CN112636625A - Improved carrier phase-shifting modulation strategy applied to MMC - Google Patents

Improved carrier phase-shifting modulation strategy applied to MMC Download PDF

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CN112636625A
CN112636625A CN202011549217.XA CN202011549217A CN112636625A CN 112636625 A CN112636625 A CN 112636625A CN 202011549217 A CN202011549217 A CN 202011549217A CN 112636625 A CN112636625 A CN 112636625A
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bridge arm
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白志红
董一诺
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Zhejiang University ZJU
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Zhejiang University ZJU
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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
    • H02M7/53—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
    • 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
    • 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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Abstract

The invention discloses an improved carrier phase-shift modulation strategy applied to MMC, which comprises the following steps: (1) the method comprises the following steps that a carrier phase shift modulation strategy is adopted as a main control strategy for an upper bridge arm or a lower bridge arm of three phases in an MMC, a corresponding capacitance voltage balance strategy is added, a control signal of a sub-module is generated, and the input or the output of the corresponding sub-module is controlled; (2) in the MMC three-phase circuit, for another bridge arm of the same phase of the bridge arm adopting a carrier phase shift modulation strategy in the step (1), calculating the number of input sub-modules by adopting a complementary rule, and selecting input or output of the sub-modules by adopting a voltage sequencing method; (3) and adopting different control strategies for an upper bridge arm and a lower bridge arm in the MMC three-phase circuit, and setting a proper period for exchanging. By utilizing the invention, under the condition of not increasing the circular current of the bridge arm, the capacitance-voltage balance effect of the sub-modules is improved, the inductive voltage of the bridge arm is reduced, the harmonic component of the output voltage is reduced, and the quality of the output voltage is improved.

Description

Improved carrier phase-shifting modulation strategy applied to MMC
Technical Field
The invention belongs to the field of modulation strategies of power electronic equipment, and particularly relates to an improved carrier phase-shifting modulation strategy applied to an MMC.
Background
In recent years, Modular Multilevel Converters (MMC) have been considered as one of the most potential power electronic conversion topologies by researchers due to their advantages of modularity, strong expansibility, good output voltage quality, etc. And is especially suitable for occasions with higher requirements on voltage and power. In addition, the MMC has already realized substantial application in the field of high-voltage flexible direct-current transmission, which shows that the MMC has great practical application value. However, the MMC has a great room for improvement in many aspects, such as a modulation strategy, and has a good research prospect.
The proper modulation strategy is the premise that the MMC works well, and has direct influence on the output voltage performance of the MMC. The modulation strategies commonly used for the MMC at present have a recent level approximation modulation strategy, a space vector modulation strategy, a carrier stacking pulse width modulation strategy and a carrier phase shift modulation strategy.
The recent level approximation modulation strategy is to approximate sinusoidal reference voltage by using a step wave instantaneous value, the principle is simple, but the method is more suitable for the condition that the number of sub-modules is large. Under the condition of less sub-modules, when a nearest level approximation method is adopted, the low-frequency harmonic content of the output waveform is high due to the fact that the number of steps of the output voltage waveform is too small, and the waveform distortion is serious.
The space vector modulation strategy is mainly to approximate the voltage modulation wave by using the nearest space voltage vector, but the synthesis process of the voltage space vector is more complicated. And when the number of the sub-modules is increased, the complexity of the space vector synthesis is increased sharply, so that the method is not suitable for a high-voltage high-power system.
The strategy of carrier wave laminated pulse width modulation is to utilize N same carrier wave laminated to compare with modulation wave to generate control signal to control the input or cut-out of corresponding sub-modules. However, the switching frequency and power of the sub-modules are related to the corresponding carriers, and the switching frequency and power of the sub-modules may be very different from one carrier to another. This may cause a large sub-module capacitor voltage ripple and sub-module power imbalance, which may affect the good operation of the MMC.
The carrier phase shift modulation strategy is to compare N carriers with phase difference of 2 pi/N with modulation wave to generate control signal to control the input or cut-out of corresponding sub-modules.
For example, chinese patent document No. 201810175304.X discloses a double voltage-sharing coefficient voltage balancing method suitable for MMC adopting carrier phase shift modulation, which obtains voltage-sharing coefficients k1i and k2i by using an average value of capacitance voltage of each submodule and voltage of a bridge arm where the submodule is located, achieves capacitance voltage balancing in an uncontrollable pre-charging stage by finely adjusting a duty ratio of an MOSFET in an energy-taking power supply according to k1i, and achieves controllable pre-charging and capacitance voltage balancing in a normal operation stage by finely adjusting a duty ratio of an IGBT in the submodule according to k2 i.
However, under the carrier phase shift strategy, the sub-module capacitance voltage balance needs a complex control strategy, and the voltage balance control strategy can cause distortion of output voltage and change of the number of input sub-modules, which results in generation of a large bridge arm inductance induction voltage.
Disclosure of Invention
The invention provides an improved carrier phase-shift modulation strategy applied to MMC, which uses the existing hardware to improve the capacitor voltage balance effect of a submodule, reduce the inductive voltage of a bridge arm, reduce the harmonic component of output voltage and improve the quality of the output voltage under the condition of not increasing the circular current of the bridge arm.
An improved carrier phase shift modulation strategy applied to MMC comprises the following steps:
(1) the method comprises the following steps that a carrier phase shift modulation strategy is adopted as a main control strategy for an upper bridge arm or a lower bridge arm of three phases in an MMC, a corresponding capacitance voltage balance strategy is added, a control signal of a sub-module is generated, and the input or the output of the corresponding sub-module is controlled;
(2) in the MMC three-phase circuit, for another bridge arm of the same phase of the bridge arm adopting a carrier phase shift modulation strategy in the step (1), calculating the number of input sub-modules by adopting a complementary rule, and selecting input or output of the sub-modules by adopting a voltage sequencing method;
(3) and adopting different control strategies for an upper bridge arm and a lower bridge arm in the MMC three-phase circuit, and setting a proper period for exchanging.
The specific process of the step (1) is as follows:
(1-1) adopting a carrier phase shift modulation strategy for an upper bridge arm or a lower bridge arm of three phases in the MMC to obtain a reference modulation signal of a corresponding sub-module;
(1-2) calculating correction quantity through a capacitance voltage balance strategy according to the specific condition of the capacitance voltage value of each submodule; superposing the correction quantity on the reference modulation signal obtained in the previous step to be used as a final modulation signal of the submodule;
and (1-3) finally, inputting the obtained final modulation signal of each submodule into a carrier phase-shifting modulation module, comparing the final modulation signal with a corresponding carrier to generate a control signal of the submodule, and controlling the input or the output of the submodule.
In the step (1-2), the specific process of calculating the correction amount through the capacitance-voltage balance strategy is as follows:
the capacitance voltage of the sub-module is obtained through sampling, and the capacitance voltage and the rated voltage of the sub-module capacitance are obtained
Figure BDA0002857361800000031
Comparing to obtain an error amount, inputting the error amount into a P controller, and calculating to obtain a preliminary correction amount;
and when the current direction of the bridge arm is positive, multiplying the initial correction by +1, and when the current direction of the bridge arm is negative, multiplying the initial correction by-1 to obtain a result as a final correction.
In the invention, each submodule is provided with two IGBTs, two diodes which are respectively connected with the IGBTs in an anti-parallel mode and an energy storage capacitor which is connected with the diodes in a parallel mode; and each submodule is controlled to be switched into or out of a circuit by controlling the on or off of the two IGBTs.
The input quantity of the submodules of the upper bridge arm and the lower bridge arm is kept constant N, and the pulse width modulation multi-level voltage close to a sine wave is output on the alternating current side by controlling the change of the number of the input submodules of the upper bridge arm and the lower bridge arm through a control signal.
The specific process of the step (2) is as follows:
(2-1) calculating the number n of sub-modules of a bridge arm in an input state by adopting a carrier phase shift modulation strategy in the step (1)i1(i ═ u, v, w), where u, v, w are the three phases respectively;
(2-2) for the other three bridge arms in the three-phase circuit, calculating the number of input submodules by adopting a complementary rule, wherein the formula is ni2(i＝u,v,w)＝N-ni1(i＝u,v,w)；
(2-3) n to be input in three bridge arms not adopting the carrier phase shift modulation strategyi2And (i, u, v, w) the submodules are determined by combining a capacitor voltage sequencing strategy and the current direction of the bridge arm.
The specific implementation process of the step (2-3) is as follows: if the current direction of the bridge arm is positive, namely the sub-module capacitor is in a charging state, selecting n with the lowest voltage according to the sorting result of the sub-module capacitor voltagei2(i ═ u, v, w) a submodule investment; if the current direction of the bridge arm is negative, namely the sub-module capacitor is in a discharging state, selecting n with the highest voltage according to the sorting result of the sub-module capacitor voltagei2And (i ═ u, v, w) investment.
Preferably, in the step (3), the suitable period is selected from a range of one to several tens of periods of the modulated wave in integer multiple, and the specific period duration T needs to be combined with an actual application scenario and MMC platform parameters.
The factors to be considered for setting the specific period duration T include: the capacitance value of the submodule capacitor of the MMC platform, the bridge arm current in a normal running state, and the fluctuation amplitude and speed of the submodule capacitor voltage.
And setting a proper period, and exchanging different control strategies adopted by the upper bridge arm and the lower bridge arm in the MMC three-phase circuit, so that the consistency of the working states of the upper bridge arm and the lower bridge arm can be ensured.
Compared with the prior art, the invention has the following beneficial effects:
1. the number of the submodules which are put into the circuit at the same time is strictly controlled to be N, so that the inductive pulse voltage of the bridge arm is greatly reduced, and the interphase circulating current is also reduced.
2. The invention can realize the good operation of the MMC without adding an additional hardware circuit and only by adjusting a modulation strategy.
3. Compared with the traditional carrier phase-shifting strategy, the sub-module capacitor voltage balance method has the advantages that the sub-module capacitor voltage balance effect is obviously improved, and the carrier phase-shifting strategy has the advantage in the output performance aspect.
4. The invention has high and more concentrated output voltage harmonic component times, and is convenient for filter design or does not need additional filtering equipment.
5. The invention has strong adaptability and is easy to expand. The improved optimization method for the carrier phase shift modulation strategy is also suitable for the invention; the optimized improvement strategy for the voltage sequencing method is also applicable to the invention. The present invention is a top-level modulation strategy with great potential for improvement.
Drawings
FIG. 1 is a three-phase MMC topology circuit;
FIG. 2 is a control block diagram of an improved carrier phase shift modulation strategy applied to MMC;
fig. 3 is a schematic diagram of a principle that a carrier phase shift modulation strategy (only an upper bridge arm or a lower bridge arm employs carrier phase shift modulation at a certain time) is employed in a j-phase (j is u, v, w) when the strategy is implemented specifically;
fig. 4 is a schematic diagram of the principle of applying a complementary rule and a voltage sequencing strategy (only the upper arm or the lower arm of j phase adopts the control mode at a certain time) in j phase (j is u, v, w) when the strategy is implemented specifically;
FIG. 5 is a waveform of capacitance voltage of 3 sub-modules of a u-phase upper bridge arm and a u-phase lower bridge arm when the MMC works at 50 Hz;
FIG. 6 is a 1.0-1.2s three-phase load voltage waveform when the MMC is operated at 50 Hz;
FIG. 7 is a result of FFT analysis of u-phase load voltage waveform when the MMC is operated at 50 Hz;
FIG. 8 is a 1.0-1.2s three-phase load current waveform when the MMC is operated at 50 Hz;
FIG. 9 shows induced voltage waveforms of 1.0-1.2s three-phase upper bridge arm inductors when the MMC is operated at 50 Hz;
FIG. 10 shows induced voltage waveforms of 1.0-1.2s three-phase lower bridge arm inductors when the MMC is operated at 50 Hz.
Detailed Description
The invention will be described in further detail below with reference to the drawings and examples, which are intended to facilitate the understanding of the invention without limiting it in any way.
As shown in FIG. 1, each phase of the three-phase MMC topology structure has an upper part and a lower partThe total of 6 bridge arms of the two bridge arms are provided, each bridge arm is provided with N sub-modules and a bridge arm inductor L, and the voltage of a direct-current side bus is Udc. Each submodule is provided with two IGBTs, two diodes which are connected with the IGBTs in an anti-parallel mode, and an energy storage capacitor which is connected with the diodes in parallel. And each submodule is controlled to be switched into or out of a circuit by controlling the on or off of the two IGBTs. When T is1Is turned on and T2Turning off, the submodule throw-in circuit outputs capacitance voltage v to the outsidecpji(vcnji) (ii) a When T is1Is turned off and T2When the circuit is conducted, the submodule switching-out circuit outputs voltage 0 to the outside. The input quantity of the upper bridge arm submodule and the lower bridge arm submodule is kept to be constant N so as to maintain the stability of the capacitance voltage and the direct current voltage of the submodules. Meanwhile, the number of input submodules of the upper bridge arm and the lower bridge arm is controlled to change through a control signal, so that the pulse width modulation multilevel voltage close to a sine wave is output at an alternating current side.
As shown in fig. 2, a control block diagram of an improved carrier phase-shift modulation strategy applied to MMC is shown, wherein j (j ═ u, v, w) is given. As known from a control block diagram, at a certain moment, a bridge arm of each three-phase circuit in the MMC calculates a control signal of the submodule by adopting a carrier phase-shifting modulation method. And the control signals of the other three corresponding bridge arm sub-modules are generated by a complementary rule and a voltage sequencing condition.
Taking u-phase as an example, at a certain moment, the upper u-phase bridge arm is controlled by adopting a carrier phase-shifting modulation strategy, and the lower u-phase bridge arm is controlled by adopting a complementary rule and voltage sequencing. The carrier phase shift modulation strategy is matched with a capacitance voltage balance strategy, and N modulation waves are used for comparing with carriers corresponding to each submodule to obtain a control signal of each submodule.
Firstly, neglecting the induced voltage on the bridge arm inductance, and according to kirchhoff's voltage law, obtaining a u-phase upper bridge arm reference voltage expression as follows:
Figure BDA0002857361800000061
wherein u ispurefIs a reference voltage of the U-phase upper arm, UdcIs a DC side bus voltage, Eurefcos (ω t) is an ideal value of the u-phase output voltage.
The reference voltage of the upper bridge arm is reduced to N sub-modules, and a reference modulation signal of each sub-module of the upper bridge arm can be obtained as follows:
Figure BDA0002857361800000071
wherein D ispurefA reference modulation signal for the u-phase upper bridge arm submodule,
Figure BDA0002857361800000072
is the u-phase modulation ratio.
After the reference modulation signal of each sub-module is obtained through the carrier phase shift modulation strategy, the correction quantity calculated through the capacitance voltage balance strategy is added according to the specific situation of the capacitance voltage value of each sub-module.
Taking the first submodule of the u-phase upper bridge arm as an example, the capacitor voltage of the first submodule is obtained through sampling and is vcpu1And the rated voltage of the sub-module capacitor is compared with the rated voltage
Figure BDA0002857361800000073
Comparing to obtain error amount, inputting the error amount into P controller to obtain initial correction amount Dp1. When the current direction of the bridge arm is positive, the correction quantity is multiplied by +1, and when the current direction of the bridge arm is negative, the correction quantity is multiplied by-1; the final correction signal + D will be obtainedp1or-Dp1And superposing the reference modulation signal of the submodule obtained in the front as a final modulation signal of the submodule. And finally, inputting the obtained final modulation signals of the N sub-modules into a carrier phase shift (CPS-PWM) modulation module, and comparing the final modulation signals with corresponding carriers to generate sub-module driving signals.
As shown in fig. 3, the basic principle of the carrier phase shift modulation strategy is disclosed. The sinusoidal modulation signals are present in the figure, and it can be seen from the figure that the sinusoidal modulation signals of the upper and lower bridge arms differ by 180 °. A series of fixed angle differences are required to complete carrier phase shift modulationAnd each submodule corresponds to one triangular carrier. Mutual displacement between sub-module carriers of the same bridge arm
Figure BDA0002857361800000074
And the carriers of the sub-modules corresponding to the upper bridge arm and the lower bridge arm are the same. The modulation signal is compared with the carrier signal during modulation, and when the modulation signal is greater than the triangular carrier signal, the sub-module T1Conduction, T2And (4) switching off, connecting the sub-module capacitor into the main circuit, and otherwise, cutting off the sub-module. Therefore, the corrected sine modulation signal is compared with the triangular carrier to obtain the driving signal of each submodule, and the u-phase upper bridge arm is controlled to carry out a carrier phase shift modulation operation mode with a voltage balance effect.
As shown in fig. 4, the control principle of the lower arm in u phase is shown. Firstly, calculating n of bridge arm input submodule in real timei1The number of (i ═ u, v, w) is given by the formula ni2(i＝u,v,w)＝N-ni1(i ═ u, v, w) calculating the number n of submodules to be put into the circuit by the lower bridge armi2(i＝u,v,w)。
And then if the current direction of the bridge arm is positive, namely the sub-module capacitor is in a charging state, selecting n with the lowest voltage according to the sorting result of the sub-module capacitor voltagei2(i ═ u, v, w) a submodule investment; if the current direction of the bridge arm is negative, namely the sub-module capacitor is in a discharging state, selecting n with the highest voltage according to the sorting result of the sub-module capacitor voltagei2And (i ═ u, v, w) investment.
And as shown in the control block diagram of fig. 2, selecting a proper period T to periodically exchange different control strategies adopted by the upper and lower bridge arms in the u-phase circuit of the MMC. In the application example, simulation parameters and actual operation conditions are considered, and the exchange period T is set to be 5 modulation wave periods. After 5 modulation wave periods, the lower bridge arm is controlled by adopting a carrier phase-shifting strategy, and the upper bridge arm calculates the number of input sub-modules by adopting a complementary rule and selects according to a voltage sequencing result and the current direction of the bridge arm.
When the induced voltage on the bridge arm inductance is neglected, according to kirchhoff's voltage law, the expression of the reference voltage of the u-phase lower bridge arm can be obtained as follows: :
Figure BDA0002857361800000081
wherein u isnjrefIs a U-phase lower bridge arm reference voltage, UdcIs a DC side bus voltage, Eurefcos (ω t) is an ideal value of the u-phase output voltage.
The reference voltage of the lower bridge arm is reduced to N sub-modules, and a reference modulation signal of each sub-module of the lower bridge arm can be obtained as follows:
Figure BDA0002857361800000082
wherein D isnurefA reference modulation signal for the u-phase lower bridge arm submodule,
Figure BDA0002857361800000083
is the u-phase modulation ratio.
After obtaining the reference modulation signal of each sub-module, a correction quantity calculated by a capacitance-voltage balance strategy is added according to the specific situation of the capacitance voltage value of each sub-module.
Taking the first submodule of the u-phase lower bridge arm as an example, the capacitor voltage of the u-phase lower bridge arm is obtained through sampling and is vcnu1And the rated voltage of the sub-module capacitor is compared with the rated voltage
Figure BDA0002857361800000091
Comparing to obtain error amount, inputting the error amount into P controller to obtain initial correction amount Dn1. When the current direction of the bridge arm is positive, the correction quantity is multiplied by +1, and when the current direction of the bridge arm is negative, the correction quantity is multiplied by-1; the final correction signal + D will be obtainedn1or-Dn1And superposing the reference modulation signal of the submodule obtained in the front as a final modulation signal of the submodule. Finally, inputting the obtained final modulation signal of each submodule into a carrier phase shift (CPS-PWM) modulatorAnd the system module is used for comparing with the corresponding carrier wave to generate a submodule driving signal.
As shown in fig. 3, the basic principle of the carrier phase shift modulation strategy is disclosed. The sinusoidal modulation signals are present in the figure, and it can be seen from the figure that the sinusoidal modulation signals of the upper and lower bridge arms differ by 180 °. A series of triangular carriers with fixed angle difference are also needed for completing carrier phase shift modulation, and each sub-module corresponds to one triangular carrier. Mutual displacement between sub-module carriers of the same bridge arm
Figure BDA0002857361800000092
And the carriers of the sub-modules corresponding to the upper bridge arm and the lower bridge arm are the same. The modulation signal is compared with the carrier signal during modulation, and when the modulation signal is greater than the triangular carrier signal, the sub-module T1Conduction, T2And (4) switching off, connecting the sub-module capacitor into the main circuit, and otherwise, cutting off the sub-module. Therefore, the corrected sine modulation signal is compared with the triangular carrier to obtain the driving signal of each submodule, and the u-phase lower bridge arm is controlled to carry out a carrier phase shift modulation operation mode with a voltage balance effect.
As shown in fig. 4, the control principle of the upper arm in the u phase is shown. Firstly, calculating the number n of lower bridge arm input sub-modules in real timei2(i ═ u, v, w), and is represented by the formula: n isi1(i＝u,v,w)＝N-ni2(i ═ u, v, w) calculating the number n of submodules to be put into the upper bridge armi1(i＝u,v,w)。
And then if the current direction of the bridge arm is positive, namely the sub-module capacitor is in a charging state, selecting n with the lowest voltage according to the sub-module capacitor voltage sequencing resulti1(i ═ u, v, w) a submodule investment; if the current direction of the bridge arm is negative, namely the sub-module capacitor is in a discharging state, selecting n with the highest voltage according to the voltage sequencing result of the sub-module capacitori1And (i ═ u, v, w) investment.
And after 5 modulation wave periods, the control strategies are exchanged again, and the process is repeated. In this way, the consistency of the working states of the upper bridge arm and the lower bridge arm is ensured.
The simulation model parameters are shown in Table 1
TABLE 1
Figure BDA0002857361800000101
Fig. 5 shows the waveforms of the capacitance voltages of the u-phase upper and lower bridge arm 3 sub-modules when the MMC works at 50Hz, and it can be seen that the capacitance voltages of the 3 sub-modules of the same bridge arm are very close under this method. The capacitor voltage fluctuation of the upper bridge arm submodule and the lower bridge arm submodule is opposite, and the ripple wave of the upper bridge arm submodule and the lower bridge arm submodule is about 8V, which is 8 percent of the rated voltage and is less than 10 percent.
As shown in fig. 6, when the MMC is operated at 50Hz, the load voltage exhibits a better sinusoidal characteristic without an additional filtering device.
As shown in fig. 7, when the MMC is operated at 50Hz, without additional filtering device, the output voltage harmonic is mainly concentrated around 3 times of the carrier frequency, and the harmonic frequency is high and concentrated, which is easy to filter.
As shown in fig. 8, when the MMC is operated at 50Hz, the load current exhibits a better sinusoidal characteristic without passing through an additional filtering device.
As shown in fig. 9 and 10, when the MMC works at 50Hz, the amplitude of the induction voltage of the bridge arm inductance is small, and is only 20% of the capacitor voltage of the sub-module, so that the damage caused by the overlarge induction voltage of the bridge arm inductance is effectively reduced.
The embodiments described above are intended to illustrate the technical solutions and advantages of the present invention, and it should be understood that the above-mentioned embodiments are only specific embodiments of the present invention, and are not intended to limit the present invention, and any modifications, additions and equivalents made within the scope of the principles of the present invention should be included in the scope of the present invention.

Claims (9)

1. An improved carrier phase shift modulation strategy applied to MMC is characterized by comprising the following steps:
(1) the method comprises the following steps that a carrier phase shift modulation strategy is adopted as a main control strategy for an upper bridge arm or a lower bridge arm of three phases in an MMC, a corresponding capacitance voltage balance strategy is added, a control signal of a sub-module is generated, and the input or the output of the corresponding sub-module is controlled;
(2) in the MMC three-phase circuit, for another bridge arm of the same phase of the bridge arm adopting a carrier phase shift modulation strategy in the step (1), calculating the number of input sub-modules by adopting a complementary rule, and selecting input or output of the sub-modules by adopting a voltage sequencing method;
(3) and adopting different control strategies for an upper bridge arm and a lower bridge arm in the MMC three-phase circuit, and setting a proper period for exchanging.
2. The improved carrier phase-shifting modulation strategy applied to MMC of claim 1, characterized in that, the specific process of step (1) is:
(1-1) adopting a carrier phase shift modulation strategy for an upper bridge arm or a lower bridge arm of three phases in the MMC to obtain a reference modulation signal of a corresponding sub-module;
(1-2) calculating correction quantity through a capacitance voltage balance strategy according to the specific condition of the capacitance voltage value of each submodule; superposing the correction quantity on the reference modulation signal obtained in the previous step to be used as a final modulation signal of the submodule;
and (1-3) finally, inputting the obtained final modulation signal of each submodule into a carrier phase-shifting modulation module, comparing the final modulation signal with a corresponding carrier to generate a control signal of the submodule, and controlling the input or the output of the submodule.
3. The improved carrier phase shift modulation strategy applied to MMC of claim 2, wherein in step (1-2), the specific procedure of calculating the correction amount through the capacitance voltage balance strategy is as follows:
the capacitance voltage of the sub-module is obtained through sampling, and the capacitance voltage and the rated voltage of the sub-module capacitance are obtained
Figure FDA0002857361790000021
Comparing to obtain an error amount, inputting the error amount into a P controller, and calculating to obtain a preliminary correction amount;
and when the current direction of the bridge arm is positive, multiplying the initial correction by +1, and when the current direction of the bridge arm is negative, multiplying the initial correction by-1 to obtain a result as a final correction.
4. The improved carrier phase-shifting modulation strategy applied to MMC of claim 1, wherein, there are two IGBTs and two diodes connected in anti-parallel with each other, and one energy storage capacitor connected in parallel in each sub-module; and each submodule is controlled to be switched into or out of a circuit by controlling the on or off of the two IGBTs.
5. The improved carrier phase-shift modulation strategy applied to MMC of claim 1, wherein the number of input submodules of the upper bridge arm and the lower bridge arm is kept constant N, and the pulse width modulation multilevel voltage close to sine wave is output at the AC side by controlling the change of the number of the input submodules of the upper bridge arm and the lower bridge arm through control signals.
6. The improved carrier phase-shifting modulation strategy applied to MMC of claim 1, characterized in that, the specific process of step (2) is:
(2-1) calculating the number n of sub-modules of a bridge arm in an input state by adopting a carrier phase shift modulation strategy in the step (1)i1(i ═ u, v, w), where u, v, w are the three phases respectively;
(2-2) for the other three bridge arms in the three-phase circuit, calculating the number of input submodules by adopting a complementary rule, wherein the formula is ni2(i＝u,v,w)＝N-ni1(i＝u,v,w)；
(2-3) n to be input in three bridge arms not adopting the carrier phase shift modulation strategyi2And (i, u, v, w) the submodules are determined by combining a capacitor voltage sequencing strategy and the current direction of the bridge arm.
7. The improved carrier phase-shifting modulation strategy applied to MMC of claim 6, characterized in that, the specific implementation procedure of step (2-3) is as follows: if the current direction of the bridge arm is positive, namely the sub-module capacitor is in a charging state, selecting voltage according to the sorting result of the sub-module capacitor voltageLowest ni2(i ═ u, v, w) a submodule investment; if the current direction of the bridge arm is negative, namely the sub-module capacitor is in a discharging state, selecting n with the highest voltage according to the sorting result of the sub-module capacitor voltagei2And (i ═ u, v, w) investment.
8. The improved carrier phase-shifting modulation strategy applied to MMC of claim 1, wherein in step (3), said suitable period is selected from a range of one to several tens of integral multiples of the period of the modulated wave, and the specific period duration T needs to be combined with the actual application scenario and MMC platform parameters.
9. The improved carrier phase-shifting modulation strategy applied to the MMC of claim 8, wherein the specific period duration T is set by considering the capacitance value of the submodule capacitor of the MMC platform, the bridge arm current in a normal operation state, and the fluctuation amplitude and speed of the capacitor voltage of the submodule.
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