CN106026163A - MMC-based low-voltage ride through control method and system of photovoltaic grid-connected inverter - Google Patents

Abstract

The invention provides an MMC-based low-voltage ride through control method and system of a photovoltaic grid-connected inverter. On the basis of traditional inner current loop and outer voltage loop double closed-loop vector control, when a grid-connected point voltage drop is detected, a reactive current instruction value as shown in the specification is redistributed by judging the voltage drop degree to achieve reactive current injection; the capacitor voltage fluctuation range of an MMC sub-module is adjusted by optimizing bridge arm current; a new active current instruction value is introduced to limit active current; and the active current is limited, so that low-voltage ride through is achieved and safe photovoltaic grid-connected operation is ensured. Through the system and the method, capacitor voltage fluctuation is suppressed by optimizing bridge arm loop current; output reactive power is increased; and the capacitor voltage fluctuation range of the sub-module is optimized by controlling reference voltage of the sub-module, so that the problem that the capacitor voltage fluctuation of the MMC sub-module is out of the limit during reactive injection is solved; and low-voltage ride through is smoothly achieved.

Description

A kind of low voltage traversing control method based on MMC photovoltaic combining inverter and system

Technical field

The present invention relates to field of power electronics, particularly relate to the low voltage crossing of a kind of photovoltaic grid-connected inversion based on MMC Control field.

Background technology

With while mankind's socio-economic development, the energy is the most increasingly asked by necessary for human.But, it is on the one hand continuous The energy demand quickly increased, on the other hand along with the continuous exploitation of limited fossil energy, reserves constantly reduce and the most withered Exhaust.The energy demand quickly increased dependence and the environment being on the rise with limited fossil resource, to conventional fossil resource is asked Topic, it would be highly desirable to solve.For realizing Sustainable Socioeconomic Development, the exploitation of mankind's new forms of energy renewable to solar energy, wind energy etc. and Utilization becomes inevitable choice.Solar energy is made to be a kind of clean reproducible energy, is paid attention to by various countries.

Along with the quick increase of photovoltaic generation proportion, photovoltaic electric station grid connection also can bring safety and stability to Operation of Electric Systems Problem, particularly in the case of system grid connection point breaks down and causes Voltage Drop, bring low electricity to photovoltaic electric station grid connection Pressure crossing problem.This low voltage crossing problem faced now has had become as the difficult point in new-energy grid-connected technology and has ground Study carefully focus.

At present, existing domestic and foreign literature is the low voltage crossing technology to photovoltaic grid-connected inversion based on tradition VSC mostly Studying, the patent of Publication No. CN104734191 discloses a kind of based on reactive current injection grid-connected inverse Become device low-voltage ride-through method, be also that the low-voltage ride-through method to photovoltaic DC-to-AC converter based on traditional VSC is studied. MMC (modular multilevel inverter) has more advantages relative to traditional VSC (voltage source inverter), but because of MMC (mould Massing multilevel converter) submodule capacitor voltage fluctuation affected by reactive power size.One is arrived when MMC sends reactive power During definite value, submodule capacitor voltage fluctuation can exceed that setting, thus limit MMC and send reactive power size.Therefore, exist When MMC photovoltaic inverting system uses the low voltage traversing control method that reactive power is injected, need first to overcome MMC submodule electricity Hold voltage pulsation, the problem preventing device failure, send reactive power ability improving MMC inverter, thus realize stable, peace Full low voltage crossing.

Summary of the invention

For solving the problem of MMC submodule capacitor voltage fluctuation, the present invention proposes the low voltage crossing control of a kind of improvement Method processed and the system of correspondence, to realize photovoltaic inversion low voltage crossing based on MMC, it is ensured that photovoltaic safety grid-connection runs.Tool Body is as follows:

The low voltage traversing control method of a kind of photovoltaic combining inverter based on MMC, in current inner loop outer voltage On two close cycles vector controlled basis, when detecting that grid-connected point voltage falls, by judging that Voltage Drop degree is divided again Join referenced reactive current valueRealize reactive current to inject；And adjust MMC submodule capacitor voltage by optimizing bridge arm current Fluctuation range；It is re-introduced into new watt current command value to limit watt current；Limit watt current thus realize low-voltage and wear More, it is ensured that photovoltaic safety grid-connection runs.

Further, the method is by adding idle outer shroud, and computing module 1 and control logic 1 three part realize again Distribution reactive current value command valueWherein, idle outer shroud is for when photovoltaic inverting system is properly functioning；Control logic 1 to use In judging whether electrical network falls, judge that system is in normal operating condition and is in fault by Voltage Drop degree In the case of subnormal voltage operation state, and switch the corresponding method of operation；Computing module 1 is for according to grid voltage sags journey Degree, calculates again and distributes new referenced reactive current valueThe corresponding method of operation is, if photovoltaic grid-connected inverting system is normal During operation, take the idle command value of idle outer shroudAs output, run with unity power factor；If grid-connected point voltage falls When falling, locking is idle outer shroud, computing module 1 calculates reactive current value command valueInverter is made to send out by injecting reactive current Go out reactive power, it is achieved dynamic reactive enabling capabilities, thus realize the low voltage crossing of photovoltaic parallel in system.

The perunit value U of grid-connected point voltage is judged by controlling logic 1_TResiding scope, to referenced reactive current valueCarrying out redistributing, distribution method is as follows

$\left\{\begin{array}{c}{i}_q^{ref}=1.5\&times;(0.9-U_T)I_N,(0.2\&le;U_T\&le;0.9)\\ i_q^{ref}=1.05\&times;I_N,(U_T<0.2)\\ i_q^{ref}=0,(U_T>0.9)\end{array}\right.$

In formula, U_TRepresent the perunit value of grid-connected point voltage, I_NRepresent the rated current of inverter.

Keeping that through-put power is constant, submodule capacitance and DC voltage constant in the case of, MMC submodule electric capacity Voltage pulsation is mainly by system power factorImpact, system power factor and inverter send reactive power and are inversely proportional to.? During photovoltaic inversion low voltage crossing, the power factor of system can be caused to reduce after injecting reactive power by reactive current, MMC Submodule capacitor voltage undulate quantity is very big, exceedes the maximum of regulation fluctuation, causes damaging device.

Further, two frequency multiplication capacitance voltages are reduced or eliminated by optimization brachium pontis circulation to send out to increase MMC inverter Go out reactive power ability, and ensure MMC submodule capacitor voltage undulate quantity within allowed band, concrete:

In electrical network, there are A, B, C three-phase, using A phase as representing, in like manner can obtain B, C phase；Former MMC bridge arm current I_caSize is:

In formula, I_dcFor DC side electric current, I_2fFor secondary circulation amplitude.

By bridge arm current I_caBy obtaining new brachium pontis circulation after formula optimizationOptimizing formula is:

In formula, I_dcFor DC side electric current, K is voltage modulated coefficient, I_mFor output Fundamental-frequency Current amplitude；And after being optimized MMC submodule capacitor voltage:

In formula,For submodule capacitor voltage in MMC inverter,For lower submodule capacitor voltage, U_dcFor DC side Voltage, P_dcFor DC side power, K is voltage modulated coefficient.

Additionally, in grid-connected inverting system based on MMC, submodule capacitor voltage undulate quantity has a below equation:

${\mathrm{\&Delta;u}}_{smc\_pa}\&ap;\frac{\&Integral;P_{pa}dt}{{\mathrm{NCU}}_{cref}}$

In formula, electric capacity is C；Upper brachium pontis voltage fluctuation of capacitor amount is Δ u_{smc_pa}；Upper brachium pontis active power is P_pa；

Submodule capacitor voltage is by electric capacity reference voltage U_crefWith undulate quantity Δ u_smcComposition, by introducing submodule electric capacity Reference voltage U_crefRegulation coefficient λ adjusts submodule electric capacity reference voltage U_cref, control MMC submodule capacitor voltage further Fluctuation range, the relational expression after introducing is as follows:

λU_cref+Δu_smc≤u_max

In formula, λ is reference voltage regulation coefficient, and span is 0.9～1.1, u_maxFor the capacitance voltage upper limit.Therefore, front State on summary of the invention, it is also possible to control to adjust MMC submodule by controlling MMC submodule capacitor voltage fluctuation range further Voltage fluctuation of capacitor scope.

Further, the present invention, also by adding computing module 2 and controlling logic 2, limits watt current；Wherein, mould is calculated Block 2 is used for calculating the newest watt current command valueControl logic 2 and be used for the newest watt current command valueWith With outer voltage watt current command valueSize, and switch the corresponding method of operation, i.e. computing module 2 calculates the newest Watt current command valueAfter, judge if controlling logic 2It is less thanTake the watt current instruction of outer voltage And lockingWatt current instruction according toOutput；If controlling logic 2 to judgeIt is more than or equal toTime, by logic control Molding block judge after switch the method for operation, watt current instruction according toOutput, blocking voltage outer shroud, thus realize photovoltaic also The low voltage crossing of net system.

When the grid-connected voltage of photovoltaic parallel in system falls, it is stipulated that and site electric current i not can exceed that rated current I_N 1.5 times, calculate new watt current value command valueFormula is as follows:

$i_d^{ref}=\sqrt{{\left(1.5I_N\right)}^2-{\left(i_q^{ref}\right)}^2}$

Wherein, i_dRepresent watt current, i_qRepresent reactive current, I_NRepresent rated current.

On this basis, the invention allows for photovoltaic combining inverter low voltage ride through control system based on MMC, should System includes photovoltaic array PV, Boost chopper circuit, MMC inverter；Wherein, photovoltaic array PV uses disturbance observational method, it is achieved Maximum power output；Boost chopper circuit is for boosting photovoltaic output voltage；MMC inverter is for by DC inversion Become exchange；Native system uses above-mentioned photovoltaic combining inverter low voltage traversing control method, it is achieved low voltage crossing controls.

Beneficial effect:

1) by optimizing brachium pontis loop current suppression voltage fluctuation of capacitor, increase output reactive power, efficiently solve idle The low-voltage ride-through method injected is applied to the problem that the fluctuation of MMC submodule capacitor voltage is out-of-limit；

2) by controlling submodule reference voltage, optimize submodule capacitor voltage fluctuation range, make MMC submodule electric capacity electricity Pressure fluctuation range is controlled further, thus prevents the fluctuation of MMC submodule capacitor voltage out-of-limit further；

3) optimize brachium pontis circulation and two kinds of methods of submodule reference voltage or a combination thereof method reduces submodule electric capacity Voltage pulsation, method the most effectively, easily realize, and need not increase extra outer power equipment, low cost；

4) by adding new watt current command valueRedistribute watt current command value to limit idle injection Time watt current, it is ensured that system run safety；

5) on the basis of the two close cycles vector control system of traditional current inner loop outer voltage, noted by reactive current Enter, limit watt current and the method adjusting MMC submodule capacitor voltage fluctuation range, it is not necessary to increase extra outer power equipment Device, can be achieved with the low voltage crossing of photovoltaic inverting system based on MMC, it is ensured that photovoltaic safety grid-connection runs.

Accompanying drawing explanation

The low voltage ride through system block diagram of Fig. 1 photovoltaic combining inverter based on MMC

Low-voltage control module block diagram in the low voltage ride through system of Fig. 2 photovoltaic combining inverter based on MMC

The flow chart of the low-voltage ride-through method of Fig. 3 photovoltaic combining inverter based on MMC

Fig. 4-1 grid-connected point voltage analogous diagram based on MMC

Fig. 4-2 uses the output current simulations figure of the MMC inverter of the low voltage traversing control method of the present invention

Fig. 4-3 reactive current actual value and command value analogous diagram

The reactive power analogous diagram of Fig. 4-4 photovoltaic combining inverter based on MMC output

Fig. 4-5 is not optimised submodule capacitor voltage analogous diagram

Submodule capacitor voltage fluctuation analogous diagram after Fig. 4-6 optimization

Embodiment

This low voltage traversing control method providing a kind of photovoltaic combining inverter and system, in traditional current inner loop On the two close cycles vector controlled basis of outer voltage, when detecting that grid-connected point voltage falls, injected by reactive current, control The mode that system module capacitance scope range of the fluctuation of voltage and restriction watt current combine, it is achieved low voltage crossing, it is ensured that photovoltaic safety It is incorporated into the power networks.Detailed description of the invention is as follows:

When detecting that grid-connected point voltage falls, by redistributing referenced reactive current valueRealize reactive current Inject；By adding new watt current command valueRedistribute watt current value command value to limit watt current；And By optimizing bridge arm current and/or controlling submodule reference voltage method adjustment MMC submodule capacitor voltage fluctuation range；Thus Realize low voltage crossing, it is ensured that photovoltaic safety grid-connection runs.

On the basis of original current inner loop outer voltage, by adding idle outer shroud, computing module 1 and controlling logic 1 Three parts realize redistributing reactive current value command valueWherein, idle outer shroud is for normally transporting at photovoltaic inverting system During row；Control logic 1 to be used for judging whether electrical network falls, judge that system is in by Voltage Drop degree properly functioning State is in the subnormal voltage operation state under failure condition, and switches the corresponding method of operation；Computing module 1 is for basis Grid voltage sags degree, calculates again and distributes new referenced reactive current value

When electrical network is properly functioning, take the idle command value of idle outer shroudAs output, now reactive power command value Q^refIt is set to 0, to realize the operation of MMC inverter unity power factor；When electrical network falls, redistribute new idle Current instruction valueRealize reactive current to inject, be used for making MMC inverter send corresponding reactive power；Control logic 1 part It is mainly used to judge whether electrical network falls, and switches to the corresponding method of operation according to judgement.

Such as Fig. 2, carrying out practically mode is: when the photovoltaic grid-connected inverting system of MMC is properly functioning, pass through Logic control module Switch 1 after judgement to be connected with switch 2, take the idle command value of idle outer shroudAs output, existing MMC inverter unit power because of Number runs.When grid-connected point voltage falls, switching the method for operation by logic control 1 after being judged, switch 1 is connected with switch 3, Locking is idle outer shroud, and according to new referenced reactive current valueMMC is made to send reactive power by injecting reactive current, real Existing dynamic reactive enabling capabilities, thus realize the low voltage crossing of photovoltaic parallel in system.

In view of photo-voltaic power generation station accesses power system technology provision discussion, judge grid-connected electricity by controlling logic 1 The perunit value U of pressure_TResiding scope, to referenced reactive current valueCarrying out redistributing, distribution method is as follows:

$\left\{\begin{array}{c}{i}_q^{ref}=1.5\&times;(0.9-U_T)I_N,(0.2\&le;U_T\&le;0.9)\\ i_q^{ref}=1.05\&times;I_N,(U_T<0.2)\\ i_q^{ref}=0,(U_T>0.9)\end{array}\right.$

In formula, U_TRepresent is the perunit value of grid-connected point voltage, I_NRepresent is the rated current of MMC inverter.

Relative to traditional VSC, there is more advantages, but because the fluctuation of MMC submodule capacitor voltage is the biggest by reactive power Little impact, when MMC sends reactive power arrival certain value, submodule capacitor voltage fluctuation may exceed setting, thus Limit MMC and send reactive power size.Therefore, at Voltage Drop, when MMC sends reactive power, submodule electric capacity electricity should be reduced Pressure can produce fluctuation, makes MMC send more reactive power.To this end, the invention provides two kinds of methods, i.e. optimize bridge arm current and The method controlling submodule reference voltage, or adjust MMC submodule capacitor voltage fluctuation range by a combination thereof, to ensure nothing Normally sending of merit power, optimizes the most further, to guarantee to export OPTIMAL REACTIVE POWER current instruction valueThe most such as Under:

Embodiment 1, the method optimizing bridge arm current:

In electrical network, there are A, B, C three-phase, using A phase as representing, in like manner can get respective formula and the respective value of B, C phase； MMC submodule capacitor voltage size is:

In formula, U_cpaFor submodule capacitor voltage, U in A phase before optimizing_cnaFor the lower submodule capacitor voltage of A phase before optimizing, U_dc For DC voltage, P_dcFor DC side power, K is voltage modulated coefficient,For power factor.

The situation constant from above formula analysis, submodule capacitance constant in holding through-put power and DC voltage Under, submodule capacitor voltage fluctuation is main by power factorImpact, power factor the least expression MMC sends reactive power more Greatly, then fundamental frequency and the two frequency multiplication voltage fluctuation of capacitors of submodule capacitor voltage are the biggest.During photovoltaic inversion low voltage crossing, MMC inverter needs to send reactive power, the power factor of system can be caused to reduce, thus submodule capacitor voltage undulate quantity is very Greatly, the maximum that regulation fluctuates can be can exceed that, cause damaging device, so must during the low voltage crossing of photovoltaic parallel in system Submodule capacitor voltage must be kept to fluctuate within prescribed limit, meet the requirement less than submodule electric capacity reference voltage 10%.

A phase former MMC bridge arm current I_caSize is:

In formula, I_dcFor DC side electric current, I_2fFor secondary circulation amplitude.

By brachium pontis circulation I_caRound definite value and obtain new brachium pontis circulationFor:

The corresponding submodule capacitor voltage after be derived by optimization:

From above formula analysis, by the brachium pontis circulation after optimizingMake in submodule capacitor voltage without two frequency multiplication ripples Move, and fundamental frequency fluctuation reduces the most accordingly, although create frequency tripling fluctuation, but numerical value is at least below original capacitance voltage two The 1/3 of double-frequency fluctuation amount.Therefore, can effectively reduce submodule capacitor voltage fluctuation after brachium pontis circulation optimization.

Embodiment 2, the method controlling submodule reference voltage:

When the submodule quantity of MMC arrives some, there is a below equation for submodule capacitor voltage undulate quantity:

${\mathrm{\&Delta;u}}_{smc\_pa}\&ap;\frac{\&Integral;P_{pa}dt}{{\mathrm{NCU}}_{cref}}$

In formula, N represents brachium pontis submodule number, U_crefRepresenting submodule reference voltage, C is electric capacity, Δ u_{smc_pa}In expression Brachium pontis voltage fluctuation of capacitor amount, P_paBrachium pontis active power in expression.

Knowable to above formula analysis, in upper brachium pontis submodule number, electric capacity and power one timing, submodule reference voltage is U_cref Change can shadow play voltage fluctuation of capacitor, i.e. submodule reference voltage U_crefBecome big, then voltage fluctuation of capacitor Δ u_{smc_pa}Reduce； Submodule reference voltage U_crefDiminish, then voltage fluctuation of capacitor Δ u_{smc_pa}Become big.Submodule capacitor voltage is by electric capacity reference voltage U_crefWith undulate quantity Δ u_smcComposition, i.e. U_cref+Δu_smc.Therefore, it is possible to by controlling submodule electric capacity reference voltage U_crefControl Undulate quantity Δ u processed_smc, to ensure that submodule capacitor voltage is in the reasonable scope.

To this end, the present invention controls capacitance voltage reference value size by introducing reference voltage regulation coefficient λ, obtain with public affairs Formula:

λU_cref+Δu_smc≤u_max

In formula, λ is reference voltage regulation coefficient；u_maxFor the capacitance voltage upper limit；By analysis and simulation, λ value is at 0.9- Between 1.1.

Embodiment 3, the combination of two kinds of methods:

That is, optimization bridge arm current and control submodule reference voltage method are used in combination with, to control MMC submodule Voltage fluctuation of capacitor scope, to ensure normally sending of reactive power, realizes low voltage crossing smoothly.

Further, when the grid-connected point voltage using MMC photovoltaic grid-connected inverting system does not fall, and site electricity The available following formula of stream is expressed:

$i=\sqrt{i_d^2+i_q^2}$

In formula, i_dRepresent watt current, i_qRepresent reactive current.

When grid-connected point voltage falls, not can exceed that rated current I according to usage of trade regulation site electric current_N's 1.5 again.Further, being limited and be shaped with merit current value, the present invention, by adding computing module 2, is used for calculating new watt current Command valueComputing formula is as follows:

$i_d^{ref}=\sqrt{{\left(1.5I_N\right)}^2-{\left(i_q^{ref}\right)}^2}$

In formula, I_NRepresent rated current,For referenced reactive current value.

Equally, on the basis of original current inner loop outer voltage, then add new computing module 2 and control logic 2 liang Individual part.Limit watt current command valueAlso site electric current can be limited in 1.5 times of rated current I_NWithin, to ensure light Overhead utility is safe and reliable grid-connected；Control logic 2 to be mainly used to compare watt current command valueRefer to outer voltage watt current Make valueSize, with judge and site electric current whether more than 1.5I_N, thus switch to the corresponding method of operation.

Such as Fig. 2, carrying out practically mode is: gather watt current command valueAnd according to referenced reactive current valueMeter New watt current command valueIf controlling the watt current command value that logic 2 is the newestMeritorious electricity with outer voltage Stream command valueSize, and switch the corresponding method of operation；That is, watt current value command value is calculatedAfter, if new has Merit current instruction valueMore than the watt current command value connecting outer voltageSwitch 5 is connected with switch 4, power taking pressure The watt current instruction of outer shroudAnd lockingWatt current instruction according toOutput；If controlling new the having of logical judgment Merit current instruction valueLess than or equal to the watt current command value connecting outer voltageTime, switch 5 is connected with switch 6, And new watt current command valueAs output, blocking voltage outer shroud, it is limited in 1.5I with realization site electric current_NWithin, Thus realize the low voltage ride-through function of photovoltaic parallel in system.

In sum, control brachium pontis loop current suppression voltage fluctuation of capacitor, increase output reactive power, change submodule electric capacity Reference voltage is mainly for optimizing submodule capacitor voltage fluctuation range, and both approaches united application is more for MMC output Reactive power there is preferable effect.

It addition, present invention also offers the photovoltaic grid-connected inversion low voltage crossing of based on MMC corresponding for said method System, as it is shown in figure 1, this system includes photovoltaic array PV, Boost chopper circuit, MMC inverter；Wherein, photovoltaic array PV adopts Use disturbance observational method, it is achieved maximum power output；Boost chopper circuit is for boosting photovoltaic output voltage；The MMC change of current Device is for becoming exchange by DC inversion；Native system uses above-mentioned photovoltaic combining inverter low voltage traversing control method, it is achieved low Voltage ride-through controls.

According to the method described in above-described embodiment 3, the low voltage crossing built in Matlab/Simulink simulation software Control system, parameter such as table 1 below, emulation use MMC photovoltaic parallel in system such as Fig. 1,

Photovoltaic array parameter name	Numerical value	MMC parameter name	Numerical value
				Intensity of illumination	25℃	Dc bus rated voltage	800V
Temperature	1000W/m3	AC network rated voltage	380V
				Open-circuit voltage	44.5V	MMC brachium pontis submodule number	4
Short circuit current	8.2A	The voltage rating of submodule	200V
				Maximum power point voltage	35V	Carrier frequency	3kHz
Maximum power point electric current	7.86A	Submodule capacitance	1mF
				Photovoltaic array series connection number	16	Brachium pontis inductance value	5mH
Photovoltaic array parallel connection number	114	Connect reactance value	2mH
				Rated capacity	50kW	Equivalent resistance	0.01Ω

Table 1 emulates major parameter

There is three phase short circuit fault at 1.3s in networked system site, 1.925s failure vanishes, when the fault of system maintains Between be 0.625s, the fault resstance size of setting is 0.0013 Ω, and earth resistance size is 0.001 Ω, and three-phase voltage is down to volume Determining about the 20% of voltage, 1.925s three-phase voltage starts to recover.Native system rated current is 80A.

By Fig. 4-1 and Fig. 4-2, grid-connected point voltage, electric current recover normal, and photovoltaic plant achieves low voltage crossing.And by Fig. 4-2 understands, and site electric current is successfully limited in 1.5 times of rated current, i.e. 120A, it is ensured that photovoltaic system safe operation.Figure 4-3 is reactive current actual value and command value, when grid voltage sags, calculates idle command value, wherein dotted line in Fig. 4-3 Representing referenced reactive current value, solid line represents reactive current actual value, and as can be seen from the figure reactive current actual value can be accurate Really trace command value, reactive current value is become 80A from the 0A of properly functioning period, successfully realizes the injection of reactive current, and by Fig. 4-4 understands, and MMC inverter sends reactive power and is about 8.5kvar.Analyzed from Fig. 4-5, send reactive power at MMC Period, it is about 22.5% without optimizing submodule capacitor voltage fluctuation range, is far longer than the setting of MMC inverter 10%, by Fig. 4-6 analyzes and understands, and uses after optimizing after submodule capacitor voltage fluctuation, and submodule fluctuation range is about 7.5%, meets MMC The properly functioning requirement of inverter.

Above example is only the technological thought that the present invention is described, it is impossible to limit protection scope of the present invention with this, every The technological thought proposed according to the present invention, any change done on the basis of technical scheme, each fall within scope Within.

Claims (9)

1. a low voltage traversing control method based on MMC photovoltaic combining inverter, it is characterised in that: at current inner loop voltage On the two close cycles vector controlled basis of outer shroud, when detecting that grid-connected point voltage falls, by judging that Voltage Drop degree is come Redistribute referenced reactive current valueTo realize reactive current injection, and adjust MMC submodule by optimizing bridge arm current Voltage fluctuation of capacitor scope, and it is re-introduced into new watt current command value to limit watt current.

Low voltage traversing control method the most according to claim 1, it is characterised in that: by adding idle outer shroud, calculating Module (1) and the logic (1) three that controls partly realize redistributing reactive current value command valueWherein, idle outer shroud is used for Output when photovoltaic inverting system is properly functioning is arranged；Control logic (1) to be used for judging whether electrical network falls, pass through voltage Fall degree and judge that system is in the subnormal voltage operation state that normal operating condition is under failure condition, and switch phase The method of operation answered；Computing module (1) for according to grid voltage sags degree, calculates again and distributes new reactive current and refer to Make valueThe corresponding method of operation is, if photovoltaic grid-connected inverting system is properly functioning, to take the idle command value of idle outer shroud As output, run with unity power factor；If grid-connected point voltage falls, locking is idle outer shroud, computing module (1) is counted Calculate reactive current value command valueInverter is made to send reactive power by injecting reactive current, it is achieved dynamic reactive supports.

Low voltage traversing control method the most according to claim 1, it is characterised in that: by judging grid-connected point voltage Perunit value U_TResiding scope, to referenced reactive current valueRedistributing, distribution method is as follows:

$\left\{\begin{array}{c}{i}_q^{ref}=1.5\&times;(0.9-U_T)I_N,(0.2\&le;U_T\&le;0.9)\\ i_q^{ref}=1.05\&times;I_N,(U_T<0.2)\\ i_q^{ref}=0,(U_T>0.9)\end{array}\right.$

In formula, U_TRepresent the perunit value of grid-connected point voltage, I_NRepresent the rated current of inverter.

Low voltage traversing control method the most according to claim 1, it is characterised in that: keeping, through-put power is constant, sub Module capacitance value and DC voltage constant in the case of, the fluctuation of MMC submodule capacitor voltage is main by system power factorImpact, system power factor and inverter send reactive power and are inversely proportional to, and during photovoltaic inversion low voltage crossing, inject Reactive power, can cause the power factor of system to reduce, and MMC submodule capacitor voltage undulate quantity is very big, exceedes regulation fluctuation Maximum, causes damaging device；Two frequency multiplication capacitance voltages are reduced or eliminated to increase MMC inverter by optimizing brachium pontis circulation Send reactive power ability, and ensure MMC submodule capacitor voltage undulate quantity within allowed band, concrete:

In electrical network, there are A, B, C three-phase, using A phase as representing, in like manner can obtain B, C phase；Former MMC bridge arm current I_caSize is:

In formula, I_dcFor DC side electric current, I_2fFor secondary circulation amplitude,

By bridge arm current I_caBy obtaining new brachium pontis circulation after formula optimizationOptimizing formula is:

In formula, I_dcFor DC side electric current, K is voltage modulated coefficient, I_mFor output Fundamental-frequency Current amplitude；

And the MMC submodule capacitor voltage after being optimized:

In formula,For submodule capacitor voltage in MMC inverter,For lower submodule capacitor voltage, U_dcFor DC voltage, P_dcFor DC side power, K is voltage modulated coefficient.

Low voltage traversing control method the most according to claim 1, it is characterised in that: submodule capacitor voltage undulate quantity has Below equation:

${\mathrm{\&Delta;u}}_{smc\_pa}\&ap;\frac{\&Integral;P_{pa}dt}{{\mathrm{NCU}}_{cref}}$

In formula, electric capacity is C；Upper brachium pontis voltage fluctuation of capacitor amount is Δ u_{smc_pa}；Upper brachium pontis active power is P_pa；Submodule electric capacity Voltage is by electric capacity reference voltage U_crefWith undulate quantity Δ u_smcComposition, by introducing submodule electric capacity reference voltage U_crefRegulation coefficient λ adjusts submodule electric capacity reference voltage U_cref, control MMC submodule capacitor voltage fluctuation range, the pass after introducing further It is that formula is as follows:

λU_cref+Δu_smc≤u_max

In formula, λ is reference voltage regulation coefficient, u_maxFor the capacitance voltage upper limit.

Low voltage traversing control method the most according to claim 5, it is characterised in that: reference voltage regulation coefficient λ takes Value scope is 0.9～1.1.

Low voltage traversing control method the most according to claim 1, it is characterised in that: by add computing module (2) and Control logic (2), limit watt current；Wherein, computing module (2) is used for calculating new watt current command valueControl is patrolled Collect (2) and be used for the newest watt current command valueWith with outer voltage watt current command valueSize, and switch The corresponding method of operation；That is, watt current value command value is calculatedAfter, judge if controlling logic (2)It is less thanLocking meter Calculating module (2), watt current command value is according to the watt current command value of outer voltageOutput；If controlling logic (2) to judgeIt is more than or equal toTime, watt current instructs according to new watt current command valueOutput, blocking voltage outer shroud.

Low voltage traversing control method the most according to claim 7, it is characterised in that: when the grid-connected electricity of photovoltaic parallel in system Hair Fixer is raw when falling, it is stipulated that and site electric current i not can exceed that rated current I_N1.5 times, new watt current value command value Computing formula is as follows:

$i_d^{ref}=\sqrt{{\left(1.5I_N\right)}^2-{\left(i_q^{ref}\right)}^2}$

In formula, i_dRepresent watt current, i_qRepresent reactive current, I_NRepresent rated current.

9. a photovoltaic combining inverter low voltage ride through control system based on MMC, it is characterised in that: this system includes photovoltaic Array PV, Boost chopper circuit, MMC inverter；Wherein, photovoltaic array PV uses disturbance observational method, it is achieved peak power is defeated Go out；Boost chopper circuit is for boosting photovoltaic output voltage；MMC inverter is for becoming exchange by DC inversion；This is System uses any one photovoltaic combining inverter low voltage traversing control method described in claim 1 to 8, it is achieved low-voltage is worn More control.