金玉小区4台630KVA箱变供电：由6KV电缆线路利用原有电杆敷设1100m提供。原有线路还有2000米架空线路，架空线为钢芯铝绞线LGJφ185。630KVA变压器高压侧额定电流Ie1=60.6A，供电变电所运行参数如下：
箱变计算负荷如下：1#箱变功率Pjs=456KW，2#箱变功率Pjs=498KW，3#箱变功率Pjs=463KW，4#箱变功率Pjs=452KW，cosφ=0.85，单台变压器高压电缆引线为YJV-6-3×35，4台630KVA箱变总电流Ie1=240A。原架空线还带有少量负荷：400KVA、50KVA、20KVA各一台。请确定新增电缆的型号及截面。

        1 计算过程

        1.1 电力电缆选型，主要依据是GB50217－94《电力工程电缆设计规范》和《高压电缆选用导则》DL401—2002。应考虑最大载流量。 
        应计算三种情况：持续运行载流量、周期运行(应考虑负荷曲线)载流量、事故紧急运行或过负荷运行时的载流量。 
        1.1.1 预期的相间或相对地短路时流过的对称和不对称的短路电流。 
        1.1.2 短路电流最长持续时间。 
        1.1.3 电缆线路压降。 
        1.2 根据环境与敷设条件选型 电力电缆选型的考量，主要依据是GB50217－94《电力工程电缆设计规范》。其主要思想是对电缆型号的选择，应在满足电缆敷设场合技术要求的前提下，兼顾我国电缆工业发展的技术政策。即：线芯以铝代铜、绝缘层以橡塑代油浸纸、金属护套以铝代铅以及在外护层上发展橡塑护套或组合护套等。考虑到：最近几年，裸铜加权平均价上涨200％，而与此同时，铝材加权平均价仅上涨30％；铜材的导电率高，20℃时的电阻率为1.72×106 Ω/cm，铝线芯20℃时的电阻率2.82×106Ω/cm约为铜的1.68倍；载流量相同时，铝线芯截面约为铜的1.5倍。采用铜线芯损耗比较低，铜材的机械性能优于铝材，延展性好，便于加工和安装。抗疲劳强度约为铝材的1.7倍。但铝材比重小，在单位长度电阻值相同的情况下，铝导体截面应为铜导体的140％，铝线芯的质量仅为铜的42％，而采购价仅为铜芯缆的30％。
        铝芯电缆与铜芯电缆相比明显较轻，且较为经济。
        综合比较各种因素，一般考虑：①架空输电线路、②较大截面的中频线路，宜采用铝芯电缆。又因本线路已有三分之二段为185mm2钢芯铝绞线，且6～35KV交联聚乙烯绝缘聚氯乙烯护套电力电缆，介质损耗低，性能优良，结构简单，制造方便，外径小，质量轻，载流量大，敷设方便，不受高差限制，耐腐蚀，做终端和中间接头较简便。（最新数据：YJV-6-3×120电缆近似重量：5340Kg/Km，电缆出厂价格：364，000元/Km；YJLV-6-3×180电缆近似重量：3946Kg/Km，电缆出厂价格：120，000元/Km。）
        故6KV电压等级铝芯聚氯乙烯交联绝缘类电力电缆（YJLV）为首选。

      Jinyu District 4 sets 630KVA Power Box changed: by the use of the original pole 6KV laying of cable lines to provide 1100m. Original lines are 2000 meters overhead lines, overhead line ACSR LGJφ185. 630KVA transformer high side current rating Ie1 = 60.6A, power substation operating parameters are as follows:
Box Change load calculation is as follows: 1 # box variable power Pjs = 456KW, 2 # box changes the power Pjs = 498KW, 3 # box changes the power Pjs = 463KW, 4 # box changes the power Pjs = 452KW, cosφ = 0.85, a single high voltage transformer cable lead to YJV-6-3 × 35,4 sets 630KVA box changes the total current Ie1 = 240A. Also with a small amount of overhead lines of the original load: 400KVA, 50KVA, 20KVA each one. Make sure the new cable type and cross section.

        A calculation

        1.1 Selection of power cables, mainly based on GB50217-94 "Cable design electrical engineering" and "high voltage cable guide for the selection" DL401-2002. Consideration should be given the maximum carrying capacity.
        Should be calculated in three ways: continue to run carrying capacity, cycle to run (should consider load curve) contains traffic accident emergency operation or overload the carrying capacity of run-time.
        1.1.1 the expected white or relatively short circuit flow through the symmetrical and asymmetrical short circuit current.
        1.1.2 Short-circuit current longest duration.
        1.1.3 cable line voltage drop.
        1.2 According to the environmental conditions of selection and laying of power cable selection considerations, mainly based on the GB50217-94 "Cable design of electrical engineering." The main idea is the choice of cable types, cable laying should be occasion to meet the technical requirements of the premise, taking into account the development of China's cable industry's technology policy. Namely: copper wire core with aluminum on behalf of the insulation layer rubber on behalf of the oil impregnated paper, metal sheathed with aluminum on behalf of the lead and the outer sheath on the development of rubber jacket or a combination of jacket and so on. Taking into account: In recent years, a weighted average of bare copper rose 200%, while the weighted average price of aluminum rose only 30%; high conductivity of copper, 20 ℃ when the resistance was 1.72 × 106 Ω / cm, aluminum core 20 ℃, the resistivity of 2.82 × 106Ω/cm about 1.68 times that of copper; carrying capacity the same, the aluminum core cross-section of copper is about 1.5 times. Copper core with low loss, the mechanical properties of copper than aluminum, ductile, easy processing and installation. Fatigue strength of aluminum is about 1.7 times. However, a small proportion of aluminum, the same resistance per unit length in the case of aluminum conductor cross section should be 140% copper conductor, copper aluminum core quality is only 42% of the purchase price is only 30% of copper cable .
        Aluminum cable with significantly lighter than copper cables, and more economical.
        Comprehensive comparison of various factors, general considerations: ① overhead transmission lines, ② large section of the IF line, should use aluminum cable. Because of this line segment has two-thirds to 185mm2 ACSR, and 6 ~ 35KV XLPE insulated PVC sheathed power cable, low dielectric loss, good performance, simple structure, convenient manufacture, OD Small, light weight, large carrying capacity, easy installation without elevation limit, corrosion resistance, make the terminal more simple and middle joints. (Latest data: YJV-6-3 × 120 lines approximate weight: 5340Kg/Km, cable prices: 364,000 yuan / Km; YJLV-6-3 × 180 lines approximate weight: 3946Kg/Km, cable prices: 120 , 000 yuan / Km.)
        Therefore 6KV voltage conductor PVC insulated electric power cables linked (YJLV) first choice.