Ferranti effect

Named after its discoverer, Sebastian Ziani de Ferranti Ferranti effect occurs in high voltage networks, where long overhead lines or cable lines with high operational capacity are relieved suddenly by turning off the power switch on the decrease end, or an unloaded at the end of the overhead line or cable line is turned on. Ferranti discovered during the commissioning of the power plant in Deptford in 1890 at the functional tests (load shedding or restarting ) betriebsfrequente voltage increases to the overhead line feeders.

General

Due to the Ferranti effect occur betriebsfrequente voltage increases due to the capacitive charging current of the reactance ( reactance ) of the overhead line or cable line depend. In order for the mains voltage UE is on the unloaded end of the line by following factor greater than the mains voltage at the feed U.S.:

With

ω with the capacitance C ', inductance L', the angular frequency and the cable length l, neglecting the resistive component (R = 0), the result is approximately the factor of voltage increase:

The voltage increases to take the longer the unloaded transmission line or cable line l and the resulting increase in capacitive reactance XC of the line. The inductive reactance XL affects, as well as the resistive coating of the pipe, reducing from. When switching an unloaded overhead line or cable line still occur in addition transients ( switching surges ). Has the same time still a single-pole and earth fault compensated by the phase-earth fault, the voltage peaks are considerably increased by the additional earth fault, which can lead to the destruction of the plant.

Countermeasures

To reduce the Ferranti effect cross shunt reactors are ( charging current reactors ) are used. These chokes are per conductor connected to ground and increase the inductive reactance of the line and act as a reducing effect on the voltage overshoot. With the choice of coil power, the temporary increase in voltage can be determined. The use of cross- shunt reactors at the beginning and the end of the conduit not only results in a reduction of the Ferranti effect, but also brings about a compensation of the capacitive charging current for the low load operation.

In addition, be used so-called entrainment circuits. These are circuits allow simultaneous switch off both power switches at the beginning and end of a transmission line.