Motor and pump units can start either directly across the line (utility) or through a VFD. If the power system is weak (high impedance and relatively low available MVA), the result will be limitations on the maximum size of motor that may be started directly across the line. Currently some available power systems are not capable of starting large motors across the line and therefore VFD systems are required.
A VFD can be used for starting purposes or to operate a motor/pump unit (unit) continuously. Liquids pipelines have been using VFDs for unit starting purposes and for pressure control while either one of the units connected to the VFD or while there is a dedicated VFD per unit.
One VFD can be used to operate all the units. The VFD starts the motor, ramps it up, and synchronizes it with the utility to switch the motor from VFD operation to utility operation. The VFD follows the same sequence with the next unit. The VFD also switches the motor from utility operation to VFD operation. With pumps in series, one unit can remain connected to the VFD for pressure control in liquids pipelines.
Each unit can also be controlled by a dedicated VFD, allowing independent control of each unit for start-up and speed control. Both single VFD or dedicated VFD per unit offer advantages. Benefits of using a single VFD for all units are cost savings for some applications, a backup start across the line when allowed by the power system, and station pressure control. The benefits of a dedicated VFD per unit application include more flexibility in flow and pressure control, ease of expansion without interrupting existing operations for an extended period, energy efficiency, and redundancy.
Motor protection schemes for the above two VFD applications are discussed and compared in this paper. In order to maintain a safe and reliable pipeline system, comprehensive motor protection functions are provided to the pump unit. Although the principles are the same, there are some differences between the two motor protection applications.
For a dedicated VFD per unit application, the VFD protection/control circuitry normally provides motor protection because the motor is always connected to the VFD. RTDs over-temperature protection is not typically a standard feature of VFDs, however, it can be provided by proper PLC programming or the VFD incoming feeder protection relay.