JPH0670662B2 - Power cable ground fault prediction method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a prediction method for preventing a ground fault accident in a power cable system.

[Prior Art] (1) Regarding the power cable line, voltage and current are recorded on the chart paper by using GPT and ZCT, and the record is examined at the time when the ground fault relay operates to discover the ground fault phase.

(2) Alternatively, the abnormality is detected by measuring the partial discharge current.

[Problems to be Solved by the Invention] (1) Since the investigation is conducted after a ground fault accident occurs,
There is no way to prevent a power outage.

(2) The partial discharge current detection method is difficult because it is weak against external noise in the actual line.

[Object of the Invention] A precursor phenomenon of a ground fault of a power cable was confirmed by an experiment.

The present invention is based on a new recognition of this physical phenomenon, and provides a method of preventing an accident that predicts a ground fault.

[Means for Solving the Problems] As shown in FIGS. 2a and 2b, (1) Partial discharge and external noise are generated with respect to the current (or zero-phase current) flowing in the ground wire 14 of the power cable, for example, the CV cable 12. (2) Always measure the current flowing through the ground current in the ground wire 14 of the CV cable 12 or the zero-phase current in the non-grounded system, (3) ) A measuring instrument having a resolution of 1 msec or more is used for current measurement, and an alarm is issued as needed when the detected current exceeds the threshold value.

[Explanation] Regarding the new precursor phenomenon: In Fig. 1a, a small diameter through hole is made in the insulating layer of the 6.6 kVCV cable 12 for testing to simulate a water tree defect.
Apply the regular ground voltage (6.6 / √3 = 3.8kV) of 6kV system.

In air, this voltage does not lead to dielectric breakdown against this defect, but if the dielectric strength is lowered by injecting water, etc., it will occur after a short time ground fault occurs. Due to the Joule heat, the water evaporates and the insulation is restored instantaneously.

Then, it was found that by repeating this, deterioration of the defective portion progressed and finally a complete ground fault equivalent to a ground fault accident was made.

That is, it is considered that such an intermittent ground fault phenomenon occurs when the same cable deterioration (progress of the water tree) occurs also in the actual line.

Figure 1b is an example of the waveform of the ground fault phenomenon confirmed in the experiment.

In the experiment, the current flowing through the ground wire 14 in Fig. 1a was taken out by CT16, the voltage was taken out by PT18, and the waveform was recorded on a waveform recording device (synchroscope etc.) 20 capable of high speed processing.

The observed data is on the order of several A, which is much larger than the partial discharge current and noise current. These partial discharge currents are so weak that it is difficult to measure them in-situ, but the current based on the precursor phenomenon observed this time is at a level at which in-situ measurement is possible.

However, since it is difficult to measure with a conventional recording apparatus that is premised on the current at the time of ground fault, and the phenomenon itself is high-speed, more precise measurement is required.

[Embodiment] [1] Configuration: Fig. 2a shows a case where each CV cable 12 is used, and Fig. 2b shows a case where a three-core cable is used collectively.

Note that 16a in FIG. 2b is a ZCT.

The circuit itself is no different from the conventional case.

However, in the conventional case, since the detection level of the signal is set according to the large current of the ground fault, it cannot be detected by the conventional measuring instrument. Therefore, the threshold value is set at a level much lower than that in the conventional ground fault current detection and larger than the ground current flowing due to partial discharge and external noise.

Also, since the speed of the phenomenon is high, it is necessary to use a measuring instrument having a resolution of 1 A or more at the current level and several 100 μsec to 1 msec or more in accuracy.

22 is a computer.

[2] Action: The case of a 6.6 kVCV cable line will be described.

(1) As described above, the waveform recording device 20 measures the current and voltage,
Always measure.

(2) The threshold level is set as described above, and an alarm is issued when the current exceeds that value. Actually, the following is done.

(3) When the detected signal exceeds the above threshold level, a trigger is triggered by this signal and the current / voltage waveform for about 0.1 sec is stored in the computer 22.

(4) This waveform is used as a reference to make a determination, and various alarms are issued accordingly.

That is, there are emergency alerts (if there is no time to wait, there may be simultaneous interruptions), emergency alerts (if not so high, but at high risk), caution alerts (if you look at the situation for a while).

(5) In some cases, the waveform symbols stored in the computer 22 in (3) above may be sent to a database or sent to a larger computer to process signals from a large number of lines at once.

[Advantageous effects of the invention] (1) The present invention provides an instantaneous ground fault that recovers in a very short time without a power outage with a cable, which occurs prior to a ground fault accident. The magnitude of the zero-phase current in the ground system is on the order of several A, much larger than the power that flows due to partial discharge and external noise, and its waveform is also steep in pulse nature. Therefore, the ground fault is predicted by utilizing a natural phenomenon (precursor phenomenon) that no one has used.

(2) In the present invention, the threshold value is set to a level larger than the ground current due to partial discharge and external noise, and the measurement is performed using a measuring instrument with a resolution of 1 msec or more. In the field measurement, it is possible to detect the above-described precursory phenomenon which is clearly distinguished from external noise and has a pulse-like waveform at high speed.

Then, by detecting this precursory phenomenon, it is judged that there is a risk of a ground fault, and for example, the subsequent observation is strengthened, or in some cases, measures such as replacing the cable are taken to prevent a power failure accident. You will be able to prevent it.

BRIEF DESCRIPTION OF THE DRAWINGS The drawings are all related to embodiments of the present invention, FIG. 1a is an explanatory diagram of an experimental circuit for confirming an intermittent ground fault phenomenon, FIG. 1b is an intermittent ground fault observed waveform diagram, and FIG. 2a. And FIG. 2b are explanatory views of different embodiments. 12: Power cable, 14: Ground wire 16: CT, 18: PT 20: Waveform recorder, 22: Computer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takashi Maruyama 1-5-1 Kiba, Koto-ku, Tokyo Fujikura Electric Line Co., Ltd. (72) Inventor Masao Shiseki 1-5-1, Kiba, Koto-ku, Tokyo No. Fujikura Electric Wire Co., Ltd. (56) References JP-A-55-9151 (JP, A), SAIKAI 58-119772 (JP, U)

Claims (1)

[Claims] 1. A threshold value is set to a level higher than a value of a ground current of a power cable flowing due to internal partial discharge and external noise or a zero-phase current in an ungrounded system, and the resolution is 1 msec. A ground fault prediction method for a power cable, which constantly measures the ground current of the power cable of the power cable line or the zero-phase current in a non-grounded system with the above measuring device.