Grid inertia: why it matters in a renewable world

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 known as grid
inertia.

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 and improve the overall resilience of a
renewable-focused power network.

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 the grid acts like a
shock absorber and slows the rate of change.

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 frequency
within its normal range of variation.”

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 or below that, trouble
results.

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 area have to go without power. This is
known as load shedding.

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, voltage drops, power lines overheat, and blackouts can occur,” said
Chamberlin.

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.

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.

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.

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.

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.

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.

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 disengages once the
generator is synchronized with the grid.

“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.”

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 compared to the expense of adding
static VAr compensators.

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.

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