CN103048547A - 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 flexible DC power transmission Parameters design of smoothing reactor

Technical field

The invention belongs to flexible DC power transmission (VSC-HVDC) technical field, be specifically related to the Parameters design that a kind of flexible DC power transmission is used 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 the fields such as new-energy grid-connected and passive load power supply.Smoothing reactor is one of visual plant in the flexible direct current converter station, and its parameter directly affects harmonic characteristic, control response speed and the failure restraint ability of change of current system.In existing Technology of HVDC based Voltage Source Converter pertinent literature, have no the relevant report of smoothing reactor Parameters design, and in conventional direct current transportation, following factor is mainly considered in the design considerations of smoothing reactor:

The one, suppress DC current ascending velocity primary design inductance value during according to system's generation disturbance, because in conventional direct current transportation, when a converter bridge breaks down, because the voltage of fault bridge will equal zero within a period of time in about 3/8 cycle continuously, therefore the corresponding decline of its DC voltage, and DC current rises, within a period of time of voltage drop, Current rise, other of transverter perfect bridge with commutation repeatedly, the rising of DC current is elongated the commutation time, if the time is elongated ground too much, will cause perfecting bridge commutation failure occurs in succession.The commutation failure of secondary will make the voltage of transverter further decline to a great extent, DC current rises quickly, the result of vicious cycle often makes accident expand to all bridges of this utmost point, thus DC line only the surplus next utmost point send electricity, transmission power is reduced to half before the accident.Therefore, must manage to reduce the increase of the DC current that causes owing to a bridge commutation failure.Calculating formula of reduction for the flat ripple reactance that reaches this purpose is $L_d=\frac{{\mathrm{\ΔU}}_d}{{\mathrm{\ΔI}}_d}\mathrm{\Δt}=\frac{{\mathrm{\ΔU}}_d(\mathrm{\β}-1-{\mathrm{\γ}}_{\min })}{{\mathrm{\ΔI}}_d\×360f}.$

The 2nd, determine inductance value according to the requirement that reduces the electric current and voltage ripple in the DC line.In conventional direct current transportation, after smoothing reactor was selected according to the requirement that suppresses the fault current ascending velocity, because it also plays flat ripple effect simultaneously, thereby the ripple of DC line voltage and current was generally all smaller.But also need consider the problem of following two aspects: 1. ripple need be set up DC filter in case of necessity to the electromagnetic induction of parallel communication line induction noise, particularly current ripples of DC line; During 2. little electric current, the discontinuous problem of current waveform.

The parameter designing of smoothing reactor is similar to the subject matter of conventional direct current transportation consideration in the flexible DC power transmission engineering, but there is not the problem of commutation failure in flexible DC power transmission, other failure mechanisms also are different from conventional direct current transportation, and the calculation method of parameters that therefore suppresses the fault current ascending velocity is not identical yet.In addition, because the flexible DC power transmission engineering in general adopts cable line, need not consider that the dc voltage and current harmonic wave is to the interference problem of communication line, even adopted overhead transmission line, because the PWM that Technology of HVDC based Voltage Source Converter adopts modulation or the SWM modulator approach of approaching based on sine, the voltage current waveform quality of transverter alternating current-direct current side output is all better, need not consider that generally harmonic wave suppresses 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 the Parameters design that a kind of flexible DC power transmission is used smoothing reactor, the system resonance that can avoid circuit parameter to cause.

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

(1) determines input parameter, comprise brachium pontis inductance value L _Arm, line inductance value L _Line, submodule capacitance C ₀With brachium pontis submodule quantity n;

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

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

(4) according to the condition that produces resonance, determine flat ripple reactance inductance value.

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

When the DC bipolar short trouble occured, submodule capacitor discharge electric current was:

$i=e^{-\frac{t}{{\mathrm{\&tau;}}_1}}\left[\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{<figure-callout\; id="2"\; label="eq\; U\; dc"\; filenames="HDA00002545688000012.png"\; state="\{\{state\}\}">eq</figure-callout>}}}{U}_{\mathrm{dc}}^{}{}_2^{}}\sin (\mathrm{\&omega;t}+\mathrm{\&beta;})\right];$

Current-rising-rate is:

$\frac{\mathrm{di}}{\mathrm{dt}}=\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}[-\frac{1}{{\mathrm{\&tau;}}_1}{e}^{-\frac{t}{{\mathrm{\&tau;}}_1}}\sin (\mathrm{\&omega;t}+\mathrm{\&beta;})+{\mathrm{\&omega;e}}^{-\frac{t}{{\mathrm{\&tau;}}_1}}\cos (\mathrm{\&omega;t}+\mathrm{\&beta;})];$

In the formula, L _EqBe the equivalent inductance value that the reactance according to brachium pontis reactance peace ripple determines, L _Eq=L _Arm+ 3L _s, L _ArmBe brachium pontis reactance inductance value; C _EqBe equivalent brachium pontis submodule capacitance; U _DcBe dc voltage value; τ ₁Time constant for the discharge current decay;

Be limited under the value of equivalent reactance:

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

Be limited under the value of flat ripple reactance:

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

Wherein, the described condition according to the 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, timeconstantτ≤τ then ₀, be limited on the flat ripple inductance value that dynamic responding speed requires:

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

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

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

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

Therefore:

$L_{\mathrm{sl}}\&NotEqual;\frac{2n}{{3\mathrm{<figure-callout\; id="2"\; label="h"\; filenames="HDA00002545688000012.png"\; state="\{\{state\}\}">h</figure-callout>}}^2{\mathrm{\&omega;}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}};$

In the formula, L _Sl=L _s+ L _Line, L _sBe smoothing reactor inductance, L _LineBe the substitutional connection inductance;

R _Sl=R _s+ R _Line, R _sBe smoothing reactor resistance, R _LineBe the circuit equivalent resistance.

Therefore, the inductance value of smoothing reactor satisfies:

$L_s\&NotEqual;\frac{2n}{{3\mathrm{<figure-callout\; id="2"\; label="h"\; filenames="HDA00002545688000012.png"\; state="\{\{state\}\}">h</figure-callout>}}^2{\mathrm{\&omega;}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}}-L_{\mathrm{line}}.$

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

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

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

1, method of the present invention need not consider that commutation failure, Communication Jamming and harmonic wave suppress problem, and is therefore simple;

2, the present invention has considered the inhibition requirement of fault current escalating rate, and capacitor discharge Current rise speed is effectively suppressed, and has guaranteed that device uses safety, has alleviated the requirement to the protection system design;

3, the present invention has considered the requirement of system's dynamic responding speed, has guaranteed the automatic control characteristic of flexible DC power transmission system, has reduced simultaneously the cost of smoothing reactor;

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

5, the present invention is by getting rid of flexible DC power transmission system resonance loop tuning-points, makes the natural mode shape of flexible DC power transmission system avoid the harmonics frequency of fundamental frequency and transverter, the system resonance of avoiding circuit parameter to cause.

Description of drawings

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

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

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

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 DC power transmission current conversion station, and the effect in flexible direct current converter station mainly contains:

1, the escalating rate of fault current limiting;

2, prevent that the steep wave shock wave that DC line or direct current place produce from entering the valve Room, thereby make converter valve avoid suffering superpotential stress and damage;

3, the ripple in the smooth direct current electric current.

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

Flexible DC power transmission system for modularization multi-level converter (MMC) topology, the DC bipolar short trouble comprises two processes, the one, before the converter blocking, the both sides current conversion station all injects short-circuit current by submodule bottom diode to short dot, is equivalent to three-phase shortcircuit.Simultaneously, the submodule capacitor is by the IGBT discharge on top, and discharge loop as shown in Figure 1.The brachium pontis electric current is the stack of ac short circuit electric current and submodule capacitor discharge current, in half cycle, reach peak value, converter blocking after several milliseconds, the DC bipolar short trouble enters second process, at this moment, the submodule capacitor stops discharge, but the ac short circuit electric current still forms short circuit by the trouble spot.In the phase one; the converter valve because fault current is flowed through; the electric current that this moment, valve bore is the combination of alternating current and capacitor discharge electric current; wherein the capacitor discharge electric current plays decisive role; short-circuit current before the locking is mainly determined by the capacitor discharge electric current; and the size of capacitor discharge electric 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 the capacitor discharge electric current; when total reactance value hour; capacitor discharge Current rise speed is very fast; high to the protection rate request, therefore need appropriate design brachium pontis reactance and flat ripple reactance value to reach the failure restraint requirement.Wherein, the value of brachium pontis reactor determines that to the requirement of power delivery the requirement that therefore the DC bipolar short trouble is suppressed is satisfied by the design of smoothing reactor according to the flexible DC power transmission system.

Because inductance value is larger, the response time of flexible DC power transmission system is longer, therefore smoothing reactor also should be taken into account the dynamic responding speed of system when satisfying the failure restraint requirement, also should check simultaneously selected smoothing reactor inductance value and brachium pontis reactance, line reactance, brachium pontis electric capacity, line capacitance does not form resonance.

The topmost parameter of smoothing reactor is its inductance value, suppresses direct current harmonic wave and the angle that suppresses fault current from smoothing reactor, and its inductance value should be tending towards higher value, but can not be too large.Because inductance value is too large, produce easily superpotential during operation, the control response speed of DC transmission system is descended, and the investment of smoothing reactor also increase.Therefore the inductance value of smoothing reactor is in that satisfy should be as small as possible under the prerequisite of main performance requirements.For in the existing document without the introduction of flexible DC power transmission smoothing reactor Parameters design, and conventional direct current transportation smoothing reactor design concept is different from the characteristics of flexible DC power transmission, and it is as follows that present embodiment has proposed a kind of flexible DC power transmission smoothing reactor Parameters design:

1, the escalating rate of fault current limiting

This moment, the capacitor discharge electric current can be estimated by equivalent second-order circuit shown in Figure 2.In the voltage-source type transverter of MMC topological structure, smoothing reactor be connected on the direct-current polar can establishment DC bipolar short trouble the fault current ascending velocity.When the DC bipolar short trouble occured, submodule capacitor discharge electric current was:

$i=e^{-\frac{t}{{\mathrm{\&tau;}}_1}}\left[\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{<figure-callout\; id="2"\; label="eq\; U\; dc"\; filenames="HDA00002545688000012.png"\; state="\{\{state\}\}">eq</figure-callout>}}}{U}_{\mathrm{dc}}^{}{}_2^{}}\sin (\mathrm{\&omega;t}+\mathrm{\&beta;})\right];$

Current-rising-rate is:

$\frac{\mathrm{di}}{\mathrm{dt}}=\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}[-\frac{1}{{\mathrm{\&tau;}}_1}{e}^{-\frac{t}{{\mathrm{\&tau;}}_1}}\sin (\mathrm{\&omega;t}+\mathrm{\&beta;})+{\mathrm{\&omega;e}}^{-\frac{t}{{\mathrm{\&tau;}}_1}}\cos (\mathrm{\&omega;t}+\mathrm{\&beta;})];$

Wherein, L _Eq=L _Arm+ 3L _s, L _ArmBe brachium pontis reactance inductance value, C _EqBe equivalent brachium pontis submodule capacitance.As seen, increased smoothing reactor inductance value L _sAfter, the escalating rate of fault current has descended.Therefore be limited under the value of equivalent reactance:

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

Be limited under the value of flat ripple reactance:

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

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 DC power transmission system dynamic responding speed is slower, and therefore, the inductance value of smoothing reactor can not be excessive, otherwise flexible DC 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, τ≤τ then ₀, therefore satisfy on the flat ripple inductance value that dynamic responding speed requires being limited to:

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

3, avoid resonance

Because being to guarantee each facies unit, the characteristics of MMC topology have fixing electric capacity number to drop into, therefore for each brachium pontis, the capacitance size that drops into is fixing, simultaneously because 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 dropped into or oscillation of power occurs, resonance probably occured, if do not controlled, be difficult to suppress the decay of this vibration, oscillating current can cause the further imbalance of capacitance voltage simultaneously, probably aggravates this vibration.

The inductance value of smoothing reactor should be avoided the harmonics frequency of fundamental frequency and transverter.The submodule number that facies unit drops into is constant to be n, represents overtone order with h, when cable line more in short-term, line mutual-ground capacitor is much smaller than the current conversion station equivalent capacity, can ignore, according to shown in Figure 3, the resonance equivalent impedance loop is between standing:

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

Therefore have:

$L_{\mathrm{sl}}\&NotEqual;\frac{2n}{{3\mathrm{<figure-callout\; id="2"\; label="h"\; filenames="HDA00002545688000012.png"\; state="\{\{state\}\}">h</figure-callout>}}^2{\mathrm{\&omega;}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}};$

In the formula, L _Sl=L _s+ L _Line, L _sBe smoothing reactor inductance, L _LineBe the substitutional connection inductance;

R _Sl=R _s+ R _Line, R _sBe smoothing reactor resistance, R _LineBe the circuit equivalent resistance.

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

$L_s\&NotEqual;\frac{2n}{{3\mathrm{<figure-callout\; id="2"\; label="h"\; filenames="HDA00002545688000012.png"\; state="\{\{state\}\}">h</figure-callout>}}^2{\mathrm{\&omega;}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}}-L_{\mathrm{line}};$

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

Step 1: determine input parameter, comprise brachium pontis inductance value L _Arm, line inductance value L _Line, submodule capacitance C ₀, brachium pontis submodule quantity n;

Step 2: according to the inhibition requirement of fault current escalating rate, determine that the lower of smoothing reactor inductance value is limited to

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

Step 3: according to the requirement of direct current dynamic responding speed, determine smoothing reactor on be limited to

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

Step 4: according to the requirement of avoiding resonance, namely

The flat ripple reactance inductance value that causes resonance between the feasible regions that eliminating is determined by step 2 and 3.

Should be noted that at last: 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 the present invention is had been described in detail, those of ordinary skill in the field are to be understood that: still can make amendment or be equal to replacement the specific embodiment of the present invention, and do not break away from any modification of spirit and scope of the invention or be equal to replacement, it all should be encompassed in the middle of the claim scope of the present invention.

Claims (5)

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

(1) determines input parameter, comprise brachium pontis inductance value L _Arm, line inductance value L _Line, submodule capacitance C ₀With brachium pontis submodule quantity n;

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

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

(4) according to the condition that produces resonance, determine flat ripple reactance inductance value.

2. Parameters design as claimed in claim 1 is characterized in that, the described rejection condition according to the fault current escalating rate of step (2) determines that the step of smoothing reactor inductance value lower limit comprises:

When the DC bipolar short trouble occured, submodule capacitor discharge electric current was:

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

Current-rising-rate is:

$\frac{\mathrm{di}}{\mathrm{dt}}=\sqrt{\frac{C_{\mathrm{eq}}}{L_{\mathrm{eq}}}{U}_{\mathrm{dc}}^2}[-\frac{1}{{\mathrm{\&tau;}}_1}{e}^{-\frac{t}{{\mathrm{\&tau;}}_1}}\sin (\mathrm{\&omega;t}+\mathrm{\&beta;})+{\mathrm{\&omega;e}}^{-\frac{t}{{\mathrm{\&tau;}}_1}}\cos (\mathrm{\&omega;t}+\mathrm{\&beta;})];$

In the formula, L _EqBe the equivalent inductance value that the reactance according to brachium pontis reactance peace ripple determines, L _Eq=L _Arm+ 3L _s, L _ArmBe brachium pontis reactance inductance value; C _EqBe equivalent brachium pontis submodule capacitance; U _DcBe dc voltage value; τ ₁Time constant for the discharge current decay;

Be limited under the value of equivalent reactance:

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

Be limited under the value of flat ripple reactance:

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

3. Parameters design as claimed in claim 1 is characterized in that, the described condition according to the 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, timeconstantτ≤τ then ₀, be limited on the flat ripple inductance value that dynamic responding speed requires:

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

4. 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 reactance inductance value is:

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

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

Therefore:

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

In the formula, L _Sl=L _s+ L _Line, L _sBe smoothing reactor inductance, L _LineBe the substitutional connection inductance;

R _Sl=R _s+ R _Line, R _sBe smoothing reactor resistance, R _LineBe the circuit equivalent resistance.

Therefore, the inductance value of smoothing reactor satisfies:

$L_s\&NotEqual;\frac{2n}{{3h}^2{\mathrm{\&omega;}}_0^2{C}_0}-\frac{4}{3}{L}_{\mathrm{arm}}-L_{\mathrm{line}}.$

5. Parameters design as claimed in claim 1 is characterized in that, the expression formula of step (3) time constant is:

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

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