Transcript Integration and Testing of Energy Storage with

Flexible AC Transmission Systems:
Placement, Control, and Interaction
Mariesa L. Crow
University of Missouri-Rolla
EPRI/NSF Workshop on Global Dynamic Optimization
Flexible AC Transmission Systems
Alternating current
transmission systems
incorporating power
electronics-based and other
static controllers to enhance
controllability and increase
power transfer capability
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“Without fundamental research in this area, very little use
will be made with full confidence of the real opportunities
offered by FACTS devices. For the time being we only
have limited examples, entirely based on simulation,
which demonstrate that fast regulation of reactive
compensation on a transmission grid could be very
useful in the future. Because of this, there may exist an
immediate danger of uncoordinated system-wide fast
regulation via FACTS devices which could become
detrimental to system integrity under certain operating
conditions.”
Marija Ilic, “Fundamental engineering problems and opportunities in operating power transmission grids of the future” Int'l
Journal of Electrical Power & Energy Systems, vol. 17, no. 3, pp. 207-214, June 1995.
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Constraints on Useable
Transmission Capacity
• Dynamic:
– Transient and dynamic stability
– Subsynchronous oscillations
– Dynamic overvoltages and undervoltages
– Voltage collapse
– Frequency collapse
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• Static:
– Uneven power flow
– Excess reactive power flows
– Voltage capability
– Thermal capability
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FACTS Controllers
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Static VAR Compensator - SVC
Thyristor Controlled Series Compensator - TCSC
Thyristor Controlled Phase Angle Regulator - TCPAR
Static Synchronous Compensator - StatCom
Solid State Series Compensator - SSSC
Unified Power Flow Controller - UPFC
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SVC
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Thyristor Controlled Series
Compensator (TCSC)
TCSC
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StatCom
• shunt device
• lower rated components since only carry a fraction of
the line current
• impacts bus voltage and reactive power support
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SSSC
• series device
• must have higher rated transformer and devices
• impacts active power flow
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UPFC
• combination of StatCom and SSSC
• may control voltage, impedance, and angle
• impacts active and reactive power flow in line
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UPFC Topology
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Placement and Coordination
of FACTS Devices
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Long-term Control
-Power Flow control
-FACTS scheduling
-Economics
Dynamic Control
-System oscillation damping
-Voltage stability
-FACTS “ringing”
Local Control
-Control target acquisition
-Power electronics topology
-Modulation strategies
time
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Is there a
one-size-fits-all
controller?
Steady-State
Power Flow Control
•
•
•
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UPFC
SSSC
TCSC
TCPAR
These devices can affect active power flow
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Approaches
• Sensitivity analysis
   b
bij
t
t   g
g ij
t
t
Where  is the change in power transfer
capacity in response to an addition of t
compensation in line i-j with admittance
bij+j gij and b and g are sensitivity
parameters
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• Optimization (optimal power flow) with
genetic algorithms to minimize some
cost function
– Generation costs
– Congestion
– Problem is nonlinear, non-smooth, and
non-convex
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• Max-flow (graph theory) uses forward
and backward labeling from source to
sink to dynamically determine line flows
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Issues and Challenges
• Dynamic Coordination of FACTS
settings
– Security
– Economics
– Droop
• Hierarchical or local control of FACTS?
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Dynamic Control
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•
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transient stability improvement
inter-area oscillation damping
voltage collapse avoidance
subsynchronous resonance mitigation
Each control objective will (possibly)
require a different FACTS placement
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Issues
• Most dynamic control development has
concentrated on SMIB or very small two-area
systems
• How is control implemented in a large
nonlinear interconnected dynamic network?
– FACTS-FACTS interaction
– FACTS-generator interaction
• Hardware/field verification limited
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Challenges to FACTS
Implementation
• Unbalanced operation
• Harmonics
• Integration of Energy Storage (BESS,
SMES, flywheels)
• Power electronic topologies
• Power electronics devices
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StatCom/BESS
voltages
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active power
SSSC/BESS
voltages
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active power
Issues
• Most work considers only:
– Simplified topologies
– UPFC = variable impedance
– StatCom = PV bus
– Three-phase balanced operation
– No harmonics
– Simulation based
– Isolated performance (no interactions)
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Cascaded Converter
Advantages
• Use fewer components to
achieve the same number of
levels
• Has modularized circuitry which
makes packaging possible
• Does not have balancing
problem when with batteries
Disadvantages
• Needs separate DC sources for
active power conversion
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Diode-Clamped
Advantages
The harmonic content decreases as the
number of levels increases, thus reducing
the size of filters
Efficiency is high since devices are
switched at the fundamental frequency
It is easy to realize bi-directional active
power flow with a BESS or other energy
storage system
Disadvantages:
Requires a large number of high power
clamping diodes if the number of levels is
high
A high voltage rating is required for the
blocking diodes
There is potentially a voltage balancing
problem
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Food for thought
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Long-term Control
-Power Flow control
-FACTS scheduling
-Economics
Dynamic Control
-System oscillation damping
-Voltage stability
-FACTS “ringing”
Local Control
-Control target acquisition
-Power electronics topology
-Modulation strategies
time
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• Conventional eigenvalue analysis cannot
predict the high frequency self-modes of the
several FACTS devices embedded in a large
power system network.
• High frequency control interactions among
the several FACTS devices must be checked
using an EMTP-type program
• A promising technique is based on the use of
high frequency eigenvalue calculation using
Generalized Switching Function Models for
the different FACTS devices under
consideration.
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Series controllers
• low loop impedance - the series controllers
may experience a very strong interaction, and
therefore these controllers must be designed
in a coordinated way - the main linking
variable among the series controllers is the
ac current
• high loop impedance - no control interactions
may be expected among series controllers
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HVDC
• HVDC converters embedded in a large
network will not experience control
interactions if the transference
impedances between their commutation
busbars are high. This means that, in
this case, the dc control design of each
station can be based exclusively on the
Short-Circuit Ratio (SCR) at its ac
connection point.
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SCADA systems
• Dedicated SCADA systems will have to be
developed if global control of multiple FACTS
controllers is desired.
• Currently available SCADA systems have a
refresh rate of 1 second (maximum). This is
sufficient for steady-state control dispatch of
FACTS controllers.
• However, this is completely inadequate for
dynamic control, especially if we consider that
high frequency modes (10-100 Hz) may occur
on FACTS assisted power systems
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Discussion
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