Network-following converter control method for composite direct current power synchronization

Abstract

The application discloses a grid-following converter control method of composite direct-current power synchronization and belongs to the technical field of wind power grid-connected converter control. The application solves the problem of low stability caused by sub-synchronous oscillation of the traditional grid-following converter control method in the prior art; the application collects the direct-current power of the grid-following converter, sequentially passes the direct-current power through a lead / lag link and an integral controller, introduces the obtained compensation term into a d-axis current reference instruction value output by a direct-current voltage loop, and influences the d-axis current and q-axis voltage; the influenced q-axis voltage is taken as a phase-locked loop input, a grid-connected angle directly controlled by a proportional integral controller of the phase-locked loop link is output, and a small signal value of an angle additionally introduced by the composite direct-current power synchronization is equivalent to a grid-constructing direct-current inertia synchronization link introduced in the grid-following phase-locked loop. The application improves the adaptability of the grid-following control system to a weak power grid and can be applied to converter control.

Description

TECHNICAL FIELD

[0001] The present application relates to a grid-following converter control method, in particular to a grid-following converter control method with composite direct-current power synchronization, and belongs to the technical field of wind power grid-connected converter control. BACKGROUND

[0002] With the development of renewable energy, distributed energy is mainly connected to the grid through a three-phase converter. At present, the control methods of grid-connected converters mainly include two types: grid-following control based on vector control and grid-forming control based on virtual synchronization, droop control and direct-current voltage self-synchronization. Grid-following control (GFL, Grid-Following) uses a phase-locked loop (PLL, phase-locked loop) as a synchronization unit to ensure that the voltage and angle of the inverter are synchronized with the AC grid. However, due to the gradual increase in the installed capacity of renewable energy generation and the weak grid characteristics caused by the long transmission line required for distributed power generation, the voltage at the grid connection point is prone to disturbance, which causes the phase-locked loop, a voltage-type synchronization method, to deviate or even fail, thereby causing sub-synchronous oscillation accidents in the grid-connected system to occur frequently, seriously affecting the safe and stable operation of the grid. Therefore, it is necessary to suppress sub-synchronous oscillation.

[0003] The improved method based on grid-following control can be divided into optimizing control parameters and changing control structure. Optimizing parameters is easy to implement, but its improvement effect is usually limited by the balance between steady-state margin and dynamic performance. Changing the control structure mainly improves the structure of the current inner loop and the synchronization unit, but the structure is mostly complex. At the same time, the existing methods for improving the stability of grid-connected converters under weak grids are mostly grid-forming control. The implementation of grid-forming control can be roughly divided into two approaches. In addition to using virtual synchronization and other methods to simulate the rotor motion equation of a synchronous generator, the dynamic characteristics of the direct-current capacitor can also be used to build the inertia relationship between the direct-current voltage and the grid frequency. The simulated inertia of the direct-current capacitor provides inertia support for the grid, thereby simulating the power angle characteristics of the synchronous generator and achieving the self-synchronization of the inverter and the grid. However, the essence of direct-current inertia synchronization control belongs to grid-forming control, which cannot improve the stability of grid-following converters under weak grids without changing the nature of grid-following control.

[0004] In summary, there is a need for a grid-following converter control method with composite grid-forming control ideas that is easy to implement and has obvious effects. SUMMARY

[0005] The following presents a simplified summary of the application in order to provide a basic understanding of some aspects of the application. This summary is not an extensive overview of the application. It is not intended to identify key or critical elements of the application or to delineate the scope of the application. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

[0006] In view of this, in order to solve the problems of low stability and complex control structure caused by subsynchronous oscillation of the conventional converter control method based on grid-following control in the prior art, the application provides a grid-following converter control method based on composite direct-current power synchronization.

[0007] The technical scheme is as follows: the grid-following converter control method based on composite direct-current power synchronization comprises the following steps:

[0008] S1. Collecting voltage signals, current signals and direct-current voltages of the grid-following converter, and calculating direct-current power on the direct-current side of the grid-following converter;

[0009] S2. Introducing the direct-current power on the direct-current side of the grid-following converter into the d-axis current reference instruction value output by the direct-current voltage loop in sequence through a lead / lag element and an integral controller to obtain a compensation term, and obtaining a new d-axis current reference value;

[0010] S3. The compensation term has an influence on the d-axis current through a proportional integral element of the current loop, and a new d-axis current is obtained according to the new d-axis current reference value, and the q-axis voltage is further influenced based on the relationship between the grid voltage, the grid impedance and the grid-connected point voltage to obtain an influenced q-axis voltage;

[0011] S4. Taking the influenced q-axis voltage as the input of a phase-locked loop, and directly controlling the grid-connected angle output by a proportional integral controller of the phase-locked loop element, and through theoretical analysis, the small signal value of the angle additionally introduced by the composite direct-current power synchronization in the output grid-connected angle is equivalent to introducing a grid-constructing direct-current inertia synchronization element in the grid-following phase-locked loop, so as to realize composite control.

[0012] Further, in S1, the direct-current power P on the direct-current side of the grid-following converter is represented as: dc

[0013] P dc = I dc *u dc - 1.5(u d *i d + u q *i q )

[0014] Wherein, u d is the voltage signal U​abc the d-axis direct current component of the voltage signal U q abc the q-axis direct current component of the voltage signal U d abc the d-axis direct current component of the current signal I q abc the q-axis direct current component of the current signal I dc the output current of the direct current source dc the direct current bus voltage

[0015] Further, in the S2, the direct current power P dc passes through a lead / lag element and an integral controller in sequence to obtain a compensation term, i.e. the output of the direct current power steady-state deviation after passing through the lead / lag element and the integral controller The compensation term is introduced into the d-axis current reference value output by the direct current voltage loop to obtain a new d-axis current reference value , which affects the output of the current and angle of the inverter;

[0016] The output quantity is represented as:

[0017] (1)

[0018] wherein, is the instantaneous value of the direct current power, is the steady-state value of the direct current power, is the first time constant of the lead / lag element, is the second time constant of the lead / lag element, is the proportional coefficient of the compound power synchronous control, represents a differential operator;

[0019] The new d-axis current reference value is represented as:

[0020] (2)

[0021] wherein, is the d-axis current reference value.

[0022] Further, in the S3, the traditional current closed loop is adopted to control the inverter, and the decoupled d-axis current and q-axis current are controlled separately. The proportional integral element of the current loop causes the d-axis current to change with the new d-axis current reference value to obtain a new d-axis current , and the compensation term and the new d-axis current are represented in a small signal form to obtain an improved compensation term small signal component and the d-axis small signal component of the grid-connected current after introducing the compound power synchronous control​​​ ;

[0023] New d-axis current is expressed as:

[0024] (3)

[0025] Improved compensation term small signal component is expressed as:

[0026] (4)

[0027] wherein, is a direct current power small signal component;

[0028] The d-axis small signal component of the grid-connected current after introducing the composite power synchronization control is expressed as:

[0029] (5)

[0030] wherein, is a d-axis small signal component of the original grid-connected current;

[0031] Based on the relationship between the grid connection point voltage and the line impedance and the grid voltage, ignoring the line resistance and the small signal value of the grid voltage, according to the q-axis small signal component of the original grid connection point voltage , the affected q-axis voltage is expressed in a small signal form, and the q-axis small signal component of the grid connection point voltage after introducing the composite power synchronization control is obtained ;

[0032] (6)

[0033] wherein, is a grid frequency, is a line impedance;

[0034] The q-axis small signal component of the grid connection point voltage after introducing the composite power synchronization control is expressed as:

[0035] (7).

[0036] Further, in the S4, the q-axis small signal component of the grid connection point voltage after introducing the composite power synchronization control is taken as the input of the phase-locked loop, and the grid connection angle directly controlled by the proportional-integral controller of the phase-locked loop link is output, the output grid connection angle is expressed in a small signal form, and the composite direct current power synchronization output angle small signal form is obtained , and the direct current inertia synchronization output angle small signal form is obtained ;

[0037] Composite direct current power synchronization output angle small signal form is expressed as:

[0038] (8)

[0039] (9)

[0040] wherein, is a phase-locked loop output angle small signal form, is a composite direct current power synchronization additional introduced angle small signal form, is a proportional parameter of the phase-locked loop, is an integral parameter of the phase-locked loop, is a set item, ;

[0041] Direct current inertia synchronization output angle small signal form is expressed as:

[0042] (10)

[0043] wherein, is a power grid angular frequency reference value, is a proportional parameter of the direct current inertia synchronization, is an integral parameter of the direct current inertia synchronization, is a direct current voltage steady-state value, is a direct current bus capacitance;

[0044] Referring to formulas (9) and (10), when the second time constant of the lead / lag element is equal to the ratio of the proportional parameter of the phase-locked loop and the integral parameter of the phase-locked loop , ignoring , the composite direct current power synchronization additional introduced angle small signal value is equivalent to introducing a network type direct current inertia synchronization element in the grid-connected type phase-locked loop.

[0045] The beneficial effects of this invention are as follows: The composite DC power synchronization grid-connected control method of this invention mainly addresses the subsynchronous oscillation problem of grid-connected converters under weak power grids. Based on the grid-connected control structure, it incorporates the concept of composite DC inertial synchronization grid-connected control, introducing the DC-side power steady-state deviation into the DC voltage loop. This achieves composite DC power synchronization in grid-connected control, improving the stability of the grid-connected converter under weak power grids. The improved strategy of this invention has a simple structure, significant improvement effect, fewer design parameters, and a significant effect on suppressing subsynchronous oscillations. On the one hand, it ensures the stability of the system under weak power grids, making grid-connected converters more stable. The grid-connected control system maintains stable operation under experimental conditions with low short-circuit ratios. On the other hand, it can broaden the PLL bandwidth range of the grid-connected control system under weak power grids, ensuring stable operation of the system under high PLL bandwidths and also showing good suppression effect under non-unity power factor grid connection conditions. This invention combines grid-connected control with DC inertial synchronous control strategy, and improves the adaptability of the grid-connected control system to weak power grids by improving the control adjustment grid connection angle, ensuring the stability of the grid-connected control system under weak power grids, and enabling the grid-connected control system to maintain stable operation under experimental conditions with low short-circuit ratios. Attached Figure Description

[0046] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0047] Figure 1 A flowchart illustrating the control method for a grid-connected converter with composite DC power synchronization;

[0048] Figure 2 Here is a block diagram of a grid-type converter;

[0049] Figure 3 The control block diagram for a grid-connected converter with composite DC power synchronization;

[0050] Figure 4 A schematic diagram illustrating the impact of grid-connected converter control for composite DC power synchronization on the grid-connected system;

[0051] Figure 5 The equivalent block diagram of the phase-locked loop control in a grid-type converter;

[0052] Figure 6 The equivalent block diagram of DC inertial synchronization control in a grid-type converter;

[0053] Figure 7 Equivalent block diagram for grid-connected converter control for composite DC power synchronization. Detailed Implementation

[0054] In order to make the technical solutions and advantages of the embodiments of the present application clearer, the exemplary embodiments of the present application are further described in detail below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0055] Reference Figures 1-7 The control method of the grid-connected type converter with composite direct current power synchronization includes the following steps:

[0056] S1. Collecting the voltage signal, current signal and direct current voltage of the grid-connected type converter, and calculating the direct current power of the direct current side of the grid-connected type converter;

[0057] S2. The direct current power of the direct current side of the grid-connected type converter is sequentially passed through the lead / lag element and the integral controller to obtain a compensation term, which is introduced into the d-axis current reference instruction value output by the direct current voltage loop to obtain a new d-axis current reference value;

[0058] S3. The compensation term has an impact on the d-axis current through the proportional integral element of the current loop, and a new d-axis current is obtained according to the new d-axis current reference value, and the affected q-axis voltage is obtained based on the relationship between the grid voltage, the grid impedance and the grid-connected point voltage;

[0059] S4. The affected q-axis voltage is taken as the input of the phase-locked loop, and the grid-connected angle directly controlled by the proportional integral controller of the phase-locked loop element is output, and through theoretical analysis, the small signal value of the angle extra introduced by the output grid-connected angle is equivalent to introducing the grid type direct current inertia synchronization element in the grid-connected type phase-locked loop, so as to realize composite control.

[0060] Further, in S1, the voltage signal U abc , the current signal I abc and the direct current voltage U dc of the grid-connected type converter are collected, and the direct current power P dc of the direct current side of the grid-connected type converter is calculated.

[0061] The direct current power P dc of the direct current side of the grid-connected type converter is represented as:

[0062] P dc = I dc *u dc - 1.5(u d *i d + u q *i q )

[0063] wherein u d is the d-axis DC component of the voltage signal U abc , u q is the q-axis DC component of the voltage signal U abc , i d is the d-axis DC component of the current signal I abc , i q is the q-axis DC component of the current signal I abc , I dc is the DC current source output current, u dc is the DC bus voltage.

[0064] Further, in the S2, the DC power P dc passes through a lead / lag element and an integral controller in sequence to obtain a compensation term, i.e. the output quantity of the DC power steady-state deviation after passing through the lead / lag element and the integral controller , the compensation term is introduced into the d-axis current reference instruction value output by the DC voltage loop (the main control element of the improvement strategy) to obtain a new d-axis current reference value , which affects the current and angle output of the inverter;

[0065] The output quantity is represented as:

[0066] (1)

[0067] wherein, is the instantaneous value of the DC power, is the steady-state value of the DC power, is the first time constant of the lead / lag element, is the second time constant of the lead / lag element, is the compound power synchronous control proportional coefficient, is the differential operator;

[0068] The new d-axis current reference value is represented as:

[0069] (2)

[0070] wherein, is the d-axis current reference value.

[0071] Further, in the S3, the conventional current closed loop is adopted to control the inverter, the d-axis current and the q-axis current after decoupling are controlled separately, the proportional integral element of the current loop causes the d-axis current to change with the new d-axis current reference value to obtain a new d-axis current , and the compensation term and the new d-axis current are represented in small signal form to obtain an improved compensation term small signal component and the d-axis small signal component of the grid-connected current after introducing the compound power synchronization control ;

[0072] the new d-axis current is expressed as:

[0073] (3)

[0074] the improved compensation term small signal component is expressed as:

[0075] (4)

[0076] wherein, is the DC power small signal component;

[0077] the d-axis small signal component of the grid-connected current after introducing the compound power synchronization control is expressed as:

[0078] (5)

[0079] wherein, is the d-axis small signal component of the original grid-connected current;

[0080] based on the relationship between the grid-connected point voltage and the line impedance and the grid voltage, ignoring the line resistance and the grid voltage small signal value, according to the q-axis small signal component of the original grid-connected point voltage , the affected q-axis voltage is expressed in small signal form, and the q-axis small signal component of the grid-connected point voltage after introducing the compound power synchronization control is obtained ;

[0081] (6)

[0082] wherein, is the grid frequency, is the line impedance;

[0083] the q-axis small signal component of the grid-connected point voltage after introducing the compound power synchronization control is expressed as:

[0084] (7).

[0085] Further, in the S4, the q-axis small signal component of the grid-connected point voltage after introducing the compound power synchronization control as the input of the phase-locked loop, the grid-connected angle directly controlled by the proportional-integral controller of the phase-locked loop link is expressed in small signal form, and the compound DC power synchronization output angle small signal form is obtained and the output angle small signal form of the direct current inertia synchronization is obtained ;

[0086] the composite direct current power synchronization output angle small signal form is expressed as

[0087] (8)

[0088] (9)

[0089] wherein, is the phase-locked loop output angle small signal form, is the angle small signal form introduced by the composite direct current power synchronization, is a proportional parameter of the phase-locked loop, is an integral parameter of the phase-locked loop, is a set item, ;

[0090] the direct current inertia synchronization output angle small signal form is expressed as

[0091] (10)

[0092] wherein, is a grid angle frequency reference value, is a proportional parameter of the direct current inertia synchronization, is an integral parameter of the direct current inertia synchronization, is a direct current voltage steady-state value, is a direct current bus capacitance;

[0093] Referring to formula (9) and (10), the angle small signal value introduced by the composite direct current power synchronization control is approximately equal to the output angle small signal value of the direct current inertia synchronization control, when the ratio of the second time constant of the lead / lag element to the proportional parameter of the phase-locked loop and the integral parameter of the phase-locked loop is equal, and is ignored, the angle small signal value introduced by the composite direct current power synchronization is equivalent to introducing a grid-forming direct current inertia synchronization element in the grid-following phase-locked loop, achieving the effect of the composite grid-forming control in the grid-following control, and further improving the adaptability of the system to a weak grid;

[0094] Specifically, referring to Figure 2 , V abc is a grid-connected point voltage, i abc is a grid-connected point current, P is a converter output active power, the corresponding arrow thereof represents a power flow direction, L is a filter inductance, Z gV is the line impedance, θ is the phase-locked loop output angle, and V is the line impedance. dcref For the DC voltage setpoint, i ref E is the setpoint for the grid-side current loop. ref E is the reference value for the output voltage of the grid-side converter. αβ E is the reference value for the output voltage of the grid-side converter. ref The components of V in the stationary coordinate system after coordinate transformation dc C is the DC bus voltage. dc It is a DC capacitor;

[0095] refer to Figure 3 The steady-state deviation of the DC-side power is introduced into the DC voltage loop through a lead / lag circuit and an integrator. d_ref i is the active current reference value. q_ref This is the reference value for reactive current, u d Voltage signal U abc The d-axis DC component, u q Voltage signal U abc The q-axis DC component, U dcref For DC voltage reference value, PI udc For DC voltage, replace the proportional-integral (PI) stage;

[0096] refer to Figure 4 DC power affects the q-axis voltage at the grid connection point through the voltage-current loop, which in turn affects the output of the phase-locked loop;

[0097] refer to Figure 5 , among which, U qref U is the steady-state value of the q-axis voltage. q Let θ be the q-axis voltage, α0 be the steady-state value of the grid angular frequency, and θ be the angular frequency. ref The output angle is the steady-state value, and θ is the output of the phase-locked loop;

[0098] refer to Figure 6 , where Δθ AIsynC The small-signal form of the angle introduced for the synchronous synchronization of composite DC power is α=ω. b / U dc C dc ;

[0099] refer to Figure 7 The improved control synchronization unit combines grid-following control and DC inertial synchronization control, ultimately achieving composite control and effectively improving the stability of grid-following converters under weak power grid conditions. Specifically, θ ref To output the steady-state value of the angle, α' = k ipll ω g L g .

[0100] While the application has been described in accordance with a limited number of embodiments, these are merely illustrative of the many possible embodiments of the application. Other embodiments can be devised without departing from the scope of the application as described herein. Additionally, it is intended that the description set forth herein should not be construed as limiting but merely as illustrative of the presently preferred embodiments of the application. Many modifications and variations to the embodiments described herein will be apparent to those of ordinary skill in the art, and it is intended that the application encompass all such modifications and variations as fall within the scope of the appended claims. Accordingly, the application is not to be limited by the specific examples described herein, but only by the scope of the appended claims.

Claims

1. A control method for a grid-connected converter with composite DC power synchronization, characterized in that, Includes the following steps: S1. Collect the voltage signal, current signal and DC voltage of the grid-connected converter, and calculate the DC power on the DC side of the grid-connected converter; S2. The DC power on the DC side of the grid converter is passed through the lead / lag circuit and the integral controller in sequence to obtain the compensation term. This term is then introduced into the d-axis current reference command value output by the DC voltage loop to obtain a new d-axis current reference value. S3. The compensation term affects the d-axis current through the proportional-integral link of the current loop. Based on the new d-axis current reference value, the new d-axis current is obtained. The q-axis voltage is affected by the relationship between the grid voltage, grid impedance and grid connection point voltage. The affected q-axis voltage is obtained. S4. The affected q-axis voltage is used as the input of the phase-locked loop. The proportional-integral controller of the phase-locked loop directly controls the grid connection angle of the output. Through theoretical analysis, the small angle signal value introduced by the composite DC power synchronization in the output grid connection angle is equivalent to the introduction of a grid-type DC inertial synchronization link in the grid-type phase-locked loop, so as to realize composite control. In step S4, the q-axis small-signal component of the grid connection point voltage after composite power synchronization control is introduced. As the input to the phase-locked loop (PLL), the proportional-integral controller (PIC) directly controls the grid-connected angle of the output. The output grid-connected angle is represented in small-signal form, yielding the small-signal form of the composite DC power synchronous output angle. And obtain the small signal form of the output angle of DC inertial synchronization. ; Composite DC power synchronous output angle small signal form Represented as: (8) (9) in, The phase-locked loop outputs a small-signal angle. The small angle signal form is introduced as an additional feature for composite DC power synchronization. These are the proportional parameters of the phase-locked loop. For the integral parameters of the phase-locked loop, For setting items, , For the power grid frequency, For line impedance, To improve the small-signal component of the compensation term, For small-signal components of DC power, The first time constant of the lead / lag element. The second time constant of the lead / lag element. This is the proportional coefficient for composite power synchronous control; DC inertial synchronous output angle small signal form Represented as: (10) in, This is the reference value for the angular frequency of the power grid. The proportional parameter for DC inertial synchronization. These are the integral parameters for DC inertial synchronization. This is the steady-state value of the DC voltage. For DC bus capacitors; Referring to equations (9) and (10), when the second time constant of the lead / lag element... The value of is related to the proportional parameter of the phase-locked loop. Integral parameters of phase-locked loop When the ratio is equal, ignore The additional small angle signal value introduced by the composite DC power synchronization is equivalent to introducing a grid-type DC inertial synchronization link into the grid-type phase-locked loop.

2. The control method for a grid-connected converter with composite DC power synchronization according to claim 1, characterized in that, In S1, the DC power P on the DC side of the grid converter... dc Represented as: P dc = I dc *in dc - 1.5(in d *and d + in q *and q ) Among them, u d Voltage signal U abc The d-axis DC component, u q Voltage signal U abc The q-axis DC component, i d For current signal I abc The d-axis DC component, i q For current signal I abc The q-axis DC component, I dc The output current of the DC current source is u. dc This is the DC bus voltage.

3. The control method for a grid-connected converter with composite DC power synchronization according to claim 2, characterized in that, In S2, the DC power P dc The output quantity, which is the DC power steady-state deviation, is obtained after passing through the lead / lag stage and the integral controller. The compensation term is then introduced into the d-axis current reference command value output by the DC voltage loop to obtain a new d-axis current reference value. This affects the inverter's current and angle output; Output Represented as: (1) in, This represents the instantaneous value of DC power. This represents the steady-state value of DC power. The first time constant of the lead / lag element. The second time constant of the lead / lag element. Here, s represents the proportional coefficient for composite power synchronization control, and s denotes the differential operator: New d-axis current reference value Represented as: (2) in, This is the reference value for the d-axis current.

4. The control method for a grid-connected converter with composite DC power synchronization according to claim 3, characterized in that, In step S3, a traditional current closed-loop control is used to control the inverter's current, separately controlling the decoupled d-axis current and q-axis current. The proportional-integral (PI) element of the current loop causes the d-axis current to change with the new d-axis current reference value, thus obtaining a new d-axis current. The compensation term and the new d-axis current are represented in small-signal form, yielding the small-signal component of the improved compensation term. and the d-axis small-signal component of the grid-connected current after the introduction of composite power synchronization control ; New d-axis current Represented as: (3) Improved compensation term for small signal components Represented as: (4) in, This represents the small-signal component of DC power. The d-axis small-signal component of the grid-connected current after introducing composite power synchronous control Represented as: (5) in, This represents the small-signal component of the original grid-connected current along the d-axis. Based on the relationship between grid connection point voltage, line impedance, and grid voltage, and ignoring the line resistance and small-signal value of grid voltage, the small-signal component of the original grid connection point voltage is used. The affected q-axis voltage is represented in small-signal form, thus obtaining the q-axis small-signal component of the grid connection point voltage after introducing composite power synchronization control. ; (6) in, For the power grid frequency, Line impedance; The small-signal q-axis component of the grid connection point voltage after introducing composite power synchronous control Represented as: (7)。

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