Method for Determining the Dielectric Loss Factor of Distribution Cable Insulation for Online Monitoring

Abstract

This invention discloses a method for determining the dielectric loss factor of power distribution cable insulation for online monitoring. Based on the given information, the method obtains the following: [formula missing - likely a formula for calculating the dielectric loss factor]. By solving the inverse cosine function, [formula missing - likely a formula for calculating the dielectric loss factor] is obtained, which is the phase difference between [formula missing]. The complex effective values ​​of the voltage at the cable's starting end and the voltage at the cable's ending end are obtained. Based on [formula missing], [formula missing] is obtained, and the phase difference β between [formula missing] and [formula missing] is calculated. Based on [formula missing] and β, δ is calculated, and the tangent value tanδ is obtained, which is the desired dielectric loss factor of the power distribution cable insulation. This invention provides a method for online monitoring of the dielectric loss factor of power distribution cable insulation and provides necessary technical support for determining the insulation performance of power distribution cables. It overcomes the drawback of large errors when using the difference between the currents at the beginning and end of the cable to determine the dielectric loss factor, and can be applied in engineering.

Description

Technical Field

[0001] This invention relates to a method for determining the dielectric loss factor of power distribution cable insulation for online monitoring, belonging to the field of power distribution network automation technology. Background Technology

[0002] Currently, the total length of distribution cables (voltage level 35kV and below) in operation in China exceeds one million kilometers, playing a crucial role in the entire power grid. However, with increasing service life, under the long-term influence of electric and thermal fields, the cable insulation exhibits polymer degradation and defects, and begins to age, with some even showing more severe signs of aging such as electrical treeing. Furthermore, distribution cables generally do not include radially water-blocking metal sheaths; if exposed to a humid environment for extended periods, water trees can form in the insulation under an electric field. All these factors contribute to an increase in the resistive current of the cable insulation, i.e., an increase in the dielectric loss factor.

[0003] In China, researchers have primarily focused on theoretical studies of online monitoring of insulation dielectric loss factor for power transmission cables (110kV and above). For example, scholars at Wuhan University have established an equivalent model of cross-connected cable systems and proposed a method for separating leakage current from sheath current, thus improving the online monitoring method for relative changes in interphase dielectric loss in cross-connected systems without requiring a reference voltage signal. However, this method, considering cross-connection, requires a large number of current transformers (CTs), resulting in significant errors.

[0004] Existing studies on dielectric loss factor are only relevant to transmission cables and are theoretical. For distribution cables, the voltage is generally lower and the length is shorter, resulting in smaller capacitive and resistive currents flowing through the cable insulation, which can lead to significant errors in the dielectric loss factor. Furthermore, considering engineering costs, the accuracy of current transformers (CTs) used to measure cable current in practical engineering is limited, which also increases the measurement error of dielectric loss in distribution cables. Summary of the Invention

[0005] Objective: To overcome the shortcomings of existing technologies in determining the insulation dielectric loss factor of operating power distribution cables with low accuracy, this invention provides a method for determining the insulation dielectric loss factor of power distribution cables for online monitoring.

[0006] Technical solution: To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0007] A method for determining the dielectric loss factor of power distribution cable insulation for online monitoring includes the following steps:

[0008] Obtain the complex RMS value of the current at the starting end of the cable. Complex RMS value of current at cable end .

[0009] according to , ,get and Phase difference θ, according to ,get amplitude .

[0010] according to , ,calculate Where U is the cable phase voltage, C is the cable capacitance per unit length, and L is the cable length. It is the angular frequency of the voltage.

[0011] according to ,θ, ,get , , .

[0012] according to , , ,get By solving the inverse cosine function, we obtain , for and Phase difference.

[0013] Obtain the complex RMS value of the starting voltage of the cable Complex RMS value of cable end voltage .

[0014] according to , ,get + ,according to Then obtain ( + )and The phase difference β.

[0015] according to 1. Find β, find δ, and find the tangent value tanδ, which is the required dielectric loss factor of the power distribution cable insulation.

[0016] As the preferred solution , =100π.

[0017] As the preferred solution , , .

[0018] As the preferred solution .

[0019] As the preferred solution .

[0020] As a preferred embodiment, the cable is a medium- or high-voltage power distribution network cable (110, 63, 35, 10kV).

[0021] As a preferred option, the cable is a cable that is in operation.

[0022] Beneficial effects: The method for determining the dielectric loss factor of power distribution cable insulation for online monitoring provided by this invention has the following advantages:

[0023] (1) The parameters of the first and last current, voltage amplitude and phase angle related to the calculation of dielectric loss factor are introduced and specific expressions are proposed. This provides a method for online monitoring of dielectric loss factor of power distribution cable insulation and provides necessary technical support for determining the insulation performance of power distribution cable.

[0024] (2) This invention only requires the phase difference between the first and last currents in a complex space, which overcomes the disadvantage of large error in obtaining the loss factor of cable insulation by using the difference between the first and last currents, and can be applied in engineering. Attached Figure Description

[0025] Figure 1 This is a schematic diagram showing the complex RMS values ​​of current and voltage at the beginning and end of the cable. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments.

[0027] like Figure 1 As shown, the complex RMS values ​​of the current and voltage at the cable's starting end are respectively and The complex RMS values ​​of the terminal current and voltage are respectively and The complex effective value of the total current (including capacitive and resistive current) passing through the cable insulation is = - The amplitudes corresponding to the above complex effective values ​​are respectively , , , , .make The initial phase is 0. and The phase difference is θ. and The phase difference is α. + and The phase difference is β.

[0028] Generally, the dielectric loss factor of a cable insulation is expressed as the total current passing through the insulation. = - The complementary angle of the phase difference between the voltage and the phase difference; while the voltage can usually be expressed as the average of the complex effective values ​​of the voltage at the beginning and the end, i.e. ( + Therefore, some scholars have suggested using CTs and PTs (voltage transformers) at the beginning and end of the cable to monitor current and voltage, and to achieve online monitoring of the cable insulation dielectric loss factor. However, in practical engineering, the CTs used for monitoring cables are affected by measurement accuracy, often causing significant errors. For example, for shorter 10kV distribution cables, 0.5-class CTs are frequently used in engineering projects. Studies have shown that CTs used for current testing at the beginning and end of the cable can cause significant errors, with a difference of ( ). The error is even greater than that of... The calculated amplitude values ​​are close. Therefore, because and With similar amplitudes, the method of analyzing the insulation dielectric loss factor by measuring the difference between the first and last current measurements is not feasible in practical applications due to limitations in CT test accuracy.

[0029] Based on this, this paper proposes a method that does not depend on the measured amplitude. A more accurate method for determining the dielectric loss factor of power distribution cable insulation for online monitoring is described below.

[0030] According to the Law of Cosines of Triangles, we can obtain:

[0031] (1)

[0032] Combining the triangle sine theorem,

[0033] (2)

[0034] After solving, we can obtain

[0035] (3)

[0036] In the formula, , , , .

[0037] because It can be converted into:

[0038] (4)

[0039] in, It can be represented as

[0040] (5)

[0041] but

[0042] (6)

[0043] Since α and θ are very small under normal circumstances, ,so .

[0044] because Since the value is not negative, formula (3) can be transformed into:

[0045] (7)

[0046] Substitute the relevant parameters , , ,get By solving the inverse cosine function, we can obtain and Phase difference α.

[0047] The voltage can be measured separately using a voltage transformer (PT) and a current transformer (CT). , and Seeking + Then obtain ( + )and The phase difference β.

[0048] Substitute α and β into formula (8) to obtain δ. The tangent value tanδ is the required loss factor of the insulation medium of the power distribution cable.

[0049] δ= (8)

[0050] Example:

[0051] (1) Relevant parameters of power distribution cables and total insulation current of cables

[0052] For a 10kV cross-linked polyethylene (XLPE) insulated cable, with a phase voltage U=6kV and a length L=1km, the cable conductor cross-section is 120mm². 2 The cable insulation has an inner radius R1 = 7.2 mm and an outer radius R2 = 11.7 mm. Considering the relative permittivity of XLPE... Then the dielectric constant F / m. According to the cable capacitance calculation formula, the capacitance per unit length is:

[0053] =260 (pF / m)

[0054] The dielectric loss factor (tanδ) of an insulation cable includes resistive and capacitive components. The ratio of the resistive to capacitive components is the dielectric loss factor. According to national standards, the tanδ of a 10kV cable insulation should be ≤8×10⁻⁶. -3 Then the resistive component can be ignored. Thus, The amplitude can be approximated as:

[0055] (A)

[0056] (2) Calculation of CT and PT test values ​​and phase angle α of online monitoring device

[0057] The complex effective value of the cable current at the first end is taken as: =80A, 0° (the first value is the amplitude, the second is the initial phase angle), the final current complex effective value is taken as: =79.6A, 0.005°; the effective value of the first and last voltages is taken as: =5.98kV, -90.01° =5.99kV, -90.03°.

[0058] Will , Current amplitude and and Substituting the phase angle θ into equation (7) , , It can be obtained and The phase angle α ≈ -0.4°.

[0059] (3) Calculation of dielectric loss factor of cable insulation

[0060] Calculations show that + and The phase difference is β = -90.02°.

[0061] Thus, according to formula (8), δ = 0.38°, and the dielectric loss factor of this cable insulation is calculated as follows:

[0062]

[0063] This invention introduces parameters related to the calculation of dielectric loss factor, such as the current and voltage amplitude and phase angle at the beginning and end of the cable, and proposes specific expressions, providing necessary technical support for online monitoring of the dielectric loss factor of power distribution cable insulation. This invention overcomes the shortcomings of using the difference between the currents at the beginning and end of the cable insulation to obtain the dielectric loss factor, which has large errors, and can be applied in engineering.

[0064] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for determining the dielectric loss factor of power distribution cable insulation for online monitoring, characterized in that: Includes the following steps: Obtain the complex RMS value of the current at the starting end of the cable. Complex RMS value of current at cable end ; according to , ,get and Phase difference θ, according to ,get amplitude ; according to , ,calculate Where U is the cable phase voltage, C is the cable capacitance per unit length, and L is the cable length. It is the voltage angular frequency; according to ,θ, ,get , , ; according to , , ,get By solving the inverse cosine function, we obtain , for and Phase difference; where, This represents the complex effective value of the total current passing through the cable insulation. = - ; Obtain the complex RMS value of the voltage at the starting end of the cable. Complex RMS value of cable end voltage ; according to , ,get + ,according to Then obtain ( + )and The phase difference β; according to 1. Calculate β, find δ, and find the tangent value tanδ, which is the required dielectric loss factor of the power distribution cable insulation. ， ， ； ； ； 。 2. The method for determining the dielectric loss factor of power distribution cable insulation for online monitoring according to claim 1, characterized in that: The cable is a medium- or high-voltage power distribution network cable.

3. The method for determining the dielectric loss factor of power distribution cable insulation for online monitoring according to claim 1, characterized in that: The cable in question is a cable that is currently in operation.

4. The method for determining the dielectric loss factor of power distribution cable insulation for online monitoring according to claim 1, characterized in that: =100π。

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