A method for suppressing low-frequency ripple of DC side voltage of parallel active power filter

Abstract

The invention discloses a method for suppressing low-frequency ripple of the DC side voltage of a parallel active power filter, and belongs to the field of power electronic control. The method comprises the following steps: Step 1: connecting a bidirectional Buck type active power decoupling circuit in parallel to the DC side of a conventional parallel active power filter; Step 2: transferring the low-frequency fluctuation power of the DC side of the active power filter to an auxiliary capacitor by controlling the bidirectional Buck circuit, thereby realizing the suppression of low-frequency ripple of the DC side voltage of the active power filter. Based on the active power decoupling technology, the invention suppresses the 6-fold, 12-fold, 18-fold and 24-fold fluctuations of the DC side voltage of the parallel active power filter, thereby eliminating the low-pass filter of the DC side voltage control link of the active power filter and improving the response speed of the system. In addition, in the case of small fluctuations in the DC side voltage, a small-capacitance film capacitor can be used to replace the traditional electrolytic capacitor to improve the reliability of the system.

Description

Technical Field

[0001] The invention relates to the field of power electronic control, and in particular to a method for suppressing low-frequency ripple of a DC side voltage of a parallel active power filter. Background Art

[0002] With the development of power electronics technology, many nonlinear loads such as rectifiers, inverters, uninterruptible power supplies, arc furnaces, etc. in the distribution system are increasing, and the harmonic pollution of the power grid is becoming more and more serious. Harmonics can cause problems such as transformer overheating, protection equipment failure, harmonic resonance and communication network interference. In addition, with the development of science and technology and industry, large-capacity reactive equipment such as high-power motors and transformers have appeared in the power system, resulting in low power factor of the power grid. Low power factor will lead to increased line losses, reduced power grid efficiency, and even grid collapse. Shunt active power filter (SAPF) can effectively solve the harmonic pollution of the power grid, improve the power factor of the power grid, improve the power quality of the power grid, and it is easy to disconnect from the power grid in the event of a fault without affecting the operation of other equipment. It is currently widely used in power systems.

[0003] The DC side voltage of SAPF has inherent low-frequency fluctuation problems such as 6 times, 12 times, and 18 times. The traditional approach is to connect a large-capacitance electrolytic capacitor in parallel on the DC side of SAPF to reduce the fluctuation. However, the large-capacitance electrolytic capacitor affects the response speed of the system. In addition, the fluctuating current frequently charges and discharges the capacitor, which accelerates the volatilization of the electrolyte and the aging of the capacitor, seriously affecting the reliability of the device. At the same time, in the DC side voltage control link, in order to prevent the DC side fluctuating voltage from affecting the SAPF output current, a low-pass filter with a very low cut-off frequency needs to be added, which greatly reduces the response speed of the system.

[0004] At present, a small number of scholars have introduced a resonant controller to suppress the fluctuation of the SAPF DC side voltage, but the effect is minimal and it will also have a certain impact on the SAPF output current. From the perspective of topological structure, the present invention transfers the SAPF DC side fluctuation power to the auxiliary capacitor based on active power decoupling technology, thereby realizing the suppression of the SAPF DC side voltage fluctuation. The SAPF DC side voltage finally contains only DC components and high-order harmonic components, so that a small-capacitance film capacitor can replace the traditional large-capacitance electrolytic capacitor to increase the reliability of the device; at the same time, the low-pass filter is eliminated in the DC side voltage control link to speed up the response speed of the system. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a method for suppressing low-frequency ripple of the DC side voltage of a parallel active power filter based on active power decoupling technology. The specific technical implementation plan is as follows:

[0006] A method for suppressing low-frequency ripple of DC side voltage of a parallel active power filter comprises the following steps:

[0007] Step 1: Connect a bidirectional Buck type active power decoupling circuit in parallel on the DC side of the traditional parallel active power filter;

[0008] Step 2: By controlling the bidirectional Buck circuit, the low-frequency fluctuation power on the DC side of the active power filter is transferred to the auxiliary capacitor, thereby achieving low-frequency ripple suppression of the DC side voltage of the active power filter.

[0009] Furthermore, the specific method of connecting the bidirectional Buck type active power decoupling circuit in parallel in step 1 is:

[0010] Two fully controlled devices T1 and T2 and a decoupling inductor L are added to the DC side of the traditional parallel active power filter. r and two film capacitors C r and C d After T1 and T2 are connected in series, both ends are connected to the DC bus. r Passing by L r Connected to the midpoint of T1 and T2 to form a bidirectional Buck type active power decoupling circuit, C d Used to replace traditional electrolytic capacitors.

[0011] Furthermore, the control method of the bidirectional Buck type active power decoupling circuit in step 2 is:

[0012] The decoupling inductor current i Lr With its given value i Lr * The difference is normalized after passing through the multi-PR controller to obtain the modulation wave u of the bidirectional Buck type active power decoupling circuit. f , according to u f T1 and T2 are driven based on PWM technology.

[0013] Furthermore, the decoupling inductor current given value i is obtained. Lr * The specific method is:

[0014] The auxiliary capacitor voltage u r With its given value u r * The difference is obtained after the PI controller r * ; DC side current i dc After passing through the bandpass filter, we get d * ;change i r* and i d * Adding them together gives the given value of the decoupling inductor current i Lr * .

[0015] Furthermore, the Buck type active power decoupling circuit modulation wave u is obtained. f The specific method is:

[0016] The output u of the current controller will be ro Subtract the auxiliary capacitor voltage u r , the difference is then divided by the DC side voltage u dc , the quotient is the modulation wave u of the Buck type active power decoupling circuit f .

[0017] Furthermore, the multi-PR controller of the current control link specifically includes:

[0018] A proportional controller, a resonant controller with a resonant frequency of 600π, a resonant controller with a resonant frequency of 1200π, a resonant controller with a resonant frequency of 1800π and a resonant controller with a resonant frequency of 2400π.

[0019] Furthermore, the bandpass filter of the decoupling current given value calculation link specifically includes:

[0020] A bandpass filter with a resonance frequency of 600π, a bandpass filter with a resonance frequency of 1200π, a bandpass filter with a resonance frequency of 1800π and a bandpass filter with a resonance frequency of 2400π.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1) The present invention can well suppress the low-frequency ripple of the DC side voltage of the active power filter, thereby replacing the traditional large-capacitance electrolytic capacitor with a small-capacitance film capacitor, improving the response speed of the system and increasing the reliability of the device;

[0023] 2) The present invention suppresses the fluctuation of the DC side voltage of the active power filter based on the active power decoupling technology, eliminates the low-pass filter in the DC side voltage control link, and improves the response speed of the system. In addition, an independent active power decoupling circuit is adopted, and its control is separate from the control of the active power filter, which has the characteristics of simple control and easy implementation. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is an active power filter DC side voltage low-frequency ripple suppression circuit and control principle diagram.

[0025] Figure 2 This is the voltage change on the DC side of the active power filter before and after the active power decoupling control is put into operation.

[0026] Figure 3 This is the grid current change before and after the active power decoupling control is put into use.

[0027] Figure 4 This is the FFT analysis of the DC side voltage when active power decoupling control is not enabled.

[0028] Figure 5 FFT analysis of DC voltage when active power decoupling is used to suppress 6-fold frequency fluctuations.

[0029] Figure 6 FFT analysis of DC voltage when active power decoupling is used to suppress 6-fold and 12-fold frequency fluctuations.

[0030] Figure 7 FFT analysis of DC voltage is performed when active power decoupling is used to suppress 6x, 12x, 18x and 24x frequency fluctuations. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. However, it should be understood that the present invention can be implemented in various forms, and some exemplary and non-limiting embodiments shown in the accompanying drawings and described below are not intended to limit the present invention to the specific embodiments described.

[0032] Figure 1 The figure shows the SAPF system and control principle diagram with DC side voltage low-frequency ripple suppression function of the present invention, including a three-phase power grid, a harmonic load, a three-phase filter inductor, a three-phase inverter and a bidirectional Buck type active power decoupling circuit. The bidirectional Buck type active power decoupling circuit specifically includes two fully controlled devices T1 and T2, a decoupling inductor L r and a film capacitor C r After T1 and T2 are connected in series, both ends are connected to the DC bus. r Passing by L r Connected to the midpoint of T1 and T2 to form a bidirectional Buck type active power decoupling circuit. In addition, a small-capacitance film capacitor C is used on the DC side. d To replace the traditional electrolytic capacitor. The Buck type active power decoupling circuit is connected in parallel on the DC side of the SAPF. Its control is independent of the control of the SAPF and will not affect the operation of the SAPF.

[0033] The working principle of the present invention is as follows:

[0034] The present invention is based on independent active power decoupling technology, and controls the bidirectional Buck type active power decoupling circuit to transfer the 6-fold, 12-fold, 18-fold and 24-fold frequency fluctuation power on the DC side of the SAPF to the auxiliary capacitor C r The ripple voltage of the corresponding order in the DC side voltage of the SAPF is eliminated, so that the DC side voltage of the SAPF contains only DC components and other higher-order harmonic components with smaller amplitudes, and the small-capacitance film capacitors are used to replace the traditional large-capacitance electrolytic capacitors, thereby increasing the reliability of the device. At the same time, the low-pass filter is eliminated in the DC side voltage control link to improve the response speed of the system.

[0035] Ideally, the grid connection point voltage only contains positive sequence fundamental components. Assuming that the initial phase angle of the A-phase grid voltage is 0, the three-phase voltage at the grid connection point can be expressed as:

[0036]

[0037] Among them, U s For u sx (x=a,b,c). Considering only the 5th, 7th, 11th, 13th, 17th, 19th, 23rd, and 25th harmonics with larger amplitudes in the load current, the expression of the SAPF output three-phase current can be set as:

[0038]

[0039] According to the instantaneous power theory, the three-phase instantaneous power output of SAPF is:

[0040] E3: p=p a +p b +p c =u sa i oa +u sb i ob +u sc i oc

[0041] Combining E1, E2 and E3, the instantaneous power on the AC side of the SAPF can be calculated as:

[0042]

[0043] E4:

[0044] From E4, we can see that the instantaneous power on the SAPF AC side contains 6th, 12th, 18th and 24th order fluctuating powers. According to the law of conservation of energy, there will also be corresponding number of fluctuating powers on the SAPF DC side, resulting in corresponding number of low-frequency ripples on the DC side voltage. In order to suppress the DC side voltage fluctuation, it is necessary to transfer these fluctuating powers to the auxiliary capacitor by controlling the bidirectional Buck circuit.

[0045] Figure 1 The SAPF system and control principle diagram of the present invention with the function of suppressing low-frequency ripple of DC side voltage, and the specific implementation of the low-frequency ripple suppression method thereof include:

[0046] By controlling the bidirectional Buck circuit, the low-frequency fluctuation power on the DC side of the active power filter is transferred to the auxiliary capacitor, thereby achieving low-frequency ripple suppression of the DC side voltage of the active power filter.

[0047] Furthermore, the control method of the bidirectional Buck type active power decoupling circuit is:

[0048] The decoupling inductor current i Lr With its given value i Lr * The difference is normalized after passing through the multi-PR controller to obtain the modulation wave u of the bidirectional Buck type active power decoupling circuit. f , according to u f T1 and T2 are driven based on PWM technology.

[0049] Furthermore, the decoupling inductor current given value i is obtained. Lr * The specific method is:

[0050] The auxiliary capacitor voltage u r With its given value u r * The difference is obtained after the PI controller r * ; DC side current i dc After passing through the bandpass filter, we get d * ;change i r * and i d * Adding them together gives the given value of the decoupling inductor current i Lr * .

[0051] Furthermore, the Buck type active power decoupling circuit modulation wave u is obtained. f The specific method is:

[0052] The output u of the current controller will be ro Subtract the auxiliary capacitor voltage u r , the difference is then divided by the DC side voltage u dc , the quotient is the modulation wave u of the Buck type active power decoupling circuit f .

[0053] Furthermore, the multi-PR controller of the current control link specifically includes:

[0054] A proportional controller, a resonant controller with a resonant frequency of 600π, a resonant controller with a resonant frequency of 1200π, a resonant controller with a resonant frequency of 1800π and a resonant controller with a resonant frequency of 2400π.

[0055] Furthermore, the bandpass filter of the decoupling current given value calculation link specifically includes:

[0056] A bandpass filter with a resonance frequency of 600π, a bandpass filter with a resonance frequency of 1200π, a bandpass filter with a resonance frequency of 1800π and a bandpass filter with a resonance frequency of 2400π are used to decouple the double frequency power, thereby reducing the double frequency fluctuation of the DC side voltage of the H-bridge module.

[0057] The control method is designed according to the above process, and a simulation experiment is performed using Matlab / Simulink to verify the effectiveness of the present invention. The simulation parameters are shown in Table 1:

[0058] Table 1 Simulation parameters

[0059]

[0060] Figure 2 The SAPF DC side voltage u is shown dcChanges: the active power decoupling circuit was not put into operation before 0.1s, the active power decoupling circuit was put into operation at 0.1s, and only the 6-fold frequency fluctuation controller was put into operation from 0.1s to 0.2s, the 6-fold frequency and 12-fold frequency fluctuation controllers were put into operation from 0.2s to 0.3s, and all controllers were put into operation after 0.3s. It can be seen that when the active power decoupling circuit is not put into operation, the DC side voltage has a large fluctuation, and the fluctuation amplitude is about 20V; the voltage fluctuation decreases from 0.1s to 0.2s, and the fluctuation amplitude is about 8V; the DC side voltage fluctuation further decreases from 0.2s to 0.3s, and the fluctuation is about 3V at this time; the DC side voltage is almost a straight line from 0.3s to 0.4s, containing only some high-order harmonics, and the fluctuation amplitude is about 1V. It can be seen that the SAPF DC side voltage fluctuation suppression method of the present invention can well suppress the SAPF DC side voltage fluctuation, and the DC side voltage responds very quickly. In addition, according to the traditional SAPF capacitance value, the DC side capacitance value is about 5000 μF, while the DC side capacitance value of the SAPF system of the present invention is only 150 μF, which is about 3% of the traditional SAPF capacitance value.

[0061] Figure 3 The change of the grid current is shown. The DC side voltage control link does not add a low-pass filter. Before 0.1s, due to the large fluctuation of the DC side voltage, the SAPF injects new harmonic current into the grid, and the grid current is seriously distorted at this time; 0.1s to 0.2s, as the DC side voltage fluctuation decreases, the grid current distortion also decreases; 0.2s to 0.3s, the DC side voltage fluctuation further decreases, and the grid current distortion also further decreases; 0.3s to 0.4s, the grid current is almost close to a sine wave. It can be seen that the voltage fluctuation degree of the SAPF DC side seriously affects the quality of the grid current. The SAPF DC side voltage fluctuation suppression method of the present invention can eliminate the low-pass filter of the DC side voltage control link, while ensuring the quality of the grid current.

[0062] Figure 4 The FFT analysis of the DC side voltage before 0.1s is given. It can be seen that the DC side voltage contains large 6th, 12th, 18th and 24th harmonics at this time, which is consistent with the theoretical analysis.

[0063] Figure 5 The FFT analysis of the DC side voltage when the 6-fold frequency fluctuation controller is put into use from 0.1s to 0.2s is given. It can be seen that the 6 fluctuations in the DC side voltage are suppressed at this time.

[0064] Figure 6 The FFT analysis of the DC side voltage when the 6-fold and 12-fold frequency fluctuation control controllers are put into use from 0.2s to 0.3s is given. It can be seen that the 6th and 12th frequency fluctuations in the DC side voltage are suppressed at this time.

[0065] Figure 7 The FFT analysis of the DC side voltage when all fluctuation controllers are put into operation from 0.3s to 0.4s is given. It can be seen that the 6th, 12th, 18th and 24th fluctuations in the DC side voltage are all suppressed at this time, which illustrates the effectiveness of the SAPF DC side voltage low-frequency ripple suppression method of the present invention.

[0066] The above embodiments are preferred embodiments of the present invention and are possible examples of embodiments, which are presented only for a clear understanding of the principles of the present invention. Many changes and modifications can be made to the above embodiments without departing from the spirit and principles of the technology described in the present invention. For those skilled in the art, several improvements and modifications can be made without departing from the principles of the present invention, and these changes and modifications should also be considered as the scope of protection of the present invention.

Claims

1. A method for suppressing low-frequency ripple of DC side voltage of a parallel active power filter, characterized in that: The following steps are involved: Step 1: Connect a bidirectional Buck type active power decoupling circuit in parallel on the DC side of the traditional parallel active power filter; Step 2: By controlling the bidirectional Buck circuit, the low-frequency fluctuation power on the DC side of the active power filter is transferred to the auxiliary capacitor, thereby realizing the suppression of the low-frequency ripple of the DC side voltage of the active power filter; The specific method of connecting the bidirectional Buck type active power decoupling circuit in parallel in step 1 is: Two fully controlled devices T1 and T2 and a decoupling inductor L are added to the DC side of the traditional parallel active power filter. r and two film capacitors C r and C d After T1 and T2 are connected in series, both ends are connected to the DC bus. r Passing by L r Connected to the midpoint of T1 and T2 to form a bidirectional Buck type active power decoupling circuit, C d Used to replace traditional electrolytic capacitors; The control method of the bidirectional Buck type active power decoupling circuit in step 2 is: The decoupling inductor current i Lr With its given value i Lr * The difference is normalized after passing through the multi-PR controller to obtain the modulation wave u of the bidirectional Buck type active power decoupling circuit. f , according to u f Drive T1 and T2 based on PWM technology; The modulation wave u of the bidirectional Buck type active power decoupling circuit is obtained f The specific method is: The output u of the current controller ro Subtract the auxiliary capacitor voltage u r , the difference is then divided by the DC side voltage u dc , the quotient is the modulation wave u of the Buck type active power decoupling circuit f ; Get the given value i of the decoupling inductor current Lr * The specific method is: The auxiliary capacitor voltage u r With its given value u r * The difference is obtained after the PI controller r * ; DC side current i dc After passing through the bandpass filter, we get d * ;change i r * and i d * Adding them together gives the given value of the decoupling inductor current i Lr * .

2. A method for suppressing low-frequency ripple of DC side voltage of a parallel active power filter as claimed in claim 1, characterized in that: The multi-PR controller of the current control link specifically includes: A proportional controller, a resonant controller with a resonant frequency of 600π, a resonant controller with a resonant frequency of 1200π, a resonant controller with a resonant frequency of 1800π and a resonant controller with a resonant frequency of 2400π.

3. A method for suppressing low-frequency ripple of DC side voltage of a parallel active power filter as claimed in claim 1, characterized in that: The bandpass filter of the decoupling current given value calculation link specifically includes: A bandpass filter with a resonance frequency of 600π, a bandpass filter with a resonance frequency of 1200π, a bandpass filter with a resonance frequency of 1800π and a bandpass filter with a resonance frequency of 2400π.

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