Microgrids gain popularity due to their economical and environmental benefits along with low power losses and smaller infrastructure. However, it has several operational challenges such as power quality, power system instability, reliability, and protection issues. Microgrid protection strategy is a prime issue for the reliable operation of the microgrid. The microgrid protection scheme must meet the essential conditions for grid-connected and islanded operational modes. This paper presents a comprehensive review and comparative analysis of protection schemes and their implementation challenges for different microgrid architectures with various operational requirements. The challenges associated with the implementation of microgrid protection schemes are identified and discussed in detail. Furthermore, various simulation studies have been conducted to demonstrate the microgrid protection challenges associated with different modes of operation. This paper presents key information to researchers and protection engineers to identify microgrid protection challenges and their mitigation approaches. 

Microgrid Protection Challenges and Mitigation Approaches–A Comprehensive Review

Review of current regulations, available technologies, and future trends in the green shipping industry

Onboard DC Grid is a novel, new technology which is a further development of utilizing DC links that already exists in all propulsion and thruster drives, accomplishing for usually more than 80% of the electrical power consumption on electric propulsion vessels. This extension means that we keep all the good and well proven products used in today’s electric ships like AC generators, inverter modules, AC motors, etc. The main AC switchboard and transformers are not longer needed. The result is a more flexible power and propulsion system. Further Onboard DC Grid enables a combination of power sources and energy storage. Onboard DC Grid is suitable for vessels with total installed power of up to 20MW and operates at 1000V DC on the main bus. Typical target vessel is Offshore Support Vessels (OSVs), but any other vessel type using low voltage electric distribution would also be in the target range. For DP operation this approach gives several benefits. Firstly, the power network is no longer fixed at 60Hz. This means that an additional freedom of controlling the generator engine speed is present, giving the possibility to run engines efficiently even at 50% loading or lower. Today’s discussion of operating the power plant with open or closed bustie breakers can then be closed. Secondly, use of energy storage gives a possibility to level out the power variations on the engines even if the thruster loads are varying significantly due to vessel movements in bad weather conditions. This does not only contribute to increased fuel saving, but equally important would be the increased DP performance by the fact that the dynamic response of the thrusters would be independent from the engine dynamics. Today each thruster will experience ramp limits in power changes due to limitations in engines, however the energy storage take most of these power variations and hence reduce these limitations to a minimum. To conclude Onboard DC Grid is suited for vessels with total installed power up to about 20MW. It is flexible with respect to use of various power and fuel sources, and it gives clear benefits for vessels operating in DP, with respect to fuel consumption but also with respect to dynamic performance of the thruster system. Click below to:

http://dynamic-positioning.com/proceedings/dp2011/abstract_power_hansen.pdf

Session - Power Title: Onboard DC Grid for Enhanced DP Operation in Ships

The galvanic isolation of solid-state transformers (SSTs) is typically realized with a medium-frequency (MF) magnetic-core transformer (MCT). Previous demonstrations indicate that achieving highly power-dense and lightweight MCTs imposes several challenges on the design because of stringent requirements related to insulation and cooling. This work investigates the achievable efficiency of an optimized 166 kW/7 kV air-core transformer (ACT), which is a core part of a dc transformer (DCX), i.e., an unregulated dc–dc SST with a voltage scaling defined by the transformer turns ratio. The ACT features relatively low complexity of the construction, comparably high coupling values, and high efficiency. Modeling, optimization, and construction of the realized ACT are explained and guidelines regarding insulation, cooling, and shielding of the magnetic stray flux are discussed in detail. Furthermore, the prototype is experimentally validated to demonstrate its full functionality. In the investigated DCX, which is based on a series resonant converter (SRC) topology, the realized ACT is found to achieve a full-load efficiency of 99.5% and an unprecedented gravimetric power density of 16.5 kW/kg. With the use of 10 kV SiC MOSFETs, the complete DCX is estimated to reach an efficiency of 99% at 166 kW output power. 

Design and Experimental Analysis of 166 kW Medium-Voltage Medium-Frequency Air-Core Transformer for 1:1-DCX Applications

The galvanic isolation of solid-state transformers (SSTs) is typically realized with a medium-frequency (MF) magnetic-core transformer (MCT). Previous demonstrations indicate that achieving highly power dense and lightweight MCTs imposes several challenges on the design because of stringent requirements related to insulation and cooling. This work investigates the achievable efficiency of an optimized 166kW / 7 kV air-core transformer (ACT), which is a core part of a DC Transformer (DCX), i.e., an unregulated DC-DC SST with a voltage scaling defined by the transformer turns ratio. The ACT features relatively low complexity of the construction, comparably high coupling values, and high efficiency. Modeling, optimization, and construction of the realized ACT are explained and guidelines regarding insulation, cooling, and shielding of the magnetic stray flux are discussed in detail. Furthermore, the prototype is experimentally validated to demonstrate its full functionality. In the investigated DCX, which is based on a series resonant converter (SRC) topology, the realized ACT is found to achieve a full-load efficiency of 99.5% and an unprecedented gravimetric power density of 16.5 kW/kg. With the use of 10 kV SiC MOSFETs, the complete DCX is estimated to reach an efficiency of 99% at 166kW output power.

Recent trends in shipboard power distribution network design, such as the transition from ac to dc, result in new challenges on the issue of distribution network protection. Devices such as a solid-state bus-tie switch provide the first line of defense against the propagation of low-impedance faults across the distribution system by quickly separating the faulty portion of the network and preventing additional energy to be fed into the fault. This article proposes a new topology of a solid-state bus-tie switch for shipboard power distribution networks. The topology is thoroughly described considering its operating principles, and a small-scale prototype has been used to verify its operational performances. Current interruption capabilities are demonstrated, both in the open- and closed-loop operations, with fault detection and decision algorithms implemented on a stand-alone controller.

/pdf/solid-state-bus-tie-switch-for-shipboard-power-distribution-40ies4ruf9.pdf

Solid-State Bus-Tie Switch for Shipboard Power Distribution Networks

Power distribution networks for marine applications have experienced significant changes in recent years with the evolution of DC technologies. Some low-voltage DC solutions have already been employed for low power ships with their main benefits and there are a lot of research activities to adopt these technologies to high power ships. This paper presents current technologies on marine DC power distribution networks in terms of system architectures, power equipment and system protection. In addition, several developments for marine applications which may be needed in future are discussed.

Marine DC Power Distribution Networks

The use of dc shipboard power distribution networks has been shown to offer several advantages over their ac counterparts. The reduction of the size and number of system components, the increased simplicity of operation and fuel efficiency of the system, and convenient integration of energy storage systems offer significant economical benefits to the ship operator. Solid-state bus-tie switches are widely regarded as being an indispensable component in dc shipboard power distribution networks, for their role as first line of defense against faults. Nevertheless, these devices are not limited exclusively to protection, but can provide additional functionality thanks to their design. This article shows how a solid-state bus-tie switch can be employed for soft-starting an unenergized section of a shipboard power distribution network, eliminating the need for additional components aimed specifically at this task. A technique is suggested and its performance is experimentally verified in a laboratory scaled-down model of a dc shipboard network. The additional functionality is achieved exclusively on a control basis, without modification of the switch topology initially intended purely for system protection.

/pdf/solid-state-technology-for-shipboard-dc-power-distribution-1t45leb199.pdf

Solid-State Technology for Shipboard DC Power Distribution Networks

The desire to increase the voltage of DC shipboard power distribution networks to the medium voltage level derives from the pressure to reduce the operating costs of such systems by increasing their efficiency. Solid state bus-tie switches are accepted to be an essential component of such installations at the low voltage level, as they allow system reconfiguration and prevent fault propagation through ultrafast fault current identification and interruption. Nevertheless, the lack of standardisation in medium voltage DC shipboard power systems hinders the development of such technologies as custom, ad hoc solutions must be found according to the selected voltage level. This paper presents a solid state bus-tie switch topology that is scalable in both power and voltage rating and relies exclusively on existing, commercially available technologies. This provides a simple, readily employable solution with the flexibility needed to bridge the technological gap in the time required for medium voltage system operating voltages to become standardised. This paper presents the prototype of the bus-tie switch and validates its scalability through extensive experimental tests.

/pdf/four-quadrant-bus-tie-switch-for-protection-of-shipboard-4s9ol5gf4o.pdf

Four Quadrant Bus-Tie Switch for Protection of Shipboard Power Systems

Bus tie switch and bus tie switch apparatus

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