Engineers: Voltage mismatch caused Washington ferry blackout

A design flaw in a phase loss relay was one of the causes of a Washington State Ferries (WSF) vessel losing power off Bainbridge Island last summer.

On July 29, as the 460-foot Tacoma approached the Bainbridge Island dock, a voltage surge occurred that caused a brief flash explosion. It resulted in major damage to two generators, leading to a blackout.

It will cost about $1.8 million to repair the diesel-electric boat, which is expected to be operating this spring, said Marta Coursey, spokeswoman for Washington’s Department of Transportation.

The surge was not contained because of a design problem in the Siemens F-14 phase loss relay, according to a report issued by the transportation department. The Siemens part type is 7TU9914. 

Tacoma runs on two generators.When the ferry comes to a dock, a third generator is put online as backup, said the October 2014 report, written by Raeey Regassa of Siemens Industry. 
In the July 29 incident, Tacoma’s third generator, Generator-4, was put online when the ferry docked at Seattle.
While docked, Generator-4 was then taken offline and there was a voltage surge due to the opening of the breaker that connects Generator-4 to the rest of the system. The voltage surge damaged all three surge limiters and two of the three fuses of Generator-4. The surge limiters are rated for a continuous operating voltage of 3.6 kv, but the rest of the vessel’s system is rated at 4.16 kv, a contributing factor in the blackout. The loss of fuses meant there was a voltage mismatch between Generator-4 and the rest of the 4.16-kv system, the casualty report said. This mismatch caused a voltage dip on the 4.16-kv system and Generator-1 and Generator-3 got out of synchronization.

The phase loss relay in the Generator-4 cubicle should have detected the phase loss and cut off Generator-4 from the vessel’s 4.16-kv system, the report said. However, the relay’s design meant it could detect phase loss when either all three fuses were damaged or if just one fuse was damaged, but not when two of the three fuses were damaged, which is what happened on July 29.

Siemens made four recommendations in its casualty report. The surge arresters need to be replaced in order to match the 4.16-kv voltage rating of the rest of the system. Siemens prefers that the F14 phase loss relay be replaced with a modern relay that has both phase and voltage comparison capabilities. Because of lack of space, the recommendation is to reconfigure the relay.
The synchronizer relay (SPM) on Tacoma checked phase match between two lines, but not voltage magnitude match. Siemens recommends installing a Woodward 9905-003 SPM that can check phase and voltage match. If there were an SPM relay on Generator-4 that checked voltage magnitude match on July 29, it would not have allowed the generator to hook up with the rest of the system, the report said.

Cover plates were missing on both switchboards. Cable entries to the switchboards need to be sealed.

Tacoma is one of the agency’s three Jumbo Mark II-class vessels. “WSF is making casualty prevention modifications to the F14 phase loss relay on the Puyallup and Wenatchee based on Siemens’ recommendations,” said Coursey. Puyallup and Wenatchee are the other two Jumbo Mark II-class vessels. The cost will be $682,000 per vessel. 

The transportation department said the Siemens-authored report was reviewed by DEI Electrical Consultants of Spokane Valley, Wash.

By Professional Mariner Staff