CN103048547B - Parameter designing method for smoothing reactor used for flexible direct-current power transmission - Google Patents

Abstract

The invention discloses a parameter designing method for a smoothing reactor used for flexible direct-current power transmission. The parameter designing method for a smoothing reactor used for the flexible direct-current power transmission comprises the following steps of: (1) determining input parameters which comprise a bridge arm inductance value Larm, a line inductance value Lline, a submodule capacitance value C0 and the amount n of bridge arm submodules; (2) according to the inhibitory condition of the rate of ascent of fault current, determining the lower limit of the inductance value of the smoothing reactor; (3) according to the condition of the direct-current dynamic response speed, determining the upper limit of the inductance value of the smoothing reactor; and (4) according to the condition for generating resonance, determining a smoothing reacting inductance value. According to the parameter designing method for the smoothing reactor used for the flexible direct-current power transmission, which is disclosed by the invention, system resonance caused by a circuit parameter can be avoided.

Description

A kind of Parameters design of flexible DC power transmission smoothing reactor

Technical field

The invention belongs to flexible DC power transmission (VSC-HVDC) technical field, be specifically related to a kind of Parameters design of flexible DC power transmission smoothing reactor.

Background technology

Technology of HVDC based Voltage Source Converter is application performance flexibly, makes it interconnected at urban distribution network, and there is extremely wide application prospect in new-energy grid-connected and the passive load field such as to power.Smoothing reactor is one of visual plant in flexible direct current converter station, its parameter directly affect commutation system harmonic characteristic, control response speed and failure restraint ability.In existing Technology of HVDC based Voltage Source Converter pertinent literature, have no the relevant report of smoothing reactor Parameters design, and in customary DC transmission of electricity, following factor is mainly considered in the design considerations of smoothing reactor:

One is according to suppressing DC current ascending velocity primary design inductance value during system generation disturbance, due in customary DC transmission of electricity, when a converter bridge breaks down, because the voltage of fault bridge will equal zero continuously within a period of time in about 3/8 cycle, therefore the corresponding decline of its DC voltage, and DC current rises, fall under voltage, electric current rise a period of time in, other of transverter perfect bridge by commutation repeatedly, the rising of DC current makes the commutation time elongate, if the time elongates ground too much, will cause perfecting bridge and in succession commutation failure occur.The commutation failure of secondary declines to a great extent further by making the voltage of transverter, DC current rises quickly, the result of vicious cycle often makes accident expand to this extremely all bridge, so that DC line only surplus next pole power transmission, and transmission power is reduced to the half before accident.Therefore, the increase reducing the DC current caused due to a bridge commutation failure must be managed.In order to the flat ripple reactance reaching this object calculates formula of reduction be $L_d=\frac{{\mathrm{\ΔU}}_d}{{\mathrm{\ΔI}}_d}\mathrm{\Δt}=\frac{{\mathrm{\ΔU}}_d(\mathrm{\β}-1-{\mathrm{\γ}}_{\min })}{{\mathrm{\ΔI}}_d\×360f}.$

Two is determine inductance value according to the requirement of the electric current and voltage ripple reduced in DC line.In customary DC transmission of electricity, after smoothing reactor is selected according to the requirement of suppression fault current ascending velocity, because it also plays flat ripple effect simultaneously, thus the ripple of AC line voltage and electric current is general all smaller.But also need the problem considering following two aspects: 1. ripple is to the electromagnetic induction of communication line induction noise, particularly current ripples that DC line walks abreast, needs if desired to set up DC filter; 2. during small area analysis, the discontinuous problem of current waveform.

In flexible DC power transmission engineering, parameter designing and the customary DC of the smoothing reactor subject matter considered of transmitting electricity is similar, but there is not the problem of commutation failure in flexible DC power transmission, other failure mechanisms are also different from customary DC transmission of electricity, therefore suppress the calculation method of parameters of fault current ascending velocity also not identical.In addition, because flexible DC power transmission engineering in general adopts cable line, do not need to consider that DC voltage and current harmonic wave is to the interference problem of communication line, even if having employed overhead transmission line, the PWM adopted due to Technology of HVDC based Voltage Source Converter or the SWM modulator approach of approaching based on sine, the voltage current waveform quality that transverter alternating current-direct current side exports is all better, does not generally need to consider harmonics restraint problem.In addition, there is not the problem of discontinuous current in flexible DC power transmission DC side electric current yet.

Summary of the invention

For the deficiencies in the prior art, the invention provides a kind of Parameters design of flexible DC power transmission smoothing reactor, the system resonance that circuit parameter causes can be avoided.

The Parameters design of a kind of flexible DC power transmission smoothing reactor provided by the invention, its improvements are, described method comprises the steps:

(1) determine input parameter, comprise brachium pontis inductance value L _arm, line electricity inductance value L _line, submodule capacitance C ₀with brachium pontis submodule quantity n;

(2) according to the rejection condition of fault current escalating rate, the lower limit of smoothing reactor inductance value is determined;

(3) according to the condition of direct current dynamic responding speed, the upper limit of smoothing reactor inductance value is determined;

(4) according to the condition producing resonance, flat ripple reactive inductor value is determined.

Wherein, the described rejection condition according to fault current escalating rate of step (2), determine that the step of smoothing reactor inductance value lower limit comprises:

When there is DC bipolar short trouble, submodule capacitance discharge current is:

$i=e^{-\frac{t}{{\mathrm{\τ}}_1}}\left[\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}\sin (\mathrm{\ωt}+\mathrm{\β})\right];$

Current-rising-rate is:

$\frac{\mathrm{di}}{\mathrm{dt}}=\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+{\mathrm{\ωe}}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})];$

In formula, L _eqfor the equivalent inductance value determined according to the reactance of brachium pontis reactance peace ripple, L _eq=L _arm+ 3L _s, L _armfor brachium pontis reactive inductor value; C _eqfor equivalent brachium pontis submodule capacitance; U _dcfor DC voltage value; τ ₁for the time constant of discharge current decay;

The value lower limit of equivalent reactance is:

$L_{\mathrm{eq}\_\min }=\frac{C_{\mathrm{eq}}{U}_{\mathrm{dc}}^2{[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+{\mathrm{\ωe}}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})]}^2}{{\left(\frac{\mathrm{di}}{\mathrm{dt}}\right)}^2};$

The value lower limit of flat ripple reactance is:

$L_{s\_\min }=\frac{C_{\mathrm{eq}}{U}_{\mathrm{dc}}^2{[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+{\mathrm{\ωe}}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})]}^2}{3{\left(\frac{\mathrm{di}}{\mathrm{dt}}\right)}^2}-\frac{L_{\mathrm{arm}}}{3}.$

Wherein, the described condition according to direct current dynamic responding speed of step (3), determine that the step of the smoothing reactor inductance value upper limit comprises:

If the time constant threshold value that dynamic responding speed requires is τ ₀, then timeconstantτ≤τ ₀, the flat ripple inductance value upper limit that dynamic responding speed requires is:

$L_{s\_\max }=\frac{{\mathrm{\τ}}_{0}R}{2}-\frac{L_{\mathrm{arm}}}{3}.$

Wherein, the described condition according to producing resonance of step (4), determine that the step of flat ripple reactive inductor value is:

The submodule number that facies unit drops into is constant is n, and represent overtone order with h, between standing, resonance equivalent impedance loop is:

$Z\left(h\right)=2[\frac{R_{\mathrm{eq}}}{3}+j(\frac{{2\mathrm{h\ω}}_0{L}_{\mathrm{arm}}}{3}-\frac{n}{{3\mathrm{h\ω}}_0{C}_0})]+R_{\mathrm{sl}}+{\mathrm{jh\ω}}_0{L}_{\mathrm{sl}};$

Therefore:

$L_{\mathrm{sl}}\≠\frac{2n}{{3h}^2{\mathrm{\ω}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}};$

In formula, L _sl=L _s+ L _line, L _sfor smoothing reactor inductance, L _linefor substitutional connection inductance;

R _sl=R _s+ R _line, R _sfor smoothing reactor resistance, R _linefor line equivalent resistance.

Therefore, the inductance value of smoothing reactor meets:

$L_s\≠\frac{2n}{{3h}^2{\mathrm{\ω}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}}-L_{\mathrm{line}}.$

Wherein, the expression formula of step (3) time constant is:

$\mathrm{\τ}=\frac{\frac{2L_{\mathrm{arm}}}{3}+{2L}_s}{R}.$

Compared with the prior art, beneficial effect of the present invention is:

1, method of the present invention does not need to consider commutation failure, Communication Jamming and harmonics restraint problem, therefore simple;

2, contemplated by the invention the suppression requirement of fault current escalating rate, capacitance discharge current ascending velocity is effectively suppressed, ensure that device use safety, alleviate the requirement to protection system design;

3, contemplated by the invention the requirement of system dynamic responding speed, ensure that the automatic control characteristic of flexible direct current power transmission system, reduce the cost of smoothing reactor simultaneously;

4, the present invention has taken into account suppression and the requirement of system dynamic responding speed of fault current escalating rate, makes overall system performance reach optimum;

5, the present invention is by getting rid of flexible direct current power transmission system resonant tank tuning-points, makes the natural mode shape of flexible direct current power transmission system avoid the harmonics frequency of fundamental frequency and transverter, avoids the system resonance that circuit parameter causes.

Accompanying drawing explanation

Fig. 1 is the discharge loop schematic diagram of transverter provided by the invention.

Fig. 2 is that bipolar short trouble provided by the invention discharges equivalent second-order circuit figure.

Fig. 3 is the equivalent circuit diagram in system resonance loop provided by the invention.

Embodiment

Below in conjunction with accompanying drawing, the specific embodiment of the present invention is described in further detail.

Smoothing reactor is one of important equipment of flexible direct current transmission converter station, and the effect in flexible direct current converter station mainly contains:

1, the escalating rate of fault current limiting;

2, the steep wave shock wave preventing DC line or direct current place from producing enters the valve Room, thus makes converter valve avoid suffering superpotential stress and damaging;

3, the ripple in smooth direct current electric current.

DC bipolar short trouble is one of fault that flexible direct current power transmission system is the most serious, and when there is DC bipolar short trouble, fault current development is rapid, fault curre is comparatively large, and equipment bears harsh current stress.Bipolar short trouble is often caused by the failure of insulation between two-wire line, current most of flexible DC power transmission engineering adopts cable line to connect, in order to save transmission of electricity corridor, both positive and negative polarity cable distribution distance is very near, probably dug disconnected formation short circuit simultaneously, simultaneously pole line also may form bipolar short circuit due to branch etc., as shown in Figure 1.

For the flexible direct current power transmission system of modularization multi-level converter (MMC) topology, DC bipolar short trouble comprises two processes, one is before converter blocking, and both sides current conversion station all injects short-circuit current by submodule bottom diode to short dot, is equivalent to three-phase shortcircuit.Meanwhile, submodule capacitor is discharged by the IGBT on top, and discharge loop as shown in Figure 1.Bridge arm current is the superposition of alternating short-circuit current and submodule capacitor discharge current, peak value is reached in half cycle, converter blocking after several milliseconds, DC bipolar short trouble enters second process, now, submodule capacitor stops electric discharge, but alternating short-circuit current still forms short circuit by trouble spot.In the first phase, because fault current flows through converter valve, the electric current that now valve bears is the combination of alternating current and capacitance discharge current, wherein capacitance discharge current plays decisive role, short-circuit current before locking determines primarily of capacitance discharge current, and the size of capacitance discharge current and speed depend on brachium pontis reactance peace ripple reactance value, the reactance of brachium pontis reactance peace ripple has obvious inhibiting effect to capacitance discharge current, when total reactance value is less, capacitance discharge current ascending velocity is very fast, high to protection rate request, therefore the reactance of appropriate design brachium pontis and flat ripple reactance value is needed to reach failure restraint requirement.Wherein, the value of brachium pontis reactor is determined according to the requirement of flexible direct current power transmission system to power delivery, is therefore met by the design of smoothing reactor the requirement that DC bipolar short trouble suppresses.

Because inductance value is larger, the response time of flexible direct current power transmission system is longer, therefore smoothing reactor also should take into account the dynamic responding speed of system while meeting failure restraint requirement, also should check selected smoothing reactor inductance value and brachium pontis reactance, line reactance, brachium pontis electric capacity, line capacitance do not form resonance simultaneously.

The topmost parameter of smoothing reactor is its inductance value, and suppress direct current harmonic wave from the angle of smoothing reactor and suppress fault current, its inductance value should be tending towards higher value, but can not be too large.Because inductance value is too large, easily produces superpotential during operation, the control response speed of DC transmission system is declined, and the investment of smoothing reactor also increases.Therefore the inductance value of smoothing reactor should be as small as possible under the prerequisite meeting main performance requirements.For in existing document without the introduction of flexible DC power transmission smoothing reactor Parameters design, and customary DC transmission of electricity smoothing reactor design concept is different from the feature of flexible DC power transmission, it is as follows that the present embodiment proposes a kind of flexible DC power transmission smoothing reactor Parameters design:

1, the escalating rate of fault current limiting

Now capacitance discharge current can be estimated by the equivalent second-order circuit shown in Fig. 2.In the voltage source converter of MMC topological structure, smoothing reactor is connected on the fault current ascending velocity that direct-current polar effectively can suppress DC bipolar short trouble.When there is DC bipolar short trouble, submodule capacitance discharge current is:

$i=e^{-\frac{t}{{\mathrm{\τ}}_1}}\left[\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}\sin (\mathrm{\ωt}+\mathrm{\β})\right];$

Current-rising-rate is:

$\frac{\mathrm{di}}{\mathrm{dt}}=\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+{\mathrm{\ωe}}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})];$

Wherein, L _eq=L _arm+ 3L _s, L _armfor brachium pontis reactive inductor value, C _eqfor equivalent brachium pontis submodule capacitance.Visible, add smoothing reactor inductance value L _safter, the escalating rate of fault current have dropped.Therefore the value lower limit of equivalent reactance is:

$L_{\mathrm{eq}\_\min }=\frac{C_{\mathrm{eq}}{U}_{\mathrm{dc}}^2{[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+{\mathrm{\ωe}}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})]}^2}{{\left(\frac{\mathrm{di}}{\mathrm{dt}}\right)}^2};$

The value lower limit of flat ripple reactance is:

$L_{s\_\min }=\frac{C_{\mathrm{eq}}{U}_{\mathrm{dc}}^2{[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+{\mathrm{\ωe}}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})]}^2}{3{\left(\frac{\mathrm{di}}{\mathrm{dt}}\right)}^2}-\frac{L_{\mathrm{arm}}}{3};$

2, direct current dynamic responding speed

As shown in Figure 1, circuit time constant r is circuit equivalent resistance.Flat ripple reactance value is larger, and time constant is larger, and flexible direct current power transmission system dynamic responding speed is slower, and therefore, the inductance value of smoothing reactor can not be excessive, otherwise flexible direct current power transmission system DC side dynamic responding speed can not meet the demands.

If the time constant threshold value that dynamic responding speed requires is τ ₀, then τ≤τ ₀, the flat ripple inductance value upper limit therefore meeting dynamic responding speed requirement is:

$L_{s\_\max }=\frac{{\mathrm{\τ}}_{0}R}{2}-\frac{L_{\mathrm{arm}}}{3}.$

3, resonance is avoided

Feature due to MMC topology is to ensure that each facies unit has fixing electric capacity number to drop into, therefore for each brachium pontis, the capacitance size dropped into is fixed, simultaneously due to the existence of brachium pontis reactance peace ripple reactance, whole current conversion station has an intrinsic oscillation frequency, and this frequency is determined by the size of electric capacity and inductance.Because the resistance of inductance is very little, when system drops into or oscillation of power occurs, probably resonance occurs, if do not controlled, be difficult to the decay suppressing this vibration, oscillating current can cause the further imbalance of capacitance voltage simultaneously, probably aggravates this vibration.

The inductance value of smoothing reactor should avoid the harmonics frequency of fundamental frequency and transverter.The submodule number that facies unit drops into is constant is n, and represent overtone order with h, when cable line is shorter, line mutual-ground capacitor is much smaller compared with current conversion station equivalent capacity, can ignore, and according to Fig. 3, between standing, resonance equivalent impedance loop is:

$Z\left(h\right)=2[\frac{R_{\mathrm{eq}}}{3}+j(\frac{{2\mathrm{h\ω}}_0{L}_{\mathrm{arm}}}{3}-\frac{n}{{3\mathrm{h\ω}}_0{C}_0})]+R_{\mathrm{sl}}+{\mathrm{jh\ω}}_0{L}_{\mathrm{sl}};$

Therefore have:

$L_{\mathrm{sl}}\≠\frac{2n}{{3h}^2{\mathrm{\ω}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}};$

In formula, L _sl=L _s+ L _line, L _sfor smoothing reactor inductance, L _linefor substitutional connection inductance;

R _sl=R _s+ R _line, R _sfor smoothing reactor resistance, R _linefor line equivalent resistance.

Therefore, for not causing resonance, the inductance value of smoothing reactor should meet:

$L_s\≠\frac{2n}{{3h}^2{\mathrm{\ω}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}}-L_{\mathrm{line}};$

In sum, the Parameters design of a kind of flexible DC power transmission smoothing reactor that the present embodiment provides, concrete implementation step is as follows:

Step 1: determine input parameter, comprises brachium pontis inductance value L _arm, line electricity inductance value L _line, submodule capacitance C ₀, brachium pontis submodule quantity n;

Step 2: according to the suppression requirement of fault current escalating rate, determines that the lower limit of smoothing reactor inductance value is

$L_{s\_\min }=\frac{C_{\mathrm{eq}}{U}_{\mathrm{dc}}^2{[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+{\mathrm{\ωe}}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})]}^2}{3{\left(\frac{\mathrm{di}}{\mathrm{dt}}\right)}^2}-\frac{L_{\mathrm{arm}}}{3};$

Step 3: according to the requirement of direct current dynamic responding speed, determines that the upper limit of smoothing reactor is

$L_{s\_\max }=\frac{{\mathrm{\τ}}_{0}R}{2}-\frac{L_{\mathrm{arm}}}{3};$

Step 4: according to the requirement avoiding resonance, namely the flat ripple reactive inductor value causing resonance between the feasible region that eliminating is determined by step 2 and 3.

Finally should be noted that: above embodiment is only in order to illustrate that technical scheme of the present invention is not intended to limit, although with reference to above-described embodiment to invention has been detailed description, those of ordinary skill in the field are to be understood that: still can modify to the specific embodiment of the present invention or equivalent replacement, and not departing from any amendment of spirit and scope of the invention or equivalent replacement, it all should be encompassed in the middle of right of the present invention.

Claims (4)

1. a Parameters design for flexible DC power transmission smoothing reactor, is characterized in that, described method comprises the steps:

(1) determine input parameter, comprise brachium pontis inductance value L _arm, line electricity inductance value L _line, submodule capacitance C ₀with brachium pontis submodule quantity n;

(2) according to the rejection condition of fault current escalating rate, the lower limit of smoothing reactor inductance value is determined;

(3) according to the condition of direct current dynamic responding speed, the upper limit of smoothing reactor inductance value is determined;

(4) according to the condition producing resonance, flat ripple reactive inductor value is determined;

The described rejection condition according to fault current escalating rate of step (2), determine that the step of smoothing reactor inductance value lower limit comprises:

When there is DC bipolar short trouble, submodule capacitance discharge current is:

$i=e^{-\frac{t}{{\mathrm{\τ}}_1}}\left[\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}\sin (\mathrm{\ωt}+\mathrm{\β})\right];$

Current-rising-rate is:

$\frac{\mathrm{di}}{\mathrm{dt}}\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+\mathrm{\ω}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})];$

In formula, L _eqfor the equivalent inductance value determined according to the reactance of brachium pontis reactance peace ripple, L _eq=L _arm+ 3L _s, L _armfor brachium pontis reactive inductor value; C _eqfor equivalent brachium pontis submodule capacitance; U _dcfor DC voltage value; τ ₁for the time constant of discharge current decay; T is discharge time, and ω is angular frequency, and β is initial phase angle; The value lower limit of equivalent reactance is:

$L_{\mathrm{eq}\_\min }=\frac{C_{\mathrm{eq}}{U}_{\mathrm{dc}}^2{[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+\mathrm{\ω}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})]}^2}{{\left(\frac{\mathrm{di}}{\mathrm{dt}}\right)}^2};$

The value lower limit of flat ripple reactance is:

$L_{s\_\min }=\frac{C_{\mathrm{eq}}{U}_{\mathrm{dc}}^2{[-\frac{1}{{\mathrm{\τ}}_1}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\sin (\mathrm{\ωt}+\mathrm{\β})+\mathrm{\ω}{e}^{-\frac{t}{{\mathrm{\τ}}_1}}\cos (\mathrm{\ωt}+\mathrm{\β})]}^2}{3{\left(\frac{\mathrm{di}}{\mathrm{dt}}\right)}^2}-\frac{L_{\mathrm{arm}}}{3};$

In formula, R is circuit equivalent resistance.

2. Parameters design as claimed in claim 1, is characterized in that the described condition according to direct current dynamic responding speed of step (3) determines that the step of the smoothing reactor inductance value upper limit comprises:

If the time constant threshold value that dynamic responding speed requires is τ ₀, then timeconstantτ≤τ ₀, the flat ripple inductance value upper limit that dynamic responding speed requires is:

$L_{s\_\max }=\frac{{\mathrm{\τ}}_{0}R}{2}-\frac{L_{\mathrm{arm}}}{3}.$

3. Parameters design as claimed in claim 1, is characterized in that, the described condition according to producing resonance of step (4), determines that the step of flat ripple reactive inductor value is:

The submodule number that facies unit drops into is constant is n, and represent overtone order with h, between standing, resonance equivalent impedance loop is:

$Z\left(h\right)=2[\frac{R_{\mathrm{eq}}}{3}+j(\frac{2h{\mathrm{\ω}}_0{L}_{\mathrm{arm}}}{3}-\frac{n}{3h{\mathrm{\ω}}_0{C}_0})]+R_{\mathrm{sl}}+\mathrm{jh}{\mathrm{\ω}}_0{L}_{\mathrm{sl}};$

Therefore:

$L_{\mathrm{sl}}\≠\frac{2n}{{3h}^2{\mathrm{\ω}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}};$

In formula, L _sl=L _s+ L _line, L _sfor smoothing reactor inductance, L _linefor substitutional connection inductance;

R _sl=R _s+ R _line, R _sfor smoothing reactor resistance, R _linefor line equivalent resistance, R _eqfor equivalent bridge wall submodule resistance, ω ₀for subharmonic angular frequency;

Therefore, the inductance value of smoothing reactor meets:

$L_s\≠\frac{2n}{{3h}^2{\mathrm{\ω}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}}-L_{\mathrm{line}}.$

4. Parameters design as claimed in claim 2, it is characterized in that, the expression formula of step (3) time constant is:

$\mathrm{\τ}=\frac{\frac{2L_{\mathrm{arm}}}{3}+{2L}_s}{R};$

In formula, R is circuit equivalent resistance, L _sfor smoothing reactor inductance.