Control method of composite direct-current power synchronization grid-following type converter

Abstract

The invention discloses a control method for a composite direct-current power synchronization grid-following type converter, and belongs to the technical field of wind power grid-connected converter control. The problem that in the prior art, a traditional converter control method based on network tracking type control is low in stability due to subsynchronous oscillation easily generated under a weak power grid is solved. According to the method, the direct-current power of the grid-following type converter is collected and sequentially passes through a lead / lag link and an integral controller, then an obtained compensation item is introduced to a d-axis current reference instruction value output by a direct-current voltage loop, d-axis current and q-axis voltage are influenced, the influenced q-axis voltage serves as phase-locked loop input, and the phase-locked loop is connected with a phase-locked loop. An output grid-connected angle is directly controlled by a proportional-integral controller of a phase-locked loop link, and an angle small signal value additionally introduced by composite direct-current power synchronization is equivalent to that a network-forming type direct-current inertia synchronization link is introduced into a network-following type phase-locked loop. The method improves the adaptability of the following network type control system to the weak power grid, and can be applied to converter control.

Description

Technical Field

[0001] The invention relates to a grid-connected converter control method, in particular to a composite direct current power synchronous grid-connected converter control method, and belongs to the technical field of wind power grid-connected converter control. Background Art

[0002] With the development of renewable energy, distributed energy is mainly connected to the grid through three-phase converters. At present, there are two main control methods for grid-connected converters: grid-following control based on vector control and grid-building control based on virtual synchronization, droop control, and DC voltage self-synchronization. Grid-following control (GFL) uses a phase-locked loop (PLL) as a synchronization unit to ensure that the voltage and angle of the inverter are synchronized with the AC grid. However, due to the increasing installed capacity of renewable energy power generation and the weak grid characteristics caused by the longer transmission lines required for distributed power generation, the voltage at the grid connection point is prone to disturbances, resulting in deviations or even failure of the phase-locked loop belonging to the voltage-type synchronization method, which in turn causes frequent subsynchronous oscillation accidents in the grid-connected system, seriously affecting the safe and stable operation of the power grid. Therefore, it is necessary to suppress subsynchronous oscillations.

[0003] Improvement methods based on grid-following control can be divided into optimizing control parameters and changing control structure. Optimizing parameters is easy to achieve, but its improvement effect is usually limited by the balance between steady-state margin and dynamic performance. Changing the control structure is mainly to improve the current inner loop and the synchronization unit structure, but the structures are mostly complex. At the same time, the existing methods to improve the stability of grid-connected converters under weak power grids are mostly grid-forming control. The implementation of grid-forming control can be roughly divided into two ways. In addition to simulating the rotor motion equation of the synchronous generator by virtual synchronization and other methods, the dynamic characteristics of DC capacitors can also be used to construct the inertial relationship between DC voltage and grid frequency. The simulated inertia of DC capacitors provides inertia support for the grid, thereby enabling the system to simulate the power angle characteristics of the synchronous generator and realize autonomous synchronization between the inverter and the grid. However, the essence of DC inertial synchronization control belongs to grid-forming control, and it is impossible to improve the stability of the grid-following converter under weak power grid without changing the properties of the grid-following control structure.

[0004] In summary, a composite DC power synchronous grid-following converter control method with a composite grid-forming control concept, which is easy to implement and has obvious effects, is needed. Summary of the invention

[0005] A brief overview of the present invention is provided below in order to provide a basic understanding of certain aspects of the present invention. It should be understood that this overview is not an exhaustive overview of the present invention. It is not intended to identify key or important parts of the present invention, nor is it intended to limit the scope of the present invention. Its purpose is merely to present certain concepts in a simplified form as a prelude to a more detailed description discussed later.

[0006] In view of this, in order to solve the problems of low stability and complex control structure caused by subsynchronous oscillations easily generated in weak power grids in the traditional converter control method based on grid-following control in the prior art, the present invention provides a grid-following converter control method with composite DC power synchronization.

[0007] The technical solution is as follows: A control method for a grid-following converter with composite DC power synchronization includes the following steps:

[0008] S1. Collect the voltage signal, current signal and DC voltage of the grid-type converter, and calculate the DC power on the DC side of the grid-type converter;

[0009] S2. The DC power on the DC side of the grid-following converter is sequentially passed through the lead / lag link and the integral controller to obtain a compensation term, which is introduced into the d-axis current reference command value output by the DC voltage loop to obtain a new d-axis current reference value;

[0010] S3. The compensation term affects the d-axis current through the proportional integral link of the current loop, and a new d-axis current is obtained according to the new d-axis current reference value. The q-axis voltage is further affected based on the relationship between the grid voltage, the grid impedance and the grid connection point voltage, and the affected q-axis voltage is obtained;

[0011] S4. The affected q-axis voltage is used as the phase-locked loop input, and the output grid-connected angle is directly controlled through the proportional-integral controller of the phase-locked loop. Through theoretical analysis, the angle small signal value additionally introduced by the composite DC power synchronization in the output grid-connected angle is equivalent to the introduction of a grid-forming DC inertia synchronization link in the grid-following phase-locked loop, thereby realizing composite control.

[0012] Furthermore, in S1, the DC power P on the DC side of the grid-type converter dc It is expressed as:

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

[0014] Among them, u d is the voltage signal Uabc The d-axis DC component, u q is the voltage signal U abc The q-axis DC component, i d is the current signal I abc The d-axis DC component, i q is the current signal I abc The q-axis DC component, I dc is the output current of the DC current source, u dc is the DC bus voltage.

[0015] Furthermore, in S2, the DC power P dc After passing through the lead / lag link and the integral controller in sequence, the compensation term is obtained, that is, the output x obtained after the DC power steady-state deviation passes through the lead / lag link and the integral controller. The compensation term is introduced into the d-axis current reference command value output by the DC voltage loop to obtain a new d-axis current reference value i dref ', affecting the output of the inverter current and angle;

[0016] The output x is expressed as:

[0017]

[0018] Among them, P dc is the instantaneous value of DC power, P dcref is the steady-state value of DC power, τ1 is the first time constant of the lead / lag link, τ2 is the second time constant of the lead / lag link, k is the proportional coefficient of the composite power synchronous control, and s represents the differential operator;

[0019] New d-axis current reference value i dref ' is expressed as:

[0020] i dref '=i dref +x (2)

[0021] Among them, i dref is the d-axis current reference value.

[0022] Furthermore, in S3, the conventional current closed loop is used to control the current of the inverter, and the d-axis current and the q-axis current after decoupling are controlled separately. The proportional integral link of the current loop causes the d-axis current to change with the new d-axis current reference value, and a new d-axis current i d ', and the compensation term and the new d-axis current are expressed in small signal form, and the improved compensation term small signal component Δx and the d-axis small signal component Δi of the grid-connected current after the introduction of composite power synchronization control are obtained. d ';

[0023] The new d-axis current i d' is expressed as:

[0024] i d '=i dref ' (3)

[0025] The small signal component Δx of the improved compensation term is expressed as:

[0026]

[0027] Among them, ΔP dc is the small signal component of DC power;

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

[0029] Δi d '=Δi d +Δx (5)

[0030] Among them, Δi d is the d-axis small signal component of the original grid-connected current;

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

[0032] Δu q =-ω g L g Δi d (6)

[0033] Among them, ω g is the grid frequency, L g is the line impedance;

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

[0035] Δu q '=-ω g L g (Δi d +Δx) (7).

[0036] Furthermore, in S4, the q-axis small signal component Δu of the grid connection point voltage after the composite power synchronization control is introduced q'As the input of the phase-locked loop, the proportional-integral controller of the phase-locked loop directly controls the output grid-connected angle, and the output grid-connected angle is expressed in the form of a small signal to obtain the composite DC power synchronous output angle small signal form Δθ', and obtain the DC inertia synchronous output angle small signal form Δθ synC ;

[0037] The composite DC power synchronous output angle small signal form Δθ' is expressed as:

[0038]

[0039] Among them, Δθ pll Δθ is the phase-locked loop output angle small signal form, x k is the additional angle signal introduced by the composite DC power synchronization. ppll is the proportional parameter of the phase-locked loop, k ipll is the integral parameter of the phase-locked loop, H(s) is the setting item,

[0040] DC inertia synchronous output angle small signal form Δθ synC It is expressed as:

[0041]

[0042] Among them, ω b is the grid angular frequency reference value, k pv is the proportional parameter of DC inertia synchronization, k iv is the integral parameter of DC inertia synchronization, U dc is the steady-state value of DC voltage, C dc is the DC bus capacitor;

[0043] Referring to equations (9) and (10), when the value of the second time constant τ2 of the lead / lag link is equal to the proportional parameter k of the phase-locked loop ppll and the integral parameter k of the phase-locked loop ipll When the ratio is equal and H(s) is ignored, the additional small angle signal value introduced by the composite DC power synchronization is equivalent to introducing a DC inertia synchronization link of the grid-following type phase-locked loop.

[0044] The beneficial effects of the present invention are as follows: the grid-connected control method of the composite DC power synchronization of the present invention is mainly aimed at the sub-synchronous oscillation problem of the grid-connected converter under weak power grids. On the basis of the grid-connected control structure, the composite DC inertial synchronization constructs the idea of ​​grid-connected control, introduces the DC side power steady-state deviation into the DC voltage loop, realizes the composite DC power synchronization in the grid-connected control, and improves the stability of the grid-connected converter under weak power grids; the improvement strategy of the present invention has a simple structure, obvious improvement effect, fewer design parameters, and obvious effect on suppressing sub-synchronous oscillations. On the one hand, it ensures the stability of the system under weak power grids, and makes the grid-connected converter The type control system maintains stable operation under the experimental condition of low short-circuit ratio. On the other hand, it can widen the phase-locked loop bandwidth range of the grid-following control system under weak power grid, which can ensure the stable operation of the system at high phase-locked loop bandwidth, and also has a good suppression effect under the condition of non-unit power factor grid connection. The present invention combines the grid-following control with the DC inertia synchronization control strategy, and improves the adaptability of the grid-following control system to weak power grids by improving the control and adjusting the grid connection angle, thereby ensuring the stability of the grid-following control system under weak power grids, and enabling the grid-following control system to maintain stable operation under the experimental condition of low short-circuit ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0046] Figure 1 A flow chart of a control method for a grid-following converter for composite DC power synchronization;

[0047] Figure 2 It is the structural block diagram of the grid-following converter;

[0048] Figure 3 The control block diagram of the grid-following converter for composite DC power synchronization;

[0049] Figure 4 Schematic diagram of the effect of grid-connected converter control with composite DC power synchronization on the grid-connected system;

[0050] Figure 5 It is the equivalent block diagram of the phase-locked loop control in the grid-following converter;

[0051] Figure 6 The equivalent block diagram of DC inertia synchronous control in grid-type converter is shown in FIG.

[0052] Figure 7 This is the equivalent block diagram of the grid-following converter control for composite DC power synchronization. DETAILED DESCRIPTION

[0053] In order to make the technical solutions and advantages of the embodiments of the present invention more clearly understood, the exemplary embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than an exhaustive list of all the embodiments. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0054] refer to Figure 1-Figure 7 The present embodiment is described in detail, and the control method of the grid-following converter with composite DC power synchronization specifically includes the following steps:

[0055] S1. Collect the voltage signal, current signal and DC voltage of the grid-type converter, and calculate the DC power on the DC side of the grid-type converter;

[0056] S2. The DC power on the DC side of the grid-following converter is sequentially passed through the lead / lag link and the integral controller to obtain a compensation term, which is introduced into the d-axis current reference command value output by the DC voltage loop to obtain a new d-axis current reference value;

[0057] S3. The compensation term affects the d-axis current through the proportional integral link of the current loop, and a new d-axis current is obtained according to the new d-axis current reference value. The q-axis voltage is further affected based on the relationship between the grid voltage, the grid impedance and the grid connection point voltage, and the affected q-axis voltage is obtained;

[0058] S4. The affected q-axis voltage is used as the phase-locked loop input, and the output grid-connected angle is directly controlled through the proportional-integral controller of the phase-locked loop. Through theoretical analysis, the angle small signal value additionally introduced by the composite DC power synchronization in the output grid-connected angle is equivalent to the introduction of a grid-forming DC inertia synchronization link in the grid-following phase-locked loop, thereby realizing composite control.

[0059] Furthermore, in S1, the voltage signal U of the grid-connected converter is collected. abc , current signal I abc and DC voltage U dc , the DC power P on the DC side of the grid-following converter is calculated dc ;

[0060] The DC power P on the DC side of the grid-following converter dc It is expressed as:

[0061] P dc =I dc *u dc -1.5(u d *i d +u q *i q )

[0062] Among them, u d is the voltage signal U abc The d-axis DC component, u q is the voltage signal U abc The q-axis DC component, i d is the current signal I abc The d-axis DC component, i q is the current signal I abc The q-axis DC component, I dc is the output current of the DC current source, u dc is the DC bus voltage.

[0063] Furthermore, in S2, the DC power P dc After the lead / lag link and the integral controller in turn, the compensation term is obtained, that is, the output x obtained after the DC power steady-state deviation passes through the lead / lag link and the integral controller. The compensation term is introduced into the d-axis current reference command value output by the DC voltage loop (the main control link of the improvement strategy) to obtain a new d-axis current reference value i dref ', affecting the output of the inverter current and angle;

[0064] The output x is expressed as:

[0065]

[0066] Among them, P dc is the instantaneous value of DC power, P dcref is the steady-state value of DC power, τ1 is the first time constant of the lead / lag link, τ2 is the second time constant of the lead / lag link, k is the proportional coefficient of composite power synchronization control, and s is the differential operator;

[0067] New d-axis current reference value i dref ' is expressed as:

[0068] i dref '=i dref +x (2)

[0069] Among them, i dref is the d-axis current reference value.

[0070] Furthermore, in S3, the conventional current closed loop is used to control the current of the inverter, and the d-axis current and the q-axis current after decoupling are controlled separately. The proportional integral link of the current loop causes the d-axis current to change with the new d-axis current reference value, and a new d-axis current i d ', and the compensation term and the new d-axis current are expressed in small signal form, and the improved compensation term small signal component Δx and the d-axis small signal component Δi of the grid-connected current after the introduction of composite power synchronization control are obtained. d ';

[0071] The new d-axis current i d ' is expressed as:

[0072] i d '=i dref ' (3)

[0073] The small signal component Δx of the improved compensation term is expressed as:

[0074]

[0075] Among them, ΔP dc is the small signal component of DC power;

[0076] The d-axis small signal component Δi of the grid-connected current after the introduction of composite power synchronization control d ' is expressed as:

[0077] Δi d '=Δi d +Δx (5)

[0078] Among them, Δi d is the d-axis small signal component of the original grid-connected current;

[0079] Based on the relationship between the grid connection point voltage, line impedance and grid voltage, ignoring the line resistance and the small signal value of the grid voltage, the q-axis small signal component Δu of the original grid connection point voltage is calculated. q , the affected q-axis voltage is expressed in the form of a small signal, and the q-axis small signal component Δu of the grid connection point voltage after the introduction of composite power synchronization control is obtained q ';

[0080] Δu q =-ω g L g Δi d (6)

[0081] Among them, ω g is the grid frequency, L g is the line impedance;

[0082] The q-axis small signal component Δu of the grid connection point voltage after the introduction of composite power synchronization control q ' is expressed as:

[0083] Δu q '=-ω g L g (Δi d +Δx) (7).

[0084] Furthermore, in S4, the q-axis small signal component Δu of the grid connection point voltage after the composite power synchronization control is introduced q'As the input of the phase-locked loop, the proportional-integral controller of the phase-locked loop directly controls the output grid-connected angle, and the output grid-connected angle is expressed in the form of a small signal to obtain the composite DC power synchronous output angle small signal form Δθ', and obtain the DC inertia synchronous output angle small signal form Δθ synC ;

[0085] The composite DC power synchronous output angle small signal form Δθ' is expressed as:

[0086]

[0087] Among them, Δθ pll Δθ is the phase-locked loop output angle small signal form, x k is the additional angle signal introduced by the composite DC power synchronization. ppll is the proportional parameter of the phase-locked loop, k ipll is the integral parameter of the phase-locked loop, H(s) is the setting item,

[0088] DC inertia synchronous output angle small signal form Δθ synC It is expressed as:

[0089]

[0090] Among them, ω b is the grid angular frequency reference value, k pv is the proportional parameter of DC inertia synchronization, k iv is the integral parameter of DC inertia synchronization, U dc is the steady-state value of DC voltage, C dc is the DC bus capacitor;

[0091] Referring to equations (9) and (10), the angle small signal value introduced by the composite DC power synchronous control is similar to the output angle small signal value of the DC inertia synchronous control. When the value of the second time constant τ2 of the lead / lag link is equal to the proportional parameter k of the phase-locked loop ppll and the integral parameter k of the phase-locked loop ipll When the ratio is equal, H(s) is ignored, and the additional angle small signal value introduced by the composite DC power synchronization is equivalent to introducing a DC inertia synchronization link of the grid-forming type into the grid-following type phase-locked loop, achieving the effect of the composite grid-forming control in the grid-following type control, thereby improving the system's adaptability to weak power grids.

[0092] Specifically, refer to Figure 1 , V abc is the grid connection point voltage, i abc is the grid-connected point current, P is the active power output by the converter, the corresponding arrow indicates the direction of power flow, L is the filter inductance, Z gis the line impedance, θ is the phase-locked loop output angle, V dcref is the given value of DC voltage, i ref is the grid-side current loop given value, E ref is the grid-side converter output voltage reference value, E αβ is the grid-side converter output voltage reference value E ref The component in the stationary coordinate system after coordinate transformation, V dc is the DC bus voltage, C dc is the DC capacitance;

[0093] refer to Figure 2 , the DC side power steady-state deviation value is introduced into the DC voltage loop through the lead / lag link and integrator, i d_ref is the active current reference value, i q_ref is the reactive current reference value, u d is the voltage signal U abc The d-axis DC component, u q is the voltage signal U abc The q-axis DC component, U dcref is the DC voltage reference value, PI udc Convert DC voltage to proportional integral (PI) link;

[0094] refer to Figure 3 , the DC power affects the q-axis voltage of the grid connection point through the voltage and current loop, and then affects the output of the phase-locked loop;

[0095] refer to Figure 4 , where U qref is the steady-state value of the q-axis voltage, U q is the q-axis voltage, is the steady-state value of the grid angular frequency, θ ref is the steady-state value of the output angle, θ is the phase-locked loop output;

[0096] refer to Figure 5 , where Δθ AIsynC It is the angle small signal form introduced by the composite DC power synchronous equivalent, α=ω b / U dc C dc ;

[0097] refer to Figure 6 The improved control synchronization unit combines the synchronization units of grid-following control and DC inertia synchronization control, and finally realizes compound control, which effectively improves the stability of grid-following converter under weak power grid. ref is the steady-state value of the output angle, α'=k ipll ω g L g .

[0098] Although the present invention has been described according to a limited number of embodiments, it will be apparent to those skilled in the art, with the benefit of the above description, that other embodiments may be envisioned within the scope of the invention thus described. In addition, it should be noted that the language used in this specification is selected primarily for readability and teaching purposes, rather than for explaining or defining the subject matter of the present invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the present invention is illustrative, not restrictive, with respect to the scope of the present invention, which is defined by the appended claims.

Claims

1. A control method for a grid-following converter with composite DC power synchronization, characterized in that: The following steps are involved: S1. Collect the voltage signal, current signal and DC voltage of the grid-type converter, and calculate the DC power on the DC side of the grid-type converter; S2. The DC power on the DC side of the grid-following converter is sequentially passed through the lead / lag link and the integral controller to obtain a compensation term, which is 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, and a new d-axis current is obtained according to the new d-axis current reference value. The q-axis voltage is further affected based on the relationship between the grid voltage, the grid impedance and the grid connection point voltage, and the affected q-axis voltage is obtained; S4. The affected q-axis voltage is used as the phase-locked loop input, and the output grid-connected angle is directly controlled through the proportional-integral controller of the phase-locked loop. Through theoretical analysis, the angle small signal value additionally introduced by the composite DC power synchronization in the output grid-connected angle is equivalent to the introduction of a grid-forming DC inertia synchronization link in the grid-following phase-locked loop, thereby realizing composite control.

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

3. The control method of a composite DC power synchronous grid-following converter according to claim 2, characterized in that: In S2, the DC power P dc After passing through the lead / lag link and the integral controller in sequence, the compensation term is obtained, that is, the output x obtained after the DC power steady-state deviation passes through the lead / lag link and the integral controller. The compensation term is introduced into the d-axis current reference command value output by the DC voltage loop to obtain a new d-axis current reference value i dref ', affecting the output of the inverter current and angle; The output x is expressed as: Among them, P dc is the instantaneous value of DC power, P dcref is the steady-state value of DC power, τ1 is the first time constant of the lead / lag link, τ2 is the second time constant of the lead / lag link, k is the proportional coefficient of the composite power synchronization control, and s represents the differential operator: New d-axis current reference value i dref ' is expressed as: i dref '=i dref +x (2) Among them, i dref is the d-axis current reference value.

4. The control method of a composite DC power synchronous grid-following converter according to claim 3, characterized in that: In S3, the conventional current closed loop is used to control the current of the inverter, and the d-axis current and the q-axis current after decoupling are controlled separately. The proportional integral link of the current loop makes the d-axis current change with the new d-axis current reference value, and the new d-axis current i d ', and the compensation term and the new d-axis current are expressed in small signal form, and the improved compensation term small signal component Δx and the d-axis small signal component Δi of the grid-connected current after the introduction of composite power synchronization control are obtained. d '; The new d-axis current i d ' is expressed as: i d '=i dref ' (3) The small signal component Δx of the improved compensation term is expressed as: Among them, ΔP dc is the small signal component of DC power; The d-axis small signal component Δi of the grid-connected current after the introduction of composite power synchronization control d ' is expressed as: Δi d '=Δi d +Δx (5) Among them, Δi d is the d-axis small signal component of the original grid-connected current; Based on the relationship between the grid connection point voltage, line impedance and grid voltage, ignoring the line resistance and the small signal value of the grid voltage, the q-axis small signal component Δu of the original grid connection point voltage is calculated. q , the affected q-axis voltage is expressed in the form of a small signal, and the q-axis small signal component Δu of the grid connection point voltage after the introduction of composite power synchronization control is obtained q '; Thu q =-ω g L g Yes d (6) Among them, ω g is the grid frequency, L g is the line impedance; The q-axis small signal component Δu of the grid connection point voltage after the introduction of composite power synchronization control q ' is expressed as: Thu q '=-ω g L g (Not d +Δx) (7).

5. The control method of a composite DC power synchronous grid-following converter according to claim 4, characterized in that: In S4, the q-axis small signal component Δu of the grid connection point voltage after the composite power synchronous control is introduced q 'As the input of the phase-locked loop, the proportional-integral controller of the phase-locked loop directly controls the output grid-connected angle, and the output grid-connected angle is expressed in the form of a small signal to obtain the composite DC power synchronous output angle small signal form Δθ', and obtain the DC inertia synchronous output angle small signal form Δθ synC ; The composite DC power synchronous output angle small signal form Δθ' is expressed as: Among them, Δθ pll Δθ is the phase-locked loop output angle small signal form, x k is the additional angle signal introduced by the composite DC power synchronization. ppll is the proportional parameter of the phase-locked loop, k ipll is the integral parameter of the phase-locked loop, H(s) is the setting item, DC inertia synchronous output angle small signal form Δθ synC It is expressed as: Among them, ω b is the grid angular frequency reference value, k pv is the proportional parameter of DC inertia synchronization, k iv is the integral parameter of DC inertia synchronization, U dc is the steady-state value of DC voltage, C dc is the DC bus capacitor; Referring to equations (9) and (10), when the value of the second time constant τ2 of the lead / lag link is equal to the proportional parameter k of the phase-locked loop ppll and the integral parameter k of the phase-locked loop ipll When the ratio is equal and H(s) is ignored, the additional small angle signal value introduced by the composite DC power synchronization is equivalent to introducing a DC inertia synchronization link of the grid-following type phase-locked loop.

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