DE9207800U1 - Measurement filter - Google Patents

Description

92 G 3 3 O 9 EN

Siemens AG

Measurement filter
5

The invention relates to a measurement filter for a periodic signal with a fundamental frequency.

In order to measure a periodic quantity with a fundamental frequency, e.g. the alternating voltage of a network, it is known to pass a measured value signal derived from the quantity through a measured value filter. The frequency response of the measured value filter corresponds to the magnitude of a sine wave. At the fundamental frequency, the frequency response has a zero point. In order to achieve this frequency response, two samples of the periodic quantity are used - namely ^ and (/7-32). The measured value filter has steep edges in the vicinity of the fundamental frequency, which are actually desired. However, with dynamic changes in the fundamental frequency, considerable measured value errors occur. To compensate, the sampling rate must therefore be adjusted for the samples in accordance with the change in the fundamental frequency.

However, this can only be done with a delay, which means that the measured value is distorted at the first moment of the fundamental frequency change. Furthermore, experts reject such tracking of the sampling rate for special applications (e.g. fault recording in the energy supply sector).

The invention is based on the object of providing a measurement value filter in which tracking of the sampling rate can be dispensed with.

Wof/Doe / 04.06.1992·---"'-"-

92 G 3 3 O 9 EN

Based on the known frequency response, the inventor determined that its sharp curve in the vicinity of the fundamental frequency is responsible for the resulting measurement errors.

The problem is solved with the features of claim 1.

In this way, the frequency response of the measurement filter is flat near the fundamental frequency, so that the transmission behavior remains almost constant for small frequency changes. DC components and integer harmonics are suppressed. In this simple way, there is no need to adjust the sampling rate, so that the transmission behavior for small frequency changes, such as is common in three-phase power grids, remains within a very narrow tolerance band.

Advantageous embodiments of the invention are specified in the subclaims. The filter element of the measured value filter preferably has a frequency response corresponding to the relationship in claim 3. Such a filter element can be implemented particularly easily if it comprises a computer according to claim 6 and is designed as a digital filter.

The invention and further advantages are explained in more detail below using an exemplary embodiment. Shown are:

FIG 1 is a block diagram for a measurement filter, FIG 2 is the frequency response of the filter element for π = 16, FIG 3a and 3b are the step response behavior of the filter element

and
FIG 4a and Ab the response of the filter element at

a sinus jump.

92 G 3 3 O 9 EN

The block diagram according to FIG 1 shows a measurement filter 1 to which an electrical signal 3 can be fed. This signal 3 can be, for example, a current or a voltage from a power supply network. The entire measurement value acquisition and processing shown here can be designed as single or multi-phase. The electrical signal 3 essentially consists of a periodic basic signal, e.g. 50 Hz basic frequency, and may have superimposed interference, e.g. high-frequency interference or DC signals.

The measured value filter 1 comprises a sampling device 5 which samples the signal 3 at specified times or phase angles. The sampled phase angle-equal values are fed to a filter element 7. An output signal 3a is then available at the output of the filter element 7, and thus at the output of the measured value filter 1, which can be fed to further measured value processing, e.g. limit value processing. Depending on the design of the filter element 7, its output can have an output signal 3a continuously or only when the stationary state of the input variable changes.

The filter element 7 has a frequency response which has a double zero at the .-λ fundamental frequency. This frequency response is shown in FIG 2 for η = 16, "n" represents the number of samples per period, according to the equation η = -γ- , where T _fl is the sampling period and T the ¹ A ^H

Period duration of the fundamental frequency of the signal to be filtered. The frequency response (magnitude) of the filter element 7 is determined by the relationship

I F (./L) I =2 Icos (n ._n~) - 1 | 35

S2G3 3 0 9EN

described. In the area of the fundamental frequency, the curve of the filter element 7 is very flat, so that with small frequency changes of the fundamental frequency of the electrical signal 3, the transmission behavior remains approximately constant. The filter element 7 has double zeros for f = p. fn with P = 0,1,2,..., whereby DC components and integer harmonics are suppressed.

In the case of dynamic changes in the signal 3, the measured value filter 1 can also be used for two periods as a so-called delta monitor, so that in the stationary state the output signal of the filter is zero. In the case of deviations from the fundamental frequency, an adjustment of the sampling frequency is not necessary. The measured value filter 1 with the filter element 7 is preferably constructed as a digital filter with a computer. The discrete filter equation is preferably used in this.

= x

in a program. The corresponding transfer function of the z-transform is:

F(z) = 1 - 2 . z" ⁿ + z" ²ⁿ = (lz" ⁿ ) (lz- ⁿ ). The resulting frequency response (magnitude) is: IF (XL) I = 2 I cos (n·-TL) _ 1 I

If the fundamental frequency deviates by +/- 2.5 Hz, errors of up to 31% occur with the conventional filter in the steady state. With the measurement value filter 1 proposed here, the measurement error is reduced to less than 2.5%.

92 G 3 3 0 9EN

FIGS 3a and 3b together show the step response of the measurement filter 1. FIG 3a shows the input signal and FIG 3b the output signal.

FIGS 4a and 4b show a similar view, but here a sinusoidal value is specified as the input signal.

A preferred application of the measured value filter 1 is in protection devices for medium, high and extra-high voltage systems. Protection devices equipped with such measured value processing can react almost error-free - even in the case of rapid network frequency reductions, which occur, for example, in weak networks due to short circuits or overloads. Over- or under-functions are largely avoided. Due to the constant sampling frequency in relation to additional functions, e.g. fault recording, such protection devices can also be fully integrated as part of a system integration in a control system for switchgear or a protection data processing according to the VDEW/ZVEI recommendations for a standardized serial interface.

When the measurement value filter 1 is implemented as a digital filter, the amount of computer time required can be kept to a minimum, so that a discussion about processor performance is unnecessary. A software function for adaptive sampling rate adjustment can be omitted. The constant sampling rate also makes it possible to easily implement relative times, which are required for message processing.

Claims (7)

Protection claims 1. Measurement value filter (1) for an electrical signal (3) which essentially consists of a periodic basic signal with a known basic frequency and possibly superimposed interference, whereby in three different periods of the electrical signal, values with the same phase angle are sampled and fed to a filter element (7), whereby the filter element (7) has a frequency response which has a double zero at the basic frequency. 2. Measurement filter according to claim 1, wherein the frequency response has a sinusoidal shape and a minimum is located at the fundamental frequency. 3. Measurement filter according to claim 1 or 2, wherein the frequency response has a course according to the relationship |F (Jl) |=2 |cos(n*-fL) - 1|. 4. Measurement value filter according to one of claims 1 to 3, wherein three values with the same phase angle are sampled in directly successive periods. 5. Measurement filter according to one of claims 1 to 4, wherein the signal (3) is an alternating signal of a three-phase network. 6. Measurement filter according to one of claims 1 to 5, wherein the filtered signal (3a) is fed to a limit value processing. 7. Measurement filter according to one of the preceding claims, wherein the filter element (7) comprises a computer and is designed as a digital filter. Wof/Doe / 14.08.1992