CN113489049A - Grid-connected inverter grid-side current control method - Google Patents
Grid-connected inverter grid-side current control method Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/01—Arrangements for reducing harmonics or ripples
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
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- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/40—Arrangements for reducing harmonics
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Abstract
The invention discloses a grid-connected inverter network side current control method, which is characterized in that according to an equivalent topological graph of an LCL type inverter, a filter inductor L1Branch circuit and filter capacitor CfThe branch being connected in series to a controlled source (alphav)pcc‑igZv) Wherein igZvSimulated damping ZvCan suppress the resonance peak of the inverter and the controlled voltage source alpha vpccThe influence of the harmonic voltage of the power grid on the grid-connected current can be resisted. Is connected in series with the inductor L1The controlled source of the branch can be integrated into the inverter control circuit and connected in series with the capacitor CfThe controlled source on the branch needs to be realized by adding an active compensation device. The output impedance transfer function of the inverter can be deduced according to the overall control block diagram of the inverter, and parameter design is carried out based on a passive theory. Thereby widening the passive range of the inverterAnd the output impedance strength of the inverter is enhanced so as to improve the stability of the inverter system and the capability of inhibiting the power grid disturbance.
Description
Technical Field
The invention belongs to the technical field of photovoltaic grid-connected inverter control, and relates to a grid-side current control method of a grid-connected inverter.
Background
In recent years, with the shortage of fossil energy and the increasingly serious problem of environmental pollution, renewable energy technology is greatly developed and popularized, and an inverter is a key component of grid-connected power generation of new energy, so that the function of the inverter is very important. In order to attenuate high-frequency switching harmonics generated by the on-off of a power device, the output of the grid-connected inverter is connected with a power grid through an LC or LCL filter, but the resonance peak of the LCL may cause grid-connected current oscillation and even cause the grid-connected inverter to be unstable. Especially under weak grid conditions, coupling resonance between the inverter and the grid may occur due to interaction of the inverter output impedance and the grid impedance.
In addition, due to penetration of harmonic voltage of a higher-level power grid and access of various nonlinear loads of the current-level power grid, abundant background harmonic waves are contained in the voltage of a public access Point (PCC) under the condition of a weak power grid. The background harmonic may cause distortion of the grid-connected inverter's grid-connected current, increasing the grid-connected current Total Harmonic Distortion (THD).
Therefore, certain measures need to be taken to suppress the interaction resonance of the LCL filter and the grid and the influence of grid voltage harmonics on the grid-connected current. The problem of resonance suppression can be solved by introducing passive damping or active damping, the passive damping is simple to realize and high in reliability, and has a good suppression effect on the resonance, but the loss of a system can be increased. The traditional active damping scheme is sensitive to filter parameters and power grid impedance changes, and usually, an additional sensor is needed, so that the system cost is increased, and the system operation reliability is reduced. The voltage harmonic suppression problem can be solved by a repetitive controller, a harmonic compensator or a power grid voltage feedforward technology. The power grid voltage feedforward control algorithm is simple, the harmonic suppression range is wide, meanwhile, the impact current in the starting process can be prevented, the steady-state tracking error is reduced, the dynamic performance of the system is improved, and the method is widely applied. However, under weak grid conditions, due to interaction between the inverter and grid impedance, grid voltage feed-forward introduces additional positive feedback of grid-side current, which negatively affects system stability. In order to solve the problems, the invention provides a network side current control method based on a passive theory aiming at an LCL type grid-connected inverter under the condition of a weak power grid.
Disclosure of Invention
In order to solve the problems, a network side current control method of the LCL type grid-connected inverter based on the passive theory under the weak power network is provided, and a low-cost active compensation device is connected in series with a capacitor branch of an LCL filter, so that the inverter output impedance is fully passive and has high impedance strength, and the stability of the inverter under the weak power network and the suppression capability of power network background harmonic waves are improved. The technical scheme of the invention is a grid-connected inverter grid-side current control method, which comprises the following steps:
s1, merging into controlled sources: according to the equivalent topological graph of the LCL inverter, controlled sources alphav are respectively connected in series in a filter inductance branch and a filter capacitance branchpcc-igZvWhere α is a voltage feedforward coefficient, igFor grid-connected current, vpccFor grid-connected voltage, igZvSimulated damping ZvSuppressing the resonance peak of inverter itself, and controlled voltage source alpha vpccThe influence of the harmonic voltage of the power grid on the grid-connected current is resisted; integrating a controlled source connected in series with a filter inductance branch circuit into a control loop of an inverter, and performing feedback control through the voltage of a power grid and the current of the power grid side; a controlled source connected in series with the filter capacitor branch circuit is externally provided with an active compensation device;
s2, adding an active compensation device and control: the compensator comprises two low-voltage MOSFET switch tubes, a direct-current side capacitor, a voltage stabilizing tube and a resistor, wherein the two low-voltage MOSFET switch tubes are connected with the direct-current side capacitor, the direct-current side capacitor is connected with the voltage stabilizing tube and the resistor which are connected in series in parallel, and the branch circuit formed by connecting the voltage stabilizing tube and the resistor in series limits the amplitude of bus voltage in the starting process of the circuit.
Preferably, the additional active compensation device and the control comprise the following steps:
s21, the harmonic voltage feedforward of the power grid is used for enhancing the output impedance of the harmonic frequency of the inverter and converting the voltage v of the grid-connected pointpccThe voltage filtered by the trap with the center frequency of 50Hz is provided for the PWM modulator;
s22, grid-connected current igThe negative feedback is used as active damping to restrain the resonance of the filter, and the current error signal of the main inverter is used as a control quantity;
s23, feedforward of the compensator bus voltage is used to ensure proper operation of the compensation circuit.
Preferably, the method further comprises the step of controlling the main inverter: the main inverter adopts a quasi-PR controller, and the reference current is obtained by multiplying the cosine of the phase of the grid voltage output by the SOGI-PLL and the current amplitude.
Preferably, the method further comprises enabling the inverter to output an impedance transfer function Zo(s) ensuring inverter output impedance passivity, i.e. output impedance phase satisfaction
Preferably, the method further comprises defining the phase margin as PM according to the constraint condition
And designing control parameters.
The invention has the following beneficial effects: the invention provides a passive theory-based LCL type grid-connected inverter network side current control method under a weak power network, which takes the concept of increasing the degree of freedom of closed-loop control and takes a low-cost active compensation device as a means to widen the passive area of the output impedance of an inverter, so that an inverter system can still have stronger robustness when the impedance of a power network is changed in a large range under the condition of the weak power network. Meanwhile, the output impedance amplitude of the inverter is increased, the influence of background harmonic waves of a power grid can be effectively inhibited, and the quality of the network access current is improved. In addition, the active compensation device adopted by the invention does not need a passive element, has small volume, requires a semiconductor switch device with low voltage resistance and has low circuit cost.
Drawings
Fig. 1 is an equivalent topology diagram of an LCL type filter inverter in an embodiment of the present invention;
FIG. 2 is a schematic diagram illustrating the structure and control of an active compensation device according to an embodiment of the present invention;
FIG. 3 is a block diagram of the overall control of the grid-connected inverter in the embodiment of the invention;
FIG. 4 is a graph of inverter output impedance Bode for grid side current control in the prior art;
FIG. 5 is a graph of the output impedance Bode of the grid side current controlled inverter in an embodiment of the present invention;
FIG. 6 shows a grid-connected current and PCC voltage waveforms controlled by a grid-side current under a condition of a grid impedance of 10m in the prior art;
fig. 7 shows grid-connected current and PCC voltage waveforms controlled by a grid-side current under a condition of a grid impedance of 10mh according to an embodiment of the present invention;
fig. 8 shows grid-connected current and PCC voltage waveforms controlled by grid-side current under the condition that the grid impedance is 15mh and the PCC point is added with a nonlinear load according to the embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
On the contrary, the invention is intended to cover alternatives, modifications, equivalents and alternatives which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, certain specific details are set forth in order to provide a better understanding of the present invention. It will be apparent to one skilled in the art that the present invention may be practiced without these specific details.
Fig. 1 is an equivalent topology diagram of an LCL type filter inverter. Wherein i1Representing the inverter-side current, icRepresenting the filter capacitance current, igRepresenting grid-connected current, v1Representing the output voltage, v, of the inverter legspccRepresents a common terminal (PCC) grid voltage, i.e., a grid-tie point voltage;
fig. 2 is a schematic diagram of the structure and control of the active compensation device. Wherein S1、S2Is a low-voltage MOSFET switching tube, CdcRepresenting the capacitance on the DC side, vdcRepresenting the DC-side capacitor voltage, vpnRepresenting the voltage injected into the filter capacitor branch; s1And S of the drain terminal2Source end connection, S1Source terminal and DC measuring capacitor CdcPositive electrode connection of S2Drain terminal and DC measuring capacitor CdcIs connected to the negative electrode of (1).
Fig. 3 is a block diagram of the overall control of the inverter system. Wherein ImRepresenting the reference current amplitude, alpha representing the voltage feedforward coefficient, Gpll(s) denotes the loop gain of the phase locked loop, GPR(s) denotes a quasi-PR controller, Gd(s) denotes the delay introduced by the digital controller, GBPF(s) denotes a band-pass filter, GBSF(s) denotes a band elimination filter, Zg(s) represents the grid impedance;
the grid-connected inverter grid-side current control method provided by the embodiment of the invention comprises the following steps of:
s1, merging into controlled sources: according to the equivalent topological graph of the LCL inverter, controlled sources alphav are respectively connected in series in a filter inductance branch and a filter capacitance branchpcc-igZvWhere α is a voltage feedforward coefficient, igFor grid-connected current, vpccFor grid-connected voltage, igZvSimulated damping ZvTo suppress the resonance peak of the inverter itself, and to control the voltage source alphavpccThe influence of the harmonic voltage of the power grid on the grid-connected current is resisted; integrating a controlled source connected in series with a filter inductance branch circuit into a control loop of an inverter, and performing feedback control through the voltage of a power grid and the current of the power grid side; a controlled source connected in series with the filter capacitor branch circuit is externally provided with an active compensation device;
s2, adding an active compensation device and control: the compensator comprises two low-voltage MOSFET switch tubes, a direct-current side capacitor, a voltage stabilizing tube and a resistor, wherein the two low-voltage MOSFET switch tubes are connected with the direct-current side capacitor, the direct-current side capacitor is connected with the voltage stabilizing tube and the resistor which are connected in series in parallel, and the branch circuit formed by connecting the voltage stabilizing tube and the resistor in series limits the amplitude of bus voltage in the starting process of the circuit.
The additional active compensation device and the control method comprise the following steps:
s21, the harmonic voltage feedforward of the power grid is used for enhancing the output impedance of the harmonic frequency of the inverter and converting the voltage v of the grid-connected pointpccThe voltage filtered by the trap with the center frequency of 50Hz is provided for the PWM modulator;
s22, grid-connected current igAs a resonant oscillation of the active damping pair filterLine suppression, using the current error signal of the main inverter as a control quantity;
s23, feedforward of the compensator bus voltage is used to ensure proper operation of the compensation circuit.
Also includes main inverter control: the main inverter adopts a quasi-PR controller, and the reference current is obtained by multiplying the cosine of the phase of the grid voltage output by the SOGI-PLL and the current amplitude.
Further comprising making the inverter output impedance transfer function Zo(s) ensuring inverter output impedance passivity, i.e. output impedance phase satisfaction
Further comprising defining the phase margin as PM according to the constraint condition
And designing control parameters.
In a specific embodiment, in order to suppress the resonance of the LCL filter and to improve the ability of the inverter to withstand grid disturbances, according to fig. 1, at the filter inductance L1Branch circuit and filter capacitor CfThe branch being connected in series to a controlled source (alphav)pcc-igZv) Wherein igZvSimulated damping ZvCan suppress the resonance peak of the inverter and the controlled voltage source alpha vpccThe influence of the harmonic voltage of the power grid on the grid-connected current can be resisted. Is connected in series at L1Controlled source of tributary (α v)pcc-igZv) The control loop of the inverter can be integrated, and the control loop is realized through grid voltage and grid side current feedback control. And is connected in series to the capacitor CfControlled source on branch (α v)pcc-igZv) This is achieved by the addition of active compensation means.
In a specific application example, the structure and control of the active compensation device are as follows: the schematic diagram of the active compensation device is shown in FIG. 2, the left half part is the circuit schematic diagram of the active compensation device, and compensationThe device is connected in series with a filter capacitor CfAnd (4) branching. Wherein S is1、S2Representing two low-voltage MOSFET switching tubes, CdcThe direct current side capacitor, a branch circuit formed by connecting a voltage stabilizing tube and a resistor in series is used for limiting the amplitude of the bus voltage in the starting process of the circuit. Because the compensation device only needs to process harmonic impedance, the switch tube S1、S2Low voltage MOSFETs may be selected. Meanwhile, because the circuit is connected in series with the capacitor branch circuit, the capacity needing to be processed is very small, and the influence on the efficiency of the inverter can be almost ignored.
The right half part is an active compensation device control schematic diagram, and specifically comprises three control links: 1) grid harmonic voltage feed forward is used to enhance the output impedance at the inverter harmonic frequency. Concretely, the voltage v of the grid-connected point is measuredpccThe voltage filtered by the trap with the center frequency of 50Hz is provided for the PWM modulator; 2) grid-connected current igAs active damping, suppresses the resonance of the filter. The method comprises the following steps that a current error signal of a main inverter is used as a control quantity; 3) feed-forward of the compensator bus voltage is used to ensure proper operation of the compensation circuit.
In a specific application example, the main inverter is controlled as follows: because the current reference signal is a sine quantity, in order to reduce steady-state error, the main inverter adopts a quasi-PR controller, and the reference current is obtained by taking cosine and current amplitude I from the phase position of the grid voltage output by the SOGI-PLLmAnd multiplying the two to obtain the product.
In a specific application example, the control parameters are designed as follows: in conjunction with the above-described available system overall control block diagram, as shown in fig. 3, further, an inverter output impedance transfer function Z that employs the proposed control method can be derivedo(s)。
Further, the inverter output impedance needs to be passivity-guaranteed, i.e. the output impedance phase is satisfied
The system can remain stable regardless of changes in the grid impedance. Is composed ofEnsuring the output impedance of the inverter to be fully passivity, reserving a certain phase margin, and if the phase margin is defined as PM, determining the phase margin according to constraint conditions
The control parameters can be reasonably designed.
And (3) substituting the known system parameters into the output impedance transfer function of the inverter, for example, taking the phase margin PM as 90 degrees, and carrying out parameter design according to the principle to obtain the proportionality coefficient KpThe Bode plot is plotted for 50 with the feedforward coefficient α being 0.5, as shown in fig. 5. Fig. 4 is a graph of the output impedance Bode of a conventional grid-side current-controlled inverter. Compared with the traditional network side current control method, the method provided by the invention ensures the sufficient passivity of the inverter output impedance in the ultra-wide frequency range, greatly improves the amplitude of the inverter output impedance, realizes the stability of the inverter under different power grid conditions, and improves the capability of the inverter to resist power grid disturbance. Fig. 6 and 7 are graphs of waveforms of grid-connected current and PCC voltage using conventional grid-side current control and proposed grid-side current control, respectively, in case of weak grid. It can be seen from the figure that the grid-connected current and the PCC voltage of the inverter controlled by the traditional grid-side current under the weak grid are severely oscillated, the THD of the grid-connected current is as high as 15.52%, and the upper limit of the grid-connected current is far beyond the 5% upper limit specified by GB/T37408 and 2019. The oscillation is caused by that the phase difference at the intersection frequency of the grid impedance and the inverter output impedance approaches 180 degrees, and the passivity is not enough. Under the same weak grid condition, the grid-connected current sine degree of the grid-side current control method is good, and the THD is only 0.93%. Further, as shown in fig. 8, after the grid impedance is increased to 15mh and a 150W rectification load is added to the PCC point, the grid voltage distortion is increased, but the sine degree of the grid-connected current is still high, and the THD value is only 2.43%. The stability of the inverter under the weak grid and the suppression capability of the inverter on the grid disturbance can be improved by the grid side current control method.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (5)
1. A grid-connected inverter grid-side current control method is characterized by comprising the following steps:
s1, merging into controlled sources: according to the equivalent topological graph of the LCL inverter, controlled sources alphav are respectively connected in series in a filter inductance branch and a filter capacitance branchpcc-igZvWhere α is a voltage feedforward coefficient, igFor grid-connected current, vpccFor grid-connected voltage, igZvSimulated damping ZvSuppressing the resonance peak of inverter itself, and controlled voltage source alpha vpccThe influence of the harmonic voltage of the power grid on the grid-connected current is resisted; integrating a controlled source connected in series with a filter inductance branch circuit into a control loop of an inverter, and performing feedback control through the voltage of a power grid and the current of the power grid side; a controlled source connected in series with the filter capacitor branch circuit is externally provided with an active compensation device;
s2, adding an active compensation device and control: the compensator comprises two low-voltage MOSFET switch tubes, a direct-current side capacitor, a voltage stabilizing tube and a resistor, wherein the two low-voltage MOSFET switch tubes are connected with the direct-current side capacitor, the direct-current side capacitor is connected with the voltage stabilizing tube and the resistor which are connected in series in parallel, and the branch circuit formed by connecting the voltage stabilizing tube and the resistor in series limits the amplitude of bus voltage in the starting process of the circuit.
2. The method of claim 1, wherein the additional active compensation device and control comprises the steps of:
s21, the harmonic voltage feedforward of the power grid is used for enhancing the output impedance of the harmonic frequency of the inverter and converting the voltage v of the grid-connected pointpccThe voltage filtered by the trap with the center frequency of 50Hz is provided for the PWM modulator;
s22, grid-connected current igThe negative feedback is used as active damping to restrain the resonance of the filter, and the current error signal of the main inverter is used as a control quantity;
s23, feedforward of the compensator bus voltage is used to ensure proper operation of the compensation circuit.
3. The method of claim 1, further comprising a master inverter control, the master inverter employing a quasi-PR controller, the reference current being obtained by multiplying a grid voltage phase cosine and a current amplitude output by the SOGI-PLL.
4. The method of claim 1, further comprising having an inverter output impedance transfer function Zo(s) ensuring inverter output impedance passivity, i.e. output impedance phase satisfaction
5. The method of claim 4, further comprising defining the phase margin as PM according to a constraint
And designing control parameters.
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Cited By (3)
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| CN113890033A (en) * | 2021-10-26 | 2022-01-04 | 深圳市锐风电子科技有限公司 | Active damping circuit of active power filter and control method thereof |
| CN115632399A (en) * | 2022-10-17 | 2023-01-20 | 东南大学 | Band-pass filter and advanced correction feedback type active system and damping method |
| CN115954833A (en) * | 2023-03-13 | 2023-04-11 | 石家庄科林电气股份有限公司 | Photovoltaic inverter grid-connected surge impact protection method and device and terminal equipment |
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