Active power filter control method and system suitable for frequency fluctuation

Abstract

The invention discloses an active power filter control method and system suitable for frequency fluctuation, and the method employs an FIR filter based on a Lagrange interpolation method to correct the decimal part of the sampling times, and employs a variable bandwidth adjustment internal model control (VBAI) technology to improve the bandwidth after frequency fluctuation. The problem that the control effect is seriously reduced due to the fact that traditional repeated control sampling times change caused by power grid frequency fluctuation is effectively solved, and control gain stability under frequency fluctuation is guaranteed.

Description

Technical Field

[0001] The invention relates to power electronic control technology, in particular to an active power filter control method and system suitable for frequency fluctuation. Background Art

[0002] Active power filter (APF) is a key power quality device used to compensate for grid harmonics, which can effectively control harmonic pollution. APF generally contains active components that use the inverter output as a power source. By detecting the load harmonic current and applying a specific algorithm, the output voltage value of the inverter can be controlled to generate a compensation current with the same amplitude and complementary phase as the harmonic current to achieve the purpose of filtering. At present, the most commonly used APF is the shunt active power filter (SAPF). APF mainly includes the main circuit, detection circuit, control circuit and drive circuit. For a three-phase distribution network, the main circuit is generally a three-phase thyristor bridge circuit, the detection circuit is ip-iq detection based on the instantaneous power method, the control circuit often uses PR and hysteresis control, and the drive circuit includes SPWM and SVPWM modulation circuits. The control circuit involves the core part of the APF work and is the most important link for it to exert its filtering effect. At present, the research on APF control technology under stable grid frequency is relatively sufficient, but there are still few APFs that can achieve high stability control technology under industrial control changes, especially grid frequency fluctuations, resulting in defects such as incomplete filtering of harmonics and slow response of APF when frequency fluctuates.

[0003] Repetitive control technology (RC) can be well applied in APF because it can achieve error-free tracking of periodic reference signals. Figure 1 As shown in the figure, the theory of efficient tracking and control of periodic signals is based on the internal model principle. The "internal model" in the internal model principle refers to a mathematical dynamic model of an external signal embedded in a feedback loop. The "internal model" describes the dynamic characteristics of the external signal. The so-called external signal includes both the input signal and the disturbance signal. When an accurate "internal model" is added to the feedback path of a closed-loop control system, theoretically the control system can control the input signal and the disturbance signal at the same time, and can also accurately track the input signal and eliminate the disturbance signal.

[0004] from Figure 2Mathematical analysis shows that the "inner model" system has N poles, and all of the N poles are located on the unit circle. According to the necessary and sufficient conditions for determining the stability of discrete systems, the poles located on the unit circle are critical stability poles, and the entire system is in a critically stable state. Once the load changes, the entire system is likely to become unstable. This is why typical repetitive control has poor robustness, especially when the grid frequency changes, which leads to a reduction in the overall APF effect. Figure 1 It is also shown that the repetitive control determines from the basic structure that its output signal always has a period of hysteresis, and this hysteresis is unavoidable, which has a relatively large impact on the dynamic performance of the APF when the operating conditions change (especially the grid frequency changes).

[0005] Based on this, some improved technologies are proposed as follows:

[0006] The prior art indicates that a quadratic compensator C2(z) can be connected in series with the inner model part. C2(z) is generally a constant less than 1 or a low-pass filter to achieve the shift of the poles inwards of the unit circle to improve the stability of the system. Figure 3 This solution only improves the stability of the traditional repetitive control of a single channel, but cannot improve the stability of the repetitive control when the grid frequency fluctuates, which will cause insufficient system compensation and instability.

[0007] Other studies have pointed out that in order to improve the response speed of repetitive control, the dynamic response can be improved by connecting the PI controller in parallel to the entire system, and the dynamic response can be improved by increasing the proportional coefficient, such as Figure 4 shown.

[0008] The existing solution will show poor adaptability when facing grid frequency fluctuations. Specifically, after the grid frequency changes, the number of IRC sampling will change. For example, if the sampling frequency is 10kHz and the grid frequency fluctuates to 49.6Hz, the number of samplings N = 10k / 49.6 = 201.61. IRC cannot control the decimal part of N, resulting in control of the sampling number that is still an integer multiple, and the control effect will be seriously reduced. Figure 5 It represents the amplitude-frequency characteristic of the control system. Taking the green line where the 9th harmonic is located as an example, during normal operation, the frequency of IRC at the 9th harmonic is 50*9=450Hz, and the control gain at this time is 35.38dB. When the fundamental frequency drops to 49.6Hz, the 9th harmonic frequency is 49.6*9=446.4Hz, and the gain at this time drops to 7.79dB, and the filtering effect on harmonics is seriously reduced. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide an active power filter control method and system suitable for frequency fluctuations in view of the shortcomings of the existing technology, so as to solve the problem that the control effect is seriously reduced due to the change of the traditional repeated control sampling times caused by the frequency fluctuation of the power grid, and ensure the stability of the control gain under frequency fluctuations.

[0010] In order to solve the above technical problems, the technical solution adopted by the present invention is: an active power filter control method suitable for frequency fluctuations, comprising the following steps:

[0011] S1, detect the harmonic current in the distribution network and determine whether there is harmonic current; if so, proceed to step S2;

[0012] S2. Calculating a current command value using the harmonic current;

[0013] S3, modulating the current command value through SVPWM to obtain a switch signal, and using the switch signal to control the on and off of the active power filter switch tube;

[0014] in,

[0015] The current command value I ref The calculation formula is: Among them, I Lh is the detected harmonic current; G FVBAF (z) is the transfer function of the current tracking control link, D(z) is the disturbance signal, T(z) is the repetitive control compensator, G(z) is a constant, R(z) represents the detected harmonic current, G pi (z) is a PI controller;

[0016]

[0017] K 00 , K 01 , K 10 and K 11 is the proportionality coefficient, K Icc1 and K Icc2 represents the proportion of odd and even bandwidth control, C2(z) is the quadratic compensator, N is the number of sampling times, and C2(z) is the quadratic compensator.

[0018] The present invention adopts the FIR filter (AIMSF technology) based on Lagrange interpolation method to correct the decimal part of the sampling times, which effectively solves the problem of serious reduction in control effect caused by changes in the number of traditional repeated control sampling due to grid frequency fluctuations, and ensures the stability of control gain under frequency fluctuations.

[0019] Among them, f sw represents the sampling frequency, fs is the fundamental frequency.

[0020] The secondary compensator C2(z) is a constant less than 1 or a low-pass filter.

[0021] w0=2π / T0.

[0022] K 00 With K 11 The sum is 1.

[0023] After using the switch signal to control the on and off of the active power filter switch tube, the method further includes:

[0024] S4. Continue to detect harmonic current. If the harmonic current does not exist, end; if not, return to S2.

[0025] In the present invention, T(z)=Kz n S(z), K is the proportionality coefficient, z n is the phase advance link coefficient, S(z) is the filter link function, In the present invention, z n For a simple advance effect, in implementation, n Considered as a constant, the size of the advance is determined by n.

[0026] As an inventive concept, the present invention also provides an active power filter control system suitable for frequency fluctuations, including a memory, a processor and a computer program stored in the memory; the processor executes the computer program to implement the steps of the above method.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The present invention effectively solves the problem of serious reduction in control effect caused by changes in the number of traditional repeated control sampling due to grid frequency fluctuations, and ensures the stability of control gain under frequency fluctuations;

[0029] 2. Using the decoupling concept, harmonic sampling is divided into odd harmonic sampling and even harmonic sampling for parallel control. Under ideal conditions, the response speed can be increased by 2 times, improving the dynamic performance of the system when the frequency fluctuates.

[0030] 3. When the grid frequency fluctuates, the control bandwidth can be significantly increased, such as within the 50th harmonic

[0031] The bandwidth of (50*50=2.5kHz) will be increased, which improves the stability of the system and enables the APF to maintain a good compensation effect when the grid frequency changes. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1It is the inner model of the traditional IRC control;

[0033] Figure 2 It is the IRC control principle structure;

[0034] Figure 3 IRC control principle structure to improve dynamic performance;

[0035] Figure 4 This is the principle structure diagram of the parallel APF;

[0036] Figure 5 Control gain analysis for repetitive control under frequency variation;

[0037] Figure 6 The schematic diagram of adaptive internal model sampling frequency control (AIMSF) for embedding FIR filter into the repetitive control forward channel;

[0038] Figure 7 is the gain after adopting the AIMSF technology;

[0039] Figure 8 This is the decoupling schematic diagram of the inner model;

[0040] Fig. 9 Schematic diagram of the bandwidth control internal model added for improvement;

[0041] Fig.10 This is the control principle diagram of Kicc for controlling bandwidth;

[0042] Fig.11 This is a circuit diagram of an embodiment of the present invention;

[0043] Fig.12 This is a control principle diagram of an embodiment of the present invention;

[0044] Fig.13 The filtering effect of FVBAF-RC-APF under standard working conditions (50Hz) of the embodiment of the present invention; (a) before filtering, (b) after filtering, (c) current waveforms before and after filtering;

[0045] Fig.14 Analysis of filtering effectiveness under distribution network frequency fluctuations; (a) IRC-APF compensation effect at 49.5Hz, (b) FVBAF-RC-APF compensation effect at 49.5Hz;

[0046] Fig.15 Analysis of filtering effectiveness under frequency fluctuation of distribution network; (a)f L =49.5Hz grid side current THD analysis, (b)f H =50.05Hz grid side current THD analysis;

[0047] Fig.16This is the compensated grid current when simulating grid frequency fluctuation; (a) the simulated grid frequency is 49.5 Hz, (b) the simulated grid frequency is 50.5 Hz;

[0048] Fig.17 This is the FVBAF-RC-APF workflow diagram. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0050] Example 1

[0051] The embodiment of the present invention designs a parallel active power filter, which mainly includes a harmonic current detection link, a current tracking control link, a PWM switch drive link and a current conversion main circuit link. Fig.11 As shown. The main power conversion circuit is a full-bridge circuit with parallel energy storage elements on the DC side. Because the working state of the circuit will undergo changes in inversion and rectification, it is called a converter. The principle of the parallel APF is: first, the harmonic current on the load side is sampled through the harmonic current detection link to extract the load harmonic current. The current tracking control link obtains the compensation command current based on the load harmonic current value and the DC side capacitor voltage value, and compares the current with the actual output current of the converter for control. The control technology adopts the variable bandwidth adaptive sampling frequency repetitive control (VBAF-RC) technology proposed in the embodiment of the present invention to obtain a PWM control signal pulse to control the on and off of the main circuit switch tube. At this time, the converter acts as an inverter and outputs a compensation current that is equal to the harmonic current in magnitude and opposite in phase to achieve harmonic cancellation.

[0052] The embodiment of the present invention proposes a variable bandwidth adjustment internal model control (VBAI) cascaded adaptive internal model sampling frequency control (AIMSF) technology, namely, a variable bandwidth adaptive sampling frequency repetitive control technology (VBAF-RC), which is specifically described as follows:

[0053] 1. The finite impulse response filter (FIR) based on Lagrange interpolation method is often used in some filter design scenarios with irregular frequency response requirements, such as multi-band filters or filters with special frequency response shapes. The fractional part of the sampling number is the typical form of the special frequency response, so this filter can be used to correct the sampling number.

[0054] Figure 6 It means embedding the FIR filter into the adaptive internal model sampling control technology (AIMSF) of the repeated control forward channel, and cascading the FIR filter into the forward channel of the control system to achieve gain correction. Still taking the 9th harmonic as an example, Figure 7 The blue line in the amplitude-frequency curve is the gain after using the adaptive internal model sampling frequency control technology (AIMSF), which obviously clamps the weak gain of 7.79dB back to 35dB, ensuring the stability of the control gain under frequency fluctuations.

[0055] When the grid frequency changes, especially when the grid frequency decreases, the introduction of the adaptive internal model sampling frequency control technology (AIMSF) will cause the internal model cycle delay to become longer, and the PI controller parameters originally used by IRC to adjust the dynamic performance will not be in the original optimal state, which reflects the slow dynamic performance of the entire system when the frequency fluctuates. Using the decoupling idea, the harmonic sampling is divided into odd harmonic sampling and even harmonic sampling, and they are controlled in parallel. Because the number of parallel sampling is reduced by half, the response speed can be increased by 2 times under ideal conditions, such as Figure 8 shown.

[0056] Further research found that the adaptive internal model sampling frequency control technology (AIMSF) also has serious attenuation in higher harmonic frequency bands and when the grid frequency is too large. The embodiment of the present invention proposes a variable bandwidth adjustment internal model control (VBAI) technology to force the gain of IRC in the harmonic frequency band to increase, that is, to widen the control bandwidth. The added control structure is as follows Fig. 9As shown in the figure, the added structure can be considered as a "special internal model", which essentially uses RC to control RC itself. The transfer function of the structure is analyzed and the internal control coefficient (K Iccn ,n=0,1), by adjusting KIccn, the width of the gain bandwidth can be dynamically adjusted. The control structure is as follows Fig.10 shown.

[0057] exist Fig. 9 In the , odd harmonic control and even harmonic control channels are each connected in parallel with a bandwidth control channel, namely odd bandwidth control and even bandwidth control, forming a four-mode composite repetitive control link. These four modules are essentially the transformation of the internal model, that is, adding a "special internal model" to achieve the purpose of RC controlling RC. Each of the four channels has an odd and even control coefficient, the odd control coefficient K0 = K 00 +K 01 , even control coefficient K1=K 11 +K 10 , where K 01 With K 10 Also related to the internal control coefficient K ICC1 and K ICC2 They work together to achieve bandwidth control of odd and even frequencies.

[0058] Kicc1 and Kicc2 represent the proportion of odd-order and even-order bandwidth control. In order to maintain the stability of the system, Kicc1 or Kicc2 should be greater than 0.5. If the value of Kicc continues to increase, the bandwidth between odd-order and even-order harmonics will also increase. When Kicc approaches infinity, it may even cause the gain of the entire frequency band to present a straight line, that is, the bandwidth is infinite. Of course, the control effect is lost at this time.

[0059] K00, K01, K11 and K10 represent the proportional coefficients of the three channels. K00 and K10 determine the filtering ratio of odd harmonics and even harmonics respectively. When the odd harmonic content is greater than the even harmonic content, K00>K10 can be set. Generally speaking, the sum of K00 and K10 is 1. K01 and K10 determine the bandwidth ratio of odd harmonics and even harmonics respectively, and can also affect the size of the gain. When K01>K0, the gain of odd harmonics increases and the gain of even harmonics is weakened. When K01>>K10, it may even cause even harmonics to no longer have positive gain.

[0060] Fig.10It shows the change of control bandwidth when Kicc1 and Kicc2 are equal and change according to 10, 2.5 and 1.0. Obviously, as Kicc increases, the control bandwidth at the frequency point will increase significantly, and the bandwidth up to the 50th harmonic (50*50=2.5kHz) will be raised. The bandwidth of the whole system will also increase, greatly improving the stability when the frequency fluctuates. Fig.10 As shown in (a) and (b), if the value of K01 or K10 is changed, the bandwidth control strength of Kicc1 and Kicc2 can be achieved. When K01=1 and K10=20, the odd-order bandwidth is narrowed and the even-order bandwidth is widened.

[0061] Based on the above improvements, the complete diagram of the core circuit design proposed in the embodiment of the present invention is as follows: Fig.11 As shown in the figure, the corresponding complete core control technology, namely the four-mode bandwidth variable adaptive sampling frequency repetitive control technology (FVBAF-RC) principle structure diagram is shown in Fig.12 shown.

[0062] Fig.12It represents the control principle structure of the four-mode bandwidth variable and adaptive sampling times type repetitive control technology (FVBAF-RC). It has four forward channels of repetitive control "inner models". The channels where K00 and K01 are located jointly form the odd harmonic control, and the channels where K10 and K11 are located jointly form the even harmonic control. When the input signal R(z) of the system enters, it is first responded by the PI control link, which controls the signal immediately. After delaying half a fundamental wave period, the repetitive control "inner model" starts to take effect. Each channel contains specific links. C2(z) is a secondary compensator, which is generally a constant less than 1 or a low-pass filter, used to stabilize the poles of the "inner model" control link. FIR is a finite impulse response filter based on the Lagrange linear interpolation method, used to track the fractional part of the sampling times caused by frequency deviation. Kicc is the harmonic bandwidth control coefficient. By adjusting the relative magnitudes of Kicc1 or Kicc2, the increase or decrease of the odd and even bandwidths can be achieved. When Kicc1 > Kicc2, the bandwidth at the odd frequency increases; when Kicc1 < Kicc2, the bandwidth at the even frequency increases. Except for the odd and even bandwidth control channels, the Kicc values in other channels are mainly used to resist excessive changes in peak gain. T(z) is the repetitive control compensator, mainly used to compensate the amplitude-frequency and phase-frequency characteristics of the controlled object G(z) to meet the zero gain and zero phase shift of the system. Therefore, its composition generally includes a proportional link, a phase lead link, and a filtering link, etc. D(z) is the disturbance signal, which is generally directly added to the output signal C(z), and then the output signal is transmitted back to the input signal through the negative feedback link. By continuously adjusting the value of the deviation signal E(z) to approach 0, the stable control of the input signal can be achieved. Through the continuous control of the PI controller and the FVBAF-RC controller, generally, good tracking control of the input signal can be completed within less than 10 fundamental wave periods.

[0063] The embodiment of the present invention has four forward channels of repetitive control "inner models", K 00 and K 01 in the channels where they are located jointly form the odd harmonic control, K 10 and K 11 in the channels where they are located jointly form the even harmonic control. When the input signal R(z) of the system enters, it is first responded by the PI control link, which controls the signal immediately. After delaying half a fundamental wave period, the repetitive control "inner model" starts to take effect. Each channel contains specific links. C2(z) is a secondary compensator, which is generally a constant less than 1 or a low-pass filter, used to stabilize the poles of the "inner model" control link.

[0064] FIR is a finite impulse response filter based on the Lagrange linear interpolation method, used to track the fractional part of the sampling times caused by frequency deviation, K Iccis the harmonic bandwidth control coefficient, by adjusting K Icc1 or K Icc2 The relative size of K can realize the increase or decrease of odd and even bandwidth. Icc1 >K Icc2 When K Icc1 <K Icc2 When , the bandwidth at the even frequency increases. Except for the odd and even bandwidth control channel, K in other channels Icc The value is mainly to resist excessive changes in peak gain. T(z) is a repetitive control compensator, which is mainly used to compensate the amplitude-frequency and phase-frequency characteristics of the control object G(z) to meet the zero gain and zero phase shift of the system. Therefore, its composition generally includes a proportional link, a phase advance link, and a filtering link.

[0065] D(z) is a disturbance signal, which is usually added directly to the output signal C(z), and then the output signal is transmitted back to the input signal through the negative feedback link. The value of the deviation signal E(z) is continuously adjusted to approach 0 to achieve stable control of the input signal. Through the continuous control of the PI controller and the FVBAF-RC controller, generally, the input signal can be better tracked and controlled in less than 10 fundamental cycles.

[0066] right Fig.12 The transfer function expression of the FVBAF-RC control link is shown as

[0067]

[0068] In formula (1), K 00 , K 01 , K 10 and K 11 Represents the proportional coefficients of the four channels, K Icc1 and K Icc2 represents the proportion of odd-even bandwidth control, C2(z) is the quadratic compensator, N is the number of sampling times of repeated control, and its calculation formula is

[0069]

[0070] f in formula (2) sw represents the sampling frequency, f s is the fundamental frequency.

[0071] In the continuous time domain, when the quadratic compensator C2(z) is close to 1, the transfer function of the odd and even harmonic control link can be expressed as

[0072]

[0073] Taylor expansion is performed on equation (3), and we get

[0074]

[0075] In formula (4), T0 is the fundamental period, w0 is the angular frequency of the fundamental signal, and the two satisfy the relationship w0 = 2π / T0. From formula (4), it can be found that the FVBAF-RC controller has poles at the DC or integer multiples of the fundamental frequency, and the gain of all odd harmonics and even harmonics is infinite. In theory, all harmonics can be tracked and controlled without static error.

[0076] K 00 With K 11 Determine the filtering ratio of odd harmonics and even harmonics respectively. When the odd harmonic content is greater than the even harmonic content, K 00 >K 11 Generally speaking, K 00 With K 11 The sum is 1. When K 11 When =0, the even-order bandwidth control fails and the system is equivalent to a pure odd-order harmonic control link.

[0077] K 01 and K 10 They determine the bandwidth ratio of odd harmonics and even harmonics respectively, and can also affect the gain. 01 >K 10 When K 01 >>K 10 It may even cause the even harmonics to no longer have positive gain.

[0078] Equation (5) gives the transfer function expansion of the traditional IRC. Combining equation (4), it can be seen that the error convergence speed of FVBAF-RC control is half of that of IRC under ideal conditions.

[0079]

[0080] Depend on Fig.12 The error transfer function of the system is obtained as

[0081]

[0082] G in formula (6) FVBAF (z) is the transfer function of the four-mode bandwidth variable adaptive sampling repetitive control link expressed by equation (1), R(z) represents the input signal, D(z) is the disturbance signal, T(z) is the repetitive control compensator, and G(z) is the control object. Therefore, the characteristic polynomial of the system is

[0083] Δ=1+[G pi (z)+G FVBAF(z)T(z)]G(z) (7)

[0084] Assumptions

[0085]

[0086] Formula (8) can actually be understood as an equivalent control model when only the PI controller is in effect, and the FVB AF-RC controller does not work. Then formula (7) can be expressed as

[0087] Δ=[1+G pi (z)G(z)]·[1+G FVBAF (z)T(z)G s (z)] (9)

[0088] Therefore, the stability condition of the FVBAF-RC system is that all solutions of equation (10) are within the unit circle, which is the stability condition of the system.

[0089]

[0090] Substituting the stability condition into equation (1) and equation (8) for mathematical analysis, it can be obtained that the coupling relationship between the parameters should satisfy the following equation:

[0091]

[0092] K in formula (11) RC represents the total equivalent gain of repetitive control, Indicates the system phase angle The cosine value of Represents the equivalent control model G s According to the stability condition, the parameters of the aforementioned FVBAF-RC transfer function are designed. By substituting appropriate control parameters, the amplitude-frequency response and phase-frequency curve of the equivalent control object can be obtained. From the figure, it can be seen that the gain of the control object in the medium and low frequency bands is very close to 0dB, and there is no obvious phase lag, while there is a relatively high negative gain in the high frequency band. At this time, although the phase has a large lead, the impact on the system is small due to the serious gain attenuation, and the comprehensive control effect is good.

[0093] The design difficulty of the embodiment of the present invention is that it is difficult to widen the bandwidth at the odd and even specific frequencies in the four-mode variable bandwidth adaptive sampling frequency repetitive control technology (FVBAF-RC). Therefore, it is difficult for traditional IRC control to maintain the original control gain when the grid frequency changes, thereby affecting the control effect. The bandwidth control structure proposed in the embodiment of the present invention is designed based on the "internal model" structure unique to the repetitive control itself. While maintaining good control of the periodic signal, simple and convenient bandwidth adjustment can be achieved by simply adjusting the internal control coefficient.

[0094] Fig.12 The meanings of the symbols involved are shown in Table 1.

[0095] Table 1 Meaning of each part of the current tracking control link

[0096]

[0097]

[0098] Fig.12 The transfer function shown in is:

[0099]

[0100] The above expression represents the relationship between the output signal and the input signal. The output signal C(z) can be obtained by multiplying the input signal R(z) by the above transfer function. This expression can characterize all the properties of the current tracking control link proposed in the embodiment of the present invention.

[0101] The mathematical relationships of each link are further explained as follows:

[0102]

[0103] Fig.12 The process is briefly described as follows:

[0104] ① The detected harmonic current is taken as the input signal, and negative feedback is calculated with the output signal of the previous cycle (initial 0) to obtain the deviation signal E(z), which is sent to step ②;

[0105] ②E(z) is sent to the FVBAF-RC control link, through which the preliminary command signal Y(z) (main command signal) is obtained and sent to step ③;

[0106] ③ The initial command signal Y(z) passes through the repeated control compensator T(z), and the obtained signal is added with another command signal (not the main command signal, which can be ignored) obtained by the PI controller, and acts on the controlled object together, and is added with other disturbance signals to obtain the final control current command value, which is output to the drive circuit and fed back to the initial control stage (before the deviation signal). The end condition of the feedback is that the deviation signal is 0. It is impossible for the deviation to reach 0 in actual operation, so the control will continue.

[0107] Simulation experiment based on Matlab / Simulink platform

[0108] The simulation parameters are shown in Table 2 below.

[0109] Table 2 Complete simulation circuit diagram

[0110]

[0111]

[0112] Fig.13 It shows the compensation effect of the designed FVBAF-RC-APF on the grid-side current when the grid frequency is 50Hz. At 0.4s, the grid frequency fluctuates and decreases from 50Hz to 49.5Hz. As can be seen from the figure, the APF reduces the load-side harmonics from THDi=21.44% to about 2.31% in less than 10 working cycles (10*1 / 50s), and operates stably under standard working conditions.

[0113] Fig.14 It shows the compensation effect of the traditional IRC-APF and the FVBAF-RC-APF designed by the present invention on the grid side current when the grid frequency is reduced to 49.5Hz. Fig.14 It can be seen from (a) that when the grid frequency of IRC-APF decreases from 50Hz to 49.5Hz at 0.3s, the response time becomes longer and the control effect becomes worse. THDi increases from the original stable compensation of 3.56% to 9.18%, which exceeds the upper limit of 5% stipulated in national standards such as "Power Quality: Public Grid Harmonics (GB / T14549-1993)", "Power Quality: Public Grid Interharmonics (GB / T24337-2009)" and "Grid Operation Guidelines (GB / T 31464-2015)", and the effect of harmonic filtering is relatively unsuccessful. Fig.14 It can be seen from (b) that the FVBAF-RC-APF designed in the embodiment of the present invention can still ensure stable compensation within 10 working cycles under the condition of frequency fluctuation, and the harmonic THDi is reduced to 3.57, which only increases by 1.26%, and has a good compensation effect.

[0114] The THD analysis of the FVBAF-RC-APF designed in the embodiment of the present invention is performed when the grid frequency decreases to 49.5 Hz and increases to 50.5 Hz. Fig.15 When the grid frequency drops to 49.5 Hz, the current THDi in steady state is only 2.47%, and when the grid frequency rises to 50.5 Hz, the current THDi in steady state is only 2.13%, indicating that the APF designed in the embodiment of the present invention has good adaptability to grid frequency fluctuations.

[0115] The FVBAF-RC-APF designed in the embodiment of the present invention was simulated by a physical experiment on the RT-LAB platform to further verify its applicability in engineering applications. Fig.15 As shown, through the simulated grid generator, the simulated grid frequency is reduced to 49.5Hz and increased to 50.5Hz. At this time, the grid side current after FVBAF-RC-APF filtering is as follows: Fig.16 As shown in the figure, no matter whether the grid frequency is rising or falling, the grid-side current sinusoidality detected by the oscilloscope is good, indicating that the FVBAF-RC-APF invented in practical applications has good applicability to wide frequency fluctuations, high system operation stability, and certain production application value.

[0116] As mentioned above, the grid harmonic current under frequency fluctuation conditions can be compensated stably and efficiently. Specifically, the APF system after using the embodiment of the present invention can stably reduce the harmonic THD value of the grid current and ensure that the sinusoidality of the current waveform is at a high level.

[0117] The variable bandwidth adaptive sampling frequency repetitive control (VBAF-RC) technology has the following advantages:

[0118] 1. Adapt to grid frequency fluctuations: Adopt FIR filter (AIMSF technology) based on Lagrange interpolation method to correct the fractional part of sampling times, effectively solve the problem of serious reduction of control effect caused by changes in sampling times of traditional repeated control due to grid frequency fluctuations, and ensure the stability of control gain under frequency fluctuations. For example, when the grid frequency fluctuates from 50Hz to 49.6Hz, the gain of traditional control technology drops sharply at the 9th harmonic, while the use of AIMSF technology can clamp the gain back to a higher level, ensuring effective filtering of harmonics.

[0119] 2. Improve the dynamic performance of the system: By using the decoupling concept, the harmonic sampling is divided into odd harmonic sampling and even harmonic sampling for parallel control. Under ideal conditions, the response speed can be increased by 2 times, thus improving the dynamic performance of the system during frequency fluctuations.

[0120] 3. Enhance system stability: Variable bandwidth adjustment internal model control (VBAI) technology uses RC to re-control RC itself by adding a special internal model structure, and introduces an internal control coefficient to adjust the gain bandwidth. When the grid frequency fluctuates, the control bandwidth can be significantly increased. For example, the bandwidth up to the 50th harmonic (50*50=2.5kHz) will be increased, which improves the stability of the system and enables the APF to maintain a good compensation effect when the grid frequency changes. For example, when the grid frequency fluctuates within the range of 50Hz±0.5Hz, this technology can enable the APF to stably compensate for the grid harmonic current, reduce the harmonic THD value of the grid current, and ensure that the sinusoidality of the current waveform is high.

[0121] 4. Flexible control method: By adjusting parameters such as the internal control coefficient, odd-order control coefficient and even-order control coefficient, the control of harmonics of different frequencies can be achieved simply and conveniently, including the adjustment of filtering ratio, bandwidth ratio and gain size to adapt to different grid conditions and load characteristics.

[0122] Example 2

[0123] Embodiment 2 of the present invention provides a terminal device corresponding to the above-mentioned embodiment 1. The terminal device may be a processing device for a client, such as a mobile phone, a laptop computer, a tablet computer, a desktop computer, etc., to execute the method of the above-mentioned embodiment.

[0124] The terminal device of this embodiment includes a memory, a processor, and a computer program stored in the memory; the processor executes the computer program in the memory to implement the steps of the method in the above-mentioned embodiment 1.

[0125] In some implementations, the memory may be a high-speed random access memory (RAM), and may also include a non-volatile memory, such as at least one disk memory.

[0126] In some other implementations, the processor may be a central processing unit (CPU), a digital signal processor (DSP), or other general-purpose processors of various types, which are not limited herein.

[0127] Example 3

[0128] Embodiment 3 of the present invention provides a computer-readable storage medium corresponding to the above embodiment 1, on which a computer program / instruction is stored. When the computer program / instruction is executed by a processor, the steps of the method of the above embodiment 1 are implemented.

[0129] Computer readable storage media can be tangible devices that hold and store instructions used by instruction execution devices. Computer readable storage media can be, for example, but not limited to, electronic storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any combination thereof.

[0130] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of complete hardware embodiments, complete software embodiments, or embodiments in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The scheme in the embodiments of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0131] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0132] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0133] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0134] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A control method for an active power filter suitable for frequency fluctuations, characterized in that: The following steps are involved: S1. Detect the harmonic current in the distribution network and determine whether there is harmonic current; If yes, go to step S2; S2. Calculating a current command value using the harmonic current; S3, modulating the current command value through SVPWM to obtain a switch signal, and using the switch signal to control the on and off of the active power filter switch tube; in, The current command value I ref The calculation formula is: Among them, I Lh is the detected harmonic current; G FVBAF (z) is the transfer function of the current tracking control link, D(z) is the disturbance signal, T(z) is the repetitive control compensator, G(z) is a constant, R(z) represents the detected harmonic current, G pi (z) is the PI controller, z is the complex frequency variable of the active power filter; K 00 , K 01 , K 10 and K 11 is the proportionality coefficient, K Icc1 and K Icc2 represents the proportion of odd and even bandwidth control, C2(z) is the quadratic compensator, N is the number of sampling times, and C2(z) is the quadratic compensator.

2. The active power filter control method applicable to frequency fluctuation according to claim 1, characterized in that: Among them, f sw represents the sampling frequency, f s is the fundamental frequency.

3. The active power filter control method applicable to frequency fluctuation according to claim 1, characterized in that: The secondary compensator C2(z) is a constant less than 1 or a low-pass filter.

4. The active power filter control method applicable to frequency fluctuation according to claim 3, characterized in that: w0=2π / T0.

5. The active power filter control method applicable to frequency fluctuation according to claim 1, characterized in that: K 00 With K 11 The sum is 1.

6. The active power filter control method applicable to frequency fluctuation according to claim 1, characterized in that: After using the switch signal to control the on and off of the active power filter switch tube, the method further includes: S4. Continue to detect harmonic current. If the harmonic current does not exist, end; if not, return to S2.

7. The active power filter control method applicable to frequency fluctuation according to claim 1, characterized in that: T(z)=Kz n S(z), K is the proportionality coefficient, z n is the phase advance link coefficient, S(z) is the filter link function, 8. An active power filter control system suitable for frequency fluctuations, comprising a memory, a processor and a computer program stored in the memory; characterized in that: The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 7.

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