JP3018705B2 - Digital relay for protection of AC filter equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital relay for protecting an AC filter equipment, and more particularly, to an overcurrent flowing in an AC filter equipment provided for removing a high frequency component superimposed on a current flowing in a power system. The present invention relates to a digital relay for protecting AC filter equipment, which is used for detecting AC filter equipment and protecting the AC filter equipment.

[0002]

2. Description of the Related Art AC filter equipment is mainly composed of a coil, a reactor, and a resistor connected in series, and has a low impedance for a specific high frequency component and a high impedance for other high frequency components. Things. Therefore, the AC filter equipment is installed in parallel with the power system and has a function of flowing only high-frequency components contained in the current flowing in the power system to the ground.

[0003] In some cases, an overcurrent flows from the power system to the AC filter equipment. If this state is left unattended, the AC filter equipment will be overloaded, so it is necessary to provide a circuit breaker to cut off the inflow of current. However, the overcurrent flowing from the power system includes a frequency other than the frequency shunted by the AC filter equipment, for example, a fundamental frequency, and the circuit breaker operates in response to the fundamental wave component or the like. Not preferred.

Therefore, it is necessary for the protection relay that operates the circuit breaker to accurately extract the frequency components shunted by the AC filter equipment and to extract other frequency components as little as possible. An analog high-pass filter including a capacitor, a resistor, and the like is used as a protection relay to block unnecessary frequency components.

[0005]

However, when an analog filter is used, the characteristics of the filter may change due to the aging of the characteristics of the capacitor and the resistor and the temperature. Further, it has been difficult to completely remove high-frequency components close to the fundamental wave, for example, the fifth harmonic by using an analog filter. For this reason, an active filter using an operational amplifier and having a sharp cutoff characteristic has been used, but the circuit becomes complicated, and the characteristic changes due to aging of the characteristics of the capacitor and the resistor and a temperature change. The problem still remained.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to provide a digital relay for protecting an AC filter facility which reliably blocks a fundamental wave component and is less likely to undergo aging and temperature changes. And

[0007]

In order to achieve the above object, a digital relay for protecting an AC filter according to the present invention sets a frequency .omega. Corresponding to the frequency of a fundamental wave included in a target wave, and has a period of 2.pi. / Ω is 2k (k is an integer of 1 or more
A predetermined number p (p is an integer of 1 or more ) from sampling means for sampling the target wave at each time point equally divided and sample values im (m is an integer indicating a sampling point) sampled by the sampling means ) , Three equally spaced sample values im-p, im, im + p
And first operation means for calculating a value of ιm = im + p−2cos (pπ / k) im + im−p ‥‥ (1), and ιm obtained by the above first operation means for each m A second calculating means for calculating the effective value by using the second calculating means; and a comparing means for comparing the effective value output by the second calculating means with the reference value. The result of the comparison by the comparing means is output by the second calculating means. It operates when the effective value exceeds the reference value.

[0008]

[Operation] The instantaneous value i (t) of the target wave is converted to the fundamental wave component In (n = 1).
And the harmonic component In (n = 2,3, ‥‥), i (t) = ΣIn sin ( nωt + θn ) ‥‥ (2). Σ (sum) is taken for n. n is the order, ω is the frequency of the fundamental wave of i (t), and θn is the phase of the nth order wave.

Here, if i (t) is sampled at each time point when 2π / ω is equally divided into 2k periods, im = i (2mπ / 2kω) = {In sin ( nmπ / k + θn ) } (3 ).

Further, im + p is im + p = ΣIn sin [ n (m + p) π / k + θn ] ‥‥ (4) im-p is im-p = ΣIn sin [ n (mp) π / k + θn ] ‥‥ (5).

Now, consider only the s-order wave (s is a natural number).
When the above equation (1) is calculated, ιm = im + p−2 cos (pπ / k) im + im−p = Is sin [ s (m + p) π / k + θs ] −2 cos (pπ / k) Is sin ( smπ / k + θs ) + Is sin [ s (mp) π / k + θs ] = 2 Is sin ( smπ / k + θs ) cos (spπ / k) −2 Is sin ( smπ / k + θs ) cos (pπ / k ) = 2Is sin ( sm / k + θs ) × [ cos (spπ / k) −cos (pπ / k) ] ‥‥ (6)

In the equation (6), 2 [ cos (spπ / k) −cos (pπ /
k) ] is considered to be the filter gain of the s-order wave, and is represented by G (s). Assuming a fundamental wave s = 1,
The gain G (1) is Therefore, it can be seen that the first calculating means for calculating the above equation (1) functions as a filter for removing the fundamental wave.

Assuming the s-th harmonic (s is a predetermined number selected from natural numbers of 2 or more), the gain G (s) can be maximized by selecting the number p.
It can also be seen that the first computing means can act as a harmonic pass filter. FIG. 2 shows that one cycle of the target wave is 1
The target wave is sampled at each time point by dividing into 20 equal parts, and when the predetermined number p is set to 10, the gain G (s) = 2 [ cos (10sπ / 60) −cos (10π / 60) ] It is a graph which shows a calculation result. The vertical axis represents the absolute value of the gain G (s), and the horizontal axis represents the harmonic order s.

From the figure, it can be seen that the filter is a pass filter in the vicinity of the fundamental wave and the sixth harmonic. FIG. 3 shows the gain G (s) = 2 [ cos (5sπ / 60) when one cycle of the target wave is divided into 120 equal parts and the target wave is sampled at each time point and a predetermined number p is set to 5. ) -Cos (5π / 60) ] . As can be seen from the figure, it can be seen that the filter functions as a pass filter in the vicinity of the fundamental wave and the 12th harmonic.

Therefore, a digital relay is realized by performing an arithmetic operation using the first arithmetic means and obtaining an effective value based on the output signal, excluding a fundamental wave and extracting only a harmonic of a desired order. be able to.

[0016]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 is a block diagram showing an installation state of a digital relay for protecting AC filter equipment. An AC filter equipment 22 having a capacitor C, a reactor L, and a resistor R for flowing a specific harmonic is connected between the distribution line 21 and the ground, and the current flowing through the AC filter equipment 22 is detected by a current transformer 23. Is done.

The current detected by the current transformer 23 is input to the digital relay 1 for protecting the AC filter equipment. The digital relay 1 converts the output voltage i (t) of the auxiliary transformer 11 for reducing the output voltage of the current transformer 23, the attenuator 12, and the output voltage i (t) of the attenuator 12 (proportional to the current flowing through the filter 22) for a predetermined time Δt (Δt is, for example, It is set to 0.000139 seconds.In the case of a frequency of 60 Hz, it corresponds to a phase angle of 3 ° and k = 60.) A sample-and-hold circuit 13 for sampling each time, and an A / D converter for digitally converting an analog value sampled and held A memory 15 for storing the digitally converted sample values (referred to as im), and three equally spaced sample values using a predetermined number p (p = 5 in this embodiment). im-5, i
m, im + 5 is selected, and the first arithmetic unit 16 and the first arithmetic unit 16 that calculate a value of ιm = im + 5−2 cos (5π / 60) im + im−5 (7) Using ιm,

[0018]

(Equation 1)

A second calculating unit 17 for calculating an effective value according to
And a comparison unit 18 for comparing the effective value calculated by the second calculation unit 17 with a reference value. The operation of the digital relay 1 will be described with reference to the waveform diagram of FIG. Note that the horizontal axis in the figure represents time t, and the vertical axis represents the output voltage i of the attenuator 12.
(t). The sample value im is sampled every fixed time Δt and stored in the memory 15 every moment. First
The calculation unit 16 calculates the value im sampled at an arbitrary time t = tm, the value im + 5 sampled five cycles after that, and the value im− sampled five cycles before.
5 is read from the memory 15 and ιm is calculated based on the above equation (7). In this case, since cos (5π / 60) is a constant, it is not necessary to calculate each time data is read from the memory 15, and the above equation (7) can be calculated in a very short time. the above
As described with reference to FIG. 3, the fundamental wave component included in the data ιm calculated by the equation (7) is cut off, and the harmonics near the twelfth order remain as they are.

The second calculating section 17 calculates an effective value based on the above equation (8) using the value ιm calculated in the first calculating section 16. Then, the comparing unit 18 compares the effective value with a reference value, and if the effective value exceeds the reference value, outputs an operation output of a circuit breaker (not shown) in order to secure the safety of the AC filter equipment 22. . Thus, the current flowing through the AC filter equipment 22 can be cut off. In this case, a timed relay may be attached to output the operation signal only when the state where the effective value is equal to or more than the reference value continues for a predetermined time.

As described above, by calculating equation (7), it is possible to extract only the harmonic components near the twelfth order without any fundamental wave component.
With the digital relay configured in this way,
For example, a protection relay of a filter facility that shunts the eleventh harmonic and the thirteenth harmonic can be configured.

The present invention is not limited to the above embodiment. For example, the predetermined number p is not limited to 5, and can be set to an arbitrary number according to the order of harmonics to be shunted by the filter equipment. For example, when p = 10, it can be applied to the filter equipment for the fifth shunt as described with reference to FIG. Further, in the above embodiment, the sampling interval Δt is a time corresponding to the phase angle of 3 °, but is not limited to this. Generally, the smaller the sampling phase angle is, the higher harmonics can be handled (sampling theorem). However, it is necessary to shorten the calculation time, and the larger the sampling phase angle is, the longer the calculation time is. However, the order of harmonics that can be processed decreases.

Various other design changes can be made without changing the gist of the present invention.

[0024]

As described above, according to the digital relay for protecting AC filter equipment of the present invention, three sample values im-p, i at regular intervals are obtained by using the set predetermined number p.
By selecting m, im + p and calculating a value of ιm = im + p−2 cos (pπ / k) im + im−p, a frequency corresponding to the predetermined number p without any fundamental wave component is included. Data including components
m is obtained. Therefore, p is set according to the harmonic order to be shunted by the AC filter equipment, and this data
If the protection digital relay is configured using the above, the AC filter equipment can be suitably protected.

[Brief description of the drawings]

FIG. 1 is a block diagram of a digital relay for protecting AC filter equipment.

FIG. 2 is a graph showing gain characteristics of a digital filter included in the present invention.

FIG. 3 is a graph showing gain characteristics of a digital filter included in the present invention.

FIG. 4 is a graph illustrating a sampling method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Digital relay for protection of AC filter equipment 13 Sample hold circuit 16 1st operation part 17 2nd operation part 18 Comparison part

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H02H 3/02 G01R 19/00 G01R 21/133 H02H 3/52 H02J 3/01

Claims (1)

(57) [Claims] 1. A frequency ω corresponding to a frequency of a fundamental wave included in a target wave is set, and one period 2π / ω is set to 2k (k is 1 or more).
A predetermined number p (p is 1 or more ) from sampling means for sampling the target wave at each time point equally divided by the sampling means, and sample values im (m is an integer indicating a sampling point) sampled by the sampling means. using integer), first to select three sample values im-p in equal intervals, im, the im + p, calculates the ιm = im + p-2cos ( pπ / k) im + im-p becomes a value Calculating means; second calculating means for calculating an effective value for each m by using lm obtained by the first calculating means; and comparing the effective value output by the second calculating means with a reference value. Means for operating when the effective value output by the second calculating means as a result of the comparison by the comparing means exceeds a reference value.