An LCL filter control method of a light storage direct-flexible converter control system

Abstract

The application belongs to the field of light storage direct flexible converter control, and particularly relates to an LCL filter control method of a light storage direct flexible converter control system. The method converts grid-connected current or average current into components in a stationary coordinate system through Clark transformation; calculates the difference between the components and reference current and inputs the PR controller; and controls the inverter switch tube IGBT to output inverter current through the SVPWM modulation algorithm. The application can significantly improve the current quality of the system under the fluctuation of flexible load, inhibit the direct current component and simplify the complexity of the system by optimizing the constraint relationship between the bandwidth of the PR controller and the crossing frequency.

Description

Technical Field

[0001] This invention belongs to the field of optical-storage-direct-flexible converter control technology, specifically relating to an LCL filter control method for an optical-storage-direct-flexible converter control system. Background Technology

[0002] In inverter algorithms, three-phase AC voltage is often acquired and then subjected to Clark and Park transformations before PID control. This method is used in both single-stage and two-stage photovoltaic-storage-DC-flexible converters. Many methods exist for improving PID controllers, such as: CN201210243296 - Single-stage photovoltaic inverter stable MPPT control system and method, which uses a current loop to control quality, resulting in significant errors; it employs LC filtering, but is not suitable for LCL filtering control; CN201910191987 - SOGI-based control method for photovoltaic-storage-DC-flexible converters without phase-locked loops under unbalanced grids, which applies LCL filtering and introduces SOGI; however, the lack of a phase-locked loop increases the risk of unreliable grid connection quality; CN201910513476 - A PIR optimization control method based on LCL converters, but its control accuracy is not high and its response speed is not ideal. Especially in the context of a photovoltaic-storage-DC-flexible (PEDF) system, the DC microgrid bus voltage fluctuates frequently, and the branches integrate energy storage batteries and flexible power consumption equipment, causing the converter to face more complex dynamic load demands. Traditional single LCL control strategies are insufficient to achieve efficient and stable suppression of complex harmonics and DC injection while ensuring the response speed of flexible dispatching. The overall control level urgently needs optimization. Summary of the Invention

[0003] To address the above problems, this application proposes an LCL filter control method for a photovoltaic-storage DC-flexible converter control system. The method includes the following steps for implementing the control of the inner current loop: Obtain grid-connected current or average current i 12 The grid-connected current Or the average current i 12 The grid-connected current is transformed into a stationary coordinate system using Clark transformation.

[0004] Calculate the grid-connected current and reference current in the stationary coordinate system. The difference is input to the PR controller; The SVPWM modulation algorithm generates a switching control signal to control the IGBT switching transistors of the converter to output inverter current.

[0005] The inverter current, after passing through an LCL filter, becomes the grid-connected current. Output to the power grid; The transfer function of the PR controller As shown in the following formula:

[0006] in, This represents the transfer function of the PR controller. This indicates the amplification gain of the PR controller. Indicates gain. ω represents the bandwidth, s represents the differential operator, k represents the number of terms in the polynomial, and ω represents the AC frequency.

[0007] This application simplifies system complexity and suppresses harmonics. By optimizing the bandwidth and crossover frequency constraints of the PR controller, it significantly improves the current tracking quality of the converter under dynamic scheduling of the optical-storage-DC-flexible system and suppresses the DC component of the LCL filter output. Attached Figure Description

[0008] Figure 1 Dynamic model of the current loop; Figure 2 : Dynamic model of voltage switching; Figure 3 : Control principle under one implementation method; Figure 4 : Control principle under one implementation method. Detailed Implementation

[0009] The following embodiments further illustrate the content of the present invention, but should not be construed as limiting the present invention. Any modifications or substitutions made to the methods, steps, or conditions of the present invention without departing from the spirit and essence of the invention are within the scope of the present invention.

[0010] The principle of LCL filter control method in some implementations of the optical-storage-direct-to-flexible converter control system is as follows: Figure 1 The steps to achieve control of the inner current loop include: Obtain grid-connected current or average current i 12 The grid-connected current Or the average current i 12 The grid-connected current is transformed into a stationary coordinate system using Clark transformation.

[0011] Calculate the grid-connected current and reference current in the stationary coordinate system. The difference is input to the PR controller; The SVPWM modulation algorithm generates a switching control signal to control the IGBT switching transistors of the converter to output inverter current.

[0012] The inverter current, after passing through an LCL filter, becomes the grid-connected current. Output to the power grid; The transfer function of the PR controller As shown in the following formula:

[0013] in, This represents the transfer function of the PR controller. This indicates the amplification gain of the PR controller. Indicates gain. ω represents the bandwidth, s represents the differential operator, k represents the number of terms in the polynomial, and ω represents the AC frequency.

[0014] Some specific implementation methods,

[0015] in, Let be the transfer function of the PR controller, s be the differential operator, k be the number of terms in the polynomial, and ω be the AC frequency. Reference current. The method for obtaining it includes the following steps: The reference current is calculated using the system's flexible power demand command or the output power and output voltage of the MPPT module. .

[0016] bandwidth Set according to the following formula:

[0017] in, Indicates bandwidth. This is the transition time for the harmonic controller.

[0018] Some implementations use a voltage feedforward control strategy for outer-loop voltage control, including the following steps: The DC distribution bus voltage or power supply voltage is obtained through the sampling and conditioning circuit.

[0019] Record the peak grid-connected current in the stationary coordinate system

[0020] The reference voltage is calculated via instructions from the flexible dispatch center or the MPPT module.

[0021] The PI controller of the outer voltage loop calculates the With the The difference is used to obtain the reference value of the peak grid-connected current in the stationary coordinate system using the following formula.

[0022] ω represents the frequency of alternating current. The crossover frequency of the outer voltage loop Not lower than the above maximum value

[0023]

[0024] in, The output current of the photovoltaic power source. This is the lowest voltage of the power supply branch.

[0025] In some implementations, the amplification gain of the PR controller Desired current loop crossover frequency The relationship is expressed as follows:

[0026] Wherein, represents the amplification gain of the PR controller. L 1 and L 2_min These are the first inductor and the second inductor of the LCL filter, respectively. This indicates the duty cycle of the converter's switching transistor. This is the maximum voltage of the power supply branch. This indicates the current loop crossing frequency. The resonant point of the PR controller includes the fundamental frequency of the power grid, the third harmonic, or other odd harmonics that are not integer multiples of 3.

[0027] In some specific implementations, the current loop crossover frequency is constrained by the following formula:

[0028] in, The current loop crossover frequency is [value missing]. The resonant frequency of the LCL filter is... The switching frequency of the IGBT; Preferably, the crossover frequency is the value closest to the resonant frequency of the LCL filter within a selectable range.

[0029] Some implementation methods of control such as Figure 2 It also includes the following steps: The grid-connected current is collected through a sampling and conditioning circuit. and the capacitor current of the LCL filter

[0030] The capacitor current i c and the grid-connected current The data is sent to the average value calculation module, which calculates the average current i using the following formula. 12 Then, the average current i 12 Send to the Clark transformation module.

[0031] The average value calculation module calculates the average current i using the following formula. 12 :

[0032] Where parameters a and b are preset values, i 12 Where is the average current and ic is the capacitor current. This is the grid-connected current.

[0033] Some implementations of a two-stage LCL optical-storage direct-to-flexible converter include: a boost circuit, a converter switching transistor IGBT, an LCL filter, and a sampling conditioning circuit.

[0034] A control system for a two-stage LCL optical-storage direct-to-flexible converter in some implementations is characterized in that the control system includes a Clark conversion module, a Clark inverse conversion module, a PR module, a PI module, and an SVPWM modulation module implemented based on a DSP. Example

[0035] The polynomial expression for an LCL filter is:

[0036] The expression for the delay Del(s) is as follows:

[0037] in, This is the delay of the digital controller; in commonly used DSPs, this delay is equal to the switching cycle. Or half of the switching cycle .

[0038] The model of the current loop is as follows Figure 3 The open-loop gain of the current loop is expressed as follows:

[0039] At steady-state operating point With photovoltaic power supply voltage Proportional. When using voltage feedforward control. With the maximum value of photovoltaic power supply voltage Proportional. Expressed as follows:

[0040] Under normal circumstances Select the PR controller; its transfer function expression is:

[0041] in, It is the frequency of the power grid. Design a reasonable amplification gain. The desired current loop crossover frequency can be obtained. If voltage feedforward control is used, the current loop crossover frequency is Its expression is as follows: [L=Ψ / I, the relationship between inductance and frequency] In the formula, To find the minimum value, another constraint needs to be considered:

[0042] on the one hand, Limited by the switching frequency of the voltage source inverter (VSI), which is determined by the transfer function of the digital delay. This is caused by... On the other hand... The frequency cannot be higher than the resonant frequency ωp of the LCL filter, as shown below; otherwise, it will cause stability problems.

[0043]

[0044] The resonant part has gain It can effectively track the current reference value. Generally, the resonant point is placed at the fundamental frequency of the power grid (k=1), the third harmonic (k=3), or other odd harmonics that are not integer multiples of 3 (k=5, 7, 11, 13, ...) to reduce the harmonic distortion of the injected power grid current.

[0045] When the power grid is unbalanced, both the positive and negative sequence components of the injected current contain third harmonics. Placing the harmonic point at the third harmonic location has a beneficial effect. The method is to set it according to the following formula:

[0046] Make the transition time of all harmonic controllers the same. The choice of gain should aim for a high gain at the resonant center frequency. This requires reducing the gain at higher-order center frequencies to improve the stability of the current loop.

[0047] In this example, the parameters of the VSI current controller are expressed as follows:

[0048] In the formula, the harmonic components have the same bandwidth (0.1Hz) and the same transition time. The lower the bandwidth of the harmonic controller, the more stable the current control loop. High-order resonant point. Set below other resonant points To ensure the stability of the current loop, a relatively high crossover frequency, close to the resonant frequency of the LCL filter, was also designed in this example to reduce the distortion of the injected grid current. Therefore, the selection of the high-order resonant point is quite precise.

[0049] Table 1 Open-loop gain at maximum and minimum L2 values

[0050]

[0051] Delayed value retrieval in calculation The table gives Stability parameters at that time. The design takes into account As can be seen from the table, as L2 decreases, All will decrease, and, The phase delay of the filter does not have much impact on the phase margin. The worst-case scenario occurs...

[0052] Open-loop gain at the center frequency of the resonant controller Therefore, the frequency of the injected grid current here follows the reference value well. Furthermore, the injected grid current is less affected by grid voltage fluctuations, i.e.

[0053] The small-signal model is established for the transfer function between the peak injected grid current and the voltage of the power supply branch in the αβ coordinate system as follows:

[0054] The dynamic model of the voltage loop can be derived from... Figure 4 express.

[0055] The current control loop is simulated as a first-order low-pass transfer function. It has poles at a certain point, and the transfer function is Equation A at low frequencies. A simplified version. Note the voltage loop crossover frequency. To design below To achieve stable operation, the following formula can be used for voltage control. The design typically uses a PI controller. The challenge of the design lies in how to pass the poles in open-loop operation. Compensation for the first-order low-pass transfer function, Must be designed to be higher The maximum value, As shown in the following formula,

[0056] In a flexible photovoltaic storage environment, if If the voltage is high enough, the above formula can be easily satisfied. In this case, the DC bus can serve as a buffer for energy between the optical storage branch and the converter.

[0057] Assuming the operating conditions satisfy the above formula, at frequencies close to When, equation B can be simplified to

[0058] This greatly simplifies the design of the voltage control loop. The PI controller can be represented by the following formula, and the voltage control loop can be further represented as follows:

[0059] For VSI in this example, the voltage control loop is expressed as follows:

[0060] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. The above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An LCL filter control method for a control system of an optical-storage DC-flexible converter, characterized in that, The steps for implementing current inner loop control in the method include: Obtain grid-connected current or average current i 12 The grid-connected current Or the average current i 12 The grid-connected current is transformed into a stationary coordinate system using Clark transformation. and ; Calculate the grid-connected current and reference current in the stationary coordinate system. The difference is input to the PR controller; The SVPWM modulation algorithm generates a switching control signal to control the IGBT switching transistors of the converter to output inverter current. ; The inverter current, after passing through an LCL filter, becomes the grid-connected current. Output to the power grid; The transfer function of the PR controller As shown in Equation I: Mode in, This represents the transfer function of the PR controller. This indicates the amplification gain of the PR controller. Indicates gain. ω represents the bandwidth, s represents the differential operator, k represents the number of terms in the polynomial, and ω represents the AC frequency.

2. The control method as described in claim 1, characterized in that, Voltage outer loop control using a voltage feedforward control strategy includes the following steps: Obtain the voltage of the optical storage DC-flexible bus or the power supply voltage. ; Record the peak value of the grid-connected current; The reference voltage is calculated via flexible dispatch instructions or the MPPT module. ; Calculation using a PI controller and The difference is used to obtain the reference value for the peak current.

3. The control method as described in claim 1, characterized in that, The amplification gain of the PR controller With current loop cross frequency The relationship is expressed as follows: Wherein, L1 and L represent the amplification gain of the PR controller. 2_min These are the first inductor and the second inductor of the LCL filter, respectively. This indicates the duty cycle of the converter's switching transistor. This is the maximum voltage of the power supply branch. This indicates the current loop crossing frequency.

4. The control method as described in claim 3, characterized in that, The current loop crossing frequency is constrained by the following formula: in, The current loop crossover frequency is [value missing]. The resonant frequency of the LCL filter is... The switching frequency of the IGBT; Preferably, the crossover frequency is the value closest to the resonant frequency of the LCL filter within a selectable range.

5. The control method as described in claim 1, characterized in that, The reference current It is calculated by dividing the output power by the output voltage using the system flexible power demand command or MPPT output power.

6. The control method as described in claim 1, characterized in that, The resonant point of the PR controller includes the fundamental frequency of the power grid, the third harmonic, or other odd harmonics that are not integer multiples of 3.

7. The control method as described in claim 1, characterized in that, The bandwidth Set according to the following formula: Mode in, Indicates bandwidth. This is the transition time for the harmonic controller.

8. The control method as described in claim 1, characterized in that, The transfer function of the PR controller specifically includes the transfer function defined by the following formula: Mode Among them, G i (s) is the transfer function of the PR controller, s is the differential operator, k represents the number of terms in the polynomial, and ω represents the AC frequency.

9. The control method as described in claim 1, characterized in that, The method further includes the following steps: The grid-connected current #imgpt38# and the capacitor current ic of the LCL filter are acquired through a sampling and conditioning circuit. The capacitor current ic and the grid-connected current #imgpt39# are sent together to the average value calculation module, which calculates the average current i using the following formula. 12 Then, the average current i 12 Send to the Clark transformation module.

10. The control method as described in claim 9, characterized in that, The average value calculation module calculates the average current i using the following formula. 12 : #imgpt40# Where parameters a and b are preset values, i 12 The current is the average value, ic is the capacitor current, and #imgpt41# is the grid-connected current.

11. A photoelectric storage DC-flexible converter and its control system, characterized in that, It includes a boost circuit, an IGBT, an LCL filter, and a control board carrying a processor, wherein the processor performs the method according to any one of claims 1-10.

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