IEEE Power & Energy Magazine - March/April 2021 - 43

Effect of Renewables on Transient Stability
As defined in the " Operational Security Definitions and
Criteria " section, transient stability has two components:
synchronous plant rotor angle stability (as a set of synchronous machines connected to the system) and nonsynchronous
plant fault ride-through performance (as a set of converterbased energy sources connected to the system). To qualitatively demonstrate how rotor angle stability is affected by a
significant penetration of renewables, we will use the power
angle characteristic of a synchronous machine, as illustrated
in Figure 4.
The electromagnetic power of the machine changes sinusoidally with the rotor angle. In a steady state, the electromagnetic power of the machine is balanced by the mechanical power of the turbine (operating point A in Figure 4).
During a disturbance (power unbalance), the rotor angle
changes as the rotor speed deviates from the synchronous
speed. As a measure of rotor angle stability, we can use the
concept of " synchronizing torque. " It shows how much the
rotor angle changes for the same variation in the power from
the steady-state operating point of the machine. A higher
synchronizing torque gives more stability to the synchronous plant because it encourages synchronous machines to
spin together at the same frequency. As the machine gets
more loaded (the operating point moves from A to B), the
synchronizing torque diminishes. This decrease contributes
march/april 2021

to a reduction in the transient stability of the machine to
a disturbance that perturbs the rotor angle. That is, as the
machine moves to the new operating point, the forces trying
to resynchronize it to that operating point will be weaker.
Similarly, if the maximum electromagnetic power of the
machine decreases (see the orange curve in Figure 4), it has
the effect of decreasing the synchronizing power and results
in a reduction in the transient stability of the machine.
Now, consider what initially happens when only a small
portion of the synchronous generation is replaced by converter-based renewables. If the remaining synchronous
generators are less loaded than normal, their synchronizing
torque increases. This improves the transient stability of the
system. However, a further increase of renewables requires
a progressive decommitting of the conventional units. The
equivalent power angle characteristic will mirror the orange
curve in Figure 4, with operating at point C. The maximum
electromagnetic power of the synchronous machine will be
reduced because it will be more difficult to " push " power
from the remaining synchronous machines to the loads, due
to the increased reactance of the now longer paths and the
poorer voltage support from the now prevailing nonsynchronous generation. As discussed previously, having less
electromagnetic power will cause a reduction of the synchronizing torque, and at very high renewable penetration
levels, the transient stability deteriorates rapidly because
the remaining synchronous units are forced to be highly
loaded. Note, however, that recent advances in converterbased grid connection applications based on insulated gate
bipolar transistor technology will enable engineers to counteract these phenomena.
The " All Island TSO Facilitation of Renewables Study "
(see Eirgrid 2010 in the " For Further Reading " section)
estimated the reduction of the critical clearing time (the
maximum time allowed for protection to clear a three-phase
fault at the terminals of a synchronous generator before the

Increased Loading

B Maximum Electromagnetic Power
Power

have high levels of renewables. One example is the European
Union Flexturbine project (2016-2018).
Existing transmission and distribution networks were
designed using planning criteria based on a limited number
of power flow patterns. These patterns were derived from the
fundamental paradigm of unidirectional power flow from large
generators through the transmission system to the distribution
system. The latter has been considered " passive " and was
designed to convey electricity from bulk transmission system
supply points to loads. With the advent of renewable and other
distributed generation, this is no longer the case. Transmission and distribution systems now experience multidirectional
power flows, depending on the amount of instant renewable
generation in different parts of the system. The power flow
magnitude can vary, and so can its direction. When wind generation is low, power flows from the transmission system to the
distribution system, while during periods of high wind generation, energy may flow in the opposite direction, from the
distribution system to the transmission system.
It is well known that conventional power stations have been
built close to load centers. In contrast, renewable generation is
typically located far from load centers. Therefore, for example,
if there is a lot of wind generation, power flows can reverse and
reach significant values that were not originally anticipated for
the transmission circuits. Such circuits can be highly loaded,
and they can overload if there are outages in parallel circuits.
Therefore, thermal security must be monitored in real time to
avoid cascading outages due to overload conditions.

Normal Loading

Rotor Angle

figure 4. A power angle characteristic with normal and
increased loading (operating points A and B) and a reduced
power angle characteristic (operating point C).
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IEEE Power & Energy Magazine - March/April 2021

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