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On board hybrid ships, DC grids help reducing system losses compared to AC grids. But they are more difficult to design for safe short circuit protection and to limit the short circuit energy requires more sophisticated protection devices

There is no doubt today, that hybrid propulsion and power generation systems help make a vessel extremely energy efficient compared[ds_preview] to classic pure diesel-mechanical driven systems. But integrating electrical systems on board is also demanding and it calls for the right Know-how to master the perfect and hassle-free interaction of the systems: DC grids can be much more energy efficient regarding system losses. But a short circuit current in a AC system will pass through zero Amps twice each period (100 times @50Hz). Basically, what’s happening inside an AC circuit breaker is that the short circuit is being cut off when it passes through zero Amps. A short circuit in a DC system will never pass through zero Amps. It will in very short time become very high, and stay high as long as there is energy available to feed the short circuit. If a traditional (slow) circuit breaker is used to cut off a DC short circuit current the result may be an arc inside the circuit breaker which can damage the whole system and protection selectivity can be difficult to achieve.

Fast cut-off in microseconds

In addition, there is a common requirement in marine that a failure in one part of the system shall not affect the whole system. To solve that requirements, a fast DC-current tie-breaker is needed to isolate the healthy part from the faulty part. Today such a product is not available as a standard component on the market. For that purpose, the Vacon DCGuard is a semiconductor protection device that can detect and cut off any DC faulty currents and isolate the faulty part of the system in microseconds. In case of a short circuit, the DCGuard will disconnect the faulty DC bus from the healthy DC bus so the remaining grid will maintain fully operational. During the fault the feeding DC grid will see only a small dip in DC voltage. The DCGuard is already marine type approved by DNV GL and covers the whole range from 3A to 4140A. The type approval is useful when asking for system approval as there is as of today no relevant standard for such application as DCGuard.

Technology already proves its reliability

On the IJ River GVB´s (public transportation company of Amsterdam) hybrid ferries powered by VACON drives enjoy 24/7 uptime, smaller generator size and therefore help improve air quality, produce less noise and deliver easy maneuverability. GVB has a policy of reducing the emissions and environmental impact from its ferries, trams, buses and cars to an absolute minimum. So, when GVB ordered two new ferries from Holland Shipyard they decided to use battery hybrid technology to improve fuel efficiency and reduce pollution.

Peak shaving reduces generator size

The ferries operate 24 hours 365 days per year. It takes about four minutes to cross the IJ River and the ferries are docked for only about 2 minutes before departing on the next trip. A 100% battery-powered electric ferry was not possible as the time spent docked in port was too short to charge the batteries from a shore power connection. Instead, GVB decided to use electric propulsion with diesel-powered generators and Lithium-Ion batteries to shave the power peaks. This allowed them to downsize the generators, as these batteries enable the generators to run very efficiently with an almost constant load. To reduce the remaining air pollution the diesel engines are fitted with efficient exhaust gas cleaning systems, also known as SCR (selective catalytic reactor), which remove toxic gases and particles.

To help improve performance by making better use of energy, hybridization combined with energy storage is increasingly being introduced into marine systems. DC/DC converters can be used to connect sources such as batteries, super capacitors, fuel cells and solar panels. To protect the DC grid, an extremely fast circuit breaker is indispensable.


Helge Vandel Jensen