Nonlinear control method for three-phase voltage source inverter

Abstract

The invention belongs to the technical field of inverters, and particularly relates to a non-linear control method of a three-phase voltage source type inverter, which comprises the following steps of: detecting an inductive current ILabc, and detecting an output voltage Voabc; converting the abc coordinate system into an alpha beta coordinate system to obtain IL alpha beta and Vo alpha beta; setting a voltage outer ring K2 and a current inner ring parameter K1; obtaining a filter inductance current reference value # imgabs0 # through a voltage outer loop, inputting the current reference value into a current inner loop for control, and obtaining an inverter modulation signal Sinvx (x = alpha, beta) under an alpha beta coordinate; and converting the modulation signal to be Sinvz (z = a, b, c) under an abc coordinate, and outputting a PWM driving signal of an inverter switching device in a carrier modulation mode so as to drive the inverter to output a three-phase alternating current. The problem of buffeting caused by a sliding mode item in current inner loop control is effectively solved, a nonlinear control item is introduced, the robustness of the system is enhanced, and the inverter can compensate for low-order or high-order harmonic voltage.

Description

Technical Field

[0001] The invention belongs to the technical field of inverters, and in particular relates to a nonlinear control method for a three-phase voltage source inverter. Background Art

[0002] For three-phase voltage source inverters, Zhao Ensheng of the University of Electronic Science and Technology of China designed a control method for passive voltage control outer loop and passive current control inner loop in his paper "Research on Multi-scale Instability Mechanism and Robust Passive Control Method of Isolated Microgrid". Compared with the traditional proportional integral double closed-loop control method, the passive control method is insensitive to parameter perturbations. dc , when the filter inductance or capacitance parameters change within a certain range, the passive control method can still work normally.

[0003] However, when the inverter is connected to nonlinear loads such as uncontrolled rectifier bridges and LED lights, the nonlinear current caused by the load causes the inverter output voltage to be distorted, which is a common problem. The traditional proportional-integral dual closed-loop control method and the passive dual closed-loop control method cannot compensate for the distorted output voltage well, and there is also a jitter problem caused by the sliding film term in the current inner loop control. The traditional proportional resonant control method can compensate for limited harmonic voltages such as the 5th, 7th, and 11th order, but it cannot compensate for higher harmonic voltages because the control system does not have a sufficiently high control bandwidth. Summary of the invention

[0004] The present invention aims to provide a nonlinear control method for a three-phase voltage source inverter to solve the chattering problem caused by the sliding film term in the current inner loop control, so as to realize the compensation of low-order and high-order harmonic voltages.

[0005] In order to achieve the above object, the present invention provides a nonlinear control method for a three-phase voltage source inverter, comprising the following steps:

[0006] Voltage and current detection: detect the inductor current I Labc , detect output voltage V oabc ;

[0007] Coordinate system conversion: Convert the abc coordinate system to the αβ coordinate system to obtain the I required for double closed-loop control Lαβ 、V oαβ ;

[0008] Parameter setting: set the voltage outer loop K2 and current inner loop parameter K1 to optimize the system control effect;

[0009] Voltage outer loop control: The filter inductor current reference value is obtained through the voltage outer loop The control equation of the voltage outer loop is:

[0010]

[0011] Current inner loop control: The current reference value is input into the current inner loop control, and the inverter modulation signal S under the αβ coordinates is obtained through the inner loop control. invx(x＝α,β) , where the control equation of the inner current loop is:

[0012]

[0013] Three-phase AC output: Convert the modulated signal to S in abc coordinates invz(z＝a,b,c) , the PWM drive signal of the inverter switching device is output through carrier modulation to drive the inverter to output three-phase AC power.

[0014] Optionally, in the voltage and current detection step, a filter inductor current sensor is used to detect the inductor current I Labc , use the filter capacitor voltage sensor to detect the output voltage V oabc .

[0015] Optionally, in the parameter setting step, the current inner loop parameter K1 and the voltage outer loop parameter K2 are set using the filter KVL equation:

[0016]

[0017] Optionally, in the parameter tuning step, according to the Input-to-State Stability (ISS) theory, a Lyapunov function is constructed, and the Lyapunov function is:

[0018]

[0019] The ISS theoretical expression is:

[0020]

[0021] Where e is the voltage error, r is the parasitic resistance, and Δ(t) is the external disturbance.

[0022] Optionally, in the parameter tuning step, the system differential equation is linearized near the steady-state operating point, and the linearized system differential equation becomes:

[0023]

[0024] Its time constant is:

[0025]

[0026] The working principle of the present invention is that the filter inductor current sensor detects the inductor current I Labc, the filter capacitor voltage sensor detects the output voltage V oabc By converting the abc coordinate system into the αβ coordinate system, we can obtain the I required by the control method. Lαβ 、V oαβ The filter inductor current reference value is obtained through the voltage outer loop. The current reference value is input into the current inner loop, and the absolute value of the voltage error is Introduced into the current inner loop and used Instead of the original passive control method inner loop Solve the problem of chattering caused by the synovial term. Where sign is the sign function, when hour when hour The modulation signal S under the αβ coordinates is obtained by the current inner loop control invx(x＝α,β) , convert the modulated signal to the abc coordinate as S invz(z＝α,b,c) Finally, the PWM drive signal of the inverter switch device is output through carrier modulation. Through the action of the PWM drive signal, the inverter outputs three-phase AC power.

[0027] The beneficial effects of this solution are:

[0028] 1. No matter how the load changes and switches, no matter how the output current is distorted, when the filter parameters, i.e., the LC inductor and capacitor parameters, fluctuate by 50%, the quality of the inverter output voltage is very high, and the total harmonic distortion rate is controlled below 0.5%. When the filter parameters are perturbed by 30%, the total harmonic distortion rate of the output voltage can still be limited to less than 3% when non-linear loads such as uncontrolled rectifier bridges are connected.

[0029] 2. It is only necessary to adjust the two control gains K1 and K2 of the voltage and current double closed loop to achieve the global stability of the system, and the parameter design is simple. Parameter setting can be achieved by simulation or by constructing a Lyapunov function. For the specific process, refer to the parameter setting steps in the embodiment.

[0030] 3. This method has very strong versatility and applicability and can be used in any three-phase voltage source inverter, including but not limited to two-level three-phase voltage source inverter, three-level three-phase voltage source inverter and multi-level three-phase voltage source inverter.

[0031] 4. Based on the passive double closed-loop inverter, the absolute value of the voltage error is introduced, which is different from the way that the sliding film surface and the absolute value term are consistent in the traditional super-helical sliding film control method. The jitter caused by the sliding film term in the traditional sliding film control is reduced, and the outer loop voltage error is introduced into the nonlinear control part, which can further increase the robustness of the system and enable the control method to better compensate for the harmonic voltage.

[0032] 5. Use the inductor current I Labc Replacement LC filter output current I oabc , reducing sensors and auxiliary circuits, and involving fewer control parameters, simplifying system design. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is an overall control framework of a nonlinear control method for a three-phase voltage source inverter in an embodiment of the present invention;

[0034] Figure 2 A system control block diagram of a nonlinear control method for a three-phase voltage source inverter according to an embodiment of the present invention;

[0035] Figure 3 A topological structure of a three-phase voltage source inverter in an embodiment of the present invention;

[0036] Figure 4 It is an output performance simulation of a nonlinear control method for a three-phase voltage source inverter in an embodiment of the present invention when filter parameters are not perturbed;

[0037] Figure 5 The output voltage tracking effect and quality under filter parameter perturbation of a nonlinear control method for a three-phase voltage source inverter in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The following is further described in detail by specific embodiments

[0039] Example

[0040] This embodiment is basically Figure 1 and Figure 2 As shown in the figure: A nonlinear control method for a three-phase voltage source inverter. The basic parameters verified by the experiment are as follows:

[0041] ω=2πf o , f o =50Hz, V dc =800V, f sw =100KHz, L1=340μH, L2=22μH, C=4.7μF, r snubber =0.55Ω, r =0.2Ω, Cdc1 =C dc2 =600μF.

[0042] ω is the output voltage angular frequency, f o is the output voltage frequency, V dc DC voltage, Reference voltage, f sw Switching frequency, L1 and L2 are filter inductors, r snubber is the Snubber resistor, r is the parasitic resistance of the filter inductor L1, C dc1 , C dc2 DC side capacitor, K1, K2 controller parameters

[0043] The whole method includes the following steps:

[0044] Voltage and current detection: The filter inductor current sensor detects the inductor current I Labc , the filter capacitor voltage sensor detects the output voltage V oabc ;

[0045] Coordinate system conversion: Convert the abc coordinate system to the αβ coordinate system to obtain the I required for double closed-loop control Lαβ 、V oαβ ;

[0046] Parameter setting: Set the current inner loop parameter K1 and the voltage outer loop parameter K2; the specific process is as follows:

[0047] Filter KVL equation:

[0048]

[0049] In this embodiment, no power control loop is included, so for the control method in this embodiment, the conditions of the α axis and the β axis are the same and independent of each other, so the subscripts α and β are omitted in the stability proof. Combine the current inner loop control equation and equation (1):

[0050]

[0051] Voltage error Combine the voltage outer loop control equation and equation (2):

[0052]

[0053] Substitute the actual reference voltage into equation (3), where ω=2πf,f=50Hz:

[0054]

[0055] In equation (4), both the linear part and the nonlinear part have sine or cosine disturbance terms. Due to the presence of the disturbance term -0.458cos(ωt) in the sign function of the nonlinear part, the system can only guarantee bounded stability, that is, the error e is allowed to fluctuate in a very small range, that is, |e(t)|≤∈, where ∈ is a given positive constant.

[0056] The following proves the bounded stability of the system of differential equations:

[0057] In the Input-to-State Stability (ISS) theory, when When , the system is stable in the input state, that is, the influence of the disturbance on the system state e(t) is bounded. This indicates that the system has robust stability in the presence of disturbances. Among them, α(e) is a positive definite function, which represents the attenuation term of the system state, and γ(|d|) is a positive definite function about the disturbance d(t).

[0058] Define the Lyapunov function of the system:

[0059]

[0060] but Substituting formula (5) into:

[0061]

[0062] The disturbance term 0.458cos(ωt) in the sign function sign is regarded as an external disturbance:

[0063] sign(K2e-0.458cos(ωt))≤sign(K2e)+Δ(t) (7)

[0064] Among them, |Δ(t)|≤1, and the maximum amplitude of the disturbance term in the linear term is 0.049+0.091=0.14. Therefore, after separating the disturbance term:

[0065]

[0066] Then the system attenuation term α(e)=(1+rK2)e 2 +K1e 2 , the disturbance term amplitude γ(|d|)=0.14|e|+K1e 2 |Δ(t)|, so as long as the amplitude of the attenuation term is greater than the amplitude of the disturbance term, the system is globally bounded and stable, that is:

[0067] (1+rK2+K1-K1|Δ(t)|)e 2 ≥0.14|e| (9)

[0068] The parasitic resistance r = 0.2. Since |e(t)|≤∈, we can assume ∈ = 0.1. Then we can deduce:

[0069]

[0070] From formula (10), we can get that the smaller the error precision ∈, the larger the required K2 will be. When ∈ = 0.1, K2 = 2. However, due to the limitation of system bandwidth and the influence of delay link in DSP, K1 and K2 need to be further optimized.

[0071] Next, the system differential equation is linearized near the steady-state operating point, and the value range of K1 and K2 is given in combination with the system control bandwidth.

[0072] Since it has been proved that the system is still stable under the influence of disturbance terms, and the linearization analysis is mainly to optimize performance, the disturbance terms are ignored. The differential equation of the linearized system becomes:

[0073]

[0074] This is a first-order linear system with the time constant:

[0075]

[0076] In order to ensure that the delay has an acceptable effect on the system phase at the bandwidth frequency, usually f BW ×t d <0.1, while Considering that the actual control system delay in this study is 15μs, the system time constant can be calculated to be about 23.9μs. Substituting K2=2 in the stability analysis into equation (12), K1=27 can be calculated.

[0077] Voltage outer loop control: I Lαβ 、V oαβ The filter inductor current reference value is obtained through the voltage outer loop The control equation of the voltage outer loop is:

[0078]

[0079] Current inner loop control: Set the current reference value Input to the current inner loop control method, and obtain the inverter modulation signal S in the αβ coordinate system through the inner loop control method invx(x＝α,β) , where the control equation of the inner current loop is:

[0080]

[0081] Three-phase AC output: Convert the modulated signal to the abc coordinate system as S invz(z＝a,b,c), the PWM drive signal of the inverter switching device is output through carrier modulation to drive the inverter to output three-phase AC power.

[0082] The inverter topology used in the experiment is as follows: Figure 3 As shown. This embodiment uses a 10KW GaN / Si hybrid ANPC inverter prototype to verify the proposed control method. The solid ellipse frame is GaN Mosfet, and the dotted rectangular frame is Si Mosfet. It should be pointed out that, on the one hand, since the control method proposed in this embodiment does not involve a modulation module, it is also applicable to other three-phase voltage source inverters, including two-level inverters and other types of three-level inverters such as TNPC inverters or NPC inverters. On the other hand, the ANPC prototype used in this embodiment uses an LCL filter. Since this embodiment adds a snubber resistor to the filter capacitor of the LCL filter, the resonance peak of the LCL is effectively suppressed, and in the derivation and simulation process of the above control method equation, only the current on the filter inductor L1 and the voltage on the filter capacitor C are used, and the filter inductor L2 is not involved. Therefore, it is reasonable and effective for this embodiment to use a GaN / Si hybrid ANPC with a snubber resistor to experimentally verify the proposed control method.

[0083] Figure 4 It is the output effect of the inverter when the filter parameters are not perturbed. Figure 4 In a, at 0-0.05s, the inverter load is a nonlinear load composed of a three-phase uncontrolled rectifier bridge, an inductor, a capacitor, and a resistor; at 0.05-0.1s, it is a nonlinear load in parallel with an unbalanced load; at 0.1-0.15s, it is an unbalanced load; at 0.15-0.2s, it is a resistive balanced load. It can be found that different load types will cause different output current distortions. For example, at 0-0.05s, the load current is an intermittent pulse current, at 0.05-0.1s, it is an unbalanced pulse current, and at 0.1-0.15s, it is an unbalanced current. Regardless of how the load conditions change and switch, the three-phase voltage source inverter using the control method proposed in this embodiment can always guarantee a very high output voltage quality. Figure 4 d shows that the total harmonic distortion of the output line voltage during the period 0-0.2s is 0.14%. Figure 4 b is a partial enlarged diagram of the inverter output voltage following the reference voltage. It can be found that the error between the output voltage and the reference voltage is about 0.5V; Figure 4 c shows that the inverter output inductor current follows the reference inductor current, and the tracking error is within 1 A, which is consistent with the bounded stability in the stability analysis. The simulation verifies that the proposed nonlinear dual closed-loop control method has a good complementary effect on harmonic voltages under various load conditions.

[0084] Figure 5(a)(b) shows the waveform of the inverter output voltage following the reference voltage under filter parameter perturbation. As the filter parameters change more, the output voltage contains more high-frequency components. This is mainly due to the degradation of the filter performance. The control method proposed in this embodiment can still track the reference voltage well, proving that the proposed control method is highly robust to filter parameter perturbations. Figure 5 When (c) and (d) correspond to (a) and (b) respectively, Figure 4 Under working conditions, the total harmonic distortion rate of the inverter line voltage. When the filter parameters are perturbed by 50%, the control method proposed in this embodiment can still control the THD at 0.72%, which is lower than the total harmonic distortion rate (THD) of the grid voltage specified by the national standard and IEEE standard of 5%, and the control effect is better.

Claims

1. A nonlinear control method for a three-phase voltage source inverter, characterized in that: The following steps are involved: Voltage and current detection: detect the inductor current I Labc , detect output voltage V oabc ; Coordinate system conversion: Convert the abc coordinate system to the αβ coordinate system to obtain the I required for double closed-loop control Lαβ 、V oαβ ; Parameter setting: set the voltage outer loop K2 and current inner loop parameter K1 to optimize the system control effect; Voltage outer loop control: The filter inductor current reference value is obtained through the voltage outer loop The control equation of the voltage outer loop is: Current inner loop control: The current reference value is input into the current inner loop control, and the inverter modulation signal S under the αβ coordinates is obtained through the inner loop control. invx(x＝α,β) , where the control equation of the inner current loop is: Three-phase AC output: Convert the modulated signal to S in abc coordinates invz(z＝a,b,c) , the PWM drive signal of the inverter switching device is output through carrier modulation to drive the inverter to output three-phase AC power.

2. The nonlinear control method of a three-phase voltage source inverter according to claim 1, characterized in that: In the voltage and current detection step, the filter inductor current sensor is used to detect the inductor current I Labc , use the filter capacitor voltage sensor to detect the output voltage V oabc .

3. The nonlinear control method of a three-phase voltage source inverter according to claim 2, characterized in that: In the parameter setting step, the filter KVL equation is used to set the current inner loop parameter K1 and the voltage outer loop parameter K2:

4. The nonlinear control method of a three-phase voltage source inverter according to claim 3, characterized in that: In the parameter tuning step, according to the input-to-state stability (ISS) theory, the Lyapunov function is constructed. The Lyapunov function is: The ISS theoretical expression is: Where e is the voltage error, r is the parasitic resistance, and Δ(t) is the external disturbance.

5. The nonlinear control method of a three-phase voltage source inverter according to claim 4, characterized in that: In the parameter tuning step, the system differential equation is linearized near the steady-state operating point, and the linearized system differential equation becomes: Its time constant is:

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