From Short-Circuit Currents

5.6 Handling of neutrals on HV-side and LV-side of transformers

Special attention must be placed while selecting the type of neutral handling on HV-side and LV-side of transformers. The neutral earthing on one side of the transformer has an influence on the system performance on the other side, in case of earth-faults or single-phase short-circuits as the voltages in the zero-sequence component are transferred from one side of the transformer to the other. The neutral earthing of a 110/10-kV-transformer (vector group Yyd) according to Figure 5.17 is taken as an example. It is assumed [15] that X C0/ R 0 = 0.1 ?0.05, first value is applied for systems with overhead lines, second value with cables.


Figure 5.17: Transformation of voltage in the zero-sequence component of transformers in the case of single-phase faults. (a) Equivalent circuit diagram in RYB-system and (b) equivalent circuit diagram in the system of symmetrical components

The impedances Z E1 and Z E2 as per Figure 5.17 representing the earthing are different depending on the type of neutral earthing. In case of a single-phase fault in the high-voltage system (110 kV), the voltage U 0 in the zero-sequence component is transferred to the medium-voltage system (10 kV) with the same amount. Similar consideration indicates that the voltage in the zero-sequence component is transferred to the HV-side in case of a single-phase fault in the LV-system. In both cases, a fault current is measured in the system, having no fault.

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Topics of Interest

5.7 Examples 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...

5.3 Power system having earthing with current limitation 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...

5.4 Power system with isolated neutral The operation of power systems with isolated neutrals is applicable to systems with nominal voltages up to 60 kV, however the main application is seen in power...

5.2 Power system with low-impedance earthing Low-impedance earthing is applied in medium-voltage and high-voltage systems worldwide with nominal voltages above 10 kV. Power systems having nominal...

5.1 General The theoretical approach to calculate short-circuit (s.-c.) currents with symmetrical components in general and especially in the case of single-phase short-circuit was explained in...