The Smart GridEnabling Energy Efficiency and

Demand Response

Clark W. Gellings

Brevard Community College

ETP1400 Distributed Electrical Power Generation and Storage

Bruce Hesher

433-5779

Chapter 5: DC Distribution and the Smart Grid

Thomas Edison's 19th century eclectic distribution system relied on direct current (DC) power generation, delivery, and use. It turned out to be impractical and uneconomical largely because technology of the 19th century, DC power generation was limited to relatively low voltages and DC current could not be transferred over mile.

Direct current is a continuous flow of current in one direction only. It is produced by generators such as fuel cells, photovoltaic cells, and batteries. DC current flows from a the more positive voltage to the lesser (actual electron flow is from negative to positive).

Alternating Current (AC) changes direction at regular intervals (60Hz for U.S. power distribution).

DC and AC Waveforms

In the below waveforms, the vertical axis is voltage and horizontal axis is time. The green waveform is sine wave (AC). The red waveform is constant value (DC).

60Hz AC waveform There are number of aspects of a sine wave that are relevant to AC power distribution:

Vrms

Vp

-Vp

Vp = 1.414Vrms

Vrms = .707Vp

t = 1/f = 1/60Hz = 16.6ms

Vpp = 2Vp = 2.828Vrms

A true sine wave contains only the base frequency (there are no harmonics)!

See Fourier Series and www.falstad.com/fourier

Note: 60Hz 120Vrms power has a peak value of 170Vp and a peak-to-peak value of 340Vpp!

AC vs. DC Power: An Historic Perspective

Edison’s DC power system had several limitations:

A practical distance of about 1 mile.

Inefficient DC to DC voltage changing.

Separate lines for devices at different voltages.

Significant power losses.

The Westinghouse/Tesla polyphase (AC) system overcame most of them. AC to AC voltage changing using William Stanley’s transformers was easy and it enabled long distance power transfer at high voltages to avoid power loss.

Edison contended that high voltage was too dangerous.

War of Currents

Read the article hyperlinked to the picture below.

Transformers Transform the Power Delivery System

By using transformers, the voltage can be stepped up to high levels so that electricity can be distributed at low current with low losses. Transformers can be used a different parts of a system to help minimize losses and are the enabling technology that made AC the preferred power distribution method. Note: if DC is placed on the primary side of a transformer there will be no output on the secondary side. Transformers only conduct AC.

William Stanley On 20March1886, William Stanley demonstrated the first complete system of high voltage Alternating Current transmission, consisting of generators, transformers and high-voltage transmission lines. His system allowed the distribution of electrical power over wide areas. He used the system to light offices and stores along the main street of Great Barrington, Massachusetts - the location of his West Avenue family home.

Stanley's transformer design became the prototype for all future transformers, and his AC distribution system formed the basis of modern electrical power distribution. He was the first person to make an electrical transformer.

Centralization Dictates AC Instead of DC

If the power generation is going to be centralized the distribution method needs to be AC. AC has lower losses, longer distance, only one current path (not one for each voltage). Some power sources can’t be distributed: hydro-electric dams and large wind farms are examples.

In 1895 engineers built a power plant at Niagara Falls to supply power to Buffalo New York some 20 miles away. It worked well and used AC with transformers. It was a very visible example of using AC and set the precedent for future centralized power plants with AC distribution.

Additionally, Tesla invented an AC motor that meant end users needed AC.

Benefit and Drivers of DC Power Delivery Systems

Increasingly more equipment runs on DC requiring rectification of AC power. Electronic devices run on DC. DC power at one voltage is now easily change to another voltage by integrated circuits. AC to DC conversion costs power. Most distributed generation systems produce DC power.

AC to DC power supply

If the power source is DC and the Device is DC no losses are incurred in conversion. This requires the voltages to be the same.

DC to ACThere are 2 commonly used methods to convert DC to AC, an H-bridge and PWM controller or an oscillator. The H-bridge and PWM are used for power applications like inverters; the oscillator is used for generating electronic signals.

An H-bridge, with 2 sets of switches (usually power transistors) are toggled on and off every 8.3ms to route the current in one direction then the other resulting in a 60Hz square wave output. A PWM controller is then used to reshape the voltage into a sine wave. When the duty cycle is highest the voltage is peak; when it is lowest the voltage is zero.

Powering Equipment and Appliances with DC

Many energy consuming devices use DC. A lot of electronic devices are powered by DC even though they process AC signals (radios waves, data, and etc.). DC can be precisely regulated (filtered) to provide the clean power needed by electronic devices. Air conditioners are now being made to run on DC using variable speed drives and powered by photovoltaic (PV) power. AC to DC adapters also consume power. Many common devices could go DC in the future.

Equipment Compatibility EPRI Solutions examined the compatibility of some common devices with DC power delivery on 2002:

• Switch mode power supplies, including computers.

• Fluorescent lighting with electronic ballasts.

• CFL bulbs.

• Electric baseboard and water heating units.

• Uninterruptable power supplies (UPS).

• Adjustable speed motor drives.

These devices represent a large percentage of the electrical load and could potentially be powered by DC.

Data Centers and Information Technology (IT) Loads p106

Data center (server farms) are one of the nearest-term applications for DC power. These facilities are strong candidates for DC power due to the availability of products that could enable implementation and the improvement in the bottom line of the e-commerce site of companies, organizations, etc. A number of energy research and consulting groups have pulled resources on a project at Sun Microsystems in Newark, California to investigate operating a data center on DC.

• Can DC powered servers and racks be made from existing components?

• What is the level of performance (uptime, reliability, etc.) compared to using AC.

• What are the efficiency gains by eliminating the multiple conversions steps in a AC powered data center.

Your Future Neighborhood Adding DC power systems to our homes, office buildings, or commercial facilities offers the potential for improvements in energy-delivery efficiency, reliability, power quality, and cost of operation.

A DC powered home

A DC inductive charging pad

Potential Future Work and Research

Technology advance indicate that there is significant opportunity for certain DC based application that can have significant power savings. There are some obstacles that need to be overcome to make DC systems viable.

• The business case for DC is not yet clear.

• Existing equipment has and AC plug and internal power supply (AC to DC).

• More field testing needs to be done on data centers before there lessons can be applied to other businesses.

Conclusion

As the smart grid evolves, it may be appropriate to rethink the wider use of DC power distribution in buildings.

Power going to the building will probably remain AC.

Large inter-utility power transfers are already going DC.

Links between the major service areas (Eastern, Western, and ERCOT) are AC-DC-AC.