Ferranti effect

The Ferranti effect , named after its discoverer Sebastian Ziani de Ferranti , occurs in high-voltage networks when long overhead lines or cable sections with high operating capacity are suddenly relieved at the consumer end by switching off the circuit breaker, or when an unloaded overhead line or cable section is suddenly switched on is charged. When the Deptford power station was commissioned in 1890, Ferranti noticed during the functional tests (load shedding or restarting) that the voltage increases at the overhead line outlets, which were caused by this effect.

General

Ferranti effect for a 380 kV line at 50 Hz (with L = 1.01 mH / km and C = 11.48 nF / km)

length	Cant
100 km	0.6%
200 km	2.3%
300 km	5.4%
400 km	9.9%

By the Ferranti effect power-frequency voltage peaks occur as a result of the capacitive charging current of the reactance of the overhead line or cable line depend (reactance). This means that the mains voltage U _E at the unloaded end of the line is greater than the mains voltage at the feed point U _S by the following factor :

${\displaystyle {\frac {U_{E}}{U_{S}}}=\left(1+{\frac {1}{2}}{\frac {R+\mathrm {j} X_{L}}{\mathrm {j} X_{C}}}\right)^{-1}}$

With

${\displaystyle X_{C}={\frac {-1}{\omega C'l}};\quad X_{L}={\omega L'l}}$

With the capacitance per unit length C ', inductivity per unit length L', the angular frequency ω and the line length l , neglecting the ohmic component ( R = 0), the factor of the voltage increase results approximately:

${\displaystyle {\frac {U_{E}}{U_{S}}}=\left(1-{\frac {L\cdot C\cdot \omega ^{2}}{2}}\right)^{-1}}$

The voltage increases increase the longer the unloaded overhead line or cable section l and the resulting increasing capacitive reactance X _{C of} the line. The inductive reactance X _L , like the ohmic coating on the line, has a reducing effect. When connecting an unloaded overhead line or cable section, additional transient processes (switching overvoltages) occur. If a single-pole phase -to-earth fault, compensated for by the earth fault compensation, is also effective, the excess voltage is increased considerably by the additional earth fault factor, which can lead to the destruction of system parts due to overvoltage.

Countermeasures

Cross- compensation chokes (charging current chokes) are used to reduce the Ferranti effect . These chokes are connected to earth for each conductor and increase the inductive reactance of the line and thus have a reducing effect on the excess voltage. The temporary voltage increase can be determined with the selection of the coil power. The use of cross-compensation inductors at the beginning and the end of the line not only reduces the Ferranti effect, but also compensates for the capacitive charging current during low-load operation.

In addition, so-called driving circuits are used. These are circuits that enable both circuit breakers at the beginning and end of an overhead line to be switched off at the same time .

literature

• Harald Koettnitz, Gert Winkler, Klaus-Dieter Weßnigk: Fundamentals of electrical operating processes in electrical energy systems . VEB German publishing house for basic industry, Leipzig 1986, ISBN 3-342-00087-2 .