Optimized pulse timing correction method and application based on flux deviation and potential offset

Abstract

The present invention discloses a method for optimizing pulse timing correction based on stator flux deviation and midpoint potential offset. Utilizing the concept of model predictive control, the stator flux deviation and midpoint offset problem in a midpoint-clamped inverter based on optimized pulse width modulation is transformed into a quadratic function optimization problem with inequality constraints, which is numerically solved using the active set method. The method is also disclosed for use in a permanent magnet synchronous motor driven by a three-level inverter. This patented method combines optimized pulse width modulation of a midpoint-clamped inverter with a model predictive algorithm, enabling the inverter output to have both good low-order harmonic suppression characteristics and fast dynamic response and midpoint balancing capabilities without increasing the number of optimized pulses.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-power frequency converters and related motor control technologies, and specifically relates to an optimized pulse timing correction method based on stator flux deviation and midpoint potential offset, and a permanent magnet synchronous motor driven by a three-level inverter. Background Art

[0002] Under the same operating conditions, the neutral-point clamped inverter has the advantages of higher output voltage sinusoidality and lower power device voltage compared to the ordinary two-level structure. Therefore, it is widely used in medium-voltage and high-power drive applications.

[0003] As drive system power levels continue to increase, the switching frequency of power devices will be strictly limited to a few hundred hertz to reduce switching losses. Traditional pulse-width modulation technologies (such as SVPWM) operating at low switching frequencies suffer from factors such as dead-zone effects and asymmetry between the positive and negative half-cycles of the voltage pulse, which can lead to increased low-order harmonics in the current and reduced control performance.

[0004] Optimized pulse-width modulation (PWM) ensures good voltage or current harmonic characteristics at low switching frequencies, and has been increasingly used in fields such as rail traction and ship propulsion in recent years. However, the optimized pulse timings derived from steady-state solutions do not account for the sudden changes in the stator flux of AC motors under dynamic conditions, nor do they account for shifts in the inverter midpoint. Directly applying optimized PWM to high-performance dynamic control systems can easily lead to PWM pulse disturbances, inconsistent voltage loading on power transistors, and potentially overcurrent or even damage to components.

[0005] Model Predictive Control (MPC), an advanced control strategy, was first developed in the late 1970s. With the development of digital signal processing technology, MPC has gradually demonstrated its superior performance in the field of power electronics. Model Predictive Control combines a discrete mathematical model to predict the state of a variable within a step size range. By optimizing the objective function, the reference value of the desired controlled variable is integrated with the predicted value. Each possible state is evaluated, and the set of states that minimizes the optimization objective function is selected as the final output. MPC has the advantages of a simple principle, the ability to simultaneously optimize multiple control objectives, and rapid dynamic response. However, existing optimized pulse width modulation technology, when applied to neutral-point-clamped inverters and their AC motor drives, generally employs a neutral-point balancing control method with redundant small vector replacement. This method does not alter the harmonic elimination characteristics and is applicable under both steady-state and dynamic conditions. However, this approach does increase the switching frequency to a certain extent.

[0006] Currently, it is not possible to control the midpoint balance of a midpoint-clamped inverter and track the stator flux of a motor without changing the number of pulses. Summary of the Invention

[0007] One of the purposes of the present invention is to provide an optimized pulse timing correction method based on stator flux deviation and midpoint potential offset, in view of the fact that the existing optimized pulse width modulation technology is difficult to take into account both its steady-state and dynamic performance, and when applied to a midpoint clamped inverter and its AC motor drive, it is often difficult to solve the problems of stator flux deviation, optimized pulse number, midpoint balance, low-order harmonics, etc.

[0008] The technical solution adopted by the present invention to solve the technical problem is: an optimized pulse timing correction method based on stator flux deviation and midpoint potential offset, the steps are:

[0009] S1, using a prediction model containing the quadratic function J(Δt) to calculate the predicted values ​​of the stator flux deviation, midpoint potential offset, and switching time correction at future moments;

[0010] The quadratic function

[0011] Where stator flux deviation ψ s,err =[ψ sα,err ,ψ sβ,err ] T It is obtained by comparing the given value and actual value of the motor stator flux;

[0012] The stator flux correction value ψ s,corr (Δt)=[ψ sα,corr ,ψ sβ,corr ] T ;

[0013] In the formula, the three-phase switching time correction value Δt=[Δt a1 ,Δt b1 ,Δt c1 ] T is a,b,c, t x1 and Respectively represent the actual switching moment and the nominal switching moment in the current state;

[0014] Where v n,err 、v n,corr (Δt) is the DC bus midpoint potential offset and correction value of the midpoint clamped inverter; λ v ,λ u is the weight factor of the midpoint potential and the switching time correction;

[0015] S2, using the active set method to solve the constraints, includes the following two steps:

[0016] S21, ignoring the constraints of the switching time, according to the formula Δt=-H -1 c. Solve the correction value of the unconstrained switching time of three phases a, b, c;

[0017] S22, add constraints to the unconstrained switching moments as follows;

[0018]

[0019] The constraint condition means that the actual switching time after correction can only be advanced to the current sampling time kT s , can only be postponed to the next nominal switching time

[0020] S23, for the switching moments that violate the constraints, i.e., the so-called valid constraints, the following operations are performed:

[0021] First, add constraints to limit the unconstrained switching moments, form the final solution for these switching moments, and then calculate the Δu corresponding to these switching moments. x1 , Δs x1 Set to zero and update the matrix H. Finally, calculate the flux correction and midpoint potential correction generated by these switching moments and update the matrix y err , c;

[0022] Repeat steps S21 to S23 until the solution remains unchanged.

[0023] Furthermore, the first term in the quadratic function J(Δt) of the prediction model can be expressed as:

[0024] J1(Δt)=|ψ s,err -ψ s,corr (Δt)| 2 =|ψ s,err +WΔt| 2 ,

[0025] in,

[0026] For Δu x1 (x=a,b,c) is defined as follows: If the actual switching time t x1 (x=a,b,c) makes the phase voltage increase (0→v dc / 2 or -v dc / 2→0), then Δu x1 =1; if the actual switching time t x1 (x=a,b,c) makes the phase voltage decrease (v dc / 2→0 or 0→-v dc / 2), then Δu x1 =-1.

[0027] Furthermore, the second term in the quadratic function J(Δt) of the prediction model can be expressed as:

[0028] J2(Δt)=λ v (v n,err -v n,corr (Δt) 2 =λ v (v n,err +w T Δt) 2 ,

[0029] in, C is the upper or lower half DC bus capacitance, i sx (x=a,b,c) is the three-phase load current;

[0030] Δs x1 (x=a,b,c) is defined as follows: If the actual switching time t x1 (x=a,b,c) makes the absolute value of the phase voltage decrease (v dc / 2→0 or -v dc / 2→0), then Δs x1 =1; if the actual switching time t x1 (x=a,b,c) makes the absolute value of phase voltage increase (0→v dc / 2 or -v dc / 2→0), then Δs x1 =-1.

[0031] Going further, let Q=diag([1,1,λ v ]), then the quadratic function J(Δt) can be expressed as

[0032]

[0033] Furthermore, let H = 2V T QV+λ u I,c=2V T Q T y err , and ignoring the constant term, the quadratic function J(Δt) can be expressed as

[0034] A second object of the present invention is to provide the above-mentioned optimized pulse timing correction method for a permanent magnet synchronous motor driven by a three-level inverter.

[0035] The beneficial effects of the present invention are as follows: the present invention utilizes the idea of ​​model predictive control to transform the stator flux deviation and midpoint offset problem in a midpoint clamped inverter based on optimized pulse width modulation into a quadratic function optimization problem with inequality constraints, and numerically solves the problem through the effective set method; the quadratic function simultaneously penalizes the correction amounts of the stator flux deviation, midpoint offset and switching time, and considers that the correction amount at the switching time is limited by the constraints of the current sampling time and the next switching time; in addition, different weight factors are selected to limit each correction amount, so as to achieve rapid tracking of the stator flux and balance of the midpoint potential while maintaining the detuning characteristics of the optimized pulse width modulation steady-state switching time as much as possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a principle block diagram of the correction method of the present invention;

[0037] Figure 2 It is the dynamic adjustment process of the midpoint potential voltage under rated speed and rated torque;

[0038] Figure 3 The stator flux trajectory and three-phase current waveform when the motor load torque increases from half load to full load;

[0039] Figure 4 It is the output voltage waveform before and after the switching moment correction when the motor load torque increases from half load to full load. DETAILED DESCRIPTION

[0040] To further illustrate the purpose and technical solutions of the present invention, the present invention will be further described in detail below with reference to the specific embodiments of the accompanying drawings. The following embodiments are only used to illustrate the present invention and do not constitute a limitation of the present invention.

[0041] The present invention discloses an optimized pulse timing correction method based on stator flux deviation and midpoint potential offset, which is mainly applicable to the low switching frequency and high performance optimization control of high-power inverters and AC motors. The principle block diagram is as follows: Figure 1 As shown in Figure 2, the correction of the optimized pulse time is transformed into the following quadratic function optimization problem with inequality constraints.

[0042] S1, using a prediction model including a quadratic function J(Δt) to calculate the predicted values ​​of the stator flux deviation, midpoint potential offset, and switching time correction at future moments.

[0043] The quadratic function is as follows:

[0044]

[0045] Add the following constraints:

[0046]

[0047] The constraint condition means that the actual switching time after correction can only be advanced to the current sampling time kT s , can only be postponed to the next nominal switching time

[0048] Stator flux deviation ψ in quadratic function s,err =[ψ sα,err ,ψ sβ,err ] T The correction value of the stator flux is obtained by comparing the given value of the motor stator flux with the actual value; s,corr (Δt)=[ψ sα,corr ,ψ sβ,corr ] T ;Three-phase switching time correction value Δt=[Δt a1 ,Δt b1 ,Δt c1 ] T is a, b, c, where t x1 and Respectively represent the actual switching time and nominal switching time in the current state; where v n,err 、v n,corr (Δt) is the DC bus midpoint potential offset and correction value of the midpoint clamped inverter; λ v ,λ u is the weight factor of the midpoint potential and the switching moment correction. The selection principle of this weight factor should first ensure that the tracking control of the stator flux is at the highest priority, the balance control of the midpoint potential is second, and finally the correction degree of the switching moment is limited to avoid excessive correction.

[0049] S2, using the effective set method developed for solving quadratic function optimization problems to solve the constraints, includes the following two steps:

[0050] S21, ignoring the constraints of the switching time, according to the formula Δt=-H -1 c. Solve the correction value of the unconstrained switching time of three phases a, b, c;

[0051] S22, adding the constraint conditions mentioned in step S1 to the unconstrained switching moment;

[0052] S23, for the switching moments that violate the constraints, i.e., the so-called effective constraints, the following operations are performed: first, constraints are added to limit the unconstrained switching moments, forming the final solution for these switching moments, and then the Δu corresponding to these switching moments is calculated. x1 , Δs x1 Set to zero and update the matrix H. Finally, calculate the flux correction and midpoint potential correction generated by these switching moments and update the matrix yerr , c;

[0053] Repeat steps S21 to S23 until the solution remains unchanged. Usually, two iterations are enough to satisfy the condition.

[0054] The first term in the quadratic function J(Δt) of the prediction model can be expressed as:

[0055] J1(Δt)=|ψ s,err -ψ s,corr (Δt)| 2 =|ψ s,err +WΔt| 2 ,

[0056] in, For Δu x1 (x=a,b,c) is defined as follows: If the actual switching time t x1 (x=a,b,c) makes the phase voltage increase (0→v dc / 2 or -v dc / 2→0), then Δu x1 =1; if the actual switching time t x1 (x=a,b,c) makes the phase voltage decrease (v dc / 2→0 or 0→-v dc / 2), then Δu x1 =-1.

[0057] The second term in the quadratic function J(Δt) of the prediction model can be expressed as:

[0058] J2(Δt)=λ v (v n,err -v n,corr (Δt) 2 =λ v (v n,err +w T Δt) 2 ,

[0059] in, C is the upper or lower half DC bus capacitance, i sx (x=a, b, c) is the three-phase load current (positive is the current flowing out of the inverter); x1 (x=a,b,c) is defined as follows: If the actual switching time t x1 (x=a,b,c) makes the absolute value of the phase voltage decrease (v dc / 2→0 or -v dc / 2→0), then Δs x1 =1; if the actual switching time t x1 (x=a,b,c) makes the absolute value of phase voltage increase (0→vdc / 2 or -v dc / 2→0), then Δs x1 =-1.

[0060] make Q=diag([1,1,λ v ]), then the quadratic function J(Δt) can be expressed as

[0061]

[0062] Furthermore, let H = 2V T QV+λ u I,c=2V T Q T y err , and ignoring the constant term, the quadratic function J(Δt) can be expressed as

[0063]

[0064] The method of the present invention combines the optimized pulse width modulation of the neutral point clamped inverter with a model prediction algorithm, which can maintain a low harmonic distortion rate of the output without increasing the number of optimized pulses, and realize the control of the inverter neutral point balance and the tracking of the motor stator flux.

[0065] A three-level inverter with a permanent magnet synchronous motor load is used for simulation verification.

[0066] Figure 2 The dynamic adjustment process of the mid-point potential voltage is shown in Figure 1 when the weight factor is set to 0.015 and the initial value of the mid-point potential offset is set to 0.05 times the DC bus voltage under rated speed and rated torque.

[0067] Figure 3 The stator flux trajectory and three-phase current waveforms obtained using the method proposed in this invention when the motor load torque increases from half to full load are shown in the simulation results. The simulation results show that both the midpoint potential, the stator flux, and the phase currents reach steady state within one fundamental wave cycle, demonstrating rapid dynamic response.

[0068] Figure 4 Figure 3 shows the output voltage waveform within one fundamental wave period before and after the pulse timing is corrected using the method proposed in this invention when the motor load torque is increased from half load to full load. The solid line is the switching timing calculated in the steady state before correction, and the dotted line is the final switching timing after correction based on the stator flux deviation and midpoint offset. It can be seen that the steady-state switching angle is continuously corrected during the dynamic control process to achieve rapid tracking of the stator flux and adjustment of the midpoint potential.

[0069] In summary, the optimized pulse timing correction method based on stator flux deviation and midpoint potential offset proposed in the present invention can ensure that the inverter output still has good stator flux tracking and midpoint balancing capabilities at low switching frequency.

[0070] The present invention is not limited to the above-mentioned optimal implementation mode. Any person skilled in the art can derive other variations and improved products under the guidance of the present invention. However, regardless of any changes in its shape or structure, any technical solution that is the same or similar to that of the present application falls within the scope of protection of the present invention.

Claims

1. An optimized pulse timing correction method based on flux deviation and potential offset is characterized by: Steps S1, using a quadratic function J ( Δt ) prediction model calculates the predicted values ​​of stator flux deviation, midpoint potential offset and switching time correction at future moments; The quadratic function ; Where stator flux deviation It is obtained by comparing the given value and actual value of the motor stator flux; The correction value of stator flux is ; In the formula, the three-phase switching time correction value is , , and Respectively represent the actual switching moment and the nominal switching moment in the current state; In the formula 、 is the DC bus midpoint potential offset and correction value of the midpoint clamped inverter; v ,λ u is the weight factor of the midpoint potential and the switching time correction; S2, use the active set method to solve the constraints: S21, ignoring the constraints of the switching time, according to the formula Solution a , b , c Correction value of three-phase unconstrained switching time; S22, add the following constraints to the unconstrained switching time: ， The actual switching time after correction can only be advanced to the current sampling time at most. kT s , can only be postponed to the next nominal switching time ; S23, for the switching moments that violate the constraints, first add constraints to limit the unconstrained switching moments, form the final solution for these switching moments, and then calculate the Δ u x1 , Δs x1 Set to zero and update the matrix H Finally, calculate the flux correction and midpoint potential correction generated by these switching moments and update the matrix y err 、 c ; Repeat steps S21 to S23 until the solution remains unchanged.

2. The optimized pulse timing correction method based on flux deviation and potential offset according to claim 1 is characterized in that: The prediction model quadratic function J ( Δt ) is expressed as: ,in, ; is defined as follows: If the actual switching time Increase the phase voltage ( or ), then Δ u x1 =1; if the actual switching moment causes the phase voltage to decrease ( or ), then Δ u x1 =-1.

3. The optimized pulse timing correction method based on flux deviation and potential offset according to claim 2 is characterized in that: The quadratic function of the prediction model J ( Δt ) is expressed as: , in, , C is the upper or lower half DC bus capacitance value, is the three-phase load current; is defined as follows: If the actual switching time Reduce the absolute value of the phase voltage ( or ), then Δs x1 =1; if the actual switching moment causes the absolute value of the phase voltage to increase ( or ), then Δs x1 =-1.

4. The optimized pulse timing correction method based on flux deviation and potential offset according to claim 3 is characterized in that: make , , , then the quadratic function J ( Δt ) is represented as = , make , , and ignoring the constant term, the quadratic function J ( Δt ) is expressed as .

5. The optimized pulse timing correction method based on flux deviation and potential offset as claimed in claim 1 is used for a permanent magnet synchronous motor driven by a three-level inverter.

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