CN104237745A - Method for judging abnormal commutation failure of multi-feed high-voltage DC transmission system - Google Patents

Abstract

The invention discloses a method for judging abnormal commutation failure of a multi-feed high-voltage DC transmission system. The method comprises the steps of S100, determining the failure type of the multi-feed high-voltage DC transmission system, S200, calculating a failure level under the failure type determined in the step S100, S300, performing Fourier analysis on the waveform in a cycle after the failure by use of the voltage waveform data of and reducing a far-end commutation bus under the failure level calculated in the step S200, and calculating the percentage of reduction of the voltage amplitude, S400, calculating the voltage and the value of harmonic distortion reference (VHDR) under the failure level obtained in the step S300, S500, obtaining the failure level at which the probability of abnormal commutation failure is 0.5 by simulation, and after the steps S300 and S400 are carried out, obtaining the VHDR at the moment, and S600 determining the probability of abnormal commutation failure.

Description

The abnormal commutation failure decision method of Multi-infeed HVDC system

Technical field

The present invention relates to the abnormal commutation failure decision method of a kind of Multi-infeed HVDC system.

Background technology

In transverter, the converter valve exiting conducting recovers blocking ability as failed within a period of time of reverse voltage effect, or commutation process does not carry out complete during reverse voltage, then when threshold voltage becomes forward, switched phase by the valve of commutation by the original predetermined valve exiting conducting, this situation is called commutation failure.Due to rectifier converter valve is in reverse voltage after switch off current in the long period under, so only when trigger circuit break down, just can there is commutation failure in rectifier.Straight-flow system major part commutation failure is all occur in inverter side, and commutation failure is one of modal fault type of inverter side.Therefore the impact between straight-flow system and inverter side AC system is also mainly concentrated on the research of commutation failure.In Multi-infeed HVDC system, receiving end AC system breaks down the while of may causing multiple-circuit line system or in succession commutation failure occurs, can DC system locking be caused time serious, direct current transmission interruption in power, finally threaten the safe and stable operation of whole system.

The essence of commutation failure is that the extinguish angle γ of converter valve is less than critical extinction angle γ _min.Tradition differentiates that the method for commutation failure is minimum voltage landing method, and the size between being landed by the landing and the minimum voltage occurred needed for commutation failure of comparing commutation voltage judges whether converter valve commutation failure occurs.But this method have ignored the impact of wave form distortion on commutation failure, to such an extent as to has relatively large deviation to the judgement of commutation failure result.

Different and conventional commutation failure situation; Multi-infeed HVDC transmission system there will be abnormal commutation failure phenomenon; and weigh abnormal commutation failure degree by means of only total harmonic distortion THD and can not intactly characterize, lack the method for the abnormal commutation failure order of severity (i.e. abnormal commutation failure possibility) of a kind of accurate characterization at present.

Summary of the invention

For solving the deficiency that prior art exists; the invention provides a kind of comprehensively wave form distortion and voltage magnitude and reduce by two kinds of factors; commutation failure situation can be reacted exactly, characterize the abnormal commutation failure decision method of Multi-infeed HVDC system of the abnormal commutation failure order of severity.

For achieving the above object, concrete scheme of the present invention is as follows:

The abnormal commutation failure decision method of a kind of Multi-infeed HVDC system, comprises the following steps,

S100: the fault type determining Multi-infeed HVDC system; Wherein, the fault type of Multi-infeed HVDC system comprises the fault such as single-phase earthing and three-phase shortcircuit;

S200: in determining step S100 when Multi-infeed HVDC system fault type,

Calculate the horizontal FL of corresponding failure _i(i=1,2......n):

${\mathrm{FL}}_i=\frac{U^2}{Z_{\mathrm{fault}}\·P_{\mathrm{dc}}}---\left(1\right)$

In above formula, Z _fault=ω L _fault, U is change of current bus rated voltage, Z _faultimpedance ground, L _faultit is equivalent grounded inductor value; P _dcbe straight-flow system transmission rated power, i represents the integer being greater than 1;

S300: calculate the horizontal FL of fault by step S200 _i(i=1,2......n), utilizes utility appliance to be captured in the horizontal FL of fault _ifar-end change of current busbar voltage Wave data under (i=1,2......n), and Fourier analysis is carried out to the waveform of one-period after fault, and the number percent that calculating voltage amplitude reduces;

Wherein, Fourier analysis obtains harmonic content, then obtain harmonic distortion reference quantity HDR,

$\mathrm{HDR}=\mathrm{DC}+\sqrt{{\left(H_2\right)}^2+{\left(H_3\right)}^2}---\left(2\right)$

Wherein, DC is DC component, H ₂for secondary harmonic component, H ₃for third harmonic content, because second harmonic and third harmonic content are far above other subharmonic, total harmonic distortion THD can be characterized with the two;

The number percent that voltage magnitude reduces is set to Δ V,

$\mathrm{\ΔV}=\frac{V_0-V_f}{V_0}\×100\%---\left(3\right)$

Wherein, V ₀change of current busbar voltage amplitude during for normally running, V _ffor change of current busbar voltage amplitude in the cycle after fault;

S400: when the Multi-infeed HVDC system fault type that step S100 determines, calculates the horizontal FL of fault _ivoltage corresponding to (i=1,2......n) and harmonic wave distortion reference amount, wherein voltage and harmonic wave distortion reference amount VHDR _i(i=1,2......n) represents:

VHDR _i＝ΔV+HDR (4)

S500: when the Multi-infeed HVDC system fault type that step S100 determines, the horizontal FL of fault when showing that abnormal commutation failure possibility is 0.5 by PSCAD emulation; Voltage again when to calculate abnormal commutation failure possibility be 0.5 to step S300 and step S400 and harmonic wave distortion reference amount VHDR ₀;

S600: when the Multi-infeed HVDC system fault type that step S100 determines, judge abnormal commutation failure possibility;

The standard of the abnormal commutation failure possibility of the judgement in described step S600 is:

Work as VHDR _i=VHDR ₀time, abnormal commutation failure possibility is 0.50; Namely VHDR is worked as _ivalue more than VHDR ₀time, there is abnormal commutation failure larger; Work as VHDR _ivalue lower than VHDR ₀time, there is abnormal commutation failure possibility less;

Wherein, when the probable value of abnormal commutation failure equals 1, commutation failure must be caused under the fault level of its correspondence; When the probable value of abnormal commutation failure is less than 0.5, define the fault horizontal FL corresponding with it _iunder (i=1,2......n), exception throw commutation failure may be less; When the probable value of abnormal commutation failure is greater than 0.5, define the fault horizontal FL corresponding with it _jthe possibility of lower exception throw commutation failure is comparatively large, wherein j=1,2 ... .n, j be greater than 1 integer.

Described abnormal commutation failure is: when Multi-infeed HVDC transmission system at the same time or in succession commutation failure, along with this earth fault level (FL) increases, far-end commutation failure possibility curve first increases rear minimizing, and then increases, and occurs abnormal occurrence;

Wherein, local commutation failure possibility refers to that the possibility of commutation failure occurs current conversion station 1, and far-end commutation failure possibility refers to that the possibility of commutation failure occurs current conversion station 2; This earth fault level refers to the fault degree of the change of current bus broken down.

The method for expressing of described abnormal commutation failure possibility size is, for each fault level (FL ₁, FL ₂... ..FL _n) get 100 points at a cycle equal intervals, calculate the point that abnormal commutation failure occurs and the ratio of always to count as at the horizontal FL of fault by PSCAD software _ithere is the probability of abnormal commutation failure under (i=1,2......n), be abnormal commutation failure possibility.

Beneficial effect of the present invention:

For commutation failure phenomenon abnormal in Multi-infeed HVDC system, solve the voltage and harmonic wave distortion reference amount of weighing abnormal commutation failure degree, inherently react the abnormal commutation failure phenomenon of multi-infeed HVDC system, for the research of multi-infeed systems commutation failure provides reference; And comprehensive wave form distortion and voltage magnitude reduce by two kinds of factors, can react commutation failure situation exactly.

Accompanying drawing explanation

Fig. 1 is that voltage magnitude reduces initiation commutation failure schematic diagram;

Fig. 2 is that voltage over zero displacement causes commutation failure schematic diagram;

Fig. 3 is that voltage waveform distortion causes commutation failure schematic diagram;

Fig. 4 is multi-feed high voltage direct current model schematic;

Fig. 5 is multi-infeed systems abnormal commutation failure phenomenon schematic diagram;

Fig. 6 (a) is at fault level 0.13 time A, B, C three-phase fault voltage oscillogram;

Fig. 6 (b) is at fault level 0.25 time A, B, C three-phase fault voltage oscillogram;

Fig. 6 (c) is at fault level 0.5 time A, B, C three-phase fault voltage oscillogram;

Fig. 7 (a) is at fault level 0.13 time B phase voltage harmonic content distribution plan;

Fig. 7 (b) is at fault level 0.25 time B phase voltage harmonic content distribution plan;

Fig. 7 (c) is at fault level 0.5 time B phase voltage harmonic content distribution plan;

Fig. 8 is process flow diagram of the present invention;

Fig. 9 is the relation schematic diagram of commutation failure possibility and VHDR.

Embodiment

The abnormal commutation failure decision method of a kind of Multi-infeed HVDC system, comprises the following steps,

S100: the fault type determining Multi-infeed HVDC system; Wherein, the fault type of Multi-infeed HVDC system comprises the fault such as single-phase earthing and three-phase shortcircuit;

S200: in determining step S100 when Multi-infeed HVDC system fault type,

Calculate the horizontal FL of corresponding failure _i(i=1,2......n):

${\mathrm{FL}}_i=\frac{U^2}{Z_{\mathrm{fault}}\·P_{\mathrm{dc}}}---\left(1\right)$

In above formula, Z _fault=ω L _fault, U is change of current bus rated voltage, Z _faultimpedance ground, L _faultit is equivalent grounded inductor value; P _dcbe straight-flow system transmission rated power, i represents the integer being greater than 1;

S300: calculate the horizontal FL of fault by step S200 _i(i=1,2......n), utilizes utility appliance to be captured in the horizontal FL of fault _ifar-end change of current busbar voltage Wave data under (i=1,2......n), and Fourier analysis is carried out to the waveform of one-period after fault, and the number percent that calculating voltage amplitude reduces;

Wherein, Fourier analysis obtains harmonic content, then obtain harmonic distortion reference quantity HDR,

$\mathrm{HDR}=\mathrm{DC}+\sqrt{{\left(H_2\right)}^2+{\left(H_3\right)}^2}---\left(2\right)$

Wherein, DC is DC component, H ₂for secondary harmonic component, H ₃for third harmonic content, because second harmonic and third harmonic content are far above other subharmonic, total harmonic distortion THD can be characterized with the two;

The number percent that voltage magnitude reduces is set to Δ V,

$\mathrm{\ΔV}=\frac{V_0-V_f}{V_0}\×100\%---\left(3\right)$

Wherein, V ₀change of current busbar voltage amplitude during for normally running, V _ffor change of current busbar voltage amplitude in the cycle after fault;

S400: when the Multi-infeed HVDC system fault type that step S100 determines, calculates the horizontal FL of fault _ivoltage corresponding to (i=1,2......n) and harmonic wave distortion reference amount, wherein voltage and harmonic wave distortion reference amount VHDR _i(i=1,2......n) represents:

VHDR _i＝ΔV+HDR (4)

S500: when the Multi-infeed HVDC system fault type that step S100 determines, the horizontal FL of fault when showing that abnormal commutation failure possibility is 0.5 by PSCAD emulation; Voltage again when to calculate abnormal commutation failure possibility be 0.5 to step S300 and step S400 and harmonic wave distortion reference amount VHDR ₀;

S600: when the Multi-infeed HVDC system fault type that step S100 determines, judge abnormal commutation failure possibility;

The standard of the abnormal commutation failure possibility of the judgement in described step S600 is:

Work as VHDR _i=VHDR ₀time, abnormal commutation failure possibility is 0.50; Namely VHDR is worked as _ivalue more than VHDR ₀time, there is abnormal commutation failure larger; Work as VHDR _ivalue lower than VHDR ₀time, there is abnormal commutation failure possibility less;

Wherein, when the probable value of abnormal commutation failure equals 1, commutation failure must be caused under the fault level of its correspondence; When the probable value of abnormal commutation failure is less than 0.5, define the fault horizontal FL corresponding with it _iunder (i=1,2......n), exception throw commutation failure may be less; When the probable value of abnormal commutation failure is greater than 0.5, define the fault horizontal FL corresponding with it _jthe possibility of lower exception throw commutation failure is comparatively large, wherein j=1,2 ... .n, j be greater than 1 integer.

Described abnormal commutation failure is: when Multi-infeed HVDC transmission system at the same time or in succession commutation failure, along with this earth fault level (FL) increases, far-end commutation failure possibility curve first increases rear minimizing, and then increases, and occurs abnormal occurrence;

Wherein, local commutation failure possibility refers to that the possibility of commutation failure occurs current conversion station 1, and far-end commutation failure possibility refers to that the possibility of commutation failure occurs current conversion station 2; This earth fault level refers to the fault degree of the change of current bus broken down.

The method for expressing of described abnormal commutation failure possibility size is, for each fault level (FL ₁, FL ₂... ..FL _n) get 100 points at a cycle equal intervals, calculate the point that abnormal commutation failure occurs and the ratio of always to count as at the horizontal FL of fault by PSCAD software _ithere is the probability of abnormal commutation failure under (i=1,2......n), be abnormal commutation failure possibility.

1 commutation failure principle

In converter valve commutation process, change of current busbar voltage waveform plays a crucial role, and stable voltage waveform makes commutation normally carry out, and the waveform changed during fault may cause the generation of commutation failure.Two reasons are generally considered: one is that voltage magnitude reduces, and two is voltage over zero displacements when analyzing commutation failure theoretically.And the impact of voltage over zero displacement on commutation failure is very little, so the mechanism of commutation failure depends primarily on the reduction of voltage magnitude under unbalanced fault.In fact, also having a kind of more complicated reason to cause commutation failure to occur, is exactly voltage waveform distortion.This reason is difficult to analyze from quantitative angle, because may also can with zero crossing displacement while voltage waveform distortion, and distortion be not occur by specific rule.

1.1 voltage magnitudes reduce

The commutation toothed area of commutation process voltage available-time represents, as shown in Figure 1, when normally running, inverter side Trigger Angle is α, and angle of overlap is μ, extinguish angle γ, and has α+μ+γ=π.Commutation area is S ₁, during fault, (here for three-phase fault) change of current busbar voltage amplitude reduces.At the certain (S of commutation toothed area ₁=S ₂), the IGBT group moment, when constant, commutation time lengthening, as Fig. 1 shows, angle of overlap μ was increased to μ ', and extinguish angle γ is reduced to γ '.

When commutation voltage is lower than certain critical value, γ < γ ₀, cause commutation failure, if critical pressure drop is Δ U, then have,

$\mathrm{\&Delta;U}=1-\frac{I_d^{\&prime;}}{I_d}\frac{(I_d/I_{\mathrm{dFL}})\&CenterDot;X_{\mathrm{cpu}}}{(I_d/I_{\mathrm{dFL}})\&CenterDot;X_{\mathrm{cpu}}+\cos {\mathrm{\&gamma;}}_0-\cos \mathrm{\&gamma;}}---\left(5\right)$

Wherein,

I _ddC current before-fault;

I ' _ddC current after-fault;

I _dFL-rated direct current;

X _cpu-converter power transformer impedance perunit value;

γ-critical extinction angle setting valve;

γ ₀-commutation failure critical extinction angle (generally getting 7 °-10 °).

1.2 voltage over zero displacements

When there is unbalanced fault, voltage magnitude decreases, and commutation voltage zero crossing in advance, because commutation toothed area S is constant, causes converter valve extinguish angle γ to reduce equally simultaneously, and commutation failure occurs, as shown in Figure 2,

Critical pressure drop is:

$\mathrm{\&Delta;U}=1-\frac{I_d^{\&prime;}}{I_d}\frac{(I_d/I_{\mathrm{dFL}})\&CenterDot;X_{\mathrm{cpu}}}{(I_d/I_{\mathrm{dFL}})\&CenterDot;X_{\mathrm{cpu}}+\cos {\mathrm{\&gamma;}}_0-\cos \mathrm{\&gamma;}}---\left(6\right)$

Compared with (5) formula, many displacement angle in (6) formula denominator system commutation failure critical pressure drop is reduced, adds commutation failure possibility to a certain extent.In view of system is all full-load run, now I under normal circumstances _d/ I _dFL=1.0.

Formula (5) and formula (6) can be reduced to:

$\mathrm{\&Delta;U}=1-\frac{I_d^{\&prime;}}{I_d}\frac{X_{\mathrm{cpu}}}{X_{\mathrm{cpu}}+\cos {\mathrm{\&gamma;}}_0-\cos \mathrm{\&gamma;}}---\left(7\right)$

1.3 voltage waveform distortion

When an error occurs, change of current busbar voltage is no longer stable sine wave, and distortion to a certain degree can occur.This distortion includes voltage magnitude and reduces and voltage over zero displacement.Zero crossing displacement is not the main cause causing commutation failure, many faults are while voltage waveform distortion, amplitude reduce, move after zero crossing, can reduce theoretically commutation failure occur possibility, but in fact to stop commutation failure there is no very large contribution.Fig. 3 gives the distorted waveform of change of current busbar voltage,

After fault, voltage waveform distorts as seen from Figure 3, and voltage magnitude reduces, and zero crossing is subjected to displacement.Because voltage waveform distortion is random, formula (5) and (6) no longer applicable, need to judge from the angle of harmonic wave.

Embodiment

The model of many feed-ins shown in Fig. 4 schematic drawing, two straight-flow systems are CIGRE master pattern in PSCAD, direct current transmission power P _dcfor 1000MW, DC voltage is 500kV; Receiving end AC system electric pressure is 230kV.Y1, y2 are two Inverter Station, Z _f1and Z _f2the filter impedance of Inverter Station change of current bus bar side, Z _s1and Z _s2two AC system equiva lent impedances respectively, Z _tieit is mutual impedance between two AC system.Straight-flow system 1 change of current bus generation earth fault, there is commutation failure in current conversion station y1, when fault level exceedes certain value, current conversion station y2 can be caused also commutation failure to occur.It is relevant whether current conversion station y2 can there is the commutation failure type that is not only out of order with straight-flow system 1 change of current bus, also with its to connect AC system intensity relevant.

Under many feed-ins model as described in Figure 4, get three kinds of typical fault levels, 0.13,0.25,0.5 time inverter side change of current bus-bar fault voltage waveform:

Far-end transverter A, B, C three-phase voltage waveform when thick dot-and-dash line is normal operation in Fig. 6, far-end change of current bus three-phase voltage waveform when thin dashed line is this earth fault.As can be seen from Fig. 6 (a), under minor failure, when this earth fault level is 0.13, false voltage waveforms amplitude reduces and distort (B phase is the most obvious), can know that commutation failure possibility is now about 0.8 from Fig. 5.When this earth fault level is increased to 0.25, false voltage waveform is with normally to run waveform basically identical, and now commutation failure possibility is about 0.4, and the possibility of generation commutation failure is smaller.Under catastrophic failure, when this earth fault level is 0.5, there is obviously distortion in voltage waveform, and B phase amplitude seriously reduces, and now the possibility of far-end commutation failure is 1.0.Can know that minor failure and catastrophic failure all can cause voltage waveform to distort, and cause far-end transverter commutation failure.

Get far-end change of current bus B phase voltage waveform data under three kinds of typical fault levels respectively, Fourier analysis is carried out to the waveform of one-period after fault and obtains harmonic content distribution as shown in Figure 7.Obtain voltage DC component and two, third harmonic content is as shown in table 1.

DC component and two, third harmonic content under table 1 three kinds of fault levels

After fault, in the cycle, B phase voltage peak value is respectively 170.4kV, 166.7kV and 150.7kV as shown in Figure 7, total harmonic distortion is respectively 20.86%, 10.66% and 28.44%, obviously along with the increase of fault level, voltage magnitude reduces gradually, there is ANOMALOUS VARIATIONS in harmonic content, especially shows in low-order harmonic and DC component.As shown in Table 1, time minor failure (FL=0.13), DC component accounts for 3.373% of first-harmonic content, and is only 0.15% under 0.25 fault level, simultaneously second harmonic and third harmonic total content the former also far more than the latter.Larger DC component and harmonic content hinder the normal commutation of converter valve, can cause the generation of commutation failure.Time catastrophic failure (FL=0.5), although DC component is less, harmonic content is very large, simultaneously voltage magnitude degradation, and now commutation failure possibility is 1.

Increasing by three kinds of fault levels, is 0,0.3,0.4 respectively.Analyze voltage magnitude, total harmonic distortion, DC component and harmonic content under all fault levels, number percent that voltage magnitude reduces and commutation failure possibility, result is as shown in table 2 and table 3.

Voltage magnitude and total harmonic distortion under each fault level of table 2

DC component and two, third harmonic content and Δ V under each fault level of table 3

Substantially become positive correlation from the table 2 abnormal commutation failure phenomenon that can visually see with voltage total harmonic distortion after fault, but can not intactly characterize this phenomenon by means of only THD, also need the reduction considering voltage magnitude.

The order of severity of abnormal commutation failure is into positively related with voltage with harmonic wave distortion reference amount VHDR, namely the value of VHDR is larger, abnormal commutation failure is more serious, and commutation failure possibility is the possibility that abnormal commutation failure occurs herein, is also the order of severity of abnormal commutation failure; Possibility is larger, and the order of severity is larger.

Under often kind of fault level, the numerical value of VHDR is as shown in table 3.

Show that shown in the curve map 9 that VHDR changes with commutation failure possibility, can find out: the value of VHDR is larger, the possibility of commutation failure is larger by last two column datas of table 3; As VHDR=19.82%, commutation failure possibility is 0.50; When the value of VHDR (is about 20%) more than 19.82%, the possibility that commutation failure occurs is larger; When VHDR value lower than 20% time, commutation failure possibility is less.

Can see that VHDR is consistent with the change of commutation failure possibility by table 3 and Fig. 9; and combine wave form distortion because of it and voltage magnitude reduces by two kinds of factors; more can react commutation failure situation exactly; the order of severity of abnormal commutation failure can be characterized, i.e. the possibility of abnormal commutation failure.

Claims (5)

1. the abnormal commutation failure decision method of Multi-infeed HVDC system, is characterized in that, comprise the following steps,

S100: the fault type determining Multi-infeed HVDC system; Wherein, the fault type of Multi-infeed HVDC system comprises the fault such as single-phase earthing and three-phase shortcircuit;

S200: in determining step S100 when Multi-infeed HVDC system fault type,

Calculate the horizontal FL of corresponding failure _i(i=1,2......n):

${\mathrm{FL}}_i=\frac{U^2}{Z_{\mathrm{fault}}\&CenterDot;P_{\mathrm{dc}}}$

In above formula, Z _fault=ω L _fault, U is change of current bus rated voltage, Z _faultimpedance ground, L _faultit is equivalent grounded inductor value; P _dcbe straight-flow system transmission rated power, i represents the integer being greater than 1;

S300: calculate the horizontal FL of fault by step S200 _i(i=1,2......n), utilizes utility appliance to be captured in the horizontal FL of fault _ifar-end change of current busbar voltage Wave data under (i=1,2......n), and Fourier analysis is carried out to harmonic distortion reference quantity HDR to the waveform of one-period after fault, and the percent delta V that calculating voltage amplitude reduces;

S400: when the Multi-infeed HVDC system fault type that step S100 determines, calculates the horizontal FL of fault _ivoltage corresponding to (i=1,2......n) and harmonic wave distortion reference amount, wherein voltage and harmonic wave distortion reference amount VHDR _i(i=1,2......n) represents:

VHDR _i＝ΔV+HDR

S500: when the Multi-infeed HVDC system fault type that step S100 determines, the horizontal FL of fault when showing that abnormal commutation failure possibility is 0.5 by PSCAD emulation; Voltage again when to calculate abnormal commutation failure possibility be 0.5 to step S300 and step S400 and harmonic wave distortion reference amount VHDR ₀;

S600: when the Multi-infeed HVDC system fault type that step S100 determines, judge abnormal commutation failure possibility.

2. the abnormal commutation failure decision method of the Multi-infeed HVDC system as described in right 1, it is characterized in that, the harmonic distortion reference quantity HDR that in described step S300, Fourier analysis obtains is,

$\mathrm{HDR}=\mathrm{DC}+\sqrt{{\left(H_2\right)}^2+{\left(H_3\right)}^2}$

Wherein, DC is DC component, H ₂for secondary harmonic component, H ₃for third harmonic content, because second harmonic and third harmonic content are far above other subharmonic, total harmonic distortion THD can be characterized with the two;

The percent delta V that voltage magnitude reduces is:

$\mathrm{\&Delta;V}=\frac{V_0-V_f}{V_0}\&times;100\%$

Wherein, V ₀change of current busbar voltage amplitude during for normally running, V _ffor change of current busbar voltage amplitude in the cycle after fault.

3. the abnormal commutation failure decision method of the Multi-infeed HVDC system as described in right 1, is characterized in that, the standard of the abnormal commutation failure possibility of the judgement in described step S600 is:

Work as VHDR _i=VHDR ₀time, abnormal commutation failure possibility is 0.50; Namely VHDR is worked as _ivalue more than VHDR ₀time, there is abnormal commutation failure larger; Work as VHDR _ivalue lower than VHDR ₀time, there is abnormal commutation failure possibility less;

Wherein, when the probable value of abnormal commutation failure equals 1, commutation failure must be caused under the fault level of its correspondence; When the probable value of abnormal commutation failure is less than 0.5, define the fault horizontal FL corresponding with it _iunder (i=1,2......n), exception throw commutation failure may be less; When the probable value of abnormal commutation failure is greater than 0.5, define the fault horizontal FL corresponding with it _jthe possibility of lower exception throw commutation failure is comparatively large, wherein j=1, and 2 ... .n, j be greater than 1 integer.

4. the abnormal commutation failure decision method of the Multi-infeed HVDC system as described in right 1, it is characterized in that, described abnormal commutation failure is: when Multi-infeed HVDC transmission system at the same time or in succession commutation failure, along with this earth fault level (FL) increases, far-end commutation failure possibility curve first increases rear minimizing, and then increase, there is abnormal occurrence;

Wherein, local commutation failure possibility refers to that the possibility of commutation failure occurs current conversion station 1, and far-end commutation failure possibility refers to that the possibility of commutation failure occurs current conversion station 2; This earth fault level refers to the fault degree of the change of current bus broken down.

5. the abnormal commutation failure decision method of the Multi-infeed HVDC system as described in right 1, it is characterized in that, the method for expressing of described abnormal commutation failure possibility size is, for each fault level (FL ₁, FL ₂... ..FL _n) get 100 points at a cycle equal intervals, calculate the point that abnormal commutation failure occurs and the ratio of always to count as at the horizontal FL of fault by PSCAD software _ithere is the probability of abnormal commutation failure under (i=1,2......n), be abnormal commutation failure possibility.