A method and system for complex coefficient droop constant frequency grid-connected cooperative control

Abstract

The embodiment of the application relates to the technical field of micro-grid operation control, in particular to a complex coefficient droop fixed-frequency grid-connection collaborative control method and system, wherein the method comprises the following steps: adopting complex coefficient droop control to fix the output voltage frequency of a converter to a rated frequency; detecting the output voltage frequency of the converter, and obtaining a first frequency deviation by subtracting the rated frequency; determining a first droop amount according to the first frequency deviation and a first droop coefficient; detecting the output voltage change rate of the converter, and determining a second droop amount according to a second droop coefficient; superimposing the two to obtain a first power instruction value; obtaining a second frequency deviation based on the first power instruction value and the output frequency; inputting the output value of a controller to obtain a second power instruction value by superimposing the first power instruction value; and controlling the power output of the converter according to the second power instruction value. The application realizes fixed-frequency grid-connection without a phase-locked loop and power static deviation tracking through complex coefficient droop and frequency synchronous collaborative control.

Description

Technical Field

[0001] This application relates to the field of microgrid operation control technology, and in particular to a complex coefficient drooping fixed frequency grid-connected collaborative control method and system. Background Technology

[0002] Microgrids, as an important form of efficient integration of distributed power sources into the distribution network, have widely adopted traditional droop control due to its advantages such as no communication required and plug-and-play functionality. Traditional droop control simulates the external characteristics of a synchronous generator, achieving power control by adjusting the relationship between active power and frequency, and reactive power and voltage amplitude. However, this control method has inherent drawbacks. Active power regulation inevitably causes frequency deviation, and the impedance-to-inductance ratio of the line leads to coupling between active and reactive power. Especially in low-voltage microgrids where line resistance cannot be ignored, the accuracy and dynamic performance of power distribution are significantly degraded.

[0003] When a microgrid switches from islanded mode to grid-connected mode, the converter needs to maintain frequency and phase synchronization with the main grid. Existing grid-connection synchronization technologies typically rely on phase-locked loops (PLLs) or frequency-locked loops (LLLs) to obtain grid voltage information. However, PLLs suffer from slow dynamic response and frequent phase-locking failures in weak grid or grid harmonic environments. Furthermore, they increase the complexity of the control system and its dependence on grid voltage measurements, reducing the microgrid's autonomy and reliability. Simultaneously, during grid-connected operation, the converter output power needs to accurately track dispatch commands. Traditional droop control suffers from steady-state errors, failing to achieve zero steady-state error power control and affecting the accuracy of power exchange between the microgrid and the grid.

[0004] On the other hand, converter control strategies with active support capabilities are receiving increasing attention in order to cope with grid frequency fluctuations and voltage changes. However, in existing solutions, active frequency droop and reactive voltage droop are usually coupled, and reactive power is significantly affected by line impedance and load changes, making it difficult to accurately control at a given value in steady state. This limits the microgrid's support effect on the grid and its ability to optimize power quality. Summary of the Invention

[0005] To address this, the present invention provides a complex coefficient droop fixed-frequency grid-connected coordinated control method and system to overcome the problems of instability of phase-locked loops in weak grid environments and frequency deviation and power steady-state error in traditional droop control.

[0006] To achieve the above objectives, this invention provides a complex coefficient drooping fixed-frequency grid-connected coordinated control method. It includes:

[0007] Step S1: Use complex coefficient droop control to fix the converter output voltage frequency to the rated frequency;

[0008] Step S2: Detect the inverter output voltage frequency, calculate the first frequency deviation by subtracting the inverter output voltage frequency from the rated frequency, and determine the first droop amount based on the first frequency deviation and a preset first droop coefficient.

[0009] Step S3: Detect the rate of change of the converter output voltage, determine the second droop amount based on the rate of change of voltage and the preset second droop coefficient, and superimpose the first droop amount and the second droop amount onto the externally given power command value to obtain the first power command value;

[0010] Step S4: Based on the first power command value and the frequency of the converter output voltage, a second frequency deviation is obtained. The second power deviation is input to the controller to obtain an output value. The output value is used as a correction value and superimposed on the first power command value to obtain the second power command value.

[0011] Step S5: Control the power output of the converter according to the second power command value.

[0012] Further, in step S1, the complex coefficient droop control applies the complex power droop to the converter output voltage phasor by constructing a complex droop coefficient. The complex droop coefficient includes line impedance information, and its phasor form is expressed as follows:

[0013] ;

[0014] in, Let R be the real droop factor, and X be the line resistance and reactance, respectively, and Z be the line impedance magnitude.

[0015] Furthermore, in step S2, the detection of the converter output voltage frequency does not rely on a phase-locked loop or a frequency-locked loop, but rather uses the dq-axis voltage reference value output by the power control loop to generate the converter output voltage frequency. The calculation formula for the converter output voltage frequency is as follows:

[0016] ;

[0017] in, and This is the reference value for the dq-axis voltage output of the power control; This is the angular frequency gain coefficient; ω is the rated frequency of the power grid; f is the frequency of the frequency synchronization control output, which is the power grid frequency in steady state; ω is the angular frequency corresponding to the power grid frequency.

[0018] Furthermore, the controller is a proportional-integral controller, and the output is a voltage reference value correction; the proportional-integral controller is designed in conjunction with the low-pass filter characteristics of the power measurement circuit, so that the closed-loop transfer function from the first power command value to the actual output power of the converter has a gain of 1 in steady state.

[0019] Furthermore, the closed-loop transfer function is:

[0020] ;

[0021] The transfer function C(s) is expressed as:

[0022] ;

[0023] Where s is the complex frequency variable of the Laplace transform, and U is the amplitude of the converter output voltage. This refers to the gain coefficient in the power calculation stage. Here, T represents the line impedance magnitude, and T is the power filtering time constant. The coefficients of the closed-loop characteristic polynomial of the converter in the synchronous rotating coordinate system are... For the real and imaginary part correlation parameters of the complex droop coefficient, This is the proportionality coefficient. The integral coefficients satisfy the following in steady state: Further, in step S3, the first droop is determined based on the first frequency deviation between the converter output voltage frequency and the rated frequency and a preset first droop coefficient, and the second droop is determined based on the converter output voltage change rate and a preset second droop coefficient. The droop equation is expressed as: ;

[0024] in, This is a reference value for active power. This is a reference value for reactive power. , These represent the rated active power and rated reactive power, respectively, and ω is the converter output angular frequency. The rated angular frequency, the This is the first frequency deviation; The preset first droop coefficient, The preset second droop coefficient; The error signal before the proportional-integral controller in the d-axis voltage channel is physically equivalent to the rate of change of the converter output voltage, i.e., the... Corresponding to the rate of change of the converter output voltage, and in steady state This makes the second droop amount zero.

[0025] Furthermore, the relationship between the error signal and the rate of change of the converter output voltage is as follows:

[0026] ;in, Here, U is the integral coefficient of the proportional-integral controller in power point tracking control, and U is the amplitude of the converter output voltage. Based on this relationship, the reactive power droop equation is equivalent to:

[0027] ;

[0028] That is, the reactive power reference value is determined by the rate of change of the converter output voltage. In steady state, the rate of change of voltage is zero, and the reactive power reference value is exactly equal to the rated reactive power.

[0029] Further, in step S5, the second power command value is converted into a dq-axis voltage reference value via a power tracking control loop. The dq-axis voltage reference value is simultaneously input to a frequency synchronization control loop. The frequency synchronization control loop multiplies the ratio of the q-axis component to the d-axis component in the dq-axis voltage reference value by an angular frequency gain coefficient and then superimposes it onto the rated angular frequency. The phase angle of the converter output voltage is then output via an integrator.

[0030] Further, in step S3, the second droop is determined based on the converter output voltage change rate and a preset second droop coefficient. The converter output voltage change rate is characterized by an error signal before the proportional-integral controller in the d-axis voltage channel. The error signal is the difference signal at the input of the proportional-integral controller. The first droop is superimposed on the rated active power to obtain an active power reference value, and the second droop is superimposed on the rated reactive power to obtain a reactive power reference value. The active power reference value and the reactive power reference value are input to the controller as components of the first power command value.

[0031] A complex coefficient drooping fixed-frequency grid-connected coordinated control system includes:

[0032] The frequency fixing module is used to fix the frequency of the converter output voltage to the rated frequency using complex coefficient droop control.

[0033] The frequency detection and first droop module is used to detect the frequency of the converter output voltage, obtain the first frequency deviation by subtracting the frequency of the converter output voltage from the rated frequency, and determine the first droop amount based on the first frequency deviation and a preset first droop coefficient.

[0034] The voltage detection and second droop module is used to detect the rate of change of the converter output voltage, determine the second droop amount based on the rate of change of voltage and the preset second droop coefficient, and superimpose the first droop amount and the second droop amount onto the externally given power command value to obtain the first power command value.

[0035] The power point tracking control module is used to obtain a second frequency deviation based on the first power command value and the converter output voltage frequency, input the second frequency deviation into the controller to obtain an output quantity, and use the output quantity as a correction quantity to be superimposed on the first power command value to obtain the second power command value.

[0036] The power output control module is used to control the power output of the converter according to the second power command value.

[0037] Compared with existing technologies, the advantages of this invention are as follows: The complex coefficient droop control in this invention avoids the frequency deviation problem of traditional droop control, achieving constant-frequency operation of isolated microgrids with superior dynamic performance. The frequency synchronization control of this invention eliminates the need for phase-locked loops or frequency-locked loops, achieving frequency synchronization between the converter and the grid using only local power control information, thus improving the autonomy and reliability of the microgrid. Power point tracking control eliminates steady-state errors during grid connection, achieving zero steady-state error power point tracking. The active power frequency droop and reactive power voltage change rate droop in the upstream stage enable the converter to have active frequency support and voltage change response capabilities, and reactive power can be precisely controlled in steady state, solving the problem of inaccurate reactive power control under traditional droop control strategies.

[0038] Furthermore, the frequency synchronization control loop does not rely on phase-locked loops or frequency-locked loops, nor does it require additional measurement of grid voltage. It can achieve complete synchronization between the converter output voltage frequency and the grid frequency in steady state by using only the dq-axis voltage reference value output by the power control loop itself. This simplifies the control system structure and improves the autonomy and reliability of the microgrid's grid-connected operation.

[0039] Furthermore, the steady-state active power output of the converter is equal to the difference between the rated active power and the frequency droop, and the steady-state reactive power output of the converter is equal to the rated reactive power. In steady state, the active and reactive power outputs of the converter do not affect each other, that is, the active and reactive power are decoupled in steady state. Therefore, this invention also achieves precise control of the converter's output reactive power. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of power transmission in a converter.

[0041] Figure 2 This is a block diagram for the droop control of complex coefficients;

[0042] Figure 3 This is a block diagram of the phasor form of the complex coefficient droop control;

[0043] Figure 4 This is a diagram illustrating the overall structure of the coordinated control of grid connection synchronization and power point tracking.

[0044] Figure 5 This is a block diagram of frequency synchronization control.

[0045] Figure 6 Here is a block diagram of the power point tracking control.

[0046] Figure 7 To reconstruct the control diagram considering the coordinated control of frequency synchronization control and preceding droop control;

[0047] Figure 8 This is a schematic diagram of a complex coefficient drooping fixed-frequency grid-connected coordinated control system. Detailed Implementation

[0048] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0049] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0050] Please see Figures 1-8 As shown, Figure 1 This is a schematic diagram of power transmission in a converter. Figure 2 This is a block diagram for the droop control of complex coefficients; Figure 3 This is a block diagram of the phasor form of the complex coefficient droop control; Figure 4 This is a diagram illustrating the overall structure of the coordinated control of grid connection synchronization and power point tracking. Figure 5 This is a block diagram of frequency synchronization control. Figure 6 Here is a block diagram of the power point tracking control. Figure 7 To reconstruct the control diagram considering the coordinated control of frequency synchronization control and preceding droop control; Figure 8 This is a schematic diagram of a complex coefficient drooping fixed-frequency grid-connected coordinated control system.

[0051] The technical solution provided in this application includes the following steps:

[0052] Step S1: Use complex coefficient droop control to fix the converter output voltage frequency to the rated frequency;

[0053] Step S2: Detect the inverter output voltage frequency, calculate the first frequency deviation by subtracting the inverter output voltage frequency from the rated frequency, and determine the first droop amount based on the first frequency deviation and a preset first droop coefficient.

[0054] Step S3: Detect the rate of change of the converter output voltage, determine the second droop amount based on the rate of change of voltage and the preset second droop coefficient, and superimpose the first droop amount and the second droop amount onto the externally given power command value to obtain the first power command value;

[0055] Step S4: Based on the first power command value and the frequency of the converter output voltage, a second frequency deviation is obtained. The second frequency deviation is input to the controller to obtain an output value. The output value is used as a correction value and superimposed on the first power command value to obtain the second power command value.

[0056] Step S5: Control the power output of the converter according to the second power command value.

[0057] In this embodiment of the invention, for example, Figure 1 The diagram shows the power transfer of the converter. The converter is connected to the PCC through line impedance. The phasor equations for the power injected into the PCC and the output power of the converter can be obtained, expressed as:

[0058] ,

[0059] ;

[0060] in, The power injected into the converter by PCC, S = P + jQ, is the output power of the converter. For the voltage phasor of PCC, The output voltage phasor of the converter. The current phasor flowing through the tie line between the converter and the PCC. The comma indicates conjugation.

[0061] The current conjugate on the tie line is then expressed as:

[0062] ;

[0063] Where Z is the line impedance, Z = R + jX.

[0064] Therefore, the specific forms of the power injected into the PCC by the converter and the output power of the converter can be obtained as follows:

[0065] ,

[0066] ;

[0067] With the PCC voltage as the d-axis, then we have , Considering (R+jX)(R-jX)=Z², where Z is the magnitude of the line impedance, the injected power into the PCC can be expressed as:

[0068] ;

[0069] The impedance ratio matrix Zratio is:

[0070] ;

[0071] This allows for the construction of a droop control strategy with a complex droop coefficient, while integrating the power filtering stage into the control strategy.

[0072] ;

[0073] in, The real droop coefficient, together with the impedance ratio matrix Zratio, forms the complex droop coefficient. and These are the rated values ​​of the d-axis and q-axis components of the converter output voltage, respectively. and These are the rated active power and rated reactive power of the converter, respectively; T is the reciprocal of the filter cutoff frequency.

[0074] The control block diagram in the form of a complex coefficient droop control matrix can be obtained for the converter, such as... Figure 2 As shown, and These are the d-axis and q-axis components of the voltage reference value sent into the inner loop voltage control, respectively.

[0075] The phasor form is expressed as:

[0076] ;

[0077] in, This represents the difference between the d-axis output voltage and the d-axis rated voltage of the converter. This represents the difference between the q-axis output voltage and the q-axis rated voltage of the converter. This is the difference between the converter's output active power and its rated active power. This is the difference between the output reactive power of the converter and the rated reactive power.

[0078] The complex power phasor form is expressed as:

[0079] ;

[0080] in, This is the complex droop coefficient.

[0081] The complex coefficient droop control in this invention avoids the frequency deviation problem of traditional droop control, enabling constant-frequency operation of isolated microgrids with superior dynamic performance. The frequency synchronization control of this invention eliminates the need for phase-locked loops or frequency-locked loops, achieving frequency synchronization between the converter and the grid solely using local power control information, thus improving the autonomy and reliability of the microgrid. Power point tracking control eliminates steady-state errors during grid connection, achieving zero steady-state error power point tracking. The upstream active power frequency droop and reactive power voltage change rate droop enable the converter to have active frequency support and voltage change response capabilities, and reactive power can be precisely controlled in steady state, solving the problem of inaccurate reactive power control under traditional droop control strategies.

[0082] like Figure 3 The diagram shown is a phasor-form control block diagram for complex coefficient droop control, with rated complex power. The difference between the complex power S output by the converter and the complex droop coefficient is in the complex droop coefficient. And the conjugate value of the voltage reference value generated under the action of the low-pass filter and fed into the inner loop voltage control. .

[0083] like Figure 4 The diagram shown is an overall structural diagram of the grid-connected synchronization and power point tracking coordinated control of the invention.

[0084] like Figure 5 The diagram shown is a block diagram of frequency synchronization control. The angular frequency of the output of the frequency synchronization control is expressed as:

[0085] ;

[0086] in, and This is the reference value for the dq-axis voltage output of the power control; This is the angular frequency gain coefficient; ω is the rated frequency of the power grid; f is the frequency of the frequency synchronization control output, which is the power grid frequency in steady state; ω is the angular frequency corresponding to the power grid frequency.

[0087] The output angular frequency of the frequency synchronization control, under the action of the integrator, generates the phase angle of the frequency synchronization control output, expressed as:

[0088] ;

[0089] Where θ is the phase angle of the frequency synchronization control output.

[0090] To eliminate the steady-state error in converter output power caused by complex coefficient droop control during microgrid grid connection, a PI controller is introduced into the control strategy, resulting in power point tracking control, expressed as:

[0091] ;

[0092] in, This refers to the proportional coefficient of the PI controller in power point tracking control. This refers to the integral coefficient of the PI controller in power point tracking control.

[0093] Figure 6 The diagram shows the power point tracking (PPT) control block diagram. Similar to complex coefficient droop control, the power filtering stage is integrated into the PPT control, making the entire control structure more compact. This is a reference value for the active power output of the converter. This is a reference value for the reactive power output of the converter.

[0094] The open-loop transfer function of power point tracking control is expressed as:

[0095] ;

[0096] The closed-loop transfer function for further power point tracking control is expressed as:

[0097] ;

[0098] The transfer function C(s) is expressed as:

[0099] ;

[0100] Where s is the complex frequency variable of the Laplace transform, and U is the amplitude of the converter output voltage. This refers to the gain coefficient in the power calculation stage. Here, T represents the line impedance magnitude, and T is the power filtering time constant. , , The coefficients of the closed-loop characteristic polynomial of the converter in the synchronous rotating coordinate system are... ,

[0101] For the real and imaginary part correlation parameters of the complex droop coefficient, This is the proportionality coefficient. The integral coefficients satisfy the following in steady state: .

[0102] From this, we can analyze the power point tracking performance of the power point tracking control. It can be seen that when the system is in steady state, i.e., s=0, It can achieve zero steady-state error tracking of the power reference value.

[0103] Power point tracking (PPT) can eliminate steady-state errors in converter output power, but it reduces the converter's ability to support the grid. To improve the converter's grid support, frequency droop control can be used to obtain a reference value for active power. Considering the uncertainty of the converter's output voltage in steady state and power coupling caused by line impedance, the frequency corresponds to the rate of change of the phase angle. Similar to frequency active power droop, the error signal before the d-axis voltage channel PI controller can be used. (This value is 0 in steady state) The reactive power reference value is obtained by droop control instead of voltage droop control, and its control form is expressed as:

[0104] ;

[0105] in, This is the active power droop factor. This is the reactive power droop factor.

[0106] The droop amount obtained from droop control is applied to the rated power to obtain the power reference value, which, together with the output power feedback, forms a closed-loop system.

[0107] Ignoring the dynamic process of the inner loop voltage control, since the rate of change of U is... The rate of change is the same, therefore the error signal Rate of change of converter output voltage U The relationship between them is as follows:

[0108] ;

[0109] Considering Since it is a constant, it can be simplified to a form that includes the rate of change of voltage as follows:

[0110] ;

[0111] The above equation is essentially a differential equation concerning the d-axis component of the converter output voltage. When the rate of change of the converter output voltage is... When the solution to the differential equation is given by:

[0112] ;

[0113] Since the system was in steady state before the voltage change, Therefore, the constant C is 0, and thus The relationship between U and U can be expressed as follows:

[0114] ;

[0115] As can be seen from the above With voltage change rate Based on the equivalence between them, the droop equation for the converter output power can be obtained as follows:

[0116] ;

[0117] It can be seen that the converter using the control strategy of this invention has frequency support and voltage change response capabilities. Its output active power is related to the frequency offset and the active power droop coefficient, while its output reactive power is related to the voltage change rate, the integral coefficient of the PI controller, and the reactive power droop coefficient.

[0118] Considering the relationship between frequency synchronization control and droop control, the reconfiguration diagram of grid synchronization and power point tracking coordinated control is obtained as follows: Figure 7 As shown, the droop information for active and reactive power comes from the other channel, respectively.

[0119] according to Figure 7 The matrix relationship containing the converter's output voltage and output power is obtained, expressed as:

[0120] ;

[0121] Expanding this gives us the matrix relationship between voltage deviation and power deviation, expressed as:

[0122] ;

[0123] The simplified relationship between voltage deviation and power deviation is expressed as:

[0124] ;

[0125] in:

[0126] ;

[0127] Multiply both sides of the voltage deviation and power deviation equation by the impedance ratio matrix. Considering that the inverse of the impedance ratio matrix is ​​itself, the matrix transfer function from voltage deviation to power deviation is obtained as follows:

[0128] ;

[0129] When the system is in steady state, that is, when s=0, we have:

[0130] ;

[0131] When the system is in steady state, the matrix transfer function described above can be expressed as:

[0132] ;

[0133] Therefore, the output power of the converter in steady state can be obtained as:

[0134] ;

[0135] It is evident that the steady-state active power output of the converter equals the difference between the rated active power and the frequency droop, while the steady-state reactive power output equals the rated reactive power. In steady state, the active and reactive power outputs of the converter do not affect each other, meaning there is steady-state decoupling between active and reactive power. Therefore, this invention also achieves precise control of the converter's output reactive power.

[0136] Specifically, in step S1, the complex coefficient droop control applies the complex power droop to the converter output voltage phasor by constructing a complex droop coefficient. The complex droop coefficient contains line impedance information, and its phasor form is expressed as follows:

[0137] ;

[0138] in, Let R be the actual droop factor, and X be the line resistance and reactance, respectively, and Z be the line impedance modulus.

[0139] Specifically, in step S2, the detection of the converter output voltage frequency does not rely on a phase-locked loop or a frequency-locked loop, but rather uses the dq-axis voltage reference value output by the power control loop to generate the converter output voltage frequency. The calculation formula for the converter output voltage frequency is as follows:

[0140] ;

[0141] in, and This is the reference value for the dq-axis voltage output of the power control; This is the angular frequency gain coefficient; ω is the rated frequency of the power grid; f is the frequency output of the frequency synchronization control, which is the power grid frequency in steady state; ω is the angular frequency corresponding to the power grid frequency; θ is the phase angle of the frequency synchronization control output.

[0142] Specifically, the controller is a proportional-integral controller, and the output is a voltage reference value correction. The proportional-integral controller is designed in conjunction with the low-pass filter characteristics of the power measurement circuit, so that the closed-loop transfer function from the first power command value to the actual output power of the converter has a gain of 1 in steady state.

[0143] Specifically, the closed-loop transfer function of the power point tracking control is:

[0144] ;

[0145] The transfer function C(s) is expressed as:

[0146] ;

[0147] This is the proportionality coefficient. Here, T is the integral coefficient, and T is the power filtering time constant; in steady state, it satisfies... .

[0148] Specifically, in step S3, the first droop is determined based on the first frequency deviation between the converter output voltage frequency and the rated frequency and a preset first droop coefficient, and the second droop is determined based on the converter output voltage change rate and a preset second droop coefficient. The droop equation is expressed as follows: ;

[0149] in, This is a reference value for active power. This is a reference value for reactive power. , These represent the rated active power and rated reactive power, respectively, and ω is the converter output angular frequency. The rated angular frequency, the This is the first frequency deviation; The preset first droop coefficient, The preset second droop coefficient; The error signal before the proportional-integral controller in the d-axis voltage channel is physically equivalent to the rate of change of the converter output voltage, i.e., the... Corresponding to the rate of change of the converter output voltage, and in steady state This makes the second droop amount zero.

[0150] Specifically, the relationship between the error signal and the rate of change of the converter output voltage is as follows:

[0151] ;

[0152] in, Here, U is the integral coefficient of the proportional-integral controller in power point tracking control, and U is the amplitude of the converter output voltage. Based on this relationship, the reactive power droop equation is equivalent to:

[0153] ;

[0154] That is, the reactive power reference value is determined by the rate of change of the converter output voltage. In steady state, the rate of change of voltage is zero, and the reactive power reference value is exactly equal to the rated reactive power.

[0155] Specifically, in step S5, the second power command value is converted into a dq-axis voltage reference value via a power tracking control loop. The dq-axis voltage reference value is simultaneously input to a frequency synchronization control loop. The frequency synchronization control loop multiplies the ratio of the q-axis component to the d-axis component in the dq-axis voltage reference value by an angular frequency gain coefficient and then superimposes it onto the rated angular frequency. The phase angle of the converter output voltage is then output via an integrator.

[0156] In this embodiment of the invention, the frequency synchronization control loop does not rely on a phase-locked loop or a frequency-locked loop, nor does it require additional measurement of the grid voltage. It can achieve complete synchronization between the converter output voltage frequency and the grid frequency in steady state by using only the dq-axis voltage reference value output by the power control loop itself. This simplifies the control system structure and improves the autonomy and reliability of the microgrid's grid-connected operation.

[0157] Specifically, in step S3, the second droop is determined based on the converter output voltage change rate and a preset second droop coefficient. The converter output voltage change rate is characterized by an error signal before the proportional-integral controller in the d-axis voltage channel. The error signal is the difference signal at the input of the proportional-integral controller. The first droop is superimposed on the rated active power to obtain an active power reference value, and the second droop is superimposed on the rated reactive power to obtain a reactive power reference value. The active power reference value and the reactive power reference value are input to the controller as components of the first power command value.

[0158] In this embodiment of the invention, since the second droop is based on the voltage change rate rather than the voltage amplitude itself, the droop is zero when the steady-state voltage remains unchanged. This ensures that the reactive power output by the converter is precisely equal to the rated reactive power in steady state, thereby achieving complete decoupling of active and reactive power in steady state and solving the problem of inaccurate reactive power control in traditional droop control. Simultaneously, the first droop is determined based on the frequency deviation, enabling the converter to have active frequency support capability and respond to grid frequency fluctuations.

[0159] like Figure 8 As shown, Figure 8 This is a schematic diagram of a complex coefficient drooping fixed-frequency grid-connected collaborative control system according to an embodiment of the present invention.

[0160] Specifically, a complex coefficient drooping fixed-frequency grid-connected collaborative control system includes:

[0161] The frequency fixing module is used to fix the frequency of the converter output voltage to the rated frequency using complex coefficient droop control.

[0162] The frequency detection and first droop module is used to detect the frequency of the converter output voltage, obtain the first frequency deviation by subtracting the frequency of the converter output voltage from the rated frequency, and determine the first droop amount based on the first frequency deviation and a preset first droop coefficient.

[0163] The voltage detection and second droop module is used to detect the rate of change of the converter output voltage, determine the second droop amount based on the rate of change of voltage and the preset second droop coefficient, and superimpose the first droop amount and the second droop amount onto the externally given power command value to obtain the first power command value.

[0164] The power point tracking control module is used to obtain a second frequency deviation based on the first power command value and the frequency of the converter output voltage, input the second frequency deviation into the controller to obtain an output, and use the output as a correction amount to be superimposed on the first power command value to obtain the second power command value.

[0165] The power output control module is used to control the power output of the converter according to the second power command value.

[0166] In this embodiment, the first droop coefficient corresponds to the active power droop coefficient. The second droop coefficient corresponds to the reactive power droop coefficient. The active power droop coefficient The reactive power droop coefficient is determined based on the frequency support requirements of the converter and the stability of grid-connected operation. The angular frequency gain coefficient is determined based on the converter's voltage change response requirements and grid-connected operation stability. Specifically, this is done when determining the angular frequency gain coefficient. The proportional coefficient of the proportional-integral controller in power point tracking control and integral coefficient After considering the line impedance, a small-signal model of the grid-connected coordinated control system is established, and a value is selected that ensures all characteristic roots of the closed-loop system lie in the left half-plane of the complex plane. and This is to meet the requirements for stable system operation.

[0167] In a specific parameter setting, take , , , , The converter output voltage is 311V. In Example 1, in an application scenario where the converter is connected to the PCC via a biased inductive line, the line impedance is taken as... The remaining control structures and parameters are set as described above. The converter operates according to the above-described complex coefficient droop fixed-frequency grid-connected collaborative control method. It achieves frequency synchronization with the grid through frequency synchronization control, and responds to grid frequency changes and output voltage changes through active frequency droop and reactive voltage change rate droop, respectively. In Example 2, under another line impedance condition, the line impedance is taken as 0.942 + j × 0.0942 Ω, and the remaining control structures and parameters are set as described above. The converter still operates according to the above-described complex coefficient droop fixed-frequency grid-connected collaborative control method. Through the analysis of the closed-loop system stability, it is determined that the control parameters can meet the grid-connected operation requirements under the corresponding line impedance condition, thus demonstrating that the control method can be applied to grid-connected operation scenarios with different line impedance ratios.

[0168] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A complex coefficient drooping fixed-frequency grid-connected coordinated control method, characterized in that, include: Step S1: Use complex coefficient droop control to fix the converter output voltage frequency to the rated frequency; Step S2: Detect the inverter output voltage frequency, calculate the first frequency deviation by subtracting the inverter output voltage frequency from the rated frequency, and determine the first droop amount based on the first frequency deviation and a preset first droop coefficient. Step S3: Detect the rate of change of the converter output voltage, determine the second droop amount based on the rate of change of voltage and the preset second droop coefficient, and superimpose the first droop amount and the second droop amount onto the externally given power command value to obtain the first power command value; Step S4: Based on the first power command value and the frequency of the converter output voltage, a second frequency deviation is obtained. The second frequency deviation is input to the controller to obtain an output value. The output value is used as a correction value and superimposed on the first power command value to obtain the second power command value. Step S5: Control the power output of the converter according to the second power command value.

2. The method according to claim 1, characterized in that, In step S1, the complex coefficient droop control applies the complex power droop to the converter output voltage phasor by constructing a complex droop coefficient. The complex droop coefficient contains line impedance information, and its phasor form is expressed as follows: ； in, Let R be the real droop factor, and X be the line resistance and reactance, respectively, and Z be the line impedance magnitude.

3. The method according to claim 1, characterized in that, In step S2, the detection of the converter output voltage frequency does not rely on a phase-locked loop or a frequency-locked loop, but rather uses the dq-axis voltage reference value output by the power control loop to generate the converter output voltage frequency. The calculation formula for the converter output voltage frequency is as follows: ； in, and This is the reference value for the dq-axis voltage output of the power control; This is the angular frequency gain coefficient; ω is the rated frequency of the power grid; f is the frequency of the frequency synchronization control output, which is the power grid frequency in steady state; ω is the angular frequency corresponding to the power grid frequency.

4. The method according to claim 1, characterized in that, The controller is a proportional-integral controller, and the output is a voltage reference value correction. The proportional-integral controller is designed in conjunction with the low-pass filter characteristics of the power measurement circuit, so that the closed-loop transfer function from the first power command value to the actual output power of the converter has a gain of 1 in steady state.

5. The method according to claim 4, characterized in that, The closed-loop transfer function is: ； The transfer function C(s) is expressed as: ； Where s is the complex frequency variable of the Laplace transform, and U is the amplitude of the converter output voltage. This refers to the gain coefficient in the power calculation stage. Here, T represents the line impedance magnitude, and T is the power filtering time constant. The coefficients of the closed-loop characteristic polynomial of the converter in the synchronous rotating coordinate system are... For the real and imaginary part correlation parameters of the complex droop coefficient, This is the proportionality coefficient. The integral coefficients satisfy the following in steady state: .

6. The method according to claim 1, characterized in that, In step S3, the first droop is determined based on the first frequency deviation between the converter output voltage frequency and the rated frequency and a preset first droop coefficient, and the second droop is determined based on the converter output voltage change rate and a preset second droop coefficient. The droop equation is expressed as: ; in, This is a reference value for active power. This is a reference value for reactive power. , These represent the rated active power and rated reactive power, respectively, and ω is the converter output angular frequency. The rated angular frequency, the This is the first frequency deviation; The preset first droop coefficient, The preset second droop coefficient; The error signal before the proportional-integral controller in the d-axis voltage channel is physically equivalent to the rate of change of the converter output voltage, i.e., the... Corresponding to the rate of change of the converter output voltage, and in steady state This makes the second droop amount zero.

7. The method according to claim 6, characterized in that, The relationship between the error signal and the rate of change of the converter output voltage is as follows: ； in, Here, U is the integral coefficient of the proportional-integral controller in power point tracking control, and U is the amplitude of the converter output voltage. Based on this relationship, the reactive power droop equation is equivalent to: ； That is, the reactive power reference value is determined by the rate of change of the converter output voltage. In steady state, the rate of change of voltage is zero, and the reactive power reference value is exactly equal to the rated reactive power.

8. The method according to claim 1, characterized in that, In step S5, the second power command value is converted into a dq-axis voltage reference value via a power tracking control loop. The dq-axis voltage reference value is simultaneously input to a frequency synchronization control loop. The frequency synchronization control loop multiplies the ratio of the q-axis component to the d-axis component in the dq-axis voltage reference value by an angular frequency gain coefficient and then superimposes it onto the rated angular frequency. The phase angle of the converter output voltage is then output via an integrator.

9. The method according to claim 1, characterized in that, In step S3, the second droop is determined based on the converter output voltage change rate and a preset second droop coefficient. The converter output voltage change rate is characterized by an error signal before the proportional-integral controller in the d-axis voltage channel. The error signal is the difference signal at the input of the proportional-integral controller. The first droop is superimposed on the rated active power to obtain an active power reference value, and the second droop is superimposed on the rated reactive power to obtain a reactive power reference value. The active power reference value and the reactive power reference value are input to the controller as components of the first power command value.

10. A complex coefficient drooping fixed-frequency grid-connected collaborative control system, characterized in that, include: The frequency fixing module is used to fix the frequency of the converter output voltage to the rated frequency using complex coefficient droop control. The frequency detection and first droop module is used to detect the frequency of the converter output voltage, obtain the first frequency deviation by subtracting the frequency of the converter output voltage from the rated frequency, and determine the first droop amount based on the first frequency deviation and a preset first droop coefficient. The voltage detection and second droop module is used to detect the rate of change of the converter output voltage, determine the second droop amount based on the rate of change of voltage and the preset second droop coefficient, and superimpose the first droop amount and the second droop amount onto the externally given power command value to obtain the first power command value. The power point tracking control module is used to obtain a second frequency deviation based on the first power command value and the converter output voltage frequency, input the second frequency deviation into the controller to obtain an output quantity, and use the output quantity as a correction quantity to be superimposed on the first power command value to obtain the second power command value. The power output control module is used to control the power output of the converter according to the second power command value.

Images