Aug 02 07:20 AM PDT

Low-Voltage Direct Current (LVDC) Microgrids – Becoming a Reality

To understand the distribution and delivery of DC (direct current) in microgrid applications, a useful analogy is to first think of how DC works at the printed circuit board (PCB) level. Direct current, although pico in nature, is distributed on the PCB in various voltages for different functions to power the components within the application. Now, imagine DC power distribution on a “higher” plane and you have the concept of DC distribution at the microgrid; either at the building or at room level, where it powers applications inside the premises.

With the inevitable full implementation of the Smart Grid, the move to a smart building approach is now here. But, when considering low-voltage DC (LVDC) distribution for a microgrid many questions arise, such as: How do we take a system approach to LVDC power delivery? Who will develop and manage the enabling standards road map? Who will develop the specific codes and standards?

Residential and Commercial Use of DC Power

Taking a look at the grid and considering how to make it “smart” requires breaking things down into pieces to better understand the whole concept. The block diagram below shows the layers involved: from delivering LVDC power and controlling it to how different elements of the application relate to each other. The diagram of the different layers within the entire power grid starts with the national grid and moves down to the room-level microgrid.

Now you may ask, just what voltages are in play when we’re talking about microgrids? As of this writing, for room microgrids 24Vdc and 48Vdc (48Vdc for LCD TV and similar application power requirements) get the play and for building microgrid services and data centers, 380~400Vdc gets the play. For outdoor applications, 24Vdc up to 380Vdc are in use.

The rewards of using DC versus AC (alternating current) for microgrids may be dramatic, enabling consumers to better manage and control their energy costs, by integrating the next generation of plug-in electric vehicles, for example, by improving efficiency, and by providing a better way to harness renewable energy.

One group, the EMerge Alliance, is working on DC voltage standards for 24Vdc and 380Vdc. (Information about the alliance can be found at www.emergealliance.org, including the group’s involvement in making use of LVDC easier to implement through standards.)

Above is an example of an industrial application provided by the EMerge Alliance. Consumer applications are also possible, such as through wall drops to implement LVDC into the building.

380Vdc Data Center Activity

There is currently wide industry involvement and cooperation to develop multiple 380Vdc demos for data center microgrids for:
• University campuses
• Electric utilities
• Telecom industry

Organizations that are harmonizing multiple 380Vdc specification efforts include:
• DC power partners joining the EMerge Alliance
• European Telecommunications Standards Institute
• International Electrotechnical Commission (SG4)

Many manufacturers are working together to develop approaches for IT equipment as well as facility equipment

One of the reasons for using high-voltage DC (HVDC) and LVDC is the ease in which renewable energies can be integrated into a DC-operated microgrid. Here is a summary of the benefits of using 380Vdc versus AC:

• Higher reliability
-Fewer conversions translates to fewer points of failure
• Higher Efficiency
-Results in higher efficiency power supplies and UPSs
-No PDU transformer needed
• Smaller size
• Better power quality
• Easier integration of renewable energy
• Easier integration of energy storage

Other DC Applications

The EMerge Alliance members are also working on two other voltages for DC-powered applications, including:

• 380Vdc Uses
-Telecom central offices (operating today at 48Vdc)
-Variable speed drives (washers, dryers, air conditioning units)
-Other home appliances (ranges and ovens)
-“Rapid chargers” for plug-in electric vehicles
• 24Vdc Uses
-Lighting
-Consumer electronics (e.g. TVs, PCs, projectors)

DC Challenges

Although LVDC distribution for the microgrid clearly offers many benefits, it also presents significant challenges, including:

• Standardizing the DC voltages
-24Vdc & 380Vdc appear to be the leading candidates
• Safety agency approval/listing (e.g., UL)
-DC and AC products both need such approvals
• Paradigm Shift
-Moving back to some of Thomas Edison’s original DC concepts
-We are accustomed to the “AC” world today
• Vendor Selection
-Fewer DC vendors available (compared to the many AC vendors to choose from) — at least for the present

DC-Powered Home – Fantasy or Future Reality?

Let’s look at some examples of applications powered by DC. Below I list examples in my own home, and I’m sure you can imagine a variety of potential applications in your home not to mention in your office environment.

My home already has many DC-powered products with “external” power supplies, including:

• My laptop computer (20 V – 4.5 A)
• My cell phone charger (5 V – 550 mA)
• My wife’s cell phone charger (5 V – 350 mA)
• Wireless internet router power supply (5 V – 2.5 A)
• Cable modem (12 V – 750 mA)
• My electric razor (12 V – 400 mA)
• Multiple-powered USB port hubs (5 V – 3.8 A)
• Battery-operated vacuum cleaner (10 V – 250 mA)

Additionally, my home already has many DC-powered products with “internal” power supplies, such as:

• Blue-ray DVD player (22W)
• Four satellite set-top boxes, or STBs (6.5 V – 500 mA)
• Two cable STBs (4.2 A)
• Bose home theater system (300 W)
• Four LCD TVs; 65” (525 W), 52” (375 W), 40” (255 W), 36” (175W)

My home also has many very large AC-powered loads, including:
• Microwave oven (240 V – 8.3 kW)
• Dishwasher (120 V – 11 A)
• Toaster (120 V – 1050 W)
• Coffee pot (120 V – 1100 W)
• Clothes washer (120 V – 7 A)
• Electric clothes drier (240 V – 5600 W)
• Refrigerator (120 V – 8.3 A)
• Electric stove top (240 V – 8.8 kW)
• Electric oven (240 V – 3600 W)

In summary, the implementation of LVDC microgrids is beginning to take shape with the installation of a variety of applications, from lighting to security cameras for drop ceilings going directly to the room or cube level. Plus, the growing availability of alternative energies that produce DC voltages and the adaptability of all types of energy storage devices give the LVDC microgrid implementation a strong basis to grow on.

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