THE CLOSED BUS TIE BREAKER CHALLANGE

The majority of integrated power and propulsion vessels are depending on closed bus tie breaker operation for optimal use of their electric power plant. Closed bus tie breaker operation is also the preferred mode to most system operators since this gives a better operational availability and less running hours of the main engines. However, over the last decade, this same industry has become more aware of single point / common source failures which can lead to full black-out or loss of all thrusters when the bus tie breaker is closed.
 
Examples of common-source power faults that may occur:

• Over / under voltage
• Over / under frequency
• Unballanced reactive power
• AVR/ Excitation fault
• Over load
• Short circuit – Earth circuit
• Unbalanced voltage
• Light Arc
• Voltage distortion/ EMC

IS AN OPEN BUS TIE BREAKER SAFER?

The recognition of the above mentioned common source failure modes when running with the bus tie breaker closed, has led to a prolonged discussion in the industry.
The simple question above has been raised, which there unfortunately, isn’t always a clear answer to.
 

UNCERTAINTY FACTORS

Depending on the design, there may be quite a few common source faults which can blackout a vessel / rig, even when the bus tie breaker is in open position. Open bus tie breaker cannot be regarded as 100% safe in all cases.
 
Lately, we have also seen quite a few inadequate attempts from various vendors to solve the closed bus tie breaker problem. Some of these half-done solutions are even class approved.
When the FMEA reveals lack of sufficient protection for closed bus tie breaker operation, or when in doubt, it is normal to implement a DP procedure stating;
“When operating in DP2/3 mode, the bus tie breaker must be opened”
 
This is the industry’s most common reaction to the problem, and is the easiest way around a complicated topic. Unfortunately this simplified conclusion almost certainly results in an inadequate perception of the failure mode and power safety situation for the vessel.
 
The open / closed bus tie discussion is truly a headache for the industry, and leads to a uncertainty factor which will not just fade away.