Wind-solar hybrid generation system multi-time scale collaborative low voltage ride through control method and device

Abstract

The application provides a kind of wind and light power generation system multi-time scale collaborative low voltage ride through control method and device, steps include: collecting active power and real-time capacity data, and calculating virtual impedance control parameter;According to data, the positive sequence voltage instruction value is obtained, and the reactive power distribution coefficient is calculated.The above parameters and instructions are issued to each wind and light power generation unit, and the centralized collaborative parameter configuration before fault is carried out;When entering low control mode and starting virtual node control, each wind and light power generation unit dynamically adjusts positive sequence reactive current instruction and negative sequence reactive current instruction.The application can realize optimal regulation and control from global perspective at lower cost by combining global centralized cooperation before fault with distributed dynamic coordination during fault, and improve the fault voltage support capability of wind and light power generation system;Further, the non-fault phase overvoltage problem can be avoided under dynamic regulation and control, and the program has the advantages of portability and the like, and has high practical application value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy power supply, in particular to a wind-solar power generation system multi-time scale collaborative low voltage ride through control method and device. BACKGROUND

[0002] With the increase of energy demand, the traditional energy system is limited by resource constraints and has not been able to meet the actual demand. With the development of new energy technology, the future development direction of the energy system is gradually introducing new energy power supply design.

[0003] At present, a large number of 1000 MW large-scale new energy bases have been newly built in some sandy and barren areas, but due to the lack of synchronous generators in the local area, the system voltage support capability is not enough, so the wind-solar power generation system connected to the weak power grid system has become a typical scenario of new energy grid connection in such areas.

[0004] In the scenario of weak support on the system side, the system voltage during the fault is greatly reduced, which may lead to large-scale disconnection of new energy and subsequent chain accidents. One of the important reasons for such serious accidents is that the current low voltage ride through specification designed for strong grid scenarios is not enough to cope with the large-scale wind-solar power generation system connected to the weak grid scenario, and there is a lack of sufficient voltage support capability during system short-circuit fault.

[0005] In the prior art, the fault control strategy of the wind-solar power generation unit that has been put into operation in engineering usually refers to the current standard, and the reactive power output of the new energy is only related to the degree of PCC point alternating voltage drop, which can be regarded as a voltage-controlled current source without constraints. This is a single-target control method, which has the advantages of small calculation amount, high safety and fast response speed, but it is difficult to flexibly tap the voltage support capability of the wind-solar power generation unit in different fault scenarios. With the continuous decline of the strength of the opposite side system, the system response is increasingly affected by the output of the new energy, and various safety constraint boundaries must be considered in the fault control strategy. Therefore, in recent years, new fault controllable boundaries of the new energy have been constructed by continuously enriching the controllable constraint conditions of the new fault control strategy of the wind-solar power generation unit. However, the above methods only aim at the improvement of the low penetration capability of a single machine, and do not consider the differences between widely dispersed massive units. With the continuous expansion of the scale of the wind-solar power generation system and the continuous reduction of the grid side strength, different wind-solar power generation units experience different voltage drop levels, and the fault state difference and interactive coupling characteristics become more and more significant. The traditional method gives the same low penetration control mode and parameters to the massive units, and cannot fully utilize the controllable resources of shallow voltage drop to support the units with deep voltage drop.

[0006] Therefore, some studies also put forward the multi-machine coordinated low penetration control strategy of wind-solar power generation system, including centralized control and distributed control. Among them, the centralized control collects the state information of each unit in the wind-solar power generation system by a coordinated controller after the fault occurs and issues a global optimal decision. However, considering that the short-circuit fault usually lasts for tens to hundreds of milliseconds, such centralized optimization control has high requirements for measurement, calculation and communication configuration, and has poor engineering practicability. The distributed control makes each unit self-adaptively coordinated to low penetration output without communication through virtual node control, proportional integral control and other methods. However, pure distributed control lacks unified deployment from a global perspective, and the coordination mechanism relies on closed-loop feedback, which will lead to transient overvoltage of non-fault phase and inefficient use of controllable reactive resources when the electrical distance of each unit is large.

[0007] Therefore, in the face of large-scale wind-solar power generation system connected to weak grid, the existing low penetration control method is difficult to fully explore the fault transient voltage support capability of the wind-solar power generation system while meeting the safety control constraints, and ensure the safe operation of the system. SUMMARY

[0008] The present application provides a method and device with simple execution and fast response, which can fully explore the fault transient voltage support capability of the wind-solar power generation system while meeting the safety control constraints, and ensure the safe operation of the system.

[0009] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

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

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

[0012] The virtual impedance control parameters of the wind-solar power generation unit are calculated according to the equivalent impedance of the grid side;

[0013] The expected positive sequence voltage regulation instruction value of the wind-solar power generation system outlet after the fault occurs is calculated according to the real-time input capacity and the reference parameters of the power system; based on the expected positive sequence voltage regulation instruction 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, the expected positive sequence voltage instruction value of each wind-solar power generation unit after the fault is calculated;

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

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

[0016] When the wind-solar power generation system detects that the grid point voltage drop meets the low-pass starting condition, the low-pass control mode is entered and the virtual node control is started, and 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 and collaborative parameter configuration.

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

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

[0019] The voltage constraint condition is that the voltage of the non-fault phase does not exceed the safe upper limit.

[0020] Optionally, in the virtual node control, the following steps are included:

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

[0022] Optionally, the expected positive sequence voltage regulation instruction value at the late fault occurrence of the wind-solar power generation system outlet is calculated The formula is:

[0023]

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

[0025] Optionally, the expected positive sequence voltage instruction value after failure of each wind-solar power generation unit is calculated, and the formula is:

[0026]

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

[0028] Optionally, the period of centralized cooperative parameter configuration before failure is consistent with the period of collecting the active power output and real-time capacity of each wind-solar power generation unit.

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

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

[0031] Optionally, the wind-solar power generation unit dynamically adjusts according to the centralized cooperative parameter configuration, including the following steps:

[0032] First, the positive sequence reactive current instruction is used to raise the fault phase voltage, and then the negative sequence reactive current instruction value is dynamically calculated when the non-fault phase voltage triggers the safety upper limit, to obtain the negative sequence reactive current instruction;

[0033] The non-fault phase voltage is limited to be lower than the safety upper limit by the negative sequence reactive current instruction.

[0034] On the other hand, the application also provides a wind-solar power generation system multi-time scale cooperative low voltage ride through control device, which comprises a processor and a memory; the memory stores computer readable instructions, and the processor executes the computer readable instructions to perform the wind-solar power generation system multi-time scale cooperative low voltage ride through control method.

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

[0036] The application can realize optimal regulation and control in a global perspective, maximally improve the fault voltage support capability of the wind-solar power generation system, avoid overvoltage of the non-fault phase under dynamic regulation, and has the advantages of fast calculation speed, low application cost, high program portability and high practical application value by combining global centralized cooperation before failure with distributed dynamic coordination during failure at a low measurement, calculation and communication cost. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments will be briefly introduced. Obviously, the drawings in the following description only constitute some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0038] Figure 1 The low voltage ride through control flowchart in the embodiment of the present application;

[0039] Figure 2 The topology diagram of the large-scale wind-solar power generation system in the embodiment of the present application;

[0040] Figure 3 The phasor trajectory diagram of the voltage controllable boundary of the wind-solar power generation system in the embodiment of the present application;

[0041] Figure 4 The principle diagram of the improved virtual node control strategy in the embodiment of the present application;

[0042] Fig. 5(a) is the voltage per unit waveform diagram of each phase after the voltage of phase b of unit 1 is lower than the off-grid threshold in the traditional method in the specific embodiment;

[0043] Fig. 5(b) is the voltage per unit waveform diagram of each phase after the voltage of phase b of unit 1 is lower than the off-grid threshold in the method of the present application in the specific embodiment;

[0044] Fig. 5(c) is the voltage per unit waveform diagram of each phase of unit 2 under the condition of Fig. 5(a) in the traditional method;

[0045] Fig. 5(d) is the voltage per unit waveform diagram of each phase of unit 2 under the condition of Fig. 5(b) in the method of the present application;

[0046] Fig. 5(e) is the voltage per unit waveform diagram of each phase of unit 3 under the condition of Fig. 5(a) in the traditional method;

[0047] Fig. 5(f) is the voltage per unit waveform diagram of each phase of unit 3 under the condition of Fig. 5(b) in the method of the present application;

[0048] Fig. 6(a1) is a voltage support performance diagram of the existing centralized coordinated low voltage ride through method under the condition of different electrical distance differences;

[0049] Fig. 6(a2) is a voltage support performance diagram of the existing centralized coordinated low voltage ride through method under the condition of different short circuit ratios;

[0050] Fig. 6(b1) is a voltage support performance diagram of the existing distributed coordinated low voltage ride through control method under the condition of different electrical distance differences;

[0051] Fig. 6(b2) is a voltage support performance diagram of the existing distributed coordinated low voltage ride through control method under the condition of different short circuit ratios;

[0052] Fig. 6(c1) is a voltage support performance diagram of the control method in the embodiment of the present application under the condition of different electrical distance differences;

[0053] Fig. 6(c2) is a voltage support performance diagram of the control method in the embodiment of the present application under the condition of different short circuit ratios. DETAILED DESCRIPTION

[0054] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0055] It should be noted that similar reference numerals and letters indicate similar items in the following drawings, and thus, once an item is defined in one drawing, it need not be further defined and explained in subsequent drawings.

[0056] In the description of the present application, the meaning of "a plurality of" is two or more than two, unless otherwise explicitly specified and limited.

[0057] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connection" and the like should be understood broadly, for example, can be fixed connection, can be detachable connection, or integral; can be directly connected, or indirectly connected through an intermediate medium; can be the internal communication of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0058] It is worth mentioning that the method used in the present application is a conventional method unless otherwise specified; the raw materials and devices used are conventional commercially available products unless otherwise specified, and their sources are not limited.

[0059] In one aspect, as shown in Figure 1 , the embodiment provides a wind-solar power generation system multi-time scale collaborative low voltage ride through control method, which mainly includes the following steps:

[0060] First, as shown in Figure 2 , a typical large-scale wind-solar power generation system is constructed into a weak power grid system, and the new energy is cut off during the fault period Power outer ring, only the current inner ring is retained. In order to realize the independent control of positive and negative sequence power, the positive and negative sequence loops adopt the fixed d-axis voltage control strategy, and the positive and negative sequence components are extracted by the time delay cancellation method. The wind-solar power generation system outlet short circuit ratio is adjustable in the range of 1.5-10. The 0.38 / 35kV transformer T1 in the station is connected to the Dyn11 group, and the 35kV / 220kV step-up main transformer T2 is connected to the YNd11 group. In the large-scale wind-solar power generation system, there are two ways of series connection and parallel connection between different wind-solar power generation units, as shown in Figure 2 , three wind-solar power generation units, wherein the connection relationship between unit 1 and units 2 and 3 is parallel connection, and the lengths of the collection lines are different, the connection relationship between units 2 and 3 is series connection, and the distance from unit 2 to the grid connection point of the wind-solar power generation system is farther than that from unit 3. Figure 2 In Figure 2 , 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 station. The subscript g represents the variable on the equivalent grid side. U, Z represent the voltage and resistance on the node and line, and T represents the transformer. WT represents the wind turbine.

[0061] In the normal operation stage of the wind-solar power generation system, the active power output and real-time capacity of each wind-solar power generation unit are periodically collected, and the equivalent impedance on the grid side is calculated; wherein the three-phase voltage and current data of the PCC point are collected, and the Clark transformation is carried out to obtain the voltage v α , v β current components i α , i β . The current components are further transformed by Park transformation and sequence component separation control to obtain 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 coordination controller is activated every 10 minutes to collect the current active power 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 on the grid side is calculated using an online impedance calculator, and the result is expressed as R. g +jL g .

[0063] The virtual impedance control parameters of the wind and solar power generation units are calculated based on the equivalent impedance on the grid side. Specifically, the collaborative controller located at the grid connection point of the wind and solar power generation system updates the virtual impedance control parameters of each wind and solar power generation unit every 10 minutes based on the currently collected equivalent impedance on 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 taken as 3, and ω is the angular frequency corresponding to the fundamental frequency of the power system. Online impedance estimation technology is relatively mature, so its impedance estimation method will not be elaborated further, and it is assumed that the grid-side impedance is known. In the virtual impedance control parameters, the virtual impedance parameter is inversely proportional to the grid strength; specifically, this parameter changes with the grid strength. 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. Conversely, 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 under different scenarios.

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

[0067]

[0068] In the formula, For node ijk Phase voltage, take a, b or c; superscript “+ / -” represents positive / negative sequence component; γ i.j.k γ i.j.k Phase difference representing positive and negative sequence voltages, α Ti Phase shift representing positive and negative sequence voltages generated by substation main transformer, α i.j.k Phase shift representing positive and negative sequence voltages generated by substation main transformer, α

[0069] Phase voltage The amplitude depends on two types of variables: sequence voltage V i.j.k + , V i.j.k - and its phase difference γ i.j.k γ i.j.k . Amplitude and phase difference γ i.j.k are related to the current output of wind-solar generation units and electrical distance. It can be inferred that the phase voltage of wind-solar generation units decreases monotonically with the increase of electrical distance. In addition, the electrical distance between different sub-stations in a wind-solar generation system is much larger than that between single wind-solar generation units within a sub-station. Therefore, can be regarded as almost the same in a single sub-station, but significantly different between sub-stations. γ i.j.k depends on many factors, including phase deviation caused by line voltage drop, angular deviation caused by multi-stage transformer. Therefore, the overall phase distribution in the wind-solar generation system is disordered, with values ranging from 0 to 2π. According to the voltage amplitude and phase distribution within the wind-solar generation system, the potential trajectory of the abc phase voltage vector in a single sub-station appears as an ellipse on the geometric plane defined as the voltage controllable boundary of the sub-plant. Multiple voltage controllable boundaries of different sub-stations appear as multiple nested ellipses, as Figure 3 shown, 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 generation system. The prior art only considers the maximum phase voltage safety of a single wind-solar generation unit, and its constraint equation can be marked by the pink dot in Figure 3 . Obviously, these points are not the potential maximum phase voltage in the wind-solar generation system. In order to ensure voltage safety, the constraint equation should be located at the top of the major axis of each ellipse, as shown by the yellow dot in Figure 3 . According to Figure 3 , the maximum and minimum expectations of the phase voltage in the entire system can be represented as:

[0070]

[0071] In the formula, V ph.max and V ph.minVmaxand Vmindenote the upper and lower limits of the phase voltage allowed by the power system in normal operation, which are set as 1.1pu and 0.9pu according to existing standards; the subscripts "fur" and "nea" denote the wind and solar power generation units farthest and nearest to the PCC bus in the wind-solar power generation system.

[0072] The expected positive sequence voltage regulation instruction value at the outlet of the wind-solar power generation system after the fault occurs is calculated according to the real-time input capacity and the reference parameters of the power system; specifically, the most conservative case is assumed in the coordination to solve the complex voltage controllable boundary in the process of cooperative low voltage ride through. This case represents the most unfavorable difference in the amplitude of the phase voltage at the grid connection point of each wind-solar power generation unit, and the maximum difference in the amplitude between the farthest and nearest wind-solar power generation units. The most conservative case assumes that each wind-solar power generation unit outputs the maximum positive sequence reactive current and does not output any negative sequence reactive current, thereby causing the maximum voltage drop on the line. Therefore, while performing step 2), the cooperative controller collects the real-time input capacity of each wind-solar power generation unit, calculates the expected positive sequence voltage regulation instruction value at the outlet of the wind-solar power generation system after the fault occurs, and the calculation formula is:

[0073]

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

[0075] Based on the expected positive sequence voltage regulation instruction 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, the expected positive sequence voltage instruction value of each wind-solar power generation unit after the fault is calculated; specifically, it is Figure 2 The expected positive sequence voltage instruction value after the fault is allocated to each wind-solar power generation unit of the three wind-solar power stations in the formula, which is used to provide an upper limit of the controllable voltage constraint of each unit after the fault, and the calculation formula is:

[0076]

[0077] In the formula, V +* is the positive sequence voltage regulation instruction value matrix, is the desired positive sequence voltage command value of the nth wind-solar power generation unit on the mth collection line in the later stage of fault; is the rated current value matrix of the wind-solar power generation unit, is the matrix X co is the rated current value of the nth wind-solar power generation unit represented by the corresponding row X co is the reactance per unit matrix X m.i is the reactance per unit of the ith section of the mth collection line.

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

[0079] At the same time of the above solving and calculating steps, the reactive power distribution coefficient is calculated according to the active power output and the real-time capacity; specifically, the reactive power distribution coefficient is calculated and distributed by the cooperative controller according to the current active power output and the rated capacity of each wind-solar power generation unit in each substation, which is used to distribute the reactive power output task for each unit after fault, and the calculation formula is:

[0080]

[0081] In the formula, ζ i represents the reactive power distribution coefficient of the ith unit in the wind-solar power generation system, P i represents the current active power output of the unit, S i represents the current rated capacity of the unit. Among them, considering that the types of power sources in a substation are similar and the electrical distance is not large, the current active power output and capacity of a substation can be calculated as a whole unit, instead of solving the values of all wind-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-solar generation unit, and can optimize the low penetration reactive power output ratio of each unit after the fault occurs. This parameter can allocate low penetration reactive power according to the current reactive controllable capacity of each wind-solar generation unit after the fault, that is, the unit with large reactive controllable capacity provides more voltage support capability, and the unit with small reactive controllable capacity provides less voltage support capability. Compared with the existing method which does not consider the difference between the controllable capacities of wind-solar units, the present application can realize more efficient utilization of controllable resources of variable frequency devices.

[0083] The virtual impedance control parameters, positive sequence voltage instruction values and reactive power distribution coefficients are sent to each wind-solar generation unit for centralized and collaborative parameter configuration before the fault. Optionally, the period of centralized and collaborative parameter configuration before the fault is consistent with the period of collecting the active power output and real-time capacity of each wind-solar generation unit, and the collection period is preferably 5-10 minutes, and the present embodiment is preferably executed once every 10 minutes. The centralized and collaborative process before the fault is defined as being executed once every 10 minutes, which is different from the existing method which starts to cooperate after the fault occurs. The method of the present embodiment can realize low penetration power coordination with a global perspective at a lower cost. Because these parameters only depend on the system operating state and the active power output of the wind-solar generation unit, they change relatively slowly and do not need to be updated in real time. This centralized and collaborative control before the fault has a lower cost, and can ensure safe and efficient coordinated control with a global perspective.

[0084] When the wind-solar generation system detects that the grid-connected point voltage drop meets the low penetration starting condition, such as a short-circuit fault occurs in the grid, taking the BCG fault as an example, other faults are the same, Figure 2 The fault position is indicated. The system enters the low penetration control mode and starts the virtual node control, at this time each wind-solar generation unit enters the virtual node control state, that is, each wind-solar generation unit dynamically adjusts the positive sequence reactive current instruction and the negative sequence reactive current instruction according to the centralized and 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 instruction, and when the non-fault phase voltage triggers the safety upper limit, the negative sequence reactive current instruction value is dynamically calculated to obtain the negative sequence reactive current instruction, and the non-fault phase voltage is limited to be lower than the safety upper limit through the negative sequence reactive current instruction.

[0085] Each wind and solar power generation unit monitors the positive and negative sequence voltages at the grid connection point in real time. Based on the negative sequence voltage value at the time of the fault and the real-time positive sequence voltage value, the negative sequence voltage command value in the virtual node control is calculated, and voltage constraints are set in the calculation of the negative sequence voltage command value. Specifically, a predefined cooperative low-voltage ride-through control coefficient is used, and the wind and solar power generation units dynamically adjust their output current commands during the adaptive cooperative process after the fault occurs, thus eliminating the need for communication. To maximize voltage support efficiency, this embodiment considers the dynamic power allocation of each wind and solar power generation unit, including two issues: 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. This embodiment adds a reactive power allocation coefficient ζ to the traditional virtual node control. The principle of virtual bus control is that the wind and solar power generation units are connected to a network with a virtual internal impedance Z. v Connected to the virtual power grid, such as Figure 4 As shown. Each wind and solar power generation unit monitors its own grid connection point's positive and negative sequence voltage values ​​in real time, 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 -* V represents the virtual node control negative sequence voltage command value for the i-th wind and solar power generation unit. i. - (0 + () represents the negative sequence voltage value at the grid connection point immediately after the fault occurs, V i + (t) represents the real-time measured positive-sequence voltage value at the grid connection point. The optimal positive and negative sequence ratio of reactive power output for wind and solar power generation units was analyzed during the command value calculation. The analysis results show that to maximize the increase in fault phase voltage amplitude within a limited controllable reactive power capacity, positive-sequence reactive power should be generated as much as possible, and negative-sequence reactive power output should only be used when the voltage safety boundary is triggered. Based on this principle, a dynamic optimal positive and negative sequence ratio control was proposed, whereby the negative-sequence voltage command is dynamically adjusted according to the change in the positive-sequence voltage at the grid connection point of the wind and solar power generation unit, maximizing the fault phase voltage while ensuring the safety of the non-fault phase voltage. This efficient utilization of controllable reactive power capacity allows this embodiment to have a stronger fault voltage support efficiency compared to existing methods.

[0088] After the wind-solar 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] wherein, I Q.i +* represents the given positive sequence reactive current command value of the i th wind-solar generation unit during low penetration, and Δt represents the running interval of the wind-solar generation unit controller.

[0091] The above design introduces and improves the virtual node control. After the fault occurs, each wind-solar generation unit enters the virtual node control state, and the virtual node control can realize distributed cooperation without communication. Each unit dynamically adjusts under the running parameters given by the foregoing centralized cooperation, and finally reaches the optimal low penetration running state of the entire wind-solar generation system. Compared with the traditional virtual node control strategy, the embodiment embeds a reactive power distribution coefficient, and it can be seen from the formula that the coefficient determines the response speed of the virtual node control of the unit. The greater the reactive power distribution coefficient, the faster the response speed of the virtual node control. The embodiment adjusts the size of the reactive power distribution coefficient of each unit to regulate the speed of the low penetration reactive response of each node, thereby indirectly allocating the low penetration reactive output of each unit after the fault occurs. Compared with the existing method, the controllable resources of reactive power can be fully utilized without occupying the resources of high active power wind-solar generation units.

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

[0093]

[0094] wherein, the variable of the negative sequence control loop has the same meaning as the foregoing formula, and it can be seen that the value of ζ controls the response speed of the wind-solar generation unit output command. According to the ratio of the active power output between different wind-solar generation units, different ζ are allocated to each unit, which can adaptively adjust the response speed of the unit low voltage penetration control, thereby indirectly allocating the reactive power ratio of each wind-solar generation unit. The value of ζ is calculated in advance according to the ratio between the reactive controllable capacity of each wind-solar generation unit and the reactive controllable capacity of the entire wind-solar generation system in the foregoing step.

[0095] wherein, the voltage constraint condition is that the non-fault phase voltage does not exceed the safe upper limit. Based on the virtual impedance control parameter and the reactive power distribution coefficient, and the negative sequence voltage command value, the positive sequence reactive current command and the negative sequence reactive current command are generated, and it is checked in real time whether the positive sequence reactive current command and the negative sequence reactive current command satisfy 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 Q.i+* (t) and I Q.i -* After (t), substitute it into the following inequality and determine whether it holds true; the inequality expression is:

[0096]

[0097] In the formula, and I lim.i γ represents the safe upper limit of the active power second harmonic oscillation amplitude and the output phase current amplitude allowed by the hardware of the i-th wind and solar power generation unit. i This represents the phase difference between the positive and negative sequence voltages at the grid connection point of the wind and solar power generation unit, and is the positive sequence active current command; I α1,i I α2,i Iβ 1,i Iβ 2,i The positive and negative sequence current components in the αβ coordinate system are obtained through 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 operating limit. In the formula, let the positive and negative sequence active current commands I... P.i +* Set to zero.

[0098] If the conditions are met, the virtual node control process continues, and the positive-sequence active current command I... P.i +* Generate in real time according to the following formula:

[0099]

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

[0101] If the conditions are not met, the control process will be stopped, and the positive and negative sequence reactive current commands I will be sent. Q.i +* and I Q.i -* The value of the previous operation step size is fixed and remains unchanged, and the positive sequence active current command I... P.i +* Always set to zero.

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

[0103] In another aspect, the embodiment also provides a wind-solar power generation system multi-time scale collaborative low voltage ride through control device, which is a collaborative controller for performing control operations of the embodiment, mainly comprising a processor and a memory; the memory stores computer readable instructions, and the processor executes the instructions to perform the wind-solar power generation system multi-time scale collaborative low voltage ride through control method; further comprising a low voltage ride through module, configured to perform control operations of each unit after a fault, that is, 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 collaborative parameter configuration. The low voltage ride through module is embedded in the controller of each wind-solar power generation unit.

[0104] Embodiment 1;

[0105] Referring to a wind-solar power generation system in a certain place, a topological structure as shown in Figure 2 is constructed, and the specific parameters of the wind-solar power generation system are as shown in Table 1:

[0106] Table 1

[0107]

[0108]

[0109] In order to prove the performance advantage of the method of the embodiment compared with the traditional method, the performance comparison under the BCG fault is given as shown in Figures 5(a)-5(f) , and specifically, in the above set of FIG. 5, the voltage lifting performance of the traditional method and the method of the embodiment is compared. Among them, the test system comes from Figure 2 , which contains 3 wind-solar units, namely unit 1, unit 2 and unit 3. As can be seen from FIG. 5(a), FIG. 5(c) and FIG. 5(e), the voltage support capability of the traditional method is insufficient, after the fault occurs, unit 1 first detects that the b-phase voltage is lower than the off-grid threshold, unit 1 off-grid leads to further reduction of the reactive power shortage in the system, resulting in the b-phase voltage of unit 3 being lower than the off-grid threshold, unit 3 off-grid, and further triggering a chain effect, causing unit 2 to also eventually off-grid. Therefore, the traditional method may cause the above-mentioned post-fault wind-solar power cascading off-grid problem due to insufficient voltage support capability. In contrast, as can be seen from FIG. 5(b), FIG. 5(d) and FIG. 5(f), the method of the embodiment coordinates the wind-solar power under different fault states, fully excavates the wind-solar power under shallow voltage drop to support the units under deep voltage drop, and realizes the improvement of the voltage support capability of the entire wind-solar power generation system. The three wind-solar units do not reach the off-grid threshold and still remain safe and on-grid operation. In summary, the voltage support capability of the method of the embodiment compared with the traditional method is fully verified.

[0110] Further, in the above example, the wind-solar power generation system adopts a low voltage ride through control strategy, with a reactive power support coefficient K = 1.5. The fault off-grid voltage threshold of the wind turbine is set to 0.2 p.u. In the traditional method, there is a lack of coordination, so multiple wind-solar power generation units in different fault states cannot maximize the lifting of the fault phase voltage at the grid connection point. Therefore, the wind-solar power generation units in the first substation detect that the b-phase voltage has fallen below the off-grid threshold, and the wind turbine exits operation. This action further weakens the reactive power support capability of the wind-solar power generation system, prompting the wind-solar power generation units in the third substation to reach their off-grid threshold, ultimately leading to the cascading off-grid of all units in the wind-solar power generation system. In contrast, the proposed coordinated low voltage ride through control takes full advantage of the voltage support capability of each wind-solar power generation unit from the beginning, enabling units further away from the fault to provide additional reactive power and assisting units closer to the fault, without the occurrence of cascading off-grid accidents under fault, demonstrating stronger fault voltage support capability.

[0111] In addition, Table 2 also comprehensively tests various fault conditions, in which the percentage increase in fault voltage support capability of the proposed method compared to the traditional method is listed.

[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 voltage at the grid connection point of the corresponding wind-solar power generation system by about 0.1 pu. In contrast, the low voltage ride through control in this embodiment exhibits stronger voltage support capability: when the fault voltage drops to no less than 0.3 pu or more, the voltage support provided is more than 110% higher than that of the traditional method.

[0115] To further verify the wide applicability in various wind-solar power generation system scenarios, Figures 6(a1)-6(c2) The influence of different wind-solar power generation systems accessing different short circuit ratios (ranging from 10.0 to 1.5) and different maximum electrical distance differences between wind-solar power generation units (ranging from 3 kilometers to 76 kilometers) under BC fault is discussed. And the Figure 2 The C-phase voltage at the 35kV side of the grid connection point (PCC2) of the unit 2 with the farthest electrical distance is taken as an example, and the waveforms under the existing centralized coordinated control method, the distributed coordinated control method and the three methods of the present application are compared as Figures 6(a1)-6(c2)It can be seen from FIG. 6(a1) and FIG. 6(a2) that the centralized coordinated control cannot keep the healthy phase voltage of unit 2 safe when maximizing the current output of the wind-solar generation units under low SCR conditions (SCR < 2.9), and is only suitable for strong grid scenarios. At the same time, although the distributed coordinated control in FIG. 6(b1) and FIG. 6(b2) can work well under high SCR (SCR > 5.0) or small electrical distance (< 29 km), it ignores the disorderly fluctuations of multiple wind-solar generation units during the fault transient, resulting in short-term overvoltage problems in certain cases of large electrical distance (> 29 km) and low SCR (< 2.9). In contrast, the present application in FIG. 6(c1) and FIG. 6(c2) can maximize the voltage support capability of the entire wind-solar generation system, suitable for different system short-circuit ratios and unit electrical distance differences, and has wider adaptability.

[0116] Finally, it should be noted that the above is only used to illustrate the technical solutions of the present application, and is not a limitation on the scope of protection of the present application. Simple modifications or equivalent replacements of the technical solutions of the present application made by those skilled in the art do not deviate from the essence and scope of the technical solutions of the present application.

Claims

1. A method for multi-time scale collaborative low voltage ride through control of a wind-solar hybrid power system, comprising the following steps: periodically collecting active power output and real-time injected capacity of each wind-solar hybrid power unit and calculating equivalent impedance of a grid side during a normal operation stage of the wind-solar hybrid power system; calculating virtual impedance control parameters of the wind-solar hybrid power unit according to the equivalent impedance of the grid side; calculating an expected positive sequence voltage regulation instruction value of an outlet of the wind-solar hybrid power system after a fault occurs based on real-time injected capacity and power system reference parameters, and calculating an expected positive sequence voltage instruction value of each wind-solar hybrid power unit after the fault occurs based on the expected positive sequence voltage regulation instruction value after the fault occurs, inherent parameters and rated parameters of a collection line where each wind-solar hybrid power unit is located; calculating a reactive power distribution coefficient based on the active power output and the real-time injected capacity; downloading the virtual impedance control parameters, the positive sequence voltage instruction value and the reactive power distribution coefficient to each wind-solar hybrid power unit to perform centralized collaborative parameter configuration before the fault occurs; when the wind-solar hybrid power system detects that a voltage drop of a grid connection point meets a low ride through starting condition, entering a low ride through control mode and starting virtual node control, and each wind-solar hybrid power unit dynamically adjusts positive sequence reactive current instruction and negative sequence reactive current instruction according to the centralized collaborative parameter configuration. In the virtual node control, the following steps are included: 2.The method of claim 1, wherein: each wind-solar hybrid power unit detects real-time positive sequence voltage and negative sequence voltage of the grid connection point, calculates a negative sequence voltage instruction value in the virtual node control based on the negative sequence voltage value at the time when the fault occurs and the real-time positive sequence voltage value, and sets a voltage constraint condition in the negative sequence voltage instruction value; the voltage constraint condition is that a non-fault phase voltage does not exceed a safe upper limit. In the virtual node control, the following steps are also included:

3. The method of claim 2, wherein the method further comprises: each wind-solar hybrid power unit generates the positive sequence reactive current instruction and the negative sequence reactive current instruction based on the virtual impedance control parameters, the reactive power distribution coefficient and the negative sequence voltage instruction value, and real-time checks whether the positive sequence reactive current instruction and the negative sequence reactive current instruction meet hardware safe upper limits of active power oscillation amplitude and phase current amplitude, if yes, continues to execute the virtual node control process, and if not, keeps the reactive current instruction of the last step and sets the positive sequence active current instruction to zero. the expected positive sequence voltage instruction value of each wind-solar hybrid power unit after the fault occurs is calculated by the following formula: 4.The method of claim 1, wherein: Computing the expected positive sequence voltage regulation command value at the export of a wind power generation system after a fault occurs The formula is: In the formula, V is the reference value of voltage regulation of the wind-solar power generation system, V ph.max V is the upper limit of the phase voltage allowed by the power system during normal operation, V ph.min V is the lower limit of the phase voltage allowed by the power system during normal operation, X i.j X is the reactance unit value of the jth segment of the ith collection line in the wind-solar power generation system, I N.i.j X is the current rated value of the wind-solar power generation unit on the jth segment of the ith collection line in the wind-solar power generation system; m is the number of collection lines in the wind-solar power generation system, and n is the maximum number of wind-solar power generation units connected to each collection line.

5. The method of claim 4, wherein the method further comprises: the period of the centralized collaborative parameter configuration before the fault occurs is consistent with the period of collecting the active power output and the real-time injected capacity of each wind-solar hybrid power unit. In the formula, V +* is a positive sequence voltage regulation instruction value matrix, is the expected positive sequence voltage instruction value of the nth wind-solar power generation unit on the mth collection line after failure; is a rated current value matrix of the wind-solar power generation unit, is the matrix X co is the rated current value of the nth wind-solar power generation unit on the collection line represented by the corresponding row; X co is a reactance per unit matrix, X m.j is the reactance per unit of the ith section of the mth collection line. 6.The method of claim 1, wherein: the equivalent impedance of the grid side is solved by an online impedance calculator. 7.The method of claim 1, wherein: in the virtual impedance control parameters, the virtual impedance parameter is inversely proportional to the grid strength. 8.The method of claim 1, wherein: the dynamic adjustment of each wind-solar hybrid power unit according to the centralized collaborative parameter configuration includes the following steps: 9.The method of claim 1, wherein: firstly, the fault phase voltage is lifted through the positive sequence reactive current instruction, and when the non-fault phase voltage triggers the safe upper limit, the negative sequence reactive current instruction value is dynamically calculated to obtain the negative sequence reactive current instruction; the non-fault phase voltage is limited to be lower than the safe upper limit through the negative sequence reactive current instruction. ​ 10. A wind-solar hybrid power system multi-time scale collaborative low voltage ride through control device, characterized in that, The wind-solar power generation system multi-time scale collaborative low voltage ride through control method comprises a processor and a memory; the memory stores computer readable instructions; when the processor runs the computer readable instructions, the wind-solar power generation system multi-time scale collaborative low voltage ride through control method of any one of claims 1 to 9 is executed.

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