CN113131453B - Single-ended traveling wave protection method for flexible direct current transmission line - Google Patents

Abstract

The invention discloses a single-ended traveling wave protection method for flexible direct current transmission lines. The method is based on the waveform characteristics of three types of preceding traveling wave faults, namely, intra-area fault, out-of-area fault and lightning disturbance. Determine the fault time and timing starting point; if and only if the valley point is detected by the traveling wave before the first-mode fault within the setting time of the valley point, and the zero-crossing point cannot be detected by the traveling wave before the first-mode fault within the zero-crossing setting time, the protection Identified as a zone fault. The protection method is reliable in principle, simple in criterion, fast in action, and has strong resistance to transition resistance, lightning strike and noise interference.

Description

A single-ended traveling wave protection method for flexible DC transmission lines

technical field

The invention belongs to the technical field of control and protection of flexible direct current transmission systems, and in particular relates to a single-ended quantity protection method for flexible direct current transmission lines.

Background technique

The flexible DC transmission based on the voltage source converter (VSC) of the fully controlled switching device has the advantages of flexible control and high power quality. Effectively alleviate the intermittent and random impact of wind, light and other new energy power generation on the power grid. However, the limitation of the current capacity of the fully controlled device also puts forward higher requirements for the protection technology. The existing research believes that the protection needs to complete the fault identification within 3ms, and only the single-ended quantity protection is expected to meet the action speed requirements.

The principle of single-ended quantity protection of flexible HVDC transmission lines relies on the analysis of the fault traveling wave process. It is an effective way to construct the protection criterion to extract the fault information carried by the fault traveling wave using appropriate mathematical methods. The mainstream fault information extraction methods can be divided into frequency domain method and time domain method.

In the prior art, the frequency domain method is based on the blocking effect of line boundary elements such as current-limiting reactors on the high-frequency components of the fault traveling wave head, and uses mathematical methods such as wavelet transform and S transform to quantify the high-frequency information carried by the fault traveling wave. It has good resistance to transition resistance, but the high-frequency information carried by the lightning current wave head is similar to the short-circuit fault traveling wave in the area, and this type of protection is difficult to deal with lightning strike interference.

In the prior art, the time domain method uses numerical features or waveform features to extract fault information, and the time domain protection based on numerical features mainly uses differential or integral algorithms to extract fault features. Cope with high frequency interference. The time domain protection based on waveform characteristics is based on a complete fault loop analysis. Compared with the time domain protection method based on numerical characteristics, its principle is more complete and reliable, because the method has nothing to do with the transition resistance in theory, so this type of protection It has a good ability to withstand transition resistance, but the lightning strike interference is similar to the steepness of the fault traveling wave head of the short-circuit fault in the area, so that the above scheme may malfunction.

Therefore, in order to deal with the shortcomings of the existing technology, it is necessary to study a single-ended protection of flexible DC transmission lines that can withstand lightning strikes.

Purpose of invention

The purpose of the present invention is to deal with the deficiencies of the prior art, and to provide a single-ended quantity protection method for a flexible DC transmission line that is resistant to lightning strike interference, and specifically to provide a single-ended quantity protection method for a flexible DC transmission line. The method uses the valley time to eliminate out-of-area faults, and at the same time uses the zero-crossing time to eliminate lightning strike interference, and finally effectively identifies the short-circuit faults in the area. The method is reliable in principle, simple in criterion, and improves the ability of the single-ended traveling wave protection to withstand lightning strike interference.

SUMMARY OF THE INVENTION

The present invention provides a single-ended traveling wave protection method for flexible direct current transmission lines. The method is based on the forward traveling wave waveform characteristics of the flexible direct current transmission lines. The flexible direct current transmission lines are a kind of true bipolar two ends. A flexible DC transmission system comprising two parallel connected transmission lines, one of which is an overhead line and the other is also an overhead line;

The protection method includes the following steps:

Step 1. Detect the waveforms of the three types of pre-fault traveling waves of the flexible DC transmission line, namely, the intra-area fault, the out-of-area fault, and the lightning strike interference, respectively, and perform sampling. If the protection meets the protection shown in formula (1), the protection starts Criterion,

为当前采样时刻下的电压梯度值，Δ_set为启动门槛值；Then the protection starts, and the point where the line voltage floats lower than the normal voltage for the first time within 0.2ms before and after the protection start time is marked as the timing starting point t ₀ . In formula (1),

is the voltage gradient value at the current sampling time, and _Δset is the startup threshold value;

Step 2. Taking the timing starting point t ₀ described in step 1 as the benchmark, if the valley value point of the traveling wave u _f1 before the first-mode fault cannot be detected in t _1,set , it is determined as a fault in the area, and the protection is reset;

Step 3. Taking the timing starting point t ₀ described in step 1 as the benchmark, if the valley point of the traveling wave u _f1 before the first-mode fault is detected in t _1,set , and the first mode is detected in t _2,set . The zero-crossing point of the traveling wave u _f1 before the fault is judged as lightning strike interference, and the protection is reset;

Step 4. Taking the timing starting point t ₀ described in step 1 as the benchmark, if the valley point of the traveling wave u _f1 before the first-mode fault is detected in t _1,set , and at the same time, no one can be detected in t _2,set . If the zero-crossing point of the traveling wave u _f1 before the mode fault, it is judged as a short-circuit fault in the area, and the protection action is performed.

的计算公式如式(2)所示：Preferably, in step 1, the

The calculation formula of is shown in formula (2):

In the formula, u(k-j) is the voltage sampling value before the jth sampling period.

Preferably, in step 2, the calculation formula of the traveling wave u _f1 before the one-mode fault is as shown in formula (3):

where u ₁ , i ₁ are expressed as shown in formula (4):

In the formula, u ₁ , i ₁ are the fault-mode voltage and fault-mode current; u _p , u _n , _{i p} , in are the positive and negative fault voltages and currents measured at the protection installation; Z _c1 is the transmission line The one-mode wave impedance of ;

The determination method of the valley point is shown in formula (5):

In the formula, t ₁ is the determined valley point time, and T _s is the sampling interval.

Preferably, in step 3, the method for determining the zero-crossing point is as shown in formula (6):

In the formula, t ₂ is the zero-crossing time of the judgment, and T _s is the sampling interval.

Description of drawings

In order to illustrate the technical solutions of the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained from these drawings without any creative effort.

Figure 1 is a schematic diagram of the topology of a true bipolar two-terminal flexible DC transmission system;

Figure 2 is the waveform diagram of three typical pre-fault traveling waves;

Figure 3 shows the action results of the single-pole grounding fault under different transition resistances at the remote end;

Figure 4 shows the action results under different fault types;

Figure 5 shows the action of protection under noise;

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

1 is a schematic topology diagram of a true bipolar two-terminal flexible DC transmission system to which the method of the present invention is applicable. The true bipolar two-terminal flexible DC transmission system includes two parallel-connected transmission lines, one of which is an overhead line, The other is an overhead line;

Figure 2 shows the waveform diagrams of three typical pre-fault traveling waves, namely, the waveform diagrams of the pre-fault traveling waves in the area of faults, out-of-area faults, and lightning strike interference. The steepness of the wave head and the zero-crossing point of the traveling wave before the fault can represent the drop of the wave tail of the first traveling wave of the fault. Under three different traveling waves before the fault, the protection action is as follows:

1) In case of short-circuit fault outside the zone, the valley point of the traveling wave before the fault cannot be detected before the valley point setting value, and the protection is reset.

2) When lightning strikes interfere, the valley point of the traveling wave before the fault can be detected before the setting value of the valley point, and the zero-crossing point of the traveling wave before the fault can be detected before the setting value of the zero-crossing point, and the protection is reset.

3) When there is a short-circuit fault in the area, the valley point of the traveling wave before the fault can be detected before the setting value of the valley point, and the zero-crossing point of the traveling wave before the fault cannot be detected before the setting value of the zero-crossing point, and the protection action.

Figures 3 and 4 show the protection actions under different fault distances, transition resistances and fault types. It can be seen that the protection is not affected by the short-circuit fault type and transition resistance. Hysteresis, but still within the protection threshold, the protection operates reliably.

The single-ended quantity protection method for flexible DC transmission lines according to the present invention will be described in detail below with reference to the embodiments.

Example 1

Figure 1 shows the construction of a ±500kV true bipolar flexible DC transmission system in PSCAD/EMTDC. The length of the DC line is 500km, the overhead line is frequency-dependent model, the lightning protection line is not eliminated, the tower model uses the multi-wave impedance model, and 3 towers are set at the lightning strike; the protection sampling frequency is 50kHz. The fault-mode traveling wave reaches the protection installation in 0ms.

In the simulation, the fault types in the internal fault setting are positive grounding, negative grounding, and bipolar short circuit; the fault distances are 250km, 450km; the transition resistances are 0Ω, 300Ω, and 500Ω; the external faults are set to the exit of the current limiting reactor near the converter station side Metallic fault; lightning strike interference is 25kA lightning strike wire interference.

Figures 3 and 4 show the action results of the protection under different fault conditions. It can be seen that the protection still has high sensitivity when the remote end of the zone is high resistance, and the protection can reliably eliminate lightning strike interference. The noise with a signal-to-noise ratio of 30db is added to the protection sampling data when the fault occurs outside the zone, and the corresponding protection action is shown in Figure 5.

It can be seen that the disturbance generated by the noise forms a valley point (interference point) at the third sampling point, but it does not meet the determination method of the valley point, that is, formula (5), so it will not be judged as a valley. point. The protection cannot detect the valley point before the setting value of the valley point, identify the fault outside the zone, and do not act reliably. After many tests, the protection algorithm can withstand 30db noise.

By adopting the single-ended traveling wave protection method based on the waveform characteristics of the preceding traveling wave in a flexible direct current transmission system of the present invention, the following beneficial effects can be obtained:

(1) The protection method is reliable in principle and simple in criteria, and only two time criteria can be used to improve the protection capability, and the realization difficulty is low.

(2) The protection principle uses two different waveform information to eliminate the faults outside the area and the interference of lightning strikes, and has strong ability to withstand transition resistance and anti-interference.

It should be noted that the content not described in detail in the embodiments of the present invention belongs to the prior art known to those skilled in the art.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. Substitutions should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims (4)

1. A single-ended traveling wave protection method for a flexible DC transmission line, the method is based on the waveform characteristics of the forward traveling wave of the flexible DC transmission line, and the flexible DC transmission line is a true bipolar flexible DC transmission line. The DC power transmission system includes two parallel connected power transmission lines, one of which is an overhead line and the other is also an overhead line, characterized in that the protection method includes the following steps: Step 1. Detect the waveforms of the three types of pre-fault traveling waves of the flexible DC transmission line, namely, the intra-area fault, the out-of-area fault, and the lightning strike interference, respectively, and perform sampling. If the protection meets the protection shown in formula (1), the protection starts Criterion,

Then the protection starts, and the point where the line voltage floats lower than the normal voltage for the first time within 0.2ms before and after the protection start time is marked as the timing starting point t ₀ . In formula (1),

is the voltage gradient value at the current sampling time, and _Δset is the startup threshold value; Step 2. Taking the timing starting point t ₀ described in step 1 as the benchmark, if the valley value point of the traveling wave u _f1 before the first-mode fault cannot be detected in t _1,set , it is determined as a fault in the area, and the protection is reset; Step 3. Taking the timing starting point t ₀ described in step 1 as the benchmark, if the valley point of the traveling wave u _f1 before the first-mode fault is detected in t _1,set , and the first mode is detected in t _2,set . The zero-crossing point of the traveling wave u _f1 before the fault is judged as lightning strike interference, and the protection is reset; Step 4. Taking the timing starting point t ₀ described in step 1 as the benchmark, if the valley point of the traveling wave u _f1 before the first-mode fault is detected in t _1,set , and at the same time, no one can be detected in t _2,set . If the zero-crossing point of the traveling wave u _f1 before the mode fault, it is judged as a short-circuit fault in the area, and the protection action is performed. 2 . The single-ended traveling wave protection method for flexible DC transmission lines according to claim 1 , wherein in step 1, the

The calculation formula of is shown in formula (2):

In the formula, u(k-j) is the voltage sampling value before the jth sampling period. 3. a kind of single-ended traveling wave protection method for flexible DC transmission line according to claim 1, is characterized in that, in step 2, the calculation formula of traveling wave u _f1 before one mode fault is as formula (3) shown:

where u ₁ , i ₁ are expressed as shown in formula (4):

In the formula, u ₁ , i ₁ are the fault-mode voltage and fault-mode current; u _p , u _n , _{i p} , in are the positive and negative fault voltages and currents measured at the protection installation; Z _c1 is the transmission line The one-mode wave impedance of ; The determination method of the valley point is shown in formula (5):

In the formula, t1 is the determined valley point time, and Ts is the sampling interval. 4. A single-ended traveling wave protection method for a flexible DC transmission line according to claim 1, characterized in that, in step 3, the determination method of the zero-crossing point is as shown in formula (6):

In the formula, t2 is the determined zero-crossing time, and Ts is the sampling interval.

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