Versatile pumping
AC drive converterstor oil pumping systems

State-of-the-art AC drive converters are also replacing the variable-speed mechanicaI devices which have been used for a long time on pumps for pumping oil and gas. Simovert Masterdrives is establishing itself in all of the usual applications due to a raft of benefits which it offers.

Up until now, pumps which were used to pump crude oil and natural gas, from sources remote from threephase line supplies, were generally powered by diesel engines. The speed, and therefore the flow rates, were adjusted by using an electro-mechanical coupling between the motor, gearbox and drive pulleys. Using Simovert Masterdrives AC drives, this can be realized with a substantially higher functionality and in a significantly more user-friendly so that the pumping equipment be quickly adapt to changing local situations. Three basic pump types are used under the tough conditions met in oil field applications.

Horsehead pumps…
Clearly conjure up the image of oil fields with pumps. They are generally used for flow rates below 150 m³/day and pump depths less than 1500 m.
The oil which is pumped is simultaneously used as the lubricating agent for the pump. For extremely high viscosities, it can occur that the pump chamber is not completely filled, which over time can damage the pump and even fracture the bore rods. This can be prevented by using pressure sensors in the bore hole, which can be much more simply integrated in the overall system when AC drive converters are used.
Generally, these pumps have a cyclic motion which is influenced by the geometry, the counter weights and the bore rods. The motor itself is driven by the counter weights or from the bore rods at two positions in this cycle. If the counter weights are optimally balanced, the motor current characteristic is similar when descending and when ascending. If the motor is driven, there is an imbalance in the voltage DC link, whereby the VDmax function in the drive converter prevents erroneous tripping and the previously used braking resistors are not required: The system becomes simpler and operates more reliably. "Kinetic buffering" ensures that also for brief voltage drops (in individual cases up to 10 seconds), oil is still pumped.
Many pumping companies use the simple direction of rotation reversal possible with AS drive converters when maintaining the bore rods so that they can be quickly re-coupled without having to use a crane. Masterdrives also makes it a lot simpier to adapt the speed, as neither pulleys, belts nor gearboxes have to be changed-over. Not only this, but the speed control range is significantly wider than that possible when using mechanical systems.
Existing conventional solutions can be simply retrofitted with AC drive converters and for new applications, favorably-priced induction motors can be used.

Electric submersible pumps ...
comprise a special induction motor (45 to 1000 kW), an incoming transformer to couple to the line supply and a starter or AC drive converter for the closed-loop control. They are primarily used for flow rates exceeding 200 m³/day and pump depths exceeding 1500 m.
However, even when the oil flow rates decrease, the submersible pumps do not have to be replaced by horsehead pumps, if they are being fed from an AC drive converter as these allow parameters to be changed to adapt the system to the actual situation. The cable between the drive converter and submersible pump can be several kilometers long if a sinusoidal filter is provided at the drive converter output. A transformer associated with the pump supplies the power. Furthermore, when Masterdrives drives are used, additional valves to limit the tlow rate for submersible pumps are no longer required, as the pumping operation adapts itself to the required flow which also reduces the mechanical and electrical stressing on the units.

Progressing cavity pumps ...
with outputs from 11 to 200 kW are mainly used in bore holes with flow rates exceeding 200m3/day and for drilling depths less than 2000 m.
They can be directly coupled through belts or gearboxes so that they can be adapted to different conditions. Progressing cavity pump systems are positive displacement pumps with a worm-type rotor, which rotates eccentrically in a stator. The drive converter can run-up a rotor in regenerative feedback operation smoothly using the "restart-on-the-fly" function. A rotor, when braking generates a torque which opposes the norm al direction of rotation, so that in several cases, a braking unit with braking resistors is required to prevent overloading the voltage DC link and tripping due to overvoltage.
If the torque limit of a progressing cavity pump is exceeded, the technology controller automatically reduces the speed and increases the torque to prevent an overload condition. The drive converter then automatically goes back into the normal operating mode. If the overload condition occurs long enough, an alarm is output, the AC drive converter is smoothly stopped and releases at the end, the "DC link overvoltage protection" in order to prevent that the rotor excessively regenerates back into the drive. Also for this pump design, the controlled power-on and power-off by the AC drive converter protects and reduces the stressing on the mechanical pump components. In addition to the specific advantages for the variable pump systems, the general, well-proven features of Simodrive Masterdrives, known from other applications, apply such as:
- The motor and mechanical system are protected by powering-up and powering-down the pumps in a controlled fashion
- Using the DC voltage link, the line supply always sees the high power factor of the AC drive converter
- The system can be simply integrated into local or remote monitoring systems
- The "automatic powering-up" function af ter a line supply fault means that service personnel do not have to be involved to get the system re-started
- The drive converter supplies the controllogic of the voltage DC link, so that the units still continue to function even during power failures
- Several drive converters of the same type can be extremely simply parameterized using an operator control interface (aPIS) or using the Simovis software
- The "operating hours counter" function simplifies service/maintenance planning
- Lower costs for installing protective devices
- High dynamic performance at load changes or changing pump conditions.