CN103199511B - VSC-HVDC power transmission line pilot protection method based on model parameter identification - Google Patents

Abstract

The invention provides a VSC-HVDC power transmission line pilot protection method based on model parameter identification. According to the VSC-HVDC power transmission line pilot protection method based on the model parameter identification, the capacitance of a capacitor model with positive external fault equivalence is identified to be positive, and a current and voltage derivative correlation coefficient is one, the capacitance of a capacitor model with negative internal fault equivalence is identified to be negative, and a current and voltage derivative correlation coefficient is negative one. The fault in an internal area and the fault in an external area can be distinguished by judging whether the identified capacitance or the correlation coefficient is positive and negative of. Theoretical analysis and PSCAD simulation tests prove that the VSC-HVDC power transmission line pilot protection method based on the model parameter identification has the advantages that compensation of capacitance current is not needed, the principle is simple, realization is easy, influence of transition resistance, fault types, fault positions and control methods are avoided, and fault in the internal area and the fault in the external area can be distinguished fast and reliably under various working conditions. The VSC-HVDC power transmission line pilot protection method based on the model parameter identification is mainly used for pilot protection of a VSC-HVDC power transmission line.

Description

Based on the VSC-HVDC electric transmission line longitudinal protection method of model and parameters identification

Technical field

The present invention relates to a kind of relay protection method of power system, be specifically related to a kind of VSC-HVDC electric transmission line longitudinal protection method based on model and parameters identification.

Background technology

Voltage source converter type direct current (Voltage Source Converter HVDC, VSC-HVDC) transmission system adopts full-controlled switch device and high-frequency PWM modulation technique, is a kind of flexible, efficient direct current transmission and distribution technology.It has, and passive inverter, independently control are meritorious and idle, trend is reversed without the need to changing polarity of voltage, without the need to features such as a large amount of filtering and reactive power compensators, have broad application prospects in fields such as renewable energy source power, island with power, urban electricity supply, asynchronous Power System Interconnection, multi-terminal HVDC transmissions.

DC power transmission line is general longer, and failure rate is high, a set ofly perfects reliable relaying protection to ensureing that the safe operation of whole system has important meaning.But, current DC power transmission line relaying protection also exist theoretical incomplete, there is no blanket setting principle, only depend on simulation result and carry out the problem of adjusting etc., thus the reliability that result in DC line protection is not high.

In recent years, the protection philosophy based on Model Identification obtains attention and development.Document (Automation of Electric Systems; 2008; 32 (24): 30-34.) a kind of electric transmission line longitudinal protection method based on Model Identification is proposed; the method is equivalent to inductor models troubles inside the sample space; external area error is equivalent to capacitor model, distinguishes internal fault external fault by computation model error.

Main protection in existing VSC-HVDC circuit adopts traveling-wave protection mostly; traveling-wave protection requires high to sample frequency, protects insensitive under high transition resistance, and current differential protection is as the backup protection detecting high transition resistance; but be subject to line distribution capacitance impact, there is the drawback that responsiveness is slow.

Summary of the invention

The object of the invention is to propose a kind ofly require low, quick action, resistance to transition resistance ability are strong, reliability the is high VSC-HVDC electric transmission line longitudinal protection method based on model and parameters identification to sample frequency.

For achieving the above object, present invention employs following technical scheme:

This longitudinal protection method adopts Time-Domain algorithm; differential voltage and differential current is calculated according to the electric current at DC line two ends and voltage; described differential voltage refers to DC line both end voltage fault component sum; differential current refers to DC line two ends current failure component sum; if differential voltage and differential current meet the constraints of positive capacitor model; then be judged to be external area error, if differential voltage and differential current meet the constraints of negative capacitance model, be then judged to be troubles inside the sample space.

The electric current at described DC line two ends is to flow to circuit for positive direction from converter.

The concrete steps of described longitudinal protection method are as follows:

Step one, in current conversion station, with predetermined sampling rate, synchronized sampling is carried out to the direct current at DC line end points place and direct voltage, then by analog to digital converter, the sample direct voltage that obtains and direct current are converted to digital quantity, utilize difference algorithm to calculate corresponding fault component to digital quantity;

Step 2, carry out low-pass filtering treatment to the fault component obtained and obtain corresponding low frequency fault component Δ u, Δ i, then obtains differential voltage and differential current according to formula (4):

$\left\{\begin{array}{c}\mathrm{\Δ}{i}_{\mathrm{cd}}=\mathrm{\Δ}{i}_M+\mathrm{\Δ}{i}_N\\ \mathrm{\Δ}{u}_{\mathrm{cd}}=\mathrm{\Δ}{u}_M+\mathrm{\Δ}{u}_N\end{array}\right.---\left(4\right)$

, utilize two point value differential formulas to ask for the derivative value of differential voltage, then utilize least square method to identify electric capacity, or, ask for the coefficient correlation between the derivative value of differential voltage and differential current, in formula (4), Δ i _cdrepresent differential current, Δ u _cdrepresent differential voltage;

Step 3, compares the electric capacity identified or the coefficient correlation sought out with corresponding electric capacity setting value or coefficient correlation setting value, thus failure judgement type, if troubles inside the sample space, protection sends actuating signal fast.

The determination methods of described fault type is:

Determination methods one: if formula (10) is set up, be then troubles inside the sample space; Otherwise, be then external area error;

$C_j=\frac{\underset{i=1}{\overset{K}{\mathrm{\&Sigma;}}}\mathrm{\&Delta;}{i}_{\mathrm{cd}}\left(i\right)*\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}}\left(i\right)}{\underset{i=1}{\overset{K}{\mathrm{\&Sigma;}}}\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}}{\left(i\right)}^2}<C_{\mathrm{set}}---\left(10\right)$

In formula (10), K is the sampled point number in 5ms, C _jfor identifying the capacitance obtained, C _setfor the electric capacity setting value of setting, electric capacity setting value is generally taken as-800 to 0;

Determination methods two:

If formula (11) is set up, be then troubles inside the sample space, otherwise, be then external area error;

$\mathrm{\&rho;}(\mathrm{\&Delta;}{i}_{\mathrm{cd}},\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}})<{\mathrm{\&rho;}}_{\mathrm{set}}---\left(11\right)$

In formula (11): ρ _setfor the coefficient correlation setting value of setting, coefficient correlation setting value is generally taken as 0, and data window is taken as 5ms.

Beneficial effect of the present invention is:

The present invention is on the basis of Model Identification thought, in conjunction with the feature of VSC-HVDC two ends bulky capacitor in parallel, by differentiating the positive and negative of the capacitance that identifies or electric current and voltage derivative coefficient correlation, can effectively distinguish in district, external area error, overcome in traditional traveling-wave protection to sample frequency require high, high transition resistance is insensitive, be subject to external interference impact shortcoming, the method is carried out in the time domain, desired data window is short, quick action and not by the impact of distributed capacitance, resistance to transition resistance ability is strong, there is absolute selectivity, the present invention can both be quick under various operating mode, sensitive, reliably distinguish polar curve access area internal fault and external area error, significantly improve the reliability of power supply.

Accompanying drawing explanation

Fig. 1 is the structure principle chart of VSC-HVDC transmission line; In Fig. 1: M is rectifier terminal (being called for short M end or M side), N is inversion end (being called for short N end or N side); u _mp, u _mnbe respectively M and hold the positive and negative electrode voltage surveyed; i _mp, i _mnbe respectively M and hold the positive and negative electrode electric current surveyed; u _np, u _nnfor N holds the positive and negative electrode voltage surveyed; i _np, i _nnfor N holds the positive and negative electrode electric current surveyed; G1, G2 are respectively the AC power of M end and N end; T1, T2 are respectively the converter transformer of M end and N end; Electric current and voltage reference direction as shown in Figure 1.

Fault component network figure when Fig. 2 is VSC-HVDC circuit external fault; In Fig. 2: uf is equivalent fault component voltage source; L, R, C are inductance, resistance and the electric capacity lumped parameter that circuit π model is corresponding; C _pfor the bulky capacitor in parallel at circuit two ends; R _sN, L _sNfor N holds converter equivalent resistance and inductance; Δ i _m', Δ i _n' be respectively the current failure component that circuit M holds and N holds;

The condenser network illustraton of model of equivalence when Fig. 3 is VSC-HVDC circuit external fault; In Fig. 3: Δ i _cd, Δ u _cdbe respectively corresponding differential current and differential voltage, C is the corresponding electric capacity lumped parameter of circuit π model.

Fault component complementary network when Fig. 4 is VSC-HVDC line-internal fault; In Fig. 4: u _ffor equivalent fault component voltage source; R _sM, L _sMbe respectively M and hold converter equivalent resistance and inductance, R _sN, L _sNn holds converter equivalent resistance and inductance respectively; C _pfor the bulky capacitor in parallel at circuit two ends; C is the electric capacity lumped parameter that circuit π model is corresponding; R _m, L _mfor fault point to hold with M between resistance lumped parameter corresponding to circuit π model and inductance lumped parameter; R _n, L _nfor fault point to hold with N between resistance lumped parameter corresponding to circuit π model and inductance lumped parameter; Δ i _sM, Δ i _sNbe respectively the current failure component flowing into M end and N end converter; Δ i _cM, Δ i _cNbe respectively the current failure component flowing into M end and N end shunt capacitance; The reference direction of electric current and voltage as shown in Figure 4.

The condenser network illustraton of model of equivalence when Fig. 5 is VSC-HVDC line-internal fault; In Fig. 5: Δ i _cd, Δ u _cdbe respectively corresponding differential current and differential voltage, C _pfor the bulky capacitor in parallel at circuit two ends.

Simulation result during positive pole span M end 30km place 300 Ω transition resistance fault in Tu6Wei district;

Simulation result during positive pole span M end 270km place metallic earthing fault in Tu7Wei district;

Simulation result when Fig. 8 is DC line generation metallic earthing fault outside M petiolarea;

Simulation result when Fig. 9 is generation metallicity three-phase ground short trouble in alternating current circuit outside N petiolarea.

Embodiment

Below in conjunction with accompanying drawing, the present invention will be further described.

VSC-HVDC transmission system is by VSC converting plant, and VSC Inverter Station and DC power transmission line three part are formed.Convert alternating current is direct current by converting plant, and transmission line is by the Inverter Station of DC power transmission to opposite end, and DC power conversion is alternating current by Inverter Station.Core content of the present invention is for VSC-HVDC transmission line provides the protection of fast and reliable.

Voltage source converter type DC power transmission line two ends are parallel with bulky capacitor, and in fault, occur moment, current failure component is bulky capacitor discharging current in parallel mainly, and system side shows as capacitance characteristic.Feature accordingly, the present invention proposes a kind of VSC-HVDC electric power line longitudinal coupling protection new principle based on model and parameters identification.External fault is equivalent to positive capacitor model by this principle, and the capacitance identified is that just electric current and voltage derivative coefficient correlation are 1; Internal fault is equivalent to negative capacitor model, and the capacitance identified is negative, and electric current and voltage derivative coefficient correlation are-1.By differentiating the positive and negative of the capacitance that identifies and coefficient correlation, can distinguish in district, external area error.Theory analysis and PSCAD emulation experiment show, this principle is without the need to building-out capacitor electric current, and principle is simple, be easy to realize, not by transition resistance in principle, fault type, the impact of abort situation and control mode, under various operating mode all can in the differentiation district of fast and reliable, external area error.The principle of the invention is mainly used in VSC-HVDC electric power line longitudinal coupling protection.

The present invention is a kind of VSC-HVDC electric power line longitudinal coupling protection new departure, and it is on the basis of Model Identification thought, in conjunction with the feature of VSC-HVDC two ends bulky capacitor in parallel, proposes a kind of DC power transmission line pilot protection new method utilizing model and parameters identification.The method, by differentiating the positive and negative of the capacitance that identifies or electric current and voltage derivative coefficient correlation, can be distinguished in district, external area error effectively, specifically comprises the following steps:

Step one, the mathematical principle of correlation analysis: correlation analysis is used for degree of correlation between description two variable or multiple variable, and wherein Linear correlative analysis is used to the linear correlation degree of expression two variable, and usual correlation coefficient ρ is as numerical indication.Known by document (protecting electrical power system and control, 2008,36 (13): 16-20), for two energy types variable x (t), y (t), its coefficient correlation can be expressed as:

${\mathrm{\&rho;}}_{\mathrm{xy}}=\frac{{\&Integral;}_{-\&infin;}^{\&infin;}x\left(t\right)y\left(t\right)\mathrm{dt}}{\sqrt{{\&Integral;}_{-\&infin;}^{\&infin;}{x}^2\left(t\right)\mathrm{dt}}\sqrt{{\&Integral;}_{-\&infin;}^{\&infin;}{y}^2\left(t\right)\mathrm{dt}}}---\left(1\right)$

After discretization:

${\mathrm{\&rho;}}_{\mathrm{xy}}=\frac{\underset{k=1}{\overset{N}{\mathrm{\&Sigma;}}}x\left(k\right)y\left(k\right)}{\sqrt{\underset{k=1}{\overset{N}{\mathrm{\&Sigma;}}}{x}^2\left(k\right)\underset{k=1}{\overset{N}{\mathrm{\&Sigma;}}}{y}^2\left(k\right)}}---\left(2\right)$

Correlation coefficient ρ is nondimensional numerical value, and-1≤ρ≤1, if ρ=1, then there is positive linear relationships between two amounts; If ρ=-1, then there is negative linear relationship between two amounts; If ρ=0, be then not related to completely between two amounts.

Step 2, in current conversion station, with predetermined sampling rate, synchronized sampling is carried out to the direct current at the end points place of DC line, direct voltage, and by modulus converter A/D, gathered direct voltage and direct current are converted to digital quantity at local terminal, then utilize difference algorithm calculating voltage failure of the current component.

Step 3, low-pass filtering treatment is carried out to the fault component obtained and obtains corresponding low frequency fault component Δ u, Δ i, then differential voltage and differential current is calculated, two point value differential formulas are utilized to ask for the derivative value of differential voltage, then least square method is utilized to identify electric capacity, or, ask for the coefficient correlation between the derivative value of differential voltage and differential current.

External fault is held for M, its fault component network as shown in Figure 2, circuit adopts π model equivalent circuit, converter show as perception (Tang Guangfu. based on the high voltage dc transmission technology [M] of voltage source converter. Beijing: China Electric Power Publishing House, 2010:2-36.), resistance and inductance can be equivalent to.

If electric current positive direction be converter effluent to circuit, the circuit theory from basic:

$\left\{\begin{array}{c}\mathrm{\&Delta;}{i_M}^{\&prime;}=\mathrm{\&Delta;}{i}_M-C\frac{\mathrm{d\&Delta;}{u}_M}{\mathrm{dt}}\\ \mathrm{\&Delta;}{i_N}^{\&prime;}=\mathrm{\&Delta;}{i}_N-C\frac{\mathrm{d\&Delta;}{u}_N}{\mathrm{dt}}\\ \mathrm{\&Delta;}{i_M}^{\&prime;}+\mathrm{\&Delta;}{i_N}^{\&prime;}=0\end{array}\right.---\left(3\right)$

Definition differential current, differential voltage are as follows:

$\left\{\begin{array}{c}\mathrm{\&Delta;}{i}_{\mathrm{cd}}=\mathrm{\&Delta;}{i}_M+\mathrm{\&Delta;}{i}_N\\ \mathrm{\&Delta;}{u}_{\mathrm{cd}}=\mathrm{\&Delta;}{u}_M+\mathrm{\&Delta;}{u}_N\end{array}\right.---\left(4\right)$

Then during DC power transmission line external area error, fault component current differential equation is:

$\mathrm{\&Delta;}{i}_{\mathrm{cd}}=C\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}}---\left(5\right)$

Analysis mode (5), can be equivalent to positive condenser network model, as shown in Figure 3 by DC line external fault conditions.

From formula (5), the electric capacity identified is positive line mutual-ground capacitor value, and the coefficient correlation of differential current and differential voltage derivative is:

$\mathrm{\&rho;}(\mathrm{\&Delta;}{i}_{\mathrm{cd}},\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}})=1---\left(6\right)$

During DC power transmission line internal fault, fault additivity network as shown in Figure 4.

System failure moment, bulky capacitor in parallel can discharge to fault point, produces very large impulse current, shows as Δ i in fault component complementary network _cM, Δ i _cNrespectively much larger than Δ i _sM, Δ i _sN, therefore system side can be equivalent to bulky capacitor in parallel.

For Fig. 4, ignore converter shunting Δ i _sM, Δ i _sN, can be obtained by basic circuit theory:

$\left\{\begin{array}{c}-\mathrm{\&Delta;}{i}_M=\mathrm{\&Delta;}{i}_{\mathrm{cM}}=C_p\frac{\mathrm{d\&Delta;}{u}_M}{\mathrm{dt}}\\ -\mathrm{\&Delta;}{i}_N=\mathrm{\&Delta;}{i}_{\mathrm{cN}}=C_p\frac{\mathrm{d\&Delta;}{u}_N}{\mathrm{dt}}\end{array}\right.---\left(7\right)$

Definition according to differential current and differential voltage can obtain:

$-\mathrm{\&Delta;}{i}_{\mathrm{cd}}=C_p\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}}---\left(8\right)$

Analysis mode (8), can be equivalent to negative capacitance circuit model by line-internal malfunction, as shown in Figure 5.

From formula (8), the electric capacity identified is the negative value of circuit two ends bulky capacitor in parallel, and differential current and differential voltage derivative coefficient correlation are

$\mathrm{\&rho;}(\mathrm{\&Delta;}{i}_{\mathrm{cd}},\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}})=-1---\left(9\right)$

Step 4, the criterion of structure pilot protection, compares with setting value, thus failure judgement.

Electric capacity polarity criterion is as follows:

$C_j=\frac{\underset{i=1}{\overset{K}{\mathrm{\&Sigma;}}}\mathrm{\&Delta;}{i}_{\mathrm{cd}}\left(i\right)*\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}}\left(t\right)}{\underset{i=1}{\overset{K}{\mathrm{\&Sigma;}}}\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}}{\left(i\right)}^2}<C_{\mathrm{set}}---\left(10\right)$

In formula (10), K is the sampled point number in 5ms, and Cj identifies the capacitance obtained, C _setfor the electric capacity setting value of setting, electric capacity setting value is generally taken as-800 ~ 0;

Relevant function method criterion is as follows:

$\mathrm{\&rho;}(\mathrm{\&Delta;}{i}_{\mathrm{cd}},\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}})<{\mathrm{\&rho;}}_{\mathrm{set}}---\left(11\right)$

In formula: ρ _setfor the coefficient correlation setting value of setting, be generally taken as 0; Data window is taken as 5ms.

Two kinds of criterions do not have the difference of essence, can differentiate separately, also by or door differentiate, what meet (10) or (11) can be determined as troubles inside the sample space, otherwise is external area error.

The present invention carries out after only needing measuring junction electric parameters processing the electric capacity calculating and identify correspondence again, or calculates coefficient correlation and then judge internal fault external fault.Be summarised as following some:

(1) in current conversion station, with predetermined sampling rate, synchronized sampling is carried out to the direct current at the end points place of DC line, direct voltage, utilizes difference algorithm calculating voltage failure of the current component.

(2) carry out low-pass filtering treatment to the fault component of the electric current and voltage obtained, extract low frequency component Δ u, Δ i, recycling formula (4) (5) (7) (8) identifies electric capacity C in conjunction with least-squares algorithm.Or utilize the fault component of trying to achieve to try to achieve corresponding coefficient correlation in conjunction with difference algorithm according to formula (2) again.

(3) utilize formula (10) or (11) as criterion, these two kinds of criterions meet any one can judge it is troubles inside the sample space, sends fault actions signal fast.

Emulation experiment

As shown in Figure 1, power system capacity is 60MW to ± 60kV bipolar VSC-HVDC transmission system simulation model, and line length is 300km, carries out electromagnetic transient simulation with PSCAD, carries out data processing with MATLAB.

In simulation model, circuit adopts J.Marti variable element cable model frequently.Control system is the two close cycles tandem PI controller based on " Direct Current Control ", and M side adopts determines active power and determines Reactive Power Control strategy, and N side adopts the control strategy determined direct voltage and determine reactive power.The bulky capacitor in parallel of both positive and negative polarity is all taken as 1000 μ F, and data sampling rate is 10kHz.System breaks down when 2.5s, and trouble duration is 0.1s.The cut-off frequency of low pass filter is 150Hz.In order to take into account reliability and rapidity, data window is taken as 5ms, setting value C _setbe set as 0, ρ _setbe set as 0, provide partial simulation result below.

When in district, positive pole span M holds 30km place 300 Ω transition resistance fault, simulation result is as Fig. 6;

When in district, positive pole span M holds 270km place metallic earthing fault, simulation result is as Fig. 7;

Simulation result outside M petiolarea during DC line generation metallic earthing fault is as Fig. 8;

Simulation result outside N petiolarea during alternating current circuit generation metallicity three-phase ground short trouble is as Fig. 9.

Known at different lane place internal faults and external area error from above simulation result, the electric capacity obtained according to differential current and differential voltage identification and coefficient correlation positive and negative, reliably can distinguish district's internal and external fault fast.The troubles inside the sample space of fault pole is equivalent to the external area error perfecting pole, and the electric capacity calculated is positive line mutual-ground capacitor, and differential current and differential voltage derivative coefficient correlation are also 1, therefore energy split pole of the present invention action.

Claims (2)

1., based on a VSC-HVDC electric transmission line longitudinal protection method for model and parameters identification, it is characterized in that, comprise the following steps:

This longitudinal protection method adopts Time-Domain algorithm, differential voltage and differential current is calculated according to the electric current at DC line two ends and voltage, described differential voltage refers to DC line both end voltage fault component sum, differential current refers to DC line two ends current failure component sum, if differential voltage and differential current meet the constraints of positive capacitor model, then be judged to be external area error, if differential voltage and differential current meet the constraints of negative capacitance model, be then judged to be troubles inside the sample space;

The concrete steps of described longitudinal protection method are as follows:

Step one, in current conversion station, with predetermined sampling rate, synchronized sampling is carried out to the direct current at DC line end points place and direct voltage, then by analog to digital converter, the sample direct voltage that obtains and direct current are converted to digital quantity, utilize difference algorithm to calculate corresponding fault component to digital quantity;

Step 2, carry out low-pass filtering treatment to the fault component obtained and obtain corresponding low frequency fault component Δ u, Δ i, then obtains differential voltage and differential current according to formula (4):

$\left\{\begin{array}{c}\mathrm{\&Delta;}{i}_{\mathrm{cd}}=\mathrm{\&Delta;}{i}_M+\mathrm{\&Delta;}{i}_N\\ \mathrm{\&Delta;}{u}_{\mathrm{cd}}=\mathrm{\&Delta;}{u}_M+\mathrm{\&Delta;}{u}_N\end{array}\right.---\left(4\right)$

, utilize two point value differential formulas to ask for the derivative value of differential voltage, then utilize least square method to identify electric capacity, or, ask for the coefficient correlation between the derivative value of differential voltage and differential current, in formula (4), Δ i _cdrepresent differential current, Δ u _cdrepresent differential voltage;

Step 3, compares the electric capacity identified or the coefficient correlation sought out with corresponding electric capacity setting value or coefficient correlation setting value, thus failure judgement type, if troubles inside the sample space, protection sends actuating signal fast;

The determination methods of described fault type is:

Determination methods one: if formula (10) is set up, be then troubles inside the sample space; Otherwise, be then external area error;

$C_j=\frac{\underset{i=1}{\overset{K}{\mathrm{\&Sigma;}}}\mathrm{\&Delta;}{i}_{\mathrm{cd}}\left(i\right)*\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}}\left(\text{i}\right)}{\underset{i=1}{\overset{K}{\mathrm{\&Sigma;}}}\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}}{\left(i\right)}^2}<C_{\mathrm{set}}---\left(10\right)$

In formula (10), K is the sampled point number in 5ms, C _jfor identifying the capacitance obtained, C _setfor the electric capacity setting value of setting, electric capacity setting value is taken as-800 to 0;

Determination methods two:

If formula (11) is set up, be then troubles inside the sample space, otherwise, be then external area error;

$\mathrm{\&rho;}(\mathrm{\&Delta;}{i}_{\mathrm{cd}},\frac{\mathrm{d\&Delta;}{u}_{\mathrm{cd}}}{\mathrm{dt}})<{\mathrm{\&rho;}}_{\mathrm{set}}---\left(11\right)$

In formula (11): for the coefficient correlation of differential current and differential voltage derivative, ρ _setfor the coefficient correlation setting value of setting, coefficient correlation setting value is taken as 0, and data window is taken as 5ms.

2. a kind of VSC-HVDC electric transmission line longitudinal protection method based on model and parameters identification according to claim 1, is characterized in that: the electric current at described DC line two ends is to flow to circuit for positive direction from converter.