Variable Freq. Generator (VFG)

The most common alternative for High-Line power in an oilfield is the use of a portable generator (Gen-Set) in combination with either a Fixed-Speed Drive (FSD or Switch Board) or a Variable-Speed Drive (VSD). Both of these configurations, however, have limitations. In order to overcome these limitations, a novel type of Gen-Set, called a Variable-Frequency Generator (VFG), was designed and built with the capabilities of starting large motors and of operating them at a variable speed and voltage. The VFG is an engine-driven medium or low voltage generator that provides electrical power at a variable frequency and a variable voltage with a perfect sine wave without harmonic distortions. This technical paper reviews the factory- and field-test data of a 660[kW]/825[kVA] VFG system powering a 760[HP] ESP. The paper also describes in detail the benefits of a VFG system as compared to a Gen-Set/VSD or Gen-Set/FSD systems.

Where is the nearest power grid to supply high-voltage power for artificial lift in an oilfield? On many occasions, the grid infrastructure is either not available or requires a major capital expenditure to extend it to a remote location. The most common alternative is to use a portable generator (Gen-Set) in combination with either a Fixed-Speed Drive (FSD) or a Variable-Speed Drive (VSD). Both of these configurations, however, have limitations. On the one hand, a Gen-Set/FSD system that consists of an 855[HP] diesel engine and a 660[kW] generator can barely start the motor of a 400[HP] Electrical Submersible Pump (ESP). While larger motors can be operated using this system, getting them started is difficult due to a high inrush current. On the other hand, an equivalent Gen-Set/VSD system can start and run a 760[HP] ESP; however, the VSD is not a preferred load for the generator and produces ‘dirty’ power that is harmful to the ESP. In order to overcome these limitations, a novel type of Gen-Set, called a Variable-Frequency Generator (VFG), was designed and built with the capabilities of starting large motors and of operating them at a variable speed and voltage. The VFG is an engine-driven medium-voltage generator that provides electrical power at a variable frequency and a variable voltage with a perfect sine wave without harmonic distortions. It has a proven thirteen-year track record with both diesel and natural-gas engines, operating continuously in extremely hot desert environments, in extremely wet jungle environments and cold Northern Alberta and British Columbia winters without requiring the replacement of any major components.

The benefits of a VFG system include:  

• Outputs a pure sinusoidal waveform without harmonics across the entire ranges of  variable speed and voltage. This results in less electrical and mechanical stresses on the motor, the Motor Lead Extension, the main cable and the wellhead penetrator, all of which help to extend the ESP’s run life.
• Increases production by more than 7% when compared with a conventional Gen-Set/VSD system with the same load.
• Reduces motor amperage by more than 4% due to the output of a perfect sine wave.
• Increases surface electronics’ run-life, even in hot desert environments. The outputted pure-sinusoidal waveform significantly reduces wear on all electrical and mechanical components, resulting in an extended Mean-Time-Between-Failures (MTBF), decreased down-time and lower maintenance costs when compared with a Gen-Set/VSD system. Specifically, the need for electrical and electronic spare parts can be reduced by approximate 90%.
• Provides a higher electrical efficiency, lower energy consumption and competitive initial and operating costs when compared with a Gen-Set/VSD with the same load. Specifically, it provides proven fuel cost savings when a Bi-fuel (natural gas / diesel) system is used in conjunction with a diesel engine, and it eliminates fuel cost when field gas is used with a gas engine.
• Eliminates the need for an ESP-rated transformer in case of system utilizing Medium Voltage Generator.
• Fully exploits both the engine and generator’s ratings in order to meet the system’s power requirements, which reduces the initial cost by eliminating the need for an oversized Gen-Set. It is customary to oversize a Gen-Set by approximately 300% for applications where a three-phase electric motor needs to be started Directly-on-Line (DOL) with a Gen-Set/FSD system. For example, an 85[HP] motor requires a 257[kVA] Gen-Set/FSD. The same motor would still require a 50% oversized Gen-Set/VSD system. However, field tests have demonstrated that a VFG utilizing an engine with 2250[HP] available for a 2000[kVA] medium-voltage generator is capable of starting and operating a 2000[HP] motor.
• Can continuously operate as low as 35% of the engine’s load factor, while Gen-Set/VFD systems are limited to operating only between 85% and 100% of their rated power.

Introduction

This technical paper reviews the factory- and field-test data of a 660[kW]/825[kVA] VFG system powering a 760[HP] ESP. The paper also describes in detail the benefits of a VFG system as compared to a Gen-Set/VSD system. The tested VFG system consisted of the following major components:  

• Diesel engine rated for 855[HP] at 1800[rpm]
  • Operates at any speed from 900[rpm] to 1950[rpm] (30[Hz] – 65[Hz])
• Synchronous generator rated for 660[kW], 825[kVA] and 131[A]
  • Produces a voltage in the range of 1.2[kV] to 4.0[kV]
  • Starts a motor at low frequencies and operates it at any frequency between 30[Hz] to 65[Hz].
  • Specialized electronic motor controller with Soft-Start capability.

Shop Testing

Prior to field installation, a shop test was conducted on the VFG in order to evaluate both its different motor-starting methods and its generator-to-motor loading characteristics. A 640[HP] (KMH-J 640HP/60Hz/3470V/117A) ESP motor coupled to a 12000[bpd] (105KC12000) centrifugal pump was selected for this test. In addition, the motor and generator were connected together with 6500[ft] of #1AWG round main cable in order to simulate the expected distance between the two in the field. 

A 50[Hz] start mode with a 77% Soft-Start (2226V) resulted in the ESP system starting and stabilizing in three seconds. The inrush current during the motor start-up was measured at 2.8 times the nameplate amps.

A ramp-up start mode that was initiated at 40[Hz] with an 88% Soft-Start, while the generator was accelerated from 900[rpm] to 1500[rpm], resulted in a stabilization time of only two seconds and an inrush current of 2.4 times the nameplate amps.

Once the ESP motor was started, its frequency was varied from 30[Hz] to 65[Hz] by adjusting the speed of the diesel engine. Upon the completion of the ESP motor’s starting and running evaluations using the VFG system, it was sent for field testing.   

Field Testing    

Equipment at Test Site:  

• ESP motor, 760[HP], 3760[V], 123[A]
• 16000[bpd] centrifugal pump
• 850[kVA] Gen-Set/VSD system

Measurements at Test Site while connected to Gen-Set/VSD:  

• Maximum achieved operating speed: 57[Hz] 
• Fluid pumped at 57[Hz]: 18250[bpd]
• ESP motor pulled 114[A] at the above conditions

The ESP was then disconnected from the Gen-Set/VSD and connected to the VFG. It is important to note that there were no changes made below the wellhead, only to the surface generator. Measurements at Test Site while connected to VFG:  

• With VFG operating at 57[Hz]:
  • 18250[bpd] of pumped fluid 
  • ESP motor pulled only 109[A]   
• With VFG operating at 114[A] load from ESP motor:
  • 19550[bpd] of pumped fluid at 59[Hz]   

Test point #1 was for a constant-speed comparison between the Gen-Set/VSD and the VFG. When the ESP motor was driven by the Gen-Set/VSD, it used more power than when driven by the VFG. This was verified by using a constant speed of 57[Hz] for both systems. The measured amperage was at 114[A] for the Gen-Set/VSD and 109[A] for the VFG, resulting in a 4.5% reduction of the amperage load. Test point #2 was for a constant-amperage comparison between the Gen-Set/VSD and the VFG. The VFG was sped up to 59[Hz] in order for the motor to draw the same amperage that it did when the Gen-Set/VSD was at 57[Hz]. Using the data from the Field Production Report, this increase in speed was related directly to an increase in production from 18250[bpd] to 19550[bpd], which is an incremental increase of 1250[bpd] or 7.1%.

The incremental increase of revenue in one year, assuming a 7% production increase after upgrading from a Gen-SET/VFD system to a VFG, justifies the upgrade. 

A VFG system is capable of starting and operating an ESP motor without the requirement of oversizing either the engine or generator. When comparing the VFG to the Gen-Set/VSD used in the field test, the ‘clean’ sine-wave power supplied by the VFG resulted in a reduction in the ESP motor’s current for a given operating speed. Conversely, if the well is capable of greater production for a given current limit, then the motor speed can be increased, resulting in additional fluid production. The clean sine wave produced by the VFG has no harmonics or voltage spikes. It is anticipated that this ‘clean’ power will contribute to longer ESP run lives due to reduced electrical stresses on the motor, the Motor Lead Extension, the main cable and the wellhead penetrator. It is also demonstrated that a VFG will have lower maintenance costs than that of the Gen-Set/VSD because the former is less susceptible to high ambient temperature.   

SPE166191 "Variable Speed Generator" SPE Annual Caspian Technical Conference and Exhibition, Baku, Azerbaijan; November 2015   

"Variable Speed Generator" 20th International Petroleum and Natural Gas Congress and Exhibition, Ankara, Turkey; May 2015   

SPE166191 “High Pressure Multi-Stage Centrifugal Pump for 10,000PSI Frack-Pump - HPHPS FRAC Pump” Annual Technical Conference and Exhibition, New Orleans; October 2013   

“HPHPS used to support shale gas fracturing in Horn River Basin” SPE-Gulf Coast / ESP Workshop, Houston; April 2013   

"Variable Speed Generator" Society of Petroleum Engineers - Artificial Lift Technical Exchange Meeting, Bahrain; March 2009   

"Variable Speed Generator" Society of Petroleum Engineers - SPE-Gulf Coast / ESP Workshop, Houston; April 2005