Parallel APF harmonic suppression method based on SVPWM for charging pile grid connection

Abstract

The invention discloses a parallel APF harmonic suppression method based on SVPWM for charging pile grid connection. The parallel APF harmonic suppression method comprises the following steps that an uncontrolled direct current charging pile typical model is built; nonlinear load harmonic waves in the typical model of the uncontrolled direct current charging pile are detected; taking the chaos sequence as a spread spectrum signal, and superposing the spread spectrum signal on a reference frequency signal to replace a fixed switching frequency of SVPWM (Space Vector Pulse Width Modulation) so as to suppress high-frequency noise waves of an APF (Active Power Filter) system; and a PI control strategy based on a chaos period is adopted to generate a compensation current to treat the detected nonlinear load harmonic waves. And the harmonic suppression effect is enhanced.

Description

Technical Field

[0001] The present invention relates to the field of charging piles and power quality management, and in particular to a parallel APF harmonic control method based on SVPWM modulation for grid-connected charging piles. Background Art

[0002] With the increasing popularity of electric vehicles, the development of electric vehicle charging stations has gradually become one of the key technologies for the continuous progress of its industry. However, since electric vehicle charging stations involve energy conversion and voltage rise and fall, it is inevitable to introduce a large number of power electronic converters; on the other hand, the electric vehicle charging process is regarded as a nonlinear load, so the large-scale construction of charging stations will inevitably affect the power quality of the distribution network, including voltage sag, three-phase imbalance, harmonic pollution and voltage flicker, which may cause distribution network failure or even shutdown in severe cases. Harmonic content is one of the important indicators of power quality. In electric vehicle charging stations, the harmonic content is affected by many factors such as the type, number, charging topology and battery status of charging piles, and has significant diversity and complexity. Therefore, effectively identifying and preventing the power quality problems of the distribution network caused by electric vehicle charging stations is of great significance to the development of the entire electric vehicle industry.

[0003] Nowadays, a common way to solve the problem of harmonic pollution is to install compensation equipment. This type of equipment is roughly divided into two types: one is a passive power filter (PPF), and the other is an active power filter (APF). Among them, APF is the most commonly used harmonic elimination device. With the continuous development of the power system, especially the impact of the large-scale access of charging piles on the power grid, the performance requirements of APF are getting higher and higher. The current APF equipment must not only improve the steady-state and dynamic compensation accuracy, but also have the characteristics of high efficiency, high reliability, and high adaptability. At the same time, in order to ensure the long-term stable operation of APF, sufficient thermal design and protection measures are required to extend the life of the device. In addition, it is also necessary to consider the mutual negative impact of APF and other power quality management devices on the work, so as to ensure the stable and reliable operation of the entire power system.

[0004] Although harmonic control and related APF technologies are developing rapidly, the current harmonic detection technology has problems of slow detection speed or low detection accuracy. At the same time, the harmonic control effect is poor. The distortion rate after harmonic control is mostly above 5%, which will still have a great impact on the power quality. Therefore, a more reliable control method is needed for the harmonic problems caused by the grid connection of charging piles. Summary of the invention

[0005] The purpose of the present invention is to solve the deficiencies of the above-mentioned prior art, thereby providing a parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection, thereby enhancing the harmonic control effect.

[0006] A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection includes the following steps:

[0007] Build a typical model of an uncontrolled DC charging pile;

[0008] Detect nonlinear load harmonics in a typical model of an uncontrolled DC charging pile;

[0009] The chaotic sequence is used as a spread spectrum signal and superimposed on the reference frequency signal to replace the fixed switching frequency of SVPWM to suppress the high-frequency noise of the APF system.

[0010] A PI control strategy based on chaotic cycle is used to generate compensation current to control the detected nonlinear load harmonics.

[0011] After building a typical model of an uncontrolled DC charging pile, the DC-DC power converter in the uncontrolled DC charging pile model is equivalent to a nonlinear resistor whose resistance value is determined by the input voltage and current of the actual charging pile, and the harmonic characteristics of the uncontrolled DC charging pile are analyzed.

[0012] Step S2 is specifically as follows:

[0013] An improved pq current detection method is used to detect nonlinear load harmonics in uncontrolled DC charging piles.

[0014] The improved pq harmonic current detection method includes fundamental positive sequence voltage extraction, Clarke transformation, pq transformation, low pass filter (LPF) elimination, pq inverse transformation and Clarke inverse transformation.

[0015] The improved pq current detection method is used to detect the nonlinear load harmonics in the uncontrolled DC charging pile, specifically:

[0016] Extract the asymmetric three-phase voltage of the power grid a 、e b 、e c The fundamental positive sequence component of is converted into two-phase voltage using Clarke transformation;

[0017] The three-phase current i of the power grid a 、i b 、i c Carry out Clarke transformation to two phases, and perform pq transformation on the two-phase voltage converted by Clarke transformation to determine the active power p and reactive power q;

[0018] LPF is used to remove the AC components of active power p and reactive power q, and active power DC component p' and reactive power AC component q' are obtained. The active power DC component p' and reactive power AC component q' are transformed by Clarke inverse transformation to obtain the fundamental component i of the three-phase current. af 、i bf 、i cf ;

[0019] With three-phase current i a 、i b 、i c Subtract the fundamental component i af 、i bf 、i cf Calculate the harmonic current and realize the detection of harmonic current.

[0020] The chaotic sequence is used as a spread spectrum signal and superimposed on the reference frequency signal to replace the fixed switching frequency of SVPWM, which is expressed as:

[0021] f(t)=f s +Δf=k1v s +k2v2

[0022] Among them, f s represents the reference switch operating frequency, Δf represents the additional chaotic sequence signal, v s represents the fixed voltage that determines the operating frequency of the reference switch, v2 represents the spread spectrum signal voltage, and k1 and k2 are the corresponding amplification factors.

[0023] The PI control strategy based on chaotic cycle is as follows:

[0024] The continuously changing control period T generated by chaotic frequency modulation c The PI controller of the APF system is introduced, and the PI controllers of the current inner loop and the voltage outer loop are designed.

[0025] The PI controller transfer function of the inner current loop is Φ i for:

[0026]

[0027] Where K pi is the proportional coefficient of the PI controller of the current inner loop, T ii is the integral time constant of the PI controller of the current inner loop, K PWM is the amplification factor of the modulation link, and R is the output impedance of the APF.

[0028] The PI controller transfer function of the voltage outer loop is Φ v for:

[0029]

[0030] Where K pv is the proportional coefficient of the PI controller of the current outer loop, T iv is the integral time constant of the PI controller of the current outer loop, and C is the output capacitive reactance of the APF.

[0031] Beneficial effects of the present invention: The present invention establishes a typical model of an uncontrolled DC charging pile, and uses an equivalent model method to make the DC-DC power converter in the charging pile equivalent to a nonlinear resistor, and the resistance value is determined by the input voltage and current of the actual charging pile, making it more in line with the actual situation; the pq harmonic current detection method is improved to make harmonic detection more accurate; the chaotic frequency modulation strategy is used to pre-process the harmonics, and the harmonic control effect is enhanced; the chaotic frequency modulation strategy is combined with PI control to improve the harmonic control effect. The effectiveness of this solution is verified by controlling harmonics through simulation experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is the harmonic control flow chart of the present invention;

[0033] Figure 2 is a topological diagram of a DC charging pile of the present invention;

[0034] Figure 3 is an equivalent circuit diagram of the charging pile of the present invention;

[0035] Figure 4 It is the harmonic waveform before treatment analyzed by the simulation of the present invention;

[0036] Figure 5 It is a block diagram of the improved pq harmonic detection method of the present invention;

[0037] Figure 6 It is a block diagram of the chaotic frequency modulation principle of the present invention;

[0038] Figure 7 It is the current inner loop control block diagram of the present invention;

[0039] Figure 8 It is a block diagram of voltage outer loop control of the present invention;

[0040] Fig. 9 It is the distortion rate after harmonic control of the present invention. DETAILED DESCRIPTION

[0041] Aiming at the problem of power quality degradation caused by charging piles being connected to the grid, the present invention proposes a parallel APF harmonic control method based on SVPWM modulation for charging piles being connected to the grid. The specific flow chart is as follows: Figure 1 shown.

[0042] Step 1: For the uncontrolled DC charging pile, a typical model of the uncontrolled DC charging pile is established;

[0043] Step 2: Use the improved pq current detection method to detect the nonlinear load harmonics in the uncontrolled DC charging pile;

[0044] The pq harmonic current detection method was improved. First, the fundamental positive sequence voltage was extracted, and then the accurate harmonic detection results were obtained through Clarke transformation, pq transformation, low-pass filter (LPF) elimination, pq inverse transformation and Clarke inverse transformation. Figure 5 The improved pq harmonic detection method of the present invention is shown in FIG. According to the block diagram, the improved pq harmonic detection method first needs to detect the asymmetric three-phase voltage e a 、e b 、e c The fundamental positive sequence component is extracted and then used in the calculation. Then, the three-phase current i a 、i b , i are converted into two phases by Clarke transformation, and then converted into two-phase voltage by pq transformation to determine the active power p and reactive power q, and then the AC component is removed by low-pass filter (LPF) to obtain the DC component p', q'. Then p' and q' are used to obtain the three-phase current i a 、i b 、i c The fundamental component i af 、i bf 、i cf , which can be expressed as:

[0045]

[0046] Finally, the three-phase current i a 、i b 、i c Subtract the fundamental component i af 、i bf 、i cf The harmonic current can be obtained and accurate detection of the harmonic current can be achieved.

[0047] Step 3: Use the chaotic sequence as a spread spectrum signal and superimpose it on the reference frequency signal to replace the fixed switching frequency of SVPWM and suppress the high-frequency noise of the APF system;

[0048] In the process of applying chaotic carrier frequency modulation to SVPWM to control harmonics, the chaotic sequence is used as a spread spectrum signal and superimposed on the reference frequency signal to form a chaotic carrier, which together with the modulated wave becomes the control signal of SVPWM, thereby replacing the fixed switching frequency of SVPWM, thereby expanding the harmonic frequency bandwidth and reducing the EMI of the system. The block diagram of the chaotic frequency modulation principle is shown in the figure. Figure 6 After chaotic frequency modulation, the carrier frequency changes from constant to chaotic within a certain range, which can be described by mathematical formula:

[0049] f(t)=f s +Δf=k1v s +k2v2 (3)

[0050] Among them, f s represents the reference switch operating frequency, Δf represents the additional chaotic sequence signal, v s represents the fixed voltage that determines the reference switch operating frequency, v2 represents the spread spectrum signal voltage, and k1 and k2 are the corresponding amplification factors. When chaotic frequency modulation is applied, the switching frequency changes from a constant frequency f s Transformed into a chaotic frequency that changes with the chaotic sequence signal Δf, at the reference frequency f s The main harmonic frequency in the corresponding Fourier transform changes from the previous nf s Convert to n(f s +Δf)(n=2,3,…), the distribution of harmonic frequency points is thus transformed into a band shape, thereby suppressing the amplitude of the harmonics.

[0051] Step 4: Use the PI control strategy based on chaotic cycle to generate compensation current to control the detected nonlinear load harmonics.

[0052] The continuously changing control period T generated by chaotic frequency modulation c It is introduced into the PI control system of APF, and on this basis, the current inner loop and voltage outer loop PI controllers are designed to reduce the harmonic distortion rate (THD) of APF through chaotic frequency modulation. PWM represents the amplification factor of the modulation link,

[0053] The current inner loop PI control block diagram based on the chaotic frequency modulation strategy established by the present invention is as follows: Figure 7 As shown, its transfer function is:

[0054]

[0055] Where K pi is the proportional coefficient of the controller, T ii is the integral time constant of the controller, K PWM is the amplification factor of the modulation link, T cis the control period of the inner current loop.

[0056] The voltage outer loop PI control block diagram based on the chaotic frequency modulation strategy established by the present invention is as follows: Figure 8 As shown, its transfer function is:

[0057]

[0058] Where K pv is the proportional coefficient of the controller, T iv is the integral time constant of the controller.

[0059] Example

[0060] DC charging pile topology Figure 2 As shown, it includes an AC-DC rectifier, a passive filter, a DC-DC converter and an output filter device.

[0061] In order to analyze the harmonic characteristics of the charging pile, the charging pile topology is simplified, and the DC-DC power converter in the charging pile is equivalent to a nonlinear resistor through the equivalent model method. The resistance value is determined by the input voltage and current of the actual charging pile. The equivalent circuit of the electric vehicle charging pile is as follows: Figure 3 As shown. In order to fit the actual charging station equipment, the three-phase line voltage is set to 400V, the filter inductor L = 1mH, and the filter capacitor C = 1μF. Based on this, the present invention simulates and analyzes its harmonic characteristics. The pulsation number of the three-phase uncontrolled rectifier is 6, so the theoretical analysis shows that the harmonic frequency generated is mainly 6k±1 times (where k is a positive integer). A three-phase uncontrolled charging pile is built in MATLAB's simulink, and the AC side current is analyzed by FFT. The results are as follows: Figure 4 As shown in the figure, the total harmonic distortion rate of the AC side current is 22.28%, and the 5th, 7th, 11th, and 13th harmonics account for a large proportion. The simulation results are consistent with the conclusion that the harmonic frequency is mainly 6k±1.

[0062] The harmonics are controlled by the method of the present invention. The APF switching frequency is set to 10kHz, the DC side voltage is 800V, and a three-phase fully controlled rectifier bridge is used to generate compensation current to enter the grid. The harmonic distortion rate after compensation is as follows: Fig. 9 As shown in the figure, the harmonic distortion rate of the charging station is reduced from 22.28% without APF device to 0.62%, which shows the excellent effect of APF on harmonic control of charging station.

[0063] The present invention is not limited to the above-mentioned embodiments. For ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention. The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection, characterized in that: The following steps are involved: Build a typical model of an uncontrolled DC charging pile; Detect nonlinear load harmonics in a typical model of an uncontrolled DC charging pile; The chaotic sequence is used as a spread spectrum signal and superimposed on the reference frequency signal to replace the fixed switching frequency of SVPWM to suppress the high-frequency noise of the APF system. A PI control strategy based on chaotic cycle is used to generate compensation current to control the detected nonlinear load harmonics.

2. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection according to claim 1, characterized in that: After building a typical model of an uncontrolled DC charging pile, the DC-DC power converter in the uncontrolled DC charging pile model is equivalent to a nonlinear resistor whose resistance value is determined by the input voltage and current of the actual charging pile, and the harmonic characteristics of the uncontrolled DC charging pile are analyzed.

3. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection according to claim 1 or 2, characterized in that: Step S2 is specifically as follows: An improved pq current detection method is used to detect nonlinear load harmonics in uncontrolled DC charging piles.

4. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection according to claim 3, characterized in that: The improved pq harmonic current detection method includes fundamental positive sequence voltage extraction, Clarke transformation, pq transformation, low pass filter (LPF) elimination, pq inverse transformation and Clarke inverse transformation.

5. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection according to claim 4, characterized in that: The improved pq current detection method is used to detect the nonlinear load harmonics in the uncontrolled DC charging pile, specifically: Extract the asymmetric three-phase voltage of the power grid a 、e b 、e c The fundamental positive sequence component of is converted into two-phase voltage using Clarke transformation; The three-phase current i of the power grid a 、i b 、i c Carry out Clarke transformation to two phases, and perform pq transformation on the two-phase voltage converted by Clarke transformation to determine the active power p and reactive power q; LPF is used to remove the AC components of active power p and reactive power q, and active power DC component p' and reactive power AC component q' are obtained. The active power DC component p' and reactive power AC component q' are transformed by Clarke inverse transformation to obtain the fundamental component i of the three-phase current. af 、i bf 、i cf ; With three-phase current i a 、i b 、i c Subtract the fundamental component i af 、i bf 、i cf Calculate the harmonic current and realize the detection of harmonic current.

6. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection according to claim 1, characterized in that: The chaotic sequence is used as a spread spectrum signal and superimposed on the reference frequency signal to replace the fixed switching frequency of SVPWM, which is expressed as: f(t)=f s +Δf=k1v s +k2v2 Among them, f s represents the reference switch operating frequency, Δf represents the additional chaotic sequence signal, v s represents the fixed voltage that determines the operating frequency of the reference switch, v2 represents the spread spectrum signal voltage, and k1 and k2 are the corresponding amplification factors.

7. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection according to claim 1, characterized in that: The PI control strategy based on chaotic cycle is as follows: The continuously changing control period T generated by chaotic frequency modulation c The PI controller of the APF system is introduced, and the PI controllers of the current inner loop and the voltage outer loop are designed.

8. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection according to claim 7, characterized in that: The PI controller transfer function of the inner current loop is Φ i for: Where K pi is the proportional coefficient of the PI controller of the current inner loop, T ii is the integral time constant of the PI controller of the current inner loop, K PWM is the amplification factor of the modulation link, and R is the output impedance of the APF.

9. A parallel APF harmonic control method based on SVPWM modulation for charging pile grid connection according to claim 7 or 8, characterized in that: The PI controller transfer function of the voltage outer loop is Φ v for: Where K pv is the proportional coefficient of the PI controller of the current outer loop, T iv is the integral time constant of the PI controller of the current outer loop, and C is the output capacitive reactance of the APF.

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