CN110112988B - Model prediction control method, controller and system for three-level variable frequency speed control system - Google Patents
Model prediction control method, controller and system for three-level variable frequency speed control system Download PDFInfo
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- CN110112988B CN110112988B CN201910414915.XA CN201910414915A CN110112988B CN 110112988 B CN110112988 B CN 110112988B CN 201910414915 A CN201910414915 A CN 201910414915A CN 110112988 B CN110112988 B CN 110112988B
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- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- H02P27/00—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage
- H02P27/04—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using variable-frequency supply voltage, e.g. inverter or converter supply voltage
- H02P27/06—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using variable-frequency supply voltage, e.g. inverter or converter supply voltage using dc to ac converters or inverters
- H02P27/08—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using variable-frequency supply voltage, e.g. inverter or converter supply voltage using dc to ac converters or inverters with pulse width modulation
- H02P27/12—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using variable-frequency supply voltage, e.g. inverter or converter supply voltage using dc to ac converters or inverters with pulse width modulation pulsing by guiding the flux vector, current vector or voltage vector on a circle or a closed curve, e.g. for direct torque control
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Abstract
The disclosure provides a model prediction control method, a controller and a system of a three-level variable frequency speed control system. The model predictive control method of the three-phase three-level variable frequency speed control system comprises the following steps: dividing space voltage vectors corresponding to the three-phase three-level frequency converter topology into 12 sectors; determining a sector to which a current output voltage vector of the three-phase three-level frequency converter belongs; determining a space voltage vector required by a current sector capable of realizing balance control of midpoint voltage on a direct current side according to the sign of the difference value of upper side capacitor voltage and lower side capacitor voltage of a three-phase three-level frequency converter topology and the direction of each phase current; selecting a space voltage vector which minimizes the cost function from space voltage vectors required by the current sector; the cost function is a module value of the difference between components of the space voltage vector and the current output voltage vector on an alpha beta axis respectively; and the selected space voltage vector is utilized to control the on-off state of the three-phase three-level frequency converter, so that the frequency conversion and speed regulation control of the motor is realized.
Description
Technical Field
The disclosure belongs to the field of power electronic control, and particularly relates to a model predictive control method, a controller and a system for a three-level variable frequency speed control system.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
The alternating current motor is widely applied to the field of alternating current speed regulation of fans, pumps and the like due to the advantages of simple structure, durability, reliability, low cost, easy manufacture and the like. Early alternating current speed regulation systems adopt modes such as rotor circuit series resistance and stator circuit voltage reduction to realize a speed regulation function, but the methods have large power consumption and poor control effect. With the development of power electronic technology, the frequency converter with the alternating-current variable-frequency speed regulation function plays an important role in an alternating-current speed regulation system, and is particularly suitable for alternating-current speed regulation occasions with the purposes of energy conservation and consumption reduction.
In frequency converters, three-phase two-level inverters are a common circuit topology. Compared with a two-level topological structure, the diode clamping three-phase three-level frequency converter has the advantages of more output levels, less output waveform harmonic waves, small voltage and current stress of a switching tube and the like, and is widely applied to the field of high-voltage high-power alternating current speed regulation. However, in the three-level topology, there are two capacitors on the dc side, and the voltages on the two capacitors must be balanced by a control method or a modulation method when the three-level topology is applied. Unbalanced dc side capacitor voltage not only reduces the performance and efficiency of the system, but also reduces the lifetime of the switching device. The inventor finds that in a three-level frequency converter topological structure of a three-level frequency conversion speed regulation system, when a motor is in no-load, light load, low rotating speed and direct current braking, if the balance control of the midpoint voltage on the direct current side is realized by using redundant small vectors, the influence of the small vectors on the midpoint potential is uncertain, and the selection of proper small vectors to control the midpoint voltage balance is difficult.
Disclosure of Invention
In order to solve the above problem, a first aspect of the present disclosure provides a model predictive control method for a three-level variable frequency speed control system, which considers the magnitude of the dc-side capacitor voltage and the direction of each phase current to achieve control of the dc-side midpoint potential balance and variable frequency speed control of a motor when operating in various operating conditions simultaneously.
In order to achieve the purpose, the following technical scheme is adopted in the disclosure:
a three-level variable frequency speed control system model prediction control method, the three-level variable frequency speed control system includes three-phase three-level frequency converter and motor, the control method includes:
dividing space voltage vectors corresponding to the three-phase three-level frequency converter topology into 12 sectors;
determining a sector to which a current output voltage vector of the three-phase three-level frequency converter belongs;
determining a space voltage vector required by a current sector capable of realizing balance control of midpoint voltage on a direct current side according to the sign of the difference value of upper side capacitor voltage and lower side capacitor voltage of a three-phase three-level frequency converter topology and the direction of each phase current;
selecting a space voltage vector which minimizes the cost function from space voltage vectors required by the current sector; the cost function is a module value of the difference between components of the space voltage vector and the current output voltage vector on an alpha beta axis in the three-phase static coordinate system respectively;
and the selected space voltage vector is utilized to control the on-off state of the three-phase three-level frequency converter, so that the frequency conversion and speed regulation control of the motor is realized.
In order to solve the above problem, a second aspect of the present disclosure provides a controller, which considers the voltage of a capacitor on a dc side and the current direction of each phase, so as to realize control of neutral point potential balance on the dc side and variable frequency speed control of a motor when operating in various operating conditions simultaneously.
In order to achieve the purpose, the following technical scheme is adopted in the disclosure:
a controller is connected with a three-phase three-level frequency converter, and the three-phase three-level frequency converter is connected with a motor; the controller includes:
the sector dividing module is used for dividing space voltage vectors corresponding to the three-phase three-level frequency converter topology into 12 sectors;
the sector determining module is used for determining a sector to which a current output voltage vector of the three-phase three-level frequency converter belongs;
the sector required space voltage vector determining module is used for determining a space voltage vector required by the current sector capable of realizing balanced control of the midpoint voltage of the direct current side according to the sign of the difference value of the upper side capacitor voltage and the lower side capacitor voltage of the three-phase three-level frequency converter topology and the direction of each phase current;
the space voltage vector screening module is used for selecting a space voltage vector which enables the cost function to be minimum from space voltage vectors required by the current sector; the cost function is a module value of the difference between components of the space voltage vector and the current output voltage vector on an alpha beta axis in the three-phase static coordinate system respectively;
and the variable-frequency speed regulation control module is used for controlling the switching state of the three-phase three-level frequency converter by utilizing the selected space voltage vector to realize the variable-frequency speed regulation control of the motor.
In order to solve the above problem, a third aspect of the present disclosure provides a three-level variable frequency speed control system, which considers the magnitude of the dc side capacitor voltage and the direction of each phase current to realize the control of the dc side midpoint potential balance and the variable frequency speed control of the motor when operating in various working conditions.
In order to achieve the purpose, the following technical scheme is adopted in the disclosure:
a three-level variable frequency speed control system comprises the controller.
The beneficial effects of this disclosure are:
(1) according to the method for dividing the space voltage vector into 12 sectors, the calculation times of the cost function in one switching period are reduced from 27 times to 4 times, the calculation amount of the model predictive control method of the three-level variable-frequency speed control system is greatly reduced, and the control efficiency is improved; and the model predictive control principle is adopted, so that the response speed is high, simple and understandable.
(2) The cost function disclosed by the invention is a module value of the difference between the space voltage vector and the current output voltage vector on the alpha-beta axis in the three-phase static coordinate system, so that the introduction of a weight coefficient in the cost function is avoided, and the complexity of the algorithm is reduced.
(3) When determining the space voltage vector required by the current sector capable of realizing balanced control of the midpoint voltage on the direct current side, the method not only considers the capacitance voltage on the direct current side, but also considers the current direction of a small vector, so that the midpoint potential balance can be effectively controlled when the motor operates under normal working conditions, and an effective control effect on the midpoint voltage balance on the direct current side can be achieved when the motor operates under the abnormal working conditions of no-load and light-load operation, low-rotation-speed operation, even direct current braking and the like.
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 main circuit diagram of a diode clamping three-level ac speed regulation system according to an embodiment of the present disclosure;
fig. 2 is a three-level space voltage vector diagram provided by an embodiment of the present disclosure;
fig. 3 is an equivalent circuit diagram of an asynchronous motor provided by an embodiment of the present disclosure;
fig. 4 is a block diagram of voltage-transformation, frequency-conversion and speed-regulation (VVVF) control provided in the embodiment of the present disclosure;
FIG. 5(a) is a graph of the effect of small vectors on the midpoint voltage provided by embodiments of the present disclosure example 1;
FIG. 5(b) is a graph of the effect of small vectors on the midpoint voltage provided by embodiments of the present disclosure example 2;
FIG. 5(c) is a plot of the effect of small vectors on the midpoint voltage provided by embodiments of the present disclosure example 3;
FIG. 5(d) is an example 4 of the effect of small vectors on the midpoint voltage provided by an embodiment of the present disclosure;
fig. 6 is a control flow chart of a model predictive control method for a three-level variable frequency speed control system according to an embodiment of the present disclosure.
Detailed Description
The present disclosure is further described with reference to the following drawings and examples.
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.
Interpretation of terms:
VVVF is an abbreviation for Variable Voltage and Variable Frequency, meaning: the variable voltage, variable frequency, that is, variable frequency governing system. The inverter controlled by VVVF is connected with a motor, and the purposes of keeping constant magnetic flux (approximately represented by back electromotive force/frequency) and controlling the rotating speed (proportional to the frequency) of the motor are achieved by simultaneously changing the frequency and the voltage, so that the inverter is mostly applied to frequency converters and belongs to the field of industrial automation.
As shown in fig. 1, the control object of the present embodiment is a diode clamped three-level inverter system.
The system comprises the following parts:
(1) the main circuit is a diode clamping three-level inverter and has 12 IGBT switching tubes and 6 clamping diodes in total;
(2) the direct current side comprises two series capacitors, a neutral point O is formed between the two capacitors, the middle of two clamping diodes of each phase bridge arm is connected with the neutral point O, and the topology can generate + Udc/2、-UdcThe three levels of/2 and 0; wherein, UdcIs the dc input voltage of the three-level inverter.
(3) The alternating current side connects the three-phase alternating current asynchronous motor.
In a three-phase three-level topology, each phase produces 3 states, and the three phases have 27 states in total, and the switching function is defined as follows:
the switching function and bridge arm output voltage and output state are shown in table I:
TABLE I bridge arm output Voltage, output State, and switching function relationship
Wherein u ispAnd unThe physical meanings of (a) represent the upper capacitor voltage and the lower capacitor voltage, respectively.
Space voltage vectors corresponding to the three-phase three-level topology are shown in fig. 2, and the space voltage vectors can be divided into large vectors, medium vectors, small vectors and zero vectors according to the magnitude of the vectors, as shown in table II. The small vector and the medium vector have influence on the DC side midpoint potential, and the medium vector has uncertain influence on the midpoint potential, so that the small vector is used for realizing the balance control of the midpoint potential.
TABLE II space Voltage vector Classification
The three-level frequency converter converts the direct-current voltage into three-phase adjustable alternating-current voltage so as to control the rotating speed of the motor. The embodiment adopts a VVVF alternating-current speed regulation control strategy.
When the number n of pole pairs of the motorpAt a certain time, the synchronous speed n of the motor1And the frequency f of the output voltage of the frequency converter1The relationship of (1) is:
the effective value of each phase of stator electromotive force of the three-phase asynchronous motor is as follows:
Eg＝4.44f1NKΦm (3)
wherein E isg: the air gap flux induces an effective value of electromotive force in each phase of the stator; n: the number of turns of each phase of winding of the stator in series connection; k: stator fundamental winding coefficients; phim: air gap flux per stage.
Fig. 3 is an equivalent circuit diagram of an asynchronous motor. Wherein U isr *Is the stator voltage (i.e. the converter output voltage). When neglecting the voltage drop on the stator resistance and the stator leakage reactance, there are
Namely:
as shown in the formula (4), the VVVF variable frequency speed control mode is usually ensured
The value of (A) is a fixed value, and the magnetic flux of each air gap is constant. By varying stator voltage
Change f1The purpose of frequency conversion speed regulation is realized. This control is based on neglecting the voltage drop across the stator resistance and the leakage reactance, but at low frequencies these voltage drops cannot be neglected. In this case, low frequency compensation is required, i.e. the frequency converter should have a dc output function.
Compared with the speed regulation method of the stator series resistance, the speed regulation method of the motor has the advantages of low loss and good energy-saving effect by adopting a variable voltage variable frequency speed regulation mode.
FIG. 4 is a block diagram of variable voltage, variable frequency and variable speed control. As can be seen from fig. 4, given the rotational speed of the electric machine, the inverter output voltage required at this time can be derived from equations (2) and (4). The output voltage control of the three-level frequency converter is realized by controlling a switching function, and further the voltage-transformation frequency-conversion speed regulation control of the motor is realized. The three-level frequency converter not only needs to realize frequency conversion speed regulation control, but also needs to realize the balance control of the midpoint voltage at the direct current side.
When the frequency converter is operated at a higher power factor, the negative reduction vector reduces the lower side capacitor voltage and the positive reduction vector reduces the upper side capacitor voltage. However, when the frequency converter operates in a low-power situation, such as no-load, light-load or direct-current braking operation, the influence of the small vector on the central potential is reversed. The effect of the small vector on the midpoint potential is related to the direction of current flow for the small vector, as shown in fig. 5(a) -5 (d). When u is shown in FIG. 5(a)p>unWhen i is not present, if ia>0, POO should be selected to reduce the upper side capacitance voltage; when u is shown in FIG. 5(b)p>unWhen i is not present, if ia<0, ONN should be chosen to reduce the upper side capacitance voltage; when u is shown in FIG. 5(c)p<unWhen i is not present, if ia>0, the reduction ONN should be selected to reduce the lower side capacitance voltage; when u is shown in FIG. 5(d)p<unWhen i is not present, if ia<0, POO should be selected to reduce the lower side capacitance voltage.
In this case, not only the upper and lower capacitor voltages on the dc side but also the direction of the small vector current need to be considered, thereby achieving effective control of the midpoint voltage.
When a control method of model prediction is adopted to realize the control of the output voltage of the three-level frequency converter and the balance control of the midpoint voltage of the direct current side, the traditionally defined cost function is as follows:
wherein u isαβAnd
the space voltage vector and the component of the output voltage of the frequency converter on an alpha beta axis in a three-phase static coordinate system are used, and lambda is a weight coefficient; k is the kth space voltage vector.
According to the formula (5), the space voltage vector which enables the value function to be minimum is selected by calculating the value function of the 27 space voltage vectors, and the space voltage vector is used as the output voltage of the frequency converter at the moment to control the switching state of the three-level frequency converter, so that the frequency converter is controlled.
As can be seen from the above analysis, in the calculation of the switching function, 27 times of the cost function and 27 times of the upper and lower capacitance voltage estimation values need to be calculated. In addition, the selection of the weight coefficient affects the system control performance, and the selection process is cumbersome.
To reduce these tedious and tedious calculations, the present embodiment divides the space vector into 12 sectors based on the large vector and the medium vector, as shown in fig. 2. The number of calculations of the cost function is thus reduced from 27 to 4.
In addition, the embodiment also realizes the balance control of the midpoint voltage through the redundant positive and negative small vectors, and at the moment, the use of a weight coefficient is avoided, and the calculation times of the cost function are reduced to 4. It should be noted that, in the present embodiment, when selecting the small vector, not only the voltage value of the dc-side capacitor but also the current direction corresponding to the small vector need to be considered. The cost function defined in this embodiment is
From the above analysis, a vector summary of the cost function needed to be calculated in different sectors is shown in table III.
TABLE III space Voltage vectors required in different sectors
Since the current direction of the small vector is taken into account in the selection of the small vector, the influence of the small vector on the center potential can be determined. The small vector selected at this time can effectively control the midpoint voltage on the direct current side. The method can effectively control the midpoint voltage of the direct current side even in light load, load shedding, low rotating speed and direct current brake application occasions with low power factors.
As shown in fig. 6, the method for model predictive control of a three-level variable frequency speed control system according to this embodiment includes:
step 1: and dividing space voltage vectors corresponding to the three-phase three-level frequency converter topology into 12 sectors.
Specifically, the space voltage vector corresponding to the three-phase three-level frequency converter topology is divided into 12 sectors by the maximum modulus and the space voltage vector boundary next to the maximum modulus, as shown in fig. 2.
Step 2: and determining the sector to which the current output voltage vector of the three-phase three-level frequency converter belongs.
And step 3: and determining a space voltage vector required by a current sector capable of realizing balance control of the midpoint voltage on the direct current side according to the sign of the difference value of the upper side capacitor voltage and the lower side capacitor voltage of the three-phase three-level frequency converter topology and the direction of each phase current.
According to the table III, space voltage vectors required by different sectors can be determined, and in the process of determining the space voltage vectors, the signs of the difference values of the upper side capacitor voltage and the lower side capacitor voltage of the three-phase three-level frequency converter topology and the directions of the currents of all phases are considered, so that the neutral point voltage on the direct current side can be controlled in a balanced manner, and the stability of the three-level frequency conversion speed regulation system is improved.
And 4, step 4: selecting a space voltage vector which minimizes the cost function from space voltage vectors required by the current sector; the cost function is a module value of the difference between components of the space voltage vector and the current output voltage vector on an alpha beta axis in the three-phase static coordinate system respectively;
and 5: and the selected space voltage vector is utilized to control the on-off state of the three-phase three-level frequency converter, so that the frequency conversion and speed regulation control of the motor is realized.
In the specific implementation, the VVVF method is utilized to ensure that the magnetic flux of each air gap of the motor is unchanged, and the frequency of the output voltage is changed by changing the output voltage of the three-phase three-level frequency converter, so that the purpose of frequency conversion and speed regulation is achieved.
The precondition of the VVVF method is as follows: neglecting the voltage drop of the stator resistance and the stator leakage reactance of the motor.
In the embodiment, by the method of dividing the space voltage vector into 12 sectors, the calculation times of the cost function in one switching period are reduced from 27 times to 4 times, so that the calculation amount of the model predictive control method of the three-level variable-frequency speed control system is greatly reduced, and the control efficiency is improved; and the model predictive control principle is adopted, so that the response speed is high, simple and understandable.
The cost function of the embodiment is a module value of the difference between the space voltage vector and the current output voltage vector on the alpha-beta axis in the three-phase static coordinate system, so that the introduction of a weight coefficient in the cost function is avoided, and the complexity of the algorithm is reduced.
In the embodiment, when the space voltage vector required by the current sector capable of realizing balanced control of the midpoint voltage on the direct current side is determined, the capacitance voltage on the direct current side is considered, and the current direction of a small vector is also considered, so that the midpoint potential balance can be effectively controlled when the motor operates under normal working conditions, and an effective neutral point voltage balance control effect can be achieved when the motor operates under the abnormal working conditions of no-load and light-load operation, low-rotation-speed operation, even direct current braking and the like.
In another embodiment, a controller is also provided that is coupled to a three-phase three-level inverter that is coupled to a motor.
The controller includes:
(1) the sector dividing module is used for dividing space voltage vectors corresponding to the three-phase three-level frequency converter topology into 12 sectors;
specifically, in the sector division module, the space voltage vector corresponding to the three-phase three-level frequency converter topology is divided into 12 sectors by the maximum modulus value and the space voltage vector boundary next to the maximum modulus value, as shown in fig. 2.
(2) The sector determining module is used for determining a sector to which a current output voltage vector of the three-phase three-level frequency converter belongs;
(3) and the space voltage vector determination module required by the sector is used for determining the space voltage vector required by the current sector capable of realizing balanced control of the midpoint voltage of the direct current side according to the sign of the difference value of the upper side capacitor voltage and the lower side capacitor voltage of the three-phase three-level frequency converter topology and the direction of each phase current.
According to the table III, space voltage vectors required by different sectors can be determined, and in the process of determining the space voltage vectors, the signs of the difference values of the upper side capacitor voltage and the lower side capacitor voltage of the three-phase three-level frequency converter topology and the directions of the currents of all phases are considered, so that the neutral point voltage on the direct current side can be controlled in a balanced manner, and the stability of the three-level frequency conversion speed regulation system is improved.
(4) The space voltage vector screening module is used for selecting a space voltage vector which enables the cost function to be minimum from space voltage vectors required by the current sector; the cost function is a module value of the difference between components of the space voltage vector and the current output voltage vector on an alpha beta axis in the three-phase static coordinate system respectively;
(5) and the variable-frequency speed regulation control module is used for controlling the switching state of the three-phase three-level frequency converter by utilizing the selected space voltage vector to realize the variable-frequency speed regulation control of the motor.
Specifically, in the variable-frequency speed regulation control module, the VVVF method is utilized to ensure that the magnetic flux of each pole of the air gap of the motor is unchanged, and the frequency of the output voltage is changed by changing the output voltage of the three-phase three-level frequency converter, so that the purpose of variable-frequency speed regulation is achieved.
The precondition of the VVVF method is as follows: neglecting the voltage drop of the stator resistance and the stator leakage reactance of the motor.
In another embodiment, a three-level ac frequency conversion speed regulation system is further provided, which includes the controller described above.
The three-level alternating current variable frequency speed control system has the advantages of more alternating voltage output levels, less harmonic content and small voltage stress of the switching tube, and is particularly suitable for high-voltage and high-power application occasions.
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.
Claims (3)
1. A three-level variable frequency speed control system model prediction control method is provided, the three-level variable frequency speed control system comprises a three-phase three-level frequency converter and a motor, and the control method is characterized by comprising the following steps:
dividing space voltage vectors corresponding to the three-phase three-level frequency converter into 12 sectors; determining a sector to which a current output voltage vector of the three-phase three-level frequency converter belongs;
determining a space voltage vector required by a current sector capable of realizing balance control of midpoint voltage on a direct current side according to the sign of the difference value of upper side capacitor voltage and lower side capacitor voltage of the three-phase three-level frequency converter and the direction of each phase current;
selecting a space voltage vector which minimizes the cost function from space voltage vectors required by the current sector; the cost function is a module value of the difference of the components of the space voltage vector required by the current sector and the current output voltage vector on the axis of the two-phase static coordinate system respectively;
the selected space voltage vector is utilized to control the on-off state of the three-phase three-level frequency converter, so that the frequency conversion and speed regulation control of the motor is realized, and the method specifically comprises the following steps: the variable-voltage variable-frequency speed regulation method is utilized to ensure that the magnetic flux of each pole of air gap of the motor is unchanged, and the frequency of output voltage is changed by changing the output voltage of the three-phase three-level frequency converter, so that the purpose of variable-frequency speed regulation is realized; the precondition of the variable voltage variable frequency speed regulation method is as follows: neglecting voltage drop on a motor stator resistor and a stator leakage reactance;
the DC voltage is changed into three-phase adjustable AC voltage through a three-phase three-level frequency converter, so as to control the rotating speed of the motor, the rotating speed of the motor is fixed by adopting a voltage transformation frequency conversion speed regulation mode, the required output voltage of the inverter is obtained through calculation, the output voltage control of the three-phase three-level frequency converter is realized by controlling a switching function, and further the voltage transformation frequency conversion speed regulation control of the motor is realized;
dividing space voltage vectors corresponding to the three-phase three-level frequency converter into 12 sectors according to the maximum module value and the space voltage vector boundary which is only next to the maximum module value;
the three-level variable frequency speed control system model prediction control method considers the voltage of a capacitor at the direct current side and the current direction of each phase so as to realize the control of the neutral point potential balance at the direct current side and the variable frequency speed control of a motor when the three-level variable frequency speed control system operates under various working conditions simultaneously; in different sectors, the required space voltage vector is:
wherein u ispIs the upper side capacitance voltage unIs the lower side capacitance voltage iaFor phase A current, ibFor phase B current, icThe phase C current.
2. A controller is connected with a three-phase three-level frequency converter, and the three-phase three-level frequency converter is connected with a motor; characterized in that, the controller includes:
the sector dividing module is used for dividing space voltage vectors corresponding to the three-phase three-level frequency converter into 12 sectors;
the sector determining module is used for determining a sector to which a current output voltage vector of the three-phase three-level frequency converter belongs;
the sector required space voltage vector determining module is used for determining a space voltage vector required by the current sector capable of realizing balanced control of the midpoint voltage of the direct current side according to the sign of the difference value of the upper side capacitor voltage and the lower side capacitor voltage of the three-phase three-level frequency converter and the direction of each phase current;
the space voltage vector screening module is used for selecting a space voltage vector which enables the cost function to be minimum from space voltage vectors required by the current sector; the cost function is a module value of the difference of the components of the space voltage vector required by the current sector and the current output voltage vector on the axis of the two-phase static coordinate system respectively;
the variable-frequency speed regulation control module is used for controlling the on-off state of the three-phase three-level frequency converter by utilizing the selected space voltage vector to realize the variable-frequency speed regulation control of the motor;
in the sector dividing module, dividing space voltage vectors corresponding to the three-phase three-level frequency converter into 12 sectors according to the maximum module value and a space voltage vector boundary which is only next to the maximum module value;
the variable-voltage variable-frequency speed regulation method is utilized to ensure that the magnetic flux of each pole of air gap of the motor is unchanged, and the frequency of output voltage is changed by changing the output voltage of the three-phase three-level frequency converter, so that the purpose of variable-frequency speed regulation is realized;
the precondition of the variable voltage variable frequency speed regulation method is as follows: neglecting voltage drop on a motor stator resistor and a stator leakage reactance;
the controller considers the voltage of the capacitor at the direct current side and the current direction of each phase so as to realize the control of the neutral point potential balance at the direct current side and the variable frequency speed regulation control of the motor when various working conditions run simultaneously; in different sectors, the required space voltage vector is:
wherein u ispIs the upper side capacitance voltage unIs the lower side capacitance voltage iaFor phase A current, ibFor phase B current, icThe phase C current.
3. A three-level variable frequency governor system including a controller as claimed in claim 2.
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