CN108448923B - Frequency conversion control method for realizing soft switch of three-phase inverter - Google Patents
Frequency conversion control method for realizing soft switch of three-phase inverter Download PDFInfo
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- CN108448923B CN108448923B CN201810309735.0A CN201810309735A CN108448923B CN 108448923 B CN108448923 B CN 108448923B CN 201810309735 A CN201810309735 A CN 201810309735A CN 108448923 B CN108448923 B CN 108448923B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0003—Details of control, feedback or regulation circuits
- H02M1/0012—Control circuits using digital or numerical techniques
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0048—Circuits or arrangements for reducing losses
- H02M1/0054—Transistor switching losses
- H02M1/0058—Transistor switching losses by employing soft switching techniques, i.e. commutation of transistors when applied voltage is zero or when current flow is zero
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- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
- Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
- Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes
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Abstract
The invention discloses a frequency conversion control method for realizing soft switching of a three-phase inverter, belonging to the non-isolated high-frequency power conversion direction in the field of power electronics. The invention predicts the three-phase inverter side inductance current pulsation by using the sampled three-phase inverter grid current and voltage and the calculated modulation wave information. In each space vector sector, the switching frequency is adjusted to ensure that the magnitude of the current pulsation of the phase of the inductance which is the most difficult to realize the soft switching in the three-phase inverter is kept in a state of just realizing the soft switching, thereby reducing the switching loss. In this case, soft switching of the other phase is performed most of the time. The last phase has a constant switching state in the whole sector, and soft switching is not required. The invention aims to solve the technical problems of realizing wide-range soft switching of all switching tubes of a three-phase inverter through full-digital frequency conversion control, reducing current pulsation and circulating current loss of the three-phase inverter under light load and improving the conversion efficiency and power density of the three-phase inverter.
Description
Technical Field
The invention relates to a frequency conversion control method applied to a three-phase voltage source type inverter (rectifier), belonging to the non-isolated high-frequency power conversion direction in the field of power electronics.
Background
Three-phase voltage source inverters are widely applied to various industrial equipment and civil devices, such as Uninterruptible Power Supplies (UPS), electric vehicle charging piles, photovoltaic grid-connected systems and the like. The most common topology is a two-level three-phase bridge circuit. The novel rectifier has a simple and symmetrical structure, can realize seamless switching between inversion and rectification, and is mature and reliable in control nowadays through long-time development. However, for the upper and lower switching tubes of each bridge arm, only one of the switching tubes can realize soft switching-on at any moment according to the flow direction of the inductive current of the switching tube, which greatly increases the switching loss. In high power applications, the switching frequency has to be set very low to reduce the switching losses. By doing so, the alternating current distortion rate is increased, and a filter is required to be added for compensation, so that the dynamic response speed of the inverter is reduced, the whole system is large in size, and the power density is reduced.
In order to reduce the switching loss of the inverter, researchers have proposed a number of methods for soft switching of the inverter, including adding an auxiliary circuit to each leg on the dc side or the ac side, so that the direction of the superimposed current can be changed at each switching cycle. When the upper tube is switched on, inductive current flows into the middle point of the bridge arm, and when the lower tube is switched on, the inductive current flows out of the middle point of the bridge arm, so that the upper tube and the lower tube can realize soft switching. The method of adopting the auxiliary circuit can increase the cost and the volume of the inverter and generate extra loss, so the method without the auxiliary circuit is simpler, more reliable and more efficient.
The most direct method is to design the inductance value at the inverter side to be very small, and increase the current ripple to naturally realize soft switching. However, in order to avoid unnecessary excessive current ripple under different loads, frequency conversion control is required to reduce the circulating current loss. The method is used in a single-phase inverter, and the switching-on and switching-off of a switching tube are controlled by adopting an inductive current zero-crossing detection circuit. For example: IEEE Journal of emitting and Selected Topics in Power electronics 'Digital-Based interleaving Control for GaN-Based MHz CRM (transistor-capacitor power factor controller)' published in 2016, a method for detecting zero crossing of current is adopted to realize soft switching and frequency conversion Control of the device, two bridge arm interleaving controls can counteract partial current pulsation, and switching loss is very small even if the switching frequency reaches MHz.
The method can be applied to a single-phase inverter, however, in a three-phase inverter, because three-phase currents are mutually coupled, the current waveform is influenced by the switching state of each bridge arm, and it is difficult to ensure that all switching tubes can realize soft switching at the same time. Therefore, a special modulation mode is needed, so that only two bridge arms are in a high-frequency switching state at the same time, and one bridge arm keeps the switching state constant. Therefore, whether the switch tube in the high-frequency switch can realize soft switching or not only needs to be considered. In "Critical-mode-based soft-switching modulation for three-phase inverters" published in 2017 at ieee Energy Conversion Convergence and Exposure (ECCE) ("energy Conversion society)," [ Critical current mode switching modulation for three-phase inverters ] a current zero-crossing detection method is still adopted, but since three-phase switching frequencies are uniform, it is necessary to select one phase to synchronize the switching frequencies. The modulation mode is DPWM (discontinuous space vector), and the bridge arm interleaving mode is still adopted. However, complexity is increased due to the need for both digital control and analog zero-crossing detection circuitry.
Disclosure of Invention
The invention aims to solve the problem that the traditional three-phase voltage source inverter (because the inverter can generally realize the rectification function, the inverter can be only written as the inverter) can not realize the wide-range soft switching of all switching tubes during the unit power factor operation, and provides a frequency conversion control method for realizing the soft switching of the three-phase inverter.
The purpose of the invention is realized by the following technical scheme.
The invention discloses a frequency conversion control method for realizing soft switching of a three-phase inverter, which predicts the side inductance current pulsation of the three-phase inverter by using the three-phase power grid current and voltage obtained by sampling and the calculated modulation wave information. In each space vector sector, the switching frequency is adjusted to ensure that the magnitude of the current pulsation of the phase of the inductance which is the most difficult to realize the soft switching in the three-phase inverter is kept in a state of just realizing the soft switching, thereby reducing the switching loss. In this case, soft switching of the other phase is performed most of the time. The last phase has a constant switching state in the whole sector, and soft switching is not required.
The invention discloses a frequency conversion control method for realizing three-phase inverter soft switching, which is used for realizing three-phase inverter soft switching. The three-phase inverter needs to operate in a unity power factor inversion or rectification mode. The three-phase inverter is used as a main circuit of a three-phase inverter control system, and the three-phase inverter control system further comprises a digital controller, a sampling circuit and a driving circuit. The sampling circuit is used for sampling three-phase voltage and current instantaneous value v on the alternating current sidea,vb,vc,ia,ib,ic。
Preferably, the filter is of LCL structure, in which three phases are invertedA transformer side inductance of L1A capacitance value of C and an inductance value of L on the AC side2。
The invention discloses a frequency conversion control method for realizing soft switching of a three-phase inverter, which comprises the following steps:
step one, sampling three-phase power grid voltage va，vb，vcAnd three-phase network current ia，ib，icAnd filtering the instantaneous value after the switching ripple wave. Due to the symmetry of the power grid, in order to reduce cost, only any two-phase value in three-phase power grid voltage and current needs to be sampled, and the other phase voltage and current can be obtained through other two phases. And tracking the voltage phase of the power grid in real time through a digital control circuit.
And step two, d/q axis conversion is carried out on the three-phase power grid current, and closed-loop control is carried out on the grid-connected current by adopting a digital controller. In order to avoid the resonance problem of the LCL filter, active damping control, namely a band-pass digital filter, can be adopted to filter out the three-phase modulation wave ma,mb,mcOf the resonant frequency component. And five-segment space vector modulation is adopted. Wherein the five-segment space vector modulation can be selected from one of the following two methods: the zero vector of the first type is that the upper tube of the three-phase bridge is completely conducted, and the zero vector of the second type is that the lower tube of the three-phase bridge is completely conducted. Three-phase modulation wave information m obtained by utilizing internal closed-loop control of digital controllera,mb,mcCalculating the switching frequency fsThe calculation formula is
Wherein IbiasThe bias current of the soft switch is determined according to the output capacitance of the switch tube. For the first five-segment space vector modulation, the state of each sector is shown in table one,
table-phase states of a first space vector
| Sector area | 1 | 2 | 3 | 4 | 5 | 6 |
| x corresponding phase | c | c | a | a | b | b |
| Phase-invariant of switch state | a | b | b | c | c | a |
| Soft switch partial loss phase | b | a | c | b | a | c |
For the second type of five-segment space vector modulation, the state of each sector is shown in table two,
table two second space vector phase states
Step three, adjusting carrier frequency in a digital controller of the digital control circuit, wherein the carrier frequency is the calculated switching frequency f of the step twosModulation ratio independent of switching frequency fsInfluence. And transmitting the obtained switching signals to a driving circuit and driving corresponding switching tubes, namely realizing wide-range soft switching of all the switching tubes by full-digital frequency conversion control.
Has the advantages that:
1. the invention discloses a frequency conversion control method for realizing the soft switch of a three-phase inverter, which realizes the soft switch of the three-phase inverter through frequency conversion control, so that the need of an additional auxiliary circuit, a sensor and a current zero-crossing detection circuit is avoided.
2. The invention discloses a frequency conversion control method for realizing soft switching of a three-phase inverter, and switching frequency fsGiven by the proposed frequency calculation formula (1), it can be seen from the calculation formula (1) that, under any fixed operating conditions, the switching frequency f within one power frequency periodsThe variation range is only about 1.5 times, and the device is suitable for working under wide load and wide voltage range.
4. The invention discloses a frequency conversion control method for realizing soft switching of a three-phase inverter, which can greatly reduce the switching loss of the three-phase inverter, improve the conversion efficiency and further improve the switching frequency because wide-range soft switching of all switching tubes of the three-phase inverter is realized and the three-phase inverter has one-phase bridge arm switching tube to be inactive in a half power frequency period.
5. The invention discloses a frequency conversion control method for realizing soft switching of a three-phase inverter, which can improve the cut-off frequency of the inverter by increasing the current pulsation by adopting the inductance value of a smaller filter, and greatly improve the switching frequency, so that the dynamic response effect of the system is better, the stability is not influenced by frequency conversion control, and the normal work of the inverter and the realization of the soft switching can still be ensured under the conditions of sudden load addition and sudden load reduction.
Drawings
FIG. 1 is a main circuit of a three-phase voltage source inverter;
FIG. 2 is a block diagram of the frequency conversion control of the soft switch of the three-phase inverter;
FIG. 3 shows a trend of switching frequency variation within a power frequency period;
FIG. 4 is a graph of the relationship between the highest and lowest switching frequencies within a power frequency period and the inverter power and DC voltage;
fig. 5 is a typical waveform over a power frequency period.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and examples. The technical problems and the advantages solved by the technical solutions of the present invention are also described, and it should be noted that the described embodiments are only intended to facilitate the understanding of the present invention, and do not have any limiting effect.
Example 1:
in the embodiment, the storage battery is a direct-current power supply of a three-phase inverter, the direct-current voltage of the storage battery is 350-400V, the alternating-current power grid is 110V in a three-phase voltage effective value, the rated power is 3500W, unit power factor grid-connected charging and discharging can be achieved, and the storage battery can be used for an uninterruptible power supply or an electric vehicle charger.
The main circuit structure of the three-phase inverter is shown in fig. 1. The three-phase half-bridge switching device comprises 6 switching tubes, and SiC devices are selected for all the switching tubes. The middle points of the three-phase bridge are respectively connected with the LCL filter. Inverter side inductance value of L1Capacitance value is C, and grid side inductance value is L2. Design of the lowest switching frequency fsWith a limit of 100 kHz. Therefore, the inverter has the switching frequency f at the lowest voltage of 350V and the maximum load of 3500WsShould be 100kHz, L can be obtained1The inductance value of (3) is 10 muH. The capacitance value C is required to be as small as possible so as to reduce reactive current, reactive power is generally designed to be less than 2% of active power, and in practice, a high-frequency capacitor of 4.7 mu F is selected as the capacitor. The grid side inductor aims at reducing the distortion of grid-connected current, and the inductance value L is required to be lower than the current distortion rate of 5 percent due to higher switching frequency2Only 20 muh is required. Due to the adoption of frequency conversion control, in practice, the current pulsation is far less than 5%. In this embodiment, the digital controller is selected as a TMS320F28335 chip.
Fig. 2 is a block diagram of frequency conversion control for implementing soft switching of a three-phase inverter, which is disclosed in this embodiment, and the method for implementing frequency conversion control for implementing soft switching of a three-phase inverter disclosed in this embodiment includes the following specific implementation steps:
step one, sampling three-phase power grid voltage va，vb，vcAnd three-phase network current ia，ib，icThe instantaneous value after the switching frequency component is filtered out. The grid voltage is phase-locked by a digital controller.
And step two, d/q axis conversion is carried out on the three-phase power grid current, and closed-loop control is carried out on the grid-connected current by adopting a digital controller. In order to avoid the resonance problem of the LCL filter, active damping control, namely a band-pass digital filter, can be adopted to filter out the three-phase modulation wave ma,mb,mcOf the resonant frequency component. And five-segment space vector modulation is adopted. Wherein the five-segment space vector modulation can be selected from one of the following two methods: the zero vector of the first type is that the upper tube of the three-phase bridge is completely conducted, and the zero vector of the second type is that the lower tube of the three-phase bridge is completely conducted. Three-phase modulation wave information m obtained by utilizing internal closed-loop control of digital controllera,mb,mcThe switching frequency f can be calculated according to equation (1)sThe calculation formula is
Wherein IbiasThe soft switch bias current I is used for realizing the bias current of the soft switch, considering that the junction capacitance of a selected device is small, but in order to ensure that the soft switch can not be lost when the load is dynamically cut off, the bias current IbiasDesigned to be 2A. The corresponding state of each sector is determined according to the first table and the second table.
Step three, adjusting the carrier frequency in the digital controller into the calculated switching frequency f of the step twosModulation ratio independent of switching frequency fsInfluence. The obtained switching signals are transmitted to a driving circuit and drive corresponding switching tubes, the dead time is set to be 100ns, namely, wide-range soft switching of all the switching tubes is realized through full-digital frequency conversion control, and each sector is provided with one switching tube which cannot realize soft switching in an interval of about 10 degrees.
After the frequency conversion control is adopted, the change trend of the switching frequency in one power frequency period is shown in fig. 3, and the visible frequency is continuous and smooth. Under different voltage and load conditions, the maximum and minimum switching frequencies within the power frequency cycle are shown in fig. 4, and it can be seen that the lighter the load, the higher the voltage, and the higher the switching frequency. And the switching frequency varies only about 1.5 times under any conditions.
Fig. 5 shows the waveform of a 350V dc input in inversion mode during a power frequency cycle. The inductor current on the inverter side has a high current ripple, but the grid current has little ripple. Also in the first sector (0 to 60 degrees), the c-phase current envelope is fixed at 2A, consistent with the design. All the switching tubes of the c phase can completely realize soft switching. While phase b can not realize soft switching only by switching tubes within the interval of about 50 to 60 degrees, and all devices can realize soft switching at other times. Since the a-phase switching state remains constant, no consideration is given to its soft switching implementation. In other sectors, the relationship between the three phases alternates. The average number of soft switching is achieved for all switches is about 95.8% of the total switching number.
The above detailed description is intended to illustrate the objects, aspects and advantages of the present invention, and it should be understood that the above detailed description is only exemplary of the present invention and is not intended to limit the scope of the present invention, and any modifications, equivalents, improvements and the like within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (1)
1. A frequency conversion control method for realizing soft switching of a three-phase inverter comprises a direct-current power supply, a three-phase bridge circuit and a filter, wherein each phase of bridge circuit comprises an upper group of switching tubes and a lower group of switching tubes, driving signals of the upper group of switching tubes and the lower group of switching tubes are complementary, and the driving signals contain certain dead time; the three-phase inverter needs to operate in a unit power factor inversion or rectification mode; the three-phase inverter is used as a main circuit of a three-phase inverter control system, and the three-phase inverter control system also comprises a digital control circuit, a sampling circuit and a driving circuit; the sampling circuit is used for sampling three-phase voltage and current instantaneous value v on the alternating current sidea,vb,vc,ia,ib,ic(ii) a The method is characterized in that: the method comprises the following steps:
sampling an instantaneous value of a power grid voltage and a power grid current after filtering a switching ripple wave; sampling any two-phase value in the voltage and current of the three-phase power grid, and solving the other phase voltage and current through other two phases; tracking the voltage phase of the power grid in real time through a digital control circuit;
performing d/q axis decomposition on the three-phase power grid current, and adopting grid-connected current closed-loop control and five-segment space vector modulation; wherein the five-segment space vector modulation is selected from one of the following two methods: the zero vector of the first type is that the upper tubes of the three-phase bridge are all conducted, and the zero vector of the second type is that the lower tubes of the three-phase bridge are all conducted; three-phase modulation wave information m obtained by utilizing internal closed-loop control of digital controllera,mb,mcCalculating the switching frequency fsThe calculation formula is
Wherein IbiasThe bias current of the soft switch is determined according to the output capacitance of the switch tube; for the first five-segment space vector modulation, the state of each sector is shown in table one,
table-phase states of a first space vector
For the second type of five-segment space vector modulation, the state of each sector is shown in table two,
table two second space vector phase states
Step three, adjusting carrier frequency in a digital controller of the digital control circuit, wherein the carrier frequency is the calculated switching frequency f of the step twosModulation ratio independent of switching frequency fs(ii) an effect; transmitting the obtained switching signals to a driving circuit and driving corresponding switching tubes, namely realizing wide-range soft switching of all the switching tubes through full-digital frequency conversion control;
the filter has LCL structure with inductance value L at three-phase inverter side1A capacitance value of C and an inductance value of L on the AC side2。
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