CN113809948B - Feedback current compensation method for grid-connected inverter under current sampling condition of shunt - Google Patents
Feedback current compensation method for grid-connected inverter under current sampling condition of shunt Download PDFInfo
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- H—ELECTRICITY
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- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
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- 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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Abstract
The invention discloses a feedback current compensation method of a grid-connected inverter under the condition of current sampling of a shunt, which comprises the following steps: step 1, establishing a mathematical model of a grid-connected interface circuit according to a main circuit structure of the grid-connected interface circuit; step 2, determining the nonlinear range (-I) of the current x ，I x ) Given current I ref Determining a phase angle switching point in a current period by using an angle switching formula; step 3: according to the three-phase current waveform of the inverter system, reducing the parameters of the controller in the forward channel when the three-phase current waveform is in a nonlinear range, compensating the amplitude and the phase of a given current, and further increasing the inertia characteristics of the controller; step 4: and after the nonlinear range current is compensated according to the phase angle switching point, the nonlinear range current is smoothly switched at the switching point, and finally, the current distortion problem of a feedback channel when the current passes through the zero point is improved. The invention solves the nonlinear problem of feedback channel current zero crossing point distortion caused by shunt resistor sampling in the grid-connected inverter in the prior art.
Description
Technical Field
The invention belongs to the technical field of grid-connected current waveform control, and particularly relates to a feedback current compensation method of a grid-connected inverter under the condition of sampling current of a shunt.
Background
In recent years, efficient use and distribution of renewable energy sources provides a solid and reliable point of exertion for the long-standing development of micro-grids. In a micro-grid system, the power electronic technology is an important medium for realizing electric energy exchange and interaction, and the reliable operation of the power electronic technology is one of important guarantees for the safe operation of the micro-grid system. In a direct current micro-grid system, a grid-connected interface circuit is used as an electric energy interaction bridge between a public power grid and a micro-grid, and plays a vital role in the stability of the voltage of a public direct current bus and the quality of the electric energy of grid connection.
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 general types of 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 the 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 implementing digital compensation on a current feedback channel, thereby improving the grid-connected current waveform control effect and the grid-connected power quality.
Disclosure of Invention
The invention aims to provide a feedback current compensation method for a grid-connected inverter under the current sampling condition of a shunt, and solves the problem of nonlinearity of feedback channel current zero crossing point distortion caused by shunt resistance sampling of the grid-connected inverter in the prior art.
The technical proposal adopted by the invention is that,
a feedback current compensation method of a grid-connected inverter under the condition of current sampling of a shunt comprises the following steps:
step 1, establishing a mathematical model of a grid-connected interface circuit according to a main circuit structure of the grid-connected interface circuit;
step 2, determining the nonlinear range (-I) of the current x ，I x ) Given current I ref Determining a phase angle switching point in a current period by using an angle switching formula;
step 3: according to the three-phase current waveform of the inverter system, reducing the parameters of the controller in the forward channel when the three-phase current waveform is in a nonlinear range, compensating the amplitude and the phase of a given current, and further increasing the inertia characteristics of the controller;
step 4: and after the nonlinear range current is compensated according to the phase angle switching point, the nonlinear range current is smoothly switched at the switching point, and finally, the current distortion problem of a feedback channel when the current passes through the zero point is improved.
The present invention is also characterized in that,
the grid-connected interface circuit main circuit structure in the step 1 comprises a three-phase full-bridge inverter and a direct-current bus voltage U dc Grid voltage e and filtered powerInductance constitutes grid-connected inverter, DC bus capacitor C dc The positive electrode and the negative electrode are respectively connected with a three-phase inverter bridge formed by IGBT switching tubes;
the mathematical model of the grid-connected interface circuit is as follows:
wherein i is d 、i q Representing the inductance current in the two-phase rotation d and q coordinate systems, u d 、u q Representing the output voltage of the bridge port of the inverter under the two-phase rotation d and q coordinate systems, e d 、e q The grid voltages in the two-phase rotation d and q coordinate systems are represented, respectively, w represents the grid rotation angular frequency.
The switching and calculating modes of the switching angle in the step 2 are as follows: according to the current set point I ref Current amplitude I corresponding to the switching point in the current period x The phase angles of each phase switching point of the three-phase load current of the inverter system are calculated as follows:
θ AN the current switching angle of the a phase in one period is represented by n=1, 2,3,4, and the current switching angle of the a phase in one period is four, k is k when a=1 and n=1 x When a=1 and n=2, =0, k x When a= -1 and n=3, = 1, k x When a= -1 and n=4, = 1, k x ＝2；θ BN The current switching angle of the B phase in one period is represented by n=1, 2,3,4, and the current switching angle of the B phase in one period is four, k is k when b=1 and n=2 x When b=1 and n=4, =0, k x When b= -1 and n=1, = 1, k x When b= -1 and n=3, = 1, k x ＝2；θ CN The current switching angle of the C phase in one period is represented by n=1, 2,3,4, and the current switching angle of the C phase in one period is four, k is k when c=1 and n=2 x When c=1 and n=4, =1, k x ＝2，When c= -1 and n=1, k x When c= -1 and n=3, = 1, k x ＝2。
The controller parameter transformation mainly follows: linear region execution scaling coefficient value k+k 0 Non-linear region execution scaling coefficient value k 0 The main process is as follows:
to be used forFor one period, from θ A1 To->In the process, the proportionality coefficient A is formed by k 0 Gradually approach k+k 0 From->To theta C1 The scaling factor A is again from k+k 0 Infinite approximation k 0 ，θ C1 To theta C2 Is a nonlinear region, the current proportionality coefficient A of the region is kept to be k 0 Unchanged from theta C2 To theta B1 The interval proportionality coefficient A is also defined by k 0 Approximation k+k 0 And from k+k 0 Gradually get close to k 0 The process is then cycled. It follows that the controller scaling factor a can be divided into two: (1) a is represented by k 0 Gradually approach k+k 0 This process is referred to as rising edge A of A 1 The method comprises the steps of carrying out a first treatment on the surface of the (2) A is from k+k 0 Infinite approximation k 0 This process is referred to as falling edge A of A 2 . Wherein A is 1 、A 2 The expressions of (2) are shown as the formulas (5) and (6), respectively, wherein θ A1 Indicating the point at which the inverter system leaves the non-linearity, θ C1 Represents the start point of phase C entering the nonlinear region, k 1 The value range of (1) is (0, 5), a is the proportionality coefficient of the calculation formula of the controller, and e represents the natural base number.
Wherein A is 1 For the rising edge scaling factor, A 2 Is the falling edge proportionality coefficient;
in step 3, the amplitude and phase compensation method is to compensate the current amplitude in the nonlinear region in order to achieve smooth switching between the linear region and the nonlinear region, and to suppress steady-state errors caused by the reduction of the proportional parameters of the controller by giving the current amplitude with the same magnitude as the linear region in the nonlinear region, so that the inverter system can meet the control requirements in both the linear region and the nonlinear region. According to the characteristics of the controlled object of the inverter system, the controlled object comprises an integral link, so that the output of the inverter system has 90-degree phase lag, and when the inverter system is compensated in a nonlinear region, the phase of a given current needs to be advanced by 90 degrees, so that the advanced compensation of the current in the nonlinear region is realized.
The invention has the beneficial effects that
According to the invention, on the basis of adopting shunt resistor sampling as current detection, when the three-phase current is in a nonlinear range, given current amplitude and phase are compensated, so that the influence of nonlinear factors of feedback channel current on a forward channel controller is reduced. The essence is to reduce the influence of nonlinear factors in the shunt resistor sampling channel due to resistor voltage detection on the forward channel of the control inverter system.
Drawings
FIG. 1 is a flow chart of a method for compensating feedback current of a grid-connected inverter under the condition of current sampling of a shunt according to the invention;
FIG. 2 is a schematic diagram of a main circuit of a grid-connected inverter in a feedback current compensation method of the grid-connected inverter under the condition of current sampling of a shunt;
FIG. 3 is a control block diagram of a dq rotating coordinate system grid-connected inverter in a feedback current compensation method of the grid-connected inverter under the current sampling condition of a shunt;
FIG. 4 is a schematic diagram of three-phase AC angle switching calculation in a feedback current compensation method of a grid-connected inverter under the current sampling condition of a shunt;
FIG. 5 is a schematic diagram showing the change of the scaling factor of the forward channel controller in the feedback current compensation method of the grid-connected inverter under the current sampling condition of the current divider;
FIG. 6 is a control block diagram of a grid-connected inverter system in a method for compensating feedback current of the grid-connected inverter under the condition of current sampling by a shunt according to the present invention;
Detailed Description
The invention will be described in detail below with reference to the drawings and the detailed description.
A feedback current compensation method for grid-connected inverter under the condition of current sampling of a shunt is shown in fig. 1, and specifically comprises the following steps:
step 1, establishing a mathematical model of a grid-connected interface circuit according to a main circuit structure of the grid-connected interface circuit;
step 2, determining the nonlinear range (-I) of the current x ，I x ) Given current I ref Determining a phase angle switching point in a current period by using an angle switching formula;
step 3: according to the three-phase current waveform of the inverter system, reducing the parameters of the controller in the forward channel when the three-phase current waveform is in a nonlinear range, compensating the amplitude and the phase of a given current, and further increasing the inertia characteristics of the controller;
step 4: and the 12 switching points in a single period are obtained through a phase angle switching formula, and the nonlinear-range current is subjected to smooth switching at the switching points after being compensated, so that the current distortion problem of a feedback channel when the current passes through a zero point is finally improved.
The step 1 is specifically that,
the main circuit part is topological as shown in fig. 2, and comprises a three-phase full-bridge inverter and a direct-current bus voltage U dc Grid-connected inverter formed by grid voltage e and filter inductance, and direct-current bus capacitor C dc The positive and negative poles are respectively connected with a three-phase inverter bridge formed by IGBT switching tubes. In a three-phase three-wire system, the inverter system has only two degrees of freedom, and therefore, the system can be analyzed and designed in a two-phase stationary coordinate system. Quiet three phasesThe conversion of the voltage equation (1) in the stop coordinate system (abc) to the two-phase stationary coordinate system (αβ) yields the formula (2):
in the formula (1), e a 、e b 、e c Is three-phase network voltage, L is filter inductance, u a 、u b 、u c Three-phase bridge voltage i for inverter output a 、i b 、i c And respectively representing three-phase load currents of the three-phase grid-connected inverter system, and t represents time.
From equation (2), it is known that the ac component is still contained in the two-phase vertical coordinate system, and to further simplify the design of the control inverter system, the two-phase stationary coordinate system may be further mapped to the two-phase rotating coordinate system (dq). Wherein i is α 、i β Representing the inductance current in two-phase stationary alpha and beta coordinate systems, u α 、u β Respectively represent the output voltage of the bridge port of the inverter under two-phase stationary alpha and beta coordinate systems, e α 、e β Representing the grid voltages in the two-phase stationary alpha and beta coordinate systems, respectively. The ac quantity in the αβ coordinate system is converted into the dc quantity in the dq coordinate system as shown in the formula (3).
I in formula (3) d 、i q Representing the inductance current in the two-phase rotation d and q coordinate systems, u d 、u q Representing the output voltage of the bridge port of the inverter under the two-phase rotation d and q coordinate systems, e d 、e q The grid voltages in the two-phase rotation d and q coordinate systems are represented, respectively, w represents the grid rotation angular frequency. 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. 3.
Because the nonlinear problem of feedback channel current zero crossing point distortion caused by shunt resistor sampling exists in the existing grid-connected inverter, when the current is in a nonlinear range, the inertia of a control system near a current detection distortion position is improved through compensation of given current amplitude and phase, and further the influence of feedback channel nonlinear factors on the steady-state characteristics of the control system is reduced.
The forward channel is used as a part of the front stage of the system signal flow and comprises a plurality of contents such as sensing, amplifying, conditioning and conditioning of the signal, and the like. The stability of the control system depends on various influencing factors, and the anti-interference performance is the primary index for measuring the stability of the system. The forward channel is used as a signal acquisition conversion channel controlled by a computer, the possibility of external interference is greatest, and generally when a signal feedback channel has a problem, a sensor inputs a deviation signal into a system, so that an error occurs in a controller, and the anti-interference design of the forward channel is the most important link of the forward channel design.
The step 2 is specifically as follows:
FIG. 4 is a schematic diagram showing phase angle switching of three-phase inductor current according to a given current value I ref Current amplitude I corresponding to the switching point in the current period x The phase angles of each phase switching point of the three-phase load current of the inverter system are calculated as follows:
θ AN the current switching angle of the a phase in one period is represented by n=1, 2,3,4, and the current switching angle of the a phase in one period is four, k is k when a=1 and n=1 x When a=1 and n=2, =0, k x When a= -1 and n=3, = 1, k x When a= -1 and n=4, = 1, k x ＝2；θ BN The current switching angle of the B phase in one period is represented by n=1, 2,3,4, and the current switching angle of the B phase in one period is four, k is k when b=1 and n=2 x When b=1 and n=4, =0, k x When b= -1 and n=1, = 1, k x When b= -1 and n=3, = 1, k x ＝2；θ CN The current switching angle of the C phase in one period is represented by n=1, 2,3,4, and the current switching angle of the C phase in one period is four, k is k when c=1 and n=2 x When c=1 and n=4, =1, k x When c= -1 and n=1, = 2, k x When c= -1 and n=3, = 1, k x ＝2；
The step 3 is specifically as follows:
the controller parameter transformation mainly follows: linear region execution scaling coefficient value k+k 0 Non-linear region execution scaling coefficient value k 0 . The two-region scaling factor conversion follows the periodic transformation schematic of the controller scaling factor a in fig. 4, and the main process is as follows:
FIG. 5 is a schematic diagram showing the periodic transformation of the proportional coefficient A of the controllerFor one period, from θ A1 To->In the process, the proportionality coefficient A is formed by k 0 Gradually approach k+k 0 From->To theta C1 The scaling factor A is again from k+k 0 Infinite approximation k 0 ，θ C1 To theta C2 Is a nonlinear region, the current proportionality coefficient A of the region is kept to be k 0 Unchanged from theta C2 To theta B1 The interval proportionality coefficient A is also defined by k 0 Approximation k+k 0 And from k+k 0 Gradually get close to k 0 This process is repeated later. It follows that the controller scaling factor a can be divided into two: (1) a is represented by k 0 Gradually approach k+k 0 This process is referred to as rising edge A of A 1 The method comprises the steps of carrying out a first treatment on the surface of the (2) A is from k+k 0 Infinite approximation k 0 This process is referred to as falling edge A of A 2 . Wherein A is 1 、A 2 The expressions of (2) are shown as the formulas (5) and (6), respectively, wherein θ A1 Represents the point of departure from non-linearity, θ C1 Represents the start point of phase C entering the nonlinear region, k 1 The value range of (1) is (0, 5), a is the proportionality coefficient of the calculation formula of the controller, and e represents the natural base number.
Wherein A is 1 For the rising edge scaling factor, A 2 Is the falling edge proportionality coefficient;
amplitude and phase compensation: according to the above description, in order to achieve smooth switching between the linear region and the nonlinear region, the current amplitude should be compensated in the nonlinear region, and in the nonlinear region, steady-state errors caused by the reduction of the proportional parameters of the controller are suppressed by giving the current amplitude with the same magnitude as the linear region, so that the inverter system can meet the control requirements in both the linear and nonlinear regions. According to the characteristics of the controlled object of the inverter system, the controlled object comprises an integral link, so that the output of the inverter system has 90-degree phase lag, and when the inverter system is compensated in a nonlinear region, the phase of a given current needs to be advanced by 90 degrees, so that the advanced compensation of the current in the nonlinear region is realized.
FIG. 6 is a control block diagram of a grid-tie inverter system in which the controlled object isThe first-order inertial link existing in the feedback channel can be equivalent to G L (s). Since most digital control at present usually has a delay link, the delay includes a PWM transfer delay G pwm (s) and sampling computation delay->Two parts; when the voltage v of the common coupling point of the inverter system pcc After the current control loop is introduced, the output current of the inverter 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 disturbance, where k g Is a feedforward coefficient; the nonlinear problem of feedback channel current zero crossing distortion caused by shunt resistance sampling is represented by n, F(s) represents compensation of given current amplitude and phase of an inverter system while weakening the action of a controller on the inverter system, and the transfer function of the F(s) channel is as follows>G c (s) is the transfer function of the inverter system irrespective of the feed-forward of the grid voltage, which can be expressed as +.>
When the current waveform is in the linear region, F(s) =0; while the current waveform is in the nonlinear region, the proportional controller G c (s)＝k 0 Smaller scale parameters reduce G c The control duty ratio of the(s) channel to the inverter system reduces the influence of nonlinear factors on the output characteristic of the inverter system, and F(s) is introduced to compensate the inverter system because the controlled object of the inverter system has an integration linkI.e. there is a phase lag of 90 deg., so the compensation element F(s) current amplitude should be equal to the given value of the controller and lead by 90 deg. to compensate for its lag.
Claims (2)
1. The feedback current compensation method for the grid-connected inverter under the condition of current sampling of the current divider is characterized by comprising the following steps:
step 1, establishing a mathematical model of a grid-connected interface circuit according to a main circuit structure of the grid-connected interface circuit;
the grid-connected interface circuit main circuit structure in the step 1 comprises a three-phase full-bridge inverter and a direct-current bus voltage U dc Grid-connected inverter formed by grid voltage e and filter inductance, and direct-current bus capacitor C dc The positive electrode and the negative electrode are respectively connected with a three-phase inverter bridge formed by IGBT switching tubes;
the mathematical model of the grid-connected interface circuit is as follows:
wherein i is d 、i q Representing the inductance current in the two-phase rotation d and q coordinate systems, u d 、u q Representing the output voltage of the bridge port of the inverter under the two-phase rotation d and q coordinate systems, e d 、e q The power grid voltages under two-phase rotation d and q coordinate systems are respectively represented, and w represents the power grid rotation angular frequency;
step 2, determining the nonlinear range (-I) of the current x ，I x ) Given current I ref Determining a phase angle switching point in a current period by using an angle switching formula;
the switching and calculating modes of the switching angle in the step 2 are as follows: according to the current set point I ref Current amplitude I corresponding to the switching point in the current period x The phase angles of each phase switching point of the three-phase load current of the inverter system are calculated as follows:
θ AN the current switching angle of the a phase in one period is represented, n=1, 2,3,4, and the current switching angle of the a phase in one period is four, when a=1 and n=1, k x When a=1 and n=2, =0, k x When a= -1 and n=3, = 1, k x When a= -1 and n=4, = 1, k x ＝2；θ BN The current switching angle of the B phase in one period is represented by n=1, 2,3,4, and the current switching angle of the B phase in one period is four, k is k when b=1 and n=2 x When b=1 and n=4, =0, k x When b= -1 and n=1, = 1, k x When b= -1 and n=3, = 1, k x ＝2；θ CN The current switching angle of the C phase in one period is represented by n=1, 2,3,4, and the current switching angle of the C phase in one period is four, k is k when c=1 and n=2 x When c=1 and n=4, =1, k x When c= -1 and n=1, = 2, k x When c= -1 and n=3, = 1, k x ＝2；
Step 3: according to the three-phase current waveform of the inverter system, reducing the parameters of the controller in the forward channel when the three-phase current waveform is in a nonlinear range, compensating the amplitude and the phase of a given current, and further increasing the inertia characteristics of the controller;
the controller parameter transformation mainly follows: linear region execution scaling coefficient value k+k 0 Non-linear region execution scaling coefficient value k 0 Nonlinear region execution scale coefficient value k 0 The main process is as follows:
to be used forFor one period, from θ A1 To->In the process, the proportionality coefficient A is formed by k 0 Gradually approach k+k 0 From->To theta C1 The scaling factor A is again from k+k 0 Infinite approximation k 0 ，θ C1 To theta C2 Is a nonlinear region, the current proportionality coefficient A of the region is kept to be k 0 Unchanged from theta C2 To theta B1 Interval ofThe scaling factor A is also defined by k 0 Approximation k+k 0 And from k+k 0 Gradually get close to k 0 The process is repeated afterwards; the controller scaling factor a is divided into two: (1) a is represented by k 0 Gradually approach k+k 0 This process is referred to as rising edge A of A 1 The method comprises the steps of carrying out a first treatment on the surface of the (2) A is from k+k 0 Infinite approximation k 0 This process is referred to as falling edge A of A 2 Wherein A is 1 、A 2 The expressions of (2) are shown as the formulas (5) and (6), respectively, wherein θ A1 Indicating the point at which the inverter system leaves the non-linearity, θ C1 Represents the start point of phase C entering the nonlinear region, k 1 The value range of (1) is (0, 5), a is the proportionality coefficient of the calculation formula of the controller, and e represents the natural base number;
wherein A is 1 For the rising edge scaling factor, A 2 Is the falling edge proportionality coefficient;
step 4: and after the nonlinear range current is compensated according to the phase angle switching point, the nonlinear range current is smoothly switched at the switching point, and finally, the current distortion problem of a feedback channel when the current passes through the zero point is improved.
2. The method for compensating feedback current of grid-connected inverter under the condition of current sampled by a current divider as claimed in claim 1, wherein in the step 3, the compensation method of amplitude and phase is to compensate the current amplitude in a nonlinear region in order to achieve smooth switching between the linear region and the nonlinear region, and when in the nonlinear region, steady-state errors caused by the reduction of proportional parameters of a controller are restrained by giving the current amplitude with the same magnitude as that of the linear region, so that the inverter system achieves control requirements in both linear and nonlinear regions; according to the characteristics of the controlled object of the inverter system, the controlled object comprises an integral link, so that the output of the inverter system has 90-degree phase lag, and when the inverter system is compensated in a nonlinear region, the phase of a given current needs to be advanced by 90 degrees, so that the advanced compensation of the current in the nonlinear region is realized.
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