Page 254 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 251

current. A temperature-sensing device embedded in bearings gives
adequate warning.
9. Loss of synchronism and field failure in synchronous motor protection
Synchronous machines subjected to sudden mechanical overloads should
be protected against loss of synchronism, which can also be caused by a
reduction of field current or supply voltage.
Where the overloads period is short a loss of synchronism relay can be
arranged to disconnect the field supply only, so that the machine runs as an
induction motor during overload then the relay resynchronizes the machine
automatically after overload periods.
Field failure relay should be provided for synchronous machines larger
than about 500 HP which are not provided with loss of synchronism protec-
tion, preventing overheating of the rotor after continuous operation as an
induction motor. An undercurrent relay in the field circuit gives satisfactory
protection.
Damper winding thermal protection may be used as an alternative to field
failure protection if the machine is required to run for long periods as an
induction motor.

18.9 Switchgear (Busbar) Protection

18.9.1 IMPORTANCE OF BUSBARS
Busbars are the most important component in a distribution network. They
can be open busbars in an outdoor switch yard, up to several hundred volts,
or inside a metal-clad cubicle restricted within a limited enclosure with
minimum phase-to-phase and phase-to earth clearances. We come across
busbars which are insulated, as well as those which are open and are
normally in small-length sections interconnected by hardware.
They form an electrical “node” where many circuits come together, feed-
ing in and sending out power (see Fig. 18.9.1).
From Fig. 18.9.1, it is very clear that for any reason the busbars fails, it
could lead to a shutdown of all distribution loads connected through them,
even if the power generation is normal and the feeders are normal.

18.9.2 BUSBAR PROTECTION
Busbars are frequently left without protection because it is very rare to have
faults, especially metal-clad switchgear, and it is protected by backup
protection, it can be protected by a separate busbar protection but it is very
expensive due to the cost of CTs and relays. Also, any false operation will
cause the complete system to trip.