IEEE Power & Energy Magazine - November/December 2015 - 84

Frequency Response and
Synthetic Inertial Control
Frequency response is the overall response of a power system
to small, routine fluctuations in frequency and also large,
sudden mismatches between generation and load that may
result from generation or transmission tripping offline. The
loss of a large central station generating plant is of most concern. When total demand exceeds total generation, system
frequency drops. Traditionally, the inertia of synchronous
machines helps retard the frequency decline, providing an
opportunity for generating units with governors to increase
power output to stabilize the system before a frequencybased disturbance could otherwise occur.
Among power system operators and utilities, there is a concern regarding the degradation of frequency response in North
America during the past two decades. The decline has resulted
from various factors, including the withdrawal of primary or
governor response shortly after an event, the lack of in-service
governors on conventional generation, and the unknown and
changing nature of load frequency characteristics. Large penetrations of inverter-based (or nonsynchronous) generation technologies further complicate this issue. Synchronous machines
always contribute to system inertia, and some fraction of the
synchronous generation in operation at any time has governor
controls enabled. By contrast, wind and PV plants, in different ways, can provide "synthetic system inertia" through a fast
frequency response (FFR) with a power electronic converter to
simulate inertial response, and a slower frequency response can
be provided through electronic governor action. When wind
and PV generation displace conventional synchronous generation, the mix of the remaining synchronous generators changes,
and there is the potential to adversely impact overall frequency
response if good engineering practice is not followed.

The impact of nonsynchronous generation on frequency
stability may appear more quickly in relatively small grids
with high penetrations of wind because a relatively low wind
capacity level can comprise a larger percentage of demand
than in a larger grid. For example, the combined Ireland and
Northern Ireland power system has developed a system of
nonsynchronous penetration (SNSP) ratio to help identify
operating limits. Eirgrid currently limits SNSP to less than
50%. In the future, EirGrid expects to raise that limit to 75%.
This is an issue that much larger systems will face in future
scenarios of high penetrations of wind and solar.
Recent research by the National Renewable Energy Laboratory and GE showed that systemwide frequency response
can be maintained with high levels of wind and solar generation when local stability, voltage, and thermal problems are
addressed using traditional transmission system reinforcements (e.g., transformers, shunt capacitors, and local lines).
The analysis also showed that the limited application of nontraditional but commercially available frequency-responsive
controls on wind, PVs, concentrating solar power plants, and
energy storage are equally effective at improving minimum
frequency and settling frequency and therefore overall frequency response. For example, Figure 4 shows the benefits
of dynamic response from VG by comparing the Western
Interconnection's frequency for several scenarios studied
in "Western Wind and Solar Integration Study Phase 3." All
traces are for the sudden trip of two of the Palo Verde generators (2,756 MW) under stressful light spring load conditions
when there is less additional generation available to respond.
In Figure 4(a), the blue base case has 26 GW of renewables.
Doubling the wind and solar production to more than 50 GW
(red high-mix case) leaves the characteristic of the system response to this large generation trip event essentially

60.1

60.1
Light Spring Base Case
Light Spring High-Mix

59.9
59.8
59.7

59.9
59.8
59.7
59.6

59.6
59.5

Light Spring High-Mix
Light Spring High-Mix with Governor
Controls on Utility-Scale PV Plants

60
Frequency (Hz)

Frequency (Hz)

60

0

10

20

30
Time (s)
(a)

40

50

60

59.5

0

10

20

30
Time (s)

40

50

60

(b)

figure 4. Western Electricity Coordinating Council frequency response to the loss of two Palo Verde units for Light Spring
conditions. (a) The base case compared to a high mix of wind and solar. (b) High-mix with and without frequency controls.
84

ieee power & energy magazine

november/december 2015



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