阅读说明：本技术 一种lcl型并网单逆变器电流反馈有源阻尼方法 (LCL type grid-connected single inverter current feedback active damping method ) 是由 于晶荣 陈壮深 周仁友 张刚 于 2020-12-12 设计创作，主要内容包括：本发明公开了一种LCL型并网单逆变器电流反馈有源阻尼方法,本发明采用逆变器侧电流作为单电流反馈点,通过比对滤波电容两端并联电阻的无源阻尼方案,构建了基于逆变器侧电流反馈的等效有效阻尼方法。相比于现有有源阻尼方法,该方法在不增加传感器的情况下,使系统更易于稳定,同时不会影响系统的带宽。另外,从阻尼系数ζ的角度出发,给出了该方法的参数设计依据。仿真结果验证了本发明提出的有源阻尼控制方法的有效性和可行性。(The invention discloses an LCL type grid-connected single inverter current feedback active damping method, which adopts inverter side current as a single current feedback point and constructs an equivalent effective damping method based on inverter side current feedback by comparing a passive damping scheme of resistors connected in parallel at two ends of a filter capacitor. Compared with the existing active damping method, the method enables the system to be more stable without adding a sensor, and meanwhile, the bandwidth of the system is not affected. In addition, from the perspective of damping coefficient ζ, the parameter design basis of the method is given. The simulation result verifies the effectiveness and feasibility of the active damping control method provided by the invention.)

1. The LCL type grid-connected single inverter current feedback active damping method is characterized in that the alternating current side of the LCL type grid-connected single inverter is connected with a power grid through an LCL filter;

taking the output current i1 of the inverter as a feedback current;

the expression of the feedback function a is:

k is a proportionality coefficient;

at this time, the output current i of the LCL filter₂And an input voltage u_invThe transfer function between is:

wherein the content of the first and second substances,intrinsic resonance peaks determined for the LCL filter's own parameters. Zeta is the damping coefficient;

the relationship between the proportionality coefficient k and the damping coefficient ζ is:

wherein, K_pwmThe equivalent gain of PWM takes the values as follows:

wherein, U_dcIs a DC side voltage, V_triIs the peak of the carrier.

2. The LCL type grid-connected single inverter current feedback active damping method according to claim 1, wherein when the feedback function A is equivalent to a resistor R connected in parallel at two ends of a filter capacitor_c2And in time, the values of the parameter k and the damping coefficient zeta are respectively as follows:

R_c2the two ends of the filter capacitor are equivalently connected with resistors in parallel;

thus R_c2Is determined by the output current i of the LCL filter after the feedback function A is introduced₂And an input voltage u_invThe damping coefficient zeta newly added in the transfer function between the two damping coefficients is as follows:

s is a complex variable.

3. The LCL type grid-connected single inverter current feedback active damping method according to claim 1 or 2, characterized in that ζ takes a value of 0.707.

Technical Field

The invention relates to a current feedback active damping method for an LCL type grid-connected single inverter.

Background

The continuous aggravation of global energy crisis and environmental problems stimulates the rapid development of renewable energy grid-connected technologies such as wind power and photovoltaic. As an important supplement to the traditional large-scale centralized wind-solar power station, the distributed power generation technology of renewable energy sources draws wide attention at home and abroad. The grid-connected inverter plays a role in transferring energy from a distributed power source to a parallel power grid as an important bridge and a link for connecting the power grid and renewable energy sources, can effectively reduce the impact of a renewable energy source grid-connected unit on the power grid, improves the power supply reliability and the electric energy quality, and is generally favored.

In a large-scale renewable energy-based distributed power generation station, a three-phase grid-connected inverter is mainly adopted, and inversion is usually realized by adopting a PWM (pulse width modulation) control strategy, so that switching harmonics inevitably exist in the output voltage of the grid-connected inverter, and a proper output filter needs to be selected in order to suppress the switching harmonics in grid-connected current. The output filter mainly has two types of L type and LCL type, and compared with the L type filter, the LCL filter contains a filter capacitor C, so that on the premise of realizing the same filter effect, the total amount in the LCL filter is less than the inductance of the L filter, therefore, the LCL filter has smaller volume and lower cost. However, the frequency response of the LCL filter has an inherent resonance peak due to its own parameters.

In order to suppress the intrinsic resonance peak and ensure the stability of the system, a virtual impedance method is generally adopted. The virtual impedance method is generally classified into two types: one is based on state variable feedback and one is based on a wave trap.

The literature indicates that the virtual impedance method based on the wave trap cannot be considered for the robustness and bandwidth optimization of the system at the same time, and the bandwidth selection of the system is limited, so the research direction in recent years is mainly the virtual impedance method based on state variable feedback.

In the equivalent circuit of the LCL filter, sampling points which can be used for state variable feedback comprise inverter current, network side current, capacitance current and capacitance voltage. However, according to the number of sensors used, the virtual impedance method based on state variable feedback can be divided into two categories: the first type is that an additional sensor is needed besides a necessary current outer ring and a phase-locked loop sensor; the second category is that no additional sensors are required.

The first category of solutions requires additional sensors with additional losses, and therefore the current research direction is focused on the second category of solutions. In consideration of the specific implementation form of the control algorithm, the feedback state variable mainly adopted by the second type of scheme at present is the grid-side current, namely, the grid-side current feedback active damping method (GCFAD).

There is a document that proposes GCFAD introducing a second order differential from the viewpoint of increasing the system damping coefficient and adding a quadratic term to the open-loop characteristic polynomial. The GCFAD method introducing the second order differential is essentially equivalent to the method of connecting resistors at two ends of a filter capacitor in parallel, so that a good resonance peak suppression effect can be obtained. However, the introduction of a second order differential controller in the control loop not only makes it difficult to implement in engineering, but also amplifies high frequency harmonic currents. For the purpose of eliminating the above effects, it is further proposed in the literature to replace the second order derivative controller with a first order high pass filter approximation. However, the following documents further indicate: the GCFAD introduced into the first-order high-pass filter can well inhibit LCL resonance peak when the resonance frequency is greatly shifted, and the system bandwidth is reduced.

At present, the feedback variable mainly adopted by the second type of scheme is the grid-side current, which is considered for the concrete implementation form of the control algorithm, however, no good solution exists up to now. Therefore, it is necessary to consider converting the feedback variables into other feedback variables.

Since the inverter side of the LCL is equipped with current sensors for protection, the inverter current feedback can save additional current sensors of the control loop compared to the grid side current feedback. Furthermore, there are references which indicate: for a power grid side current feedback control system, in order to ensure the stability of the system under the condition of weak power grid, an additional damping function must be added into the system. And the inverter current feedback control system has an inherent damping characteristic. Therefore, the inverter current feedback control system is more stable and simpler than a power grid side current control system in terms of control stability.

In recent years, documents for inhibiting the LCL inherent resonance peak by taking the inverter current as a feedback variable at home and abroad are not abundant, and a single inverter current feedback resonance inhibition method for reducing resonance frequency deviation and power grid impedance influence is proposed in documents. However, this control method is equivalent to an inductor series resistance, an inductor, on the inverter side, and this solution has a limited suppression effect on the intrinsic resonance peak and still reduces the system bandwidth.

At present, no scholars propose a more optimal control scheme. Therefore, a suppression scheme for effectively suppressing the intrinsic resonance peak of the grid-connected inverter system based on inverter current feedback is needed to be designed.

Disclosure of Invention

The invention aims to solve the technical problem of providing an LCL type grid-connected single inverter current feedback active damping method, which enables a system to be more stable without adding a sensor and does not influence the bandwidth of the system.

The technical solution of the invention is as follows:

a LCL type is incorporated into the power networks the single inverter current feedback active damping method, the alternating current side of said LCL type is incorporated into the power networks the single inverter connects through LCL filter; description of the circuit: the ABC end on the alternating current side of the three-phase inverter is connected with the ABC end of the power grid through the L1 and the L2 which are connected in series respectively; and the middle point of each series branch (i.e. the connection point of L1 and L2) is connected to a common point through a capacitor Cf, see the system structure diagram;

taking the output current i1 of the inverter as a feedback current; feedback current i₁Loaded at a given place after being processed by a feedback function A, namely loaded at K_pwmAnd a front side. Feedback current is processed by feedback function A and then is compared with given current i of the inverter_1refSuperposing;

the expression of the feedback function a is:

k is a proportionality coefficient;

at this time, the output current i of the LCL filter₂And an input voltage u_invThe transfer function between is:

wherein the content of the first and second substances,intrinsic resonance peaks determined for the LCL filter's own parameters. Zeta is the damping coefficient;

the relationship between the proportionality coefficient k and the damping coefficient ζ is:

wherein, K_pwmThe equivalent gain of PWM takes the values as follows:

wherein, U_dcIs a DC side voltage, V_triIs the peak value of the carrier;

when the feedback function A is equivalent to the parallel connection of a resistor R at the two ends of the filter capacitor_c2And in time, the values of the parameter k and the damping coefficient zeta are respectively as follows:

R_c2the two ends of the filter capacitor are equivalently connected with resistors in parallel;

thus R_c2Is determined by the output current i of the LCL filter after the feedback function A is introduced₂And an input voltage u_invThe damping coefficient zeta newly added in the transfer function between the two damping coefficients is as follows:

L₁and L₂The inductance values of the first filter inductor and the second filter inductor of the LCL filter respectively, and C is the capacitance value of the filter capacitor of the LCL filter;

s is a complex variable;

and zeta is 0.707. In order to achieve both the resonance peak suppression effect and the dynamic performance, the zeta value in engineering application can be 0.707.

Has the advantages that:

the existing active damping scheme cannot obtain good balance between the suppression effect of the inherent resonance peak of the LCL and the bandwidth. Aiming at the problem, the inverter side current is used as a single current feedback point, and an equivalent effective damping method based on the inverter side current feedback is constructed by comparing a passive damping scheme of resistors connected in parallel at two ends of a filter capacitor. Compared with the existing active damping method, the method enables the system to be more stable without adding a sensor, and meanwhile, the bandwidth of the system is not affected. In addition, from the perspective of damping coefficient ζ, the parameter design basis of the method is given. The simulation result verifies the effectiveness and feasibility of the active damping control method provided by the invention.

The invention provides an improved single-inverter side current feedback active damping method, which has the following advantages:

1) the control system only needs to sample the side current of the inverter, and a sensor is not needed to be added, so that the engineering cost can be saved.

2) Making the system easier to stabilize without affecting the bandwidth of the system.

3) The controller parameters can be flexibly designed according to the damping coefficient ζ.

Drawings

Fig. 1 is a diagram of a three-phase LCL inverter system, wherein fig. 1(a) is a diagram of the system structure and fig. (b) is a single-phase equivalent circuit;

FIG. 2 is a block diagram of current closed loop control;

FIG. 3 is a LCL filter control block diagram;

FIG. 4 is a simplified control block diagram;

FIG. 5 is a circuit diagram corresponding to six basic passive damping methods;

wherein:

FIG. 5(a) is a schematic circuit diagram of the series resistance on inductor L1;

FIG. 5(b) is a circuit schematic of the series resistance across inductor L2;

FIG. 5(c) is a circuit schematic of the series resistance on capacitor Cf;

FIG. 5(d) is a circuit schematic of a parallel resistor across inductor L1;

FIG. 5(e) is a circuit schematic of a parallel resistor across inductor L2;

FIG. 5(f) is a schematic circuit diagram of a resistor connected in parallel across capacitor Cf;

fig. 6 is a bode diagram of six passive damping, the bode diagrams of fig. 6(a) - (f) corresponding to the circuits of fig. 5(a) - (f).

FIG. 7 shows a resistor R connected in parallel across a filter capacitor C_c2An equivalent control block diagram of (a);

FIG. 7(a) shows the parallel resistance R of the filter capacitor_c2FIG. 7(b) is an equivalent transformation diagram;

FIG. 8 is a graph of the parallel resistance of the filter capacitor versus the open loop Bode using the feedback function A;

FIG. 9 is a control block diagram of the system; FIGS. 9(a) and (b) are open-loop and closed-loop control block diagrams, respectively;

FIG. 10 is a closed loop bode plot of the filter capacitor parallel resistance and the virtual parallel resistance;

FIG. 11 is a comparison of control methods; fig. 11(a) is an overall Bode diagram of the original LCL resonance peak, and fig. 11(b) is a partial Bode diagram of the original LCL resonance peak.

FIG. 12 is a graph of the actual effect of various control methods; fig. 12(a) to (f) are graphs showing effects obtained when the virtual parallel 5 Ω resistance, ζ ═ 0.6, ζ ═ 0.707, ζ ═ 0.8, and ζ ═ 1, respectively, are set without any control method.

Detailed Description

The invention will be described in further detail below with reference to the following figures and specific examples:

example (b):

modeling of LCL type grid-connected inverter system

Fig. 1 shows a voltage-type grid-connected inverter system structure using LCL-type filter filtering. In the figure: the inverter side inductor L1, the filter capacitor C and the grid side inductor L2 form an LCL type filter, i₁And i₂Respectively inverter current and grid-connected current u_cIs the voltage across the filter capacitor; lg is the grid impedance. U shape_dcIs a direct current side voltage; u. of_invAnd u_gRespectively the inverter side voltage and the grid voltage. The single-phase equivalent circuit is shown in fig. 1 (b).

And modeling analysis is carried out on the single-phase equivalent circuit diagram, so that the current closed-loop control block diagram of the figure 2 can be obtained.

When the parasitic resistance of the LCL filter itself is not considered:

K_PWMequivalent gain of PWM, G_PIFor PI controller, the expression is

K_PIs a proportionality coefficient, K_iIs an integral coefficient.

u_mIs the output of the PI controller.

Since the natural resonant peak of the LCL is only related to the parameters of the LCL, the LCL filter part of the single-phase equivalent circuit can be simplified to the control block diagram shown in fig. 3:

in FIG. 3, the output current i₂And an input voltage u_invThe transfer function between is:

wherein the content of the first and second substances,i.e. the natural resonance peak determined by the LCL filter parameters itself.

Improved single inverter side current feedback active damping method

Because the main control block diagram adopts the current i of the inverter₁As sampling points, for the purpose of reducing the sensors, the inverter current i is considered here₁A feedback control function is constructed for the feedback variable. From the above analysis, the inherent resonance peak of the LCL filter is only related to the parameters of the LCL filter, so the main control block diagram of FIG. 2 is simplified, and the control block diagram of the feedback function A is introduced as shown in FIG. 4, where u is shown_vIs the a output of the feedback function.

Equation (3) is the transfer function after the feedback function a is introduced.

K_PWMThe equivalent gain of PWM takes the values as follows:

wherein E is the DC voltage source voltage, V_triIs the peak of the carrier.

3.1 virtual parallel resistance scheme

In fact, the feedback function a can be derived from the point of view of active damping. Listed in fig. 5 are the six most basic passive damping schemes.

As can be seen from the bode diagram of fig. 6:

FIGS. 6(a), (b) show that the inductance L₁、L₂When the series resistor is connected, the low-frequency component of the system can be inhibited, and meanwhile, the inhibition effect on the resonance peak is poor.

FIGS. 6(c), (e) show that the inductance L₂When the parallel resistor and the filter capacitor are connected in series with the resistor, the high-frequency component of the system can be amplified.

FIG. 6(d) shows that the inductance L₁The parallel resistor amplifies both the high frequency component and the low frequency component near the resonance peak, and the suppression effect on the resonance peak is poor.

FIG. 6(f) shows that a resistor R is connected in parallel across the filter capacitor C_c2In the process, the low-frequency segmentation and the high-frequency segmentation of the system are not influenced, and the resonance peak can be well inhibited. At the same time, with R_c2The increase in value has a reduced effect on the low and high frequency segments.

The part is equivalent to a resistor R connected in parallel at two ends of a filter capacitor_c2The derivation of the feedback function a is performed in view of the active damping control scheme of (1). Considering the equivalent parallel resistance R preliminarily_c2The resistance value of (2) is 5 Ω.

The following transfer functions can be obtained from the control block diagrams of fig. 7(a) and (b), respectively:

i₂refers to the current flowing through the net side inductor L2.

By comparing the formula (4) with the formula (5), it can be obtained that after the equivalent transformation is performed, the expression of the feedback function a should be:

the effectiveness of the control scheme of the parallel resistance of the filter capacitor and the control scheme adopting the feedback function A on the suppression effect of the inherent resonance peak is verified through simulation as follows:

as can be seen from fig. 8, the control scheme of the parallel resistor on the filter capacitor completely coincides with the open-loop transfer function using the feedback function a in the amplitude-frequency characteristic and the phase-frequency characteristic, which indicates that the control effect of the virtual parallel resistor on the filter capacitor can be obtained using the feedback function a.

Problems with virtual parallel resistance schemes

Fig. 9 is a control block diagram of the system, and analysis of the closed-loop transfer function of the system can find that there is a large difference between the two schemes of the virtual parallel resistance and the actual parallel resistance, which is mainly due to the introduction of the inverter current i₁The difference in transfer function caused after the feedback loop.

Fig. 10 is a closed loop Bode of two systems, as can be seen: the control scheme of the virtual parallel resistor does not have good effect on inhibiting the resonance peak.

General form of virtual parallel resistance scheme

The control scheme of the virtual parallel resistor has a poor effect of suppressing the resonance peak in the closed loop, and therefore, it is necessary to further analyze the effectiveness of the control scheme.

The foregoing equation (2) shows that the output current i₂And an input voltage u_invThe transfer function between consists of the product of the integral term and the undamped second order system, which means that the natural resonance peak is caused by the undamped second order system. Therefore, the stability of the system can be improved by increasing the damping coefficient ζ, i.e. the formula (2) needs to be modified:

to analyze the stability of the system, it can be explained from the following two theoretical perspectives:

when the stability analysis is carried out by using the Laus-Helverz stability criterion, the newly added quadratic term in the open-loop characteristic polynomial can make the data in the first column of the Laus table be positive numbers, so that the system tends to be more stable.

From the stability point of view of the second-order control system, the damping coefficient of the system can be increased by the measure. The system is changed from an undamped state to an underdamped state or even a critical damping state, so that the pole of the closed loop is forced to be far away from the right half plane of the s domain, and the system is more stable.

Analyzing the essence of equation (4), it can be found that: the virtual parallel resistance scheme of 3.1 is essentially a new quadratic term in the open-loop characteristic polynomial.

Comparing equation (3) with equation (7) yields: to ensure that the damping coefficient of the system is increased, the feedback function a should take the form:

wherein k is a proportionality coefficient, and the relationship between the proportionality coefficient and the damping coefficient of the system is shown in formula (9):

equations (8) and (9) are general forms of the feedback function A, and equation (6) is a virtual parallel resistor R across the filter capacitor_C2In a special form thereof.

From the equations (6) and (8), the equivalent is to connect a resistor R in parallel across the filter capacitor_c2And in time, the values of the parameter k and the damping coefficient zeta are respectively as follows:

relationship between damping coefficient and stability of system

In order to improve the suppression effect on the resonance peak on the basis of the control scheme, the parameter k of the feedback function a can be further improved, so that the damping coefficient ζ of the system is improved.

According to the above, when the virtual parallel resistor R_c2When the value is 5 Ω, the following equation (10) can be obtained: the damping of the equivalent second-order system is 0.4714, which means that the system is an underdamped system and shows stable system characteristics.

Therefore, in order to further improve the stability of the system, the gain of the feedback function is modified based on the purpose of increasing the damping coefficient of the system, so as to achieve the purpose of optimal damping of a second-order system.

The value of the proportional coefficient k changing with the change of the damping coefficient zeta can be calculated by the formula (6), in order to verify the relation between the damping coefficient zeta and the suppression effect of the LCL inherent resonance peak, the damping coefficient zeta is set to be 0.6-1 in FIG. 11, and a comparison graph of a control method for revising the damping coefficient and a control method for virtually connecting 5 omega resistors in parallel is shown.

From fig. 11, the following conclusions can be drawn: as the damping coefficient ζ increases, the suppression effect in the vicinity of the original LCL intrinsic resonance peak also increases, and the low-frequency and high-frequency characteristics of the system are not affected, which means that the bandwidth of the system is not affected.

Zeta value of 0.707 can be taken when applied in engineering by integrating dynamic performance and resonance peak suppression capability of the system.

Simulation results and analysis

In order to verify the effectiveness and feasibility of the proposed active damping control method, a simulation model of a single inverter current feedback system is built through Matlab/Simulink.

Fig. 12 shows the values of THD when the system is in the absence of any control method, virtually parallel to the 5 Ω resistance, and adjusting the damping coefficient ζ, respectively.

Fig. 12 shows that the grid-side current i is due to the presence of the system's natural resonance peak when no control method is taken₂The THD of (a) is 78.13%, while when the control method of the virtual parallel 5 Ω resistance which was originally proposed is adopted，i₂The THD of (D) was reduced to 6.02%. And as ζ increases from 0.6 to 1, i₂The THD of the product is respectively reduced to 4.90%, 4.21%, 3.73% and 3.02%. The simulation results are in accordance with the conclusions drawn in fig. 11.

Conclusion

The existing active damping scheme cannot obtain good balance between the resonance peak suppression effect and the bandwidth, so that the invention derives a single inverter current feedback control method which effectively suppresses the inherent resonance peak of the LCL inverter and does not influence the system bandwidth from the angle of increasing the damping coefficient zeta. The control method is not like a conventional network side current feedback active damping method, and can amplify high-frequency harmonic current. And unlike the improved method of introducing a first-order high-pass filter network side current feedback active damping and the existing method of single inverter current feedback resonance suppression, the method needs to move the inherent resonance peak to achieve good suppression effect. The control method can be equivalent to a passive damping scheme of connecting resistors in parallel at two ends of a filter capacitor, so that the influence on low-frequency and high-frequency bands is small while a good resonance peak suppression effect is achieved, and the bandwidth of a system cannot be influenced.

By analyzing the zeta value actually introduced by the virtual parallel resistance scheme, the suppression effect on the LCL intrinsic resonance peak is enhanced along with the increase of zeta. Finally, the superiority of the method is verified through simulation.