Siemens Industry, Inc.
Power Technology
Issue 113
Transient Overvoltages during System Restoration Following a Black
Start
Daniel W. Durbak
Senior Staff Consultant
daniel.durbak@siemens.com
The various overvoltage stresses found during system restoration following a black
start include steady state overvoltages that can last indefinitely (minutes), and transient
overvoltages that have a limited duration. Switching surges are one type of transient overvoltage which
have high frequency (100 Hz to 10 kHz) components that decay quickly (milliseconds). Switching surge
waveforms typically contain only one, or just a few, predominant peaks. Another type of transient
overvoltage is a temporary overvoltage (TOV); TOVs are of longer duration, from 50 milliseconds to
seconds, and may have hundreds of high magnitude peaks.
Since the risk of flashover in electrical insulation for transformers, shunt reactors, circuit breakers, and
cables increases with magnitude and duration, the overvoltage must be compared to the corresponding
voltage withstand rating of each component. For example, a 200 percent temporary overvoltage with a
duration of 1 second may be a high risk for a transformer, while a 200 percent switching surge with a
duration of 20 milliseconds may be an acceptable risk.
Simulation of switching events with an electromagnetic transient program (EMTP) can provide insight into
the expected transient overvoltages during system restoration. The three-phase EMTP models used for
the analysis include the black start generators, large power transformers with core saturation, lines,
cables, circuit breakers, surge arresters and loads.
Energizing a large transformer during system restoration can result in higher TOVs than energizing during
times of normal operation. TOV results are sensitive to several parameters, including the source
impedance, system capacitances, transformer inrush current, load levels and circuit breaker closing times
(switching angles). During the first steps of system restoration following a black start, a transmission
system will generally be relatively weak with relatively high equivalent system inductances because few
generators are on-line. Thus, the first system resonant frequency can be much lower than during normal
system operation. Large capacitances from EHV transmission lines also contribute to the low resonant
frequencies. Low levels of load reduce the damping and prolong the duration of the TOV. Sufficient load
is also required to maintain stability of the turbines and steady state voltages within emergency limits.
Top-down versus bottom-up restoration sequence options impact TOV results. Top-down system options
restore the system from the highest (top) transmission voltage down to the distribution circuits. Bottom-up
options restore the system from lower voltage circuits prior to the highest voltage circuits. The simplified
single line diagrams of Figures 1 and 2 show systems in the early stages of restoration with the black
start generator unit modeled on the left. The circuit breakers in these figures would be open initially, and
then be energized in sequence from left to right. Energizing the generator step-up (GSU) transformer (on
left) should not be a significant overvoltage concern.
Power Technology
August 2012
345 kV
1
115 kV
2
4
3
Z
C
2
C
2
Figure 1 – Top-Down Restoration Option:
Energize 345 kV Line before Significant Load
115 kV
1
2
345 kV
3
4
C
2
Z
C
2
Figure 2 – Bottom-Up Restoration Option:
Energize Local 115 kV Circuits and Load before 345 kV Circuits
In the top-down option, the high side of the GSU is a 345 kV bus. Energizing Breaker 2 in Figure 1
creates a switching surge on the 345 kV line, where the highest overvoltage occurs at the open end, at
Breaker 3. Surge arresters at the end of the line, or pre-insertion resistors in circuit breakers, are usually
effective in limiting the peak of the switching surges. These types of switching events can initiate
significant power frequency overvoltages if the generator cannot absorb all of the capacitive vars created
by the energized system and effectively regulate the voltage at its terminal, or if the Ferranti rise at the
end of the line is excessive.
Significant TOVs are possible when Breaker 3 in Figure 1 closes to energize the large transformer on the
right. At this step in the restoration sequence, the only load on the system is the auxiliary load at the
generator terminal, which is relatively small and may not be large enough to significantly dampen any
TOV from harmonic resonances. One major drawback of a top-down restoration option is that the load
cannot be significantly increased at this step. For a top-down option to be viable from TOV perspectives,
the first 345 kV overhead circuits energized prior to load must be kept as short as possible. Short lines
have a low total capacitance (C), which yields higher resonant frequencies with the equivalent system
inductance back to the source. Higher resonant frequencies usually result in lower TOV magnitudes.
In the bottom-up option shown in Figure 2, the 115 kV bus is the high side of the GSU. The load restored
from the 115 kV system is significantly larger than the auxiliary load at the generator terminal. Closing
Breaker 2 to pick up the transformer to the 345 kV system does not usually initiate a significant TOV
concern when the load on the 115 kV system is high enough to dampen the transients. Energizing
Breaker 3 in Figure 2 creates a switching surge on the 345 kV line, where the highest overvoltage is on
the open end, at Breaker 4. These types of switching events are generally not prone to a significant TOV
with the assumption that the generator can absorb all of the capacitive vars created by the energized
system. However, significant TOV events are possible when other large transformers are energized from
the 345 kV system.
In summary, TOVs resulting from energizing large transformers can be a critical concern of the overall
electrical assessment of a system restoration plan following a black start. Increasing loads may help
reduce high magnitude TOVs. Bottom-up restoration options may pose less of a risk for high magnitude
TOVs. Top-down restoration options can also be viable when energizing short distances of EHV lines
prior to energizing loads.
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