Sometimes, the critical component in an artificial lift system is not located downhole in the well but at the surface. A case in point is variable speed drive (VSD) technology for extending electrical submersible pump life while cutting operating costs. A new generation of low-voltage Sine Wave Drives (SWD) that uses integral-output sine wave filter technology for operating both electrical submersible and surface pumps to provide robust, economical new solutions
is available.

Complying with strict Institute of Electrical and Electronics Engineers (IEEE) 519 1992 guidelines, the SpeedStar 519 SWD variable speed drive increases system efficiency and reduces harmonic reflection back to the power supply while prolonging run life of the electrical submersible and surface pumps it drives.

Because the new sine wave drive performs equally well when driving submersible pumps as when driving surface pumps, logistical issues at the surface facility are simplified. And because a single type of variable speed drive works for both applications, maintenance, operator training, and spare parts inventories are made more cost-effective.

The key enabler of the new technology’s performance improvements is a drive input integral 18-pulse phase-shifting autotransformer. This innovative feature typically reduces reflected current harmonic distortion to 5%, compared to 25% current harmonic distortion experienced by conventional VSD systems. As a result, power system efficiency is significantly improved, and the package footprint of surface equipment is reduced by eliminating the external phase shifting transformer required with conventional VSD systems.

Common problems eliminated

The new variable speed drive was designed taking into account the most common pump problems encountered in the field, and solutions are integrated into the system in anticipation of those issues. For example, the drive can provide a safe restart of a spinning motor, or deliver a rocking start to free pumps stuck from sand, scale, or debris. It also offers load-side, phase-to-phase, short-circuit protection. Recognizing that electrical loading during pump startup is one of the principle causes of reduced run life, the system has been engineered to deliver higher torque per ampere. This reduces motor heating and torque pulsation, both of which adversely impact equipment run life. By limiting the in-rush current during startup to a maximum of 1.5 times the operating current, all downstream components benefit, including connectors, splices, cables, and electrical insulation.

Under normal operation, superior motor control circuitry keeps the equipment operating at peak efficiency. This includes the ability to maintain unity power factor regardless of loading, mitigation of output harmonics, and speed controls to maintain constant load or pressure. The end result is reduced power consumption through maximum effective use of available power. For example, by integrating the phase-shifting auto transformer into the drive, only three power phase conductors are required. This eliminates the need for an external phase-shifting input transformer, which contributes to reduced installation costs and a smaller surface footprint size while gaining 1% in power consumption efficiency.

Incremental savings add up

The incremental gains in power consumption efficiency can add up to significant savings. For example, a producer in Latin America requested an analysis for 39,390 kilovolt ampere (kVA) units, each housed in its own weatherproof enclosure. The comparison was done for the 12-pulse, 480-volt systems versus an 18-pulse, 480-volt system. Requirements called for a displacement power factor of 0.95 with a total power factor of 0.90. All equipment was required to be warranted for its first 24 months of operation. Schlumberger conducted a physical and electrical analysis of the two systems and used its 18-pulse system to compare to the 12-pulse requirement.

After ensuring that the 18-pulse drive met all the customer requirements, a cost-value analysis was run. It showed that for the unit cost (VSD vs. VSD), the 18-pulse SpeedStar 519 SWD was higher than the 12-pulse VSD, but the cost of the total system package was lower because the sine wave drive did not require an external phase shift transformer. However, the big difference came when the power consumption and efficiency were plotted for the integral 18-Pulse Phase Shifting Auto transformer.

Total harmonic distortion for both current and voltage are within specifications at all speeds for a typical 18-pulse phase-shifting autotransformer. As shown in Figure 2, an essentially constant input power factor curve at all speeds yields maximum efficiency, with near perfect efficiency when operating at maximum speed.

The new sine wave VSD equipment proved to be 0.9% more efficient than the standard 12-pulse system, amounting to an annual energy saving of US $80,000 at an electric cost of $0.10/kW hour. More importantly, the new equipment has a superior power factor of 0.985 regardless of loading. The reactive power factor savings from having a steady power factor of 0.98 compared to 0.90 is 92 kVA reactive (kVAr). This eliminates the need for an oversized generator and provides possible savings of up to $950,000 in annual penalties and power charges at $0.03/kVAr hour.

Finally, installation costs are less with the next-generation VSD technology because of the basic three-phase input power cable hookup and the fact that the drive is housed in a single unit as opposed to a two-piece unit. The new, 18-pulse system is IEEE 519 compliant and carries a five-year warranty compared to the two-year warranty required.

The new equipment comes in a single-piece, TUV third-party certified, IP 56 weatherproof enclosure certification for its 454 kVA size. Other sizes will undergo similar TUV IP 56 certification. All power electronics are protected from the environment. For this application, the case became very clear: sine wave VSD technology was the preferred solution based not only on total cost but also on operating cost and efficiency, overall performance, maintenance, and logistics.

Operational benefits pay off

The SpeedStar 519 SWD technology is easy to operate because it does not require wellsite tuning. The patented integral output filter pushes the resonance band of the downhole system below the drive’s carrier frequency, thereby eliminating excessive resonance and minimizing voltage overshoots. This benefit is realized in spite of variations in cable length, motor type, or the number of transformer taps. The near-sinusoidal output wave form reduces voltage stress levels, motor temperature, and vibration, all of which are known contributors to shortened run life.

Also, the drive and the system are protected against lightning strikes or voltage surges with an inline surge protection device, much more effective than parallel Metal Oxide Varistors.

Should the producer elect to equip pumps with Phoenix downhole monitoring systems and/or espWatcher surveillance and control systems, there is no modification to the surface unit. The new sine wave drive uses the UniConn universal site controller interface for all data acquisition requirements. Data are multiplexed up the existing power cable, so no additional wellhead or pump porting is required. Operation is intuitive to the producer’s needs. Instead of varying input parameters to try to achieve desired results, the producer only has to enter the desired result such as target load or pressure, and the system automatically adjusts itself to achieve them. This feature is invaluable in wells with varying conditions, such as gassy wells or those with viscous oil, and helps operators maximize uptime on wells that could otherwise be problematic to produce.

Since all Schlumberger pumps are compatible with producers’ supervisory control and data acquisition (SCADA) systems or commercial continuous monitoring and surveillance services, operators are able to focus on wells that are underperforming or trending outside of predetermined norms. These wells are automatically detected in the system, and alarms alert experts who diagnose the problem. Most of the time, they are able to remediate the situation remotely by adjusting motor speed or power, or in the case of incipient failure, shutting the pump down before collateral damage occurs. The result is greater uptime, optimized production efficiency, and reduced operating and maintenance costs.