CN110880790A - Control method of grid-connected power converter with LCL topological structure - Google Patents
Control method of grid-connected power converter with LCL topological structure Download PDFInfo
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Abstract
The invention relates to new energy, such as photovoltaic grid-connected technology and stability research, in particular to a control method of a grid-connected power converter with an LCL topological structure. Solves the problem that the traditional control method needs to additionally collect the capacitance currenti c As a signal for a control circuit, an additional sensor is required, hardware cost is increased, and control effect is not ideal. The invention directly adoptsi 2 As a harmonic source, a signal of a control circuit is formed, and the purpose of suppressing harmonic waves can be achieved without adopting a traditional strategyi c And the sensor is omitted, the hardware cost is saved, and the performance is better. The invention does not need additional sensor to collect capacitance currenti c The harmonic closed-loop feedback control is carried out simultaneously when grid-connected current single-loop feedback is carried out, the direct control is carried out in the control theory, the stability is higher than that of the traditional harmonic control method, and the high-quality current grid connection can be still realized when the power grid is distorted.
Description
Technical Field
The invention relates to new energy, such as photovoltaic grid-connected technology and stability research, in particular to a control method of a grid-connected power converter with an LCL topological structure.
Background
The development of new energy sources such as fans and photovoltaic devices relieves the crisis of fossil energy. With the continuous improvement of the power generation permeability of new energy, a large number of power electronic converters are needed to serve as interfaces between the new energy and a traditional power grid, however, when the IGBT and the PWM serve as control devices and control modes of the power electronic converters, high-frequency sub-switching harmonics of thousands of hertz can be generated in the operation process, and the quality of grid-connected current is difficult to guarantee. In addition, leakage inductance and line impedance of a transformer in the power system can make a Point of Common Coupling (PCC) Point weak, that is, a weak grid, and threatens stable operation of converter control. Due to the existence of a weak power grid, low-order harmonic current generated by a nonlinear load in the power distribution network forms background harmonic voltage at a PCC point through equivalent line impedance, so that the voltage quality of the PCC point is more complicated. Therefore, how to enhance the robust control of the grid-connected converter under the complex working conditions and still ensure the high quality of the grid-connected current is one of the problems to be researched urgently in recent years.
The grid-connected power converter with the LCL topological structure is widely applied to practical engineering due to excellent high-frequency attenuation characteristics, and when a PCC (point of common coupling) is distorted, a traditional control method needs to additionally acquire a filter capacitor current i in the LCL topological structurecAs a signal for a control circuit, an additional sensor is required, hardware cost is increased, and a control effect, which ensures high quality of grid-connected current, is not desirable.
Disclosure of Invention
The invention solves the need of the traditional control methodAdditional acquisition of the capacitive current icAs a signal of a control circuit, an additional sensor is needed, the hardware cost is increased, and the control effect is not ideal, a control method of a grid-connected power converter with an LCL topological structure is provided, and the control method does not need to adopt a capacitance current icWhile directly using the net side current i2As a harmonic source, a signal of a control circuit is formed, so that the aim of suppressing harmonic waves can be achieved, and compared with the traditional control method, i is not required to be adoptedcAnd the sensor is omitted, the hardware cost is saved, and the performance is better.
The invention is realized by adopting the following technical scheme: the control method of the grid-connected power converter with the LCL topological structure is realized by the following steps:
1) collecting converter DC side capacitor voltage UDCAnd the command value U of the DC side capacitor voltageDC *Making a difference, and forming a d-axis current command value i controlled by a current inner loop through a PI controllerd*；
2) Collecting voltage U of PCC pointpccAnd phase information thereof, and simultaneously collecting network side current i2Combined with a voltage UpccPhase information of (a) will be i2Obtaining dq axis current i through abc/dq0 coordinate transformationdAnd iq(ii) a In order to ensure that the converter is operated at a unit power factor (namely, the reactive power is zero), a q-axis current command value i is setqSetting the d-axis current command value i to zerodCommand value of current of x and q axes iqRespectively connected with the collected d-axis current idCollected q-axis current iqMake a difference to obtain Δ id＝id*－idAnd Δ iq＝iq*－iqThen passes through a proportional resonance controller GPR+HCObtaining:
umdq(s)＝(Δid+Δiq)·GPR+HC(s)
in the above formula,. DELTA.idAnd Δ iqObtaining a voltage modulation wave signal u after passing through a proportional resonant controllermdqRepresenting the voltage modulated wave in dq coordinate system, the symbol m represents the modulated wave (Modulation wave), s is laplace operator,where j is an imaginary unit and w is an angular frequency (where w is an independent variable and varies over the entire frequency domain, characterizing the frequency domain characteristics of the system), where G isPR+HCThe expression of (a) is:
in the formula, kPThe gain of the controller as a whole is characterized for the scaling factor, where 0.5, k is takenRCharacterizing the gain of the fundamental resonance term for the fundamental resonance controller coefficients, here taken to be 200, w0At the fundamental angular frequency of 2 pi f0，f0Is the fundamental frequency, f0＝50Hz，kiCoefficient of harmonic resonance controller, kiIs 10, wiAt harmonic angular frequency of 2 pi fiI is the number of harmonics, fiIs (2n +1) f0N is a positive integer, n is 2,3, 4;
3) combined with voltage UpccPhase information of (2) proportional resonant controller GPR+HCOutput value u ofmdqCarrying out dq0/abc coordinate transformation to obtain a voltage signal under an abc coordinate system;
4) the voltage signal obtained in the step 3) and the collected voltage U are comparedpccThe difference is made to form a modulated signal (this step is a voltage feedforward strategy, which aims to accelerate the response speed of the controller), and then the modulated signal passes through a delay link (any control system includes a delay link, generally 1.5 sampling periods, denoted as e)1.5sTsWhere s is Laplace operator, Ts is sampling period of control system), kpwmThe modulation link obtains a final pulse trigger signal; wherein k ispwmThe gain coefficient of the converter is a value which is half of the voltage of the direct current side, and represents the connection between the alternating current side and the direct current side of the converter.
The invention does not need an additional sensor to collect the capacitance current icThe harmonic closed-loop feedback control is carried out simultaneously when grid-connected current single-loop feedback is carried out, the direct control is carried out in the control theory, the stability is higher than that of the traditional harmonic control method, and the high-quality current grid connection can be still realized when the power grid is distorted.
Acquisition of i by conventional methodscAs a source of harmonics, i.e. acquisition icAs a signal for the control circuit, intended to acquire icOn one hand, harmonic suppression and damping suppression are performed, and on the other hand, two functions are simultaneously realized by two controllers, and in fact, when certain delay conditions are met or various parameters of the LCL structure are reasonably designed (parameter design reference of the LCL structure [1]]) The stable operation of the LCL type grid-connected converter can be ensured without a damping suppression strategy, and the stable operation of the converter under the condition of no damping strategy is realized. Without using icAnd directly adopt i2As a harmonic source, a signal of a control circuit is formed, and the purpose of suppressing harmonic waves can be achieved without adopting i compared with the traditional strategycAnd the sensor is omitted, the hardware cost is saved, and the performance is better.
[1] LCL filter parameter optimization design considering inverter side current feedback influence is given by Zujingming, Jilin, Kudzuvine, Shaohuojun [ J ]. China Motor engineering report (17): 4656-4664.
The superiority and feasibility of the method of the invention compared with the traditional method are proved as follows:
first, fig. 2 shows a net side current i of the LCL topology2A harmonic impedance model of the loop; FIG. 3 shows the filter capacitor current i of the LCL topologycA harmonic impedance model of the loop; wherein, CPR、LPRAnd CHC、LHCIs a proportional resonant controller GPR+HCThe impedance model, the converter and the power grid of (1) are respectively equivalent to a current source and a voltage source, namely, the LCL topological structure can be divided into two loops, namely a loop of a network side inductor L2+ a loop of a converter side inductor L1, also called a network side current i2Loop or Loop a Loop, and network side inductor L2+ filter capacitor C Loop, also called filter capacitor current icLoop or Loop B Loop.
When the PCC point is used as an excitation source, the control characteristics of the converter are considered, so that an impedance frequency domain characteristic diagram of two loops can be obtained, as shown in fig. 4, an x line is the impedance frequency domain characteristic of Loop a, each group of peaks represents one group of resonance controllers, the invention takes three groups of resonance controllers as an example, the harmonic waves are respectively fundamental waves, 5 th harmonic waves and 7 th harmonic waves, and the resonance frequencies are respectively fundamental waves, 5 th harmonic waves and 7 th harmonic waves50Hz, 250Hz, 350 Hz; line is the impedance frequency domain characteristic of Loop B, and curve peak is L in Loop B2And C, and C.
Generally, the PCC point background harmonic voltage is formed by the current generated by the nonlinear load in the distribution network flowing through the weak network impedance, and the harmonic number contained is the same as the nonlinear current harmonic, and is (6n ± 1, n is 1,2,3 …), and the higher the harmonic number is, the less the content is; as shown in FIG. 4, Loop A has high admittance capability to the harmonic within 750Hz, and Loop B has high admittance capability to the harmonic above 750 Hz; generally, the low-order harmonic wave does not exceed 13, so the background harmonic wave mainly flows through Loop A, the harmonic wave with the resonant frequency (above 1000 Hz) mainly flows through Loop B, and the invention uses the current of the Loop A Loop, namely the network side current i2And collecting and controlling the harmonic source.
Secondly, the traditional grid-connected power converter with the LCL topological structure collects capacitance current when performing harmonic wave processing, and is originally used as an active damping loop. And when the digital delay meets a certain condition, the system can be stable without an active damping system, namely a single-ring damping strategy. The open-loop frequency domain characteristics of the harmonic control loop of the conventional method and the method of the present invention are shown in fig. 5; zg1The curve is the open-loop frequency domain characteristic, Z, of the down-converter of the conventional methodg2The curve is the open-loop frequency domain characteristic of the converter after the method of the invention is adopted, f0At the fundamental frequency, i.e. 50Hz, fHAt harmonic frequencies, i.e. 250Hz, 350Hz, 450Hz, fsrLoop B series resonance frequency. As can be seen from fig. 5, in the conventional method, the filter capacitor and the network side inductor generate series resonance, and generate infinite gain for a certain high-frequency subharmonic, which affects the stability of the system.
Fig. 6 shows a root trace diagram of the closed-loop system when the digital delay time is considered and the traditional capacitance current is taken as a sampling harmonic source, and fig. 7 is a root trace diagram of the closed-loop system when the method of the invention, namely the network side current is taken as the sampling harmonic source; it should be noted that the control system considering the delay is a discrete control system, the root locus is stable when falling in the unit circle, unstable when falling outside the unit circle, and singleThe critical stability is found on the bit circle. In FIG. 6, a portion of the root trace falls outside the unit circle, illustrating the capacitor current icThe sampling of (2) can cause instability hidden danger to the system; fig. 7 illustrates that the curve of the root locus falls within the unit circle while satisfying a sufficiently large gain, which means that the method of the present invention contributes to the stability of the system.
Fourthly, fig. 8 is an experimental waveform diagram, after the method of the present invention is enabled: for convenience of comparison, the waveform of the current on the converter side, the waveform of the current on the network side and the waveform of the voltage on the PCC adopt an A-phase waveform, the condition of adopting the traditional method before enabling is the condition of adopting the method of the invention after enabling. As can be seen from fig. 8: the method provided by the invention obviously improves the grid-connected current, enables the waveform to be more sinusoidal and stable, and verifies the correctness of the analysis.
Drawings
FIG. 1 is a schematic diagram of a control method according to the present invention;
FIG. 2 shows the net side current i of the LCL topology2A harmonic impedance model of the loop;
FIG. 3 shows the filter capacitor current i of the LCL topologycA harmonic impedance model of the loop;
FIG. 4 is a graph of the frequency domain characteristics of the impedance of a two-loop, conventional method and the method of the present invention;
FIG. 5 is a graph of the open-loop frequency domain characteristics of a harmonic control loop for a two-loop, conventional method and the method of the present invention;
FIG. 6 is a closed loop root trace diagram of a conventional method;
FIG. 7 is a closed loop root trace diagram of the method of the present invention;
fig. 8 shows the converter side current, grid side current, and grid side voltage waveforms enabled by the method of the present invention.
Detailed Description
The control method of the grid-connected power converter with the LCL topological structure is realized by the following steps:
1) collecting converter DC side capacitor voltage UDCAnd the command value U of the DC side capacitor voltageDC *Making a difference, and forming a d-axis current controlled by a current inner loop through a PI controllerInstruction value id*；
2) Collecting voltage U of PCC pointpccAnd phase information thereof, and simultaneously collecting network side current i2Combined with a voltage UpccPhase information of (a) will be i2Obtaining dq axis current i through abc/dq0 coordinate transformationdAnd iq(ii) a In order to ensure that the converter is operated at a unit power factor (namely, the reactive power is zero), a q-axis current command value i is setqSetting the d-axis current command value i to zerodCommand value of current of x and q axes iqRespectively connected with the collected d-axis current idCollected q-axis current iqMake a difference to obtain Δ id＝id*－idAnd Δ iq＝iq*－iqThen passes through a proportional resonance controller GPR+HCObtaining:
umdq(s)＝(Δid+Δiq)·GPR+HC(s)
in the above formula,. DELTA.idAnd Δ iqObtaining a voltage modulation wave signal u after passing through a proportional resonant controllermdqRepresenting a voltage Modulation wave under dq coordinate system, wherein the symbol m represents the Modulation wave, s is laplacian operator, and s is jw, where j is an imaginary unit, w is an angular frequency (where w is an independent variable, and the variation range is the whole frequency domain, representing the frequency domain characteristic of the system), and G is the frequency domain characteristic of the systemPR+HCThe expression of (a) is:
in the formula, kPThe gain of the controller as a whole is characterized for the scaling factor, where 0.5, k is takenRCharacterizing the gain of the fundamental resonance term for the fundamental resonance controller coefficients, here taken to be 200, w0At the fundamental angular frequency of 2 pi f0，f0Is the fundamental frequency, f0＝50Hz，kiCoefficient of harmonic resonance controller, kiIs 10, wiAt harmonic angular frequency of 2 pi fiI is the number of harmonics, fiIs (2n +1) f0N is a positive integer, n is 2,3, 4;
3) bonding ofVoltage UpccPhase information of (2) proportional resonant controller GPR+HCOutput value u ofmdqCarrying out dq0/abc coordinate transformation to obtain a voltage signal under an abc coordinate system;
4) the voltage signal obtained in the step 3) and the collected voltage U are comparedpccThe difference is made to form a modulated signal (this step is a voltage feedforward strategy, which aims to accelerate the response speed of the controller), and then the modulated signal passes through a delay link (any control system includes a delay link, generally 1.5 sampling periods, denoted as e)1.5sTsWhere s is Laplace operator, Ts is sampling period of control system), kpwmThe modulation link obtains a final pulse trigger signal; wherein k ispwmThe gain coefficient of the converter is a value which is half of the voltage of the direct current side, and represents the connection between the alternating current side and the direct current side of the converter.
Claims (2)
1. A control method of a grid-connected power converter with an LCL topological structure is characterized by comprising the following steps:
1) collecting converter DC side capacitor voltage UDCAnd the command value U of the DC side capacitor voltageDC *Making a difference, and forming a d-axis current command value i controlled by a current inner loop through a PI controllerd*；
2) Collecting voltage U of PCC pointpccAnd phase information thereof, and simultaneously collecting network side current i2Combined with a voltage UpccPhase information of (a) will be i2Obtaining dq axis current i through abc/dq0 coordinate transformationdAnd iq(ii) a In order to ensure that the converter operates at a unit power factor, a q-axis current command value i is setqSetting the d-axis current command value i to zerodCommand value of current of x and q axes iqRespectively connected with the collected d-axis current idCollected q-axis current iqMake a difference to obtain Δ id＝id*－idAnd Δ iq＝iq*－iqThen passes through a proportional resonance controller GPR+HCObtaining:
umdq(s)＝(Δid+Δiq)·GPR+HC(s)
in the above formula,. DELTA.idAnd Δ iqObtaining a voltage modulation wave signal u after passing through a proportional resonant controllermdqRepresenting a voltage modulated wave in dq coordinate system, the symbol m represents the modulated wave, s is laplacian operator, s ═ jw, where j is an imaginary unit, w is an angular frequency, where G isPR+HCThe expression of (a) is:
in the formula, kPThe gain of the controller as a whole is characterized for the scaling factor, where 0.5, k is takenRCharacterizing the gain of the fundamental resonance term for the fundamental resonance controller coefficients, here taken to be 200, w0At the fundamental angular frequency of 2 pi f0，f0Is the fundamental frequency, f0＝50Hz，kiCoefficient of harmonic resonance controller, kiIs 10, wiAt harmonic angular frequency of 2 pi fiI is the number of harmonics, fiIs (2n +1) f0N is a positive integer, n is 2,3, 4;
3) combined with voltage UpccPhase information of (2) proportional resonant controller GPR+HCOutput value u ofmdqCarrying out dq0/abc coordinate transformation to obtain a voltage signal under an abc coordinate system;
4) the voltage signal obtained in the step 3) and the collected voltage U are comparedpccMaking difference to form modulation signal, then making the modulation signal pass through delay element kpwmThe modulation link obtains a final pulse trigger signal; wherein k ispwmIs the gain factor of the converter, and has a value of half the dc side voltage.
2. The method for controlling the grid-connected power converter with the LCL topology structure as claimed in claim 1, wherein the delay element in the step 4) is 1.5 sampling periods denoted as e1.5sTsWherein s is a Laplace operator, and Ts is a sampling period of the control system.
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