IEEE Power & Energy Magazine - January/February 2014 - 40

several hundred horsepower to several thousand horsepower.
Fuel and feed-water pump motors and forced- and induceddraft fan motors belong to this large-horsepower group. it
is this latter group that presents the greatest challenge to
the reactive power resources available in any well-designed
black-start plan.
the method used for starting up these large motors
is often a hard start, which applies full line-to-line voltage across the motor terminals. occasionally, motors may
also be soft-started by applying a reduced voltage during
the starting period. it is therefore extremely important to
properly identify the motor-starting method since this will
greatly affect the depth of the dip in voltage resulting from
the black-start process.
accurate motor data are vital for conducting dynamic
studies to verify the viability of a given black-start process.
the information needed to establish the dynamic model for
the large induction motors participating in the black-start process includes the inertia of the motor plus its mechanical load,
the starting or locked rotor torque, the starting or locked rotor
current and associated power factor, the pull-out torque, the
full-load torque, and the full-load current and its associated
power factor. all of these data should be at rated voltage and
frequency. From these motor performance data, parameters
for the stator and rotor circuits are estimated. the dynamic
model for these large induction motors should include both
inertial and rotor circuit flux dynamics. this dynamic
model must closely match the speed-torque characteristic of
the motors, particularly at starting, pull-out, and full-load
operating points. in addition, it is important to include the
mechanical load damping effect in the inertial model of the
mechanical load, which for most centrifugal pumps and fans
follows a quadratic speed-torque characteristic.
the motor-starting sequence is another variable that must
be verified in any black-start process. the feasibility of plant
start-up can be tested by dynamically simulating the various
motor-starting sequences. the sequence must also accommodate the start-up requirements of the plant, which may
require certain motors to be started before others.
the voltage dip caused by starting these large induction
motors must be accurately quantified. this is because the
motors already online have magnetic contactors that open at

950 MVA
Load

Initial Inrush
Incandescent Lamp Filaments
Reach Operating Temperature
Small Motors up to Speed

330 MVA
180 MVA
100 MVA
0 0.5 s

Large Motors
up to Speed

Diversity in Load
Cycling Begins

30 min
Time After Pickup

figure 4. Load variation following cold load pickup.
40

ieee power & energy magazine

approximately 80% of the nominal bus voltage. ieee standard 399-1997 recommends a minimum terminal voltage of
80% of rated voltage. occasionally there may be magnetic
contactors that can hold their contacts with voltages as low
as 70% of the rated value; the number of cycles for which
this operating condition can be sustained is low, however. in
addition, the life expectancy of the insulation of the stator
and rotor windings is reduced as a result of the large currents
circulating through these windings. in such situations, the
motor manufacturer must be consulted to avoid motor damage such as shorted turns or even catastrophic motor failure.
Undervoltage protection settings should also be verified to
avoid the opening of circuit protection caused by undervoltage relay action.
the accelerating time period required by an induction
motor depends in great measure on the combined inertia of
the motor and its mechanical load. the longer the accelerating period, the higher the heating experienced by the stator
and rotor windings. when accelerating periods last a few
tens of seconds, motor manufacturer data on allowed motor
heating should be consulted to avoid a significant loss of useful operating life of the winding insulating material.

Self-Excitation
as noted above, energization of a transmission line or cable
will result in a rise in voltage along the line or cable due to
charging currents. the charging requirements can be large
enough to result in the bsUs absorbing reactive power. there
is the potential for self-excitation if the charging current is
large relative to the size of the generating unit. the result can
be an uncontrolled rise in voltage that could result in equipment failure. such an undesirable operating condition may
occur when the effective charging capacitive reactance of
the transmission system used in the black-start operation, as
seen by the bsU, is less than the q-axis generator reactance
Xq. in generating units with no negative field current capability, self-excitation cannot be controlled by the excitation
system, and thus the machine terminal voltage rises almost
instantaneously for cases where the effective capacitive reactance is less than the d-axis reactance Xd. generator excitation systems with negative field current capability delay but
do not prevent the onset of self-excitation. it is worth noting
that most generating units installed in the last 40 years do not
have negative field current capability. it is therefore extremely
important to verify the reactive power capability of the bsU
when operated at a leading power factor.
self-excitation can also occur from the load end for the
inadvertent loss of supply resulting from the opening of a
transmission line or cable at the sending end that leaves the
line connected to a large motor or a group of motors.

System Stability
power system stability was defined in 2004 by the ieee/
CigrÉ Joint task Force on stability terms and definitions
as "the ability of an electric power system, for a given initial
january/february 2014

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