CN110736905A - Insulation aging evaluation method for 110kV XLPE high-voltage cable - Google Patents

Abstract

The invention discloses a method for evaluating the insulation aging of 110kV XLPE high-voltage cables. First, the polarization-depolarization current (PDC) is used to test the polarization current and depolarization current of the high-voltage cable in the polarization process and the depolarization process. Then, the low-frequency dielectric loss factor under the corresponding polarization voltage is obtained according to the obtained polarization current and depolarization current, and the variation of the low-frequency dielectric loss factor under different polarization voltages is used as the characteristic quantity to characterize the aging of the cable insulation. By evaluating the insulation performance, the invention can realize efficient diagnosis of the insulation aging state of the high-voltage cable, and can more accurately reflect the insulation aging state of the high-voltage cable.

Description

A 110kV XLPE high-voltage cable insulation aging assessment method

technical field

The invention belongs to the field of electrical technology, relates to a power cable insulation diagnosis technology, and in particular relates to a 110kV XLPE high-voltage cable insulation aging evaluation method.

Background technique

Under the combined action of many factors such as mechanical stress, moisture, temperature and electric field, the insulation performance of the insulation body and accessories of XLPE power cables will gradually deteriorate, which will lead to a decrease in the insulation margin of the cable. Under the action of impulse voltage, the cable may be damaged. Insulation breakdown, causing electrical accidents. In order to monitor the status of the cable insulation, periodic insulation diagnosis of the cable is necessary.

The insulation thickness of high-voltage power cables is relatively thick, the rated voltage level is higher, and the structure is different from that of medium-voltage cables. Therefore, traditional medium-voltage cable insulation diagnostic techniques (such as insulation resistance and absorption ratio measurement, power frequency dielectric loss factor measurement, leakage Current measurement, etc.) are not effective in high-voltage cables. At present, there is no recognized effective method for insulation diagnosis of high-voltage cables. Scholars at home and abroad have also made many attempts. The isothermal relaxation current method has been used in the insulation diagnosis of high-voltage cables, but it is necessary to fit and analyze the isothermal relaxation current curve, and the actual operation is complicated and unreliable. In addition, some scholars have proved that the low-frequency factor of high-voltage cables has a positive correlation with the insulation aging state, but their research is tested by a broadband dielectric impedance spectrum tester, which takes a long time to test and has problems in diagnostic efficiency.

SUMMARY OF THE INVENTION

Aiming at the technical status quo of the lack of an efficient diagnosis method for high-voltage power cable insulation aging at present, the purpose of the present invention is to provide a 110kV XLPE high-voltage cable insulation aging evaluation method, which can be used in a short power cable offline time without destroying the cable insulation. In the case of , make an effective diagnosis of its insulation aging.

The inventive idea of the present invention is: due to the high rated voltage level and large capacitance of the high-voltage cable, it is very difficult to actually detect the AC medium loss of the high-voltage cable. The low-frequency dielectric loss characteristics of the cable can also be obtained by using the polarization-depolarization current method under DC voltage. The study found that the dielectric loss of power cables may not have a positive correlation with the degree of insulation aging. It is not ideal to diagnose cable insulation aging solely based on dielectric loss. However, there are differences in the calculated dielectric loss factors of the aged power cables under different polarization voltages, and the variation is positively correlated with the aging time. Therefore, the variation of the dielectric loss factor obtained by the PDC test under different polarization voltages can help to judge the insulation aging of the power cable.

Based on the above inventive idea, the method for evaluating the insulation aging of 110kV XLPE high-voltage cables provided by the present invention includes the following steps:

S1 measures the polarization current of the high-voltage cable to be measured in the polarization process and the depolarization current of the depolarization process under the first polarization voltage and the second polarization voltage, respectively;

S2 calculates the corresponding low-frequency dielectric loss factor when the first polarization voltage and the second polarization voltage are respectively applied to the high-voltage cable according to the following formula:

In the formula, σ ₀ is the electrical conductivity of the cable insulation, ε ₀ is the vacuum permittivity, ε _∞ is the optical frequency permittivity, ε'(ω) is the polarization strength, ε"(ω) is the dielectric loss, χ' (ω) is the real part of the repolarization rate χ(ω) of the insulating medium, and χ”(ω) is the imaginary part of the repolarization rate χ(ω) of the insulating medium;

S3 obtains the low-frequency dielectric loss factor variation Δtanδ corresponding to the first polarization voltage and the second polarization voltage, and uses the low-frequency dielectric loss factor variation Δtanδ as the characteristic quantity Δtanδ characterizing the insulation aging of the high-voltage cable, Δtanδ=tanδ ₂ -tanδ ₁ tanδ ₁ and tanδ ₂ are the first polarized electricity, respectively,

The low frequency dielectric loss factor corresponding to the voltage and the second polarization voltage;

S4 evaluates the insulation aging degree of the high-voltage cable based on the characteristic quantity Δtanδ.

In the above-mentioned 110kV XLPE high-voltage cable insulation aging evaluation method, the purpose of step S1 is to measure the polarization current i _pol and the depolarization process during the polarization process by applying the polarization voltage to the high-voltage cable to be tested and the short-circuit grounding depolarization process after the polarization is completed. current i _depol . In order not to affect the insulation of the high-voltage cable, in the present invention, the first polarized voltage applied to the high-voltage cable ranges from 1 to 4 kV, and the second polarized voltage ranges from 3 to 6 kV. Especially when the difference between the second polarization voltage and the first polarization voltage is 2kV, the stability of the dielectric loss variation of the power cable under different high voltages is better.

In the above-mentioned 110kV XLPE high-voltage cable insulation aging evaluation method, the purpose of step S2 is to obtain the corresponding low-frequency dielectric loss factors when different high voltages are applied to the high-voltage cable.

Due to the conductivity σ ₀ of the high voltage cable insulation:

In the formula, U ₀ is the polarization voltage applied to the high-voltage cable, ε ₀ is the vacuum dielectric constant, i _pol (t _final ) represents the polarization current after the polarization voltage is applied to the high-voltage cable for a set time, i _depol (t _final ) represents the depolarization current of the high-voltage cable after a set time during the depolarization process, the geometric capacitance of the _{C 0} cable, rs _s is the inner radius of the cable shielding layer, and rc is the cable core radius. Polarization and depolarization times are long enough to enable cable insulation diagnostics. Research shows that when both the polarization time and the depolarization time are about 100s, the insulation diagnosis of the cable can be effectively completed. In order to improve the diagnosis efficiency, the polarization time and the depolarization time are set to be 100s in the present invention.

Then, the dielectric response function f(t) can be calculated from the electrical conductivity of the high-voltage cable insulation:

where i _pol is the polarization current measured during the polarization process of applying polarization voltage to the high-voltage cable.

The present invention assumes that the analyzed high-voltage cable insulating medium is linearly uniform and isotropic, then the complex polarizability χ of the medium can be obtained by Fourier transform of the medium response function f(t):

where ω is the angular frequency; χ' is the real part of the repolarization rate of the insulating medium, which reflects the ability of the medium to bind charges; χ" is the imaginary part of the repolarization rate of the insulating medium, which reflects the polarization loss.

Then the frequency domain expression of polarization current is:

In the formula, ε is the relative permittivity, U(ω) is the sinusoidal excitation voltage applied to the insulating medium in the frequency domain, ε=ε′(ω)-jε″(ω), ε'(ω)=ε _∞ +χ'(ω) is the polarization strength, is the dielectric loss (including conductivity loss and polarization loss), and ε _∞ is the optical frequency dielectric constant.

便可计算得到对高压电缆施加不同极化电压时对应的低频介质损耗因数。Therefore, by defining

The corresponding low-frequency dielectric loss factors can be calculated when different polarization voltages are applied to the high-voltage cable.

In the above-mentioned 110kV XLPE high-voltage cable insulation aging assessment method, in step S4, for a new unused cable, the low-frequency dielectric loss factor change Δtanδ corresponding to the first polarization voltage and the second polarization voltage (that is, the characteristic quantity that characterizes the cable insulation aging. ) is close to 0, and this characteristic quantity Δtanδ increases linearly with the thermal aging of the cable. Therefore, according to the above steps S1-S3, the cable to be tested is periodically obtained to obtain the characteristic quantity Δtanδ, and the insulation aging degree of the high-voltage cable to be measured can be evaluated according to the obtained characteristic quantity.

Compared with the prior art, the 110kV XLPE high-voltage cable insulation aging evaluation method of the present invention has the following beneficial effects:

1. The present invention first tests the polarization current and depolarization current of the high-voltage cable in the polarization process and the depolarization process through the polarization-depolarization current (PDC), and then obtains the polarization current and the depolarization current according to the Obtain the low-frequency dielectric loss factor under the corresponding polarization voltage, and then use the change of the low-frequency dielectric loss factor under different polarization voltages as the characteristic quantity to characterize the aging of the cable insulation, and evaluate the insulation performance of the high-voltage cable. Short (about 100s), so it can achieve efficient diagnosis of the aging state of high-voltage cable insulation.

2. Due to the good test stability of the low-frequency dielectric loss factor variation under different polarization voltages, it can more accurately reflect the insulation aging state of high-voltage cables, and has high sensitivity, so it has a wide range of applications in the field of insulation aging diagnosis. prospect.

3. The present invention has a lower test voltage through polarization-depolarization current (PDC), and is less destructive to cable insulation than partial discharge test and withstand voltage test.

Description of drawings

Fig. 1 is a schematic diagram of a high-voltage cable polarization-depolarization current test of the present invention;

In the figure, 1- high voltage DC power supply, 2- SPDT relay, 3, current limiting resistor, 4- cable sample, 41- shielding ring, 42- cable copper layer, 5- picoammeter, 6- upper computer.

Figure 2 shows the variation curve of the low-frequency dielectric loss factor tanδ with the polarization voltage of the new XLPE cable and the thermally aged XLPE cable sample.

Figure 3 shows the change curve of the low-frequency dielectric loss factor tanδ and the low-frequency dielectric loss factor variation Δtanδ of the XLPE cable sample with aging time.

Detailed ways

The present invention will be specifically described below through the examples. It is necessary to point out that the present examples are only used to further illustrate the present invention, but should not be construed as limiting the protection scope of the present invention. Those skilled in the art can The content of the present invention makes some non-essential improvements and adjustments to the present invention.

Example

In this example, seven 110kV XLPE 50cm short cable samples were selected for accelerated thermal aging experiments, of which one 110kV XLPE 50cm short cable sample was used as a control, and the remaining cables were subjected to 10 days, 20 days, 30 days, 40 days, 50 days and 60 days respectively. days of accelerated thermal aging. In addition, a 220kV XLPE high-voltage cable that has been running for 12 years was selected for verification experiments.

The accelerated thermal aging test of short cable samples was carried out in a constant temperature and humidity aging box. The specific steps are as follows: put the short cable sample into the constant temperature and humidity aging box, set the temperature and humidity to 130°C and 0% respectively, and take out the cable after continuous thermal aging until the corresponding aging days.

Polarization-depolarization current (PDC) tests were then performed on 1 new cable, 6 thermally aged cables, and 1 retired cable. The test schematic is shown in Figure 1. The test device includes a high-voltage DC power supply 1, a SPDT relay 2, a current limiting resistor 3, a picoammeter 5 and a host computer 6. One end of the high-voltage DC power supply 1 is connected to the SPDT relay 2 through a wire. The other end is connected to one end of the picoammeter 5 through the wire, the other end of the picoammeter is connected to the cable copper layer 42 of the cable sample 4 through the wire, and the knife of the SPDT relay 2 is connected to the current limiting resistor through the wire. 3 is connected at one end, the other end of the current limiting resistor 3 is connected with the cable core of the cable sample 4, the contact b of the SPDT relay 2 and the shielding ring 41 of the cable sample 4 are grounded through the wire, the high voltage DC power supply 1, the SPDT relay 2 and the picoammeter 5 are connected to the host computer 6 through the transmission line, and the host computer 8 controls the high-voltage DC power supply and the SPDT relay, and collects the picoammeter data in real time.

The polarization-depolarization current testing process of the cable sample using the above-mentioned test device is as follows: control the SPDT relay through the host computer, when the knife is connected to the contact a, apply a polarization voltage to the cable sample through the high voltage DC power supply ( 1kV, 2kV, 3kV, 3kV, 4kV, 5kV and 6kV) to carry out the polarization process, after a period of polarization time t ₁ (100s in this embodiment), the SPDT relay is controlled to switch the blade to the contact b, Both ends of the insulating medium are grounded and discharged through a current-limiting resistor to perform a depolarization process, and the depolarization duration is t ₂ (100s in this embodiment). The picoammeter measures polarization current and depolarization current during polarization and depolarization, respectively.

According to the Nyquist sampling theorem, the value range of ω is related to the sampling rate of the current signal. In this embodiment, the sampling rate is 1 Hz, so ω is set to a maximum of 0.5 Hz. Therefore, this embodiment calculates the dielectric loss factor at 0.1 Hz. Using the formulas (2)-(4) given above, the electrical conductivity σ ₀ of the high-voltage cable insulation of the cable sample, the real part χ' (0.1Hz) and the imaginary part χ" ( 0.1Hz), and then calculate the dielectric loss factor tanδ _0.1Hz at 0.1Hz frequency according to formula (1).

The tanδ _0.1Hz changes of the new cable and the cable after thermal aging for 5 days under different polarization voltages are shown in Fig. 2. It is evident that the tanδ _{0.1 Hz} of the new cable does not change with the polarization voltage. After thermal aging, the tanδ _0.1Hz of the cable increases correspondingly as the polarization voltage increases. This is the theoretical basis for the present invention to use Δtanδ as the aging diagnosis criterion.

Then use the formula Δtanδ=tanδ ₂ -tanδ ₁ to calculate the dielectric loss factor variation Δtanδ of the cable sample under different polarization voltages to evaluate its insulation aging state.

Table 1 shows the change of Δtanδ under different polarization voltage differences for cable samples thermally aged for 5 days. It can be seen from Table 1 that when the selected polarization voltage difference is 2kV, the variance of its Δtanδ value is significantly smaller than that when the polarization voltage difference is 1kV and 3kV, indicating that the test results are relatively stable. In addition, when the polarization voltage is too low, the polarization degree of the insulation medium of the high-voltage cable may be too low and affect the test results; when the polarization voltage is too high, a small amount of space charge injection may be generated, which also affects the test results. Therefore, the present invention selects the tanδ _0.1Hz difference Δtanδ obtained by testing under the polarization voltages of 4kV and 2kV, and further studies its change with insulation aging time.

Table 1 Variation of low frequency (0.1Hz) dielectric loss factor of cable samples aged for 5 days under different polarization voltages

The 0.1Hz dielectric loss factor tanδ _0.1Hz of the cable sample under 2kV polarization voltage and the 0.1Hz dielectric loss factor tanδ _0.1Hz difference Δtanδ of the cable sample under 4kV and 2kV polarization voltage change with the aging time of cable insulation as shown in Figure 3 Show. The 0.1Hz dielectric loss factor tanδ and the variation Δtanδ of the new cable without aging treatment are close to 0. With the increase of thermal aging time, the 0.1Hz dielectric loss factor tanδ of cable samples does not have a positive correlation with the degree of insulation aging. However, with the increase of thermal aging time, the 0.1Hz dielectric loss factor variation Δtanδ of the cable samples showed a linear increase. Therefore, the dielectric loss factor variation Δtanδ at 0.1 Hz frequency of the medium and high voltage cable sample of the present invention can be used to characterize the aging of the cable insulation and be used to judge the aging state of the cable insulation, and greatly improve the accuracy and efficiency of diagnosis.

The following takes a 220kV XLPE high-voltage cable that has been running for 12 years as an example, combined with the 110kV XLPE high-voltage cable insulation aging evaluation method provided by the present invention, to diagnose its insulation aging state, which specifically includes the following steps:

S1 measures the polarization current and the depolarization current of the high-voltage cable to be tested under 2kV and 4kV, respectively; the polarization time and the depolarization time are respectively 100s;

S2 calculates the corresponding low-frequency dielectric loss factor when the first polarization voltage and the second polarization voltage are respectively applied to the high-voltage cable according to the following formula:

In the formula, σ ₀ is the electrical conductivity of the cable insulation, ε ₀ is the vacuum permittivity, ε _∞ is the optical frequency permittivity, ε'(ω) is the polarization strength, ε"(ω) is the dielectric loss, χ' (ω) is the real part of the repolarization rate χ(ω) of the insulating medium, and χ”(ω) is the imaginary part of the repolarization rate χ(ω) of the insulating medium.

Using the formulas (2)-(4) given above, the electrical conductivity σ ₀ of the insulation of the high-voltage cable, the real part χ' (0.1Hz) and the imaginary part χ" (0.1Hz) of the repolarization rate of the insulating medium can be calculated. , and then calculate the 0.1Hz dielectric loss factor tanδ _0.1Hz of the high-voltage cable according to formula (1).

The final calculation yields tanδ ₁ =tanδ _0.1Hz (2kV)=3.9%, and tanδ ₂ =tanδ _0.1Hz (4kV)=5.5%.

S3 obtains the low-frequency dielectric loss factor variation Δtanδ corresponding to the 2kV polarization voltage and the 4kV polarization voltage, and uses the low-frequency dielectric loss factor variation Δtanδ as the characteristic quantity Δtanδ characterizing the insulation aging of the high-voltage cable.

Δtanδ=tanδ ₂ −tanδ ₁ =1.6%.

S4 evaluates the insulation aging degree of the high-voltage cable based on the characteristic quantity Δtanδ.

It can be seen from the previous analysis that the characteristic quantity Δtanδ of the high-voltage power cable increases linearly with the thermal aging of the cable. Therefore, according to the above steps S1-S3 of the cable to be measured, the change trend of the characteristic quantity Δtanδ can be obtained, and then the insulation aging degree of the high-voltage cable to be measured can be evaluated according to the obtained characteristic quantity.

For the above-mentioned 220kV XLPE high-voltage cable that has been in operation for 12 years, since the calculated characteristic quantity Δtanδ is only 1.6%, which is relatively small, it shows that the insulation aging problem of this high-voltage cable is not serious.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Thus, provided that these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims (5)

1. a 110kV XLPE high-voltage cable insulation aging assessment method is characterized in that comprising the following steps: S1 measures the polarization current of the high-voltage cable to be measured in the polarization process and the depolarization current of the depolarization process under the first polarization voltage and the second polarization voltage, respectively; S2 calculates the corresponding low-frequency dielectric loss factor when the first polarization voltage and the second polarization voltage are respectively applied to the high-voltage cable according to the following formula:

In the formula, σ ₀ is the electrical conductivity of the cable insulation, ε ₀ is the vacuum permittivity, ε _∞ is the optical frequency permittivity, ε'(ω) is the polarization strength, ε"(ω) is the dielectric loss, χ' (ω) is the real part of the repolarization rate χ(ω) of the insulating medium, and χ”(ω) is the imaginary part of the repolarization rate χ(ω) of the insulating medium; S3 obtains the low-frequency dielectric loss factor variation Δtanδ corresponding to the first polarization voltage and the second polarization voltage, and uses the low-frequency dielectric loss factor variation Δtanδ as the characteristic quantity Δtanδ characterizing the insulation aging of the high-voltage cable, Δtanδ=tanδ ₂ -tanδ ₁ tanδ ₁ and tanδ ₂ are the first polarized electricity, respectively, The low frequency dielectric loss factor corresponding to the voltage and the second polarization voltage; S4 evaluates the insulation aging degree of the high-voltage cable based on the characteristic quantity Δtanδ. 2 . The method for evaluating insulation aging of 110kV XLPE high-voltage cables according to claim 1 , wherein the value range of the first polarization voltage is 1-4kV, and the value range of the second polarization voltage is 3-6kV. 3 . 3. The method for evaluating insulation aging of 110kV XLPE high-voltage cables according to claim 2, wherein the difference between the second polarization voltage and the first polarization voltage is 2kV. 4. according to the described 110kV XLPE high-voltage cable insulation aging assessment method described in any one of claim 1 to 3, it is characterized in that the electrical conductivity of high-voltage cable insulation is obtained by following formula calculation:

In the formula, U ₀ is the polarization voltage applied to the high-voltage cable, ε ₀ is the vacuum dielectric constant, i _pol (t _final ) represents the polarization current after the polarization voltage is applied to the high-voltage cable for a set time; i _depol (t _final ) represents the depolarization current of the high-voltage cable after a set time during the depolarization process, _{C 0} is the geometric capacitance of the oil-paper insulation, rs _s is the inner radius of the cable shielding layer, and rc is the cable core radius. 5. according to the described 110kV XLPE high-voltage cable insulation aging assessment method of claim 4, it is characterized in that described insulating medium repolarization rate χ(ω) is calculated by following formula:

where f(t) is the medium response function,

where i _pol is the polarization current measured during the polarization process of applying polarization voltage to the high-voltage cable.

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