In modern marine and offshore electrical applications, the increase in installed power and the growing integration and simultaneity of electrical networks of different types pose increasingly significant challenges in managing the maximum prospective short-circuit currents. In particular, in high-energy-availability systems, traditional switchboard design can become technically complex (with required performance levels for installed devices exceeding what is available on the market) and/or economically unsustainable.
Shore Connection and Vessel Retrofit
A typical case is represented by Shore Connection applications in ship retrofit projects: during the connection and transfer phase to the shore electrical grid, the simultaneous presence of onboard generation and shore supply can lead to a significant increase in available short-circuit power, with fault current levels exceeding those foreseen in the original system design and therefore surpassing the maximum ratings of existing switchboards.
In marine installations, it is also common to find main switchboards designed for medium-to-high short-circuit levels (e.g. 31.5 kA or 40 kA), while some commercial equipment or retrofit components in secondary distribution systems use standard devices certified for lower values (e.g. 20 kA). In these cases, the use of current-limiting reactors allows the impedance of the circuit to be increased, thereby reducing the downstream fault level within the limits that lower-rated equipment can withstand.
Current-Limiting Reactors for Short-Circuit Current Reduction
From a technical standpoint, the reactor limits both the RMS value of the short-circuit current and its peak dynamic value, significantly reducing thermal and electrodynamic stresses on busbar systems and on switching and interruption devices. This approach mitigates system stress levels and enables the use of standard equipment even in networks characterized by high available short-circuit power.
However, it is important to consider the continuous power dissipation caused by the reactor during normal operation (no-fault conditions), as it is a passive component with non-negligible impedance.
This solution is particularly valued in marine retrofit projects, where available space and downtime are extremely limited and where the removal of existing equipment and installation of new components is often difficult due to the lack of sufficiently large openings or passages, making hull cutting unavoidable.
Fault Current Limiter
In even more demanding applications, such as offshore platforms or industrial plants with hundreds of megawatts of installed power, prospective short-circuit current levels can far exceed the standard capabilities of medium-voltage circuit breakers available on the market, which are typically limited to 50 kA.
Beyond this threshold, switchgear requires reinforced supporting structures, more robust mechanical anchoring systems for power conductors, larger busbars, and special-purpose equipment, leading to significant impacts in terms of cost, footprint, and construction complexity.
In these scenarios, “Fault Current Limiter” devices are applied: ultra-fast systems capable of intercepting and limiting short-circuit current before it reaches its theoretical peak value.
Their operating principle is based on the rapid interruption of the fault circuit through the triggering of a small explosive charge contained within the device, which interrupts the current rise within a few milliseconds from fault detection.
In addition to limiting the electrodynamic effects of fault current, FCL devices drastically reduce thermal stresses, the interruption duty required from circuit breakers, and the overall size and structural requirements of the switchboard.
Under normal operating conditions (no short circuit), the FCL behaves as a conductor with negligible impedance, comparable to the main power circuit conductors.
This approach makes it possible to keep short-circuit levels within limits compatible with standardized equipment, avoiding the need for special switchboards that would be economically unsustainable.
Reactors and FCLs in Combination
The combination of current-limiting reactors and Fault Current Limiter devices represents one of the most effective strategies today to ensure reliability, safety, and optimized investment in high-power electrical systems, both in the marine sector and in the most complex offshore and industrial applications.