CN113541522B - A control method for realizing full-range soft switching of four-quadrant operation of three-phase inverter - Google Patents

Abstract

The invention relates to a control method for realizing four-quadrant operation full-range soft switching of a three-phase inverter, which comprises the following steps of constructing a converter system, wherein the converter system comprises a three-phase inverter topology, a digital controller, a sampling circuit and a driving circuit; decomposing the phase current of the load to obtain components of a d axis and a q axis for current closed-loop control; DPWM is adopted for modulation to obtain a modulation wave; sampling the load phase current, the load phase voltage and the direct current bus voltage, and updating the carrier period in real time by the digital controller; the driving circuit drives the corresponding switching devices to realize soft switching in the whole range of all the switching devices. The critical switching frequency curve of the soft switch is realized by analyzing the switching devices under different power factors, and the minimum critical switching frequency curve is used as the switching frequency curve of all the switching devices for realizing the soft switch, so that the problem of high switching loss of the three-phase inverter when the three-phase inverter operates under different power factors is solved, and the switching frequency is higher.

Description

A control method for realizing full-range soft switching of three-phase inverter four-quadrant operation

technical field

The invention belongs to the non-isolated high-frequency power conversion direction in the technical field of power electronics, and particularly relates to a control method for realizing full-range soft switching of four-quadrant operation of a three-phase inverter.

Background technique

Three-phase inverters are widely used in various industrial equipment and civil devices, such as photovoltaic systems, static var generators or AC asynchronous motors, etc. Most of these applications need to consume reactive power, so three-phase inverters are required. The inverter can have the ability to generate or absorb reactive power, that is, the three-phase inverter is required to work under the condition of non-unity power factor. In the traditional three-phase inverter, since half of the switching devices are in a hard switching state, the switching loss of the switching devices is large, so the switching frequency of the switching devices is low. Due to the lower switching frequency of the switching devices and the larger size of the passive components, the power density of the inverter is also lower. The soft switching technology can make the switching device work in a higher frequency environment, thereby greatly reducing the volume and weight of passive components, ensuring that the efficiency and power density of the inverter can be at a higher level.

The soft switching of traditional three-phase inverter switching devices is mostly realized with the help of auxiliary network, which can be divided into resonant DC loop (RDCL) and auxiliary resonant commutator pole (ARCP) according to the location of the auxiliary network. Although the zero-voltage turn-on (ZVS) of the main switching device can be achieved with the aid of the auxiliary network, due to the increase in the number of passive components and switching devices in the auxiliary network, the control strategy of the inverter will become complicated and the power of the entire inverter will be increased. Density also cannot be kept at a high level. For soft switching of the upper and lower switching devices on the same bridge arm, the most direct method is to increase the inductor current ripple on the inverter side, so that the inverter inductor current can flow in both directions within one switching cycle, so as to satisfy the The soft-switching requirements of the upper and lower switching devices on the same bridge arm, the aforementioned methods for realizing soft-switching have been used by scholars in single-phase inverters or power factor correction (PFC) circuits. For a three-phase inverter, since the three-phase currents are coupled and the inductor current ripple of each phase is affected by the remaining two phases, it is difficult to achieve soft switching of the switching devices of the three-phase inverter.

In view of the coupling problem of three-phase currents of three-phase inverters, some scholars propose to connect the neutral point of the filter capacitor with the neutral point of the DC bus, thereby realizing the decoupling between the three-phase currents, but the three-phase current The sum is always zero, so a special modulation strategy is also required to make the control of the three-phase inverter easier.

"Critical-Conduction-Mode-Based Soft-Switching Modulation for Three-Phase PV Inverters With Reactive Power Transfer Capability" discloses a three-phase photovoltaic inverter reactive power transfer soft-switching technology based on current critical conduction mode, The decoupling of three-phase current is realized by discontinuous pulse width modulation (DPWM), and the soft switching of switching devices is realized by CRM (critical conduction mode) control method, but the variation range of switching frequency is wide; in order to reduce the variation of switching frequency range, the document adopts the frequency synchronization (Fs sync) method. Since the zero-crossing time of the inductor current on the three-phase inverter side is not synchronous, the switching device corresponding to the first zero-crossing phase and the switching device corresponding to the second zero-crossing phase are controlled to be turned off at the same time. Although the frequency synchronization can be achieved under this control method , but will cause the current to appear intermittent. In the case of non-unity power factor, this paper only analyzes the power factor from leading 0.8 to lagging 0.8, and soft switching of switching devices under the full power factor range needs to be realized.

SUMMARY OF THE INVENTION

Aiming at the deficiencies of the prior art, the technical problem to be solved by the present invention is to provide a control method for realizing full-range soft switching of four-quadrant operation of a three-phase inverter.

The technical scheme adopted by the present invention to solve the technical problem is:

A control method for realizing full-range soft switching of four-quadrant operation of a three-phase inverter, characterized in that the method comprises the following steps:

Step 1. Build a converter system. The converter system includes a three-phase inverter topology, a digital controller, a sampling circuit and a drive circuit; the three-phase inverter topology includes a DC bus capacitor, a three-phase half-bridge circuit and an LCL filter. , each phase bridge arm contains two switching devices with complementary driving signals;

Step 2: Decompose the load phase current to obtain d-axis and q-axis components, which are used for current closed-loop control; use DPWM modulation to obtain modulated waves;

Step 3: The sampling circuit samples the load phase current, the load phase voltage and the DC bus voltage, and the digital controller updates the carrier cycle in real time according to formula (14); the corresponding switching devices are driven by the driving circuit to realize all switching devices at any power factor. soft-switching in the full range below;

Among them, V _rms is the RMS value of the load phase voltage, M is the modulation ratio, I _rms is the RMS value of the load phase current, I _bias is the bias current, and L ₁ is the inverter side inductance.

The expression of the inverter side inductance L ₁ is:

In formula (15), f _smin is the minimum switching frequency.

In DPWM modulation, one power frequency cycle includes six sectors, each sector occupies 60°, and the modulated wave of each sector consists of two parts, and the midpoint of the sector is the dividing point; among them, the first sector The modulated wave expressions in the zones 0-30° and 30°-60° are equations (5) and (6), respectively:

Among them, m _a , m _c represent a and c-phase modulated waves, m _b1 , m _b2 represent b-phase modulated waves at 0 to 30° and 30° to 60°, and the values of sita ₁ and sita ₂ are 0 to 60°, respectively. 30° and 30°～60°;

The three-phase critical switching frequency expressions of the first sector at 0 to 30° and 30° to 60° are equations (8) and (9), respectively:

In equations (8) and (9), f _sa and f _sc represent the critical switching frequencies of phases a and c, f _sb1 and f _sb2 represent the critical switching frequencies of phase b at 0° to 30° and 30° to 60°, and V _dc is the DC bus voltage, v _a , v _b , _vc are the instantaneous values of the load phase voltage, i _a2 , i _b2 , and i _c2 are the instantaneous values of the load phase current;

The switching frequency curve under each power factor has the same minimum value at both ends of the sector, and the minimum value among all the minimum values is taken as the switching frequency of the switching device to meet the requirements of all switching devices under any power factor. soft switching requirements.

Compared with the prior art, the beneficial effects of the present invention are:

1. In the present invention, through the soft switching method based on current pulsation prediction, all switching devices can achieve full-range soft switching without an auxiliary network. In addition, under the current ripple prediction soft switching method, the inductor current on the inverter side flows in both directions, so the current ripple is large, so as to ensure that the switching device can achieve soft switching, and has the characteristics of high switching frequency, which reduces the passive components. The volume and weight enable the power density of the three-phase inverter to be kept at a high level.

2. In order to ensure that all switching devices can achieve full-range soft switching, the present invention adopts discontinuous five-segment space vector modulation, and analyzes the critical switching frequency curve of switching devices to achieve soft switching under different power factors, and finds the minimum The critical switching frequency curve is the lowest critical switching frequency curve, and the lowest critical switching frequency curve is used as the switching frequency curve for all switching devices to achieve soft switching. The efficiency of the device is kept at a high level.

3. For specific working conditions (such as DC bus voltage, grid-side phase voltage and output power have been determined), the switching frequency will not change with the switching of the power factor. Specifically, whether it is jumping between power factors under inductive load (such as switching power factor from 0.8 to 0.5), or switching between power factors under capacitive load (such as switching power factor from -0.8 to -0.5) Still jumping each other under inductive and capacitive loads (such as switching the power factor from -0.8 to 0.5 or 0.5 to -0.8), the switching frequency is constant. After the DC bus voltage, grid-side phase voltage and power factor are determined, when switching under different power levels, the switching frequency will decrease with the increase of output power.

4. The present invention does not need to sample the inductor current on the inverter side, nor does it need an auxiliary network to implement soft switching, and the volume of passive components is relatively small, so the cost of the three-phase inverter is low.

Description of drawings

Fig. 1 is the topology structure diagram of the three-phase inverter of the present invention;

2 is a schematic diagram of the discontinuous five-segment space vector modulation of the present invention;

FIG. 3 is a graph of switching frequency in the first sector under unity power factor;

FIG. 4 is a graph of switching frequency in the first sector under different power factors;

Figure 5 is a graph of the switching frequency in the first sector with a power factor of 0.8;

FIG. 6 is a graph of the critical switching frequency in the first sector under different power factors;

Fig. 7 is a power factor of 0.8, a three-phase inverter side inductor current waveform diagram and a three-phase grid current waveform diagram within one power frequency cycle.

Detailed ways

The technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments, but this does not limit the protection scope of the present application.

The present invention relates to a control method (method for short) for realizing full-range soft switching of four-quadrant operation of a three-phase inverter, and the method comprises the following steps:

Step 1. Build a control system. The control system includes a three-phase inverter topology, a digital controller, a sampling circuit and a drive circuit; the three-phase inverter topology includes a DC power supply, a three-phase half-bridge circuit and a filter. The bridge circuit includes two switching devices with complementary driving signals;

Step 2, decomposing the load phase current; in order to realize the decoupling of the three-phase current, the discontinuous five-segment space vector (DPWM) modulation is adopted;

Use formula (1) to calculate the d-axis and q-axis components of the load phase current;

Among them, γ is the phase angle of the load phase voltage; i _α and i _β are the α-axis and β-axis components of the load phase current, which satisfy formula (2):

In formula (2), the expressions of the instantaneous values i _a2 , i _b2 and i _c2 of the load phase current are:

为负载相电压与负载相电流之间的相位差；I_rms为负载相电流的RMS(均方根)值，其表达式为：where θ is the sector angle,

is the phase difference between the load phase voltage and the load phase current; I _rms is the RMS (root mean square) value of the load phase current, and its expression is:

In formula (4), S is the output power, V _rms RMS value of load phase voltage;

As shown in Figure 2, a power frequency cycle contains six sectors, each sector occupies 60°, the modulated wave of each sector consists of two parts, and the midpoint of the sector is the dividing point; The amplitude of one-phase modulated wave is 0 or 1, so it is only necessary to control the other two-phase switching devices; taking the first sector as an example, at 0-30° of the first sector, the modulated wave of the a-phase It is fixed to zero, so the switching states of the two switches of phase a are constant; at 30° to 60° of the first sector, the modulation wave of phase c is fixed to 1, so the switching states of the two switches of phase c are constant; Changes in phase sequence, the characteristics of modulated waves in other sectors are similar to those in the first sector, and will not be repeated here;

The modulated wave expressions of the first sector 0-30° and 30°-60° are respectively equations (5) and (6):

Among them, m _a , m _c represent a and c-phase modulated waves, m _b1 , m _b2 represent b-phase modulated waves at 0 to 30° and 30° to 60°, and the values of sita ₁ and sita ₂ are 0 to 60°, respectively. 30° and 30°～60°; M is the modulation ratio, and its expression is:

Among them, V _dc is the DC bus voltage;

The three-phase critical switching frequency expressions of the first sector at 0 to 30° and 30° to 60° are equations (8) and (9), respectively:

In equations (8) and (9), f _sa and f _sc represent the critical switching frequencies of phases a and c, and f _sb1 and f _sb2 represent the critical switching frequencies of phase b at 0° to 30° and 30° to 60°; I _bias is the bias current, which is determined according to the output capacitance of the switch tube; v _a , v _b , and v _c are the instantaneous values of the load phase voltage, and their expressions are:

In the same way, the modulated waves of the remaining sectors can be obtained, which belongs to the conventional technical means in the art, and will not be repeated here.

Step 3, using the sampling circuit to sample the load phase current, load phase voltage and DC bus voltage, and the digital controller updates the carrier cycle;

For formulas (8) to (9), it is applicable to both unity power factor and non-unity power factor; Figure 3 shows the three-phase switching frequency of the first sector obtained under the unity power factor under the current ripple prediction soft-switching method In the graph, there is a critical switching frequency curve with a lower frequency, and the frequency of this critical switching frequency curve is always the smallest in the entire sector, so this critical switching frequency curve can be used as the switching frequency curve to realize all switching devices. Full range of soft switches;

As shown in Figure 4, by analyzing the switching frequency curve in the first sector by implementing soft switching of each phase bridge arm under different power factors, it can be seen that the switching frequency curve under non-unitary power factor is no longer symmetrical about the midpoint, but Each switching frequency curve has the same minimum value at both ends of the sector, and the minimum value under different power factors is different; in order to meet the soft switching requirements of all switching devices under any power factor, all the minimum values The minimum value is taken as the switching frequency of the switching device. Figure 5 shows the switching frequency curve in the first sector with a power factor of 0.8. Similar to the switching frequency curve under unity power factor, the switching frequency curve in the first sector under non-unitary power factor also exists by f _sb1 The critical switching frequency curve formed by and f _sa , Figure 6 is the critical switching frequency curve diagram in the first sector under different power factors;

的取值为60°；将sita₁＝0代入式(5)m_b1的表达式，则有：It can be seen from Figure 6 that when the power factor is 0.5, there is a minimum value in the minimum value of the switching frequency. At this time, the values of θ and sita ₁ are both zero.

The value of is 60°; substituting sita ₁ =0 into the expression of formula (5)m _b1 , there are:

代入式(3)i_b2的表达式，则有：Set θ=0,

Substitute into the expression of formula (3)i _b2 , there are:

Substitute θ=0 into the expression of formula (10)v _b , there are:

Substitute equations (11) to (13) into the f _sb1 expression of equation (8), and after simplification, the general switching frequency formula of equation (14) is obtained:

In formula (14), L ₁ is the inverter side inductance, and its expression is:

In formula (15), f _smin is the minimum switching frequency;

The sampling circuit collects the load phase voltage, load phase current and DC bus voltage in real time and inputs them to the digital controller. The digital controller updates the carrier cycle in real time according to formula (14), and drives the corresponding switching devices through the driving circuit to realize the full range of all switching devices. soft switch inside.

Example 1

As shown in Figure 1, this embodiment uses a two-level three-phase voltage source inverter topology, and the filter is an LCL filter; the switching device uses SiC MOSFET, the model is C3M0060065k; the battery is used as the DC power supply of the three-phase inverter. , the DC bus voltage V _dc is 400V; the load is the grid, the RMS value of the load phase voltage V _rms is 110V, the rated power is 3300VA, and the minimum switching frequency f _smin is 100kHz. When the three-phase inverter is under non-unity power factor condition, the analysis under inductive load and capacitive load is similar, and the inductive load is used as an example to illustrate.

The filter capacitor of the LCL filter is C, the AC side inductance is L ₂ , the bias current I _bias is 2A, and the inverter side inductance L ₁ is calculated by formula (15) to be 10 μH; for the filter capacitor C, the control flow filter The reactive current of the capacitor C is less than 2% of the active current, and the value of the filter capacitor C is 4.7μH. Considering the voltage drop of the AC side inductor L ₂ and the current ripple of the inverter side and AC side inductors, the value of the AC side inductor L ₂ is 25μH, and the model of the digital controller is TMS320F28379S.

The operation structure of the three-phase inverter is simulated under all the above parameters, and the power factor is 0.8 as shown in Figure 7, the inductor current waveform of the three-phase inverter side in one power frequency cycle and the three-phase power grid are obtained. Current (AC side current) waveform diagram. As can be seen from the figure, whether it is the three-phase inverter side inductor current or the AC side current, except for the phase difference, the three-phase waveforms are the same, and the inverter side inductor current ripple is about 2 to 3 times the peak value of the AC side current. times, the inductor current ripple on the inverter side is large, and the soft switching of the switch tube can be realized. At any time, the inductor current on the inverter side is greater than the bias current (2A). In addition, the inductor current on the inverter side is greater than the set bias current before and after dynamic switching, so all switches can achieve full-range soft switch.

What is not described in the present invention applies to the prior art.

Claims (3)

1. A control method for realizing full-range soft-switching of three-phase inverter four-quadrant operation, characterized in that the method comprises the following steps: Step 1. Build a converter system. The converter system includes a three-phase inverter topology, a digital controller, a sampling circuit and a drive circuit; the three-phase inverter topology includes a DC bus capacitor, a three-phase half-bridge circuit and an LCL filter. , each phase bridge arm contains two switching devices with complementary driving signals; Step 2: Decompose the load phase current to obtain d-axis and q-axis components, which are used for current closed-loop control; use DPWM modulation to obtain modulated waves; Step 3: The sampling circuit samples the load phase current, the load phase voltage and the DC bus voltage, and the digital controller updates the carrier cycle in real time according to formula (14); the corresponding switching device is driven by the driving circuit to realize the full range of all switching devices. soft switch;

Among them, V _rms is the RMS value of the load phase voltage, M is the modulation ratio, I _rms is the RMS value of the load phase current, I _bias is the bias current, and L ₁ is the inverter side inductance. 2. The control method for realizing full-range soft switching of three-phase inverter four-quadrant operation according to claim 1, wherein the expression of the inverter side inductance L ₁ is:

In formula (15), f _smin is the minimum switching frequency. 3. The control method for realizing full-range soft-switching of three-phase inverter four-quadrant operation according to claim 1, characterized in that, in DPWM modulation, one power frequency cycle includes six sectors, and each sector occupies six sectors. 60°, the modulated wave of each sector is composed of two parts, and the midpoint of the sector is used as the dividing point; among them, the modulated wave expressions of the first sector 0-30° and 30°-60° are respectively ( 5), (6):

Among them, m _a , m _c represent a and c-phase modulated waves, m _b1 , m _b2 represent b-phase modulated waves at 0 to 30° and 30° to 60°, and the values of sita ₁ and sita ₂ are 0 to 60°, respectively. 30° and 30°～60°; The three-phase critical switching frequency expressions of the first sector at 0 to 30° and 30° to 60° are equations (8) and (9), respectively:

In equations (8) and (9), f _sa and f _sc represent the critical switching frequencies of phases a and c, f _sb1 and f _sb2 represent the critical switching frequencies of phase b at 0° to 30° and 30° to 60°, and V _dc is the DC bus voltage, v _a , v _b , _vc are the instantaneous values of the load phase voltage, i _a2 , i _b2 , and i _c2 are the instantaneous values of the load phase current; The switching frequency curve under each power factor has the same minimum value at both ends of the sector, and the minimum value among all the minimum values is taken as the switching frequency of the switching device to meet the requirements of all switching devices under any power factor. soft switching requirements.

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