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1. GRID INERTIA
WHY IT
MATTERS IN A
RENEWABLE
WORLD

2. •General Electric will demolish the 750 MW Inland
Empire Energy Center (IEEC) in California that has
20 years remaining in its useful life. With solar and
wind dominating the grid, the plant has been
deemed uneconomical after operating well below
capacity for several years. The site will be used for a
new battery storage facility.
•Good news in terms of a zero-carbon future, right?
Maybe not. A rush to retire such units may impair
the ability of the grid to accept more solar and wind
resources in the future. Why? It’s all about a factor

3. • A power network without inertia is one that is unstable, suffers
from issues of power quality and is susceptible to blackouts.
The primary mechanism for providing inertia is via the presence
of heavy rotating equipment such as steam turbines and gas
turbines driving generators and rotating generators.
• Efforts to decommission such equipment and replace them with
renewable resources, while well intended, could inadvertently
hamper the creation of the robust and reliable renewable grid of
the future. Additionally, failure to invest in aging
turbomachinery in an effort to achieve environmental targets
could backfire. Operators could be forced to continue to
operate dirty generating resources to provide grid stability and
inertia when a small upgrade could greatly reduce emissions

4. NEWTON AND GRID INERTIA
• Sir Isaac Newton noted that unless acted upon by an
external force, an object at rest remains at rest and an
object in motion continues to move in a straight line with
constant speed. Another way to state this might be
resistance to change.
• In an electric system, the energy contained in generators
and motors at power stations and industrial facilities
provides inertia as they rotate at the same frequency as the
electricity grid. This effectively acts as a buffer against
rapid change. If demand for power spikes, the frequency of
the grid tends to decrease. Having a lot of rotating mass on

5. • Solar, on the other hand, is connected to the grid without
rotating mass. Even massive wind turbines fail to provide
the necessary stability as they are not directly connected
to the grid. Instead, a frequency converter between the
wind turbine and electricity grid prevents the kinetic
energy of the wind turbine’s rotating mass from providing
inertia during periods of frequency change.
• “When inertia decreases, sudden changes in frequency
caused by a change in electricity consumption or
production are faster and larger,” said Minna Laasonen,
senior advisor at Fingrid, the transmission operator in
Finland. “This means that it is more difficult to keep the

6. • What’s the big deal? A surge of renewables onto a grid
without sufficient rotating mass could cause serious
problems: power being cut in certain areas in an effort to
bring demand back in line with supply; and large power
plants getting disconnected from the grid to prevent them
becoming overloaded.
• The key to understanding this is frequency i.e. the speed
of the grid. Some parts of the world such as the U.S.
operate at a frequency of 60 Hz. Other parts operate at 50
Hz. Taking a simplified view of things, this is a measure of
how fast electrons are moving along an alternating current
wire. 60 times a second (60 Hz) or fifty times a second (50
Hz) is the frequency of the grid. If it rises too much above

7. • Looking on a smaller scale, household fuses and circuit
breakers are there to prevent a frequency overload. Operate too
many appliances, devices and gadgets on one circuit and
everything is shut down. This prevents damage to equipment
and wiring.
• It’s the same on a power network. If everyone turns on their
energy hungry devices (air conditioning, heating, etc.) at the
same time, frequency drops. If there’s more supply than
demand, frequency rises. System operators, then, are engaged
in a constant frequency balancing act. In extreme cases,
utilities lighten the load to avoid damaging grid equipment by
disconnecting neighborhoods. This remedial step might keep
rest of the network in operation. But those in the disconnected

8. • You can also get a domino effect. If frequency goes out of control,
one part of the system has to shut down. This causes severe strain
on the rest of the network. If not dealt with, cascading outages lead
to a major blackout such as that experienced in the Northeast of the
U.S. in 2003. 60 million people ended up without electricity. In the
summer of 2019, major blackouts in New York City and the U.K.
further emphasize the need for greater grid resilience.
• Donald Chamberlin, a retired electrical engineer who worked for a
utility in New England for 42 years, explained some of the
drawbacks in how the grid is evolving. If power is mainly coming
from solar panels and wind farms, and local generating facilities are
taken off line, it certainly lowers emissions. But it also removes the
necessary sources of reactive power.
• “Without enough reactive power, transmission capacity is reduced,

9. REACTIVE POWER
• Electricity is a complex subject. And one of the more obscure aspects
is the difference between real and reactive power. Real power (or
effective power) delivers energy from the generation source to the load
and is measured in volts, amps and watts.
• Reactive power, on the other hand, does no actual work. It is measured
in volt amperes reactive (VArs). It is the form of electricity which
creates or is stored in the magnetic field surrounding a piece of
equipment. Reactive power can be positive or negative. The amount of
current in a device impacts the amount of reactive power needed. If you
double the amount of power being consumed in an area, the reactive
power consumed quadruples. Reactive power consumption, therefore,
is a vital aspect of managing the network. This is typically done by
adding reactive compensating devices.

10. • Another factor is that reactive power does not travel as
far as real power. When the generator is near the load, the
same power generators that supply real current can
supply reactive current. Long transmission lines
operating at heavy loads consume VArs. This leads to
conductor heating and voltages falling.
• Reactive current, therefore, is best provided by sources
close to power loads to reduce the amount of reactive
current that has to be carried by the delivery system. A
lower reactive current demand on the delivery system
allows it to carry more real current. This helps the utility
to maintain its service voltage within required limits.

11. •Reactive power devices, then, must be placed nearer
the load to correct the power factor and avoid damage
to equipment. Low voltage can cause electric system
instability or collapse, damage to motors and the
failure of electronic equipment. High voltage can
exceed the insulation capabilities of equipment and
cause dangerous electric arcs.

12. A VARIETY OF TECHNOLOGIES ARE USED TO
STABILIZE VOLTAGE AND PREVENT ITS DECAY OR
COLLAPSE.
THESE INCLUDE:
CAPACITORS
•Capacitor banks can supply reactive power when
needed, but cannot absorb it. This means they can
supply lagging VArs only. This limits their role in
voltage regulation. One advantage is that they are
relatively inexpensive and easy to maintain.

13. STATIC VAR COMPENSATORS
Static VAr compensators are really higher-tech
capacitors; i.e. they are electronically switched with
instantly acting solid-state devices. They experience
severe output reduction under depressed voltage
conditions since their output is a function of the
square of the voltage at their terminals. Capacitors and
Static VAr compensators should always play a role in
grid stability but they are not enough.

14. SYNCHRONOUS CONDENSERS
• The term “condenser” is applied to rotating machines that
only supply reactive current. Unlike capacitors and static
VAr compensators, synchronous condensers are dynamic
sources as their output can change quickly to match
reactive power need. Since condensers are large rotating
generators, they add stored energy in the form of inertia
to the electric system. This property is useful in handling
transient conditions such as temporary short circuits and
momentary disruptions. This inertia is especially useful
for low inertia power sources such as photovoltaic cells
and wind turbines.

15. • Another advantage to using generators on the grid is that they
can be adapted to produce both reactive and real power as
needed. If the generator is needed suddenly for peaking power,
it can provide it rapidly. Otherwise, it is used to maintain the
proper voltage by supplying reactive power. Most generators
already have automatic voltage regulators that cause the
reactive power output to increase or decrease to control
voltages: putting lagging VArs onto the system under
conditions of low voltage/heavy load and absorbing leading
VArs under conditions of high voltage/light load.
• The ability to switch from peaking generator to synchronous
condenser is achieved by placing a synchronous self-shifting
(SSS) clutch between the turbine and generator. When the
power turbine is shut down, the clutch automatically

16. •“With the introduction of large-scale wind
farms whose power output can vary widely, it is
important to react quickly to changing
conditions,” said Chamberlin. “Wind turbine
generators are built to be lightweight with low
inertia, adding to the need for the inertial
properties of synchronous condensers.”

17. PLAN WISELY
• With wind and solar flooding onto the grid, and coal and natural
gas power plants retirements being announced on a regular
basis, there is a desire to decommission these units as rapidly as
possible. As a symbol of a new era, it may seem prudent to
flatten aging plants and erect battery storage facilities in their
place, as is being proposed in California. Yet such a strategy may
be short-sighted.
• The grid must be stable and controllable. The rush to add more
wind and solar without accounting for reactive power resources
lowers grid resilience. Retired generators and rarely used
peaking units can each supply hundreds of MegaVArs. Since
they have already been paid for, capital costs are negligible

18. •When plans are being drawn up to close
aging or poorly utilized facilities, therefore,
it may be wise to evaluate the benefit of
using its assets for synchronous
condensing. If the equipment doesn’t
include a clutch, it can be retrofitted at low
cost.