CN115765508A - Prediction control method for equivalent space vector model of modular multilevel converter - Google Patents
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Abstract
The invention discloses a prediction control method for an equivalent space vector model of a modular multilevel converter, which comprises the following steps: regarding each bridge arm of the modular multilevel converter as a whole, providing a group of equivalent space vectors suitable for the modular multilevel converter, and reducing the number of voltage vectors required to be evaluated by the model predictive control method in unit time; predicting output current according to the equivalent space vector, and performing current control by adopting model prediction; selecting an optimal equivalent space vector and calculating the action time of the optimal equivalent space vector; according to the calculated time, adding a compensation term in a discrete domain on the equivalent space vector dwell time, and reducing the tracking error of the output current; controlling the bridge arm energy and the circulation by adopting proportional-integral control and proportional-integral resonance control; and calculating a bridge arm modulation reference signal of the modular multilevel converter, and realizing a modulation link by adopting carrier phase shift modulation. The method and the device provided by the invention do not need to add an additional hardware circuit, and compared with the traditional model prediction control method, the method and the device can obviously reduce the calculation amount required by the controller and improve the tracking control precision of the controller.
Description
Technical Field
The invention relates to the technical field of model predictive control of modular multilevel converters, in particular to a method for predicting and controlling an equivalent space vector model of a modular multilevel converter.
Background
Modular Multilevel Converters (MMC) have advantages of modularity, expandability, low switching frequency, good harmonic performance, etc., are widely concerned by the industry, and are applied to application occasions such as High Voltage Direct Current (HVDC) systems, motor drives, battery Energy Storage Systems (BESS), power Electronic Transformers (PET), static synchronous reactive compensators (STATCOM), etc.
The MMC has a complex topological structure, and needs more control targets (including output current, circulation current, capacitance voltage and the like) for realizing the normal operation of the MMC. In order to achieve the above object, a Proportional-integral (PI) controller and a Proportional-integral-resonant (PIR) controller having a cascade structure are generally used, which have problems of complicated control structure and slow dynamic response speed. To improve the system dynamic response speed, a Model Predictive Control (MPC) is increasingly applied to the MMC.
In the process of implementing the invention, the inventor finds that at least the following disadvantages and shortcomings exist in the prior art:
1. the MMC sub-modules and the switch devices are numerous, and the number of voltage vectors needing to be evaluated in a unit period is large, so that the calculation burden of an MMC control system is heavy, and the cost of a controller is high;
2. the MMC has numerous internal passive devices and sensors, and model parameters are not matched, so that the tracking error of the model prediction control method is large, and the control precision is low.
The two limitations limit the popularization and application of model prediction control in MMC.
Disclosure of Invention
The invention provides a modular multilevel converter equivalent space vector model prediction control method, which is characterized in that each bridge arm of an MMC is regarded as a whole, a group of equivalent space vectors suitable for the MMC are provided, and the quantity of voltage vectors required to be evaluated by the model prediction control method in unit time is reduced; the tracking error of the output current is reduced by adding a compensation term under a discrete domain on the equivalent space vector dwell time, which is described in detail in the following:
in a first aspect, a modular multilevel converter equivalent space vector model prediction control method includes:
regarding each bridge arm of the modular multilevel converter as a whole, providing a group of equivalent space vectors suitable for the modular multilevel converter, and reducing the number of voltage vectors required to be evaluated by the model predictive control method in unit time;
predicting output current according to the equivalent space vector, and performing current control by adopting model prediction;
selecting an optimal equivalent space vector and calculating the action time of the optimal equivalent space vector;
according to the calculated time, adding a compensation term in a discrete domain on the equivalent space vector dwell time, and reducing the tracking error of the output current;
controlling the bridge arm energy and the circulation by adopting proportional-integral control and proportional-integral resonance control;
and calculating a bridge arm modulation reference signal of the modular multilevel converter, and realizing a modulation link by adopting carrier phase shift modulation.
Wherein, regarding each bridge arm of the modular multilevel converter as a whole, a group of equivalent space vectors suitable for the modular multilevel converter are provided as follows:
each leg of the modular multilevel converter is considered as a whole and is defined as an equivalent leg unit. In each equivalent bridge arm unit, the capacitance voltage is equal to the capacitance voltage v on the direct current side dc The output voltage is defined as u xj Where x = { u, l }, j = { a, b, c }. Each equivalent bridge arm unit comprises an equivalent switch S eq1 And equivalent switchS eq2 . When S is eq1 Is in a conducting state and S eq2 When the bridge arm unit is in the off state, the equivalent bridge arm unit is in the on state, and the output voltage is v dc (as 1); when S is eq1 In an off state and S eq2 When the bridge arm unit is in a conducting state, the equivalent bridge arm unit is in a cutting-off state, and the output voltage is v dc (0).
According to the switching state of the equivalent bridge arm unit and a three-phase equivalent control loop thereof, 8 equivalent switch combinations exist in the three-phase MMC system, and the equivalent switch combinations respectively correspond to 8 equivalent space vectors. 8 space vectors are defined as U 0 ～U 7 . The 6 non-zero vectors can equally divide the α β reference plane into 6 parts (SectorI-SectorVI). In each period, 6 non-zero vectors are evaluated, and the output current of the modular multilevel converter can be controlled by selecting two optimal space vectors and calculating the action time of the two optimal space vectors.
The predicting the output current according to the provided equivalent space vector, and the current control by adopting model prediction specifically comprises the following steps:
the prediction model of the modular multilevel converter can be expressed as:
in the formula i α And i β The components of the output current in the alpha and beta axes, u α And u β The components of the equivalent output voltage on the alpha and beta axes, u gα And u gβ The components of the grid voltage on the alpha and beta axes, respectively.
Based on the above equation, the model of the modular multilevel converter output current can be further discretized into:
through the equation, the output current under 6 non-zero space vectors can be predicted
U Ω ＝(U 1 ,U 2 ,U 3 ,U 4 ,U 5 ,U 6 )
In the formula of U Ω Is a set of 6 equivalent space vectors.
The selecting the optimal equivalent space vector and calculating the action time specifically comprises the following steps:
to select the optimal vector, the objective function can be defined as:
in the formula, J is an objective function,
and
can be expressed by lagrange third order interpolation as:
at U Ω In the method, two non-zero vectors with the minimum objective function value are selected as the optimal equivalent space vector and are marked as U opt1 And U opt2 。
The principle of the computation of the optimum equivalent space vector's action time is shown in fig. 5. The final voltage in each period comprises two optimal equivalent space vectors (U) opt1 And U opt2 ) And zero vector (U) 0 Or U 7 ). The predicted current vector under the vector action can be calculated and solved by the formula (6), and is expressed as delta I opt1 ,ΔI opt2 And Δ I 0 . The action time corresponding to each predicted current vector can be respectively represented as t opt1 ,t opt2 And (T) s -t opt1 -t opt2 )。
Decomposing the three space vectors and the predicted current vectors thereof into an alpha-axis and beta-axis coordinate system to obtain a simultaneous equation:
the action time of two optimal equivalent space vectors obtained by simultaneously solving the equation is as follows:
in the formula u opt1α And u opt1β Are respectively U opt1 The components on the alpha and beta axes, u opt2α And u opt2β Are respectively U opt2 The components on the alpha and beta axes.
Wherein, according to the calculated time, adding a compensation term in a discrete domain on the equivalent space vector dwell time specifically comprises:
to eliminate the errors introduced by the model uncertainty for MMC control, a compensation term is added to the action time of two optimal space vectors as follows:
the expression of the action time of each optimal equivalent space vector is
T opt1 ＝t opt1 +t com1 ,T opt2 ＝t opt2 +t com2
In a second aspect, a modular multilevel converter equivalent space vector model predictive control apparatus includes:
the equivalent space vector module is used for regarding each bridge arm of the modular multilevel converter as a whole and providing a group of equivalent space vectors suitable for the modular multilevel converter;
the current control module is used for predicting output current according to the equivalent space vector and performing current control by adopting model prediction;
the optimal vector selection module is used for selecting an optimal equivalent space vector and calculating the action time of the optimal equivalent space vector;
the compensation module is used for adding a compensation item in a discrete domain on the equivalent space vector dwell time according to the calculated time;
and the integral control module comprises bridge arm energy control, current conversion control and modulation links.
The technical scheme provided by the invention has the beneficial effects that:
1. the method for predicting and controlling the equivalent space vector model of the modular multilevel converter reduces the number of voltage vectors required to be evaluated by the method for predicting and controlling the model in unit time by providing the equivalent space vector;
2. according to the modular multilevel converter equivalent space vector model prediction control method, the compensation item in a discrete domain is added to the delay time of the equivalent space vector, so that the tracking error of the output current is reduced;
3. the modular multilevel converter equivalent space vector model prediction control method can improve the dynamic response capability and improve the steady-state performance;
4. the method for predicting and controlling the equivalent space vector model of the modular multilevel converter does not need to add an additional hardware circuit, and compared with the traditional model prediction control method, the method can obviously reduce the calculated amount required by the controller, improve the tracking control precision of the controller and improve the steady-state performance.
In order to further verify the effectiveness and the practicability of the provided modular multilevel converter equivalent space vector model prediction control method, experiments are carried out through a laboratory three-phase modular multilevel converter prototype. The prototype used is shown in figure 6. The experimental parameters are shown in table 4. The AC output end of the MMC is connected to a three-phase programmable AC power supply, and the DC side of the MMC is connected to a DC power supply. The dSPACE 1006 is used as a central controller, and the PSS15S92F6-AG (Intelligent Power supply Module) is used as a switching device for the power module. The experimental results are given by the waveforms collected by the oscilloscope.
Drawings
FIG. 1 is a schematic diagram of a modular multilevel converter topology and equivalent circuit;
wherein, diagram (a) is a modular multilevel converter topology; the diagram (b) is a single-phase alternating current equivalent circuit; fig. (c) shows a single-phase dc equivalent circuit.
FIG. 2 is an equivalent bridge arm unit of the modular multilevel converter and an equivalent space vector diagram thereof;
wherein, the diagram (a) is a three-phase equivalent alternating current circuit; FIG. (b) shows an equivalent bridge arm unit; the diagram (c) is an equivalent space vector.
FIG. 3 is a block diagram of the overall control of a modular multilevel converter;
FIG. 4 is a schematic diagram of an optimal equivalent space vector selection process;
FIG. 5 is a schematic diagram of the calculation of optimal equivalent space vector function time;
FIG. 6 is a prototype diagram of a three-phase modular multilevel converter;
FIG. 7 is a waveform diagram of an experiment of a modular multilevel converter;
the graph (a) is output current, the graph (b) is output current THD, the graph (c) is a-phase output current and a reference value thereof, the graph (d) is a-phase bridge arm current and a circulating current, the graph (e) is a-phase bridge arm output voltage reference, the graph (f) is a-phase bridge arm voltage, the graph (g) is output line voltage, the graph (h) is capacitance voltage, the graph (i) is a switching signal, and the graph (j) is execution time of the proposed algorithm.
Detailed description of the invention
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in further detail below.
In order to solve the problems in the background art, improve the dynamic response capability, improve the steady-state performance, reduce the calculated amount required by the controller and improve the tracking control precision, the embodiment of the invention takes the MMC structure as a research object and develops an MMC equivalent space vector model prediction control method.
Example 1
A modular multilevel converter equivalent space vector model prediction control method comprises the following steps:
step 101: regarding each bridge arm of the modular multilevel converter as a whole, providing a group of equivalent space vectors suitable for the modular multilevel converter, and reducing the number of voltage vectors required to be evaluated by the model predictive control method in unit time;
step 102: predicting output current according to the equivalent space vector, and performing current control by adopting model prediction;
step 103: selecting an optimal equivalent space vector and calculating the action time of the optimal equivalent space vector;
step 104: according to the calculated time, adding a compensation term in a discrete domain on the equivalent space vector dwell time, and reducing the tracking error of the output current;
step 105: controlling the bridge arm energy and the circulation by adopting proportional-integral control and proportional-integral resonance control;
step 106: calculating a bridge arm modulation reference signal of the modular multilevel converter, and realizing a modulation link by adopting carrier phase shift modulation;
step 107: and the effectiveness of the model predictive control method is verified through experimental tests.
Example 2
The scheme in embodiment 1 is further described below with reference to specific calculation formulas, drawings and examples, and is described in detail below:
step 201: regarding each bridge arm of the modular multilevel converter as a whole, providing a group of equivalent space vectors suitable for the modular multilevel converter, and reducing the number of voltage vectors required to be evaluated by the model predictive control method in unit time;
the three-phase MMC has a topology as shown in fig. 1 (a), and each phase includes an upper bridge arm and a lower bridge arm, and each bridge arm includes N sub-modules. Upper bridge arm and lower bridge armThrough bridge arm inductance and L arm Are connected. The direct current port of the MMC is connected with a direct current source, and the alternating current port of the MMC is connected with a power grid through a filter inductor.
The MMC has a single-phase equivalent circuit as shown in FIG. 1 (b), and its control model can be expressed as
In the formula i j To output a current u j Is the AC equivalent output voltage u shown in FIG. 1 (b) uj And u lj Bridge arm voltages u of the upper bridge arm and the lower bridge arm respectively gj For grid voltage, j indicates phase sequence, j = { a, b, c }; l is eq And R eq Respectively, the equivalent inductance and the equivalent resistance of the upper alternating current loop.
The output current, the equivalent inductance and the equivalent resistance can be expressed specifically as
i j ＝i uj -i lj ,L eq ＝L+(L arm /2),R eq ＝R+(R arm /2) (2)
In the formula i uj And i lj Bridge arm currents R of the upper bridge arm and the lower bridge arm in FIG. 1 (b) are shown respectively arm R is the equivalent resistance of the bridge arm filter inductor.
An MMC equivalent single-phase DC circuit is shown in FIG. 1 (c), and its control model can be expressed as
2R arm i cirj +2L arm (di cirj /dt)＝v dc -(u lj +u uj )＝-2u cirj (3)
In the formula u cirj Is the voltage reference of the circulating current controller, v dc Is a DC side voltage i cirj Is a circulating current, which can be defined as
i cirj ＝(i uj +i lj )/2 (4)
In order to reduce the number of voltage vectors that need to be evaluated, the present invention considers each leg of the MMC as a whole, which is defined asAnd an effective bridge arm unit. Based on the simplified single-phase ac circuit of fig. 1 (b), an equivalent ac control circuit of three phases is shown in fig. 2 (a). Defining the equivalent bridge arm cell is shown in fig. 2 (b). In each equivalent bridge arm unit, the capacitance voltage is equal to the capacitance voltage v on the direct current side dc The output voltage is defined as u xj Where x = { u, l }, j = { a, b, c }. Each equivalent bridge arm unit comprises an equivalent switch S eq1 And an equivalent switch S eq2 . When S is eq1 Is in a conducting state and S eq2 When the bridge arm unit is in the off state, the equivalent bridge arm unit is in the on state, and the output voltage is v dc (note 1); when S is eq1 In an off state and S eq2 When the bridge arm unit is in a conducting state, the equivalent bridge arm unit is in a cutting-off state, and the output voltage is v dc (0).
Let u uj And u lj The equivalent bridge arms respectively have the working modes shown in table 1, and the equivalent bridge arms are mainly divided into the following two types: when the equivalent upper bridge arm unit is in an input state and the equivalent lower bridge arm unit is in an output state, the output voltages of the upper bridge arm unit and the lower bridge arm unit are respectively v dc And 0. At this time, the AC side equivalent output voltage of the MMC is-v dc And/2, the switch state is marked as 0. On the contrary, when the equivalent upper bridge arm unit is in the cut-off state and the equivalent lower bridge arm unit is in the put-in state, the output voltages of the upper bridge arm unit and the lower bridge arm unit are respectively 0 and v dc . At this time, the AC side equivalent output voltage of the MMC is v dc And/2, the switch state is marked as 1. It should be noted that, during normal operation of the MMC, only the two switch states exist in the equivalent circuit. In other switching states, the dc source will be in a short circuit or reverse charge state.
TABLE 1 working mode of equivalent bridge arm unit and output voltage of each phase
According to the on-off state of the equivalent bridge arm unit and the three-phase equivalent control loop thereof, the three-phase MMC system existsAnd 8 equivalent switch combinations respectively corresponding to the 8 equivalent space vectors. 8 space vectors are defined as U 0 ～U 7 The specific switching states are shown in table 2.
TABLE 2 switching states of equivalent space vectors
As shown in fig. 2, among the space vectors, 6 non-zero vectors may equally divide the α β reference plane into 6 parts. In each period, 6 non-zero vectors are evaluated, and the output current of the MMC can be controlled through selection of two optimal space vectors and calculation of action time of the two optimal space vectors.
Step 202: predicting output current according to the equivalent space vector, and performing current control by adopting model prediction;
the MMC model prediction control method mainly comprises three parts, namely a model prediction current control link, a bridge arm energy control link, a circulation control link and a modulation link based on an equivalent space vector, as shown in figure 3. In FIG. 3, i α (k) And i β (k) The components of the output current at time k on the alpha and beta axes, u gα (k) And u gβ (k) The components of the grid voltage at time k on the alpha and beta axes, i α (k + 1) and i β (k + 1) the components of the output current on the alpha and beta axes when k +1, respectively,
and
the components of the output current reference value on the alpha axis and the beta axis at the time k +1 respectively,
and
the components of the output current reference value on the alpha axis and beta axis are respectively given at time k,J(U 1 )～J(U 6 ) Is an objective function, U, of MMC under 6 non-zero vectors opt1 And U opt2 For two selected optimal voltage vectors, t opt1 (k) And t opt2 (k) Are respectively U opt1 And U opt2 Action time at time k, t com1 (k) And t com2 (k) For a compensation term of the action time, T opt1 And T opt2 For the final action time, v SMujp And v SMlj The capacitor voltages v of the upper bridge arm submodule and the lower bridge arm submodule respectively SMujp And v SMlj The sum of the capacitor voltages of the sub-modules of the upper bridge arm and the lower bridge arm is theta j Is the phase angle, u, of the grid voltage cirj Is the voltage reference of the circulating current controller, m uj And m lj Which are respectively the reference values of the modulation signals of the upper bridge arm and the lower bridge arm.
The prediction model of MMC can be expressed as:
in the formula i α And i β The components of the output current in the alpha and beta axes, u α And u β The components of the equivalent output voltage on the alpha and beta axes, u gα And u gβ The components of the grid voltage on the alpha and beta axes, respectively.
Based on the above equation, the model of the MMC output current can be further discretized into:
through the above equation, the output current under 6 non-zero space vectors can be predicted.
U Ω ＝(U 1 ,U 2 ,U 3 ,U 4 ,U 5 ,U 6 ) (7)
In the formula of U Ω The components projected on the α and β axes are set to 6 equivalent space vectors as listed in table 3.
TABLE 3 equivalent space vector projection Components
Step 203: selecting an optimal equivalent space vector and calculating the action time of the optimal equivalent space vector;
the optimal equivalent space vector selection process is shown in fig. 4. To select the optimal vector, the objective function can be defined as:
in the formula, J is an objective function,
and
can be expressed by lagrange third order interpolation as:
at U Ω In the method, two non-zero vectors with the minimum objective function value are selected as the optimal equivalent space vector and are marked as U opt1 And U opt2 。
The principle of the computation of the optimum equivalent space vector's action time is shown in fig. 5. The final voltage in each period comprises two optimal equivalent space vectors (U) opt1 And U opt2 ) And zero vector (U) 0 Or U 7 ). The predicted current vector under the vector action can be calculated and solved by the formula (6), and is expressed as delta I opt1 ,ΔI opt2 And Δ I 0 . The action time corresponding to each predicted current vector can be respectively represented as t opt1 ,t opt2 And (T) s -t opt1 -t opt2 )。
Decomposing the three space vectors and the predicted current vectors thereof into an alpha-axis and beta-axis coordinate system to obtain a simultaneous equation:
the action time of two optimal equivalent space vectors obtained by simultaneously solving the equation is as follows:
in the formula u opt1α And u opt1β Are respectively U opt1 The components on the alpha and beta axes, u opt2α And u opt2β Are respectively U opt2 The components on the alpha and beta axes.
Step 204: according to the calculated time, adding a compensation term in a discrete domain on the equivalent space vector dwell time, and reducing the tracking error of the output current;
to eliminate the errors introduced by the model uncertainty for MMC control, a compensation term is added to the action time of two optimal space vectors as follows:
and finally, the expression of the action time of each optimal equivalent space vector is as follows:
T opt1 ＝t opt1 +t com1 ,T opt2 ＝t opt2 +t com2 (15)
in addition, to suppress the zero sequence voltage component, the zero vector to be injected and its acting time can be expressed as:
T 0 ＝T 7 ＝0.5T MMC_0 ＝0.5(T s -T opt1 -T opt2 ) (16)
the equivalent on-time of a three-phase cell can be expressed as:
according to table 2, the on-time of each bridge arm can be calculated as:
T uj ＝T s -T j ,T lj ＝T j (18)
step 205: controlling the bridge arm energy and the circulation by adopting proportional-integral control and proportional-integral resonance control;
the bridge arm energy control and circulation control links adopt traditional PI control and PIR control.
Step 206: calculating a bridge arm modulation reference signal of the modular multilevel converter, and realizing a modulation link by adopting carrier phase shift modulation;
after the voltage reference of the circulation loop is obtained by adopting the traditional circulation controller, the final modulation reference of each bridge arm can be calculated as follows:
m xj ＝m xj_i +m cirj ＝T xj /T s +(u cirj /v dc ) (19)
in the formula, m xj_i And m xj_i The voltage modulation references of the alternating current loop and the direct current loop are respectively.
The MMC can be controlled by using a carrier phase shift modulation method.
Step 207: and the effectiveness of the model predictive control method is verified through experimental tests.
To further verify the effectiveness of the proposed control method, a three-phase MMC platform shown in fig. 6 was used to perform experimental verification, and the main loop parameters are listed in table 4.
TABLE 4 Experimental parameters
The experimental results of the MMC under the proposed control method are shown in fig. 7. As shown in fig. 7 (a), the amplitude of the output current of the MMC stabilizes around 4A. As shown in fig. 7 (b), the total harmonic distortion of the output current is about 3.04%, and the harmonic characteristics thereof are good. The a-phase output current and its reference value are shown in fig. 7 (c), which has higher tracking accuracy. The bridge arm current and the circulating current of the phase a are shown in fig. 7 (d), wherein the amplitude of the bridge arm current is about 2A, and the circulating current suppression effect is good. The reference values of the bridge arm modulation signals and the actual measurement values of the bridge arm output voltages of the a-phase are shown in fig. 7 (e) and 7 (f). It can be seen that the reference value of the bridge arm modulation signal is a typical space vector control waveform, and the maximum value of the number of input sub-modules is 4. The output line voltage of MMMC is shown in fig. 7 (g), and the balance effect between the three phases is good. The waveform of the capacitor voltage of the phase a is as shown in fig. 7 (h), the capacitor voltages of the upper and lower arms are balanced, and the average value is stabilized at about 30V. As shown in fig. 7 (i), the proposed control method has a fixed switching frequency of about 5kHz due to the application of the carrier phase shift modulation scheme. The total execution time of the proposed control algorithm is approximately 42 mus as shown in fig. 7 (j).
In summary, the modular multilevel converter equivalent space vector model prediction control method has the following advantages:
1. the method for predicting and controlling the equivalent space vector model of the modular multilevel converter reduces the number of voltage vectors required to be evaluated by the method for predicting and controlling the model in unit time by providing the equivalent space vector;
2. according to the modular multilevel converter equivalent space vector model prediction control method, the compensation item in a discrete domain is added to the equivalent space vector dwell time, so that the tracking error of the output current is reduced;
3. the modular multilevel converter equivalent space vector model prediction control method can improve the dynamic response capability and improve the steady-state performance;
4. the method for predicting and controlling the equivalent space vector model of the modular multilevel converter does not need to add an additional hardware circuit, and compared with the traditional model prediction control method, the method can obviously reduce the calculated amount required by the controller, improve the tracking control precision of the controller and improve the steady-state performance.
A modular multilevel converter equivalent space vector model prediction control device comprises:
the equivalent space vector module is used for regarding each bridge arm of the modular multilevel converter as a whole and providing a group of equivalent space vectors suitable for the modular multilevel converter;
the current control module is used for predicting output current according to the equivalent space vector and performing current control by adopting model prediction;
the optimal vector selection module is used for selecting an optimal equivalent space vector and calculating the action time of the optimal equivalent space vector;
the compensation module is used for adding a compensation item in a discrete domain on the equivalent space vector dwell time according to the calculated time;
the integral control module comprises bridge arm energy control, current conversion control and modulation links;
the execution main bodies of the modules and units can be computers, single-chip microcomputers, microcontrollers and other devices with calculation functions, and in specific implementation, the execution main bodies are not limited in the embodiment of the invention and are selected according to requirements in practical application.
In the embodiment of the present invention, except for the specific description of the model of each device, the model of other devices is not limited, as long as the device can perform the above functions.
Those skilled in the art will appreciate that the drawings are only schematic illustrations of preferred embodiments, and the above-mentioned serial numbers of the embodiments of the present invention are only for description and do not represent the merits of the embodiments.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (6)
1. A modular multilevel converter equivalent space vector model prediction control method is characterized by comprising the following steps:
regarding each bridge arm of the modular multilevel converter as a whole, providing a group of equivalent space vectors suitable for the modular multilevel converter, and reducing the number of voltage vectors required to be evaluated by the model predictive control method in unit time;
predicting output current according to the equivalent space vector, and performing current control by adopting model prediction;
selecting an optimal equivalent space vector and calculating the action time of the optimal equivalent space vector;
according to the calculated time, adding a compensation term in a discrete domain on the equivalent space vector dwell time, and reducing the tracking error of the output current;
controlling the bridge arm energy and the circulation by adopting proportional-integral control and proportional-integral resonance control;
and calculating a bridge arm modulation reference signal of the modular multilevel converter, and realizing a modulation link by adopting carrier phase shift modulation.
2. The method for predictive control of the equivalent space vector model of the modular multilevel converter according to claim 1, wherein the bridge arms of the modular multilevel converter are regarded as a whole, and a set of equivalent space vectors suitable for the modular multilevel converter is provided specifically as follows:
each leg of the modular multilevel converter is considered as a whole and is defined as an equivalent leg unit. In each equivalent bridge arm unit, the capacitance voltage is equal to the capacitance voltage v on the direct current side dc The output voltage is defined as u xj Where x = { u, l }, j = { a, b, c }. Each equivalent bridge arm unit comprises an equivalent switch S eq1 And an equivalent switch S eq2 . When S is eq1 Is in a conducting state and S eq2 When the bridge arm unit is in the off state, the equivalent bridge arm unit is in the on state, and the output voltage is v dc (note 1); when S is eq1 In an off state and S eq2 When the bridge arm unit is in a conducting state, the equivalent bridge arm unit is in a cutting-off state, and the output voltage is v dc (0).
According to the switching state of the equivalent bridge arm unit and a three-phase equivalent control loop thereof, 8 equivalent switch combinations exist in the three-phase MMC system, and the equivalent switch combinations respectively correspond to 8 equivalent space vectors. 8 space vectors are defined as U 0 ～U 7 . The 6 non-zero vectors may equally divide the α β reference plane into 6 parts. In each period, 6 non-zero vectors are evaluated, and the output current of the modular multilevel converter can be controlled by selecting two optimal space vectors and calculating the action time of the two optimal space vectors.
3. The method for model predictive control of an equivalent space vector of a modular multilevel converter according to claim 1, wherein the predicting the output current according to the proposed equivalent space vector, and the current control using model prediction specifically comprises:
the prediction model of the modular multilevel converter can be expressed as:
in the formula i α And i β The components of the output current in the alpha and beta axes, u α And u β The components of the equivalent output voltage on the alpha and beta axes, u gα And u gβ The components of the grid voltage on the alpha and beta axes, respectively.
Based on the above equation, the model of the modular multilevel converter output current can be further discretized into:
through the equation, the output current under 6 non-zero space vectors can be predicted
U Ω ＝(U 1 ,U 2 ,U 3 ,U 4 ,U 5 ,U 6 )
In the formula of U Ω Is a set of 6 equivalent space vectors.
4. The method for predictive control of an equivalent space vector model of a modular multilevel converter according to claim 1, wherein the selecting of the optimal equivalent space vector and the calculating of the action time thereof are specifically as follows:
to select the optimal vector, the objective function can be defined as:
in the formula, J is an objective function,
and
can be expressed by lagrange third order interpolation as:
at U Ω In the method, two non-zero vectors with the minimum objective function value are selected as the optimal equivalent space vector and are marked as U opt1 And U opt2 。
The principle of the computation of the optimum equivalent space vector's action time is shown in fig. 5. The final voltage in each period comprises two optimal equivalent space vectors (U) opt1 And U opt2 ) And zero vector (U) 0 Or U 7 ). The predicted current vector under the vector action can be calculated and solved by the formula (6), and is expressed as delta I opt1 ,ΔI opt2 And Δ I 0 . The action time corresponding to each predicted current vector canAre respectively represented by t opt1 ,t opt2 And (T) s -t opt1 -t opt2 )。
Decomposing the three space vectors and the predicted current vectors thereof into an alpha-axis and beta-axis coordinate system to obtain a simultaneous equation:
the action time of two optimal equivalent space vectors obtained by simultaneously solving the equation is as follows:
in the formula u opt1α And u opt1β Are respectively U opt1 The components in the alpha and beta axes, u opt2α And u opt2β Are respectively U opt2 The components on the alpha and beta axes.
5. The method for predicting and controlling the equivalent space vector model of the modular multilevel converter according to claim 1, wherein the adding of the compensation term in the discrete domain to the equivalent space vector dwell time according to the calculated time is specifically:
to eliminate the errors introduced by the model uncertainty for MMC control, a compensation term is added to the action time of two optimal space vectors as follows:
the expression of the action time of each optimal equivalent space vector is
T opt1 ＝t opt1 +t com1 ,T opt2 ＝t opt2 +t com2
6. An equivalent space vector model predictive control device of a modular multilevel converter, which is characterized by comprising:
the equivalent space vector module is used for regarding each bridge arm of the modular multilevel converter as a whole and providing a group of equivalent space vectors suitable for the modular multilevel converter;
the current control module is used for predicting output current according to the equivalent space vector and performing current control by adopting model prediction;
the optimal vector selection module is used for selecting an optimal equivalent space vector and calculating the action time of the optimal equivalent space vector;
the compensation module is used for adding a compensation item in a discrete domain on the equivalent space vector dwell time according to the calculated time;
and the integral control module comprises a bridge arm energy control link, a current conversion control link and a modulation link.
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