Method for evaluating space charge accumulation based on two-terminal current

Abstract

The present application belongs to the field of electrical insulation material monitoring, and particularly relates to a DC electric field space charge evaluation scheme for insulation materials. The method can evaluate the space charge accumulation condition of insulation materials in operation without directly measuring the space charge, by measuring and recording the current and its curve between the two ends of the materials, and can evaluate the insulation condition of the insulation materials. Compared with the existing space charge measuring device, the present application has the advantages of low cost and simple equipment, and can overcome the shortcomings of the current space charge measuring device that cannot evaluate the space charge accumulation condition of insulation materials in operation.

Description

Technical Field

[0001] This invention belongs to the field of electrical insulation material monitoring, and specifically relates to a scheme for evaluating the space charge of DC electric field in insulation materials. Background Technology

[0002] The significant accumulation of space charge in insulating materials under high DC fields has multiple impacts on insulation. Firstly, the accumulation of space charge can increase the local electric field to eight times that of the applied electric field. Furthermore, the significant electric field distortion effect of space charge makes the calculation and design of the actual electric field in cable insulation difficult. Additionally, with the slow accumulation of space charge, after long-term operation, the point of maximum electric field stress shifts from the interface to the interior of the insulation. Simultaneously, the presence of space charge accelerates cable dendration and aging. Therefore, assessing the space charge in insulation under high DC fields is a crucial issue in determining cable quality and the suitability of a material for DC cable insulation.

[0003] To date, obtaining information related to space charge in insulating materials primarily utilizes pulsed electroacoustic (PEA) devices or pressure wave (PWP) devices. While these two methods can effectively reflect space charge distribution, they are relatively expensive and unsuitable for measuring space charge distribution in operational equipment. This new method, however, can obtain necessary information on space charge accumulation by measuring the conductivity current of the insulating material. It is not only less expensive and simpler to use, but also suitable for assessing space charge accumulation in operating equipment. Summary of the Invention

[0004] One of the objectives of this invention is to assess the accumulation of space charge in insulating materials under a DC electric field.

[0005] The second objective of this invention is to evaluate the accumulation of space charge in insulating materials under a DC electric field, thereby determining the insulation performance of the insulating material and whether the insulating material is suitable for continued operation.

[0006] The third objective of this invention is to evaluate in real time the accumulation rate of space charge in insulating materials under a DC electric field, and thereby monitor the abnormal state of the insulating materials.

[0007] The specific scheme of this invention for evaluating space charge in insulating materials under a DC electric field is as follows: First, an experiment is conducted in advance by applying a DC voltage to both ends of the insulating material for a certain period of time. The limiting voltage (the maximum voltage U that allows the conductive current through the insulating material to reach a steady state without discharge) is obtained through testing. max ), at the limiting voltage U max Under the given conditions, the current across the insulating material is measured and recorded, denoted as I. max1 I max2, until the two currents basically reach a steady state. The time length required to reach the steady state is denoted as t0. Integrate the difference between I max1 , I max2 within the time t0 to obtain the total amount of limiting space charge Q max , and record the maximum value of |I max1 - I max2 | as ΔI max . Apply a DC voltage across the insulating material. Similarly, record the currents I1 and I2 at both ends within the time t0. Integrate the difference between I1 and I2 to obtain the total amount of space charge Q. Determine the situation of space charge accumulation based on the comparison relationship between the total amount of space charge Q and Q max .

[0008] The corresponding relationship between the comparison relationship of the total amount of space charge Q and the total amount of limiting space charge Q max and the space charge accumulation state is as follows: If |Q / Q max | ≥ A1, it means that there is a relatively large amount of space charge accumulation in the insulating material, approaching the limit that the insulating material can withstand, and it is not suitable for continued operation; if A2 ≤ |Q / Q max | < A1, it means that there is a certain amount of space charge accumulation inside the insulating material, which has a greater impact on the insulating material, but the insulation of the insulating material can still continue to withstand; if |Q / Q max | < A2, it means that there is little or no space charge accumulation inside the insulating material.

[0009] For common thermoplastic polymer insulating materials such as polyethylene, cross-linked polyethylene, and polypropylene, A1 and A2 can take A1 = 0.8 and A2 = 0.5.

[0010] The absolute value of the difference between I1 and I2, |I1 - I2|, reflects the speed of space charge quantity accumulation. Record the value of |I1 - I2| as ΔI. Determine the abnormal state of the insulating material based on the comparison relationship between the value of ΔI and ΔI max .

[0011] The corresponding relationship between the comparison relationship of the value of ΔI and ΔI max and the abnormal state is as follows: If ΔI / ΔI max ≥ B1, it means that the current charge accumulation speed is very fast, there is an abnormal state, and the insulating material needs to be detected; if B2 ≤ ΔI / ΔI max < B1, it means that the charge accumulation speed exceeds the normal speed, and an abnormal state warning needs to be given; if ΔI / ΔI max < B2, it means that the charge accumulation speed is normal.

[0012] For common thermoplastic polymer insulating materials such as polyethylene, cross-linked polyethylene, and polypropylene, B1 and B2 can take B1 = 3 and B2 = 1.

[0013] Compared with existing space charge measurement devices, this invention has the advantages of low cost, simple equipment, and simple measurement method. It can also overcome the shortcomings of current space charge measurement devices that cannot assess the accumulation of space charge in insulating materials that are in operation. Attached Figure Description

[0014] Figure 1 It is the charging current curve of the insulating material under the limiting voltage.

[0015] Figure 2 It is the discharge current curve of the insulating material under the limiting voltage.

[0016] Figure 3 It is the charge accumulation curve of the insulating material under the limiting voltage.

[0017] Figure 4 This is the charging current curve of sample X.

[0018] Figure 5 This is the discharge current curve of sample X.

[0019] Figure 6 This is the charging current curve of the Y sample.

[0020] Figure 7 This is the discharge current curve of the Y sample.

[0021] Figure 8 This is the charging current curve of sample Z.

[0022] Figure 9 This is the discharge current curve of sample Z. Detailed Implementation

[0023] Common thermoplastic polymer insulating material samples were selected, with X, Y, and Z being identical samples. A1 = 0.8, A2 = 0.5, B1 = 3, and B2 = 1. The same insulating materials as X, Y, and Z were tested under different voltages (field strengths). The voltage duration t0 should ensure that the conduction current reaches a steady state. The limiting voltage was obtained experimentally. The electric field corresponding to the limiting voltage is 670 kV / mm. The current with the higher value on both sides is called the charging current, and the current on the other side is called the discharging current. The charging and discharging currents of the insulating material under the limiting electric field for time t0 and their curves were recorded. The recorded charging current curve is shown in the figure below. Figure 1 As shown, the discharge current curve is as follows: Figure 2 As shown, t0 is 2h. ΔI is recorded. max 1.55×10 -8 A. By integrating the difference between the charging current and the discharging current at the limiting voltage, the charge accumulation curve is obtained, and the limiting charge Q is obtained. max =1.92×10 -5 C, the charge accumulation curve is as follows Figure 3 As shown.

[0024] The charging current curve of sample X is shown in the figure below. Figure 4 As shown, the discharge current curve is as follows: Figure 5 As shown, with a time length of t0, the total charge accumulation Q is obtained by integration. A =1.58×10 -5 C, |Q A / Q max If |≥0.8, then the space charge accumulation in the insulating material is relatively large, and the maximum recorded value of ΔI is 1.14×10 -8 A, during time t0, ΔI / ΔI max <1, the charge accumulation rate is normal.

[0025] The charging current curve of the Y sample is shown in the figure below. Figure 6 As shown, the discharge current curve is as follows: Figure 7 As shown, with a time length of t0, the total charge accumulation Q is obtained by integration. B =1.28×10 -5 C, 0.5 ≤ |Q B / Q max If | < 0.8, then a certain amount of space charge accumulates inside the insulating material, and the maximum value of ΔI is recorded as 1.22 × 10. -8 A, during time t0, ΔI / ΔI max <1, the charge accumulation rate is normal.

[0026] The charging current curve of sample Z is shown in the figure below. Figure 8 As shown, the discharge current curve is as follows: Figure 9 As shown, with a time length of t0, the total charge accumulation Q is obtained by integration. C =1.75×10 -6 C, |Q C / Q max If | < 0.5, then there is little or no space charge accumulation inside the insulating material, and the maximum recorded value of ΔI is 3.89 × 10 -10 A, during time t0, ΔI / ΔI max <1, the charge accumulation rate is normal.

Claims

1. A method for assessing the DC space charge of insulating materials, which can be used to evaluate the accumulation of space charge in insulating materials, characterized in that: First, preliminary experiments were conducted to obtain the limiting voltage. At the limiting voltage, the current curves across the terminals were recorded, and the total space charge was calculated. Q max Record | I max1 - I max2 The maximum value is | Similarly, record the current curves at both ends of the insulating material after applying a DC voltage, and calculate the charge accumulation using the current curves at both ends. Q Real-time recording I 1, I The absolute value of the difference | I 1- I 2|, through charge quantity Q and the pre-determined limit of total space charge Q max The comparison relationship is used to determine the space charge accumulation state of the insulating material, through To determine the rate of space charge accumulation and the abnormal state of the insulating material, including the total amount of space charge. Q With the total amount of space charge Q max The correspondence between the comparison relationship and the space charge accumulation state is as follows: If | Q / Q max | If A1, the insulating material has accumulated a large amount of space charge, approaching its withstand limit, and is not suitable for continued operation; if A2... | Q / Q max | A1 indicates a certain amount of space charge accumulation within the insulating material, which significantly impacts the material's performance, but the insulation can still withstand the load; if | Q / Q max | A2 indicates that there is little or no space charge accumulation inside the insulating material. The value and The comparison relationship and the correspondence between abnormal states are as follows: If B1 indicates a rapid charge accumulation rate, suggesting an abnormal state that requires testing of the insulating material; if B2... B1 indicates that the charge accumulation rate exceeds the normal rate, requiring an abnormal state warning; if B2 indicates that the space accumulation rate of the insulating material is normal.

2. The space charge assessment method according to claim 1, characterized in that, The limiting voltage is specifically defined as follows: by conducting experiments on the target insulating material and applying different DC voltages, the limiting voltage is the maximum voltage at which the current in the insulating material can reach a steady state without breakdown after the voltage is applied.

3. The space charge assessment method according to claim 1, characterized in that, The method for recording the current curves at both ends comprises the following steps: applying a DC voltage to the insulating material and recording the conductivity current and its curve within that time range. I 1(t), I 2(t), whose time length t0 is the time when the two currents basically reach steady state under the limiting voltage.

4. The space charge assessment method according to claim 1, characterized in that, The method of calculating the accumulated amount using the current curves at both ends. Q , Q max The calculation method, specifically the steps, are as follows: Calculate the current curves at both ends of the insulating material. I 1(t), I 2(t), with a time length of t0, the charge accumulation is obtained by integrating the difference between the two currents, where . Q max This represents the amount of charge accumulation calculated under the limiting voltage.

5. The space charge assessment method according to claim 1, characterized in that, The total space charge Q With the total amount of space charge Q max The correspondence between the comparative relationship and the space charge accumulation state is as follows: For the three types of thermoplastic polymer insulating materials, polyethylene, cross-linked polyethylene, and polypropylene, A1 and A2 are taken as A1=0.8 and A2=0.5, if | Q / Q max | If the value is 0.8, the insulating material will accumulate a large amount of space charge, approaching its withstand limit, and will not be suitable for continued operation; if it is 0.5... | Q / Q max | If the value is 0.8, then a certain amount of space charge accumulates inside the insulating material, which has a significant impact on the insulating material, but the insulation of the insulating material can still withstand the load; Q / Q max | If the value is 0.5, then there is little or no space charge accumulation inside the insulating material.

6. The space charge assessment method according to claim 1, characterized in that, The aforementioned The value and The ratio relationship and the comparison relationship with the abnormal state are as follows: For the three types of thermoplastic polymer insulation materials, polyethylene, cross-linked polyethylene and polypropylene, B1 and B2 are taken as B1=3 and B2=1.

3. If the current charge accumulation rate is very fast, an abnormal state exists, and the insulating material needs to be tested; if 1 3. If the charge accumulation rate exceeds the normal rate, an abnormal state warning is required. 1 indicates that the accumulation rate of insulating material in space is normal.

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