CN113644683B - 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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- CN113644683B CN113644683B CN202110819903.2A CN202110819903A CN113644683B CN 113644683 B CN113644683 B CN 113644683B CN 202110819903 A CN202110819903 A CN 202110819903A CN 113644683 B CN113644683 B CN 113644683B
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Classifications
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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
-
- 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
-
- 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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- Engineering & Computer Science (AREA)
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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 a switching angle is calculated according to an intersection point of a current nonlinear range and three-phase current on the basis of taking shunt resistance sampling as current detection, a VSG controller with larger control inertia is adopted under the condition that the system is greatly influenced by sampling nonlinearity, a PI controller is adopted to control in consideration of dynamic response requirements of the system in the linear range, and traditional Proportional Integral (PI) control is combined with Virtual Synchronous Generator (VSG) control, so that the problem of grid-connected current control quality reduction caused by feedback channel current zero-crossing nonlinear distortion caused by shunt resistance sampling in the existing grid-connected inverter is solved.
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
Along with the acceleration of industrialization progress, the energy problem is the focus of attention of the national first. In the novel energy structure, the power generation technology of clean energy will become the mainstream trend of the development of the power industry, and the research of the distributed power generation technology has an important pushing effect on the development of the clean energy technology. The grid-connected inverter is used as an interface circuit of the distributed power supply grid-connected power generation, plays an important role in improving the power quality and the grid-connected efficiency, and therefore the output current of the 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 generally selected to ensure the electric energy quality of the grid-connected current. Current control of the converter, whether current source or voltage source converters, is typically achieved using inductive current feedback control, as the current inner loop control gain determines the allowable bandwidth of the multiple loop control system. And secondly, the optimal design of current sampling is realized. 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 the stable operation of the system. Accurate current sampling can reduce the probability of problems such as overcurrent and short circuit in the operation process of the system, ensure that the controller can send 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, certain error can occur in the control signal of the system, so that the control of the grid-connected current is not satisfactory, and 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 carries out optimal design on a sampling circuit to reduce the volume and the cost, and three-phase current sampling is taken as an indispensable condition for system control, and the proportion of the sampling circuit in the cost of the whole system is not neglected. There are three common current sampling circuits: (1) a hall current sensor is employed. The modularization of the Hall current sensor is more and more mature, but the problems of gain and overlarge volume caused by mismatching of the two current sensors exist; (2) a current transformer is used. The current transformer can directly measure a circuit with relatively high voltage to realize effective isolation, but has the problem of direct current component detection error; (3) and sampling by adopting a shunt resistor. The shunt resistor sampling detection has the advantages of low cost and simplicity, but the nonlinear problem can occur during sampling, and the sampling precision is required to be improved. The invention aims to reduce the nonlinear problem caused by shunt resistance sampling by adding an improvement measure for controller switching into a grid-connected converter system, thereby improving the grid-connected current waveform control effect and the grid-connected power quality.
Disclosure of Invention
The invention aims to provide a controller switching method for improving the current quality of a grid-connected inverter, which solves the problem of reduced control quality of grid-connected current 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 according to a grid-connected inverter main circuit structure and analyzing system characteristics;
step 2: PI parameters and VSG control parameters are selected according to system characteristics, and the stability of the system under control is analyzed, so that the system can be stably connected with the grid and has enough stability margin;
step 3: according to the nonlinear range of the system, calculating the switching angle of the three-phase current according to an angle switching formula, and calculating the i under the rotating coordinate system of the system at the moment d And i q Extracting components to serve as current feedback quantity of VSG control;
step 4: programming PI control and VSG composite control strategies to realize automatic circulation and smooth switching of grid-connected current and eliminate current distortion of a feedback channel when current passes through zero.
The invention is also characterized in that:
the main circuit structure in step 1 comprises a DC bus voltage U dc Grid-connected inverter composed of grid voltage e and filter.
The filter comprises a DC bus capacitor C dc DC bus capacitor C dc Three-way switching tubes of IGBT are respectively connected with the anode and the cathode of the IGBT, wherein the IGBT switching tubes are packaged anti-parallel diodes or are provided withAn anti-parallel diode characteristic turn-off power switching device.
Step 3, selecting a current-type VSG control strategy, wherein the active power P corresponding to the system e And the expression of reactive power Q is shown in formula 4:
wherein w is the actual angular frequency, w 0 Is fundamental wave angular frequency, J is rotational inertia, D is damping coefficient, D p 、D q Droop coefficients, V, of P-f control, Q-U control, respectively d For the voltage of the d-axis component, s is a differential operator, K is the inertia coefficient of the reactive power loop, P ref 、Q ref Is the active power and reactive power given by the system, U N Is the rated voltage of the system, and the output power P of the system is calculated according to the instantaneous theoretical formula e And Q has the formula shown in formula 5:
in dq coordinate system, since unit power factor control is adopted, let V be q =0, the equation (5) can be simplified to equation 6, and a control block diagram of the current-type VSG can be obtained based on this;
in step 3, in any phase current period, according to the current given value I ref Current amplitude I corresponding to the switching point x Calculating the system switching angle, wherein the calculation formula is shown as formula 7:
the beneficial effects of the invention are as follows: the invention discloses a controller switching method for improving current quality of a grid-connected inverter, which combines traditional Proportional Integral (PI) control with Virtual Synchronous Generator (VSG) control, and solves the problem of reduced control quality of grid-connected current caused by nonlinear distortion of a feedback channel current zero-crossing point due to shunt resistance sampling 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 schematic diagram of a grid-tie inverter main circuit of the present invention;
FIG. 3 is a dq rotational coordinate system grid-tie inverter control block diagram;
FIG. 4 is a VSG control block diagram of a grid-tie inverter;
FIG. 5 is a schematic diagram of a three-phase AC current angle switching calculation;
fig. 6 is a block diagram of a composite control structure of a grid-connected inverter.
Detailed Description
The invention will be described in detail below with reference to the drawings and the detailed description.
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 point (namely points A1-A4, B1-B4 and C1-C4 shown in fig. 4) of a current nonlinear range and three-phase current, a VSG controller with larger control inertia is adopted under the condition that the system is greatly influenced by sampling nonlinearity, and a PI controller is adopted to control in consideration of the dynamic response requirement of the system in a linear range, so that the VSG and PI control are required to be switched, and the method is implemented according to the following steps as shown in fig. 1:
step 1: establishing a mathematical model of a grid-connected interface circuit according to a grid-connected inverter main circuit structure and analyzing system characteristics;
step 2: PI parameters and VSG control parameters are selected according to system characteristics, and the stability of the system under control is analyzed, so that the system can be stably connected with the grid and has enough stability margin;
step 3: according to the nonlinear range of the systemThe angle switching formula calculates the switching angle of the three-phase current, and the switching angle is equal to i in the rotating coordinate system of the system d And i q Extracting components to serve as current feedback quantity of VSG control;
step 4: according to the steps, programming is carried out on PI control and VSG composite control strategies, automatic circulation and smooth switching of grid-connected current are achieved, the problem of current distortion of a feedback channel when current passes through a zero point is solved, a current sampling channel is optimized, grid-connected efficiency and electric energy quality are further improved, and feasibility of an algorithm is verified;
the main circuit part is topological as shown in fig. 1, and comprises a direct current bus voltage U dc The grid voltage e and the filter constitute a grid-connected inverter. DC bus capacitor C dc The anode and the cathode are respectively connected with three-way switching tubes of the IGBT, wherein the IGBT switching tubes are packaged anti-parallel diodes or turn-off power switching devices with anti-parallel diode characteristics. In the case of a three-phase equilibrium, the system has only two degrees of freedom, so that the control process can convert a three-phase system into a two-phase system. The system voltage equation (1) under the three-phase static coordinate system is converted into the two-phase static coordinate system to obtain the formula (2):
as can be seen from the equation (2), the system still contains an ac component in the two-phase vertical coordinate system, and in order to achieve no-static-difference control of the system, it is necessary to convert the ac quantity in the αβ coordinate system into the dc quantity in the dq coordinate system, as shown in the equation (3). Thus, a control block diagram of the three-phase grid-connected inverter under a two-phase rotation coordinate system can be obtained, as shown in fig. 2.
Because of the nonlinear problem of feedback channel current zero crossing distortion caused by shunt resistor sampling in the existing grid-connected inverter, the invention combines the traditional proportional integral control (PI) with the Virtual Synchronous Generator (VSG) control. The VSG control is to simulate the model and the active frequency modulation and reactive voltage regulation characteristics of the synchronous generator by controlling the converter so as to obtain the operation mechanism and the system output external characteristics similar to the traditional synchronous generator. According to the characteristics of moment of inertia and damping coefficient of the VSG in the control process, the control has corresponding inertia characteristics. The invention utilizes VSG to control inertia to restrain influence of non-linearity of shunt resistance sampling on the system in the non-linearity range. A current-type VSG control strategy is selected, wherein the active power P corresponding to the system e And the reactive power Q is expressed as formula (4).
Wherein w is the actual angular frequency, w 0 The fundamental wave angular frequency is represented by J, the moment of inertia and D, the damping coefficient. D (D) p 、D q The droop coefficients of the P-f control and the Q-U control are respectively shown. V (V) d For the voltage of the d-axis component, s is a differential operator, K is the inertia coefficient of the reactive power loop, P ref 、Q ref Is the active power and reactive power given by the system, U N Is the rated voltage of the system. According to the instantaneous theoretical formula, the output power P of the system e And Q is represented by formula (5).
In dq coordinate system, since unit power factor control is adopted, let V be q When=0, equation (5) can be simplified to equation (6), and a control block diagram of the current-mode VSG obtained based on this equation is shown in fig. 3.
Due to the inherent characteristics of the resistance sampling, nonlinear characteristics can exist in the current sampling, nonlinear factors can exist in the feedback link due to the nonlinearity of the current sampling, and the slope change of the system near the zero crossing point is large, so that a forward channel of the system is introduced, and the characteristics of the system are deteriorated. When the PI+VSG composite control method is adopted, the control is switched to VSG control in a nonlinear interval of the system, PI control is adopted outside the interval, the nonlinearity of the system can be eliminated through switching every by every switching angle, and the slope change at the zero crossing point of the system is greatly reduced. The composite control can well eliminate nonlinearity of the system and improve the output waveform of the system.
Wherein FIG. 4 shows waveforms of three-phase inductor current, wherein A 1 -A 4 Represents the switching point of phase A, B 1 -B 4 Represents the switching point of phase B, C 1 -C 4 Representing the switching point of phase C. In any phase current period, according to the current given value I ref Current amplitude I corresponding to the switching point x Calculating the system switching angle, wherein the calculation formula is as formula (7):
from the formula (7), it can be deduced that the controller is switched 12 times in one current period, the current is divided into a linear region and a nonlinear region according to the switching angle, the system enters the nonlinear region from the linear region, and i at the switching point is extracted d And i q The value is used as feedback of VSG current inner loop and is equal to i in the whole nonlinear range d And i q Value run, allowing the system to take advantage of the inertia of VSG control and i over a linear range d And i q Values to eliminate system non-linearities. I.e. as shown in fig. 4, the C-phase output current passes through the switching point C 1 When the system is in the nonlinear region from the linear region, extracting C 1 Point i d And i q As C 1 To C 2 VSG current in the interval is fed back to suppress miningSample nonlinearity effects on the system.
FIG. 5 is a block diagram of a compound control architecture for a grid-tie inverter, wherein the controlled object isBecause the feedback channel has a first-order inertia link, the feedback channel can be equivalent to G L (s). The digitally controlled delay includes PWM pass delay G pwm (s) and sampling calculation delay G d (s) two parts, generally considered as sampling computation delay of one sampling period; when the common coupling point voltage v pcc After the current control loop is introduced, the output current of the system is influenced, and v can be increased to inhibit the negative effect of grid disturbance on the output current of the inverter pcc Feedforward suppresses grid voltage disturbances, where k g Is a feed forward coefficient.
Claims (3)
1. The controller switching method for improving the current quality of the grid-connected inverter is characterized by comprising the following steps of:
step 1: establishing a mathematical model of a grid-connected interface circuit according to a grid-connected inverter main circuit structure and analyzing system characteristics;
step 2: PI parameters and VSG control parameters are selected according to system characteristics, and the stability of the system under control is analyzed, so that the system can be stably connected with the grid and has enough stability margin;
step 3: according to the nonlinear range of the system, calculating the switching angle of the three-phase current according to an angle switching formula, and for the switching angle of the three-phase current, under the rotating coordinate system of the systemi d Andi q extracting components to serve as current feedback quantity of VSG control;
the step 3 is specifically to select a current-type VSG control strategy, wherein the active power corresponding to the systemP e And reactive powerQThe expression of (2) is shown as the expression (4):
（4）
in the method, in the process of the invention,wfor the actual angular frequency of the light source,w 0 for the fundamental angular frequency of the wave,Jin order for the moment of inertia to be of interest,Din order to be a damping coefficient,D p 、D q respectively isP-fControl(s),Q-UThe sag factor of the control is controlled such that,V d is thatdThe voltage of the axis component is such that,sis a differential operator which is a function of the differential operator,Kis the inertia coefficient of the reactive power loop,P ref 、Q ref is the active power and reactive power given by the system,U N is the rated voltage of the system, and the output power of the system is calculated according to the instantaneous theoretical formulaP e AndQthe expression of (2) is shown as the formula (5):
（5）
at the position ofdqIn the coordinate system, since unit power factor control is adopted, it is provided thatV q =0, the formula (5) can be simplified into the formula (6), and a control block diagram of the current-type VSG can be obtained based on the formula (6);
（6）；
in the step 3, in any one phase current period, according to the current set valueI ref Current amplitude corresponding to the switching pointI x Calculating the system switching angle, wherein the calculation formula is as formula (7):
（7）；
step 4: programming PI control and VSG composite control strategies, switching to VSG control in a nonlinear interval of the system, and adopting PI control outside the interval to realize automatic circulation and smooth switching of grid-connected current and eliminate current distortion of a feedback channel when current passes through a zero point.
2. According to claim 1The controller switching method for improving the current quality of the grid-connected inverter is characterized in that the main circuit structure in the step 1 comprises direct current bus voltageU dc Grid voltageeAnd a grid-connected inverter formed by the filters.
3. The method for switching a controller to improve current quality of a grid-tied inverter of claim 2, wherein the filter comprises a dc bus capacitorC dc DC bus capacitorC dc The anode and the cathode of the IGBT are respectively connected into three-way switching tubes of the IGBT, wherein the IGBT switching tubes are packaged anti-parallel diodes or turn-off power switching devices with anti-parallel diode characteristics.
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| CN111446716A (en) * | 2020-03-31 | 2020-07-24 | 东南大学 | Harmonic voltage and harmonic current suppression method for VSG grid-connected system |
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| US6879053B1 (en) * | 2002-10-22 | 2005-04-12 | Youtility, Inc. | Transformerless, load adaptive speed controller |
| CN107732978A (en) * | 2017-09-23 | 2018-02-23 | 天津大学 | A kind of inverter system that power distribution strategies are improved based on virtual synchronous generator |
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