IEEE Power & Energy Magazine - March/April 2021 - 41

Power system stability can be assessed by considering its
various forms, which include the following:
✔ Steady-state stability: This is the ability of a system
to arrive at a safe steady state without disconnecting
customers following the outage of one (N-1) or more
(N-k) generator, transmission line, transformer, and
load components. This includes the following:
* Thermal constraints: These involve the requirement
to maintain power flows in the elements of a system,
below their thermal ratings in all N-1 contingencies
and credible N-k contingencies; these ratings can be
variable, e.g., seasonal or so-called dynamic, whereby they change more frequently (e.g., every hour) depending on the load and weather conditions.
* Steady-state voltage stability: The is the ability of a
system to maintain secure voltage levels and voltage
gradients during all N-1 contingencies and credible
N-k contingencies.
✔ Transient stability: This concerns the ability of a system to keep its components connected following large
disturbances, such as faults in the transmission system
that are cleared by protection, sudden trips of large
generators, the disconnection of large loads, and so
forth. Transient stability further includes
* Synchronous plant stability: This describes this
ability of a system to maintain its synchronism (rotor angle stability).
* Nonsynchronous plant stability: This is the ability of a system to keep other sources of generation
connected. It is also called fault ride-through capability.
✔ Oscillatory stability: This is the ability of a system
to quickly damp out the power and frequency oscillations that can occur in the power generation. Oscillations can be triggered after small and large disturbances that cause variations in the load, generation
(especially wind, which has intrinsic variability), automatic control device settings, and so on. Oscillatory
stability includes the following:
* Small signal stability: This is the ability of a system to maintain the synchronism of its synchronous
plants and keep its nonsynchronous plants connected following small disturbances.
* Control mode stability: This describes the ability of
a system to dampen oscillations related to turbines
and their controls.
✔ Frequency stability: This is the ability of a system to
maintain its frequency within safe limits and to
return to the permitted frequency range following
disturbances.
Each of the operational security components can be
assessed by a set of security criteria applied to analyze the
results of simulations based on a power system model and a
scenario containing particular conditions and disturbances.
If the modeled system performance parameters are within
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an established set of security criteria, the scenario is considered secure. If at least one security criterion is violated, the
scenario is considered unsecure, and corrective measures
will need to be taken. A version of a set of operational security criteria is proposed as follows:
✔ Thermal criterion: Transmission lines, cables, and
transformers must be loaded at less than 100% of their
nominal thermal ratings in base case conditions and
at less than 110% in N-1 contingency conditions. For
transformers, which have a greater short-term load capacity, 130% may be acceptable.
✔ Voltage stability criterion: Voltage stability must be
maintained in base-case conditions and all studied
contingency conditions. Steady-state voltage levels
at all HV buses within the transmission system (e.g.,
110 kV and higher) must be within limits: 0.95-1.1 per
unit in base case conditions and 0.9-1.12 per unit in
N-1 contingency conditions.
✔ Synchronous plant transient stability criterion: The
maximum rotor angles of synchronous generators
must be within limits to stay in synchronism. This
corresponds to a requirement that the maximum angle
separation of any two synchronous generators in one
island must be less than 360º.
✔ Frequency stability criterion: In the event of tripping
one generator or a three-phase fault with a forced outage of an element (transmission line or cable), the frequency deviation must be lower than 1 Hz when the
duration of the frequency deviation is longer than 0.5 s.
✔ Oscillatory stability criterion: The oscillation damping coefficient can be assessed through small-signal
analysis and as an additional outcome of the time domain simulation analysis. The damping factor and the
exponential decay rate of an oscillation following an
event are related, and the larger the damping coefficient is, the higher the damping factor will be. The
damping coefficient must be greater than 0.05.
✔ Additional criteria:
* System nonsynchronous penetration (SNSP) level:
This concept is valid for a synchronous system as
a whole, and it was developed in parallel with the
" All Island TSO Facilitation of Renewables Studies " (see Eirgrid 2010 in the " For Further Reading "
section). The objective of the SNSP metric is to
have an index that, at a high level, captures a range
of operational issues. It shows a percentage of the
nonsynchronous generation (including net imports
via the HVdc interconnectors, if any) in the total
generation in a synchronous power system. HVdc
interconnectors are included since, when they are
importing power, they displace generation located
in the system that would otherwise be operating.
At a constant demand, an increase in nonsynchronous generation would equate to an increase in the
SNSP. The maximum level of the SNSP can be
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