JP2562490B2 - Insulation performance test method for plastic insulated cables - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] Factors that lower the insulation performance of a plastic insulated cable include protrusions protruding from the inner and outer conductive layers into the insulator, separation of the inner and outer conductive layers from the insulator, Initial defects such as foreign substances and voids mixed in the insulator, external damage, incompatibility of accessories, etc., or subsequent defects such as water deterioration caused by long-term use and heat deterioration are considered. There are various levels of these defects, some of which significantly deteriorate the insulation performance of the cable, and some of which do not affect the actual operation of the cable at all.

Therefore, it is necessary to guarantee the long-term life of the cable by judging whether it is a cable that can withstand long-term actual operation (sound cable) or a cable that does not withstand (defective cable) for the cable to be shipped or the cable currently in use. Sex comes out.

The present invention provides a test of the insulation performance of such a cable,
It relates to a reliable method in a short time.

[Conventional technology and its problems]

Conventionally, an AC withstand voltage method and a DC withstand voltage method are widely known as insulation performance testing methods for plastic insulated cables.

The AC withstand voltage method is usually performed in a factory after a frame test of an AC cable, that is, after manufacturing the cable and before shipping.
On the other hand, the DC withstand voltage method is used not only for frame tests of DC cables, but also for field completion tests of AC and DC cables and insulation evaluation tests during actual operation. The DC withstand voltage method is used in the field test instead of the AC withstand voltage method because the locally laid cable has a long length and a large load capacity.
This is because it is difficult to prepare a large-scale large-capacity test transformer that can apply a high AC voltage of 50 Hz or 60 Hz.

By the way, in the case of conducting a test by the DC withstand voltage method, in the case of a cross-linked polyethylene insulated cable when a plastic insulated cable is used, if the selection of the test voltage at that time is not appropriate, even if a tree (local dielectric breakdown) occurs from the defective part, However, the progress of the insulation may stop, and the entire path of the insulator may not be destroyed. It is presumed that this is because space charges are accumulated at the tip of the tree when a high DC voltage is applied, which causes the progress of the tree to stop due to electrolytic relaxation.

If the tree does not progress and the entire path of the insulator is not destroyed, the cable will not change in appearance and the cable will be used as a non-defective item for actual use. The cable used for actual use in such a state is highly dangerous because the tree is likely to develop during operation and cause dielectric breakdown.

[Means for solving the problem and its action]

SUMMARY OF THE INVENTION The present invention provides a more reliable method for testing the insulation performance of a plastic insulated cable in view of the above problems of the prior art. The method applies a high DC voltage to the plastic insulated cable, When deciding whether or not to actually use the product, after applying the DC high voltage, apply AC or its half-wave rectified high voltage whose peak value is higher than the normal ground voltage peak value of the cable and whose frequency is lower than the commercial frequency. It is a feature.

In this way, when a tree is generated from the defective part in the DC withstanding voltage test, the low frequency high voltage is applied thereafter to ensure that the tree is advanced and the cable insulation is destroyed all the way. This makes it possible to more reliably screen for healthy cables and defective cables.

Generally, the progress of the tree is caused by the discharge energy in the tree tube, and this discharge is less likely to occur with a direct current or impulse voltage than with an alternating voltage because of the effect of space charge. With an AC voltage, partial discharge in the tree tube easily occurs. However, in the case of a plastic insulated cable, this partial discharge may not last long. It is because part of the plastic is decomposed by the hourly discharge when the tree progresses,
This is because the gas pressure increases and the gas pressure in the tree line increases. It is well known from Paschen's law that the discharge stops when the gas pressure increases.

But at commercial frequencies, especially when the applied voltage is high,
Partial discharge becomes intense. In this case, the decomposition gas pressure is momentarily increased at the tip of the tree and the discharge is likely to stop, but a new branch of the tree is likely to be formed near the root of the tree. As a result, the overall shape of the tree becomes a round shape (Marimo-shaped tree) as if it were "Marimo".
Therefore, at the commercial frequency as a whole, the linear propagation speed of the tree toward the conductor side or the sheath side is not so large.

On the other hand, when a low frequency high voltage is applied, partial discharge is not as intense as when a commercial frequency high voltage is applied, but there is enough time for the decomposed gas to diffuse, so the gas pressure in the tree tube does not increase, Energy due to partial discharge is likely to concentrate at the tip of the tree, and the propagation direction of the tree becomes linear toward the conductor side or the sheath side. Although the form of this tree is called a tree-shaped tree, if the applied voltage is the same, the growth speed of the tree in the direction of shorting the insulation distance is faster than the commercial frequency. Of course, the influence of space charge as in the case of DC voltage or impulse voltage is small.

Therefore, if a low-frequency high-voltage is applied after the special current withstanding voltage test, if there is a tree that has been generated during the DC withstanding voltage test and its progress has stopped, ensure that it is dielectrically broken down in a short time. Is possible. Of course, the low-frequency high-voltage is an alternating current, but the effect is the same even if it is a half-wave rectified waveform. Further, if the peak value of the low frequency high voltage is equal to or higher than the regular ground voltage of the cable, it is convenient because the actual operating voltage of the cable can be guaranteed. The applied waveform does not necessarily have to be a sine wave as described later, but can be a quasi-triangular wave or another waveform.

Another advantage of applying a low frequency high voltage is that the generator can be simplified. The size of an AC high voltage generator is usually represented by the capacity W, the larger it is, the larger the device is.

W = I × V (VA) (1) where I is the charging current and V is the rated voltage. When the charging current I is a sine wave alternating current, I = jωCV (A) (2) where ω = 2πf (f is frequency) C: load capacitance.

Therefore, the capacity W can be reduced by lowering the frequency f. Ie 50Hz commercial frequency and 0.1Hz
With the low frequency of, its capacity will be 500 times different,
It turns out that the low frequency method is much more advantageous for applying high voltage to the cable in the field. Furthermore, if the waveform is changed to a quasi-triangular wave, by adding a circuit that repeatedly charges and discharges the DC high voltage generator, it becomes possible to share the DC high voltage generator and the low frequency high voltage generator, which is economical. Also, it is possible to shorten the test time.

〔Example〕

 Examples of the present invention will be described below.

First, a DC high voltage application test is performed to determine whether the plastic insulated cable is non-defective or withstands subsequent practical use. This test is the same as before,
As shown in FIG. 1, the sheath of the cable 1 is grounded, and the conductor is connected to the DC high voltage generator 2 through the protective resistor 3. Incidentally, 4 is a cable terminating portion. Cables with significant defects in this test will experience a dielectric breakdown.

Next, a low-frequency high-voltage application test is performed on the cable that has passed the above test. In this embodiment, as shown in FIG. 2, the power from the commercial frequency power source 5 was converted into a low frequency by the frequency converter 6 and applied to the cable 1 as a high voltage having a frequency lower than the commercial frequency. The boosting to the high voltage may be performed before or after the frequency conversion. A healthy cable does not cause dielectric breakdown by the application of this low frequency high voltage, but a cable in which a tree is generated inside the insulator in the above DC high voltage application test promotes tree growth in this test. Therefore, there are some things that lead to dielectric breakdown. Therefore, a cable that passes this test can be guaranteed to be a healthy cable that does not cause dielectric breakdown due to the progress of the tree during actual use.

Table 1 shows the data of the applied voltage and the applied time when the test method of the present invention is applied to the field completion test of the crosslinked polyethylene insulated cable.

The applied voltage and time for the DC high voltage application test (DC withstand voltage test) are user standards that conform to the electrical equipment standards.

The applied voltage and time of the low frequency high voltage application test were determined as follows. That is, this applied voltage V and time t
Is the insulation thickness d and the tree growth rate v (= dx / dt, 0 ≦ x
≦ d). The tree growth rate v tends to increase as the applied voltage V increases, and also varies depending on the insulating material. Therefore, a treeing needle electrode 8 having a tip radius of curvature of 2 μm is inserted into a cross-linked polyethylene block 7 as shown in FIG. Apply a high voltage,
We investigated the rate of tree growth. According to this, 20KV
(0 to peak value), it was found that it takes about 1 minute for the tree to propagate a distance of 1 mm (average value of 5 experiments).
The values in Table-1 are obtained by calculating the applied voltage and time of the low frequency high voltage for each rated voltage from the experimental results. The time is advanced by 10 minutes.

Of course, the values in Table-1 are only examples, and it goes without saying that appropriate values should be selected depending on the type and composition of the insulating material.

In order to make the test method of the present invention more highly reliable, the leakage current of the cable is measured when a high DC voltage is applied, or the partial discharge is measured when a low frequency high voltage is applied. It is also effective to implement them in combination.

〔The invention's effect〕

In the conventional DC withstanding voltage test, there was a risk of causing a dielectric breakdown during actual use operation because a defect was generated by the test and remained, but after the DC withstanding voltage test as in the present invention, low frequency high voltage When the voltage application test is performed, the defects generated in the DC withstanding voltage test can be propagated to cause dielectric breakdown, and thus more reliable cable screening can be performed. In addition, since the low-frequency high voltage is applied, the tree growth speed is fast, and the test can be performed in a relatively short time, and since the capacity of the high-voltage generator is small, the test can be performed with a small and inexpensive device. It is extremely convenient for field tests.

[Brief description of drawings]

1 is an explanatory diagram of a DC withstanding voltage test, FIG. 2 is an explanatory diagram of a low frequency high voltage application test, and FIG. 3 is an explanatory diagram showing an experiment for determining an applied voltage and time of a low frequency high voltage application test. Is. 1: Plastic insulated cable, 2: DC high voltage generator,
3: Protection resistor, 4: Cable termination, 5: Commercial frequency power supply,
6: Frequency converter.

Claims (1)

(57) [Claims] 1. A high voltage direct current is applied to a plastic insulated cable to determine whether or not the cable is actually used,
After the DC high voltage is applied, an insulation performance test method for a plastic insulated cable, characterized in that an AC or a half-wave rectified high voltage whose peak value is equal to or higher than the normal ground voltage peak value of the cable and whose frequency is lower than the commercial frequency .