimated value of the voltage measurement delay can be obtained.

[0125] According to step 3, the voltage measurement delay estimation value obtained in step 2 is substituted into the low-order delay compensation control, thereby outputting a reactive power compensation instruction. Combined with the vector control system of the doubly-fed wind turbine, this method can effectively suppress the overvoltage problem at the grid connection point. Figure 7 The comparison results before and after the introduction of the delay estimation compensation strategy are shown. If the delay evaluation compensation strategy is not added, that is, only the 10ms delay caused by the voltage half-cycle effective value detection algorithm is preliminarily considered for compensation, the peak value of the grid connection point voltage is 1.112pu. After the delay estimation compensation strategy is added, the peak value of the grid connection point voltage drops by 2.6% to 1.082pu. The simulation results show that this control method can effectively suppress the overvoltage problem caused by insufficient delay estimation accuracy, and also improve the safety margin of the doubly fed wind turbine generator, ensuring its stable operation in the face of low voltage ride-through faults.

[0126] The above contents are further detailed descriptions of the present invention in combination with specific / preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, they can also make several substitutions or modifications to these described embodiments without departing from the concept of the present invention, and these substitutions or modifications should be regarded as belonging to the protection scope of the present invention.

[0127] Parts of the present invention that are not described in detail belong to the well-known technologies of those skilled in the art.

Claims

1. A low voltage ride through control method considering voltage measurement delay estimation, characterized in that: The steps include: Step 1: Mathematically model the typical low voltage ride-through control system of the doubly fed wind turbine, establish the active and reactive output models and obtain the restriction conditions of the reactive output of the doubly fed wind turbine; Step 2: Use the voltage transformer measuring end and oscilloscope to collect the voltage signal of the grid-connected point of the doubly-fed wind turbine before passing through the voltage measurement device in the case of a low voltage ride-through fault of the doubly-fed wind turbine, simulate the voltage signal after passing through the voltage measurement device, and accurately estimate the voltage measurement delay based on the voltage signals before and after; Step 3: Based on the voltage measurement delay estimate obtained in step 2, a low-order delay compensation optimization control method is used to compensate for the phenomenon that the reactive power command lags behind the grid connection point voltage change caused by the voltage measurement device delay, thereby optimizing the reactive power command, preventing reactive power overgeneration, and suppressing grid connection point overvoltage.

2. A low voltage ride through control method considering voltage measurement delay estimation according to claim 1, characterized in that: Step 1: The typical low voltage ride-through control system of the doubly-fed wind turbine is referred to as the doubly-fed wind power system, which includes a doubly-fed wind turbine, a grid-side filter, a grid-side converter, a machine-side converter, a voltage measurement device and a reactive voltage control system thereof, wherein the voltage measurement device is a voltage measurement device for the voltage controller end; mathematical modeling is performed on the doubly-fed wind power system to obtain a low voltage ride-through control simulation model of the doubly-fed wind turbine under typical vector control based on stator voltage orientation, and the following active and reactive output models are established: Among them, I rd ,I rq is the dq axis component of the rotor current, L s is the stator inductance, L m is the mutual inductance, w g is the synchronous electrical angular velocity, U sd and U sq is the dq axis component of the stator voltage, U s is the grid voltage; At the same time, according to the limitations of the maximum rotor current and the maximum stator current, the limiting conditions of the reactive power output of the doubly fed wind turbine are obtained: Q≤min(Q max1 ,Q max2 ) Among them, I smax is the maximum stator current, I rmax is the maximum rotor current.

3. A low voltage ride through control method considering voltage measurement delay estimation according to claim 1 or 2, characterized in that: Step 2: Step (2-1): Construct a voltage signal output model suitable for the voltage measurement device of the doubly fed wind turbine grid connection point, wherein the input signal is the voltage signal of the doubly fed wind turbine grid connection point before the voltage measurement device in the case of low voltage ride-through fault of the doubly fed wind turbine collected by the voltage transformer measuring end and the oscilloscope, and this is used as the input U of the grid connection point voltage reference signal pcc_ref , the output signal is the grid-connected point voltage signal after passing through the voltage measurement equipment. Since the grid-connected point voltage is in the form of a sine wave, a three-parameter sine wave combined with a least squares fitting method is used to simulate the output of the grid-connected point voltage signal after being sampled by an analog-to-digital converter (ADC). The three-parameter fitting function is as follows: U pcc_fit (t n )=A0cos(2πf0t n )+B0sin(2πf0t n )+C0 Among them, U pcc_fit (t n ) is the simulated voltage signal output, A0, B0, C0 are the parameters to be fitted, f0 is the nominal frequency of the power grid, t n is the time of the nth sampling point, and the fitted signal can be converted into the form of amplitude A and phase φ by the following formula: A0cos(2πf0t)+B0sin(2πf0t)+C0=A cos(2πf0t+φ)+C0 in, φ=arctan(B0,A0) Thus, the input and output signals of the grid connection point voltage of the double-fed wind power system before and after passing through the voltage measurement equipment are obtained; Step (2-2): Establish a voltage measurement equipment delay estimation strategy, by performing a cross-correlation function analysis on the input and output signals of the grid-connected point voltage of the doubly-fed wind power system obtained in step (2-1) before and after passing through the voltage measurement equipment, and using a cosine interpolation fitting method to further accurately estimate the voltage measurement equipment delay. The cross-correlation function calculation method is as follows: where x c (τ) is the cross-correlation function, Upc c _ ref [t n ] is the grid voltage signal that has not been collected by the voltage measurement equipment, U pcc_fit (t n +τ) is the analog voltage measurement signal output after least square method and three-parameter fitting, that is, the grid-connected point voltage signal after passing through the voltage measurement equipment, τ is the time interval of each sampling point, that is, the delay time; By finding the cross-correlation function x c (τ) Maximum peak value, determine the horizontal coordinate τ at the peak moment max The position of the horizontal coordinate τ max The cross-correlation function value x between two adjacent positions c (τ max -1) and x c (τ max +1) uses cosine interpolation fitting to achieve accurate estimation of delay. The delay after interpolation fitting can be calculated using the following formula: where t d * is the estimated delay, i.e., the estimated value of the voltage measurement delay, floor(m / 2) is half of the number of sampling samples, m is the number of sampling samples, dT is the sampling rate of the voltage measurement device, and α and β are intermediate variables.

4. The low voltage ride through control method considering voltage measurement delay estimation according to claim 3, characterized in that: Step 3: If the actual delay caused by voltage measurement is t d , the voltage measurement delay caused by it is If you want to completely eliminate this delay, you should add a time compensation term However, this compensation term can only compensate for microsecond delays, while the voltage measurement time is tens of milliseconds. Therefore, a low-order Taylor expansion method is used to expand the delay and then compensate instead of adding a time compensation term. The expanded result is: Among them, y(t d ,s) is the high-order error term of Taylor expansion, s is the Laplace operator, t d The delay that needs to be compensated; Because the actual voltage measurement delay t d It cannot be obtained accurately, so the delay estimate t obtained in step (2-2) is used. d * As the target value of delay compensation; for reactive power command, since the signal collected from the grid connection point is discontinuous, the reactive power command Q(t) needs to be discretized, and the discretized power command expression is: Where Q(t) is the reactive power command, and its subscript represents the reactive power command value under different sampling delays, τ s is the sampling period; Then the optimized reactive power command Q after voltage measurement delay compensation is * ref (t) is: in As power command compensation after voltage measurement delay compensation link; Considering the output reactive power restriction of the doubly-fed wind turbine in step 1, the optimized reactive power command needs to meet the following requirements: Q * ref (t)≤min(Q max1 ,Q max2 )。 5. A low voltage ride through control method considering voltage measurement delay estimation according to claim 4, characterized in that: Taking into account the limited computing resources and processing speed in the actual control system, a second-order delay compensation method is adopted. That is, the third-order term and the subsequent remainder are ignored for the obtained discretized power instruction, and only the second-order compensation term is retained. The reactive power output instruction is optimized while considering the voltage measurement delay and the maximum reactive power output limit.

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