Page 348 - Handbook of Electrical Engineering

PROTECTIVE RELAY COORDINATION 335

to open the incomer and bus-section circuit breakers so that the fault does not develop and spread as
a fire or blast along the busbars.

12.6.2 Overcurrent Protection

It is not normally necessary to provide overcurrent protection in the bus-section circuit because the
presence of overcurrent, not caused by an in-zone fault, would be detected by an outgoing circuit
relay. For the busbar to be overloaded the outgoing system must also be overloaded. Introducing an
overcurrent relay in the bus-section circuit will add complication to the coordination of the incoming
and outgoing relays, since a time margin is necessary between each relay. In systems where there
are large induction motors the coordination can already be awkward to achieve.

12.6.3 Undervoltage Protection

If the busbars are being operated at an unusually low voltage then the consumers may attempt to
consume their full power. If this happens they will take in more than their rated current, which is a
potentially damaging condition. If the switchboard supplies one or more large induction motors then
during their starting process they will draw a heavy current. Should the motor experience difficulty
during starting then a prolonged period of high current will occur and this could cause a depression
in the busbar voltage. Such a depression may adversely affect other consumers.
Undervoltage operation is undesirable and therefore a suitable relay (27) with a time delay is
often used, especially in high voltage switchboards. A similar problem can arise with main generator
switchboards. If a generator is suddenly tripped then the remaining generators must try and supply
the load. Each of these generators will experience a sudden increase in current and a drop in terminal
voltage. The load will react to the drop in voltage. The automatic voltage regulators will try and
restore the voltage. If the load is predominately induction motors then they will all try and accelerate
back to their normal speed. The acceleration will be accompanied by an increase in their reactive
current which will aggravate the volt-drop and delay the voltage recovery. If the depression is more
than at least 20% and lasts for more than 0.2 to 0.5 seconds then there is a risk that the system of
induction motors will fail to recover, see also sub-section 7.6.1.
The (27) relays should have an adjustable voltage range to cover for the 80% voltage condition,
typically 50% to 100%. The relay should have a time delay that is adjustable up to at least 0.5 second.
The relay may be set to trip all the outgoing circuits on its section of busbars. Alternatively
a more selective method can be used in which the largest consumers are tripped initially. If the
initial tripping fails to produce a good recovery then a second level of tripping may be used for the
remaining consumers.
During the studies that are usually carried out for system stability, starting large motors, loosing
a generator etc., a study of undervoltage (and overvoltage) should be included. Several scenarios
should be considered so that a good compromise between voltage depression and its duration can be
found for setting the (27) relays.
Undervoltage schemes are often included in the reacceleration control systems of individual
motors or groups of motors. However, these are more appropriately considered in motor protection
rather than busbar protection schemes.