power distribution network

The vessel’s Power Distribution Network is a highly redundant, layered distribution system for delivering power from generation and auxiliary power sources to vessel systems.

The system is made up of a network of electrical conduits which link distribution nodes, creating a web-like distribution network that provides multiple paths between power sources and consuming systems. Power can be routed around the network to meet operating requirements and to bypass damaged conduits.

The network also transforms power to the voltages and frequencies required by various vessel systems.

Structure

The Power Distribition Network is layered, with the first layer (the primary distribution network) providing maximum distribution capacity between primary generation sources and first-layer distribution nodes. Some high-consumption systems such as propulsion are supplied directly from the primary network.

From the first-layer distribution nodes, a second layer (the secondary distribution network) uses a range of smaller-capacity conduits to distribute power to distribution boards closer to the systems consuming it.

Distribution Nodes

The power delivered to each conduit is determined by its supplying node(s). Each node performs power conversion and also manages the current being delivered to each of its downstream conduits. The current available for each node to distribute downstream is determined by the current arriving from its supply (upstream) conduits.

Conduits

The voltage and current that can be handled by each conduit is limited by the physical parameters of the conduit (for example the size of the cable cores making up the conduit, their insulation and shielding, etc).

Conduits are typically made up of smaller cables run in parallel to provide the required capacity rather than a single large cable. This increases installation flexibility as large cables are more difficult to install in confined spaces.

The system must be carefully managed to ensure conduits are not overloaded and thereby damaged.

Conduits typically have significant redundant capacity over that needed for their nominal distribution role. This allows alternative routing of power around damaged conduits (in the case of battle damage, for example).

Primary Distribution Network (1DN)

The AC output of the main power generation system is immediately rectified to medium voltage (1000V) DC using twelve-pulse passive rectifiers. The use of this kind of rectifier minimises any harmonics induced in the generator or any downstream AC feeds. This is distributed via the Primary Distribution Network (1DN), consisting of a relatively small number of high-capacity conduits linking generation systems to layer-one distribution nodes and directly to propulsion systems.

The network includes redundant conduits which are installed along physically different routes throughout the vessel to minimise the effect of damage on the network’s minimum delivery capacity.

Secondary Distribution Network (2DN)

The Secondary Distribution Network (2DN) consists of a large number of lengthy mid-capacity conduits linking distribution nodes with distribution boards supplying vessel systems.

The DC supply from the 1DN is inverted to AC at the distribution node. Depending on the downstream requirement, inversion will be to either high-frequency (400Hz) or low frequency (50Hz) AC.

400Hz Secondary Distribution Network

The high-frequency 400Hz 2DN is an AC system rated at approximately 440 volts.

High-frequency AC (HFAC) power offers advantages over standard frequency AC in a number of applications:

  • Electric equipment can be much smaller and lighter. For example, doubling frequency generally permits electric machines to be 75% smaller. Other grid components (such as transformers, filters and circuit breakers) can also be smaller.
  • Electric motors can achieve higher speeds. High-speed induction motors can be directly used for compressors, high pressure pumps and turbines.
  • Acoustic noise is reduced dramatically due to a higher frequency mechanical vibration.
  • Harmonics in HFAC systems are at a higher frequency and so are more easily removed by filters.

Challenges to using HFAC include:

  • Skin effect: at higher frequencies the distribution of electrical activity within the conductor moves to the surface of the conductor (the ‘skin effect’). As the core of the conductor is no longer being used for electrical transmission, impedance is increased. In addition, electrical activity at the surface of a conductor is more susceptible to inductive reactance.
  • Grid Safety: circuit breakers must react faster to overload conditions in high frequency transmission scenarios to prevent damage.

50Hz Secondary Distribution Network

The standard frequency 50Hz 2DN is a three-phase AC system rated at 240 volts. Most supplied systems use a single phase, with the use of individual phases limiting cross-system interference.

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