| 摘要 |
In earth fault protection for a generator in unit connection by measuring the voltage between the star point of the generator and earth, the star point additionally being loaded with an operating-frequency displacement voltage, the problem is that the displacement voltage has to have high amplitudes for the earth fault current detection because of the operating-frequency interference voltages which necessitate that the sensitivity is set comparatively low. To obtain a useful current (Iv) allocated to the displacement voltage (Uv), which is representative of an earth fault, from interference currents (Is) of any type within certain limits and also of operating frequency, the vector of the displacement voltage (Uv) is switched to and fro between two positions (Uv1, Uv2) in the sense of a phase oscillation. The corresponding useful currents (Iv1, Iv2) can be detected with a large signal/noise ratio by phase-selective demodulation or rectification by means of a multiplier (11), which is supplied with the measurement current and a signal corresponding to the phase modulated displacement voltage (Uv). <IMAGE> |
