IEEE Power & Energy Magazine - March/April 2021 - 44

Synchronizing Power

generator starts slipping poles) with an increasing penetration of wind generation. Based on this estimation and the
fact that clearance time is inversely related to synchronizing
power, the latter can be presented as in Figure 5. It is seen
from the figure that the rotor angle stability of the system
slightly improves until the wind penetration increases by up
to 50%. Then, it starts declining, and it quickly deteriorates
at wind penetration levels of 80% and higher. Therefore,
monitoring the remaining synchronous plants' transient

20
40
60
80
Nonsynchronous Plant Penetration (%)

100

figure 5. The synchronizing power versus nonsynchronous
plant penetration level (derived from the critical clearance
time studies in " All Island TSO Facilitation of Renewables " ;
see Eirgrid 2010 in the " For Further Reading " section).

V
A
B

security (i.e., rotor angle stability) in real time will be even
more critical with higher levels of renewable penetration.
To investigate how the transient security of nonsynchronous
plants (the ability to ride through a three-phase fault) evolves
with increasing renewable generation, consider how threephase fault characteristics evolve. The most economical way of
connecting renewable sources to a synchronous system is via
power electronic converters. This has two main implications:
the mechanical system inertia is increasingly depleted when
renewable penetration increases, and the short circuit current
contribution from renewable sources is limited to their maximum load current. The first implication will affect a system's
frequency security; the second causes voltage dips associated
with faults in the transmission system to propagate more widely
within a system that has reduced short circuit levels. This has a
direct impact on the fault ride-through performance of nonsynchronous sources of generation. The bigger part of the system
sees a voltage dip, and a larger proportion of the nonsynchronous generation can trip on undervoltage protection.

Effect of Renewables on Voltage Stability
In the early days, renewable generators did not have any voltage and reactive power control capability. Soon, as renewable
nonsynchronous generators displaced conventional ones, it
was recognized that conventional generators' voltage control capabilities were being depleted. Grid codes began to be
updated to introduce at least some conventional generationlike functionality into a nonsynchronous generation portfolio.
For example, the Irish grid code requires that all transmission-connected wind power systems be able to control voltage
and/or reactive power at their terminals. The best existing
technology for this task involves connecting renewables to the
grid via a fully rated converter. This enables modulating
the grid-side, three-phase voltage according to the required
reactive power. However, most distribution-connected
renewable sources are still fixed-power-factor machines that
do not regulate voltage. This means that voltage regulation

P
(a)

45,000

40,000
Inertia (MW)

A′
B′

35,000
30,000
25,000

P
(b)

figure 6. The idealized power voltage characteristic of a
transmission corridor between two parts of an ac power
system with (a) a low penetration and (b) high penetration
of renewables.
44

ieee power & energy magazine

20,000

20
30
40
50
Wind Generation (%)

figure 7. System inertia reduction as the renewable penetration rises in the single synchronous power system of
Ireland and Northern Ireland (Source: Eirgrid).
march/april 2021

IEEE Power & Energy Magazine - March/April 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2021