Single-phase inverter control method applicable to nonlinear load and control digitization method

Abstract

The present invention discloses a control method and a control digitization method for a single-phase inverter that can carry a nonlinear load. By using the filtered capacitor current with limited amplitude as the current inner loop feedback, the capacitor current can be quickly controlled, and due to the differential effect of the capacitor current, the output voltage can be corrected in advance, so that the inverter has a better load carrying capacity. In addition, the current limiter can limit the current of the inverter to play a protective role. It adds output voltage feedforward to the traditional quasi-PR control voltage outer loop PI control current inner loop control method, reduces the influence of static errors when the current loop regulator tracks the AC signal, and speeds up the system response speed. The present invention also discloses a method for quasi-PR control digitization, which greatly facilitates the digital implementation of quasi-PR control in practical applications.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics, and in particular relates to a control method and a digital control method for a single-phase inverter capable of carrying a nonlinear load. Background Art

[0002] Inverters are necessary equipment in renewable energy power generation systems and will directly affect the efficiency and power quality of the power generation system. When the inverter carries a nonlinear load, harmonic currents will be generated, and harmonic voltages will be formed on the inverter output impedance, resulting in a high harmonic content in the inverter output voltage, which will cause serious damage to the power quality of the power supply system. In addition, when the load changes suddenly, the output current will change, which is a disturbance signal for the control system and will affect the output voltage of the inverter. It is difficult to quickly compensate for the disturbance caused by the output current change using traditional inductor current as the current inner loop control. Therefore, there is an urgent need for an inverter control method with strong load capacity and nonlinear load capability.

[0003] At present, in order to solve the above problems, many academic papers and patents have been studied and proposed corresponding solutions, such as:

[0004] In the patent application number 2013104228205, entitled "A control method and control system for a single-phase inverter", Feng Ningbo et al. improved the traditional voltage-current dual-loop control method by adding the inverter's equivalent internal resistance voltage drop feedforward, thereby improving the dynamic response of the system output voltage. However, in practical applications, the equivalent internal resistance of the inverter is not easy to measure and is not universal.

[0005] In the article titled "A Practical Feedforward Application in Inverters with Nonlinear Loads", authors Yin Qiuyang and others studied a feedforward control method suitable for inverters with nonlinear loads. The full feedforward transfer function of the load current is established, and the impact of the 1.5-beat delay caused by digital control is considered. From the perspective of the inverter output impedance, the optimal approximation method is used to determine the feedforward control parameters. In practical applications, since the full feedforward function contains nonlinear links, it is difficult to achieve in the digital control process and can only be solved by approximate methods.

[0006] R.-J.Wai et al. published “Design of high-performance stand-alone and grid-connected inverter for distributed generation applications” in IEEE Transactions on Industrial Electronics, 2013: 1542-1555. The article uses a sliding mode control strategy to control the inverter. The inverter system has good dynamic response performance, but the control strategy has high requirements on the accuracy of the inverter mathematical model. If the accuracy of the inverter mathematical model cannot meet the requirements, the use of this control method will affect the stability of the system.

[0007] Therefore, there is an urgent need for a single-phase inverter control method suitable for carrying nonlinear loads and a control digitization method to solve the current problems of slow response speed, weak load carrying capacity and digital implementation of single-phase inverters that can carry nonlinear loads. Summary of the invention

[0008] In view of this, the purpose of the present invention is to provide a single-phase inverter control method and a control digitization method suitable for carrying nonlinear loads, so as to solve the technical problems of slow response speed, weak load capacity and realization of control digitization of the single-phase inverter control system.

[0009] The present invention is applicable to a single-phase inverter control method capable of carrying a nonlinear load, comprising:

[0010] The output current, inductor current and output voltage of the single-phase inverter are sampled to obtain an output current sampling value, an inductor current sampling value and an output voltage sampling value. The output voltage sampling value is passed through an output voltage low-pass filter to obtain an output voltage filter output value. The inductor current sampling value is passed through an inductor current low-pass filter to obtain an inductor current filter output value. The output current sampling value is passed through an output current low-pass filter to obtain an output current filter output value.

[0011] Subtracting a given reference voltage value from the output value of the output voltage filter to obtain an input value of a voltage loop, and the input value of the voltage loop is passed through a quasi-PR controller to obtain an output value of the voltage loop;

[0012] The output value of the output current filter is input into the limiter for processing to obtain the limiter output value, and the output value of the inductor current filter is subtracted from the limiter output value to obtain the feedback value of the current loop;

[0013] The output value of the voltage loop is subtracted from the feedback value of the current loop to obtain the input value of the current loop, and the input value of the current loop is passed through a PI controller to obtain the output value of the current loop;

[0014] The output voltage filter output value is multiplied by the digital signal processor gain coefficient to obtain a voltage feedforward value; the current loop output value and the voltage feedforward value are summed and multiplied by the switch tube gain coefficient, and the obtained product is subtracted from the output voltage sampling value to obtain a digital signal processor input value; after pulse width modulation by the digital signal processor, a sinusoidal pulse width modulation signal is obtained;

[0015] The sinusoidal pulse width modulation signal is amplified by the switch tube of the single-phase inverter to obtain a high-frequency DC square wave high voltage;

[0016] The high-frequency DC square wave high voltage is filtered by the filter inductor and filter capacitor of the single-phase inverter to obtain a sinusoidal output voltage.

[0017] The method of digitizing the control of the aforementioned quasi-PR controller of the present invention includes defining coefficients:

[0018]

[0019] Where w0 is the center angular frequency, w c is the fundamental angular frequency, t s is the sampling time, k p is the proportional coefficient, osgx, osgy and temp are the intermediate coefficients, b0, b1, b2, a1 and a2 are the discrete coefficients;

[0020] Define the current loop number as i, the intermediate variables w[i], w[i+1] and w[i+2] are all zero, and the input data and output data of the quasi-PR controller are processed as follows:

[0021] Step S1, the intermediate variable w[i+2] is equal to the current input value X[i] of the quasi-PR controller plus a1 multiplied by the intermediate variable w[i+1] plus a2 multiplied by the intermediate variable w[i];

[0022] Step S2, the current output value Y[i] of the quasi-PR controller is equal to b0 multiplied by the intermediate variable w[i+2] plus b1 multiplied by the intermediate variable w[i+1] plus b2 multiplied by the intermediate variable w[i];

[0023] Step S3, the intermediate variable w[i] is equal to the intermediate variable w[i+1];

[0024] Step S4, the intermediate variable w[i+1] is equal to the intermediate variable w[i+2];

[0025] Step S5: The number of loops i is equal to i plus 1, and the process returns to step S1.

[0026] Beneficial effects of the present invention:

[0027] 1. The present invention is applicable to a single-phase inverter control method that can carry nonlinear loads. By introducing output voltage feedforward, the influence of static errors when the current loop regulator tracks the AC signal is reduced, thereby accelerating the system response speed.

[0028] 2. The present invention is applicable to a single-phase inverter control method that can carry nonlinear loads. By using the filtered capacitor current with limiting as the current inner loop feedback, the capacitor current can be quickly controlled, and due to the differential effect of the capacitor current, the output voltage can be corrected in advance, so that the inverter has a better load-carrying capacity. In addition, the current limiter can limit the current of the inverter and play a protective role.

[0029] 3. The method of digitizing the control of the quasi-PR controller of the present invention realizes the discretization of the quasi-PR control, which greatly facilitates the digital realization of the quasi-PR control in practical applications.

[0030] 4. The present invention is applicable to a single-phase inverter control method and a control digitization method that can carry nonlinear loads, and the parameters of the quasi-PR controller and the PI controller can be directly debugged and studied in MATLAB / Simulink. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a model diagram of the single-phase inverter control method system and algorithm debugging.

[0032] Figure 2 This is a single-phase inverter system and control block diagram.

[0033] Figure 3 To achieve the equivalent quasi-PR digital map.

[0034] Figure 4 For the first embodiment, the capacitor current i c is the feedback value of the current loop f Single-phase inverter output voltage waveform.

[0035] Figure 5 This is an output voltage waveform diagram of Example 1 with the time axis enlarged around 0.1s.

[0036] Figure 6 The output value i of the inductor current filter used in the second embodiment is L_f is the feedback value of the current loop f Single-phase inverter output voltage waveform.

[0037] Figure 7 This is the output voltage waveform diagram of Example 2 with the time axis enlarged around 0.1s.

[0038] Figure 8 This is a waveform diagram of the output voltage of the single-phase inverter with nonlinear load in Example 3.

[0039] Fig. 9 This is a waveform diagram of the output current of the single-phase inverter with nonlinear load in Example 3.

[0040] Fig.10 This is an FFT analysis diagram of the output voltage of the single-phase inverter in Example 3.

[0041] Fig.11 This is a waveform diagram of the output voltage of the single-phase inverter before and after the linear parallel nonlinear load in Example 4.

[0042] Fig.12 This is a waveform diagram of the output current of the single-phase inverter before and after the linear parallel nonlinear load in Example 4. DETAILED DESCRIPTION

[0043] The present invention is further described below in conjunction with the accompanying drawings and embodiments, but the present invention is not limited to the embodiments.

[0044] Embodiment 1: Figure 2 As shown, the structure of the single-phase inverter described in this embodiment includes a single-phase full bridge composed of switching tubes, a filter inductor connected in series with the AC output side of the unidirectional full bridge, and a filter capacitor connected in parallel with the AC output of the unidirectional full bridge, the DC input end of the unidirectional full bridge is connected to the DC source, and the AC output end of the unidirectional full bridge is connected to the load.

[0045] like Figure 2 As shown, the control method of a single-phase inverter with nonlinear loads in this embodiment includes:

[0046] The output current, inductor current and output voltage of the single-phase inverter are sampled to obtain the output current sampling value i o , inductor current sampling value i L And the output voltage sampling value v o , output voltage sampling value v o After passing through the output voltage low-pass filter LPF1, the output voltage filter output value v is obtained o_f , the inductor current sampling value i L After passing through the inductor current low-pass filter LPF2, the inductor current filter output value i is obtained. L_f， Output current sampling value i o After passing through the output current low-pass filter LPF3, the output current filter output value i is obtained. o_f .

[0047] The given reference voltage value v ref The output voltage filter output value v o_f The input value v of the voltage loop is obtained by subtracting in , the input value of the voltage loop v inAfter the quasi-PR controller, the voltage loop output value v is obtained out .

[0048] The output current filter output value i o_f After the input limiter is processed, the limiter output value i is obtained o_l , the inductor current filter output value i L_f The limiter output value i o_l The difference is used to obtain the feedback value i of the current loop f .

[0049] The output value of the voltage loop v out The feedback value of the current loop i f The input value i of the current loop is obtained by subtracting in , the input value i of the current loop in After the PI controller, the output value of the current loop is obtained. out .

[0050] The output voltage filter output value v o_f Multiply by the DSP gain factor Then the voltage feedforward value v is obtained f ; The current loop output value i out With the voltage feedforward value v f The sum is multiplied by the switch tube gain coefficient K pwm , multiply the obtained product by the output voltage sampling value v o The difference is made to obtain the input value of the digital signal processor; after the digital signal processor pulse width modulation, a sinusoidal pulse width modulation (SPWM) signal is obtained.

[0051] The sinusoidal pulse width modulation signal is amplified by the switch tube of the unidirectional full bridge to obtain a high-frequency DC square wave high voltage.

[0052] The high-frequency DC square wave high voltage is filtered by the filter inductor and filter capacitor of the single-phase inverter to obtain a sinusoidal output voltage, that is, the output voltage of the single-phase inverter.

[0053] The reference voltage v in this embodiment ref It is a sinusoidal signal with a voltage peak of 120V and a frequency of 50Hz. The load is a linear load with an initial resistance of 30 ohms. A 30 ohm resistor is connected in parallel at 0.1s. Figure 4 The output value i of the inductor current filter used in this embodiment is L_f The output current filter output value i o_f The output value of the limiter after passing through the limiter is subtracted to obtain the current value (i.e. the capacitor current i c ) is the feedback value of the current loop i f Single-phase inverter output voltage waveform. Figure 5 for Figure 4 The output voltage waveform is magnified on the time axis around 0.1s.

[0054] The method of digitizing the control of the quasi-PR controller described in the above embodiment includes defining coefficients:

[0055]

[0056] Where w0 is the center angular frequency, w c is the fundamental angular frequency, t s is the sampling time, k p is the proportional coefficient, osgx, osgy and temp are the intermediate coefficients, b0, b1, b2, a1 and a2 are the discrete coefficients;

[0057] Define the current loop number as i, the intermediate variables w[i], w[i+1] and w[i+2] are all zero, and the input data and output data of the quasi-PR controller are processed as follows:

[0058] Step S1, the intermediate variable w[i+2] is equal to the current input value X[i] of the quasi-PR controller plus a1 multiplied by the intermediate variable w[i+1] plus a2 multiplied by the intermediate variable w[i];

[0059] Step S2, the current output value Y[i] of the quasi-PR controller is equal to b0 multiplied by the intermediate variable w[i+2] plus b1 multiplied by the intermediate variable w[i+1] plus b2 multiplied by the intermediate variable w[i];

[0060] Step S3, the intermediate variable w[i] is equal to the intermediate variable w[i+1];

[0061] Step S4, the intermediate variable w[i+1] is equal to the intermediate variable w[i+2];

[0062] Step S5: The number of loops i is equal to i plus 1, and the process returns to step S1.

[0063] The above method can simply realize the digitization of the quasi-PR controller and facilitate the modification of k in the quasi-PR controller. p and k r The coefficient is used to obtain the output value Y[i] of the quasi-PR controller through the input value X[i] of the quasi-PR controller.

[0064] Embodiment 2:

[0065] The difference between this embodiment and the first embodiment is that the feedback value i of the current loop is f Change to the inductor current filter output value i L_f . Figure 6 The output value i of the inductor current filter used in this embodiment isL_f is the feedback value of the current loop f Single-phase inverter output voltage waveform. Figure 7 for Figure 6 Zoom in on the output voltage waveform on the time axis around 0.1s.

[0066] comprehensive Figure 4 , 5 , 6, 7 can be concluded that the output value i of the inductor current filter is L_f is the feedback value of the current loop f When the load switches from light load to heavy load, the single-phase inverter output voltage adjustment time is longer, and by using the filtered limited capacitor current i c As the current inner loop feedback value i f When the load switches from light load to heavy load, the capacitor current can be quickly controlled, and due to the differential effect of the capacitor current, the output voltage can be corrected in advance. The adjustment time of the output voltage of the single-phase inverter is very short, which makes the inverter have better load capacity. In addition, the current limiter can limit the current of the inverter and play a protective role.

[0067] Embodiment three:

[0068] The difference between this embodiment and the first embodiment is that the linear load is changed into a nonlinear load, and the nonlinear load is a single-phase uncontrolled rectifier circuit, and the parameters of the single-phase uncontrolled rectifier circuit are a DC filter capacitor of 50 μF, a DC filter inductor of 50 μH, and a DC end load resistance of 5 ohms. Figure 8 , Fig. 9 The following is a waveform diagram of output voltage and output current of a single-phase inverter with a nonlinear load according to an embodiment of the present invention. Fig.10 This is a diagram showing the FFT analysis results of the output voltage of a single-phase inverter with a nonlinear load according to an embodiment of the present invention.

[0069] When the single-phase inverter carries a nonlinear load, the output voltage coincides with the given reference voltage waveform. When the FFT analysis is performed, the maximum frequency is considered to be 1000Hz, the output voltage THD value is 0.02%, and the output voltage total harmonic distortion is extremely low. The present invention uses quasi-PR control to track the given reference voltage without static error, and can offset the harmonic components caused by the nonlinear load of the inverter.

[0070] Embodiment 4:

[0071] The difference between this embodiment and the first embodiment is that the linear load is changed to a resistance load with an initial value of 30 ohms. At t=0.1s, the load end is connected in parallel with a nonlinear load, and the nonlinear load is a single-phase uncontrolled rectifier circuit. The parameters of the single-phase uncontrolled rectifier circuit are a DC filter capacitor of 50μF, a DC filter inductor of 50μH, and a DC end load resistor of 5 ohms. Fig.11 , Fig.12The output voltage and output current waveforms of the single-phase inverter with linear load and non-linear load in the embodiment of the present invention are shown.

[0072] When a single-phase inverter is connected with a linear load or a linear load is connected in parallel with a non-linear load, the output voltage coincides with the given reference voltage waveform. When a non-linear load is connected in parallel, the output voltage has almost no fluctuation and the output current is smooth. The system has strong stability and very rapid dynamic adjustment.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, and should be included in the scope of the claims of the present invention.

Claims

1. A single-phase inverter control method applicable to nonlinear loads, characterized in that: include: The output current, inductor current and output voltage of the single-phase inverter are sampled to obtain an output current sampling value, an inductor current sampling value and an output voltage sampling value. The output voltage sampling value is passed through an output voltage low-pass filter to obtain an output voltage filter output value. The inductor current sampling value is passed through an inductor current low-pass filter to obtain an inductor current filter output value. The output current sampling value is passed through an output current low-pass filter to obtain an output current filter output value. Subtracting a given reference voltage value from the output value of the output voltage filter to obtain an input value of a voltage loop, and the input value of the voltage loop is passed through a quasi-PR controller to obtain an output value of the voltage loop; The output value of the output current filter is input into the limiter for processing to obtain the limiter output value, and the output value of the inductor current filter is subtracted from the limiter output value to obtain the feedback value of the current loop; The output value of the voltage loop is subtracted from the feedback value of the current loop to obtain the input value of the current loop, and the input value of the current loop is passed through a PI controller to obtain the output value of the current loop; The output voltage filter output value is multiplied by the digital signal processor gain coefficient to obtain a voltage feedforward value; the current loop output value and the voltage feedforward value are summed and multiplied by the switch tube gain coefficient, and the obtained product is subtracted from the output voltage sampling value to obtain a digital signal processor input value; After pulse width modulation by a digital signal processor, a sinusoidal pulse width modulation signal is obtained; The sinusoidal pulse width modulation signal is amplified by the switch tube of the single-phase inverter to obtain a high-frequency DC square wave high voltage; The high-frequency DC square wave high voltage is filtered by the filter inductor and filter capacitor of the single-phase inverter to obtain a sinusoidal output voltage.

2. A method for digitizing the control of the quasi-PR controller of claim 1, characterized in that: Includes definition coefficients: Where w0 is the center angular frequency, w c is the fundamental angular frequency, t s is the sampling time, k p is the proportional coefficient, osgx, osgy and temp are the intermediate coefficients, b0, b1, b2, a1 and a2 are the discrete coefficients; Define the current loop number as i, the intermediate variables w[i], w[i+1] and w[i+2] are all zero, and the input data and output data of the quasi-PR controller are processed as follows: Step S1, the intermediate variable w[i+2] is equal to the current input value X[i] of the quasi-PR controller plus a1 multiplied by the intermediate variable w[i+1] plus a2 multiplied by the intermediate variable w[i]; Step S2, the current output value Y[i] of the quasi-PR controller is equal to b0 multiplied by the intermediate variable w[i+2] plus b1 multiplied by the intermediate variable w[i+1] plus b2 multiplied by the intermediate variable w[i]; Step S3, the intermediate variable w[i] is equal to the intermediate variable w[i+1]; Step S4, the intermediate variable w[i+1] is equal to the intermediate variable w[i+2]; Step S5: The number of loops i is equal to i plus 1, and the process returns to step S1.

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