CN115864885B - Topological structure of hybrid modular multilevel converter and regulation and control method thereof - Google Patents
Topological structure of hybrid modular multilevel converter and regulation and control method thereof Download PDFInfo
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Abstract
The invention provides a topological structure of a hybrid modular multilevel converter and a regulating method thereof, wherein the topological structure comprises an upper bridge arm and a lower bridge arm which are connected in series, and the upper bridge arm and the lower bridge arm comprise a high-frequency module and a low-frequency module; the high-frequency module adopts a three-phase four-bridge arm topology, each phase of the high-frequency module is provided with a high-frequency sub-module, and the three phases are connected with a converter sub-module in parallel; the low-frequency module adopts a three-phase topology, and each phase of the low-frequency module is provided with N-1 low-frequency submodules in cascade connection; the high-frequency sub-module is a half-bridge converter formed by SiCMOSFET devices, and the converter sub-module and the low-frequency sub-module are both half-bridge converters formed by Si IGBT devices. According to the invention, the high-frequency component of the output voltage is fixed on the SiC MOSFET device, the low-frequency component is fixed on the Si IGBT device, the advantages of low switching loss of the SiC device and low on-state loss of the Si device are fully exerted, and the device efficiency of the modularized multi-level converter can be effectively improved.
Description
Technical Field
The invention relates to the technical field of power electronic conversion, in particular to a topological structure of a hybrid modular multilevel converter and a regulating and controlling method thereof.
Background
The modularized multi-level converter (Modular Multilevel Converter, MMC) is widely applied to the fields of flexible direct current transmission, electric transmission, grid-connected converters and the like due to the advantages of high modularization degree, strong expansibility, low output waveform harmonic content and the like.
As shown in fig. 1, the conventional MMC is mainly composed of Si-based Insulated Gate Bipolar Transistors (IGBT) with low power density, and common modulation methods include a recent level approximation (Nearest Level Modulation, NLM) and carrier phase-shift pulse width modulation (Carrier Phase Shifting Pulse Width Modulation, CPS-PWM). NLM switching frequency is low, and the switching loss of the device is less, but under the condition that the number of submodules is less, the lower harmonic content of MMC output waveforms is higher, and the requirements on filtering parameters are higher. In comparison, CPS-PWM equivalent switching frequency is high, low-order harmonic content of MMC output waveform is low, but device switching frequency is high, and switching loss of the device is larger.
With the deep research and development of wide bandgap semiconductors and the manufacturing process thereof, new generation wide bandgap semiconductor devices represented by SiC-based Metal oxide semiconductor field effect transistors (Metal-Oxide Semiconductor Field-Effect Transistor, MOSFETs) have been commercialized. Compared with Si IGBT, the SiC MOSFET has lower switching frequency and lower switching loss, and can greatly improve the overall performance of the device. However, the manufacturing cost of the SiC MOSFET is 5-8 times that of the Si IGBT with the same capacity, and the large-scale application of the SiC MOSFET in MMC is severely restricted.
Disclosure of Invention
The invention provides a topological structure of a hybrid modular multilevel converter and a regulating and controlling method thereof, which aim to solve the problems of high switching loss, high power density and high cost of the traditional MMC.
In order to solve the technical problems, the invention adopts the following technical methods: a topological structure of a hybrid modular multilevel converter and a regulation and control method thereof comprise an upper bridge arm and a lower bridge arm which are connected in series, wherein the upper bridge arm comprises a high-frequency module and a low-frequency module which are sequentially connected in series from top to bottom, and the lower bridge arm comprises a low-frequency module and a high-frequency module which are sequentially connected in series from top to bottom; the high-frequency module adopts a three-phase four-bridge arm topology, each phase of the high-frequency module is provided with a high-frequency sub-module, and the three phases are connected with a converter sub-module in parallel; the low-frequency module adopts a three-phase topology, and each phase of the low-frequency module is provided with N-1 low-frequency submodules in cascade connection; the high-frequency sub-module is a half-bridge converter formed by SiCMOSFET devices, and the converter sub-module and the low-frequency sub-module are both half-bridge converters formed by Si IGBT devices.
Preferably, the direct current sides of the high-frequency modules of the upper bridge arm and the lower bridge arm are connected with capacitors in parallel
The direct current side of each low-frequency sub-module is respectively connected with a capacitor in parallel>
。
As another aspect of the invention, a method for regulating and controlling a topological structure of a hybrid modular multilevel converter, wherein the low-frequency module adopts a nearest level approach to modulate and output a step wave voltage
The high-frequency module adopts unipolar PWM modulation to output shaping voltage +.>
Step wave voltage->
And shaping voltage->
Output voltage +.>
Output from the midpoint of the upper and lower legs, +.>
Representation->
Phase or->
Phase or->
And (3) phase (C).
Further, the low frequency module outputs a step wave voltage
When (1): firstly, calculating the number of low-frequency submodules required to be put into each moment of an upper bridge arm and a lower bridge arm according to a modulation voltage reference value; then the upper bridge arm and the lower bridge arm respectively output the ladder wave voltage of the upper bridge arm by adopting a nearest level approximation method>
And lower bridge arm ladder wave voltage->
The method comprises the steps of carrying out a first treatment on the surface of the Finally, the lower bridge arm ladder wave voltage is +>
And upper bridge arm ladder wave voltage->
Taking average value after difference to form step wave voltage +.>
And outputting.
Still further:
1) Outputting the ladder wave voltage of the upper bridge arm
When (1):
firstly, calculating the number of low-frequency submodules required to be input at each moment of an upper bridge arm according to an upper bridge arm modulation voltage reference value in the formula (1) and the formula (2)
；
In the method, in the process of the invention,
modulating a voltage reference value for an upper bridge arm; />
Modulating a voltage reference value for a lower bridge arm; />
The high-voltage direct-current side voltage is the high-voltage direct-current side voltage of the hybrid modular multilevel converter; />
Representing taking an integer to the positive infinity direction; />
The reference value of the capacitor voltage at the direct current side of the low-frequency sub-module and the high-frequency module;
then the last level approximation method is adopted to output the ladder wave voltage of the upper bridge arm
As in equation (3);
2) Outputting the step wave voltage of the lower bridge arm
When (1):
firstly, calculating the number of low-frequency submodules required to be put into each moment of a lower bridge arm according to a lower bridge arm modulation voltage reference value in the formula (1) and the formula (4)
；
Then the last level approximation method is adopted to output the lower bridge arm step wave voltage
As in equation (5);
3) The step wave voltage
The calculation formula of (2) is as follows:
further, the high frequency module outputs a shaped voltage
When the high frequency module outputs the voltage reference value
And triangle carrier->
The following comparison is performed to output +.>
Three levels;
1) When (when)
When (1):
if it is
Output->
The method comprises the steps of carrying out a first treatment on the surface of the If->
Outputting 0;
2) When (when)
When (1):
if it is
Output->
The method comprises the steps of carrying out a first treatment on the surface of the If->
Outputting 0; />
The high frequency module outputs a voltage reference value
Output voltage for hybrid modular multilevel converter
With low frequency module ladder voltage +>
Is the difference between (a):
still further, the low frequency modules of the upper bridge arm and the lower bridge arm adopt a sequencing voltage equalizing method to realize voltage stabilization.
Preferably, the high-frequency module includes four working states, namely:
working state 1: upper switch tube of converter sub-module
Switch on and off>
Turn-off, high voltage DC side current
Wherein the inflow high-voltage direct current side is negative, and the outflow high-voltage direct current side is positive; at this time, when the upper switch tube of the high-frequency sub-module
Opening upThe high-frequency module outputs 0 level, and the high-frequency module bypasses the capacitor; lower switch tube of high-frequency submodule
Opening, if the upper bridge arm outputs +.>
Level, if in lower bridge arm, high frequency module outputs +>
The level, the high-frequency module capacitor discharges;
working state 2: upper switch tube of converter sub-module
Switch on and off>
Turn-off, high voltage DC side current
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the upper switching tube of the high frequency sub-module is +.>
Opening, wherein the high-frequency module outputs 0 level, and the high-frequency module bypasses the capacitor; lower switching tube of high-frequency submodule>
Opening, if the upper bridge arm outputs +.>
Level, if in lower bridge arm, high frequency module outputs +>
The level, the high-frequency module capacitor charges;
working state 3: lower switch tube of current conversion sub-module
Switch on and switch on tube->
Turn-off, high voltage DC side current
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the upper switching tube of the high frequency sub-module is +.>
Opening, if the upper bridge arm outputs +.>
Level, if in lower bridge arm, high frequency module outputs +>
The level, the high-frequency module capacitor discharges; lower switch tube of high-frequency submodule
Opening, wherein the high-frequency module outputs 0 level, and the high-frequency module bypasses the capacitor;
working state 4: lower switch tube of current conversion sub-module
Switch on and switch on tube->
Turn-off, high voltage DC side current
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the upper switching tube of the high frequency sub-module is +.>
Opening, if the upper bridge arm outputs +.>
Level, if in lower bridge arm, high frequency module outputs +>
The level, the high-frequency module capacitor charges; lower switch tube of high-frequency submodule
And (3) switching on, outputting 0 level by the high-frequency module, and bypassing the capacitor of the high-frequency module.
Preferably, the working states 1 and 2 of the high-frequency module are defined as a mode 1, and the working states 3 and 4 are defined as a mode 2;
1) The voltage equalizing process of the high-frequency module of the upper bridge arm comprises the following steps:
step S11, calculating the number of low-frequency submodules required to be put into each moment of the upper bridge arm through the step (2)
；
Step S12, calculating each of the previous time and the previous time
Comparing, judging +.>
Whether a change occurs; if the three phases are not changed, the voltage equalizing operation is not cut in, and the step S11 is performed; if there is any one of the three phases +.>
When the pressure changes, the pressure equalizing operation is switched into step S13;
step S13, judging the high-voltage direct-current side current
Positive and negative of (a);
if it is
Judging whether there is one or several phases of +.>
If so, enter mode 1 and calculate +.>
The method comprises the steps of carrying out a first treatment on the surface of the If not, then judging the upper bridge arm high frequency module direct currentSide capacitor voltage->
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the upper bridge arm +.>
If yes, enter mode 2 and calculate +.>
If not, enter mode 1 and calculate +.>
；
If it is
Judging whether there is one or several phases of +.>
If so, enter mode 1 and calculate +.>
The method comprises the steps of carrying out a first treatment on the surface of the If not, then judge the DC side capacitor voltage of the upper bridge arm high frequency module +.>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the upper bridge arm +.>
If yes, enter mode 1 and calculate +.>
If not, enter mode 2 and calculate +.>
；
2) The voltage equalizing process of the high-frequency module of the lower bridge arm comprises the following steps:
step S21, calculating the number of low-frequency submodules required to be put into each moment of the lower bridge arm through the step (4)
；
Step S22, calculating each of the previous time and the previous time
To judge two moments
Whether a change occurs; if the three phases are not changed, the voltage equalizing operation is not cut in, and the step S21 is performed; if there is any one of the three phases +.>
If the pressure is changed, the pressure equalizing operation is switched into step S23;
step S23, judging the high-voltage DC side current
Positive and negative of (a);
if it is
Judging whether there is one or several phases of +.>
If so, enter mode 2 and calculate +.>
If not, then judging the DC side capacitor voltage of the lower bridge arm high frequency module>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the lower bridge arm +.>
If yes, enter mode 2 and calculate using equation (9)
If not, enter mode 1 and calculate +.>
；
If it is
Judging whether there is one or several phases of +.>
If so, enter mode 2 and calculate +.>
If not, then judging the DC side capacitor voltage of the lower bridge arm high frequency module>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the lower bridge arm +.>
If yes, enter mode 1 and calculate using equation (4)
If not, enter mode 2 and calculate +.>
。
The invention provides a topological structure of a hybrid modular multilevel converter and a regulating and controlling method thereof. Compared with the traditional MMC, the topological structure provided by the invention has the advantages that the output performance can be close to that of the MMC of the full SiC MOSFET device mainly by adding two high-frequency sub-modules containing SiC MOSFETs on the upper bridge arm and the lower bridge arm, and the number of direct-current capacitors used by the traditional MMC is smaller, the power density is higher, and the device cost is lower. The modulation and voltage equalizing strategy provided by the invention can fix the high-frequency component of the output voltage to the SiC MOSFET device, fix the low-frequency component to the Si IGBT device, fully exert the advantages of low switching loss of the SiC device and low on-state loss of the Si device, and effectively improve the device efficiency of the modularized multi-level converter.
Drawings
Fig. 1 is a schematic diagram of a conventional MMC topology and its modulation principle (wherein (a) is a schematic diagram of a recent level approximation modulation, and (b) is a schematic diagram of carrier phase-shifting pulse width modulation);
fig. 2 is a schematic diagram of a hybrid modular multilevel converter topology according to the present invention;
fig. 3 is a schematic diagram of the HSM modulation of the upper arm of the HMMC;
fig. 4 is a schematic diagram of HMMC lower arm HSM modulation;
FIG. 5 is a waveform diagram of a mid-point output step wave voltage of an HMMC bridge arm;
FIG. 6 is a schematic diagram of HMMC high frequency module modulation;
fig. 7 is a schematic diagram of an operating state of a high-frequency module of an HMMC upper bridge arm (wherein, (a), (b), (c), and (d) are schematic diagrams of an operating state 1, an operating state 2, an operating state 3, and an operating state 4 of the high-frequency module of the HMMC upper bridge arm, respectively);
FIG. 8 is a schematic diagram of a high frequency module equalizing mode of the upper bridge arm of the HMMC;
FIG. 9 is a flow chart of the capacitive voltage equalizing of the high frequency module of the upper bridge arm of the HMMC;
FIG. 10 is a flow chart of the capacitance equalization of the HMMC lower bridge arm high frequency module;
FIG. 11 is a graph of HMMC a phase output voltageu ao A waveform diagram;
FIG. 12 is a graph of HMMCa phase high frequency sub-module PWM wave and HSM output waveforms (where (a) is the a phase high frequency sub-module PWM wave and (b) is the a phase HSM output waveform);
fig. 13 is a graph of HMMCa phase upper arm N-1 HSM and high frequency module dc side capacitor voltage waveforms (where (a) is a phase upper arm N-1 HSM dc side capacitor voltage waveform and (b) is a phase upper arm high frequency module dc side capacitor voltage waveform).
Detailed Description
The invention will be further described with reference to examples and drawings, to which reference is made, but which are not intended to limit the scope of the invention.
1. Hybrid modular multilevel converter topology (Hybrid MMC, HMMC)
As shown in fig. 2, the HMMC provided by the invention is composed of an upper bridge arm and a lower bridge arm which are connected in series, wherein the upper bridge arm and the lower bridge arm are symmetrically arranged, the upper bridge arm comprises a high-frequency module and a low-frequency module which are sequentially connected in series from top to bottom, and the lower bridge arm comprises a low-frequency module and a high-frequency module which are sequentially connected in series from top to bottom; the high-frequency module adopts a three-phase four-bridge arm topology, each phase of the high-frequency module is provided with a high-frequency sub-module, and the three phases are connected with a converter sub-module in parallel; the low-frequency module adopts a three-phase topology, and each phase of the low-frequency module is provided with N-1 low-frequency submodules in cascade connection; specifically, the high-frequency sub-module is a half-bridge converter formed by SiCMOSFET devices, and the converter sub-module and the low-frequency sub-module are both half-bridge converters formed by Si IGBT devices. As shown in the figure 2 of the drawings,
three-phase output voltage for HMMC; />
The high-voltage direct-current side voltage of the HMMC; />
A direct-current side capacitor of the low-frequency sub-module (HSM for short); />
A direct current side capacitor of the high frequency module; />
The reference value of the capacitor voltage at the direct current side of the HSM and the high-frequency module.
HMMC three-phase upper and lower bridge arm low frequency module structure symmetry hasThe body connection mode is as follows: taking the a-phase upper bridge arm as an example, as shown in FIG. 2, each HSM comprises a DC side capacitor
And a Si IGBT type half-bridge converter, wherein the output port 1 is formed by an upper switch tube of the Si IGBT type half-bridge converter>
Emitter node and lower switching tube of (2)>
The collector node of the (2) is connected to the last HSM or high frequency module, and the output port 2 is connected from the lower switch tube of the Si IGBT type half-bridge converter +.>
Emitter node lead-out of (a) to the next HSM or bridge arm inductanceL. DC side capacitor->
A DC side capacitor mounted on the DC side of the Si IGBT type half-bridge converter>
Upper switching tube for positive pole and Si IGBT half-bridge conversion>
The collector node of (a) is connected with the negative electrode of the lower switching tube of the Si IGBT type half-bridge conversion +.>
Is connected to the emitter node of (c). After N-1 HSMs in the upper bridge arm are connected in series, the upper bridge arm is connected with a bridge arm inductorLIs connected to the midpoint of the a-phase bridge arm.
The HMMC topology has two high-frequency modules and is symmetrical up and down in structure. Taking the upper bridge arm high frequency module as an example, as shown in FIG. 2, it is composed of a DC side capacitor
Three high-frequency sub-modules and one converter sub-module, highUpper switching tube in frequency sub-module>
And lower switch tube->
All are SiC MOSFETs, and the upper switch tube in the converter submodule is +.>
And lower switch tube->
Are all Si IGBTs. />
Source and->
The drain electrode node is connected with the low-frequency module of the a-phase upper bridge arm; />
Source and->
The drain electrode node is connected with the b-phase upper bridge arm low-frequency module; />
Source and->
The drain electrode node is connected with and connected with the c-phase upper bridge arm low-frequency module. DC side capacitor->
Positive electrode and->
The drain electrode node is connected with the negative electrode>
The source nodes are connected. In the converter submodule->
Collector and->
Drain of->
Positive electrode of the converter sub-module +.>
Emitter and->
The collector is connected with the positive pole of the system direct current side and is +.>
Emitter and->
Drain of->
Is connected to the negative electrode of the battery. />
2. Method for regulating and controlling topological structure of hybrid modular multilevel converter
The invention relates to an improvement of a topological structure regulating method of a hybrid modular multilevel converter, which mainly comprises two aspects of a regulating strategy and a voltage equalizing method, and other aspects refer to a traditional regulating technology, wherein the two aspects are specifically as follows.
1) Modulation strategy
The modulation strategy of HMMC can be divided into two parts, low frequency module modulation and high frequency module modulation. Taking the phase a as an example for analysis,
the a-phase of HMMC needs to output alternating voltage which is composed of two parts, wherein one part is the step wave voltage outputted by the low-frequency module +.>
Another part is high frequencyShaping voltage +.>
And the two parts are overlapped and then output from the midpoints of the upper bridge arm and the lower bridge arm.
1. Low frequency module modulation principle
The low-frequency module adopts the nearest level approach modulation to lead the midpoint pair of the a-phase bridge arm toOThe point outputs a step wave voltage. Each phase of the HMMC comprises an upper bridge arm and a lower bridge arm, and the expressions of the upper bridge arm modulation voltage reference value and the lower bridge arm modulation voltage reference value of the phase a are as follows:
in the method, in the process of the invention,
modulating a voltage reference value for an upper bridge arm; />
Modulating a voltage reference value for a lower bridge arm; />
The high-voltage direct-current side voltage is the high-voltage direct-current side voltage of the hybrid modular multilevel converter; />
The reference value of the capacitor voltage at the direct current side of the low-frequency sub-module and the high-frequency module; />
Is HMMC output voltage, which is output from the midpoint of the upper bridge arm and the lower bridge arm, +.>
Representation->
Phase or->
Phase or->
And (3) phase (C). Here, it is worth noting that +.>
、/>
In the meantime, substituted ++in formula (1)>
Should be the value of the previous moment.
The modulation principle of the upper bridge arm N-1 HSM is shown in figure 3, and the specific modulation process is as follows: firstly, calculating the quantity of HSMs required to be input at each moment of an upper bridge arm according to an upper bridge arm modulation voltage reference value in the (1)
The calculation formula is as follows:
in the method, in the process of the invention,
representing taking an integer in the positive infinity direction. Notably, in calculating the current moment +.>
When substituting +.>
Should be the value of the previous moment.
When the number of HSMs to be charged changes, the charging state of each HSM changes. Outputting the ladder wave voltage of the upper bridge arm by adopting a nearest level approximation method
The calculation formula is as follows: />
(3)
The modulation principle of the lower bridge arm N-1 HSM is shown in fig. 4, the modulation process is the same as that of the upper bridge arm, and the calculation formula of the number of input submodules is as follows:
outputting lower bridge arm ladder wave voltage by adopting nearest level approximation method
The calculation formula is as follows:
the low-frequency modules of the upper bridge arm and the lower bridge arm are modulated independently, and the a-phase bridge arm output step wave voltage can be obtained by combining (6)
The output step wave voltage is shown in fig. 5. />
2. High frequency module modulation principle
Taking a phase as an example for analysis, the high-frequency submodule outputs a voltage reference value
Output voltage +.>
With low frequency module ladder voltage +>
Is the difference between (a):
in the method, in the process of the invention,
is->
And (3) phase (C).
a
Phase high frequency submodule output voltage reference value +.>
And actual value +.>
As shown in fig. 6. The high-frequency submodule adopts unipolar PWM modulation to enable +.>
And triangle carrier->
The comparison can be performed to output in one period
The three levels are specifically modulated as follows:
1) When (when)
When (1):
if it is
Output->
The method comprises the steps of carrying out a first treatment on the surface of the If->
Outputting 0;
2) When (when)
When (1):
if it is
Output->
The method comprises the steps of carrying out a first treatment on the surface of the If->
Outputting 0;
the on and off of the specific devices of the high-frequency submodule depend on the on condition of the switching devices of the converter submodule and the flow direction of the current at the direct-current side.
2) Pressure equalizing method
The low-frequency modules of the upper bridge arm and the lower bridge arm of the HMMC can realize voltage stabilization by adopting a traditional ordering voltage equalizing method, and the description is omitted here. The high-frequency module can not participate in sequencing voltage equalizing together with the low-frequency HSM due to the fact that the high-frequency PWM wave is fixedly output, and the direct-current side capacitor of the high-frequency module
Self-stabilization cannot be achieved. For this purpose, a specific pressure equalizing scheme is designed.
The high-frequency module of the bridge arm is taken as an example for analysis, and 4 working states are shown in fig. 7.
Working state 1: upper switch tube of converter sub-module
Switch on and off>
Turn-off, high voltage DC side current
Wherein the inflow high-voltage direct current side is negative, and the outflow high-voltage direct current side is positive; at this time, when the upper switch tube of the high-frequency sub-module
Opening, wherein the high-frequency module outputs 0 level, and the high-frequency module bypasses the capacitor; lower switch tube of high-frequency submodule
On, high frequency module output->
Level (if in the lower bridge arm, the output of the high frequency module is + ->
Level), the high frequency module capacitance discharges.
Working state 2: upper switch tube of converter sub-module
Switch on and off>
Turn-off, high voltage DC side current
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the upper switching tube of the high frequency sub-module is +.>
Opening, wherein the high-frequency module outputs 0 level, and the high-frequency module bypasses the capacitor; lower switching tube of high-frequency submodule>
On, high frequency module output->
Level (if in the lower bridge arm, the output of the high frequency module is + ->
Level), the high frequency module capacitance charges.
Working state 3: lower switch tube of current conversion sub-module
Switch on and switch on tube->
Turn-off, high voltage DC side current
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the high frequency submoduleUpper switching tube->
On, high frequency module output->
Level (if in the lower bridge arm, the output of the high frequency module is + ->
Level), high frequency module capacitor discharge; lower switch tube of high-frequency submodule
And (3) switching on, outputting 0 level by the high-frequency module, and bypassing the capacitor of the high-frequency module. />
Working state 4: lower switch tube of current conversion sub-module
Switch on and switch on tube->
Turn-off, high voltage DC side current
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the upper switching tube of the high frequency sub-module is +.>
On, high frequency module output->
Level (if in the lower bridge arm, the output of the high frequency module is + ->
Level), high frequency module capacitor charges, when the lower switch tube of the high frequency sub-module
And (3) switching on, outputting 0 level by the high-frequency module, and bypassing the capacitor of the high-frequency module.
It can be seen that, in
And->
And when the high-frequency module is in the power-on state, the charge and discharge of the capacitor at the direct-current side of the high-frequency module can be flexibly controlled by changing the on state of the current sub-module.
The operating states 1 and 2 of the high frequency module are defined as mode 1, and the operating states 3 and 4 are defined as mode 2.
1. As shown in fig. 9, the voltage equalizing process of the high frequency module of the upper bridge arm includes:
step S11, calculating the number of low-frequency submodules required to be put into each moment of the upper bridge arm through the step (2)
；
Step S12, calculating each of the previous time and the previous time
Comparing and judging two moments
Whether a change occurs; if the three phases are not changed, the voltage equalizing operation is not cut in, and the step S11 is performed; if there is any one of the three phases +.>
When the pressure changes, the pressure equalizing operation is switched into step S13;
step S13, judging the high-voltage direct-current side current
Positive and negative of (a);
if it is
Judging whether there is one or several phases of +.>
If so, enter mode 1 and calculate +.>
The method comprises the steps of carrying out a first treatment on the surface of the If not, then judge the DC side capacitor voltage of the upper bridge arm high frequency module +.>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the upper bridge arm +.>
If yes, enter mode 2 and calculate +.>
(because the mode 2 high frequency module outputs negative polarity PWM wave, the low frequency module needs to input one more HSM as shown in FIG. 8), if not, enter mode 1, and calculate +% using equation (2)>
。
If it is
Judging whether there is one or several phases of +.>
If so, enter mode 1 and calculate +.>
The method comprises the steps of carrying out a first treatment on the surface of the If not, then judge the DC side capacitor voltage of the upper bridge arm high frequency module +.>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the upper bridge arm +.>
If yes, enter mode 1 and calculate +.>
If not, enter mode 2 and calculate +.>
；
2. As shown in fig. 10, the voltage equalizing process of the high frequency module of the lower bridge arm includes:
step S21, calculating the number of low-frequency submodules required to be put into each moment of the lower bridge arm through the step (4)
；
Step S22, calculating each of the previous time and the previous time
Comparing and judging two moments
Whether a change occurs; if the three phases are not changed, the voltage equalizing operation is not cut in, and the step S21 is performed; if there is any one of the three phases +.>
If the pressure is changed, the pressure equalizing operation is switched into step S23;
step S23, judging the high-voltage DC side current
Positive and negative of (a);
if it is
Judging whether there is one or several phases of +.>
If so, enter mode 2 and calculate +.>
If not, then judging the lower bridge arm high frequency moduleDC side capacitor voltage +.>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the lower bridge arm +.>
If yes, enter mode 2 and calculate using equation (9)
If not, enter mode 1 and calculate +.>
；/>
If it is
Judging whether there is one or several phases of +.>
If so, enter mode 2 and calculate +.>
If not, then judging the DC side capacitor voltage of the lower bridge arm high frequency module>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the lower bridge arm +.>
If yes, enter mode 1 and calculate using equation (4)
If not, enter mode 2 and calculate +.>
。
3. Simulation analysis
In order to better prove the effectiveness of the topological structure and the regulation method provided by the invention, the following verification is carried out by combining a simulation example. According to the HMMC topology shown in FIG. 2, a simulation platform is built in MATLAB/Simulink, and simulation parameters are shown in Table 1.
TABLE 1 Main simulation parameters
FIG. 11 shows HMMC a phase output voltage
Waveform of the output voltage +>
The output result is similar to the output result after the traditional carrier wave phase shift modulation, and is a high-frequency ladder wave.
Fig. 12 shows the voltage waveforms of the a-phase high frequency sub-module in the HMMC high frequency module and the voltage waveforms of the HSM in the a-phase low frequency module. It can be seen that the switching frequency of the HSM waveform is low, while the switching frequency of the high frequency sub-module is high.
Therefore, the modulation strategy provided by the invention can control the SiC MOSFET device to work in a high-frequency state and the Si IGBT device to work in a low-frequency state on the premise of ensuring the waveform quality of the HMMC output voltage, so that the switching loss of the device is reduced.
Fig. 13 shows waveforms of capacitor voltages at the direct current side of the phase a upper bridge arm N-1 HSMs and the high frequency module, and it can be seen that fluctuation ranges of all capacitor voltages are less than 5%, and the provided voltage equalizing scheme well achieves the purpose of equalizing voltage.
The foregoing embodiments are preferred embodiments of the present invention, and in addition, the present invention may be implemented in other ways, and any obvious substitution is within the scope of the present invention without departing from the concept of the present invention.
In order to facilitate understanding of the improvements of the present invention over the prior art, some of the figures and descriptions of the present invention have been simplified, and some other elements have been omitted from this document for clarity, as will be appreciated by those of ordinary skill in the art.
Claims (1)
1. The method for regulating and controlling the topological structure of the hybrid modular multilevel converter is characterized by comprising the following steps of:
the topological structure of the hybrid modular multilevel converter comprises an upper bridge arm and a lower bridge arm which are connected in series, wherein the upper bridge arm comprises a high-frequency module and a low-frequency module which are sequentially connected in series from top to bottom, and the lower bridge arm comprises a low-frequency module and a high-frequency module which are sequentially connected in series from top to bottom; the high-frequency module adopts a three-phase four-bridge arm topology, each phase of the high-frequency module is provided with a high-frequency sub-module, and the three phases are connected with a converter sub-module in parallel; the low-frequency module adopts a three-phase topology, and each phase of the low-frequency module is provided with N-1 low-frequency submodules in cascade connection; wherein N represents the sum of the numbers of the upper high-frequency submodules and the lower-frequency submodules of each phase of the upper bridge arm or the lower bridge arm; the high-frequency sub-module is a half-bridge converter formed by SiCMOSFET devices, and the converter sub-module and the low-frequency sub-module are both half-bridge converters formed by Si IGBT devices; the direct current sides of the high-frequency modules of the upper bridge arm and the lower bridge arm are connected in parallel with capacitors
The direct current side of each low-frequency sub-module is respectively connected with a capacitor in parallel>
；
The low-frequency module adopts the nearest level approximation to modulate and output the step wave voltage
The high-frequency module adopts unipolar PWM modulation to output shaping voltage +.>
Step wave voltage->
And shaping voltage->
Output voltage +.>
Output from the midpoint of the upper and lower legs, +.>
Representation->
Phase or->
Phase or->
A phase;
the low-frequency module outputs a step wave voltage
When (1): firstly, calculating the number of low-frequency submodules required to be put into each moment of an upper bridge arm and a lower bridge arm according to a modulation voltage reference value; then the upper bridge arm and the lower bridge arm respectively output the ladder wave voltage of the upper bridge arm by adopting a nearest level approximation method>
And lower bridge arm ladder wave voltage->
The method comprises the steps of carrying out a first treatment on the surface of the Finally, the lower bridge arm ladder wave voltage is +>
And upper bridge arm ladder wave voltage->
Taking average value after difference to form step wave voltage +.>
Outputting;
1) Outputting the ladder wave voltage of the upper bridge arm
When (1):
firstly, calculating the number of low-frequency submodules required to be input at each moment of an upper bridge arm according to an upper bridge arm modulation voltage reference value in the formula (1) and the formula (2)
；
In the method, in the process of the invention,
modulating a voltage reference value for an upper bridge arm; />
Modulating a voltage reference value for a lower bridge arm; />
The high-voltage direct-current side voltage is the high-voltage direct-current side voltage of the hybrid modular multilevel converter; />
Representing taking an integer to the positive infinity direction; />
The reference value of the capacitor voltage at the direct current side of the low-frequency sub-module and the high-frequency module;
then the last level approximation method is adopted to output the ladder wave voltage of the upper bridge arm
As in equation (3);
2) Outputting the step wave voltage of the lower bridge arm
When (1):
firstly, calculating the number of low-frequency submodules required to be put into each moment of a lower bridge arm according to a lower bridge arm modulation voltage reference value in the formula (1) and the formula (4)
；
Then the last level approximation method is adopted to output the lower bridge arm step wave voltage
As in equation (5);
3) The step wave voltage
The calculation formula of (2) is as follows:
the high frequency module outputs a shaping voltage
At the time, the high frequency module is output with a voltage reference value +.>
And triangle carrier->
The following comparison is performed to output +.>
Three levels;
1) When (when)
When (1):
if it is
Output->
The method comprises the steps of carrying out a first treatment on the surface of the If->
Outputting 0;
2) When (when)
When (1):
if it is
Output->
The method comprises the steps of carrying out a first treatment on the surface of the If->
Outputting 0;
the high frequency module outputs a voltage reference value
Output voltage of hybrid modular multilevel converter>
With low frequency module ladder voltage +>
Is the difference between (a):
the low-frequency modules of the upper bridge arm and the lower bridge arm adopt a sequencing voltage equalizing method to realize voltage stabilization;
the high-frequency module comprises four working states, namely:
working state 1: upper switch tube of converter sub-module
Switch on and off>
Turn-off, high-voltage DC side current +.>
Wherein the inflow high-voltage direct current side is negative, and the outflow high-voltage direct current side is positive; at this time, when the upper switch tube of the high-frequency sub-module
Opening, wherein the high-frequency module outputs 0 level, and the high-frequency module bypasses the capacitor; lower switch tube of high-frequency submodule
Opening, if the upper bridge arm outputs +.>
Level, if in lower bridge arm, high frequency module outputs +>
The level, the high-frequency module capacitor discharges;
working state 2: upper switch tube of converter sub-module
Switch on and off>
Turn-off, high-voltage DC side current +.>
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the upper switching tube of the high frequency sub-module is +.>
Opening, wherein the high-frequency module outputs 0 level, and the high-frequency module bypasses the capacitor; lower switching tube of high-frequency submodule>
Opening, if the upper bridge arm outputs +.>
Level, if in lower bridge arm, high frequency module outputs +>
The level, the high-frequency module capacitor charges;
working state 3: lower switch tube of current conversion sub-module
Switch on and switch on tube->
Turn-off, high-voltage DC side current +.>
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the upper switching tube of the high frequency sub-module is +.>
Opening, if the upper bridge arm outputs +.>
Level, if in lower bridge arm, high frequency module outputs +>
The level, the high-frequency module capacitor discharges; lower switch tube of high-frequency submodule
Opening, wherein the high-frequency module outputs 0 level, and the high-frequency module bypasses the capacitor; />
Working state 4: lower switch tube of current conversion sub-module
Switch on and switch on tube->
Turn-off, high-voltage DC side current +.>
The method comprises the steps of carrying out a first treatment on the surface of the At this time, when the upper switching tube of the high frequency sub-module is +.>
Opening, if the upper bridge arm outputs +.>
Level, if in lower bridge arm, high frequency module outputs +>
The level, the high-frequency module capacitor charges; lower switch tube of high-frequency submodule
On, high frequency modeThe block outputs 0 level, and the high-frequency module capacitor bypasses;
defining the working states 1 and 2 of the high-frequency module as a mode 1, and defining the working states 3 and 4 as a mode 2;
1) The voltage equalizing process of the high-frequency module of the upper bridge arm comprises the following steps:
step S11, calculating the number of low-frequency submodules required to be put into each moment of the upper bridge arm through the step (2)
；
Step S12, calculating each of the previous time and the previous time
Comparing, judging +.>
Whether a change occurs; if the three phases are not changed, the voltage equalizing operation is not cut in, and the step S11 is performed; if there is any one of the three phases +.>
When the pressure changes, the pressure equalizing operation is switched into step S13;
step S13, judging the high-voltage direct-current side current
Positive and negative of (a);
if it is
Judging whether there is one or several phases of +.>
If so, enter mode 1 and calculate +.>
The method comprises the steps of carrying out a first treatment on the surface of the If not, then judge the DC side capacitor voltage of the upper bridge arm high frequency module +.>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the upper bridge arm +.>
If yes, enter mode 2 and calculate +.>
If not, enter mode 1 and calculate +.>
；
If it is
Judging whether there is one or several phases of +.>
If so, enter mode 1 and calculate +.>
The method comprises the steps of carrying out a first treatment on the surface of the If not, then judge the DC side capacitor voltage of the upper bridge arm high frequency module +.>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the upper bridge arm +.>
If yes, enter mode 1 and calculate +.>
If not, enter mode2 and calculating +.>
；
2) The voltage equalizing process of the high-frequency module of the lower bridge arm comprises the following steps:
step S21, calculating the number of low-frequency submodules required to be put into each moment of the lower bridge arm through the step (4)
；
Step S22, calculating each of the previous time and the previous time
Comparing and judging two moments
Whether a change occurs; if the three phases are not changed, the voltage equalizing operation is not cut in, and the step S21 is performed; if there is any one of the three phases +.>
If the pressure is changed, the pressure equalizing operation is switched into step S23;
step S23, judging the high-voltage DC side current
Positive and negative of (a);
if it is
Judging whether there is one or several phases of +.>
If so, enter mode 2 and calculate +.>
If not, then judging the DC side capacitor electricity of the lower bridge arm high frequency modulePressure->
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the lower bridge arm +.>
If yes, enter mode 2 and calculate using equation (9)
If not, enter mode 1 and calculate +.>
；/>
If it is
Judging whether there is one or several phases of +.>
If so, enter mode 2 and calculate +.>
If not, then judging the DC side capacitor voltage of the lower bridge arm high frequency module>
Whether is larger than the average value of the capacitor voltage at the direct current side of the low-frequency submodule of the lower bridge arm +.>
If yes, enter mode 1 and calculate +.>
If not, the method comprises, if not,mode 2 is entered and +.>
。/>
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