CN112087000A - Photovoltaic flexible loop closing device and operation control method - Google Patents

Photovoltaic flexible loop closing device and operation control method Download PDF

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CN112087000A
CN112087000A CN202010985124.5A CN202010985124A CN112087000A CN 112087000 A CN112087000 A CN 112087000A CN 202010985124 A CN202010985124 A CN 202010985124A CN 112087000 A CN112087000 A CN 112087000A
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voltage source
source type
photovoltaic
converter
closing device
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CN112087000B (en
Inventor
袁宇波
葛雪峰
史明明
杨景刚
张宸宇
周琦
缪惠宇
刘瑞煌
陈烨
姜云龙
方鑫
苏伟
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State Grid Corp of China SGCC
Electric Power Research Institute of State Grid Jiangsu Electric Power Co Ltd
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State Grid Corp of China SGCC
Electric Power Research Institute of State Grid Jiangsu Electric Power 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
    • H02J3/38—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
    • H02J3/007—Arrangements for selectively connecting the load or loads to one or several among a plurality of power lines or power sources
    • H02J3/0075—Arrangements for selectively connecting the load or loads to one or several among a plurality of power lines or power sources for providing alternative feeding paths between load and source according to economic or energy efficiency considerations, e.g. economic dispatch
    • 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
    • H02J3/008—Circuit arrangements for ac mains or ac distribution networks involving trading of energy or energy transmission rights
    • 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
    • H02J2300/26—The renewable source being solar energy of photovoltaic origin involving maximum power point tracking control for photovoltaic sources
    • 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

The invention discloses a photovoltaic flexible loop closing device and an operation control method thereof, wherein the device comprises two voltage source type converters, the two voltage source type converters are respectively connected with two different alternating current systems, two circuit breakers are respectively connected to the output sides of the two voltage source type converters, the two voltage source type converters share a direct current capacitor, a photovoltaic cell panel collection direct current bus is directly connected into the two voltage source type converters through a first mechanical switch and shares the direct current capacitor, and the photovoltaic cell panel collection direct current bus is connected into a secondary power supply of the flexible loop closing device through a second mechanical switch and a DC-DC converter. The photovoltaic flexible loop closing device provided by the invention realizes intelligent interaction of power and energy between power supply sub-areas, and simultaneously provides photovoltaic alternating current interface equipment, so that the input cost of a photovoltaic grid-connected system is reduced; the novel flexible operation control strategy not only realizes photovoltaic maximum power tracking control, but also realizes comprehensive optimization of voltage and power quality and operation loss of the distribution network.

Description

Photovoltaic flexible loop closing device and operation control method
Technical Field
The invention relates to a photovoltaic grid-connected control system and method, in particular to a photovoltaic flexible loop closing device and an operation control method.
Background
In recent years, the access proportion of distributed photovoltaic in distribution networks is continuously increased, and the access of distributed photovoltaic also presents a series of challenges for the operation management of the distribution networks. Distributed photovoltaic access positions are scattered, and each distributed photovoltaic system operates in an MPPT working mode independently. Due to the independent and disordered operation of the distributed photovoltaic and the randomness and fluctuation of the output, a severe challenge is provided for the management of power and energy of the power distribution network, when the output of the distributed photovoltaic is excessive, the load rate of a distribution area is too low, and power is sent backwards to cause an overvoltage condition in serious cases. In order to avoid imbalance of distributed energy output time and spatial distribution, researchers provide a technical solution of a flexible loop closing device. The flexible loop closing device is composed of back-to-back power converters, intelligent interaction of power and energy between power supply sub-areas is achieved by connecting different power supply sub-areas, unbalanced load distribution between the power supply sub-areas is effectively solved, and reliability of operation of a distribution network is improved. At present, the flexible loop closing device still has the defects of high cost and low efficiency, and the popularization and the application of the flexible loop closing device in a distribution network are limited. Another drawback of distributed photovoltaics is that a resonance phenomenon may be caused by a large number of distributed photovoltaic interface transformations and by the coupling effect of the interface converter and the grid, which affects the stable operation of the distribution network.
In conclusion, the existing grid-connected scheme cannot comprehensively solve the problems of energy management and electric energy quality management caused by a distributed photovoltaic access distribution network, and has the defects of high cost and low efficiency of a flexible loop closing device.
Disclosure of Invention
The purpose of the invention is as follows: the invention provides a photovoltaic flexible loop closing device and an operation control method, wherein the photovoltaic flexible loop closing device not only realizes intelligent interaction of power and energy between power supply sub-areas and improves the operation economy and reliability of a distribution network, but also provides photovoltaic alternating current interface equipment and reduces the input cost of a photovoltaic grid-connected system; the operation control strategy of the photovoltaic flexible loop closing device not only realizes photovoltaic maximum power tracking control, but also realizes comprehensive optimization of voltage and power quality and operation loss of a distribution network.
The technical scheme is as follows: the technical scheme adopted by the invention is that the photovoltaic flexible loop closing device comprises two voltage source type converters, the two voltage source type converters are respectively connected with two different alternating current systems, two circuit breakers are respectively connected to the output sides of the two voltage source type converters, the two voltage source type converters share a direct current capacitor, a photovoltaic cell panel collection direct current bus is directly connected to the two voltage source type converters through a first mechanical switch (DK1) and shares the direct current capacitor, and the photovoltaic cell panel collection direct current bus is connected to a secondary power supply of the flexible loop closing device through a second mechanical switch (DK0) and a DC-DC converter.
The invention also provides an operation control method applied to the photovoltaic flexible loop closing device, when the output voltage of the photovoltaic system is smaller than a set threshold value, a light-load MPPT operation mode is adopted, otherwise, a full-load MPPT operation mode is adopted; in a light-load MPPT operation mode, the second mechanical switch is in a closed state, the first mechanical switch is in an open state, the two voltage source type converters work in a static reactive compensation mode, and MPPT tracking is carried out through the DC/DC converter; under the full-load MPPT operation mode, the first mechanical switch is in a closed state, the second mechanical switch is in an open state, the photovoltaic cell panel feeds output active power to the two alternating current system feeders through the two voltage source type converters, and MPPT tracking is carried out through the two voltage source type converters.
The MPPT tracking through the DC/DC converter means that the DC/DC converter adopts voltage outer loop-current inner loop control, wherein a voltage reference value VDCrefAfter the difference is made with the actually measured photovoltaic direct current collection bus voltage, an input current reference value I is generated through a PI regulatorDCrefInput current I of a DC-DC converterDCWith reference value IDCrefAnd after difference making, outputting a PWM (pulse width modulation) adjusting signal of the DC-DC converter through a PI (proportional-integral) regulator.
The two voltage source type converters work in a static reactive compensation mode, namely the voltage source type converters adopt d and q decoupling control, wherein the capacitor voltage VCWith a reference value VCrefAfter difference is made, an output current d-axis reference signal I is generated by a PI regulatorg，drefOutput current d-axis reference signal Ig，drefMeasured current d-axis component I of voltage source type converterg，dAfter difference is made, the d-axis component of the modulation signal of the voltage source type converter is generated through adjustment of a direct current chain PR; reactive reference value Ig，qrefReactive current component I measured with voltage source converterg，qAnd after difference is made, generating a q-axis modulation signal of the voltage source type converter through a PR demodulator. The direct current chain PR demodulator adopts a resonance controller, and the transfer function of the resonance controller is as follows:
Figure BDA0002687107100000021
wherein k isP2、kI2Respectively, a proportionality coefficient and a resonance coefficient, omega is frequency, and s represents a function GPRDependent variable after laplace transformation.
The MPPT tracking through the two voltage source type converters means that one voltage source type converter adopts a voltage control mode, and the other voltage source type converter adopts a PQ control mode.
Wherein, one of the voltage source type converters adopts a voltage control mode, and comprises the following processes: the voltage source type converter adopts d, q decoupling control, wherein the capacitor voltage VCWith a reference value VCrefAfter difference is made, an output current d-axis reference signal I is generated by a PI regulatorg，drefOutput current d-axis reference signal Ig，drefMeasured current d-axis component I of voltage source type converterg，dAfter difference is made, the d-axis component of the modulation signal of the voltage source type converter is generated through adjustment of a direct current chain PR; calculating the reference value of output current and reactive reference value I of voltage source type converterg，qrefReactive current component I measured with voltage source converterg，qAnd generating a q-axis modulation signal of the voltage source type converter through a PR demodulator after the difference is made.
The other voltage source type converter adopts a PQ control mode and comprises the following processes: the voltage source type converter adopts d, q decoupling control, wherein a d-axis reference signal I of an output currentg，drefMeasured current d-axis component I of voltage source type converterg，dAfter difference is made, the d-axis component of the modulation signal of the voltage source type converter is generated through adjustment of a direct current chain PR; calculating the reference value of output current and reactive reference value I of voltage source type converterg，qrefReactive current component I measured with voltage source converterg，qAnd generating a q-axis modulation signal of the voltage source type converter through a PR demodulator after the difference is made.
Calculating the output current reference value of the voltage source type converter, comprising the following processes: obtaining an output power reference value of a back-to-back voltage source type converter by minimizing a target function of distribution network voltage and loss, and calculating to obtain a corresponding output current reference value of the back-to-back voltage source type converter, wherein the target function W of the distribution network voltage and loss is as follows:
W＝αf(PV1_ref，PV2_ref，QV1_ref，QV2_ref)+βg(PV1_ref，PV2_ref，QV1_ref，QV2_ref)
the constraint conditions are as follows:
Figure BDA0002687107100000031
wherein the functions f and g are evaluation functions of distribution network loss and node voltage; alpha and beta are distribution network loss and node voltage weight coefficients respectively, and alpha + beta is 1; pV1_ref、PV2_refRespectively representing active power reference value, Q of two voltage source type convertersV1_ref、QV2_refRespectively representing reactive power reference values of the two voltage source type converters; sNThe rated power of the voltage source type converter; pPVThe photovoltaic output power.
Has the advantages that: compared with the prior art, the invention has the following advantages: the photovoltaic type flexible loop closing device comprises two voltage source type converters and a photovoltaic grid-connected system, wherein a photovoltaic battery is connected to the public direct-current bus side of the voltage source type converters and is connected with a secondary power supply of the flexible loop closing device through a DC-DC converter. The flexible loop closing device not only realizes intelligent interaction of power and energy between power supply sub-regions, improves the economical efficiency and reliability of distribution network operation, but also provides photovoltaic alternating current interface equipment, and reduces the input cost of a photovoltaic grid-connected system. According to the operation control strategy of the photovoltaic flexible loop closing device, when the photovoltaic is started at low voltage, the photovoltaic is not connected with the side of the common direct current bus, and a secondary power supply of equipment is charged through a DC-DC converter; under the photovoltaic MPPT operation mode, the tracking control of the maximum photovoltaic power is realized by flexibly controlling the direct-current bus voltage of the hybrid flexible loop closing device, and the comprehensive optimization of the voltage and electric energy quality and the operation loss of a distribution network is realized by scheduling the active power of each port of the flexible loop closing device.
Drawings
FIG. 1 is a schematic structural view of a photovoltaic type flexible loop closing device according to the present invention;
fig. 2 is a schematic structural diagram of the photovoltaic flexible loop closing device according to the present invention in a light-load MPPT operation mode;
fig. 3 is a control schematic diagram of a DC-DC converter of the photovoltaic flexible loop closing device in a light-load MPPT operation mode according to the present invention;
fig. 4 is a control schematic diagram of a voltage source type converter of the photovoltaic type flexible loop closing device in a light load MPPT operation mode according to the present invention;
fig. 5 is a schematic structural diagram of a photovoltaic-type flexible loop closing device according to the present invention in a full-load MPPT operation mode;
fig. 6 is a control schematic diagram of a voltage source type converter of the photovoltaic type flexible loop closing device in a full-load MPPT operation mode, which includes (a) a control schematic diagram of the voltage source type converter VSC1 and (b) a control schematic diagram of the voltage source type converter VSC 2.
Detailed Description
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
The photovoltaic type flexible loop closing device comprises two voltage source type converters, wherein a photovoltaic cell is connected to the public direct current bus side of the voltage source type converters and is connected with a secondary power supply of the flexible loop closing device through a DC-DC converter. The flexible loop closing device not only realizes intelligent interaction of power and energy between power supply sub-areas, but also improves the operation economy and reliability of the distribution network; meanwhile, photovoltaic alternating current interface equipment is provided, so that the investment cost of a photovoltaic grid-connected system is reduced; the invention provides a photovoltaic flexible loop closing device operation control strategy, when photovoltaic is started at low voltage, the photovoltaic is not connected with the side of a common direct current bus, and a secondary power supply of equipment is charged through a DC-DC converter; under the photovoltaic MPPT operation mode, the tracking control of the maximum photovoltaic power is realized by flexibly controlling the direct-current bus voltage of the hybrid flexible loop closing device, and the comprehensive optimization of the voltage and electric energy quality and the operation loss of a distribution network is realized by scheduling the active power of each port of the flexible loop closing device.
Fig. 1 is a schematic structural diagram of a photovoltaic flexible loop closing device, which includes two Voltage Source Converters (VSC), two different ac systems are respectively connected to the VSC1 and the VSC2, the ac output side includes two ac breakers and one mechanical disconnector, the breakers AK1 and AK2 are respectively connected to the output sides of the VSC1 and the VSC2, the VSC1 and the VSC2 share a DC capacitor C, the photovoltaic cell collection DC bus is directly connected to the shared DC capacitor C through the mechanical switch DK1, and the photovoltaic power collection DC bus is connected to a secondary power Source of the flexible loop closing device through the mechanical switch DK0 and the DC-DC Converter.
The photovoltaic cell is a controlled current source, and the output current of the photovoltaic cell is related to the voltage; therefore, Maximum Power Point Tracking (MPPT) can be achieved by regulating the output voltage of the photovoltaic cell. The MPPT working point voltage of the photovoltaic cell is lower because the illumination intensity is extremely low in the morning or in the night before the night. Therefore, in order to maintain the wide voltage range operation of the photovoltaic cell, the photovoltaic grid-connected system mostly adopts a two-stage converter, namely, the two-stage converter consists of a front-stage DC-DC converter and a rear-stage DC-AC converter. The operation of the photovoltaic grid-connected system comprises two modes of light-load MPPT operation and full-load MPPT operation, and the operation modes of the two modes are detailed below.
Mode 1: light load MPPT operation
Under the MPPT working mode of the photovoltaic cell, the output voltage is less than UthDefined as light load MPPT operation. In a light-load MPPT operation mode, the mechanical switch DK0 is equal to 1, the DK1 is equal to 0, and the DK0 is equal to 1, which indicates that the mechanical switch DK0 is in a closed state, otherwise, the mechanical switch DK0 is in an off state.
The photovoltaic cell charges the secondary energy storage power supply through the DC-DC converter, the VSC1 and the VSC2 work in a STATCOM mode, and the whole operation structure of the photovoltaic flexible loop closing device is shown in fig. 2.
Fig. 3 shows the operation principle of MPPT of the DC/DC converter in this mode of operation.
WhereinVDCrefReference voltage, V, output for MPPT controllerbusThe dc bus voltage is collected for the photovoltaic cells. The DC-DC converter being controlled by a voltage outer loop-current inner loop, i.e. a voltage reference VDCrefAfter the difference is made with the actually measured photovoltaic direct current collection bus voltage, an input current reference value I is generated through a PI regulatorDCrefIn which IDCThe current injected into the DC-DC converter for the photovoltaic collection bus, i.e. the input current of the DC-DC converter.
Input current I of DC-DC converterDCWith reference value IDCrefAnd after difference making, outputting a PWM (pulse width modulation) adjusting signal of the DC-DC converter through a PI (proportional-integral) regulator.
The voltage source type converters VSC1 and VSC2 work in a static reactive compensation mode; taking the voltage source converter VSC1 as an example, as shown in fig. 4, the voltage source converter VSC1 adopts d, q decoupling control and capacitance voltage VCWith a reference value VCrefAfter difference is made, an output current d-axis reference signal I is generated by a PI regulatorg1，dref. Output current d-axis reference signal Ig1，drefMeasuring the d-axis component I of the current with the VSC1 of the voltage source converterg1，dAfter the difference is made, the d-axis component of the modulation signal of the voltage source converter VSC1 is generated by PR adjustment. The direct-current capacitor voltage control realizes the active power exchange control of the voltage source type converter VSC1 and the power grid, and realizes the integral regulation and control of the capacitor voltage and the energy of the voltage source type converter VSC 1.
The DC link voltage control adopts a resonance controller, and the transfer function is as follows:
Figure BDA0002687107100000051
wherein k isP2、kI2Respectively, the ratio and the resonance coefficient, omega is the frequency, and s represents the function GPRDependent variable after laplace transformation.
Reactive reference value Ig1，qrefMeasuring reactive current component I with voltage source converter VSC1g1，qAnd after the difference is made, a q-axis modulation signal of the voltage source type converter VSC1 is generated through a PR demodulator.
Mode 2: full load MPPT operation
Under the mode of a photovoltaic cell full-load MPPT working mode, the output voltage is greater than Uth. In the full-load MPPT operating mode, DK0 is 0 and DK1 is 1.
The photovoltaic cell feeds output active power to the alternating current system feeders 1 and 2 through the voltage source type converters VSC1 and VSC2, and the whole operation structure of the photovoltaic type flexible loop is shown in fig. 5.
MPPT control mode:
the output power of the voltage source converter VSC1 is SVSC1The active power and the reactive power are respectively PVSC1、QVSC1And has the following components:
Figure BDA0002687107100000052
the output power of the VSC2 of the voltage source type converter is SVSC2Active power and reactive power are respectively PVSC2、QVSC2And has the following components:
Figure BDA0002687107100000053
if neglecting the loss of VSC1, VSC2, then there are:
PVSC1+PVSC2＝PPV
wherein P isPVFor photovoltaic output power, i.e.:
PPV＝VbusIPV
photovoltaic DC collection bus voltage Vbus＝VC。
The voltage source converter VSC1 operates in a voltage control mode by controlling a common DC capacitor VCAnd the tracking control of the maximum photovoltaic power is realized by the voltage. The VSC1 outputs active power reference value PV1_refA reference value of reactive power of QV1_ref。
The VSC2 is operated in PQ power mode and has a reference value of output power PV2_ref、QV2_ref. By determining the active power and reactive power reference values of the VSC1 and the VSC2, the output current reference values of the corresponding VSC1 and the VSC2 are calculated.
Fig. 6 is a schematic diagram of a control system for a voltage source converter in full-load MPPT mode of operation.
Further, the reference values of the active power and the reactive power output by the voltage source converters VSC1 and VSC2 can be obtained by the following methods:
by optimizing the output power of the voltage source type converters VSC1 and VSC2, the following objective function can be minimized, and the comprehensive management of the voltage and the loss of the distribution network can be realized.
W＝αf(PV1_ref，PV2_ref，QV1_ref，QV2_ref)+βg(PV1_ref，PV2_ref，QV1_ref，QV2_ref)
Wherein f and g are functions for evaluating the loss of the distribution network and the voltage of the node; α and β are distribution network loss and node voltage weighting coefficients, respectively, and α + β is 1. PV1_ref、PV2_refRespectively representing active power reference value, Q of two voltage source type convertersV1_ref、QV2_refRespectively representing the reactive power reference values of the two voltage source type converters. And the output power of the voltage source converters VSC1 and VSC2 meets the following constraint conditions:
Figure BDA0002687107100000061
wherein SNThe rated power of the voltage source converters VSC1 and VSC2 can be obtained by combining the above two equations, so that the output active power and reactive power reference values of the voltage source converters VSC1 and VSC2 can be obtained. By determining the active power and reactive power reference values of the VSC1 and the VSC2, the output current reference values of the corresponding VSC1 and the VSC2 are calculated.
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 the like) 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 flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams 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.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.

Claims (10)

1. The utility model provides a flexible ring device that closes of photovoltaic type which characterized in that: the device comprises two voltage source type converters, wherein the two voltage source type converters are respectively connected with two different alternating current systems, two circuit breakers are respectively connected to the output sides of the two voltage source type converters, the two voltage source type converters share a direct current capacitor, a photovoltaic cell panel collects a direct current bus and is directly connected to the two voltage source type converters through a first mechanical switch (DK1) and shares the direct current capacitor, and the photovoltaic cell panel collects a direct current bus and is connected to a secondary power supply of the flexible loop closing device through a second mechanical switch (DK0) and a DC-DC converter.
2. An operation control method applied to the photovoltaic type flexible loop closing device of claim 1, characterized in that: when the output voltage of the photovoltaic system is smaller than a set threshold value, the photovoltaic type flexible loop closing device adopts a light-load MPPT operation mode, otherwise, a full-load MPPT operation mode is adopted; in a light-load MPPT operation mode, the second mechanical switch is in a closed state, the first mechanical switch is in an open state, the two voltage source type converters work in a static reactive compensation mode, and the photovoltaic type flexible loop closing device carries out MPPT tracking through the DC/DC converter; under the full-load MPPT operation mode, the first mechanical switch is in a closed state, the second mechanical switch is in an open state, the photovoltaic cell panel feeds and outputs active power to the two alternating current system feeders through the two voltage source type converters, and the photovoltaic type flexible loop closing device performs MPPT tracking through the two voltage source type converters.
3. The operation control method of the photovoltaic type flexible loop closing device according to claim 2, characterized in that: the photovoltaic flexible loop closing device carries out MPPT tracking through a DC/DC converter, and comprises the following stepsThe process: the DC-DC converter is controlled by a voltage outer loop and a current inner loop, wherein the voltage reference value VDCrefAfter the difference is made with the actually measured photovoltaic direct current collection bus voltage, an input current reference value I is generated through a PI regulatorDCrefInput current I of a DC-DC converterDCWith reference value IDCrefAnd after difference making, outputting a PWM (pulse width modulation) adjusting signal of the DC-DC converter through a PI (proportional-integral) regulator.
4. The operation control method of the photovoltaic type flexible loop closing device according to claim 2, characterized in that: the two voltage source type converters work in a static reactive compensation mode and comprise the following processes: the voltage source type converter adopts d, q decoupling control, wherein the capacitor voltage VCWith a reference value VCrefAfter difference is made, an output current d-axis reference signal I is generated by a PI regulatorg，drefOutput current d-axis reference signal Ig，drefMeasured current d-axis component I of voltage source type converterg，dAfter difference is made, the d-axis component of the modulation signal of the voltage source type converter is generated through adjustment of a direct current chain PR; reactive reference value Ig，qrefReactive current component I measured with voltage source converterg，qAnd after difference is made, generating a q-axis modulation signal of the voltage source type converter through a PR demodulator.
5. The operation control method of the photovoltaic type flexible loop closing device according to claim 4, characterized in that: the PR demodulator adopts a resonance controller, and the transfer function of the resonance controller is as follows:
Figure FDA0002687107090000011
wherein k isP2、kI2Respectively, a proportionality coefficient and a resonance coefficient, omega is frequency, and s represents a function GPRDependent variable after laplace transformation.
6. The operation control method of the photovoltaic type flexible loop closing device according to claim 2, characterized in that: the photovoltaic type flexible loop closing device carries out MPPT tracking through two voltage source type converters, namely one voltage source type converter adopts a voltage control mode, and the other voltage source type converter adopts a PQ control mode.
7. The operation control method of the photovoltaic type flexible loop closing device according to claim 6, characterized in that: one of the voltage source type converters adopts a voltage control mode and comprises the following processes: the voltage source type converter adopts d, q decoupling control, wherein the capacitor voltage VCWith a reference value VCrefAfter difference is made, an output current d-axis reference signal I is generated by a PI regulatorg1，drefOutput current d-axis reference signal Ig1，drefMeasured current d-axis component I of voltage source type converterg1，dAfter difference is made, the d-axis component of the modulation signal of the voltage source type converter is generated through adjustment of a direct current chain PR; calculating the reference value of output current and reactive reference value I of voltage source type converterg1，qrefReactive current component I measured with voltage source converterg1，qAnd generating a q-axis modulation signal of the voltage source type converter through a PR demodulator after the difference is made.
8. The operation control method of the photovoltaic type flexible loop closing device according to claim 6, characterized in that: the other voltage source type converter adopts a PQ control mode and comprises the following processes: the voltage source type converter adopts d, q decoupling control, wherein a d-axis reference signal I of an output currentg2，drefMeasured current d-axis component I of voltage source type converterg2，dAfter difference is made, the d-axis component of the modulation signal of the voltage source type converter is generated through adjustment of a direct current chain PR; calculating the reference value of output current and reactive reference value I of voltage source type converterg2，qrefReactive current component I measured with voltage source converterg2，qAnd generating a q-axis modulation signal of the voltage source type converter through a PR demodulator after the difference is made.
9. The operation control method of a photovoltaic-type flexible loop closing device according to claim 7 or 8, characterized in that: the method for calculating the output current reference value of the voltage source type converter comprises the following steps: obtaining an output power reference value of a back-to-back voltage source type converter by minimizing a target function of distribution network voltage and loss, and calculating to obtain a corresponding output current reference value of the back-to-back voltage source type converter, wherein the target function W of the distribution network voltage and loss is as follows:
W＝αf(PV1_ref，PV2_ref，QV1_ref，QV2_ref)+βg(PV1_ref，PV2_ref，QV1_ref，QV2_ref)
the constraint conditions are as follows:
Figure FDA0002687107090000021
wherein the functions f and g are evaluation functions of distribution network loss and node voltage; alpha and beta are distribution network loss and node voltage weight coefficients respectively, and alpha + beta is 1; pV1_ref、PV2_refRespectively representing active power reference value, Q of two voltage source type convertersV1_ref、QV2_refRespectively representing reactive power reference values of the two voltage source type converters; sNThe rated power of the voltage source type converter; pPVThe photovoltaic output power.
10. A computer-readable storage medium storing a computer-executable program comprising the following controls: when the output voltage of the photovoltaic system is smaller than a set threshold value, the photovoltaic type flexible loop closing device adopts a light-load MPPT operation mode, otherwise, a full-load MPPT operation mode is adopted; in a light-load MPPT operation mode, the second mechanical switch is in a closed state, the first mechanical switch is in an open state, the two voltage source type converters work in a static reactive compensation mode, and the photovoltaic type flexible loop closing device carries out MPPT tracking through the DC/DC converter; under the full-load MPPT operation mode, the first mechanical switch is in a closed state, the second mechanical switch is in an open state, the photovoltaic cell panel feeds and outputs active power to the two alternating current system feeders through the two voltage source type converters, and the photovoltaic type flexible loop closing device performs MPPT tracking through the two voltage source type converters.
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