A midpoint potential balance control method based on a notch filter and a compensator

Abstract

The application provides a midpoint potential balance control method based on a notch filter and a compensator, and belongs to the technical field of converters, and comprises the following steps: determining a difference signal of a positive midpoint bus capacitor voltage signal and a negative midpoint bus capacitor voltage signal of a three-level converter DC side, and inputting the difference signal into a notch filter for processing; inputting the processed difference signal as a negative feedback signal into a compensator together with a zero reference signal; dividing a compensator output signal by a specific proportional coefficient as a zero sequence modulation signal, and superimposing the zero sequence modulation signal into three-phase modulation signals of a current converter, and generating a PWM signal according to the three-phase modulation signals superimposed with the zero sequence modulation signal, so as to control the midpoint potential balance of the three-level converter DC side; the application generates a zero sequence modulation signal by using a notch filter and a compensator, and superimposes the zero sequence modulation signal on three-phase alternating current modulation signals, so as to control the current on the neutral line, and the problem of midpoint potential imbalance of the three-level converter DC side capacitor can be solved.

Description

Technical Field

[0001] This invention belongs to the field of converter technology, specifically relating to a midpoint potential balance control method based on a notch filter and a compensator. Background Technology

[0002] In a three-level converter, the parameters of components such as switching transistors and DC-side capacitors are different, and sensors also have certain measurement deviations. In addition, due to the truncation and rounding errors during digital programming, the current on the DC side neutral line of the three-level converter will inevitably have a DC component. This will cause the voltage on the DC-side voltage divider capacitor to deviate from its rated value. The imbalance of the DC-side voltage will further cause the imbalance of the AC-side current and generate a DC bias, thus affecting the normal and stable operation of the system. Summary of the Invention

[0003] To address the aforementioned shortcomings of existing technologies, this invention provides a midpoint potential balance control method based on a notch filter and a compensator to solve the aforementioned technical problems.

[0004] This application provides a midpoint potential balance control method based on a notch filter and a compensator, including:

[0005] The difference signal between the positive bus capacitor voltage signal and the negative bus capacitor voltage signal on the DC side of the three-level converter is determined, and the difference signal is input into a notch filter for processing.

[0006] The processed difference signal is used as a negative feedback signal and input together with the zero reference signal into the compensator;

[0007] The output signal of the compensator is divided by a specific scaling factor to obtain a zero-sequence modulation signal, which is then superimposed on the three-phase modulation signal of the current converter. A PWM signal is generated based on the three-phase modulation signal superimposed with the zero-sequence modulation signal to control the DC side neutral point potential balance of the three-level converter.

[0008] The mathematical model of the controlled object is derived by multiplying the three-phase modulation signal with superimposed zero-sequence modulation signal by a specific scaling factor and considering the dynamic characteristics of the DC side capacitor voltage, the actual DC component of the difference signal can be obtained.

[0009] The difference signal is obtained by superimposing the third harmonic component on the actual DC component of the difference signal, thereby forming a closed-loop control.

[0010] Furthermore, the specific scaling factor is the fundamental amplitude of the current AC-side current of the converter. The product of the power factor cosγ

[0011] Furthermore, the derivation of the mathematical model of the controlled object includes:

[0012] Calculate the product of the three-phase modulation signal superimposed with the zero-sequence modulation signal and a specific scaling factor;

[0013] Let the product be multiplied by Obtain the DC component of the DC side neutral current;

[0014] The DC component of the DC-side neutral current multiplied by the transfer function between that DC component and the DC component of the difference signal.

[0015] Furthermore, the transfer function of the notch filter is: ω0 represents the power frequency angular frequency.

[0016] Furthermore, the compensator employs a PI controller, and the transfer function of the compensator is: Where, k p k is the proportional coefficient of the compensator. i This is the integral coefficient of the compensator.

[0017] Furthermore, the transfer formula for the open-loop control of this method, formed by the notch filter, the compensator, and the dynamic characteristics of the DC-side capacitor voltage, is as follows: ω0 represents the power frequency angular frequency.

[0018] The beneficial effects of this invention are that the midpoint potential balance control method based on notch filters and compensators provided by this invention uses notch filters and compensators to generate a zero-sequence modulation signal, which is then superimposed on the three-phase AC modulation signal to control the current on the neutral line. This solves the problem of unbalanced midpoint potential of the DC-side capacitors in three-level converters, ultimately achieving the goal of controlling the DC-side capacitor voltage balance. Furthermore, the design principle of this invention is reliable, the structure is simple, and it has very broad application prospects. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a commonly used three-level converter topology diagram in existing technology;

[0021] Figure 2 A schematic diagram of the entire closed-loop control process of a control method provided in an embodiment of the present invention;

[0022] Figure 3A comparison of simulation results of the DC-side voltage balancing effect before and after a control method provided in an embodiment of the present invention. Detailed Implementation

[0023] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0024] This invention superimposes a zero-sequence modulation signal on a three-phase AC modulation signal to control the neutral current, ultimately achieving the goal of controlling the DC-side capacitor voltage balance.

[0025] Three-phase AC modulation signal m a m b m c Based on this, a zero-sequence modulation signal m0 is superimposed, as shown in the following formula:

[0026]

[0027]

[0028]

[0029] Typically, the neutral current i np Three-phase modulation signal m a m b m c Three-phase alternating current i a i b i c and the switching function f a f b f c It has the following relationship:

[0030] i np (t)=-[f a (t)+f b (t)+f c (t)] (4)

[0031] f a (t)=m a (t)i a (t)[sgn(m a )-sgn(-m a (5)

[0032] fb (t)=m b (t)i b (t)[sgn(m b )-sgn(-m b (6)

[0033] f c (t)=m c (t)i c (t)[sgn(m c )-sgn(-m c (7)

[0034] For a three-phase balanced signal, f a (t)+f b (t)+f c The sum of the fundamental frequency components and harmonic components in the positive and negative sequences of (t) is 0, so only the DC component remains. Therefore, according to formula (4), the neutral current i np DC component i np0 The relationship is as follows:

[0035] i np0 (t)=-3f a0 (t) (8)

[0036] The harmonic component f a0 DC component f in (t) a0 It can be calculated using the following formula:

[0037]

[0038] By simplification, we can obtain f a0 DC component f in (t) a0 The expression is:

[0039]

[0040] in, γ represents the amplitude of the alternating current, γ is the phase difference between the output voltage and current of the converter, and cosγ represents the power factor of the converter's output power.

[0041] Furthermore, substituting formula (10) into formula (8), we can obtain the DC side neutral current i. np DC component i np0 The expression is:

[0042]

[0043] DC-side positive neutral bus capacitor voltage V1, DC-side negative neutral bus capacitor voltage V2, and DC-side neutral current i np DC component inp0 The relationship is as follows:

[0044]

[0045] Where <>0 indicates that the DC component of the signal is taken, and C is the capacitance value between the positive and negative DC bus.

[0046] By performing a Laplace transform on both sides of the differential equation (12), the DC component of the difference between the DC-side positive bus capacitor voltage V1 and the DC-side negative bus capacitor voltage V2 can be obtained. <v1-v2>The transfer function of the zero-sequence modulated signal m0 is:

[0047]

[0048] The above describes the derivation process based on the dynamic characteristics of the DC-side capacitor voltage. Based on the mathematical model of the controlled object obtained above, such as... Figure 2 As shown in the figure, this application provides a method for controlling the midpoint potential balance based on a notch filter and a compensator.

[0049] The difference signal between the positive bus capacitor voltage signal V1 and the negative bus capacitor voltage signal V2 on the DC side of the three-level converter is determined, and the difference signal is input into a notch filter for processing.

[0050] The processed difference signal is used as a negative feedback signal and input together with the zero reference signal into the compensator;

[0051] The output signal of the compensator is divided by a specific scaling factor to obtain a zero-sequence modulation signal, which is then superimposed on the three-phase modulation signal of the current converter. Based on the three-phase modulation signal with the superimposed zero-sequence modulation signal, a suitable modulation method is selected to generate a PWM signal, which is used to generate the drive signal for the converter switching transistor, thereby controlling the DC side midpoint potential balance of the three-level converter.

[0052] The three-phase modulated signal, superimposed with the zero-sequence modulated signal, is multiplied by a specific scaling factor. Based on the dynamic characteristics of the DC-side capacitor voltage, the DC component of the difference signal is re-determined, and the transfer function is: C is the capacitance value of the busbar;

[0053] The DC component of the difference signal is superimposed with harmonic components. Then, the notch filter is input again to form a closed-loop control.

[0054] Derive the mathematical model of the controlled object, calculate the product of the three-phase modulation signal superimposed with the zero-sequence modulation signal and a specific scaling factor; multiply this product by... The DC component of the DC-side neutral current is obtained; the DC component of the DC-side neutral current is multiplied by the transfer function between the DC component and the DC component of the difference signal.

[0055] The notch filter is used to remove the third harmonic component from the difference signal between the DC side positive bus capacitor voltage signal V1 and the negative bus capacitor voltage signal V2, so as to avoid its impact on the performance of the control system.

[0056] The transfer function expression of a notch filter is shown below:

[0057]

[0058] In the above formula, ω0 represents the power frequency angular frequency, which is 314 rad / s.

[0059] If the DC side midpoint potential of the three-level converter is balanced, the difference between the DC side positive and negative bus capacitor voltage signals V1 and V2 should be 0. Therefore, the input reference signal is zero, and the control objective is to make the difference between the average values ​​of the DC side positive and negative bus capacitor voltages of the three-level converter zero.

[0060] The error signal 'e', ​​obtained by subtracting the input reference signal and the feedback signal, is processed by a compensator to adjust the system bandwidth and ensure the stability margin of the control system. This allows the three-level converter's DC-side midpoint potential balance control to possess good steady-state and transient performance. The compensator's transfer function is:

[0061]

[0062] The output of the compensator needs to be divided by a specific proportional coefficient. The selection of this proportional coefficient is related to the fundamental amplitude of the current AC side current of the current converter and the power factor, and is used to offset the multiplier variables in the mathematical model of the controlled object.

[0063] according to Figure 2 The control system block diagram shown can be represented by the following open-loop transfer function:

[0064]

[0065] Selecting the system cutoff frequency ω c For a frequency of 0.3ω0, the open-loop transfer function at the cutoff frequency should satisfy |l(jω0)|. c |=1. The phase margin at the cutoff frequency is selected as 60 degrees. Based on this, the specific control parameters of the compensator can be calculated. The relevant calculation method has been published and will not be repeated here.

[0066] like Figure 3 As shown, the simulation demonstrates the effect of DC-side voltage balancing on the front and rear sides using the notch filter and compensator-based midpoint potential balance control method provided in this embodiment. In the simulation model, the capacitance of the positive and negative capacitors on the DC side of the three-level converter is 12mF, the DC voltage is 1200V, and the AC grid voltage is 690V. The simulation sets that no DC-side voltage balancing control measures are used before 0.5 seconds, and the midpoint potential balance control method based on the notch filter and compensator provided in this embodiment is used at 0.5 seconds.

[0067] The entire simulation process is as follows: At 0.05 seconds, the PCS is powered on to establish AC voltage; at 0.2 seconds, the first load with a power factor of 0.8 is applied. From the simulation waveform, it can be seen that the voltages on the DC side positive and negative capacitors are basically balanced; at 0.4 seconds, the load is switched to a load with a power factor of 0.6. At this time, it can be clearly seen that the voltages of the positive and negative capacitors gradually deviate from the rated value of 600V and have a further diverging trend; at 0.5 seconds, the midpoint potential balance control method based on notch filter and compensator provided in this embodiment is enabled. It can be seen that the voltages of the positive and negative capacitors converge rapidly, achieving the expected control effect.

[0068] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the invention should also be covered within the protection scope of the invention. Therefore, the protection scope of the invention should be determined by the scope of the claims.

Claims

1. A method for controlling the midpoint potential balance based on a notch filter and a compensator, characterized in that, include: The difference signal between the positive bus capacitor voltage signal and the negative bus capacitor voltage signal on the DC side of the three-level converter is determined, and the difference signal is input into a notch filter for processing. The processed difference signal is used as a negative feedback signal and input together with the zero reference signal into the compensator; The output signal of the compensator is divided by a specific scaling factor to become a zero-sequence modulation signal, which is then superimposed on the three-phase modulation signal of the current converter. A PWM signal is generated based on the three-phase modulation signal superimposed with the zero-sequence modulation signal to control the DC side neutral point potential balance of the three-level converter. The mathematical model of the controlled object is derived by multiplying the three-phase modulation signal with superimposed zero-sequence modulation signal by a specific scaling factor, and considering the dynamic characteristics of the DC side capacitor voltage, the actual DC component of the difference signal can be obtained. The difference signal is obtained by superimposing the third harmonic component on the actual DC component of the difference signal, thereby forming a closed-loop control. The specific scaling factor is the fundamental amplitude of the current AC side current of the current converter. and power factor product ; The derivation of the mathematical model of the controlled object includes: Calculate the product of the three-phase modulation signal superimposed with the zero-sequence modulation signal and a specific scaling factor; Let the product be multiplied by 6 / π This yields the DC component of the DC side neutral current. The DC component of the DC-side neutral current multiplied by the transfer function 1 / between that DC component and the DC component of the difference signal. Cs C is the capacitance value of the bus.

2. The method according to claim 1, characterized in that, The transfer function of the notch filter is: , It represents the power frequency angular frequency.

3. The method according to claim 1, characterized in that, The compensator employs a PI controller, and its transfer function is: ,in, The proportional coefficient of the compensator. This is the integral coefficient of the compensator.

4. The method according to claim 2 or 3, characterized in that, The transfer formula for the open-loop control of this method, formed by the notch filter, the compensator, and the dynamic characteristics of the DC-side capacitor voltage, is as follows: 。

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