IEEE Power & Energy Magazine - January/February 2014 - 58

With the implementation of such decision support tools,
the power grid will be better prepared and equipped
to handle extreme events.
Assessment of Power System Conditions

✔ place AGC control in manual mode if it has not auto-

Following a major power system disturbance, the first priority is to assess system conditions. This critical task involves
answering several questions:
✔ What are the statuses and characteristics of all generating units online or tripped off-line?
✔ What are the statuses of neighboring power systems
and tie lines?
✔ What, if any, system load is still being served?
✔ If portions of the system are still energized, are frequency, voltage, and equipment loadings within acceptable limits?
✔ What are the boundaries of any islanded systems?

matically tripped to manual mode
✔ address any power system overloads quickly, so that
all equipment is operating within acceptable limits
✔ establish appropriate levels of operating reserve as
soon as possible.
For generators that have recently tripped, operators must
✔ establish, if possible, a source of on-site power for
each generator's auxiliary loads
✔ ensure that the generator has safely shut down (a list of
items to check for each generator should be included
in the generator's restoration plan)
✔ review any generator alarms for relevant information
✔ determine whether the generator is available for
restart.
For generators with black-start capability, operators must
✔ review any generator alarms for relevant information
✔ check that the generator is available for restart
✔ start any necessary on-site emergency generation for
the generator's auxiliary equipment
✔ prepare the generator for a black start.
For generating units that are not in service, operators must
✔ review any generator alarms for relevant information
✔ check the availability of the generator for start-up.

Identifying Electrical Islands
Methods are needed to accurately and reliably identify the
extent of electrical islands. The identification can be accomplished using a combination of breaker and switch statuses
with phasor measurements.
In the first pass, the electrical islands can be determined
by running a conventional network topology program on
the measured breaker and switch positions. In the second
pass, dynamic PMU quantities can be compared over time
and the buses that are in the same electrical island can be
identified.
EMS overview displays have the ability to show different islands in different colors. Warnings can be provided in
the event that a system operator selects a breaker that will
connect two islands. Summary displays can also show the
amount of generation capacity, which generators are connected, and the load that is connected in each island.

Topology and State Estimation
A successful restoration plan relies on accurately tracking network topology. Topology error identification has historically
been performed manually as a post processing step following
the convergence or nonconvergence of the state estimator. The
errors in breaker and switch status can be detected by means
of highly reliable PMU data. As a result, operators can avoid
mistakes that may lead to another outage event.

Preparation of Subsystems
Before starting the actual restoration of the power system it
is necessary to take several preliminary steps. For generation that has remained online, operators must
✔ adjust the voltage and frequency of online generators
so they are within acceptable limits
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Dead Bus Clearing
In the dead bus-clearing phase, the circuit breakers connecting two dead buses are "all opened," with the goal of ensuring that equipment can later be energized without any risk
of inadvertently connecting unplanned load or line charging.
For a large system, this phase may take several hours. Some
utilities have implemented special relays in the substations
or group control procedures on their EMSs to minimize the
time and operator involvement for this step.

Restoration Building Blocks
A restoration building block is a minimal configuration with
a stable source of power. One example is a black-start unit
or an energized interconnection point with some associated transmission that can serve as a viable subsystem for
rebuilding the overall power system.
A restoration building block should have the following
characteristics:
✔ a black-start capability sufficient to restore light and
power to critical equipment
✔ the ability to match generation to load within frequency limits
january/february 2014

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2014