IEEE Power & Energy Magazine - March/April 2017 - 72

As we add more VRE into power systems, the grid of the
future will have many more inverter-based generators
and be much more distributed than the current power system.
only reactive power but can also provide the inertia and
fault characteristics of synchronous generators. Denmark
has recently installed several of these systems to provide
both fault current and inertia to the grid, with good results.
Other options may be to eliminate the use of overcurrent
protection schemes and develop
and implement more advanced
protection schemes that use cur230
rent differential or other methods
to detect and clear faults. Besides
220
fault current provision, synchronous condensers bring two other
210
important benefits to the system:
200
mechanical inertia and voltage
control. Many integration studies
190
have looked into the possibility of
converting large, retiring thermal
180
plants into synchronous condens170
ers for the above purposes.
0
200
400
600
800
1,000
1,200
1,400
Another protection scheme also
Relative Time (s)
used in the grid is based on speAvailable MW
Commanded MW
cial relays that measure the rate of
Minimum Allowed MW
Measured MW
change of frequency (ROCOF).
The controllers in ROCOF relays
figure 11. PV plant participation in AGC.
examine the derivative of the frequency to determine if a fault is
occurring on the grid. With less
system inertia, the rapid decline or
12
rise in frequency during an abnormal event may require changes to
10
the ROCOF settings or even the
Synchronous Generator's Fault Current Is 6x Rated
elimination of their use.
Power (MW)

be designed to continue to provide current for several cycles
through a fault condition.
One possible solution for the lack of high fault currents
is again the use of synchronous condensers to provide fault
current. Synchronous condensers are machines that provide

8
6

Inverter that
Disconnects
as Quickly as
Possible

Synchronous Generator
Inverter
Inverter with Ride-Through

4
2
0
-2
-4

Inverter that
Provides Seven Cycles
of Ride-Through

figure 12. Fault currents compared to time for a synchronous generator, an inverter
with rapid disconnect, and an inverter with ride-through capability.
72

ieee power & energy magazine

Unintentional
Islanding
The risk of unintentional islanding
is an important consideration when
examining power systems with
imbedded distributed generation.
As power systems become much
more distributed in nature, there
are potentially many more parts of
the grid that could create electrical islands during faults. The solution at low penetrations has been
to require active anti-islanding
techniques such as those used in
march/april 2017



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2017

IEEE Power & Energy Magazine - March/April 2017 - Cover1
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IEEE Power & Energy Magazine - March/April 2017 - Cover3
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