Page 236 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 235

18.6.11.3 Autoreclosing and Other Equipment
Different types of autoreclosing equipment have been developed to meet the
varying requirements on the dead time and number of reclosing shots:
1. One high-speed reclosing shot;
2. One high-speed reclosing shot followed by one or more delayed reclosing
shots;
3. One delayed reclosing shot;
4. Several delayed reclosing shots.
The equipment is supplied as separate units or incorporated in the line
protections. It can be easily supplemented and adapted for different breaker
schemes, for example, the one-and-a-half breaker scheme and the double
breaker scheme.
In addition, other, more sophisticated types of equipment have been
developed for automatic system restoration after a disturbance. Such equip-
ment may sometimes work together with synchro-check and synchronizing
equipment to restore the network.
Automatic equipment has also been developed for automatic load restora-
tion after load shedding.

18.6.11.4 Distribution Networks with Radial Feeders
Problems of system stability or synchronizing requirements do not occur with
radial feeders with load only and therefore they can easily be provided with
three-phase autoreclosing equipment. This gives certain benefits such as short
outage times and the possibility to save on personnel. If instantaneous tripping
and high-speed autoreclosing are utilized, damage to the line in connection with
a transient fault can be limited. When feeders consist partly of underground
cables, the suitability of autoreclosing should be considered, since the faults
occurring in a cable are generally permanent. If there is only a small risk of the
cable being damaged by excavators, autoreclosing may nevertheless be justified,
since cable faults do not arise as frequently as faults on overhead lines.

18.6.11.5 Autoreclosing in Strong High-Voltage Networks

If the network is sufficiently strong, three-phase high-speed autoreclosing
can be utilized without the synchronism or stability being disturbed. Simple
equipment without facilities for checking the voltage, phase angle, and
frequency difference can then be used. Protective relays, breakers, etc., will
be simpler than for single-phase autoreclosing and no zero-sequence and
negative-sequence currents are obtained during the dead time. When only
three-phase delayed autoreclosing is utilized, no problems are experienced
with the deionizing time and breaker operating time. If oscillations occur in