LC / LCL type single-converter-side current control method

Abstract

The application discloses an LC / LCL type single-converter-side current control method, which comprises the following steps: firstly, collecting the voltage of the capacitor in the LC / LCL filter, locking the phase by using a phase-locked loop, and passing through an active damping module; then, passing the obtained capacitor voltage through a digital differentiator to obtain an estimated capacitor current; secondly, collecting the current of the converter-side inductor in the LC / LCL filter, and subtracting the converter-side inductor current from the estimated capacitor current to obtain an estimated grid-side output current; thirdly, subtracting the estimated grid-side output current from a current controller reference signal to obtain a current controller input error signal, and sending the calculated error signal into the current controller; and finally, subtracting the output of the active damping module from the output of the current controller to obtain a PWM wave modulation instruction signal.

Description

Technical Field

[0001] This invention belongs to the field of grid-connected converter control technology, and more specifically, relates to a passive LC / LCL type single converter side current control method with enhanced current quality. Background Technology

[0002] With the widespread application of power electronics technology in various fields related to power systems, modern society has increasingly higher requirements for the safety, stability, and power supply quality of power systems. As the power conversion interface between distributed generation and the grid, grid-connected converters have been widely researched and applied. Compared to L-type filters, LC / LCL filters are more widely used due to their superior switching ripple attenuation characteristics. In LC / LCL grid-connected converters, two current feedback control modes are typically used: converter-side current feedback (ICF) and grid-side output current feedback (GCF). Compared to GCF, ICF can directly feedback the inverter-side current without additional sensors, facilitating overcurrent protection and reducing system cost and size, making it more attractive in industrial applications. Furthermore, ICF offers advantages in f... s The inherent damping characteristics of the GCF type at frequencies below / 6 can ensure system stability under weak grid conditions, while the GCF type requires additional damping.

[0003] Output current quality is a crucial issue for grid-connected converters, attracting significant research attention. High-gain controllers, offering high-precision control, are widely used in grid-connected converters with stringent power quality requirements. Examples include proportional-resonant (PR) and repetitive controllers. High-gain controllers have proven effective in ensuring power quality in GCF-type grid-connected converters because they directly control the grid-side output current. However, high-gain controllers are insufficient for ICF-type converters to achieve high power quality output because the grid-side output current is indirectly controlled, allowing harmonic currents to freely flow into the filter capacitors. To improve the output power quality of ICF-type grid-connected converters, researchers have proposed grid voltage feedforward and capacitor voltage feedforward strategies to suppress interference from the grid voltage. However, the voltage feedforward scheme cannot handle nonlinear factors originating from the grid-connected converter itself. Additionally, a capacitor current compensation scheme based on a differentiator is proposed. This scheme inherits the advantages of the feedforward strategy while addressing the system's own nonlinearity; however, it uses multiple resonant controllers, inevitably increasing the controller's computational burden. Furthermore, although some progress has been made in improving the output power quality of ICF-type grid-connected converters, the impact of the proposed methods on the external stability of the system is rarely discussed in research. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide an LC / LCL type single converter side current control method. By utilizing the high precision performance of the repetitive controller, the external stability of the system is guaranteed through passive design, and no additional sensors are required.

[0005] To achieve the above-mentioned objective, the present invention provides a current control method for an LC / LCL type single converter, characterized by comprising the following steps:

[0006] (1) Acquire the capacitor voltage in the LC / LCL filter Phase-locked loop is used to lock the phase, and the phase is locked to the phase using a phase-locked loop. conduct Coordinate transformation, to obtain Capacitor voltage in coordinate system ;

[0007] (2) Collect the current of the converter-side inductor in the LC / LCL filter. ,right conduct Coordinate transformation, to obtain Converter-side inductor current in coordinate system ;

[0008] (3) Convert the capacitor voltage The estimated capacitance current is obtained through the digital differentiator D. Then, the inductor current on the converter side... With the estimated capacitor current By subtracting the values, we can obtain the estimated output current. ;

[0009] (4) Set the reference current of the current controller With the estimated output current The difference is used to obtain the input error signal of the current controller. Then the error signal The current output control quantity is obtained through the current controller. ;

[0010] (5) Set the capacitor voltage The input is given to the active damper, through which the capacitor voltage is... Multiplying by the active damping transfer function, we get Active damping feedback quantity in coordinate system ;

[0011] (6) The output control quantity of the current controller With active damping feedback Subtract, and then pass the result through Inverse coordinate transformation yields the modulation command value. ;

[0012] (7) The PWM module modulates the modulation command value. The corresponding switching control signals of the switching transistors are obtained and used to control the turn-on and turn-off of each IGBT in the active power filter.

[0013] The objective of this invention is achieved as follows:

[0014] This invention discloses a current control method for a single converter side of an LC / LCL filter. First, the voltage of the capacitor in the LC / LCL filter is acquired, and phase-locked loop (PLL) is used for phase locking, followed by an active damping module. The obtained capacitor voltage is then passed through a digital differentiator to obtain an estimated capacitor current. Next, the current of the converter-side inductor in the LC / LCL filter is acquired. The difference between the converter-side inductor current and the estimated capacitor current is calculated to obtain an estimated grid-side output current. The difference between the current controller reference signal and the estimated grid-side output current is calculated to obtain the current controller input error signal, which is then sent to the current controller. The current controller output control quantity and the output of the active damping module are subtracted to obtain a PWM wave modulation command signal.

[0015] Meanwhile, the LC / LCL type single converter side current control method of the present invention also has the following beneficial effects:

[0016] (1) The present invention retains the inherent overcurrent protection capability of the ICF type LC / LCL grid-connected converter.

[0017] (2) The design of capacitor current estimation based on digital differentiator and current control based on repetitive controller in this invention provides an economical solution for ICF type LC / LCL grid-connected converter and has good harmonic suppression performance.

[0018] (3) This invention extends the passive region of the ICF type LC / LCL grid-connected converter to near the Nyquist frequency, thereby enhancing the passivity and improving the stability of the system under complex power grid conditions.

[0019] (4) The current control loop designed in this invention consists of a digital differentiator and a current controller. The current controller is an insertion repetitive controller. Its two important parameters, namely the repetitive controller gain and the number of lead beats of the lead element, are designed according to the internal stability design and external stability design of the system, respectively. The digital differentiator is a digital differentiator based on backward Euler plus lead compensator, which has a good differential fitting effect. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a current control method for an LC / LCL type single converter side according to the present invention;

[0021] Figure 2 This is a block diagram of an insert-repeat controller;

[0022] Figure 3 These are experimental waveforms of the grid-connected converter using the present invention under weak grid conditions;

[0023] Figure 4 These are experimental waveforms of a grid-connected converter without using an insert repetitive controller under grid voltage distortion conditions.

[0024] Figure 5 The experimental waveforms are obtained by using an inserted repetitive controller but without using the estimated grid-side output current feedback method under grid voltage distortion conditions in a grid-connected converter.

[0025] Figure 6 These are experimental waveforms of a grid-connected converter using an inserted repetitive controller and an estimated grid-side output current feedback method under grid voltage distortion conditions. Detailed Implementation

[0026] The specific embodiments of the present invention will now be described with reference to the accompanying drawings to enable those skilled in the art to better understand the invention. It should be particularly noted that in the following description, detailed descriptions of known functions and designs that might obscure the main content of the invention will be omitted here.

[0027] Example

[0028] Figure 1 This is a schematic diagram of a passive LC / LCL type single converter side current control design method for enhancing current quality and current performance, according to the present invention.

[0029] In this embodiment, as Figure 1 As shown, the grid-connected converter consists of two parts: the main circuit and the control system. Figure 1 The dotted line frame is the control area.

[0030] The main circuit consists of converter main circuit 1 and LCL filter 2. Converter main circuit 1 is connected to the power grid through LCL filter 2, thus forming a complete grid-connected converter main circuit.

[0031] The control section consists of: PLL phase-locked loop module 3, active damper 4, current controller 5, digital differentiator 6, and PWM module 7, which constitute the control section of the grid-connected converter.

[0032] The entire grid-connected converter system includes the main circuit section and the control section mentioned above. The internal delay of the system includes two parts: calculation delay and modulation delay. The total delay is 1.5 times the sampling period. The port output admittance Y of the system is the ratio of the output current to the capacitor voltage.

[0033] The following is combined Figure 1 The present invention will be described in detail below, specifically including the following steps:

[0034] S1. Acquire the capacitor voltage in the LCL filter. Phase-locked loop is used to lock the phase, and the phase is locked to the phase using a phase-locked loop. conduct Coordinate transformation, to obtain Capacitor voltage in coordinate system .

[0035] S2. Collect the current of the converter-side inductor in the LCL filter. ,right conduct Coordinate transformation, to obtain Converter-side inductor current in coordinate system .

[0036] S3, obtain the capacitor voltage The estimated capacitance current is obtained through the digital differentiator D. Then, the inductor current on the converter side... With the estimated capacitor current By subtracting the values, we can obtain the estimated output current. .

[0037] S4. Transfer the current controller reference signal. With the estimated output current The difference is used to obtain the input error signal of the current controller. The calculated error signal The current output control quantity is obtained through the current controller. .

[0038] S5, change the capacitor voltage The input is given to the active damper, through which the capacitor voltage is... Multiplying by the active damping transfer function, we get Active damping feedback quantity in coordinate system .

[0039] S6, output control quantity from the current controller Active damping feedback quantity Subtract, then pass the result through Inverse coordinate transformation yields the modulation command value. .

[0040] S7, the PWM module uses the modulation command value obtained in step S6. The corresponding switching control signal of the switching transistor is obtained, and then the switching control signal is used to control the turn-on and turn-off of each IGBT in the active power filter.

[0041] In this embodiment, the following is combined with Figure 2 The current controller (insertion repetitive controller) used is described in detail.

[0042] The transfer function of the insertive repetitive controller is:

[0043] ;

[0044] in, For proportional gain; For low-pass filter ,constant and The values ​​are 0.5 and 0.25 respectively; To control the gain repeatedly, This is the ratio of the sampling frequency to the power grid's base frequency. For compensators, for a purely leading element , This refers to the number of beats ahead.

[0045] and The design of the two parameters must satisfy both the internal and external stability of the system. The internal stability must satisfy:

[0046] ;

[0047] in

[0048] , ;

[0049] in This represents the internal system delay, calculated as 1.5 times the sampling period. ; The system cutoff angular frequency is 3491 rad / s. After internal stability design, The internal stability of the system can only be satisfied when the value is between 2 and 4.

[0050] External stability satisfies the following optimization problem:

[0051] ;

[0052] in Obtained under conditions of internal stability The upper boundary, This represents the system's port output admittance. After solving the optimization problem... It is possible When the value is 4, the maximum value of 0.3 is obtained; when the maximum value is obtained, the optimal system response speed under this scheme can be obtained.

[0053] The active damping module used is

[0054] ;

[0055] in For active damping gain, For low-pass filters; where, It is an adjustable constant ranging from 0 to 1. The design meets the following conditions:

[0056] ;

[0057] in

[0058] ;

[0059] The solution was obtained by plotting the graph using MATLAB software. A value of 0.6 can be used to obtain a full-frequency passive circuit under the condition of using an ideal differentiator.

[0060] The transfer function of the digital differentiator used is:

[0061] ;

[0062] in, This represents the value of the capacitor in the LCL filter. Let z be the sampling period, and z denote the Z-domain operator. It is an adjustable constant ranging from 0 to 1. This example uses... A value of 0.7 yields a better differential fitting result. This avoids the influence of noise amplification near the Nyquist frequency while also balancing the impact of the digital differentiator on the passivity of the system in the high-frequency range.

[0063] With the above design, the system's passivity is greatly enhanced, enabling it to be passive below 0.43 times the Nyquist frequency. This allows it to maintain stability under complex grid conditions such as strong and weak grids, while simultaneously improving the output power quality of LC / LCL grid-connected converters with single-converter side current feedback.

[0064] Figure 3 These are experimental waveforms of the grid-connected converter using the present invention under weak grid conditions. Figure 3 The experimental waveforms show that the system remains stable under weak power grid conditions;

[0065] Figure 4These are experimental waveforms of a grid-connected converter without using an insert repetitive controller under grid voltage distortion conditions.

[0066] Figure 5 The waveforms are experimental waveforms of a grid-connected converter using an inserted repetitive controller but without using an estimated output current feedback method under grid voltage distortion conditions.

[0067] Figure 6 These are experimental waveforms of a grid-connected converter using an inserted repetitive controller and an estimated output current feedback method under grid voltage distortion conditions.

[0068] By comparison Figure 4 and Figure 5 The waveforms show that the output current distortion is severe without the insertion repetitive controller; however, the harmonic quality is significantly improved after enabling the insertion repetitive controller. Nevertheless, because the output current is indirectly controlled, grid current harmonics can freely flow into the filter capacitor, so the output current is still distorted.

[0069] Compare Figure 5 and Figure 6 As can be seen from the waveform diagram, by comparison, after using the inserted repetitive controller and the estimated output current feedback method, the output current is controlled to be sinusoidal, and the harmonics of the output current are lower than the standard line of IEEE-1547.

[0070] Although the illustrative specific embodiments of the present invention have been described above to enable those skilled in the art to understand the invention, it should be understood that the invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of the present invention are protected.

Claims

1. A method for controlling the current on the side of an LC or LCL type single converter, characterized in that, Includes the following steps: (1) Acquire the capacitor voltage in the LC or LCL filter. Phase-locked loop is used to lock the phase, and the phase is locked to the phase. conduct Coordinate transformation, to obtain Capacitor voltage in coordinate system ; (2) Collect the current of the converter-side inductor in the LC or LCL filter. ,right conduct Coordinate transformation, to obtain Converter-side inductor current in coordinate system ; (3) Convert the capacitor voltage The estimated capacitance current is obtained through the digital differentiator D. Then, the inductor current on the converter side... With the estimated capacitor current By subtracting the values, we can obtain the estimated output current. ; (4) Set the reference current of the current controller With the estimated output current The difference is calculated to obtain the input error signal of the current controller. ; Then the error signal The current output control quantity is obtained through the current controller. ; (5) Convert the capacitor voltage The input is given to the active damper, through which the capacitor voltage is... Multiplying by the active damping transfer function, we get Active damping feedback quantity in coordinate system ; (6) The output control quantity of the current controller With active damping feedback Subtract, and then pass the result through Inverse coordinate transformation yields the modulation command value. ; (7) The PWM module modulates the modulation command value. The corresponding switching control signals of the switching transistors are obtained and used to control the turn-on and turn-off of each IGBT in the active power filter. The current controller is an insertion repetitive controller, and its transfer function is: ； in, The gain is proportional; s represents the S-domain operator; For a low-pass filter, its transfer function is: ,constant and Satisfying Relationships ; To control the gain repeatedly, This is the ratio of the sampling frequency to the power grid's base frequency. For a compensator, its transfer function is: , This refers to the number of shots ahead; Among them, parameters and The design must satisfy both the internal and external stability of the system. Internal stability must satisfy: ； in, For angular frequency variables, Sampling period, intermediate variable , satisfy: ， ； in, Represents the internal latency of the system. This is the system cutoff angular frequency; External stability satisfies the following optimization problem: ； in Obtained under conditions of internal stability The maximum value, Indicates the system's port output admittance. The Nyquist sampling angular frequency, It is the imaginary unit.

2. The current control method for an LC or LCL type single converter side according to claim 1, characterized in that, The transfer function of the digital differentiator D is: ； in, The capacitance value in the LC / LCL filter. Let z be the sampling period, and z denote the Z-domain operator. It is an adjustable constant ranging from 0 to 1.

3. The current control method for an LC or LCL type single converter side according to claim 1, characterized in that, The transfer function of the active damper is: ； in, For active damping gain, It is a low-pass filter. It is an adjustable constant ranging from 0 to 1; Among them, active damping gain The following two conditions must be met: ； in, Angular frequency variable, intermediate variable The expression is: ； in, The capacitance value in the LC / LCL filter. This represents the value of the converter-side inductance in the LC / LCL filter.

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