Low-voltage ride-through control response acceleration method for wind-solar power supply of weak power grid

Abstract

The invention provides a low-voltage ride-through control response acceleration method for a wind-light power supply of a weak power grid. The method comprises the following steps: when the voltage of a grid-connected point meets a starting condition of a low-voltage ride-through mode, acquiring a voltage phase value of a first grid-connected point, namely a phase-locked loop voltage phase value extracted by a phase-locked loop and calculated by a current control loop; obtaining a second grid-connected point voltage phase value according to the grid-connected point voltage and the grid-connected point current, and obtaining a transient phase-locking deviation value by subtracting the first grid-connected point voltage phase value from the second grid-connected point voltage phase value; and judging whether to start a compensation control mode or not based on the output current instruction value and the transient phase-locking deviation value, and performing low-voltage ride through control according to the compensation control mode when the compensation control mode is started. The method is high in calculation speed and low in application cost, the problems that the weak power grid new energy power supply low-voltage ride-through power response speed is low, and the voltage supporting capacity is insufficient within tens of milliseconds in the fault transient state are effectively solved, and accurate and efficient compensation of the transient deviation of the phase-locked loop is achieved by optimizing the output current instruction calculation process of the low-voltage ride-through module.

Description

Technical Field

[0001] The present invention relates to the technical field of automation of power transmission systems, and particularly to a method for accelerating the low-voltage ride-through control response of wind and solar power sources in a weak power grid. Background Art

[0003] In a weak grid scenario, when a short-circuit fault occurs and is cleared, the amplitude and phase of the grid voltage fluctuate violently for a long time. A large number of weak grid fault recording data show that in order to maintain stable synchronization in a weak grid, the existing new energy power generation units often set the bandwidth of the phase-locked loop (PLL) below 20 Hz, resulting in a significant transient phase-locked deviation during low-voltage ride-through (hereinafter referred to as "LVRT"), slow response of the output active and reactive power, and it is difficult to achieve effective voltage support within dozens of milliseconds of the fault transient. At the same time, the current standard only focuses on the current response speed of LVRT, and the existing research in the field of LVRT also often only focuses on the improvement of voltage support ability, lacking clear specifications and in-depth research on the response speed of LVRT active and reactive power, further restricting the fault disturbance adaptation ability of new energy in a weak grid. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the present invention provides a method for accelerating the low-voltage ride-through control response of wind and solar power sources in a weak power grid, which has a fast calculation speed, low application cost, and effectively solves the problems of slow low-voltage ride-through power response speed of new energy power sources in a weak power grid and insufficient voltage support ability within dozens of milliseconds of the fault transient. By optimizing the calculation process of the output current command of the low-voltage ride-through module, precise and efficient compensation of the transient deviation of the phase-locked loop PLL is achieved.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for accelerating the low-voltage ride-through control response of wind and solar power sources in a weak power grid, comprising the following steps: When the grid connection point voltage meets the low-voltage ride-through mode startup condition:

[0006] Obtain the grid connection point voltage, grid connection point current, output current command value, and the first grid connection point voltage phase value respectively, where the first grid connection point voltage phase value is the phase-locked loop voltage phase value obtained by the current control loop extracted by the phase-locked loop;

[0007] Obtain the second grid connection point voltage phase value according to the grid connection point voltage and the grid connection point current;

[0008] Obtain the transient phase-locked deviation amount by taking the difference between the first grid connection point voltage phase value and the second grid connection point voltage phase value;

[0009] Judge whether to start the compensation control mode based on the output current command value and the transient phase-locked deviation amount:

[0010] When the compensation control mode is started, low voltage ride-through control is performed according to the compensation control mode;

[0011] When the compensation control mode is not started, low voltage ride-through control is performed according to the low voltage ride-through mode.

[0012] In some embodiments, the output current command value includes a d-axis output current command value and a q-axis output current command value;

[0013] The steps for obtaining the output current command value are as follows:

[0014] Obtain the effective value of the grid-connected point voltage, the reactive power support coefficient, and the maximum current limiting value;

[0015] Obtain the q-axis output current command value according to the effective value of the grid-connected point voltage and the reactive power support coefficient;

[0016] Obtain the d-axis output current command value according to the maximum current limiting value and the q-axis output current command value.

[0017] In some embodiments, the steps for obtaining the second grid-connected point voltage phase value according to the grid-connected point voltage and the grid-connected point current are as follows:

[0018] Perform Clark transformation on the grid-connected point voltage and the grid-connected point current respectively to obtain the α-axis grid-connected point voltage, the β-axis grid-connected point voltage, the α-axis grid-connected point current, and the β-axis grid-connected point current;

[0019] Perform Park transformation and sequence component separation control on the α-axis grid-connected point current and the β-axis grid-connected point current respectively to obtain the d-axis grid-connected point positive sequence current, the q-axis grid-connected point positive sequence current, the d-axis grid-connected point negative sequence current, and the q-axis grid-connected point negative sequence current;

[0020] Obtain the first norm value of the grid-connected point voltage according to the α-axis grid-connected point voltage and the β-axis grid-connected point voltage;

[0021] Substitute the first norm value of the grid-connected point voltage into the fourth-order DPR algorithm to obtain the second grid-connected point voltage phase value.

[0022] In some embodiments, the steps for determining whether to start the compensation control mode based on the output current command value and the transient phase-locked deviation amount are as follows: construct a compensation control criterion;

[0023] Judge whether the compensation control criterion is established according to the output current command value and the transient phase-locked deviation amount;

[0024] When the compensation control criterion is established, start the compensation control mode;

[0025] When the compensation control criterion is not established, do not start the compensation control mode.

[0026] In some of these embodiments, the compensation control criterion includes a first compensation control criterion and a second compensation control criterion. The first compensation control criterion is the compensation control criterion during the fault occurrence stage, and the second compensation control criterion is the compensation control criterion during the fault clearing stage.

[0027] In some of these embodiments, when the weak grid wind-solar power generation unit is not equipped with an online impedance estimator:

[0028] The first compensation control criterion is:

[0029]

[0030] The second compensation control criterion is:

[0031]

[0032] Where Δθ is the transient phase-locked deviation, is the d-axis output current command value, is the q-axis output current command value.

[0033] In some of these embodiments, when the weak grid wind-solar power generation unit is equipped with an online impedance estimator:

[0034] The first compensation control criterion is:

[0035]

[0036] The second compensation control criterion is:

[0037]

[0038] Where is the d-axis output current command value, is the q-axis output current command value, R g is the equivalent resistance on the grid side, X g is the equivalent reactance on the grid side, and Δθ is the transient phase-locked deviation.

[0039] In some of these embodiments, the steps of low voltage ride-through control according to the compensation control mode are:

[0040] When the compensation control criterion is satisfied, the updated output current command value is obtained according to the compensation control operation formula; the compensation control operation formula is:

[0041]

[0042] Where is the d-axis output current command value, is the q-axis output current command value, Δθ is the transient phase-locked deviation, and t is the current time;

[0043] Input the updated output current command value into the current inner loop of the grid-connected controller for low voltage ride-through control.

[0044] In some embodiments, set the initial value of the compensation control flag bit to 1. When the grid connection point voltage meets the low voltage ride-through mode start condition, the value of the low voltage ride-through flag bit is 1;

[0045] Perform an OR logic operation on the compensation control flag bit and the low voltage ride-through flag bit;

[0046] When the result of the OR logic operation is 1, perform low voltage ride-through control using the low voltage ride-through control response acceleration method for the weak grid wind-solar power source.

[0047] In some embodiments, after the value of the low voltage ride-through flag bit becomes 0, after a preset delay, the value of the compensation control flag bit becomes 0.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] 1. By optimizing the calculation process of the output current command of the low voltage ride-through module, the present invention realizes the accurate and efficient compensation of the transient deviation of the phase-locked loop PLL.

[0050] 2. During the compensation process of the low voltage ride-through output current command, the present invention combines the high-order DPR fast deviation quantity acquisition algorithm and the phased dynamic optimization compensation strategy to shorten the low voltage ride-through response time of the weak grid new energy power generation unit to within 35 ms, and the response speed is increased by more than 40% compared with the existing method, effectively solving the problems of slow low voltage ride-through power response speed and insufficient voltage support ability within dozens of milliseconds of the fault transient of the weak grid new energy power source.

[0051] 3. The present invention has the advantages of fast calculation speed, low application cost, high program portability, etc., and has high practical application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 is a flow schematic diagram of a method for accelerating the low voltage ride-through control response of a weak grid wind-solar power source according to the present invention;

[0053] Figure 2 is a fault topology loop diagram of a new energy power generation unit accessing a weak grid in an embodiment of the present invention;

[0054] Figure 3 is a geometric principle schematic diagram of the high-order DPR algorithm in an embodiment of the present invention;

[0055] FIG. 4(a) shows the influence of the leading deviation on the output current component of the new energy unit in the embodiment of the present invention, and FIG. 4(b) shows the influence of the lagging deviation on the output current component of the new energy unit;

[0056] Figure 5 is the sectional compensation control principle in the fault transient stage in the embodiment of the present invention;

[0057] Figure 6 is the transient fast low voltage ride-through control flow chart proposed in the embodiment of the present invention;

[0058] Figure 7 is the comparison of the effective value waveforms of the interconnection point voltage under three control methods in the embodiment of the present invention;

[0059] Figure 8 is the comparison of the speed advantages of the phased dynamic compensation control strategy in the embodiment of the present invention. Detailed implementation manners

[0060] Aiming at the problems in the prior art, the existing methods for improving the power response speed under disturbances can be divided into three categories: optimization and improvement methods, new control methods, and compensation control methods. The optimization and improvement methods improve the response speed by optimizing the PLL bandwidth design without changing the overall control structure. However, relying solely on parameter adjustment means to improve the response speed has limited effects and may cause problems such as a decrease in stability. The new control methods introduce more complex synchronization control technologies, but usually require re-designing the overall structure and control parameters of the inverter, with high complexity and large implementation costs, making it difficult to be widely applied to the transformation and upgrading of existing equipment. The compensation control methods add a feed-forward or compensation branch inside the PLL. Compared with the first two types of methods, it not only effectively improves the response speed but also ensures good engineering practicability. However, the existing compensation control ideas usually use a sliding window algorithm to extract the transient deviation amount, which has an inherent delay, and only adopts a static compensation strategy without targeted dynamic optimization design for the fault disturbance scenario, which may lead to problems such as a decrease in voltage support ability during the fault transient period and transient overvoltage during the fault clearing period, and it is difficult to fully exploit the potential of accelerating the low voltage ride-through response.

[0061] Therefore, in order to improve the low voltage ride-through response speed of weak grids, the existing methods all need to pay a certain price, including significantly changing the control structure or sacrificing the fault voltage support ability. Even so, the existing methods still fail to fully exploit the low voltage ride-through response speed potential of new energy power sources in weak grids, and there is an urgent need to study a low-cost and high-performance low voltage ride-through control strategy.

[0062] To clearly illustrate the technical features of this solution, the following will describe in detail the implementation manner of this application in combination with the accompanying drawings and embodiments, so as to fully understand how this application uses technical means to solve technical problems and the implementation process of achieving corresponding technical effects and implement accordingly. Each feature in the embodiments of this application and in the embodiments can be combined with each other on the premise of not conflicting, and the formed technical solutions are all within the protection scope of this application.

[0063] In the low-voltage ride-through control response acceleration method for weak-grid wind-solar power sources proposed in this disclosure, the weak-grid wind-solar power generation unit adopts a grid-following control strategy. The low-voltage ride-through control during a fault includes two tasks: First, increase reactive power as soon as possible and reduce active power after the fault occurs to support the system voltage; Second, restore to the active power output before the fault as soon as possible after the fault is cleared. Taking a single wind-solar power generation unit connected to a weak grid as an example, the fault topological loop is as Figure 2 shown. In the figure, U g represents the equivalent voltage on the grid side, I g represents the equivalent current on the grid side, Z g represents the equivalent impedance on the grid side, I f represents the short-circuit current at the fault point, R f represents the transition resistance at the fault point, U o represents the voltage at the point of common coupling, I o represents the current at the point of common coupling, and PCC represents the point of common coupling.

[0064] Referring to Figure 1 , the embodiments of this disclosure provide a low-voltage ride-through control response acceleration method for weak-grid wind-solar power sources, including the following steps: When the voltage at the point of common coupling meets the start condition of the low-voltage ride-through mode, start the low-voltage ride-through mode:

[0065] Before the fault occurs, the wind-solar power generation unit usually adopts maximum power point tracking control and only outputs active power. Therefore, the phase of the voltage at the point of common coupling is slightly ahead of the phase of the grid voltage, which can be expressed as:

[0066]

[0067] In the formula, is the phase of the equivalent voltage at the point of common coupling before the fault, θ g is the phase of the equivalent voltage on the grid side, is the active power output before the fault, is the voltage at the point of common coupling before the fault;

[0068] When Figure 2 a fault occurs at any position, a dq coordinate system is established with the equivalent voltage U g on the grid side as the reference, and the transient topological equation is:

[0069]

[0070] Wherein, U gd is the d-axis component of the equivalent voltage on the grid side, U od is the d-axis component of the grid-connected point voltage, U oq is the q-axis component of the grid-connected point voltage, L g1 is the inductance on the near power source side of the fault point, L g2 is the inductance on the far power source side of the fault point, I gd is the d-axis component of the equivalent current on the grid side, I gq is the q-axis component of the equivalent current on the grid side, R g1 is the resistance on the near power source side of the fault point, R g2 is the resistance on the far power source side of the fault point, I od is the d-axis component of the grid-connected point current, I oq is the q-axis component of the grid-connected point current, ω is the angle corresponding to the power frequency of the system, K is the reactive power support coefficient, and t is time;

[0071] In a strong grid scenario, the resistance and inductance parameters in the formula are very small, the short-circuit current transient process is short, and the phase of the grid-connected point voltage approximately jumps stepwise to the following state instantaneously when a fault occurs:

[0072]

[0073] Wherein, Δθ o is the phase jump difference of the grid-connected point voltage phasor before and after the fault occurs, Z g2 is the equivalent impedance on the side of the fault point far from the power source, is the phase angle of the equivalent impedance on the side of the fault point far from the power source; due to the small d-axis component I od of the grid-connected point current during the fault, the third term in formula (4) can be ignored. Combining formulas (2) and (3), the phase of the grid-connected point voltage lags behind the grid voltage phase when a fault occurs, and leads when the fault is cleared.

[0074] In a weak grid scenario, the resistance and inductance parameters in the transient topology equation cannot be ignored. The amplitude and phase fluctuations of the grid-connected point voltage cannot be regarded as stepwise jumps, and there is an obvious transient process. In formula (2), due to the existence of the fault point transition resistance R f , the d-axis component I od of the grid-connected point current, the q-axis component I oq of the grid-connected point current and the d-axis component I gd of the equivalent current on the grid side, the q-axis component I gq of the equivalent current on the grid side have non-linear implicit dependencies on each other, and a completely explicit analytical solution cannot be derived. However, this disclosure only focuses on the trend of grid strength and transient deviation. Therefore, taking a fault occurring at the outlet near the power source as an example for analysis, the conclusions obtained are also applicable to other fault locations. The variable d-axis component I of the grid-connected point currentod The q-axis component I of the current at the grid connection point oq Substituting it as an input condition parameter, the transient expressions of the dq currents on the grid side can be solved as follows:

[0075]

[0076]

[0077] In the formula, is the d-axis component of the current flowing through the fault point during the steady state, is the q-axis component of the current flowing through the fault point during the steady state, α is the first intermediate expression, α = (R g +R f ) / L g , Δ d is the difference between the d-axis components of the current flowing through the fault point at the moment of fault occurrence and during the fault steady state, Δ q is the difference between the q-axis components of the current flowing through the fault point at the moment of fault occurrence and during the fault steady state, is the d-axis component of the current flowing through the fault point at the moment of fault occurrence, is the q-axis component of the current flowing through the fault point at the moment of fault occurrence, L g is the equivalent inductance on the grid side;

[0078] The transient expression of the phase jump of the grid connection point voltage obtained from Equation (6) is:

[0079]

[0080] It can be seen from Equation (7) that the exponential term e -αt determines the transient duration of the phase jump of the grid connection point voltage. The degree of fault voltage drop, i.e., the d-axis component U gd of the equivalent voltage on the grid side and the output current of the new energy source, i.e., the d-axis component I od of the current at the grid connection point and the q-axis component I oq of the current at the grid connection point jointly determine the transient swing oscillation amplitude of the grid connection point voltage phase. The weaker the grid strength, the more severe the degree of fault voltage drop, and the larger the q-axis component of the new energy output current, the longer the transient process of the grid connection point voltage phase and the more significant the swing oscillation amplitude. In engineering, a synchronous reference frame phase-locked loop PLL is widely used in new energy power sources to maintain synchronization under disturbances, and the control transfer function is:

[0081]

[0082] In the formula, k p is the proportional coefficient of the SRF-PLL, k i is the integral coefficient of the SRF-PLL, and s is the Laplace transform operator;

[0083] Assume that Δθ of is the step jump generated by the phase of the grid connection point voltage, then the transient phase expression of the phase-locked loop PLL can be solved from Equation (8):

[0084]

[0085] In the formula, C is the second intermediate expression,

[0086] The exponential term in Equation (9) shows that there is a delay in the control response of the phase-locked loop PLL, and the lower the control bandwidth, that is, the smaller the proportional coefficient k of the SRF-PLL p the more significant the response delay. From the above formula, it can be seen that when a weak grid fault occurs and is cleared, it will cause large and persistent swings in the phase of the grid connection point voltage. Due to the poor response tracking speed of the low-bandwidth phase-locked loop PLL, when the swing time constant of the grid connection point voltage phase is less than the response time constant of the phase-locked loop PLL, the control coordinate system of the phase-locked loop PLL will be difficult to track the system phase for a long time. The dq-axis output current command values of the new energy generation unit during low voltage ride-through are not equal to the actual output active and reactive current components, resulting in a slow response of the low ride-through power.

[0087] In summary, it is necessary to determine the start and block of the compensation control before performing the compensation control mode. Therefore, an "OR" logical operation is performed on the two flag bits, the compensation control flag bit LV de and the low voltage ride-through flag bit LV. If the result is 1, the compensation control mode proposed in this disclosure is started; otherwise, the traditional low voltage ride-through control mode is adopted.

[0088] Obtain the grid connection point voltage, grid connection point current, output current command value, and the first grid connection point voltage phase value respectively. The first grid connection point voltage phase value is the phase-locked loop voltage phase value calculated by the current control loop extracted by the phase-locked loop;

[0089] Obtain the second grid connection point voltage phase value according to the grid connection point voltage and the grid connection point current;

[0090] When a fault occurs and is cleared, the initial phase of the grid connection point voltage is unknown and may be accompanied by frequency fluctuations. The traditional sliding window accumulation algorithm often requires a certain data window to extract phasors, so there is an inherent response delay. To overcome this limitation, a high-order DPR algorithm is introduced. The core idea of this algorithm is to use the linear combination of base phasors to analyze the instantaneous phase in the one-norm space, without accumulating the data of the time window, and can realize the dynamic phasor phase calculation with almost zero delay for both amplitude and frequency fluctuations.

[0091] In some of these embodiments, the step of obtaining the second grid connection point voltage phase value according to the grid connection point voltage and the grid connection point current is:

[0092] The grid connection point voltage and the grid connection point current are respectively subjected to Clark transformation to obtain the grid connection point voltage v on the α-axis α 、the grid connection point voltage v on the β-axis β 、the grid connection point current i on the α-axis α and the grid connection point current i on the β-axis β ;

[0093] The grid connection point current i on the α-axis α and the grid connection point current i on the β-axis β are respectively subjected to Park transformation and sequence component separation control to obtain the positive sequence current of the grid connection point on the d-axis the positive sequence current of the grid connection point on the q-axis the negative sequence current I of the grid connection point on the d-axis d - and the negative sequence current of the grid connection point on the q-axis

[0094] According to the grid connection point voltage v on the α-axis α and the grid connection point voltage v on the β-axis β the one-norm value of the grid connection point voltage is obtained; the one-norm value of the grid connection point voltage is:

[0095]

[0096] In the formula, ||v||1 is the one-norm value of the grid connection point voltage, v α1 is the one-norm value of the grid connection point voltage on the α-axis, v β1 is the one-norm value of the grid connection point voltage on the β-axis;

[0097] Introduce the one-norm calculation in mathematics, perform one-norm operation on the three-phase voltage of the grid connection point collected, that is, the αβ-axis components of the grid connection point voltage, so as to transform the voltage variable into the norm domain, which is convenient for quickly and without delay extracting the transient phase of the phasor in it;

[0098] In the first quadrant of the one-norm space, different phases correspond to unique mapping coordinates v β1 , and the phase of the grid connection point voltage can be approximately represented by the ordinate, as shown in Figure 3 the left side. Unfold and straighten the edge of the closed one-norm space, and the first-order δ space and the first-order DPR algorithm can be obtained, as shown in Figure 3 the right side. On this basis, using the periodicity and odd symmetry of the error function of the first-order DPR algorithm, a base pair can be constructed near π / 8, and further extended to a fourth-order DPR algorithm through the linear combination of multiple groups of base pairs. Substitute the one-norm value of the grid connection point voltage into the fourth-order DPR algorithm to obtain the second grid connection point voltage phase value: the algorithm process is:

[0099]

[0100]

[0101] Wherein, K1 is the first intermediate variable, K2 is the second intermediate variable, v α1.s is the third intermediate variable, v β1.s is the fourth intermediate variable, Bias is the fifth intermediate variable, θ o is the voltage phase value of the second grid connection point, that is, θ o is the transient voltage phase value of the grid connection point extracted by the fourth-order DPR algorithm;

[0102] A high-order DPR algorithm is introduced to calculate the transient phase information without delay. Its core principle is to split the dynamic phasor in the one-norm space into a linear combination of a series of basis vectors, and realize the extraction of transient phase information with zero transient lag and continuous phase resolution through the high-order linear superposition of the basis vectors, which is especially suitable for scenarios that require fast vector / phasor measurement. Compared with the transient deviation extraction algorithm used in the existing algorithms, the extraction speed of the algorithm used in the present invention is reduced from the original 15 - 20 ms to within 3 ms, providing a basis for fast low-voltage ride-through control.

[0103] According to the voltage phase value θ of the first grid connection point PLL and the voltage phase value θ of the second grid connection point o the transient phase-locked deviation Δθ is obtained by taking the difference; the voltage phase value θ of the first grid connection point PLL is the voltage phase value of the grid connection point calculated by the current control loop extracted from the phase-locked loop;

[0104] Δθ(t) = θ o (t) - θ PLL (t);

[0105] Based on the output current command value and the transient phase-locked deviation, it is judged whether to start the compensation control mode:

[0106] In some of the embodiments, the output current command value includes the d-axis output current command value and the q-axis output current command value The output current command value for low-voltage ride-through is calculated according to the voltage dip at the grid connection point, and the calculation method refers to the specifications for low-voltage ride-through of wind-solar power plants in national standards GB / T 19963-2021 and GB / T 19964-2024:

[0107] The steps to obtain the output current command value are as follows:

[0108] Obtain the effective value U of the grid connection point voltage of 、the reactive power support coefficient K and the maximum current limit value I lim ;

[0109] According to the effective value U of the grid connection point voltage of and the reactive power support coefficient K, the q-axis output current command value is obtained

[0110]

[0111] According to the maximum current limiting value I lim and the q-axis output current command value obtain the d-axis output current command value

[0112]

[0113] The output current of the new energy power source will generate a voltage drop on the line impedance, thereby raising the grid connection point voltage. The expression for this process is:

[0114]

[0115] In the formula, I P is, I Q is;

[0116] On the geometric plane, the influence of the transient phase-locked loop deviation on the actual output current component of the new energy power generation unit is shown in Figures 4(a) and (b). When the phase-locked loop PLL phase is ahead of the grid connection point voltage phase, the transient phase-locked loop deviation will increase the actual output active component and decrease the reactive component. Considering that the reactance in the system is usually much larger than the resistance, this usually reduces the transient voltage rise effect. Conversely, when it is lagging, it will reduce the active component and increase the reactive component, thereby strengthening the transient voltage rise effect. Further deriving the line voltage drop equation, the expression for the grid connection point voltage rise effect considering the influence of the transient phase-locked loop deviation is as follows:

[0117]

[0118] Considering that the square root term in the above formula usually has a small influence on the grid connection point voltage amplitude, this part is ignored, and the expression for the difference between the actual amplitude of the voltage rise and the expected amplitude of the control with respect to Δθ is obtained:

[0119]

[0120] It can be seen from the above formula that different Δθ values will affect the positive and negative of ΔU o During the fault occurrence stage and the fault clearing stage, the positive and negative effects of Δθ on U o can be judged in real time, so as to input the compensation control in segments. When Δθ has a positive effect, the compensation control mode should be exited. Conversely, when Δθ has a negative effect, the compensation control mode should be input. Taking the fault occurrence stage as an example, the phasor relationship in the above is as Figure 5As shown, under the control objective of maximizing the boost of the grid connection point voltage, there is an optimal deviation angle, and its value depends on the ratio of the grid-side equivalent impedance. Taking the optimal deflection angle as the axis of symmetry, the "compensation area" and the "non-compensation area" can be divided. When the new energy power generation unit operates in the "non-compensation area", the transient phase-locked deviation helps to improve the low-voltage ride-through response speed, and the compensation control mode should be exited. When operating in the "compensation area", the transient phase-locked deviation will slow down the low-voltage ride-through response speed, and the compensation control mode should be engaged.

[0121] Similarly, in order to avoid overvoltage at the grid connection point during the fault clearing stage, ΔU in the above formula o is minimized, so that by partitioning and inputting the compensation control for the "compensation area" and "non-compensation area" corresponding to the fault clearing stage, the active power recovery speed can be accelerated without causing transient overvoltage.

[0122] In some embodiments, the step of determining whether to start the compensation control mode based on the output current command value and the transient phase-locked deviation amount is: constructing a compensation control criterion;

[0123] In some embodiments, the compensation control criterion includes a first compensation control criterion and a second compensation control criterion. The first compensation control criterion is the compensation control criterion during the fault occurrence stage, and the second compensation control criterion is the compensation control criterion during the fault clearing stage.

[0124] According to two scenarios of whether an online impedance estimator is installed in the project, the corresponding adaptation modes of the present invention are respectively proposed:

[0125] When the weak grid wind-solar power generation unit is not equipped with an online impedance estimator:

[0126] The first compensation control criterion is:

[0127]

[0128] The second compensation control criterion is:

[0129]

[0130] In the formula, Δθ is the transient phase-locked deviation amount, is the d-axis output current command value, is the q-axis output current command value.

[0131] In some embodiments, when the weak grid wind-solar power generation unit is equipped with an online impedance estimator:

[0132] The first compensation control criterion is:

[0133]

[0134] The second compensation control criterion is:

[0135]

[0136] Wherein, is the d-axis output current command value, is the q-axis output current command value, R g is the equivalent resistance on the grid side, X g is the equivalent reactance on the grid side, and Δθ is the transient phase-locked deviation.

[0137] Such as Figure 6 The fast low-voltage ride-through control flow chart shown. When the new energy power generation unit enters the low-voltage ride-through state according to the national standard requirements, the control method proposed in this disclosure is started. During the fault occurrence stage, it is detected in real time whether the first compensation control criterion is satisfied. If so, the low-voltage ride-through output current command value is compensated. If not, no compensation control measures are taken. After the fault clearing stage, in order to prevent the transient overvoltage caused by the rapid recovery of active power in the previous few cycles, within the first 60 ms after exiting the low-voltage ride-through state, the active power command is directly switched to the value before the fault without delay, and the proposed compensation control remains in the startup state. It is detected in real time whether the second compensation control criterion is satisfied, and the compensation control is also input or exited according to the above logic, that is, if so, the low-voltage ride-through output current command value is compensated. If not, no compensation control measures are taken.

[0138] This disclosure first proposes a low-ride control strategy with segmented instruction compensation and defines the operation criterion for segmented input and output of compensation. When the first compensation control criterion is satisfied during the fault occurrence stage or the second compensation control criterion is satisfied during the fault clearing stage, the compensation control mode is input, otherwise the compensation control mode is exited. The segmented dynamic compensation control strategy proposed by the present invention can maximize the low-ride response speed of new energy power sources without sacrificing voltage support ability and avoiding transient overvoltage compared with the traditional static compensation control strategy.

[0139] It should be noted that when the weak grid wind-solar power generation unit is equipped with an online impedance estimator, the equivalent positive-sequence voltage on the grid side is calculated once after entering the low-voltage ride-through state The equivalent negative-sequence voltage on the grid side And the equivalent impedance R g +jL g , L g is the equivalent inductance on the grid side. If the weak grid wind-solar power generation unit is not equipped with an online impedance estimator, this step operation does not need to be performed.

[0140] Judge whether the compensation control criterion is established according to the output current command value and the transient phase-locked deviation;

[0141] When the compensation control criterion is established, start the compensation control mode;

[0142] When the compensation control criterion is not satisfied, the compensation control mode is not started;

[0143] When the compensation control mode is started, low voltage ride-through control is performed according to the compensation control mode; the steps of performing low voltage ride-through control according to the compensation control mode are as follows:

[0144] When the compensation control criterion is satisfied, an updated output current command value is obtained according to the compensation control operation formula; the compensation control operation formula is:

[0145]

[0146] In the formula, is the updated d-axis output current command value, is the updated q-axis output current command value, is the d-axis output current command value, is the q-axis output current command value, Δθ is the transient phase-locked loop deviation, and t is the current time;

[0147] The updated output current command value is input into the current inner loop of the grid-connected controller for low voltage ride-through control;

[0148] When the compensation control mode is not started, low voltage ride-through control is performed according to the low voltage ride-through mode;

[0149] When the compensation control criterion is not satisfied, an updated output current command value is obtained according to the low voltage ride-through control operation formula; the low voltage ride-through control operation formula is:

[0150]

[0151] In the formula, is the updated d-axis output current command value, is the updated q-axis output current command value, is the d-axis output current command value, is the q-axis output current command value;

[0152] The updated output current command value is input into the current inner loop of the grid-connected controller for low voltage ride-through control.

[0153] The present disclosure first proposes an idea of compensating for the fault transient deviation by correcting the low voltage ride-through output current command, and defines the criterion for the compensation control to be put into operation, and can compensate for the transient deviation of the phase-locked loop through the real-time correction of the low voltage ride-through output current command. This idea only needs to change the output current command during the low voltage ride-through control, and does not need to add a compensation branch inside the phase-locked loop like the traditional method, avoiding the modification of the original control structure and having better engineering practicability.

[0154] After the grid-connected voltage detected by the wind and solar power generation unit drops to meet the low-voltage ride-through start condition, it enters the low-voltage ride-through mode, and the low-voltage ride-through flag bit LV is set to 1. At the same time, a delay holder is turned on, and the preset duration is delayed. Usually, the preset duration is 60 ms, and the output compensation control flag bit LV is output de , with an initial value of 1. Only after 60 ms after the low-voltage ride-through flag bit LV is set to 0, the compensation control flag bit LV de is set to 0. During the entire low-ride period, in order to quickly increase the reactive power to raise the grid-connected voltage, the new energy power source, that is, the wind and solar power generation unit, usually cuts off the power outer loop according to national standards and only retains the current inner loop. The output current command value is given in real time by the low-voltage ride-through control module. During the fault clearing stage, in order to avoid transient overvoltage at the grid connection point in the first few cycles, the low-voltage ride-through control module will set the q-axis current command to zero and only slowly restore the d-axis output current command value to the pre-fault state at a speed of about 0.2 - 0.8PN / s. One cycle refers to 20 ms.

[0155] In some of the embodiments, the initial value of the set compensation control flag bit LV de is 1. When the grid-connected voltage meets the low-voltage ride-through mode start condition, the value of the low-voltage ride-through flag bit LV is 1;

[0156] An OR logic operation is performed on the compensation control flag bit LV de and the low-voltage ride-through flag bit LV;

[0157] When the result of the OR logic operation is 1, the low-voltage ride-through control response acceleration method for the weak grid wind and solar power source is used for low-voltage ride-through control.

[0158] In some of the embodiments, after the value of the low-voltage ride-through flag bit LV is 0, after the preset duration is delayed, the value of the compensation control flag bit LV de is 0.

[0159] When the grid-connected voltage drops to meet the low-voltage ride-through start condition, it enters the low-voltage ride-through control mode. At this time, the value of the low-voltage ride-through flag bit LV is 1, and at the same time, a delay holder is turned on, and the output compensation control flag bit LV de and the initial value is 1. Only after the value of the low-voltage ride-through flag bit LV is 0, after the preset duration is delayed, the value of the compensation control flag bit LV de is 0. Preferably, the preset duration is 60 ms. Based on the low-voltage ride-through flag bit LV, the present invention newly proposes a compensation control flag bit LV based on a delay holder de , which forms a "delay hold" logic operation with the original low-voltage ride-through flag bit LV, so as to realize the whole process function of low-voltage ride-through from fault occurrence to fault clearing.

[0160] Calculation example

[0161] To further illustrate the performance of the present invention, a research calculation example is built with reference to the transmission system of a certain photovoltaic power station as shown below. The specific system parameters are shown in Table 1 and Table 2. Figure 2 As shown. The specific system parameters are shown in Table 1 and Table 2.

[0162] Table 1 Parameters of new energy conversion equipment and filters

[0163]

[0164] Table 2 Other parameters such as line and transformer impedance

[0165]

[0166] To verify the performance of the method proposed in the present disclosure, a comprehensive comparative analysis is carried out with engineering methods and typical literature in this section. Currently, the SRF-PLL synchronization control structure is commonly used in engineering and follows the low-voltage ride-through control requirements of national standards. The existing compensation control method aims at the transient phase-locking deviation problem under generalized disturbances, collects transient deviation amounts based on a fitting algorithm and adopts a static feed-forward compensation strategy in the phase-locked loop PLL, which is a relatively representative research result of compensation control in recent years. In addition, to highlight the advantage of the method proposed in the present disclosure in rapid fault recovery, all three methods immediately resume the active current output after detecting the fault clearance. The three methods will be compared from the following four performance indicators:

[0167] Fault occurrence stage indicators: 1) The response speed of the grid connection point voltage support; 2) The voltage rise amplitude at the grid connection point at 30 ms after the fault.

[0168] Fault clearance stage indicators: 3) The peak value of the transient overvoltage at the grid connection point; 4) The duration of the transient overvoltage at the grid connection point.

[0169] Figure 7 Among them, Urms1 represents the effective value waveform of the grid connection point voltage of the traditional method; Urms2 represents the effective value waveform of the grid connection point voltage of the existing compensation control method; Urms3 represents the effective value waveform of the grid connection point voltage of the method proposed in the present disclosure. From Figure 7It can be seen that the transient voltage support response time of the engineering method is as high as 152.2 ms. The grid connection point voltage at 30 ms after the fault is only 0.02 p.u., which is lower than the wind turbine disconnection threshold and may lead to the failure of crossing through the fault. After the fault is cleared, the transient phase-locked deviation aggravates the reactive power surplus, resulting in the engineering method being unable to immediately resume active power output, which will trigger a transient overvoltage with a maximum peak value of 1.29 pu and a duration of 42.5 ms, also leading to the failure of crossing through the fault. Although the existing compensation control method adopts compensation control inside the phase-locked loop (PLL), due to the failure to formulate a phased compensation strategy for the dynamic characteristics of the fault disturbance scenario and the inherent delay of the fitting algorithm, the response speed is still insufficient. The transient voltage support response time of the fault is as high as 36.7 ms, and the grid connection point voltage at 30 ms is only 0.13 pu, still lower than the wind turbine disconnection threshold. When the fault is cleared, the active power output still cannot be immediately restored, which will cause a transient overvoltage problem with a maximum peak value of 1.19 pu and a duration of 33.1 ms. Compared with the above two methods, the method proposed in the present disclosure compensates for the transient phase-locked deviation by dynamically optimizing the dq-axis current command value, that is, the idea of the output current command value in stages, significantly improving the low voltage ride-through response speed of the wind-solar power generation unit in a weak grid. The transient voltage support response time of the fault is shortened to 20.5 ms, which is 86.5% higher than that of the engineering method and 44.1% higher than that of the existing compensation control method. The grid connection point voltage at 30 ms after the fault is raised to 0.24 p.u., which is 84.6% higher than that of the existing compensation control method and higher than the wind turbine disconnection threshold. When the fault is cleared, the method proposed in the present disclosure can quickly restore the active power output before the fault without delay and will not trigger a transient overvoltage.

[0170] Secondly, taking the same 80% voltage dip condition as an example, the rapidity advantage of the phased dynamic compensation control strategy proposed in the present disclosure compared with the traditional static compensation control is compared. To control variables, the following four strategy configurations are selected for comparison: 1) no compensation control, 2) traditional DFT algorithm + traditional static compensation control, 3) fourth-order DPR algorithm + traditional static compensation control, 4) fourth-order DPR algorithm + phased dynamic compensation control. Figure 8 The comparison results of the grid connection point voltage under different strategy configurations are shown.

[0171] Figure 8 Among them, Urms1 represents the control strategy without compensation, Urms2 represents the traditional DFT algorithm + static compensation control, Urms3 represents the fourth-order DPR algorithm + static compensation control, Urms4 represents the fourth-order DPR algorithm + phased dynamic compensation control. From Figure 8It can be seen that when the voltage drops to 0.2 pu, although the control configuration relying solely on the DPR algorithm + static compensation can quickly raise the grid connection point voltage after the fault, it does not optimize the compensation switching mechanism according to the control objective, resulting in a decline in voltage support ability, which is only 0.22 pu, even lower than the 0.4 pu that can be maintained without compensation. This phenomenon is particularly obvious in the weak grid scenario. After the fault is cleared at 0.2 s, a transient overvoltage as high as 1.23 pu is also triggered, reducing the active power output recovery speed of the new energy power source. In contrast, the proposed staged dynamic compensation control optimizes the compensation switching mechanism according to the control objective, complements the DPR algorithm, does not sacrifice voltage support ability during the fault disturbance stage, and at the same time avoids the transient overvoltage problem after the fault is cleared, maximizing the overall response speed of the low voltage ride-through control.

[0172] It can be seen from this embodiment that compared with the control strategy of the current national standard specifications and the existing latest compensation control method, the present disclosure shortens the low voltage ride-through response time of the new energy power generation unit in the weak grid to within 35 ms, and the response speed is increased by more than 40% compared with the existing method, effectively solving the problems of slow low voltage ride-through power response speed and insufficient voltage support ability within dozens of milliseconds of the fault transient of the new energy power source in the weak grid. Moreover, the present invention has the advantages of fast calculation speed, low application cost, high program portability, etc., and has high practical application value.

[0173] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than limiting the protection scope of the present invention. Any simple modification or equivalent replacement of the technical solution of the present invention by those of ordinary skill in the art does not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for accelerating the low-voltage ride-through control response of a weak-grid wind-solar power source, characterized in that: It includes the following steps: When the grid connection point voltage meets the low voltage ride-through mode startup condition: Obtain the grid connection point voltage, grid connection point current, output current command value, and the first grid connection point voltage phase value respectively. The first grid connection point voltage phase value is the phase value of the voltage of the phase-locked loop obtained by the current control loop calculation of the phase-locked loop; Obtain the second grid connection point voltage phase value according to the grid connection point voltage and the grid connection point current; Obtain the transient phase-locked deviation by taking the difference between the first grid connection point voltage phase value and the second grid connection point voltage phase value; Based on the output current command value and the transient phase-locked deviation, determine whether to start the compensation control mode: When the compensation control mode is started, perform low voltage ride-through control according to the compensation control mode; When the compensation control mode is not started, perform low voltage ride-through control according to the low voltage ride-through mode.

2. The low-voltage ride-through control response acceleration method for wind-solar power sources in a weak power grid according to claim 1, wherein: The output current command value includes the d-axis output current command value and the q-axis output current command value; The step of obtaining the output current command value is: Obtain the effective value of the grid connection point voltage, the reactive power support coefficient, and the maximum current limiting value; Obtain the q-axis output current command value according to the effective value of the grid connection point voltage and the reactive power support coefficient; Obtain the d-axis output current command value according to the maximum current limiting value and the q-axis output current command value.

3. The low-voltage ride-through control response acceleration method for the weak grid wind-solar power source according to claim 1, characterized in that: The step of obtaining the second grid connection point voltage phase value according to the grid connection point voltage and the grid connection point current is: Perform Clark transformation on the grid connection point voltage and the grid connection point current respectively to obtain the α-axis grid connection point voltage, β-axis grid connection point voltage, α-axis grid connection point current, and β-axis grid connection point current; Perform Park transformation and sequence component separation control on the α-axis grid connection point current and the β-axis grid connection point current respectively to obtain the d-axis grid connection point positive sequence current, q-axis grid connection point positive sequence current, d-axis grid connection point negative sequence current, and q-axis grid connection point negative sequence current; Obtain the first norm value of the grid connection point voltage according to the α-axis grid connection point voltage and the β-axis grid connection point voltage; Substitute the first norm value of the grid connection point voltage into the fourth-order DPR algorithm to obtain the second grid connection point voltage phase value.

4. The low-voltage ride-through control response acceleration method for the weak-grid wind-solar power source according to claim 1, wherein: The step of determining whether to start the compensation control mode based on the output current command value and the transient phase-locked deviation is: construct a compensation control criterion; Judge whether the compensation control criterion holds according to the output current command value and the transient phase-locked deviation; When the compensation control criterion holds, start the compensation control mode; When the compensation control criterion does not hold, do not start the compensation control mode.

5. The low-voltage ride-through control response acceleration method for weak-grid wind-solar power sources according to claim 4, characterized in that: The compensation control criterion includes the first compensation control criterion and the second compensation control criterion. The first compensation control criterion is the compensation control criterion in the fault occurrence stage, and the second compensation control criterion is the compensation control criterion in the fault clearing stage.

6. The low-voltage ride-through control response acceleration method for weak-grid wind-solar power sources according to claim 5, characterized in that: When the weak grid wind-solar power generation unit is not equipped with an online impedance estimator: The first compensation control criterion is: The second compensation control criterion is: where Δθ is the transient phase-locked deviation amount is the d-axis output current command value is the q-axis output current command value 7. The low-voltage ride-through control response acceleration method for the weak-grid wind-solar power source according to claim 5, wherein: When the weak grid wind-solar power generation unit is equipped with an online impedance estimator: The first compensation control criterion is: The second compensation control criterion is: wherein, is the d-axis output current command value, is the q-axis output current command value, R g is the equivalent resistance on the grid side, X g is the equivalent reactance on the grid side, and Δθ is the transient phase-locked deviation.

8. The low-voltage ride-through control response acceleration method for weak-grid wind-solar power sources according to any one of claims 5-7, characterized in that: The step of performing low voltage ride-through control according to the compensation control mode is: When the compensation control criterion is satisfied, an updated output current command value is obtained according to the compensation control operation formula; the compensation control operation formula is: wherein, is the d-axis output current command value, is the q-axis output current command value, Δθ is the transient phase-locked deviation amount, and t is the current time; The updated output current command value is input into the current inner loop of the grid-connected controller for low-voltage ride-through control.

9. The low-voltage ride-through control response acceleration method for a weak-grid wind-solar power source according to claim 1, wherein: Set the initial value of the compensation control flag bit to 1. When the grid connection point voltage meets the low-voltage ride-through mode startup condition, the value of the low-voltage ride-through flag bit is 1; Perform an OR logic operation on the compensation control flag bit and the low-voltage ride-through flag bit; When the result of the OR logic operation is 1, the low-voltage ride-through control response acceleration method for the weak-grid wind-solar power source is adopted for low-voltage ride-through control.

10. The low-voltage ride-through control response acceleration method for weak-grid wind-solar power sources according to claim 9, wherein: After the value of the low-voltage ride-through flag bit becomes 0, after a preset time delay, the value of the compensation control flag bit becomes 0.

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