In May of 2017, Ingeteam Inc. was awarded a project to supply a medium voltage Variable Frequency Drive (VFD) for the purpose of starting a series of four LNG compressors. The installation took place at an existing LNG plant belonging to Caprock Midstream, where there were already compressors that were being driven by natural gas engines. 

The tentative goal is to ultimately remove all natural gas engines from the plant, and substitute their production capabilities with electrical motor-driven compressors. The VFD system was successfully commissioned in August of 2017 and has been in successful operation since.

Due to the remote location, the poor grid quality, and the large inrush current associated with the direct on-line connection of electrical motors, VFDs are becoming more commonly used as motor starters. By nature a VFD is able to limit the inrush current being supplied to the motor to a minimal percentage of the rated current of the motor, allowing it to have a small impact on the grid upon starting, while also being able to start the unloaded compressor with the time window specified by the operator. This is something that soft-starters cannot accomplish, since they are not able to reduce the inrush current as much as a VFD. 

Additionally, the VFD has the capability to vary the rotational speed of the motor by delivering a variable frequency output. This allows the system operator to operate at reduced levels during times when full load production is not desired. 

The VFD will start the motor by gradually increasing the output frequency until the motor is spinning at the rated speed. Once the rated speed is achieved, the VFD will synchronize the output phases of the VFD with the grid making it a “bump-less” transfer, leaving the electrical motor connected to the grid for full speed operation. This process can then be repeated for all four of the motors. 

Borja Sanchez | IMD Sales Manager