Multi-mode switching control method suitable for grid-connected isolation inverter

Abstract

The invention discloses a multi-mode switching control method suitable for a grid-connected isolation inverter, and the method comprises the steps: in each half power grid voltage period, according to a power grid instantaneous voltage phase Theta g outputted by a phase-locked loop in the isolation inverter, the phase Theta g is within [0, pi], then the phase Theta g is compared with given power grid voltage phases Theta g1 and Theta g2, the Theta g1 and Theta g2 meet the condition that Theta g1 is smaller than or equal to Theta g2, and the Theta g1 is smaller than or equal to Theta g2; a variable-frequency discontinuous conduction mode, namely a variable-frequency DCM mode, is adopted in the ranges of 0 < theta g < theta g 1 and pi-theta g 2 < theta g < pi, a main switching tube of a flyback converter in the isolation inverter is controlled to be switched on when the period is larger than a given switching period and the drain-source voltage valley value, and a critical conduction mode, namely a BCM mode, is adopted in the range of theta g 1 < theta g < pi-theta g 2. And controlling a main switch tube of the flyback converter to be switched on at the drain-source voltage valley value. Different working modes are adopted under different output voltages, so that the switching loss of the isolation inverter in a full-load range is reduced, and the energy conversion efficiency of the isolation inverter is greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converter control, and in particular to a multi-mode switching control method suitable for a grid-connected isolation inverter. Background Art

[0002] In the grid connection of new energy and energy storage power stations, isolated converters play a very important role. Flyback converter + DC / AC polarity conversion circuit is one of the current mainstream isolated inverter topologies. The high-frequency flyback converter circuit mainly realizes electrical isolation and grid-connected current closed-loop control. It is stable and reliable, simple to control, reduces power loss and reduces costs. The polarity conversion circuit converts the industrial frequency sinusoidal half-wave current into industrial frequency full-wave current without high-frequency switching loss. The filter circuit then absorbs harmonics to obtain a grid-connected current that meets the grid connection requirements. The switching loss of MOSFET accounts for a large proportion of the losses of single-stage isolated inverters. Since the main switch tube of the traditional flyback inverter works in a hard switching state, there will be a large voltage overshoot when the switch is turned off, and the energy conversion efficiency is not high. Therefore, how to improve the efficiency is currently a research hotspot for flyback isolation inverters. Summary of the invention

[0003] The purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and to propose a multi-mode switching control method suitable for a grid-connected isolated inverter. The method has simple overall control, can reduce the voltage stress of the switch tube, can realize quasi-resonant soft switching, and adopts variable frequency discontinuous conduction mode control near the zero point of the grid voltage to improve the energy conversion efficiency. The switching efficiency is improved by realizing quasi-resonant soft switching and DCM variable frequency control.

[0004] To achieve the above object, the technical solution provided by the present invention is: a multi-mode switching control method suitable for a grid-connected isolated inverter, wherein in each half grid voltage cycle, according to the grid instantaneous voltage phase θ output by the phase-locked loop inside the isolated inverter g , where the phase θ g In [0, π], the phase with the given grid voltage θ g1 and θ g2 Comparing them, the two satisfy θ g1 ≤θ g2 , when 0<θ g <θ g1 and π-θ g2 <θ e In the range of θ<π, the variable frequency discontinuous conduction mode, namely the variable frequency DCM mode, is used to control the main switch tube of the flyback converter in the isolation inverter to be turned on when the period is greater than the given switching period and the drain-source voltage is at the valley value, and in θ g1 <θ g <π-θ g2Within the range, the critical conduction mode, namely the BCM mode, is adopted to control the main switch tube of the flyback converter to turn on at the valley value of the drain-source voltage.

[0005] Furthermore, the grid voltage phase θ at the switching time g1 and θ g2 Equal or unequal, and satisfy:

[0006]

[0007] Where N T is the transformer ratio of the flyback converter, V in is the input voltage of the isolation inverter, V gm is the output sinusoidal voltage amplitude.

[0008] Furthermore, the BCM mode adopts average current control or peak current control, and the control process includes the following steps:

[0009] S1, calculate the primary reference current i ref (θ g ): Let the reference current of the main switch be The parameters A and B in the formula are obtained from the following formula:

[0010]

[0011] Where, L tot ≈L m is the primary magnetizing inductance of the flyback converter transformer, C tot ≈C s is the equivalent absorption capacitance of the primary side of the flyback converter, P g To isolate the output power of the inverter, V in is the input voltage of the isolation inverter, V g is the effective value of the grid-connected voltage, N is the primary-to-secondary transformation ratio of the transformer;

[0012] S2. Calculate the delay time. In the equivalent circuit of the isolated inverter, the delay time t of the circuit can be obtained by resonance. d_BCM (θ g ) is equal to the unit delay time t d_T (θ g ):

[0013]

[0014] S3. Calculate the opening time t on (θ g ), the volt-second balance of the primary magnetizing inductance of the flyback converter can be obtained:

[0015]

[0016] S4. Calculate the off time t off (θ g ), the volt-second balance of the primary magnetizing inductance of the flyback converter can be obtained:

[0017]

[0018] S5, let the switching cycle T sw_BCM (θ g )for:

[0019] T sw_BCM (θ g ) = t on (θ g )+t off (θ g )+t d_BCM (θ g )

[0020] The above formula can be combined to obtain the turn-on time, turn-off time and delay time of the main switch tube, so that the main switch tube can be turned on at the valley value of the drain-source voltage, reducing the switching loss in the BCM mode.

[0021] Furthermore, the variable frequency DCM mode is implemented by average current control or peak current control. In the variable frequency DCM mode, the main switch tube is controlled to be turned on at the bottom of the drain-source voltage resonance while limiting the switching period to be greater than a certain limit value. The control process includes steps S1 to S4 and the following step S6:

[0022] S6. According to the reference current, delay time, on-time and off-time obtained in steps S1 to S4, the delay time t under the variable frequency DCM mode is obtained. d_DCM (θ g ) and the switching period T sw_DCM (θ g ):

[0023]

[0024] T sw_DCM (θ g ) = t on (θ g )+t off (θ g )+t d_DCM (θ g )

[0025] Where n is a counting variable with an initial value of 1, which is accumulated by 1 in the cycle judgment cycle; the switch cycle T is judged sw_DCM (θ g ) is less than the set minimum period T DCM_min , if it is less than the minimum period TDCM_min , then let: n = n + 1, repeat the judgment until the period is greater than the minimum period T DCM_min , control the main switch tube to conduct;

[0026] By combining the above formulas, the switching cycle meets the requirements, and the turn-on time, turn-off time and delay time of the main switch tube under the variable frequency DCM mode can be obtained, thereby realizing the variable frequency DCM mode and reducing the switching loss.

[0027] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0028] 1. Frequency conversion switch of variable frequency discontinuous conduction mode (i.e. variable frequency DCM mode) at both ends of the sinusoidal half wave.

[0029] The present invention controls the flyback converter to adopt a variable frequency discontinuous conduction mode with a lower switching frequency near the zero crossing point of the output sinusoidal voltage, so that the main switch tube works at a lower switching frequency, thereby reducing the voltage stress and switching loss of the main switch tube.

[0030] 2. Zero voltage switching is achieved.

[0031] The present invention designs circuit parameters, derives appropriate main switch on and off times, controls the main switch to be turned on at the valley value of the drain-source voltage, and achieves zero voltage turn-on of the main switch near the output sinusoidal peak, thereby greatly reducing the switching loss of the circuit and improving the energy conversion efficiency.

[0032] 3. Simple control method and excellent performance.

[0033] The multi-mode switching control method implemented in the present invention has efficient power conversion capability and stable output current waveform. The inverter is small in size, light in weight, has good reliability and electrical isolation performance, high in power generation efficiency, has obvious economic benefits, and is suitable for various types of isolated inverter systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Flow chart of the method of the present invention.

[0035] Figure 2 Schematic diagram of the interval of the method of the present invention.

[0036] Figure 3 Schematic diagram of the drain-source voltage and drain current of the main switch tube in half a grid cycle.

[0037] Figure 4 is the voltage V of the main switch tube in BCM mode ds and drain current I d Waveform diagram of .

[0038] Figure 5is the voltage V of the main switch tube in variable frequency DCM mode ds and drain current I d Waveform diagram of . DETAILED DESCRIPTION

[0039] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0040] like Figure 1 As shown, this embodiment discloses a multi-mode switching control method suitable for a grid-connected isolated inverter. The method is to control the instantaneous voltage phase θ of the grid output by the internal phase-locked loop of the isolated inverter in each half grid voltage cycle. g , where the phase θ g In [0, π], the phase with the given grid voltage θ g1 and θ g2 Comparing them, the two satisfy θ g1 ≤θ g2 , when 0<θ g <θ g1 and π-θ g2 <θ g In the range of θ<π, the variable frequency discontinuous conduction mode (i.e., variable frequency DCM mode) is adopted to control the main switch tube of the flyback converter in the isolation inverter to be turned on when the period is greater than the given switching period and when the drain-source voltage is at the valley value, and in θ g1 <θ g <π-θ g2 Within the range, the critical conduction mode (i.e., BCM mode) is adopted to control the main switch tube of the flyback converter to turn on at the drain-source voltage valley.

[0041] The isolation inverter samples the grid voltage and sends it to the phase-locked loop, and determines which working mode to adopt based on the sampled grid phase.

[0042] The grid voltage phase θ at the switching time of the above multi-mode switching control method is g1 and θ g2 Can be equal or unequal, and satisfy:

[0043]

[0044] Where N T is the transformer ratio of the flyback converter, V in is the input voltage of the isolation inverter, V gm is the output sinusoidal voltage amplitude, Figure 2 Schematic diagram of phase switching interval.

[0045] The BCM mode is used at the sinusoidal peak of the grid cycle. If the switching period T sw_BCM (θg )for:

[0046] T sw_BCM (θ g ) = t on (θ g )+t off (θ g )+t d_BCM (θ g )

[0047] According to the volt-second balance principle of the primary magnetizing inductance of the flyback converter:

[0048]

[0049] Where V in is the output voltage of the isolated inverter, V g is the effective value of the grid voltage, N is the transformer ratio in the flyback converter, then the delay time t d_BCM (θ g ) is equal to the unit delay time t d_T (θ g ):

[0050]

[0051] Where, L tot ≈L m is the primary magnetizing inductance of the flyback converter transformer, C tot ≈C s is the equivalent absorption capacitance of the primary side of the flyback converter circuit. Combining the following equations, we can get the turn-on time t on (θ g ) and the turn-off time t off (θ g )for:

[0052]

[0053] The reference current is The parameters A and B in the formula can be obtained as follows:

[0054]

[0055] Where P g To isolate the output power of the inverter, V in is the input voltage of the isolation inverter, V g is the effective value of the grid-connected voltage.

[0056] Combining the above formula, we can get the turn-on time, turn-off time and delay time, and the switching cycle T can be obtained by comprehensive arrangement. sw_BCM (θ g )for:

[0057]

[0058] The main switch tube is controlled to turn on at the valley value of the drain-source voltage. At this time, the voltage across the main switch tube is zero, thereby realizing quasi-resonant soft switching, reducing switching losses in the BCM mode, and greatly improving the energy conversion efficiency.

[0059] At both ends of the sinusoidal half-wave, the output power of the circuit is small. Controlling the main switch tube to work in the variable frequency DCM mode can enable the main switch tube to be turned on at the bottom of the drain-source voltage resonance. At the same time, the switching period is limited to be greater than a certain threshold, thereby reducing the switching frequency, reducing losses, and improving the efficiency of the isolation inverter.

[0060] Let the switching period T sw_DCM (θ g )for:

[0061] T sw_DCM (θ g ) = t on (θ g )+t off (θ g )+t d_DCM (θ g )

[0062] The reference current, turn-on time, and turn-off time of this mode are the same as those of the BCM mode, and the appropriate delay time can make the switching cycle greater than the threshold while driving the main switch to turn on at the drain-source voltage valley. Under quasi-resonant soft switching, the circuit loss can be reduced and the efficiency can be improved. The delay time t of the low-frequency switch of the main switch is d_DCM (θ g )for:

[0063]

[0064] In the formula, n is a counting variable with an initial value of 1, which is accumulated by 1 in the cycle judgment cycle. Then the switching cycle T sw_DCM (θ g )for:

[0065]

[0066] When the switching cycle is greater than the set minimum cycle T DCM_min That is, when the switching frequency is less than a given threshold, the main switch tube is controlled to turn on at the drain-source voltage valley, which can reduce the switching frequency and reduce the loss.

[0067] The turn-on time and turn-off time can be known, and the main switch tube can be controlled to turn on at the drain-source voltage valley, reducing losses and improving efficiency.

[0068] The simulation experiment was carried out in MATLAB / Simulink, and the running time was set to 0.08s. The input voltage of the simulated isolated inverter was 37V, the primary and secondary turns ratio of the flyback converter transformer was 1:6, the total resonant inductance of the primary side was 3uH and the total resonant capacitance was 151.26uF after conversion. The grid voltage amplitude was 311V, the frequency was 50Hz, and the output capacitance was 0.66uF. The control method adopted the average current method, and the delay time td0 under the BCM mode was 0.672us, which was also half the resonant period of the variable frequency DCM mode. The minimum switching period of the given variable frequency DCM mode was 10us. The mode switching parameters θ1 were set to 0.15 and θ2 to 2.97.

[0069] The drain-source voltage and drain current of the main switch tube of the isolation inverter in half a grid cycle are measured as follows: Figure 3 As shown, the voltage V of the main switch tube in BCM mode in this example is ds and drain current I d The waveform is as Figure 4 As shown, with the increase of transmission power, V ds The peak value of is also increasing. The voltage V at the main switch tube in variable frequency DCM mode ds The waveform and drain current T d The waveform is as Figure 5 In the variable frequency DCM mode, when the excitation current drops to zero, the primary circuit of the flyback inverter begins to resonate. Since the minimum period of the given main switch is 10us, the main switch is turned on only when the switching period of the main switch is greater than the threshold and the drain-source voltage is at the bottom of the resonance, which can reduce the switching loss of the switch.

[0070] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A multi-mode switching control method suitable for a grid-connected isolated inverter, characterized in that: This method is to use the grid instantaneous voltage phase θ output by the internal phase-locked loop of the isolation inverter in each half grid voltage cycle. g , where the phase θ g In [0,π], the phase with the given grid voltage θ g1 and θ g2 Comparing them, the two satisfy θ g1 ≤θ g2 , when 0<θ g <θ g1 and π-θ g2 <θ g In the range of θ<π, the variable frequency discontinuous conduction mode, namely the variable frequency DCM mode, is used to control the main switch tube of the flyback converter in the isolation inverter to be turned on when the period is greater than the given switching period and the drain-source voltage is at the valley value, and in θ g1 <θ g <π-θ g2 Within the range, the critical conduction mode, namely the BCM mode, is adopted to control the main switch tube of the flyback converter to turn on at the valley value of the drain-source voltage.

2. A multi-mode switching control method suitable for a grid-connected isolated inverter according to claim 1, characterized in that: The grid voltage phase θ at the switching moment g1 and θ g2 Equal or unequal, and satisfy: Where N T is the transformer ratio of the flyback converter, V in is the input voltage of the isolation inverter, V gm is the output sinusoidal voltage amplitude.

3. A multi-mode switching control method applicable to a grid-connected isolated inverter according to claim 2, characterized in that: The BCM mode adopts average current control or peak current control, and the control process includes the following steps: S1, calculate the primary reference current i ref (θ g ): Let the reference current of the main switch be The parameters A and B in the formula are obtained from the following formula: Where, L tot ≈L m is the primary magnetizing inductance of the flyback converter transformer, C tot ≈C s is the equivalent absorption capacitance of the primary side of the flyback converter, P g To isolate the output power of the inverter, V in is the input voltage of the isolation inverter, V g is the effective value of the grid-connected voltage, N is the primary-to-secondary transformation ratio of the transformer; S2. Calculate the delay time. In the equivalent circuit of the isolated inverter, the delay time t of the circuit can be obtained by resonance. d_BCM (θ g ) is equal to the unit delay time t d_T (θ g ): S3. Calculate the opening time t on (θ g ), the volt-second balance of the primary magnetizing inductance of the flyback converter can be obtained: S4. Calculate the off time t off (θ g ), the volt-second balance of the primary magnetizing inductance of the flyback converter can be obtained: S5, let the switching cycle T sw_BCM (θ g )for: T sw_BCM (i g )=t on (i g )+t off (i g )+t d_BCM (i g ) The above formula can be combined to obtain the turn-on time, turn-off time and delay time of the main switch tube, so that the main switch tube can be turned on at the valley value of the drain-source voltage, reducing the switching loss in the BCM mode.

4. A multi-mode switching control method applicable to a grid-connected isolated inverter according to claim 3, characterized in that: The variable frequency DCM mode is implemented by average current control or peak current control. In the variable frequency DCM mode, the main switch tube is controlled to be turned on at the bottom of the drain-source voltage resonance while limiting the switching period to be greater than a certain limit value. The control process includes steps S1 to S4 and the following step S6: S6. According to the reference current, delay time, on-time and off-time obtained in steps S1 to S4, the delay time t under the variable frequency DCM mode is obtained. d_DCM (θ g ) and the switching period T sw_DCM (θ g ): T sw_DCM (i g )=t on (i g )+t off (i g )+t d_DCM (i g ) Where n is a counting variable with an initial value of 1, which is accumulated by 1 in the cycle judgment cycle; the switch cycle T is judged sw_DCM (θ g ) is less than the set minimum period T DCM_min , if it is less than the minimum period T DCM_min , then let: n=n+1, repeat until the period is greater than the minimum period T DCM_min , control the main switch tube to conduct; By combining the above formulas, the switching cycle meets the requirements, and the turn-on time, turn-off time and delay time of the main switch tube under the variable frequency DCM mode can be obtained, thereby realizing the variable frequency DCM mode and reducing the switching loss.

Images