High resistance grounding systems for oil well electrical systems

#1. Field of the Invention

#The subject matter of this invention relates generally to high resistance grounded systems and more particularly to high resistance grounding schemes for oil well electrical systems.

#2. Description of the Prior Art

#For a better understanding of the invention reference may be had to the preferred embodiment thereof shown in the accompanying drawings in which;

#FIG. 1 shows a schematic view, partially broken away, of an electric oil well down hole pump system being driven by a surface power source with down hole signal sensing system and the signal blocker system of the present invention;

#FIG. 2 shows a signal blocking arrangement similar to that shown in FIG. 1 for a low voltage embodiment of the invention;

#Referring now to the drawings and FIG. 1 in particular, FIG. 1 schematically shows an oil well system 10, which includes an above ground power supply system 11 the heart of which is a Wye connected power transformer secondary winding 12 (transformer) which provides power to a down hole electrical oil well pump 14 having Wye connected oil well pump motor windings 15. There is provided an electrically conductive well casing or inner production pipe 16 which traverses from the surface S above ground to the down hole region DH over a distance D. Interconnected with the windings 15 of the down hole electrical oil well pump 14 is a signal source 17. The signal source provides electrical signals to the neutral N′ of the windings 15, at one terminal thereof and is interconnected with the conductive oil well casing 16 at the other terminal thereof. Above the surface S is a signal receiver 18. The windings 12 and the windings 15 each have phase lines A, B and C which are interconnected to terminals X, Y and Z and X′, Y′ and Z′ respectively, between which generally traverse the long lines 19 from the surface S to down hole region DH. Signals ia, ib and ic flow in lines A, B and C respectively between the signal source 17 and the signal receiver 18. Return or ground current ig flows between signal receiver 18 to signal source 17 through the casing 16 of the well. The signals ia, ib and ic may be direct current, pulsed DC or 200 kilohertz ac signals, for example. It is not unusual for the distance D to be approximately 8,000 feet or greater. The signal source 17 may provide information related to well pressure, temperature and vibration for example, from the down hole region DH to surface S by way of the signals ia, ib and ic. The pump motor 14 represented by the windings 15 may be a 60 or 700 horsepower motor of the high voltage variety operating between, for example, 1100 volts and 4600 volts. The signals ia, ib and ic generally range from 4 to 20 milliamps. The power supply system 11 may supply power in the magnitude of 12 hundred to 5 kilovolts at a frequency of 40 to 90 hertz. Generally, the lines 19 may drop 600 volts between the surface S and the down hole region DH due to their excessive length.

#The transformer 12 has a neutral N capable of having a common mode voltage V disposed therebetween and ground. In an ideal situation where the power loads are balanced, the voltage V is generally zero volts. The neutral N is interconnected by way of a line to point U of a high resistance grounding system HRG. High resistance grounding system HRG may comprise a step down transformer 20 having the primary P thereof connected at one terminal thereof to the point U and the other terminal thereof connected to the input terminal 23 of a signal blocker 24. The other side or secondary S of the transformer 20 has connected thereacross a high resistance impedance or resistor R and parallel therewith a relay control device 22. Between terminal 23 and ground G of the system signal blocker 24 are interconnected an electronic control system 26 (to be described in greater detail with respect to FIG. 8) for the system signal blocker, a normally opened relay 30 and pair of oppositely disposed or inverse connected silicon controlled rectifiers or gated control devices 28A and 28B. The control system 26 interconnects with the gated silicon controlled rectifier 28B interconnected with the control system 26 by way of terminals F and H the voltage across which determine the firing status of silicon controlled rectifier 28B. On the other hand the silicon controlled rectifier 28A is controlled by the terminals J and K in the control system 26, which also has a voltage impressed thereacross which controls the firing status of the silicon controlled rectifier 28A. The series combination of the resistive element R28 and capacitive element C29 will operate with inductor LI to form an RLC circuit that limits the first derivative of system voltage with respect to time, that is the rate of rise of voltage across the silicon controlled rectifiers with respect to time. Said in another way, it is a high frequency filter for variable speed drives, in the event that variable speed drives are being utilized. In normal operation, when the voltage V is at approximately zero volts, the voltage on the primary P of the transformer 20 is essentially at zero and therefor the resistive value R is essentially out of the circuit, the normally opened relay contact 30 remains open and the oppositely disposed silicon controlled rectifiers 28A and 28B remain in a nonconductive state. Thus no electrical current flows between the neutral N of the transformer 12 and ground G, thus the resistive value R appears transparent or nonexistent to the voltage on the neutral N of the transformer 12. However, if an arcing ground fault AF occurs in a first portion of the electrical system, such as is shown in the down hole region DH, between line C and the casing 16, for example, it has a tendency to reduce the voltage between line C and ground and thus increase the voltage between line A and ground and line B and ground. The increased voltage on lines A and B has a tendency to escalate with the arcing ground fault condition, causing the voltage V on the neutral N of the transformer 12 in a second portion of the electrical system with respect to ground G to obtain some nonzero value. However, the primary P of the transformer 20 sees this voltage increase and reflects it through to the resistive value R. In addition, control system 26 senses the voltage between the point 23 and ground G and actuates the oppositely disposed silicon controlled rectifiers 28A and 28B to conduct. This places the resistance R in the circuit between the neutral N and ground G. This limits the current and damp the voltage V at the neutral N to a maximum of a line-to-neutral magnitude. At this point in time, the signals ia, ib and ic are damped out or shorted out by the presence of resistive value R, but this is of no great consequence, as it is more desirable to cure the effects of the arcing ground fault at this time to prevent damage to down hole components. The voltage across the primary P of the transformer 20, which is reflected into the secondary S thereof actuates the relay control device 22, which with an appropriate time delay, closes the normally opened contact 30, thus eliminating the need for the control system to continue to control the silicon controlled rectifiers 28A and 28B to conduct and increasing the electronics' life.

#The natural effect of all this is to increase the serviceability of the entire system 10, prevent the destruction of the down hole motor 14 and the sensors as indicated at 17. In an embodiment of the invention, the net resistive value seen between the neutral N and the ground G may be 130 ohms. The time delay provided by the relay control 22 may be 1.5 seconds. The grounding resistor R may be approximately 6 to 7 ohms. The secondary of the transformer S may be connected to a warning system (not shown) which directly or remotely indicates to personnel that an arcing ground fault has occurred or is occurring. The control system 26 for the signal blocker 24 provides a significant portion of the present invention. Its construction and use will be described hereinafter with respect to FIG. 8.