Cable water tree severity assessment method

Abstract

The invention belongs to the field of power transmission and transformation equipment operation and maintenance, and particularly relates to a cable water tree severity assessment method. The method comprises the following steps: firstly, carrying out a partial discharge test on a cable by using damped oscillation wave equipment, when partial discharge is found, observing whether the partial discharge is changed by changing direct-current pre-pressurization time, and if the partial discharge is obviously changed, considering that water tree defects exist in the cable; then, recording different direct-current pre-pressurization times and corresponding partial discharge amounts, and drawing an incidence relation curve of the direct-current pre-pressurization times and the partial discharge amounts; and finally, evaluating the severity of the water tree according to the slope of the incidence relation curve. By means of the method, the length of a small number of water trees can be evaluated, the problem that an existing ultralow frequency method cannot detect the small number of water trees is solved, and compared with pure partial discharge detection, water tree severity evaluation based on the length of the water trees is achieved. The method is simple and reliable.

Description

Technical Field

[0001] The invention belongs to the field of power transmission and transformation equipment operation and maintenance, and in particular to a cable water tree severity assessment method. Background Art

[0002] With the continuous development of urban power grids, the cabling rate has increased year by year. As the last kilometer of power supply, the reliability of 10kV cables directly affects the reliability of power supply. At present, most 10kV cables are directly buried or laid in pipes, and they are operated in a humid environment for a long time. Water tree defects will be formed under the action of external force damage, rat and insect bites, etc., which will lead to cable breakdown and cause power outages. The current water tree severity assessment generally adopts the ultra-low frequency dielectric loss measurement method, but it is only effective for large-area and large-number water tree defects. The breakdown caused by water trees mainly depends on the length of the water tree, not the number. For the small-area and small-number water tree defects that mainly exist in the current cables, the ultra-low frequency dielectric loss detection method is no longer applicable, and it is urgent to carry out targeted water tree severity assessment method research. Summary of the invention

[0003] The object of the present invention is to provide a cable water tree severity assessment method suitable for assessing the length of a small number of water trees in view of the above-mentioned shortcomings.

[0004] The technical solution of the present invention is: a method for evaluating the severity of cable water tree, which is characterized by the following steps: first, a damped oscillation wave device is used to perform a partial discharge test on the cable. When partial discharge is found, the DC pre-voltage time is changed to observe whether the partial discharge changes. If a significant change occurs, it is considered that there is a water tree defect in the cable; then, different DC pre-voltage times and their corresponding partial discharge amounts are recorded, and a correlation curve between the DC pre-voltage time and the partial discharge amount is drawn; finally, the severity of the water tree is evaluated according to the slope of the correlation curve.

[0005] The slope is the average of the slopes of the curves, and the severity of the water tree is evaluated based on the average value: if the average slope is ≤1 pc / s, the severity of the water tree is level 1; if the average slope is >2 pc / s, <3 pc / s, the severity of the water tree is level 2; if the average slope is ≥3 pc / s, the severity of the water tree is level 3; among them, level 1 is the lightest and level 3 is the most severe.

[0006] The working principle is: the damped oscillation wave method is the main method currently used for cable partial discharge detection. By changing the DC pre-stressing time of the damped oscillation voltage, different amounts of charges can be injected into the cable water tree channel. The electric field formed by these charges will be superimposed on the electric field of the damped oscillation voltage to stimulate partial discharge, thereby discovering water tree defects. The longer the water tree is, the easier it is to cause cable breakdown, and the higher the severity of the water tree. The longer the water tree is, the longer it takes to grow, and the better the conductivity of its internal channel. Under the same DC pre-stressing time, the more charges are injected, and the greater the amount of partial discharge stimulated. Therefore, the severity of the cable water tree can be evaluated based on the correlation between the DC pre-stressing time and the amount of partial discharge.

[0007] Beneficial effects of the present invention: 1. A cable water tree severity assessment method based on the correlation between DC pre-stressing time and partial discharge is provided. By recording different DC pre-stressing times and their corresponding partial discharge amounts, a correlation curve between DC pre-stressing time and partial discharge amount is drawn, and the severity of the water tree is assessed according to the slope of the correlation curve. This can achieve an assessment of the length of a small number of water trees, fundamentally solving the problem that the existing ultra-low frequency method cannot detect a small number of water trees. Compared with simple partial discharge detection, this method achieves a water tree severity assessment based on the length of the water tree. 2. The method is simple and reliable.

[0008] The embodiments of the present invention will be described in further detail below with reference to examples. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a flow chart of the invention.

[0010] Figure 2 It is a curve diagram showing the correlation between DC pre-voltage time and partial discharge amount. DETAILED DESCRIPTION

[0011] See also Figure 1 , 2 , a method for evaluating the severity of cable water tree: the defective local discharge amount changes with the pre-voltage time → a significant change occurs, a correlation curve between the DC pre-voltage time and the local discharge amount is drawn → the slope of the correlation curve between the DC pre-voltage time and the local discharge amount is calculated → the severity of the water tree is evaluated according to the slope.

[0012] Specifically, a damped oscillation voltage is used to perform a partial discharge test on the test cable. When a partial discharge is found, first evaluate whether it is a water tree defect. At the stable starting voltage of the partial discharge, extend the DC pre-voltage time. The definition of the DC pre-voltage time is as follows: Figure 2 shown. Figure 2It is a curve diagram of the correlation between DC pre-voltage time and partial discharge. The definition of DC pre-voltage time, where the time difference Δt between boosting to the test voltage and starting oscillation is the DC pre-voltage time. Preferably, the DC pre-voltage time is not longer than 60s. During this time, when the partial discharge amount changes significantly with the extension of the DC pre-voltage time, it can be identified as a water tree defect.

[0013] Then, at the local discharge stable starting voltage, at different DC pre-voltage times, the local discharge amount caused by the water tree defect is recorded, and a correlation curve between the DC pre-voltage time and the local discharge amount is drawn. Preferably, the DC pre-voltage time is 0s, 15s, 30s, 45s and 60s respectively.

[0014] Then, the slope of the curve of the relationship between the DC pre-voltage time and the partial discharge amount is calculated. Preferably, the drawn curve is first fitted using an exponential function, and then the fitted curve is derived to obtain the slope of the curve. Preferably, the number of points of the fitting curve and the slope of the curve should be no less than 100 points.

[0015] Finally, the slope of the curve is averaged. The severity of the water tree is evaluated based on the average value. Preferably, the severity of the water tree is evaluated using the data in Table 1, where level 1 is the lightest and level 3 is the most severe.

[0016] Table 1

[0017] The above description is only a specific implementation mode of the present invention, and various examples do not constitute a limitation on the essential content of the present invention.

Claims

1. A cable water tree severity assessment method, characterized in that The steps are as follows: first, use a damped oscillation wave device to perform a partial discharge test on the cable. When partial discharge is found, change the DC pre-voltage time to observe whether the partial discharge changes. If there is a significant change, it is considered that there is a water tree defect in the cable; then, record different DC pre-voltage times and their corresponding partial discharge amounts, and draw a correlation curve between the DC pre-voltage time and the partial discharge amount; finally, evaluate the severity of the water tree according to the slope of the correlation curve.

2. A cable water tree severity assessment method according to claim 1, characterized in that The slope is the average of the slopes of the curves, and the severity of the water tree is evaluated based on the average value: if the average slope is ≤1 pc / s, the severity of the water tree is level 1; if the average slope is >2 pc / s, <3 pc / s, the severity of the water tree is level 2; if the average slope is ≥3 pc / s, the severity of the water tree is level 3; among them, level 1 is the lightest and level 3 is the most severe.

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