ABB 615 series Technical Manual page 400

Section 4
Protection functions
394
an alternative solution to traditional residual current-based ground-fault protection
functions, for example the IoCos mode in 67/51N, 67/50N. Main advantages of 21YN
include versatile applicability, good sensitivity and easy setting principles.
Residual overvoltage condition is used as a pickup condition for the admittance-based
ground-fault protection. When the residual voltage exceeds the set threshold Voltage
pickup value, a ground fault is detected and the neutral admittance calculation is released.
In order to guarantee a high security of protection, that is, avoid false pickups, the Voltage
pickup value setting must be set above the highest possible value of VG during normal
operation with a proper margin. It should consider all possible operation conditions and
configuration changes in the network. In ungrounded systems, the healthy-state VG is
typically less than 1%×Vph (Vph = nominal phase-to-ground voltage). In compensated
networks, the healthy-state VG may reach values even up to 30%×Vph if the network
includes large parts of overheadlines without a phase transposition. Generally, the highest
VG 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 trip condition for the admittance protection enables a
multistage protection principle. For example, one instance of 21YN could be used for
alarming to detect faults with a high fault resistance using a relatively low value for the
Voltage pickup value setting. Another instance of 21YN could then be set to trip with a
lower sensitivity by selecting a higher value of the Voltage pickup value setting than in the
alarming instance (stage).
To apply the admittance-based ground-fault protection, at least the following network
data are required:
• System grounding method
• Maximum value for VG during the healthy state
• Maximum ground-fault current of the protected feeder when the fault resistance Rf is
zero ohm
• Maximum uncompensated ground-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 grounding resistor in the case of a high-resistance
grounded system
• Knowledge of the magnitude of VG as a function of the fault resistance to verify the
sensitivity of the protection in terms of fault resistance
Figure 188 shows the influence of fault resistance on the residual voltage magnitude in
ungrounded and compensated networks. Such information should be available to verify
the correct Voltage pickup value setting, which helps fulfill the requirements for the
sensitivity of the protection in terms of fault resistance.
1MAC059074-MB A
615 series ANSI
Technical Manual