In a review of the Overal 22 kV Single Line Diagram of a project, it was pointed out by one of the reviewers the absence of a busbar protection (i.e. high impedance differential protection) for the 22 kV switchboards. The question is, is it really needed? Earth fault protection is already included. The client is also adamant on having high impedance protection but it was investigated anyway.
Busbars have often been left without specific protection,for one or more of the following reasons[1]:
a. the busbars and switchgear have a high degree of reliability, to the point of being regarded as intrinsically safe
b. it was feared that accidental operation of busbar protection might cause widespread dislocation of the power system, which, if not quickly cleared, would cause more loss than would the very infrequent actual bus faults
c. it was hoped that system protection or back-up protection would provide sufficient bus protection if needed
The risk of a fault occurring on modern metal-clad gear is very small, but it cannot be entirely ignored. An uncleared fault may lead to the complete loss of the station by fire resulting to widespread and prolonged supply interruption.
Single phase to earth faults comprise the majority of bus faults. However, a large proportion of busbar faults result from human error rather than the failure of switchgear components. Modern metal-clad switchgear with full phase segregation, only earth faults are possible, and a protection scheme need to have earth fault sensitivity only.
An enquiry to Vendors were sent out for them to provide pricing for the 22 kV Main Switchboard which have 2 incomers, 1 bus tie and 20 outgoing feeders. The additional cost is an enormous $600K. Now comes another question, is it really worth it?
Your answer may be as good as mine, it depends on the required reliability and availability of your electrical system.
References:
[1] Areva Network Protection & Automation Guide Chapter 15