IEC 60885-3:2015 pdf download – Electrical test methods for electric cables – Part 3: Test methods for partial discharge measurements on lengths of extruded power cables
1 Scope This part of IEC 60885 specifies the test methods for partial discharge (PD) measurements on lengths of extruded power cable, but does not include measurements made on installed cable systems. Reference is made to IEC 60270 which gives the techniques and considerations applicable to partial discharge measurements in general. 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60270:2000, High-voltage test techniques – Partial discharge measurements 3 Terms, definitions and symbols 3.1 Terms and definitions For the purposes of this document, the terms and definitions given in IEC 60270 apply.
4 Overview 4.1 General Partial discharge measurements shall be carried out using the test techniques specified in IEC 60270. 4.2 Object The object of the test is to determine the discharge magnitude, or to check that the discharge magnitude does not exceed a specified value, at a specified voltage and a declared minimum sensitivity. 4.3 Problem of superposition of travelling waves for long lengths Short lengths of cable behave in the same way as a single capacitor in that the discharge magnitude can be measured directly by considering the cable as a single capacitor. However longer cables behave like a transmission line and PD pulses travel away from their source in both directions along the cable, in the form of a wave. On reaching the remote end from the measuring equipment, the pulse will be reflected with the same polarity if the end is open circuit. The reflected pulse will then travel back along the length of cable and arrive at the detector at a time after the directly received pulse. If the time between the arrival of the two pulses is short (the time difference depending on the length of the cable) then the detection instrument may give a false response, indicating either a larger or smaller magnitude of discharge than was actually the case. The methods detailed in this standard allow correct measurement of partial discharges under these conditions. Figures 1 to 4 illustrate the behaviour of travelling waves and possible superposition effects.
5 Partial discharge tests 5.1 Test apparatus 5.1 .1 Equipment The equipment consists of a high-voltage alternating voltage supply having a rating adequate to energise the length of cable under test, a voltmeter for high voltages, a measuring circuit, a discharge calibrator, a double pulse generator and, where applicable, a terminal impedance or reflection suppressor. All components of the test equipment shall have a sufficiently low noise level to achieve the required sensitivity. The frequency of the test supply shall be in the range 45 Hz to 65 Hz with a waveshape approximating to a sinusoid with the ratio of peak to r.m.s. values being equal to √2 with a maximum tolerance of 5 %. 5.1 .2 Test circuit and instruments The test circuit includes the high voltage power supply, test object, the coupling capacitor and the HV and PD measuring equipment. The measuring circuit consists of the measuring impedance (input impedance of the measuring instrument and the input unit which is selected to match the cable impedance), the connecting lead and the measuring instrument. The measuring instrument or detector includes a suitable amplifying device, an oscilloscope, or other instrument to indicate the existence of partial discharges and to measure the apparent charge. The measuring system shall comply with IEC 60270. 5.1 .3 Double pulse generator A double pulse generator is an instrument producing two equal pulses (with the same apparent charge) following each other within a time interval which can be varied between 0,2 µs to 1 00 µs. The rise time of the pulses shall not exceed 20 ns (1 0 % to 90 % of peak value); the time between 1 0 % values of the front and the tail shall not exceed 1 50 ns. The pulses may be synchronized with the power frequency.