System and method for an AC powered downhole gauge with capacitive coupling

1. A method of powering a motor and an instrument of a submersible pump assembly, comprising:

(a) providing a three conductor power cable from a surface location to the motor, which has a Y-point on a lower end of the motor;

(b) electrically coupling a capacitor to the Y-point and a side of the capacitor opposite the Y-point to the instrument;

(c) producing three phase motor power for the motor and simultaneously producing single phase instrument power on the three conductor power cable, the instrument power being at a higher frequency and lower voltage than the motor power, the instrument power voltage existing at the Y-point and passing through the capacitor to the instrument, the motor power having a motor voltage that is zero at the Y-point;

(d) in the event one of the conductors of the power cable shorts to ground, which causes the motor voltage to occur at the Y-point, an impedance of the capacitor increases due to the lower frequency of the motor power than the instrument power, thereby passing to the instrument a reduced level of motor voltage; and

(e) bi-directionally transmitting data on the three conductor power cable between the surface location and the instrument by modulating the instrument power, interrupting a frequency of the instrument power, or shifting a frequency of the instrument power.

2. The method according to claim 1 , further comprising prior to step (c), applying between ground and each conductor of the three conductor power cable a DC test voltage, which is blocked from the instrument by the capacitor.

3. The method according to claim 1 , wherein step (b) further comprises:

coupling the instrument to the capacitor via a transformer, the transformer and the capacitor being selected to provide a low impedance path for the instrument power and a high impedance path for the motor power.

4. The method according to claim 1 , wherein the instrument comprises:

at least one gauge subassembly for measuring an operating characteristic or controlling a device; and step (c) further comprises:

communicatively coupling a signaling unit to the at least one gauge subassembly for transmitting data on the three conductor power cable.

5. The method of claim 1 , wherein step (c) comprises:

providing the instrument power with a voltage that is approximately a fraction 1/N of the motor power voltage and a frequency that is approximately a multiple N of the motor power frequency.

6. An electrical submersible pump system comprising:

a three conductor power cable extending from a surface location into a conduit;

an electrical submersible pump motor within the conduit and coupled to the three conductor power cable, the motor having windings at a lower end joined to form a Y-point;

a capacitor and transformer coupled to the Y-point;

an instrument coupled to the transformer, which is connected through the capacitor to the Y-point;

a power source at the surface coupled to the three conductor power cable, the power source producing three phase motor power to operate the motor and producing single phase instrument power for simultaneous transmission on the conductors of power cable, wherein the instrument power has a voltage that is approximately a fraction 1/N of the motor power voltage and a frequency that is approximately a multiple N of the motor power frequency;

a controller within the instrument modulating AC current drawn by the instrument from the instrument power to transmit data on the three conductor power cable to the surface location; and

one or more signaling units separately coupled to the three conductor power cable and transmitting data over the three conductor power cable to the instrument either by temporarily shifting a frequency of the instrument power or by temporarily interrupting tin the instrument power; and

the capacitor being sized so that when the Y-point has a neutral motor power voltage and the instrument power is transmitted over the three conductor cable, the capacitor passes the instrument power at the instrument power frequency to operate the instrument, and so that when a conductor of the three conductor power cable shorts to ground, creating motor power voltage at the Y-point, the impedance of the capacitor increases to pass a fraction of the motor power voltage at the motor power frequency to operate the instrument.

7. The system according to claim 6 , wherein the capacitor blocks from the instrument high DC test voltage during testing of the three conductor power cable insulation.