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

Abstract

The present 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 DC side split capacitor 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 the auxiliary capacitor by controlling the active power decoupling 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 present invention suppresses the 6-fold, 12-fold, 18-fold and 24-fold fluctuations of the DC side voltage of the parallel active power filter. When the DC side voltage fluctuation is small, a thin-film capacitor with a small capacitance can be used to replace the traditional electrolytic capacitor, thereby improving the reliability of the system. In addition, the low-pass filter of the DC side voltage control link of the active power filter is omitted, thereby improving the response speed 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] Active power filters take into account the functions of fast harmonic compensation and reactive power compensation, and are important devices in power quality management. They can well compensate for harmonics caused by nonlinear loads such as various rectifiers and inverters, and improve power quality. Among them, shunt active power filters (SAPF) are easily disconnected from the power grid when a fault occurs without affecting the operation of other equipment, and are currently widely used in power systems.

[0003] The load harmonics in the three-phase symmetrical power grid mainly include 6k+1 (k=1,2,…(natural number)) positive sequence harmonics and 6k+5 (k=0,1,…(natural number)) negative sequence harmonics. When SAPF compensates for these harmonics, it will cause 6k (k=1,2,…(natural number)) fluctuations in its DC side voltage. The magnitude of the DC side voltage fluctuation greatly affects the quality of the SAPF output current. In order to suppress the DC side voltage fluctuation, the traditional approach is to connect a large-capacitance electrolytic capacitor in parallel on the SAPF DC side to slow down the fluctuation. However, the electrolytic capacitor has the defects of easy volatility of electrolyte and short life. Especially when the fluctuating voltage is large, the fluctuating current repeatedly charges and discharges the electrolytic capacitor, accelerating the aging of the capacitor. The electrolytic capacitor needs to be replaced frequently, affecting the reliability of the device. At the same time, the large-capacitance electrolytic capacitor greatly affects the response speed of the DC side voltage. In addition, in order to prevent the DC side fluctuating voltage from affecting the SAPF output current, a low-pass filter with a very low cutoff frequency needs to be added, which further 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 DC side split capacitor active power decoupling circuit in parallel to the DC side of the conventional parallel active power filter;

[0008] Step 2: By controlling the active power decoupling circuit of the DC side split capacitor, the 6-fold, 12-fold, 18-fold and 24-fold frequency fluctuation power of the DC side of the active power filter is transferred to the auxiliary capacitor, thereby realizing the low-frequency ripple suppression of the DC side voltage of the active power filter.

[0009] Furthermore, the specific method of paralleling the DC side split capacitor active power decoupling circuit in step 1 is:

[0010] Two fully controlled switching 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 d1 and C d2 After T1 and T2 are connected in series, the two ends are connected to the positive and negative poles of the DC bus, and the auxiliary capacitor C d1 and C d2 After the series connection, the two ends are connected to the positive and negative poles of the DC bus respectively, and the decoupling inductor L r One end is connected to the midpoint of T1 and T2, and the other end is connected to C d1 and C d2 The midpoint forms a DC side split capacitor active power decoupling circuit.

[0011] Furthermore, the control method of the DC side split capacitor active power decoupling circuit in step 2 is:

[0012] The DC side voltage u of the active power filter dc Get the decoupling inductor current i Lr The given value i Lr * , use i Lr * Subtract i Lr The difference is passed through the current controller, and the output of the current controller is the output reference voltage u of the active power decoupling circuit. ro , use u ro Divided by the DC side voltage u dc Get the modulation wave u of the active power decoupling circuit f , according to u f,T1 and T2 are driven based on PWM technology.

[0013] Furthermore, the DC side voltage u dc Get the decoupling inductor current given value i Lr * The specific method is:

[0014] Use u dc Difference Δu dc After a delay of 90 degrees, Δu is obtained dc-d , Δu dc and Δu dc-d At the same time, the rotation matrix M is sent to reduce the frequency and then i is obtained d * ,i d * After passing through the R controller, the given value of the decoupling inductor current i is obtained. Lr * .

[0015] Furthermore, the decoupling current i Lr The controller specifically includes:

[0016] A proportional controller, a resonant controller with a resonant angular frequency of 300π, a resonant controller with a resonant angular frequency of 900π, a resonant controller with a resonant angular frequency of 1500π and a resonant controller with a resonant angular frequency of 2100π.

[0017] Furthermore, the rotation matrix M for reducing the frequency of the decoupling current given value is specifically:

[0018]

[0019] Where ω is the grid angular frequency.

[0020] Furthermore, the R controller for extracting the decoupling current given value specifically includes:

[0021] A resonant controller with a resonant angular frequency of 300π, a resonant controller with a resonant angular frequency of 900π, a resonant controller with a resonant angular frequency of 1500π and a resonant controller with a resonant angular frequency of 2100π.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 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;

[0024] 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

[0025] Figure 1 The present invention is an active power filter DC side voltage low-frequency ripple suppression circuit and control principle diagram.

[0026] 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.

[0027] Figure 3 This is the grid current change before and after the active power decoupling control is put into use.

[0028] Figure 4 This is the FFT analysis of the DC side voltage when active power decoupling control is not enabled.

[0029] Figure 5 FFT analysis of DC side voltage when active power decoupling control is activated.

[0030] Figure 6 This is the FFT analysis of the grid current when active power decoupling control is not enabled.

[0031] Figure 7 This is the FFT analysis of the grid current when active power decoupling control is activated.

[0032] Figure 8 This is the change in the decoupling capacitor voltage before and after the active power decoupling circuit is put into use. DETAILED DESCRIPTION

[0033] 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.

[0034] Figure 1 The figure shows the SAPF system and control principle diagram of the invention with the function of suppressing the low-frequency ripple of the DC side voltage, including a three-phase power grid, a harmonic load, a three-phase filter inductor, a three-phase inverter and a DC side split capacitor active power decoupling circuit. The DC side split capacitor active power decoupling circuit specifically includes two fully controlled switch devices T1 and T2, a decoupling inductor L r and two film capacitors C d1 and C d2After T1 and T2 are connected in series, both ends are connected to the positive and negative poles of the DC bus, C d1 and C d2 After the series connection, the two ends are connected to the positive and negative poles of the DC bus respectively, and the decoupling inductor L r One end is connected to the midpoint of T1 and T2, and the other end is connected to C d1 and C d2 The midpoint constitutes a DC side split capacitor active power decoupling circuit. The DC side split capacitor active power decoupling circuit is connected in parallel on the DC side of the SAPF, and its control is independent of the control of the SAPF and will not affect the operation of the SAPF.

[0035] The working principle of the present invention is as follows:

[0036] The present invention is based on the independent DC side split capacitor active power decoupling technology, and controls the DC side split capacitor active power decoupling circuit to transfer the 6-fold, 12-fold, 18-fold and 24-fold frequency fluctuation power of the SAPF DC side 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.

[0037] 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:

[0038]

[0039] 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:

[0040]

[0041] According to the instantaneous power theory, the three-phase instantaneous power output of SAPF is:

[0042] E3: p ac =p a +p b +p c =u sa i oa +u sb i ob +u sc ioc

[0043] Combining E1, E2 and E3, the instantaneous power on the AC side of the SAPF can be calculated as:

[0044]

[0045] From E4, we can see that the instantaneous power on the AC side of the SAPF 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 DC side of the SAPF, resulting in corresponding number of low-frequency ripples on the DC side voltage. In order to suppress the voltage fluctuation on the DC side, it is necessary to transfer these fluctuating powers to the auxiliary capacitor by controlling the active power decoupling circuit of the DC side split capacitor.

[0046] To suppress the 6-fold frequency fluctuation voltage with the largest amplitude, two decoupling capacitors C d1 and C d2 The upper voltage is modulated as:

[0047]

[0048] Among them, U dc is the voltage u at the DC side of SAPF dc The DC component, U c for u d1 and u d2 The amplitude of the AC component, δ is u d1 and u d2 The initial phase angle of the AC component relative to the A-phase grid voltage.

[0049] Based on this, the two decoupling capacitor currents and the decoupling inductor current can be expressed as:

[0050]

[0051] E7:i Lr =i d1 -i d2 =3ω(C d1 +C d2 ) c cos(3ωt+δ)

[0052] According to E5~E7, the instantaneous power in the active power decoupling circuit can be obtained as:

[0053]

[0054] Ideally, the two decoupling capacitors are exactly equal, that is, C d1 =C d2 =C d , then E8 can be simplified to:

[0055]

[0056] It can be seen that there is also a 6-fold frequency fluctuation power such as E9 in the decoupling circuit. This 6-fold frequency fluctuation power can be used to balance the 6-fold frequency fluctuation power in E4, thereby suppressing the 6-fold frequency fluctuation of the DC side voltage of the SAPF. At this time, the DC side voltage also contains 12-fold, 18-fold and 24-fold frequency fluctuation voltages. These fluctuation voltages are reduced in frequency by the rotation matrix M and become 9-fold, 15-fold and 21-fold frequency fluctuation voltages respectively. They can be suppressed by resonant controllers with resonant frequencies of 900π, 1500π and 2100π respectively.

[0057] Figure 1 The low-frequency ripple suppression circuit and control principle diagram of the SAPF DC side voltage of the present invention, and the specific implementation of the low-frequency ripple suppression method thereof include:

[0058] By controlling the active power decoupling circuit of the split capacitor on the DC side, the 6-fold, 12-fold, 18-fold and 24-fold 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.

[0059] Furthermore, the control method of the DC side split capacitor active power decoupling circuit is:

[0060] The DC side voltage u of the active power filter dc Get the decoupling inductor current i Lr The given value i Lr * , use i Lr * Subtract i Lr The difference is passed through the current controller, and the output of the current controller is the output reference voltage u of the active power decoupling circuit. ro , use u ro Divided by the DC side voltage u dc Get the modulation wave u of the active power decoupling circuit f , according to u f ,T1 and T2 are driven based on PWM technology.

[0061] Furthermore, the DC side voltage u dc Get the decoupling inductor current given value i Lr * The specific method is:

[0062] Use u dc Difference Δu dc After a delay of 90 degrees, Δu is obtained dc-d , Δu dc and Δudc-d At the same time, the rotation matrix M is sent to reduce the frequency and then i is obtained d * ,i d * After passing through the R controller, the given value of the decoupling inductor current i is obtained. Lr * .

[0063] Furthermore, the decoupling current i Lr The controller specifically includes:

[0064] A proportional controller, a resonant controller with a resonant angular frequency of 300π, a resonant controller with a resonant angular frequency of 900π, a resonant controller with a resonant angular frequency of 1500π and a resonant controller with a resonant angular frequency of 2100π.

[0065] Furthermore, the rotation matrix M for reducing the frequency of the decoupling current given value is specifically:

[0066]

[0067] Where ω is the grid angular frequency.

[0068] Furthermore, the R controller for extracting the decoupling current given value specifically includes:

[0069] A resonant controller with a resonant angular frequency of 300π, a resonant controller with a resonant angular frequency of 900π, a resonant controller with a resonant angular frequency of 1500π and a resonant controller with a resonant angular frequency of 2100π.

[0070] 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:

[0071] Table 1 Simulation parameters

[0072]

[0073] Figure 2 The SAPF DC side voltage u is shown dcChanges: the active power decoupling circuit was not put into operation before 0.1s, and the active power decoupling circuit was put into operation at 0.1s. It can be seen that when the active power decoupling circuit was not put into operation, the DC side voltage had a large fluctuation, and the fluctuation amplitude was about 20V; after the active power decoupling circuit was put into operation, the low-frequency fluctuation of the SAPF DC side voltage was suppressed, and the DC side voltage was almost a straight line, containing only some high-order harmonics, and the fluctuation amplitude was about 2V. It can be seen that the SAPF DC side voltage fluctuation suppression method of the present invention has a good suppression effect on the SAPF DC side voltage fluctuation. In addition, according to the traditional SAPF capacitor value, the DC side capacitance value is about 5000μF, and the DC side capacitance value in the SAPF system of the present invention is only 200μF, which is about 4% of the traditional SAPF capacitance value.

[0074] Figure 3 The change of 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, SAPF injects new harmonic current into the grid, and the grid current is seriously distorted at this time. After 0.1s, the active power decoupling circuit is put into use. As the DC side voltage fluctuation decreases, the grid current distortion also decreases. It can be seen that the degree of voltage fluctuation on the SAPF DC side seriously affects the quality of 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 and ensure the quality of grid current at the same time.

[0075] 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.

[0076] Figure 5 The FFT analysis of the DC side voltage after the active power decoupling circuit is put into use 0.1s later is given. It can be seen that the 6th, 12th, 18th and 24th fluctuations in the DC side voltage are suppressed at this time, indicating the effectiveness of the SAPF DC side voltage low-frequency ripple suppression method of the present invention.

[0077] Figure 6 The FFT analysis of the grid current before 0.1s is given. It can be seen that at this time, due to the large fluctuations in the DC side, the grid current contains a large number of 5th, 7th, 11th, 13th, 17th, 19th, 23rd and 25th harmonics, which is equivalent to the SAPF losing its harmonic compensation capability. The THD of the grid current is 13.64%.

[0078] Figure 7The FFT analysis of the grid current after the active power decoupling circuit is put into operation for 0.1s is given. It can be seen that at this time, due to the suppression of the DC side voltage fluctuation, the SAPF will not inject harmonics into the grid current, and can well compensate for the load harmonics, and the grid current THD = 0.72%. This also further illustrates that the ripple suppression method of the present invention can well suppress the voltage ripple on the DC side of the SAPF, eliminating the low-pass filter in the DC side voltage control link.

[0079] Figure 8 The change of the decoupling capacitor voltage is given. It can be seen that after 0.1s of inputting the active power decoupling circuit, the AC components of the two decoupling capacitor voltages have the same amplitude and a phase difference of 180 degrees, which is consistent with the theoretical analysis.

[0080] 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 parallel active power filter. It is characterized in that The following steps are involved: Step 1: Connect a DC side split capacitor active power decoupling circuit in parallel to the DC side of the conventional parallel active power filter; Step 2: By controlling the active power decoupling circuit of the DC side split capacitor, the 6-fold, 12-fold, 18-fold and 24-fold frequency fluctuation power of 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 DC side split capacitor active power decoupling circuit in parallel in step 1 is: Two fully controlled switching 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 d1 and C d2 After T1 and T2 are connected in series, the two ends are connected to the positive and negative poles of the DC bus, and the auxiliary capacitor C d1 and C d2 After the series connection, the two ends are connected to the positive and negative poles of the DC bus respectively, and the decoupling inductor L r One end is connected to the midpoint of T1 and T2, and the other end is connected to C d1 and C d2 The midpoint of constitutes a DC side split capacitor active power decoupling circuit; The control method of the DC side split capacitor active power decoupling circuit in step 2 is: dc Get the decoupling inductor current i Lr The given value i Lr *, use i Lr * minus i Lr The difference is passed through the current controller, and the output of the current controller is the output reference voltage u of the active power decoupling circuit. ro , use u ro Divided by the DC side voltage u dc Get the modulation wave u of the active power decoupling circuit f , according to u f , drive T1 and T2 based on PWM technology; The DC side voltage u dc Get the decoupling inductor current given value i Lr *The specific method is: Use u dc Difference Δu dc After a delay of 90 degrees, Δu is obtained dc-d , Δu dc and Δu dc-d At the same time, the rotation matrix M is input for down-conversion to obtain i d *,i d *After passing through the R controller, the given value of the decoupling inductor current i is obtained Lr *.

2. A method for suppressing low-frequency ripple of DC side voltage of a parallel active power filter as claimed in claim 1, It is characterized in that The current controller specifically comprises: A proportional controller, a resonant controller with a resonant angular frequency of 300π, a resonant controller with a resonant angular frequency of 900π, a resonant controller with a resonant angular frequency of 1500π and a resonant controller with a resonant angular frequency of 2100π.

3. A method for suppressing low-frequency ripple of DC side voltage of a parallel active power filter as claimed in claim 1, It is characterized in that The rotation matrix M is specifically: Where ω is the grid angular frequency.

4. A method for suppressing low-frequency ripple of DC side voltage of a parallel active power filter as claimed in claim 1, It is characterized in that The R controller specifically includes: A resonant controller with a resonant angular frequency of 300π, a resonant controller with a resonant angular frequency of 900π, a resonant controller with a resonant angular frequency of 1500π and a resonant controller with a resonant angular frequency of 2100π.

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