A kind of Modularized multi-level converter sub-module topological structure
Technical field
The present invention relates to electric power system power transmission and distribution technical field, relate to the novel submodule topology of a kind of modularization multi-level converter.
Background technology
Technology of HVDC based Voltage Source Converter is the grid-connected bottleneck solving the regenerative resources such as Large Scale Wind Farm Integration, for the capacity-increasing transformation of city high-voltage fence, Power System Interconnection and island with power provide new tool and technical scheme, there is stronger technical advantage, it is the strategic choice changing bulk power grid development pattern, listing " national energy science and technology " 12 " planning " high-performance power transmission and transformation key equipment in, is the major technologies and equipment needing focus technology to tackle key problems.
Because flexible DC power transmission grows up from the basis of customary DC transmission of electricity, therefore except advantage that customary DC technology of transmission of electricity has, flexible DC power transmission also has densification, modularized design than customary DC transmission of electricity, be easy to mobile, install, debug and safeguard, be easy to expansion and realize the advantages such as multi-terminal HVDC transmission.
MMC-HVDC (HVDC Transmission Technology based on modularization multi-level converter) is a kind of new structure in VSC-HVDC (voltage source converter based HVDC).Compare with cascaded H-bridges current transformer, MMC remains the design feature of high modularization, has again a high voltage dc bus simultaneously, can realize the four quadrant running of output voltage, output current.With traditional two, three-level current transformer compares, and MMC does not exist the problem of all the pressing of switching tube series and parallel, current-sharing.Each sub modular structure of MMC is relatively simple, controls easily, can infinite expanding, is specially adapted to HVDC field.Because two level of VSC or three level are brought up to tens level level even up to a hundred by MMC, thus on the basis maintaining withstand voltage level, increase system voltage grade, greatly reduce the switching frequency of switching device thus reduce switching loss, the output voltage waveforms more sinusoidal wave thus underproduction harmonic content further of convergence.Meanwhile, MMC adopts on all four modular technology, and its modular construction makes its extensibility strong, is convenient to realize Redundant Control, all presses at research and development, manufacture, dynamic and static state and reduces there is considerable advantage in circulation.
In engineering construction, Nanhui wind energy turbine set flexible transmission demonstration project, as China's first MMC engineering, completes the acceptance of work in July, 2011, becomes the HVDC power transmission engineering based on MMC that the first bar of China formally puts it into commercial operation.Simultaneously the Large Scale Wind Farm Integration flexible DC power transmission engineering of south net and state's net Zhoushan five terminal flexible direct current engineering are built.
Traditional MMC-HVDC engineering adopts direct current cables to transmit usually, and direct current cables safety and stability performance is better, but its price is higher, and long distance powedr transmission cost is higher.And in jumbo transmission, more adopt low price, the better overhead wire transmission of radiating effect.And in overhead wire transmission, traditional half-bridge structure inherently lacks DC Line Fault isolating power: when DC side breaks down, the antiparallel fly-wheel diode of full-controlled switch device easily forms the energy feeding loop that fault point AC system is directly communicated with, must be cut off by tripping AC circuit breaker, its shortcoming is mechanical response comparatively slow (needing 2 ~ 3 cycles at the soonest), affect the normal transmission of electric power, overcurrent and the overvoltage of converter valve device may be caused simultaneously.Often need to adopt the complementary measures such as increasing device nominal parameter, configuration high-speed by-pass switch, therefore this topology be not suitable for the overhead transmission line transmission of electricity that the temporary faults such as flashover easily occur, and need that laying involves great expense, the cable line of failure rate is low.
Therefore, need to design new sub modular structure, can effective isolated DC fault.
Summary of the invention
For described problem and the application requirement in high-power occasion above, the present invention proposes a kind of Modularized multi-level converter sub-module topological structure on the basis of original half-bridge structure, be made up of the half-bridge structure of connecting and current limliting module, new modified node method does not change the structure of original submodule, control strategy and capacitor voltage equalizing mode, improves part in series and can realize independent control.New modified node method improves the fault ride-through capacity of system, when DC side fault, and can effective isolated fault electric current.
Technical solution of the present invention: a kind of Modularized multi-level converter sub-module topological structure, is made up of half-bridge structure and current limliting block coupled in series; Described half-bridge structure comprises the first switch module, second switch module and first electric capacity of mutually connecting; The negative pole of described first switch module is connected with the positive pole of second switch module; The positive pole of described first electric capacity is connected with the positive pole of the first switch module, and the negative pole of the first electric capacity is connected with the negative pole of second switch module;
Described current limliting module is made up of the second electric capacity, diode and the 3rd switch module; The negative level of described diode is connected with the positive pole of the second electric capacity, the positive pole of diode is connected with the positive pole of the 3rd switch module, the negative pole of the 3rd switch module is connected with the negative level of the second electric capacity, and the second electric capacity of current limliting module is born level and is connected with the negative level of second switch module;
Node between the negative pole of the first switch module of described half-bridge structure and the positive pole of second switch module is the input of described topological structure, and the node between the positive pole of the 3rd switch module of current limliting module and diode cathode is the output of described topological structure.
Further, described first switch module, second switch module and the 3rd switch module are by an insulated gate bipolar transistor and a diode inverse parallel composition.
Further, under normal operation, the insulated gate bipolar transistor of the 3rd switch module in current limliting module applies pulse always, makes it be in conducting state always, thus makes the second electric capacity and diode be in short-circuit condition, on output without impact.
Based on a kind of modularization multi-level converter that Modularized multi-level converter sub-module topological structure realizes, comprise three facies units, each facies unit divides upper and lower bridge arm, each brachium pontis comprises the submodule topological structure of several series connection, and the submodule topological structure quantity of every phase upper and lower bridge arm series connection is identical; Upper and lower bridge arm is connected current limiting reactor respectively, is from top to bottom mutually often: the upper all submodules of brachium pontis, upper brachium pontis reactor, lower brachium pontis reactor, all submodules of lower brachium pontis; And the external three-phase alternating voltage in junction of every phase upper and lower bridge arm, the input of upper brachium pontis the top submodule topological structure is connected with direct-flow positive pole, and lower brachium pontis bottom submodule output is connected with direct current negative pole.
Further, in DC transmission system, when bipolar short trouble occurs DC side, first fault current detected, then close the triggering signal of all first switch modules and second switch module, the 3rd switch module signal simultaneously in cut-off current-limiting module, after electric current enters topological structure from the output of topological structure, flowed out from the input of topological structure by the diode of current limliting module and the second electric capacity, and no longer by the 3rd switch module and first electric capacity of current limliting module.This structure can cut down fault current by electric capacity, reaches and eliminates fault object.Half-bridge structure institute shunt capacitance does not flow through electric current, is protected, and making after fault can quick-recovery soon.
Further, when described fault is direct current permanent fault, detailed process is: the trigger impulse turning off insulated gate bipolar transistor in the first switch module, second switch module and the 3rd switch module, then AC circuit breaker is disconnected, overhaul, after fault restoration, carry out reclosing, then open the first switch module, second switch module and the 3rd switch module.
Further, when described fault is direct current temporary fault, detailed process is: the trigger impulse turning off insulated gate bipolar transistor in the first switch module, second switch module and the 3rd switch module, wait for that DC side fault current is after zero, again the insulated gate bipolar transistor of the first switch module, second switch module and the 3rd switch module is triggered, set up DC voltage, waiting system enters steady operation.
Technical solution of the present invention: the present invention is mainly for the bipolar short trouble of DC side, and under studying bipolar short trouble, fault current turns off mechanism, braking measure.
The invention has the advantages that: 1) in the transmitting procedure of high-voltage large-capacity, can effectively suppress DC side fault current, without the need to AC circuit breaker action; 2) this circuit can effectively protect atom module capacitance, can realize recovering fast after fault recovery; 3) compare with other topological structures, save components and parts.
Accompanying drawing explanation
Fig. 1 is that modular multilevel of the present invention improves submodule topology theory figure.
Fig. 2 is the three-phase modular multilevel current transformer schematic diagram containing novel sub modular structure of the present invention.
Fig. 3 is that under DC Line Fault of the present invention, electric current flows to schematic diagram from positive pole (1 end) inflow current.
Fig. 4 is that under DC Line Fault of the present invention, electric current flows to schematic diagram from negative pole (2 end) inflow current.
Fig. 5 is current direction schematic diagram under bipolar short circuit DC Line Fault of the present invention.
Embodiment
Below in conjunction with embodiment, the present invention is further described.
Fig. 1 is the topology of the modularization multi-level converter submodel of the improvement that the present invention proposes.Wherein 2 ends series connection one by di-cap connect again with the module of IGBT parallel connection.This structure is when normally working, and the IGBT of 2 end series connection is in conducting state always, and electric capacity is shorted, and former half-bridge sub modular structure controls according to original control method, without the need to changing original modulator approach, does not also affect submodule output voltage.When DC side breaks down, the IGBT of 2 end series connection turns off.
Fig. 2 is three-phase modular multilevel converter structure, each submodule modified model submodule composition as shown in Figure 1.
Fig. 3 breaks down when DC side, and electric current flows into from 1 end, and electric current flows through atom module capacitance C1, then flows out (not flowing through electric capacity in parallel) from 3 ends through D3.
Fig. 4 breaks down when DC side, and electric current flows into from 2 ends, and electric current flows through resistance capacitance in parallel, then flows out from 1 end through D2.This structure can cut down fault current by electric capacity, reaches and eliminates fault object.Half-bridge structure institute shunt capacitance does not flow through electric current, is protected, and making after fault can quick-recovery soon.
Be described with reference to the accompanying drawings, when running under normal condition, the IGBT (S3) of the 3rd switch in current limliting module is in normally open, and the di-cap in current limliting module does not have electric current to pass through.When bipolar short trouble occurs system, the 3rd switch S 3 in cut-off current-limiting module, fault current passes through from the upper branch road electric capacity of current limliting module, charges to current limliting module capacitance C2.
Fig. 5 is that the current direction figure when there is bipolar short trouble, figure are for A, B two-phase (often simplifying mutually with a submodule replacement).When fault time, electric current from electrical network one in opposite directions fault go out flowing, electric current, through a phase brachium pontis, then through fault point, then after an other phase brachium pontis, flows into another phase.As shown in Figure 5, electric current flows through path is D
b4-C
b2-D
b2-D
a4-C
a2-D
a2.Electric current is common crosses 2N submodule, and each submodule voltage is U
m, grid connection point is line voltage U
aB.
U
dc=NU
m
U
AB=2NU
m+2N×(U
D2+U
D4)
Can obtain:
Again because: m < 1
U
D2+U
D4<0
Wherein: U
dcit is DC voltage under normal operation; U
aand U
bthe phase voltage that three-phase alternating current side exports A phase and B phase respectively; M is modulation ratio; U
aBthe line voltage of AB phase; U
d2and U
d4diode D in submodule respectively
2and D
4both end voltage.
Known diode bears reverse voltage, and fault turns off, reliable turn-off fault current.
More than describe the preferred embodiment of the present invention in detail; but the present invention is not limited to the detail in above-mentioned execution mode, within the scope of technical conceive of the present invention; can carry out multiple equivalents to technical scheme of the present invention, these equivalents all belong to protection scope of the present invention.