WO1990005311A1

WO1990005311A1 - Method and means for testing insulation integrity on low-voltage, unshielded electrical cables

Info

Publication number: WO1990005311A1
Application number: PCT/US1989/004678
Authority: WO
Inventors: Luis G. Delvalle; Theodore W. Rose; Jean-Pierre Wolff
Original assignee: Electric Power Research Institute, Inc.
Priority date: 1988-11-03
Filing date: 1989-10-25
Publication date: 1990-05-17

Abstract

The insulation of low-voltage unshielded electrical cables (10) is tested for defects by placing the cable (10) in an atmosphere (14) of ionizable gas, applying a high-voltage transient electrical pulse from a power supply (18) to the electrical cable (10), and simultaneously applying a DC voltage from a DC power supply (20) to the electrical cable (10). The high-voltage transient electrical pulse creates an electrical field around the cable which is sufficient to cause corona discharge at a defect in the insulation of the cable. The DC voltage provides a DC current to the cable which travels along a corona discharge to ground, thereby indicating the presence of an insulation defect.

Description

METHOD AND MEANS FOR TESTING INSULATION INTEGRITY ON LOW-VOLTAGE, UNSHIELDED ELECTRICAL CABLES

Background of the Invention

This invention relates generally to the testing of insulation of electrical cables, and more par¬ ticularly the invention relates to testing of insulation integrity of low-voltage, unshielded electrical cables.

Low-voltage, unshielded electrical cables are used for instrumentation, control, and low-voltage (e.g. 600 volts) power distribution in electrical power plants and other facilities in the electrical power and other industries. In installing such cables the insulation can be damaged. Heretofore, in situ high-potential leakage and breakdown tests have been employed with such cables. However, these tests often indicate insulation breakdown failure when, in fact, the cables would have been func-tional under operating voltages and service conditions. On the other hand, these tests can give positive results, whereas subsequent movement of the cables or introduction of moisture can lead to operational failure. Also, the tests are particularly ineffec¬ tive in gauging the ability of cables to function in a high-temperature, high-pressure, moist accident environment. The ^"key difficulty with in-plant tests lies in the lack of a reference ground plane which can exist with the cables in service in a hot and moist atmosphere or during inadvertent water submergence. Heretofore, in an attempt to provide the necessary ground plane in the immediate vicinity of the cable during test, selected conduits in a plant have been filled with tap water. However, this can be damaging to cabling and equipment as well as hazardous to plant personnel. _»

Summary of the Invention

An object of the present invention is an improved method of testing the insulation of low- voltage, unshielded electrical cables.

Another object of the invention is apparatus which creates a virtual ground plane in testing low- voltage cables without requiring the use of water.

Briefly, in accordance with the invention, a cable is placed within a grounded container (such as an electrical conduit) along with an ionizable gas. Alternatively, the cable can be near any grounded object. The gas is ionized by applying high-voltage transient pulses of short duration through the cable, thereby creating an ionizing electrical field around the conductor. The ionized gas, in effect, transfers the ground plane from the container or other nearby grounded object to the surface of the insulated cable. A -second, DC potential is simultaneously applied to the cable. The DC potential can be substantially less than that used with the tradition¬ al DC high-voltage testing.

The electrical field created by the high-voltage transient pulse will be stable and uniform except in the presence of a fault in the insulation. Tho fault will distort the field, causing high field stress which produces a corona discharge to the grounded container at the fault. A defect is readily detected by DC fault current conduction sufficient to trip a circuit protective device. Alternatively, without an imposed DC voltage, the discharge to ground of the transient pulse could be detected by noting a change in the character of the excitation signal.

The invention and objects and features thereof will be more full understood from the following detailed description and appended claims when taken with the drawing.

Brief Description of the Drawing

The Figure illustrates apparatus for testing an insulated cable in accordance with one embodiment of the invention.

Detailed Description of Illustrative Embodiments

Referring to the drawing, an insulated cable 10 undergoing test is placed inside of a grounded conduit 12 which contains air or another gas 14. The conductor of cable 10 is connected through an AC coupling network 16 to a transient AC power supply 18 and a serially-connected DC power supply 20. A DC current fault indicator 22 is provided in the serial circuit for indicating a fault current. In one embodiment, a number 12 AWG cable having polymeric insulation was placed within a 2-inch diameter steel conduit 10 feet long. The cable was greater than 50 feet in length and had to be moved in sections within the conduit for testing. A pinhole default 24 was artificially induced in the insulation and held in place one-half inch from the conduit wall. The defect was readily detected by fault conduction current sufficient to trip the circuit's protective device 22.

Excitation consisted of a DC power supply of 2500 volts and a Tesla-type AC generator with an output voltage of 5 kilovolts and multifrequency content from lOKHz to 200KHZ. With the cable centered in the conduit, this excitation was insufficient to induce fault-to-ground until a quantity of ionizable argon gas was introduced. However, in the presence of the argon gas, the electrical field created by the high-voltage pulse ionizes the gas and in effect transfers the ground plane from the grounded conduit 12 to the surface of the insulation on the cable 10. The pinhole fault distorts the field, causing high field stress which produces a corona discharge through the gas surround¬ ing the cable at that point. This electrical discharge occurs along the path of least resistance between the fault and the grounded metallic surface. In the presence of the second potential, a sudden current flows through the path created by the corona discharge to ground, and sensors in the DC power supply react to the sudden current flow, terminate the test, and indicate occurrence of the fault. Thus the DC voltage serves both as an electrical stressor and an indicator signal.

The test procedure in accordance with the invention essentially establishes a ground plane at the surface of the electrical cable and promotes a fault-to-ground as an indication of a defect. The test procedure is less damaging than traditional high-potential tests, since low-energy ionizing excitation and a lower DC voltage are utilized. The test procedure combines broad band high-frequency voltage and DC high-voltage on a common conduc-tor. The test procedure can be used to detect flaws in low-voltage, unshielded cables which are used for instru-mentation, control, and low-voltage power distribution in many types of industrial facilities. These cables can be tested in the installed state, but will require that sensitive loads be disconnected before the associated cable is tested. The cables may be located in a cable tray, cable duct, or metallic conduit. The may also ge located in non- metallic conduit or routed across an open space since, in such case, the ionized sheath around the cable would serve as the return path to ground. Further, the test procedure can be used in any situation for which existing electrical techniques would be used to test a cable's voltage breakdown or leakage properties, including testing of some types of shielded cables. It is noted that the invention can be employed anywhere the cable is not in vacuum except in an application in which high explosives or explosive gas mixtures are present.

While the invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. For example, the invention is applicable to other installation configurations such as a cable tray rather than a conduit, with the ambient atmosphere as the ionized gas. Thus, various modifications and amplifications may occur to those skilled in the art without ^•departing from the true spirit and scope of the invention as defined by the appended claims.

Claims

CLAIMS :

1. A method of testing an insulated electrical cable for insulation defects comprising the steps of placing the cable near a grounded conductor in an atmosphere of ionizable gas, applying a high-voltage transient electrical pulse to the electrical cable conductor, thereby creating an electrical field around the cable and a corona discharge through the ionizable gas at an insulation defect, simultaneously applying a DC voltage to the electrical cable conductor, and sensing DC current flow through the corona discharge path to ground.

2. The method of testing as defined by claim 1 wherein said step of applying a high-voltage transient electrical pulse includes applying a pulse with a peak voltage on the order of 5000 volts with a multiple frequency content from lO Hz to 200KHZ.

3. The method of testing as defined by claim 2 wherein said step of applying DC voltage includes applying a DC voltage on the order of 2500 volts.

4. The method of testing as defined by claim 3 ' wherein said step of placing the cable in an atmosphere of ionizable gas includes placing the cable in a conduit filled with ionizable gas such as air.

5. The method of testing as defined by claim 4 wherein the ionizable gas is argon.

6. The method of testing as defined by claim 1 wherein said step of placing the cable in an atmosphere of ionizable gas includes placing the cable in a conduit filled with ionizable gas.

7. The method of testing as defined by claim 6 wherein the ionizable gas is argon.

8. Apparatus for testing the insulation of an electrical cable comprising a container of ionizable gas of suitable dimensions for receiving an electrical cable for test, a transient power supply for generating a pulse of AC voltage, a DC power supply, means for serially connecting said transient power supply, said DC power supply, and a cable for test whereby said transient power supply creates an electrical field around said cable and corona

- discharge at a. defect in the insulation of the cable, and said DC power supply provides a DC current through the cable and the corona discharge to ground.

9. Apparatus as defined by claim 8 and further including means for sensing DC current flow indica¬ tive of a flaw in insulation.

10. Apparatus as defined by claim 9 wherein said container of ionizable gas comprises a conduit.

11. Apparatus as defined by claim 10 wherein said ionizable gas is argon.

12. Apparatus as defined by claim 9 wherein said ionizable gas is ambient air.