Electric submersible pumps

1. A variable speed drive system comprising a variable speed drive circuit and a multi-phase permanent magnet motor for driving an electric submersible pump, the drive circuit being adapted to drive all of the phases of the motor simultaneously and the drive circuit comprising:

variable voltage supply means for supplying a voltage that can be varied as required;

inverter means supplied by said voltage for producing modulated waveforms having a voltage in the range 1,000 V AC-4,000 V AC to drive all of the phases of the motor simultaneously, each modulated waveform switching between an upper voltage level and a lower voltage level; and

control means for operating the inverter means so as to produce over-modulation such that each modulated waveform produced by the inverter means is distorted and is unable to follow a peak of a sinewave or other waveshape that is demanded in simple proportion and each modulated waveform produced by the inverter means has extended periods at the upper voltage level and at the lower voltage level, the control means being operable to control the speed of the motor by varying said voltage supplied by the voltage supply means to the inverter means and to permit operation of the motor at speeds above 4,500 revolutions per minute.

2. The drive system of claim 1 , wherein at lower speeds the control means is arranged to control the inverter means according to a low speed control mode in which the waveforms are linearly modulated.

3. The drive system of claim 1 , wherein the drive circuit is adapted to apply pulse width modulation.

4. The drive system of claim 1 , wherein the variable voltage supply means comprises chopper means for chopping a fixed voltage in a variable time-dependent sequence in order to supply said voltage.

5. The drive system of claim 4 , wherein the chopper means comprises:

capacitance means connected to first and second fixed supply voltage sources; and

selection means for selectively supplying the voltage defined by the first and second fixed supply voltage sources.

6. The drive system of claim 4 , wherein the chopper means is adapted to vary a duty cycle of the selection means to adjust the voltage across the capacitance means.

7. The drive system of claim 1 , wherein the variable voltage supply means comprises a poly-phase boost converter adapted to supply a difference between the upper voltage level and the lower voltage level from a poly-phase supply.

8. The drive system of claim 1 , wherein the variable voltage supply means is adapted to vary its internal frequency with output so as to improve efficiency.

9. The drive system of claim 1 , further comprising transformer means having a first secondary winding constituting a first fixed supply voltage source and a second secondary winding constituting a second fixed supply voltage source.

10. The drive system of claim 1 , wherein filter means are connected to the inverter means so as to smooth out transitions between the upper voltage level and the lower voltage level.

11. The drive system of claim 10 , wherein the filter means are adapted to supply substantially sinusoidally varying voltages to the motor.

12. The drive system of claim 1 , further comprising:

means for varying the drive current or voltage supplied to drive the motor with a fixed load while the motor is driven at a fixed speed;

means for monitoring an output power of the circuit during such variation of the drive current or voltage in order to determine a minimum output power required to drive the motor at said fixed speed; and

means for controlling the output power of the circuit in order to minimize the output power of the circuit required to drive the motor at said fixed speed.

13. The drive system of claim 1 , further comprising:

means for varying, relative to an estimated rotor position of the motor, the phase of the drive current or voltage supplied by the circuit to drive the motor while said current or voltage is held at a fixed amplitude;

means for monitoring a motor speed during such variation of the drive current or voltage in order to determine a maximum speed at which the motor can be driven by an available output power; and

means for controlling the phase of the drive current or voltage in order to maximize the motor speed.

14. A method for varying the speed of an electric submersible pump system comprising a pump and a multi-phase permanent magnet motor for driving the pump, the method comprising:

supplying a voltage to an inverter, the voltage being variable as required;

using the inverter to produce modulated waveforms having a voltage in the range 1,000 V AC-4,000 V AC for driving all of the phases of the multi-phase permanent magnet motor simultaneously, the modulated waveforms switching between an upper voltage level and a lower voltage level;

operating the inverter so as to produce over-modulation such that each modulated waveform is distorted and is unable to follow a peak of a sinewave or other waveshape that is demanded in simple proportion and such that each modulated waveform has extended periods at the upper voltage level and at the lower voltage level; and

controlling the speed of the motor by varying said voltage supplied to the inverter to permit operation of the motor at speeds above 4,500 revolutions per minute.

15. The drive system of claim 1 , further comprising means for estimating a rotor position of the motor.

16. The method of claim 14 , wherein the modulated waveforms are generated on surface and transmitted downhole to the motor.

17. The method of claim 16 , wherein the waveforms are transmitted downhole to the motor along a multi-phase power cable.

18. The drive system of claim 1 , wherein:

the drive circuit is located at surface,

the multi-phase permanent magnet motor is located downhole, and

the drive circuit and the multi-phase permanent magnet motor are connected by a multi-phase power cable.