阅读说明：本技术 一种基于预测控制的vsg延时补偿方法 (VSG delay compensation method based on predictive control ) 是由 兰征 侯长茂 何东 龙阳 曾进辉 邹彬 黄浪尘 于 2021-09-27 设计创作，主要内容包括：本发明公开了一种基于预测控制的VSG延时补偿方法,包括以下步骤:通过采样得到逆变器输出电流i-(cv)和电压v-(o),经abc/dq坐标变换后分别得到i-(cvdq)和v-(odq),经过VSG控制和电压环控制得到i-(cvdq)～(*),同时将i-(cvdq)和v-(odq)经过预测函数求出下一时刻的电流i-(cvdq)(k+1),并将i～(*)-(cvdq)(k)和i-(cvdq)(k+1)作为价值函数的输入；在事先定义好的8个开关状态下产生的参考电流和预测电流通过价值函数寻优,最后根据价值函数的输出从而选取8个开关矢量中误差最小的一组作用于变换器,形成电流闭环控制,本发明在电流内环上通过采用MPC控制替代传统PI控制,对电流做出预测,起到对延时的补偿作用。(The invention discloses a VSG delay compensation method based on predictive control, which comprises the following steps of obtaining an output current i of an inverter through sampling cv And voltage v o Respectively obtaining i after abc/dq coordinate transformation cvdq And v odq I is obtained through VSG control and voltage loop control cvdq * While simultaneously mixing i cvdq And v odq The current i at the next moment is determined by a prediction function cvdq (k +1), and mixing i * cvdq (k) And i cvdq (k +1) as an input to a cost function; the method comprises the steps of optimizing reference current and predicted current generated under 8 switch states defined in advance through a cost function, and selecting a group with the minimum error from 8 switch vectors to act on a converter according to the output of the cost function to form current closed-loop control.)

1. A VSG delay compensation method based on predictive control is characterized by comprising the following steps:

firstly, obtaining an output current i of an inverter through sampling_cvAnd voltage v_oRespectively obtaining i after abc/dq coordinate transformation_cvdqAnd v_odqAnd the voltage loop control is carried out to obtain i_cvdq ^*While simultaneously mixing i_cvdqAnd v_odqThe current i at the next moment is determined by a prediction function_cvdq(k +1), and mixing i^* _cvdq(k) And i_cvdq(k +1) as an input to a cost function;

and step two, optimizing the reference current and the predicted current generated under the condition of 8 switch states defined in advance through a cost function, and finally selecting one group with the minimum error from 8 switch vectors to act on the converter according to the output of the cost function to form current closed-loop control.

2. The VSG delay compensation method based on the predictive control of claim 1, wherein in the step one, the measured value i of the AC side current is passed_cvαβ(k) And LC filtered voltage measurement v_oαβ(k) Constructing a current prediction mathematical model to predict the current value i of the system at the moment k +1_cvαβ(k+1)。

3. The VSG delay compensation method based on the predictive control as claimed in claim 2, wherein the mathematical model of the current prediction is

4. The method of claim 3, wherein i is the delay compensation method for VSG based on predictive control_cvdqAnd (k +1) is a predicted value of the grid current at the (k +1) th sampling moment under a two-phase rotation dq coordinate system.

5. The method of claim 1, wherein in step one, the VSG control and i of the voltage loop output are adjusted^* _cvdq(k) And predicted current i at the next moment_cvdq(k +1) as an input to a cost function of g ═ i^* _cvd(k)-i_cvd(k+1)|+|i^* _cvq(k)-i_cvq(k+1)|。

6. The method of claim 5, wherein the cost function is a current reference i^* _cvdq(k) With the current i measured at the next instant_cvdqThe difference of (k +1) is taken as an absolute value.

7. The VSG delay compensation method based on the predictive control of claim 1, wherein in the second step, the VSG control delay comprises a sampling delay, an algorithm delay, a PWM delay and a physical link delay; the sampling delay, the algorithm delay, the PWM delay and the physical link delay are included in a delay link, and the delay link is arranged in front of the SPWM link to construct a total delay transfer function.

8. The method of claim 7, wherein the total delay transfer function is G(s) -G (G ═ G)_h(s)G_d(s)≈τe^-1.5τs(ii) a τ is the delay time constant, G_h(s) is a zero order keeper delay function, G_d(s) is a delay link function.

Technical Field

The invention relates to the technical field of virtual synchronous generators, in particular to a VSG delay compensation method based on predictive control.

Background

Chinese patent CN108400603A discloses a microgrid frequency difference-free switching control method based on VSG reactive power delay regulation, which includes: the method comprises the following steps of switching on and off a breaker regulated and controlled by an energy management system of the microgrid, judging the operation mode of the microgrid, and carrying out frequency difference-free switching control: when the circuit breaker is closed, grid-connected/isolated island mode switching is carried out, the reference angular frequency of the VSG and the angular frequency of the public bus are used as input of VSG control, and the control reference phase angle of the inverter is output through integral calculation; when the circuit breaker is disconnected, carrying out island/grid-connected mode switching, taking a given reference value of reactive power and reactive power measured by first-order filtering as VSG control input, and adding first-order delay after PI control to output control reference voltage; and generating a pulse signal to control the on-off of the driving switch tube through voltage and current double-loop control and space vector pulse width modulation.

In the prior art, the virtual synchronous generator technology can enable an inverter power supply to overcome the defects of small inertia and low damping. In the VSG control studied so far, the current inner loop mostly adopts a PI controller to ensure the stable output of the system. However, when the problem of digital control delay inherent in VSG control is considered, due to the existence of delay, the control accuracy and stability of the system under PI control are both reduced, and when the delay is too large, even a destabilization phenomenon may occur, which restricts the response characteristic and control accuracy of the system and affects the stability of the system.

Disclosure of Invention

The present invention aims to solve the above problems in the prior art, and provides a VSG delay compensation method based on predictive control, which predicts the current by using MPC control instead of the conventional PI control on the current inner loop, and performs the compensation function for the delay.

The purpose of the invention can be realized by the following technical scheme:

a VSG delay compensation method based on predictive control comprises the following steps:

firstly, obtaining an output current i of an inverter through sampling_cvAnd voltage v_oRespectively obtaining i after abc/dq coordinate transformation_cvdqAnd v_odqAnd the voltage loop control is carried out to obtain i_cvdq ^*While simultaneously mixing i_cvdqAnd v_odqThe current i at the next moment is determined by a prediction function_cvdq(k +1), and mixing i^* _cvdq(k) And i_cvdq(k +1) as an input to a cost function;

and step two, optimizing the reference current and the predicted current generated under the condition of 8 switch states defined in advance through a cost function, and finally selecting one group with the minimum error from 8 switch vectors to act on the converter according to the output of the cost function to form current closed-loop control.

As a further scheme of the invention: in step one, the measured value i of the current on the AC side is passed_cvαβ(k) And LC filtered voltage measurement v_oαβ(k) Constructing a current prediction mathematical model to predict the current value i of the system at the moment k +1_cvαβ(k+1)。

As a further scheme of the invention: the mathematical model of current prediction is

As a further scheme of the invention: i.e. i_cvdqAnd (k +1) is a predicted value of the grid current at the (k +1) th sampling moment under a two-phase rotation dq coordinate system.

As a further scheme of the invention: in step one, i of VSG control and voltage loop output^* _cvdq(k) And predicted current i at the next moment_cvdq(k +1) as an input to a cost function of g ═ i^* _cvd(k)-i_cvd(k+1)|+|i^* _cvq(k)-i_cvq(k+1)|。

As a further scheme of the invention: the cost function being a current reference value i^* _cvdq(k) With the current i measured at the next instant_cvdqThe difference of (k +1) is taken as an absolute value.

As a further scheme of the invention: in the second step, the VSG control delay comprises sampling delay, algorithm delay, PWM delay and physical link delay; the sampling delay, the algorithm delay, the PWM delay and the physical link delay are included in a delay link, and the delay link is arranged in front of the SPWM link to construct a total delay transfer function.

As a further scheme of the invention: the total delay transfer function is G(s) ═ G_h(s)G_d(s)≈τe^-1.5τs(ii) a τ is the delay time constant, G_h(s) is a zero order keeper delay function, G_d(s) is a delay link function.

The invention has the beneficial effects that:

compared with the traditional P I inner loop control of the VSG, the MPC current control of the VSG has the advantages of good adaptability, strong robustness and the like; the MPC current control of the VSG replaces PWM control with a simple control structure and algorithm thereof, and the problems of decoupling and parameter setting do not need to be considered; the MPC control strategy of the VSG can effectively improve the robustness of the VSG under the control delay and enhance the stability of the system; the MPC current control of the VSG replaces PWM control with a simple control structure and algorithm thereof, and the problems of decoupling and parameter setting do not need to be considered; and the MPC control of the VSG is also researched to regulate the parameter J and perform multi-target control work, so that the problem that the MPC control of the VSG is only used for a single VSG and cannot be further extended to a plurality of VSGs at present is solved.

Drawings

The invention will be further described with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of the present invention VSG delay compensation principle based on predictive control;

FIG. 2 is a VSG control architecture diagram of the present invention that considers control delay;

FIG. 3 is a graph of tau versus a characteristic root λ of the present invention_1,2、λ_3,4The root track of (2);

fig. 4 is a waveform diagram of output current of a conventional VSG control system when τ ═ 1e-4 according to the present invention;

fig. 5 is a waveform diagram of output current of a common VSG control system when τ -2 e-4 is the present invention;

fig. 6 is a waveform diagram of output current of a common VSG control system when τ is 2.5e-4 according to the present invention;

fig. 7 is a waveform diagram of the output current of the VSG-MPC control system when τ is 2.5e-4 in accordance with the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Referring to fig. 1-7, the present invention is a VSG delay compensation method based on predictive control, first obtaining an inverter output current i by sampling_cvAnd voltage v_oRespectively obtaining i after abc/dq coordinate transformation_cvdqAnd v_odqWhile simultaneously mixing i_cvdqAnd v_odqThe current icvdq (k +1) at the next time is determined by the prediction function. Output current i_cvAnd voltage v_oObtaining i through VSG control and voltage control_cvdq*. The main part of MPC control is to compensate for the delay by predicting the current at the next time, so that the current value i is predicted at the next sampling time_cvdq(k +1), according to kirchhoff's voltage law, the instantaneous voltage and current expression of the VSG controlled inverter at the time t is as follows:

the formula is arranged to obtain a discretization mathematical model as follows:

equation (2) is the measured value i of the current passing through the AC side in the k control period_cvαβ(k) And LC filtered voltage measurement v_oαβ(k) Predicting the current value i of the system at the moment k +1_cvαβ(k + 1). Transforming it into a two-phase rotating dq coordinate system is:

equation (3) is a model current prediction mathematical model. In the formula: theta is the spatial angle of the grid, i_cvdqAnd (k +1) is a predicted value of the grid current at the (k +1) th sampling moment under a two-phase rotation dq coordinate system.

Therefore, the load current i is measured at the kth sampling time_cvdq(k) And the network voltage v_odq(k) The current i at the next moment can be obtained by the prediction function formula (3)_cvdq(k +1), i x of the VSG control and voltage loop output at this time_cvdq(k) And the current i at the next time obtained by the prediction function formula (3)_cvdq(k +1) as input to the cost function. The cost function is as follows:

g＝|i^* _cvd(k)-i_cvd(k+1)|+|i^* _cvq(k)-i_cvq(k+1)| (4)

the cost function being a current reference value i_cvdq(k) With the current i measured at the next instant_cvdqThe difference of (k +1) is taken as an absolute value.

In order to control the on-off of the switching tube, 8 switching states Sk (k is 1 … n) of the inverter switching tube need to be defined in advance before prediction, after the 8 states of the switching tube are defined, the reference current and the predicted current generated in the 8 switching states are optimized through a cost function finally, and then a group with the minimum error in 8 switching vectors is selected according to the output of the cost function to act on the converter to form current closed-loop control, so that the traditional PI control and PWM modulation in the current closed-loop are replaced, and a compensation effect on time delay is achieved. The values of the basic voltage vector corresponding to the switch states in the two-phase stationary α β coordinate system are shown in the following table:

v in different switch states_cvα、v_cvβValue of

In summary, the basic operation process of the VSG delay compensation method based on predictive control is as follows: firstly, the output current i of the inverter is obtained by sampling_cvAnd voltage v_oRespectively obtaining i after abc/dq coordinate transformation_cvdqAnd v_odqAnd the voltage loop control is carried out to obtain i_cvdqAll at the same time_cvdqAnd v_odqThe current i at the next moment is determined by a prediction function_cvdq(k +1), and mixing i_cvdq(k) And i_cvdq(k +1) as input to the cost function. Then, reference current and prediction current generated in a predefined 8 switching state are optimized through a cost function, and finally, a group with the minimum error in 8 switching vectors is selected to act on a converter according to the output of the cost function to form current closed-loop control, so that the traditional PI control and PWM modulation in the current closed loop are replaced, and the time delay compensation effect is achieved; compared with the traditional P I inner loop control of the VSG, the MPC current control of the VSG has the advantages of good adaptability, strong robustness and the like; the MPC current control of the VSG replaces PWM control with a simple control structure and algorithm thereof, and the problems of decoupling and parameter setting do not need to be considered; the MPC control strategy of the VSG can effectively improve the robustness of the VSG under the control delay and enhance the stability of the system; the MPC current control of the VSG replaces PWM control with a simple control structure and algorithm thereof, and decoupling and parameter setting problems do not need to be considered.

Preferably, the parameter J can be adjusted and multi-objective controlled for MPC control of the VSG; the VSG control delay mainly comprises sampling delay, algorithm delay, PWM delay and physical link delay. The above-mentioned various delays are included in a delay link, said delay link is placed before SPWM link, tau is delay time constant, in which G_h(s) is a zero order keeper delay function, G_d(s) is a delay link function, then delay G_h(s) and a zero order keeper G_dThe transfer function of(s) is:

G_d(s)＝e^-τs (5)

the total delay transfer function is:

G(s)＝G_h(s)G_d(s)≈τe^-1.5τs (6)

therefore, a VSG state space mathematical model considering control delay is established on the basis, wherein the state space equation of each link is as follows:

(1) equation of state space of delay link

(2) State space equation for power controller

(3) State space equation of output LC filter and coupling inductor

(4) State space equations for voltage and current controllers

The VSG small signal model expression considering the control delay is as follows:

wherein, [ Delta x [ ]]＝[Δω_mΔP_outΔQ_outΔφ_dqΔγ_dqΔi_cvdqΔv_odqΔi_odqΔx_d1Δx_d2Δx_d3Δx_d4]. Eigenvalue root-trace analysis of the state space matrix a of the established state space equation is shown in fig. 3:

adopting a control variable method, keeping other parameters unchanged, and observing the characteristic root lambda influenced most by the time delay tau when the time delay tau is continuously increased_1,2、λ_3,4The characteristic root trace of (a) is shown in fig. 3, where the arrow direction is the direction of increase of τ. As can be seen from FIG. 3, as τ increases, λ_1,2、λ_3,4Etc. 2 sets of feature roots move to the right. Therefore, by analyzing the root locus of the feature root corresponding to the delay τ, we can know that the system gradually tends to be unstable as the delay increases.

The MPC compensates for the delay, and FIGS. 4-7 illustrate the output current waveforms of the conventional VSG-PI control and VSG-MPC control. As can be seen from fig. 4, when τ is 1e-4s, the grid-connected current waveform of the conventional VSG-PI control is relatively smooth, and the standard sinusoidal curve, the amplitude and the phase can better track the grid, and at this time, the grid-connected system is stable. When the delay time is increased to 2e-4s, the system is still stable, but the grid-connected current has distortion of different degrees; when the delay time is increased to 2.5e-4s, the grid-connected current waveform is unstable. In the same delay time, when VSG-MPC control is adopted, the current waveform has no large change, is very smooth and has no distortion, and the grid-connected system is always kept stable, so that the control strategy can effectively inhibit delay and improve the stability of the system.

The working principle of the invention is as follows: the method is used for solving the problem of inherent delay influence of digital control of the virtual synchronous generator, so that the response characteristic and the control precision of the system and the stability of the system are improved; on the current inner ring, the MPC control is adopted to replace the traditional PI control, the current is predicted, the compensation effect on the time delay is achieved, and firstly, the MPC control is adoptedSample the inverter output current i_cvAnd voltage v_oRespectively obtaining i after abc/dq coordinate transformation_cvdqAnd v_odqAnd the voltage loop control is carried out to obtain i_cvdq ^*While simultaneously mixing i_cvdqAnd v_odqThe current i at the next moment is determined by a prediction function_cvdq(k +1), and mixing i^* _cvdq(k) And i_cvdq(k +1) as input to the cost function. Then, reference current and prediction current generated in a predefined 8 switching state are optimized through a cost function, and finally, a group with the minimum error in 8 switching vectors is selected to act on a converter according to the output of the cost function to form current closed-loop control, so that the traditional PI control and PWM modulation in the current closed loop are replaced, and the time delay compensation effect is achieved; the VSG control delay mainly comprises sampling delay, algorithm delay, PWM delay and physical link delay, the various delays are contained in a delay link, the delay link is arranged before the SPWM link, and the regulation of the MPC control of the VSG on the parameter J and the multi-target control work are researched.

While one embodiment of the present invention has been described in detail, the description is only a preferred embodiment of the present invention and should not be taken as limiting the scope of the invention. All equivalent changes and modifications made within the scope of the present invention shall fall within the scope of the present invention.