ABB RELION REX640 Technical Manual page 480

Section 4
Protection functions
474
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
Voltage start value setting must be set above the highest possible value 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 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 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 259 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.
1MRS759142 C
REX640
Technical Manual