CN109742727B - Method for judging low-voltage 400V leakage current - Google Patents
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Abstract
The invention provides a method for judging low-voltage 400V leakage current, which is characterized in that current in a three-phase live wire and a zero line is sampled in a centralized manner through a zero-sequence current transformer, the vector sum of four-wire zero-sequence current obtained through measurement is leakage current of a low-voltage 400V circuit, the leakage current obtained through sampling is transmitted to a controller after signal conditioning and analog-to-digital conversion, the controller calculates the current variation of the leakage current by adopting a vector mutation algorithm and/or an amplitude mutation algorithm, a leakage accident is judged according to the calculation result, and if the electric shock or ground fault is judged, a power supply is disconnected. The method can quickly judge the electric leakage condition of the low-voltage 400V system, has simple and efficient calculation, can effectively prevent the phenomena of refusing action and misoperation of the electric leakage protection device, protects personal safety, and does not need to add any hardware cost.
Description
Technical Field
The invention belongs to the technical field of electrical engineering, and particularly relates to a method for judging low-voltage 400V leakage current.
Background
Along with the improvement of the automation degree and the reliability requirement of the power distribution system, the residual current protection device is used as a key protection device in the low-voltage power distribution system, the occurrence of electric leakage and electric shock accidents can be prevented, and the safety and the reliability of the 400V low-voltage system are improved.
However, the residual current protection device widely used at present can only monitor the magnitude of the total leakage current signal, and determine whether a fault exists according to the magnitude of the total leakage current. Along with the improvement of the automation degree of a power distribution system, more and more nonlinear loads are applied to a 400V low-voltage system, stray capacitors exist in the nonlinear loads, and high-order harmonics exist in zero-sequence currents flowing through the stray capacitors. During the non-linear load starting and working processes, the traditional residual current protection device may malfunction. Meanwhile, because the conventional residual current protection device can only detect the amplitude, when the phases of the transient leakage current and the steady leakage current are inconsistent, the vector sum of the zero sequence current may be smaller than the action threshold of the residual current protection device, and then the accident of protection failure occurs. In summary, it is necessary to develop a method for determining leakage current with high reliability under low voltage, so as to solve the problems of operation rejection and malfunction of the conventional leakage protection device, and improve the reliability and stability of power supply.
Disclosure of Invention
Technical problem to be solved
The invention aims to solve the technical problem that the current leakage current under the condition of low voltage 400V is easy to have the phenomena of action rejection and misoperation, and provides a judgment method for low voltage 400V leakage current protection based on a vector mutation algorithm and an amplitude mutation algorithm, so that accurate action of leakage and electric shock of a 400V low-voltage system is realized.
(II) technical scheme
In order to achieve the purpose, the invention adopts the main technical scheme that:
the invention provides a method for judging low-voltage 400V leakage current, which comprises the steps of carrying out centralized sampling on currents in a three-phase live wire and a zero line through a zero-sequence current transformer, measuring the vector sum of four-wire zero-sequence currents which is obtained, namely the leakage current of a low-voltage 400V circuit, transmitting the leakage current obtained by sampling to a controller after signal conditioning and analog-to-digital conversion, calculating the current variation of the leakage current by the controller by adopting a vector mutation algorithm, judging leakage accidents according to the calculation result, and disconnecting a power supply if the leakage current is judged to be an electric shock or a ground fault;
the vector mutation algorithm comprises the following steps: sampling to obtain the amplitude and phase components of each harmonic of the leakage current, firstly carrying out Fourier transform on current signals in two measured cycles to obtain the amplitude and phase of each harmonic, solving each harmonic vector difference in zero-sequence current before and after two periods, summing the each harmonic vector differences to obtain the zero-sequence current variable quantity in the two periods, and if the effective value of the zero-sequence current variable quantity in the two periods exceeds 30mA, determining that electric shock or grounding fault occurs.
Further, after the power supply is cut off, the following operations are further performed: recovering the power supply after three seconds, and if the effective value of the leakage current is recovered to be within 30mA, determining that the fault disappears and the line recovers to operate normally; if the effective value of the leakage current is larger than 30mA, a permanent fault is considered to exist, and a fault indication signal is sent out through a signal lamp and a buzzer.
Further, the vector mutation algorithm specifically uses the following expressions (2) to (4) to perform simplified calculation on the current variation Δ I of the zero-sequence current:
I＝I0+I1sin(wt+Φ1)+I2sin(2wt+Φ2)+...+Insin(nwt+Φn) (2)
I'＝I0'+I1'sin(wt+Φ1')+I2'sin(2wt+Φ2')+...+In'sin(nwt+Φn') (3)
ΔI＝ΔI0+ΔI1sin(wt+ΔΦ1)+ΔI2sin(2wt+ΔΦ2)+...+ΔInsin(nwt+ΔΦn) (4)
in the formula (2) -formula (4), I represents a zero-sequence current signal obtained by sampling the first two cycles, I' represents a zero-sequence current signal obtained by sampling currently, and Δ I represents a current variation of the zero-sequence current; n represents the number of harmonics; i isn、In'、ΔInN-th harmonic amplitudes representing two cycles, present and current variations, respectively, and Δ In＝In'-In；Φn、Φn'、ΔΦnThe n-th harmonic phase, Δ Φ, representing the two cyclic fronts, the current and the current variation respectivelyn＝Φn'-ΦnW represents frequency and t represents time, wherein I and I' are obtained by sampling;
and (3) obtaining an effective value of the current variation delta I through integral calculation of the current variation delta I of the zero-sequence current obtained through calculation in the formula (4), and if the effective value of the current variation delta I in the two cycles exceeds 30mA, determining that an electric shock or a ground fault occurs, and further adopting corresponding response measures.
Further, if the effective values of fundamental waves and harmonic waves of the leakage current in a steady state exceed 30mA, the leakage phenomenon is considered to exist, and the device sends an alarm signal to the outside; and calculating the transient variation of the zero sequence current of the leakage current through real-time monitoring, and if the transient variation of the zero sequence current is suddenly changed and the variation exceeds 30mA, judging that an electric shock or a ground fault occurs.
In addition, the invention also provides a method for judging the low-voltage 400V leakage current, which is characterized in that the current in a three-phase live wire and a zero line is sampled in a centralized manner through a zero-sequence current transformer, the vector sum of the four-wire zero-sequence current obtained by measurement is the leakage current of the low-voltage 400V circuit, the leakage current obtained by sampling is transmitted to a controller after signal conditioning and analog-to-digital conversion, the controller adopts an amplitude mutation algorithm to calculate the current variation of the leakage current, judges the leakage accident according to the calculation result, and disconnects the power supply if the leakage accident is an electric shock or a ground fault;
the amplitude mutation algorithm comprises the following steps: and periodically sampling the leakage current, solving the amplitude of the current difference value which is separated by two cycles, obtaining the amplitude difference by mutually solving the difference, then solving the square sum of the amplitude differences in the two cycles, defining the square amplitude difference, and if the square amplitude difference in the front and the back cycles is more than 30mA S, determining that an electric shock accident or a ground fault occurs.
Further, after the power supply is cut off, the following operations are further performed: recovering the power supply after three seconds, and if the effective value of the leakage current is recovered to be within 30mA, determining that the fault disappears and the line recovers to operate normally; if the effective value of the leakage current is larger than 30mA, a permanent fault is considered to exist, and a fault indication signal is sent out through a signal lamp and a buzzer.
Further, the amplitude jump algorithm specifically uses the following formula (5) to calculate a squared amplitude difference Δ i of the zero-sequence current, where the squared amplitude difference Δ i is the current variation of the leakage current:
in the formula (5), N is the number of sampling points in 2 periods, and the current of the Mth sampling point is recorded as iMThe amplitude of the current after the two cycles is iM+NThe difference between the current waveform amplitudes at 2 cycles before and after the current waveform is Δ iM＝iM+N-iMAnd if the delta i exceeds 30mA S, the electric leakage or electric shock fault is considered to occur, and corresponding response measures are taken.
Further, if the effective values of fundamental waves and harmonic waves of the leakage current in a steady state exceed 30mA, the leakage phenomenon is considered to exist, and the device sends an alarm signal to the outside; and calculating the transient variation of the zero sequence current of the leakage current through real-time monitoring, and if the transient variation of the zero sequence current is suddenly changed and the variation exceeds 30mA, judging that an electric shock or a ground fault occurs.
It should be noted that the two methods for judging the low-voltage 400V leakage current based on the vector mutation algorithm and the amplitude mutation algorithm can be used separately or combined together, so that the leakage current can be comprehensively and safely judged for accidents, and the reliability and the accuracy can be further improved.
(III) advantageous effects
Compared with the prior art, the invention has the following beneficial effects:
1) the judging method is simple and efficient, and the detection accuracy is high.
2) The leakage protection device can effectively prevent the phenomena of action rejection and misoperation from occurring, and the personal safety is protected.
3) The vector mutation algorithm and the amplitude mutation algorithm can be used separately and singly, and can also be used together comprehensively to judge the accident type, so that the reliability and the accuracy of judgment are improved.
Drawings
The features and advantages of the present invention will be more clearly understood by reference to the accompanying drawings, which are illustrative and not to be construed as limiting the invention in any way, and in which:
FIG. 1 is a block diagram of a leakage current protection device;
FIG. 2 is a schematic diagram of zero sequence current sampling of the leakage current protection device;
fig. 3 is a zero sequence current vector variation calculation chart of the leakage current protection device;
fig. 4 is a zero sequence current amplitude variation calculation chart of the leakage current protection device.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, fig. 1 is a block diagram of a leakage current protection device according to an embodiment of the present invention, where lines a, B, and C respectively represent three-phase live lines in a low-voltage 400V line, N represents a neutral line zero line, T represents a zero-sequence current transformer, and K represents a circuit breaker. The zero sequence current signal obtained by the detection of the zero sequence current transformer T is transmitted to the controller after passing through the conditioning circuit and the analog-to-digital conversion circuit, the controller judges whether electric leakage or electric shock accidents occur or not according to the zero sequence current signal detected by the zero sequence current transformer T, and if the electric shock or ground fault is judged, the power supply is disconnected firstly. Recovering the power supply after three seconds, and if the effective value of the leakage current is recovered to be within 30mA, determining that the fault disappears and the line recovers to operate normally; if the effective value of the leakage current is larger than 30mA, a permanent fault is considered to exist, and a fault indication signal is sent out through a signal lamp and a buzzer.
Fig. 2 is a schematic diagram of zero-sequence current sampling according to an embodiment of the present invention, in which an abscissa t represents time, an ordinate I represents zero-sequence current, and curves in a coordinate system represent waveform amplitudes of the zero-sequence current sampled at different times, where Δ t is a sampling time interval, and an effective value I of the zero-sequence current in two current cycles is calculated assuming that the sampling times in a single current cycle (e.g., 20ms) are n timeseqRepresented as described in formula (1):
when the zero sequence current effective value I is obtained by calculationeqIf the current exceeds 30mA, the fault phenomenon exists in the line, and the line fault needs to be judged and corresponding measures need to be taken. In addition, the zero sequence current sampling principle shown in fig. 2 can prevent the phenomena of malfunction and failure of the conventional leakage protection device under the action of non-sinusoidal leakage current.
Due to the existence of the nonlinear load, the inherent leakage current in the low-voltage line may have various harmonic components, so that the peak value of the leakage current is too high, and the misoperation of the leakage protection device is caused. In order to eliminate the above false operation, the effective value of the leakage current can be obtained by integrating the signal measured by the leakage protection device.
Fig. 3 is a vector diagram for calculating the zero sequence current variation of the leakage current protection device, which is used to illustrate the vector mutation algorithm. Sampling to obtain amplitude and phase components of each harmonic, firstly carrying out Fourier transform on current signals in two measured cycles to obtain the amplitude and phase of each harmonic, solving each harmonic vector difference in zero-sequence current before and after two periods, and summing the each harmonic vector differences to obtain zero-sequence current variation in the two periods, wherein I represents the zero-sequence current signal obtained by sampling the first two cycles, I' represents the zero-sequence current signal obtained by sampling at present, and delta I represents the current variation; n represents the number of harmonics; i isn、In'、ΔInN-th harmonic amplitudes representing two cycles, present and current variations, respectively, and Δ In＝In-In'；Φn、Φn'、ΔΦnN-th harmonic phases respectively representing two cycles, present and current variations, and Δ Φn＝Φn-Φn'; w represents frequency and t represents time. I and I 'are obtained by sampling, the zero sequence current variation delta I is obtained by subtracting the formula (3) from the formula (2), and in the formula (4), in order to simplify the operation, the formula I' -I is approximately equal to the continuous expression on the right side of the formula (4), so that the calculation speed is accelerated, and meanwhile, the calculation result is basically not influenced, wherein the formulas (2) - (4) are specifically as follows:
I＝I0+I1sin(wt+Φ1)+I2sin(2wt+Φ2)+...+Insin(nwt+Φn) (2)
I'＝I0'+I1'sin(wt+Φ1')+I2'sin(2wt+Φ2')+...+In'sin(nwt+Φn') (3)
ΔI＝I'-I≈ΔI0+ΔI1sin(wt+ΔΦ1)+ΔI2sin(2wt+ΔΦ2)+...+ΔInsin(nwt+ΔΦn) (4)
the zero sequence current variable quantity delta I obtained by the calculation of the formula (4) is calculated by the formula (1) to obtain an effective value of the zero sequence current variable quantity. And if the effective value of the zero sequence current variation delta I in the two cycles exceeds 30mA, determining that electric shock or grounding fault occurs, and further adopting corresponding response measures.
Fig. 4 shows a zero sequence current signal waveform in 4 adjacent cycles, which is used to explain the amplitude jump algorithm, assuming that the number of sampling points in 2 cycles is N, the upper diagram shows the sampling currents of the first two cycles, and the lower diagram shows the amplitudes of the sampling currents after the two cycles. The current at the Mth sampling point is recorded as iMThe amplitude of the current after the two cycles is iM+N. The difference between the current waveform amplitudes at 2 cycles before and after the current waveform is Δ iM＝iM+N-iM. Defining a square amplitude difference calculation method as follows:
if the delta i calculated by the formula (5) exceeds 30mA ＊ S, the electric leakage or electric shock fault is considered to occur, and corresponding response measures are taken.
In addition to the method for determining the current variation in fig. 3-4, the present embodiment further includes the following steps of monitoring and determining the steady-state and transient-state currents, and if the effective values of the fundamental wave and the harmonic wave of the leakage current in the steady state exceed 30mA, it is determined that the leakage phenomenon exists, and the device sends an alarm signal to the outside; and calculating the change vector of the zero sequence current of the leakage current through real-time monitoring, and if the transient variation of the zero sequence current is suddenly changed and the variation exceeds 30mA, judging that an electric shock or a ground fault occurs.
It should be noted that the vector mutation algorithm and the amplitude mutation algorithm of the present invention described in fig. 3-4 above may be used separately or together to determine the type of the accident, and the combined use of the two can further improve the reliability and accuracy of the leakage current determination of 400V.
According to field tests, the method for judging the leakage current protection based on the vector mutation algorithm and the amplitude mutation algorithm can quickly judge the leakage condition of the low-voltage 400V system, is simple and efficient in calculation and higher in accuracy than the common prior art, can accurately analyze the low-voltage 400V leakage condition in a qualitative and quantitative mode, provides corresponding judgment standards, and does not need to add any hardware cost.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (3)
1. A method for judging low-voltage 400V leakage current is characterized in that current in a three-phase live wire and a zero line is sampled in a centralized mode through a zero-sequence current transformer, the vector sum of four-wire zero-sequence current obtained through measurement is leakage current of a low-voltage 400V circuit, the leakage current obtained through sampling is transmitted to a controller after signal conditioning and analog-to-digital conversion, the controller calculates the current variation of the leakage current by adopting a vector mutation algorithm, a leakage accident is judged according to the calculation result, and if the leakage accident is judged to be an electric shock or a ground fault, a power supply is disconnected;
the vector mutation algorithm comprises the following steps: sampling to obtain amplitude and phase components of each harmonic of the leakage current, firstly carrying out Fourier transform on current signals in two measured cycles to obtain the amplitude and phase of each harmonic, solving each harmonic vector difference in zero-sequence current before and after two periods, summing each harmonic vector difference to obtain zero-sequence current variable quantity in the two periods, and considering that electric shock or grounding fault occurs if the effective value of the zero-sequence current variable quantity in the two periods exceeds 30 mA;
the vector mutation algorithm specifically uses the following formula (2) -formula (4) to perform simplified calculation on the current variation Δ I of the zero-sequence current:
I＝I0+I1sin(wt+Φ1)+I2sin(2wt+Φ2)+...+Insin(nwt+Φn) (2)
I'＝I0'+I1'sin(wt+Φ1')+I2'sin(2wt+Φ2')+...+In'sin(nwt+Φn') (3)
ΔI＝ΔI0+ΔI1sin(wt+ΔΦ1)+ΔI2sin(2wt+ΔΦ2)+...+ΔInsin(nwt+ΔΦn) (4)
in the formula (2) -formula (4), I represents a zero-sequence current signal obtained by sampling the first two cycles, I' represents a zero-sequence current signal obtained by sampling currently, and Δ I represents a current variation of the zero-sequence current; n represents the number of harmonics; i isn、In'、ΔInN-th harmonic amplitudes representing two cycles, present and current variations, respectively, and Δ In＝In'-In；Φn、Φn'、ΔΦnThe n-th harmonic phase, Δ Φ, representing the two cyclic fronts, the current and the current variation respectivelyn＝Φn'-ΦnW represents frequency and t represents time, wherein I and I' are obtained by sampling;
and (3) obtaining an effective value of the current variation delta I through integral calculation of the current variation delta I of the zero-sequence current obtained through calculation in the formula (4), and if the effective value of the current variation delta I in the two cycles exceeds 30mA, determining that an electric shock or a ground fault occurs, and further adopting corresponding response measures.
2. The method according to claim 1, wherein after the power supply is turned off, further performing the following operation: recovering the power supply after three seconds, and if the effective value of the leakage current is recovered to be within 30mA, determining that the fault disappears and the line recovers to operate normally; if the effective value of the leakage current is larger than 30mA, a permanent fault is considered to exist, and a fault indication signal is sent out through a signal lamp and a buzzer.
3. The method according to claim 1, wherein if the effective value of the fundamental wave and the harmonic wave exceeds 30mA in the steady state of the leakage current, the leakage phenomenon is considered to exist, and the device sends out an alarm signal to the outside; and calculating the transient variation of the zero sequence current of the leakage current through real-time monitoring, and if the transient variation of the zero sequence current is suddenly changed and the variation exceeds 30mA, judging that an electric shock or a ground fault occurs.
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| CN112345966B (en) * | 2019-08-07 | 2023-02-03 | 青岛鼎信通讯股份有限公司 | Method and device for detecting direct current leakage through residual current transformer |
| CN111864695B (en) * | 2020-03-18 | 2021-06-04 | 同济大学 | Leakage current detection protection method and device |
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| CN112510650A (en) * | 2020-11-27 | 2021-03-16 | 河北工业大学 | Electric leakage protection method and device and electric leakage multi-stage protection system |
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