IEEE Power & Energy Magazine - January/February 2014 - 37

Restoration actions performed to recover from
a blackout may vary from those determined
in the restoration studies.

overall system. the testing of line energizations is more complex, as it involves the deenergization of parts of the transmission system so they can be connected to the bsU. this must
be accomplished without any adverse impact on loads, which
may not always be possible. restoration plans that require the
energization of load at a particular step in the plan cannot be
tested beyond that step because it is never acceptable to submit loads to outage and pickup as part of a test. in addition to
any field testing performed, simulation is usually required to
verify and validate the plan. a study including both steadystate and transient analysis is therefore performed.
the restoration plans must document the various cranking paths. For example, the plans show the number and
switching sequence of transmission elements involved,
including the initial switching requirements between each
bsU and the unit to be started (i.e., its next-start unit).
in the case of a total system outage, system restoration
must begin from the bsUs, with restoration of the power
system proceeding outward toward critical system loads. as
the bsUs themselves can only supply a small fraction of the
system load, these units must be used to help start larger
units that need their station service loads to be supplied by
outside power sources. Full restoration of system load can
only occur when these larger units can come online. the
restoration plan following a system blackout should therefore include self-starting units that can be used to black-start
large, steam turbine-driven plants located electrically close
to these units. as mentioned above, another objective for
many systems is the supply of auxiliary power to nuclear
power stations in need of off-site power to supply critical
station service loads. other priority loads may include military facilities, law enforcement facilities, hospitals and other
public health facilities, and communication facilities.
the typical black-start scenario includes the self-starting unit
or units, the transmission lines that will transport the power supplied to the large motor loads in the power plant to be blackstarted, and at least three transformer units. these would include
the generator step-up transformers of the black-start generating
unit, the generator step-up transformers of the next-start unit
involved (e.g., a large steam turbine unit), and one or more auxiliary transformers serving motor control centers at the next-start
plant. the transmission lines used for the black start may be
either overhead lines or high-voltage underground cables. the
load to be black-started includes very large induction motors,
ranging from a few hundred horsepower to several thousands of
horsepower as well as plant lighting and small-motor loads.
january/february 2014

the key concerns are the control of voltage and frequency.
both voltage and frequency must be kept within a tight band
around nominal values to guard against damage to equipment and to ensure restoration progress. any equipment failure will severely hinder restoration and may require starting
over with a revised plan. system protection operations can
also occur if voltage or frequency strays outside acceptable
ranges, again with the potential to set back or stop the restoration process. the following sections give an overview of
several of the technical concerns that must be addressed.

Steady-State Concerns
the black-start plan describes the steps that the transmission
operators need to take to restore the isolated power system from
the bsU. this includes sequentially energizing transformers,
transmission lines, and, potentially, shunt compensation and
load pickup to supply power to the next-start unit auxiliary
loads, allowing their associated units to begin operation. once
larger generating units are available, higher-voltage lines can be
energized, again in a step-by-step sequence, to supply power to
major substations where load can be picked up and further interconnections made until the grid is fully restored.
the steady-state analysis of this isolated power system
includes:
✔ voltage control and steady-state overvoltage (Ferranti
effect) analysis
✔ capability of the bsUs to absorb reactive power (vars)
produced by the charging capacitance of the transmission system
✔ step-by-step simulation of the black-start plan being
tested to ensure its feasibility and compliance with
required operational limits
✔ verification of the robustness of the tested black-start
plan to ensure its ability to compensate for the unavailability of key components to be used in the plan
✔ demonstration of generation and load-matching
capability.
Voltage control analysis determines the voltage reference
set point of the black-starting generating unit and the offnominal tap setting for all transformers that are part of the
plan. this ensures proper control of voltage and provides the
needed terminal voltage to start up the large induction motor
loads at the black-started plant. transformer tap settings that
are appropriate for normal conditions that generally include
significant current flows may result in high system voltages
under the lightly loaded black-start condition. since most
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2014