A harmonic and reactive power compensation switching method for parallel active power filter

Abstract

The present invention discloses a harmonic and reactive power compensation switching method for a parallel active power filter. First, a PAPF composed of a three-level NPC inverter is connected to the common coupling point of a DC bus through an LCL filter; the minimum active power output by the DC bus within a specified grid current THD range is calculated; the reactive power actually output by the PAPF is obtained, thereby obtaining the residual capacity of the PAPF through calculation; when the minimum active power calculation is completed, the PAPF starts to compensate for the load reactive power, while comparing the residual capacity with the minimum active power, and outputting active power according to the comparison result; the output power at the DC bus is obtained according to the output active power, thereby obtaining a battery output current reference value, and the reference value is added to the PAPE harmonic suppression control algorithm; finally, the harmonic components are extracted, and tracking is performed according to the extracted harmonic components.

Description

Technical Field

[0001] The present invention relates to the technical field of parallel active filters, and in particular to a harmonic and reactive power compensation switching method for parallel active filters. Background Art

[0002] Today, power distribution systems are increasingly susceptible to various power quality issues, primarily reflected in factors such as frequency, voltage, and current. Furthermore, the widespread use of nonlinear loads has significantly increased current harmonics. Since then, passive power filters (PPFs) have been widely used to suppress current harmonics in power systems due to their low cost, simple structure, and high efficiency. However, PPFs have several drawbacks, such as poor dynamic performance, difficult-to-handle resonance phenomena, and high sensitivity to power parameters. In this regard, parallel active power filters (PAPFs) not only overcome the inherent shortcomings of PPFs but also provide greater filtering capacity, making them a very promising solution for addressing future current harmonic power quality issues. However, the popular PAPF approach was developed only to compensate for light-load harmonics. As a result, a large amount of redundant capacity remains idle, and the utilization rate of PAPF equipment has become very low.

[0003] In distribution networks, various loads are increasingly generating not only significant harmonics but also high levels of reactive power. This deteriorates power quality, negatively impacting the operation of distribution network equipment, necessitating reactive power compensation. Considering the high investment and operating costs of installing additional power quality compensators, the redundant capacity of PAPFs is highly beneficial for regulating grid power quality. This allows the inverters in PAPFs to be designed with a multifunctional design, fully utilizing their available power capacity, thereby improving equipment utilization and saving additional costs.

[0004] While the primary role of PAPF systems is to perform current harmonic compensation, they can also act as reactive power compensators. In terms of compensation strategy, PAPFs typically operate as global filters, where all harmonic components or reactive power are compensated. This is not a problem because the PAPF's rated capacity is sufficiently large. Otherwise, the multifunctional inverter (MFI) would not be able to provide satisfactory power quality compensation. Therefore, when the PAPF is multifunctional and its pre-designed power capacity cannot meet demand, it is necessary to dynamically allocate the PAPF's available power capacity to compensate for harmonic currents and reactive power. In this sense, dynamic reactive compensation capabilities and coordinated control of harmonic and reactive compensation are very important. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the present invention proposes a harmonic and reactive power compensation switching method for a parallel active filter, and analyzes a reactive power and harmonic compensation coordination method for a nonlinear load with different reactive power and different harmonic content, which ensures that the compensation effect of the nonlinear load harmonics can meet the standard while compensating the load reactive power as much as possible when the parallel inverter operates at a given capacity.

[0006] In order to solve the above technical problems, the technical solution of the present invention is:

[0007] A method for switching harmonic and reactive power compensation of a parallel active power filter, comprising the following steps:

[0008] Step 1: First, connect a PAPF consisting of a three-level NPC inverter to the common coupling point of the DC bus through an LCL filter;

[0009] Step 2: Calculate the minimum active power output by the DC bus within the specified grid current THD range. When the circuit system is started, the input flag signal flag is 1 to PAPF, and the minimum active power calculation is performed. When the grid current harmonic THD value is in the range of 3.5% to 4.5%, the active power output at the DC bus is used as the minimum active power.

[0010] Step 3: Obtain the reactive power actually output by the PAPF, and thereby calculate the remaining capacity of the PAPF;

[0011] Step 4: When the minimum active power calculation is completed, PAPF starts to compensate for the load reactive power, compares the remaining capacity with the minimum active power, and outputs the active power according to the comparison result;

[0012] Step 5: Obtain the output power at the DC bus according to the output active power, thereby obtaining a battery output current reference value, and add the reference value to the PAPE harmonic suppression control algorithm;

[0013] Step 6: Convert the load current from the abc axis to the dq0 axis, and filter out the harmonic components of the load current through a low-pass filter to obtain the fundamental component of the load current on the dq0 axis. After converting it back to the abc axis, subtract the fundamental component from the original load current to extract the harmonic components of the load current.

[0014] Step 7: After obtaining the harmonic components, the harmonic components in the opposite direction of the load current harmonic components are used as the reference value of the PAPF output current, and the PAPF output current I is controlled by the PID controller. pc Track the reference value of PAPF output current, and finally complete the harmonic compensation control of PAPF through PWM control.

[0015] Preferably, in step 2, the minimum active power is calculated as follows:

[0016] First, the active power output at the DC bus is controlled to be the PAPF rated capacity P inverter , obtain the THD value of the current grid current harmonics. If it is less than 3.5%, the value of the active power output at the DC bus is reduced by the power step ΔP (using Simulink simulation circuit system, by observing the fluctuation of the THD value of the grid current harmonics in the simulation circuit, when the fluctuation reaches a flat state, the power step ΔP is obtained through Simulink simulation circuit system). If it is greater than 4.5%, the value of the active power output at the DC bus is increased by the power step ΔP, and in this process, the value of the active power output at the DC bus is limited to 0 to the PAPF rated capacity P inverter Within this range, when the active power output at the DC bus remains constant and continuous, that is, the grid current THD value can be stabilized between 3.5% and 4.5%, the active power output at the DC bus is taken as the minimum active power.

[0017] Preferably, in step 2, during the calculation of the minimum active power, the nonlinear load side is detected by the PAPF harmonic suppression control algorithm. When the nonlinear load side does not change or the reason for the change is not due to a change in the content of the harmonic component, the minimum active power calculation is completed.

[0018] Preferably, in step 2, after the minimum active power calculation is completed, the input flag signal flag is 0, the PAPF stops the minimum active power calculation, and then detects whether the harmonic content on the load side changes through the PAPF harmonic suppression control algorithm. If it changes, the PAPF harmonic suppression control algorithm controls the inverter to stop compensating for the load reactive power, and at the same time, the input flag signal flag1 is 1 to recalculate the minimum active power;

[0019] If there is no change, continue with the subsequent steps and continue to input the flag signal flag as 0 to detect whether the harmonic content on the load side has changed.

[0020] As a preference, in step 3, the remaining capacity P remain Calculation method:

[0021]

[0022] Among them, Q PC is the reactive power actually output by PAPF, S rate is the rated power of PAPF.

[0023] Preferably, in step 4, the comparison method of the remaining capacity and the minimum active power is: if the remaining capacity is less than the minimum active power, the inverter outputs the minimum active power to compensate for the load harmonics; if the remaining capacity is greater than the minimum active power, the remaining capacity is output, and all of this remaining capacity is used to output active power.

[0024] Preferably, in step 5, the formula of the battery output current reference value is as follows:

[0025]

[0026] Among them, P out is the output power of the DC bus, V dc is the DC bus output voltage.

[0027] Preferably, the step 7 further includes, based on the harmonic component tracking, introducing the battery output current reference value i on the d-axis. dc , introduce component i on the q axis q , where component i dc Used to control the DC bus voltage to maintain stability, component i q Used to control the reactive power output of PAPF.

[0028] As a preference, the component i g How to obtain:

[0029] Input flag signal Constant to PAPF flag , according to the comparison result of the remaining capacity and the minimum active power, determine the Need flag The value of

[0030] The flag signal Need flag Transmitted to PAPF, PAPF according to the flag signal Coustant flag and flag signal Need flag Perform reactive power calculations;

[0031] Finally, the output reactive power Q of PAPF is obtained inverter (Q L , Q max ,0) and calculate the component i by the inverter reactive power output q :

[0032]

[0033] in, and are PAPF dq0 axis voltage, is the d-axis load current harmonic component.

[0034] As a preference, in step 7, determining Need flag Value method: When the remaining capacity is less than the minimum active power, Need flag The value is 1. When the remaining capacity is greater than the minimum active power, Need flag The value of is 0.

[0035] Preferably, in step 7, the reactive power is calculated as follows:

[0036] When Constant flag The value of is 1, and Need flag When the value is 1, the output reactive power Q L ;

[0037] Reactive power Q L Detect the reactive power required by the load side for PAPF;

[0038] When Constant flag The value of is 1, and Need flag When the value is 0, the output reactive power Q max ;

[0039] Q max It is the maximum reactive power that can be compensated under rated capacity and guaranteed harmonic compensation effect.

[0040] When Constant flag The value of is 0, that is, the PAPF harmonic suppression control algorithm controls the inverter to stop compensating the load reactive power and re-enter the minimum active power calculation process, then the output reactive power is 0.

[0041] Preferably, the Q max It can be obtained by the following formula:

[0042]

[0043] The present invention has the following characteristics and beneficial effects:

[0044] The above technical solution adopts a three-phase three-level neutral point clamped converter as the PAPF, which serves as the interface between the DC source side of the battery and the AC source side of the grid, improving the power quality; the photovoltaic array can be connected to the DC bus as an independent module, forming a photovoltaic energy storage module with the battery. In the control algorithm, only the photovoltaic output power needs to be taken into account, laying a certain foundation for the subsequent research on the introduction of photovoltaics; the system can fully utilize the PAPF capacity while ensuring that the apparent power flowing through the PAPF does not exceed its rated capacity, greatly protecting the PAPF; the system realizes coordinated control of harmonic compensation and reactive power compensation, which can ensure the load harmonic compensation effect while compensating the load reactive power as much as possible, reducing the reactive power required to be output on the grid side and improving its power factor. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0046] Figure 1 Schematic diagram of the structure of the three-phase three-wire PAPF system in this embodiment.

[0047] Figure 2 This is a flow chart of DC bus active power control in this embodiment.

[0048] Figure 3 This is the PAPF control flow chart in this embodiment.

[0049] Figure 4 These are the THD value, active and reactive power, and the three OPM flag signals in this embodiment.

[0050] Figure 5 In this embodiment, the power flowing through the PAPF, the power required by the load side, and the power emitted / absorbed by the grid side are shown in the three OPMs.

[0051] Figure 6 This is the harmonic compensation in the three OPMs in this embodiment.

[0052] Figure 7 1 is the variation process of the PAPF A-phase output current in the three OPMs in this embodiment.

[0053] Figure 8 are the voltages and currents of the grid, load, and PAPF in the three OPMs in this embodiment. DETAILED DESCRIPTION

[0054] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0055] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings.

[0056] On the contrary, the present invention covers any alternatives, modifications, equivalents, and solutions that fall within the spirit and scope of the present invention as defined by the claims. Furthermore, to facilitate a better understanding of the present invention, certain specific details are described in detail below in the detailed description of the present invention. Those skilled in the art will be able to fully understand the present invention without these details.

[0057] The present invention provides a harmonic and reactive power compensation switching method for a parallel active filter, which is as follows:

[0058] First, a PAPF consisting of a three-level NPC inverter is connected to the common coupling point of the DC bus through an LCL filter.

[0059] Specifically, such as Figure 1 As shown in FIG, in this embodiment, the system for implementing harmonic and reactive power compensation switching includes two subsystems, namely, a parallel active power filter (PAPF) and a DC bus. The PAPF is composed of a three-level NPC inverter, which is connected to the DC bus by an inductor L PC1 and L PC2 and capacitor C PC The LCL filter shown is connected to the common coupling point. The LCL filter is used to reduce the harmonics generated by high-frequency switching. The DC bus consists of a capacitor C in parallel with the DC power supply. dc composition.

[0060] Based on the above system, the multifunctional PAPF control principle of this embodiment mainly involves the control of the parallel NPC inverter and the DC bus.

[0061] In the DC bus control algorithm, the main function of this part is to calculate the active power output of the DC bus within the specified grid current THD range. The minimum active power is configured to obtain the required harmonic compensation performance and send a flag signal to the parallel NPC to assist in reactive power control. The DC bus algorithm control process is composed of Figure 2 shown.

[0062] Depend on Figure 2 It can be seen that the formula for obtaining the reference value of the battery output current is as follows:

[0063]

[0064] Among them, P outis the output power of the DC bus, V dc is the DC bus output voltage. Considering that the DC bus is usually connected to a battery, its charge / discharge current is mainly determined by the active power extracted from the DC bus.

[0065] It should be noted that the output power of the DC bus is the active power fed into the DC bus.

[0066] Furthermore, the control of active power is mainly divided into two parts: calculation of minimum active power and evaluation of output active power.

[0067] Calculation of minimum active power:

[0068] When calculating the minimum active power, the input is in, is the THD value of the grid current, The THD value of the harmonic components on the nonlinear load side. When the circuit system starts up, the input flag signal flag is 1 to PAPF, and the minimum active power calculation is performed. When the grid current harmonic THD value is within the range of 3.5% to 4.5%, the active power output at the DC bus is used as the minimum active power.

[0069] Specifically, such as Figure 2 As shown in the figure, the calculation method of minimum active power is:

[0070] First, the active power output at the DC bus is controlled to be the rated capacity P of the PAPF. inverter , obtain the THD value of the current grid current harmonics. If it is less than 3.5%, the value of the active power output at the DC bus is reduced by the power step ΔP. If it is greater than 4.5%, the value of the active power output at the DC bus is increased by the power step ΔP. In this process, the value of the active power output at the DC bus is limited to 0 to the rated capacity P of the PAPF. inverter Within this range, when the active power output at the DC bus remains constant and continuous, that is, the grid current THD value can be stabilized between 3.5% and 4.5%, the active power output at the DC bus is taken as the minimum active power.

[0071] It should be noted that the power step ΔP is obtained by using the Simulink simulation circuit system by observing the fluctuation of the THD value of the grid current harmonics in the simulation circuit. When the fluctuation reaches a flat state, the power step ΔP is obtained through the Simulink simulation circuit system. inverter The THD value of the current grid current harmonics can also be directly obtained from the current grid current.

[0072] During the calculation of the minimum active power, the nonlinear load side is detected through the PAPF harmonic suppression control algorithm. When there is no change on the nonlinear load side or the reason for the change is not due to the change in the content of the harmonic component, the minimum active power calculation is completed.

[0073] After the minimum active power calculation is completed, the input flag signal flag is 0, PAPF stops the minimum active power calculation, and then uses the PAPF harmonic suppression control algorithm to detect whether the harmonic content on the load side has changed. If it has changed, the PAPF harmonic suppression control algorithm controls the inverter to stop compensating the load reactive power, and at the same time the input flag signal flag1 is 1 to recalculate the minimum active power;

[0074] If there is no change, continue with the subsequent steps, and at the same time, input the flag signal flag to 0, and continue to detect whether the harmonic content on the load side has changed.

[0075] It is understandable that during the entire minimum active power calculation process, the calculation state or the detection state is always saved, and the calculation process and the non-calculation process are switched according to the detection result.

[0076] After the minimum active power calculation is complete, the control begins detecting nonlinear load changes. When the nonlinear load changes, the change is reflected in the THD value of the load's harmonic components. Therefore, the control detects changes by comparing the previous and next THD values ​​of the load's harmonic components at regular intervals. If a change occurs, the control begins to determine the cause of the load-side change. In harmonic compensation, the compensation effect is correlated with the active power. Under the same load harmonic components, sufficient active power results in good harmonic compensation, while insufficient active power results in poor compensation. Therefore, if the load-side change is caused by an increase / decrease in required active / reactive power, while the increase / decrease in required load power increases / decreases the load's fundamental component and thus decreases / increases its THD value, the harmonic content remains unchanged, and therefore the minimum active power does not need to be recalculated. As described above, if the control detects a change in the load's harmonic components, it reenters the first stage and recalculates the minimum active power based on the current minimum active power.

[0077] After completing the minimum active power calculation, the subsequent output active power evaluation is performed:

[0078] Specifically, the reactive power actually output by the PAPF is first obtained, and then the remaining capacity of the PAPF is obtained by calculation.

[0079] Remaining capacity P remain Calculation method:

[0080]

[0081] Among them, Q PC is the reactive power actually output by PAPF, S rate is the rated power of PAPF.

[0082] It can be understood that the reactive power actually output by the PAPF and the rated power of the PAPF can both be directly obtained.

[0083] It can be seen from the above formula that the remaining capacity P remain Refers to the remaining capacity of PAPF after the reactive power of the load is compensated.

[0084] It is understandable that before evaluating the active power, the flag signal flag is first detected. That is, it is determined whether the minimum active power calculation control is calculating the minimum active power. If the minimum active power is being calculated, the control will output a continuously adjusted P inverter To assist in the calculation of the minimum active power and transmit the flag signal Need to the inverter control flag 、Constant flag , among which, Need flag Used to determine whether the remaining capacity of PAPF after it compensates the reactive power of the load is no longer able to compensate the harmonics well. Constant flag This flag is used to determine whether to stop compensating for load reactive power, as this is necessary when calculating minimum active power. Therefore, if the minimum active power is being calculated, the control system uses these two flag signals to inform the inverter control to stop compensating for load reactive power. Furthermore, when the minimum active power calculation is complete, the control system begins compensating for load reactive power and compares the remaining capacity with the minimum active power. If the remaining capacity is less than the minimum active power, it indicates that compensation for load reactive power has impacted harmonic compensation. Therefore, it is necessary to output the minimum active power to prioritize harmonic compensation before compensating for load reactive power. Otherwise, the PAPF has ample remaining capacity after compensating for load reactive power, so the remaining capacity is output and used entirely for active power output.

[0085] After the minimum active power calculation and active power output judgment, the output power at the DC bus is obtained, and the i can be derived according to the battery output current reference value. dc And add it to the PAPF control.

[0086] In a further configuration of this embodiment, in the PAPF harmonic suppression control algorithm, the PAPF control function is to compensate for harmonics and load reactive power. The harmonic compensation process is divided into two parts: harmonic component extraction and harmonic component tracking.

[0087] Specifically, such as Figure 3 As shown in the figure, the harmonic component extraction method is as follows: the load current is converted from the abc axis to the dq0 axis, and the harmonic components of the load current are filtered out by a low-pass filter to obtain the fundamental component of the load current on the dq0 axis. After converting back to the abc axis, the fundamental component is subtracted from the original load current to extract the harmonic components of the load current.

[0088] After obtaining the harmonic components, it is necessary to let PAPF inject harmonic components in the opposite direction into the power grid to offset the harmonic components of the load current, thus completing the compensation of the harmonic components. Figure 3 As shown, the harmonic component in the opposite direction is used as the PAPF output current I pc The reference value is tracked by PID controller, and finally the harmonic compensation control of PAPF is completed by PWM control. In addition, based on the harmonic component tracking, the component i is introduced in the d-axis dc , introduce component i on the q axis q , where component i dc Used to control the DC bus voltage to maintain stability, component i q It is used to control the reactive power output of PAPF. The control process is as follows: Figure 3 shown.

[0089] Furthermore, PAPF reactive power output control mainly includes two parts: reactive power output judgment and reactive power output calculation. The judgment of reactive power output is mainly based on the flag signal Need flag 、Constant flag To determine the output Q L Still output Q max , Q L is the reactive power required by the load, Q max It is the maximum reactive power that can be compensated under rated capacity and guaranteed harmonic compensation effect, Q max It can be obtained by the following formula:

[0090]

[0091] Specifically, when the remaining capacity is less than the minimum active power, Need flag The value is 1. When the remaining capacity is greater than the minimum active power, Need flag The value of is 0.

[0092] When Constant flag The value of is 1, and Need flag When the value is 1, the output reactive power Q L ;

[0093] Reactive power Q LDetect the reactive power required by the load side for PAPF;

[0094] When Constant flag The value of is 1, and Need flag When the value is 0, the output reactive power Q max ;

[0095] Q max It is the maximum reactive power that can be compensated under rated capacity and guaranteed harmonic compensation effect.

[0096] When Constant flag The value of is 0, that is, the PAPF harmonic suppression control algorithm controls the inverter to stop compensating the load reactive power and re-enter the minimum active power calculation process, then the output reactive power is 0.

[0097] In the above technical solution, firstly, the flag signal Constant flag To determine whether the minimum active power is being calculated, if it is in progress, the output is 0, that is, no compensation for load reactive power. Otherwise, the flag signal Need flag To coordinate between harmonic compensation and reactive power compensation, under the condition of ensuring the harmonic compensation effect, if the remaining capacity is sufficient to compensate the load reactive power, then the output Q L To fully compensate for reactive power, if it is insufficient, the maximum reactive power Q will be output max Finally, the output reactive power Q of PAPF is obtained inverter It is understandable that the output reactive power Q of PAPF is inverter Including the following three situations Q L , Q max , 0.

[0098] Furthermore, the component i can be obtained by calculating the reactive power output q :

[0099]

[0100] in, and are the PAPFdq axis voltages, is the d-axis load current harmonic component.

[0101] Finally, this embodiment uses MATLAB / Simulink tools to perform numerical simulation tests on the proposed method according to the description of the above technical solution. As shown in Table 1, the versatility of the PAPF system and its flexible switching between functions can be highlighted by the number of its operating modes (OPMs), as described below.

[0102] Table 1

[0103]

[0104] 1) OPM I: In OPM I, the reactive power required by the load is small and less than the maximum value of reactive power compensation. In this case, the PAPF uses part of its capacity to compensate the load reactive power and the rest of its capacity is used to deliver active power.

[0105] 2) OPMII: In OPMII, the required load reactive power is too large, exceeding the maximum reactive power compensation value. In this case, PAPF prioritizes the harmonic compensation capability of the grid current to obtain a THD value of less than 5%, and the rest is used to compensate the load reactive power.

[0106] 3) OPMIII: In OPMIII, if the load impedance changes, causing its harmonic content to change, the system restarts the calculation of the minimum active power and inputs OPM I and OPM II as needed.

[0107] Therefore, this test is divided into three parts, and the rated capacity of PAPF is set to 60kVA. Figure 4 In the first part, the load side is connected to a nonlinear load with a reactive power of 5000Var and a THD of 59.11%. First, the system starts to calculate the minimum power P required for harmonic compensation. min .

[0108] During the calculation process, it is necessary to stop the harmonic compensation of the load, so Constant flag is 0, Q out is 0, such as Figure 4 (g) and (h). In the calculation, P inverter Will be based on The value is used to adjust the size. If it is greater than 4.5%, it will be enlarged, and if it is less than 3.5%, it will be reduced.

[0109] from Figure 4 As can be seen in (c), at around 0.4s, It has entered the 3.5%-4.5% range, but has not stabilized yet, so P inverter Continue adjusting until about 0.5s, It has stabilized in the range of 3.5%-4.5%, P inverter No longer changes. At the minimum power P min After the calculation is completed, the system starts to compensate for the load reactive power. flag Set to 1, Q out Start outputting power, and the remaining capacity of PAPF is used to output active power.

[0110] like Figure 4 In the second part, the load side is connected to a nonlinear load with a reactive power of 50,000Var and a THD of 13.8%. Compared with the first part, the required reactive power is greatly increased, while the harmonic content remains unchanged.

[0111] from Figure 4 (b) As can be seen, since the load harmonic content has not changed, there is no need to recalculate the minimum active power P min However, the reactive power required by the load exceeds the maximum value. flag Set to 1, therefore, the system only compensates for part of the reactive power, and its remaining capacity is P min To compensate for harmonics. In the third part, the load side is connected to a nonlinear load with a reactive power of 5000Var and a THD of 71.53%. Compared with the first part, the required reactive power remains unchanged, but the harmonic content changes. Due to the change in harmonic content, the minimum power P min Recalculation is required. During the calculation process, reactive power compensation stops. After the calculation is completed, the load reactive power is compensated and the remaining capacity is used to output active power.

[0112] like Figure 5 As shown in (b) and (e), the reactive power Q flowing through the PAPF pc The reactive power Q required by the load L To compensate, the grid side does not need to output or output less reactive power to compensate the load side, such as Figure 5 (g). In addition, Figure 5 (c) Since the rated capacity of PAPF is limited, the apparent power flowing through PAPF is within the rated capacity in the stable state, which can protect and fully utilize the inverter.

[0113] Figure 6 The harmonic compensation situation is given. In the whole process, the system can finally compensate for the harmonics and make the grid side current I s The THD value is below 5%.

[0114] The experimental parameters are shown in Table 2, and the overall process is as follows Figure 7 As shown, the process consists of three parts.

[0115] Table 2

[0116]

[0117]

[0118] Initially, the control algorithm is ineffective, and the PAPF does not compensate for load reactive power. The system then enters the first phase, where the control begins calculating the minimum active power, as reflected by the decreasing PAPF output current in the figure. After achieving the minimum active power, the system begins to compensate for reactive power and uses the remaining PAPF capacity for active power output. Because some of the capacity is used for reactive power delivery, the PAPF output current amplitude is smaller than before control began. Subsequently, the reactive power demanded by the load increases, exceeding the maximum reactive power that the PAPF can compensate. Therefore, no matter how much the reactive power demanded by the load increases, the PAPF output current amplitude can only remain at the level shown in the figure. In the third phase, the resistance of the nonlinear load is reduced to half of its original value, resulting in an increase in the harmonic content of the nonlinear load and a corresponding increase in the minimum active power. Therefore, the system begins recalculating the minimum active power. As shown in the figure, the PAPF output current continues to increase from its original amplitude until the new minimum active power value is achieved.

[0119] Figure 8 The voltage and current of the grid, PAPF, and load are shown under three different scenarios. The active and reactive power of the PAPF and load, as well as the THD of the grid current, are shown in Table 3. This shows that under different scenarios, the PAPF compensates for the load's reactive power and harmonics based on its capacity, and during this compensation process, the grid current THD remains below 5%.

[0120] Table 3

[0121]

[0122]

[0123] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. It will be apparent to those skilled in the art that various changes, modifications, substitutions, and variations of these embodiments, including components, without departing from the principles and spirit of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for harmonic and reactive power compensation switching of a parallel active power filter, characterized in that: Here are the steps: Step 1: First, connect a PAPF consisting of a three-level NPC inverter to the common coupling point of the DC bus through an LCL filter; Step 2: Calculate the minimum active power output by the DC bus within the specified grid current THD range. When the circuit system is started, the input flag signal flag is 1 to PAPF, and the minimum active power calculation is performed. When the grid current harmonic THD value is in the range of 3.5% to 4.5%, the active power output at the DC bus is used as the minimum active power. Step 3: Obtain the reactive power actually output by the PAPF, and thereby calculate the remaining capacity of the PAPF; Step 4: When the minimum active power calculation is completed, PAPF starts to compensate for the load reactive power, compares the remaining capacity with the minimum active power, and outputs the active power according to the comparison result; Step 5: Obtain the output power at the DC bus according to the output active power, thereby obtaining a battery output current reference value, and add the reference value to the PAPF harmonic suppression control algorithm; Step 6: Convert the load current from the abc axis to the dq0 axis, and filter out the harmonic components of the load current through a low-pass filter to obtain the fundamental component of the load current on the dq0 axis. After converting it back to the abc axis, subtract the fundamental component from the original load current to extract the harmonic components of the load current. Step 7: After obtaining the harmonic components, the harmonic components in the opposite direction of the load current harmonic components are used as the reference value of the PAPF output current, and the PAPF output current I is controlled by the PID controller. pc Track the reference value of PAPF output current, and finally complete the harmonic compensation control of PAPF through PWM control.

2. The method for harmonic and reactive power compensation switching of a parallel active filter according to claim 1, characterized in that: In step 2, the calculation method of the minimum active power is: First, the active power output at the DC bus is controlled to be the PAPF rated capacity P inverter , obtain the THD value of the current grid current harmonics. If it is less than 3.5%, the value of the active power output at the DC bus is reduced by the power step ΔP. If it is greater than 4.5%, the value of the active power output at the DC bus is increased by the power step ΔP. In this process, the value of the active power output at the DC bus is limited to 0 to the PAPF rated capacity P inverter Within this range, when the active power output at the DC bus remains constant and continuous, that is, the grid current THD value can be stabilized between 3.5% and 4.5%, the active power output at the DC bus is taken as the minimum active power.

3. The method for harmonic and reactive power compensation switching of a parallel active filter according to claim 2, characterized in that: In step 2, during the calculation of the minimum active power, the nonlinear load side is detected by the PAPF harmonic suppression control algorithm. When the nonlinear load side does not change or the reason for the change is not due to a change in the content of the harmonic component, the minimum active power calculation is completed.

4. The method for harmonic and reactive power compensation switching of a parallel active filter according to claim 2, wherein: In step 2, after the minimum active power calculation is completed, the input flag signal flag is 0, the PAPF stops the minimum active power calculation, and then detects whether the harmonic content on the load side changes through the PAPF harmonic suppression control algorithm. If it changes, the PAPF harmonic suppression control algorithm controls the inverter to stop compensating for the load reactive power, and at the same time the input flag signal flag1 is 1 to recalculate the minimum active power; If there is no change, continue with the subsequent steps, and at the same time, input the flag signal flag to 0, and continue to detect whether the harmonic content on the load side has changed.

5. The method for harmonic and reactive power compensation switching of a parallel active power filter according to claim 1, characterized in that: In step 3, the remaining capacity P remain Calculation method: Among them, Q PC is the reactive power actually output by PAPF, S rate is the rated power of PAPF.

6. The method for harmonic and reactive power compensation switching of a parallel active power filter according to claim 1, characterized in that: In step 4, the comparison method between the remaining capacity and the minimum active power is as follows: if the remaining capacity is less than the minimum active power, the inverter outputs the minimum active power to compensate for the load harmonics; if the remaining capacity is greater than the minimum active power, the remaining capacity is output and all of the remaining capacity is used to output active power.

7. The method for harmonic and reactive power compensation switching of a parallel active power filter according to claim 1, characterized in that: In step 5, the formula of the battery output current reference value is as follows: Among them, P out is the output power of the DC bus, V dc is the DC bus output voltage.

8. The method for harmonic and reactive power compensation switching of a parallel active power filter according to claim 7, characterized in that: The step 7 further includes introducing the battery output current reference value i on the d-axis based on the harmonic component tracking. dc , introduce component i on the q axis q , where component i dc Used to control the DC bus voltage to maintain stability, component i q Used to control the reactive power output of PAPF.

9. The method for harmonic and reactive power compensation switching of a parallel active power filter according to claim 8, characterized in that: Component i q How to obtain: Input flag signal Constant to PAPF flag , according to the comparison result of the remaining capacity and the minimum active power, determine the Need flag The value of The flag signal Need flag Transmitted to PAPF, PAPF according to the flag signal Constant flag and flag signal Need flag Perform reactive power calculations; Finally, the output reactive power Q of PAPF is obtained inverter And the component i is obtained by calculating the reactive power output of the inverter q : in, and are PAPF dq0 axis voltage, is the d-axis load current harmonic component.

10. The method for harmonic and reactive power compensation switching of a parallel active power filter according to claim 9, characterized in that: In step 7, determine the Need flag Value method: When the remaining capacity is less than the minimum active power, Need flag The value is 1. When the remaining capacity is greater than the minimum active power, Need flag The value of is 0.

11. The method for harmonic and reactive power compensation switching of a parallel active power filter according to claim 9, characterized in that: In step 7, the reactive power is calculated as follows: When Constant flag The value of is 1, and Need flag When the value is 1, the output reactive power Q L ; Reactive power Q L Detect the reactive power required by the load side for PAPF; When Constant flag The value of is 1, and Need flag When the value is 0, the output reactive power Q max ; Q max It is the maximum reactive power that can be compensated under rated capacity and guaranteed harmonic compensation effect. When Constant flag The value of is 0, that is, the PAPF harmonic suppression control algorithm controls the inverter to stop compensating the load reactive power and re-enter the minimum active power calculation process, then the output reactive power is 0.

12. The method for harmonic and reactive power compensation switching of a parallel active power filter according to claim 11, characterized in that: The Q max It can be obtained by the following formula: Among them, S rate is the rated power of PAPF.

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