CN110208649B - Commutation fault detection method and device based on AC voltage reduction speed - Google Patents

Commutation fault detection method and device based on AC voltage reduction speed Download PDF

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CN110208649B
CN110208649B CN201910472322.9A CN201910472322A CN110208649B CN 110208649 B CN110208649 B CN 110208649B CN 201910472322 A CN201910472322 A CN 201910472322A CN 110208649 B CN110208649 B CN 110208649B
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commutation
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alternating voltage
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崔勇
董明瑞
曹亮
王伟红
金敏杰
杨秀
顾丹珍
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Shanghai University of Electric Power
State Grid Shanghai Electric Power Co Ltd
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Abstract

The invention relates to a commutation fault detection method and a commutation fault detection device based on the reduction speed of alternating voltage, wherein the method comprises the following steps: step S1: obtaining a threshold value of the dropping speed of the alternating-current voltage based on the system parameters; step S2: during phase conversion, collecting alternating voltage in real time; step S3: and calculating the dropping speed of the alternating voltage, and outputting a fault signal if the dropping speed of the alternating voltage is less than a threshold value. Compared with the prior art, the invention adopts the signal of the dropping speed of the alternating voltage to realize the fault detection, and the alternating voltage is a quantity which is very easy to detect, so the fault detection difficulty is greatly reduced.

Description

Commutation fault detection method and device based on AC voltage reduction speed
Technical Field
The invention relates to a commutation fault detection technology, in particular to a commutation fault detection method and device based on the reduction speed of alternating-current voltage.
Background
In recent years, HVDC transmission projects in China are developed vigorously, the transmission capacity is gradually increased, and the impact of a commutation failure fault on an alternating current system is more and more serious. How to effectively judge the commutation failure is significant for how to take the measures of inhibiting the commutation failure.
The failure of commutation is mainly caused by the following factors: the change of direct current, the reduction of alternating voltage at a receiving end, the increase of harmonic waves of phase-changing voltage, phase angle forward shift and the like. So far, many people have conducted intensive and comprehensive research on commutation failure, and have obtained criteria such as commutation area, turn-off area, voltage sag and arc-quenching angle.
The defect that the increase of direct current is not considered by the classical arc-quenching angle criterion is overcome, and the improvement is made in the literature. The current increase after the receiving end alternating current fault is caused by the fact that the difference value of direct current voltage of a rectifying side and direct current voltage of an inverting side becomes large, and direct current voltage change is introduced into an arc extinction angle criterion. There are also relevant documents relating voltage to current and improving the voltage criterion of commutation failure by assuming that the power is constant during a transient, but there are documents that indicate that the power changes instantaneously after a fault. There are also many documents that relate the position and area where the fault occurs in the receiving-end ac system to whether the commutation failure occurs, so that it is convenient and fast to determine whether an ac fault can cause the commutation failure in practical application. For example, the detailed analysis is performed by combining sensitivity analysis and commutation failure criterion in the literature. However, the basic judgment methods used in the above documents are classical criteria that do not consider the minimum off-angle of the rise of the direct current, and have certain improvement space. For the index of the distortion of the ac bus voltage after the fault, there is also a literature quantitative analysis of the influence of the harmonic on the commutation.
For an MIDC system, documents are combined with node admittance matrixes under different frequencies to quantitatively analyze the influence of commutation failure under harmonic injection of each direct current access point; other documents indicate that the reason why the commutation of other direct current access points fails with high probability when the fault level of a certain direct current access point is small is analyzed in detail. When the receiving-end power grid has an asymmetric fault, except that the voltage is reduced, the zero point of the alternating-current voltage of the converter bus is caused to move forward at a high probability, so that the phase change failure of the inverter station is caused.
So far, there are many studies to determine whether phase commutation failure occurs by using the voltage minimum value in fault, and simulation experiments find that most of the cases are that phase commutation failure occurs only if the voltage is not reduced to the minimum value, so there is room for improvement. Other documents describe that it is a probabilistic question whether commutation failure occurs.
Disclosure of Invention
The present invention aims to overcome the defects of the prior art and provide a commutation fault detection method and device based on the reduction speed of the alternating voltage.
The purpose of the invention can be realized by the following technical scheme:
a commutation fault detection method based on the dropping speed of an alternating voltage comprises the following steps:
step S1: obtaining a threshold value of the dropping speed of the alternating-current voltage based on the system parameters;
step S2: during phase conversion, collecting alternating voltage in real time;
step S3: and calculating the dropping speed of the alternating voltage, and outputting a fault signal if the dropping speed of the alternating voltage is less than the threshold value.
The threshold specifically comprises:
Figure BDA0002081138780000021
wherein: k is a radical ofacIs a threshold value, Id0For steady-state operation of direct current, ULLFor converting the current to AC bus-line voltage,/i0Is a receiving end no-load DC voltage, gammaminAt minimum extinction angle, beta is trigger lead angle, XcNUM for leakage reactance of converter transformer1To calculate process parameters.
The calculation process parameter NUM1The method specifically comprises the following steps:
Figure BDA0002081138780000022
wherein: k is a radical ofcNUM is a correction coefficient with a value between 0 and 12For maximum possible commutation time, ω is the angular velocity of the fundamental frequency, RciEquivalent impedance on DC lines for commutation, RdIs the resistance of the dc line.
The maximum possible commutation time NUM2The method specifically comprises the following steps:
Figure BDA0002081138780000023
wherein: γ is the extinction angle and β is the trigger advance angle.
A commutation fault detection device based on an alternating voltage falling speed comprises a memory, a processor and a program stored in the memory and executed by the processor, wherein the processor executes the program to realize the following steps:
step S1: obtaining a threshold value of the dropping speed of the alternating-current voltage based on the system parameters;
step S2: during phase conversion, collecting alternating voltage in real time;
step S3: and calculating the dropping speed of the alternating voltage, and outputting a fault signal if the dropping speed of the alternating voltage is less than the threshold value.
Compared with the prior art, the invention has the following beneficial effects:
1) the signal of the dropping speed of the alternating voltage is adopted to realize fault detection, and the alternating voltage is a quantity which is very easy to detect, so that the fault detection difficulty is greatly reduced.
2) The threshold value considering the rising speed of the direct current is adopted, and the threshold value is designed by utilizing the electromagnetism conforming to the natural regulation, so that the detection accuracy is greatly improved.
Drawings
FIG. 1 is a schematic flow chart of the main steps of the method of the present invention;
FIG. 2 is a schematic diagram of a condition before a receive side AC fault occurs;
FIG. 3 is a schematic diagram of stage 1 after a fault is sensed by the receiving end AC;
FIG. 4 is an equivalent circuit diagram during commutation;
FIG. 5 is a schematic diagram of valve current during commutation;
FIG. 6 is a graph of current delta for each location when no commutation failure occurs;
FIG. 7 is a graph of current delta for each location when a commutation failure occurs;
FIG. 8 is a graph of transition resistance versus minimum extinction angle;
FIG. 9 is a graph of voltage rate of change versus minimum extinction angle.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.
According to the method, the reason for the rise of the direct current in the fault period is firstly analyzed, and accurate quantitative analysis is carried out so as to improve the accuracy of the commutation failure judgment method. Aiming at the problem that the existing commutation failure criterion based on the lowest value of commutation voltage is different from a simulation experiment, an index based on the voltage change rate in fault is provided to measure whether commutation failure occurs or not, and the rationality is verified by comparing the result of theoretical calculation with the result of the simulation experiment.
The detection method obtained by the present application is implemented by a computer system in the form of a computer program, and the corresponding apparatus includes a memory, a processor, and a program stored in the memory and executed by the processor, as shown in fig. 1, when the processor executes the program, the following steps are implemented:
step S1: obtaining a threshold value of the dropping speed of the alternating-current voltage based on the system parameters;
step S2: during phase conversion, collecting alternating voltage in real time;
step S3: calculating the reduction speed of the alternating voltage, and outputting a fault signal if the reduction speed of the alternating voltage is less than a threshold value, wherein the threshold value is specifically as follows:
Figure BDA0002081138780000041
wherein: k is a radical ofacIs a threshold value, Id0For steady-state operation of direct current, ULLFor converting the current to AC side bus voltage,/i0Is unloaded at the receiving endDC voltage, gammaminAt minimum extinction angle, beta is trigger lead angle, XcNUM for leakage reactance of converter transformer1To calculate process parameters.
Calculating a process parameter NUM1The method specifically comprises the following steps:
Figure BDA0002081138780000042
wherein: k is a radical ofcNUM is a correction coefficient with a value between 0 and 12For maximum possible commutation time, ω is the angular velocity of the fundamental frequency, RciEquivalent impedance on DC lines for commutation, RdIs the resistance of the dc line.
Maximum possible commutation time NUM2The method specifically comprises the following steps:
Figure BDA0002081138780000043
wherein: γ is the extinction angle and β is the trigger advance angle.
In addition, the alarm can be realized by sound and light alarm or remote alarm.
Before the method of the present application was obtained, the inventors carried out the following analysis:
first, transient DC current analysis is performed on HVDC system
When a fault occurs that is not severe but can cause a commutation failure, the rise in dc current can be in two stages: the method comprises the following steps that in the first stage, the time range is from the beginning of an alternating current fault to before two valves of the same bridge arm are conducted, the direct current is mainly increased due to the decrease of the no-load direct current voltage of a receiving end in the first stage, the time spent in the first stage is short, the trigger angles of a sending end and the receiving end can be considered to be unchanged, the first stage is close to a steady state, and a phase change failure process occurs in the first stage; in the second stage, the time range is a period of time after the two valves of the same bridge arm are turned on (for example, V1 and V4 are both turned on), since the two valves on the same bridge arm are both turned on, the receiving end dc side is equivalent to short circuit and grounding, and the stage that the dc current rises quickly is related to the impedance and control strategy of the dc system, which is the result of phase commutation failure in the first stage.
Before the receiving end AC fault occurs, the condition is shown in figure 2, and U is operated in a steady statec0Maintain constant, icIs equal to 0, so there is id1＝id2＝idAnd the voltage of the two ends of the power transmission and receiving are constant, and the power transmitted by the direct current system is constant. The actual system has a certain deviation from the rated value in steady state operation, so that two proportionality coefficients l exist before the no-load direct current voltage at two ends is sent and receivedr0And li0。
Then, the first stage is analyzed, and the receiving end no-load direct current voltage l is obtained after the receiving end alternating current fault detectioni0·Udi0Is reduced to li1·Udi0The dc link equivalent capacitance C is caused to discharge. The capacitor C can be equivalent to a direct current voltage source due to the short time period to be considered, and the source voltage is equal to the steady-state capacitor voltage
Figure BDA0002081138780000051
(equivalent commutation resistances on rectifying and inverting sides are considered equal Rcr＝Rci) As in fig. 3.
Fig. 2 and 3 were analyzed using the principle of superposition.
For fig. 2, in the pre-fault condition, only the power supply on the rectification side is considered, and equation (1) is obtained:
Figure BDA0002081138780000052
in fig. 3, in the post-failure stage 1, only the rectifier-side power supply is considered, and equation (2) is obtained:
Figure BDA0002081138780000053
referring to FIG. 3, in the 1 st stage after the failure, only the inverter side power supply is considered, and the formula (3) is obtained
Figure BDA0002081138780000054
Obtained by synthesizing the following formulae (1) to (3):
Figure BDA0002081138780000055
Figure BDA0002081138780000056
Figure BDA0002081138780000061
is obtained from the formulae (4) to (6) and has a terminal direct current of i'd1The capacitance current i does not change in the first transient stage after the inverter side alternating current faultc' inverter side direct current i ' is no longer zero at this stage 'd2The main cause of the rise is the rise in the capacitor current, namely:
Figure BDA0002081138780000062
let the rate of change of the AC voltage be kac，li1＝li0+kacT, the inverter side current change rate k can be obtainedidAnd kacThe relationship of (1):
Figure BDA0002081138780000063
since the actual voltage of the capacitor cannot be kept constant, the current change rate obtained by the equations (7) and (8) is large, and therefore a coefficient k between 0 and 1 is requiredcAnd (7) correcting. k is a radical ofcThe specific value of (A) is related to various parameters of a direct current system and does not change along with the severity of the fault (the value of the Cigre-Benchmark model is about k)c＝0.78)。
Figure BDA0002081138780000064
Figure BDA0002081138780000065
Second, commutation process analysis considering transients
1) Analysis of commutation process during normal operation
Fig. 4 is an equivalent circuit on the inverter side during a phase change from inverter side valve V1 to valve V3. A diagram of valve current during commutation is shown in fig. 5, where β represents the inversion side trigger advance angle and α represents the trigger delay angle.
The following equations hold for the commutation period based on KCL and KVL:
Figure BDA0002081138780000066
then equation (12) holds during commutation:
Figure BDA0002081138780000067
wherein: l iscIs a phase change inductor; u. ofaAnd ubTo commutate the bus phase voltage.
If the change of the direct current is ignored, id(t)＝Id0Then there is
Figure BDA0002081138780000068
Due to i3＝Id0-i1Therefore, in the phase commutation period (t ═ α/ω to (pi- γ)/ω), equation (12) is integrated, and there are:
Figure BDA0002081138780000071
wherein U isLLIs effective value of valve side line voltage, omega is system angular velocity, Xc＝ωLcBeta is the inverter side trigger lead angle, and gamma is the calculated arc-quenching angle.
Further obtaining:
Figure BDA0002081138780000072
and when the extinction angle is a critical value, the obtained voltage value is the lowest value of the commutation bus voltage without commutation failure.
2) Commutation process analysis taking into account transient dc current variations
The rise in current in the first phase after an HVDC system on the receiving side has an ac fault is very significant, assuming id(t)＝Id0This is not true and new analyses are required. Handle
Figure BDA0002081138780000073
And formula 2-2 are available in tandem:
Figure BDA0002081138780000074
will id-2i3Renamed as ieIf the formula (16) is changed to the formula (17). At the beginning of commutation moment i3When is equal to 0, then ie＝id0(Note i)d0And Id0Different, direct current at the start of commutation and direct current in steady state operation, respectively). After the commutation is finished, i3＝id1Then i ise＝-id1(id1A dc instantaneous value at the end of commutation).
Figure BDA0002081138780000075
Further obtain ieExpression for ω t:
Figure BDA0002081138780000076
the expression of the direct current is:
Figure BDA0002081138780000077
the joint type (18) and (19) aree＝-idMaking ω t the critical commutation angle allows a critical k between commutation failure and no commutation failureac。
Generally, the fault on the ac side of the receiving end occurring at the instant when a certain commutation process is completed has the largest influence on the next commutation failure of the same converter bridge, because the fault on the ac side has no influence on the extinction angle of the current commutation process, the output of the extinction angle (gamma) controller remains basically unchanged, and the dc current has already risen to a certain extent by the next commutation (the above assumption is also true for a 12-pulse converter). So id0Should be the direct current at the beginning of a commutation in the event of such a fault,/iIs the per unit value of the commutation bus voltage at the beginning of the next commutation. In addition, the critical phase change angle is the trigger lead angle beta minus the minimum extinction angle gammamin(the required turn-off time of the thyristor is about 400 mus, after considering the error of the series element and the influence of harmonics, gammaminAt 10 degrees).
Figure BDA0002081138780000081
Substituting formula (19) for formula (20), and substituting U withdi0＝2×1.35·ULL＝2.7·ULLThe following can be obtained:
Figure BDA0002081138780000082
Figure BDA0002081138780000083
Figure BDA0002081138780000084
finally, the method of the application is subjected to example analysis and verification
1) Introduction to the model
The analysis was verified by the Cigre-Benchmark model in PSCAD. The transmission power of the model is 1000MW, the DC voltage level is 500kV, and 12-pulse current converters are adopted on both the rectification side and the inversion side. The model parameters are as follows:
the transformation ratios of the converter transformers on the rectifying side and the inverting side are 345.0kV/213.4557kV and 230.0kV/209.2288kV respectively; the commutation reactance is Xc13.315 Ω; the inversion station is controlled by a fixed extinction angle in normal operation, the extinction angle is 15 degrees, the advance trigger angle is 38.065 degrees, and the per unit value of commutation voltage of the inversion side is li00.986 p.u.; the resistance of the DC line is Rd5 Ω; the equivalent impedance on the DC line in the phase-changing process is Rci＝25.430Ω。
2) Transient DC current change verification
Two faults are set, respectively, one of which may cause a commutation failure and the other one cannot. Then, the direct current increment (difference between the instantaneous value of the direct current and the rated value) of the receiving end and the transmitting end of the direct current system and the current on the equivalent capacitance of the direct current line are measured and compared with the theoretical calculation value of the current increment.
A240-omega three-phase symmetrical fault is set on the inversion side, 1.0s is the starting fault lasting for 0.1s, phase commutation failure cannot be caused, and the increment of each current is as shown in the following figure 6.
As can be seen from FIG. 6, when the AC fault occurs, the DC current at the inverting side is invertedStream IdThe increase in inv is indeed due to the line capacitance current IcCaused by an increase. Current I at the rectifying side of about 0.005s after the occurrence of a faultdrec begins to increase significantly while the capacitor current IcThe overall result is an inverter-side direct current Idinv continues linearly increasing in course. I isdthe term is I according to the first formula of formula (7)dinv theoretical increment which can predict I more accurately within a half perioddThe increment of inv, and commutation failure always occurs within 0.005s after ac failure, so the above theory can meet the requirements when commutation failure is studied.
A three-phase symmetrical fault of 100 Ω is set on the inversion side, 1.0s is a starting fault lasting 0.1s, which causes a commutation failure, and the increment of each current is as follows in fig. 7.
In fig. 7, the first phase of current increase corresponds to the time from the start of the fault until the valves of one leg are simultaneously conducting (1.0s to 1.0033 s). At this stage, the upper and lower valves of the same arm are not turned on, and the reason for the rise of the current is the drop of the dc current on the inverter side, as compared with Idinv and Idthe theory mentioned in the application can accurately predict the increase of the direct current on the inversion side. After 1.0033s, because the phase change of one valve is not successful before, the two valves belonging to the same bridge arm are simultaneously conducted, which is equivalent to the rapid increase of the direct current caused by the short circuit and grounding of the inversion side, and in this case, the change of the direct current is caused by a plurality of reasons, and cannot be accurately predicted.
As can be seen from fig. 6 and 7, the dc current on the inverter side increases approximately linearly within a certain time (about half cycle when a commutation failure occurs, and about before the ac fault starts to turn on the valve on the same arm when a commutation failure occurs). Thus, the actual current slope will be compared to the theoretical current slope under varying degrees of fault conditions, with the results listed in table 1.
TABLE 1 Current Change Rate Table
Figure BDA0002081138780000091
Figure BDA0002081138780000101
3) Critical voltage change rate verification causing commutation failure
In the Cigre-Benchmark model, three-phase faults are set under the conditions of different short circuit starting times (1.0 s-1.01 s, with an interval of 0.0002s) and different short circuit transition resistances (240 Ω -0 Ω, with an interval of 5 Ω) and the voltage change rate of each simulation is calculated, and the relationship between the minimum extinction angle and the transition resistance and the voltage change rate is respectively shown in fig. 8 and fig. 9.
As can be seen from fig. 8 and 9, the change law of the minimum extinction angle with the voltage change rate and the transition resistance is similar, and the frequency of the commutation failure increases from 0% to 100% as the transition resistance and the voltage change rate decrease. The phase commutation failure is not well-defined by the combination of various factors such as the start time of the failure, harmonics, etc. In addition, the harmonic wave also brings certain errors to the measurement of the voltage change rate. In order to avoid the influence of harmonic waves to a certain extent, a sample cluster under a certain fixed fault severity is selected, and if the number of samples in phase commutation failure is 3 or more, the maximum voltage change rate in the samples in phase commutation failure is considered as the critical voltage change rate.
TABLE 2 fixed Fault severity (220 Ω) Voltage Change Rate tables
Figure BDA0002081138780000102
Table 2 is a partial fixed fault severity (220 Ω) voltage change rate table (containing samples of all commutation failures at that fault severity), and it is known that the frequency of commutation failures increases significantly at voltage change rates less than-3.834 p.u./s. The critical rate of change at which commutation failure occurred was-3.762 p.u./s, as determined by equation (21), with an error of 1.88% from the actual value, which was seen to be relatively close.
Aiming at the defect that direct current is generally not considered when the phase change failure is judged at present, the method explains the reason of the rise of the direct current in the transient process and makes certain quantitative analysis. Just for the contradiction that the phase change failure occurs when the voltage is not stabilized to the lowest voltage value when the phase change failure is judged through the lowest voltage value, the application provides that whether the phase change failure occurs is judged through the voltage change rate, and the contradiction is effectively solved.
The application finds that when receiving end voltage of an HVDC system is reduced, the main reason for causing direct current to rise in a period of time at the beginning is the discharge of an equivalent capacitance of a line, and the direct current change rate is in a linear change relation with the receiving end voltage change rate in a certain period of time. The method analyzes the phase change process of the inverter in more detail under the condition of considering the transient change of the direct current, and provides a method for calculating the voltage change rate of the inverter between the phase change failure and the non-phase change failure on the basis of the critical phase change angle theory.

Claims (6)

1. A commutation fault detection method based on the reduction speed of an alternating voltage is characterized by comprising the following steps:
step S1: a threshold value for the ac voltage drop rate is derived based on system parameters,
step S2: when the phase is changed, the alternating voltage is collected in real time,
step S3: calculating the reduction speed of the alternating voltage, and if the reduction speed of the alternating voltage is smaller than the threshold value, outputting a fault signal;
the threshold specifically comprises:
Figure FDA0002995830320000011
wherein: k is a radical ofacIs a threshold value, Id0For steady-state operation of direct current, ULLFor converting the current to AC side line voltage,/i0Is a receiving end no-load DC voltage, gammaminAt minimum extinction angle, beta is trigger lead angle, XcNUM for leakage reactance of converter transformer1To calculate process parameters.
2. The commutation fault detection method based on the AC voltage reduction speed according to claim 1, wherein the calculation process parameter NUM1The method specifically comprises the following steps:
Figure FDA0002995830320000012
wherein: k is a radical ofcNUM is a correction coefficient with a value between 0 and 12For commutation time, ω is the fundamental angular velocity, RciEquivalent impedance on DC lines for commutation, RdIs the resistance of the dc line.
3. The method according to claim 2, wherein the commutation time NUM is a period of time for which the commutation is performed2The method specifically comprises the following steps:
Figure FDA0002995830320000013
wherein: γ is the extinction angle and β is the trigger advance angle.
4. A commutation fault detection device based on an ac voltage drop rate, comprising a memory, a processor, and a program stored in the memory and executed by the processor, wherein the processor executes the program to implement the following steps:
step S1: a threshold value for the ac voltage drop rate is derived based on system parameters,
step S2: when the phase is changed, the alternating voltage is collected in real time,
step S3: calculating the reduction speed of the alternating voltage, and if the reduction speed of the alternating voltage is smaller than the threshold value, outputting a fault signal;
the threshold specifically comprises:
Figure FDA0002995830320000021
wherein: k is a radical ofacIs a threshold value, Id0For steady-state operation of direct current, ULLFor converting the current to AC bus-line voltage,/i0Is a receiving end no-load DC voltage, gammaminAt minimum extinction angle, beta is trigger lead angle, XcNUM for leakage reactance of converter transformer1To calculate process parameters.
5. The commutation fault detection device of claim 4, wherein the NUM parameter is calculated1The method specifically comprises the following steps:
Figure FDA0002995830320000022
wherein: k is a radical ofcNUM is a correction coefficient with a value between 0 and 12For commutation time, ω is the angular velocity of the fundamental frequency, RciEquivalent impedance on DC lines for commutation, RdIs the resistance of the dc line.
6. The commutation fault detection device according to claim 5, wherein the commutation time NUM is equal to or greater than a predetermined value2The method specifically comprises the following steps:
Figure FDA0002995830320000023
wherein: γ is the extinction angle and β is the trigger advance angle.
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