Note: Descriptions are shown in the official language in which they were submitted.
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
SYSTEM AND METHOD FOR CONVERTERLESS OPERATION OF MOTOR-
DRIVEN PUMPS
BACKGROUND
[0001] The subject matter of this disclosure relates generally to motor-
driven
pumps, and more particularly, to a system and method for converterless
operation
of motor- driven pumps.
[0002] The conventional approach to controlling the speed of motor-driven
pumps is
through the use of a variable speed drive (VSD) that is fed by a fixed
frequency AC supply. The VSD synthesizes voltages and currents of such
frequency
as is necessary to operate the pump in the desired manner. In the oil and gas
industry,
the voltage output by the VSD is usually stepped up to a medium voltage using
a
transformer because high voltage motors are deployed in wells to reduce the
size of
the power cable needed to supply the motor.
[0003] Figure 1 illustrates a conventional system 10 that is known in the oil
and gas
industry for operating electric submersible pumps (ESPs) 12 in an off-grid
application. One or more prime movers that are directly coupled to generators
14
produce an AC voltage having a fixed frequency and amplitude to supply
electrical
loads 15. The prime movers may comprise, for example, a reciprocating engine
that is
fueled by either natural gas or diesel fuel, or a turbine. The generated AC
power is fed
to a VSD 16 that is responsible for regulating the operation of the ESPs 12
subsequent
to stepping up the AC voltage to a medium voltage level that is supplied to
ESP
motor(s) 18 via a suitable transformer 19.
[0004] There is a need in the oil and gas industry to provide a system for
operating
ESPs that is less complex, less costly, and that has a smaller footprint. A
system that
reduces the capital expense, weight and footprint size will advantageously
reduce
the time it takes to put a well into production using power generated on-site
when
compared with the time it takes to put a well into production using utility
power
because of the delays associated with getting the utility to install necessary
power
lines.
I
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
[0005] It is possible to use the natural gas produced by the well to support
operation
of the generator, thereby reducing the operating expense of the system.
Depending on
the selection of the generator and the prime mover, it may be necessary to
couple the generator and the prime mover through a gearbox. It is generally
possible
to select a gearbox with a fixed ratio, thereby avoiding the need for changing
gear
ratios during system operation.
BRIEF DESCRIPTION
[0006] According to one embodiment, a converterless motor-driven pump system
comprises:
at least one off-grid prime mover comprising a rotational driveshaft and
operating in
response to a throttle control command to control a rotation speed of the
rotational driveshaft;
at least one electric power generator driven by the at least one off-grid
prime
mover to generate AC power;
at least one variable speed motor directly powered by the at least one
electric
power generator;
at least one electric submersible pump driven by the at least one variable
speed
motor, wherein one or more operating characteristics associated with the at
least one electric submersible pump are monitored by one or more corresponding
sensors;
a system controller programmed to generate the throttle control command in
response to the one or more pump operating characteristics such that the at
least one
off- grid prime mover, the at least one electric power generator, and the at
least one
variable speed motor together operate to regulate a pressure at the inlet of
the at
least one electric submersible pump; and monitoring and protection equipment
comprising circuit breakers to ensure safety to personnel around the system,
and
to provide protection to the prime mover, generator, and variable speed motor
2
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
during system starting, or in response to equipment failure or in response to
occurrence of one or more unforeseen events.
[0007] According to another embodiment, a method of operating an electric
submersible pump comprises:
controlling a driveshaft speed of an off-grid prime mover in response to a
throttle
control command;
controlling an AC power output of an electric power generator in response to
the
driveshaft speed of the off-grid prime mover;
controlling a speed of a variable speed motor directly in response to the AC
power
output of the electric power generator; and
monitoring operating characteristics of the electric submersible pump and
generating the throttle control command in response thereto such that together
the off-
grid prime mover, the electric power generator, and the variable speed motor
operate
to regulate a pressure at an inlet to the electric submersible pump.
DRAWINGS
[0008] These and other features, aspects, and advantages of the present
invention
will become better understood when the following detailed description is read
with
reference to the accompanying drawings, wherein:
[0009] Figure 1 illustrates a conventional electrical submersible pump (ESP)
system that is known in the art;
[0010] Figure 2 illustrates a converterless ESP system according to one
embodiment;
[0011] Figure 3 is a block diagram illustrating a system controller
interfacing with
and controlling a converterless ESP system according to one embodiment; and
[0012] Figure 4 is a block diagram illustrating a method of providing off-grid
power
to a motor-driven submersible well pump according to one embodiment.
[0013] While the above-identified drawing figures set forth particular
embodiments, other embodiments of the present invention are also contemplated,
as
3
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
noted in the discussion. In all cases, this disclosure presents illustrated
embodiments
of the present invention by way of representation and not limitation. Numerous
other
modifications and embodiments can be devised by those skilled in the art which
fall
within the scope and spirit of the principles of this invention.
DETAILED DESCRIPTION
[0014] The embodiments described herein are directed to control of motor-
driven pumps in applications that are operating independently of a utility
power
grid, and combine the control of a prime mover and an AC generator to provide
substantially similar functionality as a variable speed drive (VSD) to reduce
system
complexity, cost and footprint size. Such embodiments are particularly useful
in the
oil and gas industry where the usual control objective is to regulate the
pressure at
the inlet of the motor driven submersible pump, although other control
objectives,
including without limitation, temperature, speed, or vibration can be applied
in like
fashion.
[0015] Figure 2 illustrates a converterless ESP system 20 according to one
embodiment. In this embodiment, the prime mover(s) 21 is/are directly
controlled
to regulate the pump inlet pressure. More specifically, the ESP system 20
comprises
one or more prime movers 21 that are coupled to one or more generators 22,
means 24
for electrically connecting the output of the generator(s) 22, and a motor-
driven pump
26. The prime mover 21 is typically a reciprocating engine that is fueled by
either
natural gas or diesel fuel, but is not so limited, as other types of prime
movers such as,
without limitation, turbines may also be employed as the prime mover 21.
Depending
on the selection of the prime mover 21 and the generator 22, it may be
desirable
to use a gearbox to match the shaft speeds of the prime mover 21 and
generator 22. It is preferable to use a fixed ratio gearbox to keep the system
20 as
simple as possible. The motor-driven pump 26 is typically located within a
well for
purposes of artificially lifting a fluid from the well. The fluid could be,
without
limitation, water, gas or oil in a well, or a combination thereof. It is
likely some
amount of solids, such as sand or proppant, will be entrained with the fluid.
[0016] A sensor package 28 is attached to the motor-driven pump 26 that
may comprise, for example, one or more temperature sensors and one or more
4
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
pressure sensors to provide an indication of various pump operating
temperatures and
pressures. An important pressure is the inlet pressure to the pump 26, since
this
pressure provides a direct indication of whether the well is being operated at
the
proper loading for maximizing well production. The sensor package 28 may
further
comprise one or more vibration sensors configured to monitor various pump
vibration characteristics and to provide an indication whether a predetermined
vibration level is exceeded. At least one speed sensor may be included in the
sensor
package 28 in order to accurately monitor the rotational speed of the pump.
Other types of sensors may be included in the sensor package 28 depending on
the particular application requirements.
[0017] The converterless ESP system 20 advantageously i) eliminates the need
for a variable speed drive and transformer, simplifying the system, resulting
in improved system reliability, ii) can use pumped gas via the pump 26 itself
as
the fuel to run the prime mover 22, resulting in very low fuel costs, and iii)
operates independently of a utility power grid.
[0018] It can be appreciated that there may be reasons to retain a
transformer
between the generator 22 and the motor driven pump 26. Such reasons may
include, without limitation, minimizing system cost and/or maximizing
operational
flexibility. According to one aspect, the decision to retain or remove the
transformer
from the system 20 may be made on the basis of system optimization rather than
conceptual operation of the system 20.
[0019] Figure 3 is a block diagram illustrating the flow of power and
information
for a converterless ESP system 30 according to one embodiment. The power flows
from the prime mover 21 through the generator 22 and cable 32 to the motor 34
and
subsequently the pump 26. The power between the prime mover 21 and the
generator
22 is mechanical driveshaft power, as is the power between the induction motor
34 and the pump 26. A gearbox between the prime mover 21 and the generator
22 may advantageously be employed for purposes of system optimization, as
stated
herein.
[0020] The programmable system controller 36 is responsible for monitoring the
pump operating conditions, including without limitation input and output
pressures,
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
pump temperature(s), pump vibration levels, and pump rotational speed, and
commanding the throttle position control 38 of the prime mover 21 that will
drive
the pump 26 output to the desired pump operating point in response to one or
more of the monitored operating conditions. According to one aspect, the
system
controller 36 also monitors the shaft speed of the prime mover 21 and commands
the generator exciter 39 of the synchronous generator 22 accordingly.
[0021] The programmable system controller 36 may comprise, without limitation,
one or more computers and/or data processors/devices and associated display
devices. The data processors/devices may comprise one or more CPUs, DSPs
and associated data storage devices, data acquisition devices and
corresponding
handshaking devices that may be integrated with the system controller 36
and/or
distributed throughout the converterless ESP system 30. The system controller
36 may
communicate with a remote operations center 37 that is able to monitor system
operation and modify system operating objectives without requiring action of a
local
operator.
[0022] According to another aspect, the system controller 36 monitors the
voltage,
frequency and current being supplied to the motor 34, and generates the prime
mover
throttle control command in response to the monitored information to modify
control
of the prime mover 21. The rate of change in prime mover driveshaft speed, for
example, might be controlled to maintain the generator current below a
specified
value by limiting the current being supplied by the generator 22. Such
operation can
help reduce stress on the system, thereby making the converterless ESP system
30
more reliable.
[0023] According to another aspect, the generator 22 may be a permanent
magnet generator that does not require excitation. It can be appreciated that
use of a
permanent magnet generator would further simplify the converterless ESP
system 30 without sacrificing performance.
[0024] It can be appreciated that the pump motor 34 may be any electric motor
that
can be line started, including not only induction motors, but also a special
class of
permanent magnet motors known as line-start permanent magnet motors.
6
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
[0025] In summary explanation, a converterless ESP system eliminates the
variable speed drive and, potentially, its associated transformer from a motor
driven
submersible pump system, resulting in a simpler system that reduces capital
expense,
weight and system footprint. The use of power generated on-site advantageously
reduces the time it takes to put a well into production resulting from delays
in getting
the utility to install requisite power lines. Further, the use of natural gas
produced by
the well itself advantageously reduces the operating expense.
[0026] Since the output of the generator 22 is substantially sinusoidal when
compared with the output of a variable speed drive, a filter is not required
between the
generator 22 and the motor 34. The output of a variable VSD, for example,
contains
significant high frequency content, the result of chopping up DC
voltage/current to
produce AC voltage/current. This chopping action disadvantageously creates
high
frequency components called harmonics that are detrimental to the motor
driving the
pump. A filter is usually installed between the VSD and the motor; however,
anecdotal data suggest that even such a filter may not always adequately
filter
out the harmonics, leading to accelerated aging of the insulation systems in
the
transformer 19, cable 32, and motor 34. This disadvantageously reduces the
life of the
ESP system.
[0027] A VSD also draws nonsinusoidal currents from its supply, unless an
active
front end is applied to the VSD. These resulting harmonics are detrimental to
the
generator supplying the VSD. Many system designs oversize the generator so
that it
can better tolerate the harmonic currents drawn by the VSD. Other system
designs
will use an active power filter to source the harmonic currents drawn by the
VSD,
thereby alleviating the generator from having to supply them. Either of such
approaches adds to the cost and complexity of the system.
[0028] The principles described herein with reference to the various
embodiments include reduced capital expense and more timely well production.
The off-grid converterless system embodiments advantageously allow putting a
well
into production sooner since there is frequently a substantial waiting period
for the
utility to install supply lines to the well site, as stated herein. At such
time as utility
power is available, the well operator can remove the prime mover and
generator,
replacing them with a variable speed drive and transformers if desired.
7
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
[0029] Figure 4 is a block diagram illustrating a method 40 of providing off-
grid
power to a motor-driven submersible well pump 26 according to one embodiment.
A
prime mover 21 driveshaft is coupled directly or indirectly to a generator 22;
while the generator 22 is electrically coupled to a motor that may be a line
start motor
such as an induction motor or permanent magnet motor 34 via a power cable 32
that
may be, for example, without limitation, an electrical submersible pump cable;
and the motor driveshaft is directly coupled to the submersible well pump 26,
as
represented in block 42. The prime mover 21 is turned-on to rotate its
driveshaft,
causing the generator 22 to produce AC power sufficient to power the motor 34,
that
subsequently drives the submersible well pump 26, as represented in block 44.
A
sensor package 28 that may comprise, without limitation, various pressure
sensors,
temperature sensors, vibration sensors, and speed sensors associated with the
submersible well pump 26 function to monitor operating conditions including
without limitation, pump inlet pressure, pump vibration levels, pump
rotational
speed, and temperatures at desired points associated with the submersible well
pump 26, as represented in block 46. The monitored operating data is acquired
by a
system controller 36 that determines whether the prime mover driveshaft should
be
rotating at a different speed. The system controller 36 then transmits an
appropriate
throttle control command 38 to the prime mover 21, causing the prime mover
driveshaft to rotate faster or slower as necessary to ensure the submersible
well pump
26 is operating at the desired operating point, as represented in block 48.
According to one embodiment, the system controller 36 also monitors the
rotational speed of the prime mover driveshaft via one or more speed sensors
25
associated with the driveshaft of the prime mover 21, and commands the exciter
39
of a generator 22 to supply an appropriate level of excitation to the
generator 22 when
the generator 22 is a synchronous generator, as represented in block 50.
[0030] For reasons of safety and system protection, system elements may be
included
that are responsible for monitoring the operation of the system equipment,
with means
to instruct the controller 36 to shut down the system 30 if a failure or
external event
causes an exception to intended operation. Exemplary system elements may
include,
without limitation, one or more pump pressure sensors, pump speed sensors,
pump
temperature sensors, pump vibration sensors, pump viscosity sensors, pump gas
volume fraction sensors, specific gravity sensors, motor current sensors,
motor
8
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
temperature sensors, motor voltage sensors, and motor frequency sensors. A
pump gas
volume fraction sensor, for example, may be employed to determine a volumetric
ratio of liquid versus gas that is flowing through the pump(s). External
events causing
a system shutdown may include, for example, i) a volume fraction that gets too
large,
i.e., too much gas for pump to handle, ii) a motor temperature that gets too
high, or
iii) a clogged pump, causing pump pressure to get too high. Monitored sensor
signals
are transmitted to the system controller 36 that ensures that the motor 34 and
pump 26
are operating within prescribed design, safety, specification and/or threshold
limits.
[0031] Another embodiment includes monitoring the voltage, frequency,
temperature and current being supplied to the motor 34 via the generator 22,
and
acquiring the monitored information, as represented in block 52. The acquired
motor
supply voltage, frequency, temperature and current information is used by the
system
controller 36 to determine whether the prime mover driveshaft should rotate at
a
different speed. If a different prime mover driveshaft speed is required, the
system
controller 36 transmits an appropriate throttle command 38 to the prime mover
21,
causing a change in the running speed of the prime mover 21, as represented in
block 54. This embodiment can be employed in applications where it might be of
interest to, for example, limit the current being supplied by the generator
22; so the
rate of change in prime mover speed could be controlled to keep the generator
current
less than a specified value.
[0032] Since some applications may employ a permanent magnet generator that
does not require excitation, it can be appreciated that a generator exciter
will not be
required in such applications. The use of a permanent magnet generator further
simplifies the converterless ESP system 30 without sacrificing performance, as
stated
herein.
[0033] Although particular embodiments have been described herein with
application to electric submersible pumps, the principles described herein can
just as easily be applied to other applications including without limitation,
geothermal applications. In such applications, gas turbines or reciprocating
engines
can be employed to rotate the generator.
9
CA 02924644 2016-03-17
WO 2015/041805
PCT/US2014/052590
[0034] The principles described herein can be applied to a motor generator
set
feeding a plurality of ESPs (i.e., when an existing oil field is to be
expanded to
include, for example, 50% more wells, wherein the wells are in close proximity
to
each other). The controller 36 in this application is further programmed
according to
one embodiment to provide for load balancing among the ESP motors, thereby
reducing unwanted losses.
[0035] The controller 36 may further be configured with synchronization logic
and
programmed according to yet another embodiment to generate a control signal
that activates an auxiliary/spare generator to provide a parallel operation
capability.
[0036] While the invention has been described in terms of various
specific embodiments, those skilled in the art will recognize that the
invention can be
practiced with modification within the spirit and scope of the claims.
