CN104237745B

CN104237745B - Method for judging abnormal commutation failure of multi-feed high-voltage DC transmission system

Info

Publication number: CN104237745B
Application number: CN201410550497.4A
Authority: CN
Inventors: 李宏伟; 潘志远; 牟黎; 许圆圆; 赵彤; 张黎; 邹亮
Original assignee: State Grid Corp of China SGCC; Shandong University; State Grid of China Technology College
Current assignee: State Grid Corp of China SGCC; Shandong University; State Grid of China Technology College
Priority date: 2014-10-16
Filing date: 2014-10-16
Publication date: 2015-04-15
Anticipated expiration: 2034-10-16
Also published as: CN104237745A

Abstract

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

Description

Method for judging abnormal commutation failure of multi-feed-in high-voltage direct-current power transmission system

Technical Field

The invention relates to a method for judging abnormal commutation failure of a multi-feed-in high-voltage direct-current power transmission system.

Background

In the converter, if the converter valve which is out of conduction fails to recover the blocking capability within a period of time when a reverse voltage is applied, or if the phase change process is not completed during the reverse voltage, when the valve voltage changes to the forward direction, the phase of the valve which is out of conduction is changed to the original valve which is scheduled to be out of conduction, and the condition is called phase change failure. Since the rectifier converter valve is under reverse voltage for a long time after the current is turned off, the rectifier will fail to change phase only when the trigger circuit fails. Most of phase commutation failures of the direct current system occur on the inversion side, and the phase commutation failures are one of the most common failure types on the inversion side. Therefore, the research on commutation failure mainly focuses on the influence between the direct current system and the inverter side alternating current system. In a multi-feed-in high-voltage direct-current transmission system, a failure of a receiving-end alternating-current system may cause phase commutation failure of multiple direct-current systems simultaneously or sequentially, and in severe cases, the direct-current system is locked, the direct-current transmission power is interrupted, and finally the safe and stable operation of the whole system is threatened.

The essence of the commutation failure is that the converter valve has an extinction angle gamma smaller than the critical extinction angle gamma_min. The traditional method for judging the commutation failure is a minimum voltage drop method, and whether the commutation failure occurs to the converter valve is judged by comparing the magnitude between the voltage drop of the commutation voltage and the minimum voltage drop required by the occurrence of the commutation failure. However, the method ignores the influence of waveform distortion on commutation failure, so that the judgment of the commutation failure result has larger deviation.

Different from the conventional commutation failure situation, the multi-feed direct-current power transmission system has an abnormal commutation failure phenomenon, the measured abnormal commutation failure degree cannot be completely represented only by Total Harmonic Distortion (THD), and a method for accurately representing the severity degree of the abnormal commutation failure (namely the possibility of the abnormal commutation failure) is lacked at present.

Disclosure of Invention

In order to solve the defects in the prior art, the invention provides the abnormal commutation failure judgment method of the multi-feed-in high-voltage direct-current power transmission system, which integrates two factors of waveform distortion and voltage amplitude reduction, can accurately reflect the commutation failure condition and represent the severity of the abnormal commutation failure.

In order to achieve the purpose, the invention adopts the following specific scheme:

a method for determining abnormal commutation failure of a multi-feed-in high-voltage direct-current power transmission system comprises the following steps,

s100: determining the fault type of the multi-feed-in high-voltage direct-current power transmission system; the fault types of the multi-feed-in high-voltage direct-current power transmission system comprise single-phase grounding, three-phase short circuit and the like;

s200: in case of determining the type of multi-feed hvdc transmission system fault in step S100,

calculating a respective fault level FL_i(i＝1,2......n)：

<math> <mrow> <msub> <mi>FL</mi> <mi>i</mi> </msub> <mo>=</mo> <mfrac> <msup> <mi>U</mi> <mn>2</mn> </msup> <mrow> <msub> <mi>Z</mi> <mi>fault</mi> </msub> <mo>·</mo> <msub> <mi>P</mi> <mi>dc</mi> </msub> </mrow> </mfrac> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> </mrow> </math>

In the above formula, Z_fault＝ω·L_faultU is the rated voltage of the commutation bus, Z_faultIs the ground impedance, L_faultIs the equivalent ground inductance value; p_dcIs the transmission rated power of the direct current system, i represents an integer larger than 1;

s300: the fault level FL is calculated from step S200_i(i 1,2.. n), collected at fault level FL using auxiliary equipment_i(i 1,2.. n), performing Fourier analysis on the waveform of one period after the fault, and calculating the percentage of voltage amplitude reduction;

wherein, the harmonic content is obtained by Fourier analysis, then the harmonic distortion reference value HDR is obtained,

HDR = DC + \sqrt{{(H_{2})}^{2} + {(H_{3})}^{2}} - - - (2)

wherein DC is a direct current component, H₂Is the second harmonic content, H₃The Total Harmonic Distortion (THD) can be represented by the second harmonic and the third harmonic because the contents of the second harmonic and the third harmonic are far higher than those of other subharmonics;

the percentage of the voltage amplitude reduction is set to av,

wherein, V₀Voltage amplitude, V, of the commutation bus for normal operation_fThe voltage amplitude of the commutation bus in a period after the fault;

s400: in case of the multi-feed hvdc transmission system fault type determined in step S100, a fault level FL is calculated_i(i 1,2.. n) corresponding voltage and harmonic distortion reference quantity, wherein the voltage and harmonic distortion reference quantity is VHDR_i(i 1,2.. n) denotes:

VHDR_i＝ΔV+HDR (4)

s500: multi-feed high voltage direct current transmission determined in step S100Under the condition of the fault type of the electric system, obtaining a fault level FL when the abnormal commutation failure probability is 0.5 through PSCAD simulation; then, the voltage and harmonic distortion reference value VHDR with the abnormal commutation failure probability of 0.5 is calculated in step S300 and step S400₀；

S600: under the condition of the fault type of the multi-feed-in high-voltage direct-current power transmission system determined in the step S100, judging the possibility of abnormal commutation failure;

the criterion for determining the probability of the abnormal commutation failure in step S600 is as follows:

when VHDR_i＝VHDR₀Then, the probability of abnormal commutation failure is 0.50; i.e. when VHDR_iValue of (2) exceeds VHDR₀In time, abnormal commutation failure is large; when VHDR_iIs lower than VHDR₀In time, the possibility of abnormal commutation failure is low;

when the probability value of the abnormal commutation failure is equal to 1, the commutation failure is necessarily caused under the corresponding fault level; when the probability value of the abnormal commutation failure is less than 0.5, defining a fault level FL corresponding to the abnormal commutation failure_iN) is less likely to cause an abnormal commutation failure; when the probability value of abnormal commutation failure is more than 0.5, defining the fault level FL corresponding to the abnormal commutation failure_jThe probability of causing abnormal commutation failure is high, wherein j is 1,2.

The abnormal commutation failure is as follows: under the condition that the multi-feed-in direct current power transmission system fails in phase commutation simultaneously or sequentially, along with the increase of a local Fault Level (FL), a remote phase commutation failure possibility curve is increased, then decreased, and then increased, so that an abnormal phenomenon occurs;

the local commutation failure possibility refers to the possibility of commutation failure of the converter station 1, and the remote commutation failure possibility refers to the possibility of commutation failure of the converter station 2; the local fault level refers to the degree of failure of the faulted converter bus.

The abnormal commutationThe magnitude of the failure probability is expressed for each Failure Level (FL)₁、FL₂.....FL_n) Taking 100 points at equal intervals in a cycle, calculating the ratio of the point with abnormal commutation failure to the total point number through PSCAD software as the fault level FL_iThe probability of an abnormal commutation failure occurring under (i 1,2.... n), that is, the possibility of an abnormal commutation failure.

The invention has the beneficial effects that:

solving the reference quantity of voltage and harmonic distortion for measuring the abnormal commutation failure degree aiming at the abnormal commutation failure phenomenon in the multi-feed-in high-voltage direct-current power transmission system, essentially reflecting the abnormal commutation failure phenomenon of the multi-feed-in direct-current system and providing reference for the commutation failure research of the multi-feed-in system; and two factors of waveform distortion and voltage amplitude reduction are integrated, so that the phase change failure condition can be accurately reflected.

Drawings

FIG. 1 is a schematic diagram of phase commutation failure caused by a decrease in voltage amplitude;

FIG. 2 is a schematic diagram of a phase commutation failure caused by a voltage zero crossing displacement;

FIG. 3 is a diagram illustrating phase commutation failure due to voltage waveform distortion;

FIG. 4 is a schematic diagram of a multi-feed DC power transmission model;

FIG. 5 is a schematic diagram illustrating an abnormal commutation failure in a multi-feed system;

FIG. 6(a) is a A, B, C three-phase fault voltage waveform diagram at fault level 0.13;

FIG. 6(b) is a A, B, C three-phase fault voltage waveform diagram at fault level 0.25;

FIG. 6(c) is a A, B, C three-phase fault voltage waveform diagram at fault level 0.5;

FIG. 7(a) is a plot of harmonic content of the B-phase voltage at fault level 0.13;

FIG. 7(B) is a plot of harmonic content of the B-phase voltage at fault level 0.25;

FIG. 7(c) is a plot of harmonic content of the B-phase voltage at fault level 0.5;

FIG. 8 is a flow chart of the present invention;

FIG. 9 is a schematic diagram of commutation failure probability versus VHDR.

Detailed Description

calculating a respective fault level FL_i(i＝1,2......n)：

HDR = DC + \sqrt{{(H_{2})}^{2} + {(H_{3})}^{2}} - - - (2)

the percentage of the voltage amplitude reduction is set to av,

VHDR_i＝ΔV+HDR (4)

s500: under the condition of the fault type of the multi-feed-in high-voltage direct-current transmission system determined in the step S100, obtaining a fault level FL when the abnormal commutation failure probability is 0.5 through PSCAD simulation; then, the voltage and harmonic distortion reference value VHDR with the abnormal commutation failure probability of 0.5 is calculated in step S300 and step S400₀；

wherein, the probability value when the abnormal commutation fails is equal to1, phase commutation failure is necessarily caused under the corresponding fault level; when the probability value of the abnormal commutation failure is less than 0.5, defining a fault level FL corresponding to the abnormal commutation failure_iN) is less likely to cause an abnormal commutation failure; when the probability value of abnormal commutation failure is more than 0.5, defining the fault level FL corresponding to the abnormal commutation failure_jThe probability of causing abnormal commutation failure is high, wherein j is 1,2.

The magnitude of the probability of an abnormal commutation failure is expressed for each Fault Level (FL)₁、FL₂.....FL_n) Taking 100 points at equal intervals in a cycle, calculating the ratio of the point with abnormal commutation failure to the total point number through PSCAD software as the fault level FL_iThe probability of an abnormal commutation failure occurring under (i 1,2.... n), that is, the possibility of an abnormal commutation failure.

1 principle of commutation failure

In the phase conversion process of the converter valve, the voltage waveform of the converter bus plays a key role, the stable voltage waveform enables the phase conversion to be normally carried out, and the waveform which changes in the case of a fault may cause the phase conversion failure. Theoretically, two reasons are generally considered when analyzing commutation failure: firstly, the voltage amplitude is reduced, and secondly, the voltage zero crossing point is displaced. The influence of the voltage zero-crossing displacement on the commutation failure is small, so the mechanism of the commutation failure under the asymmetric fault mainly depends on the reduction of the voltage amplitude. In fact, there is also a more complex reason why commutation failure occurs, namely voltage waveform distortion. The reason for this is difficult to quantitatively analyze, because the voltage waveform distortion may be accompanied by zero-crossing displacement, and the distortion does not occur according to a specific rule.

1.1 Voltage amplitude reduction

The commutation process can be expressed in terms of voltage-time commutation tooth area, as shown in fig. 1, and in normal operation, the flip angle on the inverting side is α, the commutation angle is μ, the extinction angle is γ, and α + μ + γ is pi. Area of commutation is S₁The amplitude of the commutation bus voltage is reduced at fault (here, three-phase fault is taken as an example). The area of the commutation teeth is constant (S)₁＝S₂) And under the condition that the trigger time of the thyristor is not changed, the commutation time is prolonged, as shown in figure 1, the commutation angle mu is increased to be mu ', and the extinction angle gamma is reduced to be gamma'.

When the phase-change voltage is lower than a certain critical value, gamma<γ₀If the critical voltage drop is set to be DeltaU, then,

wherein,

I_d-pre-fault dc current;

I′_d-post fault dc current;

I_dFL-rated direct current;

X_cpu-per unit value of converter transformer impedance;

gamma-minimum extinction angle setting value;

γ₀critical extinction angle for phase change failure (typically 7 ° -10 °).

1.2 Voltage zero crossing Displacement

When asymmetric faults occur, the voltage amplitude is reduced, meanwhile, the zero crossing point of the commutation voltage is advanced, the extinction angle gamma of the converter valve is reduced due to the constant commutation tooth area S, and commutation failure occurs, as shown in figure 2,

the critical pressure drop is:

compared with the formula (5), the denominator of the formula (6) is added by a displacement angleThe critical pressure drop of the system phase commutation failure is reduced, and the probability of phase commutation failure is increased to a certain extent. Considering that the system is normally fully loaded, I_d/I_dFL＝1.0。

Equations (5) and (6) can be simplified as:

1.3 Voltage waveform distortion

When a fault occurs, the commutation bus voltage is no longer a smooth sine wave, but rather a degree of distortion may occur. Such distortions include voltage amplitude reduction and voltage zero crossing displacement. The displacement of the zero-crossing point is not a main cause of commutation failure, and many faults move backwards at the zero-crossing point while the voltage waveform is distorted and the amplitude is reduced, so that the possibility of commutation failure can be reduced theoretically, but actually, the displacement does not greatly contribute to the prevention of the commutation failure. Figure 3 shows a distorted waveform of the commutation bus voltage,

it can be seen from fig. 3 that after a fault, the voltage waveform is distorted, the voltage amplitude is reduced, and the zero-crossing point is displaced. Since the voltage waveform distortion is irregular, equations (5) and (6) are no longer applicable and need to be determined from a harmonic point of view.

Examples

FIG. 4 is a simplified diagram of a multi-feed model, in which two DC systems are both CIGRE standard models in PSCAD, and DC transmission power P_dc1000MW, DC voltage 500 kV; the receiving end alternating current system voltage level is 230 kV. y1, y2 are two inversion stations, Z_f1And Z_f2Is the filter impedance of the inverter station converter bus side, Z_s1And Z_s2Respectively two AC systemsEquivalent impedance, Z_tieIs the coupling impedance between two ac systems. When a converter bus of the direct current system 1 has a ground fault and the converter station y1 has a phase change failure, the converter station y2 also has a phase change failure when the fault level exceeds a certain value. Whether a commutation failure will occur at the station y2 is not only related to the type of failure of the dc system 1 converter bus, but also to the strength of the ac system to which it is connected.

Under the multi-feed model shown in fig. 4, the fault voltage waveforms of the inversion-side converter bus at three typical fault levels of 0.13, 0.25 and 0.5 are taken:

in fig. 6, a thick chain line indicates a three-phase voltage waveform of the far-end converter A, B, C in normal operation, and a thin chain line indicates a three-phase voltage waveform of the far-end converter bus in local fault. As can be seen from fig. 6(a), at a slight fault, the local fault level is 0.13, the amplitude of the fault voltage waveform is reduced and distortion occurs (phase B is most obvious), and it can be seen from fig. 5 that the probability of commutation failure at this time is about 0.8. When the local fault level is increased to 0.25, the fault voltage waveform is basically consistent with the normal operation waveform, the probability of commutation failure is about 0.4, and the probability of commutation failure is relatively low. Under severe fault, when the local fault level is 0.5, the voltage waveform is obviously distorted, and the amplitude of the B phase is seriously reduced, at this time, the possibility of failure of far-end commutation is 1.0. It is known that both minor and major faults cause distortion of the voltage waveform and cause a commutation failure of the remote inverter.

The waveform data of the voltage of the far-end converter bus B phase under three typical fault levels are respectively taken, and the waveform of one period after the fault is subjected to Fourier analysis to obtain the harmonic content distribution as shown in FIG. 7. The obtained voltage dc component and the contents of the second and third harmonics are shown in table 1.

TABLE 1 DC component and second and third harmonic content at three fault levels

From fig. 7, it can be seen that the peak values of the B-phase voltage in the period after the fault are 170.4kV, 166.7kV and 150.7kV, respectively, and the total harmonic distortion is 20.86%, 10.66% and 28.44%, respectively, and it is obvious that as the fault level increases, the voltage amplitude gradually decreases, and the harmonic content is abnormally changed, especially reflected on the low-order harmonic and the dc component. As can be seen from table 1, in the case of a minor fault (FL ═ 0.13), the dc component accounts for 3.373% of the fundamental component, while at the 0.25 fault level it is only 0.15%, while the total content of the second and third harmonics is much greater for the former than for the latter. The large direct current component and the harmonic content hinder the normal phase change of the converter valve, and the phase change failure can be caused. In the case of a serious fault (FL ═ 0.5), although the dc component is small, the harmonic content is large, and the voltage amplitude is severely reduced, and the probability of commutation failure is 1.

Three fault levels were added, 0, 0.3, 0.4 respectively. The voltage amplitude, total harmonic distortion, dc component and harmonic content, percentage of voltage amplitude reduction and commutation failure probability at all fault levels were analyzed and the results are shown in tables 2 and 3.

TABLE 2 Voltage amplitude and Total harmonic distortion at various fault levels

TABLE 3 DC component, second and third harmonic content and DeltaV at each fault level

It can be seen from table 2 that the abnormal commutation failure phenomenon is substantially positively correlated with the total harmonic distortion of the voltage after the fault, but the phenomenon cannot be fully characterized by THD alone, and the reduction of the voltage amplitude needs to be considered.

The severity of the abnormal commutation failure is positively correlated with the voltage and harmonic distortion reference value VHDR, namely the larger the value of VHDR, the more serious the abnormal commutation failure is, and the commutation failure possibility is the possibility of the abnormal commutation failure, namely the severity of the abnormal commutation failure; the greater the likelihood, the greater the severity.

The values of VHDR at each fault level are shown in table 3.

From the last two columns of data in Table 3, shown in graph 9, which yields the change in VHDR with commutation failure probability, it can be seen that: the larger the value of VHDR, the greater the likelihood of commutation failure; when VHDR was 19.82%, the commutation failure probability was 0.50; when the value of VHDR exceeds 19.82% (about 20%), there is a high probability of commutation failure; when the value of VHDR is less than 20%, commutation failure is less likely.

It can be seen from table 3 and fig. 9 that VHDR is consistent with the change of the commutation failure probability, and the commutation failure situation can be reflected more accurately by combining two factors of waveform distortion and voltage amplitude reduction, and the severity of the abnormal commutation failure, that is, the probability of the abnormal commutation failure can be represented.

Claims

1. A method for determining abnormal commutation failure of a multi-feed-in high-voltage direct-current power transmission system is characterized by comprising the following steps,

s100: determining the fault type of the multi-feed-in high-voltage direct-current power transmission system; the fault types of the multi-feed-in high-voltage direct-current transmission system comprise single-phase grounding and three-phase short-circuit faults;

calculating a respective fault level FL_i(i＝1,2......n)：

s300: the fault level FL is calculated from step S200_i(i 1,2.. n), collected at fault level FL using auxiliary equipment_i(i 1,2.. n), carrying out Fourier analysis on the waveform of one period after the fault to obtain a harmonic distortion reference amount HDR, and calculating the percentage Δ V of voltage amplitude reduction;

VHDR_i＝ΔV+HDR

the harmonic distortion reference HDR obtained by the fourier analysis in step S300 is,

the percentage of voltage amplitude reduction Δ V is:

when the probability value of the abnormal commutation failure is equal to 1, the commutation failure is necessarily caused under the corresponding fault level; when the probability value of the abnormal commutation failure is less than 0.5, defining a fault level FL corresponding to the abnormal commutation failure_iN) is less likely to cause an abnormal commutation failure; when the probability value of abnormal commutation failure is more than 0.5, defining the fault level FL corresponding to the abnormal commutation failure_jThe probability of causing abnormal commutation failure is high, wherein j is 1,2, …, n, j is an integer larger than 1.

2. The method for determining the abnormal commutation failure of the multi-feed-in high-voltage direct current transmission system according to claim 1, wherein the abnormal commutation failure is: under the condition that the multi-feed-in direct current power transmission system fails in phase commutation simultaneously or sequentially, along with the increase of a local Fault Level (FL), a remote phase commutation failure possibility curve is increased, then decreased, and then increased, so that an abnormal phenomenon occurs;

3. The method according to claim 1, wherein the magnitude of the probability of an abnormal commutation failure is expressed for each Fault Level (FL)₁、FL₂.....FL_n) Taking 100 points at equal intervals in a cycle, calculating the ratio of the point with abnormal commutation failure to the total point number through PSCAD software as the fault level FL_iThe probability of an abnormal commutation failure occurring under (i 1,2.... n), that is, the possibility of an abnormal commutation failure.