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Washington State Femes — Siemens, Blohm + Voss Among Suppliers On Project (Continued from page 49) systems with DC drives. While the DC system provides all the neces- sary performance requirements, design engineers feared DC tech- nology was becoming obsolete. They began studying the market for a propulsion system that could meet their demanding parameters — to find a system which would remain viable well into the 21st Century — covering the projected 50-year lifespan of the vessels. In 1993, a design was approved utiliz- ing variable frequency AC drives. "We chose electric drive because it allowed us to use a common electric generating system for both propul- sion power and ship's hotel load," said Stephen Gleaves, electrical supervisor, vessel design. It also had similar response characteris- tics to the DC motor drives without the high maintenance requirements of DC motors. The state received 12 bids from six vendors. Only three proposed cycloconverters; all others bid LCI systems. Siemens won the contract with a $47-million bid for three shipsets. Features of the winning bid in- cluded complete system redundancy, and a higher safety factor by utiliz- ing large propulsion transformers between the generators and the drives. Theoretically, cycloconverters can operate directly off the supply line without transformers, but trans- formers isolate the drives, reducing the severity of electric faults. In addition, they allow for 12- pulse operation which reduces har- monics on the electric grid thereby eliminating the need for additional electrical noise filters. They also ac- commodate WYE-connected syn- chronous motors, resulting in a less expensive installation. Synchronous AC motors were selected over in- duction motors because of their high reliability, simplicity, and better operating characteristics with the cycloconverter drives. The complete propulsion pack- age includes diesel alternators, a propulsion switchboard, propulsion transformers, four cycloconverter drives, synchronous AC motors and the shafting system (including the propeller, control consoles, and ex- tensive alarm and monitoring sys- tems). Complete system redundancy was a maj or consideration when the state design committee made its final ven- dor selection. Siemens' package includes four complete cycloconverter systems, two per shaft, effectively giving a full standby system for each shaft — so even if one drive malfunctions, system performance is not adversely affected. To further ensure uninterrupted vessel operation, the control system is completely redundant. Officials ranked reliability factors over price in their final selection process. With safe, uninterrupted service being the main objective of the ferry system, vendor warranties were an- other practical consideration. Si- emens demonstrated a high level of confidence in its system, backing it with an 18-month warranty. The contract carries a reliability clause whereby a monetary penalty is invoked if breakdowns cause more than three missed runs in a year. The reliability, efficiency, and spe- cialized operating characteristics of cycloconverter technology is earn- ing it a place in the marine industry. Engineers and designers will be monitoring its performance on the Washington State ferries over the next few years and looking for addi- tional applications. For more information on Siemens Circle 40 on Reader Service Card Choose from one of the largest inventories in the U.S. We stock all types and sizes of chain to meet your needs, both new and used, forged and welded, domestic or foreign. 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