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IEEE Power and Energy Society General Meeting 2014
Panel on
Cyber Physical Systems Challenges for the Power Grid of the
Future
Paper Number: 14PESGM2436
Potential Developments in Distribution
Engineering
G. T. Heydt
Arizona State University
Tempe, AZ
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OVERVIEW
Distribution systems: the step child of power engineering?
High power levels and innovative technologies
Cable engineering
The role of solid state technologies
Basic impulse level
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18.4%
22.5%
28.2%
30.9%
24.6%
12.3%
16.9
46.2%
US energy use by sector 1950 – 2010
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About 0.3% reduction in
the industrial sector per
year since 1950
Deleting transportation use of energy, the
1950 to 2010 trend in use of energy in the
US is shown here
INDUSTRIAL
COMMERCIAL
16.3%
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25.6%
43.1
%
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22.4%
3
31.3%
RESIDENTIAL
The residential sector remains at just
under 1/3 of the total energy
demand, with very slow growth
ENERGY USE
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2
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ANALYSIS BY SCENARIOS
FOR THE FUTURE
GENERATION
DISTRIBUTION
LOADS
Main scenarios for DISTRIBUTION
LOADS:
1. “AS-IS” mixed electronic and ZIP
loads
2. “THE ELECTRONIC AGE”
significant increase in electronic
processing e.g. to 80% and higher
3. “INTERRUPTABLE / MANAGED”
loads are traditionally isolated from
uncertainty. In this scenario, loads may
be interrupted (e.g., many loads are
already battery supported)
4. “EFFICIENCY HARVESTING”
taking advantage of advanced
efficiencies in lighting and machines.
Each scenario has its own characteristics on susceptibility to power quality problems,
maximizing interruptability to match generation, overall system efficiency, control,
interface with the distribution system.
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The near-term (10 year) future of distribution
systems
Innovative infrastructure
• Accommodate customer DG at residential, commercial and industrial levels
perhaps to penetration to 50% by nameplate
• To accommodate EVs
• To accommodate electronic loads, possible energy storage
• Automated circuit switching (e.g., outage recovery, networking, double feed
designs)
• Continued unbundling of services, and separation of D-systems from other
electric utility infrastructures
• Better decisions on underground services and networked systems
• Increased use of networked secondaries
More control, instrumentation, automation
• Electronic controls in the distribution system (e.g., direct digital controls)
• Instrumentation in the D-system, including PMU based instrumentation
• Control of three phase feeders, switching, and var support
• Instrumentation for reinforcement of revenue recovery
DISTRIBUTION SYSTEMS
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Distributed generation
There is a continuing implementation of residential solar DG, but this is
based on net metering. If net metering is disallowed, or a D-system
connection charge is implemented, there could be a reduction in interest
in residential solar. Utility scale solar in the distribution system (e.g.,
500 kW) is a real possibility.
Innovative components
• Going to 35 kV standard three phase primaries
• Slow appearance of electronic controls (unless new resilient high
voltage electronic switches can be attained)
DISTRIBUTION SYSTEMS
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Some low probability ‘blue sky’ distribution engineering
possibilities
• High frequency secondary distribution in buildings (45 kHz, lighting)
• Polyphase distribution cables (nφ > 3)
• Innovative lighting (100% LED, or even the next generation of lighting
post-LED with relative efficacy > 15)
• 240 V secondaries in North America
• Automatic reconfiguration capabilities – a reality in some places
worldwide
DISTRIBUTION SYSTEMS
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There is a clear (but slow) migration to 35 kV distribution
primaries.
Higher distribution power levels (e.g., P >> 10 MW) may
result in innovative polyphase AC designs.
Vfn
Variable frequency distribution
systems (as in aircraft systems)
Greater reliance on cable
systems
HIGH POWER LEVELS and
INNOVATIVE TECHNOLOGIES
Ven
Van
Vdn
Vcn
Vbn
Vln
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CABLE ENGINEERING
Two phase and Edison three
wire systems
VLL
Polyphase distribution cables;
multilayered designs with low
phase-phase voltages, and high
power densities
VLN
Widespread use of 35 kV
underground cables
Vln
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SOLID STATE TECHNOLOGIES
Challenges
• Decrease losses in solid state switches (bulk resistance  I2R
loss, junction voltage  losses ~|I||E|)
• High power. Higher voltage switching at the primary distribution
level (e.g., 15 kV, 35 kV, at 1 MVA and above) – at a modest cost
• Development of GaN and other high power switches well beyond 8
kV
• Resolution of the Basic Impulse Level problem
• Fast protective systems (e.g., pilot wire protection)
• The implementation of solid state distribution transformers, at
competitive costs and competitive performance. Perhaps a hybrid
magnetic-solid state design
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BASIC IMPULSE LEVEL
•Most distribution is overhead in the US giving high exposure to
lightning
•Voltage levels are very high – in the MV range
•Direct strikes and indirect strikes (i.e., nearby strike, causing induced
voltage nearby)
•Current levels measured in kA – e.g., 10 – 70 kA and possibly higher
•Protection via static wire above; lightning arresters; metal oxide
varistors (MOVs)
•The protection either diverts strokes (static wire), or provides a
variable resistance path (R is nearly infinite up to 1.25*rated voltage,
but becomes very low when |V| exceeds 1.25*rated voltage, use
instantaneous voltage for this calculation, e.g., 34.5 kV three phase
systems gives Vbreakdown = 1.25*34.5K*sqrt(2)/sqrt(3) = ~35.2 kV)
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•A design measure to counter lightning and switching transients, the BIL is a
high voltage level used at the design stage for distribution system components
•The BIL is used to design insulation systems – under extreme lightning and
switching surge cases
•Design to the BIL is mandatory in the US
•BIL has evolved over many years of experience in T&D design and operation
•BIL is coordinated with the highest BIL levels closest to the source of surges
(i.e., near overhead lines the BIL is highest, at transformers along the line the
BIL is reduced. And the BIL is lowest in the lower voltage circuits)
•Insulation coordination is a design procedure for all insulation and dielectrics
in T&D systems to localize damage and discharge due to high voltages.
•Withstand voltage = the voltage at which breakdown will occur (zero – peak)
•The BIL is a voltage level
designed for a specific
v(t)
waveshape of high voltage surge
50 µs
– namely a 1.2 by 50
microsecond wave.
BASIC IMPULSE LEVEL
1.2 µs
Time
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• In ANSI C57 for transformers
•In a wide range of ANSI and IEEE standards for other equipment
•Typical value: 600% of crest voltage line-neutral in overhead transmission
systems
BASIC IMPULSE LEVEL
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Note 5 kV class is
5*1.414/1.732 zero-peak
line-neutral kV or 4.1 kV.
BIL for ‘normal duty’ is 60
kV which is 60/4.1 or
1470% of rating
35 kV class by the same
calculation gives BIL =
200*1.732/(35*1.414) =
700% of rating
BIL is 525% of
rating in
C57.12
BASIC IMPULSE LEVEL
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CONCLUSIONS
Challenges are expected in distribution engineering due to:
• Significant increase in electronic loads
• Inclusion of solar distributed generation
• Accommodating semiconductor based components inclusive of BIL
requirements
The following areas are identified for focused research and
development in distribution engineering:
• Inclusion of semiconductor switched controllers and protection devices
• Increased voltage and power levels in primary circuits
• Innovative cable designs including high phase order and other than 60
Hz systems
• Networked primary and secondary distribution systems
• Increased use of underground distribution