Multi-carrier frequency CPS-POD modulation method based on hybrid cascade seven-level inverter

Abstract

The invention discloses a multi-carrier frequency CPS-POD modulation method based on a hybrid cascade seven-level inverter. Aiming at the problems of current backward flow and energy feedback in a traditional hybrid modulation strategy suitable for a hybrid cascade seven-level inverter, the invention provides a carrier phase shift-positive and negative phase inversion lamination (CPS-POD) modulation method based on multi-carrier frequency. Aiming at the problem that rapidity and stability of a system are difficult to consider due to fixed virtual inertia and damping coefficient in traditional VSG control, a self-adaptive module is added on the basis, the virtual inertia and the damping coefficient are changed in real time by analyzing the oscillation process of the system and setting a condition threshold value, the control performance is greatly optimized, and the stability of the system is improved. And the system efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of hybrid cascade seven-level inverter modulation and control, and in particular to a modulation method based on multi-carrier frequency carrier phase shift-positive and negative inversion stacking and an adaptive VSG control method. Background Art

[0002] A hybrid cascade multilevel inverter mainly refers to a circuit composed of power units of different structures or different voltage levels connected in series. Compared with a cascade multilevel inverter, its advantage is that when the same number of output levels is achieved, fewer cascade modules are required and the output voltage harmonic content is lower, thereby reducing the cost of the entire system and ensuring the quality of power.

[0003] For the hybrid cascade seven-level inverter, the high-frequency modulation method is mainly used, and the high-frequency modulation can be divided into carrier phase shift (CPS-PWM), in-phase disposition (IPD), adjacent anti-phase (APOD) and positive and negative anti-phase (POD) methods. Using only one of these traditional modulation methods alone cannot make the inverter output an ideal waveform, and may even cause a large number of harmonics and current backflow and energy feedback problems.

[0004] Virtual synchronous generator technology (VSG) can give full play to the potential advantages of distributed micro-sources. In photovoltaic systems, a storage device is usually equipped on the DC side to integrate the photovoltaic and energy storage systems, and then the technology is applied to the grid-connected inverter to fully utilize photovoltaic micro-sources. Traditional VSG technology fixes the virtual inertia J and damping coefficient D, making it difficult for the system to balance rapidity and stability. Summary of the invention

[0005] The purpose of the present invention is to solve the above problems. 1. A modulation method based on multi-carrier frequency carrier phase shifting and positive and negative inversion stacking is proposed to improve the problems of current backflow and energy feedback; 2. An adaptive module is introduced into the traditional VSG control to improve the control performance and the rapidity and stability of the system.

[0006] In order to achieve the above object, the technical solution of the present invention is: a multi-carrier frequency CPS-POD modulation method based on a hybrid cascade seven-level inverter, comprising the following steps:

[0007] Step 1: Combine the carrier phase shift modulation method and the positive and negative phase reverse stack modulation method to make the output voltage and output power have the same polarity, thus avoiding the problems of current backflow and energy feedback;

[0008] Step 2: Based on the second-order synchronous generator principle and the relationship between VSG reactive power and voltage, a VSG outer loop control model is established;

[0009] Step 3: The VSG inner loop control model is established by introducing virtual inductance and quasi-proportional resonant control;

[0010] Step 4: Introduce the adaptive module into the VSG control system, and then divide the system oscillation process into four stages. The virtual inertia and damping coefficient are adjusted by comparing the virtual rotor angular velocity and the grid angular velocity and judging the positive and negative change rate of the virtual rotor angular velocity.

[0011] Furthermore, the step 1 comprises the following steps:

[0012] Step 1.1: In the carrier phase shift-positive and negative phase reverse stacking modulation method based on multiple carrier frequencies, six carriers are first given: V cr1 、V cr1- (same frequency and symmetrical about the horizontal axis), V cr2 、V cr2- (same frequency and symmetrical about the horizontal axis), V cr3 、V cr3- (same frequency and symmetrical about the horizontal axis) and V cr1 、V cr1- 、V cr3 、V cr3- The carrier frequency is V cr2 、V cr2- Then a sinusoidal modulation wave V is given. ref ;

[0013] Step 1.2: The principle of the carrier phase shift-positive and negative phase inversion stacking modulation method based on multiple carrier frequencies is as follows: the carrier frequency of the low-voltage unit H2 is twice the carrier frequency of the high-voltage unit H1. For the high-voltage unit, the output levels are -2E, 0, 2E. When the modulation wave V ref Greater than carrier V cr2 When g11 Output high level, otherwise low level. ref Less than V cr2- When g31 The output is high level, otherwise it is low level. For low voltage units, the output levels are -E, 0, E. When V m Greater than V cr1 or V cr3 When g12 The output is high level, otherwise it is low level. ref Less than V cr1- and V cr3- When g32 The output is high level, otherwise it is low level;

[0014] Step 1.3: Based on the modulation principle, the inverter output power and output voltage have the same polarity, which avoids the problems of current backflow and energy feedback.

[0015] Furthermore, the step 2 specifically includes the following steps:

[0016] Step 2.1: The mathematical model of the synchronous motor is established. Assuming that the pole pair number of the VSG is 1, the simulated rotor motion equation in the VSG control can be obtained:

[0017]

[0018] Where J is the virtual moment of inertia, D is the damping coefficient, θ is the virtual internal potential phase angle, ω and ω n are virtual angular frequency, rated angular frequency, P ref and P e Respectively represent the reference value and actual value of the active power of the power grid;

[0019] Step 2.2: The mathematical model of VSG outer loop control is established as shown below, where E m is the voltage amplitude of the virtual internal potential of VSG, k q is the droop coefficient of reactive power and voltage, Q ref and Q e They are the reference value and actual value of reactive power respectively, and E1 is the voltage reference value.

[0020]

[0021] Then the instantaneous value of the virtual internal potential is obtained as:

[0022] E ref =E m ·sinθ (3)

[0023] Furthermore, the step 3 specifically includes the following steps:

[0024] Step 3.1: Due to the internal potential E of the synchronous generator q , stator current I, machine terminal voltage U g Existence q =U g -I(r a +jωL d ) In order to make the control strategy of VSG have similar electrical characteristics of synchronous generator stator, the voltage loop of VSG introduces virtual inductance L v , the inductance and output impedance between the inverter and the grid are improved, which is helpful for the decoupling and parameter design of the VSG active and reactive loops, and also helps to suppress the formation of circulating current when multiple VSGs are operated in parallel;

[0025] Step 3.2: The control model of the VSG inner current loop is established. The current loop control of the VSG adopts quasi-proportional resonance (QPR) control. The transfer function of the quasi-PR controller is shown in the following equation.

[0026]

[0027] Among them, K p is the proportionality coefficient; K r is the resonance coefficient; ω0 is the resonance frequency, where ω0=ω n ;ω c is the bandwidth of the resonance part. Considering that the grid frequency fluctuation usually does not exceed 1Hz, we take ω c =2πrad / s.

[0028] Furthermore, the step 4 specifically includes the following steps:

[0029] Step 4.1: Phase (1) and Phase (3): In phase (1), the virtual rotor angular velocity of the virtual synchronous generator is greater than the grid angular velocity ω>ω n And the rate of change of angular velocity dω / dt>0, while ω<ω in stage (3) n And dω / dt<0, so both stages have |ω-ω n | gradually increases, which belongs to the acceleration stage of the rotor angular frequency, so it is necessary to increase J to suppress the rapid increase of ω, but the increase of J will also lead to the system overshoot σ% and the adjustment time t s Therefore, in these two stages, it is necessary to increase the virtual inertia J and also appropriately increase the damping D;

[0030] Step 4.2: Stage (2) Stage (4): In stage (2), ω>ω n But dω / dt<0, and ω<ω in stage (4) n But dω / dt>0, so both phases have |ω-ω n | gradually decreases, which belongs to the deceleration stage of the rotor angular frequency, so it is necessary to reduce the virtual inertia J to speed up |dω / dt|, so that ω can recover ω faster. n At the same time, appropriately increase the damping D to obtain a smaller overshoot σ% and adjustment time t s .

[0031] Step 4.3: Based on the adaptive control principle, the mathematical expression of the adaptive control strategy is obtained:

[0032]

[0033] Where J0 and D0 are the initial values ​​of the moment of inertia and damping coefficient, respectively. j and K dare the adaptive adjustment coefficients of the moment of inertia and damping coefficient respectively; T j is the threshold of |dω / dt|, so that J does not fluctuate frequently when the frequency fluctuates in a small range. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0035] Figure 1 is a circuit structure diagram of a hybrid cascade seven-level inverter;

[0036] Figure 2 It is a schematic diagram of a carrier phase shift-positive and negative phase inversion stacking modulation method based on multiple carrier frequencies;

[0037] Figure 3 It is the overall block diagram of VSG control;

[0038] Figure 4 It is the power angle and frequency oscillation curve of the synchronous generator;

[0039] Figure 5 It is the overall control block diagram of the hybrid cascade seven-level inverter;

[0040] Figure 6 Active power waveforms under adaptive VSG and traditional VSG control;

[0041] Figure 7 The influence of traditional VSG and adaptive VSG on frequency;

[0042] Figure 8 is the changing waveform of J and D under adaptive VSG control; DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work, any modifications, equivalent replacements, improvements, etc., shall fall within the scope of protection of this application.

[0044] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] Example 1

[0046] A multi-carrier frequency CPS-POD modulation method based on a hybrid cascade seven-level inverter includes the following steps:

[0047] Step 1: Combine the carrier phase shift modulation method and the positive and negative phase reverse stack modulation method to make the output voltage and output power have the same polarity, thus avoiding the problems of current backflow and energy feedback;

[0048] The circuit diagram of the hybrid cascade seven-level inverter is as follows: Figure 1 As shown, including switch tube S 11 ,S 31 ,S 41 ,S 21 ,S 12 ,S 32 ,S 42 ,S 22 , load R and inductor L and capacitor C. The principle diagram of the carrier phase shift-adjacent anti-phase stacking modulation method based on multiple carrier frequencies is as follows Figure 2 As shown, including carrier V cr1 、V cr1- 、V cr2 、V cr2- 、V cr3 、V cr3- , modulation wave V ref .

[0049] Step 2: Based on the second-order synchronous generator principle and the relationship between VSG reactive power and voltage, a VSG outer loop control model is established;

[0050] Step 3: The VSG inner loop control model is established by introducing virtual inductance and quasi-proportional resonant control;

[0051] VSG control block diagram Figure 3 As shown, it includes active-frequency loop, reactive-voltage loop, and voltage-current loop.

[0052] Step 4: Introduce the adaptive module into the VSG control system and divide the system oscillation process into four stages. The virtual inertia and damping coefficient are adjusted by comparing the virtual rotor angular velocity and the grid angular velocity and judging the positive and negative change rate of the virtual rotor angular velocity.

[0053] Example 2

[0054] A multi-carrier frequency CPS-POD modulation method based on a hybrid cascade seven-level inverter includes the following steps:

[0055] Step 1: Combine the carrier phase shift modulation method and the positive and negative anti-phase stacking modulation method to make the output voltage and output power have the same polarity, avoiding the problems of current backflow and energy feedback.

[0056] The specific process is as follows:

[0057] Step 1.1: In the carrier phase shift-positive and negative phase reverse stacking modulation method based on multiple carrier frequencies, six carriers are first given: V cr1 、V cr1- (same frequency and symmetrical about the horizontal axis), V cr2 、V cr2- (same frequency and symmetrical about the horizontal axis), V cr3 、V cr3- (same frequency and symmetrical about the horizontal axis) and V cr1 、V cr1- 、V cr3 、V cr3- The carrier frequency is V cr2 、V cr2- twice, then gives a sinusoidal modulation V ref ;

[0058] Step 1.2: The principle of the carrier phase shift-positive and negative phase inversion stacking modulation method based on multiple carrier frequencies is as follows: the carrier frequency of the low-voltage unit H2 is twice the carrier frequency of the high-voltage unit H1. For the high-voltage unit, the output levels are -2E, 0, 2E. When the modulation wave V ref Greater than carrier V cr2 When g11 Output high level, otherwise low level. ref Less than V cr2- When g31 The output is high level, otherwise it is low level. For low voltage units, the output levels are -E, 0, E. When V m Greater than V cr1 or V cr3 When g12 The output is high level, otherwise it is low level. ref Less than V cr1- and V cr3- When g32 The output is high level, otherwise it is low level;

[0059] Step 1.3: Based on the modulation principle, the inverter output power and output voltage have the same polarity, which avoids the problems of current backflow and energy feedback.

[0060] Step 2: Based on the principle of second-order synchronous generator and the relationship between VSG reactive power and voltage, a VSG outer loop control model is established.

[0061] The specific process is as follows:

[0062] Step 2.1: The mathematical model of the synchronous motor is established. Assuming that the pole pair number of the VSG is 1, the simulated rotor motion equation in the VSG control can be obtained:

[0063]

[0064] Where J is the virtual moment of inertia, D is the damping coefficient, θ is the virtual internal potential phase angle, ω and ω n are virtual angular frequency, rated angular frequency, P ref and P e Respectively represent the reference value and actual value of the active power of the power grid;

[0065] Step 2.2: The mathematical model of VSG outer loop control is established as shown below, where E m is the voltage amplitude of the virtual internal potential of VSG, k q is the droop coefficient of reactive power and voltage, Q ref and Q e They are the reference value and actual value of reactive power respectively, and E1 is the voltage reference value.

[0066]

[0067] Then the instantaneous value of the virtual internal potential is obtained as:

[0068] E ref =E m ·sinθ (3)

[0069] Step 3: The VSG inner loop control model is established by introducing virtual inductance and quasi-proportional resonant control.

[0070] The specific process is as follows:

[0071] Step 3.1: Due to the internal potential E of the synchronous generator q , stator current I, machine terminal voltage U g Existence q =U g -I(r a +jωL d ) In order to make the control strategy of VSG have similar electrical characteristics of synchronous generator stator, the voltage loop of VSG introduces virtual inductance L v , the inductance and output impedance between the inverter and the grid are improved, which is helpful for the decoupling and parameter design of the VSG active and reactive loops, and also helps to suppress the formation of circulating current when multiple VSGs are operated in parallel;

[0072] Step 3.2: The control model of the VSG inner current loop is established. The current loop control of the VSG adopts quasi-proportional resonance (QPR) control. The transfer function of the quasi-PR controller is shown in the following equation.

[0073]

[0074] Among them, Kp is the proportionality coefficient; K r is the resonance coefficient; ω0 is the resonance frequency, where ω0=ω n ;ω c is the bandwidth of the resonance part. Considering that the grid frequency fluctuation usually does not exceed 1Hz, we take ω c =2πrad / s;

[0075] Step 4: Introduce the adaptive module into the VSG control system, and then divide the system oscillation process into four stages. The virtual inertia and damping coefficient are adjusted by comparing the virtual rotor angular velocity and the grid angular velocity and judging the positive and negative change rate of the virtual rotor angular velocity.

[0076] The specific process is as follows:

[0077] Step 4.1: Phase (1) and Phase (3): In phase (1), the virtual rotor angular velocity of the virtual synchronous generator is greater than the grid angular velocity ω>ω n And the rate of change of angular velocity dω / dt>0, while ω<ω in stage (3) n And dω / dt<0, so both stages have |ω-ω n | gradually increases, which belongs to the acceleration stage of the rotor angular frequency, so it is necessary to increase J to suppress the rapid increase of ω, but the increase of J will also lead to the system overshoot σ% and the adjustment time t s Therefore, in these two stages, it is necessary to increase the virtual inertia J and also appropriately increase the damping D;

[0078] Step 4.2: Stage (2) Stage (4): In stage (2), ω>ω n But dω / dt<0, and ω<ω in stage (4) n But dω / dt>0, so both phases have |ω-ω n | gradually decreases, which belongs to the deceleration stage of the rotor angular frequency, so it is necessary to reduce the virtual inertia J to speed up |dω / dt|, so that ω can recover ω faster. n At the same time, appropriately increase the damping D to obtain a smaller overshoot σ% and adjustment time t s ;

[0079] Step 4.3: Based on the adaptive control principle, the mathematical expression of the adaptive control strategy is obtained:

[0080]

[0081]

[0082] Where J0 and D0 are the initial values ​​of the moment of inertia and damping coefficient, respectively. j and Kd are the adaptive adjustment coefficients of the moment of inertia and damping coefficient respectively; T j is the threshold of |dω / dt|, so that J does not fluctuate frequently when the frequency fluctuates in a small range.

Claims

1. A multi-carrier frequency CPS-POD modulation method based on a hybrid cascaded seven-level inverter, characterized in that: The following steps are involved: Step 1: Combine the carrier phase shift modulation method and the positive and negative reverse phase stacking modulation method to make the output voltage and output power have the same polarity, thus avoiding the problems of current backflow and energy feedback; Step 2: Based on the second-order synchronous generator principle and the relationship between VSG reactive power and voltage, a VSG outer loop control model is established; Step 3: The VSG inner loop control model is established by introducing virtual inductance and quasi-proportional resonant control; Step 4: Introduce the adaptive module into the VSG control system, and then divide the system oscillation process into four stages. The virtual inertia and damping coefficient are adjusted by comparing the virtual rotor angular velocity and the grid angular velocity and judging the positive and negative change rate of the virtual rotor angular velocity.

2. The multi-carrier frequency CPS-POD modulation method based on a hybrid cascaded seven-level inverter according to claim 1, characterized in that: The specific process of step 1 is as follows: Step 1.1: First, six carriers are given in the carrier phase shift-positive and negative phase inversion stacking modulation method based on multiple carrier frequencies: V cr1 、V cr1- (same frequency and symmetrical about the horizontal axis), V cr2 、V cr2- (same frequency and symmetrical about the horizontal axis), V cr3 、V cr3- (same frequency and symmetrical about the horizontal axis) and V cr1 、V cr1- 、V cr3 、V cr3- The carrier frequency is V cr2 、V cr2- Then a sinusoidal modulation wave V is given. ref ; Step 1.2: The principle of the carrier phase shift-positive and negative phase inversion stacking modulation method based on multiple carrier frequencies is as follows: The carrier frequency of the low-voltage unit H2 is twice the carrier frequency of the high-voltage unit H1. For the high-voltage unit, the output levels are -2E, 0, and 2E. When the modulation wave V ref Greater than carrier V cr2 When g11 Output high level, otherwise low level. ref Less than V cr2- When g31 The output is high level, otherwise it is low level. For low voltage units, the output levels are -E, 0, E. When V m Greater than V cr1 or V cr3 When g12 The output is high level, otherwise it is low level. ref Less than V cr1- and V cr3- When g32 The output is high level, otherwise it is low level; Step 1.3: Based on the modulation principle, the inverter output power and output voltage have the same polarity, which avoids the problems of current backflow and energy feedback.

3. The multi-carrier frequency CPS-POD modulation method based on a hybrid cascaded seven-level inverter as claimed in claim 2, characterized in that: The specific process of step 2 is as follows: Step 2.1: The mathematical model of the synchronous motor is established. Assuming that the pole pair number of the VSG is 1, the simulated rotor motion equation in the VSG control can be obtained: Where J is the virtual moment of inertia, D is the damping coefficient, θ is the virtual internal potential phase angle, ω and ω n are virtual angular frequency, rated angular frequency, P ref and P e Respectively represent the reference value and actual value of the active power of the power grid; Step 2.2: The mathematical model of VSG outer loop control is established as shown below, where E m is the voltage amplitude of the virtual internal potential of VSG, k q is the droop coefficient of reactive power and voltage, Q ref and Q e They are the reference value and actual value of reactive power respectively, and E1 is the voltage reference value. Then the instantaneous value of the virtual internal potential is obtained as:

4. The multi-carrier frequency CPS-POD modulation method based on a hybrid cascaded seven-level inverter as claimed in claim 3, characterized in that: The specific process of step 3 is as follows: Step 3.1: Due to the internal potential E of the synchronous generator q , stator current I, machine terminal voltage U g Existence q =U g -I(r a +jωL d ) In order to make the control strategy of VSG have similar electrical characteristics of synchronous generator stator, the voltage loop of VSG introduces virtual inductance L v , the inductance and output impedance between the inverter and the grid are improved, which is helpful for the decoupling and parameter design of the VSG active and reactive loops, and also helps to suppress the formation of circulating current when multiple VSGs are operated in parallel; Step 3.2: The control model of the VSG inner current loop is established. The current loop control of the VSG adopts quasi-proportional resonance (QPR) control. The transfer function of the quasi-PR controller is shown as follows: Among them, K p is the proportionality coefficient; K r is the resonance coefficient; ω0 is the resonance frequency, where ω0=ω n ;ω c is the bandwidth of the resonance part. Considering that the grid frequency fluctuation usually does not exceed 1Hz, we take ω c =2πrad / s.

5. The multi-carrier frequency CPS-POD modulation method based on a hybrid cascaded seven-level inverter as claimed in claim 4, characterized in that: The specific process of step 4 is as follows: Step 4.1: Phase (1) and Phase (3): In phase (1), the virtual rotor angular velocity of the virtual synchronous generator is greater than the grid angular velocity ω>ω n And the rate of change of angular velocity dω / dt>0, while ω<ω in stage (3) n And dω / dt<0, so both stages have |ω-ω n | gradually increases, which belongs to the acceleration stage of the rotor angular frequency, so it is necessary to increase J to suppress the rapid increase of ω, but the increase of J will also lead to the system overshoot σ% and the adjustment time t s Therefore, in these two stages, it is necessary to increase the virtual inertia J and also appropriately increase the damping D; Step 4.2: Stage (2) and Stage (4): In stage (2), ω>ω n But dω / dt<0, and ω<ω in stage (4) n But dω / dt>0, so both phases have |ω-ω n | gradually decreases, which belongs to the deceleration stage of the rotor angular frequency, so it is necessary to reduce the virtual inertia J to speed up |dω / dt|, so that ω can recover ω faster. n At the same time, appropriately increase the damping D to obtain a smaller overshoot σ% and adjustment time t s ; Step 4.3: Based on the adaptive control principle, the mathematical expression of the adaptive control strategy is obtained: Among them, J0 and D0 are the initial values ​​of the moment of inertia and damping coefficient, respectively, and K j and K d are the adaptive adjustment coefficients of the moment of inertia and damping coefficient respectively; T j is the threshold of |dω / dt|, so that J does not fluctuate frequently when the frequency fluctuates in a small range.

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