Unified control method for harmonic waves by photovoltaic inverter and active power filter

Abstract

The invention discloses a unified control method for harmonic waves by a photovoltaic inverter and an active power filter, and the method comprises the steps: detecting load reactive and harmonic current, converting the load reactive and harmonic current into reactive and harmonic compensation instruction current, and carrying out the maximum power point tracking of a solar photovoltaic array, thereby obtaining active grid-connected instruction current; and then the reactive power and harmonic compensation instruction current and the photovoltaic grid-connected instruction current can be combined, and then a current tracking control method is used for controlling a converter to inject the combined instruction current into a power grid. At night or in cloudy and rainy days, the photovoltaic grid-connected power generation system cannot work due to no electric energy output, and at the moment, the photovoltaic inverter is directly used as an active power filter to compensate reactive power and harmonic current generated by nonlinear equipment and improve the power supply performance of a power grid, so that the photovoltaic grid-connected power generation can be completed on the same device, and the power supply efficiency is improved. Harmonic suppression and reactive compensation are realized, so that a photovoltaic grid-connected power generation system always operates in an optimal economic mode, and equipment investment is effectively saved.

Description

Technical Field

[0001] The invention relates to the technical field of power system harmonic control and photovoltaic grid-connected power generation, and in particular to a unified control method for harmonics by a photovoltaic inverter and an active power filter. Background Art

[0002] With the access of distributed power sources and the widespread application of power electronic equipment, the power electronic characteristics of modern distribution networks are becoming more and more obvious, and the harmonic pollution of power grids is becoming more and more serious.

[0003] The current photovoltaic grid-connected power generation devices and active power filters (APF) each have their own shortcomings. Specifically, due to the change of sunlight during the day and night, photovoltaic grid-connected power generation devices only work during the day and need to be disconnected from the grid at night, which not only affects the utilization rate of the equipment, but also the frequent switching will affect the stability of the grid; while APF can compensate for reactive and harmonic currents, but its cost is high, its function is single, and it is difficult to promote. APF can provide reactive power and various harmonics to the grid, while photovoltaic grid-connected power generation systems only provide active power. Therefore, the two only provide different power to the grid, but have many similarities in structure and control. At this time, unified control can be implemented for both.

[0004] Therefore, how to suppress harmonics while photovoltaic grid-connected power generation, effectively reduce the impact of harmonics on the power grid, and improve the efficiency and stability of photovoltaic grid-connected power generation systems has become an urgent problem to be solved in the field of power system harmonic control and photovoltaic grid-connected power generation technology. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a unified control method for harmonics of photovoltaic inverters and active power filters, which can solve the impact of harmonic problems on power quality in existing distributed power grid systems. Through a unified control system, harmonics can be suppressed while photovoltaic grid-connected power generation is being performed, effectively reducing the impact of harmonics on the power grid and improving the efficiency and operating stability of photovoltaic grid-connected power generation systems.

[0006] In order to achieve the above object, the present invention is implemented by the following technical solutions:

[0007] The present invention is a unified control method for harmonics of a photovoltaic inverter and an active power filter, comprising the following steps:

[0008] The grid voltage phase is tracked by a digital phase-locked loop, and Clarke transformation and Parker transformation are performed on each load current to obtain instantaneous active current and instantaneous reactive current. The AC components in the instantaneous active current and instantaneous reactive current are filtered out by a low-pass filter to obtain fundamental active current DC and fundamental reactive current DC. Then, the fundamental active current DC and fundamental reactive current DC are subjected to inverse Parker transformation and inverse Clarke transformation to obtain load fundamental current, and the load fundamental current is subtracted from the three-phase load current to obtain harmonic current;

[0009] The photovoltaic inverter power is controlled based on the voltage and current double closed-loop proportional-integral control strategy, including comparing the photovoltaic grid-connected command current with the maximum power point tracking command voltage, obtaining the actual command current of the photovoltaic grid-connected through the voltage regulator, combining the actual command current with the obtained harmonic current to obtain the total command current, and obtaining the current inner loop equation of the photovoltaic inverter when it is working through feedforward control decoupling and controlling the photovoltaic inverter accordingly;

[0010] The space vector pulse width modulation technology is used to uniformly control the photovoltaic grid-connected and active power filter, including determining the relationship between the line voltage vector, switch variables and switch vector output by the photovoltaic inverter, transforming the three-phase coordinate system into a two-phase coordinate system based on the space vector, and at the same time, in each sector, selecting two adjacent voltage vectors and a zero vector to synthesize the voltage vector in the sector, and the synthesis process follows the volt-second balance principle, generating a pulse width modulation signal according to the synthesized voltage vector, and controlling the switching state of the photovoltaic inverter according to the pulse width modulation signal and generating corresponding harmonic currents to offset the harmonic currents during photovoltaic grid-connected power generation.

[0011] A further improvement of the present invention is that the calculation process of the harmonic current includes:

[0012] The calculation process of the load fundamental current is as follows:

[0013]

[0014] Among them, the calculation formula of each transformation matrix is ​​as follows:

[0015]

[0016] In the formula, i ah(t) 、i bh(t) 、i ch(t) Represents the harmonic component of the load current at time t; It represents the current passing through the low-pass filter and the photovoltaic grid-connected command current obtained through the voltage regulator (AVR); ω represents the angular frequency; t represents the time.

[0017] Subtract the load fundamental current from the three-phase load current to get the harmonic current. The corresponding calculation formula is as follows:

[0018]

[0019] In the formula, i ah 、i bh 、i ch Represents the harmonic current component; i La 、i Lb 、i Lc is the three-phase load current component; i Lfa 、i Lfb 、i Lfc Represents the fundamental component of load current.

[0020] A further improvement of the present invention is that the photovoltaic inverter power is controlled based on the voltage-current dual closed-loop proportional-integral control strategy, further comprising:

[0021] In proportional-integral control, the given reference value and the actual output value are compared and the difference is calculated. The corresponding calculation formula is as follows:

[0022] ΔU(t)=Sref(t)-U(t)

[0023] Wherein, ΔU(t) represents the difference between the given voltage reference value and the actual voltage output value at time t; Sref(t) represents the given voltage reference value at time t; U(t) represents the actual voltage output value at time t;

[0024] Through feedforward control decoupling, the current inner loop equation of the photovoltaic inverter is as follows:

[0025]

[0026] In the formula, i d1 and i q1 Indicates the actual current; and represents the reference current; p represents the Laplace transform operation; L represents the size of the filter inductor; k p represents the proportional control gain; k i represents the integral control gain; s represents the differential operator in the complex frequency domain.

[0027] A further improvement of the present invention is that the relationship between the line voltage vector, the switch variable and the switch vector output by the photovoltaic inverter is as follows:

[0028]

[0029] Where U dc Represents the capacitor voltage across the DC side; V a 、V b 、V cRepresents the three-phase voltage output by the photovoltaic inverter, where a, b, and c are the on-off states of each bridge arm; V ab 、V bc 、V ca Represents the line voltage of the three-phase inverter; the number 1 in the formula matrix means that the upper bridge arm is turned on and the lower bridge arm is turned off, while the number 0 is the opposite of the number 1.

[0030] A further improvement of the present invention is that the three-phase abc coordinate system is transformed into a two-phase αβ coordinate system, and is represented by a space vector. The corresponding formula is as follows:

[0031]

[0032] Where V α 、V β Represents the αβ coordinate components of the space voltage vector; V a 、V b 、V c Indicates the phase voltage output by the three-phase inverter.

[0033] A further improvement of the present invention is that: in each sector, two adjacent voltage vectors and a zero vector are selected to synthesize the voltage vector in the sector, and the synthesis process follows the volt-second balance principle, that is:

[0034] u*T s =u x *T x +u y *T y +u zero *T0

[0035] Where u represents the expected voltage vector, that is, the voltage vector in the sector; T s represents the sampling period; T x , T y , T0 represent the non-zero voltage vector u x , non-zero voltage vector u y , zero vector u zero In a sampling period T s The action time within which the zero vector u zero Including two zero vectors u0 and u7.

[0036] The beneficial effects of the present invention are as follows: the present invention proposes a unified control method for harmonics of a photovoltaic inverter and an active power filter, adopts a harmonic current detection principle based on instantaneous reactive power theory, and proposes a strategy for grid-connected inverter and active filter control; utilizes a digital phase-locked loop to track the grid voltage phase to ensure the accuracy of current detection; controls the converter to inject the combined command current into the grid through a current tracking control method, realizes the conversion of light energy into electrical energy, and uses the same device to perform harmonic suppression and reactive power compensation; adopts a proportional-integral control strategy and a space voltage vector modulation technology to realize the control of photovoltaic inverter power, improves the dynamic response performance of the system and the DC side voltage utilization rate, thereby solving the influence of harmonic problems on power quality in the existing distributed power grid system, and realizes the suppression of harmonics while photovoltaic grid-connected power generation through a unified control system, effectively reduces the influence of harmonics on the grid, and improves the efficiency and operation stability of the photovoltaic grid-connected power generation system. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A schematic flow chart of a unified control method for harmonics of a photovoltaic inverter and an active power filter provided in an embodiment of the present invention;

[0038] Figure 2 A block diagram of a unified control system of an active filter and a photovoltaic inverter provided in an embodiment of the present invention;

[0039] Figure 3 A schematic diagram of the working principle of the unified control strategy of the active filter and photovoltaic inverter provided in an embodiment of the present invention;

[0040] Figure 4 The embodiment of the present invention provides p -i q Principle block diagram of current detection and command current synthesis;

[0041] Figure 5 A dq component feedforward decoupling control strategy diagram provided by an embodiment of the present invention;

[0042] Figure 6 A basic structural diagram of a grid-connected inverter provided by an embodiment of the present invention;

[0043] Figure 7 A schematic diagram of sectors of a complex plane provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0045] See also Figure 1 , a unified control method for harmonics of a photovoltaic inverter and an active power filter in this embodiment includes the following steps:

[0046] S1, tracking the grid voltage phase through a digital phase-locked loop, performing Clarke transformation and Parker transformation on each load current to obtain instantaneous active current and instantaneous reactive current, filtering out the AC components in the instantaneous active current and instantaneous reactive current through a low-pass filter to obtain fundamental active current DC and fundamental reactive current DC, then performing inverse Parker transformation and inverse Clarke transformation on the fundamental active current DC and fundamental reactive current DC to obtain load fundamental current, and subtracting the load fundamental current from the three-phase load current to obtain harmonic current;

[0047] S2, controlling the power of the photovoltaic inverter based on the voltage and current dual closed-loop proportional-integral control strategy, including comparing the photovoltaic grid-connected command current with the maximum power point tracking command voltage, obtaining the actual command current of the photovoltaic grid-connected through the voltage regulator, combining the actual command current with the obtained harmonic current to obtain the total command current, obtaining the current inner loop equation when the photovoltaic inverter is working through feedforward control decoupling and controlling the photovoltaic inverter accordingly;

[0048] S3, using space vector pulse width modulation technology to uniformly control photovoltaic grid-connected and active power filters, including determining the relationship between the line voltage vector, switch variables and switch vectors output by the photovoltaic inverter, transforming the three-phase coordinate system into a two-phase coordinate system based on the space vector, and at the same time, in each sector, selecting two adjacent voltage vectors and a zero vector to synthesize the voltage vector in the sector, and the synthesis process follows the volt-second balance principle, generating a pulse width modulation signal based on the synthesized voltage vector, and controlling the switching state of the photovoltaic inverter based on the pulse width modulation signal and generating corresponding harmonic currents to offset the harmonic currents during photovoltaic grid-connected power generation.

[0049] The following is a detailed description of the design and implementation method of the unified control strategy for harmonics of the photovoltaic inverter and the active power filter (APF) provided by the present invention in conjunction with the accompanying drawings.

[0050] like Figure 2As shown in FIG. 1 , it is a block diagram of a unified control system of an active filter and a photovoltaic inverter provided in an embodiment of the present invention. By uniformly controlling the three-phase photovoltaic grid-connected power generation system and the APF, harmonic suppression can be achieved. The control strategy in this embodiment is mainly composed of maximum power point tracking (MPPT) control, i p -i q It consists of current detection, proportional integral (PI) control and space voltage vector modulation.

[0051] Combination Figure 2 In the unified control strategy of the photovoltaic inverter and APF for harmonics of the present invention, p -i q The implementation method of harmonic current detection, its specific implementation steps are as follows:

[0052] When the voltage suddenly changes, the harmonic current detected by the pq method has errors. p -i q The detection method uses a signal with the same phase as the grid voltage to replace the power supply voltage to participate in the detection operation. This method will not be affected by the sudden change of the grid voltage waveform, thereby improving the accuracy of the final result.

[0053] See also Figure 3 Regarding the working state of the inverter, when the power grid is working normally, the load harmonic current is first detected, the detection data is converted into the harmonic compensation command current, the harmonic compensation command current is combined with the photovoltaic grid-connected command current and injected into the power grid; at night or in rainy weather, the inverter acts as an APF to compensate for harmonics.

[0054] The calculation process of the harmonic current includes:

[0055] The calculation process of the load fundamental current is as follows:

[0056]

[0057] Among them, the calculation formula of each transformation matrix is ​​as follows:

[0058]

[0059] In the formula, i ah(t) 、i bh(t) 、i ch(t) Represents the harmonic component of the load current at time t; represents the current passing through the low-pass filter and the photovoltaic grid-connected command current obtained through the voltage regulator (AVR); ω represents the angular frequency; t represents the time. The harmonic current is obtained by subtracting the load fundamental current from the three-phase load current. The corresponding calculation formula is as follows:

[0060]

[0061] In the formula, i ah 、i bh 、i ch Represents the harmonic current component; i La 、i Lb 、i Lc is the three-phase load current component; i Lfa 、i Lfb 、i Lfc Indicates the fundamental component of the load current.

[0062] Specifically, Figure 4 As shown, the embodiment of the present invention provides i p -i q The block diagram of the current detection and command current synthesis principle. In the figure, the digital phase-locked loop (PLL) tracks the A-phase grid voltage u a phase to ensure the current detection accuracy. Each load current passes through C 3 / 2 and The active fundamental current is obtained by transforming In the figure is the MPPT command voltage, which is calculated by the control part of the MPPT, and and u dc The photovoltaic grid-connected command current is obtained through the voltage regulator (AVR) Instantaneous active current i p and instantaneous reactive current i q After the AC component in the low-pass filter is removed, the DC component of the fundamental active current can be obtained. and the DC component of reactive current Then perform inverse Park transform and inverse Clark transform to obtain the load current fundamental component i Lfa 、i Lfb 、i Lfc Finally, the harmonic current component i can be obtained by subtracting the fundamental component of the load current from the three-phase load current. ah 、i bh 、i ch .

[0063] The photovoltaic inverter power is controlled based on the voltage-current dual closed-loop proportional-integral control strategy, and further includes:

[0064] In proportional integral (PI) control, the given reference value and the actual output value are compared and the difference is calculated. The corresponding calculation formula is as follows:

[0065] ΔU(t)=Sref(t)-U(t)

[0066] Wherein, ΔU(t) represents the difference between the given voltage reference value and the actual voltage output value at time t; Sref(t) represents the given voltage reference value at time t; U(t) represents the actual voltage output value at time t;

[0067] The equation of the inner current loop of the photovoltaic inverter when working is obtained through feedforward control decoupling as follows:

[0068]

[0069] In the formula, i d1 and i q1 Indicates the actual current; and represents the reference current; p represents the Laplace transform operation; L represents the filter inductance; k p represents the proportional control gain; k i represents the integral control gain; s represents the differential operator in the complex frequency domain.

[0070] Specifically, in this embodiment, a dual closed-loop PI control strategy with a DC bus voltage outer loop and a current inner loop is used to achieve the control requirements of the photovoltaic inverter power. The controlled object is controlled by the proportional and integral links. The PI control can quickly adjust and reduce the deviation value ΔU(t). Figure 5 As shown in FIG. 1 , it is a dq component feedforward decoupling control strategy diagram provided by an embodiment of the present invention, and the output three-phase voltage U abc and three-phase current I abc After decomposition through coordinate transformation, the d-axis voltage and current U are obtained respectively. d ,I d and q-axis voltage and current U q ,I q , because there is a coupling relationship between the dq axis components, feedforward decoupling is required before being transmitted to the controller. d1 and i q1 More and hours, then Ldi d1 / d t >0 and Ldi q1 / d t >0, then i d1 and i q1 will increase accordingly; on the contrary, i d1 and i q1 In the figure, ω represents the angular frequency; L c Indicates the filter capacitor parameters; u d1 and u q1 Indicates the grid voltage under dq axis; u d * 、u q* Indicates the reference voltage under the dq axis.

[0071] The relationship between the line voltage vector, the switch variable and the switch vector output by the photovoltaic inverter is shown in the following formula:

[0072]

[0073] Where U dc Represents the capacitor voltage across the DC side; V a 、V b 、V c Represents the three-phase voltage output by the photovoltaic inverter, where a, b, and c are the on-off states of each bridge arm; V ab 、V bc 、V ca Represents the line voltage of the three-phase inverter; the number 1 in the formula matrix means that the upper bridge arm is turned on and the lower bridge arm is turned off, while the number 0 is the opposite of the number 1.

[0074] The three-phase abc coordinate system is transformed into a two-phase αβ coordinate system, which is represented by a space vector. The corresponding formula is as follows:

[0075]

[0076] Where V α 、V β Represents the αβ coordinate components of the space voltage vector; V a 、V b 、V c Indicates the phase voltage output by the three-phase inverter.

[0077] In each sector, two adjacent voltage vectors and a zero vector are selected to synthesize the voltage vector in the sector, and the synthesis process follows the volt-second balance principle, that is:

[0078] u*T s =u x *T x +u y *T y +u zero *T0

[0079] Where u represents the expected voltage vector, that is, the voltage vector in the sector; T s represents the sampling period; T x , T y , T0 represent the non-zero voltage vector u x , non-zero voltage vector u y , zero vector u zero In a sampling period T s The action time within which the zero vector u zeroIncluding two zero vectors u0 and u7.

[0080] Specifically, the present embodiment adopts space voltage vector modulation technology, which is a pulse width modulation method based on space voltage vector synthesis. It uses different combinations of switches in each bridge arm of the inverter and switching time matching to make the running trajectory of the space voltage vector approach a circle. Its principle is that after detecting the harmonic current, the command current signal generates a pulse width modulation (PWM) signal to control the inverter to generate the required harmonic current to offset the harmonic current in the power grid. Space vector pulse width modulation (SVPWM) has real-time control function, as well as good dynamic response performance and low-frequency on-off control effect, which can effectively reduce the switching loss of the system.

[0081] This paper adopts SVPWM to realize the unified control of photovoltaic grid connection and APF. Figure 6 As shown in FIG. 1 , it is a basic structural diagram of a grid-connected inverter provided by an embodiment of the present invention. Since the switch combination of a three-phase voltage source rectifier (VSR) is easy to obtain, its SVPWM has only 8 different switch vectors [abc] T Therefore, these 8 different switching vector combinations can be brought into the above formula to obtain the combined relationship between the AC side output phase voltage and the space vector. The combined relationship is shown in Table 1:

[0082]

[0083] Table 1

[0084] like Figure 6 As shown, it is a schematic diagram of each sector of the complex plane provided by an embodiment of the present invention. In the complex plane, the amplitude of any basic space vector is 2U dc / 3, the amplitude can be used to represent the output voltage of the three-phase voltage space vector in various switch combinations. The inverter has 8 different switch combinations (000)-(111). Each switch combination generates a voltage vector on the AC side of the inverter, so that the six non-zero vectors divide the complex plane into six sectors, and the two zero vectors are located in the center. From Table 1, it can be obtained that the amplitude of the maximum voltage vector output in the linear modulation area of ​​SVPWM is U dc / 3, and the maximum amplitude of the line voltage is U dc Therefore, the DC voltage utilization rate of SVPWM is 100%, which is a major advantage of SVPWM. It can be seen that SVPWM can meet the accuracy requirements of the entire system for pulses and has good effects.

[0085] Through the introduction of the above embodiments, the present invention adopts a unified control method for harmonics of photovoltaic inverters and active power filters, adopts the harmonic current detection principle based on instantaneous reactive power theory, and proposes a strategy for grid-connected inverter and active filtering control; uses a digital phase-locked loop to track the grid voltage phase to ensure the accuracy of current detection; controls the converter to inject the combined command current into the grid through the current tracking control method, realizes the conversion of light energy into electrical energy, and uses the same device for harmonic suppression and reactive power compensation; adopts a proportional-integral control strategy and space voltage vector modulation technology to realize the control of photovoltaic inverter power, improve the dynamic response performance of the system and the DC side voltage utilization rate, thereby solving the influence of harmonic problems on power quality in the existing distributed power grid system, and realizes the suppression of harmonics while photovoltaic grid-connected power generation through a unified control system, effectively reducing the influence of harmonics on the power grid, and improving the efficiency and operation stability of the photovoltaic grid-connected power generation system.

[0086] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as those generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have the same meaning as in the context of the prior art, and will not be interpreted with an idealized or overly formal meaning unless defined similarly here.

[0087] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A unified control method for harmonics of a photovoltaic inverter and an active power filter, characterized in that: The steps include: The grid voltage phase is tracked by a digital phase-locked loop, and Clarke transformation and Parker transformation are performed on each load current to obtain instantaneous active current and instantaneous reactive current. The AC components in the instantaneous active current and instantaneous reactive current are filtered out by a low-pass filter to obtain fundamental active current DC and fundamental reactive current DC. Then, the fundamental active current DC and fundamental reactive current DC are subjected to inverse Parker transformation and inverse Clarke transformation to obtain load fundamental current, and the load fundamental current is subtracted from the three-phase load current to obtain harmonic current; The photovoltaic inverter power is controlled based on the voltage and current double closed-loop proportional-integral control strategy, including comparing the photovoltaic grid-connected command current with the maximum power point tracking command voltage, obtaining the actual command current of the photovoltaic grid-connected through the voltage regulator, combining the actual command current with the obtained harmonic current to obtain the total command current, and obtaining the current inner loop equation of the photovoltaic inverter when it is working through feedforward control decoupling and controlling the photovoltaic inverter accordingly; The space vector pulse width modulation technology is used to uniformly control the photovoltaic grid-connected and active power filter, including determining the relationship between the line voltage vector, switch variables and switch vector output by the photovoltaic inverter, transforming the three-phase coordinate system into a two-phase coordinate system based on the space vector, and at the same time, in each sector, selecting two adjacent voltage vectors and a zero vector to synthesize the voltage vector in the sector, and the synthesis process follows the volt-second balance principle, generating a pulse width modulation signal according to the synthesized voltage vector, and controlling the switching state of the photovoltaic inverter according to the pulse width modulation signal and generating corresponding harmonic currents to offset the harmonic currents during photovoltaic grid-connected power generation.

2. The unified control method for harmonics of a photovoltaic inverter and an active power filter according to claim 1, characterized in that: The calculation process of the harmonic current includes: The calculation process of the load fundamental current is as follows: Among them, the calculation formula of each transformation matrix is ​​as follows: In the formula, i ah(t) 、i bh(t) 、i ch(t) Represents the harmonic component of the load current at time t; represents the current passing through the low-pass filter and the photovoltaic grid-connected command current obtained through the voltage regulator (AVR); ω represents the angular frequency; t represents the time; C 3 / 2 , represents the transformation matrix; i Lfa(t) 、i Lfb(t) 、i Lfc(t) Represents the fundamental component of the load current at time t; i La(t) 、i Lb(t) 、i Lc(t) is the three-phase load current component at time t; Subtract the load fundamental current from the three-phase load current to get the harmonic current. The corresponding calculation formula is as follows: In the formula, i ah 、i bh 、i ch Represents the harmonic current component; i La 、i Lb 、i Lc is the three-phase load current component; i Lfa 、i Lfb 、i Lfc Indicates the fundamental component of the load current.

3. The unified control method for harmonics of a photovoltaic inverter and an active power filter according to claim 1, characterized in that: The photovoltaic inverter power is controlled based on the voltage-current dual closed-loop proportional-integral control strategy, and further includes: In proportional-integral control, the given reference value and the actual output value are compared and the difference is calculated. The corresponding calculation formula is as follows: ΔU(t)=Sref(t)-U(t) Wherein, ΔU(t) represents the difference between the given voltage reference value and the actual voltage output value at time t; Sref(t) represents the given voltage reference value at time t; U(t) represents the actual voltage output value at time t; Through feedforward control decoupling, the current inner loop equation of the photovoltaic inverter is as follows: In the formula, i d1 and i q1 Indicates the actual current; and represents the reference current; p represents the Laplace transform operation; L represents the filter inductance; k p represents the proportional control gain; k i represents the integral control gain; s represents the differential operator in the complex frequency domain.

4. The unified control method for harmonics of a photovoltaic inverter and an active power filter according to claim 1, characterized in that: The relationship between the line voltage vector, the switch variable and the switch vector output by the photovoltaic inverter is shown in the following formula: Where U dc Represents the capacitor voltage across the DC side; V a 、V b 、V c Represents the three-phase voltage output by the photovoltaic inverter, where a, b, and c are the on-off states of each bridge arm; V ab 、V bc 、V ca Represents the line voltage of the three-phase inverter; the number 1 in the formula matrix means that the upper bridge arm is turned on and the lower bridge arm is turned off, while the number 0 is the opposite of the number 1.

5. The unified control method for harmonics of a photovoltaic inverter and an active power filter according to claim 4, characterized in that: The three-phase abc coordinate system is transformed into a two-phase αβ coordinate system, which is represented by a space vector. The corresponding formula is as follows: Where V α 、V β Represents the αβ coordinate components of the space voltage vector; V a 、V b 、V c Indicates the phase voltage output by the three-phase inverter.

6. The unified control method for harmonics of a photovoltaic inverter and an active power filter according to claim 1, characterized in that: In each sector, two adjacent voltage vectors and a zero vector are selected to synthesize the voltage vector in the sector, and the synthesis process follows the volt-second balance principle, that is: u*T s =u x *T x +u y *T y +u zero *T0 Where u represents the expected voltage vector, that is, the voltage vector in the sector; T s represents the sampling period; T x , T y , T0 represent the non-zero voltage vector u x , non-zero voltage vector u y , zero vector u zero In a sampling period T s The action time within which the zero vector u zero Including two zero vectors u0 and u7.

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