CN111064380A - Grid-connected inverter system - Google Patents
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Abstract
The invention relates to a grid-connected inverter system, which comprises a unidirectional inverter bridge, an LCL (lower control limit) type output filter, a software phase-locked loop, a combined harmonic suppression unit, an amplifier and a driving unit which are sequentially connected, wherein the LCL type output filter and the software phase-locked loop are respectively connected with a power grid, the driving unit is connected with the unidirectional inverter bridge, the combined harmonic suppression unit comprises a multi-resonance controller, a proportional-integral controller and a complete feedforward controller, one end of the multi-resonance controller is connected with the software phase-locked loop after the multi-resonance controller and the proportional-integral controller are connected in parallel, the other end of the multi-resonance controller is connected with the amplifier, one end of the complete feedforward controller is. Compared with the prior art, the invention has the advantages of improving the performance of the inverter, making up the influence of the existing semiconductor device on the performance of the inverter of an ideal device due to technical reasons, and the like.
Description
Technical Field
The invention relates to a grid-connected inverter output performance improvement control technology, in particular to a grid-connected inverter system.
Background
Based on the PWM working mode switching power amplifier, the power switching tube is driven by the driving circuit, the amplification is realized by controlling the switching of the power switching tube, the switching tube of the switching power amplifier works in a switching state, the efficiency can reach 100% theoretically, and the efficiency can also reach more than 80% in actual application. The switching power amplifier has high efficiency, and has great advantages in occasions with high requirements on volume, efficiency and power consumption. At present, a switching power amplifier, especially a third generation semiconductor device, has become a wide field of new energy conversion use, and especially has received more and more attention in the new energy field. The research focus of the flexible power amplifier is mainly focused on the modern control strategy of the switch mode power amplifying circuit and the improved topology thereof. However, the power amplifier based on the modern control strategy or the multi-level structure has certain limitations in implementation, mainly in the complexity and implementation cost of the scheme, and in specific application. The existing control methods of some switching power amplifiers improve the output distortion of the switching power amplifier to a certain extent, but all the existing control methods are based on increasing the loss of the switching power amplifier. This makes it necessary to adopt new techniques to modify and improve the non-linear characteristics of the power switches, thereby reducing their distortion and loss.
Although the nonlinearity of the power switching devices can be reduced by model correction, because the main circuit bridge arm topology of the grid-connected inverter bridge is formed by an upper power switching device and a lower power switching device, in order to prevent the direct connection phenomenon of the upper power tube and the lower power tube of the same bridge arm, a dead time must be set in a driving signal of the power switching devices. The dead zone effect may cause a large number of odd harmonics to be included in the output voltage of the inverter bridge, and these harmonics may in turn further increase the harmonic distortion degree of the grid-connected current. Therefore, harmonic distortion of grid-connected current is increased due to nonlinear factors of an inverter system, and even harmonic components of the grid-connected current exceed an allowable limit value in serious cases. In addition, the nonlinear factor of the inverter mainly has digital control delay, that is, when digital control is adopted, the microcontroller inevitably needs a period of time for A/D conversion and program code calculation, so that the control delay is generated. The digital control delay can be equivalent to a delay link serially connected in a forward channel of the system, which not only can cause the response speed of the system to be slow, but also can reduce the bandwidth and stability margin of the system, and can cause the system to be unstable in serious cases.
Therefore, due to the nonlinearity of the switching power amplifier, and due to the dead zone effect of the opening of the upper and lower bridge arms and the digital control delay, the system nonlinearity of the grid-connected inverter can cause low-order harmonic accumulation effect and distortion of grid-connected current waveform, and the output performance of the inverter is reduced.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provide a grid-connected inverter system with high grid-connected performance and good output performance.
The purpose of the invention can be realized by the following technical scheme:
a grid-connected inverter system comprises a unidirectional inverter bridge, an LCL type output filter, a software phase-locked loop, a combined harmonic suppression unit, an amplifier and a driving unit which are connected in sequence, wherein the LCL type output filter and the software phase-locked loop are respectively connected with a power grid, the driving unit is connected with the unidirectional inverter bridge,
the combined harmonic suppression unit comprises a multi-resonance controller, a proportional-integral controller and a complete feedforward controller, wherein one end of the multi-resonance controller is connected with a software phase-locked loop after being connected with the proportional-integral controller in parallel, the other end of the multi-resonance controller is connected with an amplifier, one end of the complete feedforward controller is connected with a power grid, and the other end of the complete feedforward controller is connected with a driving unit.
Further, the unidirectional inverter bridge comprises a direct current energy storage capacitor, four third-generation semiconductor power devices and four anti-parallel diodes.
Further, the coefficients of the full feedforward controller are:
wherein L isinvFor filtering electricity, CfIs a filter capacitor, Geq(s) is, GinvIs of TLPFIs a time constant.
Further, the coefficients of the multi-resonant controller are:
wherein, KmIs a resonance coefficient for multi-harmonic control.
Furthermore, the control of the working state of the unidirectional inverter bridge is realized through an impedance matching and inverter combination control strategy.
Further, the impedance matching specifically includes:
and (4) considering digital control delay and dead zone effect, constructing an impedance network model of the LCL type grid-connected inverter and realizing output impedance matching.
Further, the digital control time delay is used for a time delay link G in the output series of the capacitance current regulatordelay(s) the transfer function expression of the delay element is as follows:
wherein, TswIs a switching cycle.
Further, the dead zone effect is formed by sign (i) with an amplitude constant and a direction constant at the output voltage end of the unidirectional inverter bridgeinv) Determined error voltage ueDenotes ueThe expression of (a) is:
in the formula (I), the compound is shown in the specification,
Tswis a switching period, tdFor dead time, UdcFor the voltage across the DC storage capacitor, iinvIs an inverter current.
Further, the inverter combination control strategy specifically includes:
the method comprises the steps that a feedback current is obtained at a point of common connection PCC, the difference between a given current reference value and the feedback current respectively passes through a proportional-integral controller and a multi-resonance controller, the difference between two output results and the feedback current of a filter capacitor on an LCL type output filter is obtained, a voltage reference signal is obtained through the difference between the output result after an amplifier and the output of a complete feedforward controller, and a square wave driving pulse for driving a single-phase inverter bridge is obtained by a driving unit based on the difference between the reference voltage signal and the feedback voltage of a direct-current energy storage capacitor and a triangular wave reference signal.
Compared with the prior art, the invention has the following beneficial effects:
the invention adopts a multi-harmonic combined harmonic suppression strategy to reduce the digital control delay of the inverter and the dead zone effect of the grid-connected inverter and eliminate the deterioration of the output performance of the inverter caused by the dead zone of a power switch device, the delay of digital signal control and the like, thereby suppressing the influence of low-order harmonic on the grid-connected electric energy quality of the inverter side, reducing the harmonic content of LCL type grid-connected current and improving the grid-connected performance of the inverter
According to the invention, through the correction of a power device model and the control strategies of feedforward and multi-resonance suppression, the influence of the performance of the inverter of an ideal device (namely, the nonlinear existence of a power switching device) which cannot be realized by the existing semiconductor device due to technical reasons is compensated, and the influence of the dead zone of the power switching device, the delay of digital signal control and the like on the output performance of the grid-connected inverter is suppressed through a combined control strategy.
Drawings
FIG. 1 is a block diagram of the present invention;
fig. 2 is a mechanism of dead zone formation and nonlinear characteristic formation of the inverter;
FIG. 3 is a control block diagram of an LCL type grid-connected inverter system considering control delay and dead zone effect;
FIG. 4 is a control block diagram of an LCL type grid-connected inverter considering grid voltage feedforward;
FIG. 5 is a bode plot of output impedance with and without grid voltage feed forward;
FIG. 6 is a Bode plot of output impedance with and without the introduction of multiple resonance control;
FIG. 7 is a current gain Bode plot with and without the introduction of multiple resonance control;
FIG. 8 is an open loop Bode diagram of the system taking into account digital delay and multi-resonance control;
FIG. 9 is a Bode plot of output impedance for different harmonic rejection strategies;
FIG. 10 is an enlarged view of a portion of the amplitude frequency characteristic of FIG. 9;
fig. 11 is a current gain bode plot considering digital delay and multi-resonance control.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.
The invention provides a grid-connected inverter system, which comprises a unidirectional inverter bridge 1, an LCL (lower control limit) type output filter 2, a software phase-locked loop 3, a combined harmonic suppression unit 4, an amplifier 5 and a driving unit 6 which are sequentially connected, wherein the LCL type output filter 2 and the software phase-locked loop 3 are respectively connected with a power grid 7, the driving unit 6 is connected with the unidirectional inverter bridge 1, the unidirectional inverter bridge 1 is an inverter bridge with nonlinear correction, the combined harmonic suppression unit 4 comprises a multi-resonance controller, a proportional integral controller and a complete feed-forward controller, one end of the multi-resonance controller is connected with the software phase-locked loop 3, the other end of the multi-resonance controller is connected with the amplifier 5, the complete feed-forward controller and the proportional integral controller are connected in parallel, one end of the complete feed-forward controller is connected with the power grid. The grid-connected inverter system improves the performance of the inverter to a certain extent through the optimization of impedance matching and an inverter combination control strategy, and makes up for the influence that the performance of the inverter of an ideal device (namely, the nonlinear existence of a power switching device) cannot be realized due to the technical reason of the existing semiconductor device.
The unidirectional inverter bridge comprises a DC energy storage capacitor UdcFour third generation semiconductor power devices T1-T4 that reduce signal distortion and loss by nonlinear modification, and four anti-parallel diodes D1-D4. The LCL type output filter includes an output inductor Linv、LgAnd a filter capacitor Cf。
The innovation points of the grid-connected inverter system comprise: 1) introducing digital control delay and dead zone effect of an inverter formed by a power switch device, establishing an accurate model of the LCL type grid-connected inverter, and realizing output impedance matching of the LCL type grid-connected inverter under the feedforward control of the grid voltage; 2) the combined technology of feedforward and multi-resonance suppression is adopted for reducing the digital control delay of the inverter and the dead zone effect of the grid-connected inverter, so that the influence of low-order harmonic waves on the grid-connected electric energy quality of the inverter is suppressed, the harmonic wave content of LCL type grid-connected current is reduced, and the grid-connected performance of the inverter is improved. The method starts from a power device model, solves the distortion and loss of signal amplification of the power device, and then starts from a grid-connected inverter system formed by the power device, and adopts the technology and the algorithm for improving the output performance of the inverter, thereby being beneficial to improving the output performance of the grid-connected inverter.
The innovation point 1) realizes model correction of the grid-connected inverter semiconductor power device T1-T4, and is used for reducing distortion and loss of signals; after the innovation point 2) is utilized, the power grid obtains an input voltage signal, the frequency and the phase of the power grid are obtained through a software phase-locked loop (SOGI-PLL) technology, and a current reference value I is givengObtaining a current outer loop reference signal IgrefThe feedback current being taken from the point of common coupling PCC by igGiven a current reference value IgAnd a feedback current igPhase difference is obtained by passing through a proportional-integral controller and a multi-resonance controller, and the output result is compared with a filter capacitor C on an LCL type output filterfFeedback current icDifference by amplifying gain GicThe difference between the output result and the output of the grid voltage through the grid voltage complete feedforward control is used to obtain the voltage reference signal urThe reference voltage signal is then identical to the DC energy storage capacitor U of the inverter bridgedcThe feedback voltage difference and the triangular wave reference signal obtain square wave drive pulse for driving 4 third-generation semiconductor power devices T1-T4 of the single-phase inverter bridge to obtain inverter voltage uinvAnd an inverter current iinvInverse current iinvObtaining grid-connected current i through LCL type output filtergbGrid-connected current igbThrough the equivalent inductance L of the electric networkgridFlows into the grid Ug。
The combined output performance improvement technology for the feedforward and multi-resonance suppression of the grid-connected inverter is specifically described as follows:
1. equivalent impedance network modeling considering control delay and dead zone effect
The main circuit of the inverter shown in fig. 1 mainly comprises 4 third-generation semiconductor IGBT power devices T1-T4 and anti-parallel diodes D1-D4. Because the same bridge arm of the inverter is composed of two third-generation semiconductor IGBT power devices T1-T2 and T3-T4, in order to prevent the upper and lower IGBT power devices in the same bridge arm from being conducted simultaneously and prevent the upper and lower switching devices of the same bridge arm from being directly communicated, the direct-current energy storage capacitor U is enabled to bedcThe short circuit of (2) causes the damage of the upper and lower IGBT power devices. In order not to form the 'through' phenomenon, a dead time is required to be inserted between complementary driving signals of the IGBT power devices, namely, a dead time is set in the digital signal processor to prevent the damage of the upper IGBT power device and the lower IGBT power device, but the existence of the dead time can cause the nonlinearity of the inverter. The dead zone formation and nonlinear characteristic formation mechanism of the inverter are shown in fig. 2. In FIG. 2, with iinv>0 is an example, tdFor dead time, TswIn order to be the switching period of the switch,
and
are respectively T1、T4And T2、T3The ideal waveform of the drive signal of the driver,
and
are respectively T1、T4And T2、T3The actual drive signal waveform of (a) is,
the ideal output voltage waveform of the inverter is obtained,
the voltage waveform is actually output for the inverter. By averaging the loss of inverter output voltage waveform due to dead time during a switching cycle, an average error voltage of
In the formula:
as can be seen from equation (1), the average error voltage is related to the magnitude of the dead time, and the magnitude of the dead time is related to the nonlinear correction of the power switching device, and the better the linearity is, the smaller the dead time can be, and vice versa, and the longer the dead time is, the worse the inverter nonlinearity is, and the worse the output performance (such as harmonic characteristics) is. In addition, the inverters are controlled digitally at present, and the microcontroller inevitably needs a period of time for A/D conversion and program code calculation, thereby generating control delay. When the sampling frequency is equal to the PWM switching frequency, the PWM duty ratio updating moment has a time delay of one sampling period relative to the A/D sampling moment, and the digital control delay can be equivalent to a transfer function under a continuous domain:
the digital control delay can be equivalent to a delay link serially connected in a forward channel of the system, which not only can cause the response speed of the system to become slow, but also can reduce the bandwidth and stability margin of the system, and can cause the system to be unstable in serious cases. Therefore, the nonlinear factors of the inverter mainly include digital control delay, dead zone effect and the like, and therefore, the digital control delay and the dead zone effect are considered at the same time and are equivalent to an impedance network model of the inverter, and correction and optimization are carried out through corresponding control strategies, so that the influence of the nonlinearity of the inverter on the output performance of the inverter is necessarily reduced.
According to the inverter closed-loop control structure shown in fig. 1, and by combining the equations (1) and (2), a closed-loop control structure block diagram of the LCL type grid-connected system in consideration of the digital control delay and the dead zone effect can be obtained, as shown in fig. 3. As can be seen from FIG. 3, the digital control delay is equivalent to the fact that the output of the capacitor current regulator is serially connected into a delay element G in the system block diagramdelay(s), the dead zone effect is equivalent to that an amplitude constant and direction sign (i) are superposed on the output voltage end of the inverter bridgeinv) Determined error voltage ueU in FIG. 3cFeedback of capacitor voltage, Gig(s) is a grid-connected current controller, Zc(s) is capacitive reactance, 1/ZinvAnd(s) is the inverter side inductance inverse.
2. Combined suppression technology for reducing inverter nonlinear characteristics and improving inverter performance
The sampling error of the power grid voltage in an actual system exists, the filtering distortion error of the first-order low-pass filter also exists, so that the low-order harmonic of the power grid voltage cannot be completely eliminated by complete feedforward, and at the moment, the multi-resonance control can assist the complete feedforward to further restrain the specific low-order harmonic on the power grid side with larger influence. On the other hand, for low-order harmonic current caused by self nonlinear factors of the grid-connected inverter, the grid voltage feedforward control cannot be inhibited, and the multi-resonance control can effectively inhibit specific low-order harmonic on the inverter side. Therefore, in order to further reduce the low-order harmonic component of the network access current of the LCL type grid-connected inverter, the invention provides that the grid voltage complete feedforward and the multi-resonance control are combined, so that higher-quality electric energy is injected into a power grid, and the structure of the LCL type single-phase grid-connected inverter system adopting a combined harmonic suppression strategy is shown in figure 1, namely, the combination of the grid voltage complete feedforward control and the multi-resonance control is adopted.
2.1 inverter grid voltage feed-forward control
The capability of the grid-connected inverter to suppress the low-order harmonic wave of the grid voltage is essentially determined by the amplitude-frequency characteristic of the output impedance of a closed-loop system, so that the output impedance Z is expectedo_delayThe larger the amplitude of(s) in each frequency band, the better. The output impedance amplitude can be improved by increasing the proportionality coefficient of the outer ring grid-connected current controller, and the grid side harmonic suppression capability of the grid-connected inverter is further improved. However, too large a scaling factor may affect the stability of the system (especially the LCL grid-connected inverter) on the one hand, and may also result in too large a system bandwidth, thereby affecting the noise suppression capability.
The grid voltage feedforward control does not increase the output impedance amplitude of the inverter from the perspective of improving the parameters of the controller, but proportionally or completely feedforward the grid voltage to the output of the grid-connected current controller to counteract the harmonic disturbance of the grid voltage. The system control block diagram of the LCL type grid-connected inverter after introducing the grid voltage feedforward control is that the grid voltage feedforward control is added in figure 3, and the feedforward coefficient is Gff_LCL(s) as shown in FIG. 4.
Fig. 4 shows the output impedance transfer function Z of the grid-connected invertero_delay_ff(s) is represented by
As can be seen from equation (3), for the LCL type grid-connected inverter, if the influence of the grid harmonic on the grid-connected current is to be theoretically completely eliminated, i.e., the output impedance amplitude is infinite through the grid voltage feedforward control, the feedforward coefficient should be infinite
It can be seen that the grid voltage feed-forward control is essentially onAnd the elimination of network side harmonic waves is realized by increasing the amplitude of the output impedance to infinity. Due to Gff_LCL(s) is a second order differential transfer function, so high frequency noise interference is easily introduced in actual grid voltage feedforward control. In order to reduce the influence of the differential link, the high-frequency harmonic wave of the power grid voltage can be filtered by a first-order low-pass filter, and then the feedforward algorithm is executed. Because the use of the low-pass filter causes distortion of the grid voltage to a certain extent, the output impedance amplitude of the closed-loop system after being connected in series with the low-pass filter cannot be infinite. The complete feed-forward coefficient of the power grid voltage after the addition of the first-order low-pass filter is
Time constant TLPFDetermines the filtering performance of a first-order low-pass filter, TLPFToo small may result in undesirable high frequency components that cannot be filtered out, making implementation difficult in practical systems; and TLPFToo large can cause serious hysteresis of a complete feedforward control loop, influence amplitude improvement of output impedance and further influence harmonic suppression effect of grid-connected current. Considering that the public power grid is mainly concerned about voltage harmonics below 2kHz (within 40 times of power frequency harmonic frequency), T can be takenLPF40 mus (corresponding cut-off frequency of about 3980 Hz).
TABLE 1 System parameters of LCL-type single-phase grid-connected inverter
For the purpose of analysis and comparative study, the parameters of the main circuit and the control circuit system of the LCL type grid-connected inverter are given first, as shown in table 1. According to the parameters in the table, output impedance bode diagrams of the LCL type grid-connected inverter under the conditions of grid-free voltage feedforward, proportion feedforward and complete feedforward control can be drawn, and the output impedance bode diagrams are shown in fig. 5. In fig. 5, M denotes the amplitude,
representing the phase angle.
As can be seen from fig. 5, the proportional feedforward control can only raise the output impedance amplitude of the LCL type grid-connected inverter below about 700Hz, and the raising capability of the LCL type grid-connected inverter is weakened with the increase of the harmonic frequency, while the complete feedforward control can effectively raise the output impedance below about 2kHz, and the raising amplitude is significantly higher than that of the partial feedforward control, so that the harmonic voltage of the power grid can be better suppressed. Certainly, from practical application, because a first-order low-pass filter is connected in series in the complete feedforward link, the amplitude of the output impedance cannot be ideally increased to be large enough, and therefore the influence of the harmonic voltage of the power grid on the grid-connected current cannot be completely eliminated.
2.2 inverter Multi-resonance control
The actual power grid mainly contains low-order harmonic voltage, and harmonic components caused by nonlinearity of the grid-connected inverter are mainly low-order harmonics. Therefore, by connecting a plurality of resonance controllers tuned to the desired harmonic elimination frequency in parallel with the grid-connected current controller and setting the current of each resonance controller to zero, the control effect of suppressing the low harmonic of the grid-connected current can be realized by forcing each harmonic current to quickly track the zero value. On the basis of a proportional-integral controller, a transfer function of a grid-connected current controller after introducing a multi-resonance controller is as follows
According to the transfer function expression of the output impedance and the current gain, the grid-connected current controller not only influences the frequency characteristic of the output impedance, but also influences the frequency characteristic of the current gain. Therefore, by introducing the multi-resonance controller, not only the output impedance amplitude of the inverter at a specific frequency can be improved, but also the suppression performance of the grid-connected controller on harmonic current at the specific frequency can be improved, so that the low-order harmonics at the grid side and the inverter side can be simultaneously suppressed. Taking 3 rd, 5 th, 7 th and 9 th harmonics as examples, bode plots of the output impedance transfer function and the current gain transfer function of the LCL type grid-connected inverter are shown in fig. 6 and fig. 7, respectively, when the multi-resonance control is introduced and not introduced.
As can be seen from fig. 6, after the multi-resonance control is introduced, the gain of the amplitude of the output impedance around the frequencies of 150Hz, 250Hz, 350Hz, and 450Hz is very large, so that the interference of the harmonic voltage of the corresponding frequency on the grid side to the grid-connected current can be effectively suppressed. As can be seen from fig. 7, the amplitude of the current gain is almost 0dB around the frequencies of 150Hz, 250Hz, 350Hz and 450Hz, and the phase is almost 0 °, i.e. the resonant controller of each sub-frequency can realize fast tracking of the given signal of the sub-harmonic current (to suppress the current harmonic, the given value of each harmonic current is zero), so that the harmonic content of the grid-connected current can be reduced. Of course, multi-resonance control can only effectively suppress the network side or inverter side harmonics of a specific frequency and the vicinity thereof, and cannot suppress harmonic disturbance of other frequencies.
2.3 Combined suppression techniques
The sampling error of the power grid voltage in an actual system exists, the filtering distortion error of the first-order low-pass filter also exists, so that the low-order harmonic of the power grid voltage cannot be completely eliminated by complete feedforward, and at the moment, the multi-resonance control can assist the complete feedforward to further restrain the specific low-order harmonic on the power grid side with larger influence. On the other hand, for low-order harmonic current caused by self nonlinear factors of the grid-connected inverter, the grid voltage feedforward control cannot be inhibited, and the multi-resonance control can effectively inhibit specific low-order harmonic on the inverter side. Therefore, in order to further reduce the low-order harmonic component of the network access current of the LCL type grid-connected inverter, it is proposed to combine the complete feed-forward of the grid voltage with the multi-resonance control, so as to inject higher-quality electric energy into the grid, and the structure of the LCL type single-phase grid-connected inverter system adopting the combined harmonic suppression strategy is shown in fig. 1.
Because both the digital control delay and the multi-resonance control technology affect the stability and the dynamic and static performances of the system, the open-loop amplitude-frequency characteristics and the phase-frequency characteristics of the system need to be analyzed before the output impedance transfer function characteristics and the current gain transfer function characteristics are analyzed. Fig. 8 is a system open loop bode diagram of the LCL type grid-connected inverter under the condition of considering digital control delay and multi-resonance control. The system performance index can be obtained from fig. 8, which is shown in table 2. In Table 2, fcCut-off frequency, T, for open-loop bode diagramolFor gain at fundamental frequency, PMFor phase margin, GMIs the gain margin. As can be seen from fig. 8, the grid-connected inverter system can stably operate under four conditions. Further comparing the data in table 2, it can be seen that the multi-resonance control has very little influence on the dynamic and static performance of the system. Although the digital control delay is considered under the parameters given in table 2, the system still can be guaranteed to have good steady-state performance and dynamic response speed, but at the same time, the control bandwidth and the phase margin are obviously reduced, so the digital delay should be considered in the actual system design. Bode plots of the output impedance of the LCL grid-connected inverter for different harmonic suppression strategies are shown in fig. 9 and 10, and bode plots of the current gain are shown in fig. 11. As can be seen from fig. 9, the grid voltage feedforward control has an obvious effect of improving the output impedance amplitude near the low-order harmonic of the LCL grid-connected inverter. In addition, the high impedance characteristic of the multi-resonance control in the vicinity of the main specific harmonic frequency can effectively suppress the disturbance of the low-order harmonic. Therefore, compared with the single adoption of the power grid voltage feedforward control or the multi-resonance control, the provided combined harmonic suppression strategy can more effectively suppress the power grid harmonic voltage disturbance. As can also be seen from fig. 11, the multi-resonance control considering the digital control delay can still achieve the specific harmonic suppression effect of fast tracking on the harmonic currents around the frequencies of 150Hz, 250Hz, 350Hz, and 450Hz, thereby improving the grid-connected current quality.
TABLE 2 System Performance indicators considering digital delay and multiple resonance control
Therefore, by adopting a control strategy combining complete feed-forward of the grid voltage and multi-resonance control, the influence of nonlinear characteristics such as digital control delay and inverter dead zone on the performance of the inverter can be effectively reduced, and the injection of higher-quality electric energy into the grid by the inverter is improved.
The foregoing detailed description of the preferred embodiments of the invention has been presented. It should be understood that numerous modifications and variations could be devised by those skilled in the art in light of the present teachings without departing from the inventive concepts. Therefore, the technical solutions that can be obtained by a person skilled in the art through logic analysis, reasoning or limited experiments based on the prior art according to the concept of the present invention should be within the protection scope determined by the present invention.
Claims (9)
1. A grid-connected inverter system is characterized by comprising a unidirectional inverter bridge, an LCL type output filter, a software phase-locked loop, a combined harmonic suppression unit, an amplifier and a driving unit which are sequentially connected, wherein the LCL type output filter and the software phase-locked loop are respectively connected with a power grid, the driving unit is connected with the unidirectional inverter bridge,
the combined harmonic suppression unit comprises a multi-resonance controller, a proportional-integral controller and a complete feedforward controller, wherein one end of the multi-resonance controller is connected with a software phase-locked loop after being connected with the proportional-integral controller in parallel, the other end of the multi-resonance controller is connected with an amplifier, one end of the complete feedforward controller is connected with a power grid, and the other end of the complete feedforward controller is connected with a driving unit.
2. The grid-connected inverter system according to claim 1, wherein the unidirectional inverter bridge comprises a direct current energy storage capacitor, four third generation semiconductor power devices, and four anti-parallel diodes.
3. The grid-connected inverter system according to claim 1, wherein the coefficients of the full feedforward controller are:
wherein L isinvFor filtering electricity, CfIs a filter capacitor, Geq(s) is, GinvIs of TLPFIs a time constant.
4. The grid-connected inverter system according to claim 1, wherein the coefficients of the multi-resonant controller are:
wherein, KmIs a resonance coefficient for multi-harmonic control.
5. The grid-connected inverter system according to claim 2, wherein the control of the working state of the unidirectional inverter bridge is realized by an impedance matching and inverter combination control strategy.
6. The grid-connected inverter system according to claim 5, wherein the impedance matching specifically is:
and (4) considering digital control delay and dead zone effect, constructing an impedance network model of the LCL type grid-connected inverter and realizing output impedance matching.
7. The grid-connected inverter system according to claim 6, wherein the digital control delay is implemented as a delay element G in series with the output of the capacitor current regulatordelay(s) the transfer function expression of the delay element is as follows:
wherein, TswIs a switching cycle.
8. The grid-connected inverter system according to claim 6, wherein the dead zone effect is obtained by sign (i) with a constant amplitude and constant direction superimposed on the output voltage end of the unidirectional inverter bridgeinv) Determined error voltage ueDenotes ueThe expression of (a) is:
in the formula (I), the compound is shown in the specification,
Tswis a switching period, tdFor dead time, UdcFor the voltage across the DC storage capacitor, iinvIs an inverter current.
9. The grid-connected inverter system according to claim 5, wherein the inverter combination control strategy is specifically:
the method comprises the steps that a feedback current is obtained at a point of common connection PCC, the difference between a given current reference value and the feedback current respectively passes through a proportional-integral controller and a multi-resonance controller, the difference between two output results and the feedback current of a filter capacitor on an LCL type output filter is obtained, a voltage reference signal is obtained through the difference between the output result after an amplifier and the output of a complete feedforward controller, and a square wave driving pulse for driving a single-phase inverter bridge is obtained by a driving unit based on the difference between the reference voltage signal and the feedback voltage of a direct-current energy storage capacitor and a triangular wave reference signal.
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| CN113824129B (en) * | 2020-06-19 | 2024-04-05 | 华北电力大学（保定） | Power compensation control for improving bidirectional power stability of grid-connected converter system |
| CN111865057A (en) * | 2020-07-03 | 2020-10-30 | 南京航空航天大学 | Control method of parallel inverter unit for molding frequency-division output impedance |
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