TABLE OF CONTENTS Power systems with resonance earthing are widely in operation in Central European countries. The German power system statistic [3] indicates that 87 per cent of the MV-systems having nominal voltages U n = 10 ?30 kV and nearly 80 per cent of 110-kV-systems are operated with resonance earthing (Criteria: Total line lengths). Some MV-systems are operated with a combined scheme of resonance earthing under normal operating conditions and low-impedance earthing in case of earth- fault. Resonance earthing, therefore, is the dominating type of system earthing in Germany for power systems with voltage 10 kV up to 110 kV. In other countries such as India, South Africa and China, power systems with resonance earthing have gained an increasing importance during the last decades, however are still not so common as systems with low-voltage earthing.
Resonance earthing is realised by earthing of one or several neutrals of transformers through reactances (Petersen-coils), normally adjustable, which will be set in resonance to the phase-to-earth capacitances of the system. The principal arrangement of a power system with resonance earthing is outlined in Figure 5.10.
The impedances of transformers and lines of the positive-sequence component can be neglected compared with those of the zero-sequence component due to the order of magnitude of the impedances. The admittance of the zero-sequence...
