UDT 2020 UDT Extended Abstract Template Presentation/Panel UDT 2020 – Submarine power distribution by smart DC microgrid Peter Rampen MSc, Damen Shipyards, Gorinchem, Netherlands Abstract — DC as an electrification platform has been established onboard submarines since the very beginning – yet tomorrow’s DC grids will look and behave very differently from classical DC systems. In today’s DC applications, commercial projects have now implemented and demonstrated modular and scalable DC systems with fully controlled loads, sources and prosumers. However in these applications, DC is limited to a central hub, with centralized conversion and control. The market is now at a stage where the next generation of smart DC grids are soon realizable. Confidence in controlled DC systems has been established, and methodologies and components for DC integration are now accepted and proven. Together with semiconductor circuit breakers and next generation power electronic converters, a true vessel-wide DC power distribution grid is now possible and coupled with the extension of the grid as a data network, true controllability can be achieved. This enables system scalability as well as the potential for zonal distribution to be realized, allowing full controllability of loads and prosumers. Coupled with local power conversion and higher degrees of distributed control, robustness and reconfigurability are enhanced. This flexibility introduced by smart DC grids is not only operational, but also gives the designer more freedom in the choice and exploitation of novel type of power sources, like: variable or high speed generators and batteries. In this paper, the current experience and status of today’s DC systems is presented. The research topics which are being addressed in order to realise tomorrow’s generation of DC grids are then introduced, focusing especially on distributed control and the zonal architecture. This is backed up by the enabling technologies, and their state of maturity, demonstrating system safety, low-maintenance and compactness. Smart DC grids therefore make for not simply power delivery, but therefore true power distribution and control. By harnessing the benefits of smart DC, we can achieve system efficiency, and not just efficient systems. 1 Introduction acid batteries, in some cases in parallel with an air independent power (AIP) source. The AIP is normally Already since the introduction of electrical power rather low power and can be seen as a range extender for distributions in the late nineteenth century, submarines submerged (air independent) operation. On the surface the have been equipped with DC electric propulsion. The batteries are charged by an air dependent power (ADP) Spanish submarine Peral , commissioned in 1888, was the source. first. This submarine would nowadays be called a full- electric vessel – with batteries as the sole source of power, aft zone fwd zone 300..600Vdc and having to be charged from an external source. When fully charged it had a maximum range of 400 nmi at 3 kts. 400Vac 400Vac 50Hz 50Hz propulsion switchboard G main switchboard lead acid lead acid The early DC systems used the classical “Edison DC” 250...600Vdc 250...600Vdc PM M technology, with DC motors and generators. In the second 24Vdc 24Vdc G half of the previous century, semiconductor-based solid state converter technology was introduced (see figure 1). At first only for speed control regulators on DC motors and as rectifiers for synchronous generators, nowadays PWM- Fig. 2. Typical power configuration for present-day builds based converters are widely used on board of submarines. New developments in AIP sources, ESS with Li-ion 300..600Vdc battery technology, more electronic devices (DC by nature), power semiconductor and IT developments, 400Vac require that the presently used systems are reconsidered. In M G 50Hz G lead acid lead acid this paper a state of the art DC system is proposed using switchboard M technology that is now available at the proven concept G 115Vac 3.1 MW level. M G 400Hz First the limitations of presently used passive grids are explained, secondly several new technologies are described and finally the benefits of the proposed active Fig. 1. Typical power configuration in the late 20 th century DC grid for a submarine is presented. The main power sources for submerged operation are a very large energy storage system (ESS) consisting of lead
UDT 2020 Presentation/Panel UDT Extended Abstract Template 2 PASSIVE DC GRID LIMITATION enabling low loss and compact solid state converters and solid state circuit breakers (SSCB). Since these devices For efficiency reasons the batteries are, via a short-circuit have micro-processor based control they are by nature protection device, directly connected to the DC smart. By also implementing state of the art IT systems, a distribution system. This implies that the ESS determines smart grid can be created. In these smart grids connected the DC distribution voltage, which is depending on the devices are remotely monitored, supervised and operated. ESS state of charge (SOC) and the load current. This voltage window can be up to a factor 2 (e .g. 300…600 3.1 Solid-state circuit breakers Vdc). All equipment connected to the DC distribution therefore has to be rated for this voltage range, and as a A SSCB combines the advantage of very fast micro- consequence, this equipment has a current rating two times processor based short-circuit detection with very fast higher than that nominally required. This results in larger switching characteristic of power electronic devices, as and heavier systems, which are key parameters for a shown in figure 4. Short-circuit currents can be detected submarine. Additionally, this equipment is specially made based on a high current-slope. As a result, the short for this voltage range, and is therefore only affordable for circuit current can be switched-off within nominal the high power equipment. By numbers, most equipment operation current, and overcurrent-rated equipment is not is still supplied by AC and 24Vdc distribution, so large required. For local power distribution earth fault grid converters are installed to convert the main DC protection can be applied, making the system safer for voltage to the auxiliary AC voltage level(s). personnel. Because of the fast switching time preventing high Short-circuit protection is particularly an issue with DC currents being reached, the short-circuit energy very low, due to the absence of zero crossing. Short-circuit currents and arc flashes will be very limited. By implementing in these systems are very high and steep (see figure 3), SSCB close to equipment with large energy storage, a because in this low impedance system high amount of very safe electrical system can be created. energy is stored in converter capacitors and the batteries. For short-circuit protection very large mechanical circuit micro-processor controller breakers and special fuses are therefore needed. For today ’s configurations these devices are able to switch the short-circuit currents off safely. However when new current battery technologies and more capacitance is introduced these devices will be too slow to limit the high short current peaks. This can result in high arc-flash values, which can be dangerous for fire safety. Fig. 4. MOSFET based SSCB topology With the use of SSCBs interconnected grid topologies can be envisioned, which are reconfigurable and zonally redundant. Interconnected grids increasing the optimal use of the available sources. Despite the available types being still limited, currently the first SSCB are becoming commercial available. Some types are developed for the lighting groups (about 16A), while others are available for high power 1000Vdc systems, with ratings starting at 750A up to a few kA’s. Low power SSCBs using the state of the art semiconductors, realize an advanced circuit breaker with low losses. For high current ratings, the losses are Fig. 3. Typical short-circuit current simulation for DC system depending on the used semiconductor type. IGBT-based with high capacitance SSCBs are still having high losses, and need (relatively) high cooling capacity. IGCT-based SSCB on the other 3 Active DC grids hand promise a lower loss circuit breaker [5], although losses are still higher than a fuse or mechanical breaker. Several technological and social developments are However an SSCB has superior fault clearing and limiting driving the developments of smart DC grids. Land based characteristics compared with these devices. It is expected power is increasingly generated by renewable energy that SiC semiconductors are becoming available that will sources, decentrally connected to the grid and which are further improve high current SSCBs. by their very nature unpredictable sources. This requires resilient grids that are able to cope with dynamic 3.2 Partial rated DC converters unbalance in power demand and supply. Novel semiconductor materials like Silicon Carbide (SiC) Another technology that enables high efficiency with lower losses are now increasingly available, integration of ESS are partial power processing converters.
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