IPS-designed switchboard is an electric plant "technical marvel" for Signet Maritime Corporation's new Z-drive escort tug boats America and Pacific Star
Owners and operators of new state-of-the-art tug boats recently built by Martinac Shipbuilding of Tacoma, WA, are collectively praising the innovative electrical control systems designed and built by IPS.
The second vessel, Pacific Star, was handed over by the builder in June 2009, and Martinac's chief electrical designer Gary Farrell wrote the following on these applications.
 In the construction of 100ft, 80 ton bollard pull, Z-Drive Terminal/Escort support tug boats America and Pacific Star for Signet Maritime Corporation, J. M. Martinac Shipbuilding Corporation set out to resolve a notorious problem encountered in the design of the electric plant on tug boats. The solution came in a power management system designed and built into the main switchboard by Industrial Power Systems Inc. of Jacksonville, Florida.
Large tugs such as these generally employ a hefty winch that is either electric or electro/hydraulic. In either case this winch represents a problem in the design of the electric plant due to infrequent usage and relative size of the electrical load it comprises. General usage of this winch might be on the average of 15 minutes out of a 24 hour period yet it often constitutes an electrical load equal to or larger than all the other electrical loads on the vessel combined.
Ideally a diesel/electric generator should be sized such that the demand load is about 75% -80% of the generator sets rated capacity. More often in than not the electric plant for such vessels incorporates a generator selected to accommodate the entire load. The standing load minus the winch is roughly only 40%-50% of the generator rated capacity. Therefore, this selection creates a situation where for approximately 23 hours and 45 minutes out of the average 24 hour period the generator is under loaded. Operating a generator set in an under loaded condition like this causes the diesel engines to slobber and carbon-up. The result is excessive down time and maintenance costs on the diesel engines.
J. M. Martinac asked Industrial Power Systems to build them a switchboard that would provide a power management system that would allow a 99kW generator to be selected to accommodate a 70kW normal load demand load plus a 56kW hawser winch load. To supply a 126kW total load from a 99kW generator would require a sophisticated power management system.
First, this power management system must provide power limiting logic for the winch and load shedding capabilities when called for by the demand load on the switchboard to prevent overloading a single on line generator. Second, when this condition persists for a predetermined time frame it must start, synchronize, parallel and put on line a standby generator. Third, since an under loaded condition is undesirable, when the demand load drops into a range that can be easily handled by a single generator a parallel generator must be removed from the line. Fourth, due to manning levels on vessels of this type, the power management system must be automatic.
The switchboard was required to incorporate all controls for voltage, frequency, automatic load sharing and shedding, plus all instrumentation required by the American Bureau of Shipping. The switchboard was required to include a sizeable 480VAC distribution section, and equally sizable 208Y/120VAC distribution section. Due to limited space available in the engine room for the switchboard it was required that it could be no larger than 70"w X 72"h X 22"d. All circuit breakers in the switchboard were required to coordinate tripping and the distribution breakers were required to coordinate with downstream devices.
While all of this might seem quite impossible, it was not. An ABS-certified and attractive switchboard of those dimensions was delivered to the Martinac facility on schedule that met all of those specifications. The switchboard was subsequently load tested aboard the vessel where it was demonstrated that with only one generator on line it limited power to the hawser winch to 28kW when the total demand load reached 89kW and remained there for a period of 5 seconds. With two generators operating in parallel when the load reached 178kW this same power limiting feature was activated. It was also confirmed that with one generator on line if the load remained at 89 kW for 10 seconds the standby generator is automatically started, synchronized and put on line in parallel with the first generator.
Load shedding was provided as a failsafe back up feature to prevent tripping a generator off-line due to an overload condition. Loads on the vessel were segregated into 'Vital' and 'Non-Vital' categories. Many non-vital loads were grouped on common panels supplied from the switchboard others such as the galley range and water heater directly. Testing proved that if the frequency should drop below 58hz on a running generator the power management system trips feeder circuit breakers to all non-vital loads and activates an alarm in the central alarm and monitoring system that load shedding has occurred.
It was also demonstrated that with two generators on line and in parallel, if the demand load drops to a range between 85-60kW and remains at that magnitude for a period of 5 minutes, the power management system activates an alarm in the central alarm and monitoring system to notify the operator that one generator should be shut down. Furthermore, if the load continues to drop below 60kW and remains at that state for a period of 1 minute, a generator is automatically taken off line, placed in a cool down mode, then after 1 minute of cool down that generator is shut down.
As the chief electrical designer here at J. M. Martinac Shipbuilding Corporation I was very impressed with the switchboards designed and built by Industrial Power Systems Inc. From discussions I have had with the Owner's representative for Signet Maritime Corporation regarding these switchboards I believe the feeling is mutual. 
Gary Farrell
Martinac Shipbuilding
401 East 15th St, Tacoma, WA 98421
Tel: 253-572-4005 / Fax: 253-572-6055
www.martinacship.com
|
