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Cycloconverters Chosen For Washington State Ferries

When the Washington State Ferry System selected a synchronous, variable-speed AC motor with a cycloconverter drive for the propulsion system on its three new Jumbo Mark II ferries, the ships became the first public transportation vessels to utilize the technology.

A cycloconverter system is typically used in situations requiring medium to high power (2,000 kW to 20,000 kW), slow speed (0 to 1,000 rpm), and where excellent dynamic response and precise speed control is necessary. It can accommodate rapid changes in direction and speed, deliver full power in both forward and reverse, and can apply full torque at any rpm supplying maximum available power to the shaft.

To date, heavy industry has been the main user, but cycloconverter technology is making headway in the marine industry, first on icebreakers where its high power characteristics won favor, and on cruise liners, where the electrical load requirements exceed the propulsion load. Utilizing a single electric generating plant to supply both loads reduces the number of diesel power sources in the engine room, saving both space and money.

The Washington State ferries accommodate nearly 30 million passengers annually. The complex topography of the Puget Sound region makes ferry transportation essential for the citizens, and a challenge to system officials. Reliability, high- and low-speed maneuverability, instant dynamic response, as well as high power capabilities are required elements on the state's car ferries. The unique structure and operating practices of the vessels further complicate propulsion system designs. The ships are double-ended to eliminate time-consuming backing and turning around procedures. Cars drive straight through the vessels entering on one end, exiting on the other. All of the vessels maintain two pilothouses, two engine rooms and two self-con tained propulsion systems. At the end of each run, the captain walks across the deck, enters the opposing bridge, switches over the controls, and pulls forward out of the slip. There is a propeller at each end of the vessel, separate drives, and two motors in series on the shafting. Each ship carries four diesel generators, two per engine room. To maintain the system's tight sailing schedule, docking time must be kept to a minimum. Vessels approach the dock at full speed, stop quickly and maneuver into the narrow slips unassisted.

A double propeller/double rudder system gives them full lateral movements when needed. While underway, most of the power is delivered by the stern propeller with just enough power to the bow propeller to keep it from impeding forward progress. The bow propeller is used to stop the boat, as it can do so faster t h a n the stern propeller going in reverse.

Maneuverability is another vital characteristic of fleet vessels. With a dense urban population from Seattle to Everett on one side of the water, Bremerton and its suburbs lining the peninsula, and a string of islands in between, ferries run crosswise across the Sound while the busy shipping lanes run lengthwise, necessitating a continual crossover of commercial and transportation traffic. With one of the highest boats-percapita figures in the world, Seattle's pleasure boat fleet presents an even greater challenge to the ferries. Above all else, vessels must be reliable as even a short delay on a single vessel disrupts schedules for hours. Breakdowns can essentially strand island communities.

With one of the highest boats-per-capita figures in the world, Seattle's pleasure boat fleet presents an even greater challenge to the ferries Most of the current fleet runs on diesel-electric systems with DC drives. While the DC system provides all the necessary performance requirements, design engineers feared DC technology 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 utilizing variable frequency AC drives.

"We chose electric drive because it allowed us to use a common electric generating system for both propulsion power and ship's hotel load," said Stephen Gleaves, electrical supervisor, vessel design. It also had similar response characteristics 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 included complete system redundancy, and a higher safety factor by utilizing large propulsion transformers between the generators and the drives.

Theoretically, cycloconverters can operate directly off the supply line without transformers, but transformers isolate the drives, reducing the severity of electric faults. In addition, they allow for 12- pulse operation which reduces harmonics on the electric grid thereby eliminating the need for additional electrical noise filters. They also accommodate WYE-connected synchronous motors, resulting in a less expensive installation. Synchronous AC motors were selected over induction motors because of their high reliability, simplicity, and better operating characteristics with the cycloconverter drives.

The complete propulsion package includes diesel alternators, a propulsion switchboard, propulsion transformers, four cycloconverter drives, synchronous AC motors and the shafting system (including the propeller, control consoles, and extensive alarm and monitoring systems).

Complete system redundancy was a maj or consideration when the state design committee made its final vendor 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 another practical consideration. Siemens 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 specialized operating characteristics of cycloconverter technology is earning 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 additional applications.

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