IEEE Power & Energy Magazine - January/February 2015 - 71

to preserve grid stability. these go beyond the need described
above for reactive power compensation. In the 220-kV and
380-kV grid, the influences of the higher loads on connected
grids as well as the implications for system stability of increasing phase angle differences need to be considered. furthermore, the outage of a highly loaded line leads to an n-1 case
and can cause transients on the lines remaining in operation.
to safely avoid further outages, the defined boundary values
should not be exceeded. the adaptations necessary in the
tennet grid as well as the approach taken to determine available ampacities in certain long-term grid planning scenarios
will be described in detail in a subsequent article.

Dynamic Rating Approach Verification
when using a dynamic rating system for ohLs, the system
operator has to make sure that the clearance of the lines is
always maintained and the maximum conductor temperature is never exceeded.
to verify the reliability of the dynamic rating system,
the following approach was taken. time-resolved conductor
temperature was calculated from the data used to determine
ampacity (ambient temperature, wind speed, solar radiation,
and conductor type) and from the measured values of the actual
line current. the calculated values for conductor temperature
were then compared with measured values of conductor temperature. with a safe implementation of the dynamic rating
process, the calculated conductor temperature should be higher
than the corresponding measured value most of the time.

Transient Stability Assessment

which will not be changed by using dynamic line rating. as
shown in figure 17(a), an increase in n-1 security that covers
mainly the thermal stability of a power system does not lead
to an increase of the stability margins. In addition, the protection philosophy has to be adapted to higher nominal currents
and with regard for secure, selective, fast, and robust fault
clearing. Special attention must be given to postfault behavior
to avoid a loss of stability in the case of severe disturbances
(see figure 18). Countermeasures in the area of protection
and control as well as emergency control actions, however,
are well demonstrated and have been in operation for years.

Case 3: Impact of Reduced Inertia
on Power System Frequency
the frequency of a power system must lie within a predefined range and not deviate too far from the frequency for
which the system was designed, so that operational security
is not compromised. If the frequency is not held near its
nominal value, protection systems begin to activate to protect machinery and keep the power system operational.
Large deviations in frequency are often caused by the tripping of large production units, which results in sudden imbalances in active power. Kinetic energy is released from the
remaining machines in the system, causing the rotors of the
machines to slow down and the system frequency to decrease.
the frequency can then drop by unacceptable amounts, resulting in the disconnection of production units and loads and
producing a cascade effect that can lead to widespread power
outages. as production units become larger and larger, greater
imbalances result, with larger frequency deviations.
Large synchronous generators help the power system resist
system frequency deviations. all rotating machines contribute
to this resistance with their inertia. as renewable generation

for a flexible ac power system, dynamic line rating is a useful
approach that temporarily provides additional transmission
capacity while ensuring n-1 security with higher loading of
the transmission lines. It can be
used to support the further integration of sources of renewable
Dynamic Rating
Grid Development
energy (reS), which are often
located far away from consumers,
by optimizing grid development
+
and helping to meet the need for
new transmission corridors. the
need to maintain stability margins,
however, could limit the ability of
Stability
Stability
dynamic line rating to provide
(n -1) Limit
(n -1) Limit
additional transmission capacity (see figure 17). the transient
stability of the power system must
therefore be investigated and monThermal (n -1) Limit
Thermal (n -1) Limit
itored in real time to avoid operating the system beyond its stability
Today
Future?
Today
Future?
margins, even when n-1 security
(a)
(b)
is not a problem.
the stability of the power system exists in relationship with the figure 17. Dynamic current rating: effect of increased thermal limits on transient staresulting network impedances, bility limit with (a) no additional transmission lines and (b) additional transmission lines.
january/february 2015

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2015