IEEE Power & Energy Magazine - January/February 2014 - 42

3
2

(V)

1
0
-1
-2
-3
0.0

0.1

0.2

0.3

0.4

0.5

(s)

figure 6. An example of a TOV; voltage is expressed per
unit of nominal peak phase voltage.

switching surges are the transient overvoltages that immediately follow the opening or closing of a circuit breaker or
other switching device. switching surges have high-frequency
components (from 100 hz to 10 khz) that decay quickly, typically within two to three cycles of the power frequency, and are
followed by a normal steady-state voltage. switching surges
typically contain only one, or just a few, voltage peaks that
are of interest, as shown in Figure 5. this sample waveform is
from a simulation of the energizing of a typical overhead line.
the magnitude and wave shape of the switching surge
depends on the angle of the power frequency source voltage

Slip

Electric Torque

wave at the instant of circuit breaker closure. this requires
that many simulations be performed with various closing
times to obtain a statistical distribution of the overvoltage
results. surge arresters are effective in limiting the peak of
the switching surges.
temporary overvoltages (toVs) include many types of
events for which the voltage transient lasts longer than the
surges discussed above, exceeding the rated value for three
cycles or, potentially, significantly longer. toVs encompass
power frequency phenomena such as the Ferranti rise on an
open-ended line or cable and the overvoltage on an unfaulted
phase during a single line-to-ground fault.
toVs can also follow switching surges. For example, a
toV can result from switching circuits that saturate the core
of a power transformer, e.g., when cables and transformers are energized together. the harmonic-rich transformer
inrush currents can interact with the harmonic resonances
of the power system. the resonant frequencies are a function
of the series inductance associated with the system's shortcircuit strength and the shunt capacitances of cables and
lines. higher inductances (a property of relatively weak systems, such as those often occurring during restoration) and
higher capacitances (such as those due to long cables) yield
lower resonant frequencies and a higher chance of toVs.
Figure 6 shows an example of a toV taken from a simulation of the energization of a large transformer. like switching
surges, this type of toV can be dependent on the circuit breaker
closing times. in contrast to switching surges with one predominant peak, toVs can have hundreds of peaks, all of about the
same magnitude, if the toV lasts for several seconds.
the expected toV magnitude and duration is often a
major concern for surge arresters. Metal-oxide varistor-type
surge arresters have little effect on toVs that are below
about 1.6 per unit. silicon carbide-type surge arresters are
not affected by toV levels below the 60-hz spark-over level.
if the toV repeatedly exceeds the spark-over level, however,
then the multiple discharges may result in excessive energy
absorption and consequent arrester failure.

A Black-Start Example
Terminal Voltage

30.000

27.000

24.000

21.000

18.000

15.000

12.000

9.000

6.000

3.000

0.0

Absorbed Reactive Power

Time (s)

figure 7. Motor performance during the starting period:
motor terminal voltage, reactive power, electric torque,
and slip.
42

ieee power & energy magazine

in this section, we present a simulated black start. the scenario is one where a fast-starting, gas turbine-driven generating unit is used as a bsr to start up a combined-cycle power
plant. the black-start system consists of the bsU's step-up
transformer, underground high-voltage (hV) cables that connect the hV substation at the bsU to the hV substation at the
combined-cycle plant, and the generator step-up and auxiliary
transformers at the combined-cycle plant. both the taps of
transformers with tap-adjustment capability and the voltage
reference set point of the bsU were chosen to ensure that terminal voltages at the large induction motor units used in the
black start were close to their nominal values.
the black-start plan begins with the across-the-line starting of a 2,500-hp motor. the motor performance during
the starting period is shown in Figure 7. terminal voltage,
january/february 2014

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