IEEE Power & Energy Magazine - March/April 2021 - 42

Existing transmission and distribution networks
were designed using planning criteria based on a limited
number of power flow patterns.
established based on the comprehensive analysis of
the operational security of the synchronous system
and revised in line with improving the flexibility of
the system components (mainly generators' ability
to provide inertial and fast frequency response). At
present, the maximum SNSP limit for the synchronous system in Ireland and Northern Ireland is set
as 65% (see " Operational Constraints Update " in
the " For Further Reading " section). The maximum
SNSP is maintained by the curtailment of nonsynchronous renewable generation.
* Rate of change of frequency (RoCoF): This is the
measure of the frequency through time following the
sudden disconnection of a generator or an interconnector. The most likely time for the RoCoF to exceed
its limit occurs during the initial moments after an
event. To this end, the RoCoF is derived from the
power system swing equation and can be calculated
as the observed change in the system power multiplied by the nominal frequency and divided by
twice the postevent system inertia. At present, the
minimum and maximum RoCoF are monitored. The
minimum RoCoF could occur due to the trip of a
large generator or importing interconnector, and the
maximum RoCoF could result from the trip of an
exporting interconnector.
* System inertia: This criterion is valid for a synchronous system as a whole, but it can be used for part of
a larger synchronous system that is planned to operate as a separate synchronous system during a sys-

70
60

(%)

50
40
30
20

30 June

20 June

10 June

31 May

21 May

11 May

1 May

21 April

1 April

11 April

figure 3. The percentage of the SNSP in Ireland's total
generation in April-June 2019. (Source: Eirgrid.)
42

ieee power & energy magazine

tem separation (e.g., following a severe frequency incident). Monitoring system inertia from synchronous
generation ensures that operators always maintain a
minimum number of conventional generating units
synchronized to the power system. At present, the
minimum level of inertia for the Ireland and Northern Ireland system is 23,000 MW (see " Operational
Constraints Update " in the " For Further Reading "
section). It can be provided by a minimum number of
conventional generating units to maintain
- enough inertia to support frequency control and
stability
- sufficient synchronous torque to maintain the system strength and transient stability
- reactive power support to manage the voltage control and stability; for that purpose, units should
be selected in voltage-critical parts of the system.
* Active power operational reserve: This is a classical requirement for a rotating reserve that can be
complemented by new technologies (e.g., batteries).
* Available megawatt ramping: This is an assessment
of the total available ramping capability of an online
plant and an offline plant across various time horizons (e.g., 3 h, 8 h, and so on).

Impact of an Extremely High
Penetration of Renewable Sources
on Operational Security
The Effects of Resource Variability
Renewable resource variability has a profound impact on
operational security. Figure 3 displays the percentage of
nonsynchronous generation (mostly wind) on the island of
Ireland. The SNSP is shown in the percentage of the total
generation for April-June 2019. It is seen that such generation is extremely volatile and changes randomly from almost
zero to 65%. This means that the scheduling of the conventional plant will be significantly different from day to day
and hour to hour, causing the plant to cycle and even stop
and start much more often than during the past. More frequent cycling and operation outside the plant's most efficient
range (which is normally near the plant's full rated output
minus the reserve provision) reduces efficiency and leads to
increased wear, and thus it can reduce reliability. It is worth
mentioning that Europe is working to improve the flexibility
of existing and new gas and steam turbines and extend the
equipment's lifecycle to meet the requirements of grids that
march/april 2021

IEEE Power & Energy Magazine - March/April 2021

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