Downhole measurement tool circuit and method to balance fault current in a protective inductor

A downhole measurement tool circuit and method to balance fault current in a protective inductor, which keeps an alternating current balanced in a protective choke during a phase-to-ground fault condition in a power cable or a downhole motor of an electrical submersible pump. The downhole measurement tool circuit and method cause a conducting of current during the negative polarity voltage portions of a phase-to-ground fault condition, but do not cause a conduction of negative polarity voltage during use of a negative polarity megger.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a downhole measurement tool circuit and method to balance fault current in a protective inductor, and more particularly to a downhole measurement tool circuit and method to balance fault current in a protective inductor which keeps an alternating current balanced in a protective choke during a phase-to-ground fault condition in a power cable or a downhole motor of an electrical submersible pump.

2. Description of the Related Art

Various types of downhole equipment, such as pumps and similar devices, are used to move fluids from beneath the surface of the earth to the surface. Well known applications include oil and gas wells and water wells. A typical downhole arrangement would include a string composed of a series of tubes or tubing suspended from the surface. One type of well-known pump is a downhole electrical submersible pump. The electrical submersible pump either includes or is connected to a downhole motor which is sealed so that the whole assembly is submerged in the fluid to be pumped. The downhole motor is connected to a three-phase power source at the surface and operates beneath the level of fluid downhole in order to pump the fluid to the surface.

In the common design of many downhole measurement tools associated with an electrical submersible pump, they are connected to the Y-point of the downhole motor of the electrical submersible pump and to the ground of the downhole system, such as disclosed in U.S. Pat. No. 6,176,308, which is incorporated herein by reference. The three-phase power supply for the electrical submersible pump is isolated from the ground, and the downhole measurement tool utilizes this feature to communicate to an associated surface equipment of a downhole system by low frequency modulation of a current or voltage supplied by the associated surface equipment. The downhole measurement tool is coupled to the electrical submersible pump and used to monitor certain downhole parameters, such as pressure and temperature, of a subterranean bore-hole.

If a phase-to-ground fault occurs in the downhole motor or power cable of the downhole system, this will apply high voltage alternating current to the Y-point of the downhole motor of the electrical submersible pump, and consequently to the downhole measurement tool. To prevent damage to the downhole measurement tool during this fault condition, a protective choke is typically included in the circuitry of the downhole measurement tool, which provides a suitably high impedance to minimize the alternating current flowing from the Y-point to the ground of the downhole system through the circuitry of the downhole measurement tool.

Another typical requirement of many downhole measurement tools is to be able to use a negative polarity, direct current megger to check the electrical insulation quality of the power cable and/or downhole motor of the downhole system. For this purpose, the circuitry of the downhole measurement tool typically includes a diode, which only conducts during positive polarity voltage. However, since the diode only conducts in the positive polarity voltage, during a phase-to-ground fault of alternating current voltage, the current in the protective choke will reach a direct current level which saturates the choke, and lowers its inductance, accordingly reducing its protective function. In order to eliminate this saturation condition, a large capacitor is typically included in the circuitry of the downhole measurement tool in order to keep the choke current balanced during a phase-to-ground fault condition. The voltage and temperature ratings, plus the large physical sizes required by suitable chokes and capacitors cause them to be expensive and physically large, which may involve additional mechanical mounting considerations as described by U.S. Pat. No. 6,176,308.

It is therefore desirable to provide a downhole measurement tool circuit and method to balance fault current in a protective inductor that keeps a protective choke current balanced in the event of a phase-to-ground fault in a power cable or a downhole motor of an electrical submersible pump of a downhole system.

It is further desirable to provide a downhole measurement tool circuit and method to balance fault current in a protective inductor which eliminates the need for a large, expensive capacitor for protection against choke saturation, thereby reducing the size and cost of the downhole measurement tool.

It is still further desirable to provide a downhole measurement tool circuit and method to balance fault current in a protective inductor which causes a conducting of current during the negative polarity voltage portions of a phase-to-ground fault condition, but which do not cause a conduction of negative polarity voltage during use of a negative polarity megger.

SUMMARY OF THE INVENTION

In general, in a first aspect, the invention relates to a downhole system capable of balancing an alternating current between a power cable or a downhole motor of an electrical submersible pump and a downhole measurement tool during a phase-to-ground fault condition. The downhole system comprises an electrical coupling between a Y-point of the downhole motor of the electrical submersible pump and the downhole measurement tool and a triggerable network that selectively conducts during application of a negative polarity voltage to the downhole measurement tool based on a rate of increase of the negative polarity voltage. The downhole system may include an inductor electrically coupled to the Y-point of the downhole motor of the electrical submersible pump and the downhole measurement tool. The inductor is of selected inductance for filtering alternating current ripple voltage from the downhole motor of the electrical submersible pump to the downhole measurement tool.

The triggerable network of the downhole system may comprise a silicon-controlled rectifier and an associated resistor that form a path of current conduction for the negative polarity voltage during the phase-to-ground fault condition. The triggerable network may further comprise a gate input of the silicon-controlled rectifier coupled to a resistor and a capacitor connected in series. The silicon-controlled rectifier may be a plurality of silicon-controlled rectifiers coupled to a plurality of resistors and a plurality of capacitors connected in series. The resistors and the capacitors may provide a trigger current for the silicon-controlled rectifiers, and the resistors and the capacitors may selectively control the conduction of the silicon-controlled rectifiers. The trigger current may be based on the rate of increase of the negative polarity voltage.

In addition, the downhole system may include a diode providing a path of current conduction for a positive polarity voltage during the phase-to-ground fault condition and a plurality of zener diodes coupled in series and in a reverse-bias mode to limit the voltage resulting from the positive polarity voltage.

In general, in a second aspect, the invention relates to a downhole measurement tool having a circuit capable of balancing an alternating current during a phase-to-ground fault condition, with the downhole measurement tool capable of being coupled to a Y-point of a three-phase downhole motor of an electrical submersible pump. The circuit of the downhole measurement tool includes a triggerable network that selectively conducts during application of a negative polarity voltage to the downhole measurement tool based on a rate of increase of the negative polarity voltage and an inductor commutating circuit to limit the voltage resulting from a positive polarity voltage during the phase-to-ground fault condition.

The triggerable network of the downhole measurement tool may be constructed of a silicon-controlled rectifier and an associated resistor that form a path of current conduction for the negative polarity voltage during the phase-to-ground fault condition. The triggerable network can further comprise a gate input of the silicon-controlled rectifier coupled to a resistor and a capacitor connected in series. Additionally, the silicon-controlled rectifier can be a plurality of silicon-controlled rectifiers coupled to a plurality of resistors and a plurality of capacitors, which are connected in series. The resistors and the capacitors of the downhole measurement tool may provide a trigger current for the silicon-controlled rectifiers, and the resistors and the capacitors can selectively control the conduction of the silicon-controlled rectifiers. Further, the trigger current may be based on the rate of increase of the negative polarity voltage. Moreover, the inductor commutating circuit of the downhole measurement tool can include a diode providing a path of current conduction for a positive polarity voltage during the phase-to-ground fault condition and a plurality of zener diodes coupled in series and in a reverse-bias mode to limit the voltage resulting from the positive polarity voltage. The downhole measurement tool may also include an inductor being of selected inductance for filtering alternating current ripple voltage to the downhole measurement tool.

In general, in a third aspect, the invention relates to a method of balancing a phase-to-ground fault condition between a power cable or a three-phase downhole motor of an electrical submersible pump and a downhole measurement tool. The method comprises the steps of electrically coupling to a Y-point of the downhole motor of the electrical submersible pump to the downhole tool; selectively conducting a negative polarity voltage to the downhole measurement tool during the phase-to-ground fault condition based on a rate of increase of the negative polarity voltage; and providing a path of current conduction for a positive polarity voltage during the phase-to-ground fault condition.

The step of selectively conducting the negative polarity voltage may be via a triggerable network comprising a silicon-controlled rectifier and a network of resistors and capacitors that form a path of current conduction for the negative polarity voltage during the phase-to-ground fault condition. The method may further include the step of providing a trigger current for the silicon-controlled rectifier based on the rate of increase of the negative polarity voltage. If the rate of increase of the negative polarity voltage is sufficient, the method causes the network of resistors and capacitors to provide the trigger current sufficient to trigger the silicon-controlled rectifier thereby causing the silicon-controlled rectifier to conduct current during the negative polarity voltage of the phase-to-ground fault condition. If the rate of increase of the negative polarity voltage is insufficient, the method causes the network of resistors and capacitors to provide the trigger current insufficient to trigger the silicon-controlled rectifier whereby the silicon-controlled rectifier does not conduct current.

Other advantages and features will be apparent from the following description, and from the claims.

DETAILED DESCRIPTION OF THE INVENTION

The circuits and methods discussed herein are merely illustrative of specific manners in which to make and use this invention and are not to be interpreted as limiting in scope.

While the circuits and methods have been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the construction and the arrangement of the electrical components and steps without departing from the spirit and scope of this disclosure. It is understood that the circuits and methods are not limited to the embodiments set forth herein for purposes of exemplification.

A downhole measurement tool circuit and method10is provided herein to balance fault current in a protective inductor and relates to any electrical apparatus, in particular a downhole measurement tool12, which is protected from phase-to-ground fault currents by a protective choke14. The circuit and method10apply to the downhole measurement tool12electrically coupled through a lead20to the Y-point16of an electrical submersible pump18. The electrical submersible pump18includes a downhole motor28that has three field coils22,24and26, with each of the field coils22,24and26having a common connection at one end, the Y-point16, and their other ends are respectively coupled through leads to a source of three-phase power (not shown). The source of three-phase power produces alternating voltage on the three field power leads, which are out of phase with respect to one another by one hundred and twenty degrees.

The Y-point16of the downhole motor28of the electrical submersible pump18is electrically coupled through the lead20to one end of an inductor, i.e., the protective choke14, for filtering alternating current ripple voltage from getting to the downhole measurement tool12and the other end of the protective choke14is connected to additional circuitry, as appropriate, of the downhole measurement tool12via the circuit10provided herein. The path of positive current during normal, no-fault condition of the downhole measurement tool12flows from the protective choke14through a lead30and a diode32to the additional circuitry of the downhole measurement tool12. When a phase-to-ground fault is detected, which applies high alternating current voltage to the Y-point16, the diode32provides the path of current conduction during positive polarity voltage resulting from the phase-to-ground fault condition. An array of zener diodes34,36and38are coupled in series and clamp the voltage resulting from the positive polarity voltage to prevent damage to the additional circuitry of the downhole measurement tool. During use of a negative polarity megger (not shown), the diode32does not conduct and should be of sufficient reverse voltage rating to allow use of the megger.

The circuit and method10disclosed herein further include silicon-controlled rectifiers40and42, with associated resistor44, which form a path for current during the negative polarity portion of alternating current voltage during a phase-to-ground fault condition. Resistor44should be sized to conduct approximately the same current during the negative polarity voltage as the diode32during the positive polarity voltage. The gate inputs of the silicon-controlled rectifiers40and42are coupled to resistors46and48and capacitors50and52to form a triggerable network, which provides sufficient trigger current to the associated silicon-controlled rectifiers40and42when a negative voltage with a rapid rise time is present on the Y-point16, such as during a phase-to-ground fault condition. Silicon-controlled rectifiers40and42may be of the sensitive-gate type, such that only a small amount of gate trigger current is required to trigger the silicon-controlled rectifiers40and42into a conducting state. Resistors46and48reduce false triggering of the silicon-controlled rectifiers40and42, and capacitors50and52should be sized such that an adequate trigger current is available to the gate inputs of silicon-controlled rectifiers40and42when the phase-to-ground fault condition is present. During use of the negative polarity megger, the rate of negative polarity voltage increase is much lower, and thus the trigger current through capacitors50and52is not sufficient to provide an adequate trigger current to the gate inputs of silicon-controlled rectifiers40and42, and thus silicon-controlled rectifiers40and42do not turn on into a conducting state. Silicon-controlled rectifiers40and42and capacitors50and52should have a sufficient voltage withstand rating to allow the use of the megger direct current voltage. Additionally, capacitors54and56provide additional protection from false triggering of silicon-controlled rectifiers40and42. Similarly to capacitors50and52, capacitors54and56should have a sufficient voltage withstand rating to allow use of the megger. In addition, capacitors54and56may be electrically coupled to grounds58and60, respectively.

While the foregoing exemplification utilizes two (2) silicon-controlled rectifiers,40and42, it will be appreciated in keeping with the spirit and scope of the circuit and method10disclosed herein that the number of silicon-controlled rectifiers required is dependent on the voltage rating of the silicon-controlled rectifiers and associated electrical components. In addition, it will be appreciated that the number of silicon-controlled rectifiers required is also dependent on the desired megger voltage rating.

For purposes of exemplification and not by way of limitation, the circuit and method10disclosed herein may utilize the protective choke14having a value of approximately 150 to approximately 200 Henries and suitable for approximately 12 mA of direct current. In addition, the diode32may have a voltage rating of approximately 2,000 volts and be rated at about 0.5 amp. Moreover, each of the zener diodes34,36and38may be rated at approximately 10 volts each and be suitable for the current and power dissipation during a phase-to-ground fault condition. Again, silicon-controlled rectifiers40and42may be sensitive gate type silicon-controlled rectifiers, with each rated at about 800 volts, 2 amps, and 200 micro-amp gate current. Resistors46and48may each have a value of approximately 2400 ohm, while resistor44may have a value of approximately 1000 ohms, and be sized to dissipate adequate power during the phase-to-ground fault condition. Capacitors50,52,54and56may each be rated at approximately 0.1 uFd and 1000 volts.

Whereas, the circuits and methods have been described in relation to the drawings and claims, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the spirit and scope of this invention.