Harmonic and reactive power compensation switching method of parallel active filter

Abstract

The invention discloses a harmonic and reactive power compensation switching method for a parallel active power filter, which comprises the following steps of: firstly, connecting a PAPF (Power Active Power Filter) consisting of a three-level NPC (Non-Point-to-Point) inverter to a common coupling point of a direct-current bus through an LCL (Low Current Limit) filter; calculating the minimum active power output by the direct current bus within the specified power grid current THD range; obtaining the reactive power actually output by the PAPF, and obtaining the residual capacity of the PAPF through calculation; when the calculation of the minimum active power is completed, the PAPF starts to compensate the reactive power of the load, compares the residual capacity with the minimum active power at the same time, and outputs the active power according to the comparison result; according to the output active power, obtaining the output power at a direct current bus so as to obtain a battery output current reference value, and adding the reference value to a PAPE harmonic suppression control algorithm; and finally, extracting a harmonic component, and tracking according to the extracted harmonic component.

Description

Technical Field

[0001] The 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] Nowadays, distribution systems are increasingly susceptible to various power quality problems, which are mainly reflected in many factors such as frequency, voltage and current. In addition, the widespread use of nonlinear loads has greatly increased current harmonics. Since then, passive filters (PPF) have been widely used to suppress current harmonics in power systems due to their low cost, simple structure and high efficiency. However, PPF has some disadvantages, such as poor dynamic performance, difficult-to-handle resonance phenomenon and high sensitivity to power parameters. In this sense, parallel active power filters (PAPF) not only overcome the inherent disadvantages of PPF, but also provide greater filtering capacity, which is a very promising solution to solve future current harmonic power quality problems. However, the popular PAPF method is only developed to compensate for light load harmonics. As a result, a large amount of redundant capacity is idle and the utilization rate of PAPF equipment becomes very low.

[0003] In the distribution network, there is a trend that various loads not only bring a large amount of harmonics, but also generate high reactive power. As a result, the power quality is further deteriorated, which is harmful to the operation of equipment in the distribution network, and it is necessary to compensate the reactive power of the load. Considering the high investment and operating costs of installing additional power quality compensators, the redundant capacity of PAPF will be very beneficial to the regulation of power quality in the power grid. In this way, the inverter in PAPF can be designed in a multifunctional manner, and the effective power capacity needs to be fully utilized, thereby improving equipment utilization and saving additional costs.

[0004] Although the main role of PAPF systems is to perform current harmonic compensation, they can also act as reactive power compensators. In terms of compensation strategy, PAPF usually operates as a global filter in which all harmonic components or reactive power are compensated. This is not a problem because the rated capacity of PAPF is large enough. Otherwise, the multifunctional inverter (MFI) cannot provide satisfactory power quality compensation. Therefore, when PAPF acts as a multifunctional and its pre-designed power capacity cannot meet the demand, it is necessary to dynamically allocate the feasible power capacity of PAPF to compensate for harmonic current and reactive power. In this sense, dynamic reactive compensation capability and coordinated control of harmonic and reactive compensation are very important. Summary of the invention

[0005] In view of the shortcomings of the prior art, 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 under different reactive powers and different harmonic contents, and a parallel inverter working at a given capacity, to ensure that the compensation effect of the harmonics of the nonlinear load can meet the standard while compensating the reactive power of the load as much as possible.

[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 filter, the steps are as follows:

[0008] Step 1: 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;

[0009] Step 2, calculate the minimum active power output of 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 is calculated. When the grid current harmonic THD value is in the range of 3.5% to 4.5%, the value of the active power output at the DC bus is taken as the minimum active power.

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

[0011] Step 4: When the minimum active power is calculated, PAPF starts to compensate for the load reactive power, and 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 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 , get 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 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). 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 value of the active power output at the DC bus remains unchanged and continuous, that is, the grid current THD value can be stabilized between 3.5% and 4.5%, the value of 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, and when there is no change on the nonlinear load side 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, 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 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] Preferably, 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 the 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] Where 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] Give PAPF input flag signal Constant 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 reactive power output of the inverter to obtain the component i q :

[0032]

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

[0034] Preferably, in step 7, determining Need flag Value method: When the remaining capacity is less than the minimum active power, Need flag The value of 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 by:

[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 The reactive power required by the load side is detected 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-enters 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 is adopted, and a three-phase three-level neutral point clamped converter is used as the PAPF, which serves as an interface between the DC source side of the battery and the AC source side of the power grid, thereby improving the power quality; the photovoltaic array can be connected to the DC bus as an independent module, and form a photovoltaic energy storage module with the battery. In the control algorithm, only the photovoltaic output power needs to be considered, which lays a certain foundation for the subsequent introduction of photovoltaic research; the system can fully utilize the PAPF capacity while ensuring that the apparent power flowing through the PAPF does not exceed its rated capacity, thereby greatly protecting the PAPF; the system realizes coordinated control of harmonic compensation and reactive power compensation, and 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 drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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 creative labor.

[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 flag signals in the three OPMs in this embodiment.

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

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

[0052] Figure 7 Graph 1 shows 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 substitution, modification, equivalent method and scheme made on the essence and scope of the present invention as defined by the claims. Further, in order to make the public have a better understanding of the present invention, some specific details are described in detail in the detailed description of the present invention below. Those skilled in the art can fully understand the present invention without the description of 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] Firstly, 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, Figure 1 As shown in FIG. 1 , 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. PC1 and L PC2 And the capacitor C PC The LCL filter shown is connected to the common coupling point. The LCL filter is used to mitigate the harmonics generated by high-frequency switching. The DC bus consists of a capacitor C connected 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 of the DC bus output 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 algorithm control flow of the DC bus 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] Where 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, It is the THD value of the harmonic component on the nonlinear load side. 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 taken as the minimum active power.

[0069] Specifically, Figure 2 As shown, 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 value of the active power output at the DC bus remains unchanged and continuous, that is, the grid current THD value can be stabilized between 3.5% and 4.5%, the value of 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 It can be directly obtained from PAPF. The THD value of the current grid current harmonics can also be directly obtained from the current grid current.

[0072] In the process of calculating 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 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 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, the input flag signal flag is 0, and continue to detect whether the harmonic content on the load side has changed.

[0075] It can be understood 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] That is, after the minimum active power is calculated, the control starts to detect the change of nonlinear load. When the nonlinear load changes, its change will be reflected in the THD value of the load harmonic component. Therefore, by comparing the previous and next values ​​of the load harmonic component THD at regular intervals to detect whether it has changed, if it has changed, the control starts to determine the reason for the change on the load side. In harmonic compensation, the compensation effect is associated with active power. Under the same load harmonic component conditions, if the active power is sufficient, the harmonic compensation effect is good, and if it is insufficient, the effect is poor. Therefore, if the reason for the change on the load side is that the required active power / reactive power increases / decreases, although the increase / decrease in the power required on the load side causes the fundamental component of the load to increase / decrease, thereby causing its THD value to decrease / increase, the content of the harmonic component therein does not change, so the minimum active power does not need to be recalculated. According to the above description, if the control detects a change in the load harmonic component, it will enter the first stage again and recalculate 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 be directly obtained.

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

[0084] It can be understood 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 It is used to determine whether it is necessary to stop compensating the load reactive power, because it is necessary to stop compensating the load reactive power when calculating the minimum active power. Therefore, if the minimum active power is being calculated, the control informs the inverter control to stop compensating the load reactive power through these two flag signals. In addition, when the minimum active power is calculated, the control system will start to compensate the load reactive power and start to compare the remaining capacity and the minimum active power. If the remaining capacity is less than the minimum active power, it means that the compensation for the load reactive power has affected the effect of harmonic compensation. Therefore, it is necessary to output the minimum active power, give priority to the harmonic compensation effect, and then compensate the load reactive power. On the contrary, it means that the remaining capacity of PAPF is still sufficient after compensating the load reactive power, so the remaining capacity is output and all of this remaining capacity is used to output active power.

[0085] After the minimum active power calculation and active power output judgment, the output power at the DC bus is obtained, so that 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, 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 the harmonic components in the opposite direction into the power grid to offset the harmonic components of the load current, so as to complete 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 the PID controller, and finally the harmonic compensation control of PAPF is completed through 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, the reactive power output control of PAPF mainly includes two parts: reactive power output judgment and reactive power output calculation. The reactive power output judgment is mainly based on the flag signal Need flag 、Constant flag To determine the output Q L 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 of 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 LThe reactive power required by the load side is detected 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-enters 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 being calculated, the output is 0, that is, the load reactive power is not compensated. Otherwise, the flag signal Need flag To coordinate the harmonic compensation and reactive power compensation, if the remaining capacity is sufficient to compensate the reactive power of the load while ensuring the harmonic compensation effect, the output Q L To fully compensate for reactive power, if it is insufficient, the maximum reactive power Q is output max Finally, the output reactive power Q of PAPF is obtained. inverter It is understandable that the output reactive power Q of PAPF inverter Including the following three situations Q L , Q max , 0.

[0098] Furthermore, the reactive power output can be used to calculate the component i q :

[0099]

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

[0101] Finally, according to the records of the above technical solutions, this embodiment uses MATLAB / Simulink tools to carry out numerical simulation tests on the proposed method. As shown in Table 1, the versatility of the PAPF system and the flexible switching between its 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 very small and less than the maximum value of reactive power compensation. In this case, PAPF uses part of its capacity to compensate the load reactive power and the remaining capacity is used to deliver active power.

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

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

[0107] Based on this, 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 It can be seen from (c) that at around 0.4s, It has entered the 3.5%-4.5% range, but has not stabilized yet, so P inverter Continue to adjust 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 reactive power of the load. flag Set to 1, Q out When power output starts, 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, and 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 part of the reactive power, and its remaining capacity is P min Used to compensate harmonics. In the third part, the nonlinear load with reactive power of 5000Var and THD of 71.53% is connected to the load side. 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 outputs 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 a 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 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. The overall process is as follows Figure 7 As shown, the process consists of three parts.

[0115] Table 2

[0116]

[0117]

[0118] At the beginning, the control algorithm did not work and the PAPF did not compensate for the reactive power of the load. Then, the system entered the first part, in which the control began to calculate the minimum active power, which is also reflected by the continuous decrease of the PAPF output current in the figure. After obtaining the minimum active power, the system began to compensate for the reactive power and used the remaining PAPF capacity to output active power. Since part of the capacity is used to transmit reactive power, it can be seen from the figure that the amplitude of the PAPF output current is smaller than the amplitude before the control started. Then, the reactive power required on the load side increased, and its value exceeded the maximum reactive power that the PAPF can compensate. Therefore, no matter how the reactive power required on the load side is increased, the amplitude of the PAPF output current can only be maintained at the level shown in the figure. In the third part, the resistance value of the resistor in the nonlinear load is reduced to half of the original value, resulting in an increase in the harmonic content of the nonlinear load and an increase in the minimum active power. Therefore, the system began to recalculate the minimum active power. As shown in the figure, the PAPF output current continues to increase on the basis of the original amplitude until a new minimum active power value is obtained.

[0119] Figure 8 The voltage and current of the grid, PAPF, and load in three cases are shown. The active power and reactive power of PAPF and load and the THD value of grid current are shown in Table 3. It can be seen that under different conditions, PAPF will compensate for the reactive power and harmonics of the load according to its own capacity, and during the compensation process, the THD value of the grid current is 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. For those skilled in the art, various changes, modifications, substitutions and variations of these embodiments including components are made without departing from the principles and spirit of the present invention, and still fall within the scope of protection of the present invention.

Claims

1. A method for harmonic and reactive power compensation switching of a parallel active filter, characterized in that: Here are the steps: Step 1: 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; Step 2, calculate the minimum active power output of 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 is calculated. When the grid current harmonic THD value is in the range of 3.5% to 4.5%, the value of the active power output at the DC bus is taken as the minimum active power. Step 3: Obtain the reactive power actually output by the PAPF, and thereby obtain the remaining capacity of the PAPF by calculation; Step 4: When the minimum active power is calculated, PAPF starts to compensate for the load reactive power, and 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 PAPE 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 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 value of the active power output at the DC bus remains unchanged and continuous, that is, the grid current THD value can be stabilized between 3.5% and 4.5%, the value of the active power output at the DC bus is taken as the minimum active power.

3. The harmonic and reactive power compensation switching method 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 through the PAPF harmonic suppression control algorithm. When the nonlinear load side does not change 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.

4. The method for harmonic and reactive power compensation switching of a parallel active filter according to claim 2, characterized in that: In step 2, after the minimum active power calculation is completed, the input flag signal flag is 0, 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 the load reactive power, and 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, the input flag signal flag is 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 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 harmonic and reactive power compensation switching method of a parallel active filter according to claim 1, characterized in that: 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 the remaining capacity is used to output active power.

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

8. The harmonic and reactive power compensation switching method of a parallel active filter according to claim 7, characterized in that: The step 7 also 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.

9. The harmonic and reactive power compensation switching method of a parallel active filter according to claim 8, characterized in that: Component i q How to obtain: Give PAPF input flag signal Constant 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 (Q L , Q max , 0) and calculate the reactive power output of the inverter to obtain the component i q : in, and are the 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 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 of 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 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 The reactive power required by the load side is detected 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-enters 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 filter according to claim 11, characterized in that: The Q max It can be obtained by the following formula:

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