A three-phase cascaded H-bridge photovoltaic grid-connected system reserve power distribution method and computer equipment

Abstract

The application belongs to the technical field of photovoltaic, and discloses a kind of reserve power distribution method of three-phase cascaded H bridge photovoltaic grid-connected system, including the following steps: periodically MPPT control is carried out, obtains the maximum output power of each photovoltaic group, calculates the sum of each phase maximum output power, on the basis of meeting power reserve target value, select the phase that needs to limit power operation, calculate the output power target value of selected phase;In selected phase, select photovoltaic group as power reserve unit, calculate the output power target value of photovoltaic group as power reserve unit.The method of the application realizes the distribution of reserve power among phases and among modules in phase, can realize the balanced control of the output power of each photovoltaic group, maximally guarantees the power balance among phases and among modules in phase, thereby guarantees the balanced output of three-phase current of system, and reduces the risk of over-modulation of H bridge module.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of photovoltaic technology, and particularly relates to a reserve power distribution method for a three-phase cascaded H-bridge photovoltaic grid-connected system and a computer device. BACKGROUND

[0002] In order to directly incorporate the photovoltaic power generation system into the medium-voltage power grid, a cascaded H-bridge (CHB) multilevel inverter can be used as a grid-connected interface, and photovoltaic strings are used as DC power sources of the CHB inverter, which meets the requirement of independent power supply for the CHB topology. The CHB multilevel technology reduces the stress of the switching device, improves the quality of the inverter output voltage and current, directly connects the photovoltaic string with the DC bus, saves the DC / DC converter, and further reduces the system loss and volume. Through the control strategy improvement such as third harmonic injection, the double-frequency fluctuation of the DC bus capacitor can be effectively suppressed, thereby reducing the requirement for the capacitance value.

[0003] In the three-phase system, due to the uneven illumination or the influence of factors such as surface dust, the maximum power tracking control of each photovoltaic string causes the power imbalance between the H-bridge modules, and further causes the imbalance of the three-phase transmission power and the three-phase output current of the system. Although the imbalance of the three-phase output current can be improved by the method of zero-sequence voltage injection, when the three-phase transmission power imbalance is too high due to the limitation of the DC bus voltage, the balanced output of the three-phase current cannot be achieved. In order to maximize the capture of solar energy during the normal operation of the three-phase CHB photovoltaic grid-connected system, especially to ensure the balance of the three-phase transmission power as much as possible during the power reserve and support of the grid frequency, both the effective support of the grid frequency and the always guaranteed quality of the grid-connected current are achieved, which is the main challenge of the CHB photovoltaic grid-connected system for the grid frequency support control compared with the conventional inverter.

[0004] The existing reserve power distribution method is to use the method of balancing the power between the modules, to realize the power balance between the modules by correcting the target value of the DC bus voltage of each module, without distinguishing which phase, for example, assuming that each phase has two modules, the maximum output power of the first module and the second module of the a phase is P a1 = 10kW and P a2 = 4kW, the maximum output power of all modules of the b phase and the c phase is 8kW, that is, P b1 = P b2 = P c1 = P c2= 8kW, at this time, the maximum output powers of the three phases a, b and c are 14kW, 16kW and 16kW respectively, from the perspective of balancing the three-phase power to the maximum extent, the output power of the module in the phase b and c should be reduced first; however, the existing control strategy reduces the output power of the module a1 first after sorting all the 6 modules, which further aggravates the imbalance of the three-phase power. Therefore, the existing method cannot solve the problem of phase-to-phase power imbalance, and even when the power between the modules is balanced, it causes or aggravates the phase-to-phase power imbalance, and further causes the problem of unbalanced three-phase grid-connected current.

[0005] Therefore, how to provide a method capable of realizing phase-to-phase and intra-phase module power balance of a three-phase cascaded H-bridge photovoltaic grid-connected system is a problem to be solved at present. SUMMARY

[0006] The embodiment of the present application provides a reserve power distribution method of a three-phase cascaded H-bridge photovoltaic grid-connected system, to solve the problem that the existing method cannot solve the phase-to-phase power imbalance of the three-phase cascaded H-bridge photovoltaic grid-connected system. In order to have a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. This part is not a general review, nor is it intended to determine the key / important constituent elements or delineate the protection scope of these embodiments. Its only purpose is to present some concepts in a simple form as a prelude to the detailed description that follows.

[0007] According to a first aspect of the embodiment of the present application, a reserve power distribution method of a three-phase cascaded H-bridge photovoltaic grid-connected system is provided.

[0008] In one embodiment, a reserve power distribution method of a three-phase cascaded H-bridge photovoltaic grid-connected system includes the following steps:

[0009] Periodically perform MPPT control to obtain the maximum output power of each photovoltaic string, calculate the sum of the maximum output power of each phase, select the phase that needs to be limited in power operation on the basis of meeting the power reserve target value, and calculate the output power target value of the selected phase.

[0010] In the selected phase, select the photovoltaic string as the power reserve unit, and calculate the output power target value of the photovoltaic string as the power reserve unit.

[0011] Optionally, the step of selecting the phase that needs to be limited in power operation includes:

[0012] First, sum and sort in descending order the maximum output power of all photovoltaic strings of the three phases a, b and c respectively to obtain the sequence P

[123] ;

[0013] Then, the adjacent two power values after sorting are subtracted, and the difference is stored in an array ΔP

[12] , which represents the power that can be reserved when the maximum output power P[i] is reduced to P[i+1], where i is 1 or 2;

[0014] The minimum m value satisfying the following inequality is obtained:

[0015]

[0016] where m is 1 or 2; if m = 2 still cannot satisfy the inequality of the above formula, it is determined that the three phases of abc all need to operate in power limiting mode; P res (t) is the power reserve target value.

[0017] Optionally, the power reserve target value P res (t) is obtained based on the following formula:

[0018]

[0019] where P' res (t0) is the initial value of the power reserve corresponding to the t0 moment when the frequency support starts, ΔP s (t) is the target value of the output power increase, P' res (t) is the initial value of the reserved reserve power.

[0020] Optionally, the target value of the output power increase ΔP s (t) is calculated by the following formula:

[0021]

[0022] where k is the active droop coefficient, J is the moment of inertia, ω g (t) and ω0 are the grid angular frequency and its rated value, respectively.

[0023] Optionally, the step of calculating the output power target value of the selected phase includes:

[0024] When the selected phase that needs to operate in power limiting mode is two phases, the output power target values of the two selected phases are equal, and all photovoltaic module strings in the other unselected phase remain in MPPT mode.

[0025] Optionally, the step of calculating the output power target value of the selected phase includes:

[0026] When the selected phase that needs to operate in power limiting mode is three phases, the output power target values of the three phases are all equal.

[0027] Optionally, the step of calculating the output power target value of the selected phase includes:

[0028] selected phase x x * The calculation formula is:

[0029]

[0030] Optionally, the step of selecting photovoltaic strings as power reserve units in the selected phase includes:

[0031] According to the maximum output power of the phase and the output power target value, photovoltaic strings for limited power operation are selected by using the following formula:

[0032]

[0033] In the formula, s is an integer from 1 to n-1, P x [1…n] is a sequence obtained by descending order sorting of the maximum output power of photovoltaic strings in phase x, ΔP x [1…n-1] is the difference between adjacent powers after sorting, P xm is the sum of the maximum output power of all photovoltaic strings in phase x.

[0034] Optionally, the step of calculating the output power target value of photovoltaic strings as power reserve units includes:

[0035] The output power target value P xj * of each photovoltaic string is calculated according to the following formula:

[0036]

[0037] If s=n-1 still cannot meet the above formula, all photovoltaic strings in the phase need to operate in limited power.

[0038] According to a second aspect of the embodiment of the present application, a computer device is provided.

[0039] In some embodiments, the computer device includes a memory and a processor, the memory stores a computer program, and the processor implements the steps of the above method when executing the computer program.

[0040] The technical scheme provided by the embodiment of the present application can include the following beneficial effects:

[0041] For the common problem of inter-phase power imbalance of the cascaded H-bridge topology, the method of the present application realizes the distribution of reserve power among phases and among modules in the phase, realizes the balanced control of the output power of each photovoltaic string, maximally guarantees the power balance among phases and among modules in the phase, thereby guaranteeing the balanced output of three-phase current of the system, and reducing the risk of over-modulation of the H-bridge module.

[0042] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application. BRIEF DESCRIPTION OF DRAWINGS

[0043] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the application and serve to explain the principles of the application.

[0044] Figure 1 is a structural diagram of a three-phase cascaded H-bridge photovoltaic grid-connected system according to an exemplary embodiment;

[0045] Figure 2 is a flow chart of a reserve power distribution method of a three-phase cascaded H-bridge photovoltaic grid-connected system according to an exemplary embodiment;

[0046] Figure 3 is Figure 2 is a flow chart of step S1 according to an exemplary embodiment;

[0047] Figure 4 is Figure 2 is a flow chart of step S2 according to an exemplary embodiment;

[0048] Figure 5a is a schematic diagram of reserve power inter-phase distribution according to an exemplary embodiment;

[0049] Figure 5b is a schematic diagram of reserve power H-bridge module inter-distribution according to an exemplary embodiment;

[0050] Figure 6 is a structural schematic diagram of a computer device according to an exemplary embodiment. DETAILED DESCRIPTION

[0051] The following description and drawings are illustrative of specific embodiments thereof and are not intended to limit the scope of the embodiments. Parts and features of some embodiments can be included or substituted in or for parts and features of other embodiments. The scope of the embodiments encompassed herein includes the whole scope of the claims together with all available equivalents of the claims. In this document, the terms "first", "second", etc. are used merely to distinguish one element from another, and do not require or imply any actual relationship or order between the elements. In fact, the first element can be referred to as the second element, and vice versa. Also, the terms "comprises", "comprising", or any other variations thereof are intended to cover a non-exclusive inclusion, such that a structure, device, or apparatus that comprises a list of elements does not include only those elements but can also include other elements not expressly listed or inherent to such structure, device, or apparatus. Without further limitation, an element defined by an "includes a" statement does not exclude the presence of additional identical elements in the structure, device, or apparatus that includes the element. Various embodiments are described in progressive stages, each of which focuses on the differences from other embodiments, and the same or similar parts between various embodiments can be referred to each other.

[0052] The terms "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, as used herein, indicate relative positions or orientation relationships based on the positions or orientation relationships shown in the drawings, and are only used for the convenience of description herein and simplification of the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. In the description herein, unless otherwise specified and limited, the terms "mount", "connect", "connection" should be interpreted broadly, for example, it can be a mechanical connection or an electrical connection, it can be a communication between two elements inside, it can be a direct connection or an indirect connection through an intermediate medium, and the specific meaning of the above terms can be understood by those skilled in the art according to the specific circumstances.

[0053] In this document, the term "multiple" means two or more, unless otherwise specified.

[0054] In this document, the character " / " represents an "or" relationship between the objects before and after it. For example, A / B means A or B.

[0055] In this document, the term "and / or" is a description of the relationship between the objects, which means that there can be three relationships. For example, A and / or B means that there are three relationships of A or B, or A and B.

[0056] In the case of no conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0057] Figure 1 An embodiment of the three-phase cascaded H-bridge photovoltaic grid-connected system of the present application is shown.

[0058] In this optional embodiment, in the three-phase cascaded H-bridge photovoltaic grid-connected system, the H-bridge inverter is directly connected with the photovoltaic string to form an H-bridge module; each phase is cascaded by n H-bridge modules to form a three-phase system in star connection; C and L are the DC side filter capacitor and the grid side filter inductor respectively; I PVxj and U dcxj are the output current and the DC bus voltage of the jth H-bridge module photovoltaic string in x phase respectively (x=a, b, c; j=1, 2, …, n, same below); u x and i x are the three-phase grid voltage and current respectively. Each H-bridge module can output three levels, and the inverter AC side can output 2n+1 levels of voltage.

[0059] In this optional embodiment, in the three-phase cascaded H-bridge photovoltaic grid-connected system, the H-bridge inverter is directly connected with the photovoltaic string to form an H-bridge module; each phase is cascaded by n H-bridge modules to form a three-phase system in star connection; C and L are the DC side filter capacitor and the grid side filter inductor respectively; I PVxj and U dcxj are the output current and the DC bus voltage of the jth H-bridge module photovoltaic string in x phase respectively (x=a, b, c; j=1, 2, …, n, same below); u x and i x are the three-phase grid voltage and current respectively. Each H-bridge module can output three levels, and the inverter AC side can output 2n+1 levels of voltage. When the inverter is stably running, the AC side output voltage is:

[0060]

[0061] In the formula, m xj is the modulation factor of each H-bridge module. According to Kirchhoff's voltage and current law, the relationship between the voltage and current of the system can be expressed as

[0062]

[0063] C*dU dcxj / dt=I PVxj -m xj i x (3)

[0064] When a corresponding photovoltaic string is caused to output power to drop due to factors such as shading, surface dust, etc., the imbalance of three-phase transmission power is easy to cause the imbalance of inverter three-phase current output, so that the grid-connected current quality is poor. Although the zero sequence voltage injection method such as the fundamental frequency zero sequence voltage injection, the weighted minimum maximum zero sequence voltage injection, the third harmonic injection, the optimal zero sequence voltage injection, etc. can be used to realize the output of balanced three-phase current when the power of each phase is imbalanced, however, due to the limitation of the DC bus voltage, even if the optimal zero sequence voltage injection method with the best balancing capability is used, the inter-phase power imbalance problem within 20% can be only handled. The embodiment of the present application discloses a kind of reserve power distribution methods of three-phase cascaded H bridge photovoltaic grid-connected system, the maximum output power of each photovoltaic string is obtained by periodically performing MPPT control;Based on the obtained power value, the distribution of reserve power is realized by phase selection and output power target value calculation, power reserve unit selection and photovoltaic string output power target value calculation, the output power of each phase photovoltaic string is adjusted to make the power between inter-phase and intra-phase module balanced as far as possible, and the grid-connected current imbalance problem is solved.

[0065] Figure 2 An embodiment of the reserve power distribution method of three-phase cascaded H bridge photovoltaic grid-connected system of the present application is shown.

[0066] In this embodiment, a reserve power distribution method of three-phase cascaded H bridge photovoltaic grid-connected system includes the following steps:

[0067] Step S1, periodically perform MPPT control, for example, perform MPPT control once every interval (such as 10 min), obtain the maximum output power of each photovoltaic string, calculate the sum of the maximum output power of each phase, and select the phase that needs to be limited power operation on the basis of meeting the power reserve target value, and calculate the output power target value of the selected phase.

[0068] Step S2, in the selected phase, select the photovoltaic string as the power reserve unit, and calculate the output power target value of the photovoltaic string as the power reserve unit.

[0069] When the three-phase cascaded H bridge photovoltaic grid-connected system performs power reserve control, when the photovoltaic grid-connected system is controlled to participate in grid frequency support, based on the measured grid frequency, the target value ΔP s (t) of output power increase is calculated:

[0070]

[0071] In the formula, k is the active droop coefficient, J is the moment of inertia, ω g (t) and ω0 are the grid angular frequency and its rated value respectively.

[0072] During participating in the grid frequency support, the power reserve value of the system is the initial value P' res (t0) of the power reserve at the time t0 when the frequency support starts and the target value ΔP s (t) of the output power increase in the above formula res (t) is calculated, that is, the target value P

[0073]

[0074] P' res (t) is the initial value of the reserved reserve power.

[0075] The step S1 selects the phase that needs to be limited in power, and the step of calculating the target value of the output power of the selected phase is as shown in formula (1) and specifically includes: Figure 3

[0076] First, the maximum output power of all photovoltaic module strings of each phase abc is summed up and sorted in descending order to obtain a sequence P[1 2 3], and the difference between the adjacent two power values after sorting is calculated and stored in an array ΔP[1 2], which can be understood as the power that can be reserved when the output power of the phase with the maximum output power P[i] is reduced to P[i+1], wherein i is 1 or 2.

[0077] The target value P res (t) of the power reserve obtained based on formula (5) is used to obtain the minimum value m that satisfies the inequality (6).

[0078]

[0079] In the formula, m is 1 or 2. If the inequality (6) cannot be satisfied when m=2, it is determined that all the three phases abc need to be limited in power. When two phases are limited in power, the target values of the output power are equal, and all photovoltaic module strings in the other phase remain in the MPPT mode; when all the three phases are limited in power, the target values of the output power of the three phases are equal, thereby realizing the maximum balance of the output power among the three phases.

[0080] The calculation formula of the target value P x * of the output power of the phase x limited in power is:

[0081]

[0082] In order to realize the maximum balance of the power among the modules in the same phase and thereby reduce the risk of over-modulation of the H-bridge module, the modules limited in power in the selected phase also need to be selected, and the target value of the output power thereof needs to be calculated.

[0083] ​The step S2, in the selected phase, selects the photovoltaic string as the power reserve unit, and calculates the output power target value of the photovoltaic string as the power reserve unit, as shown in the following formula (8) and formula (9). Figure 4 Specifically, the step S2 includes the following steps:

[0084] According to the maximum output power of the phase and the output power target value, the photovoltaic string in the power limit mode is selected by using the formula (8), and the output power target value P xj * of each photovoltaic string is calculated by using the formula (9).

[0085]

[0086]

[0087] In the formula, s is an integer from 1 to n-1, P x is the maximum output power of the photovoltaic string in the x phase, ΔP x [1…n-1] is the difference between the adjacent powers after the sorting, P xm is the sum of the maximum output powers of all the photovoltaic strings in the x phase. If s=n-1 still cannot satisfy the inequality (9), it indicates that all the photovoltaic strings in the phase need to be in the power limit mode.

[0088] For example, it is assumed that the maximum output powers of all the photovoltaic strings in each phase are sorted in descending order as shown in FIG. 5(a). When m=1, the formula (6) is satisfied, that is, the a phase with the maximum output power P[1] is selected to be in the power limit mode, and the photovoltaic strings in the b and c phases remain in the MPPT mode. Then, the photovoltaic strings in the a phase that need to be in the power limit mode are selected, and the sequence of the maximum output powers of all the photovoltaic strings in the a phase sorted in descending order is shown in FIG. 5(b). It is assumed that the minimum s value satisfying the inequality (8) is 3, and the photovoltaic strings with the maximum output powers P a [1], P a [2] and P a [3] are controlled to be in the power limit mode, and the other photovoltaic strings in the phase remain in the MPPT mode. Finally, the same output power target value P aj * of the selected photovoltaic strings is obtained by using the formula (9). At this time, the reserve power value of the phase is P am -P a * , and the relationship between P x [1…n-1] is shown in the formula (10).

[0089] ΔP a [1]+2ΔP a [2]+3ΔP s =Pam -P a * (10)

[0090] In one embodiment, a computer device is provided, which may be a server, and its internal structure diagram may be as follows: Figure 6 As shown, the computer device includes a processor, memory, and a network interface connected via a system bus. The processor provides computing and control capabilities. The memory includes a non-volatile storage medium and internal memory. The non-volatile storage medium stores an operating system, computer programs, and a database. The internal memory provides an environment for the operation of the operating system and computer programs in the non-volatile storage medium. The database stores static and dynamic information data. The network interface communicates with external terminals via a network connection. When the computer program is executed by the processor, it implements the steps in the above method embodiments.

[0091] Those skilled in the art will understand that Figure 6 The structure shown is merely a block diagram of a portion of the structure related to the present invention and does not constitute a limitation on the computer device to which the present invention is applied. A specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have different component arrangements.

[0092] In one embodiment, a computer device is also provided, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above method embodiments.

[0093] In one embodiment, a computer-readable storage medium is provided having a computer program stored thereon that, when executed by a processor, implements the steps in the method embodiments described above.

[0094] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing relevant hardware through a computer program. The computer program can be stored in a non-volatile computer readable storage medium, and when executed, can include the processes of the above-mentioned embodiments. Any reference to memory, storage, database or other medium used in each embodiment of the present application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory or optical memory, etc. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration but not limitation, RAM can be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM), etc.

[0095] The present application is not limited to the structures described above and shown in the drawings, and various modifications and changes can be made without departing from the scope thereof. The scope of the present application is only limited by the appended claims.

Claims

1. A method for allocating reserve power in a three-phase cascaded H-bridge photovoltaic grid-connected system, characterized in that, Includes the following steps: MPPT control is performed periodically to obtain the maximum output power of each photovoltaic string, and the sum of the maximum output power of each phase is calculated. Based on meeting the power reserve target value, the phase that needs to be operated with limited power is selected, and the output power target value of the selected phase is calculated. Among the selected phases, photovoltaic strings are selected as power storage units, and the target output power value of the photovoltaic strings as power storage units is calculated. The step of selecting the phase that needs to be operated with limited power includes: First, the maximum output power of all photovoltaic strings in each phase abc is summed and sorted in descending order to obtain sequence P[123]; Then, the difference between the two adjacent power values ​​after sorting is calculated and stored in the array ΔP[12]. The array ΔP[12] represents the power that can be stored when the output power of the phase with the maximum output power of P[i] is reduced to P[i+1]. In the formula, i is 1 or 2. Find the smallest value of m that satisfies the following inequality: In the formula, m is 1 or 2; if the inequality in the above formula cannot be satisfied when m = 2, then it is determined that all three phases abc must operate with limited power; P res (t) represents the target value for power reserves; The step of calculating the target output power value of the selected phase includes: Selected target output power value P for phase x x * The calculation formula is: The step of selecting photovoltaic strings as power storage units from the selected phases includes: Based on the maximum output power and the target output power of this phase, the photovoltaic string for power-limited operation is selected using the following formula: In the formula, s is an integer from 1 to n-1, and P x [1…n] is the sequence obtained by sorting the maximum output power of the photovoltaic strings of phase x in descending order, ΔP x [1…n-1] represents the difference between two adjacent power values ​​after sorting, P xm It is the sum of the maximum output power of all photovoltaic strings in phase x; The step of calculating the target output power value of the photovoltaic string as a power storage unit includes: The target output power P of each photovoltaic string is calculated using the following formula. xj * : If the above formula cannot be satisfied when s = n-1, then all photovoltaic strings in that phase must operate with limited power.

2. The method for allocating reserve power in a three-phase cascaded H-bridge photovoltaic grid-connected system as described in claim 1, characterized in that, The power reserve target value P res (t) is obtained based on the following formula: In the formula, P ’ res (t0) represents the initial power reserve value at time t0, corresponding to the start of frequency support, ΔP s (t) represents the target increase in output power, P ’ res (t) represents the initial value of the reserved reserve power.

3. The method for allocating reserve power in a three-phase cascaded H-bridge photovoltaic grid-connected system as described in claim 2, characterized in that, The target value ΔP for the increase in output power s (t) is calculated using the following formula: In the formula, k is the active droop coefficient, J is the moment of inertia, and ω g (t) and ω0 are the grid angular frequency and its rated value, respectively.

4. The method for allocating reserve power in a three-phase cascaded H-bridge photovoltaic grid-connected system as described in claim 1, characterized in that, The step of calculating the target output power value of the selected phase includes: When two phases are selected as the phases that require power limiting operation, the target output power values ​​of the two selected phases are equal, and all photovoltaic strings in the unselected phase maintain MPPT mode operation.

5. The method for allocating reserve power in a three-phase cascaded H-bridge photovoltaic grid-connected system as described in claim 1, characterized in that, The step of calculating the target output power value of the selected phase includes: When the phases requiring power-limited operation are selected as three phases, the target output power values ​​of the three phases are all equal.

6. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the method according to any one of claims 1 to 5.

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