DC Power Distribution System

Surveys on power distribution lines illustrated that nearly 8% of the electricity generated is dissipated in transmission lines, while approximately 20% of the electric power is generated to satisfy peak demands only during a short-time period, which only counts for 5% of total operation time [85].

Moreover, almost 90% of power disruptions happen in power distribution lines and network [86].

To solve the high loss and interruptions, in today’s DC grid, DC distribution system and transmission lines are the most efficient approach to transmit substantial power especially for multiple inputs from renewable resources. Advanced DC devices and technologies, the increased use of renewable energy sources have also promoted demand and develop of DC power distribution system. DC power distribution system now becoming prospective future solution, for its higher efficiency and reliability.

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Fig.2.6 Comparison of AC and DC power distribution systems in an industrial application.

DC transmission lines can be suitable for installed both overhead lines and underground lines, even undersea lines. The uses of DC transmission lines have proved their higher efficiency compared to AC transmission lines [87], it is mostly due to the asynchronous property of DC distribution system.

Asynchronous lines signify no frequency specifics in the entire system, so that electric power can be transmitted between two lines or grids without any synchronization.

Two of the major concerns about the DC power distribution system are the electric shock risk and equipment protecting. The main reason from this concern is due to challenging to break a DC transmission line compared to an AC line. Therefore, switchgears such as fuses, circuit breakers, and grounding methods are required to applied in the DC power distribution system, so that the system can be protected from damage and ensure the safety. Comprehensive investigations have been conducted and published on enhancing the safety of DC power distribution systems [87], [88];

specific protections against lightning shock which may affect DC generation devices including PV generators are proposed [89]. Further, higher DC voltage levels could result in currents leakage in the

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transmission lines and lead to corrosion of the transformer saturation, underground equipment, and failures of protective equipment [90]-[92].

A comparison of AC power distribution system and advanced DC power distribution systems is conducted in Fig. 2.6 [93]. Both two types of power distribution systems are applied in a data center application. The internal components of ICT equipment including central processing unit (CPU), memory, etc., they usually operate under a relatively low DC voltage supply voltages, while the external backup storage such as battery generally operate under a medium voltage level. From the Fig.2.6, it is obvious that, with using an AC power supply, the power conversion from AC to DC is frequently experienced four times so that can be connected to equipment, would result in multiple conversion losses. In contrast, with a DC power supply, after the first conversion from AC to DC, the DC source can be connected to the storage batteries directly, and only limited power conversions are required, typically twice DC-DC conversion for voltage adjustment. As a result, DC power distribution achieve higher efficiency, and due to less power conversion stages and components, the failures and damage of DC power distribution system are reduced principally, compare to AC distribution system.

Not only the above example, but also many researches achievement and explorations investigated advantages of DC distribution system over AC distribution system. DC distribution system provides better efficiency and higher reliability with improved power quality. It reduces installation costs due to fewer power conversion stages, so as copper loss and assemble space are reduced. It enables uncomplicated integration of DC sources generated from renewable energy, and process of energy storage systems is simple. Unlike the complicated AC distribution system with synchronization, DC distribution system requires no synchronized plug and play subsystem so that the expansion of DC system from long-distance renewable generator to local small-localized consumers are facilitated.

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DC power distribution system also shows potential in telecommunication, big-computing and data centers areas, which is promising for developing smart and communication network in the future.

That is due to the high reliability of DC distribution system enables to supply critical, elaborate and sensitive loads. Moreover, the facilities in data centers require the use of batteries to maintain high reliability level, with the benefits from DC distribution system, no superfluous rectifiers and inverters are needed for charging and discharging of the batteries since the voltage bus is DC.

Large-scale computing companies have released universal standards for DC power distribution system, including 380V and 48V DC distribution architectures, which can satisfy both high and low voltage level in a data center [95] [96].

To be noticed, the 380 V DC distribution supply line is recommended to the connections in the DC grid, since high voltage is needed in transmission lines to reduce transmission losses, as power loss is proportional to the resistance and square of current and of the cables and conductors. [97] proposed a conceptual 380V DC power distribution system for home appliances. In that paper, the conversion efficiency of DC distributed system using renewable energy and energy storage system in the DC grid is nearly 8%, higher than the conversion efficiency in the conventional AC distribution system.