Coal Seam Gas training at TAFE Queensland SkillsTech: Field based Variable Speed Drive (VSD) units for CSG Well Heads

Most CSG proponents are still operating brown field sites in the Western Downs of Queensland. Brown field sites are typified by older reciprocal compressors and hydraulic packs units (HPU’s) that run on coal seam gas or diesel to drive progressive cavity pumps (PCP’s). These PCP’s bring the coal seam gas and water to the surface where it is separated. The water is sent to ponds where desalination and reverse osmosis plants purify the water for agricultural use. The gas is dried, compressed and piped to Curtis Island for the LNG (liquefied natural gas) stage.

However, the newer green field sites use more efficient centrifugal compressors which require far less maintenance. The PCP pumps are also changing and moving away from the hydraulic drives to electrical motors to pump water and gas to the surface.

Oil Lift Hydraulic Well Head

Hydraulic Packs Unit (HPU) Engine and Head

Until recently, electric induction motors have been difficult to control. Normally the motors use an on/off or ‘soft start’ control to overcome the initial rush of start-up current which is usually up to 7 times the operating current. These motors characteristically are connected to the electrical supply grid which is alternating current (AC), 3 phase, 400 volts and 50 hertz frequency. These direct connect motors are designed for full load and when the load changes or drops the motor wastes energy and becomes largely inefficient. Enter Variable Speed/Frequency Drives (VSD’s). Instead of the motor being connected directly to the supply grid, the VSD is installed between the motor and the grid.

Schneider Electric 15 KW VSD (Variable Speed Drive) Unit

The VSD unit is connected directly to the grid which is fixed-frequency (50 Hz) and fixed voltage (400 volt). The unit converts this alternating voltage via an integrated rectifier to direct current (DC). The VSD unit contains integrated power electronics and on-board microprocessors (invertors) which switch this DC on and off rapidly turning the DC back into a controllable sinusoidal-type output. This new sinusoidal waveform provides a continuously variable frequency and voltage to drive the motor. So the fixed sinewave (50 Hz from the grid) is converted into an infinitely variable controllable sinewave output. This variable output enables the VSD unit to quickly change the speed and the torque of the motor in response to the changing load of the PCP pump, offering improved efficiency. The VSD drives are configured and the parameters are matched to the size and type of the motor and field based loads. This type of variable control also extends maintenance cycles and prolongs the life of the motor. The VSD also improves power factor corrections which can result in significantly lower electricity costs for the CSG proponents.

Electric Oil Lift Well Head

VSD enable AC motors to deliver optimum torque while drawing approximately up to 50 percent of its rated operating current. SkillsTech Australia has partnered with Schneider Electric to provide introduction courses in VSDs as they apply to CSG sector and Well Heads. Email: garry.hargreaves @tafe.qld.edu.au at SkillsTech Australia.