CN111313386B

CN111313386B - Multi-terminal hybrid high-voltage direct-current line transient protection method and system

Info

Publication number: CN111313386B
Application number: CN202010221361.4A
Authority: CN
Inventors: 陈鑫全; 李海锋; 许灿雄; 梁远升; 王钢
Original assignee: South China University of Technology SCUT
Current assignee: South China University of Technology SCUT
Priority date: 2020-03-26
Filing date: 2020-03-26
Publication date: 2021-01-19
Anticipated expiration: 2040-03-26
Also published as: CN111313386A

Abstract

The invention belongs to the field of high-voltage direct-current line protection, and relates to a multi-terminal hybrid high-voltage direct-current line transient protection method, which comprises the following steps: measuring the polar line voltage and polar line current by using the protection measuring points, and calculating the voltage traveling waves of a line mode and a ground mode; performing preliminary discrimination, and starting protection when the criterion is met; identifying a forward fault and a reverse out-of-zone fault; if the fault is a positive fault, carrying out fault pole selection, otherwise, protecting against action; in the fault pole selection unit, judging whether the fault is a local pole fault or an opposite pole fault; if the local pole is in fault, identifying the fault in the area and positioning the fault line, otherwise, protecting the local pole from action; and identifying the internal fault and the external fault by using the time domain transient voltage ratio, positioning the fault line and performing corresponding protection action. The invention further provides a multi-terminal hybrid high-voltage direct-current line transient protection system. The invention solves the problem of insufficient fault line positioning and transition resistance tolerance capability of a multi-end hybrid high-voltage direct-current line.

Description

Multi-terminal hybrid high-voltage direct-current line transient protection method and system

Technical Field

The invention belongs to the field of high-voltage direct-current line protection, and relates to a multi-terminal hybrid high-voltage direct-current line transient protection method and system.

Background

With the high-speed development of power electronic technology, the multi-terminal high-voltage direct-current transmission technology draws extensive attention. Because the Line Commutated Converter (LCC) has the advantages of large transmission capacity, low manufacturing cost and the like, the LCC is widely applied to the traditional two-end direct-current transmission system. However, for a multi-drop dc transmission system, the LCC inversion station is at risk of a commutation failure. Compared with the LCC, the Modular Multilevel Converter (MMC) has the advantages of flexible control, Modular design and the like. However, in order to obtain the same transmission capacity as the LCC, the MMC requires higher construction costs. In order to meet the above challenges, a multi-terminal hybrid high-voltage direct-current transmission technology composed of LCC and MMC is a development trend of a future multi-terminal direct-current system.

It is well known that high voltage direct current line faults are the most common direct current faults in high voltage direct current transmission systems. Therefore, the protection of the dc line is essential to the stable and safe operation of the system. In a conventional double-ended dc transmission system, travelling wave protection is usually employed as line main protection. However, too high a transition resistance can seriously weaken the traveling wave characteristics, making the discrimination of faults inside and outside the zone difficult, and leading to protection failure. Therefore, the insufficient capability of tolerating the transition resistance is always a problem to be solved in the traveling wave/transient protection of the direct current line.

In recent years, multi-terminal flexible direct current systems have been rapidly developed. Unlike double-ended dc systems, multi-ended flexible dc systems prefer to use dc breakers to isolate the faulty line, so as to minimize the impact of the fault on system reliability. In order to ensure reliable opening of the circuit breaker, current limiting reactors are usually installed on both sides of the line. Therefore, the protection scheme for the multi-terminal flexible direct-current system at present mostly depends on the attenuation of the current-limiting reactor to fault traveling waves to discriminate internal and external faults and locate a fault line. This approach of using line boundary elements is similar to conventional double ended dc transmission systems. In addition, in order to extract fault traveling wave characteristics of the multi-end flexible direct current system, signal processing algorithms such as discrete wavelet transformation, mathematical morphology, convolutional neural networks and the like are often relied on. The algorithms have excellent performance and can accurately detect the arrival time and the abrupt change of the fault traveling wave. However, since most of these algorithms are too complex and have high requirements on protection devices, the practicability of the algorithms needs to be further verified. In actual engineering, the fault detection method based on the simple algorithm is often more valuable in application.

As one of the research hotspots of the high-voltage direct-current transmission technology, the multi-terminal hybrid high-voltage direct-current system is significantly different from the double-terminal direct-current transmission system and the multi-terminal flexible direct-current system. First, fault clearing of multi-terminal hybrid high voltage dc lines does not rely on high voltage dc circuit breakers, and generally employs a similar fault clearing scheme as conventional double-terminal dc transmission systems. Once a direct current fault occurs to a direct current line, the MMC inverter station blocks current injection at an alternating current side, then the LCC rectifier station controls phase shift to discharge the direct current line, and then the fault line is isolated by the fast switches at two sides of the line. Therefore, dc line protection must achieve reliable in-zone fault identification and faulty line location. However, the special structure of the T-zone in the multi-terminal hybrid hvdc line causes the fault traveling wave of different lines to be difficult to distinguish due to the lack of boundary elements between adjacent lines connected by the T-zone. In addition, the T area can attenuate and distort the refraction and reflection of fault traveling waves of the multi-end hybrid high-voltage direct-current line, so that the protection performance is reduced. Therefore, how to solve the influence of the T area on the fault traveling wave and accurately position the fault line becomes a key difficulty for protecting the multi-end hybrid high-voltage direct-current line.

The traditional traveling wave protection is directly adopted in the protection of the existing multi-end hybrid high-voltage direct-current line, and protection measuring points are arranged at two ends of each line, so that the problem that boundary elements are lacked at two sides of a T area is solved. However, the method involves cooperation among at least four measuring point information, the logic is complex, and the protection snap performance is influenced.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a transient protection method for a multi-terminal hybrid high-voltage direct-current line.

The invention further provides a multi-terminal hybrid high-voltage direct-current line transient protection system.

The transient protection method for the multi-terminal hybrid high-voltage direct-current line is realized by adopting the following technical scheme:

a transient protection method for a multi-terminal hybrid high-voltage direct-current line comprises the following steps:

measuring the polar line voltage and polar line current by using the protection measuring points, and calculating a linear mode voltage traveling wave and a ground mode voltage traveling wave;

performing primary judgment in a starting unit, and starting protection when the criterion is met;

identifying, in a fault direction identification unit, a forward fault and a reverse out-of-area fault; if the fault is a positive fault, carrying out fault pole selection, otherwise, protecting against action;

in the fault pole selection unit, judging whether the fault is a local pole fault or an opposite pole fault; if the local pole is in fault, identifying the fault in the area and positioning the fault line, otherwise, protecting the local pole from action;

in the area fault identification and fault line positioning unit, identifying an area fault and an area fault by using a time domain transient voltage ratio, and positioning a fault line;

and performing corresponding protection actions according to the processing results of the in-zone fault identification and fault line positioning units.

Preferably, the starting unit uses the change of the line mode voltage traveling wave as a protected starting unit, and specifically includes:

in the formula: u shape_refTo rated line voltage, k_vIs the coefficient of variation of voltage, t_s0The minimum sampling period of the data window, t is the time,

is a sampling period t_s0The lower line mode voltage traveling wave.

Preferably, the fault direction identifying unit can distinguish the fault direction according to the positive large value of the line current variation, and the construction criterion is as follows:

ΔI_max≥k_iI_ref

in the formula: delta I_maxThe maximum value of the line current variation is the maximum value of the fault current minus the steady-state current; i is_refFor rated line current, k_iIs the current variation coefficient.

Preferably, the fault pole selection unit utilizes the initial peak value U of the earth-mode voltage traveling wave_0maxFault pole selection is performed as follows:

k·U_0max＞k_vU_ref

in the formula: for positive electrode protection k-1, for negative electrode protection k-1; k is a radical of_vIs a voltage variation coefficient; u shape_refIs the rated line voltage.

5. The multi-end mix of claim 1The method for protecting the transient state of the high-voltage direct-current combined line is characterized in that the time domain transient state voltage ratio R_ΔUThe calculation method of (2) is as follows:

wherein the content of the first and second substances,

is a sampling period t_s1The lower time domain transient voltage index reflects the high-frequency component of the voltage traveling wave;

is a sampling period t_s2And the lower time domain transient voltage index reflects the component of the lower frequency band of the voltage traveling wave.

Preferably, the time domain transient voltage index calculation method is as follows:

in the formula (I), the compound is shown in the specification,

is the sampling period t_sThe time-domain transient voltage of the lower,

t∈[t_s,T_s]，T_sis the time length of the data window, U₁(t) is the line mode voltage travelling wave at time t, U₁(t-t_s) Is t-t_sLine mode voltage traveling wave at time.

Preferably, U₁The calculation method (t) is as follows:

in the formula of U_p(t) and U_n(t) the voltages of the positive electrodes at times tA traveling wave and a traveling wave of the cathode voltage at time t.

Preferably, in the in-area fault identification and fault line positioning unit, when a fault line is positioned, the fault line positioning is realized by additionally arranging a protection measuring point at the head end of an adjacent line; the method specifically comprises the following steps: setting a protection measuring point at the rectification side as a protection measuring point 1, and setting a protection measuring point at the head end of an adjacent line as a protection measuring point 2, wherein once the protection measuring point 2 identifies a fault of the line where the protection measuring point is located, a signal is sent to the protection measuring point 1 through inter-station communication, and if the protection measuring point 1 receives the signal, the fault line is the line where the protection measuring point 2 is located; if the protection measuring point 1 acts and no signal is received, the fault line is the line where the protection measuring point 1 is located.

The transient protection system for the multi-terminal hybrid high-voltage direct-current line is realized by adopting the following technical scheme:

a multi-terminal hybrid high voltage direct current line transient protection system, comprising:

the component calculation module is used for measuring the polar line voltage and polar line current by using the protection measuring points and calculating a linear mode voltage traveling wave and a ground mode voltage traveling wave;

the starting unit is used for carrying out preliminary discrimination and starting protection when the criterion is met;

a fault direction identifying unit for identifying a forward fault and a reverse out-of-area fault; if the fault is a positive fault, carrying out fault pole selection, otherwise, protecting against action;

the fault pole selection unit is used for judging whether the fault is a local pole fault or an opposite pole fault; if the local pole is in fault, identifying the fault in the area and positioning the fault line, otherwise, protecting the local pole from action;

the system comprises an in-zone fault identification and fault line positioning unit, a fault line identification and fault line positioning unit and a fault line identification and fault line positioning unit, wherein the in-zone fault identification and fault line positioning unit is used for identifying an in-zone fault and an out-zone fault by utilizing a time domain transient voltage ratio and positioning the fault line;

and the protection unit is used for carrying out corresponding protection actions according to the processing results of the in-zone fault identification and fault line positioning unit.

Preferably, the time domain transient voltage ratio R_ΔUThe calculation method of (2) is as follows:

wherein the content of the first and second substances,

Compared with the prior art, the invention has the following advantages and beneficial effects:

1. according to the invention, through carrying out mathematical analysis on the fault traveling wave characteristics of the multi-end mixed high-voltage direct-current line, time domain transient voltage ratio criteria of different sampling periods are constructed, the influence of transition resistance and a T area on the traveling wave characteristics can be weakened, and the internal and external fault identification is realized.

2. The invention provides a multi-terminal hybrid high-voltage direct-current line transient protection method based on single-terminal quantity protection matching by utilizing a time domain transient voltage ratio criterion, and realizes quick judgment of multi-terminal hybrid high-voltage direct-current line faults and accurate positioning of fault lines.

Drawings

Fig. 1 is a schematic diagram of a multi-terminal hybrid hvdc transmission system in accordance with an embodiment of the present invention;

FIG. 2 is a schematic diagram of an inter-zone and outer-zone fault analysis equivalent circuit in accordance with an embodiment of the present invention;

FIG. 3 shows different angular frequencies and different Rs in one embodiment of the present invention_{f_in}G_USchematic view, | schematic view;

FIG. 4 shows U at different transition resistances according to an embodiment of the present invention_{1_in}(j ω) and U_{ratio_in}A schematic diagram;

FIG. 5 shows different ω in one embodiment of the present invention₁And ω₂G below_UratioA schematic diagram;

FIG. 6 is a schematic diagram of an equivalent circuit for a T-zone fault analysis in accordance with an embodiment of the present invention;

FIG. 7 is a graph illustrating the amplitude-frequency curve of G (j ω) in accordance with an embodiment of the present invention;

FIG. 8 shows a graph of R in one embodiment of the present invention_B、L_BAnd C_BSchematic diagram of the effect on | G (j ω) |;

FIG. 9 is a schematic diagram of the intended protection ranges of

protection stations

1, 2 in one embodiment of the invention;

FIG. 10 is a schematic diagram of the cooperation of

protection stations

1, 2 in one embodiment of the present invention;

FIG. 11 is a flow chart of a transient protection method for a multi-terminal hybrid HVDC line according to an embodiment of the present invention;

FIG. 12 is a graph illustrating a comparison of time domain transient voltages under an out-of-zone fault condition in accordance with an embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to examples and drawings, but the present invention is not limited thereto.

The invention relates to a transient protection method for a multi-end mixed high-voltage direct-current line, which aims to solve the problems of inside and outside fault identification and fault line positioning of the multi-end mixed high-voltage direct-current line, and mainly utilizes traveling wave characteristics of inside and outside faults and T-area faults of the multi-end mixed high-voltage direct-current line to construct time domain transient voltage ratio criteria under different sampling periods so as to weaken the influence of the T-area and transition resistance and realize inside and outside fault identification; a multi-terminal hybrid high-voltage direct-current line transient protection method based on single-terminal quantity protection matching is provided by utilizing a time domain transient voltage ratio criterion, so that the rapid judgment of the multi-terminal hybrid high-voltage direct-current line fault and the accurate positioning of the fault line are realized.

Example 1:

a multi-terminal hybrid high-voltage direct-current transmission system based on a multi-terminal hybrid high-voltage direct-current line transient protection method is shown in fig. 1. Fig. 1 shows a typical multi-terminal hybrid hvdc system and the point of failure closely related to protection of the hvdc line (only the positive line is shown due to the bipolar symmetry). Wherein, LCC station is as the rectifier station, and two MMC stations are as the contravariant station. In fig. 1, smoothing reactors and current limiting reactors are used as boundary elements between the converter station and the line; f1, f2 and f3 are fault points of the head end, the middle end and the tail end of the direct current line AB respectively; f4, f5 and f6 are fault points of the head end, the middle end and the tail end of the direct current line BC respectively; f7, f8 and f9 are the valve-side out-of-zone points of failure of the boundary elements. Typically, a protection station 1 is installed at the outlet of the rectifier station to detect a dc fault and initiate a fault clearing process.

The transient protection method for the multi-terminal hybrid high-voltage direct-current line specifically comprises the following steps:

and S1, analyzing the characteristics of fault traveling waves inside and outside the multi-end hybrid high-voltage direct-current transmission line area.

Protection based on the initial traveling wave may provide the fastest action speed. Therefore, the present invention is studied for the initial traveling wave. In order to eliminate the influence of positive and negative direct current line coupling, the multi-terminal hybrid high-voltage direct current transmission line network can be decoupled into a ground mode network and a line mode network through Krenberg transformation, and the corresponding decoupling voltage traveling waves are as follows:

in the formula:

subscripts

1 and 0 denote line and ground modes, U, respectively₀Is a ground mode voltage traveling wave, U₁Is a line mode voltage traveling wave; u is the polar voltage traveling wave measured at the protection measuring point 1, and subscripts p and n represent the positive pole and the negative pole respectively.

Taking the in-zone fault point f6 and the out-of-zone fault point f8 as examples, fig. 2 shows an equivalent analysis circuit diagram under the in-zone and the out-of-zone faults. In the fault travelling wave analysis process, the MMC2 station is equivalent to an RLC circuit.

In FIG. 2, R_C、L_C、C_CAnd L_cl2Respectively an equivalent resistor, an inductor, a capacitor and a current-limiting reactor, U, of the MMC2 station_fSuperimposing the voltage source amplitude u for a fault_f1And u_f0Line mode voltage traveling waves and ground mode voltage traveling waves, R, at the fault point_{f_in}And R_{f_ex}Respectively an in-zone fault transition resistance and an out-of-zone fault transition resistance, Z_{cBC_1}And Z_{cBC_0}Are respectively a DC line BLine and ground mode impedance of C, U_{1_in}And U_{0_in}Respectively an in-zone fault line-mode voltage traveling wave and an in-zone fault ground-mode voltage traveling wave, U, at the C point_{1_ex}And U_{0_ex}And the traveling wave is the outside fault line mode voltage traveling wave and the outside fault ground mode voltage traveling wave at the point C. According to fig. 2(a), the traveling wave of the voltage of the in-zone fault line mode at point C is:

in the formula: z_{k_in}(jω)＝Z_{cBC_k}(jω)//(jωL_cl2+jωL_C+R_C+1jωC_C) (k is 0,1), ω is angular frequency, and j is an imaginary unit.

Similarly, according to fig. 2(b), the traveling wave in the out-of-band fault line mode at point C is:

in the formula: z_{k_ex}(jω)＝(Z_{cBC_k}(jω)+jωL_cl2)//(jωL_C+R_C+1/jωC_C) (k＝0,1)。

Will U_{1_ex}(j ω) and U_{1_in}(j ω) by making a ratio, one can obtain:

G_U(j ω) is the ratio of the traveling wave of the out-of-zone fault line mode voltage to the traveling wave of the in-zone fault line mode voltage, i.e. G_U。

As can be seen from equations (2) to (4), for the traveling wave of the low frequency band, | j ω L_cl2| tends to 0, the line mode voltage traveling wave under the fault inside and outside the area is approximately equal, | G_UI ≈ 1. And for traveling waves of high frequency band, | j ω L_cl2I is much larger than other impedances, let R_{f_in}＝R_{f_ex}Then | G_U|≈|Z_{cBC_1}/(Z_{cBC_1}+jωL_cl2)|<1, out of range fault at this timeLower U_{1_ex}Bounded element j ω L_cl2The attenuation is large. Therefore, the high-frequency component of the voltage traveling wave can be used for identifying faults inside and outside the area, which is also the main characteristic used by the traveling wave protection principle of the current direct current line.

To verify the above conclusions, the | G is adjusted without reference to the engineering parameters of the Quinula limeliophera_UQuantitative analysis was performed. Let L_cl2＝0.075H，L_C＝0.1167H，C_C＝0.3333mF，R_C＝0.2Ω，R_{f_ex}By substituting the frequency-dependent wave impedance parameter at 0 Ω, different angular frequencies and different R values can be obtained according to equations (2) to (4)_{f_in}G_UAs shown in fig. 3.

As can be seen from FIG. 3, when R is_{f_in}＝R _{f_ex}0 Ω, low frequency | G _U1, G at high frequency_U|<1, consistent with the above conclusions. Notably, the transition resistance R_{f_in}Will cause voltage traveling wave U_{1_in}Amplitude decay when R_{f_in}Greater than R_{f_ex}And | j ω L_cl2Will make | G |_U|>1, which means that with increasing transition resistance at in-zone fault, U_{1_in}Attenuation ratio of (3) to U at the time of out-of-range fault_{1_ex}The voltage traveling wave protection is more serious, so that the sensitivity of the existing voltage traveling wave protection under the high-resistance fault is insufficient.

For this purpose, it is not provided that two different angular frequencies are each ω₁And ω₂According to the formula (2), the ratio U of voltage traveling waves at different angular frequencies under the condition of the in-zone fault can be obtained_{ratio_in}As shown in formula (5):

in the formula (5), the wave impedance changes little in the middle and high frequency range and is not approximately constant, so that the voltage traveling wave ratio U of the fault in the region_{ratio_in}≈ω₂/ω₁And is not influenced by transition resistance. According to the formula (2), U under different transition resistances can be obtained by referring to engineering parameters of the Kunlullong_{1_in}(j ω) as shown in FIG. 4 (a). Get omega₂1000rad/s, according to formula(5) Can calculate the U under different transition resistances_{ratio_in}As shown in fig. 4 (b).

As can be seen in FIG. 4, U_{1_in}The attenuation of (j ω) by the transition resistance is more pronounced, in contrast to U_{ratio_in}Is hardly affected by the transition resistance. The results show that the effect of the transition resistance can be greatly attenuated by using the ratio of the voltage traveling waves at different angular frequencies.

Further, the U under the out-of-area fault_{ratio_ex}And U under intra-area fault_{ratio_in}For comparison, there are:

from the foregoing analysis, it can be seen that the higher the angular frequency, | G_UThe smaller the | is. Therefore, when ω is₂<ω₁When, there is | G_U(jω₁)|<|G_U(jω₂) L. Therefore | G_Uratio|<1, and with ω₁The higher the omega₂Smaller, | G_UratioThe smaller will be the fundamental reason for this is that the high frequency attenuation of the out-of-range fault travelling wave by the boundary element is more severe. To verify the conclusion, reference is made to the engineering parameters of Kunlilong, and R is taken_{f_ex}＝0Ω，R_{f_in}With equation (6), different ω can be obtained as 100 Ω₁And ω₂G below_UratioAs shown in fig. 5.

As can be seen from FIG. 5, at ω taken₁And ω₂Within range, | G_UratioThe minimum of | occurs at ω₁And ω₂When the phase difference is greatest, i.e. at ω₂＝10³rad/s,ω₁＝10⁵rad/s, obtained | G_UratioAnd | is minimal, which is consistent with theoretical analysis conclusions. Therefore, in a specific application, ω should be taken₁In the high frequency range, omega₂The method is in a medium-low frequency range, so that the identification of faults inside and outside a comparison area of voltage traveling waves under different angular frequencies is facilitated.

And S2, influence of the T area of the multi-end hybrid high-voltage direct-current transmission line on fault traveling waves.

Compared with the traditional direct current transmission line, the T area is the most obvious structural characteristic of the multi-end hybrid high-voltage direct current transmission line. Taking the fault point f4 at the T-zone as an example, the fault analysis equivalent circuit is shown in fig. 6 (taking a line-mode network as an example) according to peterson's law. In FIG. 6, R_B-L_B-C_BAnd L_cl1Respectively, an equivalent impedance and a current limiting reactor of the MMC1 inverter station. According to FIG. 6, U₁And u_f1Can be represented by a transfer function:

in the formula, Z_{cAB_1}(j ω) is the line mode wave impedance of the DC line AB, Z_{cBC_1}(j ω) is the line mode impedance of the dc link BC. Although the actual line wave impedance is a frequency-varying parameter, when the frequency is higher than a certain value, the variation is small. Approximating the wave impedance as a constant, i.e. Z_{cAB_1}(j ω) and Z_{cBC_1}(j ω) is represented by Z_{cAB_1}And Z_{cBC_1}The derivation can be further simplified for equation (7):

as can be seen from equation (8), G (j ω) has a minimum value at a certain angular frequency. The T region can be known to cause obvious attenuation to the voltage traveling wave, and the obvious band-stop characteristic exists. According to formula (7), referring to engineering parameters of Kunlilon, let L_B＝0.075H，C_B＝0.35mF，R_B＝0.1Ω，L_cl1The amplitude-frequency curve of G (j ω) can be calculated by substituting the frequency-dependent wave impedance parameter at 0.1H, as shown in fig. 7.

In fig. 7, the center frequency ω of the attenuation range_cIs 131.3rad/s, attenuation boundary frequency omega_hIs 2081 rad/s. When ω is<ω_hAnd the T region has obvious attenuation effect on the amplitude of the traveling wave. When ω is>ω_hAnd meanwhile, the T area has almost no influence on the amplitude of the traveling wave. Due to | Z_{cAB_1}|<|Z_{cBC_1}I, when ω>ω_hWhen, | G (j ω) | is approximately equal to0.91<1。

Further, according to the parameter range in the actual engineering, the parameter R_B、L_BAnd C_BThe effect on | G (j ω) | is shown in fig. 8(a), (b), and (c), respectively.

As can be seen in FIG. 8, with L_BIncrease of (d), decrease of attenuation range, ω_hAnd decreases. With R_BThe frequency characteristic is hardly changed. With C_BIncrease of (a), omega_hRemains unchanged, but the attenuation range increases, ω_cAnd decreases. It can be seen that even under different parameters, the band-stop characteristic of the T region is always present, and the influence of the parameters on the band-stop characteristic is small.

And S3, constructing time domain transient voltage ratios under different sampling periods.

According to the analysis of S1 and S2, the frequency characteristics of the voltage traveling wave can be used for constructing the multi-terminal hybrid high-voltage direct-current line fault identification characteristic quantity. Generally, the frequency characteristics of the voltage traveling wave can be reflected by the sampling period of the data window. According to omega_s＝2π/t_sWhen the sampling period is small, the measured voltage traveling wave frequency is high. Thus, the time-domain transient voltage can be constructed using the line-mode voltage traveling wave as follows:

in the formula (I), the compound is shown in the specification,

is the sampling period t_sThe time-domain transient voltage of the lower,

Accordingly, the sampling period t can be calculated_sTime domain transient voltage index, thereby approximately measuring the frequency component of traveling wave, have：

In the formula (I), the compound is shown in the specification,

is a sampling period t_sAnd (5) time domain transient voltage index.

Because the attenuation of the transition resistance can be effectively weakened by adopting the ratio of the voltage traveling waves at different angular frequencies, the time domain transient voltage ratio at different sampling periods is constructed as follows:

in the formula (I), the compound is shown in the specification,

high frequency components of the voltage traveling wave are reflected, thereby being beneficial to identifying faults inside and outside the area. Because the difference of low frequencies of fault traveling waves inside and outside the area is small,

reflecting the components of the lower frequency band of the voltage travelling wave. When ω is>ω_hThe T region has little influence on the amplitude of the voltage traveling wave, so that omega_s1＝2π/t_s1And ω_s2＝2π/t_s2Should be greater than omega_h. To this end, R_ΔUThe attenuation of high resistance is weakened, the band elimination characteristic of a T area is avoided, and the in-area and out-area fault identification of the multi-end mixed high-voltage direct-current line is realized.

And S4, constructing a transient protection method of the multi-terminal mixed high-voltage direct-current line.

Starting a unit:

because the propagation velocity of the line mode voltage traveling wave is higher than that of the ground mode voltage traveling wave, the change of the line mode voltage traveling wave can be used as a starting unit for protection, and the method specifically comprises the following steps:

in the formula: u shape_refTo rated line voltage, k_vIs the coefficient of variation of voltage, t_s0The minimum sampling period of the data window, t is time,

is a sampling period t_s0The lower line mode voltage traveling wave.

A fault pole selection unit:

due to the fact that under the negative pole fault, the interpolar fault and the positive pole fault, the polarity of the first peak value of the ground mode voltage traveling wave is positive, zero and negative respectively. Therefore, the initial peak value U of the earth mode voltage traveling wave can be utilized_0maxFault pole selection is performed as follows:

k·U_0max＞k_vU_ref (13)

in the formula: the positive electrode protection k is-1, and the negative electrode protection k is 1.

③ fault direction identification unit:

because the current directions of the internal fault and the reverse external fault are opposite, the fault direction can be distinguished according to the large positive value of the line current variable quantity, and the construction criterion is as follows:

ΔI_max≥k_iI_ref (14)

Fourthly, an intra-area fault identification and fault line positioning unit:

in a multi-terminal hybrid high-voltage direct-current transmission line, a rapid isolating switch is adopted to isolate a fault line after discharge. Therefore, in-zone fault identification and faulty line location are required. However, due to the lack of boundary elements between adjacent lines, there is no significant difference in the fault traveling wave characteristics at the junctions of adjacent lines. It can be seen that it is difficult to locate a faulty line by means of the protection station 1 alone. Therefore, the head end of the DC line BC can be additionally provided withProtecting station 2. The expected protection ranges for

protection stations

1, 2 are shown in figure 9. In fig. 9, f9 is the reverse out-of-zone fault for protection station 1, and f1, f2, f3, f7 and f9 are the reverse out-of-zone faults for protection station 2. These failure points can be distinguished according to a failure direction identification unit. Using constructed time-domain transient voltage ratio R_ΔUThe criteria for protecting the measuring points 1 and 2 are as follows:

for protection station 1:

R_ΔU＞R_ΔUth1＝k_relmax(R_ΔUf7,R_ΔUf8) (15)

for protection station 2:

R_ΔU＞R_ΔUth2＝k_relR_ΔUf8 (16)

in the formula, R_ΔUth1And R_ΔUth2Respectively setting protection values of the

protection measuring points

1 and 2; k is a radical of_relIs a reliability factor; r_ΔUf7And R_ΔUf8Time domain transient voltage ratios for fault points f7 and f8, respectively.

On this basis, in order to locate the faulty line, as shown in fig. 10, when the protection station 2 recognizes a fault on the dc line BC, a signal is transmitted to the protection station 1 by inter-station communication. If the protection measuring point 1 receives the signal, the fault line is a direct current line BC, otherwise, the fault line is a direct current line AB.

It is noted that the constant value of protection station 1 is determined by the out-of-range faults f7 and f8 when R is equal to_ΔUf7Less than R_ΔUf8In time, the protection range of the measuring point 1 can cover the whole lengths of the direct current line AB and the direct current line BC. In this case, the cooperation of the protection points 1 and 2 does not affect the operation speed of the protection. However, when R is_ΔUf7Greater than R_ΔUf8In the meantime, the protection range of the protection measuring point 1 can only cover the direct current line AB and part of the direct current line BC, that is, the protection measuring point 1 has a protection dead zone on the direct current line BC. When the protection dead zone breaks down, the protection needs to protect the signal of the measuring point 2 to act.

Therefore, the whole multi-end hybrid high-voltage direct-current line network can be effectively protected, a fault line can be positioned, and the influence of a protection dead zone is avoided by matching the

protection measuring points

1 and 2.

Protecting a subsequent fault clearing scheme:

further, the action result of protection will affect the system recovery in the fault clearing scheme, which is as follows:

and when the direct current line AB has a fault, the protection acts. And then the MMC station blocks the injection of alternating current, and the LCC rectifier station enables the current of the line to be reduced to zero through phase-shifting control, so that the fault line AB is isolated through a quick switch. Then, the MMC station is restarted and the control strategy is adjusted (one station is rectification and one station is inversion). And finally, the system enters an MMC-MMC two-end operation mode.

When the direct current line BC is in fault, the protection acts, and the protection measuring point 1 receives a signal of the protection measuring point 2. Similarly, the discharged fault line BC is isolated by a fast switch. Then, the MMC stations at the LCC station and the T zone are restarted, and the control strategy is adjusted. And finally, the system enters an LCC-MMC two-end operation mode.

To this end, the present invention provides a transient protection method for a multi-terminal hybrid high-voltage dc line, which is implemented based on a single-terminal protection, and a specific process is shown in fig. 11, including:

(1) and measuring the polar line voltage and polar line current by using the protection measuring points, and calculating the linear mode voltage traveling wave and the ground mode voltage traveling wave.

(2) And performing primary judgment in the starting unit, and starting protection when the criterion is met.

(3) In the fault direction identifying unit, a forward fault and a reverse out-of-area fault are identified. If the fault is a forward fault, the next step is carried out, otherwise, the protection does not act.

(4) In the fault pole selection unit, whether the fault is the local pole fault or the opposite pole fault is judged. If the local pole is in fault, the next step is carried out, otherwise, the local pole does not act.

(5) Using time domain transient voltage ratio R_ΔUAnd identifying the internal fault and the external fault and positioning the fault line. And according to the processing result, performing corresponding actions by protection.

The invention is further illustrated by a specific simulation example.

According to the system topology structure shown in the figure 1, a PSCAD/EMTDC model of the hybrid three-terminal direct-current power transmission system is built for simulation test by referring to engineering parameters of the Kunlilon. The DC link AB is 932km and the DC link BC is 557 km. The model parameters are shown in table 1.

TABLE 1 model parameters for hybrid three-terminal DC transmission systems

Parameter(s)	LCC station	MMC1 station	MMC2 station
				Rated power	8000MW	3000MW	5000MW
Rated direct current	5000A	1875A	3125A
				Rated DC voltage	±800kV	±800kV	±800kV
Control mode	Constant current control	Constant active power control	Constant DC voltage control
				Impedance of boundary element	0.15H	0.1H	0.075H

The time length of the data window is set to be 5ms, the sampling frequency is 10kHz, and the minimum sampling period is 100 mus. The specific values of the protection parameters are shown in table 2.

TABLE 2 protection parameters

Parameter(s)

k_rel

k_v,k_i

t_s1

t_s2

R_ΔUth1

R_ΔUth2

Value taking

1.2

0.08

100μs

700μs

0.26

0.42

From the PSCAD simulation results, it can be seen from FIG. 12 that when t is reached_s1<t_s2Corresponding to ω_s1>ω_s2In case of internal or external faults

The difference is relatively large, and the difference is relatively large,

with less difference, and therefore using the time-domain transient voltage ratio

The internal and external faults can be effectively discriminated and are consistent with the previous analysis conclusion.

A protection algorithm is written on an MATLAB platform, anode fault simulation data of the established PSCAD model is imported, and the obtained protection action results are shown in tables 3 and 4. The fault distance in the table is a column, the letter is the label of the fault line, and the number is the distance between the fault point and the head end of the line where the fault point is located; p is the positive line.

Table 3 protective test Point 1 action results

Table 4 protective station 2 action results

As can be seen from tables 3 and 4, the

protection measuring points

1 and 2 correctly act under different fault conditions, and the accuracy and the practicability of the positioning of the starting unit, the fault pole selection unit and the fault line are further verified. The simulation result shows that R_ΔUThe influence of the transition resistance can be weakened, and the protection sensitivity can be improved.

TABLE 5 action results of protection test point 1 under out-of-area fault

Out-of-range fault point	R_ΔU	ΔI_max/kA	Fault pole	Action results
					LCC station valve side failure f9	/	0.007	p	Do not act
MMC1 station valve side fault f7	0.1955	1.785	p	Do not act
					MMC2 station valve side fault f8	0.2169	0.763	p	Do not act

Table 5 shows that the proposed protection does not malfunction under an out-of-range fault. As can be seen from Table 5, R_ΔUf7Is less thanR_ΔUf8. Therefore, the protection range of the protection measuring point 1 can cover the whole lengths of the direct current line AB and the direct current line BC, and the action speed of protection is not influenced by the signal transmission time of the protection measuring point 2.

Example 2:

the present embodiment provides a multi-terminal hybrid high-voltage direct-current line transient protection system, the system includes:

Wherein, the time domain transient voltage ratio R_ΔUThe calculation method of (2) is as follows:

wherein the content of the first and second substances,

The method solves the problem of insufficient capacity of fault line positioning and transition resistance tolerance of the multi-end hybrid high-voltage direct-current line, and is suitable for in-region and out-region fault identification and fault line positioning of the multi-end hybrid high-voltage direct-current line.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.

Claims

1. A transient protection method for a multi-terminal hybrid high-voltage direct-current line is characterized by comprising the following steps:

according to the processing results of the in-zone fault identification and fault line positioning unit, performing corresponding protection actions;

wherein: time domain transient voltage ratio R_ΔUThe calculation method of (2) is as follows:

wherein the content of the first and second substances,

is a sampling period t_s2The lower time domain transient voltage index reflects the component of the lower frequency band of the voltage traveling wave;

the time domain transient voltage index calculation method comprises the following steps:

in the formula (I), the compound is shown in the specification,

is the sampling period t_sThe time-domain transient voltage of the lower,

2. The multi-terminal hybrid high-voltage direct-current line transient protection method according to claim 1, wherein the starting unit uses a change of a line mode voltage traveling wave as a starting unit for protection, and specifically comprises:

is a sampling period t_s0The lower line mode voltage traveling wave.

3. The multi-terminal hybrid high-voltage direct-current line transient protection method according to claim 1, wherein the fault direction identification unit is configured to distinguish the fault direction according to a positive large value of the line current variation, and the construction criterion is as follows:

ΔI_max≥k_iI_ref

4. The multi-terminal hybrid high-voltage direct-current line transient protection method according to claim 1, wherein the fault pole selection unit utilizes a ground-mode voltage traveling wave first peak value U_0maxFault pole selection is performed as follows:

k·U_0max＞k_vU_ref

5. The multi-terminal hybrid high-voltage direct-current line transient protection method according to claim 1, wherein U is₁The calculation method (t) is as follows:

in the formula of U_p(t) and U_n(t) is a positive electrode voltage traveling wave at time t and a negative electrode voltage traveling wave at time t, respectively.

6. The multi-terminal hybrid high-voltage direct-current line transient protection method according to claim 1, characterized in that in the in-zone fault identification and fault line location unit, when locating a fault line, the fault line is located by additionally arranging a protection measurement point at the head end of an adjacent line; the method specifically comprises the following steps: setting a protection measuring point at the rectification side as a protection measuring point 1, and setting a protection measuring point at the head end of an adjacent line as a protection measuring point 2, wherein once the protection measuring point 2 identifies a fault of the line where the protection measuring point is located, a signal is sent to the protection measuring point 1 through inter-station communication, and if the protection measuring point 1 receives the signal, the fault line is the line where the protection measuring point 2 is located; if the protection measuring point 1 acts and no signal is received, the fault line is the line where the protection measuring point 1 is located.

7. A multi-terminal hybrid high-voltage direct-current line transient protection system, comprising:

the protection unit is used for carrying out corresponding protection actions according to the processing results of the in-zone fault identification and fault line positioning unit;

wherein the content of the first and second substances,

in the formula (I), the compound is shown in the specification,

is the sampling period t_sThe time-domain transient voltage of the lower,