Transcript The Smart Grid Enabling Energy Efficiency and Demand Response

The Smart Grid
Enabling Energy Efficiency and
Demand Response
Clark W. Gellings
Chapter 5: DC Distribution and the Smart Grid
Brevard Community College
ETP1400 Distributed Electrical Power
Generation and Storage
Bruce Hesher
433-5779
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:
Vp
Vrms
Vp = 1.414Vrms
Vrms = .707Vp
t = 1/f = 1/60Hz = 16.6ms
Vpp = 2Vp = 2.828Vrms
-Vp
Note: 60Hz 120Vrms
power has a peak
value of 170Vp and a
peak-to-peak value of
340Vpp!
A true sine wave contains only the base frequency
(there are no harmonics)!
See Fourier Series and www.falstad.com/fourier
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: hydroelectric 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.
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.
AC to DC power supply
DC to AC
There are 2 commonly used methods to convert DC to AC,
an H-bridge and PWM controller or an oscillator. The Hbridge 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.