Boundary calculation method and execution method for active reference instruction during low voltage ride-through of constructed network type converter

Abstract

The invention provides a boundary calculation method and execution method for an active reference instruction during low voltage ride-through of a network construction type converter, and the calculation method comprises the steps: obtaining a stability domain of a system in multiple cycles, and determining a system energy conservation formula; calculating the energy change when the system passes through the UEP of the next period from the UEP of any period, and determining the potential energy variation between two adjacent UEPs in the energy change; determining periodic minimum damping dissipation energy and a corresponding trajectory, taking a condition that the potential energy variation is smaller than the periodic minimum damping dissipation energy as a system stability condition, and constructing an equal-energy function trajectory based on the system stability condition; according to the equal energy function trajectory, an expression of damping dissipation energy moving along the equal energy function trajectory is determined, and a calculation formula of the active power reference instruction boundary is determined according to the expression. According to the method, the active reference instruction boundary capable of ensuring global stability of the system can be solved, and the stability of the converter can be fully ensured.

Description

Technical Field

[0001] The present application relates to the technical field of power electronic equipment, and in particular to a boundary calculation method and an execution method of an active reference instruction during low voltage ride-through of a grid-connected converter. Background Art

[0002] At present, in the field of energy storage converters, grid-type converters are gradually gaining popularity due to their advantages such as high independence. Similar to synchronous generators, grid-type converters are also characterized by voltage source characteristics, and can build voltage-stable operation without relying on the external AC system. However, compared with synchronous generators, the control parameters of grid-type converters are more flexible, which increases the complexity of stability analysis, and due to the current carrying capacity of the device, grid-type converters usually need to operate with current limiting in the event of a fault, so that they present different dynamic response characteristics from synchronous generators during fault ride-through. Therefore, compared with synchronous generators, there are more challenges in the transient stability problem of grid-type converters.

[0003] In the related art, when studying the transient stability of grid-type converters, the same idea as that of traditional synchronous generators is usually adopted, that is, analyzing the influence of the damping coefficient on the transient stability of the converter. However, in practical applications, adjusting the damping coefficient often affects the dynamic response performance of the converter system. Therefore, how to fully ensure the stability of the grid-type converter during fault ride-through without affecting the dynamic performance of the system has become an urgent problem to be solved. Summary of the invention

[0004] The present application aims to solve one of the technical problems in the related art at least to some extent.

[0005] To this end, the first purpose of the present application is to propose a method for calculating the boundary of active reference instructions of a grid-type converter during low voltage crossing. This method can solve the boundary of active reference instructions that can ensure the global stability of the system, which is conducive to fully ensuring the stability of the converter.

[0006] The second object of the present application is to propose a method for executing an active reference instruction during low voltage ride-through of a grid-connected converter;

[0007] The third object of the present application is to provide a boundary calculation device for active reference instructions during low voltage ride-through of a grid-type converter;

[0008] The fourth object of the present application is to provide a device for executing active reference instructions during low voltage ride-through of a grid-type converter;

[0009] A fifth object of the present application is to provide a non-transitory computer-readable storage medium.

[0010] To achieve the above-mentioned purpose, the first aspect of the present application is to propose a boundary calculation method for active reference instructions of a grid-connected converter during low voltage ride-through, the method comprising the following steps:

[0011] The stability domain of the grid-connected system of the grid-connected converter is extended to obtain the stability domain of the system in multiple periods, and the energy conservation formula of the system is determined based on the periodic nature of the energy function of the system;

[0012] Based on the stable domain over the multi-period and the energy conservation formula, calculate the energy change of the system when it crosses from the unstable equilibrium point UEP of any period to the UEP of the next period, and determine the potential energy change between two adjacent UEPs in the energy change;

[0013] Determine the minimum damping dissipation energy of the period and the corresponding trajectory line, take the potential energy change amount being less than the minimum damping dissipation energy of the period as a system stability condition, and construct an equal energy function trajectory based on the system stability condition, wherein the damping dissipation energy moving along the equal energy function trajectory is less than the minimum damping dissipation energy of the period;

[0014] According to the equal energy function trajectory, an expression for the damping dissipated energy moving along the equal energy function trajectory is determined, and according to the expression, a calculation formula for the active reference instruction boundary is determined.

[0015] Optionally, in one embodiment of the present application, the stability domain of the grid-connected system of the grid-connected converter is extended to obtain the stability domain of the system over multiple cycles, including: constructing a second-order transient model of the grid-connected system of the grid-connected converter, and determining the energy function of the system state variables of the second-order transient model; extending the stability domain period within the neighborhood of the system origin to obtain the stability domain of the energy function over multiple cycles; and deriving the stability domain of the system over multiple cycles based on the stability domain of the energy function over multiple cycles.

[0016] Optionally, in one embodiment of the present application, determining the system energy conservation formula includes: converting the energy functions in different periods to the 0th period, setting the first point and the second point on the actual trajectory of the system, and calculating the path integral expression from the first point to the second point; sorting the path integral expression, and based on the kinetic energy and potential energy in the energy function, equating the sorted path integral expression to the system's energy conservation theorem expression; and converting the energy conservation theorem expression into the energy conservation formula.

[0017] Optionally, in one embodiment of the present application, the energy change of the computing system when it crosses from the unstable equilibrium point UEP of any cycle to the UEP of the next cycle includes: converting the trajectory within the any cycle to the 0th cycle, and determining the expression of the energy change according to the sorted path integral expression, wherein the energy change is equal to the difference between the potential energy change and the energy dissipated by the trajectory through the any cycle; determining the potential energy change between two adjacent UEPs in the energy change includes: calculating the potential energy change according to the active reference instruction and the angular frequency of the system stable equilibrium point.

[0018] Optionally, in one embodiment of the present application, the equal energy function trajectory passes through a first coordinate point and a second coordinate point, the vertical coordinates of the first coordinate point and the second coordinate point represent the power angle change rate and have a value of 0, the horizontal coordinate of the first coordinate point is a first parameter, and the horizontal coordinate of the second coordinate point is a second parameter, and the expression for determining the damping dissipation energy of the movement along the equal energy function trajectory includes: determining the energy conservation equation satisfied by the UEP from any point on the equal energy function trajectory to the first period; substituting the calculation formula of the second parameter into the energy conservation equation to obtain the expression of the vertical coordinate of any point; setting the vertical coordinate of any point to 0, and substituting the expression of the active reference instruction into the expression of the vertical coordinate of any point to calculate the value of the first parameter according to the obtained formula; using the value of the first parameter and the expression of the vertical coordinate of any point to determine the expression of the damping dissipation energy of the movement along the equal energy function trajectory.

[0019] Optionally, in one embodiment of the present application, the calculation formula for determining the active reference instruction boundary based on the expression includes: determining a stability criterion based on the expression of the damping dissipated energy moving along the equal energy function trajectory; and deriving the calculation formula for the active reference instruction boundary based on the stability criterion.

[0020] To achieve the above-mentioned purpose, the second aspect of the present application further proposes a method for executing an active reference instruction of a grid-connected converter during low voltage ride-through, comprising the following steps:

[0021] During the operation of the grid-type converter, it is determined in real time whether a fault occurs in the grid-connected system of the grid-type converter, and in the event of a fault, boundary parameters of an active reference instruction and a preset initial active power reference value are obtained, wherein the boundary parameters of the active reference instruction are calculated using the method described in any one of the embodiments of the first aspect;

[0022] Obtaining an active reference value and a calculation step size of the grid-connected converter at any time, and determining whether a boundary function value corresponding to the active reference value is greater than the boundary parameter;

[0023] In the case where it is greater than the boundary parameter, taking the active reference value as the maximum reference value, and updating the initial active power reference value to the maximum reference value, so as to realize the switching control of the grid-connected converter;

[0024] In the case that it is not greater than the boundary parameter, the difference between the active reference value and the calculation step length is used as the active reference value at the next moment, and the calculation and comparison judgment of the boundary function value at the next moment are repeated.

[0025] To achieve the above-mentioned purpose, the third aspect of the present application further proposes a boundary calculation device for active reference instructions of a grid-connected converter during low voltage ride-through, comprising the following modules:

[0026] The extension module is used to extend the stability domain of the grid-connected system of the grid-connected converter to obtain the stability domain of the system over multiple periods, and determine the energy conservation formula of the system based on the periodic nature of the energy function of the system;

[0027] A determination module, configured to calculate the energy change of the system when it crosses from an unstable equilibrium point UEP of any period to a UEP of the next period based on the stable domain over the multi-period and the energy conservation formula, and determine the potential energy change between two adjacent UEPs in the energy change;

[0028] A construction module is used to determine the minimum damping dissipation energy of the period and the corresponding trajectory line, taking the potential energy change amount being less than the minimum damping dissipation energy of the period as a system stability condition, and constructing an equal energy function trajectory based on the system stability condition, wherein the damping dissipation energy moving along the equal energy function trajectory is less than the minimum damping dissipation energy of the period;

[0029] A calculation module is used to determine an expression of the damping dissipated energy moving along the equal energy function trajectory according to the equal energy function trajectory, and to determine a calculation formula for an active reference instruction boundary according to the expression.

[0030] To achieve the above-mentioned purpose, the fourth aspect of the present application further proposes a device for executing active reference instructions of a grid-connected converter during low voltage ride-through, comprising the following modules:

[0031] An acquisition module, used for determining in real time during the operation of the grid-type converter whether a fault occurs in the grid-connected system of the grid-type converter, and obtaining boundary parameters of an active reference instruction and a preset initial active power reference value in the event of a fault, wherein the boundary parameters of the active reference instruction are calculated using the method described in any one of the embodiments of the first aspect;

[0032] A judgment module, used for obtaining an active reference value and a calculation step size of the grid-connected converter at any time, and judging whether a boundary function value corresponding to the active reference value is greater than the boundary parameter;

[0033] A switching module, used to use the active reference value as the maximum reference value and update the initial active power reference value to the maximum reference value when the active reference value is greater than the boundary parameter, so as to realize the switching control of the grid-connected converter;

[0034] The circulation module is used to take the difference between the active reference value and the calculation step as the active reference value at the next moment when the difference is not greater than the boundary parameter, and return to recalculate and compare the boundary function value.

[0035] In order to implement the above-mentioned embodiments, the fifth aspect embodiment of the present application also proposes a non-temporary computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, it implements a boundary calculation method of an active reference instruction of a grid-type converter during low voltage crossing in the above-mentioned first aspect embodiment, or a method for executing an active reference instruction of a grid-type converter during low voltage crossing in the above-mentioned second aspect embodiment.

[0036] The technical solution provided by the embodiment of the present application brings at least the following beneficial effects: based on the full consideration of the multi-cycle stability potential of the grid-type converter, the present application adopts the energy function method to analyze the transient energy change process of the converter, and uses the equal energy trajectory method to approximately scale the actual trajectory to obtain the active reference instruction boundary that can ensure the global stability of the system. Then, when a fault of any duration and depth occurs in the system, as long as the active reference instruction of the grid-type converter is switched to meet the calculated boundary requirements, the overall safety and stability of the system can be guaranteed. Therefore, the present application can accurately and reliably calculate the adjustment boundary of the active reference instruction and fully consider the stability of the grid-type converter in multiple cycles. When a fault is detected in real time during the operation of the grid-type converter, the active reference instruction of the converter can be quickly switched, which improves the timeliness of the active reference instruction switching and ensures the safe and stable operation of the converter in actual working conditions.

[0037] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0039] Figure 1A flow chart of a method for calculating the boundary of an active reference instruction during low voltage ride-through of a grid-connected converter proposed in an embodiment of the present application;

[0040] Figure 2 A schematic diagram of a grid-connected structure of a grid-connected converter based on virtual synchronous machine control proposed in an embodiment of the present application;

[0041] Figure 3 A schematic diagram of the principle of active reference instruction boundary calculation of a grid-type converter proposed in an embodiment of the present application;

[0042] Figure 4 A schematic diagram of a multi-period stability domain proposed in an embodiment of the present application;

[0043] Figure 5 A schematic diagram of constructing an equal energy trajectory proposed in an embodiment of the present application;

[0044] Figure 6 A flowchart of a method for executing an active reference instruction during low voltage ride-through of a grid-connected converter proposed in an embodiment of the present application;

[0045] Figure 7 A schematic diagram of the principle of switching control performed by a grid-connected converter proposed in an embodiment of the present application;

[0046] Figure 8 A schematic diagram of a specific active reference instruction constraint relationship proposed in an embodiment of the present application;

[0047] Fig. 9 A schematic diagram of the structure of a boundary calculation device for active reference instructions during low voltage ride-through of a grid-type converter proposed in an embodiment of the present application;

[0048] Fig.10 A schematic diagram of the structure of a device for executing active reference instructions during low voltage ride-through of a grid-connected converter proposed in an embodiment of the present application. DETAILED DESCRIPTION

[0049] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0050] It should be noted that in the actual application of grid-type converters, for transient stability, reducing the active reference command during a fault can play a similar role to adjusting the damping effect, and the adjustment of the active reference command only changes the system input and does not involve changes in structural parameters, so it will not affect the dynamic performance.

[0051] However, the research on reducing the active reference command during the fault in the relevant embodiments only shows that it can have a positive effect on improving the stability of the system, and does not provide a calculation method for the corresponding adjustment boundary. In addition, the research in the relevant embodiments is only aimed at the stability of the first cycle of the grid-type converter, that is, only the first swing stability is considered, and the stability of other cycles is not considered. This method is not conducive to maintaining the stability of the converter.

[0052] To this end, the present application proposes a boundary calculation method and execution method for the active reference instruction of a grid-type converter during low voltage crossing, which can solve the active reference instruction boundary that can ensure the global stability of the system, which is conducive to fully ensuring the stability of the converter.

[0053] The following describes in detail with reference to the accompanying drawings a method for calculating the boundary of an active reference instruction of a grid-type converter during low voltage ride-through and a method for executing the boundary of the active reference instruction of the grid-type converter proposed in an embodiment of the present application.

[0054] Figure 1 A flow chart of a method for calculating the boundary of an active reference instruction of a grid-connected converter during low voltage ride-through proposed in an embodiment of the present application, such as Figure 1 As shown, the method comprises the following steps:

[0055] Step S101, extending the stability domain of the grid-connected system of the grid-connected converter to obtain the stability domain of the system over multiple periods, and determining the energy conservation formula of the system based on the periodic property of the energy function of the system.

[0056] Specifically, the present application first extends the stability domain of the grid-type converter to multiple cycles, so as to fully consider the multi-cycle stability potential of the grid-type converter when calculating the boundary.

[0057] For example, the present application proposes a grid-connected converter system based on virtual synchronous machine control (hereinafter referred to as a system), the structure of which is as follows: Figure 2 As shown, the system includes several parts such as a filter link module, a bridge inverter, an inner loop control module, an active control loop and a reactive control loop. The PCC in the figure represents a common connection point.

[0058] based on Figure 2 The system shown, in one embodiment of the present application, extends the stability domain of the grid-connected system of the grid-connected converter to obtain the stability domain of the system over multiple cycles, including the following steps: first, construct a second-order transient model of the grid-connected system of the grid-connected converter, and determine the energy function of the system state variables with respect to the second-order transient model; extend the stability domain period within the neighborhood of the system origin to obtain the stability domain of the energy function over multiple cycles; derive the stability domain of the system over multiple cycles based on the stability domain of the energy function over multiple cycles.

[0059] Specifically, in this embodiment, it is assumed that Figure 2 The PCC point in the output is a constant voltage, denoted as E. Transform the system stable equilibrium point to the origin, and let x1 = δ-δ s , x2 = ω-ω0, where δ and ω are the instantaneous virtual power angle and instantaneous angular frequency, respectively, δ s and ω0 are the virtual power angle and angular frequency of the equilibrium point respectively. Then the second-order transient model of the grid-connected system of the grid-connected converter is obtained as follows:

[0060]

[0061] Among them, x1 and x2 are system state variables; J is the virtual inertia; P ref is the active power command; V g is the grid voltage; X g is the line impedance; D is the damping coefficient. Thus, the energy function V(x1,x2) about the system state variables can be obtained.

[0062] Then, record x 1a =-π-2δ s , x 2a =π-2δ s According to the periodicity of the sine function, the stable domain period in the neighborhood of the origin is extended to obtain the stable domain of the energy function V(x1,x2) in multiple periods as shown in the following formula:

[0063] x 1a +2kπ <x1<x 1b +2kπ, k∈Z (2)

[0064] Then, after the change of formula (2), the stability domain of the system over multiple periods is obtained as shown in the following formula:

[0065] δ u,k <δ<δ u,k+1 , k∈Z (3)

[0066] In order to more clearly illustrate the boundary calculation method of this application, Figure 3 The specific calculation process shown is an exemplary description. This step corresponds to Figure 3 The stability region is extended to multiple periods as shown in .

[0067] It should be noted that the active reference instruction P ref It refers to the reference value used to guide the output active power of the inverter or other power equipment in the AC circuit.

[0068] Furthermore, based on the periodic nature of the system's energy function, the system's energy conservation formula is determined.

[0069] In one embodiment of the present application, determining the energy conservation formula of the system includes the following steps: first, converting all energy functions in different periods to the 0th period, setting the first point and the second point on the actual trajectory of the system, and calculating the path integral expression from the first point to the second point; then sorting the path integral expression, and based on the kinetic energy and potential energy in the energy function, the sorted path integral expression is equivalent to the energy conservation theorem expression of the system; finally, converting the energy conservation theorem expression into the energy conservation formula.

[0070] Specifically, in the formula (3) of the above embodiment, δ u,k =(2k-1)π-δ s represents the unstable equilibrium point (UEP) of the kth cycle. For the convenience of analysis, the present application embodiment proposes Figure 4 The stability domain of the system shown in Figure 1 is in multiple periods. s,k =2kπ+δ s The stable equilibrium point (SEP) of the kth cycle is only when the virtual power angle δ does not exceed δ u,k+1 , the converter will converge to the SEP over k cycles.

[0071] Since the energy function in each cycle of the system has a periodic nature, the energy functions of different cycles can be transformed into the 0th cycle for discussion, which satisfies the following expression:

[0072] V k (x1,x2)=V(x1-2kπ,x2), (4)

[0073] (2k-1)π-2δ s <x1<(2k+1)π-2δ s ,k∈Z

[0074] Assume that point P(x 1p ,x 2p ) and Q(x 1q ,x 2q ) are two points on a certain actual trajectory. Divide the two equations of the above formula (1) for these two points and integrate along the path from point P to point Q to obtain the following formula:

[0075]

[0076] After reorganizing the above formula (5), we can get the following formula:

[0077]

[0078] Formula (6) is the rearranged path integral expression. Then, with the help of the kinetic energy V in the energy function KE and potential energy V PE The above formula (6) is equivalent to:

[0079]

[0080] Formula (7) is the energy conservation theorem of the entire system. When the system moves from point P to point Q, the difference in the system energy function is the damping dissipation energy. Formula (7) can then be transformed and derived as shown in the following formula:

[0081]

[0082] Thus, the energy conservation formula of the system was determined.

[0083] Step S102, based on the stability domain and energy conservation formula over multiple cycles, calculate the energy change of the system when it passes from the unstable equilibrium point UEP of any cycle to the UEP of the next cycle, and determine the potential energy change between two adjacent UEPs in the energy change.

[0084] Specifically, this step analyzes the energy change relationship of the system after one cycle, and calculates the energy change after one cycle based on the stability domain and energy conservation formula of the system over multiple cycles analyzed in the previous step.

[0085] In one embodiment of the present application, the energy change of the computing system when it crosses from the unstable equilibrium point UEP of any cycle to the UEP of the next cycle includes: first converting the trajectory within any cycle to the 0th cycle, and determining the expression of the energy change according to the sorted path integral expression, wherein the energy change is equal to the difference between the potential energy change and the energy dissipated by the trajectory through any cycle; determining the potential energy change between two adjacent UEPs in the energy change includes: calculating the potential energy change according to the active reference instruction and the angular frequency of the system stable equilibrium point.

[0086] Specifically, in this embodiment, referring to the above example, when the system crosses from the UEP in the kth cycle to the UEP in the k+1th cycle, the coordinates of the two points on the trajectory are recorded as δ u,k (x 1a +2kπ,x 2,k ) and δ u,k+1 (x 1b +2kπ,x 2,k+1 ). Since the above embodiment has analyzed the stability domain of the system and the periodic nature of the energy function, the trajectory in the kth period can be converted to the 0th period for discussion. Furthermore, the following formula can be obtained from the above formula (6):

[0087]

[0088] in, It represents the potential energy change between two adjacent UEPs; Represents the energy dissipated by the trajectory after the kth cycle.

[0089] Furthermore, the potential energy change between adjacent UEPs is calculated by the following formula:

[0090]

[0091] It can be seen from the above formula that the change in potential energy of the system in adjacent cycles is determined by the size of the active power command. The specific value can be obtained by multiplying the size of the active power command by 2π and then dividing it by the angular frequency ω0 of the system's stable equilibrium point.

[0092] Step S103, determine the periodic minimum damping dissipation energy and the corresponding trajectory, take the potential energy change less than the periodic minimum damping dissipation energy as the system stability condition, and construct an equal energy function trajectory based on the system stability condition, wherein the damping dissipation energy moving along the equal energy function trajectory is less than the periodic minimum damping dissipation energy.

[0093] Specifically, the above embodiment is used to explain that since there is no intersection between different trajectories, when x2(x 1b )=0, the area enclosed by the trajectory and the x-axis is the smallest, that is, the energy dissipated by damping in the cycle is the smallest, which is defined as the minimum damping dissipation energy in the cycle, recorded as The corresponding trajectory line at this time is recorded as trajectory line C min .

[0094] Furthermore, when the following formula (11) is satisfied, the kinetic energy and potential energy of the system decrease after each cycle. After several cycles, the system energy function will inevitably be less than the critical energy function value and converge to the stable equilibrium point of the last cycle. Formula (11) is as follows:

[0095]

[0096] From this we can get the condition for system stability:

[0097]

[0098] It should be noted that due to the periodic minimum damping dissipation energy The expression of is complex and related to the state variables, which is difficult to calculate directly in practical applications. Therefore, this application constructs the intermediate quantity by constructing the equal energy function trajectory. make To satisfy To meet the above system stability conditions.

[0099] The iso-energy function trajectory constructed in the present application is described in detail below.

[0100] In one embodiment of the present application, the equal energy function trajectory passes through a first coordinate point and a second coordinate point, the vertical coordinates of the first coordinate point and the second coordinate point represent the power angle change rate and the value is 0, the horizontal coordinate of the first coordinate point is the first parameter, and the horizontal coordinate of the second coordinate point is the second parameter.

[0101] For example, Figure 5 As shown, in Figure 5 Construct a line through the first coordinate point (x 1m ,0) and the second coordinate (x 1b ,0), denoted as C eq , x 1m is the first parameter, x 1b is the second parameter.

[0102] Assumption C min There exists a point P(x 1p ,x 2p ), C eq There exists a point Q(x 1q ,x 2q ), and satisfy x 1p =x 1q When x1∈[x 1a ,x 1m ), C eq does not exist; when x1∈[x 1m ,x 1b ), along point P to point δ u,1 The path satisfies the following formula:

[0103]

[0104] Because C eq The energy of each point on the 1q ,x 2q )=E(x 1b ,0), so the above formula (13) can be converted into the following formula:

[0105] E(x 1p ,x 2p )>E(x 1b ,0)=E(x 1q ,x 2q ) (14)

[0106] Expanding the above formula (14) yields the following formula:

[0107] V KE(x 2p )+V PE (x 1p )>V KE (x 2q )+V PE (x 1q ) (15)

[0108] Due to x 1p =x 1q , then V PE (x 1p )=V PE (x 1q ), namely V KE (x 2p )>V KE (x 2q ), so x 2p >x 2q Therefore, for trajectory C eq and C min For two points P and Q with the same x1 coordinate, point Q is always below point P. In other words, C eq The area enclosed by the horizontal axis is always smaller than C min Therefore, the damping dissipated energy of the system moving along the equal energy trajectory is Meet the above conditions.

[0109] Step S104, determining an expression for the damping dissipated energy moving along the equal energy function trajectory according to the equal energy function trajectory, and determining a calculation formula for the active reference instruction boundary according to the expression.

[0110] In an embodiment of the present application, determining an expression for damping dissipated energy moving along an equal energy function trajectory includes the following steps: determining an energy conservation equation satisfied by the UEP from any point on the equal energy function trajectory to the first period; substituting the calculation formula of the second parameter into the energy conservation equation to obtain an expression for the ordinate of any point; setting the ordinate of any point to 0, and substituting the expression of the active reference instruction into the expression for the ordinate of any point to calculate the value of the first parameter according to the obtained formula; using the value of the first parameter and the expression for the ordinate of any point, determining an expression for damping dissipated energy moving along the equal energy function trajectory.

[0111] Specifically, in this embodiment, on the trajectory C eq Any point (x1,x2) set on δ u,1 The energy conservation equation shown below is satisfied:

[0112]

[0113] Substitute the calculation formula of the second parameter in the above embodiment into the above formula (16), that is, x1b =π-2δ s , and then the expression of x2 is:

[0114]

[0115] Then set x2 = 0 and substitute into P ref =3EV g sinδ s / 2X g , we can get the following formula:

[0116] (cosδ s +cos(x 1m +δ s ))-sinδ s (x 1b -x 1m )=0 (18)

[0117] The first parameter x can be obtained by the above formula (18): 1m Numerical solution of .

[0118] Furthermore, combining the above calculation results, including the calculated value of the first parameter and the expression of the ordinate of any point, the expression of the damping dissipation energy along the trajectory of the equal energy function can be obtained, that is, the damping dissipation energy along the trajectory of the equal energy function can be calculated by the following formula (19):

[0119]

[0120] Furthermore, a calculation formula for the active reference instruction boundary is determined according to the expression.

[0121] In one embodiment of the present application, a calculation formula for determining the active reference instruction boundary includes: first determining a stability criterion based on an expression of damping dissipated energy moving along an equal energy function trajectory; and then deriving a calculation formula for the active reference instruction boundary based on the stability criterion.

[0122] Specifically, in this embodiment, the stability criterion shown in the following formula can be obtained from the above formula (19):

[0123]

[0124] Combined with the above formula (20), it can be inferred that the active power command boundary condition that can meet the global stability of the system can be expressed as:

[0125]

[0126] Therefore, the present application obtains a calculation formula for the active reference instruction boundary. In practical applications, the actual grid-connected converter operation data can be substituted into the above formula (21) to calculate the real-time active power instruction boundary that can meet the system stability.

[0127] In summary, the boundary calculation method of the active reference instruction of the grid-type converter during low voltage crossing of the embodiment of the present application, on the basis of fully considering the multi-cycle stability potential of the grid-type converter, adopts the energy function method to analyze the transient energy change process of the converter, and uses the equal energy trajectory method to approximately scale the actual trajectory to obtain the boundary of the active reference instruction that can ensure the global stability of the system. Furthermore, when a fault of any duration and depth occurs in the system, as long as the active reference instruction of the grid-type converter is switched to meet the calculated boundary requirements, the global safety and stability of the system can be guaranteed. Therefore, the method can accurately and reliably calculate the adjustment boundary of the active reference instruction and fully consider the stability of the grid-type converter in multiple cycles.

[0128] Based on the above embodiments, in order to be able to perform switching control according to the active reference instruction boundary calculated by the above-mentioned first aspect embodiment when the grid-type converter is crossing a low voltage, the present application also proposes a method for executing the active reference instruction when the grid-type converter is crossing a low voltage, and the method is applied to the boundary calculation method of the active reference instruction of the above-mentioned first aspect embodiment.

[0129] This execution method can be embedded in the software program of the converter control loop, and can be calculated in parallel when the converter is operating normally to determine the converter status in real time. When a system fault is detected, the entire calculation program is activated, and then this method is executed to achieve the switching of the converter active reference instruction, thereby ensuring the stable operation of the converter under actual working conditions.

[0130] Figure 6 A flowchart of a method for executing an active reference instruction during low voltage ride-through of a grid-connected converter proposed in an embodiment of the present application is shown in FIG. Figure 6 As shown, the method comprises the following steps:

[0131] Step S601, during the operation of the grid-connected converter, it is determined in real time whether a fault occurs in the grid-connected converter system, and in case of a fault, boundary parameters of an active reference instruction and a preset initial active power reference value are obtained.

[0132] Among them, the boundary parameters of the active reference instruction are calculated using the boundary calculation method of the active reference instruction during low voltage ride-through of the grid-type converter as described in any one of the embodiments of the first aspect above.

[0133] Specifically, in order to more clearly describe the specific implementation process of the active reference instruction execution method of the present application, the following is combined with Figure 7 The software program of the grid-type converter performs the switching control method flow when a system failure occurs as shown in the figure. During the operation of the grid-type converter, it is determined in real time whether the system fails. If it is determined according to the detection parameters that V abc ≤0.9V N , then continue to execute the subsequent steps, otherwise maintain the current state unchanged.

[0134] Among them, V abc The above embodiment Figure 2 The three-phase voltage on the right side of the PCC, V N It is the preset safety voltage threshold.

[0135] Further, after determining that a fault has occurred, the existing active power instruction boundary parameter K1 is obtained, and the calculation formula of the boundary parameter is the expression determined in the above-mentioned first aspect embodiment: In addition, a preset initial active power reference value of the converter is obtained as P0, and the time variable k=1 is set to perform subsequent steps.

[0136] Step S602, obtaining an active reference value and a calculation step length of the grid-connected converter at any moment, and determining whether a boundary function value corresponding to the active reference value is greater than the boundary parameter.

[0137] Specifically, assuming that at time k, the grid-connected converter active reference value is P ref k , the calculation step is ΔP ref , where k has different values ​​in different calculation rounds and is 1 in the initial round. Then the active reference value is calculated as P using the following formula ref k The corresponding boundary function value:

[0138]

[0139] Then, compare the calculated boundary function value with the calculated active power instruction boundary parameter K1 to determine whether g(P ref k )>K1.

[0140] Step S603, when it is greater than the boundary parameter, the active reference value is used as the maximum reference value, and the initial active power reference value is updated to the maximum reference value to realize the switching control of the grid-connected converter.

[0141] Specifically, if g(P ref k )>K1, remember the for And execute the reference instruction of the active power loop of the grid-forming converter, and adjust the initial active power reference value in the instruction Thus, the switching control of the grid-connected converter is completed, so that it can ensure the global stability of the system.

[0142] Step S604, when it is not greater than the boundary parameter, the difference between the active reference value and the calculation step length is used as the active reference value at the next moment, and the calculation and comparison of the boundary function value at the next moment are repeated.

[0143] Specifically, if g(P ref k )>K1, then k=k+1, P ref k+1 =P ref k -ΔP ref , and returns to step S602, according to the formula and comparison method described in step S602, recalculates the boundary function value at the next moment and compares the newly calculated boundary function value with the active power instruction boundary parameter K1, thereby looping the above steps.

[0144] In summary, the execution method of the active reference instruction of the grid-type converter during low voltage crossing of the embodiment of the present application is based on the boundary conditions of the calculated active reference instruction. When a fault of any duration and depth occurs in the system, the active reference instruction of the grid-type converter can be switched to meet the calculated boundary requirements, which can ensure the overall safety and stability of the system. Therefore, when a fault is detected in real time during the operation of the grid-type converter, the method can quickly execute the switching of the active reference instruction of the converter, improve the timeliness of the switching of the active reference instruction, and ensure the safe and stable operation of the converter in actual working conditions.

[0145] Based on the above embodiments, in order to more clearly and intuitively describe the boundary calculation method and execution method of the active reference instruction during low voltage ride-through of the grid-type converter of the present application, an exemplary description is given below using a specific embodiment in actual application.

[0146] In this embodiment, based on the active reference instruction switching boundary calculation and execution method proposed in the previous article during fault ride-through, taking a 50kW / 380V grid-connected converter with a three-phase short circuit fault at t=0.5s as an example, specific implementation steps and results are given as follows:

[0147] The first step is to obtain the bus voltage in real time through the voltage transformer, and judge whether the system has a fault based on the obtained value. If V abc ≤0.9V N , it is considered that a fault has occurred and proceed to the next step, otherwise the status remains unchanged.

[0148] The second step is to determine the active power command boundary parameter K1 according to the internal setting parameters of the converter. In this embodiment, J = 0.1 and D = 1.5 of the grid-type converter are taken, then

[0149] The third step is to speed up the convergence of the program by increasing the fixed step size ΔP. ref In addition to the numerical value, the dynamic step size is also set, such as ΔP ref =k1(V N -V abc )+k2(g(P ref k )-K1), step length ΔP ref It increases with the depth of voltage drop and decreases with the difference from the target value. It is calculated by iterative calculation of the internal setting program.

[0150] The fourth step is to adjust the reference command of the active power loop of the grid-connected converter. The switching control of the grid-connected converter is then completed, so that the system can be guaranteed to be stable.

[0151] In this embodiment, after iterative calculation, g(P ref )=K1The solution of the equation is because The value of is conservative. If the active reference command is 0.433pu, then there must be The global stability of the system is guaranteed. Through time domain simulation, we can obtain Figure 8 is the constraint relationship between the theoretical calculation value of the critical active reference instruction and the time domain simulation value. It can be seen that the active reference instruction boundary obtained in this embodiment is conservative and effective.

[0152] In order to implement the above embodiment, the present application also proposes a boundary calculation device for active reference instructions of a grid-type converter during low voltage ride-through. Fig. 9 A schematic diagram of a boundary calculation device for active reference instructions during low voltage ride-through of a grid-connected converter proposed in an embodiment of the present application is shown in FIG. Fig. 9 As shown, the device includes an extension module 100 , a determination module 200 , a construction module 300 and a calculation module 400 .

[0153] Among them, the extension module 100 is used to extend the stability domain of the grid-connected system of the grid-connected converter, obtain the stability domain of the system in multiple periods, and determine the system energy conservation formula based on the periodic nature of the system's energy function.

[0154] The determination module 200 is used to calculate the energy change of the system when it passes from the unstable equilibrium point UEP of any period to the UEP of the next period based on the stability domain and energy conservation formula over multiple periods, and determine the potential energy change between two adjacent UEPs in the energy change.

[0155] Construction module 300 is used to determine the periodic minimum damping dissipation energy and the corresponding trajectory line, take the potential energy change less than the periodic minimum damping dissipation energy as the system stability condition, and construct an equal energy function trajectory based on the system stability condition, wherein the damping dissipation energy moving along the equal energy function trajectory is less than the periodic minimum damping dissipation energy.

[0156] The calculation module 400 is used to determine the expression of the damping dissipated energy moving along the equal energy function trajectory according to the equal energy function trajectory, and determine the calculation formula of the active reference instruction boundary according to the expression.

[0157] It should be noted that the above explanation of the embodiment of the boundary calculation method of the active reference instruction during low voltage ride-through of the grid-type converter is also applicable to the device of this embodiment, and the implementation principle is the same, which will not be repeated here.

[0158] In summary, the boundary calculation device of the active reference instruction during low voltage crossing of the grid-type converter of the embodiment of the present application, on the basis of fully considering the multi-cycle stability potential of the grid-type converter, adopts the energy function method to analyze the transient energy change process of the converter, and uses the equal energy trajectory method to approximately scale the actual trajectory to obtain the boundary of the active reference instruction that can ensure the global stability of the system. Furthermore, when a fault of any duration and depth occurs in the system, as long as the active reference instruction of the grid-type converter is switched to meet the calculated boundary requirements, the overall safety and stability of the system can be guaranteed. As a result, the device can accurately and reliably calculate the adjustment boundary of the active reference instruction and fully consider the stability of the grid-type converter in multiple cycles.

[0159] In order to implement the above embodiment, the present application also proposes an execution device for an active reference instruction of a grid-connected converter during low voltage ride-through. Fig.10 A schematic diagram of a structure of an execution device for an active reference instruction of a grid-connected converter during low voltage ride-through proposed in an embodiment of the present application is shown in FIG. Fig.10 As shown, the device comprises:

[0160] The acquisition module 10 is used to determine in real time whether a fault occurs in the grid-connected system of the grid-connected converter during the operation of the grid-connected converter, and to obtain the boundary parameters of the active reference instruction and the preset initial active power reference value in the event of a fault, wherein the boundary parameters of the active reference instruction are calculated using any method as in the above-mentioned first aspect embodiment.

[0161] The judgment module 20 is used to obtain the active reference value and calculation step of the grid-connected converter at any time, and to judge whether the boundary function value corresponding to the active reference value is greater than the boundary parameter.

[0162] The switching module 30 is used to use the active reference value as the maximum reference value when it is greater than the boundary parameter, and update the initial active power reference value to the maximum reference value to achieve switching control of the grid-connected converter.

[0163] The loop module 40 is used to use the difference between the active reference value and the calculation step length as the active reference value at the next moment when the active reference value is not greater than the boundary parameter, and return to recalculate and compare the boundary function value.

[0164] In summary, the execution device of the active reference instruction during low voltage ride-through of the grid-type converter of the embodiment of the present application is based on the boundary conditions of the calculated active reference instruction. When a fault of any duration and depth occurs in the system, as long as the active reference instruction of the grid-type converter is switched to meet the calculated boundary requirements, the overall safety and stability of the system can be guaranteed. Therefore, when the device detects a fault in real time during the operation of the grid-type converter, it can quickly execute the switching of the active reference instruction of the converter, thereby improving the timeliness of the switching of the active reference instruction and ensuring the safe and stable operation of the converter in actual working conditions.

[0165] In order to implement the above-mentioned embodiments, the present application also proposes a non-temporary computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, it implements a boundary calculation method for an active reference instruction of a grid-type converter during low voltage crossing as described in any of the above-mentioned first aspect embodiments, or implements a method for executing an active reference instruction of a grid-type converter during low voltage crossing as described in any of the above-mentioned second aspect embodiments.

[0166] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0167] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of this application, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0168] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, fragment or portion of code comprising one or more executable instructions for implementing the steps of a custom logical function or process, and the scope of the preferred embodiments of the present application includes alternative implementations in which functions may not be performed in the order shown or discussed, including performing functions in a substantially simultaneous manner or in the reverse order depending on the functions involved, which should be understood by technicians in the technical field to which the embodiments of the present application belong.

[0169] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute the instructions), or in combination with these instruction execution systems, devices or apparatuses. For the purpose of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in combination with these instruction execution systems, devices or apparatuses. More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk box (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), a fiber optic device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium and then editing, interpreting or processing in other suitable ways if necessary, and then stored in a computer memory.

[0170] It should be understood that the various parts of the present application can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0171] A person skilled in the art may understand that all or part of the steps in the method for implementing the above-mentioned embodiment may be completed by instructing related hardware through a program, and the program may be stored in a computer-readable storage medium, which, when executed, includes one or a combination of the steps of the method embodiment.

[0172] In addition, each functional unit in each embodiment of the present application may be integrated into a processing module, or each unit may exist physically separately, or two or more units may be integrated into one module. The above-mentioned integrated module may be implemented in the form of hardware or in the form of a software functional module. If the integrated module is implemented in the form of a software functional module and sold or used as an independent product, it may also be stored in a computer-readable storage medium.

[0173] The storage medium mentioned above may be a read-only memory, a magnetic disk or an optical disk, etc. Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limiting the present application. A person of ordinary skill in the art may change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A method for calculating the boundary of active reference instructions of a grid-connected converter during low voltage ride-through, characterized in that: The following steps are involved: The stability domain of the grid-connected system of the grid-connected converter is extended to obtain the stability domain of the system in multiple periods, and the energy conservation formula of the system is determined based on the periodic nature of the energy function of the system; Based on the stable domain over the multi-period and the energy conservation formula, calculate the energy change of the system when it crosses from the unstable equilibrium point UEP of any period to the UEP of the next period, and determine the potential energy change between two adjacent UEPs in the energy change; Determine the minimum damping dissipation energy of the period and the corresponding trajectory line, take the potential energy change amount being less than the minimum damping dissipation energy of the period as a system stability condition, and construct an equal energy function trajectory based on the system stability condition, wherein the damping dissipation energy moving along the equal energy function trajectory is less than the minimum damping dissipation energy of the period; According to the equal energy function trajectory, an expression for the damping dissipated energy moving along the equal energy function trajectory is determined, and according to the expression, a calculation formula for the active reference instruction boundary is determined.

2. The method according to claim 1, characterized in that The method of extending the stability domain of the grid-connected system of the grid-connected converter to obtain the stability domain of the system over multiple cycles includes: Constructing a second-order transient model of a grid-connected system of a grid-connected converter, and determining the energy function of the system state variables of the second-order transient model; Extending the stable domain period in the neighborhood of the system origin to obtain the stable domain of the energy function over multiple periods; The stability domain of the system over multiple periods is derived according to the stability domain of the energy function over multiple periods.

3. The method according to claim 2, characterized in that The energy conservation formula of the system is determined, including: Convert the energy functions in different periods to the 0th period, set the first point and the second point on the actual trajectory of the system, and calculate the path integral expression from the first point to the second point; The path integral expression is sorted out, and based on the kinetic energy and potential energy in the energy function, the sorted path integral expression is equivalent to the energy conservation theorem expression of the system; The energy conservation theorem expression is converted into the energy conservation formula.

4. The method according to claim 3, characterized in that The energy change of the computing system when it passes through the UEP of any period from the unstable equilibrium point UEP of the next period includes: Convert the trajectory within any one of the cycles to the 0th cycle, and determine the expression of the energy change according to the sorted path integral expression, wherein the energy change is equal to the difference between the potential energy change and the energy dissipated by the trajectory through any one of the cycles; The determining of the potential energy change between two adjacent UEPs in the energy change includes: The potential energy variation is calculated according to the active reference command and the angular frequency of the system stable equilibrium point.

5. The method according to claim 1, characterized in that The iso-energy function trajectory passes through a first coordinate point and a second coordinate point, the ordinates of the first coordinate point and the second coordinate point represent the power angle change rate and have a value of 0, the abscissa of the first coordinate point is a first parameter, and the abscissa of the second coordinate point is a second parameter, and the expression for determining the damping dissipation energy moving along the iso-energy function trajectory includes: Determine the energy conservation equation satisfied by any point on the equal energy function trajectory to the UEP of the first period; Substituting the calculation formula of the second parameter into the energy conservation equation to obtain an expression of the ordinate of any point; The ordinate of any point is set to 0, and the expression of the active reference instruction is substituted into the expression of the ordinate of any point, so as to calculate the value of the first parameter according to the obtained formula; The expression of the damping dissipated energy moving along the equal energy function trajectory is determined by using the value of the first parameter and the expression of the ordinate of any point.

6. The method according to claim 5, characterized in that The calculation formula for determining the active reference instruction boundary according to the expression includes: Determining a stability criterion according to the expression of the damping dissipated energy moving along the equal energy function trajectory; A calculation formula for the active reference instruction boundary is derived according to the stability criterion.

7. A method for executing active reference instructions of a grid-connected converter during low voltage ride-through, characterized in that: The following steps are involved: During the operation of the grid-type converter, it is determined in real time whether a fault occurs in the grid-connected system of the grid-type converter, and in the event of a fault, boundary parameters of an active reference instruction and a preset initial active power reference value are obtained, wherein the boundary parameters of the active reference instruction are calculated by the method according to any one of claims 1 to 6; Obtaining an active reference value and a calculation step size of the grid-connected converter at any time, and determining whether a boundary function value corresponding to the active reference value is greater than the boundary parameter; In the case where it is greater than the boundary parameter, taking the active reference value as the maximum reference value, and updating the initial active power reference value to the maximum reference value, so as to realize the switching control of the grid-connected converter; In the case that it is not greater than the boundary parameter, the difference between the active reference value and the calculation step length is used as the active reference value at the next moment, and the calculation and comparison judgment of the boundary function value at the next moment are repeated.

8. A boundary calculation device for active reference instructions of a grid-type converter during low voltage ride-through, characterized in that: Includes the following modules: The extension module is used to extend the stability domain of the grid-connected system of the grid-connected converter to obtain the stability domain of the system over multiple periods, and determine the energy conservation formula of the system based on the periodic nature of the energy function of the system; A determination module, configured to calculate the energy change of the system when it crosses from an unstable equilibrium point UEP of any period to a UEP of the next period based on the stable domain over the multi-period and the energy conservation formula, and determine the potential energy change between two adjacent UEPs in the energy change; A construction module is used to determine the minimum damping dissipation energy of the period and the corresponding trajectory line, taking the potential energy change amount being less than the minimum damping dissipation energy of the period as a system stability condition, and constructing an equal energy function trajectory based on the system stability condition, wherein the damping dissipation energy moving along the equal energy function trajectory is less than the minimum damping dissipation energy of the period; A calculation module is used to determine an expression of the damping dissipated energy moving along the equal energy function trajectory according to the equal energy function trajectory, and to determine a calculation formula for an active reference instruction boundary according to the expression.

9. An execution device for active reference instructions of a grid-type converter during low voltage ride-through, characterized in that: Includes the following modules: An acquisition module, used for determining in real time whether a fault occurs in the grid-connected system of the grid-connected converter during operation of the grid-connected converter, and obtaining boundary parameters of an active reference instruction and a preset initial active power reference value in the event of a fault, wherein the boundary parameters of the active reference instruction are calculated by the method according to any one of claims 1 to 6; A judgment module, used for obtaining an active reference value and a calculation step size of the grid-connected converter at any time, and judging whether a boundary function value corresponding to the active reference value is greater than the boundary parameter; A switching module, used for, when the active power reference value is greater than the boundary parameter, taking the active power reference value as the maximum reference value, and updating the initial active power reference value to the maximum reference value, so as to realize the switching control of the grid-connected converter; The circulation module is used to take the difference between the active reference value and the calculation step as the active reference value at the next moment when the difference is not greater than the boundary parameter, and return to recalculate and compare the boundary function value.

10. A non-transitory computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by the processor, it implements a boundary calculation method for an active reference instruction of a grid-type converter during low voltage crossing as described in any one of claims 1-6, or implements a method for executing an active reference instruction of a grid-type converter during low voltage crossing as described in claim 7.

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