A cascaded microgrid frequency control circuit and its design method

Abstract

This invention discloses a frequency control circuit and design method for a cascaded microgrid. It includes a common load and multiple inverter units connected in series. Each inverter unit comprises a distributed power source, a series inverter, and a resonant circuit. The inverter unit also includes an inner-loop control circuit for synchronizing the frequency of the inverter unit according to the series inverter synchronization control logic and performing secondary frequency recovery control according to the distributed secondary frequency control logic. This method enables frequency control without communication, reducing communication costs, avoiding communication delays, packet loss, and fault risks, and improving the reliability of the cascaded microgrid system.

Description

Technical Field

[0001] This invention relates to the field of microgrid circuit design technology, and in particular to a cascaded microgrid frequency control circuit and design method. Background Technology

[0002] Microgrids have become a research hotspot due to their important role and status in the field of renewable energy. Depending on the configuration, microgrids can be divided into two types. The first type is the parallel microgrid, which has been studied for many years. Droop control and virtual synchronous generators are the most commonly used control strategies. The other type is the cascaded microgrid, a newer type compared to the former.

[0003] In recent years, the introduction of cascaded microgrids has driven the development of microgrids in high / medium voltage fields, especially in large-scale photovoltaic power generation and energy storage power stations. Traditional cascaded system control methods mostly rely on centralized control. In recent years, inspired by droop control, many decentralized control methods have been proposed to reduce communication burdens. Similarly, frequency synchronization and power balancing can be achieved automatically. Examples include inverse power factor droop control under RL loads, fP / Q droop control, and power factor angle droop control for different load characteristics. However, these methods neglect the frequency offset they cause.

[0004] Therefore, frequency recovery control is urgently needed for the operation of cascaded microgrids. The most classic method is the central control method. However, the central controller is highly dependent on communication and has a large computational load, which reduces the reliability of the system. To reduce communication dependence and localize the control algorithm, a distributed frequency control method based on local controllers and neighbor information is proposed. However, communication delays and faults still threaten the system's performance, stability, and reliability. Summary of the Invention

[0005] In order to at least partially solve the technical problems existing in the prior art, the inventors made this invention, which provides a cascaded microgrid frequency control circuit and design method through specific implementation methods.

[0006] In a first aspect, embodiments of the present invention provide a cascaded microgrid frequency control circuit, including a common load and multiple inverter units connected in series. Each inverter unit includes a distributed power source, a series inverter, and a resonant circuit. The inverter unit further includes:

[0007] The data processing circuit is used to collect the overall power factor angle, steady-state load voltage phase angle, and output current frequency of the cascaded microgrid in real time; and to collect the output voltage, output current, output voltage phase angle, and output voltage angular frequency of the inverter unit in real time.

[0008] A power calculation circuit is used to determine the active power and reactive power of each inverter unit based on the electrical parameters.

[0009] The inner loop control circuit is used to synchronize the frequency of the inverter unit according to the series inverter synchronization control logic; and to perform secondary recovery control of the frequency of the inverter unit according to the distributed secondary frequency control logic.

[0010] A phase-locked loop circuit is used to control the frequency and phase of the loop oscillation signal inside the inverter unit.

[0011] Specifically, the inner loop control circuit includes:

[0012] A synchronization control circuit is used to synchronize the frequency of the inverter unit according to the synchronization control logic of the series inverter, which is as follows:

[0013] f i =f * +sgn(Q i )m i P i

[0014] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit. i P is the droop control factor. i The output active power of the i-th inverter unit is i, where i is the inverter unit number, and i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0015] A frequency secondary recovery circuit is used to perform secondary recovery control on the frequency of the inverter unit according to the distributed secondary frequency control logic, which is as follows:

[0016]

[0017] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit. i P is the droop control factor. i Let k be the output active power of the i-th inverter unit. Ii Let f be the integral coefficient of the i-th inverter unit, s be the Laplace operator, and f be the integral coefficient of the i-th inverter unit. reff is the rated value for frequency recovery of a series-connected energy storage microgrid. I The frequency value of the output current is denoted by i, which is the inverter unit number. i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0018] Specifically, the inverter unit further includes:

[0019] A load switching circuit is used to switch between resistive-inductive loads and resistive-capacitive loads in the common load.

[0020] Secondly, embodiments of the present invention provide a design method for a cascaded microgrid frequency control circuit, comprising the following steps:

[0021] Multiple inverter units are configured, each inverter unit including a distributed power source, a series inverter, a resonant circuit, a data processing circuit, a power calculation circuit, an inner loop control circuit, and a phase-locked loop circuit; the inner loop control circuit includes a synchronization control circuit and a frequency secondary recovery circuit.

[0022] Connect the common load and multiple inverter units in series;

[0023] The process of setting up the inverter unit includes the following steps:

[0024] A data processing circuit is set up to collect the electrical parameters of the cascaded microgrid and each inverter unit in the cascaded microgrid in real time.

[0025] A power calculation circuit is set up to calculate the electrical parameters and determine the active power and reactive power of each inverter unit.

[0026] Based on the synchronization control logic of the series inverter, the synchronization control circuit is set up to synchronize the frequency of the inverter unit.

[0027] Based on the distributed secondary frequency control logic, the frequency secondary recovery circuit is set up to perform secondary recovery control on the frequency of the inverter unit.

[0028] Specifically, the synchronization control logic of the series inverter is as follows:

[0029] f i =f * +sgn(Q i )m i P i

[0030] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit.i P is the droop control factor. i Let i be the output active power of the i-th inverter unit, and i be the inverter unit number. i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0031] Specifically, the distributed secondary frequency control logic is as follows:

[0032]

[0033] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit. i P is the droop control factor. i Let k be the output active power of the i-th inverter unit. Ii Let f be the integral coefficient of the i-th inverter unit, s be the Laplace operator, and f be the integral coefficient of the i-th inverter unit. ref f is the rated value for frequency recovery of a series-connected energy storage microgrid. I The frequency value of the output current is denoted by i, which is the inverter unit number. i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0034] Specifically, setting up the inverter unit also includes the following steps:

[0035] A phase-locked loop circuit is set up to control the frequency and phase of the loop oscillation signal inside the inverter unit;

[0036] A load switching circuit is provided to switch between resistive-inductive loads and resistive-capacitive loads in the common load.

[0037] The beneficial effects of the above-described technical solutions provided in the embodiments of the present invention include at least the following:

[0038] The distributed frequency recovery control proposed in this invention relies solely on local information to restore the frequency of a cascaded microgrid system to its rated value. Frequency control requires no communication, reducing communication costs and avoiding communication delays, packet loss, and fault risks. Compared to centralized control schemes, this method employs communication-free distributed control, significantly enhancing the reliability of the cascaded microgrid system.

[0039] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and drawings.

[0040] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0041] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0042] Figure 1 This is a schematic diagram of the cascaded microgrid frequency control circuit in an embodiment of the present invention;

[0043] Figure 2 This is a schematic diagram of the cascaded microgrid system structure in an embodiment of the present invention;

[0044] Figure 3a This is a schematic diagram illustrating the frequency recovery principle when the resistive-inductive load changes in an embodiment of the present invention.

[0045] Figure 3b This is a schematic diagram illustrating the frequency recovery principle when the resistive-capacitive load changes in an embodiment of the present invention.

[0046] Figure 4a This is a schematic diagram illustrating the working performance of the frequency recovery control logic under resistive-inductive load in an embodiment of the present invention.

[0047] Figure 4b This is a schematic diagram illustrating the working performance of the frequency recovery control logic under resistive-capacitive load in an embodiment of the present invention.

[0048] Figure 5a This is a schematic diagram of the dynamic response of the control logic when the resistive-inductive load is switched to the resistive-capacitive load in an embodiment of the present invention;

[0049] Figure 5b This is a schematic diagram of the dynamic response of the control logic when the resistive-capacitive load is switched to the resistive-inductive load in an embodiment of the present invention. Detailed Implementation

[0050] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0051] To address the problems existing in the prior art, embodiments of the present invention provide a cascaded microgrid frequency control circuit and design method.

[0052] Example 1

[0053] Embodiment 1 of the present invention provides a cascaded microgrid frequency control circuit, the structure of which is as follows: Figure 1 As shown, by real-time acquisition of electrical parameters of the cascaded microgrid and each inverter unit within it, the active and reactive power data of each inverter unit are determined based on these electrical parameters. According to the distributed secondary frequency control logic, PI control is performed within each inverter unit to achieve secondary frequency recovery control. This enables the switching of resistive-capacitive or resistive-inductive loads during load switching.

[0054] Among them, the structure of a cascaded microgrid system is as follows: Figure 2 As shown, each inverter unit includes a distributed micro-source, a series inverter, and a resonant circuit. Multiple inverter units and loads constitute a cascaded microgrid system. Distributed micro-sources are also known as distributed generation (DG). The Institute of Electrical and Electronics Engineers (IEEE) defines DG as a small-capacity generator that can be connected to the grid at any location within the power system, with a capacity range of less than 10MW. The grid connection voltage level is typically connected to the various voltage levels of the distribution system.

[0055] A cascaded microgrid frequency control circuit includes a common load and multiple inverter units connected in series. Each inverter unit includes a distributed power source, a series inverter, and a resonant circuit. The inverter unit also includes:

[0056] The data processing circuit is used to collect the overall power factor angle, steady-state load voltage phase angle, and output current frequency of the cascaded microgrid in real time; and to collect the output voltage, output current, output voltage phase angle, and output voltage angular frequency of the inverter unit in real time.

[0057] A power calculation circuit is used to determine the active power and reactive power of each inverter unit based on the electrical parameters.

[0058] The inner loop control circuit is used to synchronize the frequency of the inverter unit according to the series inverter synchronization control logic; and to perform secondary recovery control of the frequency of the inverter unit according to the distributed secondary frequency control logic.

[0059] A phase-locked loop circuit is used to control the frequency and phase of the loop oscillation signal inside the inverter unit. Figure 2 In this context, PLL stands for Phase-Locked Loop, a type of feedback control circuit. It is characterized by using an externally input reference signal to control the frequency and phase of the internal oscillation signal.

[0060] Specifically, the inner loop control circuit includes:

[0061] A synchronization control circuit is used to synchronize the frequency of the inverter unit according to the synchronization control logic of the series inverter, which is as follows:

[0062] f i =f * +sgn(Q i )m i P i

[0063] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit. i P is the droop control factor. i The output active power of the i-th inverter unit is i, where i is the inverter unit number, and i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0064] A frequency secondary recovery circuit is used to perform secondary recovery control on the frequency of the inverter unit according to the distributed secondary frequency control logic, which is as follows:

[0065]

[0066] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit. i P is the droop control factor. i Let k be the output active power of the i-th inverter unit. Ii Let f be the integral coefficient of the i-th inverter unit, s be the Laplace operator, and f be the integral coefficient of the i-th inverter unit. ref f is the rated value for frequency recovery of a series-connected energy storage microgrid. I The frequency value of the output current is denoted by i, which is the inverter unit number. i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0067] Specifically, the inverter unit further includes:

[0068] A load switching circuit is used to switch between resistive-inductive loads and resistive-capacitive loads in the common load.

[0069] The control process of a cascaded microgrid frequency control circuit in this embodiment is described in detail below.

[0070] Determine the objective function for secondary frequency control, which specifically includes the following expression:

[0071]

[0072]

[0073] Where f ref f is the rated value for frequency recovery of a series-connected energy storage microgrid. i Let P be the frequency value of the i-th inverter unit. i and m i P represents the output active power and droop control coefficient of the i-th inverter unit, respectively. j and m j Let be the output power and droop control coefficient of the j-th inverter unit, respectively; t represents time; i and j are the inverter unit numbers, which take values ​​from positive integers not exceeding n, and i is not equal to j; and n represents the total number of inverter units.

[0074] The electrical parameters of the cascaded microgrid and each inverter unit in the cascaded microgrid are collected in real time; based on the electrical parameters, the active power and reactive power of each inverter unit are determined.

[0075] Specifically, the real-time acquisition of electrical parameters of the cascaded microgrid and each inverter unit in the cascaded microgrid includes the following steps:

[0076] The system collects the overall power factor angle, steady-state load voltage phase angle, and output current frequency of the cascaded microgrid in real time; it also collects the output voltage, output current, output voltage phase angle, and output voltage angular frequency of each inverter unit in the cascaded microgrid in real time.

[0077] Based on the active and reactive power of each inverter unit, a series inverter synchronization control logic is constructed; according to the series inverter synchronization control logic, the frequency of each inverter unit is synchronized.

[0078] The synchronous control logic expression for the series inverter is as follows:

[0079] f i =f * +sgn(Q i )m i P i

[0080] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit.i P is the droop control factor. i Let i be the output active power of the i-th inverter unit, and i be the inverter unit number. i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0081] Based on the electrical parameters, determine the desired output voltage phase angle model for each inverter unit;

[0082] For cascaded microgrids, each distributed generation (DG) unit has the same load current, which is an inherently common characteristic. Specifically, the expression for the desired output voltage phase angle model is:

[0083]

[0084] Where θ Ii Let be the desired output voltage phase angle of the i-th inverter unit, and atan2 be the arctangent function. For the overall power factor angle of the series energy storage microgrid system, P * max_i For the upper limit constraint of the output power of the i-th inverter unit, θ i Let θ' be the output voltage phase angle of the i-th inverter unit. load is the phase angle of the load voltage in steady state, i is the inverter unit number, i takes a value from positive integers not exceeding n, and n represents the total number of inverter units.

[0085] The desired output voltage phase angle model is linearized using a small signal to obtain the output current frequency model of each inverter unit.

[0086] Specifically, the desired output voltage phase angle model is linearized using small-signal methods to obtain the output current frequency model for each inverter unit, including the following steps:

[0087] The desired output voltage phase angle model expression is linearized using small-signal methods.

[0088] Assuming the steady-state voltage angles are the same, let θ i =θ i0 +Δθ i Substitute the desired output voltage phase angle model expression after small-signal linearization and eliminate θ i0 The output current frequency model of each inverter unit is obtained, and the expression of the output current frequency model is as follows:

[0089]

[0090] Where f Ii Let be the frequency value of the output current of the i-th inverter unit. For the overall power factor angle of the series energy storage microgrid system, P * max_i For the upper limit constraint of the output power of the i-th inverter unit, ω i Let ω be the output voltage angular frequency of the i-th inverter unit, where i is the inverter unit number and takes a value from a positive integer not exceeding n, and n represents the total number of inverter units.

[0091] As can be seen from the output current frequency model expression, the current frequency of the cascaded microgrid system represents the weighted average frequency of all units. Therefore, the inherent characteristics of the cascaded microgrid system can be utilized to adjust the frequency of the entire cascaded microgrid system by restoring the current frequency.

[0092] Based on the output current frequency model, construct a frequency offset model for each inverter unit;

[0093] Specifically, the frequency offset model expression is as follows:

[0094]

[0095] Where Δf i k is the frequency offset of the i-th inverter unit. Ii Here are the coefficients of the auxiliary controller, s is the Laplace operator, and f is the coefficient of the auxiliary controller. ref f is the rated value for frequency recovery of a series-connected energy storage microgrid. Ii Let i be the frequency value of the output current of the i-th inverter unit, where i is the inverter unit number and takes a value from a positive integer not exceeding n, and n represents the total number of inverter units.

[0096] Based on the characteristic that all inverter units in the cascaded microgrid have the same current, a distributed secondary frequency control logic is built based on the active power, reactive power and frequency offset model of each inverter unit.

[0097] Specifically, based on the characteristic that all inverter units in the cascaded microgrid have the same current, the following expression is obtained:

[0098] f I1 =f I2 =…=f In =f I

[0099] Where f I1 f is the frequency value of the first inverter unit. I2 f is the frequency value of the second inverter unit. In f is the frequency value of the nth inverter unit, where n represents the total number of inverter units. I The frequency value of the output current;

[0100] Based on the active power, reactive power, and frequency offset model of each inverter unit, a distributed secondary frequency control logic is constructed, and the expression of the distributed secondary frequency control logic is as follows:

[0101]

[0102] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit. i P is the droop control factor, inversely proportional to the DG capacity. i Let k be the output active power of the i-th inverter unit. Ii Let f be the integral coefficient of the i-th inverter unit, s be the Laplace operator, and f be the integral coefficient of the i-th inverter unit. ref f is the rated value for frequency recovery of a series-connected energy storage microgrid. I k represents the frequency value of the output current, i is the inverter unit number, i takes a value from a positive integer not exceeding n, and n represents the total number of inverter units. Ii Used for secondary recovery control of frequency.

[0103] According to the distributed secondary frequency control logic, the frequency of each inverter unit is subjected to secondary recovery control.

[0104] Through PI control, the current frequency returns to its normal value, and the frequencies of each output gate (DG) converge to a value equal to the current frequency. PI control, or proportional integral controller, is a linear controller that uses the control deviation between the given value and the actual output value as a basis. The proportional and integral components of this deviation are then linearly combined to form the control quantity, which controls the controlled object.

[0105] Under steady-state conditions, the current frequency recovers to its normal value through PI control, and the frequencies of all inverter units converge to a value equal to the current frequency. Then, the rated value f of the system frequency recovery to the series energy storage microgrid can be obtained. ref .

[0106] f1 = f2 = ... = f i =…=f n =f I =f ref

[0107] Where fi is the frequency value of the i-th series inverter unit, i is the series inverter unit number, i takes a value from a positive integer not exceeding n, and n represents the total number of series inverter units, f IThis represents the frequency value of the output current.

[0108] At the same time, since all inverter units share the same current,

[0109] Δf1=Δf2=…=Δf i =…=Δf n

[0110] m1ΔP1=m2ΔP2=…m i ΔP i =…=m n ΔP n

[0111] Where Δfi is the frequency offset of the i-th series inverter unit, i is the series inverter unit number, i takes a value from a positive integer not exceeding n, n represents the total number of series inverter units, and m i ΔPi is the droop control coefficient, and ΔPi is the power offset of the i-th series inverter unit when the series microgrid is unloaded.

[0112] Frequency recovery principle as follows Figure 3a and Figure 3b As shown, when the resistive-inductive load or resistive-capacitive load increases, the frequency moves from point a to point b. The proposed control core is to change the offset of the fP curve, so that the operating point will move from point b to the desired point c.

[0113] To more clearly verify the effect of the SoC equalization achieved by the embodiments of the present invention, a comparative case simulation analysis is presented here. A cascaded microgrid consisting of four cascaded units is established in the simulation software. The overall control strategy diagram is as follows. Figure 2 As shown. The following is an analysis of cases A and B respectively:

[0114] In Case A, the simulation results are as follows: Figure 4a and Figure 4b As shown. Figure 4a and Figure 4b The performance of the proposed control logic under resistive-inductive loads and resistive-capacitive loads are respectively represented. Figure 4a and Figure 4b The four curves correspond to the four cascaded units in the established cascaded microgrid. As can be seen from the figure, the four curves can be restored to the desired position, realizing frequency recovery control without communication, thus verifying the effectiveness of the proposed frequency recovery control logic. Its stability is not affected by the load impedance characteristics.

[0115] In Case B, the simulation results are as follows: Figure 5a and Figure 5b As shown, the dynamic response of the proposed control logic during load characteristic changes is verified. Figure 5aIn the cascaded microgrid system, the load initially operates under resistive-inductive load, and when t = 3s, the load switches to resistive-capacitive load. Figure 5a and Figure 5b The four curves in the simulation correspond to the four cascaded units in the established cascaded microgrid. The four curves can recover to the desired positions, achieving frequency recovery control without communication, maintaining system stability, and realizing frequency recovery and power sharing. The simulation results when the capacitive-resistive load is converted to an inductive-resistive load are similar to the above analysis results; the four curves can recover to the desired positions, achieving frequency recovery control without communication, maintaining system stability, and realizing frequency recovery and power sharing.

[0116] In the circuit described in this embodiment, the frequency of the cascaded microgrid system is restored to its rated value relying only on local information. Frequency control requires no communication, reducing communication costs and avoiding communication delays, packet loss, and fault risks. Compared with centralized control schemes, this method employs communication-free distributed control, which greatly enhances the reliability of the cascaded microgrid system.

[0117] Example 2

[0118] Embodiment 2 of the present invention provides a design method for a cascaded microgrid frequency control circuit, comprising the following steps:

[0119] Step S1: Set up multiple inverter units, each inverter unit including a distributed power source, a series inverter, a resonant circuit, a data processing circuit, a power calculation circuit, an inner loop control circuit, and a phase-locked loop circuit; the inner loop control circuit includes a synchronization control circuit and a frequency secondary recovery circuit;

[0120] Step S2: Connect the common load and multiple inverter units in series;

[0121] The process of setting up the inverter unit includes the following steps:

[0122] Step S11: Set up a data processing circuit to collect electrical parameters of the cascaded microgrid and each inverter unit in the cascaded microgrid in real time;

[0123] Step S12: Set up a power calculation circuit to calculate the electrical parameters and determine the active power and reactive power of each inverter unit;

[0124] Step S13: According to the synchronization control logic of the series inverter, set up the synchronization control circuit to synchronize the frequency of the inverter unit;

[0125] Step S14: According to the distributed secondary frequency control logic, set up the frequency secondary recovery circuit to perform secondary recovery control on the frequency of the inverter unit.

[0126] Specifically, the synchronization control logic of the series inverter is as follows:

[0127] f i =f * +sgn(Q i )m i P i

[0128] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit. i P is the droop control factor. i Let i be the output active power of the i-th inverter unit, and i be the inverter unit number. i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0129] Specifically, the distributed secondary frequency control logic is as follows:

[0130]

[0131] Where f i f is the frequency value of the i-th inverter unit. * For the no-load frequency value of the series microgrid, sgn represents the sign function, and Q... i Let m be the output reactive power of the i-th inverter unit. i P is the droop control factor. i Let k be the output active power of the i-th inverter unit. Ii Let f be the integral coefficient of the i-th inverter unit, s be the Laplace operator, and f be the integral coefficient of the i-th inverter unit. ref f is the rated value for frequency recovery of a series-connected energy storage microgrid. I The frequency value of the output current is denoted by i, which is the inverter unit number. i takes a value from a positive integer not exceeding n, where n represents the total number of inverter units.

[0132] Specifically, setting up the inverter unit also includes the following steps:

[0133] A phase-locked loop circuit is set up to control the frequency and phase of the loop oscillation signal inside the inverter unit;

[0134] A load switching circuit is provided to switch between resistive-inductive loads and resistive-capacitive loads in the common load.

[0135] The methods described in the above embodiments have been described in detail in the embodiments relating to the circuit, and will not be elaborated upon here.

[0136] In this embodiment, the frequency of the cascaded microgrid system is restored to its rated value relying only on local information. Frequency control requires no communication, reducing communication costs and avoiding communication delays, packet loss, and fault risks. Compared with centralized control schemes, this method employs communication-free distributed control, which significantly enhances the reliability of the cascaded microgrid system.

[0137] Any modifications, additions, and equivalent substitutions made within the scope of the principles of this invention shall still fall within the patent coverage of this invention.

Claims

1. A cascaded microgrid frequency control circuit, comprising a common load and multiple inverter units connected in series, wherein each inverter unit includes a distributed power source, a series inverter, and a resonant circuit, characterized in that, The inverter unit also includes: The data processing circuit is used to collect the overall power factor angle, steady-state load voltage phase angle, and output current frequency of the cascaded microgrid in real time; and to collect the output voltage, output current, output voltage phase angle, and output voltage angular frequency of the inverter unit in real time. A power calculation circuit is used to determine the active power and reactive power of each inverter unit based on the output voltage, output current, output voltage phase angle and output voltage angular frequency. The inner loop control circuit is used to synchronize the frequency of the inverter unit according to the series inverter synchronization control logic; and to perform secondary recovery control of the frequency of the inverter unit according to the distributed secondary frequency control logic. A phase-locked loop circuit is used to control the frequency and phase of the loop oscillation signal inside the inverter unit; The inner loop control circuit includes: A synchronization control circuit is used to synchronize the frequency of the inverter unit according to the synchronization control logic of the series inverter, which is as follows: in f i Let i be the frequency value of the i-th inverter unit. For the no-load frequency value of the series microgrid, sg n Represents a symbolic function. Q i For the first i The output reactive power of each inverter unit m i This is the droop control coefficient. P i For the first i The output active power of each inverter unit; i Inverter unit numbering, i within no more than n Take values ​​from positive integers, n Represents the total number of inverter units; A frequency secondary recovery circuit is used to perform secondary recovery control on the frequency of the inverter unit according to the distributed secondary frequency control logic, which is as follows: in f i Let i be the frequency value of the i-th inverter unit. For the no-load frequency value of the series microgrid, sg n Represents a symbolic function. Q i For the first i The output reactive power of each inverter unit m i This is the droop control coefficient. P i For the first i The output active power of each inverter unit Let be the integral coefficient of the i-th inverter unit. s For the Laplace operator, f ref This is the rated value for frequency recovery of a series-connected energy storage microgrid. f I The frequency value of the output current. i Inverter unit numbering, i within no more than n Take values ​​from positive integers, n This represents the total number of inverter units.

2. The circuit as described in claim 1, characterized in that, The inverter unit also includes: A load switching circuit is used to switch between resistive-inductive loads and resistive-capacitive loads in the common load.

3. A design method for a cascaded microgrid frequency control circuit, characterized in that, The control circuit design method, when applied to the control circuit of any one of claims 1 to 2, includes the following steps: Multiple inverter units are configured, each inverter unit including a distributed power source, a series inverter, a resonant circuit, a data processing circuit, a power calculation circuit, an inner loop control circuit, and a phase-locked loop circuit; the inner loop control circuit includes a synchronization control circuit and a frequency secondary recovery circuit. Connect the common load and multiple inverter units in series; The process of setting up the inverter unit includes the following steps: A data processing circuit is set up to collect the electrical parameters of the cascaded microgrid and each inverter unit in the cascaded microgrid in real time. A power calculation circuit is set up to calculate the electrical parameters and determine the active power and reactive power of each inverter unit. Based on the synchronization control logic of the series inverter, the synchronization control circuit is set up to synchronize the frequency of the inverter unit. Based on the distributed secondary frequency control logic, the frequency secondary recovery circuit is set up to perform secondary recovery control on the frequency of the inverter unit.

4. The design method for a cascaded microgrid frequency control circuit as described in claim 3, characterized in that, The synchronous control logic of the series inverter is as follows: in f i Let i be the frequency value of the i-th inverter unit. For the no-load frequency value of the series microgrid, sg n Represents a symbolic function. Q i For the first i The output reactive power of each inverter unit m i This is the droop control coefficient. P i For the first i The output active power of each inverter unit; i Inverter unit numbering, i within no more than n Take values ​​from positive integers, n This represents the total number of inverter units.

5. The design method for a cascaded microgrid frequency control circuit as described in claim 3, characterized in that, The distributed secondary frequency control logic is as follows: in f i Let i be the frequency value of the i-th inverter unit. For the no-load frequency value of the series microgrid, sg n Represents a symbolic function. Q i For the first i The output reactive power of each inverter unit m i This is the droop control coefficient. P i For the first i The output active power of each inverter unit Let be the integral coefficient of the i-th inverter unit. s For the Laplace operator, f ref This is the rated value for frequency recovery of a series-connected energy storage microgrid. f I The frequency value of the output current. i Inverter unit numbering, i within no more than n Take values ​​from positive integers, n This represents the total number of inverter units.

6. A design method for a cascaded microgrid frequency control circuit as described in any one of claims 3-5, characterized in that, Setting up the inverter unit also includes the following steps: A phase-locked loop circuit is set up to control the frequency and phase of the loop oscillation signal inside the inverter unit; A load switching circuit is provided to switch between resistive-inductive loads and resistive-capacitive loads in the common load.

Images