Sensorless Long Step-out Variable Speed Drives
In downhole applications, motors in the wellbore are needed for downhole pumping, compressing or blowing of well fluids to enhance fluid recovery and process flow. Downhole device operating speeds are determined by the process fluid and method of enhancement to the fluid flow, therefore the right operating speed for the specific hydraulics is necessary to optimize the process. The speed range for Upwing downhole equipment can vary from 3,000rpm for heavy oil to 50,000rpm for pure gas compression. As the operating speed increases, the value of the variable speed drive (VSD) system becomes significantly important.
By utilizing cutting-edge technology in motor drives, Upwing is able to take full advantage of the latest technology in permanent magnet (PM) motors for downhole applications. Upwing sensorless drive systems can operate and control PM motors over 60,000rpm from the surface of a well over 12,000ft downhole without sensors and able to catch the motor’s spinning rotor on the fly without bringing the complete downhole rotating system to a full stop.
Upwing’s drives are optimized with the whole system in mind. In particular, the drives have features that maximize the efficiency of the drive, such as space vector modulation and dynamic deadtime insertion in the switching patterns of the power devices. Additionally, the total harmonic distortion (THD) of the machine currents and grid side currents are controlled to a low value. This reduces the losses in the machine that is connected to the drive, which in turn, increases the efficiency of the machine. Also, dynamic operations resulting from sudden speed or torque demand are managed by sliding mode control mixed with optional back emf feedback. The VSD control hardware consists of a rugged dual core processor capable of operating up to a PWM switching frequency of 60 khz. The processor also is capable of operating at an ambient temperature of 125°C
Upwing’s VSDs are designed to drive high-speed machines and machines with low impedance. This is achieved by switching the power devices of the drive at a high frequency that is synchronized to the fundamental frequency of the machine. Consequently, the fundamental frequency of the machine can be high without incurring a large distortion in the current that is drawn by the machine.
A VSD designed for an optimized low impedance machine can drive high impedance machines without any changes. Therefore, machines running applications that require long step-outs, automatically appear as high impedance machines to the VSD output and can be driven without any major difficulty.
The distortion is further decreased by dynamically adjusting the inserted deadtime in the switching patterns of the power devices. In addition, Upwing’s high frequency drives include a simple inductance filter, which further reduces the distortion of the current that is drawn by the machine. In fact, the filter limits the maximum distortion of the machine currents. In other words, the drive does not need to rely upon the machine inductance to limit the distortion in the currents that are drawn by the machine. The combination of high switching frequency that is synchronized to the fundamental frequency of the machine, dynamic deadtime insertion, and a simple inductance filter make Upwing’s high frequency variable speed drives an ideal choice for low-impedance and high-speed machines.
Active and Passive Grid Rectifiers
Upwing’s VSD is designed with both active and passive grid rectifier options built in. Based on the grid condition, the rectifier option can be selected through the control settings. The ability to choose between these two options is a distinguishable feature of Upwing’s VSDs, which are not offered or incorporated by commercial VSD manufacturers. The advantage of an active rectifier is that it offers very low current distortion to the grid; also, it can operate at a wide input voltage range.
In remote areas, the grid is typically soft, and in most cases, is generated by simple single phase to three phase rotary converters. An active rectifier at the front end of the VSD provides low harmonics but in some cases can result in unstable input voltage. In such installations, a passive option can be enabled to prevent oscillations if the grid can tolerate higher harmonics.
Reliability and long operating life are at the forefront of importance in the design of Upwing’s power electronics drives. This is obtained by keeping the electrical and thermal stresses on the components within the drive to reasonable levels. The reliability of Upwing’s drives is further increased by reducing the parts count in the drive. In particular, the avoidance of requiring position feedback from the machine and voltage feedback for the machine greatly increases the robustness of the drive. All Upwing drives go through rigorous electrical and thermal testing before they leave our factory, minimizing manufacturing and component infant mortality issues. High reliability, reasonable stress levels and rigorous testing result in the long product life spans of our variable speed drives.
Upwing’s variable speed drives are re-programmable in order to meet the requirements of applications. This is achieved through the use of generic embedded software and unique parameter files that are customized for a particular application. Typical drive characteristics, such as current limits, overspeed limits and acceleration rates are readily changeable. For applications that require unique capabilities, the embedded software also can be modified. The ability to re-program the embedded controllers coupled with the ability to customize the drive hardware allows our drives to meet the needs of different applications.
The data interface and communication channels of Upwing’s VSDs also enable autonomous control of our downhole rotating devices. The VSD control has ground fault protection. In the event of a ground fault, the control will shut down. External protective relays, such as Beckwith or Basler protective relays, can also be used.