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CN110247419B - Control method suitable for multi-end back-to-back flexible straightening - Google Patents

Control method suitable for multi-end back-to-back flexible straightening Download PDF

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CN110247419B
CN110247419B CN201910623035.3A CN201910623035A CN110247419B CN 110247419 B CN110247419 B CN 110247419B CN 201910623035 A CN201910623035 A CN 201910623035A CN 110247419 B CN110247419 B CN 110247419B
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current
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voltage
axis
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CN110247419A (en
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王震泉
刘述军
韩志锟
鲁东海
李金科
丁继为
乐晓蓉
赵智成
冯大伟
沈涛
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China Energy Engineering Group Jiangsu Power Design Institute Co Ltd
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    • 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
    • H02J3/36—Arrangements for transfer of electric power between AC networks via a high-tension DC link
    • 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
    • H02J3/36—Arrangements for transfer of electric power between AC networks via a high-tension DC link
    • H02J2003/365—Reducing harmonics or oscillations in HVDC
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention discloses a control method suitable for multi-end back-to-back flexible-to-straight, a multi-end back-to-back flexible-to-straight system comprises 4 converters, and the control method of each converter in the multi-end back-to-back flexible-to-straight system comprises the following processes: the outer ring controller of one converter is controlled by constant direct current voltage and is matched with reactive power control to output a current reference signal, and the outer ring controller of the other converter is controlled by combining reactive power control and active power control to output the current reference signal; each converter obtains an output modulation wave signal through dead-beat direct power control, and the output signal is modulated through the signal to control the on and off of the converter, so that the control of active power and reactive power is realized. The invention adopts dead-beat direct power control in the local control of the converter station, thereby improving the speed of power response and the control precision.

Description

Control method suitable for multi-end back-to-back flexible straightening
Technical Field
The invention belongs to the technical field of flexible direct current transmission, and particularly relates to a control method suitable for multi-end back-to-back flexible direct current transmission.
Background
Europe plans to greatly expand the share of renewable energy sources to supply their power consumption. This includes not only the deployment of distributed generators, but also the integration of large amounts of offshore wind energy (mainly from the north sea) and solar energy from countries around the mediterranean sea. This situation requires a robust power transmission system, which is characterized by: long distance power transmission, strong cross-border interconnections, long cables (usually submarine cables), from which it has been known that High Voltage Direct Current (HVDC) transmission techniques have long been considered as the best option to overcome difficulties such as long cables or asynchronous connections, and new point-to-point links are being established or considered in practice. In addition, the construction of european multi-terminal high voltage direct current networks (or "super networks") is also planned in detail.
Voltage source converter high voltage direct current (VSC-HVDC) technology has significant advantages over traditional line converter (LCC) high voltage direct current technology in multi-terminal systems. However, flexible dc transmission still has many problems to be solved:
1) a voltage source converter topology structure with direct current short circuit fault current clearing capability;
2) high voltage direct current breaker technology;
3) the basic theory of the operation of the direct current power grid and a control protection technology.
And the voltage of a direct current bus of the VSC-HVDC system fluctuates, so that the system is unstable and other serious problems exist.
Disclosure of Invention
The invention aims to overcome the defects in the prior art, provides a control method suitable for multi-end back-to-back flexible straightening, and solves the technical problem that the direct-current bus voltage of a multi-end VSC-HVDC system fluctuates to further cause system instability.
In order to solve the technical problem, the invention provides a control method suitable for multi-end back-to-back flexible-to-straight, which is characterized in that a multi-end back-to-back flexible-to-straight system comprises 4 converters, the direct current side of each converter is connected in parallel with the same direct current bus, the alternating current side of each converter is respectively connected with each feeder line of a power distribution network, and the control method of each converter in the multi-end back-to-back flexible-to-straight system comprises the following processes:
s1, acquiring active power and reactive power exchanged between the current converter and the alternating current system on an alpha and beta axis;
s2, the outer loop controller of one current converter adopts constant DC voltage control and cooperates with reactive power control to output current reference signal, and the outer loop controller of the other current converter adopts reactive power control and active power control to output current reference signal;
and S3, each converter obtains an output modulation wave signal through direct power control without beat, and the output signal is modulated by the signal to control the on and off of the converter, so that the control of active power and reactive power is realized.
Further, active power P exchanged between the converter and the AC system on the alpha-beta axissAnd reactive power QsThe following were used:
Figure BDA0002126146200000021
wherein u issα，usβIs the component of the three-phase AC voltage in the α β coordinate system, isα，isβIs the component of the three-phase alternating current in the α β coordinate system.
Further, the specific process that the outer loop controller of one of the converters controls the output current reference signal by using the constant direct current voltage and matching with the reactive power control comprises the following steps:
the actual value U of the DC voltagedcWith a given value UdrefSubtracting to obtain the error value of the voltage, and regulating by PI to obtain the current reference value of the current d axis; the actual value Q of the reactive power is compared with the given value QrefAnd subtracting to obtain the power error value, and regulating by PI to obtain the current reference value of the current q axis.
Further, the specific process that the outer loop controller of the converter outputs the current reference signal by combining reactive power control and active power control includes:
the actual value Q of the reactive power is compared with the given value QrefSubtracting to obtain a power error value, and regulating by PI to obtain a current reference value of a current q axis; the actual value P of the active power is compared with the given value PrefAnd subtracting to obtain an error value of the active power, and adjusting by a PI to obtain a current reference value of a current d axis.
Further, the process of each converter obtaining the output modulation wave signal through dead-beat direct power control is as follows:
discretizing the mathematical model of the converter in the alpha beta static coordinate system and neglecting the influence of the resistance R on the alternating current side, i.e. discretizing
Figure BDA0002126146200000031
Wherein u'α(n)，u′β(n) represents the net side voltage components of the α, β axes at sample point n, respectively; t isSRepresents a sampling time; l represents inductance, Δ iα(n) represents the deviation of the current on the alpha axis at the n +1 sampling point and the n sampling point, Δ iβ(n) represents the deviation of the current on the beta axis at the n +1 sampling point and the n sampling point; u. ofα(n) represents the component of the converter AC side output voltage on the alpha axis, uβ(n) represents the component of the converter AC side output voltage on the beta axis;
active power P exchanged on the alpha-beta axis between converter and ac systemsAnd reactive power QsFormula, approximately considering the value of the network side voltage in two adjacent sampling periods TSThe internal holding is constant, and the active and reactive variable quantities of adjacent periods are obtained as follows:
Figure BDA0002126146200000032
where Δ p (n), Δ q (n) represent the deviation of the active and reactive power at sample points n +1 and n; u. uα(n) represents the component of the converter AC side output voltage on the alpha axis, uβ(n) represents the component of the converter AC side output voltage on the beta axis; delta iα(n) represents the deviation of the current on the alpha axis at the n +1 sampling point and the n sampling point, Δ iβ(n) represents the deviation of the current on the beta axis at the n +1 sampling point and the n sampling point;
the modulated wave signals under the alpha and beta coordinate systems can be obtained by combining the formula (4) and the formula (5) as follows:
Figure BDA0002126146200000041
wherein u'α(n)，u′β(n) is the component of the modulated wave signal on the α, β axis; u. ofα(n) represents the component of the converter AC side output voltage on the alpha axis, uβ(n) represents the component of the converter AC side output voltage on the beta axis; p (n), q (n) represent the active power, respectively the reactive power at sampling point n; pref(n)，QrefAnd (n) respectively represents reference active power and reactive power at a sampling point n.
Further, the mathematical model of the converter in the α β stationary coordinate system is:
the transient mathematical model of the converter in the abc coordinate system is as follows:
Figure BDA0002126146200000042
wherein ia，ib，icIs a three-phase alternating current ua，ub，ucIs a three-phase AC voltage uca，ucb，uccThe three-phase voltage at the current converter, L and R are respectively equivalent inductance and resistance in the line;
through the park coordinate system transformation, equation 1 can be expressed as:
Figure BDA0002126146200000043
wherein u isα，uβIs alpha and beta components of three-phase alternating voltage at the network side; i.e. iα，iβIs alpha and beta components of three-phase alternating current at the network side; u'α，u′βIs the alpha and beta components of the three-phase voltage at the converter.
Compared with the prior art, the invention has the following beneficial effects: the invention firstly aims at the VSC converter, dead-beat direct power control is adopted in the local control of the converter station, and the speed of power response and the control precision are improved. The feasibility and the effectiveness of the method are verified through simulation, and theoretical basis and technical support are provided for multi-end back-to-back flexible-direct coordination control.
Drawings
FIG. 1 is a diagram of a four-terminal flexible-straight system topology;
FIG. 2 is a VSC topology;
FIG. 3 is an outer loop constant voltage control diagram;
FIG. 4 is an outer loop power control diagram;
FIG. 5 is a deadbeat direct power control;
FIG. 6 is a simulation result of a conventional control strategy;
FIG. 7 is an improved control strategy simulation result.
Detailed Description
The invention is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present invention is not limited thereby.
The direct current bus voltage fluctuation of the multi-terminal VSC-HVDC system is solved, and the serious problems of system instability and the like are further caused. The invention provides a coordination control method suitable for a multi-terminal flexible direct current transmission system, which comprises the following steps: firstly, aiming at a VSC type inverter, establishing a system equivalent mathematical model, and researching the influence of system parameters in the model on the voltage fluctuation of a direct current bus; on the basis, dead-beat direct power control is adopted for converter station control, and the speed of power response and the control accuracy are improved. Through system experiment verification, the weighted average frequency control strategy can improve the stability of direct current voltage and improve the performance of the multi-terminal soft direct current device.
The invention relates to a control method suitable for multi-end back-to-back flexibility and straightness, which comprises the following processes:
the method comprises the following steps: modeling a multi-end back-to-back flexible straight system; the method comprises the steps of modeling a multi-end back-to-back flexible-straight system, analyzing a converter at one end, obtaining a two-phase system in an alpha beta static coordinate system through a kirchhoff law column equation and coordinate change, and then obtaining active power and reactive power exchanged between the converter and an alternating current system on an alpha beta axis according to an instantaneous reactive power theory. And taking the obtained active power and reactive power as input power of the step three.
The flexible direct current transmission system has three connection modes of series-parallel connection, four-end flexible direct current provided by the invention adopts a parallel connection mode, a direct current circuit link is omitted, the direct current sides of all converters are shared, the stability of the voltage of the direct current side of the system is facilitated, and the flexible direct current transmission system is simple and flexible to control and easy to expand.
The four-terminal flexible straight system topology is shown in figure 1. The multi-end back-to-back flexible direct system model is composed of 4-end flexible direct ports, direct current sides are connected in parallel to the same direct current bus, alternating current sides of the flexible direct ports are respectively connected with all feeders of a power distribution network, and flexible interconnection (alternating current-direct current-alternating current decoupling) among a plurality of feeders is achieved, so that the multi-end back-to-back flexible direct system can achieve bidirectional control of active power and reactive power, and becomes a highly integrated comprehensive energy conversion device. Under the normal operation state, a converter (VSC) at one end works under the control of a fixed direct-current voltage, and active flexible exchange and reactive independent control among feeders are realized by other ports according to an optimal operation scheduling instruction. The converter topology is shown in fig. 2, with the power injected into the ac network as the positive direction.
From fig. 2, a mathematical model of a Voltage Source Converter (VSC) system can be obtained, taking VSC1 as an example, and in order to realize decoupling control of active power and reactive power, a transient mathematical model of the VSC-HVDC system in an abc coordinate system is as follows:
Figure BDA0002126146200000061
wherein ia，ib，icIs a three-phase alternating current ua，ub，ucIs a three-phase alternating voltage uca，ucb，uccThe three-phase voltage at the converter, L and R are respectively the equivalent inductance and resistance in the line.
The three-phase system in the abc coordinate system can be converted into a two-phase system in the α β stationary coordinate system by park transformation. Equation 1 can be expressed as:
Figure BDA0002126146200000071
wherein u isα，uβFor net side three-phase AC voltage (u)a，ub，uc) α, β components of (a); i all right angleα，iβFor net side three-phase alternating current (i)a，ib，ic) α, β components of (a); u'α，u′βThree-phase voltage (u) at inverterca，ucb，ucc) α, β components of (a).
Based on the system mathematical model analysis, according to the instantaneous reactive power theory, the active power P exchanged between the current converter and the alternating current system on the alpha-beta axissAnd reactive power QsThe following were used:
Figure BDA0002126146200000072
wherein u issα，usβIs the component of the three-phase AC voltage in the α β coordinate system, isα，isβIs the component of the three-phase alternating current in the α β coordinate system.
Step two: and (4) designing an outer ring controller, and obtaining a reference value of current as the input of the step three through voltage outer ring control.
When the multi-end flexible direct current transmission system normally operates, a converter is required to be controlled by constant direct current voltage and matched with reactive power control, and in addition, the converter can be controlled by reactive power and active power.
Assuming here that the outer loop controller of the VSC1 is set to a constant dc voltage in conjunction with reactive power control, the control diagram is shown in fig. 3: the actual value U of the DC voltagedc(direct measurement by the apparatus) to a set value UdrefSubtracting the value (which is set by a user) to obtain the error value of the voltage, and regulating the error value by a PI to obtain a current reference value of a current d axis; the actual value Q of the reactive power (which is the reactive power found in equation (3)) is compared to a set value QrefAnd (a self-set value) subtracting to obtain a power error value, and regulating by PI to obtain a current reference value of a current q axis.
The outer ring controllers of other three-terminal converters (VSC2, VSC3 and VSC4) are combined into active power control and reactive power control, and the control chart is shown in FIG. 4: the actual value Q of the reactive power (which is the reactive power found in equation (3)) is compared to a set value QrefSubtracting the power value (the self-set value) to obtain a power error value, and regulating by PI to obtain a current reference value of a current q axis; the actual value P of the active power (the active power obtained in the formula (3)) is compared with the given value PrefAnd (a self-set value) subtracting to obtain an error value of the active power, and adjusting by a PI to obtain a current reference value of a current d axis.
Step three: calculating active power and reactive power according to the first step, controlling an output current reference signal by a voltage outer ring of the second step, obtaining an output modulation wave signal by 4 VSCs (VSC1, VSC2, VSC3 and VSC4) through dead beat direct power control, and controlling the switching-off and switching-on of the VSC by the output signal through SVPWM signal modulation; thus realizing the control of active power and reactive power.
Four VSCs all adopt the control mode. A deadbeat direct power control scheme for VSC is shown in figure 5. By measuring three-phase AC voltage u on the network sidea，ub，ucThree-phase alternating current ia，ib，icAnd calculating the real values of active power and reactive power through a formula (3) through coordinate transformation. Outputting modulated wave signal u 'through dead beat control'α(n)、u′β(n) PWM pulse signals can be generated by space vector modulation SVPWM, the switch tube in the VSC is controlled to act (turn-off and turn-on), and finally the given value of power (Q) is achievedref,Pref) The method can quickly and accurately track.
In the proposed control scheme, in each sampling period, the converter voltage required for the next control period is calculated from the reference power and the predicted values of the converter system model. At the start of the next sampling period, the optimum converter voltage is saved and used, so that the entire sampling period can be used to perform all calculations and compensate for control delays due to the calculations. Meanwhile, the method combines dead-beat control and direct power control, not only omits complicated phase-locked loop design and current loop decoupling control, but also solves the problem that the switching frequency of direct power control is not fixed, and can quickly realize independent and accurate adjustment of the power instruction value.
The dead beat control is performed based on the discretization process, and therefore the discretization process is required in order to achieve higher control accuracy. Discretizing equation (2) and neglecting the influence of the resistance R on the AC side, i.e.
Figure BDA0002126146200000091
Wherein u'α(n)，u′β(n) represents the net side voltage components of the α, β axes at sample point n, respectively; t isSRepresenting the sampling time (which can be set according to the actual situation); l represents inductance, Δ iα(n) represents the deviation of the current on the alpha axis at the n +1 sampling point and the n sampling point, Δ iβ(n) represents the deviation of the current on the beta axis at the n +1 sampling point and the n sampling point; u. uα(n) represents the component of the converter AC side output voltage on the alpha axis, uβAnd (n) represents the component of the output voltage of the AC side of the converter on the beta axis.
The active power P exchanged between the inverter and the AC system on the alpha-beta axis can be obtained from the formula (3)sAnd reactive power Qs. Approximately considering the value of the network side voltage in two adjacent sampling periods TSThe internal is kept unchanged, so that the active and reactive variable quantities in adjacent periods are as follows:
Figure BDA0002126146200000092
where Δ p (n), Δ q (n) represent the deviation of the active and reactive power at sample points n +1 and n; u. ofα(n) represents the component of the converter AC side output voltage on the alpha axis, uβ(n) represents the component of the converter AC side output voltage on the beta axis; Δ iα(n) represents the deviation of the current on the alpha axis at the n +1 sampling point and the n sampling point, Δ iβ(n) represents the current at the n +1 sampling point and the n sampling pointDeviation on the beta axis.
The modulated wave signals under the alpha and beta coordinate systems can be obtained by combining the formula (4) and the formula (5) as follows:
Figure BDA0002126146200000101
wherein u'α(n)，u′β(n) is the component of the modulated wave signal on the α, β axis; u. ofα(n) represents the component of the converter AC side output voltage on the alpha axis, uβ(n) represents the component of the converter AC side output voltage on the beta axis; p (n), q (n) represent the active power, respectively the reactive power at sample point n; pref(n)，Qref(n) represents the reference active power, respectively the reactive power at the sampling point n (i.e. P (n +1), Q (n +1) at the sampling point n +1 in equation (5)).
Obtaining a modulated wave signal u'α(n)、u′βAnd (n) a PWM pulse signal can be generated by space vector modulation (SVPWM), the switching tube in the VSC is controlled to act (turn-off and turn-on), and finally the given value of power is quickly and accurately tracked.
In order to further illustrate the accuracy and reliability of the method, a four-terminal flexible direct-current transmission system simulation model shown in fig. 1 is built based on Matlab/Simulink. The simulation parameters in the four-terminal flexible direct current transmission system are shown in table 1.
TABLE 1 simulation parameters
Figure BDA0002126146200000102
FIG. 6 is a simulation result of a conventional control strategy; FIG. 7 is an improved control strategy simulation result. In an initial state, the VSC1 works under the control of a constant direct current voltage, and the active power instruction values of the VSC1, the VSC2, the VSC3 and the VSC4 are-4 MW, 2MW, 3MW and-1 MW respectively. When 0.4s, the active power instruction value of the VSC3 is reduced to 1MW from 3MW, when 0.6s, the active power instruction value of the VSC2 is increased to 5MW from 2MW, and the direct-current bus voltage under the control of the traditional PI is adopted in the graph 6(b), so that the overshoot of the direct-current bus voltage is large, the adjusting time is long, and the steady-state error is large. Fig. 7(b) illustrates the dc bus voltage under the improved control strategy of the present invention. Through comparison, the improved control strategy can effectively inhibit the jump of the bus voltage when the power of the converter is changed, and maintain the stability of the direct-current voltage of the system.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and so forth) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, it is possible to make various improvements and modifications without departing from the technical principle of the present invention, and those improvements and modifications should be considered as the protection scope of the present invention.

Claims (5)

1. A control method suitable for multi-end back-to-back flexible-to-straight is characterized in that a multi-end back-to-back flexible-to-straight system comprises 4 converters, the direct current sides of the converters are connected in parallel to the same direct current bus, the alternating current sides of the converters are connected with feeders of a power distribution network respectively, and the control method of the converters in the multi-end back-to-back flexible-to-straight system comprises the following processes:
s1, acquiring active power and reactive power exchanged between the current converter and the alternating current system on an alpha and beta axis;
s2, the outer loop controller of one converter adopts constant DC voltage control and cooperates with reactive power control to output current reference signal, and the outer loop controller of the other converter adopts reactive power control and active power control to output current reference signal;
s3, each converter obtains an output modulation wave signal through dead-beat direct power control, and the output signal is modulated by the signal to control the switching-off and switching-on of the converter, thereby realizing the control of active power and reactive power;
the specific process that the outer loop controller of one converter adopts fixed direct current voltage control and cooperates with reactive power to control the output current reference signal comprises the following steps:
the actual value U of the DC voltagedcWith a given value UdrefSubtracting to obtain the error value of the voltage, and regulating by PI to obtain the current reference value of the current d axis; the actual value Q of the reactive power is compared with the given value QrefAnd subtracting to obtain the power error value, and regulating by PI to obtain the current reference value of the current q axis.
2. The method as claimed in claim 1, wherein the active power P exchanged between the inverter and the AC system on the α β axis is a real power P exchanged between the inverter and the AC system on the back-to-back soft-to-straight basissAnd reactive power QsThe following were used:
Figure FDA0003592316970000021
wherein u issα，usβIs the component of the three-phase AC voltage in the α β coordinate system, isα，isβIs the component of the three-phase alternating current in the α β coordinate system.
3. The method as claimed in claim 1, wherein the specific process of outputting the current reference signal by the outer-loop controller of the additional inverter using the combination of reactive power control and active power control includes:
the actual value Q of the reactive power is compared with the given value QrefSubtracting to obtain a power error value, and regulating by PI to obtain a current reference value of a current q axis; the actual value P of the active power is compared with the given value PrefAnd subtracting to obtain an error value of the active power, and adjusting by a PI to obtain a current reference value of a current d axis.
4. The method as claimed in claim 1, wherein the step of obtaining the output modulated wave signal from each converter through the deadbeat direct power control comprises:
discretizing the mathematical model of the converter in the alpha beta static coordinate system and neglecting the influence of the resistance R on the alternating current side, i.e. discretizing
Figure FDA0003592316970000022
Wherein u'α(n)，u'β(n) represents the net side voltage components of the α, β axes at sample point n, respectively; t isSRepresents a sampling time; l represents inductance, Δ iα(n) represents the deviation of the current on the alpha axis at the n +1 sampling point and the n sampling point, Δ iβ(n) represents the deviation of the current on the beta axis at the n +1 sampling point and the n sampling point; u. uα(n) represents the component of the converter AC side output voltage on the alpha axis, uβ(n) represents the component of the converter AC side output voltage on the beta axis;
active power P exchanged on the alpha-beta axis between converter and ac systemsAnd reactive power QsFormula, approximately considering the value of the network side voltage in two adjacent sampling periods TSThe internal holding is constant, and the active and reactive variable quantities of adjacent periods are obtained as follows:
Figure FDA0003592316970000031
where Δ p (n), Δ q (n) represent the deviation of the active and reactive power at sample points n +1 and n; u. uα(n) represents the component of the converter AC side output voltage on the alpha axis, uβ(n) represents the component of the inverter ac side output voltage on the β -axis; Δ iα(n) represents the deviation of the current on the alpha axis at the n +1 sampling point and the n sampling point, Δ iβ(n) represents the deviation of the current on the beta axis at the n +1 sampling point and the n sampling point;
the modulated wave signals under the alpha and beta coordinate systems can be obtained by combining the formula (4) and the formula (5) as follows:
Figure FDA0003592316970000032
wherein u'α(n)，u'β(n) are components of the modulated wave signal on the α, β axes; u. ofα(n) represents the component of the converter AC side output voltage on the alpha axis, uβ(n) represents the output voltage of the AC side of the converter on the beta axisA component of (a);
p (n), q (n) represent the active power, respectively the reactive power at sample point n; pref(n)，QrefAnd (n) respectively represents reference active power and reactive power at a sampling point n.
5. The method as claimed in claim 4, wherein the mathematical model of the inverter in the α β stationary coordinate system is:
the transient mathematical model of the converter in the abc coordinate system is as follows:
Figure FDA0003592316970000041
wherein ia，ib，icIs a three-phase alternating current ua，ub，ucIs a three-phase AC voltage uca，ucb，uccThe three-phase voltage at the converter, L and R are respectively equivalent inductance and resistance in the line;
through the park coordinate system transformation, formula (1) can be expressed as:
Figure FDA0003592316970000042
wherein u isα，uβIs alpha and beta components of three-phase alternating voltage at the network side; i all right angleα，iβIs alpha and beta components of three-phase alternating current at the network side; u'α，u'βIs the alpha and beta components of the three-phase voltage at the converter.
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