Siemens Aktiengesellschaft

Ulrich Baumgaertl

Holger Hochgraef

Henry Franke

Wolfgang R&ouml;hl

Manfred Schiller

Wolfgang Roehl

The circuit contains a primary coil on the core () of a magnetic circuit with an air gap and switchable into a line carrying the measurement current. A secondary compensation coil (), with an order higher winding count and mounted on the core, is connected to the output of a controllable compensation current source (). The output of a Hall element () in the air gap is connected to the control input of the compensation source. A further coil () on the core is connected to an additional controllable current generator () whose control input () is driven by a measurement device () for the compensation current via a control and evaluation circuit (). USE/ADVANTAGE&#x2014;Measuring a.c. and d.c. Can be produced at low cost and all drift occurring during measurement can be compensated.

Method for reproducing direct currents and a direct current transformer for carrying out said method

A method and circuit arrangement for measuring a current are described, which can be used in electrical engineering to precisely trigger electronic tripping devices in circuit breakers. The current is measured according to magnitude and phase in such a way that only the fundamental wave of the current between precisely determined current zero crossings is sampled at a low sampling rate, and the root mean square is calculated on the basis of the measured phase angle and amplitude. The exact phase angle is measured using a conventional PLL circuit with a downstream zero switch.

Method and circuit for measuring a current

In power circuit breakers including a current transformer with a current detection coil without any iron and including an integration circuit connected downstream from the current detection coil, the unavoidable time constant of the integration circuit can lead to spurious trappings. This is avoided by a method in which the instantaneous values of the current are detected and stored at defined periodic intervals. The stored measured values are compared on the basis of polarity and magnitude. If it is found that the stored values have the same polarity and fall monotonally, it is assumed that the DC element is originating from the integrator and a tripping inhibit signal is set in the electronic release of the power circuit breaker.

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Berlin, Germany

Ulrich Baumgaertl