Patent Publication Number: US-2015078917-A1

Title: System and method for converterless operation of motor-driven pumps

Description:
BACKGROUND 
     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. 
     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. 
       FIG. 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 . 
     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. 
     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 
     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 during system starting, or in response to equipment failure or in response to occurrence of one or more unforeseen events. 
     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 
       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: 
         FIG. 1  illustrates a conventional electrical submersible pump (ESP) system that is known in the art; 
         FIG. 2  illustrates a converterless ESP system according to one embodiment; 
         FIG. 3  is a block diagram illustrating a system controller interfacing with and controlling a converterless ESP system according to one embodiment; and 
         FIG. 4  is a block diagram illustrating a method of providing off-grid power to a motor-driven submersible well pump according to one embodiment. 
     
    
    
     While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as 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 
     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. 
       FIG. 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. 
     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 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. 
     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. 
     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 . 
       FIG. 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. 
     The programmable system controller  36  is responsible for monitoring the pump operating conditions, including without limitation input and output pressures, 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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 
     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. 
     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. 
       FIG. 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 . 
     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 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 
     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. 
     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. 
     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. 
     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. 
     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. 
     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.