4. 4
LMS100 Flexibility for Modern Power Systems
Introduction
The LMS100, now rated at up to 116 MW in power output,
was introduced into commercial operation in 2006. It is
the largest gas turbine in GE’s aeroderivative product
portfolio and is the most efficient open cycle gas turbine
available to today’s power generation industry. In addition
to high efficiency—especially important for continuous duty
applications where fuel cost is a major factor—the LMS100
is designed with features that make it suitable for cyclic
operation. The LMS100 is able to start, stop, and move
power output quickly and frequently with reduced impact
upon maintenance cost, all while maintaining emissions to
strict standards.
The LMS100 achieves its excellent efficiency by operating
at a high overall pressure ratio (up to 43:1) and a firing
temperature in excess of 2,500°F. These operational
characteristics are enabled by the use of an off-engine
intercooler which cools the air between the low- and
high-pressure compressors.
The LMS100 utilizes advanced technologies from GE’s
heavy-duty frame gas turbines as well as GE’s industry
leading aircraft engines. By incorporating an aeroderivative
core engine and a lightweight/high-strength power turbine,
the engine is able to undergo large thermal and mechanical
stress changes without a significant detrimental impact
upon the hardware.
Modern power grids incorporate a high proportion of
intermittent renewable energy sources (wind and solar) that
cause special problems in maintaining grid reliability. Power
generators have always been required to adjust output
quickly in order to match the demand upon the system,
but with the introduction of a high proportion of wind and
solar energy the “net load” that the “other” energy sources
are required to respond to can change much more rapidly
and over much wider ranges than previously. This has been
well documented by California ISO with publication of their
“duck curve.”
Ideally, the steep load ramps that the non-wind and solar
sources are required to respond to would be fully satisfied
by (1) demand management; (2) energy storage; and (3) the
most efficient fossil fuel sources, which are combined cycle
natural gas power plants. Unfortunately, to fully equip a
system in this way to achieve the required level of reliability
would be cost prohibitive, and likely result in a poor overall
environmental impact. Thus the problem of satisfying the
most severe and prolonged ramps in the system net load
can be solved by having simple-cycle gas turbines available
with the ability to start and stop quickly, and move output
at a very fast rate. With high-efficiency, excellent hot-day
power output, fast starts and the ability to provide very fast
load ramps, the LMS100 is the ideal solution to many of the
power grid reliability issues.
Abstract
Modern electrical power systems are facing unprecedented challenges from the introduction of increasing quantities of
wind and solar energy. Not only can these renewable energy sources be dispersed and remote from where the power
is needed, the renewable energy generation is typically highly variable with the time of day, month of the year, and overall
weather patterns.
In order to justify the high up-front cost of installing renewable generation, as well as to take advantage of the low operating
costs, the balance of the power system must be designed to accommodate them in both normal operation, and when things
start to go wrong. With the ability to start quickly, change output over a wide power range rapidly, burn fuel more efficiently
than any machine in its class, and stay in emissions compliance to the most stringent levels, customers and system
operators are finding the LMS100 the ideal solution to their power grid reliability needs.
Nowhere is this better illustrated than California, a state with a renewable energy target of 33 percent by 2020, and 50
percent by 2030. In this state alone, 23 LMS100 gas turbine generators have been installed over a four year period, with an
additional five units currently in construction and more sites in the planning phase. The LMS100 is truly seen as a facilitator
to enable California to meet its ambitious renewable energy goals.
5. 5
LMS100 Flexibility for Modern Power Systems
Satisfying the need for fast power
The North American Electric Reliability Corporation (NERC)
is responsible for the reliability of the bulk power grid
and ensuring compliance with the mandatory reliability
standards. NERC publishes guidelines to assist the
responsible entities, typically Balancing Authorities (BA’s), in
planning for and satisfying their reliability obligations. Part
of the reliability obligation of a BA is to maintain sufficient
reserves to handle normal, yet unpredictable variations
in load, as well as contingency events, such as the loss of
the largest generating unit in their area of responsibility.
Thus the Operating Reserves are split between Regulating
Reserves and Contingency Reserves.
Regulating Reserves are generally “spinning” (meaning
a generator is already synchronized to the grid), and
Contingency Reserves may be “spinning” or “non-spinning.”
In nearly all cases, whether spinning or non-spinning,
Contingency Reserves are currently defined as the power
available within 10 minutes of being requested.
Being obligated to the 10-minute start time requirement to
qualify as a “Non-Spinning Contingency Reserve,” GE’s gas
turbine products are designed to start and go to full output
within a 10-minute window whenever possible without
detrimental impact to the life of the equipment. All of GE’s
aeroderivative products, including the LMS100, offer a start
to full output inside 10 minutes, sometimes significantly less.
In order to satisfy fire protection safety codes all
gas turbines must, prior to “light-off,” satisfy certain
requirements so that no combustible gases are trapped in
the exhaust system. There are several ways to accomplish
this, and typically the most efficient way to do this for GE's
aeroderivative products is to conduct a “duct purge” and
“shutdown bottle test” that provide “purge credit” for an
8-day window, a credit that can be continuously renewed
by a simple unfired motoring of the core engine with its
starter motor each 8-days. With purge credit (or alternative
methods to meet the fire protection codes) the LMS100 can
be reliably started, synchronized and loaded to full output
within the 10-minute contingency reserve time period. It
should also be noted that GE is now offering an 8-minute
start to full load without parts life impact, and less than this
is possible with some impact.
Figure 1. LMS100 Five-Unit Start Demonstration.
Figure 1 shows the actual performance of a five-unit LMS100
site where the plant control system was configured to start
all units at the same time. As shown by this data, the output
actually selected that day was available in 9 minutes, with
full output being available inside 10 minutes.
GE's LMS100, first introduced in 2005.
6. 6
LMS100 Flexibility for Modern Power Systems
The importance of moving output quickly
Balancing Authorities are required to contribute, in
proportion to their size, to the maintenance of frequency
on the power grid. This is done by changing the output of
generating units in the upward and downward direction;
upward if system load is increasing, downward if system
load is decreasing. If the power generating unit is not on
a large power grid, but on an isolated system operating in
“island mode” (such as a mining operation or data center)
then the ability of the generator to move output quickly and
precisely is of even greater importance.
To have upward regulation ability the generator must be
initially set at a level below full output such that there is
“headroom” for upward regulation. All gas turbines lose
efficiency as output is reduced—but all gas turbines are not
equal in this regard. By virtue of being a very highly efficient,
high pressure ratio, three-shaft machine, with seven
variable geometry compressor airfoil stages, the LMS100
operates with superior part-load efficiency.
“Pmin
” is the term used to define the minimum steady-state
power that a generator can be “parked” at. This is typically
set by the lowest power at which the machine will satisfy
the EPA’s requirements for “criteria pollutant” emissions,
and at Pmin
the pollutant that is most challenging to meet
is carbon monoxide (CO), this being inversely proportional
to the power setting. The LMS100 can typically operate
at a Pmin
of about 25 MW (~75% below full output) while
satisfying stringent CO emissions requirements without
installation of excessive amounts of exhaust system
oxidation catalyst. Two customers use the LMS100 with a
Pmin
set as low as 17 MW.
While the machine is regulating power (upwards and
downwards) the exhaust emissions need to stay in
compliance with the EPA’s requirements, as specified in
the site’s air permit. In the case of GE’s SAC (Single Angular
Combustor) aeroderivative machines, depending upon the
site location, exhaust NOx
control is achieved by controlling
combustor flame temperatures with water injection, and
where regulations require it, further reducing NOx
with an
exhaust Selective Catalytic Reduction (SCR) unit. The SCR
utilizes a diluted ammonia solution to react the NOx
into
nitrogen and water vapor, both being normal constituents of
air. The amount of ammonia vapor fed to the unit needs
to be appropriate to the power setting and fuel flow rate,
so maintaining compliance with the NOx
requirements
requires changing power on the unit at a rate that the
SCR ammonia injection control can match. The LMS100
can continuously maintain the most stringent standards
of exhaust NOx
compliance while changing loads at a rate
in excess of 50 MW/min. As will be discussed, the LMS100
has the capability to move much faster than 50 MW/min,
but this might result in exhaust NOx
being briefly outside
the required standards. Since NOx
and CO compliance are
generally monitored on a one-hour rolling average basis
this is not likely to be a problem unless the unit is being
subjected to continuous and severe load changes over this
one-hour average emissions reporting period.
7. 7
LMS100 Flexibility for Modern Power Systems
Figure 2. MW Output Vs Time in Automatic Generation Control.
Figure 2 shows an example of an LMS100 operating in
automatic generation control (AGC) with a Pmin
set at
35 MW. This shows the amazing ramping capability of
the LMS100. Ramping of an LMS100 in AGC is normally
conducted at about 50 MW/min, although there is flexibility
in this number to suit customer needs.
As far as maximum ramp rates for contingency events
are concerned—meaning what the unit can really do if
called upon by a severe system disturbance—the LMS100’s
heritage as an aircraft-engine derived gas turbine has
clear advantages. Aircraft engines are required to be able
to make rapid and large power changes. Large power
output changes in both upward and downward directions
can be made in seconds, and despite the increased inertia
of an LMS100 power generation system compared to an
aircraft engine, the LMS100 can also move extremely fast
when necessary.
Figure 3. LMS100 Operation in Fast frequency Response.
Figure 3 shows an example of a very rapid movement in
the output of an LMS100 unit when called upon to change
power in response to an external signal (in this case a
false frequency injection test). In this real-life example,
at the time 17 seconds, the unit has been commanded to
full output outside the normal MW ramp rate, and just 6
seconds later the output is 67 MW, for an average ramp rate
over this period of 500 MW/min. Peak output in this case
was 100 MW, occurring approximately 24 seconds after
the initial signal was input to the control system. Similarly,
very rapid ramp rates can be achieved in the downward
direction, as would be required in response to a sudden loss
of load on an isolated power system.
Power output changes of the rate and magnitude shown
in Figure 3 are very unusual for a power generation
system, and if conducted on an occasional basis would
not be expected to have any significant equipment life
impact on an LMS100. If conducted on a daily basis some
life impact could be expected, and GE would review this
for the entire gas turbine and generator system. Thus
GE would not expect these rapid power excursions to
be recorded for parts life tracking unless they became a
regular operating mode of the equipment. If they were to
become a normal operating mode GE would request details
of the expected operational profile and would conduct an
engineering assessment. Normal parts life tracking for the
LMS100 consists of recording start/stop cycles and large
power output changes, and then factoring them against
the nominal 10,000 cycle published life of the rotating
structures of the engine.
8. 8
LMS100 Flexibility for Modern Power Systems
Spinning Reserve Capability
We have already discussed the ability of the LMS100
to operate at a low Pmin
while maintaining emissions
compliance with better-than-typical fuel efficiency. The
“headroom” available to full output can be used to support
the operator’s Regulating Reserve requirement, or their
Spinning Contingency Reserve requirement. The utilization
of an LMS100 in this way will allow combined cycle power
plants in the Balancing Authority (BA) to operate at full
output and peak efficiency, thus enhancing overall system
efficiency and reducing overall system emissions. If the
LMS100 is to be utilized on any particular day to satisfy the
spinning reserve requirement, there are additional options
available to enhance its contribution.
The installation of a “synchronous condensing clutch” will
allow an LMS100 generator to remain synchronized to a
grid and effectively operating as a synchronous motor
with the gas turbine “de-clutched” and shut down. Thus
the gas turbine is using zero fuel (hence zero emissions)
and electrical power consumption from parasitic loads is
low. Normally, synchronous condensers are operated in
this manner to enable “voltage support” of a section of the
power grid; the excitation of the “motor” field is adjusted
to correct the grid power factor. Even if the gas turbine/
generator is in normal power generation mode it can be
utilized for power factor correction, but in the synchronous
condenser mode this capability is enhanced.
A side-benefit to operating an LMS100 in synchronous
condensing mode is that it can be used to satisfy the
Spinning Reserve requirements of the BA. Per NERC
(Glossary of Terms Used in NERC Reliability Standards,
Updated September 24, 2014):
“Operating Reserve – Spinning Generation synchronized
to the system and fully available to serve load within
the Disturbance Recovery Period following the
contingency event.”
The LMS100 has demonstrated the ability to transition from
the synchronous condensing mode to full power generation
output within 10 minutes of being called to do so; thus the
full output of the machine is able to qualify as spinning
reserve while there is zero fuel consumption and reduced
electrical load. This offers an excellent cost savings (in terms
of fuel consumption, emissions production and wear and
tear of equipment) over operating less capable generating
units to meet spinning reserve requirements.
If a synchronous condensing clutch is not installed
almost the full output capability of the LMS100 can be
made available by leaving the gas turbine operating
at “synchronous idle” with the breaker closed and the
power output set at minimum load, about 2 to 3 MW. In
this condition, once the exhaust system oxidation and
SCR catalysts have been allowed to reach operating
temperature, the CO emissions will be at a level that meets
the most stringent permit limits, and the “unabated” (i.e.,
without water injection) gas turbine NOx
levels will be low
enough to allow the SCR to bring the exhaust stack levels
within permit limits. The fuel flow at this condition will
be approximately 6,500 lb/hr of natural gas. As the Pmin
is
raised above 3 MW the efficiency of the unit will increase
rapidly, meaning the fuel consumption will not increase in
proportion to the power output.
The ability to change fuel type
The starting and normal load ramping capabilities of the
LMS100 are very similar when operating on either natural
gas or liquid fuel, the liquid fuel typically being diesel or
kerosene. Normally liquid fuel is utilized as a back-up fuel
to natural gas, but two LMS100 installations rely solely
on diesel fuel, as do many of GE’s LM2500 and LM6000
aeroderivative units. When the LMS100 is equipped with a
dual fuel system, online transfers between fuel types can be
made in either direction (gas to liquid, or liquid to gas), at any
level of power output, up to full load. Some minor variation
in output may be expected during the fuel transfer process.
The ability to operate on liquid fuel is especially important
in the NE USA where customers are subject to wintertime
gas curtailments.
9. 9
LMS100 Flexibility for Modern Power Systems
Low emissions without using water
The LMS100 version discussed so far in this article
utilizes water injection to the combustor to control flame
temperature for NOx
emissions control. This version of the
gas turbine is the “LMS100PA.” An alternative approach to
NOx
control that does not require water injection is to use
a staged burner system to control flame temperature within
precise limits. This is the “LMS100PB,” of which two units
are operating very successfully, and have been since the fall
of 2013. At this time the LMS100PB model does not allow
control of CO emissions to the more stringent regulatory
standards below approximately 75 MW, which is generally
considered too high in Pmin
for a frequency regulation unit
in the USA. However, recent combustor development rig
testing has shown very favorable results, and GE is able to
offer a 50 MW Pmin
with stack CO emissions at 4 ppm for
the LMS100PB model for delivery in 2018. At this time there
are no plans to offer dual fuel capability for the LMS100PB
model, but dual fuel capability will remain for the LMS100PA
model, as well as the newly announced LMS100PA+, which
offers a 10% power uprate over the LMS100PA.
Choice available on generators
The LMS100 is available with a choice of generators. The
two models currently offered have different inertia and
capability curves, as well as options on cooling systems, to
suit different customer requirements.
Customers anticipating use in “island mode” should
carefully review the different load accept capabilities
of the two generators against their anticipated use. GE
has material available to assist in this assessment. The
generators also have different load rejection characteristics,
and although this is generally not a concern item, this
should still be discussed with GE, especially if the unit is to
be used in island mode operation since it can influence the
configuration of other plant systems.
Inherent to the inertia difference between the generator
models is the amount by which they support “system
inertia.” Having a higher inertia may be of additional value
when the LMS100 is employed on a small grid in which it is a
significant proportion of the total generation available.
Conclusion
The LMS100 has demonstrated outstanding flexibility
characteristics to support modern power generation
systems. As the most efficient open cycle gas turbine
power generation system available, the LMS100 can
consistently start rapidly, start and stop as frequently
as required, and ramp the power output up and down
very rapidly with reduced impact upon maintenance and
emissions. As power grid codes are evolving in response
to the impact of the growth in wind and solar energy the
LMS100 is finding a unique position in its ability to support
the reliability standards required by the evolving power
generation system.
10. 10
LMS100 Flexibility for Modern Power Systems
Philip F. Tinne
LMS100 Technical Sales Support Leader
GE Power & Water’s Distributed Power business
A mechanical engineer by training, Phil has over 37 years of experience in the gas turbine and power generation industries.
Starting his career with Rolls-Royce in England, Phil soon moved to the USA and has held a variety of positions in both
the GE Aviation and GE Power & Water businesses. Roles have included performance engineering, flight test, systems
engineering, sales and marketing and product management. After leading the LMS100 test program as Test Director, Phil
then became responsible for developing improved installation and commissioning processes as well as assuring product
reliability growth. After introduction of the first 26 LMS100 power plants to commercial operation, Phil assumed the role of
the technical sales support leader for the product line, helping the sales expansion to continue. Phil’s credentials include a
B.Sc. in Mechanical Engineering from Liverpool Polytechnic and an MBA from Xavier University.
About the author
