Apparatus and method for determining the status and phase of an electric power cable

A probe uses a small diameter insulated needle to accurately penetrate the jacket and thin outer semiconductor sheath of URD cable so as to capacitive couple to the high voltage center conductor and determine its energized status (live or dead) and phase attribute. A precision needle depth gauge, mechanical hard stop, and digital display ensures that the needle does not penetrate the thick center conductor insulation material and provides an indication of the center conductor voltage. The probe can be combined with either internal or external phase identification circuitry to determine the phase attribute of the URD cable.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of electric power distribution networks. More specifically, the present invention relates to determining the energized status and phase of underground residential distribution power cables.

BACKGROUND OF THE INVENTION

Electric power distribution networks are used by the electric utilities to deliver electricity from generating plants to customers. Although the actual distribution voltages will vary from utility to utility, in a typical network, three-phase power at high voltage (345,000 volts phase-to-phase) is delivered to multiple transmission substations at which transformers step this high voltage down to a lower three-phase voltage (69,000 volts phase-to-phase). This 69,000-volt three-phase power then feeds multiple distribution substations whose transformers further step down the voltage to the distribution voltage (12,470 volts phase-to-phase) and separate the power into three single-phase feeder cables. Typically, these feeder cables operate at 7,200 volts phase-to-ground. Each of these feeder cables branch into multiple circuits to power a plurality of local pole-mounted or pad-mounted transformers which step the voltage down to a final voltage of 120/240 volts for delivery to commercial and residential customers.

In many cases, the final 7,200-volt distribution network utilizes underground (i.e., buried) cables. These cables are typically known as Underground Residential Distribution (URD) cables. Typical URD cables are shown inFIGS. 1 and 2.

In a typical URD cable20, a center conductor22is surrounded by an inner semiconductor sheath24. Inner semiconductor sheath24serves to relieve electrical stress by spreading out and making the electric field more uniform.

Inner semiconductor sheath24is surrounded by an insulator26. Insulator26has significant high-voltage insulating properties to minimize the overall size of URD cable20. Typically, insulator26is formed of a polymeric material, such as polyethylene.

Surrounding insulator26is an outer semiconductor sheath28. Like inner sheath24, outer sheath28serves to relieve electrical stress by making the electric field more uniform. Making the electric field more uniform protects insulator26, which would otherwise be more likely to break down.

Outer semiconductor sheath28is surrounded by a shield formed of a plurality of neutral conductors30. Neutral conductors30together serve as a return line for center conductor22. In a typical three-phase system, neutral conductors30carry current resulting from any imbalance among the three phases. In a mechanical sense, neutral conductors30form a barrier to protect URD cable20from casual penetration (as with a blunt shovel). In the event of a catastrophic penetration through neutral conductors30and into or through center conductor22, neutral conductors30serve to provide a short electrical path and thereby offer some protection to a worker wielding the penetrating object.

Semiconductor layers24and28prevent high stress electric field lines from forming under each neutral conductor30. But as a side effect, semiconductor layers24and28also impede detection of the electrical field from outside of layer28.

URD cable20may be an unjacketed URD cable20′ (FIG. 1). In unjacketed URD cable20′, neutral conductors30form the outermost layer of the cable. Neutral conductors30are therefore in contact with the Earth when unjacketed URD cable20′ is buried.

URD cable20may also be a jacketed URD cable20″. In jacketed URD cable20″, neutral conductors30are surrounded by and embedded within an insulating jacket32. Whether URD cable20is jacketed or unjacketed, neutral conductors30need not be grounded, but usually are grounded at the ends.

As new customers are added, URD cable20is cut and an extension cable is spliced in to supply power to the new customer's transformer. This poses certain problems.

One problem is that there are often multiple URD cables20in a given trench, conduit, or raceway. Typically, one of these URD cables20is de-activated prior to splicing. A problem exists in determining which of these multiple URD cables20is de-energized (i.e., “dead”).

Clamp-on ammeters are occasionally used in an attempt to determine if a URD cable20is dead. Since each URD cable20carries its own return, the ammeter is used to measure differential current. But a reading of zero current may result from two very different conditions. Either the cable is in-fact a dead cable, or the cable is live but nearly perfectly balanced (outgoing current on center conductor22is equal to return current on neutral conductors30). Since one of the goals of electrical distribution is to achieve perfect balance, the value of the test becomes more meaningless as this goal is more closely achieved. Consequently, many live cables are misdiagnosed as being dead.

Another related problem is that, in a given dig, extraneous unmapped URD cables20may be present. These extraneous URD cables20may or may not be energized, and will often confuse ammeter measurements to the point where it is impossible to determine which of the URD cables20is the de-energized URD cable20to be cut and spliced.

When a URD cable20is to be cut and spliced, it is first spiked. That is, a “spike” is driven through the selected URD cable20to short neutral conductors30to center conductor22. If the spiked URD cable20is live, then spiking will create a short circuit and trip the appropriate circuit breakers. This assures that the worker will not cut into a live URD cable20.

The spiking of a live URD cable20is undesirable for several reasons. First, spiking a live URD cable20poses a risk to the worker, albeit a risk significantly less than the cutting of a live URD cable20. Second, tripping the circuit breaker causes an unnecessary power outage to all customers served by that URD cable20. Third, unnecessarily spiking a URD cable20necessitates a repair of that URD cable20. Spiking a live URD cable20, therefore, is dangerous, costly, and time consuming.

Various apparatus have been developed to identify the status, energized or de-energized (live or dead), of URD cables20. All of these apparatuses suffer from one or more deficiencies. When attempting to use such apparatuses to identify the status of a given URD cable20, there are four primary conditions:True-dead—identifying a given URD cable20as dead when it is in fact dead;False-dead—identifying a given URD cable20as dead when it is in fact live;True-live—identifying a given URD cable20as live when it is in fact live; andFalse-live—identifying a given URD cable20as live when it is in fact dead.

A false-live result may cause the worker to backtrack and double-check the removal of power from the desired URD cable20, may cause additional and unnecessary excavation, and may cause further labor and paperwork. This may result in a waste of time and resources. But a false-dead result, on the other hand, may lead to misidentification of the specific URD cable.20to be cut and spliced. This is the worst possible scenario, in that the worker would then spike a live URD cable20, believing it to be dead. As previously mentioned, spiking a live URD cable20is dangerous, costly, and time-consuming.

The only good status results are true-live and true-dead. Only such results will properly identify the specific URD cable20to be spiked, cut, and spliced, thereby safely, inexpensively, and efficiently allowing the work to proceed.

Apparatuses intended to determine status almost invariably test to determine if a URD cable20is live. No active test is performed to determine if URD cable20is dead. The presumption is, of course, that if URD cable20is not live, it is dead. This is a dangerous presumption.

If such an apparatus determines a URD cable20is live, it is often correct. That is, such an apparatus produces a reasonably reliable true-live result, with few false-live results. On the other hand, such an apparatus does not positively determine if URD cable20is dead. The apparatus can therefore only determine if URD cable20is “not-live”. URD cable20may test not-live if it is dead, or if it is live and the test fails for whatever reason, including worker error. This form of test therefore exhibits a high incidence of false-dead results. This is the worst possible scenario, in that the worker would then spike a live URD cable20, believing it to be dead.

Another problem with many apparatuses for determining the status of URD cables20is that they are cumbersome to use. Often, an apparatus (or a portion of the apparatus) must be clamped to the URD cable20under test. This requires the worker to get down into a trench or otherwise obtain direct access to and manipulate URD cable20.

Many such apparatuses are usable only with unjacketed URD cables20′. Unjacketed URD cables20′ suffer from corrosion and other factors that shorten their useful lifetimes. For this reason, the cable of choice for newer installations is almost invariably jacketed URD cable20″. In order to use an apparatus designed for unjacketed URD cable20′ with a jacketed URD cable20″, a portion of the insulating jacket32must be cut away, drilled, or otherwise penetrated. This, too, requires that the worker obtains direct access to and manipulates URD cable20.

Because the URD cables20may carry high voltage (typically 7,200 volts), any procedure requiring direct manipulation of the cable is inherently dangerous. A faulty or misidentified cable may expose the worker to high voltage, and potentially precipitate injury or death. Additionally, all procedures requiring direct manipulation of the cable are cumbersome, costly, and time-consuming. This is especially true for a jacketed URD cable20″ being tested with an apparatus intended for an unjacketed URD cable20′. When a portion of the insulating jacket32has been cut away and that URD cable20is determined to not be the URD cable20to be cut and spliced, then that URD cable20must then be repaired to protect it from corrosion and other factors that would otherwise shorten its useful lifetime. This repair is itself cumbersome, costly, and time-consuming.

Cumbersome and time-consuming procedures often inspire workers to shortcut the testing procedure. This may lead to injury or death, as well as expensive and time-consuming error.

Determining the status of a URD cable20by detecting the presence of an electric field in or around a live URD cable20is not possible since outer semiconductor sheath28completely shields the electric field.

Also, currently no apparatus exists to determine the phase attribute of URD cable in a trench.

SUMMARY OF THE INVENTION

Accordingly, it is an advantage of the present invention that an apparatus is provided for determining the status of URD cable.

It is another advantage of the present invention that an apparatus is provided that actively tests a URD cable for both a live and a dead status.

It is another advantage of the present invention that an apparatus is provided that determines the status of a URD cable while the worker is safely at a distance from the URD cable.

It is another advantage of the present invention that an apparatus is provided that communicates status to the operator using displays viewable at a distance.

It is another advantage of the present invention that an apparatus is provided that determines the status of a URD cable in an easy and straightforward manner.

It is another advantage of the present invention that an apparatus can be used to determine the phase attribute of a URD cable.

The above and other advantages of the present invention are carried out by an apparatus for determining the status of a URD cable in an electric power network operating at line frequency which is 60 Hertz in the United States (US) and 50 Hertz in many locations outside the US.

The apparatus of the current invention uses a fine (small diameter) insulated needle to accurately penetrate the thin semiconductor sheath28without entering the thick cable insulation26. This allows the needle tip to capacitively couple to the center conductor22electric field so as to determine voltage status and measure the approximate conductor voltage. The apparatus also indicates if the needle happens to contact one of the neutral conductors30. If so, the worker simply moves the apparatus a fraction of an inch to a new position and tries again.

The apparatus also includes either internal phase identification circuitry or an interface to external phase identification equipment.

The apparatus includes a cable analysis circuit coupled to the needle. A status display is coupled to the cable analysis circuit and configured to indicate the energized status, approximate voltage, and optionally the phase of the URD cable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Commonly owned U.S. Pat. No. 7,154,281 issued Dec. 26, 2006, which describes an apparatus and method for determining the status of an electric cable and U.S. Pat. No. 7,031,859 issued Apr. 18, 2006, which describes a phase identification system are incorporated herein by reference.

U.S. Pat. No. 7,154,281 describes how the energized status of URD cable20can be determined usingFIGS. 3 and 4which show schematic views of the electrical characteristics of URD cable20and an equivalent circuit of URD cable20, respectively as follows.

URD cable20has a center conductor22. When URD cable20is live, center conductor22carries current at a high voltage EL(typically 7,200 volts). This current is coupled through a cable capacitance C to outer semiconductor sheath28. A portion of this current therefore passes through a cable resistance R to form a line signal SL. Line signal SLhas a line-signal amplitude that is normally either very small, on the order of a few millivolts (when URD cable20is live) or nearly zero (when URD cable20is dead). Naturally, line signal SLis at line frequency (normally 50-60 Hz). Line signal SLforms across cable resistance R. Line signal SLis therefore present across input contact150and common (ground) contact152.

In U.S. Pat. No. 7,154,281, the energized status of URD cable20is determined by measuring line signal SLacross cable resistance R. This basic technique is also used by all the references listed in U.S. Pat. No. 7,154,281. Unfortunately, this basic technique is unreliable because line signal SLacross cable resistance R is totally dependent on the resistance of outer semiconductor sheath28which varies widely between different cable manufacturers. In some cables, line signal SLacross cable resistance R is zero even when URD cable20is energized. This results in a high incidence of false-dead results, which is the worst possible scenario, in that the worker would then spike a live URD cable20, believing it to be dead. The present invention overcomes this problem as illustrated inFIG. 5.

The apparatus of the current invention uses a fine insulated needle to accurately penetrate through the thin outer semiconductor sheath28without entering the thick cable insulation26by more than a few thousands of an inch (mils). This allows the needle tip to capacitively couple to center conductor22electric field so as to determine voltage status and measure the approximate center conductor22voltage. This eliminates false-dead results as center conductor22electric field is measured directly irrespective of the resistance of outer semiconductor sheath28.

Being insulated from and penetrating through outer semiconductor sheath28convertsFIGS. 3 and 4toFIGS. 6 and 7respectively. InFIG. 6, needle240does not see resistance R ofFIG. 3because it is insulated from outer semiconductor sheath28. InFIG. 7, line signal SLbetween needle240and ground152is open circuit and is not influenced by resistance R of outer semiconductor sheath28.

FIG. 8illustrates a perspective view of URD probe200(the apparatus of the current invention) being pressed downward against URD cable20to determine the energized status and phase of URD cable20in accordance with a preferred embodiment of the present invention.FIG. 9illustrates an internal cross sectional view of URD probe200.

Referring toFIG. 9, URD probe200enclosure295contains tube205which is rigidly affixed to tube stop210. Tube205passes through tube guide215which is rigidly affixed to enclosure295. Spring220applies pressure to tube stop210so as to press tube stop against tube guide215. Spring220is held in place using spring guide225which is rigidly affixed to enclosure295. Spring guide225also rigidly holds support rod230which in turn holds needle holder235which rigidly holds needle240. Needle holder235optimally consists of a small drill chuck so that needle240can be easily replaced without tools.

Tube shroud245is attached to tube205to protect and shield needle holder235and needle240. Shroud foot255is rigidly attached to tube shroud245and needle240passes through shroud plug250which is rigidly attached to tube shroud245. Two shroud guide pins260are rigidly attached to shroud foot255to ensure URD cable20is centered under needle240. When downward pressure is applied to enclosure295against a URD cable20placed under shroud foot255, spring220compresses which causes shroud foot255, shroud plug250, tube shroud245, tube stop210, and tube205to move upward with respect to enclosure295while support rod230, needle holder235, and needle240remain stationary with respect to enclosure295. This spring loaded mechanism causes needle240to protrude through shroud plug250and penetrate URD cable20. Thus, the portion of needle240not penetrating URD cable20is protected and shielded.

Tube shroud245to tube205mechanical connection is designed to allow tube shroud245to be easily and conveniently removed in the field without the need for tools so as to expose needle240for easy replacement by workers if needle240is damaged. In the preferred implementation, a thumb screw (not shown) is used to affix tube shroud245to tube205.

As illustrated inFIG. 8, URD probe200is pressed against URD cable20. Shroud guide pins260are placed on opposite sides of URD cable20and URD probe200is lightly twisted so that shroud guide pins260are tight against each side of URD cable20. This ensures that the center of URD cable20is aligned exactly under needle240. Shroud guide pins260are spaced wider than the largest diameter URD cable20to be probed so URD probe200can be used on many different diameter URD cables20.

Tube stop guidance pin290is rigidly affixed to tube guide215which ensures tube stop210remains aligned with tube guide215when URD probe200is lightly twisted. Tube stop210moves freely along tube stop guidance pin290as tube stop210moves vertically above tube guide215as URD probe200is pressed against URD cable20.

Tube guide215is threaded for thumb screw265and tube stop210is clearance drilled for thumb screw265. Nut270is rigidly affixed to the end of thumb screw265to limit the vertical movement of tube stop210above tube guide215. This forms a mechanical hard stop which limits the maximum protrusion of needle240through shroud plug250and thus the maximum penetration of needle240into URD cable20.

Sensor support285is either rigidly attached directly to enclosure295or to spring guide225as illustrated inFIG. 9. Sensor280is composed of sensor body282and sensor shaft284. Sensor body282is rigidly attached to sensor support285and spring loaded sensor shaft284contacts tube stop210. Tube stop210moves vertically, as URD probe200is pressed against URD cable20, which causes sensor shaft284to compress into sensor body282which varies sensor280resistance. This resistance change with tube stop210vertical movement implements an electronic needle240depth gauge which allows the operator to monitor the penetration of needle240into URD cable20.

Grip297with hole298is rigidly attached to enclosure295. Standard lineman “shotgun” insulated hot stick grip end305(inFIG. 8) is used to hold URD probe200in a safe manner and at a safe distance when URD probe200is being pressed against URD cable20.FIG. 8illustrates the position of shroud foot255and shroud guide pins260against URD cable20. Digital display325monitors needle240penetration into URD cable20. Short lamp310warns operator that needle240has contacted a neutral conductor30and URD probe200should be moved slightly to new position to avoid neutral conductor30. Hot lamp315indicates URD cable20is energized. Pushbutton320is used to turn URD probe200on and off and to set digital display325needle240depth to zero prior to pressing URD probe200against URD cable20. As digital display325, short lamp310, and hot lamp315are mounted on URD probe200, all user displays are at a safe distance from the user.

URD probe200mechanical parts can be fabricated out of commonly available materials such as aluminum, stainless steel, and plastics. Enclosure295, tube205, and tube shroud245should be fabricated using metal materials so as to maximally shield needle240and preamp232from extraneous external electric fields. Shroud plug250should be fabricated from an insulating material such as plastic.

FIG. 10illustrates a block diagram400of URD probe200cable analysis circuit402coupled to needle240and sensor280. This circuitry can be mounted at any convenient location in or on URD probe200such as on sensor support285or on enclosure295.

Needle240is connected to preamp232which is most appropriately mounted inside support tube230close to needle holder235as illustrated inFIG. 9. Sensor280and preamp232are connected to conditioning circuit410which amplifies, buffers, and scales sensor280and preamp232outputs for processor415. Processor415drives digital display325, short lamp310, and hot lamp315.

Short lamp310illuminates when the resistance between needle240and ground falls below a predetermine value. This allows URD probe200to determine the quality of connection to URD cable20by detecting if needle240input signal capacitive coupled from center conductor22is being reduced due to an adversely low resistance to ground.

Hot lamp315illuminates when needle240line voltage exceeds a predetermined minimum value. When hot light315illuminates, digital display325alternately switches between needle240depth and needle240voltage appropriately scaled to display approximate URD cable20energized voltage. By monitoring both the actual needle240voltage and resistance, URD probe200actively determines status for both a live and a dead URD cable20.

Pushbutton320turns URD probe200on/off and zeros displayed needle240depth when pressed. Battery440and power supply445supply power at the proper voltage and current levels for all URD probe200circuitry. Ground jack460is provided so a ground cable (not shown) can be connected to a convenient ground (including earth or dirt ground) when probing jacketed URD cable20″. A ground cable is optional when probing clean unjacketed URD cable20′ because shroud foot255provides a ground connection to neutral conductors30.

FIG. 11illustrates a flow diagram500of the steps used to apply URD probe200to URD cable20. The first step502is to decide if URD cable20phase attribute is to be determined in addition to the energized status. If so, step503is to select energized/phase mode if phase identification is incorporated into URD probe200or to connect external phase identification equipment to monitor jack450. Monitor jack450can be mounted at any convenient location on URD probe200enclosure295.

The next step505is to use the pushbutton320to turn on URD probe200followed by step510to zero needle240depth. Needle240depth is zeroed by hand compressing shroud foot255while observing needle240protrusion through shroud plug250. When needle240is flush with the bottom of shroud foot255, pushbutton320is momentarily pressed which sets needle240displayed depth on digital display325to zero.

The next step515is to adjust the mechanical hard stop so as to set the maximum needle240penetration depth into URD cable20. Hard stop is set by turning thumb screw265to set the height of nut270above tube stop210. Tube stop210cannot rise above tube guide215any further when tube stop210contacts nut270. Each time thumb screw265is adjusted, the user hand compresses shroud foot255while monitoring needle240depth on digital display325. Maximum depth is selected to be only slightly deeper than the expected thickness of jacket32plus outer semiconductor sheath28. Setting the hard stop correctly prevents needle240from penetrating into insulation26by more than a few thousands of an inch.

The next step520is to grip URD probe200with hot stick grip end305. For jacketed cables20″, plug in an appropriate ground cable (not shown) into URD probe200ground jack460and connect to any convenient ground reference. Simply connecting the ground cable to a screwdriver and poking it into the dirt is adequate. Slowly press URD probe200against URD cable20while monitoring needle240penetration depth on digital display325. If short light310illuminates (step525), it indicates that needle240has contacted a neutral conductor30. Step530should then be executed whereby URD probe200is moved slightly along URD cable20and probed again. Since neutral conductors30are spiral wound, moving URD probe200just a quarter inch along the top of URD cable20is usually enough to clear neutral conductor30that was just contacted.

As needle240depth increases, step535is to watch for hot light315to illuminate. At this point, further penetration of needle240into URD cable20should be avoided by reducing downward pressure on the hot stick. When hot light315illuminates, digital display325alternately switches between needle240depth and needle240voltage appropriately scaled to display approximate URD cable20energized voltage which is noted in step540. Displayed URD cable20voltage is not very precise because it is a function of needle240to center conductor22separation and to the length of needle240penetrating through outer semiconductor sheath28. Its purpose is simply to present a rough voltage magnitude indication to the user. With experience on different types of cables, the user will learn if the displayed voltage is reasonable for the type cable being probed. If the voltage appears unreasonable, step545indicates URD probe200should be moved to a different location on URD cable20and probed again.

If displayed voltage appears reasonable, step550indicates that URD cable20is energized. If hot light315is not illuminated, and if the hard stop was set to ensure that needle240completely penetrated outer semiconductor sheath28, then step555indicates URD cable20is dead (not energized).

Needle240is illustrated inFIG. 12. The portion of needle240that penetrates outer semiconductor sheath28should be covered with a high resistance insulation material243to prevent needle240from shorting to outer semiconductor sheath28. Extreme needle240tip244should not be insulated to detect when needle240contacts a neutral conductor30. Any appropriate high resistance insulation material243can be used. Only approximately the last half inch of needle240need be insulated but the entire needle240, except for the extreme tip and the portion held by needle holder235can be insulated if desired. An example of a low cost insulated needle is a Teflon coated sewing machine needle.

Although URD probe200is designed to capacitive couple to URD cable20center conductor22, it can also be used to determine the energized status of URD cable20by not penetrating outer semiconductor sheath28with needle240but instead measuring line signal SLacross cable resistance R as described in U.S. Pat. No. 7,154,281. Using URD probe200in this manner would result in the same high incidence of false-dead results as the apparatus described in U.S. Pat. No. 7,154,281 but may have application in some circumstances where penetrating outer semiconductor sheath28cannot be tolerated.

In summary, status determination URD probe200is a probing device configured to engage URD cable20and determine the status thereof. The present invention teaches a URD probe200for determining the energized status and optionally the phase of an Underground Residential Distribution (URD) cable20. URD probe200actively determines status for both a live and a dead URD cable20. URD probe200determines status for both unjacketed and jacketed URD cables20′ and20″ respectively. URD probe200displays results viewable and instantly interpretable at a distance. URD probe200determines a quality of connection to URD cable20while determining the status thereof.

Grip297is a standardized hot stick adapter used in the industry to couple to a shotgun hot stick (grip end305only shown inFIG. 8), which is an insulated extension pole. The use of a hot stick allows URD probe200to be used at a distance from the worker, as in the bottom of a deep trench. This allows the worker to determine the status of a URD cable20safely and conveniently from outside the trench without necessitating direct manipulation of URD cable20by the worker. This significantly increases ease of use and overall safety.

The methodologies used to couple hot stick grip297to insulated shotgun hot stick grip end305are commonplace in the industry and well known to those of ordinary skill in the art. These methodologies are therefore not discussed herein.

Although the preferred embodiments of the invention have been illustrated and described in detail, it will be readily apparent to those skilled in the art that various modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

The described preferred implementation of URD probe200precisely monitors and controls needle240penetration into URD cable20to a depth that just penetrates outer semiconductor sheath28without penetrating insulator26by more than a few mils. However, other types of sensors, displays, hard stop, circuitry, and overall mechanical configurations can be used to accomplish the same function as the URD probe200implementation described in this patent disclosure.