Measurement of imbalance of three=phase system - has oscillating circuit tuned to system frequency to provide output amplitude indicating imbalance

摘要

An oscillating circuit with a specified damping and tuned to the three-phase system frequency is built of active components. The system currents or voltages are so applied to it, that the oscillating circuit is excited by elimination of the positive phase sequence system components through addition of components of the negative phase sequence system. The system includes three series connected amplifier elements (P1, P2, P3) having phase shifts of 90, 90 and 180 degrees respectively. The first phase rotated value is fed to a first amplifying element (P1) via the input (E1) of the oscillating circuit. A valve 1/square root of 3 times the difference between the other two phase rotated values is fed to the second amplifying element via the input (E2) of the oscillating circuit. An arrangement (S) for picking up the output amplitude of the oscillating circuit is connected behind the third amplifying element (P3).