NB/T 10498-2021Code for Design of AC 110 kV to 500 kV Power Cable Systems for Hydropower Station (English PDF)

Foreword

This document was issued on 7 January 2021 by the National Energy Administration of the PRC and takes effect on 1 July 2021.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.140, Chinese classification P59.

It replaces DL/T 5228-2005, which is superseded.

By Announcement No. 1 of 2021, the National Energy Administration approved and issued 320 energy industry standards, among them NB/T 10498-2021, which replaces DL/T 5228-2005; the approval date is 2021-01-07 and the implementation date is 2021-07-01.

This code was revised by the drafting group in accordance with the requirements of the Notice of the National Energy Administration on Issuing the 2016 Plan for the Formulation (Revision) of Energy Sector Industry Standards (Guo Neng Ke Ji [2016] No. 238), through extensive investigation and research, careful summarisation of practical experience, reference to relevant international standards and wide solicitation of opinions.

The main technical contents of this code are: general provisions, terms and symbols, service conditions, selection of main technical parameters, type and structure, terminations and joints, earthing methods and overvoltage protection for metal sheath, auxiliary facilities, layout and laying, fire prevention, and test.

The main technical changes in this revision are: provisions on cable on-line monitoring devices have been added; provisions on cable fire prevention have been added; a new chapter 'Test' has been added.

The former contents on voltage, permissible continuous current-carrying capacity and conductor cross-section, rated short-time withstand current and insulation level have been merged into the chapter 'Selection of Main Technical Parameters', and the contents relating to equalizing conductors and oil-filled cables have been deleted.

This code is under the administration of the National Energy Administration, is proposed and routinely managed by China Renewable Energy Engineering Institute, and its specific technical contents are interpreted by the Energy Industry Hydropower Electrical Design Standardization Technical Committee (NEA/TC17).

Comments and suggestions arising during implementation should be sent to China Renewable Energy Engineering Institute (Address: No. 2 Liupukang Beixiaojie, Xicheng District, Beijing, 100120).

Chief development organization: PowerChina Chengdu Engineering Corporation Limited. Participating development organizations: PowerChina Huadong Engineering Corporation Limited; Jiangsu Ankao Zhineng Shudian Gongcheng Keji Co., Ltd.

Chief drafting staff: Li Qiang, He Xiao, Wang Yaohui, Wen Fengxiang, Yu Dan, Yu Peng, Li Yong, Feng Zhenqiu, Hou Yanshuo, Wang Jingwen, Yi Xiaojing, Chen Xiaoming, Wang Xinqi, Ju Lin.

Chief reviewers: Yu Qinggui, Pang Xiulan, Wang Jinfu, Kang Benxian, Chen Yinqi, Wu Zhongping, Zhou Huigao, Xia Fujun, Yang Jianjun, Wang Yong, Shao Guangming, Yang Junshuang, Wang Huajun, Liu Changwu, Deng Shuangxue, Xie Xiaohui, Pan Hong, Zong Wanbo, Yang Mei.

1 Scope

NB/T 10498-2021 is the Chinese design code for AC power cable systems from 110 kV to 500 kV in hydropower stations, replacing DL/T 5228-2005. In a hydropower plant the high voltage connection from the main transformers to the switchyard often cannot run as overhead line: the transformers sit in a cavern or beside a dam, and the route passes through shafts, tunnels and galleries. High voltage cable is the answer, and designing it in those conditions is a discipline of its own. The code sets the general provisions and defined terms, then the selection of cable type - cross-linked polyethylene insulated cables and their construction - and the determination of conductor size from current rating, short circuit withstand, voltage drop and economic criteria. It covers the insulation level and insulation coordination, the metallic sheath and its bonding and earthing schemes including single-point, both-end and cross bonding, and the sheath voltage limiters. Cable accessories follow: terminations to gas insulated switchgear, to transformers and to outdoor structures, and joints with their positioning. The laying conditions receive detailed treatment - in shafts and inclined tunnels, in cable galleries, in trays and on supports - with the fixing, snaking and thermal expansion arrangements, fire protection and segregation, and the overvoltage protection of the system. Monitoring and testing requirements close the code.

1.0.1 This code is formulated with a view to standardizing the design of AC 110 kV to 500 kV power cable systems for hydropower stations, so as to achieve safety and reliability, advanced technology, economic rationality, and convenience of construction and maintenance.

1.0.2 This code is applicable to the design of power cable systems with a nominal AC voltage of 110 kV to 500 kV and a frequency of 50 Hz for new, reconstructed and extended hydropower stations.

1.0.3 In addition to this code, the design of AC 110 kV to 500 kV power cable systems for hydropower stations shall also comply with the current relevant standards of the nation.

2 Terms and Symbols

2.1.1 Shielding conductor: an insulated conductor or cable running parallel to a single-core cable line and earthed at both ends, which provides a return path to the source for single-phase short-circuit current, so as to reduce the induced overvoltage on the cable metal sheath and protect the insulation of the outer sheath.

2.1.2 Slip fixing: a fixing method that allows the cable, as it expands and contracts thermally, to change its axial angle or move slightly sideways at the fixing point.

2.1.3 Rigid fixing: a clamping fixing method that does not allow any displacement of the cable due to thermal expansion and contraction.

2.1.4 Snaking: a laying method in which the cable is laid in a wave shape according to quantitative parameters, in order to reduce the axial thermal stress of the cable or to retain a reserve length.

2.2 Symbol U0: rated power-frequency r.m.s. voltage between each conductor and the screen or metal sheath, for which the cable and accessories are designed.

2.2 Symbol U: rated power-frequency r.m.s. voltage between any two conductors, for which the cable and accessories are designed.

2.2 Symbol Um: highest power-frequency r.m.s. voltage between any two conductors, for which the cable and accessories are designed.

2.2 Symbol Up1: peak value of the lightning impulse withstand voltage between each conductor and the screen or metal sheath of the cable or accessory adopted in the design.

2.2 Symbol Up2: peak value of the switching impulse withstand voltage between each conductor and the screen or metal sheath of the cable or accessory adopted in the design.

3 Service Conditions

3.1.1 The nominal system voltage and the highest system voltage shall adopt the values specified in Table 3.1.1 (kV).

Table 3.1.1: nominal system voltage 110 kV, highest system voltage 126 kV.

Table 3.1.1: nominal system voltage 220 kV, highest system voltage 252 kV.

Table 3.1.1: nominal system voltage 330 kV, highest system voltage 363 kV.

Table 3.1.1: nominal system voltage 500 kV, highest system voltage 550 kV.

3.1.2 The lightning impulse voltage and the switching impulse voltage shall be determined by the basic insulation level of the system and the requirements of insulation coordination.

3.1.3 The frequency of the system shall be 50 Hz.

3.1.4 The neutral point earthing mode of systems of 110 kV and above shall in all cases be effective earthing.

3.1.5 Cables and accessories shall meet the requirements of the altitude and seismic intensity of the place of use, and the influence of environmental conditions such as temperature, soil thermal resistance, wind speed, icing and solar radiation shall be taken into account.

3.1.6 The maximum working current of the cable shall be determined as the maximum current value under the conditions of continuous operation, emergency operation in case of fault and overload operation.

3.1.7 The symmetrical and asymmetrical short-circuit currents flowing through the cable during phase-to-phase and phase-to-earth short circuits at the head end of the cable shall be determined according to the long-term development plan of the power system in which the project is located.

3.1.8 The duration of the rated short-time withstand current may be taken as 3 s for 110 kV power cables and 2 s for power cables of 220 kV and above. The duration of the single-phase earth short-circuit current shall be determined as not less than the operating time of the first-stage backup protection of the relay protection; for cables provided with duplicated main protection, it may be determined by the operating time of the duplicated main protection.

3.2.1 The following laying conditions shall be considered in cable design: 1) route, terrain, difference in elevation and length of the cable line; 2) number of cable circuits, and the arrangement mode and spacing of cables arranged horizontally, vertically, in trefoil or otherwise; 3) earthing method of the metal sheath; 4) arrangement of cable fire prevention facilities; 5) special laying methods and other special requirements.

3.2.2 The ambient temperature for cable laying shall comply with Table 3.2.2, which gives the principle for selecting the ambient temperature for each laying method.

Table 3.2.2: underground, directly buried: local average ground temperature of the hottest month at the burial depth.

Table 3.2.2: underground, in protective pipe: local average ground temperature of the hottest month at the burial depth.

Table 3.2.2: in air, ventilated tunnel: ventilation design temperature.

Table 3.2.2: in air, non-ventilated tunnel or cable trench: average daily maximum air temperature of the hottest month plus 5 degrees C.

Table 3.2.2: in air, overhead exposed to sunshine: average daily maximum air temperature of the hottest month.

3.2.3 In addition to complying with 3.2.1 and 3.2.2 of this code, the following factors shall be considered for cables and accessories laid underground: 1) the structure of the metal sheath and the type of armour, determined by the installation conditions of direct burial or ducts; 2) the outer sheath, determined by installation conditions such as corrosion, moisture and small animals; 3) burial depth and thickness of the frozen soil layer; 4) types of soil along the cable route, such as sand, clay and artificial soil, and their thermal resistivity, stating whether these data are measured or assumed values; 5) maximum, minimum and average soil temperature at the burial depth; 6) detailed data on nearby heat sources or cables already in operation; 7) length of the cable trench or duct bank, including the distance between manholes, if any; 8) number, inner diameter and material of the ducts; 9) distance between ducts.

4 Selection of Main Technical Parameters

4.1 The voltage of the cable shall comply with Table 4.1.1 (cable voltage, kV).

Table 4.1.1: U = 110 kV, Um = 126 kV, U0 = 64 kV.

Table 4.1.1: U = 220 kV, Um = 252 kV, U0 = 127 kV.

Table 4.1.1: U = 330 kV, Um = 363 kV, U0 = 190 kV.

Table 4.1.1: U = 500 kV, Um = 550 kV, U0 = 290 kV.

4.2.1 Under service conditions, the permissible current-carrying capacity of the conductor cross-section shall not be less than the maximum continuous working current. The permissible current-carrying capacity of the conductor may be calculated by Formula 4.2.1.

Formula 4.2.1: I = { [(theta c - theta 0) - Wd[0.5T1 + (T2 + T3 + T4)]] / [RT1 + R(1 + lambda1)T2 + R(1 + lambda1 + lambda2)(T3 + T4)] } to the power 1/2.

Where: I is the permissible current-carrying capacity of the conductor (A); theta c is the maximum permissible temperature of the cable in continuous operation (degrees C), the maximum permissible conductor temperature of cross-linked polyethylene (XLPE) cables in normal operation being 90 degrees C; theta 0 is the ambient temperature of the cable in continuous operation (degrees C).

Wd is the dielectric loss of the insulation per metre (W/m); T1 is the thermal resistance of the insulation per metre of cable (K m/W); T2 is the thermal resistance per metre of the bedding between the metal sheath and the armour (K m/W); T3 is the thermal resistance of the outer sheath per metre of cable (K m/W); T4 is the external thermal resistance per metre of cable (K m/W).

R is the AC resistance per metre of cable at the maximum permissible temperature in continuous operation (ohm/m); lambda1 is the loss factor of the cable metal sheath; lambda2 is the loss factor of the cable armour.

4.2.2 The conductor cross-section shall meet the requirements of the rated short-time withstand current.

4.2.3 The cross-section for short-circuit thermal stability of the cable may be calculated by the following formulas. Formula 4.2.3-1: S not less than (square root of Qd / C) x 100.

Formula 4.2.3-2: C = square root of { [JQ / (k rho20 alpha)] ln [(1 + alpha(td - 20)) / (1 + alpha(tg - 20))] }.

Where: S is the calculated cross-section for short-circuit thermal stability (square millimetres); Qd is the thermal effect of the short-circuit current (A squared s); C is the thermal stability coefficient; J is the mechanical equivalent of heat (J/cal), taken as 4.2 J/cal.

Q is the heat capacity per unit volume of the cable conductor (cal per cubic centimetre per degree C), taken as 0.59 for aluminium conductors and 0.81 for copper conductors; k is the ratio of the AC resistance to the DC resistance of the conductor at 20 degrees C.

rho20 is the resistivity of the cable conductor at 20 degrees C (ohm square centimetre per centimetre), taken as 0.02826 x 10 to the power -4 for aluminium and 0.01724 x 10 to the power -4 for copper.

alpha is the temperature coefficient of resistance of the cable conductor at 20 degrees C (1/degrees C), taken as 0.00403 for aluminium and 0.00393 for copper; td is the maximum permissible temperature of the conductor during short circuit (degrees C), 250 degrees C for XLPE cables; tg is the maximum permissible temperature at rated load (degrees C).

4.2.4 The conductor cross-section shall be selected from the standard cross-section series; the cross-section should not be less than 240 square millimetres for 110 kV cables, 400 square millimetres for 220 kV cables, 630 square millimetres for 330 kV cables and 800 square millimetres for 500 kV cables.

4.3.1 The lightning impulse withstand voltage of cables and their terminations and joints shall be selected in accordance with Table 4.3.1; overvoltage calculation and checking of insulation coordination shall be carried out for cables of 330 kV and above.

Table 4.3.1: cable voltage U0/U (Um) 64/110 (126) kV, lightning impulse withstand voltage Up1 550 kV.

Table 4.3.1: cable voltage U0/U (Um) 127/220 (252) kV, lightning impulse withstand voltage Up1 1050 kV.

Table 4.3.1: cable voltage U0/U (Um) 190/330 (363) kV, lightning impulse withstand voltage Up1 1175 kV or 1300 kV.

Table 4.3.1: cable voltage U0/U (Um) 290/500 (550) kV, lightning impulse withstand voltage Up1 1550 kV or 1675 kV.

4.3.2 The switching impulse withstand voltage of cables of 330 kV and above and their terminations and joints shall be selected in accordance with Table 4.3.2.

Table 4.3.2: cable voltage U0/U (Um) 190/330 (363) kV, switching impulse withstand voltage Up2 950 kV.

Table 4.3.2: cable voltage U0/U (Um) 290/500 (550) kV, switching impulse withstand voltage Up2 1175 kV.

4.3.3 The power-frequency and lightning impulse withstand voltages of the cable outer sheath insulation shall be selected in accordance with Table 4.3.3; the DC withstand voltage of the outer sheath insulation should be 30 kV, applied for not less than 1 min.

Table 4.3.3: cable voltage 64/110 (126) kV, rated short-time power-frequency withstand voltage (r.m.s.) 25 kV, lightning impulse withstand voltage (peak) 37.5 kV.

Table 4.3.3: cable voltage 127/220 (252) kV, rated short-time power-frequency withstand voltage (r.m.s.) 25 kV, lightning impulse withstand voltage (peak) 47.5 kV.

Table 4.3.3: cable voltage 190/330 (363) kV, rated short-time power-frequency withstand voltage (r.m.s.) 25 kV, lightning impulse withstand voltage (peak) 62.5 kV.

Table 4.3.3: cable voltage 290/500 (550) kV, rated short-time power-frequency withstand voltage (r.m.s.) 25 kV, lightning impulse withstand voltage (peak) 72.5 kV.

5 Type and Structure

5.1.1 XLPE cables should be adopted for 110 kV to 500 kV cables.

5.1.2 The structure of an XLPE cable shall include at least the conductor, conductor screen, insulation, insulation screen, buffer layer, metal sheath and outer sheath.

5.2.1 Copper should be selected as the conductor material.

5.2.2 Cables with a conductor cross-section of 630 square millimetres and below should adopt a compacted stranded circular conductor structure; cables with a conductor cross-section of 1000 square millimetres and above should adopt a segmental conductor structure; cables with a conductor cross-section of 800 square millimetres may adopt either a compacted stranded circular conductor or a segmental conductor structure.

5.3.1 XLPE cable insulation shall be produced by a dry cross-linking process, and the conductor screen, insulation and insulation screen of the cable shall be formed by triple co-extrusion in a single operation.

5.3.2 The nominal thickness of the XLPE cable insulation shall be determined according to the power-frequency withstand and lightning impulse withstand levels, and should not be less than the values specified in Table 5.3.2.

Table 5.3.2: U0/U (Um) 64/110 (126) kV, nominal insulation thickness tn 16 mm.

Table 5.3.2: U0/U (Um) 127/220 (252) kV, nominal insulation thickness tn 24 mm.

Table 5.3.2: U0/U (Um) 190/330 (363) kV, nominal insulation thickness tn 27 mm.

Table 5.3.2: U0/U (Um) 290/500 (550) kV, nominal insulation thickness tn 30 mm.

5.3.3 The requirements for the average insulation thickness, the minimum thickness at any point and the eccentricity shall comply with Table 5.3.3.

Table 5.3.3: average insulation thickness not less than tn for 110 kV and 220 kV cables, and not less than tn for 330 kV and 500 kV cables.

Table 5.3.3: minimum thickness at any point not less than 0.95tn for 110 kV and 220 kV cables, and not less than 0.95tn for 330 kV and 500 kV cables.

Table 5.3.3: eccentricity not more than 6% for 110 kV and 220 kV cables, and not more than 5% for 330 kV and 500 kV cables.

Note 1 to Table 5.3.3: tn is the nominal insulation thickness specified in Table 5.3.2.

Note 2 to Table 5.3.3: the eccentricity is the percentage ratio of the difference between the maximum and minimum thickness measured on the same cross-section to the maximum thickness.

5.3.4 The limits for micro-voids, contaminants and protrusions in the insulation and at the insulation interfaces shall be selected in accordance with Table 5.3.4.

Table 5.3.4, 110 kV, insulation: voids larger than 0.05 mm, limit 0.

Table 5.3.4, 110 kV, insulation: voids larger than 0.025 mm and not larger than 0.05 mm, limit not more than 18 per 10 cubic centimetres.

Table 5.3.4, 110 kV, insulation: opaque contaminants larger than 0.125 mm, limit 0.

Table 5.3.4, 110 kV, insulation: opaque contaminants larger than 0.05 mm and not larger than 0.125 mm, limit not more than 6 per 10 cubic centimetres.

Table 5.3.4, 110 kV, insulation: translucent dark brown contaminants larger than 0.25 mm, limit 0.

Table 5.3.4, 110 kV, interface between semi-conductive screen and insulation: voids larger than 0.05 mm, limit 0.

Table 5.3.4, 110 kV, interface between conductor semi-conductive screen and insulation: protrusions larger than 0.125 mm into the insulation and the semi-conductive screen, limit 0.

Table 5.3.4, 110 kV, interface between insulation semi-conductive screen and insulation: protrusions larger than 0.125 mm into the insulation and the semi-conductive screen, limit 0.

Table 5.3.4, 220 kV, insulation: voids larger than 0.05 mm, limit 0.

Table 5.3.4, 220 kV, insulation: voids larger than 0.025 mm and not larger than 0.05 mm, limit not more than 18 per 10 cubic centimetres.

Table 5.3.4, 220 kV, insulation: opaque contaminants larger than 0.125 mm, limit 0.

Table 5.3.4, 220 kV, insulation: opaque contaminants larger than 0.05 mm and not larger than 0.125 mm, limit not more than 6 per 10 cubic centimetres.

Table 5.3.4, 220 kV, insulation: translucent dark brown contaminants larger than 0.16 mm, limit 0.

Table 5.3.4, 220 kV, interface between semi-conductive screen and insulation: voids larger than 0.05 mm, limit 0.

Table 5.3.4, 220 kV, interface between conductor semi-conductive screen and insulation: protrusions larger than 0.08 mm into the insulation and the semi-conductive screen, limit 0.

Table 5.3.4, 220 kV, interface between insulation semi-conductive screen and insulation: protrusions larger than 0.08 mm into the insulation and the semi-conductive screen, limit 0.

Table 5.3.4, 330 kV and 500 kV, insulation: voids larger than 0.02 mm, limit 0.

Table 5.3.4, 330 kV and 500 kV, insulation: opaque contaminants larger than 0.075 mm, limit 0.

Table 5.3.4, 330 kV and 500 kV, interface between semi-conductive screen and insulation: voids larger than 0.02 mm, limit 0.

Table 5.3.4, 330 kV and 500 kV, interface between conductor semi-conductive screen and insulation: protrusions larger than 0.05 mm into the insulation and the semi-conductive screen, limit 0.

Table 5.3.4, 330 kV and 500 kV, interface between insulation semi-conductive screen and insulation: protrusions larger than 0.05 mm into the insulation and the semi-conductive screen, limit 0.

5.4.1 For 110 kV XLPE cables with a cross-section below 500 square millimetres, the conductor screen shall be one extruded semi-conductive layer; for other XLPE cables, the conductor screen shall consist of semi-conductive tape and an extruded semi-conductive layer.

5.4.2 The insulation screen shall in all cases be one extruded semi-conductive layer.

5.5.1 A buffer layer shall be provided between the insulation screen and the metal sheath.

5.5.2 The buffer layer shall keep the insulation screen in good electrical contact with the metal sheath.

5.5.3 The thickness of the buffer layer shall be able to meet the requirement of compensating for the thermal expansion of the cable in operation.

5.6.1 The metal sheath of the cable shall be selected according to the magnitude of the asymmetrical short-circuit current passing through it, radial water-tightness and the requirements for withstanding mechanical tension and pressure.

5.6.2 The selection of the metal sheath should meet the following requirements: 1) in non-corrosive locations, a corrugated aluminium sheath or a smooth aluminium sheath may be selected for the cable; 2) underwater or in corrosive locations, a lead sheath should be selected for the cable.

5.6.3 The cross-section of the cable metal sheath shall meet the short-circuit capacity requirements for a single-phase earth fault or for earth faults occurring simultaneously on two phases at different locations, and the manufacturer shall be required to provide the corresponding calculation sheet.

5.6.4 The minimum thickness at any point of the cable lead sheath shall satisfy Formula 5.6.4: tmin not less than tn - (0.1 + 0.05tn), where tmin is the minimum thickness (mm) and tn is the nominal thickness (mm).

5.6.5 The minimum thickness at any point of the corrugated aluminium sheath shall satisfy Formula 5.6.5: tmin not less than tn - (0.1 + 0.15tn), where tmin is the minimum thickness (mm) and tn is the nominal thickness (mm).

5.7.1 The cable outer sheath shall be made of insulating polyvinyl chloride or polyethylene with good heat resistance, and its insulation level shall comply with Table 4.3.3 of this code; polyvinyl chloride may be selected for outer sheaths under general laying conditions, while polyethylene should be used for outer sheaths of cables directly buried, laid in pipes, laid where the groundwater level is high or laid at low temperature.

5.7.2 The flame retardancy of the cable outer sheath shall not be lower than flame-retardant class C. The finished cable shall be able to pass the non-propagation test specified in the current national standard GB/T 12666.2 (single wire and cable burning test methods, Part 2: horizontal burning test).

5.7.3 In addition to complying with the relevant provisions of the current national standards GB/T 2952.1 (outer covering of cables, Part 1: general), GB/T 2952.2 (Part 2: outer covering of metallic sheathed cables) and GB/T 2952.3 (Part 3: general outer covering of non-metallic sheathed cables), the surface of the cable outer sheath shall also have a uniform and firmly adhering conductive layer.

5.7.4 The minimum thickness of the cable outer sheath shall satisfy Formula 5.7.4: tmin not less than tn - (0.1 + 0.15tn), where tmin is the minimum thickness (mm) and tn is the nominal thickness (mm).

5.7.5 Where required by the laying conditions, the outer sheath shall be capable of preventing damage by termites, rodents and fungi, and the preventive additives of the outer sheath shall not be materials prohibited for environmental protection.

5.7.6 Where required by the laying conditions, the outer covering may also consist of an armour layer and an extruded polyvinyl chloride or polyethylene layer; the armour layer shall consist of non-magnetic steel tape or steel wire and shall comply with the relevant provisions of the current national standards GB/T 2952.1, GB/T 2952.2 and GB/T 2952.3.

5.7.7 The following markings shall be printed or embossed along the entire length of the cable outer sheath: 1) name of the manufacturer; 2) voltage; 3) conductor cross-section and material; 4) insulation material; 5) year of manufacture.

6 Terminations and Joints

6.1.1 The voltage of cable terminations and joints may be expressed as U0/U (Um), and shall not be lower than the voltage of the cable.

6.1.2 The insulation characteristics of cable terminations and joints shall comply with the following provisions. Item 1: the components, parts and materials of cable terminations and joints shall be convenient for site installation while meeting the design and process requirements; the insulation structure of the cable accessory shall be combined with the cable body into an inseparable whole.

6.1.2 Item 2: the unified specific creepage distance of the external insulation of cable terminations shall be selected according to the pollution class, and the highest phase voltage of the system shall be used when calculating the unified creepage distance; the unified specific creepage distance shall not be lower than the values specified in Table 6.1.2.

Table 6.1.2: pollution class a, unified specific creepage distance of external insulation 22 mm/kV.

Table 6.1.2: pollution class b, unified specific creepage distance of external insulation 27.8 mm/kV.

Table 6.1.2: pollution class c, unified specific creepage distance of external insulation 34.7 mm/kV.

Table 6.1.2: pollution class d, unified specific creepage distance of external insulation 43.3 mm/kV.

Table 6.1.2: pollution class e, unified specific creepage distance of external insulation 53.7 mm/kV.

6.1.2 Item 3: for installation sites at an altitude above 1000 m, the external insulation level of cable terminations shall be corrected for altitude in accordance with the relevant provisions of the current national standard GB/T 311.1 (Insulation co-ordination, Part 1: definitions, principles and rules).

6.1.2 Item 4: the joint casing shall be insulated from earth, and the insulation level of the casing insulation protection box shall be consistent with that of the cable outer sheath; the insulation level at the break of the metal screen of an insulating joint shall not be less than twice the insulation level of the outer sheath of the connected cable.

6.1.3 The mechanical strength of cable terminations shall comply with the following provisions: 1) the seismic design of cable terminations shall meet the relevant provisions of the current national standard GB 50260 (code for seismic design of electrical installations); 2) cable terminations directly connected to overhead lines shall be able to withstand a horizontal tension of 2 kN; 3) cable terminations arranged outdoors shall also meet the wind pressure requirements of the service environment.

6.1.4 Joints directly buried in soil shall be provided with a protective cover outside the casing insulation protection box; the protective cover shall be corrosion resistant, and measures shall be taken outside the protective cover to prevent the protection box from being subjected to force.

6.2.1 Where an insulating filler is present in a cable termination, the filler shall be compatible with the stress cone material and harmless to the cable insulation; where an insulating liquid or gas medium is present in a cable termination, a separate monitoring device should be provided for it.

6.2.2 Cable terminations connected to GIS and cable terminations connected to oil-immersed transformers should be dry-type cable terminations.

6.2.3 Cable terminations connected to GIS shall meet the following requirements. Item 1: the interface design and the division of the scope of supply between the cable termination and the GIS shall meet the technical requirements of the current national standard GB/T 22381 (connection between gas-insulated metal-enclosed switchgear for rated voltages of 72.5 kV and above and extruded insulation power cables, fluid-filled and dry-type cable terminations), and shall facilitate the construction, maintenance and testing of the cable termination.

6.2.3 Item 2: an insulating gasket shall be provided at the connection between the enclosure of the cable termination and the GIS enclosure, with protectors connected in parallel and evenly distributed around the circumference on both sides of the gasket; the insulation level of the gasket shall be consistent with that required for the cable outer sheath.

6.2.3 Item 3: cable terminations connected to GIS and arranged outdoors shall be provided with a sunshade protective cover.

6.2.4 Cable terminations connected to oil-immersed transformers shall meet the following requirements: 1) an insulating gasket shall be provided at the connection between the cable termination enclosure and the transformer enclosure, with protectors connected in parallel and evenly distributed around the circumference on both sides, and the insulation level of the gasket shall be consistent with that required for the cable outer sheath; 2) the cable termination shall be provided with measures to prevent contact between the insulating oil and the cable insulation.