Exploring dc power distribution alternatives

Engineers should know when and where the use of dc power distribution is appropriate, and when it is not. In applications where redundancy, immediate transfer capability, high availability, and energy storage are design criteria, direct current power solutions are beneficial.


Learning objectives

  • Understand that switching devices are at the heart of dc power distribution systems.

  • Identify how switching devices enable high efficiency power converters, motor drives, and UPSs.

  • Learn why project planning and modeling are worthwhile efforts.

This article has been peer-reviewed.Although dc power distribution is not as widely used as ac power distribution systems, recent developments are enabling an increased use of dc in electrical distribution systems. In applications where redundancy, immediate transfer capability, high availability, and energy storage are design criteria, dc solutions are beneficial. Also, dc power distribution systems lend themselves to solar and wind power sources for land-based applications.

Financial institutions and data centers are currently using dc in energy storage, and pilot programs have been demonstrated for dc data centers, including the Green Datacenter AG in Switzerland. The next U.S. Navy destroyer will use dc power distribution. In the current market, dc systems are available—at perhaps a higher acquisition cost compared to ac systems—but the use of dc is increasing in several critical power scenarios. Data center dc applications remain small capacity when compared to ac counterparts. In scenarios where total cost of ownership (acquisition + operating expenditure) is significantly higher than acquisition cost, added spending in acquisition cost for efficiency and key performance indicators—including seamless transition—is where dc systems have an advantage (see Figure 1). The development of power semiconductor switching devices has enabled dc power to become a practical solution. Increased use of dc products in a variety of markets is driving unit costs down for converter technology. Applications in which dc power is generated or directly used include:

  • Figure 1: The use of dc power distribution is increasing for high-reliability applications such as financial institutions and data centers. All graphics courtesy: Jacobs EngineeringSolar and wind energy

  • VFDs

  • UPSs

  • Battery technology

  • Server technology

  • Mobile phones and tablets

  • Electric vehicles

  • LED lighting.

This article examines how and where to best incorporate dc systems in critical buildings and engineering projects. It does not advocate for an entirely dc distribution system or for one system over the other; it discusses how to determine where to use each. This article focuses on where dc systems are currently used and where they are beneficial. The state of recognized standards for dc distribution and where these efforts are going in the near term are also included.

Switching devices

Switching devices are at the heart of dc power distribution systems. Power conversion from ac to dc, dc to dc, and dc to ac—including variable frequency equipment, such as VFDs—relies on semiconductor switching devices.

Higher voltage, higher power, and higher temperature semiconductor switching devices continue to be developed and are used commercially at significant production levels. High-voltage semiconductor devices allow rectification at higher ac input voltages. Multiple rectifiers in parallel allow partial load optimization of losses by turning off converters when the load demand is reduced. Higher temperature semiconductor development is also of interest because thermal management of the power converter and application of margin in the junction temperature are design objectives. Converter specification is critical for both the converter manufacturer and system integration engineer.

Detailed electrical and thermal modeling and simulation are useful system development tools. Device on-state and switching losses must be included in the models to adequately predict efficiency. Validation testing is used to confirm as-built efficiency results. IEEE and similar organizations are resource for university, government, and industry papers that include a high level of technical details on the subject of efficiency.

Inherent to the use of power electronics-based dc distribution systems is the ability to reconfigure the system with power management software. Software can control the dc bus output voltage to initiate load transfer from one dc bus to another dc bus through auctioneering diodes. Similarly, power management can be used to change the dc bus voltage of VFDs for optimal motor efficiency. Power converters also lend themselves to diagnostics, metering, and fault-current limiting.

The switching devices—including thyristors, SCRs, insulated-gate bipolar transistors (IGBTs), and material developments in silicon carbide—enable higher efficiency power converters, motor drives, and UPSs. Specification of efficient devices, optimization of switching and on-state losses, and converter topologies is critical to achieving big-picture objectives, such as through life cost and power use effectiveness.

Figure 2: Data centers use dc battery UPS systems extensively to power critical servers during ac power disturbances and source transfers.Using dc power distribution in data centers

In the data center industry, dc battery UPS systems are used extensively to power critical servers during ac power disturbances and source transfers (see Figure 2). At the core of data centers are servers and telecommunications equipment that often convert ac power to dc for loads that use 12 V dc. Installations exist that use significant battery rooms and immediately invert through a UPS up to 120/208 V ac, 240/415 V ac, or 277/480 V ac; distributing power; and then converting it back to 12 V dc through multiple converters (see Figure 3). This seems counterintuitive, creating additional losses and equipment cost. It would seem more practical to distribute the battery bank physically to the data center floor, perhaps with front-access battery racks with integrated charging located periodically within the server aisles. California Code, Section 608 provides excellent guidance for this type of installation including access, spill containment, ventilation, and safety signage.

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Anonymous , 06/25/14 06:47 AM:

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