ABB RELION REX640 Technical Manual page 509

1MRS759142 F
4.2.4.7 Application
Admittance-based earth-fault protection provides a selective earth-fault protection
for high-resistance earthed, unearthed and compensated networks. It can be
applied for the protection of overhead lines as well as with underground cables.
It can be used as an alternative solution to traditional residual current-based
earth-fault protection functions, for example the IoCos mode in DEFxPDEF. Main
advantages of EFPADM include versatile applicability, good sensitivity and easy
setting principles.
Residual overvoltage condition is used as a start condition for the admittance-
based earth-fault protection. When the residual voltage exceeds the set threshold
Voltage start value , an earth fault is detected and the neutral admittance calculation
is released. In order to guarantee a high security of protection, that is, avoid false
starts, the
of Uo 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%×Uph (Uph = nominal phase-
to-earth voltage). In compensated networks, the healthy-state Uo may reach values
even up to 30%×Uph 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
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 admittance-based earth-fault 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 Ω)
• 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 292
in unearthed and compensated networks. Such information should be available to
verify the correct
the sensitivity of the protection in terms of fault resistance.
REX640
Technical Manual
Voltage start value setting must be set above the highest possible value
Voltage start value setting. Another instance of EFPADM
shows the influence of fault resistance on the residual voltage magnitude
Voltage start value setting, which helps fulfill the requirements for
Protection functions
509