Apparatus and method of monitoring an alternating current component of a downhole electrical imbalance voltage

1. A method of monitoring a power system of an electrical submersible pump assembly, the method comprising:

monitoring an alternating current component of a downhole electrical imbalance voltage at a neutral point of a motor of an electrical submersible pump assembly, the electrical submersible pump assembly including a pump in the well bore, the motor located in the well bore and attached to the pump, a power source located at the surface, and a power cable to connect the motor to power source;

transmitting the alternating current component of the downhole electrical imbalance voltage to the surface;

wherein the step of transmitting the alternating current component of the downhole electrical imbalance voltage to the surface involves transmitting one or more of the following: a peak-to-peak electrical imbalance voltage, and an average electrical imbalance voltage;

analyzing the alternating current component of the downhole electrical imbalance voltage; and

adjusting an operation of the electrical submersible pump assembly responsive to the analyzed alternating current component of the downhole electrical imbalance voltage.

2. A method of claim 1 , wherein the step of monitoring the alternating current component of the downhole electrical imbalance voltage further includes a high-impedance potential divider capacitively coupled to the neutral point and a voltage sensor.

3. A method of claim 1 , wherein the step of analyzing the alternating current component of the downhole electrical imbalance voltage indicates an oscillating system responsive to a large peak-to-peak electrical imbalance voltage.

4. A method of claim 1 , wherein the step of analyzing the alternating current component of the downhole electrical imbalance voltage indicates a non-oscillating system responsive to a persistent electrical imbalance voltage.

5. A method of claim 1 , wherein the power source comprises a variable speed drive and an output transformer connected to the variable speed drive.

6. A method of monitoring a power system of an electrical submersible pump assembly, the method comprising:

creating a model of a power system of an electrical submersible pump assembly to predict an alternating current component of a downhole electrical imbalance voltage at a neutral point of a motor of the electrical submersible pump assembly, the electrical submersible pump assembly including a pump in the well bore, the motor located in the well bore and attached to the pump, a power source located at the surface, and a power cable to connect the motor to the power source;

measuring the alternating current component of the downhole electrical imbalance voltage at the neutral point of the motor of the electrical submersible pump assembly;

transmitting the alternating current component of the downhole electrical imbalance voltage to the surface so that the model of the power system can be verified;

wherein the step of transmitting the alternating current component of the downhole electrical imbalance voltage to the surface involves transmitting one or more of the following: a peak-to-peak electrical imbalance voltage, and an average electrical imbalance voltage;

modifying the model of the power system as necessary; and

adjusting surface currents according to the model of the power system to match downhole motor drive currents to thereby optimize the power system.

7. A method of claim 6 , wherein the step of measuring the alternating current component of the downhole electrical imbalance voltage further includes a high-impedance potential divider capacitively coupled to the neutral point and a voltage sensor.

8. A method of claim 6 , wherein the power source comprises a variable speed drive and an output transformer connected to the variable speed drive.

9. An electrical submersible pump assembly, comprising:

a pump located in a well bore,

a motor located in the well bore and attached to the pump,

a power source located at the surface of the well bore,

a power cable to connect the motor to the power source;

a downhole sensor to measure an alternating current component of a downhole electrical imbalance voltage at a neutral point of the motor;

a controller to analyze the alternating current component of the downhole electrical imbalance voltage, the controller having at least a processor and a tangible memory; and

a program product operable on the controller and stored in the memory, the program product comprising a set of instructions that, when executed by the controller, cause the controller to perform the operations of:

analyzing the alternating current component of the downhole electrical imbalance voltage responsive to a measurement from the sensor, wherein analyzing the alternating current component of the downhole electrical imbalance voltage responsive to a measurement from the sensor involves one or more of the following: a peak-to-peak electrical imbalance voltage, and an average electrical imbalance voltage, and

adjusting an operation of the electrical submersible pump assembly responsive to the analyzed alternating current component of the downhole electrical imbalance voltage.

10. An electrical submersible pump assembly of claim 9 , wherein operations further include:

creating a model of a power system of the electrical submersible pump assembly to predict the alternating current component of the downhole electrical imbalance voltage at the neutral point of the motor;

verifying the model of the power system responsive to a measurement from the sensor;

modifying the model of the power system as necessary; and

adjusting surface currents according to the model of the power system to match downhole motor drive currents to thereby optimize the power system.

11. An electrical submersible pump assembly of claim 9 , wherein the power source comprises a variable speed drive and an output transformer connected to the variable speed drive.