SANS 6291-2008 Partial discharge measurements on power cables《电力电缆的局部放电测量》.pdf

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3、damage whatsoever than may result from the use of this material or the information contain therein, irrespective of the cause and quantum thereof. ISBN 978-0-626-21061-8 SANS 6291:2008Edition 2 SOUTH AFRICAN NATIONAL STANDARD Partial discharge measurements on power cables Published by Standards Sout

4、h Africa 1 dr lategan road groenkloof private bag x191 pretoria 0001 tel: 012 428 7911 fax: 012 344 1568 international code + 27 12 www.stansa.co.za Standards South Africa SANS 6291:2008 Edition 2 Table of changes Change No. Date Scope Foreword This South African standard was approved by National Co

5、mmittee StanSA TC 66, Electric cables, in accordance with procedures of Standards South Africa, in compliance with annex 3 of the WTO/TBT agreement. This document was published in March 2008. This document supersedes SABS SM 1291:2000 (edition 1). Annex B forms an integral part of this document. Ann

6、ex A is for information only. SANS 6291:2008 Edition 2 1 Contents Page Foreword 1 Scope 3 2 Normative references 3 3 External interference 3 4 Partial discharge test on complete drum length of cable (routine test) . 4 5 Partial discharge test on cable sample lengths (type approval test). 5 Annex A (

7、informative) The design of a shielded room for partial discharge tests on power cables 6 Annex B (normative) The identification of partial discharge patterns 8 SANS 6291:2008 Edition 2 2 This page is intentionally left blank SANS 6291:2008 Edition 2 3 Partial discharge measurements on power cables 1

8、 Scope This standard specifies methods for carrying out the partial discharge test on power cables of a) complete drum lengths of cable as a routine test during manufacture, and b) sample lengths as part of a type approval test. 2 Normative references The following referenced documents are indispens

9、able for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. Information on currently valid national and international standards can be obtained from Standard

10、s South Africa. SANS 60270/IEC 60270, High-voltage test techniques Partial discharge measurements. 3 External interference The measurement of partial discharge magnitude is generally complicated by the presence of external interference, which may originate from one or more of the following sources.

11、a) Disturbances that occur when the test circuit is not energized or is energized but at zero voltage. These may result from radio transmissions, switching operations in other circuits, thyristor-controlled variable speed drives, induction furnaces, etc., and inherent noise in the test instrument it

12、self. b) Disturbances that occur only when the test circuit is energized, but which do not originate in the cable under test. These include, for example, partial discharges in the testing transformer, on the surface of HV connections, or sparking of imperfectly earthed objects in the vicinity of the

13、 test circuit. Such disturbances, which may be equivalent to individual apparent charge magnitudes of several hundred picocoulombs, may be eliminated or greatly reduced by conducting the test in a suitably shielded room (see annex A) or by the adoption of polarity discrimination methods. The basic d

14、esign of a shielded room is given in annex A. The identification of partial discharge patterns is included as annex B. SANS 6291:2008 Edition 2 4 4 Partial discharge test on complete drum length of cable (routine test) 4.1 Apparatus 4.1.1 General Local conditions will dictate whether or not it is ne

15、cessary to conduct the test in a shielded room. Where external interference is minimal, it is sometimes possible to adopt a method of detection using a balanced detection circuit, or some form of gating, or a polarity discrimination method. The background noise level of the partial discharge detecti

16、on circuit should be less than or equal to 50 % of the maximum permissible partial discharge magnitude specified. Where signal gating is employed to eliminate an unwanted signal, the gated period should not exceed 2 % of the test voltage period. If, however, several mains-synchronized interference s

17、ources are present, the blocking period may be increased to a total of 10 % of the test voltage period. 4.1.2 Test circuit One of the test circuits detailed in SANS 60270. 4.1.3 Calibrator A calibrator as detailed in SANS 60270, the calibration of which shall be certified by an accredited testing au

18、thority. 4.2 Test specimen 4.2.1 Cable A drum length of single-core or three-core cable, as required. 4.2.2 Partial discharge test terminations In the case of tests on cables that have a rated voltage of up to 33 kV, moulded rubber stress cones or a stress-relieving paint may be used, but at higher

19、voltages, deionized water terminations will probably be required. 4.3 Procedure 4.3.1 Orientation of display After having carried out complete functional checks of the test circuit, establish the position on the partial discharge detector display of the negative peak of the test waveform according t

20、o the method detailed in annex B, and rotate the ellipse so that the ellipse is oriented as shown in figure B.1. Where the detector employs a sine wave display, this procedure may be ignored. 4.3.2 Calibration With the supply to the HV source isolated, but with no safety earthing connection to the H

21、V side of the test circuit, connect the calibrator between HV and earth. Switch on the calibrator and check that calibration pulses are displayed on the elliptical trace. If the calibrator pulses are not synchronized automatically to the test waveform, adjust the pulse repetition frequency to be as

22、close as possible to twice the frequency of the supply. SANS 6291:2008 Edition 2 5 Inject calibration pulses of magnitude as close as possible to the specified maximum level, and adjust the gain of the detector amplifier to give a pulse height of about 10 mm. Switch on the internal pulse generator o

23、f the detector and inject pulses that have the same magnitude as those generated by the calibrator. Adjust the gain control of the internal pulse generator to give a pulse height on the display equal to that of the pulses generated by the calibrator. Check the linearity of the internal pulse generat

24、or by injecting pulses from the calibrator and the internal pulse generator equal to 200 % of the specified maximum partial discharge level. 4.3.3 Determination of partial discharge inception and extinction voltages Starting from zero volt or from a voltage level well below the expected partial disc

25、harge inception voltage, gradually increase the voltage until partial discharges equal to or in excess of the specified partial discharge level appear on the display. Record this voltage as the partial discharge inception voltage. Slowly increase the test voltage to the specified withstand test volt

26、age, and maintain it at that level for the specified test period. Slowly lower the test voltage until the magnitude of the partial discharges falls below the specified partial discharge level. This will normally be equal to or lower than the inception voltage. Record this voltage as the partial disc

27、harge extinction voltage. 5 Partial discharge test on cable sample lengths (type approval test) The calibration procedure and method of test will be the same as that detailed in clause 4. However, if the type approval test includes an impulse voltage test, both the test terminations and high-voltage

28、 clearances to earth shall be adequate for the impulse withstand test voltage. SANS 6291:2008 Edition 2 6 Annex A (informative) The design of a shielded room for partial discharge tests on power cables A.1 In order to be able to eliminate most of the unwanted disturbances that will occur during a pa

29、rtial discharge test on a drum length or a sample length of power cable, it will normally be necessary to conduct the test in a shielded room. A.2 The steps in A.2.1 to A.2.4 should be taken to eliminate unwanted disturbances. A.2.1 Fully insulate the test room electrically from the building in whic

30、h it is installed. Where tests are to be conducted on sample lengths of cable only, the room can be built on insulating blocks and the walls and ceiling supported by or suspended from the main building structure. Where drum lengths of cable are to be tested, the floor of the test room should be leve

31、l with the floor of the main building to allow for the easy passage of the drum into and out of the room. The room can be built on a steel floor, insulated from the floor of the main building, as shown in figure A.1. Figure A.1 Section through floor An area about 150 mm larger than the test room, bo

32、th in length and width, should be excavated to a depth of about 200 mm, and a screed of thickness 50 mm applied to the base to provide a smooth surface for the layers of polyethylene sheeting, which will insulate the test room from the main building. A.2.2 Provide a single earth point remote from th

33、e main building earth. Where possible, a separate earth should be provided and all earth connections from the supply transformer, and other parts of the HV test circuit, should be connected to this point and to this point alone. A single connection should be made from the steel floor or from the pan

34、els of the earth point. This connection should be removable so that the earth resistance and the resistance between the SANS 6291:2008 Edition 2 7 screened room and the main building can be periodically checked. Ideally, an earth rod should be driven into the ground through a hole in the floor of th

35、e test room as close as possible to the earth connection of the supply transformer. Take care to ensure that the earth rod does not make contact with any reinforcing steelwork below the screened room. A.2.3 Fully screen the walls and ceiling of the test room. The walls and ceiling should provide scr

36、eening for any airborne or radiated disturbances. Two layers of galvanized steel sheeting, each 1 mm thick and separated by a distance of 12 mm to 20 mm, have been found to provide adequate shielding. A composite sandwich that comprises a chipboard panel, with the galvanized sheeting bonded to the f

37、ront and back of the panel, has been used successfully for this purpose. Any support required for wall or ceiling panels should be provided by way of cast resin insulators to maintain the necessary electrical insulation from the main building structure. A.2.4 Filter all power supplies to test and me

38、asuring circuits. Any lead that enters the screened room acts as an antenna outside the room and as a transmitter inside the room. The power supply to the HV transformer should be isolated from the mains by means of a double wound transformer with an electrostatic screen between the windings. The su

39、pplies to interior lights, warning lights and the control circuitry to micro-switches on entry doors and emergency operation buttons should similarly be isolated from the mains. Any interior lighting in the screened room should be provided by way of normal tungsten filament lamps. Fluorescent lamps

40、should not be used. SANS 6291:2008 Edition 2 8 Annex B (normative) The identification of partial discharge patterns B.1 General The following figures illustrate some of the patterns commonly observed on the display unit of the partial discharge detector. Most partial discharge detectors present one

41、cycle of the supply voltage as an elliptical trace rather than as a sine wave. The illustrations presented may differ slightly from the illustrations obtained from the partial discharges captured as one cycle of the test waveform, generally because persistence of vision will result in an image combi

42、ning two or three cycles. B.2 Orientation of the waveform B.2.1 Orientation Figure B.1 illustrates voltage zeros that occur on the extreme left and right of the elliptical trace. Positive peak is at the top, negative peak at the bottom and rotation is clockwise. Figure B.1 Orientation of elliptical

43、trace B.2.2 Air discharge (corona) Air discharges appear on the trace as multiple spikes equally disposed about the negative peak position as shown in figure B.2. As the test voltage is raised, the spikes increase in number on both sides of the voltage peak, but remain at the same amplitude, as show

44、n in figure B.3. SANS 6291:2008 Edition 2 9 Figure B.2 Air discharge just above inception Figure B.3 Appearance of air discharges as voltage is raised If the voltage is increased further, smaller value spikes may appear on the opposite side of the ellipse, equally disposed about the positive peak po

45、sition. If the voltage is carefully lowered, the discharges will decrease in number until only one remains, at the negative peak. The above pattern may be used to identify the position of the negative peak on the trace. If a short length (300 mm) of bare copper wire is attached to one of the HV conn

46、ections and allowed to point towards the nearest earth plane, it will produce a point of high electrical stress at the tip. Air discharges will be produced at a voltage of around 10 kV. Air discharges shall be eliminated from the test circuit unless they are small enough to be ignored, which is rare

47、ly the case. They usually originate from sharp edges of fixing bolts and nuts and small diameter test leads. At higher voltages, well above inception, air discharges will be audible, and can also be observed when the test is carried out in total darkness. At these levels, ozone gas may also be detec

48、ted. Figures B.4 and B.5 indicate methods of shielding to eliminate air discharges. SANS 6291:2008 Edition 2 10 Figure B.4 Shielding of HV transformer bushing Figure B.5 Shielding with a single toroid The cross-sectional dimensions of toroids and tubular connections and clearances to earth shall be

49、adequate for the maximum test voltage to be used. As a guide, allow 10 mm/kV (r.m.s.) clearance from centre of toroid to earthed wall or ceiling of the shielded room, 0,5 mm/kV (r.m.s.) for the diameter of tubular connections, and 1 mm/kV (r.m.s.) for the diameter of the cross-section of toroids. SANS 6291:2008 Edition 2 11 B.2.3 Contact noise Contact noise, which results from poor electrical contact in one or more of the HV connections, sometimes occurs when currents of a few amperes are being drawn, for example when testing a drum length of cable. It rarely occurs d