CN112260563A - Improved model prediction control method for 2N +1 type single-phase MMC - Google Patents
Improved model prediction control method for 2N +1 type single-phase MMC Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
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Abstract
The invention discloses an improved model prediction control method for a 2N +1 type single-phase MMC. According to the method, the comparison between the capacitor voltage average value and the reference value is added into a model prediction control algorithm, and if the average value is larger than the reference value, the number of the sub-modules required to be added is shown. Conversely, if the average value is lower than the reference value, it indicates that the sub-modules invested should be reduced. So that the capacitor voltage is stabilized at the reference value without modifying the cost equation. Furthermore, using this method, the number of options traversed is reduced to a maximum of 6 during each rolling optimization.
Description
Technical Field
The invention belongs to the technical field of application of a high-voltage high-power electronic technology in a power system, and particularly relates to an improved model prediction control method for a 2N +1 type single-phase MMC (Modular Multilevel Converter).
Background
With the rapid development of power electronics and control strategies, multilevel power converters have found widespread use in power systems. The MMC has wide application prospect in high-voltage and high-power application due to the outstanding characteristics of modularization, redundancy, low harmonic distortion, high efficiency and the like.
The traditional control method is based on the closed-loop control of a traditional linear Proportional Integral (PI) controller, and can achieve a control target by adjusting control parameters. And the optimal parameters are not easy to obtain, and the factors of the switching frequency, the gain of the controller, the bandwidth and the like need to be considered. Model Predictive Control (MPC) is of great interest for its superior dynamic performance and controllability and is widely used in MMC applications. Furthermore, the MPC method is able to control multiple control targets by using a defined cost equation.
An improved MPC of MMC is provided in 'MMC model predictive control mixed with half-bridge and full-bridge submodules' published in Chinese patent No. CN109256964B, the maximum number of conducted full-bridge submodules in a negative level state is M, the voltage sum of conducted submodules of an upper bridge arm and a lower bridge arm in each phase is ensured to be a fixed value, the selected combination is N-M, when N is very large, the calculation complexity is very high, and in the transient process, voltage level jump can occur, which leads to higher dv/dt. In the article "A Voltage-Level-Based Model Predictive Control of modulated Multi Level Converter" published by IEEE Transactions on Industrial Electronics (author F. Zhang et al), a Model Predictive Control method applied to output 2N +1 Voltage Level is proposed, and the number of turn-on SMs is not fixed to N in order to generate 2N +1 output Voltage Level. Therefore, another control target, namely energy balance of the arms, needs to be added into the cost equation to ensure the capacitor voltage balance of the sub-modules, but it is not easy to accurately determine the influence factors of the bridge arm energy in the cost equation.
Disclosure of Invention
The purpose of the invention is as follows: aiming at the problems and the defects of the existing model predictive control of the MMC outputting the 2N +1 voltage level, the invention provides an improved model predictive control method for the 2N +1 type single-phase MMC, which can avoid the design of bridge arm energy balance influence factors, and can reduce the harmonic content of output current and avoid the voltage level jump in the transient process.
The technical scheme is as follows: the invention discloses an improved model prediction control method for a 2N +1 type single-phase MMC. Wherein, the AC output end of the single-phase MMC is connected with the seriesOutput inductor LsAnd an output resistor RSAnd the upper bridge arm and the lower bridge arm have the same structure and are respectively connected in series with a branch inductor L, a parasitic resistor R and N half-bridge submodules. The method comprises the following steps:
(S1) determining the number combination set of the upper and lower bridge arm conduction sub-modules for realizing 2N +1 modulation at the next moment based on the number of the upper bridge arm conduction sub-modules at the current moment;
(S2) correcting the set of the number combination of the upper and lower bridge arm conducting submodules at the next moment determined in the step (S1) according to the capacitor voltage of each submodule at the current moment;
(S3) constructing a cost equation g related to the output current and the circulation current at the next moment, and finding out the number combination which enables the cost equation g to be minimum from the number combination set of the upper and lower bridge arm conduction sub-modules at the next moment after correction to serve as the optimal number combination; the output current and the circulation current at the next moment are both related to the number of the upper and lower bridge arm conduction sub-modules at the next moment;
(S4) respectively sequencing the capacitance voltage values of the sub-modules of the upper and lower bridge arms according to the current polarities of the upper and lower bridge arms at the current moment, and determining the conduction state of each sub-module at the next moment according to the sequencing result based on the optimal number combination so as to maintain the capacitance voltage balance.
Further, the step (S1) specifically includes the steps of:
(S11) according to the current time, turning on the bridge armuThe value of (2) determines the number of the upper bridge arm breakthroughs at the next moment: when n isuWhen the value of (1) is equal to 0, the conducting number of the upper bridge arm at the next moment is selected from 0 and 1; when n isuWhen the value of the N is equal to N, the number of the upper bridge arm breakthroughs at the next moment is selected from N, N-1; when n isuWhen the value of (1) is other values, the number of the upper bridge arm breakover at the next moment has three selectable items, respectively nu-1、nu、nu+1；
(S12) determining the conduction number of the lower bridge arm at the next time according to the conduction number of the upper bridge arm at the next time; the sum of the conducting numbers of the upper bridge arm and the lower bridge arm at the next moment has three selectable items, namely N-1, N and N + 1;
(S13) obtaining a set of all possible combinations of the numbers of the upper and lower bridge arm conduction submodules at the next moment according to the determination results of the steps (S11) and (S12).
Further, the ith sub-module capacitor voltage v at the current momentci(t) and the ith sub-module capacitor voltage v at the next momentci(t+Ts) The following discrete equation for the sub-module capacitance voltage is satisfied:
wherein S isiAs a switching function of the ith sub-module, SiWhen 1, the i-th sub-module is currently charged or discharged, SiAt 0, the ith half-bridge submodule is neither charged nor discharged at the present time, TsIs the sampling period.
Further, the step (S2) specifically includes: calculating the average capacitance voltage v of each submodule at the current moment according to the capacitance voltage of each submodule at the current momentcV is to becAnd a reference capacitor voltage vrefComparing; when v iscGreater than vrefWhen the number of the conducted sub-modules is too small, the number of the conducting sub-modules of the upper bridge arm and the lower bridge arm is increased at the next moment, so that the sum of the number of the conducting sub-modules of the upper bridge arm and the conducting sub-modules of the lower bridge arm at the next moment is N +1 or N, and the average capacitance voltage of the sub-modules at the next moment is reduced; when v iscLess than vrefAnd when the number of the conducted sub-modules is too large, reducing the number of the conducted sub-modules of the upper and lower bridge arms at the next moment, and enabling the sum of the number of the conducted sub-modules of the upper and lower bridge arms at the next moment to be N-1 or N so as to increase the average capacitance voltage of the sub-modules at the next moment.
Further, in step (S3), the cost equation g is expressed as:
wherein λ is1、λ2Is an influencing factor; i.e. is *(t+Ts) Is referred to at the next momentOutputting current; i.e. is(t+Ts)、icir(t+Ts) Respectively outputting current and circulating current at the next moment; and:
wherein v isl(t+Ts)＝nlvref，vu(t+Ts)＝nuvref，vu(t+Ts) And vl(t+Ts) The output voltages n of the upper and lower bridge arms at the next moment when the capacitor voltage is balancedu、nlThe number v of the upper and lower bridge arm conducting sub-modules at the next moment respectivelyrefIs a reference capacitor voltage; i.e. is(t)、icir(t) current and circulating current are output at the current moment respectively; vdcIs a dc supply voltage.
Further, the step (S4) specifically includes: determining the current polarities of an upper bridge arm and a lower bridge arm at the current moment, and: recording the optimal number combination as (n)u *,nl *),nu *、nl *Respectively determining the number of the upper and lower bridge arm conduction sub-modules at the next moment which minimizes the cost equation g; if the current polarities of the upper bridge arm and the lower bridge arm at the current moment are positive: respectively sequencing the capacitor voltages of the upper and lower bridge arm sub-modules in ascending order, and respectively selecting the front nu *Upper bridge arm submodule and front nl *The lower bridge arm sub-modules enable the selected sub-modules to work in a conducting state at the next moment and enable the unselected sub-modules to work in a disconnecting state at the next moment; if the current polarities of the upper bridge arm and the lower bridge arm at the current moment are negative: respectively sequencing the capacitor voltages of the upper bridge arm submodule and the lower bridge arm submodule in a descending order; and respectively select the front nu *Upper bridge arm submodule and front nl *Each lower bridge arm submodule, making the selected submoduleThe module works in a conducting state at the next moment, and the unselected sub-modules work in a switching-off state at the next moment; if the current polarity of the upper bridge arm is positive at the current moment, the current polarity of the lower bridge arm is negative: sequencing the capacitor voltages of the sub-modules of the upper bridge arm in an ascending order; n before selectionu *The upper bridge arm submodules are arranged, the capacitor voltages of the lower bridge arm submodules are sorted in descending order, and the front n is selectedl *Each lower bridge arm submodule; enabling the selected sub-modules to work in a conducting state at the next moment, and enabling the unselected sub-modules to work in a switching-off state at the next moment; if the current polarities of the upper bridge arm and the lower bridge arm are negative at the current moment, the current polarities of the lower bridge arm are positive: sorting the capacitor voltages of the sub-modules of the upper bridge arm in a descending order; n before selectionu *The upper bridge arm submodules sort the capacitor voltages of the lower bridge arm submodules in ascending order, and select the front nl *Each lower bridge arm submodule; and enabling the selected sub-modules to work in a conducting state and charge the selected sub-modules at the next moment, and enabling the unselected sub-modules to work in a switching-off state at the next moment.
Has the advantages that: compared with the prior art, the invention has the following advantages:
1. currently, there is less research on the application of MPC at 2N +1 output voltage. The 2N +1 output voltage level of the MMC has better THD performance than the N +1 level.
2. And comparing the average value of the capacitor voltage with a reference value, and if the average value is larger than the reference value, indicating that the input quantity of the sub-modules needs to be increased. Conversely, if the average value is lower than the reference value, it indicates that the sub-modules invested should be reduced. So that the capacitor voltage is stabilized at the reference value without modifying the cost equation. Therefore, no bridge arm energy influence factor needs to be set, and no modulation cost equation is needed. The balance of the capacitor voltage is ensured.
3. The harmonic content of the output current is less, and voltage level jump cannot occur in the transient process. This is because the number of bridge arm switches in the present application remains substantially constant, and the number of upper and lower bridge arm switches remains constant or is selected only among adjacent numbers. Therefore, the voltage level jump can not occur when the voltage level at the current moment is ignored when the optimal switching sequence is selected like the traditional MMC model prediction control.
Drawings
Fig. 1 is a block diagram of a modular multilevel converter topology involved in the present invention;
fig. 2 is an overall control block diagram of a modular multilevel converter involved in the present invention;
FIGS. 3(a) and 3(b) are diagrams of combinations of switches of all 2N +1 levels under model predictive control and of a comparison strategy of applied capacitor voltages, respectively;
FIGS. 4(a) and 4(b) are a simulation of a comparison strategy for capacitor voltage without and a simulation of a comparison strategy for capacitor voltage with the proposed addition, respectively;
FIGS. 5(a) and 5(b) are the average and instantaneous number of the upper and lower bridge arm conducting switch sums, respectively, without and with a capacitance-voltage comparison strategy;
fig. 6 is a simulation diagram of output voltage.
Detailed Description
The present invention is further illustrated by the following figures and specific examples, it is to be understood that these examples are given solely for the purpose of illustration and are not intended to limit the scope of the invention, which is to be determined by the appended claims as a matter of routine modification by those skilled in the art.
The method is mainly used for the capacitance voltage balance control of the MMC outputting the 2N +1 voltage level under model prediction control. As shown in fig. 1, the capacitance voltage balancing method of this embodiment is based on a single-phase MMC topology structure, wherein a dc power supply is connected to an input side of the single-phase MMC, and a dc power supply voltage is VdcThe midpoint of the direct current power supply is grounded, and two ends of the direct current power supply are connected with the positive pole and the negative pole of the upper bridge arm and the lower bridge arm of the MMC. The single-phase MMC is divided into an upper arm and a lower arm, each arm is formed by connecting N identical half-bridge submodules and a branch inductor L in series, the power loss of each arm of the single-phase MMC is simulated by using a parasitic resistor R in series, and in the single-phase MMC, each half-bridge submodule consists of a half-bridge conversion unit which comprises two Insulated Gate Bipolar Transistor (IGBT) switches, an anti-parallel diode related to the Insulated Gate Bipolar Transistor (IGBT) switches and a Direct Current (DC) switchAnd a capacitor. The half-bridge sub-modules have two states depending on the switch combination. When the upper switch S1 is closed and the lower switch S2 is turned off, the sub-modules are in a closed state (i.e., a conducting state), and the output voltage u is outputsmEqual to the capacitor voltage uc. Conversely, when the upper switch S1 is turned off and the lower switch S2 is turned on, the sub-module is bypassed and the output voltage u is outputsmEqual to zero, the submodule is in an off state.
Fig. 2 shows a general control block diagram of the present invention, and three blocks on the left side in the block diagram represent discrete quantity prediction, rolling optimization process and capacitor voltage sequencing control respectively. The purpose of the discrete quantity prediction is to predict the change of the output current and the circulation current at the next moment along with the combination of different numbers of the conduction sub-modules of the upper and lower bridge arms at the next moment; in the rolling optimization process, a cost equation is constructed according to the difference between the output current and the circulation current at the next moment and respective reference values, and the number combination of the upper and lower bridge arm conduction sub-modules at the next moment, which enables the cost equation to be minimum, is found out to be used as the optimal combination, and the combination can ensure that the output current and the circulation current at the next moment are closest to the respective reference values; and the capacitor voltage sequencing control is used for sequencing the capacitor voltages of the upper and lower bridge arms respectively according to the number of the conducting sub-modules of the upper and lower bridge arms determined in the optimal combination and the polarity of the bridge arm current at the current moment so as to ensure the capacitor voltage balance of the sub-modules.
In the prediction solution of the discrete quantity, the output current, the circulation current and the sub-module capacitor voltage at the next moment need to be predicted.
Specifically, the discrete-time equation of the output current and the circulating current can be derived from the euler antecedent equation:
in the formulae (1) and (2), TsTo sample time, vl(t+Ts) Is the capacitor voltage at the next momentOutput voltage v of lower bridge arm in balanceu(t+Ts) The output voltage of the upper bridge arm is the voltage of the capacitor in voltage balance at the next moment. V if the capacitor voltage is considered to have reached equilibriumci＝vref,i＝1,2,…,N，vciThe capacitor voltage for each sub-module. Then there is vl(t+Ts)＝nl'vref，vu(t+Ts)＝nu'vref，nu'、nl' the number of the upper and lower bridge arm conduction sub-modules at the next moment, vrefIs a reference capacitor voltage; i.e. is(t)、icir(t) current and circulating current are output at the current moment respectively; vdcIs a dc supply voltage.
According to the difference of the switching functions, the discrete equation of the sub-module capacitor voltage at the next moment is as follows:
wherein S isiAs a switching function of the ith sub-module, SiWhen 1, the i-th sub-module is currently charged or discharged, SiAt 0, the ith half-bridge submodule is neither charging nor discharging at the present time.
The control targets of the MMC are: 1) outputting a current tracking reference current; 2) circulating current suppression; 3) the sub-module capacitors are voltage balanced. In order to reduce the complexity of calculation, the capacitor voltage balance is completed in a sequencing mode. A cost equation is thus constructed from the difference between the output current and the circulating current at the next instant and the respective reference values. Since the reference value of the loop current is 0, the cost equation in the model predictive control is defined as:
wherein λ is1、λ2Is an influencing factor. i.e. is *(t+Ts) The output current reference value for the next moment is determined by an artificially given sinusoidal signal, and is knownIs sinusoidal over time.
As can be seen from equations (1) and (2), in order to predict the output current and the circulating current at the next time, the number n of the upper and lower bridge arm conduction sub-modules at the next time needs to be predictedu'、nl'。
In fig. 3(a), the numbers n of upper bridge arm conduction submodules are comparedu' to find the number n of conduction submodules of the corresponding lower bridge arml', thereby achieving a 2N +1 voltage level output. Specifically, in order to realize the rapid model predictive control, the number of the bridge arms to be conducted needs to be kept basically stable, and the number of the upper bridge arm and the lower bridge arm to be conducted is kept as constant as possible or is adjacent to each other. For example, if the upper arm conduction at the current time is 3, the predicted upper arm conduction number at the next time is found under three conduction numbers of 2, 3 and 4. That is, the number of the upper bridge arms is kept unchanged or is only the adjacent number. And the lower bridge arm is obtained by subtracting the conduction number of the upper bridge arm from the total conduction number.
Therefore, firstly, the conducting number n of the upper bridge arm in the previous period is judgeduWhen n is a value ofuWhen the value of (1) is equal to 0 or N, the number of upper bridge arm breakthroughs at the next moment can only be selected from 0, 1 or N, N-1. When n isuWhen the value of (a) is other values, the number of the upper bridge arm breakover at the next moment has three selectable items (n)u-1、nu、nu+1). In order to make the output voltage level be 2N +1, the total sum of the conducting numbers of the upper bridge arm and the lower bridge arm is N-1, N and N + 1. Therefore, a set of the number combinations of all possible upper and lower bridge arm conduction sub-modules at the next moment can be obtained. Meanwhile, no matter the number N of the sub-modules, the number of the available combinations in each rolling optimization process is at most 9.
After determining the number of possible upper bridge arm conducting submodules at the next moment according to the method, a voltage comparison link is required to be added, and the sum of the number of the upper bridge arm conducting submodules and the number of the lower bridge arm conducting submodules at the next moment are corrected by comparing the average voltage of the submodules with the capacitance reference voltage, which specifically comprises the following steps:
when the average sub-module capacitance voltage is larger than the capacitance reference voltage, the number of conducted sub-modules is too small, the number of conducted sub-modules needs to be increased in the next period, the total conducting number in the next period is N +1 or N, so that the average sub-module capacitance voltage is reduced, when the average sub-module capacitance voltage is smaller than the capacitance reference voltage, the number of conducted sub-modules is too large, the number of conducted sub-modules needs to be reduced in the next period, and the total conducting number in the next period is N-1 or N, so that the average sub-module capacitance voltage is increased. And finally determining a set of the number combination of the upper and lower bridge arm conduction sub-modules at the next moment according to the corrected sum of the number of the upper and lower bridge arm conduction sub-modules at the next moment.
It can be seen from the above modification process that no matter the number N of sub-modules, the number of combinations that can be selected in the rolling optimization process is at most 6. And (3) in the rolling optimization process, replacing each number combination into the equations (1) and (2), predicting output current and circulation current at the next moment, substituting the predicted output current and circulation current at the next moment into the cost equation in the equation (4), calculating the combination which enables the cost equation g to be minimum at the current moment, and applying the combination to capacitor voltage sequencing control to determine the conduction state of each submodule at the next moment and maintain the capacitor voltage balance.
Fig. 4(a) and 4(b) are a simulation diagram of a capacitance-voltage comparison policy not added and a simulation diagram of a capacitance-voltage comparison policy added, respectively. There is a jump in the output current reference at 0.5 seconds, and it can be seen from fig. 4(a) that the capacitor voltage is stable but deviates from the reference voltage value, while fig. 4(b) that the capacitor voltage is stable and stabilizes above the reference voltage value.
Fig. 5(a) and 5(b) are the average and instantaneous number of the upper and lower bridge arm conducting switch sums, respectively, without and with a capacitance-to-voltage comparison strategy. It can be seen that the average number of switches without the capacitance voltage comparison strategy is greater than N, which is why the capacitance voltage deviates from the reference value in fig. 4 (a). The average switch number added with the capacitance voltage comparison strategy is stabilized at N, namely the sub-module capacitance voltage is balanced at the reference voltage.
Fig. 6 is a graph of output voltage after a capacitance-voltage comparison strategy is added, where N is 8, the output voltage is 17 levels, and a 2N +1 output voltage is realized. It can be seen that there are no multi-level jumps because the optimum range of the upper arm conduction number at the next instant is limited to a voltage level equal to or adjacent to the current instant.
Claims (6)
1. An improved model prediction control method for a 2N +1 type single-phase MMC is provided, wherein an alternating current output end of the single-phase MMC and an output inductor L connected in series are providedsAnd an output resistor RSThe bridge arms are connected, the upper bridge arm and the lower bridge arm have the same structure and are respectively connected with a branch inductor L, a parasitic resistor R and N half-bridge submodules in series, and each half-bridge submodule comprises 2 insulated gate bipolar transistors IGBT and 1 capacitor; the method comprises the following steps:
(S1) determining the number combination set of the upper and lower bridge arm conduction sub-modules for realizing 2N +1 modulation at the next moment based on the number of the upper bridge arm conduction sub-modules at the current moment;
(S2) correcting the set of the number combination of the upper and lower bridge arm conducting submodules at the next moment determined in the step (S1) according to the capacitor voltage of each submodule at the current moment;
(S3) constructing a cost equation g according to the difference between the output current and the circulation current at the next moment and respective reference values, and finding out the number combination which enables the cost equation g to be minimum from the corrected set of the number combinations of the upper and lower bridge arm conduction sub-modules at the next moment to serve as the optimal number combination; the output current and the circulation current at the next moment are obtained by predicting according to the number of the upper and lower bridge arm conduction sub-modules at the next moment;
(S4) respectively sequencing the capacitance voltage values of the sub-modules of the upper and lower bridge arms according to the current polarities of the upper and lower bridge arms at the current moment, and determining the conduction state of each sub-module at the next moment according to the sequencing result based on the optimal number combination so as to maintain the capacitance voltage balance.
2. The improved model predictive control method for a 2N +1 type single-phase MMC as claimed in claim 1, wherein the step (S1) comprises the steps of:
(S11) according to the current time, turning on the bridge armuDetermines the next moment of timeThe number of upper bridge arms in conduction is as follows: when n isuWhen the value of (1) is equal to 0, the conducting number of the upper bridge arm at the next moment is selected from 0 and 1; when n isuWhen the value of the N is equal to N, the number of the upper bridge arm breakthroughs at the next moment is selected from N, N-1; when n isuWhen the value of (1) is other values, the number of the upper bridge arm breakover at the next moment has three selectable items, respectively nu-1、nu、nu+1；
(S12) determining the conduction number of the lower bridge arm at the next time according to the conduction number of the upper bridge arm at the next time; the sum of the conducting numbers of the upper bridge arm and the lower bridge arm at the next moment has three selectable items, namely N-1, N and N + 1;
(S13) obtaining a set of all possible combinations of the numbers of the upper and lower bridge arm conduction submodules at the next moment according to the determination results of the steps (S11) and (S12).
3. The improved model predictive control method for a 2N +1 type single-phase MMC of claim 1, characterized in that the ith sub-module capacitor voltage v at the present momentci(t) and the ith sub-module capacitor voltage v at the next momentci(t+Ts) The following discrete equation for the sub-module capacitance voltage is satisfied:
wherein S isiAs a switching function of the ith sub-module, SiWhen 1, the i-th sub-module is currently charged or discharged, SiAt 0, the ith half-bridge submodule is neither charged nor discharged at the present time, TsIs the sampling period.
4. The improved model predictive control method for a 2N +1 type single-phase MMC as claimed in claim 1, wherein the step (S2) specifically includes:
calculating the average capacitance voltage v of each submodule at the current moment according to the capacitance voltage of each submodule at the current momentcV is to becAnd a reference capacitor voltage vrefComparing; when v iscIs greater thanvrefWhen the number of the conducted sub-modules is too small, the number of the conducting sub-modules of the upper bridge arm and the lower bridge arm is increased at the next moment, so that the sum of the number of the conducting sub-modules of the upper bridge arm and the conducting sub-modules of the lower bridge arm at the next moment is N +1 or N, and the average capacitance voltage of the sub-modules at the next moment is reduced; when v iscLess than vrefAnd when the number of the conducted sub-modules is too large, reducing the number of the conducted sub-modules of the upper and lower bridge arms at the next moment, and enabling the sum of the number of the conducted sub-modules of the upper and lower bridge arms at the next moment to be N-1 or N so as to increase the average capacitance voltage of the sub-modules at the next moment.
5. The improved model predictive control method for a 2N +1 type single-phase MMC as claimed in claim 1, wherein in step (S3), the cost equation g is expressed as:
wherein λ is1、λ2Is an influencing factor; i.e. is *(t+Ts) Reference output current for next moment; i.e. is(t+Ts)、icir(t+Ts) Respectively outputting current and circulating current at the next moment; and:
wherein v isl(t+Ts)＝nl'vref，vu(t+Ts)＝nu'vref，vu(t+Ts) And vl(t+Ts) The output voltages n of the upper and lower bridge arms at the next moment when the capacitor voltage is balancedu'、nl' the upper and lower bridge arms are conducted at the next momentNumber of submodules, vrefIs a reference capacitor voltage; i.e. is(t)、icir(t) current and circulating current are output at the current moment respectively; vdcIs a dc supply voltage.
6. The improved model predictive control method for a 2N +1 type single-phase MMC as claimed in claim 1, wherein the step (S4) specifically includes:
recording the optimal number combination as (n)u *,nl *),nu *、nl *Respectively determining the number of the upper and lower bridge arm conduction sub-modules at the next moment which minimizes the cost equation g;
if the current polarities of the upper bridge arm and the lower bridge arm at the current moment are positive: respectively sequencing the capacitor voltages of the upper and lower bridge arm sub-modules in ascending order, and respectively selecting the front nu *Upper bridge arm submodule and front nl *The lower bridge arm sub-modules enable the selected sub-modules to work in a conducting state at the next moment and enable the unselected sub-modules to work in a disconnecting state at the next moment;
if the current polarities of the upper bridge arm and the lower bridge arm at the current moment are negative: respectively sequencing the capacitor voltages of the upper bridge arm submodule and the lower bridge arm submodule in a descending order; and respectively select the front nu *Upper bridge arm submodule and front nl *The lower bridge arm sub-modules enable the selected sub-modules to work in a conducting state at the next moment and enable the unselected sub-modules to work in a disconnecting state at the next moment;
if the current polarity of the upper bridge arm is positive at the current moment, the current polarity of the lower bridge arm is negative: sequencing the capacitor voltages of the sub-modules of the upper bridge arm in an ascending order; n before selectionu *The upper bridge arm submodules are arranged, the capacitor voltages of the lower bridge arm submodules are sorted in descending order, and the front n is selectedl *Each lower bridge arm submodule; enabling the selected sub-modules to work in a conducting state at the next moment, and enabling the unselected sub-modules to work in a switching-off state at the next moment;
if the current polarities of the upper bridge arm and the lower bridge arm are negative at the current moment, the current polarities of the lower bridge arm are positive: to the upper bridgeSorting the capacitance voltages of the arm sub-modules in a descending order; n before selectionu *The upper bridge arm submodules sort the capacitor voltages of the lower bridge arm submodules in ascending order, and select the front nl *Each lower bridge arm submodule; and enabling the selected sub-modules to work in a conducting state and charge the selected sub-modules at the next moment, and enabling the unselected sub-modules to work in a switching-off state at the next moment.
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| 曹穆 等: "基于模块化多电平换流器的STATCOM模型预测控制策略", 《第十六届全国电气自动化与电控系统学术年会论文集》 * |
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