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G
Controlling Power Systems
CONTROL EQUIPMENT AND PRACTICE practices. Kundur et al. (2007) describes best practices in
detail, listing over 50 best practices.
Terrorist attacks and other disturbances can evolve into
instability in a few seconds or tens of seconds--too fast for
control room operator actions. Operators may act within a Automatic Controls
few minutes during relatively familiar events with alarms, but
Additional information is provided below on the fol-
in new situations, 15 to 30 minutes may be required to make
lowing means of automatic controls that are listed but not
assessments and act, especially if load shedding is required.
described in Chapter 6.
Thus, various types of automatic controls are required.
Improving the control of voltage and reactive power may also
require relatively low-cost high-voltage equipment additions Techniques for Shedding Load and Generation to Enhance
such as shunt capacitor banks. Power System Dynamic Response Capabilities
Automatic controls constitute one or more layers of the
Power system dynamic response following disturbances
defense in depth or multiple layers of defense principle for
can, to some degree, be separated between real (active)
preventing or mitigating blackouts. In comparison to the
power phenomena and reactive power/voltage phenomena.
addition of new transmission lines, control improvements
Real power (measured in MW) is always held in balance
can be rapidly implemented.
when the system is operating normally. A disturbance upsets
This appendix provides details on automatic controls for
this balance and initiates dynamic response from the rotating
electric power systems, including new technology and best
synchronous generators in the system. An important aspect
practices. Such best practices help power systems to survive
of the real power balance deals with the availability of spin-
major disturbance events, both at power plants and in the
ning reserve (unloaded generation synchronized and ready to
transmission network. Besides the more conventional con-
serve additional demand) and the activation of such reserves
trols, emergency controls often termed "special protection
following islanding. This would also include measures such
systems" are applied to mitigate extreme disturbance events.
as load and generator shedding. Activation of reserves at
Information technologies hold promise to advance control
power plants by prime mover/energy supply system control
capabilities in the near future.
is limited. The tendency to carry reserves on fewer units, with
In short, power system robustness, resilience, and surviv-
many units base-loaded, reduces performance. The response
ability in the face of major disturbances, including terrorist
of units is difficult to predict because power plant operators
attacks, can be increased significantly, economically, and
can select from several control modes such as traditional
rapidly by the use/addition of automatic controls. How-
governor control of speed and system frequency, MW control
ever, there are several necessary requirements, namely, (1)
override of speed control, or coordinated boiler/turbine con-
implementation of industry best practices, (2) prioritized
trol with limited speed/frequency control. System frequency
upgrading of old analog controls (and actuators such as
regulation by secondary control (automatic generation con-
generator field circuit exciters), and (3) development and
trol) or operator actions often takes tens of minutes for large
implementation of wide-area controls. North American
upsets. Operator-directed or automatic demand-side actions
Electric Reliability Council (Electric Reliability Organiza-
are potential aids during emergencies.
tion) reliability standards for automatic controls, including
With automatic underfrequency load shedding and
performance monitoring, should evolve to better reflect best
with proper coordination between power plant control and
137
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138 TERRORISM AND THE ELECTRIC POWER DELIVERY SYSTEM
protection as described below, power system survivability a wide variety of sophisticated features, such as deadbands
following real-power imbalances is quite probable. System and control mode shifting.
frequency excursions are typically limited to 1 to 2 percent
of 60 Hz. One continuing concern, however, is unnecessary
Transmission-level Power Electronic Devices and
tripping of generation during frequency excursions because
Mechanical Devices
of boiler upsets and other problems. Prioritized control and
protection improvements and modernization would reduce Transmission-level power electronic devices such as static
tripping and improve system survivability following events volt-ampere reactive (var) compensators are employed to
with load-generation imbalance. provide continuous voltage control, similar to a generator
There are, however, relatively simple and low-cost voltage regulator, and/or other functions. Mechanically con-
practices that greatly improve reliability. However, these trolled shunt capacitor/reactor banks are switched by local
practices are not always followed--the August 14, 2003, voltage relays, by SCADA operators, and sometimes by
cascading failure providing a prime example (Nedwick et emergency controls. With digital technology, there is room
al., 1995; U.S.Canada Power System Outage Task Force, for more sophisticated control similar to that possible with
2004). Best practices for voltage reactive power require power electronic devices.
modern excitation equipment at generators. Replacement
of very old equipment with modern thyristor exciters and
Local Load-shedding Practices and Techniques
digital voltage regulators will improve generator reliabil-
ity. Generator voltage regulator controls including limiter Local underfrequency load shedding is commonly
circuits should be coordinated with protective relaying. A employed at bulk power delivery substations. Underfre-
lack of coordination has contributed to the severity of black- quency load shedding generally requires islanding of a
outs. Automatic voltage regulator line drop compensation portion of the interconnection with large generation-load
or automatic transmission-side voltage control should be imbalance. In a growing number of power companies, local
considered for better regulation of the transmission network undervoltage load shedding is also employed (Taylor, 2007).
voltage profile. Also, to avoid possible blackouts during lightning storms
or other transient events, automatic reclosing or single-pole
switching is employed. Since most terrorist actions are likely
Techniques for Maintaining Proper Transmission Network
to cause permanent outages, however, automatic reclosing
Voltage Profiles
will likely be unsuccessful.
Voltage should be near the maximum of the allowed volt-
age range and should be fairly uniform at all locations. This
Special Protection Systems or Remedial Action Schemes
high, flat voltage profile reduces losses that cause heating
and sagging into trees. Extensive use of relatively low cost Another widely used class of controls is termed special
shunt capacitor banks in both transmission and distribution protection systems (SPSs) or remedial action schemes (Tay-
systems allow a high and flat voltage profile, with substantial lor, 2007). These are emergency controls that initiate pow-
reactive power reserves at generators for emergencies. Volt- erful discontinuous actions, such as controlled separation/
age and reactive power are more complicated with separate islanding, load tripping, or generator tripping at the sending
ownership of generation and transmission systems. Rigorous end of an inter-tie. Other possible actions are steam-turbine
standards with performance monitoring are required. Overly fast valving, capacitor/reactor bank switching, HVDC fast
complex payments for reactive power or reactive power power changes, and dynamic braking. At present, most of
markets should be avoided. The section titled "Examples these controls directly detect single or multiple outages and
of Voltage/Reactive Power Practice" below in this appendix then make logic decisions about whether to initiate feedfor-
describes how poor voltage/reactive power practice played a ward action. The event-based controls are often implemented
critical role in the August 14, 2003, blackout (U.S.Canada to prevent cascading for multiple outages, but are sometimes
Power System Outage Task Force, 2004). implemented even for N1 outages. Many SPSs are wide
area with outage detection at several sites, binary transfer
trip signals to logic computers perhaps at control center(s),
Primary Automatic Controls to Prevent Cascading Instability
and then transfer trip signals to power plants and substations
Primary automatic controls, which are located mainly for control action. Reliability for the mission-critical actions
at power plants, include automatic voltage regulators and must be at least as high as primary protective relaying, requir-
prime mover controls such as speed governors. Automatic ing as a minimum redundancy so that no single component
voltage regulators include functions such as power system failure will cause overall control system failure. A large-scale
stabilizers, excitation limiters, and possibly connection of SPS implementation is described below in this appendix.
line-drop compensation. Prime mover controls include speed
and power regulation. Modern controls are digital, allowing
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APPENDIX G 139
Wide-area Feedback/Response-based Controls discontinuous controls are often wide area, but could be local
(e.g., underfrequency or undervoltage load shedding).
A promising alternative or complement to local controls
or to SPS is wide-area feedback/response-based controls.
Two types of these controls are continuous feedback control, Sophisticated Control Algorithms
and discontinuous control, which take actions similar to
Sophisticated control algorithms use various techniques
those taken by SPSs. Compared to local controls, wide-area
such as adaptive or "intelligent" control as part of digital
controls provide greater observability and controllability.
control and communication capabilities. Integration with
Positive sequence, synchronized phasor measurements are
the energy management system (EMS) functions, such as
the preferred sensors for control inputs. High-speed digital/
dynamic security assessment, is possible to adapt control to
optical communications are required.
present operating conditions. The description of wide-area
controls above focuses on actions to prevent instability and
Continuous Wide-area Control controlled or uncontrolled separations and islanding. If these
actions fail, controlled separations could be initiated. This is
Continuous wide-area control is being studied by many
relatively easy for well-defined inter-ties between areas, but
utilities, vendors, and universities. Perhaps the most serious
more difficult in a highly meshed system. Adaptive islanding
work is that by Hydro Quebec for power system stabilization
is a research area. Some aspects of this concept have been
(oscillation damping improvement) through generator exci-
demonstrated recently in simulation on a large, realistic test
tation control, and through the use of static var compensators
system (Yang et al., 2006).
and other power electronic devices.
Example of Impact of Voltage/Reactive Power Practice
Wide-area Discontinuous Feedback Control
An example of the impact of voltage/reactive power
Wide-area discontinuous feedback control is based on
practices on system performance from the August 14, 2003,
power system response to disturbances rather than on direct
blackout is presented (U.S.Canada Power System Out-
detection of only certain outages, as in most SPSs. Control
age Task Force, 2004). The initial outage of the Eastlake 5
action occurs for outages anywhere in the interconnection
generator on August 14 was related to excitation equipment
that causes a threatening response. Notable is the Wide-Area
problems during production of high reactive power. (The out-
Stability and Voltage Control System (WACS) in develop-
age likely would have been avoided with modern equipment.)
ment at BPA (Taylor et al., 2005).
As an example of poor voltage/reactive power practice, Fig-
Figure G.1 shows a block diagram of power system stabil-
ures G.2 and G.3 show conditions on August 14, 2003. Figure
ity controls. The SPS path is feedforward. The continuous
G.2 shows the 345 kV voltage profile that many engineers
feedback controls are normally local and mainly at genera-
would regard as terrible, especially considering that the load
tors, but could be wide area. The feedback (response-based)
was less than 80 percent of peak summer load and that the
Power System
Disturbances
switch capacitor/reactor banks
direct
detection
Power
System
y
(SPS) trip generators/loads Dynamics
Continuous
Discontinuous Feedback
Controls controlled separation Controls
(generators)
response detection FIGURE G.1 Power system
stability controls.
(WACS)
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140 TERRORISM AND THE ELECTRIC POWER DELIVERY SYSTEM
365 13:00 EDT
360 14:00 EDT
Desired Voltage Profile 15:00 EDT
355
16:00 EDT
350
345
kV 100%
340
335
330
95%
325
320
Chamberlin
(ITC)
(ITC)
(FE)
Lemoyne
(FE)
(FE-CAA)
Harding
(FE-CAA)
(FE-CAA)
(FE-CAA)
Star
(FE-CAA)
South Canton
(AEP)
Sammis-East
(FE)
Allen Junction
Juniper
Brownstown
St. Clair
Avon Lake
FIGURE G.2 August 14, 2003, voltage profile from west to east across northern Ohio. SOURCE: U.S.Canada Power System Outage Task
Force (2004). Fig G-2.eps
FIGURE G.3 August 14, 2003, reactive power production and reserves. SOURCE: U.S.Canada Power System Outage Task Force (2004).
fig G-3
this is a "fixed image", not easy to make changes.
OCR for page 141
APPENDIX G 141
13:00 voltage profile was before any outages. Figure G.2 also BPA's two control centers. Fault tolerant (triple-redundant)
shows a more desired voltage profile of 103 percent (which programmable logic controllers are at the control centers.
could be even higher: standard voltage range is 345 kV ± 5 Each logic computer has the equivalent of around 1,000 logic
percent). Voltage at the west (left) end near Detroit is very gates to detect the many combinations of single, double, and
good. Voltage at a large Ohio River power plant on the east triple line outages in the series/parallel transmission line
end is relatively low. Despite substantial reactive power path. Commands are then sent to generating plants. Besides
reserves in the American Electric Power area (Figure G.2) hydro generation tripping in the Northwest sending end
and a 765 kV infeed, voltage at the South Canton bus is poor. to reduce power transfer, the controls also switch 500 kV
Figure G.3 shows the very low reactive power reserves at capacitor/reactor banks. If an intertie separation does occur,
power plants in the Cleveland area. Again, the correspond- controlled separations of the northern and southern portions
ing high reactive power output combined with old excita- of the western interconnection into two electrical islands
tion equipment caused the initial Eastlake 5 outage. The is initiated. Following a severe outage, control actions are
poor voltage profile contributed to lines sagging into trees executed in less than a second.
(with heating and sagging inversely proportional to voltage
squared). Although inadequately discussed in the reports
REFERENCES
on the August 14, 2003, blackout, the disaster would likely
have been avoided with many more capacitors banks in the Kundur, P., C. Taylor, and P. Pourbeik. (co-chairs and secretary). 2007.
Blackout Experiences and Lessons, Best Practices for System Dynamic
Cleveland/Akron area. The power system would have been
Performance, and Role of New Technologies. IEEE Special Publication
much more robust and resilient. 07TP190, July.
Nedwick, P., A.F. Mistr Jr., and E.B. Croasdale. 1995. "Reactive Manage-
ment: A Key to Survival in the 1990s." IEEE Transactions on Power
Example of Special Protection System Implementation Systems 10(2): 10361043.
Taylor, C.W. 2007. "Power System Stability Controls." Chapter 12, Power
Bonneville Power Administration (BPA) may have the
System Stability and Control volume of The Electric Power Engineering
world's largest implementation of SPSs. The most important Handbook. Boca Raton, Fla.: CRC Press/IEEE Press.
SPSs involve the Pacific AC and DC inter-ties, where the Taylor, C.W., D.C. Erickson, K.E. Martin, R.E. Wilson, and V.
main action is tripping of up to 2,700 MW of hydro genera- Venkatasubramanian. 2005. "WACS: Wide-Area Stability and Volt-
tion. This is for high power transfer from the Pacific North- age Control System: R&D and On-Line Demonstration." Proceedings
of the IEEE [special issue on energy infrastructure defense systems]
west to California, where the generator tripping prevents
93(5): 892906.
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over redundant microwave or fiber-optic communications to
