A grid-connected harmonic suppression method based on a single-phase current source five-level inverter

Abstract

This invention relates to a grid-connected harmonic suppression method based on a single-phase current-source five-level inverter. The method involves sampling the capacitor voltage u. c The capacitor voltage u is obtained. c fundamental component u cb The grid connection mode is determined by a high-frequency harmonic current compensation circuit, and the required feedback current i is selected based on the grid connection mode. RC The obtained sinusoidal modulation signal i s * The signal is fed into a multi-carrier PWM generator, which then controls the switching on and off of each switching device in the single-phase current-type five-level inverter circuit. This method suppresses resonance in three modes: active mode, reactive mode, and active + reactive mode, thereby achieving adjustable power factor under grid connection. It is feature-rich, has wider applicability, and ensures fast current response, further guaranteeing system response speed and improving efficiency.

Description

Technical Field

[0001] This invention relates to the field of power electronic converter technology, and in particular to a grid-connected harmonic suppression method based on a single-phase current source type five-level inverter. Background Technology

[0002] Based on the different energy storage elements on the DC side of the inverter, grid-connected inverters can be divided into voltage source inverters and current source inverters. Currently, voltage source inverters are commonly used in grid-connected inverters, but current source inverters also have many advantages, such as not needing electrolytic capacitors at the input, which increases the inverter's lifespan; eliminating the problem of direct current flow in bridge arms; and strong short-circuit protection. Using a current source five-level inverter can make the output current waveform closer to a sine wave, while each switching device only needs to bear half of the current stress, effectively improving circuit performance and reliability. However, when current source inverters are connected to the grid, their output side usually uses capacitor filtering. Since there is an equivalent inductance between the connection point and the inverter output point, when the inductive reactance of this equivalent inductance just reaches the LC resonance of the inverter, the grid voltage on the inverter output side will introduce a high-frequency resonant component. Once this high-frequency component is sampled and enters the control system, it will cause problems such as system divergence and inverter resonance. If not suppressed, it may infinitely amplify the harmonic current of the phase frequency until the system collapses.

[0003] In existing technologies, resonance suppression is generally only targeted at voltage-source grid-connected inverters. Moreover, the suppression methods are often only suitable for pure active grid-connected modes, resulting in an unadjustable power factor and relatively simple functions. In addition, existing resonance suppression methods for voltage-source inverters usually require two-stage transformation of current and voltage, which is complex in structure and has a slow current response speed. Summary of the Invention

[0004] The technical problem to be solved by this invention is to address the resonance problem generated when a single-phase current source type five-level inverter is used for grid-connected inversion. This invention provides a grid-connected harmonic suppression method that can effectively suppress resonance, allows for adjustable power factor during grid connection, and has a fast current response speed.

[0005] The technical solution adopted in this invention is a grid-connected harmonic suppression method based on a single-phase current-source five-level inverter. This method is implemented through a grid-connected system based on a single-phase current-source five-level inverter with a small current-sharing inductor. The grid-connected system includes a main circuit and a control circuit. The main circuit includes a single-phase current-source five-level inverter circuit, a CLL filter circuit, and a grid-side power supply circuit. The control circuit includes a phase-locked loop circuit, a coordinate transformation circuit for single-phase AC current, a PI-based direct current control circuit, a low-pass filter circuit for capacitor feedback voltage, a high-frequency harmonic current compensation circuit, a grid-connected current reference signal circuit, and a multi-carrier PWM generator drive circuit. The PI-based direct current control circuit includes a single-stage PI regulator. The method includes the following steps:

[0006] S1, Collect grid voltage u s As a reference signal, the reference phase angle θ is obtained after phase-locked processing by the phase-locked loop circuit. The reference phase angle θ is then sent to the coordinate transformation circuit of single-phase AC current and the PI-based direct current control circuit, respectively, for αβ-dq transformation and dq-αβ transformation.

[0007] S2, Collect grid current i s The DC current I in the dq coordinate system is obtained by performing an αβ-dq transformation through a coordinate transformation circuit for single-phase AC current. d and I q and the DC current I d and I q The signal is fed into the PI-based direct current control circuit as the negative feedback signal of the first-stage PI regulator.

[0008] S3, Set theoretical value I d * and I q * I d * I represents the active current value that needs to be injected into the power grid. q * This represents the reactive current value that needs to be injected into the power grid, represented by I. d * and I q * In the PI-based direct current control circuit, the input is respectively connected to the negative feedback signal I. d and I q The current error signals are compared and obtained. These signals are then input into a first-stage PI controller for current conversion to obtain a new dq-axis DC current I. sd and I sq After inverse transformation by dq-αβ, the single-phase alternating current i is obtained.sa ;

[0009] S4, sampling capacitor voltage u c After processing, the capacitor voltage u is obtained. c fundamental component u cb ;

[0010] S5. Determine the grid connection mode by using the given values ​​of active and reactive current in the high-frequency harmonic current compensation circuit, and select the required feedback current i based on the grid connection mode. RC The specific process is as follows: when I d * ≠0、I q * When the voltage is 0, the grid is connected in pure active power mode, and the capacitor voltage u is selected. c As feedback current i RC ; when I d * =0, I q * When the voltage is not equal to 0, the grid is connected in pure reactive power mode, and the capacitor voltage u is selected. c The high-frequency harmonic components are used as the feedback current i RC ; when I d * ≠0、I q * When the voltage is not equal to 0, the grid is connected in active + reactive power mode, and the capacitor voltage u is selected. c The high-frequency harmonic components are used as the feedback current i RC ;

[0011] S6, obtaining single-phase alternating current i sa Based on this, harmonic compensation current i is injected in reverse into the PI-based direct current control circuit. RC With single-phase alternating current i sa A comparison is made, and a new alternating current i is obtained after the comparison. s ’ After amplitude normalization, a sinusoidal modulation signal i is obtained. s * , that is, the grid-connected current reference signal;

[0012] S7. The obtained sinusoidal modulation signal i s * The signal is fed into a multi-carrier PWM generator to generate 8 SPWM signals, which are then isolated and driven to control the switching on and off of each switching device in the single-phase current-type five-level inverter circuit.

[0013] The beneficial effects of this invention are as follows: Based on the general single-phase current-source five-level inverter circuit topology, this invention reduces the current-sharing inductance value from the mH level to the uH level through high-frequency modulation, effectively improving the practicality of the current-source multilevel converter. This invention uses a single-phase current-source five-level converter based on uH-level current-sharing inductance for grid-connected inverters, which, like traditional voltage-source inverters, also generates harmonic self-resonance on the output side. This invention solves the harmonic self-resonance problem in pure active grid-connected mode (power factor of 1) by employing a high-frequency harmonic current compensation method, and also provides resonance suppression methods for both pure reactive and active + reactive modes, thereby achieving adjustable power factor during grid connection. Therefore, the single-phase current-source five-level converter and its harmonic suppression strategy proposed in this invention, in addition to being used for conventional grid-connected power generation, also have reactive power compensation functions, enriching the inverter's functionality and broadening its applicability; furthermore, this method is based on direct current control, resulting in fast current response speed, further ensuring system response speed and improving operating efficiency.

[0014] Preferably, in step S4, the capacitor voltage u is obtained through processing. c fundamental component u cb The specific method is as follows: sample capacitor voltage u c For capacitor voltage u c By performing αβ-dq transformation and dq-αβ transformation sequentially, the DC current u in the dq coordinate system is obtained. cd and u cq Then the DC current u cd and u cq After being processed by the low-pass filter in the capacitor feedback voltage low-pass filter processing circuit, u is obtained respectively. cd and u cq fundamental component u cdb and u cqb After inverse dq-αβ transformation, the capacitor voltage u is obtained. c fundamental component u cb .

[0015] Preferably, the high-frequency harmonic component is equal to the capacitor voltage u. c With fundamental component u cb difference. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a grid-connected harmonic suppression method based on a single-phase current source type five-level inverter according to the present invention.

[0017] Figure 2 In this invention, when I is set d * =15, I q* Grid current waveform when = 0;

[0018] Figure 3 In this invention, when I is set d * =0, I q * Grid-connected current waveform at -19°C;

[0019] Figure 4 In this invention, when I d * =15, I q * Grid-connected current waveform at -15°C;

[0020] As shown in the figure: 1. Single-phase current-type five-level inverter circuit; 2. CLL filter circuit; 3. Grid-side power supply circuit; 4. Phase-locked loop circuit; 5. Coordinate transformation circuit for single-phase AC current; 6. PI-based direct current control circuit; 7. Low-pass filter circuit for capacitor feedback voltage; 8. High-frequency harmonic current compensation circuit; 9. Grid-connected current reference signal circuit; 10. Multi-carrier PWM generator drive circuit. Detailed Implementation

[0021] The invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can implement it based on the description. The scope of protection of the invention is not limited to these specific embodiments.

[0022] This invention relates to a grid-connected harmonic suppression method based on a single-phase current-source five-level inverter. The method is implemented through a grid-connected system based on a single-phase current-source five-level inverter with a small current-sharing inductor. The grid-connected system includes a main circuit and a control circuit, such as... Figure 1 As shown, the main circuit includes a single-phase current-type five-level inverter circuit 1, a CLL filter circuit 2 composed of filter capacitors and lead equivalent inductance, and a grid-side power supply circuit 3. The control circuit includes a phase-locked loop circuit 4, a coordinate transformation circuit for single-phase AC current 5, a PI-based direct current control circuit 6, a low-pass filter processing circuit for capacitor feedback voltage 7, a high-frequency harmonic current compensation circuit 8, a grid-connected current reference signal circuit 9, and a multi-carrier PWM generator drive circuit 10. The PI-based direct current control circuit 6 includes a first-stage PI regulator. The method includes the following steps:

[0023] S1, such as Figure 1 As shown, the grid voltage u is collected. s As a reference signal, the grid voltage u sAfter phase-locked processing by phase-locked loop circuit 4, the reference phase angle θ is obtained. The reference phase angle θ is then sent to coordinate transformation circuit 5 for single-phase AC current and direct current control circuit 6 based on PI, respectively, for αβ-dq transformation and dq-αβ transformation.

[0024] S2, Collect grid current i s Grid current i s The DC current I in the dq coordinate system is obtained by performing αβ-dq transformation through the coordinate transformation circuit 5 for single-phase AC current. d and I q and the DC current I d and I q The signal is fed into the PI-based direct current control circuit 6 as the negative feedback signal of the PI regulator.

[0025] S3, Set theoretical value I d * and I q * I d * I represents the active current value that needs to be injected into the power grid. q * This represents the reactive current value that needs to be injected into the power grid, represented by I. d * and I q * In the PI-based direct current control circuit 6, the input is respectively connected to the negative feedback signal I. d and I q The two systems are compared, and their respective current error signals are obtained. The current error signals are then passed through a PI controller to obtain a new dq-axis DC current I. sd and I sq After inverse transformation by dq-αβ, the single-phase alternating current i is obtained. sa ;

[0026] S4, sampling capacitor voltage u c After processing, the capacitor voltage u is obtained. c fundamental component u cb ;

[0027] S5. Determine the grid connection mode by using the given values ​​of active and reactive current in the high-frequency harmonic current compensation circuit, and select the required harmonic feedback current i based on the grid connection mode. RC ;u c and u cb Used to construct harmonic feedback current, I d * and I q *Used to determine the grid connection operating mode (pure active mode or reactive mode), the specific process is as follows: When I d * ≠0、I q * When the voltage is 0, the grid is connected in pure active power mode, and the capacitor voltage u is selected. c As feedback current i RC ; when I d * =0, I q * When the voltage is not equal to 0, the grid is connected in pure reactive power mode, and the capacitor voltage u is selected. c The high-frequency harmonic components are used as the feedback current i RC The high-frequency harmonic component is the capacitor voltage u. c With fundamental component u cb The difference; when I d * ≠0、I q * When the voltage is not equal to 0, the grid is connected in active + reactive power mode, and the capacitor voltage u is selected. c The high-frequency harmonic components are used as the feedback current i RC The high-frequency harmonic component is the capacitor voltage u. c With fundamental component u cb difference.

[0028] S6, obtaining single-phase alternating current i sa Based on this, harmonic compensation current i is injected in reverse into the PI-based direct current control circuit 6. RC With single-phase alternating current i sa A comparison is made, and a new alternating current i is obtained after the comparison. s ’ After amplitude normalization, a sinusoidal modulation signal i is obtained. s * , that is, the grid-connected current reference signal;

[0029] S7. The obtained sinusoidal modulation signal i s * The signal is fed into the multi-carrier PWM generator drive circuit 10, which generates eight SPWM signals. After isolation and driving, these signals control the switching on and off of each switching device in the single-phase current-type five-level inverter circuit 1.

[0030] This invention, based on a general single-phase current-source five-level inverter circuit topology, reduces the current-sharing inductance value from the mH level to the uH level through high-frequency modulation, effectively improving the practicality of current-source multilevel converters. This invention uses a single-phase current-source five-level converter based on uH-level current-sharing inductance for grid-connected inverters, which, like traditional voltage-source inverters, also generates harmonic self-resonance on the output side. This invention solves the harmonic self-resonance problem in pure active power grid-connected mode (power factor of 1) by employing a high-frequency harmonic current compensation method, and also provides resonance suppression methods for both pure reactive power and active + reactive power modes, thereby achieving adjustable power factor during grid connection. For details, see [link to relevant documentation]. Figure 2 , Figure 3 and Figure 4 The grid-connected current waveform is shown. Therefore, the single-phase current source type five-level converter and its harmonic suppression strategy proposed in this invention, in addition to being used for conventional grid-connected power generation, also have reactive power compensation functions, enriching the inverter's functionality and making it more widely applicable; furthermore, this method is based on direct current control, with fast current response speed, further ensuring the system's response speed and improving working efficiency.

[0031] In step S4, the capacitor voltage u is obtained after processing. c fundamental component u cb The specific method is as follows: sample capacitor voltage u c For capacitor voltage u c By performing αβ-dq transformation and dq-αβ transformation sequentially, the DC current u in the dq coordinate system is obtained. cd and u cq Then the DC current u cd and u cq After passing through the low-pass filter in the capacitor feedback voltage low-pass filter processing circuit 7, u is obtained respectively. cd and u cq fundamental component u cdb and u cqb After inverse dq-αβ transformation, the capacitor voltage u is obtained. c fundamental component u cb .

[0032] This invention has conducted simulation verification for three grid-connected working modes, (1) Pure active power mode: setting I d * =15, I q * =0; (2) Pure reactive mode: set I d * =0, I q * =-19; (3) Active + Reactive Mode: Set I d* =15, I q * =-15. To facilitate comparison of control effects, the feedback current signal used for compensation is only applied to the control system at t=0.1s. For example... Figure 2 , Figure 3 and Figure 4 As shown, it can be seen that under the three grid-connected operating modes, the harmonic self-resonance problem on the output side of the single-phase current-type five-level inverter circuit 1 can be well suppressed, the grid-connected current waveform is close to a sine wave, and at the same time, the power factor can be adjusted arbitrarily during grid connection.

[0033] Based on a common circuit topology, this invention reduces the current-sharing inductance value from the mH level to the uH level through high-frequency modulation, further improving the practicality of current-source multilevel inverters. Building upon this, this invention proposes a current harmonic compensation method for a single-phase current-source five-level inverter with a small current-sharing inductance to solve the harmonic self-resonance problem on the inverter output side. This method obtains a 50H value by sampling the voltage across the filter capacitor C, which is then processed through αβ-dq transformation, low-pass filtering, and dq-αβ inverse transformation. z The high-frequency harmonic components are obtained by subtracting the fundamental voltage component from the original capacitor voltage. These high-frequency harmonic components are then used to compensate for harmonic distortion in the grid-connected current. This compensation method effectively suppresses resonance in three different grid-connected operating modes: pure active power, pure reactive power, and active power + reactive power. For details, please refer to [link to relevant documentation]. Figure 2 , Figure 3 and Figure 4 The grid-connected current waveform is shown.

Claims

1. A grid-connected harmonic suppression method based on a single-phase current source type five-level inverter, characterized in that: This method is implemented through a grid-connected system based on a single-phase current-type five-level inverter with a small current-sharing inductor. The grid-connected system based on the single-phase current-type five-level inverter with a small current-sharing inductor includes a main circuit and a control circuit. The main circuit includes a single-phase current-type five-level inverter circuit (1), a CLL filter circuit (2), and a grid-side power supply circuit (3). The control circuit includes a phase-locked loop circuit (4), a coordinate transformation circuit for single-phase AC current (5), a PI-based direct current control circuit (6), a low-pass filter circuit for capacitor feedback voltage (7), a high-frequency harmonic current compensation circuit (8), a grid-connected current reference signal circuit (9), and a multi-carrier PWM generator drive circuit (10). The PI-based direct current control circuit (6) includes a first-stage PI regulator. The method includes the following steps: S1. Collect grid voltage u s As a reference signal, the grid voltage u s The reference phase angle is obtained after phase-locking processing by the phase-locked loop circuit (4). θ Reference phase angle θ The coordinate transformation circuit (5) and the PI-based direct current control circuit (6) are respectively fed into the single-phase AC current and are used for αβ-dq transformation and dq-αβ transformation, respectively. S2, Collect grid current i s Grid current i s The DC current in the dq coordinate system is obtained by performing αβ-dq transformation through the coordinate transformation circuit (5) for single-phase AC current. I d and I q and direct current I d and I q The signal is fed into the PI-based direct current control circuit (6) as the negative feedback signal of the PI regulator; S3, Set theoretical value I d * and I q * , I d * This indicates the amount of active current that needs to be injected into the power grid. I q * This indicates the amount of reactive current that needs to be injected into the power grid. I d * and I q * The input to the PI-based direct current control circuit (6) is respectively connected to the negative feedback signal. I d and I q The two systems are compared, and their respective current error signals are obtained. The current error signals are then passed through a PI controller to obtain the new dq-axis DC current. I sd and I sq After inverse transformation by dq-αβ, a single-phase alternating current is obtained. i sa ; S4, sampling capacitor voltage u c The capacitor voltage is obtained after processing. u c fundamental component of u cb The specific process is as follows: sample capacitor voltage. u c Regarding capacitor voltage u c The DC current in the dq coordinate system is obtained by sequentially performing αβ-dq transformation and dq-αβ transformation. u cd and u cq Then the DC current u cd and u cq After passing through the low-pass filter in the capacitor feedback voltage low-pass filter processing circuit (7), the following results are obtained: u cd and u cq fundamental component of u cdb and u cqb The capacitor voltage is then obtained after inverse dq-αβ transformation. u c fundamental component of u cb ; S5. Determine the grid connection mode by using the given values ​​of active and reactive current in the high-frequency harmonic current compensation circuit, and select the required feedback current based on the grid connection mode. i RC The specific process is as follows: when I d * ≠0、 I q * When the voltage is 0, the grid is connected in pure active power mode, and the capacitor voltage is selected. u c As feedback current i RC ;when I d * =0、 I q * When the voltage is not equal to 0, the grid is connected in pure reactive power mode, and the capacitor voltage is selected. u c High-frequency harmonic components as feedback current i RC ;when I d * ≠0、 I q * When the voltage is not equal to 0, the grid is connected in active + reactive power mode, and the capacitor voltage is selected. u c High-frequency harmonic components as feedback current i RC ; S6, obtaining single-phase alternating current i sa Based on this, harmonic compensation current is injected in reverse into the PI-based direct current control circuit (6). i RC With single-phase alternating current i sa A comparison is made, and a new alternating current is obtained after the comparison. i s ’ The sinusoidal modulation signal is obtained after amplitude normalization. i s * , that is, the grid-connected current reference signal; S7. Obtain the sinusoidal modulation signal i s * The signal is fed into the multi-carrier PWM generator drive circuit (10), where eight SPWM signals are generated by the multi-carrier PWM generator. After isolation and driving, the signals control the switching on and off of each switching device in the single-phase current-type five-level inverter circuit (1).

2. The grid-connected harmonic suppression method based on a single-phase current source type five-level inverter according to claim 1, characterized in that: The high-frequency harmonic component is equal to the capacitor voltage. u c With fundamental component u cb difference.

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