Application - ABB RET670 Applications Manual

Section 3
IED application
3.5.2.1
126
Function description
Restricted earth-fault protection, low
impedance

Application

Breakdown of the insulation between a phase conductor and ground in an effectively or
low impedance grounded power system results in a large fault current. A breakdown of
the insulation between a transformer winding and the core or the tank may result in a
large fault current which causes severe damage to the windings and the transformer
core. A high gas pressure may develop, damaging the transformer tank.
Fast and sensitive detection of ground faults in a power transformer winding can be
obtained in solidly grounded or low impedance grounded networks by the restricted earth-
fault protection. The only requirement is that the power transformer winding is
connected to ground in the star point (in case of wye-connected windings) or through a
separate grounding transformer (in case of delta-connected windings).
The low impedance restricted ground fault protection REFPDIF (87N) is a winding
protection function. It protects the power transformer winding against faults involving
ground. Observe that single phase-to-ground faults are the most common fault types in
transformers. A sensitive ground fault protection is therefore desirable.
A restricted ground fault protection is the fastest and the most sensitive protection, a
power transformer winding can have and will detect faults such as:
• ground faults in the transformer winding when the network is grounded through an
impedance
• ground faults in the transformer winding in solidly grounded network when the
point of the fault is close to the winding star point.
The restricted ground fault protection is not affected, as a differential protection, with
the following power transformer related phenomena:
• magnetizing inrush currents
• overexcitation magnetizing currents
• load tap changer
• external and internal phase faults which do not involve ground
• symmetrical overload conditions
IEC 61850 IEC 60617
identification identification
REFPDIF
IdN/I
SYMBOL-AA V1 EN
1MRK504116-UUS C
ANSI/IEEE C37.2
device number
87N
Application manual