Hitachi Relion 670 Series Applications Manual page 96

Section 7
Impedance protection
The ZRWPDIS application function is designed to detect and clear the faults mentioned in
with reliability in order to maintain the system stability.
Table 14:
Earthing system
Compensated earthing systems
Solidly earthing systems
High impedance earthing systems
7.2.3.1 Compensated earthing systems
In compensated grids, a distinction must be made between the following types of faults:
• Single earth fault, short-term (< 5sec)
• Single earth fault, sustained (> 5sec) (= sustained earth fault)
• Double cross country line-to-earth faults with separate earth contact points
• Phase-to-phase short-circuit, earthed and unearthed
When a single earth fault occurs, the fault current is reduced by the earth-fault neutralizers or the
active compensation facility level decreases until the flashover is neutralized automatically. The
concerned phase is not to be tripped. However, the faulty phase has to be identified and indicated
(earth-fault protection/determination of the earth fault direction).
When a cross country phase-to-earth fault occurs, the two phases have to be handled with different
time grading so that a delayed disconnection of the L2 phase enables elimination of the fault via the
earth-fault neutralizing function. The L2 phase is tripped only if the neutralization after a cross
country line-to-earth fault and a trip of the L1 phase has failed.
Phase preference logic
In compensated earthing systems, phase preference logic is used to achieve the correct phase
selective tripping during two simultaneous single phase earth faults in different phases on different
line sections.
Due to compensated earthing, the earth faults in these systems involve very low fault currents,
typically below 25 A. At the same time, the system voltage on the healthy phase increases to phase-
to-phase voltage level since the neutral displacement is equal to the phase voltage level at a fully
developed earth fault. Increase of the healthy phase voltage together with slow tripping increases the
risk a second fault in a healthy phase and the second fault can occur at any location.
90
Earthing systems and possible faults
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Types of fault
• Single phase earth fault of short duration
• Sustained single phase earth fault
• Cross country double earth faults
• Two phase short circuit with and without earth
connection
• Single phase earth fault
• Two phase short circuit with and without earth
connection
• Single phase earth fault
• Cross country double earth faults
• Two phase short circuit with and without earth
connection
Railway application RER670
1MRK506375-UEN Rev. N
Table 14
GUID-630CAFB0-1838-4670-A8A5-20910322BEAE v3
Application manual