Fast frequency support controller of droop control type network construction converter

Abstract

The invention provides a rapid frequency support controller of a droop control type network construction converter, which comprises a differential link, a stability controller and a network construction converter system, and is characterized in that the network construction converter system comprises a network construction converter and a power-voltage formula; in the differential link, the differential component of the voltage deviation is obtained by making a difference between the reference direct-current voltage and the actual output direct-current voltage, the stability controller receives the differential component of the voltage deviation and regulates and controls the differential component in a stability domain, a control signal is output through the stability controller, and the control signal is imported into the grid-forming converter system to control the grid-forming converter system. Combining the active power output of the source side and the demand of the load side to quantify the output of the network construction converter, and iteratively outputting voltage by means of a power-voltage relation formula; according to the method, the nonlinearity of the internal state variable of the network construction converter and the deviation of the frequency deviation value are corrected, and the droop control type network construction converter is allowed to keep the maximum support for the system frequency under the condition of avoiding the direct current link voltage collapse.

Description

Technical Field

[0001] The invention belongs to the technical field of power system control, and in particular relates to a fast frequency support controller of a droop control type grid-connected converter. Background Art

[0002] As the power structure of the power system tilts towards resources such as photovoltaic and wind power that are connected to the grid using the converter interface, the proportion of synchronous motors in the power system gradually decreases, resulting in the inertia level available to the system remaining at a low level. In a grid without sufficient damping and inertia support, the system is more prone to disturbances, and indicators such as the frequency change rate and the lowest frequency point after the disturbance deteriorate. In this case, the grid-connected converter that can actively build voltage and frequency levels can provide the system with a certain inertia and fast frequency adjustment services, which is a reasonable solution to frequency stability-related issues during the transformation stage of the power system. However, since the resources connected to the DC side of the grid-connected converter have a maximum power generation capacity, when this capacity is lower than the power generation demand, it may cause the DC link voltage to collapse. This problem hinders further exploration of the fast frequency support capability of the grid-connected converter. Summary of the invention

[0003] In order to solve the fragile nature of the DC side voltage of the grid-connected converter during power output, expand the maximum power output threshold of the grid-connected converter, and thereby enhance the stable and rapid frequency adjustment service quality of the grid-connected converter, the present invention provides a rapid frequency support controller for a droop control type grid-connected converter based on a discrete nonlinear feedback control method. The controller follows a series of mechanisms for rapid frequency support, and under the coordination of the controller, provides services for system frequency stability, avoids the risk of DC side line voltage collapse, and provides a tool for the grid-connected converter to provide rapid frequency support under disturbances.

[0004] To solve the above technical problems, the present invention is implemented through the following technical solutions: a fast frequency support controller of a droop control type grid-connected converter based on a discrete nonlinear feedback control method, comprising a differential link, a stabilization controller and a grid-connected converter system, wherein the grid-connected converter system comprises a grid-connected converter and a power-voltage formula; the differential link is composed of a reference DC voltage And the actual output DC voltage v dc The differential of the voltage deviation is obtained by making a difference. The stability controller receives the differential of the voltage deviation, regulates it in the stable domain, and outputs a control signal u through the stability controller. The control signal u is introduced into the grid-connected converter system and combined with the active output P on the source side. c and load side demand P LT Quantify the grid converter output P d , and use the power-voltage relationship formula to iterate the output voltage v dc .

[0005] Specifically, the grid-connected converter system model is transformed, and the rth grid-connected converter is written in the form of a state space equation, as shown in the state space equation group (1):

[0006]

[0007] Where: x = [ P c g ω coi P τ g v dc ] T , the underlined variables are defined as deviations from the corresponding steady-state values, where P cg , P τg 、v dc They represent the active output of the grid-connected converter, the active output of the turbine and the DC voltage respectively; u is the correction control signal input to the grid-connected device; y is equal to v dc , represents the controlled output; w is equal to P LT , represents the change of load, and the value is expressed as a random disturbance vector, which represents the irregular change of load. represents the differential of x, and the superscript T represents the transpose;

[0008] The state feedback control is designed so that after w increases in steps, the output y gradually approaches a constant reference voltage R; for each pair (w, R), there exists a pair (x ss ,u ss ) depends on (w,R) and satisfies equation group (2):

[0009]

[0010] Where: x ss is the expected equilibrium point, u ss In order to maintain the steady-state control required for the equilibrium point, the desired equilibrium point x is transformed ss Mapped to a set of new equilibrium points, defined as (η ss ,ξ ss ), transforming the system of equations (1) into the conventional state space system of equations (3):

[0011]

[0012] Among them, η1, η2, ξ1, ξ2 are all (η ss ,ξ ss ) is the variable of the equilibrium point, and the variable with a superscript dot is defined as the differential of the original variable. for and The matrix of for and The matrix of is the differential of the center frequency of inertia, is the derivative of the sum of all turbine active outputs, for v dc The differential of for Further differentiation of , f1 is a function of the state variable group (x,u,w).

[0013] Furthermore, the power-voltage relationship formula is as follows:

[0014]

[0015] Where: G c is the conductance equivalent to the DC side loss in the grid converter; C c It is the DC side capacitor of the grid converter; is the maximum current on the DC side; sat is the saturation limit function, P d To build the grid converter output, The term represents the steady-state DC voltage; is the DC voltage v of the grid converter dc The complex conjugate form of k c is the droop coefficient in the DC side current reference, The output voltage v dc The differential of .

[0016] Furthermore, after the differential link of the fast frequency support controller, Figure 2 Integrator in The steps are supplemented, equations (5)-(6) are applied to transform them, and new equation variables z and e are defined as:

[0017] z=η-η ss (5)

[0018]

[0019] Among them, e0, e1, e2 are the three-level outputs of the fast frequency support controller, with relationship, v dc For v dc The deviation in steady state, For The deviation in steady state, X is the set of constant DC reference voltages. Further, the grid-connected converter system model supplemented with the control signal u is changed to:

[0020]

[0021] in: is the differential of z, and are two transformation matrices; e T The output signal matrix of the fast frequency support controller, is the corresponding grid converter n c The output signal, X T is a constant reference voltage matrix, For the grid converter n c Constant reference voltage matrix; n c is the number of grid-connected converters; f0 is the variable group (e T ,w,R T ), the variable with an overline is defined as the complex conjugate of the original variable; i represents the device connected to the i-th bus; d pc represents the grid-connected converter system droop gain, represents the droop gain of the i-th busbar of the grid-connected converter system; b is the line admittance, b i is the admittance of the ith busbar; is the maximum current on the DC side, for; b g is the sum of the network side admittance; P d P is the input power of the grid converter; c is the power on the filter side of the grid converter; is the filter side power of the i-th grid-connected converter in the network; P f is the quantitative reference power, is the differential of the quantitative reference power; Represents all grid-connected converters;

[0022] Furthermore, based on the grid-connected converter system supplemented with the control signal u in equation (7), a sliding mode control mode is added to the control system; the sliding mode surface s of a single grid-connected converter is defined as shown in equation (8):

[0023] s=k0e0+k1e1+e2 (8)

[0024] Where: k1 and k0 are the coefficients in equation (8), and the coefficients are selected so that the polynomial s 2 +k1s+k0 is transformed into Hurwitz polynomial, and substituting into e1, e2 and e0, the control signal u is defined as:

[0025]

[0026] Where: v represents the input of the next level of the stabilizing controller. When synthesizing the input u, some variables are used as feedforward items to deal with disturbances. v dc , P c , P d All of them are measurable signals. The frequency of the adjacent busbars is received through the communication module. The connectivity between the grid-connected converter and the transmission line is determined by confirming the status of the circuit breaker at the common connection point. After adding the sliding mode control mode, the droop control is rewritten as:

[0027]

[0028] Where: variables with superscript * are defined as reference values; ω is the system angular frequency; ω c is the grid converter frequency.

[0029] In the present invention, the output mechanism provides a framework for the fast frequency regulation mechanism of the grid-connected converter. The framework follows the following principles: the grid-connected converter must ensure the stability of the DC link voltage in the presence of current limitations on the AC and DC sides; quantify the power support required according to the size of the disturbance; determine whether the grid-connected converter should participate in the frequency service; minimize the steady-state frequency deviation, and improve indicators such as the frequency change rate and the lowest frequency point. Among them: the stability of the DC link voltage is achieved through the additional input of the feedback controller, the quantitative power support is achieved through the status monitoring of the adjacent busbars and the tracking of the converter reference voltage; the switch of the grid-connected converter participating in the service is achieved through the adjustment of the reference voltage; the improvement of the system frequency performance is achieved through the mode selection of the controller.

[0030] In the present invention, the construction scheme of the enhanced system includes a model conversion method of the network system, which provides a conversion method for the network system model when adding additional control items, and provides a model basis for supplementing the discrete sliding mode control with a droop controller.

[0031] In the present invention, the design scheme of sliding mode control includes an additional input based on discrete nonlinear method added to droop control. This method adds an additional input on the basis of the original droop control, which can be used to regulate the DC side voltage to ensure that the DC side voltage does not exceed the limit under high power demand. This mechanism enables the grid-connected converter to output corresponding quantized power support according to the output mechanism and frequency deviation, providing stable and fast frequency support services for disturbances of various scales of the system.

[0032] The present invention takes into account the limitation problem of the DC link current of the converter under active power demand, corrects the nonlinearity of the internal state variables of the grid-connected converter and the deviation of the frequency deviation value, and allows the droop-controlled grid-connected converter to maintain maximum support for the system frequency while avoiding the collapse of the DC link voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is the single-line diagram of the enhancement system where the grid-connected converter is located.

[0034] Figure 2 It is the control framework of the fast frequency controller. DETAILED DESCRIPTION

[0035] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] like Figure 1 As shown, a general single-line diagram of a power system circuit including a grid-connecting converter includes a grid-connecting converter group consisting of r grid-connecting converters, a load group consisting of a plurality of loads, and a synchronous generator group consisting of a plurality of synchronous generators. The grid-connecting converter group is connected to an admittance group of the power system. Figure 1 In, ω coi Represents the average frequency of the busbars connected to the load group and generator group. The power system has the following assumptions to meet the test requirements:

[0037] (1) The time constant of the equipment connected to the DC side can be ignored;

[0038] (2) The time difference between the internal AC voltage and current control loops tracking the reference value is not taken into account;

[0039] (3) The DC side power flow model can be used for AC networks;

[0040] (4) The frequency dynamics can be represented by the DC link model of the grid-connected converter and the swing equation of the synchronous generator and the turbine speed regulation equation;

[0041] (5) The synchronous generator has an ideal current limit which can be ignored.

[0042] To construct the supplementary control model, the grid-connected converter system model is first transformed, and the rth grid-connected converter is written in the form of state space equations, as shown in the state space equation group (1).

[0043]

[0044] Where: x = [ P c g ω coi P τ g v dc ] T , the underlined variables are defined as deviations from the corresponding steady-state values, where P cg , P τg 、v dc They represent the active output of the grid-connected converter, the active output of the turbine and the DC voltage respectively; u is the correction control signal input to the grid-connected device; y is equal to v dc , represents the controlled output; w is equal to P LT , represents the change of load, and the value is expressed as a random disturbance vector, which represents the irregular change of load. represents the differential of x, and the superscript T represents the transpose.

[0045] The state feedback control is designed so that after w increases in steps, the output y asymptotically approaches a constant reference voltage R. For each pair (w, R), there exists a pair (x ss ,u ss ) depends on (w,R) and satisfies equation group (2):

[0046]

[0047] Where: x ss is the expected equilibrium point, u ss In order to maintain the steady-state control required for the equilibrium point, the desired equilibrium point x is transformed ss Mapped to a set of new equilibrium points, defined as (η ss ,ξ ss ), transforming the system of equations (1) into the conventional state space system of equations (3):

[0048]

[0049] Among them, η1, η2, ξ1, ξ2 are all (η ss ,ξ ss ) is the variable of the equilibrium point, and the variable with a superscript dot is defined as the differential of the original variable. for and The matrix of for and The matrix of is the differential of the center frequency of inertia, is the derivative of the sum of all turbine active outputs, for v dc The differential of for Further differentiation of , f1 is a function of the state variable group (x,u,w).

[0050] like Figure 2 As shown, this embodiment provides a fast frequency support controller of a droop control type grid-connected converter based on a discrete nonlinear feedback control method, including a differential link, a stabilization controller and a grid-connected converter system, wherein the grid-connected converter system includes a grid-connected converter and a power-voltage formula; the differential link is composed of a reference DC voltage And the actual output DC voltage v dc The differential of the voltage deviation is obtained by making a difference. The stability controller receives the differential of the voltage deviation, regulates it in the stable domain, and outputs a control signal u through the stability controller. The control signal u is introduced into the grid-connected converter system and combined with the active output P on the source side. c and load side demand P LT Quantify the grid converter output P d , and use the power-voltage relationship formula to iterate the output voltage v dc ;

[0051] The power-voltage relationship formula is as follows:

[0052]

[0053] Where: G c is the conductance equivalent to the DC side loss in the grid converter; C c It is the DC side capacitor of the grid converter; is the maximum current on the DC side; sat is the saturation limit function, P d To build the grid converter output, The term represents the steady-state DC voltage; is the DC voltage v of the grid converter dc The complex conjugate form of k c is the droop coefficient in the DC side current reference, The output voltage v dc The differential of .

[0054] exist Figure 2 After the differential link of the fast frequency support controller shown in the figure, Figure 2 Integrator in The steps are supplemented, equations (5)-(6) are applied to transform them, and new equation variables z and e are defined as:

[0055] z=η-η ss (5)

[0056] Among them, e0, e1, e2 are the three-level outputs of the fast frequency support controller, with The relationship between e1 = ξ1-X v dc For v dc The deviation in steady state, For Deviation in steady state, X is the set of constant DC reference voltages. The grid-connected converter system model supplemented with control signal u is changed to an enhanced system:

[0057]

[0058] in: is the differential of z, and are two transformation matrices; e T The output signal matrix of the fast frequency support controller, is the corresponding grid converter n c The output signal, X T is a constant reference voltage matrix, For the grid converter n c Constant reference voltage matrix; n c is the number of grid-connected converters; f0 is the variable group (e T ,w,X T ), the variable with an overline is defined as the complex conjugate of the original variable; i represents the device connected to the i-th bus; d pc represents the grid-connected converter system droop gain, represents the droop gain of the i-th busbar of the grid-connected converter system; b is the line admittance, b i is the admittance of the ith busbar; is the maximum current on the DC side, for; b g is the sum of the network side admittance; P d P is the input power of the grid converter; c is the power on the filter side of the grid converter; is the filter side power of the i-th grid-connected converter in the network; P f is the quantitative reference power, is the differential of the quantitative reference power; Represents all grid-connected converters;

[0059] Based on the grid-connected converter system supplemented with the control signal u in equation (7), a sliding mode control mode is added to the control system to avoid the DC voltage exceeding the limit.

[0060] The sliding mode surface s of a single grid-connected converter is defined as shown in equation (8):

[0061] s=k0e0+k1e1+e2 (8)

[0062] Where: k1 and k0 are the coefficients in equation (8), and the coefficients are selected so that the polynomial s 2 +k1s+k0 is transformed into Hurwitz polynomial, and substituted into e1, e2 and e0, the output signal u of sliding mode control is defined as:

[0063]

[0064] Where: v represents the input of the next level of the stabilizing controller. When synthesizing the input u, some variables are used as feedforward items to deal with disturbances. v dc , P c , P d All of them are measurable signals. The frequency of the adjacent busbars is received through the communication module. The connectivity between the grid converter and the transmission line is determined by confirming the status of the circuit breaker at the common connection point. After adding the sliding mode control mode, the droop control can be rewritten as:

[0065]

[0066] Where: variables with superscript * are defined as reference values; ω is the system angular frequency; ω c is the grid converter frequency.

[0067] make Substitute u into equation (5); in β0 and ε are greater than 0. From equation (9), we can see that the input of u requires power, voltage and current as feedback, so the controller parameters k0, k1, k, β0, ε are selected and set. coi It is determined by the maximum allowable deviation, β0 and ε are positive parameters used to tune the controller.

[0070] In combination with the above, the specific steps of the fast frequency controller in this scheme are:

[0071] (1) The state variable x, control input u, controlled output y and position disturbance variable w in the target grid-connected converter system model are identified and written in the form shown in equation (1).

[0072] (2) The state space equation is expressed as the enhanced form shown in equation (7), vdc and The dynamics of the deviation e1 are decoupled from the dynamics of the rest of the system.

[0073] (3) According to equation (8), the sliding surface s is formed, which is determined by the relative degree of the system. The parameters k0, k1, k, β0, and ε are adjusted to correct the saturation and tracking characteristics of the controller.

[0074] (4) To ensure that the state trajectory reaches the state s = 0 in a finite time, first find the sliding surface s and determine the state of u to use the feedforward step size to offset the influence of the measurable disturbance. The case where k s is a constant.

[0075] Of course, without departing from the spirit and essence of the invention, technicians familiar with the field should be able to make various corresponding changes and modifications based on the invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the invention.

Claims

1. A fast frequency support controller for a droop control type grid-connected converter, characterized in that: The invention comprises a differential link, a stabilization controller and a grid-forming converter system, wherein the grid-forming converter system comprises a grid-forming converter and a power-voltage formula; the differential link is composed of a reference DC voltage And the actual output DC voltage v dc The differential of the voltage deviation is obtained by making a difference. The stability controller receives the differential of the voltage deviation, regulates it in the stable domain, and outputs a control signal u through the stability controller. The control signal u is introduced into the grid-connected converter system and combined with the active output P on the source side. c and load side demand P LT Quantify the grid converter output P d , and use the power-voltage relationship formula to iterate the output voltage v dc .

2. The fast frequency support controller of the droop control type grid-connected converter according to claim 1, characterized in that: The grid-connected converter system model is transformed and the rth grid-connected converter is written in the form of state space equations, as shown in the state space equation group (1): Where: x = [ P c g ω coi P τ g v dc ] T , the underlined variables are defined as deviations from the corresponding steady-state values, where P cg , P τg 、v dc They represent the active output of the grid-connected converter, the active output of the turbine and the DC voltage respectively; u is the correction control signal input to the grid-connected device; y is equal to v dc , represents the controlled output; w is equal to P LT , represents the change of load, and the value is expressed as a random disturbance vector, which represents the irregular change of load. represents the differential of x, and the superscript T represents the transpose; The state feedback control is designed so that after w increases in steps, the output y gradually approaches a constant reference voltage R; for each pair (w, R), there exists a pair (x ss ,u ss ) depends on (w,R) and satisfies equation group (2): Where: x ss is the expected equilibrium point, u ss In order to maintain the steady-state control required for the equilibrium point, the desired equilibrium point x is transformed ss Mapped to a set of new equilibrium points, defined as (η ss ,ξ ss ), transforming the system of equations (1) into the conventional state space system of equations (3): Among them, η1, η2, ξ1, ξ2 are all (η ss ,ξ ss ) is the variable of the equilibrium point, and the variable with a superscript dot is defined as the differential of the original variable. for and The matrix of for and The matrix of is the differential of the center frequency of inertia, is the derivative of the sum of all turbine active outputs, for v dc The differential of for Further differentiation of , f1 is a function of the state variable group (x,u,w).

3. The fast frequency support controller of the droop control type grid-connected converter according to claim 2, characterized in that: The power-voltage relationship formula is as follows: Where: G c is the conductance equivalent to the DC side loss in the grid converter; C c It is the DC side capacitor of the grid converter; is the maximum current on the DC side; sat is the saturation limit function, P d To build the grid converter output, The term represents the steady-state DC voltage; is the DC voltage v of the grid converter dc The complex conjugate form of k c is the droop coefficient in the DC side current reference, The output voltage v dc The differential of .

4. The fast frequency support controller of the droop control type grid-connected converter according to claim 3, characterized in that: After the differential stage of the fast frequency support controller, the integrator The steps are supplemented, equations (5)-(6) are applied to transform them, and new equation variables z and e are defined as: z=η-η ss (5) Among them, e0, e1, e2 are the three-level outputs of the fast frequency support controller, with relationship, v dc For v dc The deviation in steady state, For Deviation in steady state, X is the set of constant DC reference voltages.

5. The fast frequency support controller of the droop control type grid-connected converter according to claim 4, characterized in that: The grid-connected converter system model supplemented with the control signal u is changed to: in: is the differential of z, and are two transformation matrices; e T The output signal matrix of the fast frequency support controller, is the corresponding grid converter n c The output signal, X T is a constant reference voltage matrix, For the grid converter n c Constant reference voltage matrix; n c is the number of grid-connected converters; f0 is the variable group (e T ,w,R T ), the variable with an overline is defined as the complex conjugate of the original variable; i represents the device connected to the i-th bus; d pc represents the grid-connected converter system droop gain, represents the droop gain of the i-th busbar of the grid-connected converter system; b is the line admittance, b i is the admittance of the ith busbar; is the maximum current on the DC side, for; b g is the sum of the network side admittance; P d P is the input power of the grid converter; c is the power on the filter side of the grid converter; is the filter side power of the i-th grid-connected converter in the network; P f is the quantitative reference power, is the differential of the quantitative reference power; Represents all grid-connected converters.

6. The fast frequency support controller of the droop control type grid-connected converter according to claim 5, characterized in that: Based on the grid-connected converter system supplemented with the control signal u in equation (7), a sliding mode control mode is added to the control system; the sliding mode surface s of a single grid-connected converter is defined as shown in equation (8): s=k0e0+k1e1+e2 (8) Where: k1 and k0 are the coefficients in equation (8), and the coefficients are selected so that the polynomial s 2 +k1s+k0 is transformed into Hurwitz polynomial, and substituting into e1, e2 and e0, the control signal u is defined as: Where: v represents the input of the next level of the stabilizing controller. When synthesizing the input u, some variables are used as feedforward items to deal with disturbances. v dc , P c , P d All of them are measurable signals. The frequency of the adjacent busbars is received through the communication module. The connectivity between the grid-connected converter and the transmission line is determined by confirming the status of the circuit breaker at the common connection point. After adding the sliding mode control mode, the droop control is rewritten as: Where: variables with superscript * are defined as reference values; ω is the system angular frequency; ω c is the grid converter frequency.

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