CN113092847A - Method for predicting zero offset of fault current - Google Patents

Method for predicting zero offset of fault current Download PDF

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CN113092847A
CN113092847A CN202110350464.5A CN202110350464A CN113092847A CN 113092847 A CN113092847 A CN 113092847A CN 202110350464 A CN202110350464 A CN 202110350464A CN 113092847 A CN113092847 A CN 113092847A
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current
zero
fault
direct current
alternating current
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CN113092847B (en
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石晶
陈红坤
吴行健
陈璟瑶
李中祥
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Wuhan University WHU
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
    • G01R19/175—Indicating the instants of passage of current or voltage through a given value, e.g. passage through zero
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
    • G01R31/52—Testing for short-circuits, leakage current or ground faults

Abstract

The invention relates to a method for predicting zero offset of fault current. According to the characteristic that the fault current after filtering out the higher harmonics and the interference is combined by the standard alternating current component and the exponentially decaying direct current component, the zero crossing point of the alternating current component is estimated according to the periodicity of the alternating current component, and then zero offset caused by decaying the direct current component is considered, so that the zero crossing point of the actual fault current is estimated, the zero crossing point does not need to be corrected in application, and the method is simple and effective. In the calculation process, the total current is represented by the ordinate of one curve and is converted into the difference value of two curves, the zero crossing point of the total current is converted into the intersection point of an alternating current curve and a direct current curve, and therefore approximate solution can be carried out by means of a similar triangle. The invention clears the mathematical rule of zero offset caused by attenuated direct current component, solves the problem of fast calculation of the zero crossing point of the fault current, provides a very concise theoretical formula and provides theoretical support for the realization of the phase control on-off technology.

Description

Method for predicting zero offset of fault current
Technical Field
The switching device can be a component for rapidly switching on and off current through any arc extinguishing medium, generally a rapid vacuum switch, and can also be other devices needing accurate phase control operation.
Background
The phase selection control technology is adopted, namely, the phase when the breaker breaks the fault current is controlled, the breaker reaches the full opening distance at the position close to the current zero crossing point, and micro-arc energy of the fault current can be broken, so that the breaking capacity of the breaker is improved, the ablation of the arc on a contact is reduced, and the service life of the breaker is prolonged.
The circuit breaker phase-controlled opening sequence is shown in fig. 1. Wherein t isresPredicting algorithm response time for zero, twaitFor the waiting time between the end of zero-point prediction and the signaling of the opening action, tCBOFor the inherent operating time of the operating mechanism, tarcAs the time of arcing, tOThe total time from receiving the opening action signal to extinguishing the electric arc is the opening action time of the vacuum fast switch. t is tarcThe shorter the arcing time, the lower the arc energy, and the more favorable the switching-off. The method aims to predict the accurate time of the current zero crossing point after the fault current occurs, and the algorithm is required to be as fast as possible on the basis of ensuring the accuracy.
Generally, when a short-circuit fault occurs after the alternating current power supply operates with a load, the analysis can be carried out by using the equivalent circuit shown in fig. 2.
In fig. 2, FS is a vacuum fast switch; u (t) is the system supply voltage; u shapepmIs the system supply voltage peak value; i is1(t) is the power supply side short-circuit fault current; i isFAnd (t) is a short-circuit to ground fault current. If a short-circuit fault occurs at the time when t is 0, α is a fault initial phase angle, and ω is a power supply angular frequency. Dividing the line into two parts of a power end and a load end at the fault occurrence position, wherein the two parts are respectively represented by S and L, and omega LS+RSRepresenting the power failure impedance,. omega.LL+RLRepresenting the load impedance. Assuming that the short circuit fault occurs relatively far from the system power supply (rather than close to the generator outlet), the power supply can be considered as an infinitely high power device with zero internal impedance, which outputs a constant voltage, frequency during the short circuit.
In a stable operating state before a fault occurs, the current on the line is
I(t)＝Ipm0 sin(ωt+α-φ0) (1)
In the formula Ipm0The normal operation current amplitude before the fault occurs, phi0The voltage current phase difference (power factor angle) before the fault occurs.
After the fault occurs, the short-circuit fault current can be obtained according to kirchhoff law and Laplace transformation
Figure BDA0003002252380000021
In the formula IpmThe amplitude of the short-circuit current periodic component after the fault occurs, phi is the voltage current phase difference after the fault occurs, and tau is the decay time constant of the fault loop.
Different current values corresponding to a plurality of time points can be obtained through high-speed detection, the detection frequency is usually more than 1kHz, and at present, a plurality of methods are provided for fitting I mathematicallypmAnd τ, to which the invention is not directed. The method provided by the invention is how to calculate the time of the current zero crossing point on the basis of obtaining the two values.
Disclosure of Invention
The technical problem of the invention is mainly solved by the following technical scheme:
a method for predicting zero offset of fault current is characterized by comprising the following steps:
step 1, a detection circuit collects a current signal, a filtering unit filters higher harmonics and noise signals in the signal, whether fault current needs to be cut off or not is judged, if the fault current exists, step 2 is executed, and otherwise, current collection and judgment are repeated;
step 2, calculating formula I (t) ═ I according to the theory of short-circuit currentpm sin(ωt-θ)+De-t/τFitting and calculating the parameter in the formula, namely the AC peak value I of the fault short-circuit current by using the collected current valuepmThe AC angular frequency ω and the initial phase θ corresponding to the short-circuit time,the maximum value D of the direct current attenuation quantity, and the time constant tau of the direct current attenuation quantity;
step 3, according to the ω tzThe zero-crossing time of the periodic component is t ═ k pi (k ═ 1,2, 3.)zFor the purpose of rapidness, k is generally only 1 or 2;
step 4, calculating zero offset, wherein the total fault current corresponding to the zero crossing point moment of the alternating current component is not zero due to existence of the attenuated direct current component, the current value of the fault at the moment is delta I, and the zero crossing point moment of the fault current is tgThe method specifically comprises the steps of splitting standard alternating current and exponentially decaying direct current, converting the total current represented by the ordinate of one curve into the difference of two curves, converting the zero crossing point of the total current into the intersection point of the alternating current curve and the direct current curve, and converting the intersection point of the alternating current curve and the direct current curve by means of a similar triangle delta t-delta I/omega IpmCalculating to obtain a zero offset delta t caused by the direct current attenuation;
step 5, from tg＝tzAnd the + delta t estimates the specific moment of the current zero crossing point under the combined action of alternating current and direct current.
In the method for predicting the zero-point offset of the fault current, the alternating current zero-crossing point is calculated through the periodicity of alternating current, then the zero-point offset is calculated by considering the influence of attenuated direct current, and the specific time of the current zero-crossing point under the combined action of the alternating current and the direct current is predicted by combining the zero-point offset and the zero-point offset.
In the method for predicting the zero-point offset of the fault current, the value of the obtained predicted current zero-crossing point is similar to the actual value, even if small deviation exists, the value is a short time before the actual zero-crossing point, and the deviation does not need to be corrected, so that the time margin for improving the operation reliability is obtained.
Therefore, the invention has the following advantages: the method avoids time delay caused by online real-time solution of a nonlinear equation and high requirements on the computing power and the storage power of the detection unit, can accurately estimate the zero offset of the fault current only through one-step simple multiplication and division operation, and can be directly used without correcting a computing error. The invention also defines a simple and clear mathematical law of zero point offset caused by the attenuated direct current component.
Drawings
Fig. 1 is a schematic diagram of a phase-controlled opening sequence of a circuit breaker.
Fig. 2 is a schematic diagram of a basic model of short-circuit current.
Fig. 3 is a schematic diagram of a short circuit current waveform.
FIG. 4 is a schematic diagram of short-circuit current wave calculation under the method of the present invention.
Detailed Description
The technical scheme of the invention is further specifically described by the following embodiments and the accompanying drawings.
Example (b):
the standard sinusoidal current period is fixed, and depends on the frequency ω 2 pi f 2 pi/T of the voltage source, for example, the frequency f of the commercial power in China is 50Hz, the period T is 20ms, the commercial power in most countries in europe is 60Hz, and according to the functional expression of the alternating current, the current zero-crossing point will occur twice in one period, and the difference between the two times is T/2. In a short time after the fault, the current is not standard sinusoidal current, but is standard sinusoidal quantity plus a direct current component with exponential attenuation, and the existence of the direct current component can cause the two zero points of the current to be no longer T/2, but can generate offset, as shown in fig. 3, the phenomenon that a large half wave and a small half wave are changed alternately appears. The patent seeks to calculate this offset quickly and with high accuracy in order to signal the circuit breaker to operate.
For clarity of explanation, equation (2) may be further simplified as shown in the following equation:
Figure BDA0003002252380000031
wherein De-t/τIs a direct current component with exponential decay, D is the maximum value of the direct current decay,
Ipmsin (ω t- θ) is a standard sine function due to IpmAnd τ can be obtained by fitting a sampled current value, ω is determined by the power supply system, and θ (initial phase) at the time of short circuit (the time start point t is 0) can be easily calculated from the sampled current value) Therefore, all the parameters in the formula (3) are known quantities, and the current zero point is estimated, which is actually to find a positive value t corresponding to the formula (3) i (t) being 0, and the smaller t is, the closer t is to the short-circuit time, which is obviously a problem of solving a nonlinear equation.
(3) The zero crossing point of the AC flow in the formula can be determined according to the tzThe + θ ═ k pi (k ═ 1,2, 3.) is calculated, and for the purpose of high speed, k is generally only 1 or 2, and if the zero-crossing time of a certain periodic component is t, then it is tzThe zero-crossing time of the fault current corresponding thereto is tgThe time interval between them is Deltat, i.e. tg＝tz+ Δ t, it is difficult to estimate Δ t directly from FIG. 3, mathematically solving a non-linear equation. The method of the present invention is to calculate the attenuated DC amount in reverse direction and to convert the calculation formula from expression 3 to expression 4 in a new calculation system.
I(t)＝Ipm sin(ωt-θ)-(-De-t/τ) (4)
Illustrated schematically in FIG. 4, the AC part Ipmsin (ω t- θ) and DC exponential decay part De-t/τThe value of (a) is kept constant, and the fault current I (t) at each moment in the new calculation system is equal to the difference between the AC curve and the DC exponential decay curve, namely the difference between the ordinate of the two curves is shown in the figure.
The zero-crossing point of the alternating current component corresponds to a point P (P ') in the graph, the zero-crossing point of the fault current is the condition that the alternating current component is equal to the direct current component, and the time coordinate of the zero-crossing point of the fault current corresponds to a point N (N ') in the graph and is equal to the time coordinate of a point Q (Q '). Because the slope transformation is slow near the zero crossing point of the alternating current and can be approximated as a straight line with constant slope, and on the other hand, the attenuation time constant when the alternating current is switched on and off is far larger than the period of the alternating current, the attenuation of the direct current component is small in the short time from P to Q, the line MN can be considered to be parallel to the time axis, and the quadrangle PMNQ (P 'M' N 'Q') can be approximated as a rectangle. Let the AC zero-crossing point t for clarity of explanationzThe fault current I (t) at the moment is delta I, and the zero crossing point of the fault current and the zero crossing point of the alternating current can be estimated according to the valueThe time difference Δ t therebetween. The slope when the alternating current passes through zero is ω IpmThe line PM (P 'M') is approximately equal to NQ (N 'Q'), and has a value Δ I. The following equation holds true for triangle PQN (P ' Q ' N ').
ΔI/Δt＝ωIpm (5)
Thereby can obtain
Δt＝ΔI/ωIpm (6)
When the fault current passes through zero from the side with the same sign as the attenuated direct current component, namely the fault current experiences a large half-wave, delta t is an increment, see the quadrangle PMNQ, when the fault current passes through zero from the side with the different sign from the attenuated direct current component, namely the fault current experiences a small half-wave, delta t is an decrement, see the quadrangle P 'M' N 'Q', the calculation methods of the two have no difference.
As can be seen from the calculation process, the zero-crossing point delta t after the big half wave is estimated is slightly smaller, and the zero-crossing point delta t after the small half wave is estimated is slightly larger, so that the situation that the estimated fault zero-crossing point is slightly ahead of the actual fault zero-crossing point according to the method is just ensured, which is proper in the phase control technology, and therefore, extra correction on the calculation error is not needed. According to the method, the zero offset value of the fault current can be accurately estimated, and theoretical support is provided for the phase control on-off technology.
The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.

Claims (3)

1. A method for predicting zero offset of fault current is characterized by comprising the following steps:
step 1, a detection circuit collects a current signal, a filtering unit filters higher harmonics and noise signals in the signal, whether fault current needs to be cut off or not is judged, if the fault current exists, step 2 is executed, and otherwise, current collection and judgment are repeated;
step 2, according to short-circuit currentTheoretical calculation formula I (t) ═ Ipmsin(ωt-θ)+De-t/τFitting and calculating the parameter in the formula, namely the AC peak value I of the fault short-circuit current by using the collected current valuepmThe AC angular frequency omega, the initial phase theta corresponding to the short-circuit time, the maximum value D of the DC attenuation, and the time constant tau of the DC attenuation;
step 3, according to the ω tzThe zero-crossing time of the periodic component is t ═ k pi (k ═ 1,2, 3.)zFor the purpose of rapidness, k is generally only 1 or 2;
step 4, calculating zero offset, wherein the total fault current corresponding to the zero crossing point moment of the alternating current component is not zero due to existence of the attenuated direct current component, the current value of the fault at the moment is delta I, and the zero crossing point moment of the fault current is tgThe method specifically comprises the steps of splitting standard alternating current and exponentially decaying direct current, converting the total current represented by the ordinate of one curve into the difference of two curves, converting the zero crossing point of the total current into the intersection point of the alternating current curve and the direct current curve, and converting the intersection point of the alternating current curve and the direct current curve by means of a similar triangle delta t-delta I/omega IpmCalculating to obtain a zero offset delta t caused by the direct current attenuation;
step 5, from tg＝tzAnd the + delta t estimates the specific moment of the current zero crossing point under the combined action of alternating current and direct current.
2. The method according to claim 1, characterized in that alternating current zero crossing points are calculated through the periodicity of alternating current, then zero point offset is calculated by considering the influence of attenuated direct current, and the specific moment of the current zero crossing points under the combined action of alternating current and direct current is estimated by combining the zero point offset and the zero point offset.
3. The method according to claim 2, wherein the value of the predicted current zero-crossing is obtained to approximate the actual value, and even if there is a small deviation, the deviation is a short time before the actual zero-crossing, and the deviation is not corrected as a time margin for improving the operation reliability.
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CN114019231A (en) * 2021-11-04 2022-02-08 国网湖南省电力有限公司 Attenuated direct current extraction method for direct current electric energy metering

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CN114019231B (en) * 2021-11-04 2023-08-18 国网湖南省电力有限公司 Attenuation direct current extraction method for direct current electric energy metering

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