COMPARISON BETWEEN ELECTRICAL DRIVES IN LNG PLANT FOR
SUBSYNCHRONOUS TORSIONAL INTERACTIONS
Toshiyuki Fujii
Mitsubishi Electric Corporation, Amagasaki, Japan
Hiroyuki Masuda
Yoshihiro Ogashi
Masahiko Tsukakoshi
Makoto Yoshimura
Toshiba Mitsubishi-Electric Industrial Systems Corporation, Tokyo, Japan
KEYWORDS: variable speed drives, generators, distributed power generation, AC-DC power converters,
pulse width modulation converters, power system harmonics, torsional interactions
ABSTRACT
Electrical drives for large capacity compressors in a liquefied natural gas (LNG) plant have been investigated.
There are mainly two topologies for such an electrical drive; load commutated inverters (LCIs) and voltage
source inverters (VSIs). Due to commutation of the thyristors, LCIs produce current harmonics with variable
frequencies, and their interharmonics are significant. Therefore, torsional interactions should strongly be
concerned. The VSI systems have low harmonics of voltage and current, and become a matured and reliable
technology. So this technology is getting attractive to the very large capacity motor drives. This paper shows
characteristics of proposed variable speed drive system (VSDS) using a diode rectifier and VSIs technology
comparing with LCIs and VSIs using the PWM rectifier (active front end) for subsynchronous torsional
interactions (SSTI). Electrical damping is investigated by transient simulation using PSCAD/EMTDC. It is
found that the proposed VSDS has positive electrical damping for wide range of torsional resonant frequency.
The electrical damping is kept positive values even in case of a higher impedance of generator connection. On
the other hand, negative electrical damping characteristics are observed for the LCI and the VSI with PWM
rectifier. This study shows that the SSTI is less concerned with the proposed VSDS that would be suitable to
apply large capacity drive applications in LNG plant.
I. INTRODUCTION
The trend of natural gas production is growing in the world. It is necessary to compress the gas about one
six-hundredth of its volume by liquefaction for transportation. In the compression process, gas turbine drives
are applied so far. On the other hand, many of large capacity industrial motor drive systems are employing
electrical drive technologies nowadays in a wide variety of applications because of high performance and
efficiency. The solution is already a reliable and proven technology. Therefore, electrical drives for large
capacity compressors in a liquefied natural gas (LNG) plant will also have advantages on efficiency,
maintainability and performance compared to conventional gas turbine drives.
Such electrical drives are categorized into two types, load commutated inverters (LCIs) and voltage source
inverters (VSIs). The LCI systems have been applied to large capacity motor drives because higher ratings of
thyristors have been available. However, due to commutation process of the thyristors, LCIs produce
significant current harmonics with variable frequencies that depend on operating speed of the motor, and their
interharmonics are considerable. Therefore, torsional interactions should strongly be concerned especially for
a weak electrical power generation and distribution system such as in remote plants isolated from the
alternate current (AC) power grids. Thanks to development of large capacity self-commutated power devices
such as insulated gate bipolar transistors (IGBTs), injection enhanced gate transistors (IEGTs), gate
1
commutated turn-off thyristors (GCTs). The VSI systems have been applied to many applications in a wide
power capacity range because of its characteristics of high performance and low harmonics in voltage and
current. Especially, the IEGTs and the GCTs have large capacity at several MW per device. Then, with these
devices, the VSI can cover very large capacity comparable to the LCI. The large capacity VSI rated at several
tens of MW is already available in the market [1], [2]. These large capacity VSIs are made of small number of
devices. This feature results in high reliability. As a consequence, this technology is also attractive to the very large
capacity motor drives [1]-[4], [12]. Simple comparisons between LCIs and VSIs are summarized in Table 1.
This VSI-VSDS consists of a diode rectifier in grid side and a self-commutated inverter with a pulse width
modulation (PWM) technique [8] in motor side. This configuration is suitable for compressor drives since the
system does not require power regeneration from the motor to the grid. The diode rectifier is compact and
operates with low harmonics and high power factor compared with the thyristor rectifier in LCI drive.
In case of remote plant, in addition to the harmonics and the power factor, interactions between the
generators and the loads are also necessary to be considered. Especially, the subsynchronous torsional
interaction (SSTI) should be well considered since the phenomenon includes the mechanical system of the
generator. If it happens, the torsional resonance may damage the mechanical system. In terms of SSTI, the
VSIs with the diode rectifier have advantages compared to LCIs and even an active front end VSI which is
employing a PWM rectifier in the grid side [10]-[12]. This paper shows characteristics of the VSI-VSDS for
SSTI using the time domain simulation analysis.
Table 1. Comparisons of electrical drives for LNG plant
Items
Load Commutated Inverters
Voltage Source Inverters
Device
Rectifier: Thyristors
Inverter: Thyristors
Capacity
Reliability
Harmonics
Power Factor
SSTI
Large
Good
Large and filters necessary
Low and reactive power control necessary
Possible in case and system study necessary
Rectifier: Diode
Inverter: Self-commutated devices
(IGBTs, IEGTs, GCTs)
Large
Good
Small
High
Less concerned
II. SYSTEM CONFIGURATION
A typical system configuration of remote LNG plants is shown in Fig. 1. Several gas-turbine and steam-turbine
generators produce electric power to the electrical drive systems for variable speed motor drives. This typical
example includes four 105-MW (117-MVA) gas-turbine generators and two 90-MW steam-turbine generators.
The generators are connected to the 132-kV, 50-Hz common bus via step-up transformers and controlling
voltage level and frequency. In load side, four VSDSs are connected to the common bus to drive 80-MW
electrical motor of a compressor.
In this study, 80-MW VSI-VSDS for large capacity variable speed drives is considered. Fig. 2 shows an
example of a large capacity VSI-VSDS main circuit topology [1]. In grid side, multi-pulse diode rectifiers are
introduced in order to reduce current harmonics. The rectifiers maintain direct current (DC) capacitor voltages
for PWM control of the VSIs in order to produce low harmonic output voltage to the motor windings. The DC
capacitors suppress voltage deviation caused by DC current harmonics of the rectifiers and VSIs. Since the
capacitors are connected in parallel to the rectifiers and VSIs, harmonic currents of each side can flow
independently. The harmonic currents from the VSI to the grid can be minimized by this configuration of
2
VSI-VSDS. Such a configuration is one of the advantages of VSI topology to the LCI’s [12]. The five-level
topology is applied to inverters to have large capacity VSI. The five-level inverters can also reduce harmonics
of output voltages and currents of the motor of compressors [2]. Each phase of the inverter consists of two
legs of diode-clamp three-level GCT inverter [9]. Figure 3 shows an external view of a developed VSI system
with diode rectifiers and GCT inverters which rated output voltage is 7.2 kV and rated capacity is 30 MVA. In
order to drive 80-MW motor, four 30-MVA VSI systems are used in parallel as shown in Fig. 4 of an
arrangement design example.
Diode
Rectifier
Turbine Generator
105MW 14.5 kV / 132 kV
GT1
GT2
GT3
GT4
Voltage Source
Motor
Inverter
Compressor
80MW
G1
M1
105MW
80MW
G2
M2
105MW
80MW
G3
M3
105MW
80MW
G4
M4
90MW
ST1
G5
90MW
ST2
G6
Figure 1. Typical system configuration of LNG plants
3
U
Grid
Motor
V
W
Figure 2. Example of main circuit topology of VSI
Figure 3. External view of the five-level GCT inverter (30 MVA, 7.2 kVrms, 6(W)x1.8(D)x2.3(H)m)
4
Figure 4. An arrangement plan for the 80-MW motor drive system
III. ELECTRICAL DAMPING
Torsional resonance is an unavoidable issue in turbine generators. The resonance must be well managed and
stabilized in the system design. SSTI is one of the items of the issue to consider when an electrical system
includes large capacity power converters. SSTI is an instability phenomenon in which the damping the
resonance is decreased through interactions with the generator and the converter. The electrical damping is
widely used to analyze SSTI. Figure 5 shows a block diagram of a feedback configuration between
mechanical system of the generator and electrical system including generator and VSI-VSDSs. The inputs of
the mechanical system are deviation of mechanical and electrical torque. The output of the block is the
deviation of rotational speed. The input and output of the electrical system are deviation of speed and
electrical torque, respectively. The electrical damping is defined by real part of the transfer function of the
electrical system as shown in (1).
 ∆T 
∆e = Re e 
 ∆ω 
(1)
If the electrical damping is negative at a certain frequency, the electrical torque Te decreases when the
generator rotation speed ω increases. Since the electrical torque decreases, the generator rotation speed
further increases. This behavior possibly falls into a positive feedback loop at the frequency in the block
diagram as shown in Fig. 5. In this case, unstable oscillation may be observed in the mechanical and
electrical systems. Figure 6 shows criteria of stability for torsional resonance. The vertical axis is the damping
of the system calculated by addition of mechanical damping Dm and electrical damping De. If the damping is
positive, then the oscillation will be damped in the system. However, if the damping is negative, then the
oscillation will be increased and the mechanical shaft possibly be damaged. Usually, a mechanical damping is
positive due to losses in turbines, shafts and a generator. These losses depend on level of output power, so
the examples of mechanical damping in Fig. 6 have range of damping in positive region at frequencies of
torsional resonance (fm1, fm2). The criteria of stable torsional resonance is Dm + De > 0, so the electrical
damping can be negative when the mechanical damping is positive large. However, we will take De>0 (Dm=0)
as the criteria of electrical damping for comparison among three types of rectifier as the worst case.
5
Mechanical
DTm
Torque Deviation
Mechanical
System
Dω
Speed Deviation
DTe
Electrical
Electrical Torque
System
Deviation
 DT 
Electrical Damping: De = Re e 
 Dω 
Figure 5. Block diagram for consideration of SSTI
Mechanical Damping
D Electrical Damping
D
Damping
+
m1
0
f m1
m2
fm2
De
Frequency
Dm + De > 0 : Stable
Dm + De < 0 : Unstable
Torsional Resonance Frequency
-
Figure 6. Criteria of stability of torsional resonance
The SSTI is well known issue for LCIs drives. The controlling of firing pulse of thyristors in LCIs decrease
electrical damping to negative values due to interactions of voltage phase deviation and firing control
especially below 15 ~ 20 Hz of frequency [5]. Frequencies of mechanical resonance of the turbine generators
are in similar frequency range of negative electrical damping of LCIs. The negative electrical damping causes
undamped resonance of shaft speed of the generators which possibly leads to severe damage of the shaft.
Even the VSI employing an active front end such as PWM rectifiers may have characteristics of negative
electrical damping due to DC voltage control which orders active power of the rectifier in fast response. On
the other hand, the proposed VSI-VSDS consists of a diode rectifier in grid side. The diode rectifier has no
active control of firing timing and is not affected by the voltage phase deviation. Therefore the electrical
damping can be greater than the other type of rectifier.
IV. SIMULATION ANALYSIS
Due to nonlinear characteristics of the converter and its controller, they must be modeled in detail for SSTI
analysis. The simulation model has been built using the electromagnetic time domain transient simulation tool
PSCAD/EMTDC which is widely used in power system analysis. As VSDS model, the LCI and VSI using
PWM rectifier (active front end VSI) are also built for comparison. Figure 7 shows system configuration for
simulation analysis of electrical damping. Three circuit topologies of VSDS are shown in Fig. 8 as examples of
a VSI, an LCI and an active front end VSI. The nominal values of electrical parameters of the generator are
listed in Table 2. A single 117-MVA generator with 120-MVA transformer (8% impedance) supplies electrical
power to a single 80-MW VSI system as a worst case in terms of system stability. The terminal voltage of the
generator is controlled by the automatic voltage regulator (AVR) with the AC exciter system including the AC
machine and the diode rectifier in this case. The mechanical system is not modeled to evaluate electrical
damping since the rotational speed is the input to the electrical system as shown in the block diagram (Fig. 5)
and the electrical damping is not influenced by mechanical characteristics. The electrical damping is
measured as follows.
6
1)
Tthe rotor speed of the generator is forced to swing at a certain frequency.
2)
Perturbation of the electrical torque of the generator is measured at the frequency.
3
Electrical damping is calculated using amplitudes and phases of the rotor speed and the electrical
torque.
This process is done in the block of “Meas. Electrical Damping” in Fig. 7. In order to verify influence of the
electrical damping due to variation of the grid parameters, additional parameter case is simulated in which the
d-axis transient reactance of the generator and impedance of the transformer are set two times higher values
than the nominal as the weak electrical system. A short circuit ratio (SCR), which is an index of power system
strength, is 5.7 for the normal condition and 2.9 for the weak condition.
117MVA Generator Model
Speed
Transformer Grid
80MW
VSDS
Elec.
Torque
Meas.
Electrical
Damping
120MVA(8%)
Field
ACExciter
System
AVR
Voltage
Voltage Command
Figure 7. System configuration for simulation analysis
7
Diode
Rectifier
Voltage Source
Inverter
M
(a) VSI
Thyristor
Rectifier
Thyristor
Inverter
M
(b) LCI
Voltage Source
Rectifier
Voltage Source
Inverter
M
(c) Active front end VSI
Figure 8. Circuit topologies of VSDSs for simulation analysis
Table 2. Parameters of the generator
Parameter
Value
unit
Capacity (S)
117
MVA
Voltage (V)
14.5
kV
50
Hz
0.000894
pu
Portie Reactance (Xp)
0.113
pu
d-axis Synchronous Reactance (Xd)
1.62
pu
d-axis Transient Reactance (Xd')
0.175
pu
d-axis Subtransient Reactance (Xd'')
0.135
pu
d-axis Transient Time Constant (Tdo')
8.35
s
d-axis Subtransient Time Constant (Tdo'')
0.045
s
q-axis Synchronous Reactance (Xq)
1.58
pu
q-axis Transient Reactance (Xq')
0.333
pu
q-axis Subtransient Reactance (Xq'')
0.132
pu
q-axis Transient Time Constant (Tqo')
0.93
s
q-axis Subtransient Time Constant (Tqo'')
0.088
s
Frequency (f)
Armature Resistance (Ra)
8
The simulation results of electrical damping are shown in Fig. 9. The bald lines and dashed lines show the
characteristics of the electrical damping at nominal parameters and higher impedance condition, respectively.
The circle, triangle and square symbols show results of simulation for VSI, LCI and active front end VSI,
respectively.
The proposed VSI-VSDS has the positive electrical damping from 2 to 50 Hz of rotational speed deviation
even at the condition of higher impedance as a weak power system. The diode rectifier is not an active
rectifier which means that there is no control of switching timing of the power devices (diode). Therefore SSTI
problems are less concern with this type of VSDS. It is very attractive to install the proposed VSI-VSDS to
LNG plants because of the characteristics.
In contrast to the VSI, the electrical damping of the LCI is basically negative due to thyristor-firing scheme.
Figure 9 does not show much difference between two conditions of impedances. However, around 10 Hz of
deviation frequency, electrical damping tends to be decreased at the weaker system. The electrical damping
characteristics may be different in other design of controllers and its parameter tuning as mentioned in other
literatures such as [3]. The system with LCI-VSDS surely requires much simulation studies and carefully
designed damping control to avoid SSTI problems.
Furthermore, the VSI-VSDS using PWM rectifier (active VSI in Fig. 9) also has negative damping
characteristics in wide range of frequency. Especially from 20 to 40 Hz, quite large negative values are
observed. The DC voltage control possibly interacts with electrical torque of the generator. Because the DC
voltage control of the active rectifier is designed frequency response of 200 rad/s (31.8 Hz) in this simulation.
Therefore, damping control should be installed to the active rectifier as well as LCI systems.
SCR=5.7
SCR=2.9
VSI
Active
VSI
LCI
Figure 9. Comparison of the electrical damping
V. CONCLUSION
This paper shows characteristics of proposed VSDS using VSI and diode rectifier technology comparing with
the LCI and the active front end VSI using the PWM rectifier in terms of subsynchronous torsional interactions
(SSTI). Electrical damping is investigated by time domain transient simulation using PSCAD/EMTDC for
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80-MW drive VSDS with 117-MVA generator. It is found that the proposed VSDS has positive electrical
damping for wide range of torsional resonant frequency. Furthermore the electrical damping is kept positive
values even in case of a higher impedance of generator connection as in weak power system. The electrical
damping, on the contrary, is negative in characteristics of the LCI and the active front end VSI with PWM
rectifier. This study shows that the SSTI is less concerned with the proposed VSDS that will be suitable to
apply large capacity drive applications in remote LNG plant isolated from the AC power grid.
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