CN1635585A

CN1635585A - Suppression method of induced current in power cable sheath

Info

Publication number: CN1635585A
Application number: CNA2004100660493A
Authority: CN
Inventors: 王东海; 皇甫学政; 王春宁; 马宏忠
Original assignee: Hohai University HHU; Nanjing Power Supply Co of Jiangsu Electric Power Co
Current assignee: Hohai University HHU; Nanjing Power Supply Co of Jiangsu Electric Power Co
Priority date: 2004-12-15
Filing date: 2004-12-15
Publication date: 2005-07-06

Abstract

This invention relates to a method to use inductance to balance power cable protective layer inductance voltage and suppress the protective current, which belongs to electric engineer technique field and comprises the following steps: dividing three-phase cable into three sections with initial end three phases connected to the earth; crossing and exchanging the metal protective layer in middle part of the three-phase cable each section; computing inductance voltage on the metal layer according to the cable form and lay parameters and determining the compensation inductance to balance the voltage; connecting the compensation inductance to the third section of cable protective layer.

Description

Suppression method of induced current in power cable sheath

技术领域technical field

本发明涉及一种采用电感平衡因电缆不等长分段所引起的护层感应电压、从而抑制护层感应电流的方法，属于电气工程技术领域。The invention relates to a method for suppressing the induced current of the sheath by using inductance to balance the induced voltage of the sheath caused by sections of unequal lengths of cables, and belongs to the technical field of electrical engineering.

背景技术Background technique

众所周知，供电线路普遍采用电力电缆作为输电网络。由于输送容量的需求不同，电缆的设计也不尽相同。对于输送容量较低的线路，一般采用三芯电缆；而对于输送容量要求较大的线路，一般采用单芯电缆。As we all know, power supply lines generally use power cables as power transmission networks. Due to the different needs of the transmission capacity, the design of the cables is also different. For lines with low transmission capacity, three-core cables are generally used; for lines with large transmission capacity requirements, single-core cables are generally used.

三芯电缆的三根线芯公用同一个金属护层，当三相电流基本平衡时，三相合成电流接近于零，合成磁通也接近于零，因此，金属护层上不会有感应电压产生。只有在非对称短路时，破坏了三相电流的对称性，合成磁通不再等于零，金属护层上才有不平衡感应电压产生。The three cores of the three-core cable share the same metal sheath. When the three-phase current is basically balanced, the three-phase composite current is close to zero, and the composite magnetic flux is also close to zero. Therefore, there will be no induced voltage on the metal sheath. . Only when the asymmetrical short circuit breaks the symmetry of the three-phase current, the resultant magnetic flux is no longer equal to zero, and the unbalanced induced voltage is generated on the metal sheath.

对于单芯电缆而言，金属护层的感应电压与三芯电缆完全不同。单芯电缆的导线与金属护层的关系，可以看作一个变压器的初级绕组与次级绕组。当电缆导线通过电流时，在其周围产生磁通，磁通不仅与线芯回路相链，同时也与电缆的金属护层相链，因此在线芯和金属护层上产生感应电压。由于每根线芯专用一个金属护层，负载电流或短路电流所产生的磁通，与金属护层交链，因此金属护层上的感应电压始终存在。这种感应电压的值与线芯截面、电缆间距离、电流大小以及电缆长度有关。尤其当电缆很长时，护层上的感应电压可达到较高的数值。For a single-core cable, the induced voltage of the metal sheath is completely different from that of a three-core cable. The relationship between the conductor and the metal sheath of a single-core cable can be regarded as the primary winding and secondary winding of a transformer. When the cable wire passes current, a magnetic flux is generated around it, and the magnetic flux is not only linked with the core circuit, but also linked with the metal sheath of the cable, so an induced voltage is generated on the wire core and the metal sheath. Since each wire core has a dedicated metal sheath, the magnetic flux generated by the load current or short-circuit current is interlinked with the metal sheath, so the induced voltage on the metal sheath always exists. The value of this induced voltage is related to the cross section of the wire core, the distance between the cables, the magnitude of the current and the length of the cable. Especially when the cable is very long, the induced voltage on the sheath can reach a high value.

目前，新的电缆线路通常采用金属护层换位等措施解决金属护层的感应电压问题(参见图1)，采用换位连接法设计中，通过计算绘制出的沿电线长度对地电压分布图如图2所示。(如当电缆为等边三角形敷设时，对地电压分布即为图2所示。)At present, the new cable line usually adopts measures such as metal sheath transposition to solve the induced voltage problem of the metal sheath (see Figure 1). In the design of the transposition connection method, the voltage distribution diagram along the length of the wire to the ground is calculated and drawn as shown in picture 2. (For example, when the cable is laid in an equilateral triangle, the voltage distribution to ground is shown in Figure 2.)

但是，随着电力建设的发展，电缆线路的改造日益增多，改造后的电缆线路，其长度往往不能满足原来的三段换位要求，从而引起金属护层电压不平衡。金属护层电压不平衡不但会引起电缆线路护层环流，有些情况下护层环流的数值可达到与线芯电流相同的数量级^【1】，导致大量的能量损耗，而且有可能引起电缆发热，影响电缆寿命。因此，必须采取措施减小或消除这种护层电压的影响。此外，理论分析表明，电缆金属护层中的感应电压与电缆长度和线芯电流的乘积成正比。在电缆长度和线芯截面较大的情况下，尤其是当系统发生短路事故时，感应电压将达到危害人身安全的数值，甚至造成电缆的运行事故。为了确保值班及检修人员接触时的安全，《GB50217-94》中规定，金属护层上感应电压不得超过50V。However, with the development of electric power construction, the transformation of cable lines is increasing, and the length of the transformed cable lines often cannot meet the original three-section transposition requirements, resulting in unbalanced metal sheath voltage. The unbalanced voltage of the metal sheath will not only cause the sheath circulation of the cable line, but in some cases the value of the sheath circulation can reach the same order of magnitude as the wire core current ^[1] , resulting in a large amount of energy loss, and may cause the cable to heat up and affect Cable life. Therefore, measures must be taken to reduce or eliminate the influence of this sheath voltage. In addition, theoretical analysis shows that the induced voltage in the cable metal sheath is proportional to the product of the cable length and the core current. In the case of large cable length and core cross-section, especially when a short-circuit accident occurs in the system, the induced voltage will reach a value that endangers personal safety, and may even cause a cable operation accident. In order to ensure the safety of the on-duty and maintenance personnel when they touch it, "GB50217-94" stipulates that the induced voltage on the metal sheath must not exceed 50V.

目前，国内外普遍采用的方法是补偿电缆，即增加电缆的长度，使其长度与原三段换位法的电缆等长，并将多放的电缆圈放在电缆沟中。这种方法显然有其不足之处，不仅因增加电缆长度而增加了成本，而且多放电缆往往受到空间和敷设条件的限制。At present, the method commonly used at home and abroad is to compensate the cable, that is, to increase the length of the cable, so that its length is equal to that of the original three-section transposition method, and place the extra cable ring in the cable trench. This method obviously has its disadvantages. Not only does it increase the cost due to increased cable length, but also multi-layout cables are often limited by space and laying conditions.

检索发现，申请日为1997.12.19、申请号为97251313.2的中国专利公开了一种电缆护层保护器，它由电机、接触片、氧化锌阀片、粘结胶、硅橡胶套组成，所述氧化锌阀片两侧面通过接触片与两电机的端部接触，且通过粘结胶将其连为一整体，在粘结胶外套设有一硅橡胶套。据介绍，该保护器结构简单，加工制造容易，体积小，成型工艺良好，密封、绝缘性能优良，对电缆外层感应电压的保护性能可靠，适于使用在电力电缆中电缆护层保护器。但是，从原理看，该项保护器主要是通过释放电流进行过电压保护，而不是减小护层电流，并且在目前的高压电缆运行中，该保护器在220KV电缆投入运行时，并不能很好的起到护层保护作用，仍然导致金属护层对地放电(电压甚至高达数万伏)。The search found that a Chinese patent with an application date of 1997.12.19 and an application number of 97251313.2 discloses a cable sheath protector, which is composed of a motor, a contact piece, a zinc oxide valve piece, an adhesive glue, and a silicone rubber sleeve. The two sides of the zinc oxide valve plate are in contact with the ends of the two motors through the contact plate, and are connected as a whole through the adhesive glue, and a silicon rubber sleeve is arranged on the outer surface of the adhesive glue. According to reports, the protector has a simple structure, easy processing and manufacturing, small size, good molding process, excellent sealing and insulation performance, and reliable protection performance against induced voltage on the outer layer of the cable. It is suitable for use as a cable sheath protector in power cables. However, from the principle point of view, this protector mainly performs over-voltage protection by releasing current, rather than reducing the sheath current, and in the current high-voltage cable operation, this protector is not very effective when the 220KV cable is put into operation. Even if it plays the protective role of the sheath, it still causes the metal sheath to discharge to the ground (the voltage is even as high as tens of thousands of volts).

发明内容Contents of the invention

本发明的目的在于：针对以上现有技术存在的缺点，提出一种电缆护层感应电流的抑制方法，该方法通过采取简单易行、并且成本经济的措施，来平衡由于三段电缆不等长而产生的电缆护层过电压，从而大大减小护层环流，降低损耗，提高电缆的载流量，延长电缆的使用寿命。The object of the present invention is: aiming at the shortcomings of the above prior art, a method for suppressing the induced current of the cable sheath is proposed. The resulting overvoltage of the cable sheath greatly reduces the circulation of the sheath, reduces loss, increases the carrying capacity of the cable, and prolongs the service life of the cable.

为了达到以上目的，申请人通过模拟设计，建立了电缆的数学模型，根据电缆的电气参数，可以建立电缆的等值电路如图3所示。其中R为电缆的线芯电阻，L_i为电缆的内感，L_e为电缆的外感，R_j为电缆金属护层的电阻，C为电缆的电容，绝缘电阻Ri在此忽略不计。单相电缆的工作电感L＝L_i+L_e。对于三相电路，各电缆的工作电感也与敷设形式有关，视具体情况而定。In order to achieve the above purpose, the applicant established a mathematical model of the cable through simulation design, and according to the electrical parameters of the cable, an equivalent circuit of the cable can be established as shown in Figure 3. Where R is the core resistance of the cable, L _i is the internal inductance of the cable, _Le is the external inductance of the cable, R _j is the resistance of the cable metal sheath, C is the capacitance of the cable, and the insulation resistance Ri is ignored here. The working inductance of the single-phase cable L=L _i +L _e . For three-phase circuits, the working inductance of each cable is also related to the laying form, depending on the specific situation.

通过以上参数及模型，可从理论上计算出电缆金属护层中的感应电势。由此得出本发明电力电缆护层感应电流的抑制方法，其步骤为：Through the above parameters and models, the induced potential in the cable metal sheath can be calculated theoretically. Thus draw the suppressing method of power cable sheath induced current of the present invention, its step is:

1)将三段电缆起始端的护层三相互联接地；1) Connect the sheaths at the starting ends of the three sections of cables to each other;

2)将三相电缆各段的金属护层在中部的两换位点处交叉换位连接；2) Cross-transpose the metal sheaths of each section of the three-phase cable at the two transposition points in the middle;

3)根据电缆敷设形式和敷设参数变化，计算金属护层上的感应电压，并确定使三相感应电压基本平衡的外接补偿电感值；3) According to the cable laying form and laying parameter changes, calculate the induced voltage on the metal sheath, and determine the external compensation inductance value that makes the three-phase induced voltage basically balanced;

4)将补偿电感外接在电缆护层上，以平衡护层感应电压，抑制护层电流。4) Connect the compensation inductance externally to the cable sheath to balance the induced voltage of the sheath and suppress the sheath current.

以上计算用于抑制护层电流的外接电感值的过程可以归纳为：The above process of calculating the external inductance value used to suppress the sheath current can be summarized as:

A、计算护层感应电压A. Calculate the induced voltage of the sheath

以电缆三角形敷设为例，三相护层单位长度的感应电压Usa、Usb、Usc分别按下式计算：Taking the triangle laying of cables as an example, the induced voltages Usa, Usb, and Usc of the unit length of the three-phase sheath are calculated according to the following formula:

${\overset{\cdot &Center Dot;}{U u}}_{Sa Sa} = = - - j j {\overset{\cdot \cdot}{I I}}_{a a} {X x}_{S S},, V V / / m m$

${\overset{\cdot \cdot}{U u}}_{Sb Sb} = = - - j j {\overset{\cdot &Center Dot;}{I I}}_{b b} {X x}_{S S},, V V / / m m$

${\overset{\cdot \cdot}{U u}}_{Sc sc} = = - - j j {\overset{\cdot \cdot}{I I}}_{c c} {X x}_{S S},, V V / / m m$

其中 $X_{S} = 2 ω (\ln \frac{2 S}{D_{S}}) \times 10^{- 7} Ω / m$ in $x_{S} = 2 ω (\ln \frac{2 S}{{D.}_{S}}) \times 10^{- 7} Ω / m$

以上式中j——为复数运算因子In the above formula, j——is the complex operation factor

ω——为角频率，ω＝2πfω——is the angular frequency, ω=2πf

Ia、Ib、Ic——分别为三相电缆各相负载电流Ia, Ib, and Ic——respectively, the load current of each phase of the three-phase cable

S——电缆中心轴间距离S——distance between cable central axes

Ds——电缆护层平均直径Ds——average diameter of cable sheath

B、计算外接补偿电感值；B. Calculate the external compensation inductance value;

${X x}_{a a} = = imag imag ((\frac{(({L L}_{11} * * {\overset{\cdot \cdot}{U u}}_{Sa Sa} + + {L L}_{22} * * {\overset{\cdot &Center Dot;}{U u}}_{Sc sc} + + {L L}_{33} * * {\overset{\cdot &Center Dot;}{U u}}_{Sb Sb}))}{{\overset{\cdot &Center Dot;}{I I}}_{a a}}))$

${X x}_{b b} = = imag imag ((\frac{(({L L}_{11} * * {\overset{\cdot &Center Dot;}{U u}}_{Sb Sb} + + {L L}_{22} * * {\overset{\cdot &Center Dot;}{U u}}_{Sa Sa} + + {L L}_{33} * * {\overset{\cdot &Center Dot;}{U u}}_{Sc sc}))}{{\overset{\cdot &Center Dot;}{I I}}_{b b}}))$

${X x}_{c c} = = imag imag ((\frac{(({L L}_{11} * * {\overset{\cdot &Center Dot;}{U u}}_{Sc sc} + + {L L}_{22} * * {\overset{\cdot &Center Dot;}{U u}}_{Sb Sb} + + {L L}_{33} * * {\overset{\cdot &Center Dot;}{U u}}_{Sa Sa}))}{{\overset{\cdot &Center Dot;}{I I}}_{c c}}))$

将X_a、X_b、X_c分别除以2πf即可得到各相所补偿的电感值。Divide X _a , X _b , and X _c by 2πf to get the compensated inductance value of each phase.

以上式中In the above formula

X_a、X_b、X_c——分别为三相应补偿的电抗值X _a , X _b , X _c —respectively, the reactance values of the three-phase compensation

L₁、L₂、L₃——分别为第1、2、3段电缆的长度L ₁ , L ₂ , L ₃ ——respectively the lengths of the 1st, 2nd, and 3rd section cables

L_a、L_b、L_c——分别为三相应补偿的电感值，计算公式分别为： $L_{a} = \frac{X_{a}}{2 πf},$ $L_{b} = \frac{X_{b}}{2 πf},$ $L_{c} = \frac{X_{c}}{2 πf}$ L _a , L _b , L _c —— are the inductance values of the three corresponding compensations respectively, and the calculation formulas are: $L_{a} = \frac{x_{a}}{2 πf},$ $L_{b} = \frac{x_{b}}{2 πf},$ $L_{c} = \frac{x_{c}}{2 πf}$

f——供电频率f - power supply frequency

这样，当由于电缆线路改造开发等原因，第三段长度小于原长度时，通过计算确定所需补偿的电感参数，在电缆末端护层上联接适当大小的电感进行补偿，即可使电缆护层上的三相感应电压也尽可能平衡。结果，电缆护层中的环流将会大大减小。In this way, when the length of the third section is less than the original length due to the transformation and development of the cable line, etc., the inductance parameter to be compensated is determined by calculation, and an inductance of an appropriate size is connected to the cable end sheath for compensation, so that the cable sheath The three-phase induced voltage on the circuit is also balanced as much as possible. As a result, circulating currents in the cable sheath will be greatly reduced.

本发明打破传统思路，通过科学的理论分析，巧妙建立了数学模型，以简单实用的方法实现了电缆护层感应电流的抑制，由于电感的造价相对电缆来说便宜得多，而且也不受敷设条件的限制，因此采用本发明成本经济，切实可行。显然，与现有技术相比，本发明具有突出的进步。The invention breaks the traditional way of thinking, through scientific theoretical analysis, cleverly establishes a mathematical model, realizes the suppression of the induced current of the cable sheath in a simple and practical way, because the cost of the inductance is much cheaper than that of the cable, and it is not affected by the laying Due to the limitation of conditions, the cost of the present invention is economical and feasible. Obviously, compared with the prior art, the present invention has outstanding progress.

附图说明Description of drawings

下面结合附图对本发明作进一步的说明。The present invention will be further described below in conjunction with the accompanying drawings.

图1为现有电缆护层换位连接结构示意图。Fig. 1 is a schematic diagram of an existing cable sheath transposition connection structure.

图2为电缆三段等长时交叉换位感应电压分布图。Fig. 2 is the distribution diagram of induced voltage for cross-transposition of three sections of cable with equal length.

图3为建立电缆数学模型的等值电路示意图。Fig. 3 is a schematic diagram of an equivalent circuit for establishing a mathematical model of a cable.

图4为本发明实施例的结构示意图。Fig. 4 is a schematic structural diagram of an embodiment of the present invention.

图5为图4实施例的感应电压分布图。FIG. 5 is a distribution diagram of induced voltage in the embodiment of FIG. 4 .

图6为图4实施例的定量感应电压分布图Fig. 6 is the quantitative induced voltage distribution diagram of Fig. 4 embodiment

具体实施方式Detailed ways

实施例Example

本实施例中变电站采用单台主变容量为5万千伏安的110kV变压器，电缆每相负载电流约I＝500A，电缆中心轴间的距离S＝250mm，电缆护层平均直径D_S＝69.3mm，由三根单芯电缆组成三相回路，线路采用了等边三角形敷设。在平衡负载条件下，U_Sa、U_Sb、U_Sc三相感应电压为：In this embodiment, the substation adopts a 110kV transformer with a single main transformer capacity of 50,000 kVA, the load current of each phase of the cable is about I=500A, the distance S=250mm between the cable central axes, and the average diameter of the cable sheath D _S =69.3 mm, a three-phase loop is composed of three single-core cables, and the lines are laid in an equilateral triangle. Under the condition of balanced load, the three-phase induced voltage of U _Sa , U _Sb , U _Sc is:

U_Sa＝U_Sb＝U_Sc＝U_S＝IX_S＝500×1.1247×10^-4＝0.0621(V/m)U _Sa ＝ U _Sb ＝ U _Sc ＝ U _S ＝IX _S ＝500×1.1247×10 ^-4 ＝0.0621(V/m)

其中Xs按照公式 $X_{S} = 2 ω (\ln \frac{2 S}{D_{S}}) \times 10^{- 7} Ω / m$ 算得。由此可见看出，where Xs according to the formula $x_{S} = 2 ω (\ln \frac{2 S}{{D.}_{S}}) \times 10^{- 7} Ω / m$ count. From this it can be seen that,

当电缆很长时(实际运行中，110kV电缆线路的总长度一般可达到5～6公里)，护层感应电压将很高。而当系统发生短路事故时，感应电压的数值将会达到10³V/m数量级【2】。When the cable is very long (in actual operation, the total length of the 110kV cable line can generally reach 5-6 kilometers), the induced voltage of the sheath will be very high. When a short-circuit accident occurs in the system, the value of the induced voltage will reach the order of 10 ³ V/m [2].

敷设时，将三相电缆A、B、C分为I、II、III三段，三相电缆起始端的护层在普通型连接接头盒1处三相互联接地，之后各段电缆的金属护层在两换位点处通过绝缘接头盒2、3分别交叉换位连接。When laying, the three-phase cables A, B, and C are divided into three sections I, II, and III. The sheath at the beginning of the three-phase cable is connected to the ground at 1 of the common connection joint box, and the metal sheath of each section of the cable is The layers are respectively cross-transposed and connected through insulating joint boxes 2 and 3 at the two transposition points.

由于线路改造，三相电缆三段电缆的长度分别为500m、500m、300m。电缆为等边三角形敷设，在额定状态下运行，换位后三相感应电压分别按照以下公式计算：Due to line transformation, the lengths of the three sections of the three-phase cable are 500m, 500m, and 300m respectively. The cables are laid in an equilateral triangle and run under rated conditions. After transposition, the three-phase induced voltages are calculated according to the following formulas:

${\overset{\cdot &Center Dot;}{U u}}_{Sa Sa} = = - - j j {\overset{\cdot &Center Dot;}{I I}}_{a a} {X x}_{S S},, V V / / m m$

${\overset{\cdot &Center Dot;}{U u}}_{Sb Sb} = = - - j j {\overset{\cdot &Center Dot;}{I I}}_{b b} {X x}_{S S},, V V / / m m$

${\overset{\cdot &Center Dot;}{U u}}_{Sc sc} = = - - j j {\overset{\cdot \cdot}{I I}}_{c c} {X x}_{S S},, V V / / m m$

又根据U_a＝L₁*U_Sa+L₂*U_Sc+L₃*U_Sb And according to U _a ＝L ₁ *U _Sa +L ₂ *U _Sc +L ₃ *U _Sb

U_b＝L₁*U_Sb+L₂*U_Sa+L₃*U_Sc U _b ＝L ₁ *U _Sb +L ₂ *U _Sa +L ₃ *U _Sc

U_c＝L₁*U_Sc+L₂*U_Sb+L₃*U_Sa U _c ＝L ₁ *U _Sc +L ₂ *U _Sb +L ₃ *U _Sa

得各相护层感应电压Ua＝Ub＝Uc＝12.4166V，护层电流为76.9231A计算公式为： $I_{h} = | \frac{{\overset{\cdot}{U}}_{Sa}}{j X_{S} (L_{1} + L_{2} + l_{3})} |,$ 由于三相感应电压平衡，此时护层中无环流。若要使护层中无电流，则根据以下计算：The induced voltage of each phase sheath Ua=Ub=Uc=12.4166V, the sheath current is 76.9231A and the calculation formula is: $I_{h} = | \frac{{\overset{\cdot}{u}}_{Sa}}{j x_{S} (L_{1} + L_{2} + l_{3})} |,$ Due to the balance of the three-phase induced voltage, there is no circulating current in the sheath at this time. If there is no current in the sheath, it is calculated according to the following:

${X x}_{a a} = = imag imag ((\frac{(({L L}_{11} * * {\overset{\cdot \cdot}{U u}}_{Sa Sa} + + {L L}_{22} * * {\overset{\cdot \cdot}{U u}}_{Sc sc} + + {L L}_{33} * * {\overset{\cdot \cdot}{U u}}_{Sb Sb}))}{{\overset{\cdot \cdot}{I I}}_{a a}})) = = 0.0248 0.0248$

${X x}_{b b} = = imag imag ((\frac{(({L L}_{11} * * {\overset{\cdot \cdot}{U u}}_{Sb Sb} + + {L L}_{22} * * {\overset{\cdot \cdot}{U u}}_{Sa Sa} + + {L L}_{33} * * {\overset{\cdot \cdot}{U u}}_{Sc sc}))}{{\overset{\cdot &Center Dot;}{I I}}_{b b}})) = = 0.0248 0.0248$

${X x}_{c c} = = imag imag ((\frac{(({L L}_{11} * * {\overset{\cdot &Center Dot;}{U u}}_{Sc sc} + + {L L}_{22} * * {\overset{\cdot &Center Dot;}{U u}}_{Sb Sb} + + {L L}_{33} * * {\overset{\cdot &Center Dot;}{U u}}_{Sa Sa}))}{{\overset{\cdot &Center Dot;}{I I}}_{c c}})) = = 0.0248 0.0248$

得需补偿大小为0.0248Ω的电抗，分别除以2πf即得外接补偿电感为0.079047mh。The reactance that needs to be compensated is 0.0248Ω, and divided by 2πf respectively, the external compensation inductance is 0.079047mh.

以上计算结果证明，当电缆为对称敷设时，三相外接补偿电感值相等。之后，在第三段电缆护层终端的普通型连接接头盒4处分别通过各自的外接补偿电感接地(参见图4)。补偿后电缆护层的感应电压对地分布如图5所示。The above calculation results prove that when the cables are laid symmetrically, the three-phase external compensation inductance values are equal. Afterwards, the common connection joint box 4 at the cable sheath terminal of the third section is grounded through respective external compensation inductances (refer to FIG. 4 ). The distribution of the induced voltage of the cable sheath to the ground after compensation is shown in Figure 5.

实践证明，本实施例采用在护层末端联接电感，以补偿三段电缆不等长产生的感应电压带来的影响，从而大大减小了护层感应电压和环流，提高了电缆的载流量，延长了电缆的使用寿命，并且不受到空间和敷设条件的限制，既消除了护层感应电压的影响，又显著降低了电缆的改造成本。Practice has proved that in this embodiment, the inductance is connected at the end of the sheath to compensate the influence of the induced voltage caused by the unequal length of the three sections of cables, thereby greatly reducing the induced voltage and circulating current of the sheath, and improving the current carrying capacity of the cable. The service life of the cable is extended, and it is not limited by space and laying conditions, which not only eliminates the influence of the induced voltage of the sheath, but also significantly reduces the transformation cost of the cable.

实验证明，补偿后，本实施例基本消除了三段电缆不等长产生的影响，从而大大减小了护层感应电压和环流，提高了电缆的载流量，延长了电缆的使用寿命，并且不受到空间和敷设条件的限制，既消除了护层感应电压的影响，又显著降低了电缆的改造成本。Experiments have proved that after compensation, this embodiment basically eliminates the influence of the unequal lengths of the three sections of cables, thereby greatly reducing the induced voltage and circulating current of the sheath, increasing the ampacity of the cables, prolonging the service life of the cables, and not Limited by space and laying conditions, it not only eliminates the influence of induced voltage on the sheath, but also significantly reduces the cost of cable transformation.

附：说明书中的符号物理意义如下：Attachment: The physical meaning of the symbols in the manual is as follows:

C 电缆的电容C Capacitance of the cable

Ds 电缆护层平均直径Ds Average diameter of cable sheath

f 供电频率f power supply frequency

I 负载电流I Load Current

Ia、Ib、Ic 分别为三相电缆各相线芯电流Ia, Ib, and Ic are the core currents of each phase of the three-phase cable respectively

I_b 护层电流I _b sheath current

j 为复数运算因子j is the complex operation factor

L 单相电缆的工作电感，L＝L_i+L_e L Working inductance of single-phase cable, L=L _i +L _e

L₁、L₂、L₃ 分别为第1、2、3段电缆的长度L ₁ , L ₂ , L ₃ are the lengths of the 1st, 2nd and 3rd section cables respectively

L_a、L_b、L_c 分别为三相应补偿的电感值L _a , L _b , L _c are the inductance values of the three corresponding compensations

L_e 电缆的外感The external inductance of the L _e cable

L_i 电缆的内感Internal sense of the L _i cable

R 电缆的线芯电阻R Core resistance of the cable

Ri 绝缘电阻，在此忽略不计。Ri Insulation resistance, neglected here.

R_j 电缆金属护层的电阻R _j Resistance of cable metal sheath

S 三相电缆中心轴间距离S Distance between central axes of three-phase cables

Ua、Ub、Uc 分别为电缆三段换位后的护层感应电压Ua, Ub, and Uc are respectively the induced voltages of the sheath after the three sections of the cable are transposed

Usa、Usb、Usc 分别为三相护层单位长度的感应电压Usa, Usb, and Usc are the induced voltages per unit length of the three-phase sheath respectively

ω 为角频率，ω＝2πfω is the angular frequency, ω=2πf

X_a、X_b、X_c 分别为三相应补偿的电抗值X _a , X _b , X _c are the reactance values of the three-phase compensation

X_S 为单位长度金属护层的电抗X _S is the reactance of the metal sheath per unit length

[1]国家经济贸易委员会电力司主编，电力电缆，北京：中国电力出版社，2002年9月[1] Editor-in-chief of the Electric Power Department of the State Economic and Trade Commission, Power Cables, Beijing: China Electric Power Publishing House, September 2002

[2]刘子玉编，电气绝缘结构设计原理，上册，北京：机械工业出版设，1981年9月[2] Edited by Liu Ziyu, Design Principles of Electrical Insulation Structures, Volume 1, Beijing: Machinery Industry Press, September 1981

Claims

1. A method for suppressing the induced current of a power cable sheath, the steps of which are:

1) Connect the sheaths at the starting ends of the three sections of cables to each other;

2) Cross-transpose the metal sheaths of each section of the three-phase cable at the two transposition points in the middle;

It is characterized by:

3) According to the cable laying form and laying parameter changes, calculate the induced voltage on the metal sheath, and determine the external compensation inductance value that makes the three-phase induced voltage basically balanced;

4) Connect the compensation inductance externally to the cable sheath to balance the induced voltage of the sheath and suppress the sheath current.

2. according to the suppressing method of the described power cable sheath induction current of claim 1, it is characterized in that: described step 3) is according to cable laying form and end length laying parameter change, calculates the induced voltage on the metal sheath, and Determine the external compensation inductance value that makes the three-phase induced voltage basically balanced; the step 4) is to externally connect the compensation inductance to the sheath of the terminal cable to balance the induced voltage of the sheath and suppress the sheath current.

3. according to the suppressing method of the described power cable sheath induction current of claim 1 or 2, it is characterized in that:

A. Calculate the induced voltage of the sheath

Usa USA = = Usb USB = = {\overset{\cdot \cdot}{U u}}_{Sa Sa} = = - - j j {\overset{\cdot \cdot}{I I}}_{a a} {X x}_{S S} V V / / m m

{X x}_{S S} = = 22 ω ω ((ln ln \frac{22 S S}{{D D.}_{S S}})) \times \times 1010^{- - 77} Ω Ω / / m m

B. Calculate the external compensation inductance value;

Xa Xa = = Xb Xb = = {X x}_{c c} = = imag imag ((\frac{(({L L}_{11} * * {\overset{\cdot &Center Dot;}{U u}}_{Sc sc} + + {L L}_{22} * * {\overset{\cdot \cdot}{U u}}_{Sb Sb} + + {L L}_{33} * * {\overset{\cdot \cdot}{U u}}_{Sa Sa}))}{{\overset{\cdot \cdot}{I I}}_{c c}}))

Divide Xa, Xb, and Xc by 2πf respectively to obtain the compensated inductance value of each phase.

4. The method for suppressing the induced current of the power cable sheath according to claim 3, characterized in that: the sheath at the starting end of the three-phase cable is connected to each other at the junction box, and the metal sheath of each section of the cable is connected to each other at the joint box. The transposition point is connected through the cross transposition of the insulating joint box.