CN115037138A - Model prediction control method of star-shaped bridge arm interphase coupling type MMC (modular multilevel converter) suitable for low-frequency working condition - Google Patents

Model prediction control method of star-shaped bridge arm interphase coupling type MMC (modular multilevel converter) suitable for low-frequency working condition Download PDF

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CN115037138A
CN115037138A CN202210625136.6A CN202210625136A CN115037138A CN 115037138 A CN115037138 A CN 115037138A CN 202210625136 A CN202210625136 A CN 202210625136A CN 115037138 A CN115037138 A CN 115037138A
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bridge arm
arm
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公铮
叶玲
戴鹏
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China University of Mining and Technology CUMT
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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
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
    • H02M1/088—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters for the simultaneous control of series or parallel connected semiconductor devices
    • 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
    • H02M1/123—Suppression of common mode 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
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention discloses a model prediction control strategy of a star bridge arm interphase coupling type MMC suitable for a low-frequency working condition, which comprises the following steps of: step 1: establishing a discrete prediction model of each sub-arm and star-shaped bridge arm on a main bridge arm, and obtaining a control quantity reference value and a sampling value, thereby predicting the optimal bridge arm voltage of the sub-arm and the star-shaped bridge arm; and 2, step: calculating reference values of direct current components and high-frequency circulating current components of bridge arm currents according to power balance conditions so as to realize phase unit capacitance-voltage balance and bridge arm unit capacitance-voltage balance control; and step 3: and determining the switch state of each submodule by combining a modulation strategy and capacitance voltage balance control. The invention realizes the control of the sub-arm capacitance voltage, the star-shaped bridge arm capacitance voltage, the alternating current and the output common-mode voltage of the star-shaped bridge arm interphase coupling type MMC suitable for the low-frequency working condition, greatly reduces the number of PI regulators in a control loop, simplifies the control structure and improves the dynamic response capability of the system.

Description

Model prediction control method of star-shaped bridge arm interphase coupling type MMC (modular multilevel converter) suitable for low-frequency working condition
Technical Field
The invention belongs to the technical field of application of a multi-level converter in a high-voltage high-power alternating-current transmission system, and particularly relates to a model prediction control strategy of a star-shaped bridge arm interphase coupling type MMC suitable for a low-frequency working condition.
Background
The Modular Multilevel Converter (MMC) is one of Multilevel converters, has a highly Modular structure, good redundancy, low switching frequency and low loss, and has an obvious advantage and a wider application range compared with a traditional Multilevel Converter. At present, the MMC has been widely applied to many medium-voltage occasions, such as the field of motor driving and the field of variable-frequency speed-regulating transmission, and can realize stepless speed regulation, improve the dynamic characteristics of a system, and improve the efficiency and quality of a motor driving system. When the MMC is applied to controlling the starting or low-frequency running of a motor, the output voltage can generate distortion. Under the low-frequency working condition, the problem of large amplitude pulsation of the sub-module capacitor voltage can cause adverse effects on the operation safety and stability of the high-voltage high-power alternating-current transmission system. If a proper low-frequency suppression strategy is adopted, the problem is effectively solved, and the MMC can be well developed and applied in the field of alternating-current transmission. The conventional low-frequency suppression strategies include a high-frequency signal injection method and an additional power channel method, and can realize suppression of sub-module capacitor voltage fluctuation under a low-frequency working condition.
However, the traditional high-frequency signal injection method can cause the common-mode voltage on the output side of the converter to be increased, and the common-mode voltage can cause adverse effects on bearings and insulated windings of the motor. A front-end transformer can be added to the engineering to reduce the effect of common mode voltage on the motor, but this reduces the advantage of the modular multilevel converter over other multilevel converters. The star-shaped bridge arm interphase coupling type MMC enables output to be free of high-frequency common-mode voltage injected additionally through a branch additionally arranged at the middle points of the upper bridge arm and the lower bridge arm of each phase. The classical control method of the star-shaped bridge arm interphase coupling type MMC is based on PI closed-loop control, the number of regulators required by a control loop is large, the calculation amount is large, and parameter setting is difficult.
Disclosure of Invention
The method aims at the problems that the number of required controllers is large, parameter adjustment is complex, the calculated amount is large and the like in a control method of a star-shaped bridge arm interphase coupling type MMC based on a PI (proportional-integral) regulator. The invention provides a model prediction control strategy suitable for a star-shaped bridge arm interphase coupling type MMC, which realizes the control of the capacitance voltage of a main bridge arm sub-module, the capacitance voltage of the star-shaped bridge arm sub-module, the output current and the output common-mode voltage under the low-frequency working condition, simplifies a control loop and improves the dynamic response capability of a system.
In order to achieve the above object, the present invention provides a model prediction control strategy for a star bridge arm interphase coupling type MMC under a low-frequency working condition, which directly predicts optimal bridge arm voltages of a main bridge arm and a star bridge arm of the star bridge arm interphase coupling type MMC by using a model prediction algorithm in a current control inner loop, and determines control signals of switching devices in each submodule by combining a modulation strategy and a capacitance-voltage balance control, and specifically comprises the following steps:
s1: discretizing the sub-arm voltage by using a forward Euler method to obtain a voltage in a Kth sampling period T s The discrete domain expression of each sub-arm voltage is as follows:
Figure BDA0003676830000000011
in the formula u j (K) Is the output voltage of j phase Kth sampling period, L is the inductance value of bridge arm, i rjx (K) For the bridge arm current of each sub-arm in the Kth sampling period, i rjx (K +1) is the expected output value of the bridge arm current at the starting moment of the (K +1) th sampling period of each sub-arm, u h (K) For the high-frequency injection voltage of the kth sampling period, r is equal to p, n (p represents an upper arm, n represents a lower arm), j is equal to a, b, c (representing three phases a, b, and c), and x is equal to 1 and 2 (representing two sub-arms in each arm).
At the beginning of each sampling period, the voltage of each sub-arm, that is, the sum of the output voltages of all conducting sub-modules on the sub-arm, can be calculated according to the current of each sub-arm obtained by sampling and the reference value of the current of each sub-arm expected to be output, and thus, a discrete prediction model of the voltage of each sub-arm can be obtained as follows:
Figure BDA0003676830000000021
wherein i pj1_ref 、i pj2_ref Are respectively two upper bridge arms of j phasesReference value of sub-arm current, i nj1_ref 、i nj2_ref The reference values of the currents of the two sub-arms of the j-phase lower bridge arm are respectively, and similarly, the obtained star-shaped bridge arm voltage discrete prediction model is as follows:
Figure BDA0003676830000000022
wherein i hj_ref Is a reference value, i, of the high-frequency circulating current component of the j-phase bridge arm current hj (K) Is a high frequency circulating current of j phase Kth sampling period.
S2: according to the discrete prediction models of the sub-arms and the star-shaped bridge arms established in the S1, in order to obtain the optimal voltage value of each bridge arm at the moment K +1, the current reference value and the high-frequency circulating current reference value of each sub-arm need to be solved, and the expression of the current reference value of each sub-arm is as follows:
Figure BDA0003676830000000023
wherein i dj_ref 、i j_ref And the reference values are respectively the direct current component and the fundamental frequency component of the j-phase bridge arm current.
The fundamental frequency component reference value is a three-phase sinusoidal signal known by a prediction algorithm, the reference values of the direct current component and the high frequency component are calculated according to a power balance condition, the system energy is dispersedly stored in the sub-module capacitor, the power loss of the converter is ignored, and the obtained power balance condition is as follows:
Figure BDA0003676830000000024
phase capacitance voltage balance control enables submodule capacitance voltage mean value u of each phase Cj_av Follow capacitor voltage rating U cap . DC current component theoretical value i calculated by power balance condition dj Then, the compensation amount Delta i of the DC component generated by the phase voltage outer loop controller is superposed dj Forming a DC reference value i dj_ref (ii) a Bridge arm capacitor voltage balance control for realizingThe high-frequency current reference value i can be obtained by balancing the capacitance and the voltage of the upper and lower bridge arms and combining the power balance condition hj_ref 。
And obtaining reference value expressions of the sub-arm current and the star-shaped bridge arm current according to the calculated direct current shunt reference value, the high-frequency component reference value and the fundamental frequency component reference value, and substituting the reference value expressions into discrete prediction expressions of the sub-arm voltage and the star-shaped bridge arm voltage to obtain the optimal voltage value of each bridge arm in the next sampling period.
S3: and (3) obtaining the optimal working state of each submodule through a proper modulation method and a capacitance-voltage balance control algorithm for the optimal bridge arm voltage of each bridge arm at the K +1 th moment predicted in the S2, wherein the specific method comprises the following steps:
for two sub-arms on each bridge arm in the main bridge arm, each sub-arm is provided with N/2 half-bridge sub-modules, the condition that N is small is considered, the outer-layer phase unit adopts carrier phase shift modulation, the inner-layer sub-arm unit adopts uniform pulse width modulation, and therefore the optimal switching states [ N ] of the two sub-arms are obtained respectively optrj1 ,N optrj2 ]：
Figure BDA0003676830000000031
Wherein u is rj1_ref 、u rj2_ref Predicted voltages, U, for two sub-arms in each bridge arm respectively cap For sub-module capacitor voltage rating, D rj1 、D rj2 The duty cycles of the submodules in the PWM state in the two sub-arms are respectively.
And after obtaining the number of conducted sub-modules in each sub-arm, balancing the sub-arm sub-module capacitor voltage and determining the working state of each sub-module by using a sorting voltage-sharing algorithm, and defining the current flowing through two sub-arms in each bridge arm as i rj1 、i rj2 。
Determining the switch state of each submodule of the sub-arm I: if i rj1 More than 0, arranging N/2 sub-module capacitance voltages in the sub-arm I in ascending order, and selecting the front N optrj1 The sub-modules are in 'on' state, so that the capacitors are fully charged, and other sub-modules work in the process of switchingRemoving the state; if i rj1 Less than or equal to 0, arranging N/2 sub-module capacitance and voltage in the sub-arm I in descending order, and selecting the first N optrj1 The submodules are in the 'on' state, so that the capacitors are fully discharged, and other submodules work in the cutting-off state.
Determining the switch state of each submodule of the second submodule: if i rj2 When the voltage is more than 0, the N/2 sub-modules in the sub-arm II are arranged in an ascending order of the capacitor voltage, and the first N is selected optrj2 The sub-modules are in an 'on' state, so that the capacitors are fully charged, and other sub-modules work in an off state; if i rj2 Less than or equal to 0, arranging N/2 sub-module capacitance and voltage in the sub-arm II in descending order, and selecting the first N optrj2 The submodules are in the 'on' state, so that the capacitors are fully discharged, and other submodules work in the cutting-off state.
For a star-shaped bridge arm, each star-shaped bypass branch is provided with N/4 full-bridge submodules, carrier phase shift modulation is adopted in consideration of the condition that N is small, for the full-bridge submodules, switching devices are divided into two groups, modulation signals of the two groups of submodules are complementary, the triangular carrier phase of each submodule in the bridge arm is staggered by pi/N angles in sequence, and when a modulation signal u is modulated rsj_ref And (4) outputting 1 when the instantaneous value of the triangular carrier is larger than the instantaneous value, and otherwise, outputting 0, so that the trigger signal of the full-bridge submodule on the star-shaped bridge arm can be obtained.
Compared with the prior art, the method realizes the balance control of the capacitance voltage of the bridge arm submodule and the inhibition of the high-frequency common-mode voltage at the output side, does not need to establish a value function, saves a large amount of variable prediction calculation, enables the switching frequency of a device to be fixed, obtains better output characteristics, and improves the dynamic response capability of the system compared with the traditional control method based on a PI regulator.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a schematic diagram of a study object of a star bridge arm interphase coupling type MMC topology structure;
FIG. 3 is a single-phase equivalent circuit diagram of a star-shaped bridge arm interphase coupling type MMC;
FIG. 4 is a block diagram of a star-shaped bridge arm interphase coupling type MMC control principle based on a model prediction algorithm.
Detailed Description
The invention will be further explained with reference to the drawings.
As shown in fig. 1, the invention provides a control method based on a model prediction algorithm for a star-shaped bridge arm interphase coupling type MMC suitable for a low-frequency working condition, so as to simplify a control loop and reduce control complexity. The model prediction control method of the embodiment is based on a star-shaped bridge arm interphase coupling type MMC topological structure, as shown in fig. 2, the topology is composed of three phase units in total, and each phase is composed of an upper bridge arm, a lower bridge arm and two bypass branches, wherein the upper bridge arm, the lower bridge arm and the two bypass branches comprise N sub-modules with the same structure. Wherein the DC power supply voltage is U dc (ii) a The voltage and the current of each sub-arm of the upper bridge arm are respectively u pjx 、i pjx (ii) a The voltage and the current of each sub-arm of the lower bridge arm are respectively u njx 、i njx (ii) a The voltage and the current of the upper star-shaped bridge arm and the lower star-shaped bridge arm are respectively u rsj 、i rsj R is p, n (p represents the upper arm, n represents the lower arm), j is a, b, c (representing the three phases a, b, c), x is 1,2 (representing the two sub-arms in each arm). The middle points (p, N) of the upper and lower bridge arms of each phase are respectively connected with a bypass branch formed by N/4 full-bridge submodules, the main bridge arm is the same with the traditional MMC in topology, and the bypass branches of the three phases form star connection, so the MMC is called as a star bridge arm interphase coupling type MMC. The star-shaped bridge arm interphase coupling type MMC reduces the interphase power fluctuation by constructing an interphase power transmission channel at the midpoint of a three-phase bridge arm, realizes energy balance and simultaneously ensures that no high-frequency common-mode voltage exists on an output side.
Fig. 3 shows a single-phase equivalent circuit diagram of the star-shaped bridge arm interphase coupling type MMC. The star-shaped bypass branches equally divide an upper bridge arm and a lower bridge arm of each phase into two parts, which are called sub-bridge arms, namely each phase unit consists of 6 bridge arms including an upper sub-arm, a lower sub-arm and two star-shaped bypasses. For convenience of analysis, each bridge arm can be equivalent to a controllable voltage source, and a half-bridge submodule is adopted on each sub-arm, so that the bridge arms are equivalent to a unipolar controllable voltage source; the star-shaped bridge arm adopts a full-bridge submodule, so that the star-shaped bridge arm is equivalent to a bipolar controllable voltage source. The expressions of the upper bridge arm voltage and the lower bridge arm voltage are as follows:
Figure BDA0003676830000000041
wherein L is bridge arm inductance, u j For j-phase output voltage, i pj1 、i pj2 Are respectively the current flowing through the two sub-arms of the upper bridge arm, i nj1 、i nj2 The currents respectively flow through the two sub-arms of the lower bridge arm. The topology is used for solving the problem that the common-mode voltage of the output side is increased due to the traditional high-frequency signal injection method, so that the current flowing through each branch circuit comprises a fundamental frequency component, a high-frequency component and a direct-current component, and the expression of each component is as follows:
Figure BDA0003676830000000042
in order to avoid the high-frequency signals from appearing in the common-mode voltage, the two sub-arms belonging to the same bridge arm are injected with high-frequency voltage signals with equal magnitude and opposite directions. At this time, the expression of each sub-arm voltage is:
Figure BDA0003676830000000043
the expression of each sub-arm current is:
Figure BDA0003676830000000051
the star bypass is used for exchanging power fluctuation among three phases, and the voltages of upper and lower star bridge arms of each phase are as follows:
Figure BDA0003676830000000052
the currents of the upper star-shaped bridge arm and the lower star-shaped bridge arm of each phase are as follows:
Figure BDA0003676830000000053
to mitigate the sub-module capacitor voltage ripple at low frequency conditions, a high frequency voltage signal is injected into each sub-arm voltage. According to the relation between the output voltage amplitude and the direct current voltage, obtaining an expression of the high-frequency voltage as follows:
Figure BDA0003676830000000054
where m is the modulation ratio of the inverter, S h Representing a switching function, which may be sinusoidal or square wave in form.
The control principle block diagram of the star-shaped bridge arm interphase coupling type MMC based on the model prediction algorithm is shown in fig. 4, the control strategy uses the model prediction algorithm for a current control inner ring, firstly directly predicts the optimal bridge arm voltages of a star-shaped bridge arm interphase coupling type MMC main bridge arm and a star-shaped bridge arm, and then generates control signals of each submodule switching device by combining with a modulation strategy and voltage-sharing control.
The method specifically comprises the following steps:
s1: discretizing the sub-arm voltage by using a forward Euler method to obtain a voltage in a Kth sampling period T s The discrete domain expression of each sub-arm voltage is as follows:
Figure BDA0003676830000000055
in the formula u j (K) Is the output voltage of j-phase Kth sampling period, L is the inductance value of the bridge arm, i rjx (K) For the bridge arm current of each sub-arm in the Kth sampling period, i rjx (K +1) is the expected output value of the bridge arm current at the starting moment of the (K +1) th sampling period of each sub-arm, u h (K) For the high-frequency injection voltage of the kth sampling period, r is equal to p, n (p represents an upper arm, n represents a lower arm), j is equal to a, b, c (representing three phases a, b, and c), and x is equal to 1 and 2 (representing two sub-arms in each arm).
At the beginning of each sampling period, the voltage of each sub-arm, that is, the sum of the output voltages of all conducting sub-modules on the sub-arm, can be calculated according to the sub-arm current obtained by sampling and the reference value of the sub-arm current expected to be output, so that a discrete prediction model of the voltage of each sub-arm can be obtained as follows:
Figure BDA0003676830000000061
wherein i pj1_ref 、i pj2_ref Reference values i of currents of two sub-arms of a j-phase upper bridge arm respectively nj1_ref 、i nj2_ref And the reference values are respectively the current of two sub-arms of the j-phase lower bridge arm. Similarly, the obtained star-shaped bridge arm voltage discrete prediction model is as follows:
Figure BDA0003676830000000062
wherein i hj_ref Is a reference value, i, of the high-frequency circulating current component of the j-phase bridge arm current hj (K) Is a high frequency circulating current of j phase Kth sampling period.
S2: according to the discrete prediction models of the sub-arms and the star-shaped bridge arms established in the S1, in order to obtain the optimal voltage value of each bridge arm at the moment K +1, the current reference value and the high-frequency circulating current reference value of each sub-arm need to be solved, and the expression of the current reference value of each sub-arm is as follows:
Figure BDA0003676830000000063
wherein i dj_ref 、i j_ref And the reference values are respectively the direct current component and the fundamental frequency component of the j-phase bridge arm current.
The fundamental frequency component reference value is a three-phase sinusoidal signal known by a prediction algorithm, the reference values of the direct current component and the high frequency component are calculated according to a power balance condition, the system energy is dispersedly stored in the sub-module capacitor, the power loss of the converter is ignored, and the obtained power balance condition is as follows:
Figure BDA0003676830000000064
phase capacitance voltage balance control enables submodule capacitance voltage mean value u of each phase Cj_av Follow capacitor voltage rating U cap . DC current component theoretical value i calculated by power balance condition dj Then, the compensation amount Delta i of the DC component generated by the phase voltage outer loop controller is superposed dj Forming a DC reference value i dj_ref (ii) a The bridge arm capacitance-voltage balance control is used for realizing the balance of the upper bridge arm capacitance voltage and the lower bridge arm capacitance voltage, and the high-frequency current reference value i can be obtained by combining the power balance condition hj_ref 。
And obtaining reference value expressions of the sub-arm current and the star-shaped bridge arm current according to the calculated direct current shunt reference value, the high-frequency component reference value and the fundamental frequency component reference value, and substituting the reference value expressions into discrete prediction expressions of the sub-arm voltage and the star-shaped bridge arm voltage to obtain the optimal voltage value of each bridge arm in the next sampling period.
S3: and (3) obtaining the optimal working state of each submodule through a proper modulation method and a capacitance-voltage balance control algorithm for the optimal bridge arm voltage of each bridge arm at the K +1 th moment predicted in the S2, wherein the specific method comprises the following steps:
for two sub-arms on each bridge arm in the main bridge arm, each sub-arm is provided with N/2 half-bridge sub-modules, the condition that N is small is considered, the outer-layer phase unit adopts carrier phase shift modulation, the inner-layer sub-arm unit adopts uniform pulse width modulation, and therefore the optimal switching states [ N ] of the two sub-arms are obtained respectively optrj1 ,N optrj2 ]：
Figure BDA0003676830000000071
Wherein u is rj1_ref 、u rj2_ref Predicted voltages, U, for two sub-arms in each bridge arm respectively cap For sub-module capacitor voltage rating, D rj1 、D rj2 The duty cycles of the submodules in the PWM state in the two sub-arms, respectively.
Then obtaining the number of the conducted sub-modules in each sub-arm, and utilizingThe sequencing voltage-sharing algorithm is used for balancing the capacitance and voltage of the sub-arm sub-modules and determining the working state of each sub-module, and the current flowing through two sub-arms in each bridge arm is defined as i rj1 、i rj2 。
Determining the switch state of each submodule of the sub-arm I: if i rj1 More than 0, arranging N/2 sub-module capacitance voltages in the sub-arm I in ascending order, and selecting the front N optrj1 The sub-modules are in an 'on' state, so that the capacitors are fully charged, and other sub-modules work in an off state; if i rj1 Less than or equal to 0, arranging N/2 sub-module capacitors in the sub-arm I in a descending order, and selecting the first N optrj1 The submodules are in the 'on' state, so that the capacitors are fully discharged, and other submodules work in the cutting-off state.
Determining the switch state of each submodule of the second submodule: if i rj2 When the voltage is more than 0, the N/2 sub-modules in the sub-arm II are arranged in an ascending order of the capacitor voltage, and the first N is selected optrj2 The sub-modules are in an 'on' state, so that the capacitors are fully charged, and other sub-modules work in an off state; if i rj2 Less than or equal to 0, arranging N/2 sub-module capacitance and voltage in the sub-arm II in descending order, and selecting the first N optrj2 The submodules are in an 'on' state, so that the capacitors are fully discharged, and other submodules work in an off state.
For a star-shaped bridge arm, each star-shaped bypass branch is provided with N/4 full-bridge submodules, carrier phase shift modulation is adopted in consideration of the condition of smaller N, for the full-bridge submodules, switching devices are divided into two groups, modulation signals of the two groups of submodules are complementary, the triangular carrier phase of each submodule in the bridge arm is staggered by an angle of pi/N in sequence, and when a modulation signal u is generated rsj_ref And (4) outputting 1 when the instantaneous value of the triangular carrier is larger than the instantaneous value, and otherwise, outputting 0, so that the trigger signal of the full-bridge submodule on the star-shaped bridge arm can be obtained.

Claims (2)

1. The model prediction control strategy of the star-shaped bridge arm interphase coupling type MMC suitable for the low-frequency working condition is characterized in that a model prediction algorithm is used for a current control inner ring, the optimal bridge arm voltages of a star-shaped bridge arm interphase coupling type MMC main bridge arm and a star-shaped bridge arm are directly predicted, and then a control signal of a switching device in each submodule is determined by combining an appropriate modulation strategy and voltage-sharing control.
The method comprises the following specific steps:
s1: discretizing the sub-arm voltage by a forward Euler method, and obtaining the sub-arm current in the K +1 th sampling period T according to the sub-arm current obtained by sampling and the reference value of the sub-arm current expected to be output s The discrete prediction model of each sub-arm voltage is as follows:
Figure FDA0003676829990000011
in the formula u j (K) Is the output voltage of j phase Kth sampling period, L is the inductance value of bridge arm, i rjx (K) For the bridge arm current of each sub-arm in the Kth sampling period, i rjx_ref For the bridge arm current expected output value u of each sub-arm at the starting moment of the K +1 th sampling period h (K) For the high-frequency injection voltage of the kth sampling period, r is p, n (p represents an upper bridge arm, n represents a lower bridge arm), j is a, b, c (representing three phases a, b, and c), x is 1,2 (representing two sub-arms in each bridge arm), and similarly, the star-shaped bridge arm voltage discrete prediction model is obtained as follows:
Figure FDA0003676829990000012
wherein i hj_ref Is a reference value, i, of the high-frequency circulating current component of the j-phase bridge arm current hj (K) Is a high frequency circulating current of j phase Kth sampling period.
S2: according to the discrete prediction models of the sub-arms and the star-shaped bridge arms established in the S1, in order to obtain the optimal voltage value of each bridge arm at the moment K +1, the current reference value and the high-frequency circulating current reference value of each sub-arm need to be solved, and the expression of the current reference value of each sub-arm is as follows:
Figure FDA0003676829990000013
wherein i dj_ref 、i j_ref And the reference values are respectively the direct current component and the fundamental frequency component of the j-phase bridge arm current.
The fundamental frequency component reference value is a three-phase sinusoidal signal known by a prediction algorithm, the reference values of the direct current component and the high frequency component are calculated according to a power balance condition, the system energy is dispersedly stored in the sub-module capacitor, the power loss of the converter is ignored, and the obtained power balance condition is as follows:
Figure FDA0003676829990000014
phase capacitance voltage balance control enables submodule capacitance voltage mean value u of each phase Cj_av Follow capacitor voltage rating U cap . DC current component theoretical value i calculated by power balance condition dj Then, the compensation amount Delta i of the DC component generated by the phase voltage outer loop controller is superposed dj Forming a DC reference value i dj_ref (ii) a The bridge arm capacitance-voltage balance control is used for realizing the balance of the upper bridge arm capacitance voltage and the lower bridge arm capacitance voltage, and the high-frequency current reference value i can be obtained by combining the power balance condition hj_ref 。
And obtaining reference value expressions of the sub-arm current and the star-shaped bridge arm current according to the calculated direct current shunt reference value, the high-frequency component reference value and the fundamental frequency component reference value, and substituting the reference value expressions into discrete prediction expressions of the sub-arm voltage and the star-shaped bridge arm voltage to obtain the optimal voltage value of each bridge arm in the next sampling period.
S3: and (5) determining the working state of each submodule by using a modulation strategy and a voltage-sharing algorithm for the predicted optimal bridge arm voltage of each bridge arm at the K +1 th moment in the S2.
2. The model predictive control strategy for the star-bridge arm interphase coupling type MMC applicable to the low-frequency working condition of claim 1, characterized in that, in S3, the sub-modules are equalized in terms of capacitance and voltage and the optimal working state of each sub-module is determined, and the specific method is as follows:
the switching condition of each submodule is determined by adopting a proper modulation method and a capacitance-voltage balance control algorithm, and the specific method is as follows:
for two sub-arms on each bridge arm in the main bridge arm, each sub-arm is provided with N/2 half-bridge sub-modules, the condition that N is small is considered, the outer-layer phase unit adopts carrier phase shift modulation, the inner-layer sub-arm unit adopts uniform pulse width modulation, and therefore the number N of the sub-modules input by each sub-arm is obtained respectively optrjx According to the sub-arm current being i rjx And balancing the capacitor voltage of the sub-arm sub-modules by using a sorting voltage-sharing algorithm so as to obtain a control signal of the half-bridge sub-module in the main arm.
For the star-shaped bridge arm, each star-shaped bypass branch is provided with N/4 full-bridge submodules, and the triggering signals of the full-bridge submodules on the star-shaped bridge arm are obtained by adopting carrier phase-shift modulation in consideration of the condition that N is small.
CN202210625136.6A 2022-06-02 2022-06-02 Model prediction control method of star-shaped bridge arm interphase coupling type MMC (modular multilevel converter) suitable for low-frequency working condition Pending CN115037138A (en)

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