CN113193766A - Direct prediction control method and system for circulating current suppression of parallel converter cluster - Google Patents

Direct prediction control method and system for circulating current suppression of parallel converter cluster Download PDF

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CN113193766A
CN113193766A CN202110361613.8A CN202110361613A CN113193766A CN 113193766 A CN113193766 A CN 113193766A CN 202110361613 A CN202110361613 A CN 202110361613A CN 113193766 A CN113193766 A CN 113193766A
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voltage vector
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value
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CN113193766B (en
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张祯滨
李昱
李�真
何汉
孙远翔
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Shandong University
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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
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21—Conversion of ac power input into dc 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
    • H02M7/217—Conversion of ac power input into dc 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
    • H02M7/219—Conversion of ac power input into dc 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 in a bridge configuration
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • 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
    • H02M1/32—Means for protecting converters other than automatic disconnection

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  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

The present disclosure provides a direct prediction control method and system for circulating current suppression of a parallel converter cluster, including: obtaining a three-phase current value of the current moment of the converter cluster, and carrying out coordinate transformation and time delay compensation; predicting the current value at the subsequent moment aiming at the compensated three-phase current value, calculating cost functions according to the predicted value, comparing all the cost functions, and selecting the voltage vector which enables the value to be the minimum as an optimal voltage vector; calculating zero-sequence circulation according to the compensated three-phase current value, and selecting a zero-voltage vector according to the zero-sequence circulation direction at the current k moment; and constructing a composite voltage vector according to the obtained optimal voltage vector and the zero voltage vector to obtain a switching signal sent to the converter so as to eliminate the circulating current. On the premise of accurately controlling the output current, zero-sequence circulating current between the parallel converters can be effectively eliminated.

Description

Direct prediction control method and system for circulating current suppression of parallel converter cluster
Technical Field
The disclosure belongs to the technical fields of new energy grid connection, intelligent micro-grid, power electronic energy conversion and the like, and particularly relates to a direct prediction control method and system for circulation suppression of a parallel converter cluster.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
The construction of a low-carbon and high-efficiency energy system is the core target of energy transformation in China, and new energy access and energy transformation thereof are key means for realizing the target. In recent years, the scale of new energy is continuously enlarged, and the power level of energy conversion and transmission is continuously improved, so the demand of research and development of high-power electronic converter equipment is urgent. To solve the above problems, there are still many limitations only from the viewpoint of increasing the power level of a single machine: the expandability is poor, and the flexible expandability cannot be realized; secondly, the reliability is poor, and the system is failed due to local failure; limited by power semiconductor capacity; and fourthly, the design of a large-capacity passive component is difficult. The modularized design idea is an effective means for solving the problems, namely, a module unit with complete function and low unit capacity is designed, and flexible expansion of the system is realized through parallel operation of a plurality of modules.
The modular design concept has remarkable advantages: the method can be flexibly matched according to requirements, and the effective utilization rate of the current transformation equipment is high; secondly, the reliability is high, the system can continue to operate under the condition of removing the fault module, the fault module is convenient to replace, and the plug and play are easy; the power semiconductor is not limited by the power grade of the power semiconductor; and fourthly, the design scheme of the passive component is mature, and the cost performance is high.
As can be seen from the above discussion, the modular design has significant advantages in terms of hardware design difficulty, reliability, flexibility, and the like, and is an ideal design scheme for high-power electronic converter equipment. However, when multiple power electronic converter devices are operated in parallel, under the condition of only depending on local measurement data (no high-speed communication), each power unit is required to output power according to the capacity of the power unit, and zero sequence circulating current is eliminated. The above requirements present a great challenge to the control of a multi-module unit. The disclosed solutions are based on linear controllers, using a control framework of loop cascades.
For a three-phase alternating current-direct current (AC-DC) converter or a three-phase direct current-alternating current (DC-AC) converter, the disclosed solutions are based on a linear controller, using a control framework of loop cascades. Specifically, the method comprises the following steps: summing the collected three-phase currents to obtain an actual zero-sequence current; subtracting the actual zero-sequence current from the zero-sequence current reference value 0 to obtain an error signal; thirdly, the obtained error signal is sent to a linear controller (such as a PI controller), and zero sequence voltage compensation quantity is obtained through output of the linear controller; fourthly, the zero sequence compensation quantity is superposed with the positive sequence compensation quantity obtained by vector control, so that the output reference voltage of the converter is obtained; modulating the reference value by a PWM modulator, outputting a pulse width signal and driving a current transformer.
The scheme is that zero sequence compensation voltage is superposed on the basis of vector control, so that zero sequence circulating current is restrained. However, the disclosed scheme has the defects of slow dynamic response, difficult multi-objective control realization, difficult nonlinear constraint optimization and the like.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a direct prediction control method for circulation suppression of a parallel converter cluster, which can effectively eliminate zero-sequence circulation among parallel converters.
In order to achieve the above object, one or more embodiments of the present disclosure provide the following technical solutions:
in a first aspect, a direct prediction control method for circulating current suppression of a parallel converter cluster is disclosed, which includes:
obtaining a three-phase current value of the current moment of the converter cluster, and carrying out coordinate transformation and time delay compensation;
predicting the current value at the subsequent moment aiming at the compensated three-phase current value, calculating cost functions according to the predicted value, comparing all the cost functions, and selecting the voltage vector which enables the value to be the minimum as an optimal voltage vector;
calculating zero-sequence circulation according to the compensated three-phase current value, and selecting a zero-voltage vector according to the zero-sequence circulation direction at the current k moment;
and constructing a composite voltage vector according to the obtained optimal voltage vector and the zero voltage vector to obtain a switching signal sent to the converter so as to eliminate the circulating current.
According to the further technical scheme, coordinate transformation is carried out to convert three-phase current in a natural coordinate system into a quadrature coordinate system, so that inter-phase decoupling is achieved.
In a further technical scheme, the delay compensation is the delay compensation of one control period of the three-phase current value after the coordinate transformation, so that the delay caused by a digital controller and an analog-digital conversion process is eliminated.
According to the further technical scheme, the zero voltage vector is selected based on a zero voltage vector selection principle, and the zero voltage vector selection principle is as follows:
Figure BDA0003005774050000031
wherein the content of the first and second substances,
Figure BDA0003005774050000032
is a zero-sequence circulating current, zero-voltage vector v0Zero voltage vector v7。
According to the further technical scheme, a voltage vector output by the power converters in the parallel converter cluster is defined as a voltage vector formed by three-phase output voltages under an orthogonal coordinate system, and the voltage vector is divided into a non-zero voltage vector and a zero voltage vector according to a module value of the voltage vector.
According to the further technical scheme, the zero sequence current in the parallel converter cluster is the circulating current of the parallel converter.
According to the further technical scheme, a section of zero voltage vector is embedded into the optimal voltage vector, the output voltage vector has the freedom degree of zero sequence voltage regulation, so that zero sequence circulating current is eliminated, and the optimal voltage vector applied to the converter is selected according to the cost function minimization principle in a first time period to control grid-connected current; and in a second time period, selecting a zero-voltage vector applied to the current transformer according to the zero-sequence current direction at the current moment so as to eliminate the zero-sequence current.
In a second aspect, a direct prediction control system for circulating current suppression of a parallel converter cluster is disclosed, which includes:
a coordinate transformation and delay compensation unit configured to: carrying out coordinate transformation and time delay compensation on the obtained three-phase current value of the current transformer cluster at the current moment;
a model prediction unit configured to: predicting the current value at the subsequent moment aiming at the compensated three-phase current value;
a minimum cost function and non-zero voltage vector selection unit configured to: calculating a cost function according to the predicted value, comparing all the cost functions, and selecting a voltage vector with the minimum value as an optimal voltage vector;
a zero sequence circulating current calculating unit configured to: calculating zero-sequence circulating current according to the compensated three-phase current value;
a zero voltage vector selection unit configured to: selecting a zero voltage vector according to the zero sequence circulating current direction at the current k moment;
constructing a resultant voltage vector unit configured to: and constructing a composite voltage vector according to the obtained optimal voltage vector and the zero voltage vector to obtain a switching signal sent to the converter so as to eliminate the circulating current.
The above one or more technical solutions have the following beneficial effects:
the invention provides a direct prediction control method aiming at the parallel operation working condition of a distributed converter equipment cluster, and can effectively eliminate zero-sequence circulating current between parallel converters on the premise of accurately controlling output current.
The method completely adopts a direct prediction control framework, on one hand, the physical limit of the converter can be fully exerted, and the dynamic response is fast; on the other hand, various nonlinear constraint conditions can be flexibly processed, and multi-objective optimization is realized.
The method is based on local measurement values, data communication among converters is not needed, and pulse time sequence synchronization is not needed. The design cost is low, the universality is wide, and the method is suitable for application scenes such as an intelligent micro-grid and new energy distributed generation;
the scheme of the invention is a general scheme for the circulation suppression, and can be popularized to scenes such as parallel connection of multi-level converters, driving motors of multiple converters and the like.
Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
The accompanying drawings, which are included to provide a further understanding of the disclosure, illustrate embodiments of the disclosure and together with the description serve to explain the disclosure and are not to limit the disclosure.
Fig. 1 is a power converter circuit topology diagram of a method and system for direct predictive control of circulating current suppression for a converter cluster according to an embodiment of the present disclosure;
fig. 2 is an equivalent average model diagram of a circulation suppression direct prediction control method of a parallel converter cluster and a system power converter circuit according to an embodiment of the disclosure;
fig. 3 is a zero sequence circuit diagram of a method and a system for direct prediction control of circulating current suppression of a parallel converter cluster according to an embodiment of the present disclosure;
fig. 4 is a voltage vector timing diagram proposed by a method and a system for controlling direct prediction of circulating current suppression of a parallel converter cluster according to an embodiment of the present disclosure;
fig. 5 is a control block diagram of a method and a system for controlling direct prediction of circulation suppression of a parallel converter cluster according to an embodiment of the present disclosure.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The embodiments and features of the embodiments in the present disclosure may be combined with each other without conflict.
Example one
In order to describe the above scheme, the following description is first made for the parallel converter cluster:
referring to fig. 1, a circuit topology diagram of a power converter of a parallel converter cluster according to the present invention is shown. The power circuit mainly realizes the conversion of an electric energy form from alternating current to direct current, and the energy can realize bidirectional flow. Single-machine power converters take the three-phase two-level topology as an example, comprising a three-phase inductor La＝Lb＝LcL, parasitic resistance Ra＝Rb＝RcR; three-phase full-bridge power switch Sa，S′a，Sb，S′b，Sc，S′c(ii) a DC bus capacitor Cdc. The plurality of converters are connected in parallel, i.e. the ac side is connected to a three-phase ac bus (grid) and the dc side is connected to a dc bus. The direction of current flowing from the grid into the converter is defined as the positive direction and is marked as ia，ib，icThe output voltage of the converter with respect to the reference point is va，vb，vcThe grid voltage is ea，eb，ecThe voltage on the DC side is Vdc. Each power converter is closed-loop controlled by a local control unit, and data exchange is not needed among controllers.
For ease of explanation below, the following variables are defined.
Definition-1: the voltage vector output by the converter is defined as a voltage vector v formed by three-phase output voltages under an orthogonal coordinate system (namely an alpha beta coordinate system). The expression constituting the voltage vector is as follows:
Figure BDA0003005774050000061
wherein v isx＝SxVdcX ∈ { a, b, c }. Taking a two-level converter as an example, according to different switch states SxAll voltage vectors output by the converter can be obtained, and the total number is 8. According to the modulus value of the voltage vector v, 6 non-zero voltages v are divided1～6 Vector sum 2 zero voltage vectors v0，v7。
Definition-2: the zero sequence voltage and zero sequence current of a three-phase alternating current system are defined as follows:
vz＝(va+vb+vc)，iz＝(ia+ib+ic)
definition 2 shows that the zero-sequence current is the circulating current of the parallel converter.
Continuing with fig. 2, an averaging model of the parallel converter is shown. The expression of (d ') for the output voltage of the ac-side arm can be obtained by averaging each phase of the arm shown in fig. 1 based on the volt-second balance principle'x+dz)·Vdc. Wherein, d'xThe differential mode component of the duty ratio of the upper switching tube of the x phase, dzFor the common mode component of the duty ratio of the upper switch tube, x is formed by { a, b, c }, VdcThe value of the direct current bus voltage is obtained. The expression of the direct current side output current is (d'x+dz)·Ix. Wherein, IxThe average value of the current of the x phase on the alternating current side is shown. Through the analysis, the average model of each phase of bridge arm is adopted to replace the switch model according to the connection relation, and the average model of the variable converter is obtained. For convenience of illustration, the parallel averaging model is derived by connecting two converters in parallel. Note that the above model still applies for the case of multiple converter clusters in parallel.
Continuing to refer to fig. 3, fig. 3 is a zero sequence circuit diagram of a direct prediction control method and system for circulating current suppression of a parallel converter cluster according to the present invention. The innovation point of the invention is a method for restraining the circulation current of the parallel converter, so that the cause of the circulation current needs to be deeply analyzed. The zero sequence circuit is an effective tool for analyzing the generation of the circulating current and is explained as follows.
According to the average voltage model established in fig. 2, the zero sequence current direction is defined as fig. 2, and the alternating side group kirchhoff voltage equation is written.
Figure BDA0003005774050000071
Subscripts 1 and 2 represent variables corresponding to the current transformer 1 and the current transformer 2, respectively. Note that the definition of zero sequence current is: i is0＝I01＝-I02＝Ia1+Ib1+Ic1. Furthermore, the duty cycle constraint is d'a1+d′b1+d′c1＝d′a2+d′b2+d′c2＝0。
Adding the three formulas to obtain:
Figure BDA0003005774050000072
according to the expression, a circuit model of the zero sequence loop, namely fig. 3, can be obtained. The zero-sequence circuit comprises zero-sequence voltage, zero-sequence circuit inductance and zero-sequence circuit resistance. Wherein, the expression of the zero sequence voltage is Vdc·(dz1-dz2) (ii) a An inductance value of (L)1+L2) (ii) a A resistance value of (R)1+R2). Analyzing the zero sequence circuit model, the zero sequence voltage is the essential reason for generating the zero sequence current (circulation). Therefore, by controlling the zero sequence voltage generated by the converter, the circulating current can be completely eliminated.
Fig. 4 is a quantity timing diagram of the direct prediction control method and system for the circulating current suppression of the converter cluster. As can be seen from the analysis of fig. 3, the zero-sequence voltage is the root cause for generating the zero-sequence circulating current, and at the same time, is the only control means for eliminating the zero-sequence circulating current.
The following rules are the theoretical basis of the invention: firstly, a non-zero voltage vector can control grid-connected current, and the zero voltage vector does not influence the grid-connected current; and both the non-zero voltage vector and the zero voltage vector can generate zero-sequence voltage, so that zero-sequence current is influenced. The classic predictive control scheme only outputs one voltage vector in one control period, and the scheme cannot control zero-sequence voltage balance, namely zero-sequence circulating current cannot be eliminated. In order to solve the above problem, the present invention proposes an improved output vector method.
In connection with fig. 4, the following parameters are defined:
Tsis the control period of the system;
0 < alpha < 1 is the proportion of the application time of the preferred vector in one control period to the proportional control period of the whole control period;
αTsi.e. the time during which the preferred vector is applied within one control period.
Overall, a segment of zero voltage vector is embedded in the optimal vector. Therefore, the output voltage vector can have the freedom degree of zero sequence voltage regulation, and therefore zero sequence circulating current is eliminated. The proposed method is shown in FIG. 4(a), at α TsIn a time period, selecting an optimal voltage vector applied to the converter according to a cost function minimization principle to control grid-connected current; at (1-. alpha.) TsAnd in the time period, selecting a zero voltage vector applied to the converter according to the zero sequence current direction at the current moment so as to eliminate the zero sequence current.
On the other hand, the volt-second balance principle is suitable for the situation that the controlled object has a time constant far larger than the switching period. Considering that the grid-connected converter generally satisfies this condition, the proposed modified voltage vector method may be equivalent to fig. 4 (b). Fig. 4(b) divides the zero voltage of fig. 4(a) into two segments and outputs a symmetrical waveform.
Fig. 5 is a control block diagram of a direct prediction control method and system for current transformer cluster circulating current suppression. The main function of the method is to enable the single-machine converter to track the current reference value of the single-machine converter and eliminate zero-sequence circulating current between the parallel converters at the same time. The control scheme mainly comprises a coordinate transformation and delay compensation unit, a model prediction unit, a minimum cost function and non-zero voltage vector selection unit, a zero-sequence circulating current calculation unit, a zero-voltage vector selection unit and a synthetic voltage vector construction unit. The working principle is described in detail as follows.
The first step, the three-phase current value of the current k moment is sampled and sent to a coordinate transformation and delay compensation unit. The coordinate transformation function is to transform the three-phase current under the natural coordinate system
Figure BDA0003005774050000081
Conversion to an orthogonal coordinate system, i.e.
Figure BDA0003005774050000082
And in a coordinate system, inter-phase decoupling is realized, so that the realization of a prediction model unit is facilitated. The coordinate transformation from abc to α β 0 is realized by the following formula.
Figure BDA0003005774050000091
Wherein, Tabc-αβ0For a coordinate transformation matrix, x may represent a column vector of voltages, currents. After coordinate transformation, according to the mathematical model of the controlled object, the pair
Figure BDA0003005774050000092
And carrying out delay compensation of one control period to eliminate delay effect caused by a digital controller and an analog-digital conversion process. The delay compensation is realized by the following formula.
Figure BDA0003005774050000093
Figure BDA0003005774050000094
Figure BDA0003005774050000095
Figure BDA0003005774050000096
Wherein the content of the first and second substances,
Figure BDA0003005774050000097
to function at time k, the non-zero vector acts on α TsThe current value under the alpha beta coordinate system after the time;
Figure BDA0003005774050000098
the current value in the alpha beta coordinate system at the end of the control period,
Figure BDA0003005774050000099
to be applied in [ kT ]s，(k+α)Ts]Voltage vector over time period.
Step two, traversing 8 voltage vectors according to the three-phase current value after delay compensation, namely substituting the 8 voltage vectors into the formula in sequence, calculating a predicted current value, predicting the current value at the moment of k +2, wherein the prediction equation is as follows
Figure BDA00030057740500000910
Figure BDA00030057740500000911
Figure BDA00030057740500000912
Figure BDA00030057740500000913
It should be noted that the digital controller has a delay effect of one control period. Firstly, the delay effect of one control cycle needs to be compensated, i.e. the k +1 time is calculated according to the k time. And traversing 8 vectors based on the value at the moment k +1, calculating the current value at the moment k +2 and calculating a cost function.
Wherein the content of the first and second substances,
Figure BDA0003005774050000101
to start at time k +1, the voltage vector uα，j，uβ，jAction of alpha TsThe current value under the alpha beta coordinate system after the time;
Figure BDA0003005774050000102
the current value in the alpha beta coordinate system at the end of the control period,
Figure BDA0003005774050000103
can be derived by forward pushing uα，j，uβ，jAnd j is the voltage vector which can be output by the converter and belongs to {1, 2.. 8 }. Calculating a cost function J from the predicted values
Figure BDA0003005774050000104
And comparing all the cost functions, and selecting the voltage vector with the minimum value as the optimal voltage vector.
And thirdly, calculating zero-sequence circulating current, wherein the selection of a zero vector directly depends on the sampling current at the moment k, and no time delay effect exists, so that the expression is as follows:
Figure BDA0003005774050000105
and fourthly, selecting a zero voltage vector according to the zero sequence circulating current direction at the current k moment. Taking into account the zero voltage vector v0Corresponding zero sequence voltage value of
Figure BDA0003005774050000106
Has the effect of reducing the zero sequence current (the positive direction of the zero sequence current is defined with reference to fig. 3); corresponding, zero voltage vector v7Corresponding zero sequence voltage value of
Figure BDA0003005774050000107
Has the function of increasing the zero sequence current. Based on the above analysis, the selection principle for formulating the zero voltage vector is as follows:
Figure BDA0003005774050000108
the establishment of the principle can control the zero sequence current to be close to the zero value, and effectively eliminates the influence of circulation current.
Fifthly, constructing a composite voltage vector according to the optimal voltage vector obtained in the second step and the zero voltage vector obtained in the fourth step and the time sequence of figure 4(a) or figure 4(b), and obtaining a switching signal S sent to the convertera，Sb，Sc。
The method provided by the technical scheme of the disclosure abandons a linear control cascade loop, and can remarkably improve the dynamic performance of the converter; in addition, the advantages of inherent multi-objective optimization, easiness in adding nonlinear constraints and the like of predictive control are reserved, and the application prospect is wide.
The technical scheme of the method does not adopt a linear control framework, but adopts model predictive control to realize converter control, and the predictive control is easy to realize multi-objective optimization: only a reasonable cost function needs to be designed, so that multiple control variables are all contained in one unified cost function, and the weight coefficient is adopted to balance the weight of each control target. By solving the optimal problem, multi-objective optimization can be realized. The predictive control has the advantages of fast dynamic response, multi-objective optimization, easy addition of linear constraints and the like.
In addition, the invention provides a circulating current restraining method for realizing the parallel converter cluster based on predictive control. The application of the method can widen the application scene of predictive control, namely, the control of a single converter is expanded to the cluster control of parallel converters.
Example two
The present embodiment aims to provide a computing device, which includes a memory, a processor, and a computer program stored on the memory and executable on the processor, and when the processor executes the computer program, the steps of the control method are implemented.
EXAMPLE III
An object of the present embodiment is to provide a computer-readable storage medium.
A computer-readable storage medium, on which a computer program is stored which, when being executed by a processor, carries out the steps of the above-mentioned control method.
Example four
The present embodiment aims to provide a direct prediction control system for circulating current suppression of a parallel converter cluster, including:
a coordinate transformation and delay compensation unit configured to: carrying out coordinate transformation and time delay compensation on the obtained three-phase current value of the current transformer cluster at the current moment;
a model prediction unit configured to: predicting the current value at the subsequent moment aiming at the compensated three-phase current value;
a minimum cost function and non-zero voltage vector selection unit configured to: calculating a cost function according to the predicted value, comparing all the cost functions, and selecting a voltage vector with the minimum value as an optimal voltage vector;
a zero sequence circulating current calculating unit configured to: calculating zero-sequence circulating current according to the compensated three-phase current value;
a zero voltage vector selection unit configured to: selecting a zero voltage vector according to the zero sequence circulating current direction at the current k moment;
constructing a resultant voltage vector unit configured to: and constructing a composite voltage vector according to the obtained optimal voltage vector and the zero voltage vector to obtain a switching signal sent to the converter so as to eliminate the circulating current.
The steps involved in the apparatuses of the above second, third and fourth embodiments correspond to the first embodiment of the method, and the detailed description thereof can be found in the relevant description of the first embodiment. The term "computer-readable storage medium" should be taken to include a single medium or multiple media containing one or more sets of instructions; it should also be understood to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by a processor and that cause the processor to perform any of the methods of the present disclosure.
Those skilled in the art will appreciate that the modules or steps of the present disclosure described above can be implemented using general purpose computer means, or alternatively, they can be implemented using program code executable by computing means, whereby the modules or steps may be stored in memory means for execution by the computing means, or separately fabricated into individual integrated circuit modules, or multiple modules or steps thereof may be fabricated into a single integrated circuit module. The present disclosure is not limited to any specific combination of hardware and software.
The above description is only a preferred embodiment of the present disclosure and is not intended to limit the present disclosure, and various modifications and changes may be made to the present disclosure by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present disclosure should be included in the protection scope of the present disclosure.
Although the present disclosure has been described with reference to specific embodiments, it should be understood that the scope of the present disclosure is not limited thereto, and those skilled in the art will appreciate that various modifications and changes can be made without departing from the spirit and scope of the present disclosure.

Claims (10)

1. A direct prediction control method for circulating current suppression of a parallel converter cluster is characterized by comprising the following steps:
obtaining a three-phase current value of the current moment of the converter cluster, and carrying out coordinate transformation and time delay compensation;
predicting the current value at the subsequent moment aiming at the compensated three-phase current value, calculating cost functions according to the predicted value, comparing all the cost functions, and selecting the voltage vector which enables the value to be the minimum as an optimal voltage vector;
calculating zero-sequence circulation according to the compensated three-phase current value, and selecting a zero-voltage vector according to the zero-sequence circulation direction at the current k moment;
and constructing a composite voltage vector according to the obtained optimal voltage vector and the zero voltage vector to obtain a switching signal sent to the converter so as to eliminate the circulating current.
2. The direct prediction control method for the circulating current suppression of the parallel current transformer cluster as claimed in claim 1, wherein coordinate transformation is performed to convert three-phase current in a natural coordinate system to an orthogonal coordinate system so as to achieve inter-phase decoupling.
3. The method as claimed in claim 1, wherein the delay compensation is a delay compensation for one control period of a three-phase current value after coordinate transformation, so as to eliminate delay caused by a digital controller and an analog-digital conversion process.
4. The direct prediction control method for the circulating current suppression of the parallel current transformer cluster as claimed in claim 1, wherein the selection of the zero voltage vector is performed based on a selection principle of the zero voltage vector, and the selection principle of the zero voltage vector is as follows:
Figure FDA0003005774040000011
wherein the content of the first and second substances,
Figure FDA0003005774040000012
is a zero-sequence circulating current, zero-voltage vector v0Zero voltage vector v7。
5. The method as claimed in claim 1, wherein the voltage vector output by the power converters in the parallel converter cluster is defined as a voltage vector formed by three-phase output voltages in an orthogonal coordinate system, and is divided into a non-zero voltage vector and a zero voltage vector according to a modulus of the voltage vector.
6. The direct circulation suppression prediction control method of the parallel current transformer cluster as claimed in claim 1, wherein the zero sequence current in the parallel current transformer cluster is the circulation of the parallel current transformer.
7. The direct prediction control method for the circulating current suppression of the parallel current transformer cluster as claimed in claim 1, wherein a section of zero voltage vector is embedded in the optimal voltage vector, the output voltage vector has the freedom of zero sequence voltage regulation, thereby eliminating zero sequence circulating current, and in the first time period, the optimal voltage vector applied to the current transformer is selected according to the cost function minimization principle to control grid-connected current; and in a second time period, selecting a zero-voltage vector applied to the current transformer according to the zero-sequence current direction at the current moment so as to eliminate the zero-sequence current.
8. A direct prediction control system for circulating current suppression of a parallel converter cluster is characterized by comprising:
a coordinate transformation and delay compensation unit configured to: carrying out coordinate transformation and time delay compensation on the obtained three-phase current value of the current transformer cluster at the current moment;
a model prediction unit configured to: predicting the current value at the subsequent moment aiming at the compensated three-phase current value;
a minimum cost function and non-zero voltage vector selection unit configured to: calculating a cost function according to the predicted value, comparing all the cost functions, and selecting a voltage vector with the minimum value as an optimal voltage vector;
a zero sequence circulating current calculating unit configured to: calculating zero-sequence circulating current according to the compensated three-phase current value;
a zero voltage vector selection unit configured to: selecting a zero voltage vector according to the zero sequence circulating current direction at the current k moment;
constructing a resultant voltage vector unit configured to: and constructing a composite voltage vector according to the obtained optimal voltage vector and the zero voltage vector to obtain a switching signal sent to the converter so as to eliminate the circulating current.
9. A computing device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, wherein the steps of the control method according to any of the preceding claims 1 to 7 are implemented when the program is executed by the processor.
10. A computer-readable storage medium, on which a computer program is stored which, when being executed by a processor, is adapted to carry out the steps of the control method according to any one of the preceding claims 1 to 7.
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