1、 International Telecommunication Union ITU-T K.95TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (02/2014) SERIES K: PROTECTION AGAINST INTERFERENCE Surge parameters of isolating transformers used in telecommunication devices and equipment Recommendation ITU-T K.95 Rec. ITU-T K.95 (02/2014) i Recomm
2、endation ITU-T K.95 Surge parameters of isolating transformers used in telecommunication devices and equipment Summary Telecommunication devices and equipment can use non-linear limiting, linear attenuating, or both types of functions to mitigate surges occurring on services. Isolating transformers
3、attenuate common-mode voltage surges on a service. Recommendation ITU-T K.95 gives test methods and preferred values for the isolating transformer surge parameters that are harmonized with established international standards on insulation coordination levels and test techniques. History Edition Reco
4、mmendation Approval Study Group Unique ID*1.0 ITU-T K.95 2014-02-13 5 11.1002/1000/12128 Keywords Insulation, insulation resistance, isolation transformer, rated impulse voltage, surge, withstand voltage. _ *To access the Recommendation, type the URL http:/handle.itu.int/ in the address field of you
5、r web browser, followed by the Recommendations unique ID. For example, http:/handle.itu.int/11.1002/1000/11830-en. ii Rec. ITU-T K.95 (02/2014) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications, information and commun
6、ication technologies (ICTs). The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The
7、 World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1
8、. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a r
9、ecognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. T
10、he words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibilit
11、y that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recomm
12、endation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest information
13、and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2014 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T K.95 (02/2014) iii Table of Contents Page
14、 1 Scope 1 2 References. 1 3 Definitions 1 3.1 Terms defined elsewhere 1 3.2 Terms defined in this Recommendation . 2 4 Abbreviations and acronyms 3 5 Conventions 3 6 Surge parameters 4 6.1 Transformer surge mitigation . 4 6.2 Common-mode surges 5 6.3 Differential-mode surges 6 7 Characteristics 6 7
15、.1 Characteristic measurement 6 7.2 Inter-winding capacitance 7 7.3 Insulation resistance . 8 7.4 Signal transformer voltage-time product 10 8 Ratings 11 8.1 Rating verification 11 8.2 Rated impulse voltage 11 8.3 Signal transformer rated winding d.c. 14 Appendix I 1.2/50 impulse 16 I.1 Introduction
16、 16 I.2 Term definitions . 16 I.2 1.2/50 waveform parameters 17 Bibliography. 18 Rec. ITU-T K.95 (02/2014) 1 Recommendation ITU-T K.95 Surge parameters of isolating transformers used in telecommunication devices and equipment 1 Scope This Recommendation sets terms, test methods, test circuits, measu
17、rement procedures and preferred result values for the surge parameters of isolating transformers used in telecommunication devices and equipment. Three types of isolating transformer are covered: mains, switched mode power supply and signal. The surge parameters of the isolating transformer insulati
18、on barrier covered by this Recommendation are: rated impulse voltage input winding to output winding capacitance insulation resistance. Additional parameters for signal isolating transformers are: core saturation voltage-time product rated input winding root mean square (rms) current for a given tem
19、perature rise. This Recommendation does not cover the transformer parameters required to help ensure appropriate operation on the service e.g., signal transformer return loss. 2 References None. 3 Definitions 3.1 Terms defined elsewhere This Recommendation uses the following terms defined elsewhere:
20、 3.1.1 breakdown b-IEC 61340-1: Failure, at least temporarily, of the insulating properties of an insulating medium under electric stress. 3.1.2 clearance b-IEC/TR 60664-2-1: Shortest distance in air between two conductive parts. 3.1.3 creepage distance b-IEC/TR 60664-2-1: Shortest distance along th
21、e surface of a solid insulating material between two conductive parts. 3.1.4 electric screen b-IEC 60050-151: Screen of conductive material intended to reduce the penetration of an electric field into a given region. 3.1.5 hazard b-IEC GUIDE 116: Potential source of harm. NOTE The term hazard can be
22、 qualified in order to define its origin (e.g., electrical hazard, mechanical hazard) or the nature of the potential harm (e.g., electric shock hazard, cutting hazard, toxic hazard, fire hazard). 3.1.6 impulse withstand voltage b-IEC/TR 60664-2-1: Highest peak value of impulse voltage of prescribed
23、form and polarity which does not cause breakdown of insulation under specified conditions. 3.1.7 insulation (electrical) b-IEC 62477-1: Electrical separation between circuits or conductive parts provided by clearance or creepage distance or solid insulation or combinations of them. 2 Rec. ITU-T K.95
24、 (02/2014) 3.1.8 insulation coordination b-IEC/TR 60664-2-1: Mutual correlation of insulation characteristics of electrical equipment taking into account the expected microclimate and other influencing stresses. 3.1.9 insulation resistance b-IEC 62631-1: Resistance under specified conditions between
25、 two conductive bodies separated by the insulating material. 3.1.10 isolating transformer b-IEC 60065: Transformer with protective separation between the input and output windings. 3.1.11 pollution b-IEC 60664-1: Any addition of foreign matter, solid, liquid, or gaseous that can result in a reductio
26、n of electric strength or surface resistivity of the insulation. 3.1.12 pollution degree b-IEC 60664-1: Numeral characterizing the expected pollution of the micro-environment 3.1.13 pollution degree 1 b-IEC 61131-2: No pollution or only dry, non-conductive pollution occurs. The pollution has no infl
27、uence. 3.1.14 pollution degree 2 b-IEC 61131-2: Normally, only non-conductive pollution occurs. Occasionally, however, a temporary conductivity caused by condensation must be expected. 3.1.15 pollution degree 3 b-IEC 61131-2: Conductive pollution occurs, or dry, non-conductive pollution occurs which
28、 becomes conductive due to condensation, which is expected. 3.1.16 overvoltage b-IEC/TR 60664-2-1: Any voltage having a peak value exceeding the corresponding peak value of maximum steady-state voltage at normal operating conditions. 3.1.17 rated impulse voltage b-IEC/TR 60664-2-1: Impulse withstand
29、 voltage value assigned by the manufacturer to the equipment or to a part of it, characterizing the specified withstand capability of its insulation against transient overvoltages. 3.1.18 thermal equilibrium b-IEC 61810-1: Variation of less than 1 K between any two out of three consecutive measureme
30、nts made at an interval of 5 min. 3.1.19 thermal resistance b-IEC 62590: Quotient of the temperature difference between two specified points or regions and the heat flow between these two points or regions under conditions of thermal equilibrium. NOTE For most cases, the heat flow can be assumed to
31、be equal to the power dissipation. 3.1.20 withstand voltage b-IEC/TR 60664-2-1: Voltage to be applied to a specimen under prescribed test conditions which does not cause breakdown and/or flashover of a satisfactory specimen. 3.2 Terms defined in this Recommendation This Recommendation defines the fo
32、llowing terms: 3.2.1 common-mode surge: Surge appearing equally on all conductors of a group at a given location. NOTE 1 The reference point for common-mode surge voltage measurement can be a chassis terminal, or a local earth/ground point. NOTE 2 Also known as longitudinal surge or asymmetrical sur
33、ge. 3.2.2 component type: Identification number assigned to a component, based on an identification system used by a particular organization. NOTE Model number, product ID, product name, product number; product type, part number; part ID are used as synonyms to component type 3.2.3 differential-mode
34、 surge: Surge occurring between any two conductors or two groups of conductors at a given location. Rec. ITU-T K.95 (02/2014) 3 NOTE 1 The surge source maybe be floating, without a reference point or connected to reference point, such as a chassis terminal, or a local earth/ground point. NOTE 2 Also
35、 known as metallic surge or transverse surge or symmetrical surge or normal surge. 3.2.4 guarded measurement (three terminal network): Measurement technique that allows the direct impedance between two terminals to be measured correctly by applying a compensating voltage to the third terminal that r
36、emoves the shunting effects of any impedances to the third terminal. 3.2.5 microclimate: Climatic condition at the place where a component is installed in the product. NOTE Only the in product maximum air temperature (classes X1 to X7) and, optionally, the maximum air humidity class (classes Y1 to Y
37、4) are taken into account. 3.2.6 rated winding d.c.: Maximum winding current that will not cause the winding conductor temperature to exceed a specified increase above the ambient temperature. 3.2.7 surge: Temporary disturbance on the conductors of an electrical service caused by an electrical event
38、 not related to the service. 4 Abbreviations and acronyms This Recommendation uses the following abbreviations and acronyms: IR Insulation Resistance RMS Root-Mean-Square 5 Conventions This Recommendation uses the following symbols to represent the different transformer configurations. Figure 5-1 sh
39、ows the symbol for a two-winding transformer. K.95(14)_F5-1Figure 5-1 Symbol for a two-winding transformer Figure 5-2 shows the symbol for a two-winding transformer with instantaneous voltage polarity indicators. K.95(14)_F5-2Figure 5-2 Symbol for a two-winding transformer with polarity indication 4
40、 Rec. ITU-T K.95 (02/2014) Figure 5-3 shows the symbol for a two-winding transformer with an electric screen between the windings. K.95(14)_F5-3Figure 5-3 Symbol for a two-winding transformer with electric screen Figure 5-4 shows the symbol for a transformer centre-tapped windings. When testing is d
41、one with shorted windings the centre tap is also connected to the short, other testing is done without any connection to the centre tap terminal. K.95(14)_F5-4CTCTFigure 5-4 Transformer with centre-tapped windings When a single twisted pair signal service also carries a d.c. powering voltage a serie
42、s capacitor is needed to block the d.c. voltage and couple the signal to the transformer. To maintain circuit symmetry (i.e., balance) either each conductor has a series capacitor inserted or the transformer winding is made in two halves and a single capacitor connects the two halves together. For t
43、est purposes a transformer having the winding in two halves should have a shorting link made between the halves, so that a continuous winding is formed. The link effectively forms the CT connection shown in Figure 5-4. 6 Surge parameters 6.1 Transformer surge mitigation An isolating transformer coup
44、les a service across the transformer insulation by magnetic induction. When common-mode surges occur on the incoming service the insulation is voltage stressed. The insulation has three physical paths: transformer winding insulation: insulating material interposed between the two windings creepage d
45、istance clearance. Clearance distances should be set so that the maximum expected voltage difference does not break down the clearance. Creepage distances should be set so that the maximum expected voltage difference and pollution degree do not cause surface flashover or breakdown (tracking). Solid
46、insulation thickness should be set so that the maximum expected voltage difference does not cause breakdown. The higher frequency components of a surge impulse will be a.c. coupled by the transformer inter-winding capacitance (shown as CP-SA+ CP-SB) from one winding to the other (see Figure 6-1). In
47、 Figure 6-1, the inter-winding capacitance is shown as two lumped capacitive elements: one Rec. ITU-T K.95 (02/2014) 5 connecting the upper winding terminals, path and suffix A; and one connecting the lower winding terminals, path and suffix B. K.95(14)_F6-1WSZTZSWPCP-SBCP-SARWPPrimary winding CP-SA
48、, CP-SBPrimary to Secondary inter-winding capacitance, paths A and B WSSecondary winding ZSService source impedance R Reference plane or point ZTTerminating or load impedance Figure 6-1 Common-mode surge conditions for the transformer To reduce inter-winding capacitance a conducting electric screen
49、can be used between the windings (see Figure 6-2). In Figure 6-2 the parasitic capacitances are shown as two lumped delta configured networks: one connecting to the upper winding terminals and electric screen, path and suffix A; and one connecting to the lower winding terminals and electric screen, path and suffix B. The electric screen decouples most of the winding capacitance, shown as CP-ScreenA, CP-ScreenB, CS-ScreenAand CS-ScreenB, leaving a much smaller value o
