CN113507123B - Bidirectional interface converter optimization control method suitable for alternating current-direct current hybrid microgrid - Google Patents

Bidirectional interface converter optimization control method suitable for alternating current-direct current hybrid microgrid Download PDF

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CN113507123B
CN113507123B CN202110799323.1A CN202110799323A CN113507123B CN 113507123 B CN113507123 B CN 113507123B CN 202110799323 A CN202110799323 A CN 202110799323A CN 113507123 B CN113507123 B CN 113507123B
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bidirectional interface
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direct current
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CN113507123A (en
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高飞
郑启鹏
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Shanghai Jiaotong University
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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
    • H02J3/02—Circuit arrangements for ac mains or ac distribution networks using a single network for simultaneous distribution of power at different frequencies; using a single network for simultaneous distribution of ac power and of dc power
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/388—Islanding, i.e. disconnection of local power supply from the network
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/22—The renewable source being solar energy
    • H02J2300/24—The renewable source being solar energy of photovoltaic origin
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • 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
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers

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Abstract

A bidirectional interface converter optimization control method suitable for an alternating current-direct current series-parallel micro-grid comprises the following steps: respectively implementing droop control in an alternating current sub-network and a direct current sub-network in the alternating current-direct current hybrid micro-grid system, and transmitting the equivalent frequency value of the direct current sub-network to a controller of a bidirectional interface converter by using a communication network; the reference value of the transmission power of the bidirectional interface converter is determined after normalization calculation through the alternating current bus frequency measured by the phase-locked loop and the equivalent frequency of the direct current sub-network received by the controller; and carrying out current control on the bidirectional interface converter, and finally realizing bidirectional power coordination control of the alternating current-direct current series-parallel micro-grid. Compared with the traditional control method which utilizes the direct-current side bus voltage as the control input quantity of the bidirectional interface converter, the bidirectional interface converter has higher power transmission upper limit, has better system stability under the same system parameters, and can effectively inhibit the steady-state circulating current phenomenon caused by the impedance of a direct-current line.

Description

Bidirectional interface converter optimization control method suitable for alternating current-direct current series-parallel micro-grid
Technical Field
The invention belongs to the field of micro-grid operation control, and particularly relates to an optimization control method for a bidirectional interface converter suitable for an alternating current-direct current parallel-serial micro-grid.
Background
The microgrid comprises various distributed power supplies, energy storage, load and energy conversion devices and related monitoring and control systems, and can be used as a complex power system with various energy forms interdependent and various operation functions mutually influenced. In practical engineering, on one hand, the alternating current equipment is still the main form of the power load in the current power grid, the alternating current micro-grid starts to develop early by virtue of the natural advantages of being integrated with the power grid interface, and the operation and control technology of the alternating current micro-grid tends to be mature; on the other hand, a large number of direct-current electric equipment such as electric vehicles and direct-current lighting are widely used, and a direct-current micro-grid has the advantages of high system efficiency, high power supply quality and the like, and becomes an important development direction of the future micro-grid. A complex micro-grid structure with an alternating current and direct current source, an alternating current and direct current bus and an alternating current and direct current load coexisting exists for a long time in the future and is called an alternating current and direct current parallel-serial micro-grid. Compared with a pure direct-current micro-grid or an alternating-current micro-grid, the alternating-current and direct-current hybrid micro-grid simultaneously comprises a direct-current bus and an alternating-current bus, and each distributed power supply, load and the like can be directly connected to the corresponding bus, so that the number of power electronic converters is reduced, and the transmission efficiency of the system is improved; the system can be simultaneously connected with various distributed power sources such as distributed photovoltaic power generation, distributed wind power generation and the like, so that the power supply reliability of the system is enhanced; the system simultaneously keeps the respective original advantages of the alternating current micro-grid and the direct current micro-grid.
A typical ac-dc hybrid microgrid system consists of a dc sub-network, an ac sub-network and a bidirectional interface converter, which can act as a rectifier or an inverter according to the power direction required at each moment, with the main objective of managing the energy of different sub-networks in the ac-dc hybrid microgrid. Generally speaking, an alternating current-direct current hybrid micro-grid system has a grid-connected operation mode or an island operation mode, and in the grid-connected mode, because a main grid has a constant voltage source similar to infinite high power, a bidirectional interface converter can directly absorb energy from the grid to supply power to the micro-grid system, and the energy management is relatively simple; in the islanding mode, the bidirectional interface converter needs to adjust its own transmission power in real time to balance the load power in the ac sub-network and the dc sub-network. In an island mode, normalized autonomous control is a traditional control method of a bidirectional interface converter, the bus voltage of a direct-current sub-network and the output power of an equivalent distributed power supply in the direct-current sub-network are in a droop relation, and the bus frequency of an alternating-current sub-network and the output power of the equivalent distributed power supply in the alternating-current sub-network are in a droop relation; the bidirectional interface converter can determine a reference value of transmission power of the bidirectional interface converter after normalization calculation by measuring bus voltage of a direct current end and bus frequency of an alternating current end, and then determine a current reference value of the bidirectional interface converter according to the transmission power reference value, so that energy coordination control of the alternating current-direct current hybrid micro-grid is finally realized. The conventional normalization autonomous control method has a small stability margin, and the existence of the impedance of the direct current line can cause the deviation between the bus voltage measured by the bidirectional interface converter and the actual bus voltage of the direct current sub-network, so that a steady-state circulating current phenomenon can be generated in a system with a plurality of bidirectional interface converters connected in parallel.
Disclosure of Invention
The invention aims to provide an optimization control method of a bidirectional interface converter suitable for an alternating current-direct current series-parallel micro-grid aiming at the defects of the prior art. The method improves the droop control quantity of the direct current side on the basis of the traditional control method of the bidirectional interface converter: an equivalent frequency is generated in the direct current sub-network, the direct current equivalent frequency and the output power of the distributed power supply in the direct current sub-network are in a droop relation, and the direct current equivalent frequency value is sent to a controller of the bidirectional interface converter by means of communication to replace a direct current voltage value in a traditional control method. Compared with the traditional method, the optimization control method of the bidirectional interface converter has better stability margin, the direct current equivalent frequency is not influenced by direct current line impedance, the steady-state circulating current phenomenon generated in a parallel system of a plurality of bidirectional interface converters can be effectively inhibited, and the power coordination distribution in an alternating current-direct current parallel micro-grid system is improved.
The technical solution of the invention is as follows:
2. a bidirectional interface converter optimization control method suitable for an alternating current-direct current series-parallel microgrid is characterized by comprising the following steps:
1) the alternating current-direct current hybrid micro-grid system consists of a direct current sub-network, an alternating current sub-network and a bidirectional interface converter, wherein a direct current end of the bidirectional interface converter is connected with a direct current bus of the direct current sub-network, an alternating current end of the bidirectional interface converter is connected with an alternating current bus of the alternating current sub-network, a communication network is constructed between the direct current sub-network and the bidirectional interface converter, and droop control is respectively implemented on the alternating current sub-network and the direct current sub-network of the alternating current-direct current hybrid micro-grid system:
1.1) f-P droop control in the AC sub-network, the AC bus frequency measured by the phase-locked loop in the bidirectional interface converter is controlled by the distributed power supply in the AC sub-network, and the frequency decreases with the increase of the output power of the distributed power supply, as shown in the following formula:
fac＝fmax-KacPac
in the formula (f)acFrequency of the AC bus, fmaxIs the maximum value of the AC bus frequency, PacFor the output power, K, of a distributed power supply in an AC subnetworkacThe droop coefficient of the alternating current sub-network;
1.2) f-P droop control in the DC sub-network, so that the equivalent frequency value in the DC sub-network is reduced along with the increase of the output power of the distributed power supply along with the frequency of the AC bus, as shown in the following formula:
fdc＝fmax-KdcPdc
in the formula (f)dcFor equivalent frequencies in the DC sub-network, PdcFor the output power, K, of a distributed power supply in a DC subnetworkdcFor the droop coefficient of the dc-sub-network,
calculating to obtain the equivalent frequency value f of the DC sub-networkdcSending the data to the controller of the bidirectional interface converter through the communication network;
2) the controller of the bidirectional interface converter performs the following normalized calculation according to the alternating current bus frequency measured by the phase-locked loop and the equivalent frequency of the direct current sub-network received by the controller to determine the reference value of the transmission power of the bidirectional interface converter:
2.1) carrying out normalization calculation on the frequency of the alternating current bus and the equivalent frequency in the direct current sub-network according to the following formula to obtain a normalization calculation value of the frequency of the alternating current bus and the equivalent frequency of the direct current sub-network:
Figure BDA0003164060420000031
in the formula (f)ac,pu、fdc,puRespectively calculating the normalized values of the AC bus frequency and the DC equivalent frequency;
2.2) according to said AC bus frequency fac,puEquivalent frequency f to DCdc,puDetermining the transmission power reference value P of the bidirectional interface converter according to the following formularef：
Pref＝KBIC(fdc,pu-fac,pu)
In the formula, PrefAssuming that the power injected from the DC sub-network into the AC sub-network is positive for the transmission power reference value of the bidirectional interface converter; kBICIs the power coefficient of the bidirectional interface converter;
3) the bidirectional interface converter converts the reference value P of the transmission power of the bidirectional interface converterrefAnd determining a d-axis current reference value of the bidirectional interface converter according to the Park conversion theory as follows:
Figure BDA0003164060420000041
in the formula idrefIs a d-axis current reference value, U, in a current controller of a bidirectional interface converterdIs the d-axis component of the AC bus voltage;
4) the bidirectional interface converter is based on a d-axis current reference value idrefAnd current control is carried out, and finally bidirectional power coordination control of the alternating current-direct current series-parallel connection micro-grid system is realized.
If the load power of the alternating current sub-network is larger than the load power of the direct current sub-network, the transmission power of the bidirectional interface converter in a steady state is larger than zero, and the bidirectional interface converter absorbs energy from the direct current sub-network and injects the energy into the alternating current sub-network so as to reduce the burden of a distributed power supply in the alternating current sub-network; similarly, when the load power of the dc sub-network is greater than the load power of the ac sub-network, the transmission power of the bidirectional interface converter is less than zero in a steady state, which indicates that the bidirectional interface converter absorbs energy from the ac sub-network and injects the energy into the dc sub-network. No matter the load power condition of the alternating current sub-network and the direct current sub-network, the bidirectional interface converter can always ensure that power is absorbed from the side with the lighter load and injected into the side with the heavier load, so that power coordination control between the alternating current sub-network and the direct current sub-network is realized. For the operation of an alternating current-direct current hybrid micro-grid system, it is always desirable to enable the power output of distributed power supplies in an alternating current sub-grid and a direct current sub-grid to be the same as possible, so as to ensure that the alternating current sub-grid and the direct current sub-grid do not operate under an overload working condition for a long time, and improve the service life and the efficiency of the system. To achieve this, the power factor K of the bidirectional interface converter is requiredBICThe load power of the alternating current sub-network and the direct current sub-network is balanced as large as possible to transmit higher power, however, the alternating current-direct current hybrid micro-grid system may change from a stable state to an unstable state with the increase of the power coefficient, and therefore the upper limit of the transmission power of the bidirectional interface converter is limited by the stability constraint. The invention provides a bidirectional interface converterCompared with the traditional control method, the optimization control method can improve the upper limit of the power coefficient of the bidirectional interface converter, thereby improving the upper limit of the transmission power of the bidirectional interface converter and improving the stability of the system.
In order to ensure that a system can still perform power coordination when one bidirectional interface converter fails in an alternating-current and direct-current hybrid microgrid, a plurality of bidirectional interface converters are often operated in parallel to improve the reliability of the system, the power coefficient of each bidirectional interface converter should be proportional to the respective power class, and the bidirectional interface converter with a high power class should have a larger power coefficient to bear larger transmission power. The optimal control method for the bidirectional interface converter can be popularized to an application scene that a plurality of bidirectional interface converters are connected in parallel, and the transmission power reference value of each bidirectional interface converter can be expressed as the following formula:
Pref,k＝KBIC,k(fdc,pu-fac,pu)
in the formula Pref,kAssuming that the power injected into the alternating current sub-network from the direct current sub-network is positive for the transmission power reference value of the kth bidirectional interface converter in the system; kBIC,kThe power coefficient of the kth bidirectional interface converter in the system. Because each bidirectional interface converter in the system adopts alternating current bus frequency and direct current equivalent frequency as input control quantity, and the two droop control quantities are equal for each bidirectional interface converter, the transmission power of each bidirectional interface converter and the respective power coefficient KBIC,kIn direct proportion, the steady-state circulating current caused by the impedance of the direct current line can be effectively inhibited.
Drawings
Fig. 1 is a schematic diagram of an ac-dc hybrid microgrid according to an embodiment of the present invention.
Fig. 2 is a control block diagram of an ac sub-network voltage source in an embodiment of the invention.
Fig. 3 is a control block diagram of the dc sub-network voltage source in the embodiment of the present invention.
Fig. 4 is a flow chart of the bidirectional interface converter optimization control method of the present invention.
Fig. 5 is a control block diagram of the bidirectional interface converter in the embodiment of the present invention.
Fig. 6 is a waveform diagram of the transmission power of the interface converter of the time domain simulation in the embodiment of the present invention.
Fig. 7 is a waveform diagram of transmission power of a time-domain-simulated multi-interface converter parallel system in an embodiment of the present invention.
Detailed Description
The following detailed description of the embodiments of the present invention will be provided with reference to the drawings and examples, so that how to apply the technical means to solve the technical problems and achieve the technical effects can be fully understood and implemented. It should be noted that, as long as there is no conflict, the embodiments and the features of the embodiments of the present invention may be combined with each other, and the technical solutions formed are within the scope of the present invention.
In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details or with other methods described herein.
The alternating current-direct current series-parallel micro-grid considered in the embodiment of the invention is shown in fig. 1, a BOOST circuit 1 is used as an equivalent distributed power supply in a direct current sub-grid, and the output end of the BOOST circuit 1 and a direct current load 2 are connected to a direct current bus in parallel; an inverter 3 is used as an equivalent distributed power supply in an alternating current sub-network, and the output end of the inverter 3 is connected with an alternating current load 5 in parallel to an alternating current bus through an LC filter 4; the direct current bus and the alternating current bus are interconnected through a bidirectional interface converter 6, the DC/AC converter is used as the bidirectional interface converter, and the alternating current end of the DC/AC converter is connected with the alternating current bus after passing through a filter inductor 7.
The optimization control method is not limited to the alternating current and direct current hybrid micro-grid structure in the embodiment, and has applicability to the common alternating current and direct current hybrid micro-grid. An actual ac/dc hybrid microgrid structure may be more complex than the exemplary embodiment, for example, including a plurality of dc subnetworks and ac subnetworks, each having a corresponding dc bus and ac bus. Technicians only need to install a bidirectional interface converter between the direct current bus and the alternating current bus, and implement droop control in the direct current sub-network and the alternating current sub-network according to the method, and construct a communication network between the direct current sub-network and the bidirectional interface converter, so that the whole alternating current-direct current hybrid micro-grid can realize bidirectional power coordination control between different direct current sub-networks and alternating current sub-networks through an optimization control method of the bidirectional interface converter, and thus, the energy management of the alternating current-direct current hybrid micro-grid is optimized and improved. The alternating current-direct current hybrid micro-grid structure considered in the embodiment has universality and typicality, and has a considerable reference significance for a specific alternating current-direct current hybrid micro-grid.
In order to ensure that the frequency of the alternating-current bus and the output power of the distributed power supply in the alternating-current sub-network meet the droop relationship, the inverter in the alternating-current sub-network adopts a voltage-current double closed-loop feedforward decoupling control method under a d-q coordinate system, and a control block diagram is shown in fig. 2 and can be represented as follows:
Figure BDA0003164060420000061
Figure BDA0003164060420000062
in the formula of Udref、Ud、Uqref、UqD-axis and q-axis components of the reference value and the actual value of the alternating-current bus voltage respectively; s is the laplace operator; i.e. iod、ioqA dq axis component representing inverter output current in the ac sub-network; kpacv、KiacvProportional coefficients and integral coefficients in the voltage outer loop PI controller are respectively; i.e. idref、iqrefRespectively taking the output quantities of the d-axis voltage outer loop PI controller and the q-axis voltage outer loop PI controller as reference values of the current inner loop; kpaci、KiaciProportional coefficients and integral coefficients in the current inner loop PI controller are respectively; u shapeds,ac、Uqs,acThe output quantities of the d-axis current inner loop PI controller and the q-axis current inner loop PI controller are respectively the output quantity of the inverterAnd voltage is used as a three-phase input signal of the PWM module after being subjected to Park inverse transformation and proportion link, and the switching on and off of the switching tube are controlled.
The frequency in the alternating current sub-network and the output active power meet a droop relational expression:
Figure BDA0003164060420000071
in order to stabilize the voltage bus in the dc sub-network, the BOOST circuit adopts a voltage-current double closed-loop control method, and a control block diagram is shown in fig. 3 and can be represented as follows:
Figure BDA0003164060420000072
Figure BDA0003164060420000073
in the formula, Kiv、KpvRespectively, the integral coefficient and the proportional coefficient, K, of the voltage outer loop PI controllerii、KpiRespectively is an integral coefficient and a proportional coefficient of the current inner loop PI controller; vref、vdcRespectively a reference value and an actual value of the direct current bus voltage; i.e. iLref、iLRespectively an inner loop current reference value and an actual value of the current flowing through the inductor; d is a duty cycle signal.
Calculating an equivalent frequency value in the direct current sub-network according to the droop relation, as follows:
fdc＝fmax-KdcPdc＝fmax-Kdciovdc
in the formula ioIs the output current of the BOOST circuit.
The bidirectional interface converter is used as a bridge for connecting the ac sub-network and the dc sub-network, and needs to determine its own transmission power reference value according to the load power conditions of the ac sub-network and the dc sub-network, and the operating principle of the bidirectional interface converter is shown in fig. 4. The controller of the bidirectional interface converter measures the frequency of the alternating current bus through the phase-locked loop module, receives the direct current equivalent frequency value, and obtains the normalized value of the two frequency quantities through normalization calculation. And then obtaining a transmission power reference value of the bidirectional interface converter according to the normalized values of the alternating current bus frequency and the direct current equivalent frequency, as follows:
Pref＝KBIC(fdc,pu-fac,pu)
according to Park transformation theory, the reference value of d-axis current is calculated according to the following formula:
Figure BDA0003164060420000074
the obtained d-axis current reference value is input into a current controller to realize current control of the bidirectional interface converter, and a control block diagram of the current controller is shown in fig. 5 and can be represented by the following formula:
Figure BDA0003164060420000081
in the formula idref,ic、id,ic、iqref,ic、iq,icRespectively representing current reference values and actual values of a d axis and a q axis of the current loop; kp,ic、Ki,icProportional coefficients and integral coefficients in the current loop PI controller are respectively; u shapeds,ic、Uqs,icThe d-axis component and the q-axis component of the output voltage of the interface converter are respectively used as three-phase input signals of the PWM module after Park inverse transformation and proportion links, and the switching on and the switching off of the switching tube are controlled.
The following describes embodiments of the present invention in detail.
The rated voltage of the direct current sub-network in the embodiment is 400V, and the maximum output power of the distributed power supply is 4 kW; the rated voltage amplitude of the alternating current sub-network is 200V, the rated frequency is 50Hz, the maximum value and the minimum value of the frequency are 50.5Hz and 49.5Hz respectively, and the maximum output power of the distributed power supply is 4 kW. The power of the alternating current load is 2.4kW, the power of the direct current load is 3.6kW, and the bidirectional interface converter is required to bear half of the difference between the load powers of the alternating current sub-network and the direct current sub-network, namely 0.6kW of transmission power, in order to balance the load powers of the alternating current sub-network and the direct current sub-network. In order to compare the traditional control method with the optimized control method provided by the invention, the power coefficient of the bidirectional interface converter is continuously increased in simulation, so that the stability of the two methods is compared and judged.
The time domain simulation result of the embodiment is shown in fig. 6, fig. 6(a) is a transmission power time domain waveform diagram of the bidirectional interface converter under the traditional control method, and fig. 6(b) is a transmission power time domain waveform diagram of the bidirectional interface converter under the optimized control method. As can be seen from fig. 6(a), in the conventional control method, when the power factor increases to 4000, the transmission power waveform of the bidirectional interface converter oscillates, and the system enters an unstable state; in fig. 6(b), when the power factor of the bidirectional interface converter is increased to about 7000 under the optimized control, the system enters an unstable state, and the stability thereof is significantly improved compared with the conventional control method. As can be seen from fig. 6, the transmission power of the bidirectional interface converter is a negative value, which indicates that the bidirectional interface converter absorbs power from the ac sub-network and injects the power into the dc sub-network, thereby implementing power coordination control between the ac sub-network and the dc sub-network.
In order to illustrate the influence of the direct-current line impedance on the traditional control method, the embodiment of the invention can be popularized to a system with a plurality of bidirectional interface converters connected in parallel, the condition that two bidirectional interface converters are connected in parallel is considered in the embodiment of the invention, and the power coefficients of the two bidirectional interface converters are respectively 1000 and 2000. Line impedance is added to a direct current line in the embodiment of the invention, a traditional control method is adopted when the time is 0-0.3 s, an optimization control method is adopted after 0.3s, and the simulation time is 0.6s in total.
The time domain simulation result of the embodiment is shown in fig. 7, a traditional control method is adopted in the period of 0-0.3 s, and the voltage values of the direct current ports measured by the two bidirectional interface converters are different due to the existence of the impedance of a direct current line, so that the transmission power directions of the two bidirectional interface converters are opposite, and the steady-state circulating current phenomenon exists in the two bidirectional interface converters; after 0.3s, the bidirectional interface converter adopts the optimized control method provided by the invention, and the two droop control quantities obtained by the two bidirectional interface converter controllers are equal, so that the transmission power of the two bidirectional interface converters is in direct proportion to respective power coefficients, and the generation of steady-state circulation is effectively inhibited theoretically.
According to the embodiment, the optimization control method of the bidirectional interface converter suitable for the alternating current-direct current hybrid micro-grid, provided by the invention, replaces the direct current bus voltage in the traditional control method with the direct current equivalent frequency, can effectively improve the stability of the system, improves the upper limit value of the transmission power of the bidirectional interface converter, can effectively inhibit the steady-state circulating current phenomenon caused by the impedance of a direct current line in a system with a plurality of bidirectional interface converters connected in parallel, and improves the power coordination control effect of the alternating current-direct current hybrid micro-grid system.
While the present invention has been described in detail by the above embodiments, it should be appreciated that the above description should not be construed as limiting the invention. Various modifications and alterations to this invention will become apparent to those skilled in the art upon reading the foregoing description. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims (1)

1. A bidirectional interface converter optimization control method suitable for an alternating current-direct current series-parallel micro-grid is characterized by comprising the following steps:
1) the alternating current-direct current hybrid micro-grid consists of a direct current sub-grid, an alternating current sub-grid and a bidirectional interface converter, wherein the direct current end of the bidirectional interface converter is connected with a direct current bus of the direct current sub-grid, the alternating current end of the bidirectional interface converter is connected with an alternating current bus of the alternating current sub-grid, a communication network is constructed between the direct current sub-grid and the bidirectional interface converter, and droop control is respectively implemented on the alternating current sub-grid and the direct current sub-grid of the alternating current-direct current hybrid micro-grid:
1.1) f-P droop control in the AC sub-network, the AC bus frequency measured by the phase-locked loop in the bidirectional interface converter is controlled by the distributed power supply in the AC sub-network, and the frequency decreases with the increase of the output power of the distributed power supply, as shown in the following formula:
fac＝fmax-KacPac
in the formula (f)acFrequency of the AC bus, fmaxIs the maximum value of the AC bus frequency, PacFor the output power, K, of a distributed power supply in an AC subnetworkacThe droop coefficient of the alternating current sub-network;
1.2) f-P droop control in the DC sub-network, so that the equivalent frequency value in the DC sub-network is reduced along with the increase of the output power of the distributed power supply along with the frequency of the AC bus, as shown in the following formula:
fdc＝fmax-KdcPdc
in the formula (f)dcFor equivalent frequencies, P, in the DC sub-networkdcFor the output power, K, of a distributed power supply in a DC subnetworkdcFor the droop coefficient of the dc-sub-network,
calculating to obtain the equivalent frequency value f of the DC sub-networkdcSending the data to the controller of the bidirectional interface converter through the communication network;
2) the controller of the bidirectional interface converter performs the following normalized calculation according to the alternating current bus frequency measured by the phase-locked loop and the equivalent frequency of the direct current sub-network received by the controller to determine the reference value of the transmission power of the bidirectional interface converter:
2.1) carrying out normalization calculation on the frequency of the alternating current bus and the equivalent frequency in the direct current sub-network according to the following formula to obtain a normalization calculation value of the frequency of the alternating current bus and the equivalent frequency of the direct current sub-network:
Figure FDA0003531278370000021
in the formula, fac,pu、fdc,puRespectively calculating the normalized values of the AC bus frequency and the DC equivalent frequency;
2.2) according to said AC bus frequency fac,puEquivalent frequency f to DCdc,puThe normalized calculation value of (A) determines the transmission power reference value P of the bidirectional interface converter according to the following formularef：
Pref＝KBIC(fdc,pu-fac,pu)
In the formula, PrefAssuming that power is injected into the alternating-current sub-network from the direct-current sub-network as positive for a transmission power reference value of the bidirectional interface converter; k isBICIs the power coefficient of the bidirectional interface converter;
3) the bidirectional interface converter converts the reference value P of the transmission power of the bidirectional interface converterrefAnd determining a d-axis current reference value of the bidirectional interface converter according to the Park conversion theory as follows:
Figure FDA0003531278370000022
in the formula idrefIs a d-axis current reference value, U, in a current controller of a bidirectional interface converterdIs the d-axis component of the AC bus voltage;
4) the bidirectional interface converter is based on the d-axis current reference value idrefAnd current control is carried out, and finally bidirectional power coordination control of the alternating current-direct current series-parallel connection micro-grid system is realized.
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