CN113078674A - Novel modular photovoltaic grid-connected system based on three-port power channel, namely control method - Google Patents
Novel modular photovoltaic grid-connected system based on three-port power channel, namely control method Download PDFInfo
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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
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- 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
- H02M1/00—Details of apparatus for conversion
- H02M1/14—Arrangements for reducing ripples from dc input or output
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- 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
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
- H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
- H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
- H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
- H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
- H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
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- 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
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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- 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
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/49—Combination of the output voltage waveforms of a plurality of converters
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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
- 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
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- 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 novel modular photovoltaic grid-connected system based on a three-port power channel, namely a control method, which comprises the following steps: the front-stage three-port isolated DC/DC photovoltaic converter, the rear-stage three-phase half-bridge cascade MMC and a combination thereof. The front-stage three-port DC/DC converter is isolated by a three-winding high-frequency transformer, an input port is connected with the photovoltaic array, and two output ports are respectively connected to two sub-modules of an upper bridge arm and a lower bridge arm of each phase unit of the MMC. The half-bridge cascade structure of MMC makes output voltage be the stack of each submodule piece output voltage, and the modulation reduces output current harmonic greatly through the carrier phase shift to can the lug connection middling pressure exchange, middling pressure direct current electric wire netting. The topology can reduce the capacity of the power decoupling capacitor by tens of times through power decoupling control, and the volume of the system is greatly reduced. In addition, the photovoltaic power among the topological phase units is balanced in a form of direct current circulation through a direct current link, so that the problem of photovoltaic power mismatch is well solved.
Description
Technical Field
The invention relates to a novel Modular topological structure based on a three-port power channel, in particular to a photovoltaic power generation system for restraining direct-current side voltage fluctuation of a cascaded Modular Multilevel Converter (MMC) and balancing photovoltaic power among bridge arms of a three-phase photovoltaic grid-connected power generation system, and belongs to the technical field of power electronic power conversion.
Background
Solar energy is increasingly paid attention to people as a renewable clean energy source, and photovoltaic power stations are developing towards large-scale and grid-connected. The traditional centralized photovoltaic array is easy to cause Maximum Power Point Tracking (MPPT) to fall into a local Maximum Power Point, which results in low photovoltaic utilization rate. With the improvement of voltage and power levels, the traditional two-level and three-level voltage source type inverters can not meet the requirements. The MMC has the advantages of modularization, easiness in expansion, small output current harmonic wave and the like, can realize photovoltaic local alternating current grid connection and long-distance direct current transmission, and has a great development prospect in a large photovoltaic grid connection system. Through splitting the photovoltaic array into a plurality of small modules and connecting the small modules to each submodule of the MMC through the DC/DC converter, MPPT is realized in each module respectively, and the photovoltaic utilization rate can be effectively improved.
For a three-phase modular photovoltaic grid-connected system based on MMC, several key problems generally need to be solved.
Parasitic electricity to ground of the photovoltaic panel easily causes leakage current to the system, so an isolated photovoltaic DC/DC converter is generally required, which requires adding a high-frequency transformer to each sub-module port of the MMC, thereby suppressing the leakage current.
The output power of each submodule of the MMC has large first-frequency multiplication, second-frequency multiplication and high-frequency component fluctuation, the direct current side of each submodule is generally required to be subjected to power decoupling by using a large-capacity electrolytic capacitor, and the large-capacity electrolytic capacitor is large in size and limited in service life, so that the occupied area of the whole system is large, and the service life is short. Therefore, the power decoupling technology is needed to reduce the capacity of the power decoupling capacitor as much as possible, so that the electrolytic capacitor is replaced by a thin film capacitor with small capacity and long service life, which is mainly divided into: firstly, a power decoupling circuit is added, but the power decoupling circuit needs to be added to each port, so that the hardware implementation is more complex; secondly, power fluctuation is reduced by adopting control methods such as circulation control and the like, but for the upper and lower bridge arms of the same phase unit of the MMC, the power compensated by circulation is equal, so that the double-frequency common-mode component of the capacitance ripple of the upper and lower arms can be compensated, and the first-frequency-multiplication and third-frequency-multiplication differential-mode component ripples with larger amplitude cannot be counteracted.
Because each module of the system respectively carries out MPPT control, when the illumination intensity is uneven or the equipment parameters are different, power mismatch easily occurs between bridge arms, the current flowing into a power grid is asymmetric, and the application of the modular photovoltaic grid-connected system is greatly limited. At present, existing research is mainly solved by adopting power mismatch elimination strategies such as a fundamental frequency circulation injection method, but the control method needs to collect photovoltaic power of each module, and complexity of the system is increased. The control is complex and is open loop control, and the balance effect has errors.
In summary, the main problems of the prior art are: an MMC system usually needs to adopt an electrolytic capacitor with larger capacity and volume to perform power decoupling, so that the service life and the reliability of a photovoltaic power generation system are influenced; the modularized photovoltaic inverter is difficult to avoid the problem of power mismatch, the existing topological power mismatch elimination strategy is small in balancing capability and poor in balancing effect, and practical application of the structure is greatly limited.
In order to improve the reliability of the engineering application of the modular photovoltaic grid-connected system, the problem of photovoltaic power mismatch needs to be well solved. By adopting the novel modular topology, the photovoltaic power mismatch of each phase is automatically eliminated or a simple control strategy is adopted, so that the stable operation of the system is realized while the photovoltaic utilization rate is effectively improved. The problem that the capacitance of an alternating current-direct current power decoupling capacitor is huge must be solved to realize high efficiency and high power density of a photovoltaic grid-connected system, so that a new technology needs to be provided to reduce the required capacitance, a thin film capacitor with long service life and high reliability can be adopted to replace an electrolytic capacitor, and the service life and reliability of the system are improved while the power density of the system is improved.
Disclosure of Invention
The invention mainly aims at the design targets of realizing the miniaturization of a modular photovoltaic grid-connected inverter based on MMC and the self-balancing of power among bridge arms, and provides a novel modular photovoltaic grid-connected system based on a three-port power channel in view of the problems in the prior art, wherein the system comprises a three-port DC/DC converter with a front-stage input port connected with a photovoltaic array, a half-bridge cascade modular multilevel converter with an input end connected with two output ends of the multi-port isolated DC/DC converter, and a rear-stage output end respectively used for local alternating current grid connection and remote direct current transmission;
the three-port isolation type DC/DC converter comprises a multi-winding high-frequency isolation transformer and two output rectifier bridges, wherein an input port is connected with a photovoltaic array to jointly form a photovoltaic sub-module;
two rear-stage ports of the photovoltaic sub-modules are connected with one sub-module of an upper bridge arm of a certain phase unit of the rear-stage modular multilevel converter one by one, and the other sub-module of a lower bridge arm is connected with one sub-module of the upper bridge arm of the certain phase unit of the rear-stage modular multilevel converter; the positions of the two connected half-bridge submodules in respective bridge arms are not specific, and the insulation design optimization of the high-frequency transformer of each photovoltaic submodule can be comprehensively considered in specific selection;
the modularized multi-level circulator is a three-phase half-bridge cascade topology, each phase unit comprises an upper bridge arm and a lower bridge arm, each bridge arm is connected with a filter inductor in series and connected to one phase of an alternating current power grid, and each submodule consists of a half-bridge circuit and a power decoupling capacitor.
In the novel modular photovoltaic grid-connected system based on the three-port power channel, the three-port DC/DC photovoltaic converter part of each photovoltaic sub-module adopts a plurality of parallel connection modes to form the photovoltaic sub-modules accessed by a plurality of photovoltaic arrays, so that the increase of the photovoltaic power grade is realized.
In the novel modular photovoltaic grid-connected system based on the three-port power channel, the three-port isolation type DC/DC converter adopts a three-active-bridge (TAB) converter, the isolation transformer is a three-winding high-frequency transformer, and each port is connected with a full-bridge circuit to realize phase-shift modulation; the upper bridge arm and the lower bridge arm of each phase unit share the same photovoltaic array, and the photovoltaic power of the upper bridge arm and the lower bridge arm is automatically balanced; the power among the phase units is automatically generated through a direct current link or actively injected into direct current circulation to realize balance, and the photovoltaic power mismatch of each bridge arm of the system is eliminated.
The novel modular photovoltaic grid-connected system based on the three-port power channel comprises
Power decoupling control: photovoltaic access is realized through the three-port isolated DC/DC converter, and power channels are provided for upper and lower bridge arms of the rear-stage modular multilevel converter at the same time, so that the cancellation of capacitance-voltage ripple differential mode components of sub-modules of the upper and lower bridge arms is realized; cancellation of voltage ripple common-mode components of the sub-module capacitors is achieved through circulation control of a rear-stage modular multilevel converter, and therefore complete cancellation of low-frequency ripples of the power decoupling capacitors is achieved;
eliminating the photovoltaic power mismatch among modules in a bridge arm: by comparing the sub-module power decoupling capacitor voltage with the bridge arm average value, an adjusting coefficient with an initial value of 1 is generated through a PI controller, and an alternating current component reference value and a direct current component reference value of the output voltage of the half-bridge circuit need to be multiplied by the coefficient at the same time and added to serve as the output voltage reference after the half-bridge circuit is adjusted, so that the output power of the sub-module is adjusted, and the failure of common-mode component ripple cancellation is avoided.
Compared with the prior art, the invention at least has the following beneficial effects:
(1) based on the MMC system, photovoltaic access to a local alternating current power grid is achieved, and meanwhile, long-distance direct current power transmission is achieved. Meanwhile, the modular topological structure is composed of a plurality of photovoltaic sub-modules, is flexible in structure and easy to expand, and can realize redundancy control. Each photovoltaic sub-module carries out maximum power tracking control, the problem that the maximum point of local power occurs when a local shadow or fault occurs in a centralized structure photovoltaic system is effectively solved, and the photovoltaic utilization rate of the system is greatly improved.
(2) The three-port DC/DC converter establishes a power channel between an upper bridge arm and a lower bridge arm of each phase unit of the MMC, so that the first-frequency and third-frequency pulse power can be counteracted in a high-frequency transformer with a front-stage three-winding, and the lower bridge arm submodule can be used for counteracting the low-frequency ripple of the power decoupling capacitance differential mode component; by adopting double-frequency loop control of an MMC system, double-frequency pulsating power is counteracted among the phase units, so that the low-frequency ripple waves of power decoupling capacitor common-mode components of upper and lower bridge arm sub-modules of each phase unit are counteracted, and the low-frequency ripple waves are completely eliminated. Compared with simulation results, the capacitance value of the power decoupling capacitor can be reduced by more than 30 times. In addition, the capacitance value of the capacitor can be further reduced by improving the switching frequency of the system, and meanwhile, the bridge arm series filter inductance can be reduced by adopting carrier phase shift modulation.
(3) The upper bridge arm and the lower bridge arm of each phase unit are connected with the same photovoltaic array, so that the problem of power mismatch between the upper bridge arm and the lower bridge arm does not exist; because a common direct current link exists between the phase units, the power of the phase units can be automatically balanced in a direct current circulation mode through the direct current link, or the direct current circulation control is actively adopted to accelerate the balancing speed, so that the power mismatch between the phase units can be eliminated. Therefore, the topological structure can well solve the problem of photovoltaic power mismatch and greatly improve the stability of grid connection of the system.
Drawings
Fig. 1 is a system overall topology diagram according to an embodiment of the present invention.
Fig. 2a is an equivalent model of a three-winding transformer according to an embodiment of the present invention.
Fig. 2b is a delta equivalent circuit of a three-winding transformer according to an embodiment of the present invention.
Fig. 3a is a block diagram of power decoupling control based on TAB phase shift modulation according to an embodiment of the present invention.
Fig. 3b is a power decoupling control block diagram based on MMC double frequency loop control according to an embodiment of the present invention.
Fig. 4 is a block diagram of a power mismatch cancellation strategy according to an embodiment of the present invention.
Fig. 5 is an overall control block diagram of the embodiment of the present invention.
Fig. 6a is a power decoupling capacitor voltage simulation waveform according to an embodiment of the present invention.
Fig. 6b is a simulated waveform of the capacitor voltage of the conventional topology without the power decoupling control.
FIG. 7a is a waveform diagram of output current simulation during uneven illumination according to an embodiment of the present invention.
FIG. 7b is a simulated waveform of the system circulating current under non-uniform illumination according to an embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION
The core of the invention is to provide a modular circuit topology for a large-scale photovoltaic grid-connected system, the topology has the advantages of modularization and easy expansion, and the local alternating current grid connection is realized while the remote direct current transmission is realized. Compared with the traditional photovoltaic grid-connected system based on MMC, the capacity of the power decoupling capacitor can be reduced by tens of times, and the power density of the system is greatly improved.
The invention provides a novel modular photovoltaic grid-connected inverter topological structure based on a three-port power channel, which is characterized in that: the system comprises a three-port DC/DC converter with a front-stage input port connected with a photovoltaic array, a half-bridge cascade modular multilevel converter with an input end connected with two output ends of the multi-port isolated DC/DC converter, and a rear-stage output end used for local alternating current grid connection and remote direct current transmission
The three-port DC/DC converter integrates isolation transformers of two sub-module photovoltaic converters of an upper Bridge arm and a lower Bridge arm into a three-winding high-frequency transformer, namely, an isolation type three-port DC/DC converter such as a three-Active Bridge (TAB) is adopted. The input port is connected with the photovoltaic array to form a photovoltaic sub-module.
The modular multilevel converter is a three-phase half-bridge cascade MMC topology, and each submodule is a half-bridge circuit and is connected with a power decoupling capacitor in parallel.
The phase unit is all cascaded half-bridge submodules of the same phase of the MMC, and comprises an upper bridge arm and a lower bridge arm which are respectively connected in series with a filter inductor and connected to one phase of an alternating current power grid. The half-bridge cascade allows the upper and lower terminals of the phase unit to be connected to a medium-high voltage direct current network for long-distance direct current transmission.
The photovoltaic sub-module comprises a three-winding transformer, an auxiliary inductor is connected in series with a primary side, is connected with an H-bridge circuit and is connected to a photovoltaic array, and a photovoltaic panel end capacitor is connected in parallel to suppress voltage ripples of photovoltaic input; two windings on the secondary side are connected with an H bridge respectively. Two output ports of each photovoltaic submodule are respectively connected to two ends of one submodule power decoupling capacitor of each phase of upper and lower bridge arms of the rear-stage MMC. Since there are N sub-modules per leg, each phase of the system will contain N photovoltaic sub-modules. And the exchange of the upper and lower bridge arm power is realized while the MPPT control is realized.
The three-winding high-frequency transformer is a three-winding transformer with 1 winding on the primary side and two balanced windings on the secondary side, the two balanced windings respectively correspond to the output of two sub-modules of an upper bridge arm and a lower bridge arm of an MMC, and all the windings are wound on the same magnetic core. The device is used for electrical isolation between a primary side and a secondary side and between the secondary side and the secondary side, is used for power decoupling of alternating current and direct current power fluctuation, can provide voltage gain and reduces the number of series photovoltaic panels of input ports.
The isolated three-port DC/DC converter is isolated by a three-winding high-frequency transformer, the front stage is connected with the photovoltaic array and the resonant element, and the rear stage is rectified and output by two ports. Besides TAB, structures such as a three-port LLC resonant converter can be adopted to realize soft switching, so that the switching frequency of a front stage is improved, the size of the converter can be further reduced, and the efficiency can be improved.
Because each pair of sub-modules of the upper bridge arm and the lower bridge arm share the same photovoltaic array, power mismatch does not exist between the upper bridge arm and the lower bridge arm. The photovoltaic power among the phase units is automatically balanced through a direct current link, and even if a power mismatch elimination strategy is not additionally adopted, the power balance among the bridge arms can be realized, so that the photovoltaic converter can well solve the problem of power mismatch.
The invention also provides a control method using the topology system, which is characterized in that:
the average phase shift angle between the input port and the output port of the three-port isolated DC/DC converter is controlled by adopting an MPPT algorithm, so that the maximum power tracking of each photovoltaic submodule is realized;
the fine tuning of phase shift angle between two ports of the TAB output side adopts power decoupling control, power exchange between an upper bridge arm submodule and a lower bridge arm submodule of an MMC is actively controlled, differential mode components of capacitance ripples of the upper bridge arm submodule and the lower bridge arm submodule of each phase unit can be counteracted, low-frequency components of the ripples are completely counteracted by matching with an MMC circulating current injection strategy of interphase counteraction of common mode component ripples, and therefore the capacity of an MMC power decoupling capacitor can be reduced by tens of times, the size of a system is greatly reduced, and the service life of the system is prolonged.
Photovoltaic power among modules in each bridge arm of the MMC is used for eliminating power mismatch among the modules on the premise of meeting ripple cancellation by adjusting the amplitude of direct current and alternating current components of output voltage of each module.
The MMC adjusts the average direct current side voltage, the system equivalent output voltage and the output current of each submodule according to the magnitude of the photovoltaic power, so that a three-phase alternating current and direct current power grid connected with the MMC is matched.
The technical solutions in the embodiments of the present invention will be fully and clearly described below with reference to the embodiments of the present invention shown in the accompanying drawings. It is understood that the drawings represent only one embodiment of the invention and not all embodiments. All other embodiments obtained without inventive step for a person skilled in the art are within the scope of the present patent.
Fig. 1 is a system overall topology diagram according to an embodiment of the present invention. As shown in fig. 1, the system grid-connected part of the embodiment of the invention is mainly based on a three-phase half-bridge cascaded MMC topology, two bridge arms of each phase need to be respectively connected with a filter inductor in series, and the harmonic wave of the output current can be greatly reduced through the carrier phase-shift modulation of the cascaded sub-modules, so that the system can be directly connected to the grid without a heavy power frequency transformer. Each photovoltaic array is respectively connected with two sub-modules of upper and lower bridge arms of each phase of the MMC through a three-port DC/DC converter, so that photovoltaic access is realized. The three-port converter is isolated by adopting a three-winding transformer, the photovoltaic MPPT control can be realized through the phase-shift modulation of front and rear full bridges, and the exchange of the power of upper and lower bridge arms is realized at the same time, so that the differential mode component ripple waves of the sub-module capacitors of the upper and lower bridge arms are offset.
The three-port isolated DC/DC converter comprises a photovoltaic input end filter capacitor, a photovoltaic input full-bridge circuit, a primary side auxiliary inductor, a three-winding high-frequency isolation transformer and two secondary side rectification full-bridge circuits.
The MMC grid-connected system is provided with three phase units which are connected in parallel, each phase unit consists of an upper bridge arm and a lower bridge arm, each bridge arm is formed by connecting N half-bridge sub-modules and a filter inductor L in series, and the half-bridge sub-modules comprise a power decoupling capacitor besides a half-bridge circuit. By increasing the number of the cascade modules, the middle point of each phase unit can be directly merged into a medium-high voltage alternating current power grid, and the upper end point and the lower end point can be used for medium-high voltage direct current power transmission.
FIG. 2(a) is the equivalent model of the three-winding transformer, which mainly includes a series inductor L composed of a primary side auxiliary inductor and a leakage inductor1Leakage inductance L of two windings on the secondary side2And L3And three windings N1～N3. FIG. 2(b) shows the Δ equivalent circuit of the three-winding transformer, and the equivalent inductance between the ports can be expressed as
In the embodiment of the invention, each bridge arm of the MMC consists of three sub-modules, the switching frequency is 2kHz, and the rated value of the sub-module capacitance voltage is 1 kV; the switch frequency of the TAB photovoltaic converter is 5kHz, the TAB photovoltaic converter is connected with a photovoltaic array with the rated power of 72kW, and the output voltage of the maximum power point is 800V; the equivalent inductance L1 of the three-winding transformer is 130uH, the equivalent inductances L2 and L3 are 10uH, and the leakage inductance is 800 uH; the grid-connected direct current voltage is 3kV, and the alternating current grid voltage amplitude is 1 kV.
Taking the phase unit a as an example, the voltage ripples of the power decoupling capacitors of the upper and lower bridge arms can be respectively expressed as
Wherein IsFor grid current amplitude, θvIs a power factor angle; i is2fAnd theta2fDoubling the frequency circulation amplitude and phase of the system; m is a modulation ratio. Fig. 3 is a power decoupling control block diagram of the system according to the embodiment of the invention, which can respectively cancel the differential mode component mainly including the first frequency multiplication and the third frequency multiplication and the common mode component mainly including the second frequency multiplication in the capacitor ripples of the upper and lower bridge arm sub-modules.
The full bridge circuit of each port of the TAB converter can realize power transmission through phase-shift modulation, and the assumption is that
Is the phase shift angle, v, between port i and port jiIs the voltage of port i, so that the power transmitted from port i to port j is
Due to L1Ratio L2，L3The power exchanged between the ports 2 and 3 is sensitive to phase change, so that the power exchange of the upper and lower bridge arm sub-modules of the MMC can be realized by fine tuning the phase shift angle between the ports 2 and 3 on the premise of ensuring that the average phase shift angle between the port 1 and the ports 2 and 3 is controlled by MPPT, and the offset of differential mode component ripple is realized. Assuming a fine-tuned phase shift angle between ports 2,3 of
The power exchanged between ports 2,3 can be simply calculated as
Fig. 3(a) is a power decoupling control block diagram for cancellation of differential mode components of capacitor voltage ripples of upper and lower bridge arm sub-modules in the embodiment of the invention, and the voltage difference values of two power decoupling capacitors connected to a photovoltaic module are collected, and the first frequency multiplication component and the third frequency multiplication component in the voltage difference values are obtained byPR controller generates corresponding
Thereby realizing accurate cancellation of differential mode component ripple. By combining the actual output power of the MMC sub-modules, the power which is required to be transmitted from the upper bridge arm to the lower bridge arm by the photovoltaic sub-modules of each phase unit can be obtained
Therefore, the feedforward control quantity of the phase shift angle is calculated to accelerate the dynamic response of the system. Suppose that the power to be exchanged is p23,k(k ═ a, b, c), the feedforward amount can be calculated as
For the common mode component of the ripple, the output power of the system can be collected, and the double frequency circulation in the system is controlled to be the double frequency circulation through the double frequency circulation controller of the system
That is, the common-mode component in the equation (2) becomes zero, and fig. 3(b) is a power decoupling control block diagram of the common-mode component ripple cancellation according to the embodiment of the present invention. The outer ring controller sums the average values of the capacitance and the voltage of the upper and lower bridge arms of each phase, d and q axis components are obtained through double frequency park conversion, and reference quantities of d and q components of the inner ring current are adjusted through the PI controller until the two components are controlled to be zero; the inner ring current controller compares the system circulating current d and q components with a reference quantity, generates a reference value of the system output voltage d and q components through PI control, and generates the system output voltage reference quantity through park inverse transformation.
Fig. 4 is a power mismatch elimination strategy among the bridge arm sub-modules according to the embodiment of the present invention, and since the power mismatch elimination strategy among the bridge arms can be balanced even without an additional control strategy, the photovoltaic power among the modules only needs to be balanced. By comparing the deviation between the capacitor voltage of each module and the average value, the actual output power of each module can be balanced by adjusting the amplitude of the alternating voltage through the PI controller.
Fig. 5 is a block diagram of the overall control system according to the embodiment of the present invention, in which a voltage reference generated by a current inner loop and a circulating current voltage reference generated by a circulating current controller are added to obtain a voltage reference of each bridge arm, and a switching signal of each sub-module switching tube is generated through a power mismatch elimination strategy and carrier phase shift PWM modulation between sub-modules.
Fig. 6(a) is a simulation waveform of the capacitor ripple using the 1.1mF power decoupling capacitor according to the embodiment of the present invention. Because the power decoupling control can completely counteract the low-frequency component of the ripple, the ripple is greatly reduced, and the ripple peak-valley difference is only 25V; fig. 6(b) is a power decoupling capacitor voltage simulation waveform of the conventional MMC-based photovoltaic grid-connected system at the same power level, and under the same ripple peak-valley difference, the conventional topology requires a power decoupling capacitor up to 35mF, which is approximately 30 times larger than the novel modular photovoltaic grid-connected system based on the three-port power channel.
Since the power decoupling control completely cancels out the low frequency amount of the voltage ripple, the ripple factor will mainly depend on the switching frequency of the system. By increasing the switching frequency of the system, the capacitance value of the power decoupling capacitor can be further reduced, so that the size and the service life of the system are further optimized.
FIG. 7 shows that the initial illumination of the photovoltaic array at each phase is 1000W/m according to the embodiment of the invention2Sequentially exposed to the light of the phase unit a reduced to 600W/m2The illumination of the photovoltaic sub-modules 1 and 2 of the phase unit b is reduced to 800W/m2The output current and the system circulation current are simulated. It can be found that when uneven illumination occurs, direct current components are immediately generated by circulation among systems, so that power balance of each phase unit is realized, the output current is not influenced due to inconsistent photovoltaic power among the submodules, the output current is balanced and symmetrical, and the system can well solve the problem of power mismatch.
The preferred embodiments of the present invention have been described in detail, however, the present invention is not limited to the specific details of the above embodiments, and various simple modifications may be made to the technical solution of the present invention within the technical idea of the present invention, and these simple modifications are within the protective scope of the present invention.
It should be noted that the various technical features described in the above embodiments can be combined in any suitable manner without contradiction, and the invention is not described in any way for the possible combinations in order to avoid unnecessary repetition.
In addition, any combination of the various embodiments of the present invention is also possible, and the same should be considered as the disclosure of the present invention as long as it does not depart from the spirit of the present invention.
Claims (4)
1. The utility model provides a novel modularization photovoltaic grid-connected system based on three-port power passageway which characterized in that: the system comprises a three-port DC/DC converter with a front-stage input port connected with a photovoltaic array, a half-bridge cascade modular multilevel converter with an input end connected with two output ends of the multi-port isolated DC/DC converter, and a rear-stage output end respectively used for local alternating current grid connection and remote direct current transmission;
the three-port isolation type DC/DC converter comprises a multi-winding high-frequency isolation transformer and two output rectifier bridges, wherein an input port is connected with a photovoltaic array to jointly form a photovoltaic sub-module;
two rear-stage ports of the photovoltaic sub-modules are connected with one sub-module of an upper bridge arm of a certain phase unit of the rear-stage modular multilevel converter one by one, and the other sub-module of a lower bridge arm is connected with one sub-module of the upper bridge arm of the certain phase unit of the rear-stage modular multilevel converter; the positions of the two connected half-bridge submodules in respective bridge arms are not specific, and the insulation design optimization of the high-frequency transformer of each photovoltaic submodule can be comprehensively considered in specific selection;
the modularized multi-level circulator is a three-phase half-bridge cascade topology, each phase unit comprises an upper bridge arm and a lower bridge arm, each bridge arm is connected with a filter inductor in series and connected to one phase of an alternating current power grid, and each submodule consists of a half-bridge circuit and a power decoupling capacitor.
2. The novel modular photovoltaic grid-connected system based on three-port power channel of claim 1, characterized in that: the three-port DC/DC photovoltaic converter part of each photovoltaic sub-module adopts a plurality of parallel connection modes to form a photovoltaic sub-module accessed by a plurality of photovoltaic arrays, so that the increase of the photovoltaic power grade is realized.
3. The novel modular photovoltaic grid-connected system based on three-port power channel of claim 1, characterized in that: the three-port isolated DC/DC converter adopts a three-active-bridge (TAB) converter, an isolation transformer is a three-winding high-frequency transformer, and each port is connected with a full-bridge circuit to realize phase-shift modulation; the upper bridge arm and the lower bridge arm of each phase unit share the same photovoltaic array, and the photovoltaic power of the upper bridge arm and the lower bridge arm is automatically balanced; the power among the phase units is automatically generated through a direct current link or actively injected into direct current circulation to realize balance, and the photovoltaic power mismatch of each bridge arm of the system is eliminated.
4. The control method of the novel modular photovoltaic grid-connected system based on the three-port power channel according to claim 1, characterized in that: comprises that
Power decoupling control: photovoltaic access is realized through the three-port isolated DC/DC converter, and power channels are provided for upper and lower bridge arms of the rear-stage modular multilevel converter at the same time, so that the cancellation of capacitance-voltage ripple differential mode components of sub-modules of the upper and lower bridge arms is realized; cancellation of voltage ripple common-mode components of the sub-module capacitors is achieved through circulation control of a rear-stage modular multilevel converter, and therefore complete cancellation of low-frequency ripples of the power decoupling capacitors is achieved;
eliminating the photovoltaic power mismatch among modules in a bridge arm: by comparing the sub-module power decoupling capacitor voltage with the bridge arm average value, an adjusting coefficient with an initial value of 1 is generated through a PI controller, and an alternating current component reference value and a direct current component reference value of the output voltage of the half-bridge circuit need to be multiplied by the coefficient at the same time and added to serve as the output voltage reference after the half-bridge circuit is adjusted, so that the output power of the sub-module is adjusted, and the failure of common-mode component ripple cancellation is avoided.
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