CN107390010B - Method for rapidly detecting trailing current of current transformer - Google Patents
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Abstract
The invention discloses a method for rapidly detecting a trailing current of a current transformer, which aims to improve the detection speed of the trailing current of the current transformer. The method of the invention comprises the following steps: the method comprises the steps of obtaining parameters of a power transmission line, calculating an impedance angle and a decay time constant of a secondary circuit, collecting parameters of power supply voltage of the power transmission line before a fault moment, calculating a fault current fundamental wave effective value, and comparing the fault current fundamental wave effective value with a current constant value of failure protection. Compared with the prior art, the method has the advantages that the difference filtering is carried out twice, then the effective value of the fundamental component in the trailing current is obtained by utilizing a quarter-cycle absolute value integration mode, whether the effective value is smaller than the current return constant value of the failure protection or not is compared, namely the fault state disappears or the protection action condition is not met, the normal operation state is recovered, the time for judging the trailing current can be shortened to be within 10ms, the reliability is ensured, the failure protection action delay is shortened, and the operation risk of a power system is remarkably reduced.
Description
Technical Field
The invention relates to a relay protection method, in particular to a detection method for trailing current of a current transformer.
Background
The current power grid backbone network in China is a large alternating current-direct current hybrid power grid, and the power electronic characteristics of the power grid become more and more obvious along with the massive access of a direct current power grid system. In order to solve the problem of direct current locking caused by the failure of the circuit breaker, national power grid companies propose a solution for station domain failure, the alternating current power grid system is required to have a fault, and when the circuit breaker fails, all adjacent switches are cut off within 200ms from the moment of the fault occurrence so as to prevent multiple phase change failures of the direct current power grid system. To remove the fault within 200ms, the action delay of the failure protection must be compressed, wherein the inherent time of the switch action cannot be compressed, and only the trailing current of the current transformer CT can be used as the current criterion.
In order to prevent adverse effects that the CT trailing current may bring to the failure current criterion, the method adopted in the prior art is to filter the dc component in the current through a system filtering algorithm before performing the protection logic calculation, and then calculate the related current criterion of the failure start through a full-period fourier algorithm. However, this method has a poor filtering effect on low-frequency dc components. Based on the CT trailing current detection method, some documents propose an improved full-period Fourier algorithm, and an Mann-Morrison algorithm is adopted to improve imaginary part calculation, so that the algorithm can basically ensure that a direct-current component is completely filtered within 25ms after fault removal, but the defect that the CT trailing current detection speed is not ideal exists.
Disclosure of Invention
The invention aims to provide a method for rapidly detecting a trailing current of a current transformer, and aims to improve the detection speed of the trailing current of the current transformer.
The invention adopts the following technical scheme: a method for rapidly detecting trailing current of a current transformer comprises the following steps:
firstly, when the power transmission line of the protected section breaks down, the protection device obtains the equivalent resistance (R) of the power transmission linefault) A load resistance (R'), an equivalent inductance (L)fault) A load inductor (L'), an exciting resistor (R) of a current transformerm) Exciting reactance (X) of current transformerm) Leakage resistance (R) of primary side winding of current transformer1) Leakage resistance (R) of secondary side winding of current transformer2) Leakage reactance (X) of primary side winding of current transformer1) Leakage reactance (X) of secondary side winding of current transformer2) Load impedance (R)load) Voltage of power supply system(Em)；
Second, calculate the impedance angleAnd the decay time constant (T) of the secondary loop2)：
Thirdly, acquiring a phase angle (alpha) of the power supply voltage of the power transmission line before the fault moment and acquiring an angle of the current of the power transmission line at the fault occurrence momentTime (0) when transmission line fault occurs, and time (F) when transmission line fault is removed1) Instantaneous value i (F) of current at the time of fault removal1)；
Fourthly, calculating the effective value (I) of the fundamental wave of the fault current:
n is the number of sampling points 24 per cycle, delta t is the time difference between two adjacent sampling points of 0.833ms, i (N) is the current sampling value of the nth point after the fault current passes through the secondary differential filtering, i (0) is the current sampling value of the 0 th point after the fault current passes through the secondary differential filtering,for the second differential filtering of fault currentCurrent sampling values of the points;
and fifthly, comparing the fault current fundamental wave effective value (I) with the current constant value of the failure protection.
The current sampling value of the fault current after the secondary differential filtering is as follows:
t is the time from the time of occurrence of the fault, the first reduction factor (lambda)1) And a second reduction factor (lambda)2) Comprises the following steps:
the differential scaling factor (k) is:
time constant (T) of non-periodic component current decay1)：
Initial value (I) of non-periodic component of fault currentm0)：
Amplitude (I) of periodic component of fault currentm)：
Current amplitude (I) of secondary side of current transformerpm)：
The current sampling value of the nth point of the fault current after the secondary differential filtering is as follows:
t is the time instant of a discrete sample point n.
The current sampling value of the 0 th point after the fault current is subjected to secondary differential filtering is as follows:
t is the time of occurrence of the fault.
The fault current of the invention is subjected to secondary differential filteringThe current sample values for the points are:
t is the time of a quarter cycle of the current fundamental wave from the time of fault occurrence.
Comparing the fault current fundamental wave effective value (I) with the current constant value of the failure protection, judging whether the fault state disappears or does not meet the protection action condition, and recovering the normal operation state.
Comparing the effective value (I) of the fundamental wave of the fault current with the current constant value of the failure protection, judging whether the fault state does not disappear or the protection action condition is met, and tripping the adjacent circuit breaker by the failure protection action.
The protection device is arranged in a main control room of a transformer substation, and a current transformer for collecting fault current of a power transmission line is arranged beside a circuit breaker of a protected line.
Compared with the prior art, the method has the advantages that the current flowing through the CT is subjected to differential filtering twice, then the effective value of the fundamental component in the CT trailing current is obtained by utilizing a quarter-cycle absolute value integration mode, whether the effective value is smaller than the current return fixed value of the failure protection or not is compared, whether the failure protection action returns or not is judged, namely the fault state disappears or the protection action condition is not met, the normal operation state is recovered, the return time of a current element is short under the CT trailing condition, the trailing current judging time can be shortened to be within 10ms, the reliability is ensured, the failure protection action delay is shortened, and the operation risk of a power system is remarkably reduced.
Drawings
Fig. 1 is a T-shaped equivalent circuit diagram of a current transformer.
FIG. 2 is a CT trailing current waveform diagram according to an embodiment of the present invention.
Fig. 3 is an equivalent circuit diagram of the infinite large-capacity power supply system when a three-phase short circuit occurs.
FIG. 4 is a graph of the reduction factor after a single filtering of the present invention versus a time constant.
FIG. 5 is a graph of the reduction factor after quadratic filtering of the present invention versus time constant.
FIG. 6 is a circuit diagram of a simulation model of a simulation example of the present invention.
Fig. 7 is a waveform diagram of a recording wave of a protection device according to an exemplary simulation of the present invention.
FIG. 8 is a diagram illustrating the effective value of CT trailing current calculated according to the simulation example of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples. As shown in fig. 1, the current transformer CT has a structure similar to that of a common transformer, and is converted into a T-shaped equivalent circuit for analysis, where R is a reference voltage1Representing the leakage resistance, X, of the primary winding of a transformer (CT)1Is leakage reactance of primary winding of transformer, R2Is leakage resistance, X, of the secondary winding of the transformer2Leakage reactance, R, of secondary winding of transformermIs the excitation resistance of a transformer, XmIs the excitation reactance of a transformer, RloadIs the load impedance to which the current transformer is connected.
As shown in fig. 2, in the CT trailing current waveform of a certain transmission line fault, the occurrence time of the transmission line fault is 0, and the fault removal time of the transmission line is F1. The fault current flowing through the secondary side of the CT during the two periods from the occurrence of the fault to the fault removal and after the fault removal is analyzed below.
From the fault of the transmission line of the protected section to the protection trip command opening until the circuit breaker arc is completely extinguished (fault)Cut off), the current flowing through the secondary side of the CT is fault current if(document 1: "power grid technology", 2008 4 th, power grid short-circuit protection principle based on current change rate, Zhao Min, pages 105-108), fault current ifThe size of (A) is as follows:
in the formula (1), Im0Initial value of non-periodic component, T, representing fault current1The time constant representing the attenuation of the non-periodic component current is determined by the equivalent impedance of the power transmission line at the fault, t is the time from the fault occurrence moment, ImRepresenting the amplitude of the periodic component of the fault current, from the amplitude E of the supply voltagemAnd the equivalent impedance of the transmission line at the fault,is the angular frequency of the fault current and,the angle of the transmission line current at the moment of the fault occurrence.
When the load of a user of an electric power system with a capacity much larger than that of a user internal power supply system changes and even short circuits occur, the voltage amplitude and the frequency of a power supply bus of a power substation of the electric power system basically keep unchanged, namely, the electric power system is called an infinite large-capacity power supply, and in actual operation, when the internal impedance of the electric power system power supply does not exceed 10% of the total impedance of a short circuit loop or the capacity of the electric power system exceeds 50 times of the capacity of the user power supply system, the electric power system can be regarded as the infinite large-capacity power supply.
As shown in fig. 3, when a three-phase short circuit occurs in the infinite large-capacity power supply system, the short circuit can be equivalent to E in each phase of power transmission linem、Rfault、LfaultR ', L' are connected in series in sequence, E of three phasesmAnd circuits having respective ends connected thereto, wherein EmRepresenting the amplitude of the A-phase power supply voltage, RfaultRepresents the equivalent resistance, L, of the A-phase transmission line at the faultfaultRepresenting the equivalent inductance of the A-phase transmission line at the fault, R 'representing the load resistance of the A-phase, L' representing the load inductance of the A-phase, and the fault point K being located at LfaultAnd R'. B. And the like for the C-phase power transmission line.
Describing the acquisition of each physical quantity in the formula (1), the amplitude I of the periodic component of the fault currentm：
Time constant T of non-periodic component current decay1：
Because the current flowing through the secondary side of the CT does not suddenly change before and after the fault occurrence time, the initial value I of the non-periodic component of the fault currentm0The acquisition of (2) needs to be analyzed in combination with the state before the fault occurs.
Current i flowing through the secondary side of CT before the moment of failurefThe current amplitude I flowing through the CT secondary side before the fault momentpmAnd circuit impedance angle before fault timeInfluence, circuit impedance angle before the moment of failureIs a current ifLagging supply voltage EmThe angle of the secondary side of the CT is determined by the equivalent resistance and the equivalent reactance of the transmission line, and the current i of the secondary side of the CT is determined before the fault momentfCurrent amplitude I of CT secondary sidepmAnd circuit impedance angleComprises the following steps:
in the formula (4), α is a phase angle of the power supply voltage of the transmission line before the fault time.
After the moment of failure, the circuit in fig. 3 is divided into two separate parts, the circuit to the left of the point of failure K is still connected to the power supply, and the failure current is directed to the current of this part of the circuit from the power supply of phase a to the short-circuit point. Still taking phase a as an example, according to kirchhoff's voltage law of the circuit, the voltage relationship of the loop connected to the power supply to the left of the fault point K is expressed by a first order differential equation:
in the formula (5), LfaultdifDt is inductance LfaultThe voltage across the terminals represents the current-voltage characteristic of the inductor after the time of the fault.
According to the circuit switching law, the current in the inductor cannot change suddenly, the secondary side current flowing through the CT cannot change suddenly before and after the fault occurs, and i in the formula (5)fAnd i in formula (4)fWhen t is equal to 0, the initial value I of the non-periodic component of the fault current can be obtainedm0：
After the fault of the transmission line is cut off, as shown in fig. 1, the secondary branch formed by the CT excitation branch and the secondary side winding and the load impedance still form an electrical loop, because the instantaneous value of the current of the excitation branch and the secondary side winding at the moment of the breaker opening is not zero, the energy stored in the inductance of the CT secondary winding is gradually released through the CT excitation branch, the secondary side winding and the load impedance loop, the discharge loop is a typical first-order RL loop, and the current i flowing through the CT secondary side after the fault is cut offfComprises the following steps:
in formula (7), i (F)1) Indicating the primary side current cut-off time(F1Time), i.e. the instantaneous value of the current of the secondary side at the time of fault removal, and also the initial value of the trailing current (CT trailing current, trailing current) of the CT secondary side after fault removal, T2The time constant of the secondary loop is determined by the impedance in the secondary loop.
According to the analysis, the fault current i flowing through the secondary side of the CT from the time of the fault occurrencef(t) is:
according to the formula (8), the fault current i is obtainedf(t) effective value, then the fault current ifAnd (t) comparing the effective value with a failure protection action fixed value (current fixed value of failure protection), and if the fault current effective value is smaller than the failure protection action return fixed value, judging that the failure protection action returns (the fault state disappears or does not meet the protection action condition, and recovering to the normal operation state).
In power relay protection, an effective value of an alternating current is generally obtained by using a fourier algorithm. The Fourier algorithm is a method widely applied to the current power relay protection, and adopts orthogonal current signals as standard current signals, respectively carries out corresponding integral transformation on the standard current signals and current signals to be processed to obtain real-axis components and imaginary-axis components with the same frequency as the standard current signals in the current signals to be processed, and can obtain effective values of the frequency components in the current signals to be processed by utilizing the real-axis components and the imaginary-axis components (document 2: power automation equipment, No. 1 in 2008, filtering performance analysis of common Fourier transformation, Yao brightness, Hu Ren super, Hangjing, pages 73-76). The fundamental wave and each subharmonic in the current signal can be accurately calculated by utilizing the Fourier algorithm, and the harmonic filtering capability is strong, but the Fourier algorithm cannot filter the attenuated direct-current component. When there is an attenuated dc component (non-periodic component) in the fault current signal, the result obtained by the fourier algorithm has a large error. Thus, for fault current if(t) the CT trailing current with attenuated DC component is directly calculated by Fourier methodThe method may cause the obtained effective value of the current to exceed the fixed value of the current of the failure protection, and cause the malfunction of the failure protection.
In order to eliminate the influence of the attenuated DC component on the malfunction possibly caused by the failure protection, the invention applies to the fault current if(t) a differential filtering method is adopted, the sampled data at the current moment of the fault current minus the sampled data at the previous moment, and because the direct current component is attenuated, the direct current components in the two sampled data are not completely the same, the differential filtering cannot completely eliminate the direct current component, but can reduce the attenuated direct current component to a certain extent.
For the trailing current of the secondary side of the CT, after the fault of the power transmission line is removed, due to the nonlinear characteristic of the excitation branch, the time constant of the secondary circuit is related to the saturation degree of the CT magnetic flux, the CT structure and the iron core structure, the remanence of the excitation branch naturally attenuates with the fault current, and therefore the attenuated direct-current component of the fault current is continuous. In order to filter the attenuated DC component of the fault current to a certain extent, i of equation (8) is usedf(t) carrying out first-order derivation to obtain fault current i 'after primary differential filtering'f(t)：
Since discrete digital values are generally used for the relay protection logic determination, the first derivative calculation for the fault current of equation (8) can be simulated by a differential calculation.
The gain value after the difference calculation is carried out on the current values of two discrete adjacent sampling points, namely the ratio of the fault current amplitude after the difference to the fault current amplitude before the difference is(document 3, Relay, No. 19 2006, research on Intelligent Low-Voltage Motor protection device, easy climbing, Shiyihui, Zhang Cheng, pp 7-10), where t1、t2Respectively, representing two adjacent sample point times. To recover fault current middle periodThe original amplitude of the periodic component is divided by the gain value to obtain the value at t2Fault current after one differential filtering at the moment:
in the formula (10), if′(t2) Indicating a fault current at t2Fault current value i after one time differencef(t1) And if(t2) Are respectively shown at t1And t2Value of fault current at time if(t2)-if(t1) Is t2The fault current differential value of a point, k, represents the differential proportionality coefficient.
When t is1、t2For the time of two adjacent sampling points, if sampling point 24 of each cycle, the time difference between two adjacent sampling points is:
drive formula (11) to formula (9), fault current i 'after primary differential filtering'f(t) is:
as can be seen from equation (12), after one differential filtering, that is, one discrete derivation calculation, the amplitude of the periodic component is not changed, the attenuated dc component (non-periodic component) of the CT tail current becomes small, and the reduction coefficient λ of the attenuated dc component of the CT tail current is:
in the equation (13), T represents a decay time constant, and corresponds to the time constant T of the non-periodic component current decay before the time F11Decay time constant T of the secondary loop after time F12Sampling rate of 24 points per cycle, and making correlationAnd carrying out differential calculation between the two subsequent sampling points to obtain a relation curve between the lambda and the T. As shown in fig. 4, three points are marked in the figure, which indicate that when the decay time constant T is equal to 50ms, 100ms and 140ms, respectively, the reduction coefficients λ of the dc component of the CT tail current decay are 0.06384, 0.03192 and 0.0228, respectively, that is, the reduction coefficient λ of the dc component of the CT tail current decay decreases monotonically with the increase of the decay time constant T, that is, the larger the time constant is, the better the effect of filtering the CT tail current decay dc component is. Let t1Time t and2first reduction factor lambda of time1And a second reduction factor lambda2Comprises the following steps:
in order to further filter the attenuated DC component in the CT trailing current, the fault current i of the CT secondary side of the equation (8)f(t) further discretizing, carrying out secondary differential filtering, carrying out second-order derivation, and substituting the formula (14) to obtain fault current (CT trailing current) i ″, which is subjected to secondary differential filteringf(t)：
As shown in fig. 5, according to equation (15), a relation curve between λ and T is made, and three points are marked in the graph, which indicate that when the decay time constant T is equal to 50ms, 100ms and 140ms, respectively, the reduction coefficients are 0.004, 0.0001 and 0.0005, respectively, that is, after two times of differential filtering, the decaying dc component of the tail current is further reduced, and the decaying dc component in the tail current on the secondary side of CT is ideally suppressed. The smaller the reduction coefficient lambda of the attenuated direct current component of the trailing current is, the less the attenuated direct current component in the trailing current is, so that the error of the effective value of the fault current fundamental wave solved by adopting the Fourier algorithm is smaller, and the misoperation probability of the failure protection is smaller. Therefore, it is desirable that the reduction coefficient is as small as possible, and the effect of the reduction coefficient is more desirable. Since the reduction factor cannot be zero, it approaches zero indefinitely. Ideally is a relative concept, which is ideal relative to the effect of not filtering differentially or filtering once differentially.
And carrying out differential filtering twice on the fault current, and realizing by adopting second-order derivation operation. The non-periodic component in the fault current can be restrained to a certain extent by the primary differential filtering, and the direct-current component attenuated by the fault current can be filtered out more ideally by the secondary differential filtering.
After the secondary differential filtering, the attenuated direct current component in the trailing current of the secondary side of the CT can have an ideal filtering effect, and the accuracy of obtaining the effective value of the fault current by adopting the Fourier algorithm is higher, so that the probability of malfunction and maloperation is lower. The following describes a process of obtaining an effective value of a fundamental current by performing a second differential filtering on a trailing current on a secondary side of the CT and performing a half-cycle fourier calculation.
From the time of failure to the time needed for obtaining enough original sampling data, the time window of the half-cycle Fourier algorithm is 10ms of a half cycle, namely the integration interval of Fourier transform is 0-T/2, the real-axis component and the imaginary-axis component of the fundamental component of CT trailing current are calculated by utilizing sampling points i (n) of the half cycle, and then the effective value of the fundamental component of the CT trailing current is obtained (document 2: power automation equipment, No. 1 of 2008, common Fourier transform filtering performance analysis, Yao Liang, Hu Re super, Hang Ching, pages 73-76):
in equation (16), N represents the number of sampling points per cycle of the fundamental wave signal, i (N) (i ″)f(t)) ]is the sampled value of the current at the n-th point after the second differential filtering, a1Representing the real-axis component of the fundamental component after a half-cycle Fourier transform, b1The imaginary axis component of the fundamental component after half-cycle fourier transform is represented. Calculating the real-axis component a of the fundamental component of the CT trailing current by using a half-cycle Fourier algorithm1And the imaginary axis component b1Then, calculating an effective value I of a fundamental component in the CT trailing current:
the above calculation process: subjecting the secondary differential filtered CT trailing current i ″, obtained in the formula (15), to secondary differential filteringf(t) the formula (16) is taken in, and the real axis component a of the fundamental wave component is calculated1And the imaginary axis component b1Then, the real axis component a is calculated1And the imaginary axis component b1And (4) carrying out calculation by an expression (17) to obtain an effective value I of a fundamental component in the CT trailing current.
And judging whether to return or not according to whether the effective value I of the fundamental component in the CT trailing current meets the return condition of the failure protection current element or not. The return conditions for the failsafe current element are: and (4) judging that the failure protection action returns when the fault state disappears or does not meet the protection action condition and recovering the normal operation state if the effective value I of the fundamental wave component in the CT trailing current is less than the current return fixed value of the failure protection, (document 4: power system automation, No. 2 in 2015, 500kV circuit breaker failure considering the system stability requirement, dead zone protection optimization, Yujiang, Zhouyangyang, Chenghui, Xuguan Hu, Liyiquan, Zhangfang, pages 142-146).
For the alternating current with the frequency of 50Hz, one cycle of a current signal is 20ms, a half-cycle data window of 10ms is needed by adopting a half-cycle Fourier algorithm, namely, the effective value of the fundamental component in the CT trailing current needs 10ms for calculation, and then the judgment is carried out according to the return condition, so that the return time of the failure protection at least needs more than 10 ms.
In order to further increase the detection speed of the CT trailing current, further shorten the time delay of the malfunction protection operation, and reduce the operation risk of the power system, the operation is calculated from the rapid amplitude (document 5: protection and control of the power system, phase 11 in 2009, several improved algorithms of a half-cycle integration algorithm, waning, explain, and normal surge, pages 66-69), the effective value (effective value of a quarter-cycle) of the trailing current fundamental wave in this period is obtained by using the charge amount of the CT trailing current flowing in the quarter-cycle, the integral calculation is performed on the absolute value of the CT trailing current in the quarter-cycle, then the effective value of the trailing current is solved by using the integral value, and whether the malfunction protection operation return condition is satisfied is judged. The charge quantity s of the tail current flowing in the quarter-cycle, namely the integral value of the quarter-cycle, is as follows:
wherein
In the equations (18) and (19), β represents the initial angle of the quarter-cycle absolute value of the trailing current, which is the initial angle of the integral operation, I represents the effective value of the CT trailing current, c represents the integral solving coefficient of the quarter-cycle absolute value of the trailing current, the size of c is related to the value of β, and in the value range of β, the maximum value and the minimum value of c exist, which are respectively:
in the equation (20), the minimum value c of the integral solving coefficient is selectedminAs a standard, the effective value I of the corresponding CT trailing current at this time will become larger, and the failure protection action return condition is to determine whether the effective value of the fault current fundamental wave is smaller than the current return constant value, so that the logic determination of the return element is not affected. For the quarter-cycle integral value S in equation (18), it can be approximated by a trapezoidal rule:
in equation (21), N represents the number of sampling points 24 per cycle of the fundamental wave signal, Δ t represents the time difference between two adjacent sampling points, and i (N) (i ″)f(t)) ], i (0) is the current sample value at the point n after the fault current is subjected to the second differential filtering, i (0) is the current sample value at the point 0 after the fault current is subjected to the second differential filtering,to failure electricityStream is twice differentially filtered beforeThe current sampling value of the point, the current sampling value of the fault current after the secondary differential filtering obtains i ″, according to the formula (15)f(t), where t is the time instant of the discrete sample point n.
The effective value I of the fault current fundamental wave is reduced by the combined formulas (18) to (20):
the CT trailing current is detected, so that fundamental wave alternating current components in the CT trailing current are detected, non-periodic components of the CT trailing current are filtered, the non-periodic components can be basically filtered by adopting twice differential filtering, and for obtaining the fundamental wave alternating current components, the shorter the time window is, the faster the obtaining speed is, but the precision of the obtained fundamental wave alternating current components is reduced; conversely, the longer the time window, the slower the acquisition speed, and the higher the accuracy of the obtained fundamental wave alternating current component. By using the half-wave symmetry, i.e. quarter-cycle symmetry, of the fundamental wave alternating current component, and because the return constant value of the current returned by the failure protection action is very small, the current return element judges whether the fault current exists or not, the accuracy can be properly reduced, and the time window is set to be a quarter-cycle. Therefore, the method of the invention adopts a quarter-cycle absolute value integration mode to detect the effective value of the CT trailing current, and for 50Hz power frequency fault current, the effective value solving time window is a quarter-cycle, namely 5ms, so that the CT trailing current distinguishing time can be limited within 10 ms.
According to the method, two differential filtering are adopted for the fault current of the secondary side of the CT, the attenuated direct current component in the CT trailing current is filtered, and then an 1/4-cycle absolute value integral algorithm is adopted to obtain the fundamental wave effective value of the fault current.
And (3) carrying out simulation test on the method by using a real-time digital simulator (RTDS). As shown in fig. 6, a simulation model is constructed, wherein S1 and S2 represent power supplies of substations on two sides of the power transmission line, and the power supplies are S1 and S2 have voltage EmFor 500kV, SK1 and SK2 respectively represent outgoing switches of substations on both sides of the power transmission line, CT represents a current transformer, JAModel element simulation is adopted in this embodiment, and equivalent parameters thereof are set as follows: leakage resistance R of primary side winding of CT10.02 omega, leakage reactance X of the primary winding of CT10.6 omega, leakage resistance R of secondary side winding of CT20.5 omega, leakage reactance X of CT secondary side winding20.25 omega, CT exciting resistance Rm0.04 omega, CT excitation reactance XmAnd 45.4 omega. Equivalent load impedance R connected after CTload0.5 omega, the equivalent resistance R of the transmission line at the faultfault0.17 omega/km, equivalent inductance L of the transmission line at the faultfaultIs 1.2e-3H/km, using an equivalent resistance RfaultAnd equivalent inductance LfaultSolving the transmission line impedance angle (circuit impedance angle)Is 66 degrees; leakage reactance X using CT secondary side winding2CT excitation reactance XmAnd leakage resistance R of secondary side winding of CT2CT exciting resistor RmEquivalent load impedance RloadSolving the decay time constant T of the secondary loop2Is 140 ms. The simulation is realized by adopting a real-time digital simulation system RTDS of Mannich Toba RTDS company of Canada, with the system version being 4.007.4. Simulating fault occurrence and fault removal by using a fault simulation module of the RTDS, wherein the phase angle alpha of the power voltage at the fault moment is 1200, and the angle of the current of the power transmission line at the fault momentIs 54 degrees. When a ground fault occurs on outgoing lines of a certain 500kV transformer substation, the line protection device acts correctly, and after the fault is removed, the CT magnetic flux attenuates to generate a larger trailing current, as shown in fig. 7, recorded by the line protection deviceAnd (3) recording waveforms, wherein the sampling frequency is 24 points per cycle, and the current cutting time in a recording waveform diagram is 0.
As shown in fig. 8, the CT trailing current effective values obtained by the first differential filtering + full-period fourier algorithm, the second differential filtering + half-period fourier algorithm, and the second differential filtering + 1/4-period fourier algorithm are respectively used, the vertical axis represents the CT trailing current effective value, the horizontal axis represents time, and a CT trailing current effective value graph is drawn for comparison. It can be seen that the CT trailing current effective value time obtained by the two-time difference filtering and the half-period fourier algorithm is within 15ms, and the CT trailing current effective value time obtained by the two-time difference filtering and the 1/4-period fourier algorithm is within 10 ms. The effective values of the CT trailing currents obtained by the two-time difference filtering + half-cycle Fourier algorithm and the two-time difference filtering + 1/4-cycle Fourier algorithm are compared in table 1. The effectiveness and superiority of the method are verified by modeling simulation and comparison of results of calculating the effective value of the CT trailing current by adopting different algorithms.
Method of the invention, in the transmission line of the section to be protected, Rfault、R′、Lfault、L′、Rm、Xm、R1、R2、X1、X2、Rload、Em、T2、α、0、F1、i(F1) The method comprises the following steps:
equivalent resistance R of power transmission linefaultThe load resistance R 'is obtained by calculating the resistivity of the wire and the nominal sectional area of the current-carrying part of the wire, the load resistance R' is also obtained by calculating the resistivity of the wire and the nominal sectional area of the current-carrying part of the wire, because the fault point K divides the power transmission line into two parts, the parameter acquisition methods of the two resistors are consistent, and the equivalent inductance L of the power transmission line is equivalentfaultCalculating the radius of the wire, the mutual geometric mean distance between the wires and the relative magnetic permeability coefficient of the wire materialThe load inductance L' is obtained through the radius of the lead, the mutual geometric mean distance between the leads and the relative magnetic permeability coefficient of the lead material, and similarly, the fault point K divides the power transmission line into two parts, and the two inductance parameters are obtained in the same way (document 6: electric power system analysis, third edition, Huiyan, Wenzhong silver, Wuhan, Huazhong science and technology university Press, 2002, 1 month, 11-15).
Excitation resistor R of current transformermThe exciting reactance X of the current transformer is obtained by calculating the iron core loss (no-load loss) and the rated voltagemThe leakage resistance R of the primary side winding of the current transformer is obtained by calculating the no-load current, the rated voltage and the rated capacity1Leakage resistance R of secondary side winding of current transformer2The leakage reactance X of the primary side winding of the current transformer is obtained by calculating short-circuit loss, rated voltage and rated capacity1Leakage reactance X of secondary side winding of current transformer2Calculated by short-circuit voltage, rated voltage and rated capacity (document 6, electric power system analysis, third edition, book of honor, warming silver, Wuhan: university of science and technology publishers, 2002, 1 month, pages 26-34).
Load impedance RloadThe value is determined by the magnitude of the load impedance connected to the current transformer and can be input to the protection device in advance.
Amplitude E of the supply voltagemThe protection device can be input in advance depending on the voltage of the power supply system.
Angular frequency of fault currentIn relation to the cycle frequency, for a power frequency of 50Hz,is a constant value of 100 pi.
Using the equivalent resistance R of the transmission linefaultLoad resistor R' and equivalent inductance L of power transmission linefaultLoad inductance L', calculating the impedance angle (circuit impedance) of the transmission line according to the formula (4)Corner)
Leakage reactance X using CT secondary side winding2CT excitation reactance XmLeakage resistance R of secondary side winding of CT2CT exciting resistor RmEquivalent load impedance RloadThen, the attenuation time constant T of the secondary circuit is calculated from the equation (7)2。
Phase angle alpha of power supply voltage of power transmission line before fault moment and angle of current of power transmission line at fault occurrence momentTime 0 when transmission line fault occurs and time F when transmission line fault is removed1Instantaneous value i (F) of current at the time of fault removal1) Obtained from the record of the protection device.
In the process of the invention, Rfault、R′、Lfault、L′、Rm、Xm、R1、R2、X1、X2、Rload、Em、T2Calculated by the protection device, alpha,0、F1And i (F)1) And recording by the protection device.
According to the calculated Rfault、R′、Lfault、L′、Rm、Xm、R1、R2、X1、X2、Rload、Em、T2Recording the obtained alpha,0 and F1According to the formula (22), the protection device calculates the effective value I of the fundamental wave of the fault current, and compares the effective value I with the return constant value of the current of the failure protection. If the effective value of the fault current fundamental wave is smaller than the fixed value of the failure protection action, the failure state is judged to disappear or the protection action condition is not met, and the normal operation state is recovered. If the effective value of the fault current fundamental wave is larger than the failure protection action fixed value, the failure state is judged not to disappear or the protection action condition is met, and the failure protection action is carried out to jump off the adjacent circuit breaker.
Engineering application example: 220kV outgoing lines of a certain 220kV transformer substation have faults, a lead is LGJ-400, and the equivalent resistance R of a power transmission line at the fault positionfault0.08 omega/km, equivalent inductance L of transmission line at faultfaultThe power transmission line fault current detection method is characterized in that 1.31e-3H/km is adopted, a PRS-7792-DA-G type protection device of a Changyuan deep-Ray relay protection automation company Limited is arranged in a master control room of a 220kV transformer substation, a current transformer for collecting fault current of a power transmission line is arranged beside a circuit breaker of a protected line, calculation and judgment are realized by adopting C language with the version of V1.00, trailing current of the current transformer is rapidly detected when a fault occurs, and detected data are shown in a table 2. As can be seen from Table 2, the fundamental effective value of the fault current is obtained basically at 6ms by adopting a quarter-cycle integral fast integration algorithm, and the fundamental effective value of the fault current is calculated at 11ms by controlling a half-cycle Fourier algorithm. The method of the invention is shown to improve the reliability of the failsafe.
According to the rapid detection method for the CT trailing current, twice differential filtering is adopted before the effective value of the fundamental current is obtained, the attenuated direct current component in the CT trailing current is filtered, the precision of the effective value of the fundamental current is improved, the 1/4-cycle absolute value integration algorithm is adopted to obtain the effective value of the fundamental current of the fault current, and the time for judging the CT trailing current is limited within 10 ms. The method can ensure that the breaker can complete obtaining of the fundamental wave effective value of the trailing current within 10ms after the fault current is cut off, and further complete condition judgment of failure protection action return, namely, a CT trailing current rapid detection method is adopted, and the action delay of failure protection is compressed, so that the tripping time of the breaker is prolonged, a power system breaks down, when the breaker fails, all adjacent switches are cut off within 200ms from the fault occurrence moment, so that multiple commutation failures of a direct current network system are prevented, the action delay of the failure protection is shortened, the tripping time of the adjacent switches is prolonged, and the reliability of the failure protection is improved.
TABLE 1 CT trailing Current effective value comparison
TABLE 2 CT trailing current effective value comparison for engineering practical application
Claims (8)
1. A method for rapidly detecting trailing current of a current transformer comprises the following steps:
firstly, when the power transmission line of the protected section breaks down, the protection device obtains the equivalent resistance (R) of the power transmission linefault) A load resistance (R'), an equivalent inductance (L)fault) A load inductor (L'), an exciting resistor (R) of a current transformerm) Exciting reactance (X) of current transformerm) Leakage resistance (R) of primary side winding of current transformer1) Leakage resistance (R) of secondary side winding of current transformer2) Leakage reactance (X) of primary side winding of current transformer1) Leakage reactance (X) of secondary side winding of current transformer2) Load impedance (R)load) Voltage of the power supply system (E)m)；
Second, calculate the impedance angleAnd the decay time constant (T) of the secondary loop2)：
Is the angular frequency of the fault current;
thirdly, acquiring a phase angle (alpha) of the power supply voltage of the power transmission line before the fault moment and acquiring an angle of the current of the power transmission line at the fault occurrence momentTime (0) when transmission line fault occurs, and time (F) when transmission line fault is removed1) Instantaneous value i (F) of current at the time of fault removal1)，F1The moment of fault removal;
fourthly, calculating the effective value (I) of the fundamental wave of the fault current:
n is the number of sampling points 24 per cycle, delta t is the time difference between two adjacent sampling points of 0.833ms, i (N) is the current sampling value of the nth point after the fault current passes through the secondary differential filtering, i (0) is the current sampling value of the 0 th point after the fault current passes through the secondary differential filtering,for the second differential filtering of fault currentCurrent sampling values of the points;
and fifthly, comparing the fault current fundamental wave effective value (I) with the current constant value of the failure protection.
2. The method for rapidly detecting the tail current of the current transformer according to claim 1, wherein the method comprises the following steps: the current sampling value of the fault current after the secondary differential filtering is as follows:
t is the time from the time of occurrence of the fault, the first reduction factor (lambda)1) And a second reduction factor (lambda)2) Comprises the following steps:
the differential scaling factor (k) is:
time constant (T) of non-periodic component current decay1)：
Initial value (I) of non-periodic component of fault currentm0)：
Amplitude (I) of periodic component of fault currentm)：
Current amplitude (I) of secondary side of current transformerpm)：
3. The method for rapidly detecting the tail current of the current transformer according to claim 2, wherein the method comprises the following steps: the nth point current sampling value of the fault current after the secondary differential filtering is as follows:
t is the time instant of a discrete sample point n.
4. The method for rapidly detecting the tail current of the current transformer according to claim 2, wherein the method comprises the following steps: the current sampling value of the 0 th point after the fault current is subjected to the secondary differential filtering is as follows:
t is the time of occurrence of the fault.
5. The method for rapidly detecting the tail current of the current transformer according to claim 2, wherein the method comprises the following steps: the fault current is subjected to secondary differential filteringThe current sample values for the points are:
t is the time of a quarter cycle of the current fundamental wave from the time of fault occurrence.
6. The method for rapidly detecting the tail current of the current transformer according to claim 1, wherein the method comprises the following steps: and fifthly, comparing the fault current fundamental wave effective value (I) with the current constant value of the failure protection, judging whether the fault state disappears or does not meet the protection action condition, and recovering the normal operation state, wherein the fault current fundamental wave effective value (I) is smaller than the current constant value of the failure protection.
7. The method for rapidly detecting the tail current of the current transformer according to claim 1, wherein the method comprises the following steps: and fifthly, comparing the fault current fundamental wave effective value (I) with the current constant value of the failure protection, wherein the fault current fundamental wave effective value is larger than the current constant value of the failure protection, judging that the fault state does not disappear or the protection action condition is met, performing the failure protection action, and tripping off the adjacent circuit breakers.
8. The method for rapidly detecting the tail current of the current transformer according to claim 1, wherein the method comprises the following steps: the protection device is arranged in a main control room of a transformer substation, and a current transformer for collecting fault current of the power transmission line is arranged beside a circuit breaker of a protected line.
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