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Increase power and efficiency of LNG refrigeration compressor drivers Increase power and efficiency of LNG refrigeration compressor drivers
All-electric-driven plants can offer many benefits
Fritz Kleiner, Stefan Rausch and Juergen Knabe*
Most main refrigerant compressors in LNG liquefaction plants are driven by frame-
type gas turbines (GTs). At the high ambient temperatures prevailing at most LNG
plant locations, the rated output power of GTs must be boosted by electric motors.
These are the so-called helper drives, which at the same time serve as starter motors
for the GTs. Newer installations also utilize the same helper drives - in power ratings
up to 20 MW - in reverse operating mode as power generators in times when one of
the GTs has excess power.
As a logical consequence, major LNG plant operators and their contractors are looking into
upsizing these helper drives to fully rated electric motor drivers for the refrigerant
compressors, eliminating the GT. The first of these "all-electric-driven" LNG plants is being
designed in Germany. Many more are being considered for both land-based and floating
liquefaction plants.
With globally increasing natural gas consumption, constructing new or expanding existing
LNG plants is an attractive proposition for major oil and gas companies. Up to 40 new
world-class production trains are being planned or considered in addition to the roughly 75
trains operating or being constructed today. RealisticaIly, though, looking at today's market
potential, no more than two to three such trains are likely to be constructed every year for
the foreseeable future
All existing LNG plants are located onshore in mostly remote areas, for safety reasons, and
close to deepwater port facilities for the LNG tankers that take the finished product to their
user markets. Future LNG plants will also be constructed as floating production facilities
that are anchored close to the natural gas fields that provide their feedstock. Particularly in
these floating plants, the option of electric drivers for the refrigeration compressors is
attractive for safety reasons, efficiency and operating flexibility. But there are also significant
economic and operational advantages in onshore installations when this evolutionary
concept is employed. Converter-fed synchronous electric motors are the electric drivers of choice for these applications, starter-helper motors as well as main drivers for the
refrigerant compressors.
Starter-helper drives. Connecting a synchronous electric motor to the power system via a
frequency converter, often simply called "a drive" or "ASD" for adjustable speed drive, the
motor performs very differently from its line-connected brethren. There are no inrush
currents, no severe starting torque pulsations, no pulling out-of-step with overloading and
no acceleration problems - the behavior of a converter-fed motor becomes much like that of
a DC motor (or a steam turbine). Additionally, its speed is no longer fixed and dictated by
the power line frequency, it is adjustable and fully torque controlled - features that are
widely known in the oil and gas industry.
To a Iesser degree engineers are aware that these drives can be used without modification
to the power part in the four-quadrant operating mode. They can generate power as well
when driven by a suitable mechanical driver or be used as active brakes for the rotating
string. The only proven drive suitable for this application, and available in the power ratings
required for LNG plants, is the load commutated inverter (LCI) drive, employing simple and
reliable thyristors (SCRs) as power semiconductor switching elements.
A 2-pole brushless synchronous motor (Fig. 1), with laminated or solid cylindrical rotor, is
directly coupled to the compressor shaft and supplied with electric power from the LCI drive
to operate at the customary 3.600 rpm that the frame-type GTs and many of the
refrigeration compressors arc designed for. The motors are typically of the horizontal, totally
enclosed air-water-cooled (TEWAC) design in protection class E Ex(p) - pressurized for
operation in Zone 1 or Class 1 Div. 1 environments - and have modified brushless exciters
for LCI drive duty. Such motors have been in use aII over the industry for more than 20
years, and there is excellent operating experience in many applications. References are
available in output ratings up to 40 MW in the hydrocarbon industry.
To start up the liquefaction process, the GTs and refrigerant compressors must be brought
up to speed. Since the GT cannot start by itself, the helper motor accelerates the entire
rotating string comprised of GT-compressor(s)-motor from standstill to the firing GT speed.
Then the GT takes over speed control of the string. By supplying a steadily increasing
electric power with a constant volts-to-Hertz ratio to the motor, its speed is increased proportionally, whereby the acceleration ramp slope can be freely selected to match the
process and compressor starting needs. By virtue of the LCI principle and design of the LCI
drive, sufficient acceleration torque is always available at the motor shaft. The airgap torque
ripple produced by the LCI drive is dampened by the design and rotor inertia to uncritical
values at the compressor coupling.
Once the GT assumes mechanical power delivery and speed control of the rotating string,
the helper motor is available to boost the power output of the GT as needed - up to its
design rating.
To this end, the LCI drive remains synchronized with the motor and receives a torque
demand or speed reference signal from the GT control system, requesting the difference in
power that the compressor requires and the GT is momentarily able to supply. In modern
LCI drives, current supplied to the motor approximately equals torque delivered. By
controlling the input voltage to the LCI drive, its DC link current and, thereby, the torque is
adjusted.
Power balancing between trains. Originally, each starter-helper drive was assigned its
own frequency converter (drive) to keep the two compressor strings independent (e.g..
Oman). In later designs, the concept has been optimized using one LCI drive for two helper
motors, starting the two strings in sequence, and then using the helper function only as
needed. Realizing that power demand of the two refrigeration compressors is not normally
the same, but for standardization reasons their GT driven have equal power rating, there is
a clever possibility to make use of these differences in power output and demand of the
individual components.
In a first inscallation in Malaysia, a proprietary control concept has been implemented that
allows not only the exchange of electric power directly between the two helper motors
(generators) but also the feedback of excess power into the plant electrical system. This
allows optimizing the total electrical and mechanical power balance of the process train and
makes full use of individual equipment capabilities. Simplified block diagrams of the two
basic concepts illustrate these principles (Figs. 2 and 3).
The all-electric driven LNG plant. All gas turbines need regular maintenance. The
necessary shutdown periods, and unscheduled outages, interrupt LNG production and reduce plant productivity by as much as 15-20 days per year - to the tune of $1.5
million/year of lost revenue (plus maintenance costs). For three years, concerted efforts
have been underway by major LNG plant operators, contractors and manufacturers to
improve availability and energy efficiency of new plant designs to survive in the stiff
competition characteristic of this marker segment.
Already having large electric motor variable-speed drives as essential equipment in the
production process (the starter-helper drives), it appears logical to increase the rating of
these motors to a point where the GT is totally superfluous. and the motor powers the
refrigerant compressors directly - with soft start and adjustable throughput via speed
control. Since electric motors are practically maintenance free for life, and thus have much
better availability than gas turbines, they are principally suitable for this substitution process
- and are up to one-third less expensive. In addition. since these custom-engineered motors
are sized to match actual power and speed requirements of the compressors, the entire
liquefaction process can be optimized without regard to Standard GT sizes and speeds, and
avoid stepup gears in most cases. Finally, this design flexibility caters to the new LNG
processes and allows additional compressor vendors into the LNG arena.
From a manufacturer´s perspective, there is no reason not to build 2-pole synchronous
electric motors in power ratings up to 100 MW. Their design, materials, mechanica