CN115296554A - High-modulation-ratio hybrid MMC and control method thereof - Google Patents
High-modulation-ratio hybrid MMC and control method thereof Download PDFInfo
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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
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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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
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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
- H02M1/12—Arrangements for reducing harmonics from ac input or output
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- H—ELECTRICITY
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- 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
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without 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
- H02M7/537—Conversion of dc power input into ac power output without 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, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac power output without 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, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac power output without 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, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
- H02M7/53875—Conversion of dc power input into ac power output without 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, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with analogue control of three-phase output
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Abstract
The invention relates to a high modulation ratio hybrid MMC, which adopts a three-phase six-bridge arm structure, wherein each phase comprises an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm are connected through a middle submodule, two ends of a direct current bus are respectively connected with the upper bridge arm and the lower bridge arm of the three-phase structure, and the voltage of the direct current bus is V dc The three middle submodules respectively output voltage and current, and the voltage and the current output by the three phases are respectively connected with an inductor L in sequence ac The three alternating current power supplies are all grounded. Aiming at the problem of large power loss of a power electronic converter, the invention provides a high modulation ratio hybrid MMC, and a submodule of each bridge arm of the hybrid MMC consists of a SiCMOS MOSFET device and a plurality of SiCMOS MOSFET devicesThe SiIGBT device is formed by mixing, and high-frequency components are concentrated on the submodule adopting the SiMOSFET by utilizing the low switching loss characteristic of the SiMOSFET, so that the power loss of the MMC can be reduced.
Description
Technical Field
The invention relates to the technical field of power electronic converters, in particular to a high-modulation-ratio hybrid MMC and a control method thereof.
Background
The performance index and the economic index of the power electronic converter, which is the most commonly applied key equipment in a power distribution network, directly determine the quality of output electric energy. Compared with other converters, the modular multilevel converter MMC has the characteristics of high modularization, low output harmonic wave, easiness in expansion and the like, and is widely applied to the fields of flexible direct-current power transmission, variable-frequency speed regulation and wind power fields. At present, the Si IGBT is a power semiconductor device commonly used for medium-voltage and high-voltage MMC, the switching frequency of the power semiconductor device is low, the power density is low, the performance of the MMC is influenced by the power semiconductor device, and therefore the transmission efficiency of the MMC is influenced by the characteristics, and the power loss is increased. On the other hand, the problem that MMC needs to pay attention to is the modulation ratio that improves MMC, and higher modulation ratio can improve the direct current bus utilization ratio, and lower direct current bus utilization ratio can cause the waste of the energy, has reduced conversion efficiency. In the prior art, in order to improve the modulation ratio, a higher harmonic component is usually selected and injected, which causes that the output voltage of the MMC contains a large amount of higher harmonics, and the output voltage needs to be further optimized.
Disclosure of Invention
The invention aims to solve the technical problem of providing a high modulation ratio hybrid MMC and a control method to solve the problems of large power loss and low modulation ratio of a power electronic converter in the prior art.
In order to solve the technical problems, the technical scheme of the invention is as follows: the utility model provides a high modulation ratio hybrid MMC, its innovation point lies in: the three-phase six-bridge arm structure is adopted, each phase comprises an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm are connected through a middle sub-module, two ends of a direct current bus are respectively connected with the upper bridge arm and the lower bridge arm of the three-phase structure, and the voltage of the direct current bus is V dc The three middle submodules respectively output voltage and current, and the voltage and the current output by the three phases are respectively connected with an inductor L in sequence ac The three alternating current power supplies are all grounded.
Furthermore, the upper bridge arm and the lower bridge arm are respectively formed by connecting N bridge arm sub-modules in series, and the N bridge arm sub-modules are respectively SM 1 -SM N Each of said intermediate submodules having two endsThe bridge arm submodule SM of the bridge arm is connected with the upper bridge arm through a coupling inductor L N Bridge arm submodule SM with lower bridge arm 1 。
Furthermore, the number N of bridge arm sub-modules corresponding to the upper bridge arm or the lower bridge arm is determined by the input direct-current bus voltage and the withstand voltage level of the switching device adopted by the bridge arm sub-modules.
Further, each phase comprises a silicon carbide metal oxide semiconductor field effect transistor SiC MOSFET and a silicon insulated gate bipolar transistor Si IGBT, wherein the bridge arm submodule SM of the bridge arm on each phase 1 And bridge arm submodule SM of lower bridge arm N SiC MOSFET devices are adopted, and Si IGBT devices are adopted for the rest bridge arm sub-modules and the middle sub-modules.
Furthermore, each bridge arm submodule and the middle submodule are connected with a capacitor in parallel, and the bridge arm submodule SM of each phase of upper bridge arm 1 Bridge arm submodule SM with lower bridge arm N A full-bridge structure is adopted, and the rest bridge arm sub-modules and the middle sub-modules are in a half-bridge structure.
In order to solve the above technical problem, the present invention further provides a control method based on a hybrid MMC with a high modulation ratio, which has the innovative points that: the method specifically comprises a current inner loop control and modulation strategy and specifically comprises the following steps:
s1: controlling the current inner ring:
(1) Setting a current reference value i vd * 、i vq * The voltage of the DC bus is V dc And collecting three-phase current value i at AC side oa 、i ob 、i oc With three-phase voltage source voltage value u sa 、u sb 、u sc ；
(2) Passing the voltage value of the alternating-current side three-phase voltage source obtained in the step (1) through a phase-locked loop device to obtain a phase theta required by park transformation;
(3) Converting sinusoidal alternating current in a three-phase stationary coordinate system into direct current components in two-axis synchronous rotating coordinate systems d and q by park transformation, namely converting the alternating current side three-phase current value i acquired in the step (1) oa 、i ob 、i oc By park transformationConversion to an output variable i vd 、i vq The voltage value u of the alternating-current side three-phase voltage source collected in the step (1) sa 、u sb 、u sc Conversion into disturbance variable u by park transformation sd 、u sq ；
(4) The output variable i obtained in the step (3) vd 、i vq The current reference value i set in the step (1) vd * 、i vq * After corresponding subtraction, respectively outputting the two outputs after passing through a PI regulator, and correspondingly introducing the disturbance variable u obtained in the step (3) sd 、u sq Sum voltage feedforward amount ω Li vq 、ωLi vd To eliminate the d and q axis coupling part to obtain the control variable reference value i diffd * 、i diffq * Finally, the required three-phase voltage reference value u is obtained through a d and q inverter refa 、u refb 、u refc ；
(5) Setting the reference value of the upper bridge arm voltage as u refuj The reference value of the lower bridge arm voltage is u refwj J = a, b, c, representing the three phases a, b, c, according to the formula
And
respectively obtaining voltage reference values of three-phase upper and lower bridge arms, and taking the voltage reference values of the three-phase upper and lower bridge arms as modulation signals to enter a modulation module;
s2: modulation strategy:
(1) And (3) making a modulation strategy: the upper bridge arm and the lower bridge arm of each phase are composed of N bridge arm submodules, and the capacitor voltage of each bridge arm submodule is set to be V c Then V is c ＝V dc N, setting the reference voltage of each bridge arm as V ref The modulation strategy of the SUPWM only comprises a bridge arm sub-module SM of an upper bridge arm 1 And bridge arm submodule SM of lower bridge arm N The bridge arm sub-modules adopting the SiC MOSFET do not participate in the sequencing and selection of the capacitor voltages of the other bridge arm sub-modules any more but are fixedPWM modulation is adopted, and the triangular carrier voltage is set to be u carrier According to a triangular carrier voltage u carrier In contrast, bridge arm submodule SM for generating upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N A drive signal of (3), which generates a voltage of u PWM The rest bridge arm submodules are sorted according to the capacitance and voltage of the bridge arm submodules, sorted in ascending order by using a sorting and selecting algorithm, and k is set 1 And k 2 Representing the number of bridge arm submodules of an upper bridge arm or a lower bridge arm put into the bridge arm submodules without using SiC MOSFET, wherein k is 1 And k 2 The value of (a) is determined by the direction of bridge arm current of an upper bridge arm or a lower bridge arm and the capacitance voltage of bridge arm sub-modules, and the number n of bridge arm sub-modules which are put into the upper bridge arm or the lower bridge arm of each phase arm The input number k of bridge arm submodules without SiC MOSFET 1 Or k 2 Bridge arm submodule SM of upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N Is added to obtain, wherein k 1 And k 2 Respectively obtaining by rounding functions Floor and Ceil;
the middle sub-modules can be put into an upper bridge arm or a lower bridge arm, the putting condition is determined according to the requirements of the upper bridge arm and the lower bridge arm, if the upper bridge arm needs to put N bridge arm sub-modules, and the bridge arm sub-module SM of the upper bridge arm at the moment 1 When negative pressure is generated, the middle sub-module is put into the upper bridge arm; if the lower bridge arm needs to be put into N bridge arm submodules, and the bridge arm submodule SM of the lower bridge arm at the moment N If negative pressure is generated, the middle submodule is put into the lower bridge arm, and if the upper bridge arm and the lower bridge arm do not need to be put into the middle submodule at the moment, the middle submodule determines the switching state of the middle submodule according to the capacitor voltage balance principle;
(2) According to a modulation strategy, according to the relationship between the bridge arm voltage of the upper bridge arm or the lower bridge arm and the input bridge arm submodule, the following relational expression can be obtained, wherein j = u and w respectively represent the upper bridge arm and the lower bridge arm:
kV c ＜u refj ＜(k+1)V c
(3) Determining the number k of bridge arm sub-modules required to be put into operation by utilizing an integer function Floor or Ceil according to a modulation strategy 1 Or k 2 The selection principle is determined by bridge arm current of an upper bridge arm or a lower bridge arm and capacitor voltage of a bridge arm sub-module, and the bridge arm sub-module SM of the upper bridge arm can be obtained according to the relation between the bridge arm current or the lower bridge arm voltage and the capacitor voltage input into the bridge arm sub-module 1 Or bridge arm submodule SM of lower bridge arm N Bridge arm voltage u PWM The calculation formula is as follows:
(4) According to the modulation strategy, if the bridge arm current of the upper bridge arm or the lower bridge arm is larger than 0, and the bridge arm sub-module SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N Is less than the reference voltage of each bridge arm, the number k of bridge arm sub-modules generated by the integer function Floor is selected 1 If the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The capacitor voltage of which is greater than the reference voltage of each bridge arm, the number k of the bridge arm sub-modules generated by the rounding function Ceil is selected 2 (ii) a If the bridge arm current of the upper bridge arm or the lower bridge arm is less than 0, and the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The capacitor voltage is larger than the reference voltage of each bridge arm, the number k of the bridge arm sub-modules generated by the integer function Floor is selected 1 If the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N Is less than the reference voltage of each bridge arm, the number k of bridge arm sub-modules generated by the rounding function Ceil is selected 2 ；
According to the bridge arm submodule SM of the upper bridge arm obtained by calculation in the step (3) 1 Or belowBridge arm submodule SM of bridge arm N Bridge arm voltage u PWM Bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N In the same bridge arm, the triangular carrier wave passes through a delay module to obtain a new carrier wave signal, and the carrier wave signal is compared with the modulation signal to generate a bridge arm submodule SM of an upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The drive signal of (1);
(5) According to a modulation strategy, determining the input condition of the middle submodule at the moment according to the requirements of an upper bridge arm and a lower bridge arm, setting S to represent the switching state of the middle submodule, if the upper bridge arm and the lower bridge arm do not need to input the middle submodule at the moment, determining the switching state of the middle submodule by a self capacitance-voltage balance principle, and if the capacitance voltage of the middle submodule at the moment is larger than the average capacitance voltage of the bridge arm submodules and the output current is larger than 0, enabling the middle submodule to be in an 'on' state, and defining the mode as S = 1; if the output current is less than 0, the middle sub-module is in an off state, which is defined as an S =0 mode, if the capacitance voltage of the middle sub-module is less than the average capacitance voltage of the bridge arm sub-modules and the output current is less than 0, the middle sub-module is in an on state, and if the output current is greater than 0, the middle sub-module is in an off state.
Compared with the prior art, the invention has the beneficial effects that:
(1) Aiming at the problem of large power loss of a power electronic converter, the invention provides a high modulation ratio hybrid MMC, a submodule of each bridge arm of the hybrid MMC is formed by mixing a SiC MOSFET device and a plurality of Si IGBT devices, and high-frequency components are concentrated on the submodule adopting the SiC MOSFET by utilizing the low switching loss characteristic of the SiC MOSFET, so that the power loss of the MMC can be reduced.
(2) Aiming at the problem of low utilization rate of a direct current bus of a power electronic converter, the invention provides a high modulation ratio hybrid MMC, wherein a flexibly-input middle sub-module is added between an upper bridge arm and a lower bridge arm, so that the amplitude of output voltage is favorably improved, and the utilization rate of the direct current bus is improved.
(3) According to the high-modulation-ratio hybrid MMC provided by the invention, three middle submodules are respectively added between the upper bridge arm and the lower bridge arm of the three phases, so that the modulation ratio is improved, and meanwhile, for a three-phase MMC topology, the number of the three submodules is reduced, and the cost is saved.
Drawings
Fig. 1 is a structural diagram of a hybrid MMC with a high modulation ratio according to the present invention.
Fig. 2 is a block diagram of the current inner loop control of the high modulation ratio hybrid MMC in the present invention.
FIG. 3 is a single phase MMC circuit diagram and SUPWM modulation schematic diagram of the present invention.
Fig. 4 is a diagram illustrating the operation principle and the capacitor voltage balance principle of the middle sub-module in the present invention.
FIG. 5 is a comparison of MMC power loss when a hybrid structure and a full Si IGBT structure are respectively adopted in the embodiment.
Fig. 6 is an ac-side three-phase voltage output waveform of the hybrid MMC of the high modulation ratio in the embodiment.
Fig. 7 is an ac side three phase line voltage output waveform of a high modulation ratio hybrid MMC in an embodiment.
Fig. 8 is an ac side three-phase current output waveform of the hybrid MMC of high modulation ratio in an embodiment.
Fig. 9 is a waveform of the capacitance voltage of each submodule of the upper bridge arm of the phase a in the embodiment.
Fig. 10 is a waveform of capacitance voltage of each submodule of the phase a lower bridge arm in the embodiment.
Fig. 11 is a diagram of the derived modulation ratio m versus the number N of each bridge arm submodule in the embodiment.
Detailed Description
The invention is further described below with reference to the following figures and specific examples.
The invention provides a high modulation ratio hybrid MMC, the concrete structure of which is shown in figure 1, and a three-phase six-bridge arm structure is adopted, wherein each phase comprises an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm are connected through a middle submodule, two ends of a direct current bus are respectively connected with the upper bridge arm and the lower bridge arm of the three-phase structure, and the voltage of the direct current bus is V dc Three middle submodules respectively output voltage and current, and three-phase output voltage and currentThe currents are respectively connected with the inductors L in sequence ac The three alternating current power supplies are all grounded.
The upper bridge arm and the lower bridge arm are respectively formed by connecting N bridge arm sub-modules in series, and the N bridge arm sub-modules are respectively SM 1 -SM N Two ends of each middle submodule are respectively connected with the bridge arm submodule SM of the bridge arm through a coupling inductor L N Bridge arm submodule SM with lower bridge arm 1 。
The number N of the bridge arm sub-modules corresponding to the upper bridge arm or the lower bridge arm is determined by the input direct-current bus voltage and the withstand voltage grade of the switch device adopted by the bridge arm sub-modules, and the direct-current bus voltage is divided by the withstand voltage value of the switch device to obtain N.
Each phase comprises a silicon carbide metal oxide semiconductor field effect transistor SiC MOSFET and a silicon insulated gate bipolar transistor Si IGBT, wherein a bridge arm submodule SM of a bridge arm on each phase 1 Bridge arm submodule SM with lower bridge arm N And SiC MOSFET devices are adopted, and Si IGBT devices are adopted for the rest bridge arm sub-modules and the middle sub-modules.
Each bridge arm submodule and the middle submodule are connected with a capacitor in parallel, and the bridge arm submodule SM of each phase upper bridge arm 1 Bridge arm submodule SM with lower bridge arm N A full-bridge structure is adopted, and the rest bridge arm sub-modules and the middle sub-modules are in a half-bridge structure.
In order to solve the above technical problem, the present invention further provides a control method based on a hybrid MMC with a high modulation ratio, which is specifically divided into a current inner loop control and a modulation strategy, wherein the current inner loop control is shown in fig. 2, and the modulation strategy is shown in fig. 3, and specifically includes the following steps:
s1: current inner loop control:
(1) Setting a current reference value i vd * 、i vq * The voltage of the DC bus is V dc Collecting three-phase current value i at AC side oa 、i ob 、i oc With three-phase voltage source voltage value u sa 、u sb 、u sc ；
(2) Passing the voltage value of the alternating-current side three-phase voltage source obtained in the step (1) through a phase-locked loop device to obtain a phase theta required by park transformation;
(3) Converting sinusoidal alternating current in a three-phase stationary coordinate system into direct current components in two-axis synchronous rotating coordinate systems d and q by park transformation, namely converting the alternating current side three-phase current value i acquired in the step (1) oa 、i ob 、i oc Conversion into an output variable i by park transformation vd 、i vq The voltage value u of the three-phase voltage source at the alternating current side collected in the step (1) is calculated sa 、u sb 、u sc Conversion into disturbance variable u by park transformation sd 、u sq ；
(4) The output variable i obtained in the step (3) is processed vd 、i vq The current reference value i set in the step (1) vd * 、i vq * After corresponding subtraction, respectively outputting through a PI regulator, and correspondingly introducing the disturbance variable u obtained in the step (3) into two output quantities sd 、u sq Sum voltage feedforward amount ω Li vq 、ωLi vd To eliminate the d and q axis coupling part to obtain the control variable reference value i diffd * 、i diffq * Finally, the required three-phase voltage reference value u is obtained through a d and q inverter refa 、u refb 、u refc ；
(5) Setting the reference value of the upper bridge arm voltage as u refuj The reference value of the lower bridge arm voltage is u refwj J = a, b, c, representing the three phases a, b, c, according to the formula
And
respectively obtaining voltage reference values of the three-phase upper bridge arm and the three-phase lower bridge arm, and enabling the voltage reference values of the three-phase upper bridge arm and the three-phase lower bridge arm to serve as modulation signals to enter a modulation module;
s2: modulation strategy:
(1) And (3) making a modulation strategy: the upper bridge arm and the lower bridge arm of each phase are composed of N bridge arm submodules, and the capacitor voltage of each bridge arm submodule is set to be V c Then V is c ＝V dc V, setting the reference voltage of each bridge arm as V ref The modulation strategy of the present invention is SUPWM, the modulation principle is shown in fig. 3, fig. 3 (a) is a single-phase MMC circuit diagram, fig. 3 (b) is an upper bridge arm output voltage waveform, fig. 3 (c) is a lower bridge arm output voltage waveform, fig. 3 (d) is an enable signal of the upper and lower bridge arms, and the modulation strategy of SUPWM only has bridge arm sub-module SM of the upper bridge arm 1 Bridge arm submodule SM with lower bridge arm N The bridge arm sub-modules adopting the SiC MOSFET are adopted, the bridge arm sub-modules adopting the SiC MOSFET do not participate in sequencing and selecting of capacitance and voltage of the other bridge arm sub-modules any more, PWM modulation is fixedly adopted, and the triangular carrier voltage is set to be u carrier According to a voltage u of a triangular carrier carrier In contrast, bridge arm submodule SM for generating the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N A drive signal of (3), which generates a voltage of u PWM The rest bridge arm submodules are sorted according to the capacitance and voltage of the bridge arm submodules, sorted in ascending order by using a sorting and selecting algorithm, and k is set 1 And k 2 Represents the number of bridge arm sub-modules of an upper bridge arm or a lower bridge arm put into the bridge arm sub-modules without using SiC MOSFET, wherein k is 1 And k 2 The value of (a) is determined by the direction of bridge arm current of an upper bridge arm or a lower bridge arm and the capacitance voltage of bridge arm sub-modules, and the number n of bridge arm sub-modules which are put into the upper bridge arm or the lower bridge arm of each phase arm The input number k of bridge arm sub-modules without SiC MOSFET 1 Or k 2 Bridge arm submodule SM of upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N Is added to obtain, wherein k 1 And k 2 Respectively obtaining by rounding functions Floor and Ceil;
(2) The middle sub-module can be used as an upper bridge arm or a lower bridge arm, the input condition is determined according to the requirements of the upper bridge arm and the lower bridge arm, the working principle and the capacitance voltage balance principle of the middle sub-module are shown in fig. 4, fig. 4 (a) is the working principle, fig. 4 (b) is the capacitance voltage balance principle, and the middle sub-module can be used according to the number k of bridge arm sub-modules required to be input 1 Or k 2 The number N of bridge arm submodules and the voltage of the bridge arm submodule SM1 of an upper bridge arm or the bridge arm submodule SMN of a lower bridge armu PWM Obtaining enable signals E of the upper and lower bridge arms nP 、E nN According to the working principle and the capacitor voltage balance principle of fig. 4, the middle sub-module can be flexibly connected to the upper bridge arm and the lower bridge arm, if the upper bridge arm needs to be put into the N bridge arm sub-modules, and the bridge arm sub-module SM of the upper bridge arm is at the moment 1 If negative pressure is generated, the middle submodule is thrown into an upper bridge arm; if the lower bridge arm needs to input N bridge arm sub-modules, and the bridge arm sub-module SM of the lower bridge arm at the moment N If the negative pressure is generated, the middle submodule is put into the lower bridge arm, and if the upper bridge arm and the lower bridge arm do not need to be put into the middle submodule at the moment, the middle submodule determines the switching state of the middle submodule according to the capacitor voltage balance principle;
(2) According to a modulation strategy, according to the relationship between the bridge arm voltage of the upper bridge arm or the lower bridge arm and the input bridge arm submodule, the following relational expression can be obtained, wherein j = u and w respectively represent the upper bridge arm and the lower bridge arm:
kV c ＜u refj ＜(k+1)V c
(3) Determining the number k of bridge arm sub-modules required to be put into operation by using an integer function Floor or Ceil according to a modulation strategy 1 Or k 2 The selection principle is determined by bridge arm current of an upper bridge arm or a lower bridge arm and capacitance voltage of bridge arm sub-modules, and the bridge arm sub-modules SM of the upper bridge arm can be obtained according to the relation between the bridge arm current of the upper bridge arm or the lower bridge arm and the capacitance voltage of the bridge arm sub-modules 1 Or bridge arm submodule SM of lower bridge arm N Bridge arm voltage u PWM The calculation formula is as follows:
(4) According to the modulation schemeSlightly, if the bridge arm current of the upper bridge arm or the lower bridge arm is more than 0, and the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The capacitor voltage of the bridge arm is less than the reference voltage of each bridge arm, the number k of the bridge arm sub-modules generated by the integer function Floor is selected 1 If the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The capacitor voltage of which is greater than the reference voltage of each bridge arm, the number k of the bridge arm sub-modules generated by the rounding function Ceil is selected 2 (ii) a If the bridge arm current of the upper bridge arm or the lower bridge arm is less than 0, and the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The capacitance voltage of the bridge arm is larger than the reference voltage of each bridge arm, the number k of the bridge arm sub-modules generated by the integer function Floor is selected 1 If the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N Is less than the reference voltage of each bridge arm, the number k of bridge arm sub-modules generated by the rounding function Ceil is selected 2 ；
According to the bridge arm submodule SM of the upper bridge arm obtained by calculation in the step (3) 1 Or bridge arm submodule SM of lower bridge arm N Bridge arm voltage u PWM Bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N In the same bridge arm, the triangular carrier wave passes through a delay module to obtain a new carrier wave signal, and the carrier wave signal is compared with the modulation signal to generate a bridge arm submodule SM of an upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The drive signal of (1);
(5) According to a modulation strategy, determining the input condition of the middle submodule at the moment according to the requirements of an upper bridge arm and a lower bridge arm, setting S to represent the switching state of the middle submodule, if the upper bridge arm and the lower bridge arm do not need to input the middle submodule at the moment, determining the switching state by the principle of capacitance-voltage balance of the middle submodule, and if the middle submodule at the moment has capacitance voltage u cm Greater than the average capacitance voltage u of the bridge arm submodule cp,avg Or u cn,avg And output a current i oj If the current value is greater than 0, the middle sub-module is in an on state, and the mode is defined as an S =1 mode; if the current i is output oj Is less than 0, and is less than 0,the middle sub-module is in an "off" state, defined as S =0 mode, if the middle sub-module capacitance voltage u is present at this time cm Less than average capacitance voltage u of bridge arm submodule cp,avg Or u cn,avg And output a current i oj If less than 0, the middle sub-module is in on state, if output current i oj And if the value is larger than 0, the middle sub-module is in an off state.
In order to illustrate that the structure provided by the invention can adjust the modulation ratio, the invention explains the relationship between the modulation index m of the MMC and the N bridge arm sub-modules of each bridge arm under different mixing schemes. In six bridge arm MMC of three-phase, need satisfy between the amplitude of direct current busbar voltage and the side phase voltage of interchange:
wherein, V dc Is a DC bus voltage, V oj The amplitude of the phase voltage of the j phase on the AC side, and m is the modulation ratio (0)<m<1)。
In the invention, all bridge arm submodules are assumed to be equal, and the maximum value of the amplitude of the AC voltage which can be output is V' o And the voltage drop on the bridge arm inductance is ignored, the capacitance voltage of each bridge arm submodule and the bridge arm voltages of the upper and lower bridge arms can be obtained as follows:
the subscript uj represents a j-phase upper bridge arm, the subscript wj represents a j-phase lower bridge arm, the subscript on _ uj represents the number of sub-modules input into the j-phase upper bridge arm, and the subscript on _ wj represents the number of sub-modules input into the j-phase upper bridge arm.
In the present invention, according to the above formula, the maximum value of the ac voltage amplitude can be obtained:
at any time, the output voltage amplitude value satisfies the following relation:
V o ≤V' o
according to the output voltage amplitude relation, the following results are obtained:
can be obtained after simplification
To further describe the high modulation ratio hybrid MMC and its control method, the present invention is described below with reference to specific embodiments:
the MMC with a three-phase six-bridge arm is built, the direct-current side voltage of the simulation model is 7.5kV, and the frequency of the simulation model is 50Hz.
By simulating the MMCs with different schemes, it is verified that the hybrid MMC with a high modulation ratio in this embodiment can reduce the power loss. In the embodiment of the invention, a SiC MOSFET device with a CAS300M17BM2 model and a Si IGBT device with a 5SNG0300Q170300 model are adopted. Fig. 5 is a comparative analysis diagram of conduction loss and switching loss generated by a power semiconductor device in an MMC when a hybrid MMC with a high modulation ratio, a conventional hybrid MMC and an all-Si IGBT structure are respectively adopted under the same power level. As can be seen from fig. 5, the conduction loss in the high modulation ratio hybrid structure is not much different from that in the full Si IGBT structure, but the switching loss of the two is much different, and the power loss of the high modulation hybrid MMC is reduced by 59%, which is 27.8% lower than that of the conventional hybrid structure.
The SiC MOSFET has the characteristic of low switching loss, and can effectively reduce power loss and improve transmission efficiency when used in a converter. However, considering that the manufacturing cost of the SiC MOSFET is high, the characteristic of the SiC MOSFET is fully utilized, and only the bridge arm submodule SM of the upper bridge arm is used 1 Bridge arm submodule SM with lower bridge arm N SiC MOSFETs are adopted, and Si IGBTs are adopted in the rest bridge arm sub-modules. Fig. 6-10 present waveforms for each port of a high modulation ratio hybrid MMC, respectively, to verify the effectiveness of the proposed MMC.
The relationship between the modulation index m of the MMC and the N sub-modules of each leg under different mixing schemes is shown in fig. 11. Compared with the traditional hybrid MMC, the hybrid MMC with high modulation ratio can keep a higher modulation index m no matter how the number of bridge arm sub-modules changes. Therefore, a larger output voltage amplitude can be obtained, and the utilization rate of the direct current bus is greatly improved.
While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.
Claims (6)
1. The utility model provides a high modulation ratio hybrid MMC which characterized in that: the three-phase six-bridge arm structure is adopted, each phase comprises an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm are connected through a middle submodule, two ends of a direct current bus are respectively connected with the upper bridge arm and the lower bridge arm of the three-phase structure, and the voltage of the direct current bus is V dc The three middle submodules respectively output voltage and current, and the voltage and the current output by the three phases are respectively connected with an inductor L in sequence ac The three alternating current power supplies are all grounded.
2. The high modulation ratio hybrid MMC of claim 1, wherein: the upper bridge arm and the lower bridge arm are respectively formed by connecting N bridge arm sub-modules in series, and the N bridge arm sub-modules are respectively SM 1 -SM N Two ends of each middle sub-module are respectively connected with a bridge arm sub-module SM of the bridge arm through a coupling inductor L N And bridge arm submodule SM of lower bridge arm 1 。
3. The high modulation ratio hybrid MMC of claim 1, wherein: the number N of the bridge arm sub-modules corresponding to the upper bridge arm or the lower bridge arm is determined by the input direct-current bus voltage and the withstand voltage grade of the switching device adopted by the bridge arm sub-modules.
4. The high modulation ratio hybrid MMC of claim 1, wherein: each phase comprises a silicon carbide metal oxide semiconductor field effect transistor SiC MOSFET and a silicon insulated gate bipolar transistor Si IGBT, wherein a bridge arm submodule SM of a bridge arm on each phase 1 Bridge arm submodule SM with lower bridge arm N SiC MOSFET devices are adopted, and Si IGBT devices are adopted for the rest bridge arm sub-modules and the middle sub-modules.
5. The high modulation ratio hybrid MMC of claim 1, wherein: each bridge arm submodule and the middle submodule are connected with a capacitor in parallel, and the bridge arm submodule SM of each phase of upper bridge arm 1 Bridge arm submodule SM with lower bridge arm N A full-bridge structure is adopted, and the rest bridge arm sub-modules and the middle sub-modules are in half-bridge structures.
6. A control method based on a hybrid MMC with a high modulation ratio is characterized in that: the method specifically comprises a current inner loop control and modulation strategy, and specifically comprises the following steps:
s1: current inner loop control:
(1) Setting a current reference value i vd * 、i vq * The voltage of the DC bus is V dc Collecting three-phase current value i at AC side oa 、i ob 、i oc With three-phase voltage source voltage value u sa 、u sb 、u sc ；
(2) Passing the voltage value of the alternating-current side three-phase voltage source obtained in the step (1) through a phase-locked loop device to obtain a phase theta required by park transformation;
(3) Converting sinusoidal alternating current in a three-phase stationary coordinate system into direct current components in two-axis synchronous rotating coordinate systems d and q by using park conversion, namely converting the alternating current side three-phase current value i acquired in the step (1) oa 、i ob 、i oc Conversion to output variable i by park transformation vd 、i vq The voltage value u of the three-phase voltage source at the alternating current side collected in the step (1) is calculated sa 、u sb 、u sc Conversion into disturbance variable u by park transformation sd 、u sq ；
(4) The output variable i obtained in the step (3) vd 、i vq And the current reference value i set in the step (1) vd * 、i vq * After corresponding subtraction, respectively outputting the two outputs after passing through a PI regulator, and correspondingly introducing the disturbance variable u obtained in the step (3) sd 、u sq Sum voltage feedforward amount ω Li vq 、ωLi vd To eliminate the coupling part of d and q axes to obtain a reference value i of the control variable diffd * 、i diffq * Finally, the required three-phase voltage reference value u is obtained through a d and q inverter refa 、u refb 、u refc ；
(5) Setting the reference value of the upper bridge arm voltage as u refuj The reference value of the lower bridge arm voltage is u refwj J = a, b, c, representing the three phases a, b, c, according to the formula
And
respectively obtaining voltage reference values of three-phase upper and lower bridge arms, and taking the voltage reference values of the three-phase upper and lower bridge arms as modulation signals to enter a modulation module;
s2: modulation strategy:
(1) And (3) making a modulation strategy: the upper bridge arm and the lower bridge arm of each phase are composed of N bridge arm submodules, and the capacitor voltage of each bridge arm submodule is set to be V c Then V is c ＝V dc N, setting the reference voltage of each bridge arm as V ref The modulation strategy of the SUPWM only comprises a bridge arm sub-module SM of an upper bridge arm 1 Bridge arm submodule SM with lower bridge arm N The bridge arm sub-modules adopting the SiC MOSFET are not used for sequencing and selecting the capacitor voltages of the other bridge arm sub-modules any more, the PWM modulation is fixedly adopted, and the triangular carrier voltage is set to be u carrier According to a triangular carrier voltage u carrier In contrast, bridge arm submodule SM for generating upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N A drive signal of (1), which generates a voltage of u PWM The bridge arm sub-modules without the SiC MOSFET are sorted according to the capacitance and voltage of the bridge arm sub-modules, sorted in an ascending order by utilizing a sorting and selecting algorithm, and k is set 1 And k 2 Representing the number of bridge arm submodules of an upper bridge arm or a lower bridge arm put into the bridge arm submodules without using SiC MOSFET, wherein k is 1 And k 2 The value of (a) is determined by the direction of bridge arm current of an upper bridge arm or a lower bridge arm and the capacitance voltage of bridge arm sub-modules, and the number n of bridge arm sub-modules which are put into the upper bridge arm or the lower bridge arm of each phase arm The input number k of bridge arm submodules without SiC MOSFET 1 Or k 2 Bridge arm submodule SM of upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N Is obtained by adding the switching states of (a), wherein k 1 And k 2 Respectively obtaining by rounding functions Floor and Ceil;
the middle sub-modules can be put into the upper bridge arm or the lower bridge arm, the putting condition is determined according to the requirements of the upper bridge arm and the lower bridge arm, if the upper bridge arm needs to put into N bridge arm sub-modules, and the bridge arm sub-module SM of the upper bridge arm at the moment 1 Go out burdenPressing, and putting the middle sub-module into an upper bridge arm; if the lower bridge arm needs to input N bridge arm sub-modules, and the bridge arm sub-module SM of the lower bridge arm at the moment N If negative pressure is generated, the middle submodule is put into the lower bridge arm, and if the upper bridge arm and the lower bridge arm do not need to be put into the middle submodule at the moment, the middle submodule determines the switching state of the middle submodule according to the capacitor voltage balance principle;
(2) According to a modulation strategy, according to the relationship between the bridge arm voltage of the upper bridge arm or the lower bridge arm and the input bridge arm submodule, the following relational expression can be obtained, wherein j = u and w respectively represent the upper bridge arm and the lower bridge arm:
kV c ＜u refj ＜(k+1)V c
(3) Determining the number k of bridge arm sub-modules required to be put into operation by utilizing an integer function Floor or Ceil according to a modulation strategy 1 Or k 2 The selection principle is determined by bridge arm current of an upper bridge arm or a lower bridge arm and capacitance voltage of bridge arm sub-modules, and the bridge arm sub-modules SM of the upper bridge arm can be obtained according to the relation between the bridge arm current of the upper bridge arm or the lower bridge arm and the capacitance voltage of the bridge arm sub-modules 1 Or bridge arm submodule SM of lower bridge arm N Bridge arm voltage u PWM The calculation formula is as follows:
(4) According to the modulation strategy, if the bridge arm current of the upper bridge arm or the lower bridge arm is larger than 0, and the bridge arm sub-module SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N Is less than the reference voltage of each bridge arm, the number k of bridge arm sub-modules generated by the integer function Floor is selected 1 If the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The capacitor voltage of which is greater than the reference voltage of each bridge arm, the number k of the bridge arm sub-modules generated by the rounding function Ceil is selected 2 (ii) a If the bridge arm current of the upper bridge arm or the lower bridge arm is less than 0, and the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The capacitance voltage of the bridge arm is larger than the reference voltage of each bridge arm, the number k of the bridge arm sub-modules generated by the integer function Floor is selected 1 If the bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N Is less than the reference voltage of each bridge arm, the number k of bridge arm sub-modules generated by the rounding function Ceil is selected 2 ；
According to the bridge arm submodule SM of the upper bridge arm obtained by calculation in the step (3) 1 Or bridge arm submodule SM of lower bridge arm N Bridge arm voltage u PWM Bridge arm submodule SM of the upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N In the same bridge arm, the triangular carrier wave passes through a delay module to obtain a new carrier wave signal, and the carrier wave signal is compared with the modulation signal to generate a bridge arm submodule SM of an upper bridge arm 1 Or bridge arm submodule SM of lower bridge arm N The drive signal of (1);
(5) According to a modulation strategy, determining the input condition of the middle submodule at the moment according to the requirements of an upper bridge arm and a lower bridge arm, setting S to represent the switching state of the middle submodule, if the upper bridge arm and the lower bridge arm do not need to input the middle submodule at the moment, determining the switching state of the middle submodule by a self capacitance-voltage balance principle, and if the capacitance voltage of the middle submodule at the moment is larger than the average capacitance voltage of the bridge arm submodules and the output current is larger than 0, enabling the middle submodule to be in an 'on' state, and defining the mode as S = 1; if the output current is less than 0, the middle sub-module is in an off state, which is defined as an S =0 mode, if the capacitance voltage of the middle sub-module is less than the average capacitance voltage of the bridge arm sub-modules and the output current is less than 0, the middle sub-module is in an on state, and if the output current is greater than 0, the middle sub-module is in an off state.
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| CN115987125A (en) * | 2023-03-21 | 2023-04-18 | 湖南大学 | Mixed MMC with doubled level number and modulation method thereof |
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| CN115987125A (en) * | 2023-03-21 | 2023-04-18 | 湖南大学 | Mixed MMC with doubled level number and modulation method thereof |
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