JPH09222440A - Electricity detection method in case of system failure - Google Patents

Abstract

(57) Abstract: A DC component included in a fault current can be accurately removed by calculation. SOLUTION: The average value of each sampling value of the current for a time corresponding to one cycle at the fundamental frequency of the system is calculated as a DC component included in the fault current (see steps S1 and S2), and this is calculated from each sampling value. By subtracting, a current value that does not include a direct current component is obtained, and various electric quantities including impedance are calculated based on the current value (see steps S4 and S5). At this time, the accuracy is improved by obtaining the DC component at the time when the variation converges (see step S3).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention accurately calculates various electric quantities such as a current effective value and impedance at the time of a system failure by a digital operation even if a certain delay time is allowed although not in real time. The present invention relates to a device that is required to do so, for example, a method for detecting the amount of electricity when a system fault occurs in a fault location device.

[0002]

2. Description of the Related Art A system voltage / current during a system failure occurrence period is converted into digital data by a sampling technique and an A / D (analog / digital) conversion technique,
A variety of devices have been put to practical use, including digital relays and fault locators (fault locators), which calculate various amounts of electricity using digital arithmetic techniques to protect and control the system. Of the various electric quantity calculations performed by these devices, there are many methods that focus on the fundamental wave components of the system current and voltage quantity, but most of these calculation expressions are input quantity (voltage , Current), only the system fundamental wave component is assumed, and if the input amount includes a DC (direct current) component or a harmonic component, the calculated electricity amount will include an error.

For example, by using sampling values (= I1, I2) at two points separated by 90 ° electrical angle, the effective value Ir of the input amount
As a calculation formula for calculating ms, there is the following formula (1). Irms = √ (I1 ² + I2 ² ) (1) Now, assuming that the input amount is a sine wave represented by the following equation (2) having a magnitude A of only the fundamental wave (ω = 2πf) component, I1 = Asin ( ωt) I2 = Asin (ωt + 90 °) = Acos (ωt) (2) and substituting the equation (2) into the equation (1), the effective value is obtained from the following equation. Irms = √ (I1 ² + I2 ² ) = √ [A ² {sin ² (ωt) + cos ² (ωt)}] = A (3)

However, the input amount is D times X times A.
When C components are superposed, I1 and I2 in the equation (2) are as follows: I1 = A {sin (ωt) + X} I2 = Asin {(ωt + 90 °) + X} = A {cos (ωt) + X} ( 4) and the effective value is: Irms = √ (I1 ² + I2 ² ) = √ [A ² {sin ² (ωt) + cos ² (ωt)} = + 2X {sin (ωt) + cos (ωt)}] = A√ As shown in {1 + 2√2Xsin (ωt + 45 °)} (5), the error amount (2√2Xsin (ωt + 45 °)) that changes sinusoidally in synchronization with the fundamental frequency is included.

By the way, in an actual system fault, the fault current may include DC component as much as 100% of the fundamental wave. Therefore, in order to reduce the error amount in the equation (5), a filter circuit, For example, by providing filters BP1 and BP2 having bandpass characteristics as shown in FIG.
I am trying to attenuate the minutes.

[0006]

That is, in order to obtain a highly accurate quantity of electricity, it is better to attenuate the frequency components other than the fundamental wave component, for example, the DC component as much as possible with a filter.
It is also clear from the above equation (5). For that purpose, for example, comparing the two filters BP1 and BP2 shown in FIG. 5, since BP2 has a larger attenuation in the low frequency region, the DC component also has a larger attenuation and the error term in the equation (5) becomes smaller. Can be easily predicted. In general, when the characteristics of BP1 and BP2 are expressed by transfer function equations of the same order, B
The response of P2 is slower. This means that in the case of a fault in which the occurrence time of the system fault is short (that is, the circuit breaker is opened in a short time), the fault is eliminated before the transient change of the filter is completed, and as a result, an accurate electric quantity Calculations may be impossible due to errors due to filter transients.

For this reason, when it is assumed that the circuit breaker is opened in a short time, it is unavoidable to use a filter such as BP1 having a low attenuation in the low frequency region.
This causes a problem that the error term in the equation (5) becomes larger than that when the BP2 filter is used.
Therefore, an object of the present invention is to reduce the DC contained in the current when using a filter with low attenuation in the low frequency region.
The purpose is to reduce the electric quantity calculation error caused by the minute.

[0008]

In order to solve such a problem, according to the invention of claim 1, various electric quantities are obtained by performing predetermined digital calculations on the basis of the quantities obtained by sampling the system current and converting it into digital quantities. In obtaining, the DC component contained in the fault current at the time of system fault was calculated by taking the average of each sampling value at the fundamental frequency for one period, and was calculated from the current sampling value at each time. The direct current component is subtracted and the resulting value is used to detect various amounts of electricity. The DC component can be calculated when the fluctuation converges (claim 2). That is, the direct current (D
The C) component is not removed by using a filter but is removed by using an arithmetic operation, whereby the accuracy is improved.

[0009]

1 is a schematic flow chart showing a first embodiment of the present invention, FIG. 2 is a detailed view thereof, and FIG.
Is a waveform diagram for explaining a processing method according to the present invention, and FIG. 4 is a schematic diagram showing a system configuration to which the present invention is applied. That is, in this embodiment, as shown in FIG. 4, a digital arithmetic unit 4 is provided in the electric power system via an insulating transformer 2 and a preprocessing unit 3 (consisting of a filter and an A / D converter, etc.), where a predetermined value is set. Is calculated to reduce the DC component. Note that reference numeral 1 in FIG. 4 indicates a protection relay, which outputs a trip command to the circuit breaker (CB) when a failure is detected.
Is opened and the fault point is removed. Further, as the filter used in the pre-processing unit 3, a filter having a small attenuation in the low frequency region like BP1 in FIG. 5 is used.

FIGS. 1 and 2 show the processing procedure performed in the digital arithmetic section 4 of FIG. First, in step S1, the current data I from the failure occurrence time to the CB opening time
(T) is collected by a sampling method. In FIG. 3A, the failure occurrence time and the CB release time are t1 and t, respectively.
It is shown as 2. If the sampling interval is Δt and sampling is performed (m + 1) times during this period, then t2 = t1 + mΔt can be expressed.

In step S2, for example, times t1 and t2
With reference to a certain point in time t0, an average of n sampling values for one cycle before that is calculated, and this is calculated as a direct current (D
C) Minutes. Now, assuming that one cycle is T (= 1 / f, f: fundamental frequency of system), DC component I _DC (t0) is Becomes When this is expanded, as shown in step S2 of FIG. 2, I _DC (t0) = {I (t0) + I (t0-Δt) + I (t0-2Δt) + ... + I (t0- (n-1) Δt)} / n (7)

In the next step S3, I at each time
_DC (t) convergence calculation is performed. Specifically, as shown in step S3 of FIG. 2, the change in I _DC (t) at each time point per unit time is represented by Δ (t _x ), for example, as shown in the following equation, and Δ (t _x ) = | _{I DC (t x -Δt) -I} DC (t x) | + | I DC (t x) -I DC (t x + Δt) | ... (8) the time when this value is the minimum (t _x = t0 ' ), I
It is estimated that _DC (t) has converged. This is because the DC component contained in the fault current cannot be accurately detected due to the transient change of the filter at the time immediately after the occurrence of the fault, and the DC component is constantly changed every hour to cause a certain attenuation. This is due to the decay with a time constant. That is, as time elapses after the occurrence of the system failure, the transient change of the filter subsides, and the time change amount of DC becomes small and becomes stable. That is, the DC component is the exponential function exp (-t
/ Τ) (τ: decay time constant), [exp {-(t0-Δt) / τ} -exp {-(t
0) / τ}]> [exp {-(t0) / τ} -exp
{-(T0 + Δt) / τ}], and the amount of change decreases with the passage of time.

Then, the DC value I at the time t0 '
_DC (t0 ') is used as a correction value and subtracted from the sampling value at each time to obtain a true value (see step S4). The arithmetic expression, an I (t) ← I (t ) -I DC (t0 ') ... (9), when deploying it as shown in step S4 in FIG. 2, I (t1) ← I (t1 _{) -I DC (t0 ') I} (t1 + Δt) ← I (t1 + Δt) -I DC (t0') · · · I (t1 + mΔt) ← I (t1 + mΔt) -I DC (t0 ') ... is (10). Using the current I (t) obtained as described above, the effective value, impedance, etc. are calculated by the same arithmetic expression as in the conventional case (see step S5).

By the above, if the fault current of the system is shown as shown in FIG. 3 (a), the DC component at the convergence point (time t0 ') is obtained as shown in FIG. 3 (d). By subtracting from the current value at each time using this value, it becomes gradually stable as shown in FIG. It should be noted that FIG. 3C shows the effective current value when the process of the present invention is not performed, and it can be seen that the current is not stable over time. Further, FIG. 3B shows a waveform obtained by passing the waveform shown in FIG. 3B through the filter BP1 having the characteristic shown in FIG.

[0015]

According to the present invention, the direct current (DC) component included in the fault current is obtained by calculation instead of utilizing the filter action, and is subtracted from the current sampling value at each time point. The advantage is that a relatively accurate amount of electricity can be obtained. In particular, the DC
By obtaining the minute at the time when it is considered to converge, the accuracy can be further improved. Although the present invention can remove the influence of low-order frequency components, it cannot eliminate the influence of high-order frequency components except integer harmonics. Therefore, the pre-processing unit 3 in FIG. Of course, it is necessary.

[Brief description of drawings]

FIG. 1 is a schematic flowchart showing an embodiment according to the present invention.

FIG. 2 is a detailed flowchart of FIG.

FIG. 3 is a waveform diagram for explaining a processing method according to the present invention.

FIG. 4 is a system configuration diagram to which the present invention is applied.

FIG. 5 is an explanatory diagram for explaining an example of filter characteristics.

[Explanation of symbols]

1 ... Protective relay, 2 ... isolation transformer, 3 ... pretreatment section, 4
... Digital operation unit, CB ... Circuit breaker.

Claims (2)

[Claims] 1. A direct current component included in a fault current at the time of a system fault is calculated based on a fundamental frequency when a systematic current is sampled and converted into a digital amount to obtain various kinds of electricity by performing predetermined digital operations respectively. Calculated by averaging each sampled value for a period corresponding to 1 cycle, subtracting the calculated direct current component from the current sampled value at each time, and using the resulting value to detect various amounts of electricity. An electric quantity detection method in the event of a system failure. 2. The method for detecting an electric quantity at the time of a system failure according to claim 1, wherein the DC component is calculated at the time when the fluctuation converges.