POWER September 2020 - 40

PLANT EQUIPMENT
Putting Idle Turbine
Generators to Work
As more wind and solar generation is added to the power grid and large coal and
nuclear plants are retired, a reduction in reactive power is becoming a concern.
Underutilized gas and steam turbines can be retrofitted to offer synchronous
condensing capability, which can benefit both owners and grid operators.
Morgan Hendry
T
he many gas turbines and combined
cycle power plants deployed
in the 1990s and 2000s were expected
to enjoy life for many more years.
Yet, changes in policy and competition
from renewables have led to some units
being mothballed. In other cases, utilization
rates fall far short of those originally
envisioned. Instead of combined
cycle plants operating at baseload, they
find themselves shifted over to peaking
operation to rapidly come into action at
times when renewable resources are insufficient
to satisfy demand.
At times, these plants are hardly utilized
at all. In Germany, for example, the
Irsching Power Station was touted as a
world record holder for efficiency when
it installed new Siemens H-class units
in 2011. But German energy policy has
largely sidelined this combined cycle
facility. It loses millions of euros every
year. The utility has made several requests
to close operation at one or more
units as it receives little compensation to
have them sitting idle as standby power.
As for peaking gas turbines, many are
minimally used. Understandably, some
utilities and independent power producers
have been wondering what to do with
these assets. Shuttering is an expensive
option, both in terms of stranded assets
and demolition costs.
A better alternative may be to retain
these plants and repurpose the generators
of idle turbines as synchronous
condensers. Failure to do so could have
serious negative repercussions for the
grid. Why? Remove too many gas and
steam turbines from service and disruption
is inevitable. Power quality will drop,
and a lack of inertia will lead to instability
and potential blackouts.
Synchronous condenser conversions
have been done successfully in many areas.
In the U.S., there are a great many
examples. The Los Angeles Department
of Water and Power (LADWP) has four
40
GE LMS100 gas turbines operating primarily
as synchronous condensers but
poised to provide generation when necessary.
Numerous steam turbine generators
have been repurposed throughout
North America too, such as four 150-MW
units at BC Hydro's Burrard Generating
Station near Vancouver, British Columbia,
Canada. Around the world, there are similar
synchronous condensing examples in
countries such as the UK, Sweden, Germany,
Mexico, Canada, Morocco, Kenya,
South Africa, Iraq, Jordan, Bangladesh,
New Zealand, Australia and Ecuador.
Grid Inertia
It is important to appreciate why such
conversions should be done and how to
go about it. Generators and motors provide
inertia as they rotate at the same
frequency as the electricity grid. Thus,
the presence of gas and steam turbines
acts as a buffer against power spikes and
changes in frequency. But with wind and
solar supplanting traditional energy sources,
the grid is losing inertia. Wind doesn't
provide it, as frequency converters placed
between the wind turbine and the electricity
grid prevent the kinetic energy of
their rotating mass from providing inertia.
During the peak evening period, frequency
drops when people return from
work and turn on air conditioning, heating,
lighting, and appliances. At other periods
of the day, frequency rises if there
is an abundance of supply compared to
demand. In other words, there is a constant
balancing act taking place as system
operators try to maintain frequency
in the correct range (60 Hz for the U.S.).
If things get out of hand, utilities might
have to conduct load shedding, that is, disconnecting
neighborhoods from the grid
to avoid damaging equipment and to keep
the rest of the network in operation. In extreme
cases, cascading outages can lead
to a major blackout. The Northeast U.S. experience
just such an event in 2003, leaving
60 million people without power. On a
www.powermag.com
1. A synchronous self-shifting (SSS) clutch
is a sophisticated freewheel gear-type clutch
that engages and disengages automatically
without the use of any external control system.
Put simply, when the input side rotates
faster than the output side, the SSS clutch
will engage. Conversely, when the output
side rotates faster than the input side, the
clutch will disengage and overrun or freewheel.
Courtesy: SSS Clutch
smaller scale, New York City and London
endured blackouts during 2019.
The key concept to understand here is
the difference between real power and reactive
power. Real power (also known as
effective power) delivers energy from the
generation source to the load. It is measured
in volts, amps, and watts. Reactive
power, measured in volt-amperes reactive
(Vars), does no actual work. It could
be regarded as the form of electricity that
creates or is stored in the magnetic field
surrounding a piece of equipment.
Reactive power has several qualities. It
can be positive or negative. Doubling the
amount of power consumed, quadruples
the amount of reactive power needed.
Additionally, reactive power does not travel
well. Long transmission lines operating
at heavy loads consume Vars. Failure to
address reactive power on these lines
can lead to conductor heating and voltPOWER
| September 2020
http://www.powermag.com
POWER September 2020

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