CN115241881A - LCL type active power filter improved current control method suitable for power grid frequency fluctuation scene - Google Patents
LCL type active power filter improved current control method suitable for power grid frequency fluctuation scene Download PDFInfo
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Abstract
The invention relates to an improved current control method of an LCL type active power filter, which is suitable for a power grid frequency fluctuation scene. The main circuit is used for collecting pulse signals, and manufacturing harmonic compensation currents with equal size and opposite phases through a three-phase inverter to be injected into a power grid; the control system is used for detecting harmonic current generated by a load, obtaining a harmonic current signal to be compensated by an APF (active power filter) according to the detected load current, obtaining fundamental wave instruction current according to the detected direct current bus capacitor voltage, obtaining instruction current to be compensated by adding the fundamental wave instruction current to the harmonic wave instruction current, comparing the instruction current with actual output current of an inverter to obtain an error value to carry out current closed-loop control, combining fuzzy proportional control and quick repetitive control in a control mode, and finally modulating through a driving circuit to obtain a modulation wave. The invention can detect the harmonic current of the current based on the harmonic wave and change the magnitude of the proportional gain in real time.
Description
Technical Field
The invention relates to the technical field of LCL type three-phase parallel active power filters, in particular to an improved current control method of an LCL type active power filter, which is suitable for a power grid frequency fluctuation scene.
Background
Power electronic devices have been widely used in various aspects of power systems, so that electric energy can change its application mode to adapt to different power utilization occasions, thereby meeting the demands of people on different forms of electric energy, but also bringing a lot of harmonic pollution to the power grid. Compared with a passive Filter, a parallel Active Power Filter (SAPF) has obvious advantages in stability and flexibility compared with the passive Filter, and is the most effective method for controlling Power harmonics at present.
The connection of the parallel APF to the grid usually requires an additional filter to suppress high frequency ripples, and an L-type or LCL-type filter is usually added to suppress high frequency ripples. The LCL type filter is a three-order system, has better high-frequency inhibition performance and switch ripple inhibition than the L type filter under the condition that the total inductance value is equal, has obvious cost advantage in high-power application occasions, and simultaneously improves the dynamic performance of the system, but the LCL type filter can cause resonance peak under the conduction of special high-order current harmonic waves, so that the output current of the inverter has oscillation phenomenon, and if no measure is taken to inhibit the high-order current harmonic waves, the system is finally unstable. A common suppression method is to connect a passive resistor in series with the capacitor branch.
With the continuous and deep research of the APF system by the scholars, the topology structure and the main circuit design of the APF are basically formed, and if the system performance of the APF is further improved, research and improvement are mainly carried out on the aspect of a control strategy, wherein the current control strategy is an important means for determining the generation of current, and the quality of the network access current is determined by the current control strategy. Currently, the commonly used compensation current control method of the APF mainly includes: dead beat control, PI control, proportional resonance control, and fuzzy control. Dead beat control has extremely fast dynamic response performance and good control precision, has the defects of inherent delay problem of a control system and higher requirement on the precision of a system mathematical model, and can cause the reduction of the compensation performance of the system and even cause the instability phenomenon when the deviation of the established system mathematical model and an actual model is larger; the PI control has the advantages of simple structure, high robustness, easiness in debugging and the like, becomes the most widely applied control strategy in the industry at present, can carry out no-static-error tracking on a direct-current signal, but has lower bandwidth, so that the tracking effect on a high-frequency instruction signal is poorer; the proportional resonant controller has infinite gain at the resonant frequency point, so that the effect of no static tracking on resonant signals can be achieved, but the structure of the proportional resonant controller needs a plurality of resonant controllers which are connected in parallel for use, the system is complex, and the practical application cost is high; the fuzzy control is an intelligent control strategy obtained by describing a natural language into a control rule and analyzing experience and actual data accumulated by technicians.
Disclosure of Invention
The invention mainly aims to overcome the problem of poor compensation performance of the parallel active power filter and solve the problems of low dynamic response speed of a changed load and poor compensation precision caused by power grid frequency fluctuation in the prior art; the improved current control method for the LCL type active power filter is suitable for the power grid frequency fluctuation scene, adopts a double-loop control mode combining fuzzy proportional control and frequency self-adaptive fast repetitive control, obviously improves the compensation steady-state precision of a system and the response speed of tracking dynamic harmonic current, and realizes the control of SAPF fundamental wave and harmonic wave.
The technical problem of the invention is mainly solved by the following technical scheme:
an improved current control method for an LCL type active power filter in a power grid frequency fluctuation scene is characterized by comprising the following steps:
the main circuit collects pulse signals, and harmonic compensation currents with equal size and opposite phases are injected into a power grid through a three-phase inverter, so that the purposes of counteracting harmonic currents generated by a load and improving the quality of electric energy are achieved.
The control system is used for detecting harmonic current generated by a load, obtaining a harmonic current signal to be compensated by an APF (active power filter) according to the detected load current, obtaining fundamental wave instruction current according to the detected direct current bus capacitor voltage, obtaining instruction current to be compensated by adding the fundamental wave instruction current to the harmonic wave instruction current, comparing the instruction current with the actual output current of the inverter to obtain an error value to carry out current closed-loop control, combining fuzzy proportional control and quick repetitive control in a control mode, and finally modulating by a driving circuit to obtain a modulation wave;
the main circuit comprises:
the three-phase inverter is used for receiving the pulse signal to generate a harmonic current with the same magnitude and opposite phase with the load harmonic current to counteract harmonic components in the load current, charging a direct current voltage source, inputting the direct current voltage source to the energy storage element and outputting the direct current voltage source to the output filter. The three-phase inverter is a three-phase voltage type inverter, the energy storage element is a direct current capacitor, and the three-phase voltage type inverter generates compensating current through a pulse signal and is connected with an output filter; and the three-phase voltage type inverter is connected with the output filter in series and then is connected with the nonlinear load in parallel to be connected into a power grid.
And the output filter is used for realizing a relatively ideal switch ripple filtering effect, the input is connected with the three-phase inverter, and the output is connected with the three-phase power grid. The output filter being a higher order filter, adoptingThe LCL filter adopts a star connection method and L is combined with a passive damping mode 1 Is an inverter-side filter inductor, L 2 The passive damping resistor R and the filter capacitor C are connected in series on the filter branch.
And the energy storage element is used for providing stable direct current for the active power filter, and the output of the energy storage element is connected to the three-phase inverter.
2. The improved current control method of LCL type active power filter applicable to power grid frequency fluctuation scene as claimed in claim 1, wherein the control system comprises
Harmonic current detection circuit: and is connected with the current tracking control circuit and used for detecting harmonic current components in the load.
Direct current side capacitance voltage detection circuit: and the current tracking control circuit is connected with the voltage tracking control circuit and is used for realizing the stability control of the voltage.
A phase-locked loop circuit: and the direct current side voltage detection circuit is connected with the direct current side voltage detection circuit and is used for accurately and quickly extracting phase angle and frequency information of a power grid.
Electric wire netting side inductance current detection circuitry: and the current tracking control circuit is connected with the harmonic current command and is used for accurately tracking the harmonic current command.
The current tracking control circuit: and the circuit is connected with a driving circuit and is used for realizing the accurate tracking and quick response capability of compensating harmonic current.
3. An improved current control method of an LCL type active power filter suitable for a power grid frequency fluctuation scene is characterized by comprising the following steps of: comprises that
Step 1: acquiring state information of a parallel active power filter, and acquiring a load current i by using a voltage current sensor L Grid voltage u g DC bus capacitor voltage u dc Filter network side inductive current i 2 。
And 2, step: for the power grid voltage u obtained in the step 1 g Phase theta and frequency f of A-phase power grid voltage are extracted by using phase-locked loop circuit 0 Information;
and 3, step 3: using the voltage of the DC-side bus capacitorReference value u of dc * Subtracting the actual value u of the current bus capacitor voltage detected in the step 1 dc Obtaining a fundamental current amplitude value through a direct current capacitor voltage controller, and multiplying the fundamental current amplitude value by phase information of an A-phase power grid to obtain a fundamental current instruction;
and 4, step 4: using harmonic detection circuit to detect the load current i in step 1 L Obtaining fundamental current component by low-pass filtering after abc/dq conversion, and then using load current i L Subtracting the fundamental current component to obtain a harmonic current command i to be compensated LH-abc And according to the phase information obtained in the step 2, performing abc/dq conversion to obtain a harmonic current command i under a two-phase rotating coordinate system LH-dq ；
And 5: adding the fundamental current command to the harmonic current command to obtain a current command i to be compensated * And current control is carried out on the grid side inductive current of the active power filter in combination with a current instruction, and the current tracking control circuit inputs SVPWM (space vector pulse width modulation) to generate a modulation wave by adopting a mode of combining fuzzy proportional control and quick repetitive control, drives a power device to switch to act and generates compensation current opposite to load side harmonic wave and reactive current. The method realizes reactive power and harmonic compensation of the output current of the power grid, and specifically comprises the following steps:
by constructing a transfer function of composite current control, a subsystem under the independent action of fuzzy proportional control and rapid repetitive control is equivalently obtained, and constraint conditions meeting system stability are solved; firstly, a constraint condition is established for a subsystem of fuzzy proportional control, so that the system can effectively inhibit the inherent resonance of the LCL under a stable condition; and (3) designing a fast repetitive controller according to the A-phase power grid frequency obtained in the step (2) to adapt to the frequency fluctuation of the power grid, wherein the specific method for selecting the parameters of the current tracking control circuit in the step (5) is as follows:
step a1: comparing the harmonic current command with the actually detected power grid side inductive current to obtain a command signal of the current tracking current, as shown in formula 1:
i * ＝i r -i 2 (1)
i * as a current command signal, i r Is harmonic toWave command current i 2 Is the actually detected grid side inductor current.
Step a2: subtracting the derivative of the actually detected grid side induction current from the derivative of the harmonic instruction current in the step a1 to obtain the change rate of the current instruction signal, as shown in formula 2:
Δi * ＝Δi r -Δi 2 (2)
in the formula,. DELTA.i * For the rate of change of the current command signal,. DELTA.i r For the derivative of harmonic command current, [ delta ] i 2 Is the derivative of the actually detected grid-side inductor current.
Step a3: establishing fuzzy rules, and respectively sending the current command signals i * Rate of change Δ i of sum current command signal * As the input of the fuzzy proportional controller, the proportional gain delta K is obtained through fuzzy control rule processing P 。
Step a4: the precision of fuzzy control is realized by obtaining the proportional gain delta K in the step 3 P Plus an initial parameter K of proportional control P0 To obtain a new proportional control parameter K P As shown in equation 3:
K P ＝K P0 +ΔK P (3)
step a5: c, the new proportion control parameter K obtained in the step a4 P As a coefficient of fuzzy proportion control of the current inner loop, and the current command i obtained in the step 1 * The feedforward input is used for fuzzy proportional control of the current inner loop to improve the dynamic response speed of the system, and the transfer function is shown in formula 4:
wherein F(s) is the current inner loop transfer function in the continuous domain.
Step a6: dividing the switching frequency by f s In steps of grid frequency f 0 The sampling frequency N of the system is obtained, and a fractional hysteresis link in the current outer loop repetitive control is shown as a formula 5:
wherein z is a discrete domain operator, z -N/6 For the fractional hysteresis loop of the repetitive controller, z -p Expressed as fractional hysteresis loop z -N/6 Integer part of (b), z -q Is a fractional hysteresis loop z -N/6 And q ∈ (1, 2). Definition of fractional part z by Lagrange interpolation -q As shown in equation 6:
wherein M represents the maximum order of the lagrangian interpolation method, k represents the polynomial order of the lagrangian interpolation method, and h (k) is the coefficient of each polynomial, and the expression is shown in formula 7:
wherein j represents a non-negative integer less than k.
Step a7: b, tracking the current command signal i obtained in the step a1 * The input current is input to the current outer loop to carry out rapid repetitive control so as to improve the steady-state tracking accuracy of the system, and the transfer function of the system is shown in a formula 8:
wherein G (z) is a transfer function expression of the current outer ring fast repeated control in the discrete domain, F (z) represents a discrete domain transfer function obtained by discretizing F(s) by a zero-order keeper method, and z k For advance controller, k r The gain of the fast repetitive controller, Q (z) is the attenuation filter, H (z) is the second order low pass filter, as shown in equation 9:
in the formula, H(s) represents an expression of a second-order low-pass filter in a continuous domain, and H (z), f are obtained by converting the second-order low-pass filter into a discrete domain through bilinear transformation c To the cut-off frequency of the second order low pass filter, ξ represents the damping ratio.
The invention has the positive effects that:
1) The invention can detect the harmonic current of the current based on the harmonic wave and change the proportional gain in real time, thereby achieving the purpose of quickly responding the change of the load in the main circuit system and effectively reducing the harmonic wave content of the common point in the power grid.
2) The invention can reduce the resonance gain of non-primary subharmonic based on the fractional hysteresis link of the rapid repetitive control, avoid the amplitude amplification of the non-primary subharmonic, and simultaneously realize the reduction of the calculation amount by reducing the hysteresis order.
3) Under the condition of power grid frequency fluctuation, the inner membrane structure of the fast repetitive controller can be changed based on the real-time power grid frequency information detected by the phase-locked loop, and the frequency self-adaption of the fast repetitive controller is realized.
4) The composite control strategy of fuzzy proportion control and frequency self-adaptive fast repetitive control can effectively guarantee the stability of the system, only the initial proportion parameter of the fuzzy proportion controller and the compensator of the frequency self-adaptive fast repetitive controller are needed to be designed, the parameter is corrected through the fuzzy proportion control and fractional order hysteresis link, the dynamic and static performances of the system are improved, and the system has higher tracking precision during steady-state operation.
Drawings
Fig. 1 is a system block diagram of the improved current control method for the LCL type active power filter in the power grid frequency fluctuation scene according to the present invention.
Fig. 2 is a block diagram of the current controller of the present invention.
FIG. 3 shows the output gain Δ K of the fuzzy proportional controller P And outputting the graph.
FIG. 4 is a graph of membership function of fuzzy variable r for a fuzzy proportional controller.
Fig. 5 is a fuzzy variable Δ r membership function diagram of the fuzzy proportional controller.
FIG. 6 shows fuzzy variable Δ K of fuzzy proportional controller P A graph of membership functions.
FIG. 7 is a Nyquist plot of the outer loop fast repetitive control system characteristic equation.
Fig. 8 is a waveform diagram of the current at the load side when the frequency of the power grid fluctuates to 51 Hz.
FIG. 9 is a diagram of APF compensated grid-side current waveforms when the grid frequency fluctuates to 51 Hz.
FIG. 10 is a diagram of a power grid side current spectrum analysis after APF compensation when a power grid frequency fluctuates to 51 Hz.
Detailed Description
The technical scheme of the invention is further specifically described by the following embodiments and the accompanying drawings.
7. An improved current control method of an LCL type active power filter suitable for a power grid frequency fluctuation scene is characterized by comprising the following steps of: comprises that
Step 1: acquiring state information of a parallel active power filter, and acquiring a load current i by using a voltage current sensor L Grid voltage u g Dc bus capacitor voltage u dc Filter network side inductive current i 2 。
Step 2: for the power grid voltage u obtained in the step 1 g Phase theta and frequency f of A-phase power grid voltage are extracted by using phase-locked loop circuit 0 Information;
and step 3: using reference value u of DC-side bus capacitor voltage dc * Subtracting the actual value u of the current bus capacitor voltage detected in the step 1 dc Obtaining a fundamental current amplitude value through a direct current capacitor voltage controller, and multiplying the fundamental current amplitude value by phase information of an A-phase power grid to obtain a fundamental current instruction;
and 4, step 4: using harmonic detection circuit to detect the load current i in step 1 L Obtaining fundamental current component by low-pass filtering after abc/dq conversion, and then using load current i L Subtracting the fundamental current component to obtain a harmonic current command i to be compensated LH-abc And is combined withAccording to the phase information obtained in the step 2, abc/dq conversion is carried out to obtain a harmonic current instruction i under a two-phase rotating coordinate system LH-dq ；
And 5: adding the fundamental current command to the harmonic current command to obtain a current command i to be compensated * And current control is carried out on the grid side inductive current of the active power filter in combination with a current instruction, and the current tracking control circuit inputs SVPWM (space vector pulse width modulation) to generate a modulation wave by adopting a mode of combining fuzzy proportional control and quick repetitive control, drives a power device to switch to act and generates compensation current opposite to load side harmonic wave and reactive current. The method realizes reactive power and harmonic compensation of the output current of the power grid, and specifically comprises the following steps:
by constructing a transfer function of composite current control, a subsystem under the independent action of fuzzy proportional control and rapid repetitive control is equivalently obtained, and a constraint condition meeting the system stability is solved; firstly, a constraint condition is established for a subsystem controlled by a fuzzy proportion, so that the system can effectively inhibit LCL inherent resonance under a stable condition; and further designing a fast repetitive controller according to the A-phase power grid frequency obtained in the step 2 to adapt to the frequency fluctuation of the power grid, wherein the specific parameter selection method of the current tracking control circuit in the step 5 is as follows:
step a1: comparing the harmonic current command with the actually detected power grid side inductive current to obtain a command signal of the current tracking current, as shown in formula 1:
i * ＝i r -i 2 (1)
i * as a current command signal, i r For harmonic command currents, i 2 Is the actually detected grid side inductor current.
Step a2: subtracting the derivative of the actually detected grid side induction current from the derivative of the harmonic instruction current in the step a1 to obtain the change rate of the current instruction signal, as shown in formula 2:
Δi * ＝Δi r -Δi 2 (2)
in the formula, delta i * For the rate of change of the current command signal,. DELTA.i r For the derivative of harmonic command current, Δ i 2 For practical inspectionThe derivative of the measured grid side inductor current.
Step a3: establishing fuzzy rules, and respectively sending the current command signals i * And rate of change Δ i of current command signal * As input to the fuzzy proportional controller, their exact values are changed into fuzzy domains r and Δ r, and the domains of r and Δ r are set as follows:
r，△r＝{-6，-4，-2，0，2，4，6} (3)
r is the input linguistic variable error of the fuzzy proportional controller, and Δ r is the input linguistic variable error rate of the fuzzy proportional controller. Fuzzy sets { NB, NM, NS, ZE, PS, PM, PB } are selected, which respectively represent negative big, negative middle, negative small, zero, positive small, positive middle and positive big. Determining membership functions of output parameters of the fuzzy proportional controller and defining fuzzy universes of the membership functions, as shown in formula 4:
△K P ＝{-0.9，-0.6，-0.3,0,0.3,0.6,0.9} (4)
△K P the output proportional gain of the fuzzy proportional controller is defined as { NB, NM, NS, ZE, PS, PM, PB }, which respectively represent negative large, negative medium, negative small, zero, positive small, positive medium, positive large. Respectively inputting the error r and the error change rate delta r into a fuzzy proportional controller, and obtaining a proportional gain delta K through fuzzy control rule processing P . The fuzzy rules are shown in table 1.
TABLE 1. DELTA.K P Fuzzy control rule of
Step a4: the precision of fuzzy control is realized by using the proportional gain delta K obtained in the step 3 P Plus an initial parameter K of the proportional control P0 Thereby obtaining a new proportional control parameter K P As shown in equation 5:
K P ＝K P0 +ΔK P (5)
step a5: c, the new proportion control parameter K obtained in the step a4 P As a coefficient of current inner loop fuzzy proportional control, anThe current instruction i obtained in the step 1 * The feedforward input is used for fuzzy proportional control of the current inner loop to improve the dynamic response speed of the system, and the transfer function is shown in formula 6:
wherein F(s) is a current inner loop transfer function under a continuous domain, and s is a continuous domain operator. The sufficient conditions for system stability obtained by performing the Laus criterion on the transfer function are shown in equation 7:
a 1 a 2 -a 0 a 3 ＝(L 1 +L 2 )C 2 K P R 2 +(L 1 +L 2 ) 2 CR-L 1 L 2 CK P ＞0 (7)
simulation parameter L introduced into filter 1 ＝1.3mH、L 2 Available ratio K of =0.2mH, C =10uF P And the suppression effect of the value of the passive damping resistor R on the LCL resonance peak is shown in tables 2 and 3.
TABLE 2 suppression of LCL resonance by open-loop
TABLE 3 suppression of LCL resonance by closed loop
"-" indicates that the phase frequency of the transfer function does not cross-180 at this time. Lower values in the table indicate better suppression of LCL resonance, while also reflecting the stability of the system. According to the data scheme in the table, the damping resistor R =1 omega is selected, and the initial value of the proportional gain is K P0 And =3, the parameter change range of the current inner loop fuzzy proportion control after the fuzzy proportion control is 2.1-3.9, the stability of an inner loop system can be ensured, and the effect of inhibiting resonance is good.
Step a6: dividing the switching frequency by f s In steps of grid frequency f 0 The sampling frequency N of the system is obtained, and a fractional hysteresis link in the current outer loop repetitive control is shown as a formula 5:
wherein z is a discrete domain operator, z -N/6 For repeating fractional hysteresis of the controller, z -p Expressed as fractional hysteresis loop z -N/6 Integer part of (2), z -q Is a fractional hysteresis loop z -N/6 And q ∈ (1, 2). Definition of fractional part z by Lagrange interpolation -q As shown in equation 6:
wherein M represents the maximum order of the lagrangian interpolation method, k represents the polynomial order of the lagrangian interpolation method, and h (k) is the coefficient of each polynomial, and the expression is shown in formula 7:
wherein j represents a non-negative integer less than k. In the present case, the quadric Lagrange's interpolation method is used, and the fractional part z of the fractional hysteresis loop -q As shown in equation 8:
according to the standard of GB/T15945 ' allowable deviation of frequency of electric energy quality electric power system ' issued by the people's republic of China, the maximum fluctuation of the power grid frequency cannot exceed +/-1 Hz, the scheme takes the fluctuation of the power grid to the critical value of 51Hz as an example, and the amplitude of the characteristic frequency harmonic wave obtained by repeated control of an ideal state, a traditional rounding method and a Lagrange interpolation method designed by the scheme is respectively as follows: 33.97dB, 14.8dB and 27.4dB.
Step a7: b, tracking the current obtained in the step a1 to obtain a current command signal i * The input current is input to the current outer loop to carry out rapid repetitive control so as to improve the steady-state tracking accuracy of the system, and the transfer function of the system is shown as formula 9:
wherein G (z) is a transfer function expression of the current outer ring fast repeated control in the discrete domain, F (z) represents a discrete domain transfer function obtained by discretizing F(s) by a zero-order keeper method, and z k For advance controller, k r The gain of the fast repetitive controller, Q (z) is an attenuation filter, Q (z) =0.95 is taken in the scheme, h (z) is a second-order low-pass filter, as shown in formula 10:
in the formula, H(s) represents an expression of a second-order low-pass filter in a continuous domain, and H (z), f are obtained by converting the second-order low-pass filter into a discrete domain through bilinear transformation c To the cut-off frequency of the second order low pass filter, ξ represents the damping ratio. The scheme mainly compensates for the harmonic within 100, and the cut-off frequency is f c =4950Hz, and a damping coefficient ξ =1.414, which yields a transfer function of the second-order low-pass filter in the discrete domain as shown in equation 11:
at this time, the sufficient condition for the outer loop fast repetitive control system to be stable by the small gain principle is shown in the formula 12:
|Q(z)-z k k r H(z)F(z)|＜1,ω∈[0,π/T] (12)
the invention has the positive effects that:
1) The invention can detect the harmonic current of the current based on the harmonic wave and change the proportional gain in real time, thereby achieving the purpose of quickly responding the change of the load in the main circuit system and effectively reducing the harmonic wave content of the common point in the power grid.
2) The invention can reduce the resonance gain of non-primary subharmonic based on the fractional hysteresis link of the rapid repetitive control, avoid the amplitude amplification of the non-primary subharmonic, and simultaneously realize the reduction of the calculation amount by reducing the hysteresis order.
3) Under the condition of power grid frequency fluctuation, the inner membrane structure of the fast repetitive controller can be changed based on the real-time power grid frequency information detected by the phase-locked loop, and the frequency self-adaption of the fast repetitive controller is realized.
4) The composite control strategy of fuzzy proportion control and frequency self-adaptive fast repetitive control can effectively guarantee the stability of the system, only the initial proportion parameter of the fuzzy proportion controller and the compensator of the frequency self-adaptive fast repetitive controller are needed to be designed, the parameter is corrected through the fuzzy proportion control and fractional order hysteresis link, the dynamic and static performances of the system are improved, and the system has higher tracking precision during steady-state operation.
The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.
Claims (3)
1. An improved current control method for an LCL type active power filter under the power grid frequency fluctuation scene is characterized by comprising the following steps:
the main circuit collects pulse signals, and harmonic compensation currents with equal size and opposite phases are manufactured by a three-phase inverter and injected into a power grid, so that the purposes of canceling the harmonic currents generated by a load and improving the power quality are achieved;
the control system is used for detecting harmonic current generated by a load, obtaining a harmonic current signal to be compensated by an APF (active power filter) according to the detected load current, obtaining fundamental wave instruction current according to the detected direct current bus capacitor voltage, obtaining instruction current to be compensated by adding the fundamental wave instruction current to the harmonic wave instruction current, comparing the instruction current with the actual output current of the inverter to obtain an error value to carry out current closed-loop control, combining fuzzy proportional control and quick repetitive control in a control mode, and finally modulating by a driving circuit to obtain a modulation wave;
the main circuit comprises:
the three-phase inverter is used for receiving the pulse signal to generate a harmonic current which has the same magnitude and opposite phase with the load harmonic current to offset the harmonic component in the load current, charging the direct current voltage source, inputting the harmonic current to the energy storage element and outputting the harmonic current to the output filter; the three-phase inverter is a three-phase voltage type inverter, the energy storage element is a direct current capacitor, and the three-phase voltage type inverter generates compensating current through a pulse signal and is connected with the output filter; the three-phase voltage type inverter is connected with the output filter in series and then is connected with the nonlinear load in parallel to be accessed into a power grid;
the output filter is used for realizing ideal switch ripple filtering effect, the input is connected with the three-phase inverter, and the output is connected with the three-phase power grid; the output filter is a high-order filter, an LCL type filter is combined with a passive damping mode, the LCL filter adopts a star connection method, and L is 1 Is an inverter-side filter inductor, L 2 The passive damping resistor R and the filter capacitor C are connected in series on the filter branch circuit;
and the energy storage element is used for providing stable direct current for the active power filter, and the output is connected to the three-phase inverter.
2. The improved current control method of the LCL type active power filter applicable to the power grid frequency fluctuation scene as claimed in claim 1, wherein the control system comprises
Harmonic current detection circuit: the current tracking control circuit is connected with the load and is used for detecting harmonic current components in the load;
DC side capacitance voltage detection circuit: the current tracking control circuit is connected with the current tracking control circuit and is used for realizing the stability control of voltage;
a phase-locked loop circuit: the direct current side voltage detection circuit is connected with the direct current side voltage detection circuit and is used for accurately and quickly extracting phase angle and frequency information of a power grid;
electric wire netting side inductance current detection circuitry: the current tracking control circuit is connected with the harmonic current instruction and is used for accurately tracking the harmonic current instruction;
the current tracking control circuit: and the circuit is connected with a driving circuit and is used for realizing the accurate tracking and quick response capability of compensating harmonic current.
3. An improved current control method of an LCL type active power filter suitable for a power grid frequency fluctuation scene is characterized by comprising the following steps: comprises that
Step 1: acquiring state information of a parallel active power filter, and acquiring a load current i by using a voltage current sensor L Grid voltage u g Dc bus capacitor voltage u dc Filter network side inductive current i 2 ；
Step 2: for the power grid voltage u obtained in the step 1 g Phase theta and frequency f of A-phase power grid voltage are extracted by using phase-locked loop circuit 0 Information;
and 3, step 3: using reference value u of DC-side bus capacitor voltage dc * Subtracting the actual value u of the current bus capacitor voltage detected in the step 1 dc Obtaining a fundamental current amplitude value through a direct current capacitor voltage controller, and multiplying the fundamental current amplitude value by phase information of an A-phase power grid to obtain a fundamental current instruction;
and 4, step 4: using harmonic detection circuit to detect the load current i in step 1 L Obtaining fundamental current component by low-pass filtering after abc/dq conversion, and then using load current i L Subtracting the fundamental current component to obtain a harmonic current command i to be compensated LH-abc And performing abc/dq conversion according to the phase information obtained in the step 2 to obtain harmonics under a two-phase rotating coordinate systemWave current command i LH-dq ；
And 5: adding the fundamental current instruction to the harmonic current instruction to obtain a current instruction i to be compensated * The current tracking control circuit adopts a mode of combining fuzzy proportional control and rapid repetitive control, inputs SVPWM to generate a modulation wave, drives a power device to switch to act and generates a compensation current opposite to a load side harmonic wave and a reactive current; the reactive power and harmonic compensation of the output current of the power grid is realized, and the method specifically comprises the following steps:
by constructing a transfer function of composite current control, a subsystem under the independent action of fuzzy proportional control and rapid repetitive control is equivalently obtained, and a constraint condition meeting the system stability is solved; firstly, a constraint condition is established for a subsystem of fuzzy proportional control, so that the system can effectively inhibit the inherent resonance of the LCL under a stable condition; and further designing a fast repetitive controller according to the A-phase power grid frequency obtained in the step 2 to adapt to the frequency fluctuation of the power grid, wherein the specific parameter selection method of the current tracking control circuit in the step 5 is as follows:
step a1: comparing the harmonic current command with the actually detected power grid side inductive current to obtain a command signal of the current tracking current, as shown in formula 1:
i * ＝i r -i 2 (1)
i * as a current command signal, i r For harmonic command currents, i 2 The actually detected grid side inductive current is obtained;
step a2: subtracting the derivative of the actually detected grid side induction current from the derivative of the harmonic instruction current in the step a1 to obtain the change rate of the current instruction signal, as shown in formula 2:
Δi * ＝Δi r -Δi 2 (2)
in the formula, delta i * For the rate of change of the current command signal,. DELTA.i r For the derivative of harmonic command current, Δ i 2 Is the derivative of the actually detected grid side inductor current;
step a3: establishing a fuzzy ruleThen, respectively sending the current command signals i * And rate of change Δ i of current command signal * As the input of the fuzzy proportional controller, the proportional gain delta K is obtained through fuzzy control rule processing P ；
Step a4: the precision of fuzzy control is realized by using the proportional gain delta K obtained in the step 3 P Plus an initial parameter K of the proportional control P0 Thereby obtaining a new proportional control parameter K P As shown in equation 3:
K P ＝K P0 +ΔK P (3)
step a5: c, obtaining a new proportion control parameter K in the step a4 P As a coefficient of fuzzy proportion control of the current inner loop, and the current command i obtained in the step 1 * The feedforward input is used for carrying out fuzzy proportion control on the current inner loop so as to improve the dynamic response speed of the system, and the transfer function is shown as a formula 4:
wherein F(s) is a current inner loop transfer function under a continuous domain;
step a6: dividing the switching frequency by f s In steps of grid frequency f 0 The sampling frequency N of the system is obtained, and a fractional hysteresis link in the current outer loop repetitive control is shown as a formula 5:
wherein z is a discrete domain operator, z -N/6 For the fractional hysteresis loop of the repetitive controller, z -p Expressed as fractional hysteresis loop z -N/6 Integer part of (2), z -q Is a fractional hysteresis step z -N/6 And q ∈ (1, 2); definition of fractional part z by Lagrange interpolation -q As shown in equation 6:
wherein M represents the maximum order of the lagrangian interpolation method, k represents the polynomial order of the lagrangian interpolation method, and h (k) is the coefficient of each polynomial, and the expression is shown in formula 7:
wherein j represents a non-negative integer less than k;
step a7: b, tracking the current command signal i obtained in the step a1 * The input current is input to the current outer loop to carry out rapid repetitive control so as to improve the steady-state tracking accuracy of the system, and the transfer function of the system is shown in a formula 8:
wherein G (z) is a transfer function expression of the current outer ring fast repeated control in the discrete domain, F (z) represents a discrete domain transfer function obtained by discretizing F(s) by a zero-order keeper method, and z k For advance controller, k r The gain of the fast repetitive controller, Q (z) is the attenuation filter, H (z) is the second order low pass filter, as shown in equation 9:
in the formula, H(s) represents an expression of a second-order low-pass filter in a continuous domain, and H (z), f are obtained by converting the second-order low-pass filter into a discrete domain through bilinear transformation c To the cut-off frequency of the second order low pass filter, ξ represents the damping ratio.
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