Multi-time-scale collaborative low-voltage ride-through control method and device for wind-solar power generation system

Abstract

The invention provides a multi-time-scale collaborative low-voltage ride-through control method and device for a wind and light power generation system, and the method comprises the steps: collecting active power output and real-time input capacity data, and calculating a virtual impedance control parameter; a positive sequence voltage instruction value is calculated according to the data, and then a reactive power distribution coefficient is calculated. The above parameters and instructions are issued to each wind and light power generation unit, and centralized cooperative parameter configuration before a fault is carried out; and when the low-pass control mode is entered and virtual node control is started, each wind and light power generation unit dynamically adjusts a positive sequence reactive current instruction and a negative sequence reactive current instruction. According to the invention, through combination of global centralized coordination before a fault and distributed dynamic coordination during the fault, optimal regulation and control of a global view angle can be realized at a low cost, and the fault voltage support capability of the wind and light power generation system is improved; and furthermore, the problem of non-fault phase overvoltage can be avoided under dynamic regulation and control, and the method has the advantages of high program transportability and the like, and has relatively high practical application value.

Description

Technical Field

[0001] The present invention relates to the field of renewable energy power supply technology, and in particular to a multi-time scale coordinated low voltage ride-through control method and device for a wind-solar power generation system. Background Art

[0002] As energy demand increases, traditional energy systems are limited by resource constraints and are no longer sufficient to meet actual needs. With the development of new energy technologies, the future development direction of energy systems is gradually introducing new energy power supply designs.

[0003] At present, a large number of large-scale renewable energy bases with a capacity of tens of millions of kilowatts have been newly built in some desert areas. However, due to the lack of synchronous generators in the local area, the system voltage support capacity is insufficient. Therefore, the connection of wind and solar power generation systems to weak power grid systems has become a typical scenario for the grid connection of renewable energy in such areas.

[0004] In the weak support scenario on the system side, a significant drop in system voltage during the fault period may cause large-scale disconnection of new energy sources from the grid, and subsequently various chain accidents. One of the important reasons for such serious accidents is that the low voltage ride-through specifications currently designed for strong power grid scenarios are no longer sufficient to cope with large-scale wind and solar power generation systems connected to weak power grids, and the system lacks sufficient voltage support capabilities during short-circuit faults.

[0005] In the prior art, the fault control strategies of wind and solar power generation units that have been put into operation in engineering usually refer to the current standards. The reactive power output of new energy is only related to the degree of AC voltage drop at the PCC point, which can be regarded as a voltage-controlled flow source without constraints. This is a single-objective control method with the advantages of small calculation amount, high safety and fast response speed, but it is difficult to flexibly and fully tap the voltage support capacity of wind and solar power generation units in different fault scenarios. With the continuous decline in the strength of the opposite system, the system response is increasingly affected by the output of new energy, and various safety constraint boundaries must be taken into account in the fault control strategy. To this end, in recent years, new wind and solar power generation unit fault control strategies have continuously enriched the controllable constraints and constructed the controllable boundaries of new energy faults. However, the above methods are only aimed at improving the low-throughput capacity of a single machine, and do not take into account the differences in the widely dispersed massive units. With the continuous expansion of the scale and capacity of wind and solar power generation systems and the continuous reduction of grid-side strength, different wind and solar power generation units feel different voltage drops, and the fault state differences and interactive coupling characteristics are becoming more and more significant. The traditional method assigns exactly the same low-voltage drop control mode and parameters to a large number of units, which cannot fully utilize the controllable resources of shallow voltage drops to support units with deep voltage drops.

[0006] Therefore, some studies have also proposed multi-machine collaborative low-throughput control strategies for wind and solar power generation systems, including centralized control and distributed control. Among them, in centralized control, a collaborative controller collects the status information of each unit in the wind and solar power generation system after a fault occurs and issues a global optimal decision. However, considering that short-circuit faults usually only last for tens to hundreds of milliseconds, this centralized optimization control has extremely high requirements for measurement, calculation and communication configuration, and its engineering practicality is poor. Distributed control uses virtual node control, proportional integral control and other methods to enable each unit to adaptively and collaboratively output low-throughput without communication. However, pure distributed control lacks unified deployment from a global perspective, and the collaborative mechanism relies on closed-loop feedback. When the electrical distances of the units are very different, it will lead to transient overvoltages in the non-fault phases and inefficient utilization of reactive controllable resources.

[0007] Therefore, when facing the scenario of large-scale wind and solar power generation systems connected to weak power grids, the existing low-voltage control methods are difficult to fully explore the fault transient voltage support capacity of wind and solar power generation systems while meeting safety control constraints to ensure the safe operation of the system. Summary of the invention

[0008] In view of the problems existing in the prior art, the present invention provides a method and device with simple execution and fast response speed, which can maximize the fault transient voltage support capacity of the wind and solar power generation system while meeting the safety control constraints, and ensure the safe operation of the system.

[0009] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0010] On the one hand, the present application provides a multi-time scale coordinated low voltage ride-through control method for a wind-solar power generation system, which mainly includes the following steps:

[0011] During the normal operation of the wind and solar power generation system, the active output and real-time input capacity of each wind and solar power generation unit are periodically collected, and the equivalent impedance on the grid side is calculated;

[0012] Calculating a virtual impedance control parameter of the wind-solar power generation unit according to the grid-side equivalent impedance;

[0013] Calculate the expected positive sequence voltage control command value at the outlet of the wind-solar power generation system after a fault occurs according to the real-time input capacity and the reference parameters of the power system; calculate the expected positive sequence voltage control command value after the fault occurs for each wind-solar power generation unit based on the expected positive sequence voltage control command value after the fault occurs and the inherent parameters and rated parameters of the collection line where each wind-solar power generation unit is located;

[0014] Calculate the reactive power distribution coefficient according to the active power output and the real-time input capacity;

[0015] The virtual impedance control parameter, the positive sequence voltage command value and the reactive power distribution coefficient are sent to each wind and solar power generation unit to perform centralized coordination parameter configuration before a fault occurs;

[0016] When the wind-solar power generation system detects that the voltage drop at the grid connection point meets the low-voltage start condition, it enters the low-voltage control mode and starts the virtual node control. Each wind-solar power generation unit dynamically adjusts the positive-sequence reactive current command and the negative-sequence reactive current command according to the centralized coordination parameter configuration.

[0017] Optionally, in the virtual node control, the following steps are included:

[0018] Each wind and solar power generation unit detects the positive sequence voltage and negative sequence voltage of the grid connection point in real time, calculates the negative sequence voltage command value in the virtual node control according to the negative sequence voltage value and the real-time positive sequence voltage value at the time of the fault, and sets the voltage constraint condition in the calculation of the negative sequence voltage command value;

[0019] The voltage constraint condition is: the non-fault phase voltage does not exceed the safety upper limit.

[0020] Optionally, the virtual node control further includes the following steps:

[0021] Each wind and solar power generation unit generates a positive-sequence reactive current command and a negative-sequence reactive current command based on the virtual impedance control parameter, the reactive power distribution coefficient, and the negative-sequence voltage command value, and verifies in real time whether the positive-sequence reactive current command and the negative-sequence reactive current command meet the hardware safety upper limit of the active power oscillation amplitude and the phase current amplitude. If so, the virtual node control process continues to be executed; if not, the reactive current command of the previous step is maintained and the positive-sequence active current command is set to zero.

[0022] Optionally, calculate the expected positive sequence voltage control command value at the outlet of the wind and solar power generation system after a fault occurs The formula is:

[0023]

[0024] In the formula, is the reference value for voltage regulation of wind and solar power generation systems, V ph.max V is the upper limit of the phase voltage allowed by the power system during normal operation. ph.min is the lower limit of the phase voltage allowed by the power system during normal operation, X i.j is the reactance per unit value on the jth section of the ith collecting line in the wind-solar power generation system, I N.i.j is the current rated current value of the wind-solar power generation unit on the jth section of the i-th collecting line in the wind-solar power generation system; m is the number of collecting lines in the wind-solar power generation system, and n is the maximum number of wind-solar power generation units connected to each collecting line.

[0025] Optionally, calculate the expected positive-sequence voltage command value after the fault of each wind-solar power generation unit. The formula is as follows:

[0026]

[0027] In the formula, V +* is the positive-sequence voltage regulation command value matrix, is the expected positive-sequence voltage command value after the fault of the nth wind-solar power generation unit on the mth collector line; is the rated current value matrix of the wind-solar power generation unit, is the rated current value of the nth wind-solar power generation unit on the collector line represented by the row corresponding to the matrix X co ; X co is the per-unit reactance value matrix, and X m.j is the per-unit reactance value of the ith section of the mth collector line.

[0028] Optionally, the period for centralized collaborative parameter configuration before the fault is the same as the period for collecting the active power output and real-time input capacity of each wind-solar power generation unit.

[0029] Optionally, the equivalent impedance on the grid side is solved by an on-line impedance calculator.

[0030] Optionally, among the virtual impedance control parameters, the virtual impedance parameter is inversely proportional to the grid strength.

[0031] Optionally, the dynamic adjustment of each wind-solar power generation unit according to the centralized collaborative parameter configuration includes the following steps:

[0032] First, raise the voltage of the fault phase through the positive-sequence reactive current command. When the non-fault phase voltage triggers the safety upper limit, dynamically calculate the negative-sequence reactive current command value to obtain the negative-sequence reactive current command;

[0033] Limit the non-fault phase voltage below the safety upper limit through the negative-sequence reactive current command.

[0034] On the other hand, the present application also provides a multi-time-scale collaborative low-voltage ride-through control device for a wind-solar power generation system, which includes a processor and a memory; the memory stores computer-readable instructions, and when the processor runs the computer-readable instructions, it executes the foregoing multi-time-scale collaborative low-voltage ride-through control method for a wind-solar power generation system.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention combines the global centralized coordination before the fault with the distributed dynamic coordination during the fault, and can achieve optimal regulation from a global perspective at low measurement, calculation and communication costs, thereby maximizing the fault voltage support capacity of the wind and solar power generation system; further, it can also avoid the overvoltage problem of non-fault phases under dynamic regulation, and 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

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0038] Figure 1 is a low voltage ride through control flow chart in an embodiment of the present invention;

[0039] Figure 2 A topological diagram of a large-scale wind-solar power generation system in an embodiment of the present invention;

[0040] Figure 3 It is a phasor trajectory diagram of the voltage controllable boundary of the wind-solar power generation system in an embodiment of the present invention;

[0041] Figure 4 A schematic diagram of improving the virtual node control strategy in an embodiment of the present invention;

[0042] FIG5(a) is a waveform diagram of the per-unit voltage values ​​of each phase after the voltage of phase b of unit 1 is lower than the off-grid threshold according to the conventional method in a specific embodiment;

[0043] FIG5( b ) is a waveform diagram of the per-unit voltage of each phase after the b-phase voltage of unit 1 is lower than the off-grid threshold according to the method of the present invention in a specific embodiment;

[0044] FIG5(c) is a waveform diagram of the per-unit voltage of each phase in unit 2 using the conventional method under the conditions of FIG5(a);

[0045] FIG5(d) is a waveform diagram of the per-unit voltage of each phase in unit 2 of the method of the present invention under the conditions of FIG5(b);

[0046] FIG5(e) is a waveform diagram of the per-unit voltage of each phase in unit 3 of the conventional method under the conditions of FIG5(a);

[0047] FIG5(f) is a waveform diagram of the per-unit voltage of each phase in unit 3 of the method of the present invention under the conditions of FIG5(b);

[0048] FIG6 (a1) is a voltage support performance diagram of the existing centralized coordinated low voltage ride-through method under the condition of electrical distance difference;

[0049] FIG6 (a2) is a voltage support performance diagram of the existing centralized coordinated low voltage ride-through method under different short circuit ratio conditions;

[0050] FIG6 (b1) is a voltage support performance diagram of the existing distributed cooperative low voltage ride-through control method under the condition of electrical distance difference;

[0051] FIG6(b2) is a voltage support performance diagram of the existing distributed coordinated low voltage ride-through control method under different short circuit ratio conditions;

[0052] FIG6 (c1) is a voltage support performance diagram of the control method under the condition of electrical distance difference in an embodiment of the present invention;

[0053] FIG6 (c2) is a diagram showing the voltage support performance of the control method under different short-circuit ratio conditions in an embodiment of the present invention. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are 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.

[0055] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0056] In the description of the present invention, “plurality” means two or more than two, unless otherwise clearly and specifically defined.

[0057] In the present invention, unless otherwise clearly specified and limited, the terms "installation", "connection" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0058] It is worth noting that the methods used in the present invention are all conventional methods unless otherwise specified; the raw materials and devices used are all conventional commercially available products, and their sources are not specifically limited unless otherwise specified.

[0059] On the one hand, if Figure 1 As shown, this embodiment provides a multi-time scale coordinated low voltage ride-through control method for a wind-solar power generation system, which mainly includes the following steps:

[0060] First, if Figure 2 As shown, a typical large-scale wind-solar power generation system is constructed and connected to the weak power grid system. During the fault period, the new energy source cuts off the outer power loop and only retains the inner current loop. In order to achieve independent control of positive and negative sequence power, both the positive and negative sequence loops adopt a fixed d-axis voltage control strategy, and the positive and negative sequence components are extracted by the delayed phase cancellation method. The outlet short-circuit ratio of the wind-solar power generation system is adjustable in the range of 1.5 to 10. The connection group of the 0.38 / 35kV transformer T1 in the station is Dyn11, and the connection group of the 35kV / 220kV step-up main transformer T2 is YNd11. In large-scale wind-solar power generation systems, there are two ways of serial connection and parallel connection between different wind-solar power generation units, such as Figure 2 As shown in the figure, the connection relationship between unit 1 and units 2 and 3 is a parallel connection, and the lengths of the collection lines are different. The connection relationship between units 2 and 3 is a serial connection, and the distance from unit 2 to the grid connection point of the wind and solar power generation system is farther than that of unit 3. Figure 2 In the figure, PCCs represents the grid connection point of the entire wind-solar power generation system, and PCCi represents the grid connection point of the i-th wind-solar power generation substation. The subscript g represents the equivalent grid-side variables. U, Z represent the voltage and resistance on the node and line, T represents the transformer, and WT represents the wind turbine. Figure 2 The system composed of three wind and solar power generation units shown can represent the topology of most wind and solar power generation systems. When a wind and solar power generation unit detects a fault, a fault control function is started.

[0061] During the normal operation of the wind and solar power generation system, the active output and real-time input capacity of each wind and solar power generation unit are periodically collected, and the equivalent impedance on the grid side is calculated; the three-phase voltage and current data of the PCC point are collected, and the voltage v of the αβ axis is obtained through Clark transformation. α 、v β Current component i α 、i β The current components are then transformed by Park and the sequence components are separated and controlled to obtain the positive and negative sequence dq axis current components U d + , U q + , U d - , U q- ,I d + ,I q + ,I d - ,I q - During the normal operation of the wind and solar power generation system, the system cooperative controller is started every 10 minutes to collect the current active output of each wind and solar power generation unit and the real-time input capacity data of each wind and solar power generation unit.

[0062] The equivalent impedance of the grid side is solved by an online impedance calculator, and the result is expressed as R g +jL g .

[0063] The virtual impedance control parameters of the wind-solar power generation unit are calculated according to the equivalent impedance of the grid side. Specifically, the collaborative controller located at the grid connection point of the wind-solar power generation system updates the virtual impedance control parameters of each wind-solar power generation unit every 10 minutes according to the currently collected equivalent impedance of the grid side. The virtual impedance X v The calculation formula is as follows:

[0064] X v =λωL g ;

[0065] In the formula, λ is 3, and ω is the angular frequency corresponding to the fundamental frequency of the power system. The online impedance estimation technology is relatively mature, so its impedance estimation method is not repeated here, and the impedance on the grid side is assumed to be known by default. In the virtual impedance control parameters, the virtual impedance parameter is inversely proportional to the grid strength; specifically, this parameter changes with the strength of the grid. The greater the grid strength and the smaller the equivalent impedance, the smaller the virtual impedance parameter, and the faster the transient response speed of the virtual node control. The smaller the grid strength and the larger the equivalent impedance, the larger the virtual impedance parameter, and the slower the transient response speed of the virtual node control. This strategy of adjusting parameters based on grid strength ensures the stability of control operation in different scenarios.

[0066] Furthermore, in order to correctly coordinate the output of wind and solar power generation units under different fault conditions and avoid overvoltage in healthy phases, this embodiment proposes an analysis theory based on the vector trajectory plane from a global perspective, which can allocate the coefficient of the maximum allowable positive voltage for the subsequent LVRT steps. The relationship between the abc phase voltage and the positive and negative sequence voltage in the wind and solar power generation system is geometrically an elliptic equation, such as Figure 3 As shown, the formula is:

[0067]

[0068] In the formula, is the node ijk Phase voltage, a, b or c; superscript "+ / -" indicates positive / negative sequence component; γ i.j.k γ i.j.k Represents the phase difference between positive and negative sequence voltages, α Ti Indicates the phase shift caused by the station main transformer to the positive and negative sequence voltages, α i.j.k Indicates the phase shift caused by the box-type transformer on the node ijk to the positive and negative sequence voltages.

[0069] Phase voltage The magnitude depends on two types of variables: the sequence voltage V i.j.k + 、V i.j.k - The amplitude and phase difference γ i.j.k γ i.j.k .amplitude and phase difference γ i.j.k It is related to the current output and electrical distance of the wind-solar power generation unit. It can be inferred that the phase voltage of the wind-solar power generation unit decreases monotonically with the increase of electrical distance. In addition, the electrical distance difference between different substations in the wind-solar power generation system is much larger than the electrical distance difference between individual wind-solar power generation units in the substation. Therefore, can be considered to be almost identical within a single substation, but significantly different between substations. Then, γ i.j.k It depends on many factors, including the phase deviation caused by line voltage drop and the angle deviation caused by multi-stage transformers. Therefore, the overall phase distribution in the wind-solar power generation system is disordered, with values ​​ranging from 0 to 2π. According to the voltage amplitude and phase distribution inside the wind-solar power generation system, the potential trajectory of the abc phase voltage vector in a single substation appears as an ellipse on the geometric plane, which is defined as the voltage controllable boundary of the substation. Multiple voltage controllable boundaries of different substations appear as multiple nested ellipses, such as Figure 3 As shown in the figure, the blue and red vectors represent the positive and negative sequence voltage phasors of different nodes, respectively. The black, thick dashed ellipse represents all potential ranges of the abc phase voltage amplitude within the wind-solar power generation system. The existing technology only considers the maximum phase voltage safety of a single wind-solar power generation unit, and its constraint equation can be expressed as Figure 3 The pink dots in the figure are marked. Obviously, these points are not the potential maximum phase voltages in the wind-solar power generation system. To ensure voltage safety, the constraint equations should be located at the vertices of the major axis of each ellipse, such as Figure 3 As shown by the yellow dot in Figure 3 , the maximum and minimum expectations of the phase voltage in the whole system can be expressed as:

[0070]

[0071] Where V ph.max and V ph.minThey represent the upper and lower limits of the phase voltage allowed in the power system during normal operation, which are set to 1.1 pu and 0.9 pu according to the existing standards; the subscripts "fur" and "nea" represent the wind and solar power generation units with the maximum and minimum electrical distances from the PCCs bus in the wind and solar power generation system respectively.

[0072] Calculate the expected positive-sequence voltage regulation command value at the outlet of the wind and solar power generation system according to the real-time input capacity and the reference parameters of the power system; specifically, in order to solve the complex voltage controllable boundary in the collaborative low-voltage ride-through process, the most conservative situation is assumed in the coordination. This situation represents the most unfavorable difference in the phase voltage amplitude at the connection point of each wind and solar power generation unit, and the maximum difference in amplitude between the farthest and nearest wind and solar power generation units. Thus, in the most conservative situation, it is assumed that each wind and solar power generation unit outputs the maximum positive-sequence reactive current and does not output any negative-sequence reactive current, resulting in the maximum voltage drop on the line. Therefore, for the collaborative controller, while executing step 2), the collaborative controller collects the real-time input capacity of each wind and solar power generation unit and calculates the expected positive-sequence voltage regulation command value at the outlet of the wind and solar power generation system during the fault. The calculation formula is:

[0073]

[0074] In the formula, is the reference value for the voltage regulation of the wind and solar power generation system, X i.j is the per-unit value of the reactance on the j-th section of the i-th collector line in the wind and solar power generation system; I N.i.j is the current rated value of the wind and solar power generation unit on the j-th section of the i-th collector line in the wind and solar power generation system, i = 1, 2,..., n; m is the number of collector lines in the wind and solar power generation system, and n is the maximum number of wind and solar power generation units connected to each collector line; V ph.max and V ph.min are set to 1.1 pu and 0.9 pu respectively according to the existing standards.

[0075] Based on the expected positive-sequence voltage regulation command value during the fault and the inherent parameters and rated parameters of the collector lines where each wind and solar power generation unit is located, calculate the expected positive-sequence voltage command value of each wind and solar power generation unit after the fault; specifically, Figure 2 Allocate the expected positive-sequence voltage command value after the fault to each wind and solar power generation unit in the 3 wind and solar power generation substations in

[0076]

[0077] In the formula, V +* is the positive-sequence voltage regulation command value matrix, is the expected positive sequence voltage command value after the nth wind-solar power generation unit on the mth collection line fails; is the rated current value matrix of wind and solar power generation units, is the matrix X co The corresponding row represents the rated current value of the nth wind-solar power generation unit on the collection line; X co is the reactance per unit value matrix, X m.i is the per unit reactance value of the i-th section of the m-th collection line.

[0078] In the above steps, a multi-machine voltage controllable boundary trajectory analysis model of the wind and solar power generation system is constructed to generate the maximum positive sequence voltage command after the fault that should be set at the farthest end of the entire system. This process takes into account the influence of the electrical distance and capacity difference of different wind and solar power generation units on the amplitude and phase distribution of the voltage after the fault, and also takes into account the phase deflection effect of the transformers with different connection groups in the system on the asymmetric voltage of the system. The positive sequence voltage command designed by the analysis model can ensure that when each wind and solar power generation unit adopts virtual node control after the fault occurs, there will be no non-fault phase overvoltage on any node. For this problem, the existing methods lack sufficient analysis, so non-fault phase overvoltage may be caused in large-scale wind and solar power generation systems and weak power grid scenarios. According to the voltage drop caused by the potential maximum output after the fault, the expected positive sequence voltage command of each node in the entire wind and solar power generation system is calculated. In this step, the present invention analyzes the most conservative boundary of the voltage safety of each node in the wind and solar power generation system for the first time. Compared with the existing methods, it can ensure voltage safety under any fault conditions.

[0079] While solving the above calculation steps, the reactive power distribution coefficient is calculated according to the active power output and the real-time input capacity; specifically, the collaborative controller calculates and distributes the reactive power distribution coefficient according to the current active power output and rated capacity of the wind and solar power generation units in each substation, which is used to distribute reactive power tasks to each unit after a fault. The calculation formula is:

[0080]

[0081] In the formula, ζ i represents the reactive power distribution coefficient of the i-th unit in the wind-solar power generation system, P i Indicates the current active output of the unit, S i Indicates the current rated capacity of the unit. Considering that the types of power sources in a substation are similar and the electrical distances are not much different, a substation can be regarded as an integral unit to calculate its current active output and capacity, instead of solving the values ​​of all wind and solar power generation units.

[0082] In this step, the multi-machine reactive power distribution coefficient is defined, which is calculated by the current active power output and rated capacity of each wind and solar power generation unit, and can optimize the low-penetration reactive power output ratio of each unit after a fault occurs. This parameter can allocate low-penetration reactive power according to the current reactive controllable capacity of each wind and solar power generation unit after a fault occurs, that is, the unit with a large reactive controllable capacity provides more voltage support capacity, and the unit with a small reactive controllable capacity provides less voltage support capacity. Compared with the existing method that does not consider the difference in controllable capacity between wind and solar units, the present invention can achieve more efficient utilization of controllable resources of the converter equipment.

[0083] The virtual impedance control parameters, positive sequence voltage command value and reactive power distribution coefficient are sent to each wind and solar power generation unit to perform centralized coordination parameter configuration before the fault; optionally, the period for performing centralized coordination parameter configuration before the fault is consistent with the period for collecting the active output and real-time input capacity of each wind and solar power generation unit. The collection period range is preferably 5-10 minutes, and the present embodiment is preferably performed every 10 minutes. The centralized coordination process before the fault is defined as being performed every 10 minutes. Different from the existing method that starts coordination only after the fault occurs, the method proposed in the present embodiment can achieve low-power coordination from a global perspective at a lower cost. Because these parameters only depend on the system operating status and the active output of the wind and solar power generation units, they change relatively slowly and do not need to be updated in real time. This centralized coordinated control before the fault occurs has a low cost and can ensure safe and efficient coordinated control from a global perspective.

[0084] When the wind and solar power generation system detects that the voltage drop at the grid connection point meets the low-breakthrough start conditions, such as when a short circuit fault occurs in the power grid, take the BCG fault as an example, and other faults are the same. Figure 2 As shown in the marked fault location. The system enters the low-through control mode and starts the virtual node control. At this time, each wind and solar power generation unit enters the virtual node control state, that is, each wind and solar power generation unit dynamically adjusts the positive-sequence reactive current command and the negative-sequence reactive current command according to the centralized collaborative parameter configuration; and there is no communication between the units and the collaborative controller. Among them, the dynamic adjustment first raises the fault phase voltage through the positive-sequence reactive current command, and when the non-fault phase voltage triggers the safety upper limit, the negative-sequence reactive current command value is dynamically calculated to obtain the negative-sequence reactive current command, and the non-fault phase voltage is limited to be lower than the safety upper limit through the negative-sequence reactive current command.

[0085] Each wind and solar power generation unit real - time detects the positive - sequence voltage and negative - sequence voltage at the grid - connection point. According to the negative - sequence voltage value at the moment of fault occurrence and the real - time positive - sequence voltage value, it calculates the negative - sequence voltage command value in the virtual - node control, and sets voltage constraint conditions in the calculation of the negative - sequence voltage command value. Specifically, with a pre - defined cooperative low - voltage ride - through control coefficient, the wind and solar power generation unit dynamically adjusts its output current command during the adaptive cooperation process after the fault occurs, so communication is not required. To maximize the voltage support efficiency, this embodiment considers the dynamic power distribution of each wind and solar power generation unit, including two problems: a) the positive - and negative - sequence reactive - power ratio of a single wind and solar power generation unit; b) the reactive - power output ratio among multiple wind and solar power generation units. This optimal ratio can be achieved based on the improved virtual - bus control. In this embodiment, a reactive - power distribution coefficient ζ is newly added to the traditional virtual - node control. The principle of virtual - bus control is to assume that the wind and solar power generation unit is connected to a virtual power grid with a virtual internal impedance Z v as shown in Figure 4 . Each wind and solar power generation unit real - time detects the positive - and negative - sequence voltage values at its own grid - connection point, and calculates the negative - sequence voltage command value in the virtual - node control. The calculation formula is:

[0086] V i -* (t)=min{V i - (0 + )V ph-max - V i + (t)};

[0087] In the formula, V i -* represents the negative - sequence voltage command value of the virtual - node control of the i - th wind and solar power generation unit, V i. - (0 + ) represents the negative - sequence voltage value at the grid - connection point at the moment when the fault just occurs, and V i + (t) represents the real - time measured positive - sequence voltage value at the grid - connection point. In the calculation of the command value, the optimal positive - and negative - sequence ratio of the low - voltage - ride - through reactive - power output of the wind and solar power generation unit is analyzed. The analysis result shows that if you want to maximize the amplitude of the fault - phase voltage under the limited reactive - power controllable capacity, you should generate as much positive - sequence reactive power as possible, and only when the voltage safety boundary is triggered, cooperate with negative - sequence reactive - power output. According to this principle, a dynamic optimal positive - and negative - sequence ratio control is proposed, that is, the negative - sequence voltage command dynamically adjusts with the change of the positive - sequence voltage at the grid - connection point of the wind and solar power generation unit, which can maximize the fault - phase voltage while ensuring the safety of the non - fault - phase voltage. This efficient utilization of the reactive - power controllable capacity enables this embodiment to have stronger fault - voltage support efficiency compared with the existing methods.

[0088] After the wind and solar power generation unit enters the improved virtual node control, the following calculation control process is executed. For the positive sequence control loop, the process of the improved virtual node control strategy is as follows:

[0089]

[0090] In the formula, I Q.i +* It represents the positive sequence reactive current command value given during the low-voltage run-through of the i-th wind-solar power generation unit, and Δt represents the operation interval of the wind-solar power generation unit controller.

[0091] The above design introduces virtual node control and makes improvements. After a fault occurs, each wind and solar power generation unit enters the virtual node control state, and the virtual node control can realize distributed collaboration without communication. Each unit dynamically adjusts itself under the operating parameters given by the aforementioned centralized collaboration, and finally reaches the optimal low-throughput operating state of the entire wind and solar power generation system. Compared with the traditional virtual node control strategy, this embodiment embeds a reactive power distribution coefficient. It can be seen from the formula that this coefficient determines the response speed of the virtual node control of the unit. The larger the reactive power distribution coefficient, the faster the virtual node control response speed. This embodiment regulates the speed of the low-throughput reactive power response of each node by adjusting the size of the reactive power distribution coefficient of each unit, and then indirectly allocates the low-throughput reactive power output of each unit after the fault occurs; compared with the existing method, it can make full use of reactive controllable resources without occupying the resources of high active output wind and solar power generation units.

[0092] For the negative sequence control loop, the control process can be obtained as follows:

[0093]

[0094] In the formula, the meaning of the variables in the negative sequence control loop is the same as the previous formula. It can be seen that the value of ζ controls the response speed of the wind and solar power generation unit output command. According to the ratio of active power output between different wind and solar power generation units, different ζ is assigned to each unit, and the low voltage ride-through control response speed of the unit can be adaptively adjusted, thereby indirectly allocating the reactive power ratio of each wind and solar power generation unit. The value of ζ is calculated and issued in advance in the previous step based on the ratio between the reactive controllable capacity of each wind and solar power generation unit and the reactive controllable capacity of the entire wind and solar power generation system.

[0095] Among them, the voltage constraint condition is: the non-fault phase voltage does not exceed the safety upper limit. Each wind and solar power generation unit generates a positive-sequence reactive current command and a negative-sequence reactive current command based on the virtual impedance control parameter and the reactive power distribution coefficient, as well as the negative-sequence voltage command value, and verifies in real time whether the positive-sequence reactive current command and the negative-sequence reactive current command meet the hardware safety upper limit of the active power oscillation amplitude and the phase current amplitude; specifically, the positive and negative-sequence reactive current commands I of the wind and solar power generation unit are generated. Q.i+* (t) and I Q.i -* (t), substitute it into the following inequality to determine whether it holds true; the inequality formula is:

[0096]

[0097] In the formula, and I lim.i represents the safety upper limit of the active power double frequency oscillation amplitude and output phase current amplitude allowed by the hardware of the i-th wind and solar power generation unit, γ i It indicates the current measured difference between the positive and negative sequence voltage phases at the wind and solar power generation unit grid connection point, which is the positive sequence active current instruction; I α1,i ,I α2,i , Iβ 1,i , Iβ 2,i is the positive and negative sequence current component in the αβ coordinate system, obtained by Clark transformation. This step is to detect whether the current amplitude and active power oscillation amplitude of the wind and solar power generation unit exceed the hardware operation upper limit. In the formula, let the positive and negative sequence active current command I P.i +* Set to zero.

[0098] If the condition is satisfied, the virtual node control process will continue to be executed, and the positive sequence active current instruction I P.i +* Generate in real time as follows:

[0099]

[0100] Wherein, the A-related parameter represents the components of the positive and negative sequence voltages in the αβ coordinate system.

[0101] If it is not satisfied, the control process is stopped and the positive and negative sequence reactive current instructions I Q.i +* and I Q.i -* The value of the previous operation step is fixed and remains unchanged, and the positive sequence active current command I P.i +* Constant zero.

[0102] The above execution steps take into account the low-through control constraints of the output active power oscillation limit and phase current amplitude limit of the wind and solar power generation units, and establish constraint equations. Combined with the virtual node control strategy, it is determined in real time whether the calculated positive and negative sequence active and reactive current instructions meet the safety constraints. If so, the virtual node control strategy continues to be executed. If the safety constraints are not met, the command value on the constraint boundary is followed for execution, thereby safely executing low-through control without communication distributed control and ensuring the safety of the converter equipment.

[0103] On the other hand, the present embodiment also provides a multi-time scale coordinated low voltage ride-through control device for a wind-solar power generation system, which is a coordinated controller for executing control operations in the present embodiment, and mainly includes a processor and a memory; the memory stores computer-readable instructions, and when the processor runs the instructions, it executes the multi-time scale coordinated low voltage ride-through control method for the wind-solar power generation system; and also includes a low voltage ride-through module for executing the control operations of each unit after a fault, that is, each wind-solar power generation unit dynamically adjusts the positive sequence reactive current instruction and the negative sequence reactive current instruction according to the centralized coordinated parameter configuration. The low voltage ride-through module is embedded in the controller of each wind-solar power generation unit.

[0104] Embodiment 1;

[0105] Refer to a certain wind and solar power generation system and build Figure 2 The topology structure shown in the figure has specific parameters of the wind-solar power generation system as shown in Table 1:

[0106] Table 1

[0107]

[0108]

[0109] In order to prove the performance advantage of the method of this embodiment over the traditional method, Figure 5(a)-Figure 5(f) As shown, the performance comparison under BCG fault is given; specifically, in the above-mentioned set of Figure 5, its meaning is to compare the voltage raising performance of the traditional method and the method of this embodiment. Among them, the test system comes from Figure 2 , including 3 wind and solar units, namely unit 1, unit 2 and unit 3. As shown in Figures 5(a), 5(c) and 5(e), the voltage support capacity of the traditional method is insufficient. After the fault occurs, unit 1 first detects that the voltage of phase b is lower than the grid-off threshold. The grid-off of unit 1 leads to a further reduction in the reactive power shortage in the system, resulting in the voltage of phase b of unit 3 being lower than the grid-off threshold. Unit 3 is disconnected from the grid, which in turn triggers a chain reaction, causing unit 2 to eventually be disconnected from the grid. Therefore, due to the insufficient voltage support capacity, the traditional method may cause the problem of wind and solar power chain disconnection after the above fault. In contrast, it can be seen from Figures 5(b), 5(d) and 5(f) that the method of this embodiment fully exploits the wind and solar power under shallow voltage drop to increase reactive power to support the unit under deep voltage drop by coordinating wind and solar power under different fault states, thereby improving the voltage support capacity of the entire wind and solar power generation system. None of the three wind and solar units have reached the grid-off threshold and still maintain safe grid-connected operation. In summary, the voltage support capacity of the method of this embodiment has been fully verified compared with the traditional method.

[0110] Furthermore, in the above example, the wind-solar power generation system adopts a low voltage ride-through control strategy, and the reactive support coefficient K = 1.5. The fault disconnection voltage threshold of the wind turbine is set at 0.2pu. In the traditional method, there is a lack of coordination, so multiple wind-solar power generation units in different fault states cannot maximize the fault phase voltage of their grid connection points. Therefore, the wind-solar power generation unit in the first substation detects that the b-phase voltage has dropped below the grid disconnection threshold, and the wind turbine is out of operation. This action further weakens the reactive support capacity of the wind-solar power generation system, prompting the wind-solar power generation unit in the third substation to reach its grid disconnection threshold, and ultimately causing all units in the wind-solar power generation system to be chained off the grid. In contrast, the proposed coordinated low voltage ride-through control fully utilizes the voltage support capacity of each wind-solar power generation unit from the beginning, enabling units farther away from the fault to provide additional reactive power and provide assistance to units closer to the fault. No chain disconnection accident occurs under the fault, demonstrating a stronger fault voltage support capability.

[0111] In addition, various fault conditions are fully tested in Table 2, which lists the percentage improvement of the fault voltage support capability of the proposed method compared with the conventional method.

[0112] Table 2

[0113]

[0114] In Table 2, taking the positive sequence voltage as an example, under various fault conditions, the traditional method can only increase the grid-connected voltage of the corresponding wind-solar power generation system by about 0.1pu. In contrast, the low voltage ride-through control shown in this embodiment shows a stronger voltage support capability: when the fault voltage drops to no less than 0.3pu, the voltage support provided by the traditional method is improved by more than 110%.

[0115] In order to further verify the wide applicability in various wind and solar power generation system scenarios, Figure 6(a1)-Figure 6(c2) The influence of different short circuit ratios (shortcircuitratio) (ranging from 10.0 to 1.5) and the maximum electrical distances between different wind and solar power generation units (ranging from 3 km to 76 km) under BC faults were discussed. Figure 2 Taking the C phase voltage of the grid connection point (PCC2) of unit 2 with the longest electrical distance at the 35kV side as an example, the waveforms of the three methods of the existing centralized collaborative control method, the distributed collaborative control method and the present invention are compared as follows: Figure 6(a1)-Figure 6(c2)As shown. It can be clearly seen from Figure 6 (a1) and Figure 6 (a2) that when maximizing the current output of the wind-solar power generation unit under low SCR conditions (SCR < 2.9), the centralized cooperative control cannot maintain the healthy phase voltage safety of unit 2, and is only applicable to strong power grid scenarios. At the same time, although the distributed cooperative control in Figure 6 (b1) and Figure 6 (b2) can operate well at high SCR (SCR>5.0) or smaller electrical distances (<29km), it ignores the disordered fluctuations of multiple wind-solar power generation units during fault transients, resulting in short-term overvoltage problems in certain cases of larger electrical distances (>29km) and low SCR (<2.9). In contrast, the present invention in Figure 6 (c1) and Figure 6 (c2) can maximize the voltage support capability of the entire wind-solar power generation system, is suitable for different system short-circuit ratios and unit electrical distance differences, and has a wider adaptability.

[0116] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A multi-time scale coordinated low voltage ride-through control method for a wind-solar power generation system, characterized by: The steps include: During the normal operation of the wind and solar power generation system, the active output and real-time input capacity of each wind and solar power generation unit are periodically collected, and the equivalent impedance on the grid side is calculated; Calculating a virtual impedance control parameter of the wind-solar power generation unit according to the grid-side equivalent impedance; Calculate the expected positive sequence voltage control command value at the outlet of the wind-solar power generation system after a fault occurs according to the real-time input capacity and the reference parameters of the power system; calculate the expected positive sequence voltage control command value after the fault occurs for each wind-solar power generation unit based on the expected positive sequence voltage control command value after the fault occurs and the inherent parameters and rated parameters of the collection line where each wind-solar power generation unit is located; Calculate the reactive power distribution coefficient according to the active power output and the real-time input capacity; The virtual impedance control parameter, the positive sequence voltage command value and the reactive power distribution coefficient are sent to each wind and solar power generation unit to perform centralized coordination parameter configuration before a fault occurs; When the wind-solar power generation system detects that the voltage drop at the grid connection point meets the low-voltage start condition, it enters the low-voltage control mode and starts the virtual node control. Each wind-solar power generation unit dynamically adjusts the positive-sequence reactive current command and the negative-sequence reactive current command according to the centralized coordination parameter configuration.

2. The multi-time scale coordinated low voltage ride through control method for wind-solar power generation system according to claim 1 is characterized by: In the virtual node control, the following steps are included: Each wind and solar power generation unit detects the positive sequence voltage and negative sequence voltage of the grid connection point in real time, calculates the negative sequence voltage command value in the virtual node control according to the negative sequence voltage value and the real-time positive sequence voltage value at the time of the fault, and sets the voltage constraint condition in the calculation of the negative sequence voltage command value; The voltage constraint condition is: the non-fault phase voltage does not exceed the safety upper limit.

3. The multi-time scale coordinated low voltage ride through control method for wind-solar power generation system according to claim 2 is characterized by: In the virtual node control, the following steps are also included: Each wind and solar power generation unit generates a positive-sequence reactive current command and a negative-sequence reactive current command based on the virtual impedance control parameter, the reactive power distribution coefficient, and the negative-sequence voltage command value, and verifies in real time whether the positive-sequence reactive current command and the negative-sequence reactive current command meet the hardware safety upper limit of the active power oscillation amplitude and the phase current amplitude. If so, the virtual node control process continues to be executed; if not, the reactive current command of the previous step is maintained and the positive-sequence active current command is set to zero.

4. The multi-time scale coordinated low voltage ride through control method for wind-solar power generation system according to claim 1, characterized in that: Calculate the expected positive sequence voltage control command value at the outlet of the wind and solar power generation system after a fault occurs The formula is: In the formula, is the reference value for voltage regulation of wind and solar power generation systems, V ph.max V is the upper limit of the phase voltage allowed by the power system during normal operation. ph.min is the lower limit of the phase voltage allowed by the power system during normal operation, X i.j is the reactance per unit value on the jth section of the ith collecting line in the wind-solar power generation system, I N.i.j is the current rated current value of the wind-solar power generation unit on the jth section of the i-th collecting line in the wind-solar power generation system; m is the number of collecting lines in the wind-solar power generation system, and n is the maximum number of wind-solar power generation units connected to each collecting line.

5. The multi-time scale coordinated low voltage ride through control method for wind-solar power generation system according to claim 4 is characterized by: Calculate the expected positive sequence voltage command value after each wind and solar power generation unit fails. The formula is: Where V +* is the positive sequence voltage control command value matrix, is the expected positive sequence voltage command value after the nth wind-solar power generation unit on the mth collection line fails; is the rated current value matrix of wind and solar power generation units, is the matrix X co The corresponding row represents the rated current value of the nth wind-solar power generation unit on the collection line; X co is the reactance per unit value matrix, X m.j is the per unit reactance value of the i-th section of the m-th collection line.

6. The multi-time scale coordinated low voltage ride through control method for wind-solar power generation system according to claim 1, characterized in that: The cycle of centralized coordination parameter configuration before a fault is consistent with the cycle of collecting the active output and real-time input capacity of each wind and solar power generation unit.

7. The multi-time scale coordinated low voltage ride through control method for wind-solar power generation system according to claim 1, characterized in that: The grid-side equivalent impedance is solved by an online impedance calculator.

8. The multi-time scale coordinated low voltage ride through control method for wind-solar power generation system according to claim 1, characterized in that: Among the virtual impedance control parameters, the virtual impedance parameter is in inverse proportion to the grid strength.

9. The multi-time scale coordinated low voltage ride through control method for wind-solar power generation system according to claim 1, characterized in that: Each wind and solar power generation unit is dynamically adjusted according to the centralized coordination parameter configuration, including the following steps: First, the fault phase voltage is raised by the positive-sequence reactive current command, and when the non-fault phase voltage triggers the safety upper limit, the negative-sequence reactive current command value is dynamically calculated to obtain the negative-sequence reactive current command; The negative-sequence reactive current command is used to limit the non-fault phase voltage to be lower than the safety upper limit.

10. A multi-time scale coordinated low voltage ride through control device for a wind-solar power generation system, characterized in that: It includes a processor and a memory; the memory stores computer-readable instructions, and when the processor runs the computer-readable instructions, it executes the multi-time-scale coordinated low voltage ride-through control method for a wind-solar power generation system as described in any one of claims 1 to 9.

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