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An Approach to Improving the Physical and Cyber Security of a Bulk Power System with FACTS
Mariesa Crow*
School of Materials, Energy & Earth
Resources
University of Missouri-Rolla
Bruce McMillin
Department of Computer Science
University of Missouri-Rolla
Stan Atcitty
Power Sources Development
Department
Sandia National Laboratories+
Abstract
This paper describes an approach to improving the physical and cyber security of a bulk power transmission system
using coordinated advanced power flow controllers. The architecture of the proposed system is described and
results obtained from a laboratory setup using hardware-in-the-loop simulation are presented that show that
introduction of FACTS devices can utilize the proposed decentralized control to mitigate cascading failures in the
power system.
Background on FACTS
In a traditional vertically integrated utility structure, the scheduling of generation was the primary means for
adjusting power flow through the network. However, as the vertically integrated utility structure is replaced by open
access, this means of transmission power flow control has been lost. Thus, new controllers, based on distributed
computing techniques must be developed that will allow transmission providers direct control. The future set of
advanced controllers are called flexible AC transmission system (FACTS) devices [1].
FACTS devices are high-voltage power electronics devices that allow precise and rapid control of power.
Encouraging the use of these new technologies is essential to make better use of existing transmission facilities and
reduce the number of new facilities that are needed. Distributed computing plays an important role in leading to a
smart, switchable grid that can anticipate impending emergencies and automatically take preventive actions through
coordinating FACTS devices. Technologies such as these can protect the grid against not only traditional threats to
reliability, such as storms and other natural events, but also against deliberate disruptions such as hacking or terrorist
activity [2]. Unified power flow controllers (UPFCs) are hybrid FACTS devices that can control both active and
reactive power flow on the line and bus voltage.
*
Communication
FACTS Device
Embedded
Computer
Low Voltage Control
System
*
PowerLine
High Voltage Power
Conversion System
*
ControlledLine :
PowerLine
Figure 1
UPFC device schematic
A UPFC device consists of an embedded computer that depends on a low voltage control system for signal
processing, which, in turn, depends on a high voltage power conversion system for rapidly controlling the power
flow on the transmission line as shown in Figure 1. Each UPFC controls a single transmission line (ControlledLine)
and multiple UPFC devices can interact with each other via exchanging messages over network communication.
*
M. Crow, corresponding author, [email protected], voice: 573-341-4153, FAX: 573-341-4192. + Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under
contract DE-AC04-94AL85000.
The net effect of the UPFC devices and the power grid is that each transmission line and UPFC device is affected by
other transmission line power flows and UPFC devices.
The UPFC FACTS Interaction Laboratory (FIL)
A small laboratory power system is not capable of fully capturing the depth and breadth of large-scale power system
dynamics. For this reason, a realistically sized power system can be simulated and interfaced via a “hardware-inthe-loop” mechanism to provide voltage and power flow information to the UPFC devices at their interconnection
points (see Figure 2). A real-time time-stepped power systems simulation has been implemented in a Simulation
Engine and UPFC devices [5] have been constructed in the FIL. In the simulation, integration times are defined to
be precisely the length of “real-world” time steps. Thus, the virtual dynamics of the larger system are meaningfully
coupled to the physical hardware creating a real-time simulation. Synchronization with the physical world is done
using real-time A/D (analog/digital) and D/A (digital/analog) I/O (input/output) boards.
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Figure 2
A Hardware in the Loop (HIL) simulation
Three UPFC devices have been constructed. These are interconnected via the simulation engine that mimics the
dynamic response of a power system. The simulation engine sends frequency, voltage, and current flow
measurements to an external synchronous machine in the lab and a programmable load. The synchronous machine
and programmable load generate the physical conditions that an actual UPFC device would encounter. The UPFC
respond to these changes. The UPFC responses are fed back into the simulation as inputs. This set-up is shown in
Figure 3. The left portion of Figure 3 represents the simulation engine whereas the right portion shows the actual
hardware. The machines are not part of the simulation but are used to produce the necessary active power flow on
the lines in which the UPFC devices are placed. The programmable loads represent the load of the system “seen” by
each of the UPFC devices. These loads change depending on the placement of the UPFC devices in the simulated
power system.
Machine 1
UPFC 1
D/A output
Programmable
load
A/D input
Machine 2
UPFC 2
D/A output
Programmable
load
A/D input
Machine 3
Power System
Simulation Engine
D/A output
A/D input
Figure 3
Conceptual layout of hardware and simulation engine
UPFC 3
Programmable
load
Figure 4 shows the hardware structure of a FACTS device. The function of the Technosoft MSK2812 Digital Signal
Processing (DSP) board is to receive commands from a local computer, and transform them into low-level switching
signals. Those signals will then be sent to the driver boards for converter operation. Moreover, in order to observe
the behavior of the FACTS device and power line, one sensor board will acquire all of the online data and send them
to the DSP.
Power Line
Sensor
Data
Local
Computer
DSP
Board
Interface
Circuit
Board
From Online
Sensors
Sensor Circuit
Board
Switching
Signals
IGBT Driver
Circuit Board
(1)
Error
Error
IGBT Driver
Circuit Board
(2)
Switching
Signals
Gate
Control
Gate
Control
Six-pulse
Converter
(1)
Shunt
Transformer
Six-pulse
Converter
(2)
Series
Transformer
Figure 4
Hardware architecture of the FACTS devices
The interface board plays an important role in the FACTS device. The functions of the interface board include DSP
protection, signal isolation, input signal scaling, etc. Figure 5 illustrates the configuration of interface circuits.
Interface Circuits
DSP A/D
Input
DSP Interrupt
(synchronization)
(0V ~ 3V)
Isolation
Amplifier
(0V ~ 3V) Operational
Amplifier
Opto
Isolation
Hysteresis
Comparator
(-6V ~ 6V) Sensor
Data
Driver Board (1)
Switching Signals
DSP I/O
(PWM output)
Buffer
Opto
Isolation
Driver Board (2)
Switching Signals
DSP I/O
(Error input)
Buffer
Figure 5
Configuration of interface circuits
The pictures of the UPFC hardware are shown in Figure 6.
Opto
Isolation
Errors of Driver
Boards
Sensor Board
Power Supply
DSP Board
(Under the data cable)
Current Sensors
Interface board
Sensor Board
Shunt Transformer
Series Transformer
(consist of three single phase transformers)
Two Six-pulse converters
Two six-pulse Converters
DC Capacitors
(3000 uF)
UPFC Layout
Figure 6
Photos of FACTS hardware.
DC Sensors
Two Driving Boards
(overlapping)
Shunt and series transformer
As a part of a FACTS device, the digital signal processing (DSP) codes for converter control, data acquisition, and
Controller Area Network (CAN) communication were developed and debugged separately. Moreover, a
LabVIEWTM user interface was developed. This interface is utilized in the debugging of FACTS devices.
UPFC Control
The primary difficulty encountered in dynamic UPFC analysis is the current lack of time-scale-based controls and
decentralized operating paradigms for interacting UPFC devices. Since UPFCs are decentralized, there is justifiable
concern over whether they will cooperate or compete in system control. The most common concern in dynamic
control is how to coordinate each UPFC device in controlling power flow and whether or not UPFC devices will
“ring” against each other as each device tries to maintain operating states that possibly conflict.
The performance of two FACTS controllers was tested in the presence of a three phase to ground fault. As a first
step, a controller was designed by assuming that the network dynamics were ideally totally observable (i.e. all
system information is known). Figure 7 shows the controlled and uncontrolled dynamic frequency responses (to
conserve space not all generator responses are shown – only generators 1-4). Note that the multi-modal responses of
the generators are rapidly damped. On the other hand, Figure 8 shows the controller response when some of the
system states are not observable as would be the case in a large distributed network.
Figure 7
Performance of controllers with ideal observability
Figure 8
Performance of controller without ideal observability
However complete observability is not possible in a large geographically distributed system such as the power
network. Figure 8 shows the same modal response when the system is not completely observable. Note that while
the UPFCs are able to damp the oscillations at some of the generators, at other generators they cause the oscillations
to worsen. This is due to the interaction of the control responses.
A new distributed control was developed to mitigate the interactions between the generators. Since the system is not
completely observable, a robust control for UPFC was designed that treated the interaction between generators as a
disturbance and not a mode to be controlled. This approach yielded the improved results shown in Figure 9.
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Figure 9
Improved decentralized robust control
Cascading Failure Mitigation
A cascading failure occurs within a power system one or more lines are lost due to a contingency and the resulting
redirected power flow stresses the network. While the local dynamic power system interactions can be sufficiently
capture by direct, the communication and computational processes of the UPFC require a long-term control strategy.
The UPFC devices, themselves, communicate over an interconnected computing network to reach agreement on
how power should be routed or re-routed in the presence of a contingency. Figure 10 depicts the effectiveness of
distributed long-term control in reducing overloaded lines. The UPFC devices are placed such that they minimize
the total number of overloads under long-term control based on the maximum flow algorithm [3]. Figure 11 gives a
comparison of different methods used to place the devices
Figure 10
Relationship of number of UPFC and overloads
Figure 11
A comparison of different approaches to placement
Continuing Work
Current work involves examining other long-term control algorithms. A quadratic performance index metric
optimized by gradient descent shows promise in reducing the number of overloads, further, but its behavior over
complicated UPFC device combinations, and its distributed fault-tolerant properties need further study[4]. Ensuring
security of the information exchanged among the embedded computers in the UPFC devices requires an in-depth
security vulnerability analysis. Dynamic control algorithms are being explored that work cooperatively over a wide
range of UPFC placements and contingencies. These techniques will be explored using the FIL to uncover
additional vulnerabilities that may be present in the system.
Acknowledgements
The authors gratefully acknowledge the support of the DOE Energy Storage Program and Sandia National
Laboratories for this work.
References
[1]
IEEE Power Engineering Society FACTS Application Task Force, FACTS Applications, IEEE Publication
96TP116-0, 1996.
[2]
S. D. Wolthusen, “Asymmetric Information Warfare: Cyberterrorism Critical Infrastructures,” Proceedings
of the XV International Amaldi Conference of Academies of Science and National Scientific Societies on
Problems of Global Securit,y (Helsinki, Finland, Sept. 2003.
[3]
B. McMillin and M. L. Crow, “Fault tolerance and security for power transmission system configuration
with FACTS devices,” Proceedings of the 32nd North American Power Symposium, vol. 1, pp. 5.1-5.9,
October 2000.
[4]
A. Armbruster, B. McMillin, M. Gosnell, and M. Crow, “Power Transmission Control Using Max-Flow,”
29th Annual International Computers Software and Applications Conference, Edinburgh, U.K., July 26-28,
2005, pp. 256-263.
[5]
L. Dong, M. L. Crow, Z. Yang, C. Shen, L. Zhang, and S. Atcitty, “A Reconfigurable FACTS System for
University Laboratories,” IEEE Transactions on Power Systems, Feb. 2004, Volume: 19 , Issue: 1, pp. 120
– 128.