CN114884125B - High-stability control method of LCL type grid-connected inversion system under weak current network - Google Patents
High-stability control method of LCL type grid-connected inversion system under weak current network Download PDFInfo
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Abstract
The invention discloses a high-stability control method of an LCL grid-connected inversion system under a weak current grid, belonging to the technical field of photovoltaic grid-connected inversion control; the problem that the conventional grid-connected inverter is easy to generate unstable conditions in a weak grid environment is solved. Aiming at the influence of grid impedance increase and small signal disturbance on a phase-locked loop and an LCL filter in a grid-connected inverter system under a weak current grid, a two-grid-connected current loop feedback active damping strategy of second-order generalized integration is adopted, an adaptive second-order resonance integrator active damping feedback function is constructed, a proportional resonance function is used as a current loop controller, virtual impedance correction is carried out on a control system, and a voltage feedforward controller is designed; a bi-level self-adaptive filter based sequencing phase-locked loop is designed; through demonstration, reasonable virtual impedance correction is added, the use of a measuring device in hardware can be reduced while a good damping effect is achieved, the hardware cost is saved, and the control strategy can ensure the stability and the robustness of the system.
Description
Technical Field
The invention relates to the technical field of photovoltaic grid-connected inversion control, in particular to a high-stability control method of an LCL-type grid-connected inversion system under a weak power grid, which is suitable for the conditions of the weak power grid and high impedance grid connection.
Background
The distributed power generation system is characterized in that a power generation source, a load and a power grid are connected together through a power electronic conversion device. Due to factors such as the increase of the distributed generation permeability, the impedance of a long-distance power transmission line, leakage inductance of a transformer and the like, the rigidity of an access power grid is weakened, and the influence of the power grid impedance on a grid-connected inverter is not negligible. When a large impedance occurs in the power grid, the rigidity of the power grid is weakened. Therefore, the grid-connected inverter control design cannot match the grid-connected conditions to the traditional ideal large grid.
In general, it is considered that grid-connected systems with a system short-circuit ratio (short circuit ratio, SCR) of 10 or less are weak grids, and the smaller the ratio is, the weaker the grid strength is. The grid-connected inverter is used as a 'bridge' for connecting a distributed power generation system and a large network, is a core of a distributed power generation access power grid, whether the stable operation of the grid-connected inverter directly influences the stability of the whole system, and the stability of the grid-connected inverter plays a key role in the development of DPGS under the condition of a non-ideal power grid. The conventional distributed grid-connected inverter is designed under the condition of ideal grid parameters, the ideal grid of the alternating current grid is considered, and the impedance and fluctuation of the grid are not in the design consideration range. However, under the condition of weak current network, the impedance existing in the alternating current power grid system cannot be ignored, the power grid voltage fluctuates due to the existence of background harmonic waves, and the power grid frequency also generates certain oscillation. The control bandwidth, stability margin, open loop gain and other performances of the grid-connected inverter can be changed under the conditions, so that the output power quality of the distributed grid-connected power generation system is reduced. Even instability phenomenon can occur when the grid impedance of the grid-connected inverter is relatively large, harmonic oscillation, line heating, equipment interference, malfunction of a relay protection device and the like are caused, and even a series of serious consequences are caused.
In summary, the weak grid condition has great challenges to the stable operation of the distributed photovoltaic grid-connected inverter, and the existing grid-connected inverter control strategy cannot meet the requirements of system stability and robustness, so that the research and development of the high-efficiency and reliable grid-connected photovoltaic inverter which can adapt to the weak grid condition has great practical significance.
Disclosure of Invention
The purpose of the invention is that: the high stability control method of the LCL type grid-connected inverter system under the weak current network is provided, the problem that the conventional grid-connected inverter is easy to generate unstable conditions under the weak current network environment is solved, and the stability and the robustness of the grid-connected inverter system are improved due to high impedance and power grid fluctuation under the weak current network condition based on the active damping feedback of the self-adaptive second-order resonance integrator and the bisecond-order self-adaptive filter sequencing phase-locked loop.
In order to achieve the above purpose, the present invention adopts the following technical scheme: a high stability control method of an LCL type grid-connected inversion system under a weak current network comprises the following steps:
S1, under the weak current network condition, based on a mathematical model of a three-phase LCL type photovoltaic grid-connected inverter system, a current-voltage double-loop control strategy is adopted on the basis of considering the impedance of a power grid, a main circuit and a control structure model of the LCL type grid-connected inverter are constructed, a second-order proportional resonance controller is selected as a current outer loop control G i (S), a system delay converter G u (S) is introduced, and the impedance of the power grid system only considers the impedance L g of the power grid;
S2, according to the structural model constructed in the step S1, a control transfer model containing filter capacitance current feedback active damping is obtained, the active damping taking the traditional LCL type filter capacitance current as feedback quantity is replaced by two state quantities of a public coupling point voltage u pcc and a grid-connected current i 2, the active damping control model of the grid-connected inverter is obtained through equivalent transformation, and the grid-connected current i 2 and the public coupling point voltage u pcc are used as feedforward control of feedback quantity;
S3, according to the grid-connected inverter active damping control model which is subjected to the equivalent transformation in the step S2, a grid-connected current feedback function H g＝Hc*C(L2+Lg)s2 is obtained through calculation and is a second-order differential link, and in the filter damping design, a pure differential term cannot be realized in a physical equivalent circuit, so that a second-order resonance integrator is adopted as a feedback function H (S) of a grid-connected current active damping strategy, a grid-connected current active damping control loop model which does not consider grid voltage front feedback is obtained, and an LCL filter open-loop transfer function G LCL (S) and an active damping loop gain T ol (S) are obtained through calculation;
S4, according to the active damping control model of the grid-connected inverter which is subjected to the equivalent transformation in the step S2, a simplified equivalent control transfer model of the grid-connected inverter is obtained through simplification, a function expression of grid-connected current i 2 is obtained through calculation, the grid impedance shows induction characteristics at a relatively high frequency, Z g/Z0 in the system can meet Nyquist stability criteria according to the principle of impedance ratio criteria, the grid voltage feedforward control taking the voltage u pcc of the common coupling point as feedback quantity is substituted into the output impedance Z 0 (S) of the inverter, an equivalent current source model of the grid-connected inverter is obtained,
Adopting parallel virtual impedance Z 0 * to enable equivalent impedance Z 0'(s) output by the grid-connected inverter to trend to infinity, obtaining a current source model of parallel virtual impedance, obtaining a simplified control model added with virtual impedance after simplification, advancing a feedback action point of the parallel virtual impedance, obtaining a grid-connected inverter control model containing the parallel virtual impedance, and obtaining the calculation result according to the output impedance Z 0(s) of the inverterThe method comprises the steps of omitting the advance link in the forward moving process of the parallel virtual impedance feedback action point, finally obtaining a feedback function G V(s), and finishing feedforward impedance correction;
S5, adding a damping coefficient k into the adjusting coefficient based on the traditional second-order generalized integral filter according to the structural model constructed in the step S1, and adding 1/k into feedback to obtain an adaptive second-order generalized integral filter expression model;
s6, aiming at grid disturbance, harmonic wave and unbalance in a non-ideal power grid, based on a traditional phase-locked loop, 90-degree phase shift and filtering are realized by the self-adaptive second-order generalized integral filter in the step S5, positive sequence component extraction in the unbalanced power grid is performed by using a symmetrical component method, and finally phase locking is realized, so that a synchronous order-splitting phase-locked loop model of the bi-second-order generalized integral filter is obtained.
In the step S1, the expression of the system delay converter G u (S) is:
in the/> And G d(s) are respectively used as a calculation delay and a sampling delay, and T s is a sampling period;
the expression of the current outer loop control G i(s) is: Wherein k p is a proportionality coefficient, k r is a second-order resonance coefficient, omega 0 is a fundamental frequency of a power grid, s is an input variable after complex variable Laplacian transformation, xi 0 is an active damping coefficient, omega i is a resonance bandwidth frequency, the frequency deviation is not more than +/-0.5 Hz according to the specification in GB/T15945 standard of allowable deviation of electric energy quality power system frequency, and is adapted to fluctuation of +/-1% of the fundamental frequency of the power grid, so omega i＝0.01*2πf0 rad/s is taken.
In the step S3, in the grid-connected current feedback function H g＝Hc*C(L2+Lg)s2, H c is a filter capacitive current active damping feedback function;
the expression of the feedback function H(s) is: Where k is the gain coefficient of the feedback function, ζ is the damping coefficient, ω n is the natural frequency, s is the input variable after complex variable Laplacian transformation;
the open loop transfer function G LCL(s) has the expression:
Wherein i 2(s) is grid-connected current, u inv(s) is inverter output voltage, L 1 is inverter side inductance reactance, L 2 is grid-connected side inductance reactance, C is filter capacitance, omega r is second-order resonance frequency,/> L g is the impedance of the power grid, the general value range of L g is 0-0.1 pu, the corresponding short-circuit ratio of the power grid is 10 at maximum, and the expression of L g is/>
The loop gain expression is:
wherein K PWM is the ratio of the measured signals of the control signal domain.
In the step S4, the function expression of the grid-connected current i 2 is:
Where i ref is an inverter output reference value, H i is a current loop feedback function, and transfer functions G 1(s)、G2(s) are expressed as follows:
According to the analysis of the Norton theorem, the grid-connected inverter is equivalent to a current source parallel resistor, wherein the expression of the inverter output equivalent current i s(s) is as follows: the inverter output impedance Z 0(s) is expressed as:
According to the Nyquist stability criterion, the voltage u PCC＝i2(s)Zg(s)+ug of the public coupling point is calculated, wherein u g is the power grid voltage, and Z g is the power grid impedance; substituting the equivalent current source model into the output impedance Z 0(s) of the inverter to obtain an equivalent current source model i 2(s) of the grid-connected inverter, wherein the expression is as follows:
the theoretical value of the parallel virtual impedance Z 0 * under ideal conditions is: I.e.
The expression of the feedback function G V(s) is: l 1 is the inverter side inductance.
In step S6, the positive sequence component of the three-phase network voltage U ab Can be expressed as: Wherein the order matrix/>
Voltage in two-phase stationary coordinate systemThe expression of (2) is:
wherein [ T αβ ] is a Clark transformation matrix,/> Q=e -jπ/2, and performing 90-degree phase shift sum operation on the input voltage signal to complete extraction of the positive sequence component of the system voltage.
The beneficial effects of the invention are as follows:
1) The invention adopts a two-grid-connected current loop feedback active damping strategy of second-order generalized integration, constructs an active damping feedback function of a self-adaptive second-order resonance integrator, takes a proportional resonance function as a current loop controller, performs virtual impedance correction on a control system, designs a voltage feedforward controller, and realizes a control strategy combining the virtual impedance correction and the two-grid-connected current loop active damping; the method is characterized in that the action point of the capacitive current active damping strategy of the traditional LCL filter is moved forward and replaced by the grid-connected current and the common coupling point voltage in an equivalent way, so that the method can achieve a good damping effect, reduce the use of a measuring device in hardware and save the hardware cost.
2) According to the invention, a second-order resonance integrator active damping feedback function is selected according to an equivalent model, a proportional resonance controller is selected as a current loop controller function, and a bi-second-order adaptive filter-based split-order phase-locked loop model is designed; a simulation experiment is built in MATLAB/Simulink, and simulation results show that: the theoretical deduction and the design scheme are reasonable and effective, the control strategy can ensure the stability and the robustness of the system, and the method is suitable for grid connection conditions of a weak power grid and high impedance.
Drawings
FIG. 1 is a schematic diagram of a main circuit and a control structure of an LCL grid-connected inverter in step S1 of the present invention;
FIG. 2 is a block diagram of an active damping control model of an equivalent-conversion grid-connected inverter in step S2 of the present invention;
FIG. 3 is a block diagram of a grid-connected current active damping control loop model without consideration of grid voltage feedforward in step S3 of the present invention;
FIG. 4 is a frequency response graph of the active damping open loop transfer function of step S3 of the present invention;
FIG. 5 is a simplified equivalent control transfer model block diagram of the grid-connected inverter of step S4 of the present invention;
FIG. 6 is a graph showing the frequency response of the system output impedance Z 0 in step S4 according to the present invention;
FIG. 7 is a block diagram of an equivalent current source model of the grid-connected inverter in step S4 of the present invention;
FIG. 8 is a block diagram of a current source model with virtual impedance in parallel in step S4 of the present invention;
FIG. 9 is a simplified control model block diagram of the virtual impedance added in step S4 of the present invention;
FIG. 10 is a block diagram of a grid-connected inverter control model including parallel virtual impedances in step S4 of the present invention;
FIG. 11 is a graph of the adaptive second-order generalized integral filter expression model in step S5 of the present invention;
FIG. 12 is a graph showing the comparison of unit step response curves of the adaptive second-order generalized integral filter at different k values and at different k values in step S5 according to the present invention;
FIG. 13 is a Bode comparison plot of the adaptive second-order generalized integral filter for varying the value of ζ and for varying the value of k in step S5 of the present invention;
FIG. 14 is a diagram of a synchronous and split phase-locked loop model of the biquad generalized integral filter of step S6 of the present invention;
FIG. 15 is a frequency output comparison chart of the conventional PLL and the synchronous and split PLL in step S6 of the present invention;
FIG. 16 is a graph of grid-connected current and voltage variation during gradual change and abrupt change of output power of the grid-connected inverter in a simulation experiment according to the present invention;
FIG. 17 is a graph of the stability and modulation depth of the DC bus voltage on the grid-connected inverter side in the simulation experiment of the present invention;
FIG. 18 is a graph of current signal contrast value and grid-tie inverter power output in a controller in a simulation experiment of the present invention;
FIG. 19 is a graph showing the harmonic content of the grid-connected current of the conventional active damping control and the control strategy of the system in the simulation experiment of the present invention;
FIG. 20 is a graph comparing current and voltage output waveforms of an inverter when the impedance of a power grid of the system is increased in a conventional simulation experiment;
fig. 21 is a graph of output results of the grid-connected inverter in the semi-physical simulation experiment of the present invention.
Detailed Description
The invention is further illustrated by the following description in conjunction with the accompanying drawings and specific embodiments.
Examples: as shown in fig. 1-21, the high stability control method of the LCL grid-connected inverter system under the weak current network of the invention comprises the following steps:
S1, under the weak current network condition, based on a mathematical model of a three-phase LCL type photovoltaic grid-connected inverter system, a current-voltage double-loop control strategy is adopted on the basis of considering the impedance of a power grid, a main circuit and a control structure model (shown in figure 1) of the LCL type grid-connected inverter are constructed, wherein L 1 is the inductance reactance of the inverter side in figure 1; l 2 is grid-connected side inductance; l g is power grid impedance, R 1、R2 is parasitic resistance, u abc is inverter output voltage, u Cabc is filter capacitor voltage, u gabc is power grid side voltage, i abc is inverter side current, i Cabc is filter capacitor current, and i 2abc is grid-connected side current; considering the worst stability, the internal resistance R 1、R2 of L 1、L2 is very small and negligible, and the grid system impedance only considers the grid impedance L g.
The second-order proportional resonance controller is selected as a current outer loop control G i(s), a system delay converter G u(s) is introduced, and the expression of the system delay converter G u(s) is as follows:
in the/> And G d(s) are respectively used as a calculation delay and a sampling delay, and T s is a sampling period;
the expression of the current outer loop control G i(s) is: Wherein k p is a proportionality coefficient, k r is a second-order resonance coefficient, omega 0 is a fundamental frequency of a power grid, s is an input variable after complex variable Laplacian transformation, xi 0 is an active damping coefficient, omega i is a resonance bandwidth frequency, the frequency deviation is not more than +/-0.5 Hz according to the specification in GB/T15945 standard of allowable deviation of electric energy quality power system frequency, and is adapted to fluctuation of +/-1% of the fundamental frequency of the power grid, so omega i＝0.01*2πf0 rad/s is taken.
S2, according to the structural model constructed in the step S1, a control transfer model containing filter capacitance current feedback active damping is obtained, the active damping taking the traditional LCL type filter capacitance current as feedback quantity is replaced by two state quantities of a public coupling point voltage u pcc and a grid-connected current i 2, the grid-connected inverter active damping control model is obtained through equivalent transformation (shown in figure 2), and grid-connected current i 2 and public coupling point voltage u pcc are used as feedforward control of the feedback quantity.
In fig. 2, G i(s) is a current outer loop controller, K PWM is a ratio of measured signals in a control signal domain, H i is a current loop feedback function, H g is a grid-connected current feedback function, and G V(s) is a final feedback function.
The control method can omit a measuring device for the capacitance current of the filter, so as to save hardware and reduce cost. Moreover, the control strategy is converted from the active damping control strategy of the filter capacitive flow, and the stable bandwidth is not lost.
And S3, according to the grid-connected inverter active damping control model with the equivalent transformation in the step S2, a grid-connected current feedback function H g＝Hc*C(L2+Lg)s2 is obtained through calculation and is a second-order differential link, wherein H c is a filter capacitance current active damping feedback function, and a pure differential term cannot be realized in a physical equivalent circuit in the filter damping design, so that in order to realize the same effect as the differential term in a low frequency band, a second-order resonance integrator is adopted as a feedback function H (S) of a grid-connected current active damping strategy, and a grid-connected current active damping control loop model (shown in figure 3) which does not consider grid voltage front feedback is obtained.
The expression of the feedback function H(s) is: where k is the gain coefficient of the feedback function, ζ is the damping coefficient, ω n is the natural frequency, and s is the input variable after the complex variable laplace transform.
The open loop transfer function G LCL(s) and the active damping loop gain T ol(s) of the LCL filter are obtained through calculation, and the open loop transfer function G LCL(s) has the expression:
Wherein i 2(s) is grid-connected current, u inv(s) is inverter output voltage, L 1 is inverter side inductance reactance, L 2 is grid-connected side inductance reactance, C is filter capacitance, omega r is second-order resonance frequency,/> L g is the impedance of the power grid, the general value range of L g is 0-0.1 pu, the corresponding short-circuit ratio of the power grid is 10 at maximum, and the expression of L g is/>
The phase frequency characteristics of the system at the resonance frequency are suddenly changed, so that the worst case of the influence of the impedance of the system needs to be considered in order to ensure the stability of the system. The grid-connected inverter control system is divided into a current control loop and an active damping loop, wherein the controller G i(s) is stable under normal conditions, so that the active damping loop is key to stabilizing the whole system. According to the control block diagram of the active damping control strategy, the active damping loop gain T ol(s) can be obtained.
The loop gain T ol(s) is expressed as:
wherein K PWM is the ratio of the measured signals of the control signal domain.
The amplitude-frequency and phase-frequency characteristic curves of the active damping open-loop transfer function are shown in fig. 4, wherein the amplitude-frequency curve of the active damping loop has a higher peak value at the resonance frequency, the phase-frequency characteristic is subjected to 180 DEG phase mutation, and according to the Nyquist stability criterion, the phase-frequency characteristic curve does not cross a-180 DEG line when the amplitude of the active damping loop transfer function is 0 dB. The design of the active damping ring is thereby shown to be rational, and the choice of the damping function H(s) is also shown to be correct.
S4, according to the active damping control model of the grid-connected inverter which is subjected to the equivalent transformation in the step S2, a simplified equivalent control transfer model (shown in fig. 5) of the grid-connected inverter is obtained through simplification, a function expression of grid-connected current i 2 is obtained through calculation, and the function expression of grid-connected current i 2 is as follows:
Where i ref is an inverter output current reference value, H i is a current loop feedback function, and transfer functions G 1(s)、G2(s) are expressed as follows:
According to the analysis of the Norton theorem, the grid-connected inverter is equivalent to a current source parallel resistor, wherein the expression of the equivalent output current i s(s) of the inverter is as follows: the inverter output impedance Z 0(s) is expressed as:
The grid impedance exhibits inductive characteristics at relatively high frequencies, compromising the stable control of Z g/Z0. From the impedance ratio criterion, the system stability must meet two conditions: firstly, the grid-connected inverter system is stable under the condition of a strong power grid; and secondly, Z g/Z0 meets the Nyquist stability criterion.
As shown in the frequency response plot of the system output impedance Z 0 in fig. 6, at frequencies less than f 1, Z g is less than Z 0, where Z g/Z0 is constantly less than 1, the nyquist plot does not bypass (-1, 0), so system stability is primarily dependent on the frequency response portion of Z g being greater than Z 0. The analysis shows that the region between f 2 and f 3 is the worst case for the system stability margin, where the phase angle difference can be guaranteed to be above-180 deg., and the system can be stable.
According to an impedance ratio criterion principle, Z g/Z0 in the system can be known to meet a Nyquist stability criterion, and the voltage u PCC＝i2(s)Zg(s)+ug of the public coupling point is represented by u g, wherein the voltage is the power grid voltage, and Z g is the power grid impedance; substituting the equivalent current source model into the output impedance Z 0(s) of the inverter to obtain an expression of the equivalent current source model i 2(s) of the grid-connected inverter, wherein the expression is as follows:
The effect of the inverter system output impedance and the grid impedance on the grid-connected inverter power output can be obviously observed in fig. 7 by the grid-connected inverter equivalent current source model (shown in fig. 7).
The parallel virtual impedance Z 0 * is adopted, so that the equivalent impedance Z 0'(s) output by the grid-connected inverter tends to infinity, the influence of the grid impedance on the grid-connected inverter is weakened, a current source model of the parallel virtual impedance is obtained (shown in figure 8), a simplified control model added with the virtual impedance is obtained after simplification (shown in figure 9),
The theoretical value of the parallel virtual impedance Z 0 * under ideal conditions is: I.e.
Advancing the feedback action point of the parallel virtual impedance to obtain a grid-connected inverter control model (shown in figure 10) containing the parallel virtual impedance, and obtaining the calculation result of the output impedance Z 0(s) of the inverterThe method comprises the steps of omitting the lead link (the inverse of the lag link) in the forward moving process of the parallel virtual impedance feedback action point, and finally obtaining a feedback function G V(s) to finish feedforward impedance correction.
The expression of the feedback function G V(s) is: l 1 is the inverter side inductance.
S5, according to the structural model constructed in the step S1, a damping coefficient k is added into an adjusting coefficient based on a traditional second-order generalized integral filter, 1/k is added into feedback, and the adjustment of overshoot and stabilization time can be ensured not to interfere with each other theoretically, so that an adaptive second-order generalized integral filter expression model is obtained (as shown in fig. 11).
As shown in fig. 12, which is a comparison graph of unit step response curves of the adaptive second-order generalized integral filter at different k values and different k values when damping coefficients are adjusted, it can be seen from the comparison graph that the k value of the adaptive second-order generalized integral filter only affects overshoot, and has no effect on stable response time, and it can be seen from the comparison of (a) and (b) that the damping ratio only affects stable response time.
As shown in fig. 13, which is a bode comparison chart of the adaptive second-order generalized integral filter when the ζ value is changed and when the k value is changed, fig. (a) shows that the change of the damping coefficient can affect the amplitude values of the low frequency band and the high frequency band, and the phase frequency curve can also change obviously; whereas the change in k value in graph (b) only has an effect on the amplitude and no effect on the phase frequency characteristics. Therefore, the influence of parameters on the system needs to be comprehensively considered in the application of the system, so that the system achieves better stability.
S6, aiming at grid disturbance, harmonic and unbalance in a non-ideal power grid, based on a traditional phase-locked loop, 90-degree phase shift and filtering are realized by using the self-adaptive second-order generalized integral filter in the step S5, positive sequence component extraction in the unbalanced power grid is performed by using a symmetrical component method, and finally phase locking is realized, so that a synchronous and sequential phase-locked loop model of the bi-second-order generalized integral filter is obtained (as shown in fig. 14).
Positive sequence component of three-phase mains voltage U abc Can be expressed as: /(I)Wherein the order matrix/>
Voltage in two-phase stationary coordinate systemThe expression of (2) is:
wherein [ T αβ ] is a Clark transformation matrix,/> Q=e -jπ/2, and performing 90-degree phase shift sum operation on the input voltage signal to complete extraction of positive sequence components of the system voltage, wherein a second-order generalized integrator can realize phase angle shift and higher harmonic filtering.
As shown in fig. 15, which is a graph showing the frequency output comparison of the conventional pll and the synchronous and sequenced pll, when there is fluctuation in the grid-connected system, there is significant oscillation at the output frequency controlled by the conventional PI pll around 50 Hz. The phase-locked loop of the biquad generalized integral filter has certain fluctuation in the starting stage, but the stabilized phase-locked precision is more accurate, and the performance is obviously improved.
Simulation experiment and result analysis:
table 1 below is a table of parameter values of the grid-connected inverter when the experimental system is built.
In photovoltaic power generation, power output may be changed in various conditions, two extreme conditions of gradual change and abrupt change of power output are simulated in simulation experiments, the boosted grid-connected voltage and current output results are shown in fig. 16, grid-connected current and voltage change conditions of the grid-connected inverter during gradual change and abrupt change of output power are shown in (a) and (b), current fluctuation is smaller during abrupt change, and the results prove that the inverter has good stability and robustness.
The stability and modulation depth curve of the dc bus voltage at the grid-connected inverter side are shown in fig. 17, and it can be seen that there is good stability, in which the measured value is averaged to more clearly reflect the stability of the dc bus voltage at the inverter side, such as the small window portion in fig. 17, and the stability can be reflected.
As shown in fig. 18, the steady-state error between the command current and the output reference current in the dq coordinate system in the grid-connected inverter controller is shown in fig. (a) as a comparison result value of the current signal in the controller, and according to the change condition of the reference current in the controller, the comparison between the inverter output power graph (b) and the command value of the system controller in fig. (a), it is known that the power output is stable under various working conditions.
As shown in fig. 19, in the diagrams (a) and (b), the harmonic content analysis of the grid-connected current quality fourier of the conventional active damping control and the control strategy of the system is shown, and compared with the diagrams, the harmonic content of the grid-connected current obtained by the controller designed by the system is less, so that the effect is better.
As shown in fig. 20, the (a) and (b) in the graph are respectively comparison graphs of current and voltage output waveforms of the inverter when the impedance of the power grid of the system is increased, when the impedance of the power grid is increased, the conventional current output can periodically fluctuate, and after the active damping and feedforward compensation strategy proposed in the system is added, the output condition of the grid-connected current has no obvious fluctuation, so that the reasonable and effective system design is illustrated.
Semi-physical experiments and results:
The semi-physical experiment platform control core is a minimum system taking TMS320F28335 as a control chip, and verification of the output result of the grid-connected inverter part is carried out on the RT-LAB experiment platform.
As shown in fig. 21, the output result diagram of the grid-connected inverter in the semi-physical simulation experiment is shown, in the diagrams (a) and (b), the voltage and current result diagrams of the phase grid and the three-phase output current result diagrams on the grid side are respectively shown, in the diagram (a), the single-row voltage and the single-phase consistency of the current can be seen, only active current exists in the system, and the three-phase current output waveform in the diagram (b) has a better effect. The harmonic content diagram also shows that the grid-connected current voltage can meet the grid-connected index, and the simulation result can prove that the controller has better stability under the weak current grid.
Aiming at the influence of grid impedance increase and small signal disturbance on a phase-locked loop and an LCL filter in a grid-connected inverter system under a weak current grid, a two-grid-connected current loop feedback active damping strategy of second-order generalized integration is adopted, an adaptive second-order resonance integrator active damping feedback function is constructed, a proportional resonance function is used as a current loop controller, virtual impedance correction is carried out on a control system, and a voltage feedforward controller is designed; a bi-level self-adaptive filter based sequencing phase-locked loop is designed; through demonstration, reasonable virtual impedance correction is added, the use of a measuring device in hardware can be reduced while a good damping effect is achieved, the hardware cost is saved, and the control strategy can ensure the stability and the robustness of the system.
The foregoing is merely illustrative of the present invention and not restrictive, and other modifications and equivalents thereof may occur to those skilled in the art without departing from the spirit and scope of the present invention.
Claims (2)
1. A high stability control method of an LCL type grid-connected inversion system under a weak current network is characterized by comprising the following steps: the method comprises the following steps:
S1, under the weak current network condition, based on a mathematical model of a three-phase LCL type photovoltaic grid-connected inverter system, a current-voltage double-loop control strategy is adopted on the basis of considering the impedance of a power grid, a main circuit and a control structure model of the LCL type grid-connected inverter are constructed, a second-order proportional resonance controller is selected as a current outer loop control G i (S), a system delay converter G u (S) is introduced, and the impedance of the power grid system only considers the impedance L g of the power grid;
S2, according to the structural model constructed in the step S1, a control transfer model containing filter capacitance current feedback active damping is obtained, the active damping taking the traditional LCL type filter capacitance current as feedback quantity is replaced by two state quantities of a public coupling point voltage u pcc and a grid-connected current i 2, the active damping control model of the grid-connected inverter is obtained through equivalent transformation, and the grid-connected current i 2 and the public coupling point voltage u pcc are used as feedforward control of feedback quantity;
S3, according to the grid-connected inverter active damping control model which is subjected to the equivalent transformation in the step S2, a grid-connected current feedback function H g＝Hc*C(L2+Lg)s2 is obtained through calculation and is a second-order differential link, and in the filter damping design, a pure differential term cannot be realized in a physical equivalent circuit, so that a second-order resonance integrator is adopted as a feedback function H (S) of a grid-connected current active damping strategy, a grid-connected current active damping control loop model which does not consider grid voltage front feedback is obtained, and an LCL filter open-loop transfer function G LCL (S) and an active damping loop gain T ol (S) are obtained through calculation;
In the grid-connected current feedback function H g＝Hc*C(L2+Lg)s2, H c is a filter capacitance current active damping feedback function;
the expression of the feedback function H(s) is: Where k is the gain coefficient of the feedback function, ζ is the damping coefficient, ω n is the natural frequency, s is the input variable after complex variable Laplacian transformation;
the open loop transfer function G LCL(s) has the expression:
Wherein i 2(s) is grid-connected current, u inv(s) is inverter output voltage, L 1 is inverter side inductance reactance, L 2 is grid-connected side inductance reactance, C is filter capacitance, omega r is second-order resonance frequency,/> L g is the impedance of the power grid, the general value range of L g is 0-0.1 pu, the corresponding short-circuit ratio of the power grid is 10 at maximum, and the expression of L g is/>
The loop gain expression is:
Wherein K PWM is the ratio of the actual measurement signals of the control signal domain;
S4, according to the active damping control model of the grid-connected inverter which is subjected to the equivalent transformation in the step S2, a simplified equivalent control transfer model of the grid-connected inverter is obtained through simplification, a function expression of grid-connected current i 2 is obtained through calculation, the grid impedance shows induction characteristics at a relatively high frequency, Z g/Z0 in the system can meet Nyquist stability criteria according to the principle of impedance ratio criteria, the grid voltage feedforward control taking the voltage u pcc of the common coupling point as feedback quantity is substituted into the output impedance Z 0 (S) of the inverter, an equivalent current source model of the grid-connected inverter is obtained,
Adopting parallel virtual impedance Z 0 * to enable equivalent impedance Z' 0(s) output by the grid-connected inverter to trend to infinity, obtaining a current source model of parallel virtual impedance, obtaining a simplified control model added with virtual impedance after simplification, advancing a feedback action point of the parallel virtual impedance, obtaining a grid-connected inverter control model containing the parallel virtual impedance, and obtaining the calculation result according to the output impedance Z 0(s) of the inverterThe method comprises the steps of omitting the advance link in the forward moving process of the parallel virtual impedance feedback action point, finally obtaining a feedback function G V(s), and finishing feedforward impedance correction;
The functional expression of the grid-connected current i 2 is:
Where i ref is an inverter output current reference value, H i is a current loop feedback function, and transfer functions G 1(s)、G2(s) are expressed as follows:
According to the analysis of the Norton theorem, the grid-connected inverter is equivalent to a current source parallel resistor, wherein the expression of the inverter output equivalent current i s(s) is as follows: the inverter output impedance Z 0(s) is expressed as:
According to the Nyquist stability criterion, the voltage u PCC＝i2(s)Zg(s)+ug of the public coupling point is calculated, wherein u g is the power grid voltage, and Z g is the power grid impedance; substituting the equivalent current source model into the output impedance Z 0(s) of the inverter to obtain an equivalent current source model i 2(s) of the grid-connected inverter, wherein the expression is as follows:
the theoretical value of the parallel virtual impedance Z 0 * under ideal conditions is: I.e./>
The expression of the feedback function G V(s) is: L 1 is the inverter side inductance;
S5, adding a damping coefficient k into the adjusting coefficient based on the traditional second-order generalized integral filter according to the structural model constructed in the step S1, and adding 1/k into feedback to obtain an adaptive second-order generalized integral filter expression model;
s6, aiming at grid disturbance, harmonic waves and unbalance in a non-ideal power grid, based on a traditional phase-locked loop, 90-degree phase shift and filtering are realized by using the self-adaptive second-order generalized integral filter in the step S5, positive sequence component extraction in the unbalanced power grid is performed by using a symmetrical component method, and finally phase locking is realized, so that a synchronous and sequential phase-locked loop model of the biquad generalized integral filter is obtained;
positive sequence component of three-phase mains voltage U abc Can be expressed as: /(I)Wherein the order matrix/>
Voltage in two-phase stationary coordinate systemThe expression of (2) is:
wherein [ T αβ ] is a Clark transformation matrix,/> Q=e -jπ/2, and performing 90-degree phase shift sum operation on the input voltage signal to complete extraction of the positive sequence component of the system voltage.
2. The high-stability control method of the LCL type grid-connected inverter system under the weak power grid of claim 1, which is characterized by comprising the following steps: in the step S1, the expression of the system delay converter G u (S) is:
in the/> And G d(s) are respectively used as a calculation delay and a sampling delay, and T s is a sampling period;
the expression of the current outer loop control G i(s) is: Wherein k p is a proportionality coefficient, k r is a second-order resonance coefficient, omega 0 is a fundamental frequency of a power grid, s is an input variable after complex variable Laplacian transformation, xi 0 is an active damping coefficient, omega i is a resonance bandwidth frequency, the frequency deviation is not more than +/-0.5 Hz according to the specification in GB/T15945 standard of allowable deviation of electric energy quality power system frequency, and is adapted to fluctuation of +/-1% of the fundamental frequency of the power grid, so omega i＝0.01*2πf0 rad/s is taken.
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