Transcript Class notes (physical), part 2

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Cyber Physical Power Systems
Fall 2015
Microgrids and Smart Grids
(part 2)
© A. Kwasinski, 2015
2
Microgrids
Energy Storage
© A. Kwasinski, 2015
Energy Storage
• Uses of energy storage devices in microgrids:
• Power buffer for slow, bad load followers, power sources.
• Energy supply for renewable energy sources.
• Stability support
• Increased availability
• Power vs. Energy
dE
P
dt
• Power delivery profile: short, shallow and often energy exchanges.
• Flywheels
• Ultracapacitors
• Energy delivery profile: long, deep and infrequent energy exchanges.
• Batteries
• For the same energy variation, power is higher in short exchanges.
© A. Kwasinski, 2015
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Power vs. energy delivery profile technologies
• Ragone chart:
• More information and charts can be found in Holm et. al., “A Comparison of
Energy Storage Technologies as Energy Buffer in Renewable Energy Sources
with respect to Power Capability.”
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Power vs. energy delivery profile technologies
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Lead-acid batteries
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• Lead-acid batteries are the most convenient choice based on cost. The
technology that most of the users love to hate.
• Lead-acid batteries are worse than other technologies based on all the other
characteristics. Disposal is another important issue.
• In particular, lead-acid batteries are not suitable for load-following power buffer
applications because their life is significantly shortened when they are
discharged very rapidly or with frequent deep cycles.
© A. Kwasinski, 2015
Lead-acid batteries life
• Lead-acid batteries are very sensitive to temperature effects. It can be
expected that battery temperature exceeding 77°F (25°C) will decrease
expected life by approximately 50% for each 18°F (10°C) increase in average
temperature. [Tyco Electronics IR125 Product Manual]
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Lead-acid batteries
• Positive electrode: Lead dioxide (PbO2)
• Negative electrode: Lead (Pb)
• Electrolyte: Solution of sulfuric acid (H2SO4) and water (H2O)
H 2O
PbO2
Pb
H 2O
H 2O
H 2O
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H 2O
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Lead-acid batteries
• Chemical reaction (discharge)
2H2O H SO
2
4
2eO22PbO2
Pb2+
2H+
2H+
SO4
2-
SO42H2SO4 PbSO4
Pb2+
Pb
2e-
PbSO4
H2O
H2O
H2O
H2O
H2O
© A. Kwasinski, 2015
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Lead-acid batteries
• Chemical reaction (discharge)
• Negative electrode
• Electrolyte
• Positive electrode
Pb
Pb2+ + 2e-
Pb2+ + SO42-
PbSO4
2H2SO4
PbO2 + 4H+ + 2ePb2+ + SO42-
•Overall Pb + PbO2 + H2SO42-
4H+ + 2SO42-
Pb2+ + 2H2O
PbSO4v
2PbSO4 + 2H2O
• The nominal voltage produced by this reaction is about 2 V/cell. Cells are
usually connected in series to achieve higher voltages, usually 6V, 12 V, 24 V
and 48V.
© A. Kwasinski, 2015
Lead-acid batteries
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• As the battery discharges, sulfuric acid concentration decreases.
• At the same time, lead sulfate is deposited on the electrode plates.
• Charging follows the inverse process, but a small portion of the lead sulfate
remains on the electrode plates.
• Every cycle, some more lead sulfate deposits build up on the electrode plates,
reducing the reaction area and, hence, negatively affecting the battery
performance.
• Electrode plates sulfatation is one of the primary effects that affects battery
life.
• To avoid accelerating the sulfatation process, batteries need to be fully
charged after every discharge and they must be kept charged at a float voltage
higher than the nominal voltage. For lead acid batteries and depending their
technology the float voltage is between 2.08 V/Cell and 2.27 V/cell. For the
same reasons, lead-acid batteries should not be discharged below 1.75 V/cell
© A. Kwasinski, 2015
Lead-acid batteries capacity
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• Battery capacity is often measured in Ah (Amperes-hour) at a given discharge
rate (often 8 or 10 hours).
• Due to varying internal resistance the capacity is less if the battery is
discharged faster (Peukert effect)
• Lead-acid batteries capacity ranges from a few Ah to a few thousand Ah.
http://polarpowerinc.com/info/operation20/operation25.htm
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Lead-acid batteries capacity
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• Battery capacity changes with temperature.
http://polarpowerinc.com/info/operation20/operation25.htm
• Some manufacturers of battery chargers implement algorithms that increase
the float voltage at lower temperatures and increase the float voltage at higher
temperatures.
© A. Kwasinski, 2015
Lead-acid batteries charge
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• Methods:
• Constant voltage
• Constant current
• Constant current / constant voltage
• Cell equalization problem: as the number of cells in series increases, the
voltage among the cells is more uneven. Some cells will be overcharged and
some cells will be undercharged. This issue leads to premature cell failure
• As the state of charge increases, the internal resistance tends to decrease.
Hence, the current increases leading to further increase of the state of charge
accompanied by an increase in temperature. Both effects contribute to further
decreasing the internal resistances, which further increases the current and the
temperature….. This positive feedback process is called thermal runaway.
© A. Kwasinski, 2015
Lead-acid batteries efficiency
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• Consider that during the charge you apply a constant current IC, a voltage VC
during a time ΔTC. In this way the battery goes from a known state of charge to
be fully charged. Then the energy transferred to the battery during this process
is:
Ein = ICVC ΔTC
• Now the battery is discharged with a constant current ID, a voltage VD during a
time ΔTD. The final state of charge coincides with the original state of charge.
Then the energy delivered by the battery during this process is:
Eout = IDVD ΔTD
• So the energy efficiency is  E 
VD I D TD
 VC
VC I C TC
• Hence, the energy efficiency equals the product of the voltage efficiency and
the Coulomb efficiency. Since lead acid batteries are usually charged at the
float voltage of about 2.25 V/cell and the discharge voltage is about 2 V/cell, the
voltage efficiency is about 0.88. In average the coulomb efficiency is about
0.92. Hence, the energy efficiency is around 0.80
© A. Kwasinski, 2015
Lead-acid batteries calculations
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• Most calculations are based on some specific rate of discharge and then a linear
discharge is assumed.
•The linear assumption is usually not true. The nonlinearity is more evident for faster
discharge rates. For example, in the battery below it takes about 2 hours to discharge
the battery at 44 A but it takes 4 hours to discharge the battery at 26 A. Of course, 26x2
is not 44.
• A better solution is to consider the manufacturer discharge curves and only use a linear
approximation to interpolate the appropriate discharge curve.
• In the example below, the battery can deliver 10 A continuously for about 12 hours.
Since during the discharge the voltage is around 12 V, the power is 120 W and the
energy is about 14.5 kWh
10 A continuous
discharge curve
approximation
Discharge
limit
Nominal curve
© A. Kwasinski, 2015
Li-ion batteries
• Positive electrode: Lithiated form of a transition metal oxide (lithium cobalt
oxide-LiCoO2 or lithium manganese oxide LiMn2O4)
• Negative electrode: Carbon (C),
usually graphite (C6)
• Electrolyte: solid lithium-salt electrolytes
(LiPF6, LiBF4, or LiClO4)
and organic solvents (ether)
http://www.fer.hr/_download/repository/Li-ION.pdf
© A. Kwasinski, 2015
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Li-ion batteries
• Advantages with respect to lead-acid batteries:
• Less sensitive to high temperatures (specially with solid electrolytes)
• Lighter (compare Li and C with Pb)
• They do not have deposits every charge/discharge cycle (that’s why the
efficiency is 99%)
• Less cells in series are need to achieve some given voltage.
• Disadvantages:
• Cost
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Ni-MH batteries
• Negative electrode: Metal Hydride such as
AB2 (A=titanium and/or vanadium, B=
zirconium or nickel, modified with chromium,
cobalt, iron, and/or manganese) or AB5
(A=rare earth mixture of lanthanum, cerium,
neodymium, praseodymium, B=nickel,
cobalt, manganese, and/or aluminum)
• Positive electrode: nickel oxyhydroxide
(NiO(OH))
• Electrolyte: Potassium hydroxide (KOH)
Cobasys batteries
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Ni-MH batteries
• Advantages:
• Less sensitive to high temperatures than Li-ion and Lead-acid
• Handle abuse (overcharge or over-discharge better than Li-ion bat
• Disadvantages:
• More cells in series are need to achieve some given voltage.
• Cost
© A. Kwasinski, 2015
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Ni-Cd batteries
• Negative electrode: Cadmium (Cd) – instead of the MH in Ni-MH batteries
• Positive electrode: nickel oxyhydroxide (NiO(OH)) – the same than in Ni-MH
batteries
• Electrolyte: Potassium hydroxide
(KOH) solution
Saft batteries
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Ni-Cd batteries
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• Advantages:
• Less sensitive to high temperatures than all the other batteries
• Handle some abuse (overcharge or over-discharge better than Li-ion
bat)
• Disadvantages:
• More cells in series are need to achieve some given voltage.
• Cost
© A. Kwasinski, 2015
Battery technologies
Cobasys: “Inside the Nickel Metal Hydride Battery”
© A. Kwasinski, 2015
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Electric vs. Magnetic energy storage
• Consider that we compare technologies based on energy density (J/m3)
[ Energy]  [Work ]  [ F ][d ]  Nm  J
[ Energy density ] 
J
Nm N

 2  Pa
3
3
m
m
m
• Plot of energy density vs. length scale (distance between plates or air gap):
University of Illinois at Urbana-Champaign
ECE 468 (Spring 2004)
• Hence, magnetic energy storage (e.g. SMES) is effective for large scale
systems (higher power)
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Ultracapacitors
• Capacitors store energy in its electric field.
• In ideal capacitors, the magnitude that relates the charge generating the
electric field and the voltage difference between two opposing metallic plates
with an area A and at a distance d, is the capacitance:
C
• In ideal capacitors:
C 
Q
V
A
d
• Equivalent model of real standard capacitors:
ESR  Rw 
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 2 Rl C 2
Ultracapacitors
• Ultracapacitors technology: construction
• Double-layer technology
http://www.ultracapacitors.org/img2/ultraca
pacitor-image.jpg
•Electrodes: Activated carbon (carbon cloth, carbon black, aerogel carbon,
particulate from SiC, particulate from TiC)
• Electrolyte: KOH, organic solutions, sulfuric acid.
© A. Kwasinski, 2015
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Ultracapacitors
• Ultracapacitors technology: construction
Traditional standard
capacitor
The charge of ultracapacitors, IEEE
Spectrum Nov. 2007
Double layer
capacitor
(ultracapacitor)
Ultracapacitor with carbon
nano-tubes electrodes
A
d
• Key principle: area is increased and distance is
decreased
C 
• There are some similarities with batteries but there are
no reactions here.
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Ultracapacitors
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• Some typical Maxwell’s ultracapacitor packages:
www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultracapacitor_Equivalent_Circuit_Model.pdf
• At 2.7 V, a BCAP2000 capacitor can store more than 7000 J in the volume of
a soda can.
• In comparison a 1.5 mF, 500 V electrolytic capacitor can store less than 200 J
in the same volume.
© A. Kwasinski, 2015
Ultracapacitors
• Comparison with other capacitor technologies
www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultracapacitor_Equivalent_Circuit_Model.pdf
© A. Kwasinski, 2015
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Ultracapacitors
• Charge and discharge:
• With constant current, voltage approximate a linear variation due to a very
large time constant:
• Temperature affects the output (discharge on a constant power load):
www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultr
acapacitor_Equivalent_Circuit_Model.pdf
© A. Kwasinski, 2015
Ultracapacitors
• Aging process:
• Life not limited by cycles but by aging
• Aging influenced by temperature and cell voltage
• Overtime the materials degrade, specially the electrolyte
• Impurities reduce a cell’s life.
Linzen, et al., “Analysis and Evaluation of Charge-Balancing
Circuits on Performance, Reliability, and
Lifetime of Supercapacitor Systems”
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Ultracapacitors
• Power electronic interface:
• It is not required but it is recommended
• It has 2 purposes:
• Keep the output voltage constant as the capacitor discharges (a
simple boost converter can be used)
• Equalize cell voltages (circuit examples are shown next)
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Flywheels
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• Energy is stored mechanically (in a rotating disc)
Motor
Generator
Flywheels Energy
Systems
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Flywheels
• Kinetic energy:
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1 2
Ek  I 
2
where I is the moment of inertia and ω is the angular velocity of a rotating disc.
I   r 2 dm
• For a cylinder the moment of inertia is
1
I  r 4 a 
2
• So the energy is increased if ω increases or if I increases.
• I can be increased by locating as much mass on the outside of the disc as
possible.
• But as the speed increases and more mass is located outside of the disc,
mechanical limitations are more important.
© A. Kwasinski, 2015
Flywheels
• Still, high speed is not the only mechanical constraint
• If instead of holding output voltage constant, output power is held constant,
then the torque needs to increase (because P = Tω) as the speed decreases.
Hence, there is also a minimum speed at which no more power can be
extracted
vmax
V

• If
r
vmin
and if an useful energy (Eu) proportional to the difference between the disk
energy at its maximum and minimum allowed speed is compared with the
maximum allowed energy (Emax) then
Eu/Emax
Eu
Vr2  1

Emax
Vr2
Vr
Vr
Bernard et al., Flywheel Energy
Storage Systems In Hybrid And
Distributed Electricity Generation
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Flywheels
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• In order to reduce the friction (hence, losses) the disc is usually in a vacuum
chamber and uses magnetic bearings.
Bernard et al., Flywheel Energy
Storage Systems In Hybrid And
Distributed Electricity Generation
• Motor / generators are typically permanent magnet machines. There are 2
types: axial flux and radial flux. AFPM can usually provide higher power and are
easier to cool.
Bernard et al., Flywheel Energy Storage Systems In Hybrid And
Distributed Electricity Generation
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Power Electronic Interfaces
© A. Kwasinski, 2015
Power electronic interfaces
• Power electronic converters provide the necessary adaptation functions to
integrate all different microgrid components into a common system.
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Power electronic interfaces
• Integration needs:
• Component with different characteristics:
• dc or ac architecture.
• Sources, loads, and energy storage devices output.
• Control issues:
• Stabilization
• Operational issues:
• Optimization based on some goal
• Efficiency (e.g. MPPT)
• Flexibility
• Reliability
• Safety
• Other issues:
•Interaction with other systems (e.g. the main grid)
© A. Kwasinski, 2015
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Power electronics basics
• Types of interfaces:
• dc-dc: dc-dc converter
• ac-dc: rectifier
• dc-ac: inverter
• ac-ac: cycloconverter (used less often)
• Power electronic converters components:
• Semiconductor switches:
• Diodes
• MOSFETs
• IGBTs
• SCRs
• Energy storage elements
• Inductors
• Capacitors
• Other components:
• Transformer
• Control circuit
© A. Kwasinski, 2015
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Power electronics basics
• Types of interfaces:
• dc-dc: dc-dc converter
• ac-dc: rectifier
• dc-ac: inverter
• ac-ac: cycloconverter (used less often)
• Power electronic converters components:
• Semiconductor switches:
• Diodes
• MOSFETs
• IGBTs
Diode
• SCRs
• Energy storage elements
• Inductors
• Capacitors
• Other components:
• Transformer
IGBT
• Control circuit
© A. Kwasinski, 2015
MOSFET
SCR
Power electronics basics
• dc-dc converters
• Buck converter
Vo  DE
• Boost converter
E
Vo 
1 D
• Buck-boost converter
DE
Vo  
1 D
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Power electronics basics
• Rectifiers
v
v
v
t
t
Rectifier
Filter
© A. Kwasinski, 2015
t
Power electronics basics
• Inverters
• dc to ac conversion
• Several control techniques. The simplest technique is square wave
modulation (seen below).
•The most widespread control technique is Pulse-Width-Modulation (PWM).
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Smart grid basics
© A. Kwasinski, 2015
Smart grids
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• There are two similar but not equal approaches to the smart grid
concept.
• EU-led vision (customer and environmentally driven):
• Europe’s electricity networks in 2020 and beyond will be:
• Flexible: Fulfilling customers’ needs whilst responding to the
changes and challenges ahead;
• Accessible: Granting connection access to all network users,
particularly for renewable energy sources and high efficiency
local generation with zero or low carbon emissions;
• Reliable: Assuring and improving security and quality of supply,
consistent with the demands of the digital age;
• Economic: Providing best value through innovation, efficient
energy management and ‘level playing field’ competition and
regulation.
“European Technology Platform SmartGrids. Vision and Strategy for Europe’s Electricity Networks of the Future”
European Commission KI-NA-22040-EN-C EUR 22040
© A. Kwasinski, 2015
Smart grids
• US led vision (security
and consumer driven)
- Motivated by needs in
availability improvements
“The NETL Modern Grid Initiative A VISION FOR THE MODERN GRID”, US DOE
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The smart grid concept
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• There are many views of what is In reality, a smart grid is not a single
concept but rather a combination of technologies and methods intended
to modernize the existing grid in order to improve flexibility, availability,
energy efficiency, and costs.
• Smart Grid 1.0:
• Smart meters
• Smart Grid 2.0 (“Energy Internet”
enabler):
• advanced autonomous controls,
• distributed energy storage,
• distributed generation, and
• flexible power architectures.
• Future smart grids: Integration with other infrastructures, IoT
© A. Kwasinski, 2015
Smart grid evolution: Past
• Centralized
operation and
control
• Passive
transmission and
distribution.
• Lack of flexibility
• Vulnerable
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Smart grid evolution:
Present/immediate future
• Still primarily centralized
control.
• Limited active distribution
network (distributed local
generation and storage). Use
of virtual storage (demandresponse)
• Addition of communication
systems
• More efficient loads
• Flexibility issues
• Somewhat more robust
© A. Kwasinski, 2015
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Smart grid evolution: Future
• Distributed operation and
control
• Active distribution network
(distributed local generation
and storage).
• Integrated communications
• Advanced more efficient and
“smarter” loads
• Flexible
• More robust
• Integration with other
infrastructures (e.g. roads)
© A. Kwasinski, 2015
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Smart grids
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• Technologies and concepts:
• Distributed energy resources (generation and storage) are
fundamental parts. They provide the necessary active characteristics
to an otherwise passive grid.
• Advanced and distributed communications. All the grid components
(including loads) are able to communicate. The grid operates like a
power-Internet (distributed, multiple-redundant, interactive and
autonomous). I.e. a Power-Net.
• Intelligent metering.
• Policies and regulatory actions. Necessary to achieve integration of
all the parts. Inadequate pricing models is a significant barrier to
introduce service-based business models (vs. energy-based).
• Grid modernization.
© A. Kwasinski, 2015
The Power-Net
• DOE view for a smart grid:
- “An electrical grid is a network of
technologies that delivers
electricity from power plants to
consumers in their homes and
offices.”
• A Power-Net expands this
view based on paradigms
from the Internet, thus, being
a cyber-physical system.
• Some features compared with
conventional power grids:
more reliable, efficient, and
flexible.
© A. Kwasinski, 2015
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The Power-Net
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• Like the Internet, the Power-Net
involves diverse and redundant
path for the power to flow from
distributed generators to users.
Its control resides in autonomous
distributed agents.
• Power is generated in distributed
generators, usually from
alternative or renewable energy
sources. Power buffers are
included to match generators
and loads dynamics. Energy
buffers are added to make
variable sources dispatchable.
• Contrary to the Internet, the
Power-Net involves a local
approach for power interactions.
© A. Kwasinski, 2015
The Internet
• Desired Internet features:
• distributed and autonomous control,
• diverse information routing and redundant data or application
storage,
• performance degradation instead of full failure,
• link transmission rate control through temporary data storage
in buffers.
M  B.T
Buffer
size
Link
bandwidth
Maximum
(delay) time
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Extending the Internet into Smart grids
• Key aspect: add distributed generation (fuel cells, microturbines, PV
modules, small wind, reciprocating engines) close to the load to make
power grids distribution portion an active electric circuit.
• Autonomous and distributed controls can be implemented with DG.
• Power vs. Energy buffers:
Predicted solar radiation
Batteries
on PV module
(Energy buffer)
W  P.T
Ultracapacitors
or flywheels
(power buffer)
© A. Kwasinski, 2015
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Control and communication issues
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• Coordination is needed in order to integrate variable generation sources (such
as PV modules) in the grid.
• Centralized control requires significant communication resources (i.e., large
bandwidth spectrum allocation) which in general is not available.
• The alternative is to provide all nodes with an autonomous control that allows
controlling power interactions with the grid without dedicated communication
links. These more intelligent nodes become agents.
VS.
© A. Kwasinski, 2015
Power Supply Resilience
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• In the past, several issues were identified in conventional power grids
that affect their availability, particularly during natural disasters.
• Conventional power grids were shown to be very fragile systems.
• Some of the issues found in conventional power grids include:
• Primarily centralized control and power distribution architecture.
• Passive power distribution grid
• Lack of redundancy in most sub-transmission and distribution
paths.
• Difficulties in integrating meaningful levels of energy storage.
• Power supply issues during disasters is a grid’s problem transferred to
the load.
© A. Kwasinski, 2015
Power Supply Resilience
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• Smart grid planning for disaster resiliency must consider disaster
impact on lifelines. During disasters special attention should be paid to
dissimilar ways in which disasters affect different distributed generation
(DG) technologies.
• Renewable sources do not have lifelines but they are not dispatchable,
they are expensive, and they require large footprints.
• Most DG technologies have availabilities lower than that of the grid.
• DG needs diverse power supply in order to achieve high availabilities.
• DG provides a technological solution to the vulnerable availability point
existing in air conditioners power supply.
•DG provides the active component to grid’s distribution portion,
essential for advanced self-healing power architectures.
•Loads are a valuable asset that can by used to improve resilience.
© A. Kwasinski, 2015
Extending the Internet into Smart grids
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• Lifeline dependencies can be reduced by extended local energy
storage. Lifeline’s effects on availability can be mitigated with diverse
local power generation.
• PVs and wind do not require lifelines but their variable profile leads to
added DG or extensive local energy buffers.
• Performance degradation: voltage regulation or selective load
shedding.
• Advanced (active) distribution through power routing interfaces
© A. Kwasinski, 2015
Advanced Power Architectures
Power routers
Monitoring points
• A hybrid ac (solid lines) and dc (doted lines) architecture with
both centralized and distributed generation resources.
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Smart grids: PHEV and PEV integration
• Problem: Typical home peak power
consumption is below 5 kW. An electric
vehicle may require 1 kW to be charged in 8
hrs. or up to 8 kW for shorter charging profiles.
Also, PEV and PHEV penetration is not uniform (higher for
neighborhoods with higher economical household income).
Hence, grid’s distribution transformers can be easily overloaded
PEV and PHEV even if charging is done during nighttime.
• DG avoids overloading distribution transformers but economical
issues still need to be addressed
• Combination of DG and energy storage may be a suitable
solution.
• Need for calculating a spatially moving demand.
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Loads control
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• In the IoT loads can be controlled to improve efficiency, but
how to (globally) optimize this and how to control this?
• Other issues: protection coordination.
WIND
GENERATOR
PV MODULES
LED LIGHTS (DC)
MAIN DC BUS
REFRIGERATOR (LOAD)
ENERGY STORAGE
ELECTRIC
VEHICLE
AIR CONDITIONER
FUEL CELL
© A. Kwasinski, 2015
EPA 430-F-97-028