Page 234 - Offshore Electrical Engineering Manual

CHAPTER
4
Busbar Protection

BUSBAR FAULTS
Busbar faults in medium- and low-voltage metalclad indoor switchgear are very rare
in onshore installations. However, several incidents have occurred offshore, and in
one instance where the risk of busbar faults was considered too low to warrant the
installation of specific protection, tripping was initiated by fire detection rather than
upstream electrical protection devices. Water from leaking pipes, or from driving
rain or sea spray penetrating module walls, may enter switchgear enclosures, giving
rise to explosive faults or serious fires. Arcing faults on the busbars of low-voltage
switchgear will cause extensive damage to the switchboard and may lead to fire in an
area of the installation likely to be close to accommodation modules or the process
control room. However, the offshore installation of a full bus-zone protection scheme
with check and supervision relays may only be advisable on the main switchboards
of the largest platforms because of the weight of all the current transformers (CTs)
required and the extra space taken up by such a scheme. Nevertheless, it is important
that every part of the power system is adequately covered by the protection scheme,
and the busbars of switchboards are no exception to this rule.

OVERCURRENT AND DIRECTIONAL OVERCURRENT
PROTECTION

The overcurrent fault protection relays on the primary circuit of the transformer
will provide some protection to the secondary circuit, but earth fault protection
devices on the upstream side will not provide any secondary circuit protection
because there is normally no zero-sequence path through the transformer. If the
busbars are sectionalised, the bus-section switch may be fitted with overcur-
rent or directional overcurrent protection. Plain overcurrent relays operating
on bus-section switches cannot, of course, remove the faulted section from the
supply, unless it is being fed via another section. However, if the switchboard
is fed by plain feeders or directly by generators with overcurrent protection,
such an arrangement can provide reasonable overcurrent protection whilst pre-
serving the supplies on healthy bus sections (see Fig. 4.4.1). If the switchboard
has three sections, directional overcurrent relays, arranged to detect fault cur-
rents flowing outwards, may be fitted to the bus-section switches. Such an

Offshore Electrical Engineering Manual. https://doi.org/10.1016/B978-0-12-385499-5.00024-8 221
Copyright © 2018 Elsevier Inc. All rights reserved.