Page 285 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 279

low-voltage systems and up to 660 V systems, as well as in high-voltage
systems of more than 33 kV.
2. Resistance earthing:
Connection to earth through a resistance for the star-connected system
has the advantage of low transient overvoltages and low fault current
used in generators and in systems of 660 33 kV when the fault current
does not exceed the load current.
3. Reactance earthing:
Connection to earth through a reactance for the star-connected
system has the advantage of low fault current but a disadvantage of
high transient overvoltages used for 660 33 kV systems when the
single phase to earth fault current is less than the three-phase fault
current.
4. Petersen coil earthing:
Connection to earth through an adjustable reactance to compensate
the capacitive earth current in the system for the star-connected system is
good for transient faults.
5. Unearthed systems:

These have the disadvantage of high transient overvoltages and the fault
current will be the capacitive current.

18.12.9 COMPARISON OF HIGH-IMPEDANCE AND LOW-
IMPEDANCE DIFFERENTIAL PROTECTION

High impedance has greater stability during CT saturation of one of the
CTs for through faults than low impedance, but needs an identical CT of
Class X and cannot share the same core with other protective relays.
Mainly used in transformer (resistance earthed) REF protection schemes,
they are low-cost, simple design, easy testing, and need accurate CT and
wiring data to set the voltage setting. However, low impedance has low
stability for through faults. It is mainly used in transformer protection but
has the advantage of the ability to share the same core of the CT with other
protective relays.

18.12.10 EXAMPLES OF TRANSFORMER DIFFERENTIAL
PROTECTION CONNECTIONS

A D y11 transformer is illustrated in Fig. 18.12.10.
An auto-transformer Y y0 is illustrated in Fig. 18.12.11.