A static error compensation method for grid-connected current

Abstract

The invention discloses a static error compensation method for grid-connected current. Grid connection refers to that the output of an LCL type grid-connected inverter is connected to a grid after being filtered by an LCL filter. A virtual resistor is connected in parallel to a capacitor branch of the LCL filter. The method of the invention introduces compensation currents of capacitor voltage feedforward and instantaneous power feedforward to given values ​​of grid-connected current of a d-axis and a q-axis respectively, and coordinates and compensates the grid-connected current with a certain weight coefficient to eliminate the static error of the grid-connected current when the capacitor branch is connected in parallel with the virtual resistor. Compared with the existing static error compensation method, it does not need to introduce other controllers and is not affected by changes in grid impedance. While effectively suppressing resonance, it maintains a good static error compensation effect to ensure safe and reliable operation of the system.

Description

Technical Field

[0001] The present invention relates to the field of new energy technology, and in particular to a static error compensation method for grid-connected current. Background Art

[0002] Due to the volatility and randomness of renewable energy, it cannot be directly used as a power source and needs to be connected to the grid through an inverter. As a large number of power electronic equipment are connected to the grid, the grid begins to gradually show weak grid characteristics such as low short-circuit ratio and low inertia. When the grid-connected inverter is connected to the weak grid, the inverter's filter control loop is coupled with the grid impedance, resulting in resonance, which affects the reliable and stable operation of the grid-connected inverter and the grid.

[0003] In weak power grids, the robustness of inverters to grid impedance has been widely used by changing the inverter's own control strategy, such as active damping technology, which constructs virtual impedance by introducing state variables (such as capacitor current feedback) in the current control loop. However, under different degrees of weak power grids, the accuracy of virtual impedance is often difficult to guarantee, which will lead to poor suppression of resonance and the resonance stability of power electronic equipment at the PCC (Point of The Common Coupling) point. Therefore, research on improving virtual resistance characteristics is of great significance.

[0004] Common solutions to improve the robustness of inverters to grid impedance are generally based on improving virtual resistance characteristics, which generally include feedforward control or adaptive control. Feedforward control can improve the dynamic response and stability of the system and reduce steady-state errors, but it has high requirements for model accuracy and cannot handle unknown disturbances. Adaptive control can significantly improve the adaptability and performance optimization capabilities of the system, but the design is complex and the amount of calculation is large.

[0005] Therefore, it is necessary to design a static error compensation method for grid-connected current to solve the problems existing in the prior art. Summary of the invention

[0006] The purpose of the present invention is to solve the shortcomings in the prior art and to propose a static error compensation method for grid-connected current.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A static error compensation method for grid-connected current, wherein the grid-connected current refers to the output of an LCL type grid-connected inverter being filtered by an LCL filter and then connected to the grid, and a virtual resistor is connected in parallel to the capacitor branch of the LCL filter. The method comprises the following steps:

[0009] Step 1: Establish a mathematical model of the LCL grid-connected inverter;

[0010] Based on the single feedforward control method of filter capacitor current, the control structure block diagram when the capacitor branch is connected in parallel with a virtual resistor is derived;

[0011] Step 2: Add the compensation current corresponding to the capacitor voltage feedforward at the given current before compensation of the current loop, supplemented by the weight coefficient α; and add the compensation current corresponding to the instantaneous power feedforward, supplemented by the weight coefficient (1-α), where α is expressed by formula (11):

[0012] (11);

[0013] Among them, Δi d1 is the compensation current corresponding to the d-axis capacitor voltage feedforward, Δi q1 is the compensation current corresponding to the q-axis capacitor voltage feedforward; Δi d2 is the compensation current corresponding to the instantaneous active power feedforward, Δi q2 is the compensation current corresponding to the instantaneous reactive power feedforward;

[0014] Step 3: Calculate the compensated current setpoint.

[0015] Furthermore, the step 1 comprises:

[0016] Grid side inductance L xm The calculation formula is as follows:

[0017] (1);

[0018] Among them, L x2 is the grid-side x-phase inductance of the LCL filter, L xg is the equivalent inductance of the x-phase line of the power grid, x=a, b, c;

[0019] On the basis of grid-connected current proportional control, a virtual resistor is connected in parallel to the capacitor branch to obtain the control structure block diagram of the LCL filter.

[0020] Furthermore, the step 2 comprises:

[0021] Given active current i before compensation in the current control loop d0 * The compensation current Δi corresponding to the d-axis capacitor voltage feedforward is increased d1 , supplemented by the weight coefficient α; the total current of the capacitor branch at power frequency I c for:

[0022] (2);

[0023] Among them, U cd is the d-axis voltage on the capacitor branch, Z Cis the total impedance of the capacitor branch at the power frequency, R c is the impedance of the virtual resistor in parallel with the capacitor.

[0024] Furthermore, the I c The compensation current Δi corresponding to the d-axis capacitor voltage feedforward is obtained by filtering with a first-order low-pass filter. d1 As shown in formula (3):

[0025] (3);

[0026] Among them, ω 0 is the corner frequency of the first-order low-pass filter, and s is a complex variable used to describe the response of the system in the complex domain.

[0027] Furthermore, the reactive current i is given before compensation in the current control loop. q0 * The compensation current Δi corresponding to the q-axis capacitor voltage feedforward is increased q1 , supplemented by a weight coefficient α; the Δi q1 The calculation formula is as follows (4):

[0028] (4);

[0029] Among them, U cq is the q-axis voltage on the capacitor branch.

[0030] Furthermore, the active current i is given before compensation in the current control loop d0 * Increase the compensation current Δi corresponding to the instantaneous active power feedforward d2 , and assisted by the weight coefficient (1-α); negative feedback closed-loop control is performed on the active power required by the system.

[0031] Further, the Δi is obtained by adjusting the PI controller d2 as follows:

[0032] (5);

[0033] (6);

[0034] Among them, P * is the given value of active power, k p , k i is the proportional coefficient and integral coefficient of the PI controller, P 0 is the actual active power of the system, u d 、u q 、i d 、i qThey are the d-axis and q-axis components of the voltage and current of the power grid at the common coupling point, respectively.

[0035] Furthermore, the reactive current i is given before compensation in the current control loop. q0 * Increase the compensation current Δi corresponding to the instantaneous reactive power feedforward q2 , and supplemented by the weight coefficient (1-α); negative feedback closed-loop control is performed on the reactive power required by the system, and the Δi is obtained by adjusting the PI controller q2 as follows:

[0036] (7);

[0037] (8);

[0038] Among them, Q * is the reactive power given value, Q 0 is the actual reactive power of the system.

[0039] Furthermore, the step 3 is: obtaining the current given value after the grid-connected current static error compensation by calculation, the specific method is as follows: Combining equation (3) and equation (5), the given active current i before compensation in the current loop is d0 * Increase the Δi d1 and the Δi d2 After that, the given active current i is obtained after compensation d * , the expression is:

[0040] (9).

[0041] Furthermore, combining equations (4) and (7), the given reactive current i before compensation in the current loop is q0 * Increase the Δi q1 and Δi q2 After compensation, the given reactive current i q * , the expression is:

[0042] (10).

[0043] Compared with the prior art, the static error compensation method of grid-connected current provided by the present invention has the following advantages:

[0044] The present invention provides a static error compensation method for grid-connected current, which introduces compensation currents of capacitor voltage feedforward and instantaneous power feedforward to the given values ​​of grid-connected current of d-axis and q-axis, respectively, and coordinates and compensates the grid-connected current with a certain weight coefficient to eliminate the static error of the grid-connected current when the capacitor branch is connected in parallel with a virtual resistor. Compared with the existing static error compensation method, the method proposed by the present invention does not need to introduce other controllers, and even if the grid impedance changes, it can effectively suppress the resonance while maintaining a good static error compensation effect, thereby ensuring the safe and reliable operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0046] Figure 1 It is a schematic diagram of the grid connection of the LCL type grid-connected inverter targeted by the present invention;

[0047] Figure 2 It is the mathematical model of LCL grid-connected inverter when virtual resistance is connected in parallel with capacitor branch;

[0048] Figure 3 This is the current loop control structure block diagram of the LCL type grid-connected inverter after current static error compensation;

[0049] Figure 4 It is a flow chart of current static error compensation of LCL type grid-connected inverter.

[0050] The symbols in the figure are defined as follows:

[0051] L x1 is the x-phase inductance of the inverter side of the LCL filter; L x2 is the x-phase inductance of the grid side of the LCL filter; i x2 is the x-phase grid-connected current; L xg is the equivalent inductance of the x-phase power grid line; L xm is the total inductance of the x-phase power grid line; C x is the x-phase capacitance of the LCL filter; i cx is the x-phase capacitor current; U dc is the DC voltage at the inverter input side; u cx is the x-phase capacitor voltage; R c is a virtual resistor connected in parallel with the capacitor, and also represents the impedance of the virtual resistor; e x is the x-phase grid voltage; i d0 *is the given active current before compensation, i q0 * is the given reactive current before compensation; Δi d1 is the compensation current corresponding to the d-axis capacitor voltage feedforward; Δi q1 is the compensation current corresponding to the q-axis capacitor voltage feedforward; Δi d2 is the compensation current corresponding to the instantaneous active power feedforward; Δi q2 is the compensation current corresponding to the instantaneous reactive power feedforward; α is the weight coefficient; i d * is the given active current after compensation, i q * For the given reactive current after compensation, x=a, b, c. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Example

[0053] The present invention provides a static error compensation method for grid-connected current, which specifically comprises the following steps:

[0054] Step 1: Establish a mathematical model of the LCL grid-connected inverter.

[0055] Based on the single feedforward control method of filter capacitor current, the control structure block diagram when the capacitor branch is connected in parallel with the virtual resistor is derived, which is as follows:

[0056] See also Figure 1 , Figure 1 The topology diagram of the LCL type grid-connected inverter targeted by the present invention is shown in the figure. In the figure, the inverter can be a two-level inverter, a T-type three-level inverter, etc. The output of the inverter is filtered by the LCL filter and then connected to the grid. Each LCL includes L x1 , L x2 , C x , C x The branch is a capacitor branch, each C x Both are connected in parallel with a virtual resistor R c , or the capacitor branch is connected in parallel with a virtual resistor R c .

[0057] Figure 1 In, L x1 is the x-phase inductance of the inverter side of the LCL filter; L x2 is the x-phase inductance of the grid side of the LCL filter; L xg is the equivalent inductance of the x-phase power grid line; C xis the x-phase capacitance of the LCL filter; U dc is the DC voltage at the inverter input side; R c With capacitor C x Virtual resistance in parallel; e x is the x-phase grid voltage;

[0058] Definition of grid-side inductance L xm Mainly by L xg and L x2 It consists of two parts, and the calculation formula is as follows:

[0059] (1);

[0060] Among them, L x2 is the grid-side x-phase inductance of the LCL filter, L xg is the equivalent inductance of the x-phase line of the power grid, x=a, b, c,

[0061] See also Figure 2 , Figure 2 It is the mathematical model of LCL grid-connected inverter when the capacitor branch is connected in parallel with virtual resistance. It introduces a capacitor current feedback branch based on the traditional grid-connected current control block diagram. The grid-connected current error is calculated through the transfer function G of the PI controller. c(S) , capacitor current feedback transfer function and the mathematical model of the LCL filter form a closed-loop structure. Step 2: Add the compensation current corresponding to the capacitor voltage feedforward at the given current before compensation of the current loop, supplemented by the weight coefficient α; and add the compensation current corresponding to the instantaneous power feedforward, supplemented by the weight coefficient (1-α), α is expressed by formula (11):

[0062] (11);

[0063] Specifically:

[0064] See also Figure 3 , Figure 3 It is a block diagram of the current loop control structure of the LCL type grid-connected inverter after current static error compensation; in order to solve the problem of grid-connected current static error when the capacitor branch is connected in parallel with a virtual resistor, a more accurate grid-connected current is obtained without introducing other controllers, and a method can still maintain a good static error compensation effect after the grid impedance changes;

[0065] First, the active current i is given before compensation in the current control loop d0 * The compensation current Δi corresponding to the d-axis capacitor voltage feedforward is increased d1 , supplemented by the weight coefficient α; Δi d1 The derivation process is as follows:

[0066] Since the virtual resistance R c When it is smaller, the resonance suppression effect is good, so the total impedance Z of the capacitor branch at the power frequency is c The impedance of the virtual resistor R c Approximately equal, so the total current of the capacitor branch at the power frequency (such as 50-60Hz, etc.) is:

[0067] (2);

[0068] Among them, U cd is the d-axis voltage on the capacitor branch;

[0069] The total current Ic of the capacitor branch is filtered by a first-order low-pass filter to obtain the compensation current Δi corresponding to the d-axis capacitor voltage feedforward. d1 , the calculation formula is as follows:

[0070] (3);

[0071] Among them, ω 0 is the corner frequency of the first-order low-pass filter, and its value is ω 0 =2π×f 0 , f 0 is the corner frequency of the first-order low-pass filter; s is a complex variable used to describe the response of the system in the complex domain.

[0072] Similarly, the reactive current i is given before compensation in the current control loop q0 * The compensation current Δi corresponding to the q-axis capacitor voltage feedforward is increased q1 , supplemented by the weight coefficient α; the compensation current Δi corresponding to the q-axis capacitor voltage feedforward q1 The calculation formula is as follows:

[0073] (4);

[0074] Among them, U cq is the q-axis voltage on the capacitor branch;

[0075] Secondly, the active current i is given before compensation in the current control loop d0 * Increase the compensation current Δi corresponding to the instantaneous active power feedforward d2 , supplemented by the weight coefficient (1-α); Δi d2 The way to obtain is as follows:

[0076] The active power required by the system is negatively feedback closed-loop controlled, and the compensation current Δi corresponding to the instantaneous active power feedforward is obtained through PI controller adjustment. d2 :

[0077] (5);

[0078] (6);

[0079] Among them, P * is the given value of active power, k p , k i is the proportional coefficient and integral coefficient of the PI controller, P 0 is the actual active power of the system, u d 、u q 、i d 、i q The common coupling point u pcc The d-axis and q-axis components of the voltage and current of the power grid;

[0080] Similarly, given the reactive current i before compensation in the current control loop q0 * Increase the compensation current Δi corresponding to the instantaneous reactive power feedforward q2 , and supplemented by the weight coefficient (1-α); negative feedback closed-loop control is performed on the reactive power required by the system, and the compensation current Δi corresponding to the instantaneous reactive power feedforward is obtained by adjusting the PI controller q2 :

[0081] (7);

[0082] (8);

[0083] Among them, Q * is the reactive power given value, Q 0 is the actual reactive power of the system.

[0084] Step 3, calculate the current given value after compensation:

[0085] The current given value after grid-connected current static error compensation is obtained by the formula. The specific calculation method is as follows:

[0086] Combining equations (3) and (5), the given active current i before compensation in the current loop is d0 * Increase the compensation current Δi corresponding to the d-axis capacitor voltage feedforward d1 The compensation current Δi corresponding to the instantaneous active power feedforward d2 After that, the given active current i is obtained after compensation d * , the expression is:

[0087] (9).

[0088] Combining equations (4) and (7), the given reactive current i before compensation in the current loop is q0 * Increase the compensation current Δi corresponding to the q-axis capacitor voltage feedforward q1 The compensation current Δi corresponding to the instantaneous reactive power feedforward q2 After that, the given reactive current i after compensation is obtained. q * , the expression is:

[0089] (10).

[0090] Among them, α is the weight coefficient, α is between 0 and 1, and α can be expressed by formula (11).

[0091] (11);

[0092] Formula (11) shows that the value of α is determined according to the proportion of the voltage feedforward compensation and the power compensation in affecting the current error.

[0093] See also Figure 3 , the given active current value after compensation i d * And given reactive current value i q * The actual d-axis and q-axis current components i d 、i q Then, the error values ​​of the two are obtained by making the difference. Then, they are adjusted by their respective PI controllers, respectively combined with their respective single capacitor current feedforward I cd ,I cq And the respective grid voltage feedforward u d 、u q , get the target voltage v d 、v q . d 、v q Perform an inverse transformation of the synchronous rotating coordinates to obtain a modulation wave in a three-phase stationary coordinate system. Compared with the common triangular carrier, a three-phase pulse width modulation signal is obtained to control the corresponding switch tube.

[0094] In the aforementioned embodiment, the inverter is a three-phase inverter. In other embodiments, the inverter is replaced by a single-phase inverter and only needs to be processed as a single-phase inverter.

[0095] Therefore, the compensation method for the static error of the grid-connected current when the capacitor branch of the LCL filter is connected in parallel with the virtual resistor introduces the compensation current of the capacitor voltage feedforward and the instantaneous power feedforward to the given values ​​of the grid-connected current of the d-axis and q-axis, respectively, and is supplemented by a certain weight coefficient to coordinate the compensation of the grid-connected current to eliminate the static error of the grid-connected current when the capacitor branch is connected in parallel with the virtual resistor. Compared with the existing static error compensation method, the method proposed by the present invention does not need to introduce other controllers, and even if the grid impedance changes, it can maintain a good static error compensation effect while effectively suppressing the resonance, thereby ensuring the safe and reliable operation of the system.

[0096] The following are some basic concepts of key words involved in the present invention:

[0097] Weak power grid: Its main electrical characteristics can be summarized as low short-circuit capacity of the grid-connected system, large impedance of the line, and rich harmonic content of the background voltage at the public grid-connected point. There are endless definitions of "weak power grid", and there is no particularly clear and unified definition. In general, the strength of the three-phase power grid is expressed by the short circuit ratio (SCR), and formula (12) is the SCR calculation formula. Generally speaking, the industry believes that a power grid with an SCR less than 10 is a weak power grid.

[0098] (12);

[0099] In the formula, S ac is the short-circuit capacity of the grid, P dN is the rated power of the grid-connected inverter, |Z| is the modulus of the grid impedance at the power frequency, and U N is the effective value of the grid voltage.

[0100] Grid-connected inverter: A grid-connected inverter is a device that converts DC power into AC power and transmits it to the power grid. It is mainly used in photovoltaic power generation systems, wind power generation systems and other renewable energy power generation systems to convert the DC power generated by these systems into AC power that meets the requirements of the power grid and transmit it to the public power grid through the grid connection point. The grid-connected inverter has the function of synchronizing the voltage and frequency of the grid to ensure that the output AC power can be seamlessly combined with the power grid. In addition, it also has protection functions such as preventing islanding effects and improving power quality to ensure the power safety and stability of the power grid and the user end.

[0101] LCL filter: LCL filter is a filter used in power systems to reduce harmonics and interference in power grids or equipment such as inverters. It consists of an inductor (L), a capacitor (C) and an inductor (L), forming a series LC filter network. The function of the LCL filter is to filter out harmonics in the power system by adjusting the parameters of the inductor and capacitor and maintain the required current quality. It can effectively suppress harmonics and reduce interference in the power grid, and improve the power factor and voltage waveform quality of the power grid. Compared with other filters, LCL filters have higher harmonic suppression capabilities and response speeds, and can better adapt to changes in harmonics and interference in power systems. However, LCL filters also have some challenges, such as difficulties in parameter adjustment and inherent resonance peak suppression, which require reasonable design and optimization to play the best role.

[0102] First-order low-pass filter: A first-order low-pass filter is a basic filter used to allow low-frequency signals to pass through while attenuating high-frequency signals. It consists of a resistor (R) and a capacitor (C), usually in an RC circuit structure. In a first-order low-pass filter, the combination of resistor and capacitor determines the filter's cutoff frequency (also called corner frequency or turn frequency), which is usually expressed as: , where (f c） is the cut-off frequency, (R) is the resistance, and (C) is the capacitance. Nowadays, digital filters are mostly used to implement them.

[0103] Active damping: Active damping refers to a damping method that is achieved by adding components such as resistors to a vibration system. Usually, the controlled object vibrates, and active damping reduces the vibration amplitude through active interference.

[0104] Virtual Impedance: Virtual Impedance is a technology used in the field of power systems and power electronics. It simulates and implements the impedance characteristics in the circuit through software algorithms. Its main purposes include improving system stability, improving power quality, and achieving power sharing.

[0105] Static Error: Static Error refers to the deviation between the actual output and the expected output under steady-state conditions. It is the error that still exists in the system after a long enough period of time when the input signal remains unchanged. Static error is usually used to measure the accuracy of the control system when it reaches steady state. Static error reflects the inherent characteristics of the system, such as gain, zeros and poles.

[0106] Synchronous Rotating Reference Frame DQ Control: Synchronous Rotating Reference Frame DQ Control is a control strategy commonly used in the field of power electronics and motor control, mainly used in AC motor drive systems and converter control. Synchronous Rotating Reference Frame DQ Control: In AC motor and converter control, a coordinate system that rotates synchronously with the grid frequency is usually selected to describe the system. This coordinate system is usually called the "dq coordinate system", in which the d-axis is synchronized with the grid phase and the q-axis is perpendicular to the d-axis. The use of a synchronous rotating coordinate system can simplify the mathematical model of the system and make the control more intuitive and convenient. DQ control is a control strategy performed in the synchronous rotating coordinate system dq, which controls the three-phase AC system by transforming the variables of the system into the dq coordinate system. In the dq coordinate system, the three-phase AC system can be converted into a balanced two-phase system, in which the d-axis component represents the main component of the system, and the q-axis component represents the secondary component of the system.

[0107] Instantaneous power control: Instantaneous power control is a commonly used control method in power electronic systems, mainly used in motor drive, grid connection and other fields. Its main principle is as follows: the instantaneous active power and reactive power of the system are calculated according to the instantaneous values ​​of voltage and current, and the command values ​​of active power and reactive power are generated according to the operation requirements of the system. The calculated instantaneous power is compared with the command power, and the real-time tracking control of power is achieved by adjusting the switching state of the power conversion device. This control method has the characteristics of fast response speed and high control accuracy, which can effectively control the power flow of the power electronic system and improve the dynamic performance and stability of the system.

[0108] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A static error compensation method for grid-connected current, wherein the output of an LCL-type grid-connected inverter is filtered by an LCL filter and then connected to the grid, and a virtual resistor is connected in parallel to the capacitor branch of the LCL filter, characterized in that: The method comprises the following steps: Step 1: Establish a mathematical model of the LCL grid-connected inverter; Based on the single feedforward control method of filter capacitor current, the control structure block diagram when the capacitor branch is connected in parallel with a virtual resistor is derived; Step 2: Add the compensation current corresponding to the capacitor voltage feedforward at the given current before compensation of the current loop, supplemented by the weight coefficient α; and add the compensation current corresponding to the instantaneous power feedforward, supplemented by the weight coefficient (1-α), where α is expressed by formula (11): (11)； Among them, Δi d1 is the compensation current corresponding to the d-axis capacitor voltage feedforward, Δi q1 is the compensation current corresponding to the q-axis capacitor voltage feedforward; Δi d2 is the compensation current corresponding to the instantaneous active power feedforward, Δi q2 is the compensation current corresponding to the instantaneous reactive power feedforward; Step 3: Calculate the compensated current setpoint.

2. A method for compensating a static error of a grid-connected current according to claim 1, characterized in that: The step 1 comprises: Grid side inductance L xm The calculation formula is as follows: (1)； in, L x2 The grid side of the LCL filter x Phase inductance, L xg For the power grid x Phase line equivalent inductance, x =a, b, c; On the basis of the grid-connected current proportional control, the capacitor branch is connected in parallel with a virtual resistor, and a control structure block diagram when the capacitor branch is connected in parallel with the virtual resistor is obtained.

3. The static error compensation method of grid-connected current according to claim 2, characterized in that: The step 2 comprises: Given the active current i before compensation in the current control loop d0 * The compensation current Δi corresponding to the d-axis capacitor voltage feedforward is increased d1 , supplemented by the weight coefficient α; the total current of the capacitor branch at power frequency I c for: (2)； in, U cd is the d-axis voltage on the capacitor branch, Z C is the total impedance of the capacitor branch at the power frequency, R c is the impedance of the virtual resistor in parallel with the capacitor.

4. The static error compensation method of grid-connected current according to claim 3, characterized in that: I c The compensation current Δi corresponding to the d-axis capacitor voltage feedforward is obtained by filtering with a first-order low-pass filter. d1 As shown in formula (3): (3)； Among them, ω0 is the corner frequency of the first-order low-pass filter, and s is a complex variable used to describe the response of the system in the complex domain.

5. The static error compensation method of grid-connected current according to claim 4, characterized in that: Given reactive current i before compensation in the current control loop q0 * The compensation current Δi corresponding to the q-axis capacitor voltage feedforward is increased q1 , supplemented by a weight coefficient α; the Δi q1 The calculation formula is as follows (4): (4)； in, U cq is the q-axis voltage on the capacitor branch.

6. A method for compensating a static error of a grid-connected current according to claim 5, characterized in that: Given the active current i before compensation in the current control loop d0 * Increase the compensation current Δi corresponding to the instantaneous active power feedforward d2 , and assisted by the weight coefficient (1-α); negative feedback closed-loop control is performed on the active power required by the system.

7. A method for compensating a static error of a grid-connected current according to claim 6, characterized in that: The Δi is obtained by adjusting the PI controller d2 as follows: (5)； (6)； Among them, P * is the given value of active power, k p , k i are the proportional coefficient and integral coefficient of the PI controller, P0 is the actual active power of the system, u d 、u q 、i d 、i q They are the d-axis and q-axis components of the voltage and current of the power grid at the common coupling point, respectively.

8. The method for compensating a static error of a grid-connected current according to claim 7, characterized in that: Given reactive current i before compensation in the current control loop q0 * Increase the compensation current Δi corresponding to the instantaneous reactive power feedforward q2 , and supplemented by the weight coefficient (1-α); negative feedback closed-loop control is performed on the reactive power required by the system, and the Δi is obtained by adjusting the PI controller q2 as follows: (7)； (8)； Among them, Q * is the given value of reactive power, Q0 is the actual reactive power of the system.

9. The method for compensating a static error of a grid-connected current according to claim 8, characterized in that: The step 3 is: to obtain the current given value after the grid-connected current static error compensation by calculation. The specific method is as follows: Combining equation (3) and equation (5), the given active current i before compensation in the current loop is d0 * Increase the Δi d1 and the Δi d2 After that, the given active current i is obtained after compensation d * , the expression is: (9)。 10. A method for compensating a static error of a grid-connected current according to claim 9, characterized in that: Combining equations (4) and (7), the given reactive current i before compensation in the current loop is q0 * Increase the Δi q1 and Δi q2 After compensation, the given reactive current i q * , the expression is: (10) 。

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