IEEE Power & Energy Magazine - March/April 2021 - 39

Power system safety means that risks to life and property
associated with generation, transmission, and distribution
are satisfactorily mitigated.
The answers will be complemented by the experience that
TSOs have gained in the single interconnected synchronous
system of Ireland and Northern Ireland, which connects to
the United Kingdom through 500-MW high-voltage (HV)
dc in the north and 500-MW HVdc in the south. As of May
2018, the system's generation fleet had some 9,500 MW
of conventional resources, 4,500 MW of wind resources,
a peak demand of 6,500 MW, and a minimum demand of
2,500 MW (see " First DSA Stakeholder Workshop " in the
" For Further Reading " section). The relatively small size of
the island's synchronous system versus the increasing penetration of intermittent renewables creates a " large-scale
renewable energy penetration laboratory. " Insights from
this " laboratory " will be useful for larger systems (such as
Scandinavia's) and eventually for the European continental
synchronous system to prepare for the levels of intermittent
renewable generation penetration seen in Ireland.

Power Transmission System Operational
Security and Its Components
A large interconnected power transmission system is probably the single most complex engineering device to be created
by mankind. In many parts of the world, power transmission
systems operate for many decades, continuously supplying
electrical power to millions of customers. The goal of power
system operation is not only to reliably supply power but to
do so safely and securely. Power system safety means that
risks to life and property associated with generation, transmission, and distribution are satisfactorily mitigated through
proper design, installation, maintenance, and organizational
measures. On the other hand, the security of the power system means that the risk of customer supply interruptions is
adequately mitigated by proper planning, design, installation,
maintenance, and operation. The latter component refers to
operational security, which is the ability to retain a normal
state or return to such a state as soon as possible following a
contingency. The goal of managing operational security is to
keep a system in its normal state 99.99% of the time. Except
for the normal state, there are four others that the power system could experience or transition through if system security
has been compromised and if exceptional external events
(e.g., extreme weather conditions) have taken place. Figure 1
represents the possible power transmission system states.

operational states are modeled by three sets of equations, one differential and two algebraic. The differential set represents the dynamic behavior of a system. The
first algebraic set consists of equality constraints, which
refer to the generation-load system balance. The second
set contains inequality constraints corresponding to the
operational limits of a system. These limits include power
transfer caps, voltage upper and lower operating restrictions, frequency lower and upper operating bounds, and
other controls that may be imposed to ensure operational
security. The differences between transmission states are
determined by the validity of the constraints of sets of
algebraic equations.
The Normal State

In this state, all constraints are satisfied, which means that
the balance between the generation and the load (plus losses)
is met, there is no overloaded equipment, and voltages and
frequencies are within normal operational limits. Also, a system's reserve margins are sufficient, and a system can withstand any of the credible contingencies so that it will remain
within its operational security bounds and be brought back
to normal operation following remedial actions. The set of
credible contingencies is defined by and used for the assessment of power system operational security. For example, one
credible contingency concerns the trip of a large generator.
If such a trip causes a system frequency to deviate within the
security limits and if the system frequency returns to normal
operational limits after a remedial action (e.g., activating a

Normal State

Restorative
(Black Start)

Blackout
System Not Intact

Power Transmission System States
In any modern energy management/supervisory control
and data acquisition system, power transmission network
march/april 2021

Alert State

Emergency
(Red Alert)
System Intact

figure 1. Power transmission system states.
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IEEE Power & Energy Magazine - March/April 2021

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