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.

By Jonathan Sauer, Jacobs Engineering, Philadelphia June 16, 2014

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.

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:

  • Solar 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.

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.

The server equipment interface is the point in the data center where a dc interface standard is needed. Typical electrical performance-based interface standards should include details of the steady state, state transitions, and transient and recovery times. Power quality is important and often misunderstood. Specification of power quality interface may be project specific. Industry standards are the preferred approach for the engineer. IEEE Standard 519 is often used at the point of common coupling with the ac utility to understand the ac harmonic current and voltage requirements. Progress has been made in the development of 125, 250, and 380 to 400 V dc system standards.

Standards for 125 and 250 V dc, which are widely used in powering relaying equipment, include:

  • IEEE 399: dc load flow and short circuit analysis recommended practices

  • IEEE 485: battery sizing recommended practices

  • IEEE 946: dc system analysis methods.

The 380 to 400 V dc standards are progressing with UL listed equipment. European Telecommunications Standards Institute and International Telecommunications Union standards have been released including 240 V dc power supply systems for telecommunications. The International Electrotechnical Commission and IEEE have working groups authoring a new dc UPS standard. The 2011 edition of the National Electrical Code (NEC) includes dc arc fault protection for photovoltaic systems. Analysis programs now include arc flash for dc systems. The NEC includes guidelines on system wiring, protection, and safety for both ac and dc.

Data centers also use a significant number of variable speed driven motors for cooling pumps and fans where a redundant dc bus may be beneficial to power multiple drives. PUE calculations are optimized by reducing cooling system losses. Developing engineering concepts of operation that include assessment of partial load efficiencies and the anticipated load profiles are of interest to modeling the through-life cost of power distribution topologies. Early project phase planning and modeling can be a worthwhile time investment. When the data centers’ operating load changes, reduced pump flow and fan speeds can be provided by a power management system. The use of dc systems and VFDs is well suited for optimization of efficiency across the load range. Power management is also applied to dc bus voltage level selection for motor drives allowing for optimization of the motor efficiency. Device level efficiency and reliability can be improved by operating switches at low junction temperatures (see “Recommended practices”).

Naval power distribution

In the naval marine industry, dc power distribution systems are used for reliable power distribution to critical propulsion, steering, navigation, and weapons systems. These systems take advantage of the inherent instantaneous transition capability offered by dc power distribution. Instantaneous transitions are seamless transitions that do not impact the operation of the load. Power management systems are used to control multiple dc bus voltages. The complexity of the transfer equipment is reduced, and the reliability of diodes and IGBTs is high.

Transformerless propulsion rectifiers are key elements for removing losses, size, weight, and failure modes associated with transformers. Transformers are large and heavy. They become inefficient with multiple secondary windings and the presence of harmonic distortion. Transformer failure modes, such as insulation failure, result in significant loss of capability. In contrast, a medium voltage (MV) connected converter is small, modular, and hot swappable.

The naval marine industry propels developments in power conversion equipment with high standards of redundancy, reliability, maintainability, availability, and construction with temperature and humidity constraints.

Military standards, the American Bureau of Shipbuilding, and IEEE include standards that are evolving to adapt to the latest power conversion developments for propulsion and distribution, including dc power. The areas of specification updates are harmonic distortion, stability, fault level, conducted and radiated emissions, and arcing fault protection.

Green energy

In the green grid market, the past decade has included increased use of dc power distribution. Construction of wind turbines with dc buses, solar power dc generation, thermal management systems with many fans and pumps that may be fed from dc buses, increases in the use of LED lighting, and electric vehicle charging stations bring elements of dc distribution systems into the forefront.

Green Grid is an organization that focuses on resource efficiency in information technology and data centers. Green Grid provides guidance for determining the PUE of a data center. Simply put, the PUE is the total load of a data center divided by the load of the computer equipment. PUE would approach unity in an ideal situation. PUE of the most efficient data centers is less than 1.20. Improvement from these levels requires engineers and designers to look at new approaches to power system architectures, such as transformerless dc conversion. Specifying engineers and designers should consider the most efficient power converter topologies available down to the load.

Using dc power distribution in industry

The process and electric vehicle industries are key to developing variable speed drive and battery technologies. The process industry often requires many VFDs. The process industry also requires reliability to aid continuous production. The steel manufacturing industry often incorporates rectifiers, a dc bus, and numerous inverters. This rectifier topology is similar to that of the dc distribution systems found in the naval market. Often, the utility interface is an MV ac system. High-power rectifiers are used to create a dc bus with multiple inverters to drive the various motors involved. Harmonic filters may be necessary to meet interface standards, such as IEEE 519, at the point of common coupling. If harmonics cannot be managed with a filter, multiple secondary, phase shifting transformers may be required. The phase shifting transformers shift the harmonics to higher orders and subsequently lower magnitudes. However, transformers are not preferred from an acquisition cost and efficiency perspective.

The automotive industry has produced a reliable battery technology, which has driven down the cost of lead acid batteries. The advent of hybrid and full electric vehicles has driven the industry to pursue other higher power density battery technology. Battery technology, including nickel cadmium, nickel metal hydride, lithium ion, lithium phosphate, and lithium polymer, shows promising energy density. The Nissan Leaf uses lithium ion battery modules. Porsche’s latest 918 Spyder uses the lithium ion technology, and developments include flywheel energy storage. The advantages to energy storage in the rotating form is very short recharge following energy depletion and an inherent ability to cycle without capacity degradation, lack of hazardous chemicals, and lack of hazardous gas production. The safety aspects of the rotating energy are important and advantageous when compared to battery banks.

The dc power system architecture may vary in the applications discussed here. However, there are some common attributes. The takeaway is that components of dc power distribution are being developed simultaneously in several growing industries. The key performance indicators are reliability, redundancy, and efficiency.

Both the U.S. Navy and the data center industry have been hesitant to move away from the commercially available ac standard. The dc solutions are currently less understood by engineers and designers who are accustomed to typical ac systems. The implementation of dc systems will start where the efficiency and reliability benefits are worth the additional acquisition cost of dc systems.

Jonathan Sauer is an electrical engineer at Jacobs Engineering. He works in the global building design group where he performs electrical systems engineering for buildings and mission critical data centers. Areas of expertise include facility electrical power, backup generation, UPS, automation, networks, commutations, security, fire, lighting, and HVAC.