5.7.1 Increase of displacement voltage for systems with resonance earthing

The capacitive asymmetry k of cable systems normally is below k < 0.1%; the system damping is in the range of d ? 2 ?4% resulting in a displacement voltage which is too low for the operation of the resonance controller. For a given capacitive asymmetry of k = 0.1% and system damping d = 2% the displacement voltage will be U _NE = 0.005 * U _n/ ?3 for resonance tuning ( v = 0%); if the detuning shall be v = 5% the displacement voltage will be U _NE = 0.002 * U _n/ ?3 only ^[17]. Resonance controller normally operates sufficiently if the displacement voltage is above U _NE > 0.03* U _n/ ?3. The displacement voltage, therefore, had to be increased by installing an additional capacitor in one phase ^[18]. The required capacitance ? C _E for different parameters is given in Table 5.3 for a 20-kV-system earthed through Petersen-coil.