PHASE LOCKED LOOP BASED TORSIONAL MODE DAMPING SYSTEM AND
METHOD
BACKGROUND
TECHNICAL FIELD
Embodiments of the subject matter disclosed herein generally relate to methods
and systems and, more particularly, to mechanisms and techniques for dampening
a torsional vibration that appears in a rotating system.
DISCUSSION OF THE BACKGROUND
The oil and gas industry has a growing demand for driving various machines at
variable speeds. Such machines may include compressors, electrical motors,
expanders, gas turbines, pumps, etc. Variable frequency electrical drives increase
energy efficiency and provide an increased flexibility for the machines. One
mechanism for driving, for example, a large gas compression train is the load
commutated inverter (LCI). A gas compression train includes, for example, a gas
turbine, a motor, and a compressor. The gas compression train may include more
or less electrical machines and turbo-machines. However, a problem introduced
by power electronics driven systems is the generation of ripple components in the
torque of the electrical machine due to electrical harmonics. The ripple component
of the torque may interact with the mechanical system at torsional natural
frequencies of the drive train, which is undesirable.
A torsional oscillation or vibration is an oscillatory angular motion that may appear
in a shaft having various masses attached to it as shown for example in Figure .
Figure 1 shows a system 10 including a gas turbine 12, a motor 14, a first
compressor 16 and a second compressor 18. The shafts of these machines are
either connected to each other or a single shaft 20 is shared by these machines.
Because of the impellers and other masses distributed along shaft 20, a rotation of
the shaft 20 may be affected by torsional oscillations produced by the rotation with
different speeds of the masses (impellers for example) attached to the shaft.
As discussed above, the torsional vibrations are typically introduced by the power
electronics that drive the electrical motor. Figure , for example, shows a power
grid source (power source) 22 providing electrical power to the LCI 24, which in
turn drives the shaft 20 of the motor 14. The power grid may be an isolated power
generator. In order to damp (minimize) the torsional vibrations, as shown in Figure
2 (which corresponds to Figure 1 of U.S. Patent No. 7,1 73,399, assigned to the
same assignee as this application, the entire disclosure of which is incorporated
here by reference), an inverter controller 26 may be provided to an inverter 28 of
the LCI 24 and may be configured to introduce an inverter delay angle change
(Db) for modulating an amount of active power transferred from inverter 28 to
motor 14. Alternatively, a rectifier controller 30 may be provided to a rectifier 32
and may be configured to introduce a rectifier delay angle change (Da) for
modulating the amount of active power transferred from the generator 22 to a DClink
44 and thus to the motor 14. It is noted that by modulating the amount of
active power transferred from the generator 22 to the motor 14 it is possible to
damp the torsional vibrations that appear in the system including motor 14 and
compressor 12. In this regard, it is noted that shafts of motor 14 and gas turbine
12 are connected to each other while a shaft of generator 22 is not connected to
either the motor 4 or compressor 2.
The two controllers 26 and 30 receive as input, signals from sensors 36 and 38,
respectively, and these signals are indicative of the torque experienced by the
motor 14 and/or the generator 22. In other words, the inverter controller 26
processes the torque value sensed by sensor 36 for generating the inverter delay
angle change (Db) while the rectifier controller 30 processes the torque value
sensed by the sensor 38 for generating the rectifier delay angle change (Da) . The
inverter controller 26 and the rectifier controller 30 are independent from each
other and these controllers may be implemented together or alone in a given
system. Figure 2 shows that sensor 36 monitors a part (section) 40 of the shaft of
the motor 14 and sensor 38 monitors a shaft 42 of the power generator 22. Figure
2 also shows the DC link 44 between the rectifier 32 and the inverter 28.
However, it is possible to use other devices and methods for generating damping
in the drive train.
SUMMARY
According to one exemplary embodiment, there is a controller system connected
to a converter or placed inside the converter itself. The converter drives a drive
train including an electrical machine and a non-electrical machine. The controller
system includes an input interface configured to receive converter data related to
variables of the converter and a controller connected to the input interface. The
input interface is configured to receive a first digital signal from a first phase lock
loop device or a dynamic observer, receive the converter data from the converter,
compare the converter data with the first digital signal, generate control data for a
rectifier and/or an inverter of the converter for damping a torsional oscillation in a
shaft of the drive train based on a result of the comparison, and send the control
data to the rectifier and/or to the inverter for modulating an active power
exchanged between the converter and the electrical machine.
According to another exemplary embodiment, there is a torsional mode damping
controller system connected to a converter or placed inside the converter itself.
The converter drives a drive train including an electrical machine and a non¬
electrical machine. The controller system a first input interface configured to
receive a first digital signal from a first phase lock device or a first dynamic
observer; a second input interface configured to receive a second digital signal
from a second phase lock device or a second dynamic observer; and a controller
connected to the first and second input interfaces. The controller is configured to
receive the first and second digital signals, compare the first digital signal with the
second digital signal, generate control data for a rectifier and/or an inverter of the
converter for damping a torsional oscillation in a shaft of the drive train based on a
result of the comparison, and send the control data to the rectifier and/or to the
inverter for modulating an active power exchanged between the converter and the
electrical machine.
According to still another exemplary embodiment, there is a system for driving an
electrical machine that is part of a drive train. The system includes a rectifier
configured to receive an alternative current from a power source and to transform
the alternative current into a direct current; a direct current link connected to the
rectifier and configured to transmit the direct current; an inverter connected to the
direct current link and configured to change a received direct current into an
alternative current; an input interface configured to receive converter data related
to variables of the converter; and a controller connected to the input interface. The
controller is configured to receive a first digital signal from a first phase lock loop
device or a dynamic observer, receive the converter data from the converter,
compare the converter data with the first digital signal, generate control data for a
rectifier and/or an inverter of the converter for damping a torsional oscillation in a
shaft of the drive train based on a result of the comparison, and send the control
data to the rectifier and/or to the inverter for modulating an active power
exchanged between the converter and the electrical machine.
According to a further exemplary embodiment, there is a method for damping a
torsional vibration in a drive train including an electrical machine. The method
includes a step of receiving converter data related to variables of the converter; a
step of receiving a first digital signal from a first phase lock loop device or a
dynamic observer; a step of comparing the converter data with the first digital
signal; a step of generating control data for a rectifier and/or an inverter of the
converter for damping a torsional oscillation in a shaft of the drive train based on a
result of the comparison; and a step of sending the control data to the rectifier
and/or to the inverter for modulating an active power exchanged between the
converter and the electrical machine.
According to another exemplary embodiment, there is a method for damping a
torsional vibration in a drive train including an electrical machine. The method
includes a step of receiving a first digital signal from a first phase lock device or a
first dynamic observer; a step of receiving a second digital signal from a second
phase lock loop device or a second dynamic observer additional to the first phase
lock device or the first dynamic observer; a step of comparing the first digital signal
with the second digital signal; a step of generating control data for a rectifier and/or
an inverter of the converter for damping a torsional oscillation in a shaft of the
drive train based on a result of the comparison; and a step of sending the control
data to the rectifier and/or to the inverter for modulating an active power
exchanged between the converter and the electrical machine.
According to yet another exemplary embodiment, there is a computer readable
medium including computer executable instructions, wherein the instructions,
when executed, implement a method for damping torsional vibrations. The
computer instructions include the steps recited in the method noted in the previous
paragraph.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part of the
specification, illustrate one or more embodiments and, together with the
description, explain these embodiments. In the drawings:
Figure 1 is a schematic diagram of a conventional gas turbine connected to an
electrical machine and two compressors;
Figure 2 is a schematic diagram of a driving train including rectifier controller and
inverter controller;
Figure 3 is a schematic diagram of a gas turbine, motor and load controlled by a
controller according to an exemplary embodiment;
Figure 4 is a schematic diagram of a converter and associated logic according to
an exemplary embodiment;
Figure 5 is a schematic diagram of a converter and associated logic according to
an exemplary embodiment;
Figure 6 is a graph illustrating a torque of a shaft with disabled damping control;
Figure 7 is a graph illustrating a torque of a shaft with enabled damping control
according to an exemplary embodiment;
Figure 8 is a schematic diagram of a converter and associated logic according to
an exemplary embodiment;
Figure 9 is a schematic diagram of a controller configured to control a converter for
damping torsional vibrations according to an exemplary embodiment;
Figure 10 is a schematic diagram of a controller that provides modulation to a
rectifier according to an exemplary embodiment;
Figure 11 is a flow chart of a method that controls a rectifier for damping torsional
vibrations according to an exemplary embodiment;
Figure 12 is a schematic diagram of a controller that provides modulation to a
rectifier and an inverter according to an exemplary embodiment;
Figure 13 is a schematic diagram of voltages existent at an inverter, rectifier and
DC link of a converter according to an exemplary embodiment;
Figure 14 is a graph indicating the torsional effect of alpha and beta angle
modulations according to an exemplary embodiment;
Figure 5 is a flow chart of a method that controls an inverter and a rectifier for
damping torsional vibrations according to an exemplary embodiment;
Figure 16 is a schematic diagram of a voltage source inverter and associated
controller for damping torsional vibrations according to an exemplary embodiment;
Figure 17 is a schematic diagram of a multimass system;
Figure 18 is a schematic diagram of a control system for damping torsional
vibrations based on one phase lock loop or dynamic observer according to an
exemplary embodiment;
Figure 19 is a schematic diagram of a control system for damping torsional
vibrations based on one or more phase lock loop devices according to an
exemplary embodiment;
Figure 20 is a flow chart illustrating a method for damping torsional vibrations
based on a single phase lock loop device according to an exemplary embodiment;
and
Figure 2 1 is a flow chart illustrating a method for damping torsional vibrations
based on two phase lock loop devices according to an exemplary embodiment.
DETAILED DESCRIPTION
The following description of the exemplary embodiments refers to the accompanying
drawings. The same reference numbers in different drawings identify the same or
similar elements. The following detailed description does not limit the invention.
Instead, the scope of the invention is defined by the appended claims. The following
embodiments are discussed, for simplicity, with regard to the terminology and
structure of an electrical motor driven by a load commutated inverter. However, the
embodiments to be discussed next are not limited to such a system, but may be
applied (with appropriate adjustments) to other systems that are driven with other
devices, as for example, a voltage source inverter (VSI).
Reference throughout the specification to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic described in connection
with an embodiment is included in at least one embodiment of the subject matter
disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an
embodiment" in various places throughout the specification is not necessarily
referring to the same embodiment. Further, the particular features, structures or
characteristics may be combined in any suitable manner in one or more
embodiments.
According to an exemplary embodiment, a torsional mode damping controller may
be configured to obtain electrical and/or mechanical measurements regarding a
shaft of an electrical machine (which may be a motor or a generator) and/or a
shaft of a turbo-machine that is mechanically connected to the electrical machine
and to estimate, based on the electrical and/or mechanical measurements,
dynamic torque components and/or a torque vibration at a desired shaft location of
a drive train. The dynamic torque components may be a torque, a torsional
position, torsional speed or a torsional acceleration of the shaft. Based on one or
more dynamic torque components, a controller may adjust/modify one or more
parameters of a rectifier that drives the electrical machine to apply a desired
torque for damping the torque oscillation. As will be discussed next, there are
various data sources for the controller for determining the damping based on the
rectifier control.
According to an exemplary embodiment shown in Figure 3, a system 50 includes a
gas turbine 52, a motor 54, and a load 56. Other configurations involving a gas
turbine and/or plural compressors or other turbo-machines as load 56 are
possible. Still, other configurations may include one or more expanders, one or
more power generators, or other machines having a rotating part, e.g., wind
turbines, gearboxes. The system shown in Figure 3 is exemplary and is simplified
for a better understanding of the novel features. However, one skilled in the art
would appreciate that other systems having more or less components may be
adapted to include the novel features now discussed.
The connection of various masses (associated with the rotors and impellers of the
machines) to a shaft 58 makes the system 50 prone to potential torsional
vibrations. These torsional vibrations may twist the shaft 58, which may result in
significant lifetime reduction or even destruction of the shaft system (which may
include not only the shaft or shafts but also couplings and gearbox depending on
the specific situation). The exemplary embodiments provide a mechanism for
reducing the torsional vibrations.
To activate the motor 54, electrical power is supplied from the power grid or a local
generator 60 in case of island or island like power systems. In order to drive the
motor 54 at a variable speed, a load commutated inverter (LCI) 62 is provided
between the grid 60 and the motor 54. As shown in Figure 4 , the LCI 62 includes
a rectifier 66 connected to a DC link 68 which is connected to an inverter 70. The
rectifier 66, DC link 68, and inverter 70 are known in the art and their specific
structures are not discussed here further. As noted above, the novel features may
be applied, with appropriate changes, to VSI systems. For illustration only, an
exemplary VSI is shown and briefly discussed with regard to Figure 16. Figure 4
indicates that the current and voltage received from the grid 60 are three phase
currents and voltages, respectively. The same is true for the currents and
voltages through the rectifier, inverter and the motor and this fact is indicated in
Figure 4 by symbol 73". However, the novel features of the exemplary
embodiments are applicable to systems configured to work with more than three
phases, e.g., 6 phase and 2 phase systems.
LCI 62 also includes current and voltage sensors, denoted by a circled A and a
circled V in Figure 4. For example, a current sensor 72 is provided in the DC link
68 to measure a current ioc- Alternatively, the current in the DC link is calculated
based on measurements performed in the AC side, for example current sensors
84 or 74 as these sensors are less expensive than DC sensors. Another example
is a current sensor 74 that measures a current iat>c provided by the inverter 70 to
the motor 54 and a voltage sensor 76 that measures a voltage va c provided by the
inverter 70 to the motor 54. It is noted that these currents and voltages may be
provided as input to a controller 78. The term "controller" is used herein to
encompass any appropriate digital, analog, or combination thereof circuitry or
processing units for accomplishing the designated control function. Returning to
Figure 3 , it is noted that controller 78 may be part of the LCI 62 or may be a
standalone controller exchanging signals with the LCI 62. The controller 78 may
be a torsional mode damping controller.
Figure 4 also shows that an LCI controller 80 may receive mechanical
measurements regarding one or more of the gas turbine 52, the motor 54 and the
load 56 shown in Figure 3. The same may be true for controller 78. In other
words, controller 78 may be configured to receive measurement data from any of
the components of the system 50 shown in Figure 3 . For example, Figure 4
shows a measurement data source 79. This data source may provide mechanical
measurements and/or electrical measurements from any of the components of the
system 50. A particular example that is used for a better understanding and not to
limit the exemplary embodiments is when data source 79 is associated with the
gas turbine 52. A torsional position, speed, acceleration or torque of the gas
turbine 52 may be measured by existing sensors. This data may be provided to
controller 78 as shown in Figure 4 . Another example is electrical measurements
taken at the converter 62 or motor 54. Data source 79 may provide these
measurements to controller 78 or controller 80 if necessary.
Controller 80 may generate, based on various references 82, and a current i
received from a sensor 84, a rectifier delay angle a for controlling the rectifier 66.
Regarding the rectifier delay angle a , it is noted that LCIs are designed to transfer
active power from the grid 60 to the motor 54 or vice versa. Achieving this transfer
with an optimal power factor involves the rectifier delay angle a and the inverter
delay angle b. The rectifier delay angle a may be modulated by applying, for
example, a sine wave modulation. This modulation may be applied for a limited
amount of time. In one application, the modulation is applied continuously but the
amplitude of the modulation varies. For example, as there is no torsional vibration
in the shaft, the amplitude of the modulation may be zero, i.e., no modulation. In
another example, the amplitude of the modulation may be proportional with the
detected torsional vibration of the shaft.
Another controller 86 may be used for generating an inverter delay angle b for the
inverter 70. Modulating the inverter delay angle b results in modulating the
inverter DC voltage which causes a modulation of the DC link current and results
in an active power oscillation on the load input power. In other words, modulating
only the inverter delay angle in order to achieve torsional mode damping results in
the damping power coming mainly from the magnetic energy stored in the DC link
68. Modulation of the inverter delay angle results in rotational energy being
transformed into magnetic energy and vice versa, depending whether the rotating
shaft is accelerated or decelerated.
Further, Figure 4 shows a gate control unit 88 for the rectifier 66 and a gate control
unit 90 for the inverter 70 that directly control the rectifier and inverter based on
information received from controllers 80 and 86. An optional sensor 92 may be
located in close proximity to the shaft of the motor 54 for detecting the dynamic
torque components, e.g., a torque present in the shaft or a torsional speed of the
shaft or a torsional acceleration of the shaft or a torsional position of the shaft.
Other similar sensors 92 may be placed between motor 54 and gas turbine 52 or
at gas turbine 52. Information u regarding measured dynamic torque components
(by sensors 92) may be provided to controllers 78, 80 and 86. Figure 4 also
shows summation blocks 94 and 96 that add a signal from controller 78 to signals
generated by controllers 80 and 86.
According to an exemplary embodiment illustrated in Figure 5 , the torsional mode
damping controller 78 may receive a current ia c a voltage vabc measured at an
output 9 1 of the LCI 62 or the inverter 70. Based on these values (no information
about a measured torque or speed or acceleration of the shaft of the motor), an air
gap torque for the motor is calculated and fed into a mechanical model of the
system. The mechanical model of the system may be represented by several
differential equations representing the dynamic behavior of the mechanical system
and linking the electrical parameters to the mechanical parameters of the system.
The model representation includes, for example, estimated inertia, damping and
stiffness values (which can be verified by field measurements) and allows to
calculate the dynamic behavior of the shaft, e.g., torsional oscillations. The
needed accuracy for torsional mode damping may be achieved as mainly the
accuracy of the phase of the dynamic torque component is relevant for the
torsional mode damping, and the amplitude information or absolute torque value is
less important.
In this regard, it is noted that the air gap torque of an electrical machine is the link
between the electrical and mechanical system of a drive train. All harmonics and
inter-harmonics in the electrical system are also visible in the air-gap torque. Interharmonics
at a natural frequency of the mechanical system can excite torsional
oscillations and potentially result into dynamic torque values in the mechanical
system above the rating of the shaft. Existing torsional mode damping systems
may counteract such torsional oscillations but these systems need a signal
representative of the dynamic torque of the motor and this signal is obtained from
a sensor that effectively monitors the shaft of the motor or shaft components of the
motor, such as toothwheels mounted along the shaft of the motor. According to
exemplary embodiments, no such signal is needed as the dynamic torque
components are evaluated based on electrical measurements. However, as will
be discussed later, some exemplary embodiments describe a situation in which
available mechanical measurements at other components of the system, for
example, the gas turbine, may be used to determine the dynamic torque
components along the mechanical shaft.
In other words, an advantage according to an exemplary embodiment is applying
torsional mode damping without the need of torsional vibration sensing in the
mechanical system. Thus, torsional mode damping can be applied without having
to install additional sensing in the electrical or mechanical system as current
voltage and/or current and/or speed sensors can be made available at comparably
low cost. In this regard, it is noted that mechanical sensors for measuring torque
are expensive for high power applications, and sometimes these sensors cannot
be added to the existing systems. Thus, the existent torsional mode damping
solutions cannot be implemented for such cases as the existent torsional mode
damping systems require a sensor for measuring a signal representative of a
mechanical parameter of the system that is indicative of torque. On the contrary,
the approach of the exemplary embodiment of Figure 5 is reliable, cost effective
and allows retrofitting an existing system.
Upon receiving the current and voltage indicated in Figure 5 , controller 78 may
generate appropriate signals (modulations for one or more of D and Db) for
controlling the rectifier delay angle a and/or the inverter delay angle b. Thus,
according to the embodiment shown in Figure 5 , the controller 78 receives
measured electrical information from an output 9 1 of the inverter 70 and
determines/calculates the various delay angles, based, for example, on the
damping principle described in Patent No. 7,173,399. In one application, the delay
angles may be limited to a narrow and defined range, for example, 2 to 3 degrees,
not to affect the operation of the inverter and/or converter. In one application, the
delay angles may be limited to only one direction (either negative or positive) to
prevent commutation failure by overhead-firing of the thyristors. As illustrated in
Figure 5 , this exemplary embodiment is an open loop as corrections of the various
angles are not adjusted/verified based on a measured signal (feedback) of the
mechanical drive train connected to motor 54. Further, simulations performed
show a reduction of the torsional vibrations when the controller 78 is enabled.
Figure 6 shows oscillations 100 of the torque of the shaft of the motor 54 versus
time when the controller 78 is disabled and Figure 7 shows how the same
oscillations are reduced/damped when the controller 78 is enabled to generate
alpha modulation, for example, at time 12 s, while the mechanical drive train is
operated in variable speed operation and crossing at t = 12s a critical speed. Both
figures plot a simulated torque on the y axis versus time on the x axis.
According to another exemplary embodiment illustrated in Figure 8, the controller
78 may be configured to calculate one or more of the delay angles changes
(modulations) Da and/or Db based on electrical quantities obtained from the DC
link 68. More specifically, a current iDc may be measured at an inductor 104 of the
DC link 68 and this value may be provided to controller 78. In one application,
only a single current measurement is used for feeding controller 78. Based on the
value of the measured current and the mechanical model of the system, the
controller 78 may generate the above noted delay angle changes. According to
another exemplary embodiment, the direct current DC may be estimated based on
current and/or voltage measurements performed at the rectifier 66 or inverter 70.
The delay angle changes calculated by the controller 78 in any of the
embodiments discussed with regard to Figures 5 and 8 may be modified based on
a closed loop configuration. The closed loop configuration is illustrated by dashed
line 0 in Figure 8. The closed loop indicates that an angular position, speed,
acceleration, or torque of the shaft of the motor 54 may be determined with an
appropriate sensor 112 and this value may be provided to the controller 78. The
same is true if sensor or sensors 112 are provided to the gas turbine or other
locations along shaft 58 shown in Figure 3 .
The structure of the controller 78 is discussed now with regard to Figure 9.
According to an exemplary embodiment, the controller 78 may include an input
interface 120 (or multiple input interfaces configured to receive signals from
various components of the system) that is connected to one of a processor, analog
circuitry, reconfigurable FPGA card, etc. 122. Element 122 is configured to
receive the electrical parameters from the LCI 62 and calculate the delay angle
changes. Element 122 may be configured to store a mechanical model 128
(disclosed in more details with regard to Figure 17) and to input the electrical
and/or mechanical measurements received at input interface 120 into the
mechanical model 128 to calculate one or more of the dynamical torque
components of the motor 54. Based on the one or more dynamical torque
components, damping control signals are generated in damping control unit 130
and the output signal is then forwarded to a summation block and a gate control
unit. According to another exemplary embodiment, the controller 78 may be an
analog circuit, a reconfigurable FPGA card or other dedicated circuitry for
determining the delay angle changes.
In one exemplary embodiment, controller 78 continuously receives electrical
measurements from various current and voltage sensors and continuously
calculates torsional damping signals based on dynamic torque components
calculated based on the electrical measurements. According to this exemplary
embodiment, the controller does not determine whether torsional vibrations are
present in the shaft but rather continuously calculates the torsional damping
signals based on the calculated dynamic torque value. However, if there are no
torsional vibrations, the torsional damping signals generated by the controller and
sent to the inverter and/or rectifier are not affecting the inverter and/or rectifier, i.e.,
the angle changes provided by the damping signals are negligible or zero. Thus,
according to this exemplary embodiment, the signals affect the inverter and/or
rectifier only when there are torsional vibrations.
According to an exemplary embodiment, the direct torque or speed measurement
at the gas turbine shaft (or estimated speed or torque information in the shaft)
enables the controller to modulate an energy transfer in the LCI in counter-phase
to the torsional velocity of a torsional oscillation. Damping power exchanged
between the generator and the LCI drive may be electronically adjusted and may
have a frequency corresponding to a natural frequency of the shaft system. This
damping method is effective for mechanical systems with a high Q factor, i.e., rotor
shaft system made of steel with high torsional stiffness. In addition, this method of
applying an oscillating electrical torque to the shaft of the motor and having a
frequency corresponding to a resonant frequency of the mechanical system uses
little damping power.
Therefore, the above discussed controller may be integrated into a drive system
based on the LCI technology without overloading the drive system. This facilitates
the implementation of the novel controller to new or existing power systems and
makes it economically attractive. The controller may be implemented without
having to change the existing power system, e.g., extending the control system of
one of the LCI drives in the island network.
If the LCI operational speed and torque is varied in a large range, the
effectiveness of the torsional mode damping may depend on the grid-side
converter current control performance. The torsional mode damping operation
results in a small additional DC link current ripple at a torsional natural frequency.
As a result, there are two power components at this frequency: the intended
component due to inverter firing angle control and an additional component due to
the additional current ripple. The phase and magnitude of this additional power
component is function of system parameters, current control settings and point of
operation. These components result into a power component that is dependent on
current control and a component that is dependent on angle modulation.
According to an exemplary embodiment, two alternative ways of power modulation
may be implemented by the controller. A first way is to directly use the current
reference on the grid side (requires fast current control implementation), e.g., amodulation
with a damping component. A second way is to modulate the grid-side
and the machine-side angles, resulting into a constant dc-link current, e.g., a-b-
modulation with a damping frequency component. The current control on the gridside
is part of this damping control and therefore, the current control does not
counteract the effect of the angle modulation. In this way, the damping effect is
higher and independent from the current control settings.
According to an exemplary embodiment illustrated in Figure 10, the system 50
includes similar elements to the system shown in Figures 3 and 4 . Controller 78 is
configured to receive electrical measurements (as shown in Figures 4, 5, and 8)
and/or mechanical measurements (see for example Figures 4 and 8 or sensor 1 2
and link 110 in Figure 10) with regard to one or more of the motor 54 or load 56 or
the gas turbine (not shown) of system 50. Based only on the electrical
measurements, or only on the mechanical measurements, or on a combination of
the two, the controller 78 generates control signals for applying a-modulation to
the rectifier 66. For example, current reference modulation is achieved by amodulation
while the b angle is maintained constant at the inverter 70. The amodulation
is represented, for example, by Da in both Figures 4 and 10. It is
noted that this a-modulation is different from the one disclosed in U.S. Patent no.
7,173,399 for at least two reasons. A first difference is that the mechanical
measurements (if used) are obtained in the present exemplary embodiment from a
location along shaft 58 (i.e., motor 54, load 56 and/or gas turbine 52) while U.S.
Patent no. 7,1 73,399 uses a measurement of a power generator 22 (see Figure 2).
A second difference is that according to an exemplary embodiment, no mechanical
measurements are received and used by the controller 78 for performing the amodulation.
According to an exemplary embodiment illustrated in Figure 11, there is a method
for damping a torsional vibration in a compression train including an electrical
machine. The method includes a step 1100 of receiving measured data related to
parameters of (i) a converter that drives the electrical machine or (ii) the
compression train, a step 1102 of calculating at least one dynamic torque
component of the electrical machine based on the measured data, a step 1104 of
generating control data for a rectifier of the converter for damping a torsional
oscillation in a shaft of the compression train based on the at least one dynamic
torque component, and a step 06 of sending the control data to the rectifier for
modulating an active power exchanged between the converter and the electrical
machine.
According to another exemplary embodiment illustrated in Figure 12, system 50
may have both the rectifier 66 and the inverter 70 simultaneously controlled (i.e.,
both a-modulation and b-modulation) for damping torsional oscillations. As shown
in Figure 12, controller 78 provides modulations for both the rectifier controller 88
and the inverter controller 90. Controller 78 determines the appropriate
modulation based on (i) mechanical measurements measured by sensor(s) 112 at
one of the motor 54, load 56 and/or gas turbine 52, (ii) electrical measurements as
shown in Figures 4 , 5, and 8, or a combination of (i) and (ii).
More specifically, the a- and b-modulation may be correlated as discussed next
with reference to Figure 13. Figure 3 shows representative voltage drops across
rectifier 66, DC link 8 and inverter 70. As a result of the a- and b-modulation it is
desired that the DC link current is constant. Associated voltage drops shown in
Figure 13 are given by:
ca =k-VAcG -cos(a)
= k - VACM -cos(p), and
where VACG is the voltage amplitude of the power grid 60 in Figure 12 and VACM is
the voltage amplitude of the motor 54.
By differentiating the last relation with time and imposing the condition that the
change of the VQCL in time is zero, the following mathematical relation is obtained
between the a-modulation and the b-modulation:
d(V Dca)/dt = - k-VAcG-sin(a) and d(V DC3)/dt = - k-VAcM -sin(P), which results in
da = (VAcM-sin(p))/(V AcG-sin(a))-dp.
Based on this last relation, both the a-modulation and b-modulation are performed
simultaneously, as shown, for example, in Figure 14. Figure 14 shows the actual
torque 200 increasing around t = 1.5 seconds. It is noted that no a-modulation
202 or b-moduiation 204 is applied between t0 and t . At t-, an excitation 206 is
applied between and t2 and both modulations 202 and 204 are applied. At the
end of the time interval t - to t2 it is noted that both modulations are removed and
the oscillations of the torque 200 is exponentially decreasing because of the
inherent mechanical damping properties of the mechanical drive train. This
example is simulated and not measured in a real system. For this reason, both
modulations are strictly controlled, e.g., are started at t and stopped at t2.
However, in a real implementation of the a-modulation and b-modulation, the
modulations may be performed continuously with the amplitude of the modulation
being adjusted based on the severity of the torsional oscillations. An advantage of
this combined modulation over the b-modulation is that there is no need for phase
adaption at different operating points and the LCI control parameters may have no
effect on the damping performance. This modulation example is provided to
illustrate the effect of modulating both delay angles on the mechanical system.
The simulation result is shown using an open loop response to the mechanical
system for the torsional damping system with inverse damping performance.
According to an exemplary embodiment illustrated in Figure 15, there is a method
for damping a torsional vibration in a drive train including an electrical machine.
The method includes a step 1500 of receiving measured data related to
parameters of (i) a converter that drives the electrical machine or (ii) the drive
train, a step 1502 of calculating at least one dynamic torque component of the
electrical machine based on the measured data, a step 1504 of generating control
data for each of an inverter and a rectifier of the converter for damping a torsional
oscillation in a shaft of the drive train based on the at least one dynamic torque
component, and a step 1506 of sending the control data to the inverter and the
rectifier for modulating an active power exchanged between the converter and the
electrical machine. It is noted that the dynamic torque component includes a
rotational position, rotational speed, rotational acceleration or a torque related to a
section of the mechanical shaft. It is also noted that the expression modulating an
active power expresses the idea of modulation at an instant even if the mean
active power over a period T is zero. In addition, if a VSI is used instead of an LCI
another electrical quantity may be modified as appropriate instead of the active
power.
According to an exemplary embodiment illustrated in Figure 16, a VSI 140
includes a rectifier 142, a DC link 144, and an inverter 146 connected to each
other in this order. The rectifier 142 receives a grid voltage from a power source
148 and may include, for example, a diode bridge or an active front-end based on
semiconductor devices. The dc voltage provided by the rectifier 142 is filtered and
smoothed by capacitor C in the DC link 144. The filtered dc voltage is then
applied to the inverter 146, which may include self-commutated semiconductor
devices, e.g., Insulated Gate Bipolar Transistors (IGBT), that generate an ac
voltage to be applied to motor 150. Controllers 152 and 154 may be provided for
rectifier 142 and inverter 146, in addition to the rectifier and inverter controllers or
integrated with the rectifier and inverter controllers, to damp torsional vibrations on
the shaft of the motor 150. The rectifier controller 153 and inverter controller 155
are shown connected to some of the semiconductor devices but it should be
understood that all the semiconductor devices may be connected to the
controllers. Controllers 52 and 154 may be provided together or alone and they
are configured to determine dynamic torque components based on electrical
measurements as discussed with regard to Figures 4 and 5 and influence control
references of the build-in rectifier and inverter control, e.g., the torque or currentcontrol
reference.
According to an exemplary embodiment illustrated in Figure 17, a generalized
multimass system 160 may include "n" different masses having corresponding
moments of inertia i to Jn. For example, the first mass may correspond to a gas
turbine, the second mass may correspond to a compressor, and so on while the
last mass may correspond to an electrical motor. Suppose that the shaft of the
electrical motor is not accessible for mechanical measurements, e.g., rotational
position, speed, acceleration or torque. Further, suppose that the shaft of the gas
turbine is accessible and one of the above noted mechanical parameters may be
directly measured at the gas turbine. In this regard, it is noted that generally a gas
turbine has high accuracy sensors that measure various mechanical variables of
the shaft for protecting the gas turbine from possible damages. On the contrary, a
conventional motor does not have these sensors or even if some sensors are
present, the accuracy of their measurements is poor.
The differential equation of the whole mechanical system is given by:
J(d02/dt 2) + D (dO/dt) + KQ = Text,
where J (torsional matrix), D (damping matrix), and K (torsional stiffness matrix)
are matrices connecting the characteristics of the first mass (for example, d 0 , d 2,
k 2 , J -) to the characteristics of the other masses and Te is an externa! (net)
torque applied to the system, e.g., by a motor. Based on this model of the
mechanical system, a torque or other dynamic torque component of the "n" mass
may be determined if characteristics of, for example, the first mass are known. In
other words, the high accuracy sensors provided in the gas turbine may be used to
measure at least one of a torsional position, speed, acceleration or torque of the
shaft of the gas turbine. Based on this measured value, a dynamic torque
component of the motor ("n" mass) or another section of the drive train may be
calculated by a processor or controller 78 of the system and thus, control data may
be generated for the inverter or rectifier as already discussed above
in other words, according to this exemplary embodiment, the controller 78 needs
to receive mechanical related information from one turbo-machinery that is
connected to the motor and based on this mechanical related information the
controller is able to control the converter to generate a torque in the motor to damp
the torsional vibration. The turbo-machinery may be not only a gas turbine but
also a compressor, an expander or other known machines. In one application, no
electrical measurements are necessary for performing the damping. However, the
electrical measurements may be combined with mechanical measurements for
achieving the damping. In one application, the machine that applies the damping
(damping machine) is not accessible for mechanical measurements and the
dynamic torque component of the damping machine is calculated by mechanical
measurements performed on another machine that is mechanically connected to
the damping machine.
According to an exemplary embodiment illustrated in Figure 18, only a motor 150
and the inverter on the LCI motor side 184 are shown. A position and speed of a
shaft of the motor are variables that may be used to control the drive train. The
position of the shaft of the motor (or motor voltage vector) may be used to
accurately determine the b firing on the motor side.
The position of the shaft of the motor may be obtained from a position sensor
attached to the shaft of the motor or obtained without a direct position sensor, e.g.,
through the use of an observer model fed by electrical variables of the motor
and/or the drive itself. Similarly, the motor voltage vector may be evaluated from
motor and/or drive electrical variables (e.g., motor voltages). The motor terminal
voltage and/or current waveforms may include large noise and commutation
spikes. For this reason, the motor terminal voltages and/or currents are not used
directly to determine the motor voltage vector position or motor rotor position. In
one application, a filtering operation is applied to the motor/drive electrical
variables (for example, through the use of an observer model or by using a PLL
(phase lock loop) device).
Figure 18 shows an exemplary embodiment for explaining how firing pulses may
be generated on the motor side given that no position sensor is directly attached to
the motor. However, as noted above, it is possible to achieve the same result by
using a position sensor. Starting from electrical variables 86 coming from the
motor 182 and/or the drive train, the motor voltage vector(s) or the motor rotor
position are derived by use of an observer or PLL block 180. This instantaneous
angle 188 is fed to a modulator 190 in order to decide and send firing commands
194 to the inverter, depending also on a b command 192. Other methods may be
used to lock to motor voltage vector position or motor rotor position as would be
appreciated by those skilled in the art. Considering the exemplary embodiment
illustrated in Figure 18, dynamic characteristics of the PLL or observer 80 have to
be accurate enough to accurately track the motor voltage vector position or motor
rotor position and to avoid a reduction of a safety angle in case of a high
discrepancy between real and "observed" positions. It is noted here that the term
"observed" is used to refer not only to variables produced by a dynamic observer
but also to variables produced by a PLL system.
According to another exemplary embodiment illustrated in Figure 19 , a fully logic
method may be implemented for damping torsional oscillations in a drive train.
Figure 19 shows only the motor 150 and the LCI 62 of the drive train. With regard
the exemplary embodiment illustrated in Figure 19, the operation of PLL (or
observer) 162 is of interest. In this exemplary embodiment, the PLL (or observer)
is locked to the zero crossing of motor voltages (detected by block 160). However,
this is not meant to be limiting the type of PLL and phase demodulator that is used
or the type of dynamic observer that is used. It is noted that the PLL is considered
to be a particular type of observer. For this reason, when referring to an observer,
it could mean also a PLL system or when referring to a PLL it could also mean an
observer. Following the zero-crossing operation in the detector 160, a digital
square wave is generated and provided to the PLL/observer 162. The
PLL/observer 162 tracks this waveform (zero-crossing may be replaced with other
known methods). An output of the PLL/observer 162 is an instantaneous angle,
which may be an analog or digital variable. The instantaneous observed angle is
provided to controller 78 for determining the actuation of the angle b on the motor.
For this reason, dynamic characteristics of the PLL/observer 162 have to be
accurate enough to accurately track the motor rotor or the motor voltage vector
position and to avoid a reduction of safety angle in case of dynamic errors.
A torsional oscillation on the drive train is reflected on a speed oscillation on the
motor section and this speed oscillation is reflected on an amplitude and frequency
of the motor voltages. If the PLUobserver is accurate, this speed oscillation is
visible in the tracked signal.
Provided that the firing angle (a, b, or both of them) on the motor side is
modulated based on a reference signal provided by the PLL/observer 162, it
follows that modifying a PLL/observer transfer function may have an effect on
damping torsional oscillations. For example, assume that the desired firing angle
on the motor side is constant and that a torsional oscillation is present in a speed
oscillation on the motor section. Firing times on the output bridge are calculated
with respect to the PLL/observer output reference. If the PLL/observer does not
accurately track the motor voltage (intentionally or unintentionally) and a constant
firing angle is modulated based on the "observed angle" (which is different from
the real angle as the PLL/observer is not accurate), the real firing angle on the
motor side is modulated by the speed itself, depending on the PLL/observer
transfer function. Having a modulation of the firing angle that is a response to a
speed oscillation in the shaft may act as a damper.
Thus, Figure 19 illustrates how a digital output of the PLL/observer 162 is
compared in a controller 78 (e.g., logic block or the controller discussed above
with regard to Figure 9) with square-wave signals representative of motor
voltages. The signals representative of the motor voltages are received by
controller 78 from detector 160 along link 172. As already pointed out, zerocrossing
of voltages are not the only signals related to motor/drive variables that
may feed the PLL (or observer) 162. Then, through a digital operation, a
correction signal is generated. Such a signal could be an analog or a digital
signal. In the first case (mixed solution), the signal is sent from controller 78 along
link 174 and is used to modulate the desired a and/or b angle. In the second case
(digital solution), the digital signal is sent directly from controller 78 (see link 70 in
Figure 19) to gates of the inverter/converter to perform the commutation. It is
noted that the operation described here (comparison between the output of the
PLL/observer and input signals coming from motor) may be already embedded in
the PLL (phase comparator) or observer. Controller 78 may include a control logic
that is configured to verify that the correction signal on the b angle does not
reduce a safety margin (unipolar modulation, as the b angle can be only reduced
but not increased).
Figure 19 also shows an optional limiter 166 that limits a value of the modulation
produced by the controller 78. Optionally, a switch 168 may be provided between
the limiter 166 and the LCI 62 for switching on and off the control data generated
by the controller 78 which is provided to the LCI 62.
In one application, the algorithm described above for Figure 9 may work more
efficiently if the PLL/observer 162 does not accurately track the motor rotor or the
motor voltage vector position. Modifying the PLL/observer transfer function to
damp oscillations in an attempt to have a visible difference between "observed"
and real positions may have a series of drawbacks (including possible reduction of
safety angle).
For this reason, inside the controller 78, a second PLL/observer 164 may be
provided. In this way, the first PLL/observer 162 may be accurate enough to track
the motor rotor or the motor voltage vector position, while the second
PLL/observer 164 is configured to follow the motor rotor or voltage vector position
with a different dynamic (that now can be chosen without many restrictions). In
steady state, at constant speed, the output of the two PLlJobservers' may be the
same, while during transients (including oscillations in speed around a center
value) the outputs of the two PLL/observers are different. The result of
comparison between the two PLL/observers' may be used to implement another
damping solution.
In the case in which two PLL/observers are used, both the mixed solution is
possible (e.g., angle b is modulated by changing the b reference value) or the fully
digital solution is possible (a comparison of the two digital signals coming from the
two PLLJobservers' is digitally processed and the result of the comparison is a
command sent directly to the gate units of the inverter without any direct
modulation on the b angle reference value).
According to an exemplary embodiment, a method for damping a torsional
vibration in a drive train including an electrical machine is illustrated in Figure 20.
The method includes a step 2000 of receiving converter data related to variables
of the converter (e.g. zero crossing), a step 2002 of receiving a first digital signal
from a first phase lock loop device or a dynamic observer, a step 2004 of
comparing the converter data with the first digital signal, a step 2006 of generating
control data for a rectifier and/or an inverter of the converter for damping a
torsional oscillation in a shaft of the drive train based on a result of the
comparison, and a step 2008 of sending the control data to the rectifier and/or to
the inverter for modulating an active power exchanged between the converter and
the electrical machine. In optional steps, the control data is digital and is sent
directly to the converter for commutating gates of the converter or the control data
is analog and is used for producing s- , b- , or a- and b-modulation to be applied to
the converter modulator.
However, if two PLL (or observer) units are made available in the system, then a
different method may be implemented for damping a torsional vibration in a drive
train including an electrical machine. According to this embodiment illustrated in
Figure 2 1, the method includes a step 2100 of receiving a first digital signal from a
first phase lock device or first dynamic observer, a step 2 02 of receiving a second
digital signal from a second phase lock loop device or a second dynamic observer,
a step 2104 of comparing the first digital signal with the second digital signal, a
step 2106 of generating control data for a rectifier and/or an inverter of the
converter for damping a torsional oscillation in a shaft of the drive train based on a
result of the comparison, and a step 2108 of sending the control data to the
rectifier and/or to the inverter for modulating an active power exchanged between
the converter and the electrical machine. In optional steps, the control data is
digital and is sent directly to the converter for commutating gates of the converter
or the control data is analog and is used for producing a- , b- , or a- and b-
modulation to be applied to the converter modulator.
The disclosed exemplary embodiments provide a system and a method for
damping torsional vibrations. It should be understood that this description is not
intended to limit the invention. On the contrary, the exemplary embodiments are
intended to cover alternatives, modifications and equivalents, which are included
in the spirit and scope of the invention as defined by the appended claims. For
example, the method may be applied to other electric motor driven mechanical
systems, such as large water pumps, pumped hydro power stations, etc. Further,
in the detailed description of the exemplary embodiments, numerous specific
details are set forth in order to provide a comprehensive understanding of the
claimed invention. However, one skilled in the art would understand that various
embodiments may be practiced without such specific details.
Although the features and elements of the present exemplary embodiments are
described in the embodiments in particular combinations, each feature or element
can be used alone without the other features and elements of the embodiments or in
various combinations with or without other features and elements disclosed herein.
This written description uses examples of the subject matter disclosed to enable any
person skilled in the art to practice the same, including making and using any
devices or systems and performing any incorporated methods. The patentable
scope of the subject matter is defined by the claims, and may include other examples
that occur to those skilled in the art. Such other examples are intended to be within
the scope of the claims.

CLAIMS:
1. A torsional mode damping controller system connected to a converter or placed
inside the converter itself, said converter driving a drive train including an electrical
machine and a non-electrical machine, the controller system comprising:
an input interface configured to receive converter data related to variables of the
converter; and
a controller connected to the input interface and configured to,
receive a first digital signal from a first phase lock loop device or a dynamic
observer,
receive the converter data from the converter,
compare the converter data with the first digital signal,
generate control data for a rectifier and/or an inverter of the converter for damping
a torsional oscillation in a shaft of the drive train based on a result of the
comparison, and
send the control data to the rectifier and/or to the inverter for modulating an active
power exchanged between the converter and the electrical machine.
2. The controller system of claim 1, wherein the control data is digital and is sent
directly to a converter modulator and/or gate units for commutating gates of the
converter.
3 . The controller system of claim 1 or claim 2 , wherein the control data is analog
and is used for producing a- , b- , or a- and b-modulation to be applied to converter
modulators.
4 . The controller system of any preceding claim, wherein the variables of the
converter include a voltage or a current supplied to the motor.
5. The controller system of any preceding claim, wherein the converter data is
related to a zero value of an alternating voltage or current.
6. The controller system of any preceding claim, wherein the controller is
configured to generate the control data based only on the converter data.
7. A torsional mode damping controller system connected to a converter or placed
inside the converter itself, said converter driving a drive train including an electrical
machine and a non-electrical machine, the controller system comprising:
a first input interface configured to receive a first digital signal from a first phase
lock device or a first dynamic observer;
a second input interface configured to receive a second digital signal from a
second phase lock device or a second dynamic observer; and
a controller connected to the first and second input interfaces and configured to,
receive the first and second digital signals,
compare the first digital signal with the second digital signal,
generate control data for a rectifier and/or an inverter of the converter for damping
a torsional oscillation in a shaft of the drive train based on a result of the
comparison, and
send the control data to the rectifier and/or to the inverter for modulating an active
power exchanged between the converter and the electrical machine.
8 . A system for driving an electrical machine that is part of a drive train, the system
comprising:
a rectifier configured to receive an alternative current from a power source and to
transform the alternative current into a direct current;
a direct current link connected to the rectifier and configured to transmit the direct
current;
an inverter connected to the direct current link and configured to change a
received direct current into an alternative current;
an input interface configured to receive converter data related to variables of the
converter; and
a controller connected to the input interface and configured to,
receive a first digital signal from a first phase lock loop device or a dynamic
observer,
receive the converter data from the converter,
compare the converter data with the first digital signal,
generate control data for a rectifier and/or an inverter of the converter for damping
a torsional oscillation in a shaft of the drive train based on a result of the
comparison, and
send the control data to the rectifier and/or to the inverter for modulating an active
power exchanged between the converter and the electrical machine.
9 . A method for damping a torsional vibration in a drive train including an electrical
machine, the method comprising:
receiving converter data related to variables of the converter;
receiving a first digital signal from a first phase lock loop device or a dynamic
observer;
comparing the converter data with the first digital signal;
generating control data for a rectifier and/or an inverter of the converter for
damping a torsional oscillation in a shaft of the drive train based on a result of the
comparison; and
sending the control data to the rectifier and/or to the inverter for modulating an
active power exchanged between the converter and the electrical machine.
10. A method for damping a torsional vibration in a drive train including an
electrical machine, the method comprising:
receiving a first digital signal from a first phase lock device or a first dynamic
observer;
receiving a second digital signal from a second phase lock loop device or a
second dynamic observer additional to the first phase lock device or the first
dynamic observer;
comparing the first digital signal with the second digital signal;
generating control data for a rectifier and/or an inverter of the converter for
damping a torsional oscillation in a shaft of the drive train based on a result of the
comparison; and
sending the control data to the rectifier and/or to the inverter for modulating an
active power exchanged between the converter and the electrical machine.