ABB RED615 Technical Manual page 337

1YHT530004D05 D
615 series
Technical Manual
during normal operation with a proper margin. It should consider all possible
operation conditions and configuration changes in the network. In unearthed
systems, the healthy-state Uo is typically less than 1%xUph (Uph = nominal phase-
to-earth voltage). In compensated networks, the healthy-state Uo may reach values
even up to 30%xUph if the network includes large parts of overheadlines without a
phase transposition. Generally, the highest Uo is achieved when the compensation
coil is tuned to the full resonance and when the parallel resistor of the coil is not
connected.
The residual overvoltage-based start condition for the admittance protection
enables a multistage protection principle. For example, one instance of EFPADM
could be used for alarming to detect faults with a high fault resistance using a
relatively low value for the Voltage start value setting. Another instance of
EFPADM could then be set to trip with a lower sensitivity by selecting a higher
value of the Voltage start value setting than in the alarming instance (stage).
To apply the neutral admittance protection, at least the following network data are
required:
• System earthing method
• Maximum value for Uo during the healthy state
• Maximum earth-fault current of the protected feeder when the fault resistance
Rf is zero ohm
• Maximum uncompensated earth-fault current of the system (Rf = 0 ohm)
• Rated current of the parallel resistor of the coil (active current forcing scheme)
in the case of a compensated neutral network
• Rated current of the neutral earthing resistor in the case of a high-resistance
earthed system
• Knowledge of the magnitude of Uo as a function of the fault resistance to
verify the sensitivity of the protection in terms of fault resistance
Figure 169 shows the influence of fault resistance on the residual voltage
magnitude in unearthed and compensated networks. Such information should be
available to verify the correct Voltage start value setting, which helps fulfill the
requirements for the sensitivity of the protection in terms of fault resistance.
Section 4
Protection functions
331