CN113644683A - Controller switching method for improving current quality of grid-connected inverter - Google Patents
Controller switching method for improving current quality of grid-connected inverter 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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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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
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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
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
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Abstract
The invention discloses a controller switching method for improving current quality of a grid-connected inverter, which is characterized in that on the basis of adopting shunt resistor sampling as current detection, a switching angle is calculated according to an intersection point of a current nonlinear range and three-phase current, a VSG (voltage source generator) controller with larger control inertia is adopted under the condition that a system is greatly influenced by sampling nonlinearity, a PI (proportional integral) controller is adopted to control in a linear range in consideration of the dynamic response requirement of the system, and the traditional PI control and Virtual Synchronous Generator (VSG) control are combined to solve the problem of grid-connected current control quality reduction caused by nonlinear distortion of a feedback channel current zero crossing point due to shunt resistor sampling in the existing grid-connected inverter.
Description
Technical Field
The invention belongs to the technical field of grid-connected current waveform control, and relates to a controller switching method for improving the current quality of a grid-connected inverter.
Background
With the acceleration of the industrialization process, the energy problem firstly becomes the focus of national attention. In a novel energy structure, the power generation technology of clean energy becomes the mainstream trend of the development of the power industry, and the research of the distributed power generation technology has an important promoting effect on the development of the clean energy technology. The grid-connected inverter is used as an interface circuit of distributed power grid-connected power generation, and plays an important role in improving the quality of electric energy and grid-connected efficiency, so that the output current of a grid-connected inverter system needs to be controlled and optimized to meet the grid-connected requirement.
At present, the optimization of grid-connected current mainly has two aspects: firstly, optimizing a current control algorithm; in order to enable the total harmonic distortion rate of the grid-connected current to meet the grid-connected requirement and realize stable grid connection, current control is usually selected to ensure the power quality of the grid-connected current. Whether a current source or a voltage source converter, the current control of the converter is usually realized by adopting inductive current feedback control, because the current inner loop control gain determines the allowable bandwidth of a multi-loop control system. And secondly, the optimization design of current sampling. The current detection is used as an important link in a grid-connected inverter control system, and the improvement of the accuracy of current sampling is a solid foundation for stable operation of the system. The accurate current sampling can reduce the probability of overcurrent, short circuit and other problems in the operation process of the system, ensure that the controller can send out accurate control signals, quickly acquire related fault information and realize fault protection of the system. In the control process of the system, if the current sampling channel has deviation, the current sampling channel can cause certain error of a control signal of the system, so that the control of the grid-connected current is not satisfactory, and therefore, the key point in the whole system design of the grid-connected inverter is the accuracy of current sampling.
In order to reduce the cost and improve the cost performance of the system, the system usually optimizes the design of the sampling circuit to reduce the volume and the cost, and the sampling circuit occupies a non-negligible proportion in the whole system cost as the three-phase current sampling is an indispensable condition for system control. There are three common current sampling circuits: the Hall current sensor is adopted. The modularization of the Hall current sensor is more and more mature, but the problems of overlarge gain and volume caused by the mismatching of the two current sensors exist; and secondly, adopting a current transformer. The current transformer can directly measure a circuit with relatively high voltage, so that effective isolation is realized, but the problem of detection error of a direct current component exists; and thirdly, sampling by adopting a shunt resistor. The shunt resistor sampling detection has the advantages of low cost and simplicity, but the non-linearity problem can occur during sampling, and the sampling precision needs to be improved. The invention aims to reduce the nonlinear problem caused by shunt resistance sampling by adding an improvement measure of controller switching to a grid-connected converter system, thereby improving the grid-connected current waveform control effect and improving the grid-connected electric energy quality.
Disclosure of Invention
The invention aims to provide a controller switching method for improving the current quality of a grid-connected inverter, and solves the problem of grid-connected current control quality reduction caused by nonlinear distortion of a feedback channel current zero crossing point due to shunt resistance sampling in the grid-connected inverter in the prior art.
The technical scheme adopted by the invention is that the controller switching method for improving the current quality of the grid-connected inverter is implemented according to the following steps:
step 1: establishing a mathematical model of a grid-connected interface circuit and analyzing system characteristics according to a main circuit structure of the grid-connected inverter;
step 2: selecting PI parameters and VSG control parameters according to system characteristics, and analyzing the system stability under the control of the PI parameters and VSG control parameters to ensure that the system can be stably connected to the grid and has enough stability margin;
and step 3: according to the nonlinear range of the system, the switching angle of the three-phase current is calculated according to an angle switching formula, and the switching angle is adjusted according to the i under the rotating coordinate system of the systemdAnd iqExtracting the component as a current feedback quantity controlled by the VSG;
and 4, step 4: the PI control strategy and the VSG composite control strategy are programmed, automatic circulation and smooth switching of grid-connected current are achieved, and current distortion of a feedback channel when the current crosses zero is eliminated.
The invention is also characterized in that:
in step 1, the main circuit structure comprises a direct current bus voltage UdcAnd grid-connected inverter composed of grid voltage e and filter.
The filter comprises a DC bus capacitor CdcDC bus capacitor CdcThe positive and negative poles of the IGBT are respectively connected with three switching tubes of the IGBT, wherein the IGBT switching tubes are all turn-off power switching devices which are packaged with anti-parallel diodes or have the characteristics of the anti-parallel diodes.
where w is the actual angular frequency, w0Is the fundamental angular frequency, J is the moment of inertia, D is the damping coefficient, Dp、DqDroop coefficients, V, of P-f control and Q-U control, respectivelydVoltage as d-axis component, s is differential operator, K is inertia coefficient of reactive power loop, Pref、QrefIs the active and reactive power, U, given by the systemNIs the rated voltage of the system, and the output power P of the system according to an instantaneous theoretical formulaeAnd Q is represented by formula 5:
in dq coordinate system, V is set because of unit power factor controlqWhen the value is 0, equation (5) can be simplified to equation 6, and a control block diagram of the current-type VSG can be obtained based on this equation;
in step 3, in any phase current period, setting a value I according to the currentrefCurrent amplitude I corresponding to the switching pointxCalculating the system switching angle, wherein the calculation formula is as follows:
the invention has the beneficial effects that: the invention relates to a controller switching method for improving the current quality of a grid-connected inverter, which combines the traditional Proportional Integral (PI) control with the Virtual Synchronous Generator (VSG) control and solves the problem of grid-connected current control quality reduction caused by the nonlinear distortion of the zero crossing point of the feedback channel current due to the sampling of a shunt resistor in the existing grid-connected inverter.
Drawings
FIG. 1 is a flow chart of a controller switching method for improving current quality of a grid-connected inverter according to the present invention;
FIG. 2 is a diagram of the main circuit structure of the grid-connected inverter of the present invention;
FIG. 3 is a dq rotating coordinate system grid-connected inverter control block diagram;
FIG. 4 is a VSG control block diagram for a grid-tied inverter;
FIG. 5 is a schematic diagram of a three-phase AC current angle switching calculation;
fig. 6 is a block diagram of a grid-connected inverter composite control structure.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
The invention relates to a controller switching method for improving current quality of a grid-connected inverter, which is characterized in that on the basis of adopting shunt resistor sampling as current detection, a switching angle is calculated according to the intersection points of a current nonlinear range and three-phase currents (namely points A1-A4, B1-B4 and C1-C4 shown in figure 4), when a system is greatly influenced by sampling nonlinearity, a VSG (voltage source generator) controller with larger control inertia is adopted, and a PI (proportional-integral) controller is adopted to control in the linear range in consideration of the dynamic response requirement of the system, so that the VSG and PI control need to be switched, and as shown in figure 1, the method is implemented according to the following steps:
step 1: establishing a mathematical model of a grid-connected interface circuit and analyzing system characteristics according to a main circuit structure of the grid-connected inverter;
step 2: selecting PI parameters and VSG control parameters according to system characteristics, and analyzing the system stability under the control of the PI parameters and VSG control parameters to ensure that the system can be stably connected to the grid and has enough stability margin;
and step 3: according to the nonlinear range of the system, the switching angle of the three-phase current is calculated according to an angle switching formula, and the switching angle is adjusted according to the i under the rotating coordinate system of the systemdAnd iqExtracting the component as a current feedback quantity controlled by the VSG;
and 4, step 4: according to the steps, a PI control strategy and a VSG composite control strategy are programmed, automatic circulation and smooth switching of grid-connected current are achieved, the current distortion problem of a feedback channel when the current crosses zero is eliminated, a current sampling channel is optimized, grid-connected efficiency and power quality are improved, and feasibility of an algorithm is verified;
the main circuit part has a topology as shown in FIG. 1, which includes a DC bus voltage UdcThe grid voltage e and the filter form a grid-connected inverter. DC bus capacitor CdcThe positive pole and the negative pole are respectively connected with three paths of switch tubes of the IGBT, wherein the IGBT switch tubes are all turn-off power switch devices which are packaged with anti-parallel diodes or have the characteristics of the anti-parallel diodes. In the case of three-phase equilibrium, the system has only two degrees of freedom, so the control process can convert a three-phase system into a two-phase system. Converting the system voltage equation (1) in the three-phase static coordinate system into a two-phase static coordinate system to obtain the formula (2):
as can be seen from equation (2), the system still contains an ac component in the two-phase vertical coordinate system, and in order to implement the non-static control of the system, it is necessary to convert the ac component in the α β coordinate system into the dc component in the dq coordinate system, as shown in equation (3). Therefore, a control block diagram of the three-phase grid-connected inverter in the two-phase rotating coordinate system can be obtained, as shown in fig. 2.
Because the nonlinear problem of feedback channel current zero crossing distortion caused by shunt resistance sampling exists in the existing grid-connected inverter, the invention combines the traditional proportional-integral control (PI) with the Virtual Synchronous Generator (VSG) control. VSG control simulates the model and active frequency modulation and reactive voltage regulation characteristics of a synchronous generator by controlling a converter to obtain the operation mechanism and the external characteristics of system output similar to those of the traditional synchronous generator. According to the characteristics of the rotational inertia and the damping coefficient of the VSG in the control process, the VSG is controlled to have corresponding inertia characteristics. The invention utilizes VSG to control inertia in a nonlinear range to inhibit the influence of sampling nonlinearity of a shunt resistor on a system. The current type VSG control strategy is selected, wherein the active power P corresponding to the systemeAnd the expression of the reactive power Q is shown in formula (4).
Where w is the actual angular frequency, w0Is the fundamental angular frequency, J is the moment of inertia, and D is the damping coefficient. Dp、DqRespectively, P-f control and Q-U control droop coefficients. VdVoltage as d-axis component, s is differential operator, K is inertia coefficient of reactive power loop, Pref、QrefIs the active and reactive power, U, given by the systemNIs the nominal voltage of the system. According to the instantaneous theoretical formula, the output power of the systemPeAnd Q is represented by the formula (5).
In dq coordinate system, V is set because of unit power factor controlqIf the value is 0, equation (5) can be simplified to equation (6), and a control block diagram of the current-type VSG can be obtained based on this equation, as shown in fig. 3.
Due to the inherent characteristics of resistance sampling, current sampling has nonlinear characteristics, the nonlinearity of the current sampling causes a nonlinear factor in a feedback link, and the slope change of a system near a zero crossing point is large, so that a forward channel of the system is introduced, and the characteristics of the system are poor. When the PI and VSG composite control method is adopted, the VSG control is switched in the nonlinear interval of the system, the PI control is adopted outside the interval, the nonlinearity of the system can be eliminated by switching the switching angles one by one, and the slope change at the zero-crossing point of the system is greatly reduced. The composite control can well eliminate the nonlinearity of the system and improve the output waveform of the system.
Wherein, FIG. 4 shows a waveform diagram of three-phase inductor current, wherein A1-A4Represents the switching point of the A phase, B1-B4Represents the switching point of phase B, C1-C4Representing the switching point of phase C. In any phase current period, according to the current set value IrefCurrent amplitude I corresponding to the switching pointxCalculating the system switching angle, wherein the calculation formula is as follows (7):
the formula (7) can deduce that the controller switches 12 times in one current period, and the current is divided into a linear region and a non-linear region according to the switching angleLinear region, when the system enters the non-linear region from the linear region, extracting i at the switching pointdAnd iqValue as feedback for VSG current inner loop and using this i in the whole non-linear rangedAnd iqValue runs to allow the system to take advantage of VSG controlled inertia and i in the linear rangedAnd iqThe value to eliminate the non-linear factor of the system. That is, as shown in FIG. 4, the C-phase output current passes through the switching point C1When the system is in the linear area, the system is shown to enter the nonlinear area, and C is extracted1I of a pointdAnd iqAs C1To C2VSG current inner loop feedback quantity in the interval is adopted, so that the influence of sampling nonlinearity on the system is restrained.
FIG. 5 is a block diagram of a composite control structure of a grid-connected inverter, wherein the controlled object is
Because the feedback channel has a first-order inertia element, it can be equivalent to GL(s). The digitally controlled delay comprises a PWM transfer delay Gpwm(s) and sample computation delay Gd(s) two parts, generally considering the sample computation delay as one sample period; when the voltage v of the point of common couplingpccAfter a current control loop is introduced, the output current of the system can be influenced, and v can be increased in order to inhibit the negative effect of power grid disturbance on the output current of the inverterpccFeed-forward suppression of grid voltage disturbances, where kgIs a feed forward coefficient.
Claims (5)
1. A controller switching method for improving current quality of a grid-connected inverter is characterized by comprising the following steps:
step 1: establishing a mathematical model of a grid-connected interface circuit and analyzing system characteristics according to a main circuit structure of the grid-connected inverter;
step 2: selecting PI parameters and VSG control parameters according to system characteristics, and analyzing the system stability under the control of the PI parameters and VSG control parameters to ensure that the system can be stably connected to the grid and has enough stability margin;
and step 3: according to the non-linearity of the systemRange, calculating the switching angle of three-phase current according to the angle switching formula, and calculating the i under the rotating coordinate system of the system at the momentdAnd iqExtracting the component as a current feedback quantity controlled by the VSG;
and 4, step 4: the PI control strategy and the VSG composite control strategy are programmed, automatic circulation and smooth switching of grid-connected current are achieved, and current distortion of a feedback channel when the current crosses zero is eliminated.
2. The method for switching the controller to improve the current quality of the grid-connected inverter according to claim 1, wherein the main circuit structure in the step 1 comprises a direct current bus voltage UdcAnd grid-connected inverter composed of grid voltage e and filter.
3. The controller switching method for improving the current quality of the grid-connected inverter according to claim 2, wherein the filter comprises a direct current bus capacitor CdcDC bus capacitor CdcThe positive and negative poles of the IGBT are respectively connected with three switching tubes of the IGBT, wherein the IGBT switching tubes are all turn-off power switching devices which are packaged with anti-parallel diodes or have the characteristics of the anti-parallel diodes.
4. The method as claimed in claim 1, wherein the step 3 is to select a current-mode VSG control strategy, where the active power P corresponding to the system is selectedeAnd the reactive power Q is expressed as formula 4:
where w is the actual angular frequency, w0Is the fundamental angular frequency, J is the moment of inertia, D is the damping coefficient, Dp、DqDroop coefficients, V, of P-f control and Q-U control, respectivelydVoltage as d-axis component, s is differential operator, K is inertia coefficient of reactive power loop, Pref、QrefIs the active and reactive power, U, given by the systemNIs the rated voltage of the system, and the output power P of the system according to an instantaneous theoretical formulaeAnd Q is represented by formula 5:
in dq coordinate system, V is set because of unit power factor controlqWhen the value is 0, equation (5) can be simplified to equation 6, and a control block diagram of the current-type VSG can be obtained based on this equation;
5. the controller switching method for improving the current quality of the grid-connected inverter according to claim 1, wherein in the step 3, in any phase current period, the given value I of the current is setrefCurrent amplitude I corresponding to the switching pointxCalculating the system switching angle, wherein the calculation formula is as follows:
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