CN103199511A - 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

VSC-HVDC electric transmission line longitudinal protection method based on model parameter 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 parameter 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, be a kind of flexibly, direct current transmission and distribution technology efficiently.It has that passive inverter, independent control are meritorious and idle, trend is reversed and need not to change polarity of voltage, need not characteristics such as a large amount of filtering and reactive power compensator, is incorporated into the power networks at renewable energy power generation, fields such as isolated island power supply, urban electricity supply, asynchronous electrical network are interconnected, multiterminal direct current transportation have broad application prospects.

DC power transmission line is generally longer, the failure rate height, and a cover perfects reliable relaying protection has important meaning to the safe operation that guarantees whole system.Yet, DC power transmission line relaying protection at present exist theoretical incomplete, do not have the blanket principle of adjusting, only depend on simulation result problem such as adjust, thereby caused the reliability of DC line protection not high.

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

Traveling-wave protection is adopted in main protection in the existing VSC-HVDC circuit mostly; traveling-wave protection requires high to sample frequency, under the high transition resistance protection insensitive, current differential protection is as the backup protection that detects high transition resistance; but be subject to the line distribution capacitance influence, have the slow drawback of responsiveness.

Summary of the invention

The objective of the invention is to propose a kind of sample frequency is required low, quick action, anti-transition resistance ability is strong, reliability the is high VSC-HVDC electric transmission line longitudinal protection method based on model parameter identification.

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

This longitudinal protection method adopts Time-Domain algorithm; electric current and voltage according to the DC line two ends calculate differential voltage and differential current; 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 satisfy the constraints of positive capacitor model; then be judged to be external area error, if differential voltage and differential current satisfy the constraints of negative capacitance model, then be judged to be troubles inside the sample space.

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

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

Step 1, in current conversion station, direct current and direct voltage to DC line end points place carry out synchronized sampling with predetermined sampling rate, by analog to digital converter direct voltage and the direct current that sampling obtains is converted to digital quantity then, utilizes difference algorithm to calculate corresponding fault component to digital quantity;

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

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

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

Step 3 compares the electric capacity that identifies or the coefficient correlation that seeks out with corresponding electric capacity setting value or coefficient correlation setting value, thus the failure judgement type, and if troubles inside the sample space, actuating signal is sent in protection fast.

The determination methods of described fault type is:

Determination methods one: if formula (10) is set up, then be troubles inside the sample space; Otherwise, then be 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 the formula (10), K is the sampled point number in the 5ms, C _jBe the capacitance that identification obtains, C _SetBe the electric capacity setting value of setting, the electric capacity setting value generally is taken as-800 to 0;

Determination methods two:

If formula (11) is set up, and then is troubles inside the sample space, otherwise, then be 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 the formula (11): ρ _SetBe the coefficient correlation setting value of setting, the coefficient correlation setting value generally is 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; characteristics in conjunction with VSC-HVDC two ends and the United Nations General Assembly's electric capacity; the capacitance that identifies by differentiation or electric current and voltage derivative coefficient correlation positive and negative; can distinguish effectively in the district; external area error; overcome in traditional traveling-wave protection sample frequency is required high; high transition resistance is insensitive; be subject to external interference influence shortcoming; this method is carried out in time domain; the desired data window is short; quick action and be not subjected to the influence of distributed capacitance; anti-transition resistance ability is strong; have absolute selectivity, the present invention can both be quick under various operating modes; sensitive; distinguish polar curve access area internal fault and external area error reliably, significantly improved the reliability of power supply.

Description of drawings

Fig. 1 is the structure principle chart of VSC-HVDC transmission line; Among Fig. 1: M is rectifier terminal (being called for short M end or M side), and N is inversion end (being called for short N end or N side); u _Mp, u _MnBe respectively the positive and negative electrode voltage that the M end is surveyed; i _Mp, i _MnBe respectively the positive and negative electrode electric current that the M end is surveyed; u _Np, u _NnHold the positive and negative electrode voltage of surveying for N; i _Np, i _NnHold the positive and negative electrode electric current of surveying for N; 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; The electric current and voltage reference direction as shown in Figure 1.

Fault component network diagram when Fig. 2 is VSC-HVDC circuit external fault; Among Fig. 2: uf is equivalent fault component voltage source; L, R, C are inductance, resistance and the electric capacity lumped parameter of circuit π model correspondence; C _pAlso the United Nations General Assembly's electric capacity for the circuit two ends; R _SN, L _SNBe N end converter equivalent resistance and inductance; Δ i _M', Δ i _N' be respectively the current failure component that circuit M end and N hold;

The condenser network illustraton of model of equivalence when Fig. 3 is VSC-HVDC circuit external fault; Among 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; Among Fig. 4: u _fBe equivalent fault component voltage source; R _SM, L _SMBe respectively M end converter equivalent resistance and inductance, R _SN, L _SNN holds converter equivalent resistance and inductance respectively; C _pAlso the United Nations General Assembly's electric capacity for the circuit two ends; C is the electric capacity lumped parameter of circuit π model correspondence; R _M, L _MBe corresponding resistance lumped parameter and the inductance lumped parameter of circuit π model between fault point and the M end; R _N, L _NBe corresponding resistance lumped parameter and the inductance lumped parameter of circuit π model between fault point and the N end; Δ i _SM, Δ i _SNBe respectively the current failure component that flows into M end and N end converter; Δ i _CM, Δ i _CNBe respectively the current failure component that flows 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; Among Fig. 5: Δ i _Cd, Δ u _CdBe respectively corresponding differential current and differential voltage, C _pAlso the United Nations General Assembly's electric capacity for the circuit two ends.

Simulation result when Fig. 6 holds 30km place 300 Ω transition resistance faults for the interior positive pole span M in district;

Simulation result when Fig. 7 holds 270km place metallic earthing fault for the interior positive pole span M in district;

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

Simulation result when Fig. 9 is the outer alternating current circuit generation of N petiolarea metallicity three-phase ground short circuit fault.

Embodiment

The present invention will be further described below in conjunction with accompanying drawing.

The VSC-HVDC transmission system is by the VSC converting plant, and VSC Inverter Station and DC power transmission line three parts constitute.Converting plant is transformed to direct current with alternating current, and transmission line is transferred to the Inverter Station of opposite end with direct current, and Inverter Station is transformed to alternating current with direct current.Core content of the present invention is to provide rapid and reliable protection for the VSC-HVDC transmission line.

Voltage source converter type DC power transmission line two ends are parallel with big electric capacity, in fault take place moment, and the current failure component mainly is and the United Nations General Assembly's capacitor discharge electric current that system side shows as capacitance characteristic.Characteristics the present invention proposes a kind of VSC-HVDC electric power line longitudinal coupling protection new principle based on model parameter identification accordingly.This principle is positive capacitor model with the external fault equivalence, and the capacitance that identifies is for just, and electric current and voltage derivative coefficient correlation are 1; The internal fault equivalence is negative capacitor model, and the capacitance that identifies is for negative, and electric current and voltage derivative coefficient correlation are-1.The capacitance that identifies by differentiation and coefficient correlation positive and negative is in can distinguishing, external area error.Theory analysis and PSCAD emulation experiment show that this principle need not the building-out capacitor electric current, and principle is simple, be easy to realize, be not subjected to transition resistance on the principle, fault type, the influence of abort situation and control mode, all can rapid and reliable differentiation district under various operating modes in, external area error.The principle of the invention is mainly used in the 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 in conjunction with the characteristics of VSC-HVDC two ends and the United Nations General Assembly's electric capacity, has proposed a kind of DC power transmission line pilot protection new method of utilizing model parameter identification on the basis of Model Identification thought.The capacitance that this method identifies by differentiation or electric current and voltage derivative coefficient correlation positive and negative in can distinguishing effectively, external area error, specifically may further comprise the steps:

Step 1, the mathematical principle of correlation analysis: correlation analysis is used for describing degree of correlation between two variablees or a plurality of variable, and wherein Linear correlative analysis is to represent the linear correlation degree of two variablees, uses coefficient correlation ρ as the numerical value index usually.By document (protecting electrical power system and control, 2008,36 (13): 16-20) as can be known, for two energy type 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 the discretization:

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

Coefficient correlation ρ is nondimensional numerical value, and-1≤ρ≤1, if then there is the linear positive relation in ρ=1 between two amounts; If then there is negative linear relationship in ρ=-1 between two amounts; If ρ=0 then is not related between two amounts fully.

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

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

Holding external fault with M is example, 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.), can equivalence be resistance and inductance.

If the electric current positive direction be the converter effluent to circuit, by basic circuit theory as can be known:

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

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

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

Analysis mode (5) can be positive condenser network model with the equivalence of DC line external fault state, as shown in Figure 3.

By formula (5) as can be known, the electric capacity that identifies 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 the DC power transmission line internal fault, fault additivity network as shown in Figure 4.

System failure moment, big electric capacity in parallel can discharge to the fault point, produces very big impulse current, shows as Δ i in the fault component complementary network _CM, Δ i _CNRespectively much larger than Δ i _SM, Δ i _SNSo system side can equivalence be and the United Nations General Assembly's electric capacity.

For Fig. 4, ignore converter shunting Δ i _SM, Δ i _SN, can be got 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 get:

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

Analysis mode (8) can be the negative capacitance circuit model with the equivalence of line-internal malfunction, as shown in Figure 5.

By formula (8) as can be known, the electric capacity that identifies is the negative value of circuit two ends and the United Nations General Assembly's electric capacity, 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 the formula (10), K is the sampled point number in the 5ms, the capacitance that Cj obtains for identification, C _SetBe the electric capacity setting value of setting, the electric capacity setting value generally is taken as-800～0;

The 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 the formula: ρ _SetCoefficient correlation setting value for setting generally is taken as 0; Data window is taken as 5ms.

Two kinds of criterions do not have the difference of essence, can differentiate separately, can pass through or the door differentiation yet, satisfy can differentiating of (10) or (11) and are troubles inside the sample space, otherwise be external area error.

The present invention only needs to handle the electric capacity that correspondence is identified in calculating again after the measuring junction electric parameters, or calculates coefficient correlation and then judge internal fault external fault.Be summarised as following some:

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

(2) fault component to the electric current and voltage that obtains carries out low-pass filtering treatment, extracts low frequency component Δ u, Δ i, and recycling formula (4) (5) (7) (8) identifies capacitor C in conjunction with least-squares algorithm.Perhaps 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 satisfy anyly can judge it is troubles inside the sample space, sends the fault actions signal fast.

Emulation experiment

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

In the simulation model, circuit adopts J.Marti variable element cable model frequently.Control system is the two closed loop tandem PI controllers based on " Direct Current Control ", and the M side adopts decides active power and decide the Reactive Power Control strategy, and the N side adopts decides direct voltage and decide the control strategy of reactive power.Also the United Nations General Assembly's electric capacity of both positive and negative polarity all is 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 at 0, ρ _SetBe set at 0, provide the partial simulation result below.

Simulation result such as Fig. 6 when positive pole span M holds 30km place 300 Ω transition resistance faults in the district;

Simulation result such as Fig. 7 when positive pole span M holds 270km place metallic earthing fault in the district;

Simulation result such as Fig. 8 during the outer DC line generation metallic earthing fault of M petiolarea;

Simulation result such as Fig. 9 when metallicity three-phase ground short circuit fault takes place the outer alternating current circuit of N petiolarea.

As can be known at different lane place internal fault and external area error, the electric capacity that identification obtains according to differential current and differential voltage and coefficient correlation positive and negative can both reliably be distinguished district's internal and external fault fast from above simulation result.The troubles inside the sample space of the fault utmost point is equivalent to perfect the external area error of the utmost point, and the electric capacity that calculates is positive line mutual-ground capacitor, and differential current and differential voltage derivative coefficient correlation also are 1, so the present invention can the split pole action.

Claims (4)

1. the VSC-HVDC electric transmission line longitudinal protection method based on model parameter identification is characterized in that, may further comprise the steps:

This longitudinal protection method adopts Time-Domain algorithm; electric current and voltage according to the DC line two ends calculate differential voltage and differential current; 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 satisfy the constraints of positive capacitor model; then be judged to be external area error, if differential voltage and differential current satisfy the constraints of negative capacitance model, then be judged to be troubles inside the sample space.

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

3. according to the described a kind of VSC-HVDC electric transmission line longitudinal protection method based on model parameter identification of claim 1, it is characterized in that: the concrete steps of described longitudinal protection method are as follows:

Step 1, in current conversion station, direct current and direct voltage to DC line end points place carry out synchronized sampling with predetermined sampling rate, by analog to digital converter direct voltage and the direct current that sampling obtains is converted to digital quantity then, utilizes difference algorithm to calculate corresponding fault component to digital quantity;

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

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

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

Step 3 compares the electric capacity that identifies or the coefficient correlation that seeks out with corresponding electric capacity setting value or coefficient correlation setting value, thus the failure judgement type, and if troubles inside the sample space, actuating signal is sent in protection fast.

4. according to the described a kind of VSC-HVDC electric transmission line longitudinal protection method based on model parameter identification of claim 3, it is characterized in that: the determination methods of described fault type is:

Determination methods one: if formula (10) is set up, then be troubles inside the sample space; Otherwise, then be 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 the formula (10), K is the sampled point number in the 5ms, C _jBe the capacitance that identification obtains, C _SetBe the electric capacity setting value of setting, the electric capacity setting value generally is taken as-800 to 0;

Determination methods two:

If formula (11) is set up, and then is troubles inside the sample space, otherwise, then be 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 the formula (11): ρ _SetBe the coefficient correlation setting value of setting, the coefficient correlation setting value generally is taken as 0, and data window is taken as 5ms.

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