TABLE OF CONTENTS Earthing with current limitation can be seen in some cases as a special case of the low-impedance earthing, provided the earth-fault factor is below 1.4. Earthing with current limitation is applied in urban power systems having rated voltage U n ? 20 kV. Some applications are known in systems with nominal voltage up to 132 kV.
The criterion for the design of the earthing conditions is the value of the single- phaseshort-circuitcurrent, which can be limited to some kA(1kAor2kA)inmedium- voltage systems or to some 10 kA in high-voltage systems (e.g., below the three-phase short-circuit current). To realise the scheme of earthing with current limitation, the neutrals of some or all transformers are earthed through reactances or resistances to such an amount that the condition for the single-phase short-circuit is fulfilled. As a disadvantage it should be noted that the earth-fault factor ? might exceed the value of 1.4, which seems to be acceptable in medium-voltage systems with nominal voltages U n = 10 20 kV. In high-voltage systems with U n = 110 ?132 kV the advantages and disadvantages have to be analysed in more detail.
In order to estimate the required value of the earthing impedance, the zero- sequence impedance is considered based on Figure 5.5, indicating the ratio of I ? k1/ I ? k3 as well as the earth-fault factor ? in relation to X 0/ X 1. As an...
