IEEE Power & Energy Magazine - January/February 2014 - 41

Cold Load Pickup
the purpose of the restoration process is, of course, to
restore power supply to the loads and allow them to operate
as they did prior to the outage. but the characteristics of the
load immediately after reenergization may be quite different
than the characteristics exhibited prior to the outage.
if the load has been deenergized for several hours or more,
the inrush current upon reenergizing the load can be eight to
ten times the normal load current. the magnitude and duration
of the inrush current that flows when a feeder is reenergized
after a prolonged outage is a function of the type of load served
by the feeder. this could include: lighting; motors; and thermostatically controlled loads such as air conditioners, refrigerators, freezers, furnaces, and electric hot water heaters.
there are various components of the load that contribute to
the total inrush current. one example is the component due to
the filaments of incandescent lights. the resistance of the filament is very low until it warms to its operating temperature.
january/february 2014

(V)

operating condition, to regain a state of operating equilibrium
after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system
remains intact." this simply means that the power system
must be able to survive a disturbance and return to a sustainable operating point without a significant loss of equipment.
power system stability can be further subdivided into:
✔ Rotor angle stability: the ability of synchronous
machines of an interconnected power system to remain
in synchronism after being subjected to a disturbance
✔ Voltage stability: the ability of a power system to
maintain steady voltages at all buses in the system
after being subjected to a disturbance
✔ Frequency stability: the ability of a power system
to maintain steady frequency following a severe system disturbance resulting in a significant imbalance
between generation and load.
as discussed above, voltage and frequency control present
the greatest concerns during restoration. angular stability is
generally not a major concern in the early stages of the restoration process. when the system is being restored from a total
blackout, angular stability is assessed only when more than one
generating unit is used in the black-start plan. even when there
are multiple units connected in the early stages of a restoration
plan, the system is operating in a weakened state and stable performance is not expected for all system contingencies. Under
normal conditions, there are generally multiple transmission
paths between groups of generators such that a fault and trip of
one of these paths does not result in instability. during restoration, however, there may only be one strong path, and hence
a fault and outage of that path would cause instability. this
period of exposure to possible but unlikely events cannot limit
the restoration, however. it is simply a stage the system must
pass through to reach a more robust operating condition. the
exposure to events that could result in instability can be used as
one of the criteria with which to rank restoration alternatives.

2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0

40
(ms)

figure 5. Switching surge from an electromechanical
transients program simulation of the energizing of a typical
overhead line. The voltage is shown per unit of the line's
nominal voltage rating.

this low resistance results in a very high inrush current-up
to ten times the normal current. this high current flows for a
short period, approximately one-tenth of a second.
another component of the inrush current is the starting
of motors when the load is picked up. when a motor starts,
the current drawn will typically be five to six times normal,
until the motor reaches its operating speed. this may take
as long as several seconds for large, industrial-type motors.
a third component of inrush current is thermostatically
controlled loads, which turn on and off automatically to hold
temperature to a desired preset value. Under normal operating conditions, approximately one-third of these loads will
likely be connected at any instant in time. but after a lengthy
interruption of service, they will all have their thermostat
contacts closed, waiting to run as soon as power is restored.
as a result, these thermostatically controlled loads will be
perhaps three times greater than they normally would be for
the first half hour or so after being energized. Most thermostatically controlled loads also contain small, single-phase
motors, which will draw five or six times running current
until they are accelerated up to running speed in perhaps
one-half of a second. this results in the initial current drawn
by some thermostatically controlled loads being as high as
15 times normal current for the first one-half of a second
following energization.
a summary of the magnitude and duration of the inrush for
some of the various types of loads is shown in Figure 4.

Transient Overvoltages
restoration of the power system is performed through a
series of switching actions to sequentially reenergize system
components. energizing equipment during restoration conditions can result in higher overvoltages than during times of
normal operation. these overvoltages can lead to equipment
failure or damage that may hinder the successful implementation of the restoration plan.
transient overvoltages include temporary overvoltages,
switching surges, and lightning surges. lightning surges,
while an important design consideration, are usually not a
concern that affects the restoration of a power system.
ieee power & energy magazine

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