The main (or generator) switchboards are usually distributed or split in two, three, or four sections, in order to obtain the redundancy requirements of the vessel.
According to rules and regulations for electric propulsion, one shall tolerate the consequences of one section failing, e.g. due to a short circuit.
For strictest redundancy requirements, one shall also tolerate failure due to fire or flooding, meaning that water and fireproof dividers must be used to segregate the sections.
Configuration of Split Switchboard on Ship Electrical System
In a two-split configuration, with equally shared generator capacity and load on both sides, the maximum single failure scenario will hence be to loose 50% of generator capacity and loads.
In order to avoid a high installation costs, the system will often be split in three of four, which reduces the required additional installations. Also, change-over switches which ensures that a generator or a load can be connected to two switchboard sections will have similar cost reducing effects, e.g. for azimuth thruster
In propulsion mode, the switchboards are normally connected together, which gives the best flexibility in configuration of the power generation plant. The load transients are distributed on a large number of diesel generators, and the most optimal number of units can be connected to the network.
Another possibility is to sail with independent switchboard parts supplying two or more independent networks. In this case the ship is often assumed to be virtually blackout proof, which could be attractive in congested waters. In this operating mode one network including it’s connected propulsion units is lost if one switchboard section fails, the other, however, remaining operable.
In practice, there are also other considerations to be made in order to obtain such independence, especially all auxiliaries, such as lubrication, cooling, and ventilation must be made
independent. Also, loss of propulsion or station keeping power on one part of the system, will through control systems also have impact on the remaining parts, as the total power or thrust tends to be kept the same, e.g. for dynamic positioning.
Electrical Installation on DP Vessels
The normal operation in DP vessels, in particular for class 3 operations, is to split the network in order to be tolerant to failure of one section. However, rules and regulations now allows for operation with closed tie breakers, if the protection circuits are designed to detect and isolate faulty parts without tripping the healthy parts.
The NMD rules (Norwegian Maritime Directorate) has one of the more stricter practicing of these rules and will normally not accept connected networks in class 3 operations.
Why using High Voltage on Ship System
As the installed power increases, the normal load currents and the short circuit currents will increase. With the physical limitations on handling the thermal and mechanical stresses in bus bars and the switching capacity of the switchgear, it will be advantageous or necessary to increase the system voltage and hence reduce the current levels. Medium voltage has become a necessity to handle the increasing power demand in many applications.
Using the IEC voltage levels the following alternatives are most common selected for the main distribution system, with application guidelines from NORSOK [64]:
– 11kV: Medium voltage generation and distribution. Should be used when total installed generator capacity exceeds 20MW. Should be used for motors from 400kW and above.
– 6.6kV: Medium voltage generation and distribution. Should be used when total installed generator capacity is between 4-20MW. Should be used for motors from 300kW and above.
– 690V: Low voltage generation and distribution. Should be used when total installed generator capacity is below 4MW. Should be used for consumers below 400kW and as primary voltage for converters for drilling motors.
– For utility distribution lower voltage is used, e.g. 400/230V.
A few comments to using High Voltage on Ship are necessary;
– Where a major part of the load consists of variable speed drives with no contribution to the short circuit level, there will normally not be any problems to utilize each voltage level to significantly higher generator capacities. For optimizing the installation, one should in each case calculate load and fault currents and select the right solution.
– In ships, low voltage (690V) motors are normally used for much higher power levels than 300kW. In each case, one must consider the load current, and starting characteristics for the drive, including alternative starting methods together with a comparison on overall costs.
– 440V distribution is quite common in ship installations. A lot of ship equipment is available only in 440V, which means that it might be difficult to avoid this voltage level in ship applications.
In US, or where the ANSI standard applies, several additional voltage levels are recognized, such as:
120V, 208V, 230V, 240V, 380V, 450V, 480V, 600V, 690V, 2400V, 3300V, 4160V, 6600V, 11000V, and 13800V. 3300V is also a commonly used system voltage in IEC applications, even though not recognized.
Since the load current and fault current determine the limitation of the equipment, the actual power limits for each system voltage may deviate from these recommendations. This particularly applies to systems where a major part of the load is converter loads and does not contribute to short circuit power. Since these do not contribute to short circuit currents in the distribution system, it often allows increasing the power limits for the different voltage levels.
Safety Regulations to Work With Ship Electrical System
Safety is an issue of concern when yards and ship owners changes from low to higher voltages, often leading to a misunderstanding effort to keep voltages as low as possible. In the context of safety, it should be regarded that medium voltage switchboards is designed to prevent personnel to get contact with conductors, even in maintenance of the switchgears.
The normal and fault currents are similarly smaller, giving less forces on the conductors and cables during e.g. short circuit. Although short circuits inside the switchboards are extremely rare,
arc-proof design (IEC 60298-3) is available and will prevent person injury and limit the equipment damages if worst case should occur.
High Voltage Circuit breakers on Ship
Circuit breakers are used for connecting and disconnecting generator or load units to the switchboards, or different parts of the switchboards together. Various circuit breaker technologies are applied. Air insulated units are the traditional solution, but today rarely applied except at low voltage levels. In the commonly used SF6 and vacuum breaker technologies, the current interruption takes place in an enclosed chamber, where the first one is filled with SF6 gas, which has higher insulation strength than air, and the vacuum breaker is evacuated by air.
These designs give compact and long term reliable solutions for medium voltages. One should consider that vacuum breakers may chop the current and can cause overvoltage spikes when breaking an inductive loads with high di/dt that may require installation of overvoltage limiters.
For smaller powers, fused contactors are a cost and space beneficial alternative to the circuit breaker, and are available in air (low voltage), SF6 or vacuum insulated types. The problem with switching spikes is less with fused contactors since current interruption is softer (lower di/dt).
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