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Processes >
Midstream >
LNG Production >
E-LNG >
Power generation
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Water-steam cycles
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Gas turbines
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Steam turbines
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Synchronous generators
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Power transmission and distribution
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Stability
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Power generation - Stability
Power systems for E-LNG plants exhibit an unusually low
ratio of generating capacity to single electrical loads.
Typically, the power consumption of some very large motors
is distributed over a few generators, resulting in a low
CAPEX and a highly reliable system. The main challenge is
to maintain a stable power-system frequency under all
circumstances. Stability in this context is essentially defined
as keeping the power-system frequency within the limits
set by the gas-turbine speeds.
 Siemens’ superimposed
electrical network
monitoring and
control computer
(ENMC) monitors all
major electrical loads
in order to keep
the power-system frequency
stable. |
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The Siemens E-LNG concept is based on an n+1 powerplant design: All gas and steam turbines in service operate permanently at part load, and the n-units assume full load within a very short time in the event of an unexpected shutdown of the surplus turbogenerator. Redistributing the load onto the operational turbogenerators is a key to liquefaction- compressor stability.
Understanding the stability challenge
In-house simulation tools allow Siemens to accurately predict
and document the stability of the E-LNG power system
under single-fault conditions, within specified environmental
and operating conditions, and fuel supplied to the power
plant according to agreed parameters. Electronic variable
speed drive systems (VSDS) employed to drive the main
refrigerant compressors can function as negative spinning
reserve in the power rebalancing process; they can reduce
their speed (and thus their power consumption) instantaneously
upon the unexpected loss of a turbogenerator in
the power plant. The amount of negative reserve is determined
by the momentary compressor stability line.
Siemens’ stability solution is based on the following principal
functions: fast signal and data exchange between all
control and protection systems of the power-to-compression
system; intelligent use of positive and negative spinning
load reserves; modified gas-turbine IGV controls, pre-control
systems; and a dedicated superimposed electrical network
monitoring and control system (ENMC, patent pending).
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Mitigating side effects
Electronic VSDS are non-linear electrical loads on the electrical
system. As such they can exhibit side effects in the
form of power-line harmonics, inter-harmonics, oscillating
torques in the drive motors, and possible electromagnetic
interferences (EMC) throughout the electrical system.
Siemens has developed effective mitigation measures for
all these unwanted effects. Passive harmonic filters are
custom-engineered for each project. The effects of interharmonics
are simulated, and corrections in control strategies
are implemented if necessary. Oscillating torques in
motor-compressor strings are damped sufficiently by
design measures and rotor inertia. Effects of active and
passive electromagnetic interferences on the entire electrical
distribution system are investigated and solutions
developed at the Siemens EMC research center, the largest
of its kind in Europe.
Ensuring quality from component to system level
Full lateral and torsional analyses according to international
standards are performed in-house and documented for
each rotating string prior to detail design. API vibration criteria
and tests are generally applied to motor-compressor
strings. Electrical stability of the power system, load-flow
and short-circuit calculations, protection coordination,
and load-shedding scenarios based on specified emergency
shut-down (ESD) actions are routinely determined by
our system engineers and made available to our clients
and their consultants. |
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