CN106602600A - Single-loop control method for LC-type three-phase grid-connected inversion system - Google Patents

Abstract

The invention relates to a single-loop control method for an LC-type three-phase grid-connected inverter system. The PCI controller is suitable for a three-phase abc coordinate system and can directly control the AC signals in the three-phase abc coordinate system. Repeated control is a method based on internal The control strategy of membrane principle can reduce the influence of dead zone and periodic disturbance of grid voltage, and at the same time, it can realize the static error-free tracking of the system. The combination of the two methods can directly and effectively control the AC volume, and at the same time combine the LC filter for filtering, so that the inverter current meets the power factor grid-connected requirements, and the harmonic distortion rate meets the relevant standards. This method is single-loop control, which is directly used to control AC variables without coordinate transformation, saving the cost of the entire system, improving the quality of grid-connected current waveforms, tracking accuracy, and power factor, and ensuring the safe and stable operation of the entire control system.

Description

Single-loop control method for LC-type three-phase grid-connected inverter system

technical field

The invention relates to a power grid-connected control technology, in particular to a single-loop control method for an LC-type three-phase grid-connected inverter system based on proportional complex integral PCI and repeated parallel compound control.

Background technique

In recent years, the supply of non-renewable energy sources such as oil, coal, and natural gas has become increasingly tight, and people have paid more and more attention to the development and utilization of new energy sources such as wind energy and solar energy. However, in the process of developing and utilizing new energy, it is very important to rationally design the grid-connected inverter control system. Generally speaking, for the control of grid-connected inverters, there are mainly two indicators, one is that the controlled output current is at the same frequency and phase as the grid voltage, and the other is that the harmonic distortion rate (THD) of the controlled output current satisfies the relevant grid standards.

The controlled output current of the grid-connected inverter must pass through a filter to filter out high-order harmonics. Currently, there are three types of filters commonly used: L-type, LC-type, and LCL-type. Among them, for the L-type filter, its structure is the simplest, but the filtering effect is poor; for the LC-type filter, it is mainly suitable for dual mode, and the filter capacitor C is equivalent to the local load when connected to the grid, and No filtering effect; for the LCL type filter, the filter capacitor C acts as a high-order harmonic channel, which can eliminate high-order harmonics to a certain extent, and the filtering effect is optimal, but the LCL type filter is a third-order system. There are resonance peaks, and the gain is large at the resonance frequency, which can easily cause system instability. When the present invention adopts the current source type inverter, the signal is triggered and modulated by the pulse width modulation (PWM) signal, and at the same time, the output voltage of the circuit will generate more harmonics at the switching frequency, and the filter must be reasonably selected to eliminate the harmonics. Compared with the complex control strategy of the LCL type filter, the control strategy of the LC type filter is simple, which is conducive to the independent operation of the grid-connected inverter and is often used in engineering practice. At present, the research on the control strategy of grid-connected inverter mainly focuses on the current control type and the voltage control type. For the current control type, the control methods mainly include proportional-integral control (PI), proportional resonance control (PR) and fuzzy control, and each control method has its own advantages and disadvantages; for the voltage control type, the control There are three main methods: constant power (PQ) control, constant voltage/constant frequency (V/F) control, and droop (Droop) control. Based on the analysis of the above two aspects, it is of great theoretical and practical significance to study the LC type filter grid-connected inverter control method.

Contents of the invention

The present invention is aimed at the problems existing in the control of grid-connected inverters, and proposes a single-loop control method for LC-type three-phase grid-connected inverter systems, which can directly and effectively control the AC volume, and at the same time combine LC-type filters for filtering, Make the inverter current meet the power factor grid-connected requirements, and the harmonic distortion rate meets the relevant standards. This method is single-loop control, which is directly used to control AC variables without coordinate transformation, saving the cost of the entire system, improving the quality of grid-connected current waveforms, tracking accuracy, and power factor, and ensuring the safe and stable operation of the entire control system.

The technical solution of the present invention is: a single-loop control method for an LC-type three-phase grid-connected inverter system, the LC-type three-phase grid-connected inverter system includes a DC input voltage source U _dc , a three-phase inverter based on pulse width modulation control device, LC filter, grid line equivalent resistance R _a , R _b , R _c , a grid-side three-phase current detection transmitter, three proportional-complex-integral PCI controllers for adjusting current error signals, and three A repetitive controller used to adjust the current error signal, firstly, under the action of the three-phase current detection transmitter on the grid side, obtain the three-phase actual grid-connected current _ia , _ib , _ic , and compare the value with The corresponding three-phase command currents i _aref , i _bref , and i _cref are compared and compared to obtain the corresponding error signal; then the error signal of each item flows to a corresponding controller composed of a PCI and a repetitive control parallel compound, and is performed by the repetitive controller. The output signal obtained by control and the output signal obtained by the control of the PCI controller are added to obtain the total output signal under composite control. The total signal flows to the PWM module and triggers the modulation signal; finally, the switch drive signal is output through the PWM module, and the signal After the drive circuit controls the switching on and off of the switching tube of the grid-connected inverter, and then controls the amplitude, phase and current quality of the output current of the grid-connected inverter system.

The mathematical model of described PCI controller is:

When the system corresponds to PCI control, the closed-loop transfer function is:

Where ω is the grid frequency, ω ₀ is the given AC frequency, K is the PWM equivalent gain, I _ref (s) is the specified current reference, and I ₀ (s) is the current output;

The amplitude-frequency characteristics and phase-frequency characteristics of the closed-loop transmission system under PCI control are

The mathematical model of the repetitive controller is:

Among them, Z ^-N is the delay link, N takes the value of 200, Q(z) takes the value of 0.95, G _c (z) is the compensator, G _c (z)=K _r z ^k S(z), where K _r is the repetitive controller gain, the value is 0.7, z ^k is the leading compensation link, S(z) is the low-pass filter, and S(z)=S ₁ (z)S ₂ (z), where S ₁ (z ) is a notch filter, and S ₂ (z) is a second-order low-pass filter.

The beneficial effect of the present invention is that: the single-loop control method of the LC-type three-phase grid-connected inverter system of the present invention is applied to the three-phase static abc coordinate system, and realizes the direct optimal control of the AC signal, and at the same time, the control accuracy is higher and the power The factor is higher, the tracking effect is better, and the system reliability is stronger. It is suitable for new energy grid-connected systems such as photovoltaic power generation and wind power generation. At the same time, this control method has a wide range of applications and can be extended to other single-phase or three-phase inverters. In the converter control method.

Description of drawings

Fig. 1 is the overall structural block diagram of the single-loop control strategy of the LC type grid-connected inverter system of the present invention;

Fig. 2 is three-phase PCI controller block diagram among the present invention;

Fig. 3 is a structural block diagram of repeating controller in the present invention;

Fig. 4 shows the Bode diagram of the whole system under the action of repeated control for the present invention;

Fig. 5 shows the three-phase abc grid-connected current diagram for the present invention;

Fig. 6 is a diagram of the harmonic distortion rate of the grid-connected current under the control of the method of the present invention.

detailed description

Accompanying drawing 1 is the overall structural block diagram of the single-loop control strategy of the LC type grid-connected inverter system based on PCI and repeated parallel compound control proposed by the present invention, which mainly includes the DC input voltage source U _dc , the three-phase control strategy based on pulse width modulation Inverter, LC filter (the inductance connected to the inverter bridge side is L _a , L _b , L _c , three filter capacitors C _a , C _b , C _c ), the equivalent resistance of the grid line R _a , R _b , R _c , a grid-side three-phase current detection transmitter, three PCI controllers for regulating current error signals, and three repeating controllers for regulating current error signals. The main control process is as follows: firstly, under the action of the grid-side three-phase current detection transmitter, the three-phase actual grid-connected currents i _a , i _b , i _c are obtained, and the values are compared with the corresponding three-phase command currents i _aref , Compare i _bref and i _cref to get the corresponding error signal e _ix (x=a,b,c); then the error signal flows to the controller composed of PCI and repetitive control in parallel, on the one hand, it is controlled by the repetitive controller to obtain the output On the other hand, it is controlled by the PCI controller to obtain the output signal, and the two are added to obtain the total output signal under the composite control strategy. At the same time, the total signal flows to the PWM module and triggers the modulation signal; finally, the PWM module outputs the switch drive Signal, the signal controls the switching on and off of the switching tube of the grid-connected inverter after passing through the drive circuit, and then controls the amplitude, phase and current quality of the output current of the grid-connected inverter system.

In the whole process of presenting the invention, the actual simulation and verification of an LC type three-phase grid-connected inverter system is mainly carried out to illustrate the correctness and feasibility of the present invention. The specific simulation parameters are: DC voltage source voltage 500V, grid voltage effective value 220V, grid voltage frequency 50Hz, PWM switching frequency 10KHz, filter inductance L is 2.52mH, filter capacitor C is 20μF, grid circuit equivalent resistance R is 0.5 Ω. The reference current of phase a is 30sin(100πt)A, the reference current of phase b is 30sin(100πt-2π/3)A, and the reference current of phase c is 30sin(100πt+2π/3)A.

Accompanying drawing 2 and accompanying drawing 3 are the PCI (proportional complex integral) controller of the present invention application and repetitive controller block diagram, wherein PCI controller is applicable to the three-phase abc coordinate system, can directly control the AC under the three-phase abc coordinate system signal, and PCI controller parameters k _p =0.5, _ki =25. Repetitive control is a control strategy based on the inner membrane principle, which can reduce the impact of such as dead zone and periodic disturbance of grid voltage, and at the same time realize the static-free tracking of the system.

Accompanying drawing 2 has shown the system structural model of three-phase PCI controller, and the mathematical model of proportional complex integral control is

When the system corresponds to PCI control, the closed-loop transfer function is

Where ω is the grid frequency, ω ₀ is the given AC frequency, K is the PWM equivalent gain, I _ref (s) is the specified current reference, and I ₀ (s) is the current output.

The amplitude-frequency characteristics and phase-frequency characteristics of the closed-loop transmission system under PCI control are

Figure 3 shows the structural model of the repetitive controller, and the mathematical model of the repetitive control system is

Where Z ^-N is the delay link, N takes the value of 200, Q(z) takes the value of 0.95, G _c (z) is the compensator, and generally G _c (z)=K _r z ^k S(z), where K _r is the repetitive controller gain, the value is 0.7, z ^k is the lead compensation link, S(z) is the low-pass filter, and S(z)=S ₁ (z)S ₂ (z), where S ₁ (z) is a notch filter, and S ₂ (z) is a second-order low-pass filter. Bring in the system parameters L＝2.52mH, C＝20μF, R＝0.5Ω, and the transfer letter of the LC type controlled object is

Discretization has

The comb filter is chosen to replace the notch filter, and its transfer function is

Taking z＝e ^jθ , we can get

When S ₁ (θ)=0, a=2, S ₁ (z) has the strongest attenuation for a specific frequency, and there is

2cos(qθ)+2＝0

Pick get q=9, so

Among them, α is a constant, θ is the discrete phase angle, ω _c is the natural frequency of the system, and the parameter q will affect the notch frequency of the comb filter.

The present invention selects the second-order low-pass filter to suppress high-frequency disturbance, the cut-off frequency is 5000rad/s, and the damping coefficient is 0.707, then

Discretization has

Figure 4 shows the Bode diagram of the whole system under repeated control. It can be seen that the resonance peak of the P curve of the controlled object corresponds to the maximum attenuation value _of the notch filter S1 curve, which can better compensate the resonance peak, but the notch The high-frequency attenuation characteristics of the device are relatively poor. After the second-order low-pass filter is selected in the present invention, the high-frequency disturbance is suppressed, and the resonance peak value and phase of the controlled object are well compensated.

Figure 5 shows the grid-connected current of three-phase abc. It can be seen that after the new controller composed of PCI and repeated parallel connection, the three-phase abc grid-connected current is almost in phase with its corresponding grid voltage, which can realize the grid-connected requirement of unit power factor, and then It shows that the new control strategy proposed by the present invention has better followability.

As shown in Figure 6, the grid-connected current harmonic distortion rate diagram under control, Table 1 shows the situation of the grid-connected current harmonic distortion rate (THD) under the control of three different strategies, in which the THD value corresponding to the traditional PI control is the largest, which is 4.85%, increased to 2.44% after adding repetitive control; the THD value corresponding to the improved PCI control is reduced compared to the traditional PI control, which is 3.32%. Adding repetitive control constitutes the best control effect of the new composite controller proposed by the present invention The THD value is the smallest, reaching 1.70.

Table 1

Claims (3)

1. a kind of monocyclic control method of LC types three-phase grid-connected inverting system, it is characterised in that LC type three-phase grid-connected inverting system bags Include DC input voitage source U_dc, based on pulse width modulation control three-phase inverter, LC wave filters, power network line equivalent resistance R_a、R_b、R_c, net side three-phase current detection transmitter, three ratio plural numbers for being used to be adjusted current error signal are long-pending Divide pci controller and three repetitive controllers for being adjusted to current error signal, examine in net side three-phase current first In the presence of surveying transmitter, actual grid-connected current i of three-phase is obtained_a、i_b、i_c, and by the value and corresponding three-phase instruction current i_aref、i_bref、i_crefRelatively make the difference, obtain correspondence error signal；Then the error signal of each flow to a corresponding PCI with One Repetitive controller parallel connection is combined the controller of composition, and repeated controller is controlled acquisition output signal and Jing PCI controls Device is controlled acquisition output signal and is added, and obtains output signal total under complex controll, and total signal flows to PWM module, And trigger modulated signal；Jing PWM modules output switch drive signal after most, the signal controls combining inverter Jing after drive circuit The on and off of switching tube, and then control amplitude, phase place and the current quality of grid-connected inverting system output current.

2. the monocyclic control method of LC types three-phase grid-connected inverting system according to claim 1, it is characterised in that the PCI controls The Mathematical Modeling of device processed is：

$G_{PCI}=K_p+\frac{K_i}{s-{\mathrm{j\ω}}_0}$

Closed loop transfer function, is when system correspondence PCI is controlled：

$T\left(s\right)=\frac{I_0\left(s\right)}{I_{ref}\left(s\right)}=\frac{a_{1}s+a_0}{b_3{s}^3+b_2{s}^2+b_{1}s+b_0}$

$\left\{\begin{array}{c}{a}_1=-{\mathrm{Kk}}_p\\ a_2=-{\mathrm{Kk}}_i-{\mathrm{j\ω}}_0-{\mathrm{Kk}}_p\\ b_3=LC\\ b_2=RC-{\mathrm{j\ω}}_{0}LC\\ b_1=-{\mathrm{Kk}}_p+1+{\mathrm{j\ω}}_{0}RC\\ b_0=-{\mathrm{Kk}}_i-{\mathrm{j\ω}}_0(1+{\mathrm{Kk}}_p)\end{array}\right.$

Wherein ω be mains frequency, ω₀It is PWM equivalent gains, I for given a-c cycle, K_ref(s) be specified current flow reference quantity, I₀S () is current output；

System closed loop passes the amplitude versus frequency characte and phase-frequency characteristic of letter when PCI is controlled, respectively

$\left|T\left(s\right)\right|=\frac{K\sqrt{{k_i}^2+{k_p}^2{(\mathrm{\ω}-{\mathrm{\ω}}_0)}^2}}{\sqrt{m^2+n^2{(\mathrm{\ω}-{\mathrm{\ω}}_0)}^2}}$

$\left\{\begin{array}{c}m={\mathrm{Kk}}_i+{\mathrm{RC\ω\ω}}_0-{\mathrm{RC\ω}}^2\\ n={\mathrm{LC\ω}}^2-{\mathrm{Kk}}_p-1\end{array}\right.$

$\∠T\left(s\right)=\arctan \left(\frac{k_p(\mathrm{\ω}-{\mathrm{\ω}}_0)}{k_i}\right)-\arctan \left(\frac{({\mathrm{LC\ω}}^2-{\mathrm{Kk}}_p-1)(\mathrm{\ω}-{\mathrm{\ω}}_0)}{{\mathrm{Kk}}_i+{\mathrm{RC\ω\ω}}_0-{\mathrm{RC\ω}}^2}\right).$

3. the monocyclic control method of LC types three-phase grid-connected inverting system according to claim 1, it is characterised in that described to repeat to control The Mathematical Modeling of device processed is

Wherein Z^-NFor delay link, N values 200, Q (z) values 0.95, G_c(z) be compensator, G_c(z)=K_r·z^kS (z), Wherein K_rTo repeat controller gain, value is 0.7, z^kFor lead compensation link, S (z) is low pass filter, and S (z)=S₁ (z)S₂(z), wherein S₁(z) be trapper, S₂Z () is second-order low-pass filter.