JPH0384476A - Measurement of live-line insulation resistance - Google Patents

Abstract

PURPOSE:To perform an accurate diagnosis for a live-line insulation by arranging a sheath resistor and a local battery in addition to a circuit in parallel with a DC power supply of the bridge circuit equivalently to DC-like. CONSTITUTION:A sliding variable resistor (r) capable of reading out the resistance value is connected in series to a reference resistor R2 and these resistors are grounded through a DC current source P. Further, the reference resistor R2 and the sliding variable resistor (r) which are connected in series, are connected in parallel to the DC current source P. When the circuit is connected in such a manner the circuit can be made to an AC live-line entirely the same as the normal case for AC-like and made to be equivalent to the bridge circuit shown in the figure for DC-like. Consequently, when the sliding variable resistor (r) is adjusted while observing a deflection of galvanometer I to balance the bridge circuit, the insulation resistance R2 can be accurately obtained by the resistance value (r) of sliding variable resistor (r) through a calculating formula of Rx=(RI/r) R2.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a live wire insulation resistance measuring method for measuring the insulation resistance of the main insulation of a power cable in a live wire state.

[Prior Art] In high-voltage electric cables and the like, the insulation performance of electrical insulators deteriorates over time, which may eventually lead to a major dielectric breakdown accident. For this reason, it is necessary to accurately grasp insulation performance at all times and take measures such as replacing cables as appropriate. Various methods have been proposed in the past for determining insulation performance, but methods of measurement under live wire conditions that allow constant monitoring have been attracting particular attention. As one such method, the DC bridge method shown in FIG. 3 is known, for example, as disclosed in Japanese Patent Laid-Open Publication No. 82-299772.

In this method, the grounding transformer G connected to the high voltage bus B
The neutral point of PT is grounded by an AC grounding device 1 for alternating current, and is grounded via a reference resistor R1 for direct current. On the other hand, connect the main conductor of the cable under test to the high voltage bus B. Here, the insulation resistance R of the main conductor between the main conductor and the shielding conductor S is
x becomes the measurement target. This shielding conductor S is grounded by an AC grounder 2 in terms of AC, and is grounded via a voltmeter v1 in terms of DC. Further, the reference resistor R2 and the slide variable resistor r connected in series are connected in parallel with the voltmeter Vl. and,
The node between the reference resistor R2 and the slide variable resistor r and the non-grounded end of the reference resistor R1 are connected to the DC voltage source El via the switch Sv.

In this way, for the DC component, a bridge circuit is constructed as shown in FIG. 4 by the insulation resistor Rx, the reference resistor R1, the slide variable resistor r, and the reference resistor R2. Here, from the reading v2 of the voltmeter v2 connected in parallel across both ends of the slide variable resistor r when the bridge circuit is balanced by adjusting the slide variable resistor r, Rx= (El-V2) ・R2/V2
...The insulation resistance Rx can be determined by the calculation formula (1).

[Problems to be Solved by the Invention] However, in reality, as shown by the dotted line in FIG. The generation of the local battery Ec cannot be ignored, and in terms of direct current, these are inserted in parallel with the voltmeter v1, as shown by the dotted line in FIG. 4, and will affect the balance condition of the bridge circuit. In order to solve this problem, in the conventional method, a variable DC voltage source E2 is inserted using a detection resistor RG provided in parallel with the voltmeter Vt for cancellation.

However, in this case, during measurement, first adjust the output voltage of the variable DC voltage source E2 with the switch SW open to make the reading on the voltmeter V1 zero, thereby canceling out the influence of the local battery Ec. Close the switch S and close the bridge again.
The circuit needs to be balanced, requiring two adjustments. In addition, since there are two places to adjust, an increase in measurement errors is unavoidable.Furthermore, the values of the sheath resistance Rc and local battery Ec change over time, so when constantly monitoring, the switch SW must be turned off frequently. It has problems such as the need for opening and closing operations.

The purpose of the present invention is to solve the above-mentioned problems and to provide a live wire and insulation resistance system that eliminates the effects of deterioration of the cable sheath or the occurrence of local batteries and allows accurate measurement at all times. The objective is to provide a measurement method.

[Means for Solving the Problems] In order to achieve the above object, in the live wire insulation resistance measuring method according to the present invention, the neutral point of the grounding transformer is grounded by an AC grounder. , in terms of direct current, it is grounded via a resistor R1, and the insulation resistance R to be measured on the grounding transformer is
The shielding conductor connected via x is grounded by an AC grounder for AC, and grounded for DC via a series circuit of resistor R2 and resistor r, and the insulation resistance Rx, resistance r, and resistance R1 are grounded for DC.
, resistor R2 constitute a bridge circuit loop, a DC power supply is connected in parallel to the series circuit of the resistor R2 and the resistor r, and the series circuit of the insulation resistor R1 and the resistor R1, and the resistor R1, The bridge circuit is constructed by adjusting at least one of the resistor R2 and the resistor r so that the potential difference between the node between the resistor R2 and the resistor r and the node between the resistor Rx and the resistor R1 becomes zero. The method is characterized in that the insulation resistance Rx is measured by balancing.

[Effect] A method for measuring live wire insulation resistance having the above configuration.

Since the sheath resistance Rc and the local battery Ec are added to the circuit in parallel with the DC power supply of the bridge circuit in a DC equivalent manner, they do not affect the balance condition of the bridge.

[Example] The present invention will be explained in detail based on the example illustrated in FIGS. 1 and 2. Note that the same reference numerals as in FIGS. 3 and 4 represent the same members.

FIG. 1 is a circuit diagram for implementing the method of the present invention,
The neutral point of the grounding transformer GP connected to the high voltage bus B is grounded by the AC grounding device 1 for AC, and is grounded via the reference resistor R1 for DC. Connect the main conductor to high voltage bus B. Although the power transmission line is shown as a single solid line in Figure 881, for example, in the case of a three-core three-phase power transmission line, the insulation resistance R1 to be measured is the main conductor and shielding conductor of each phase line. This is the composite value of the insulation resistance of S. This shielding conductor S is grounded in an alternating current manner by an alternating current grounder 2, and grounded in a direct current manner through a direct current source P.

In addition, a slide variable resistor r whose resistance value can be read is connected in series to the reference resistor R2, and these are connected to the DC current source P.
Connect in parallel with Further, the node between the reference resistor R2 and the slide variable resistor r and the node between the insulation resistor Rx and the reference resistor R1 are connected via a galvanometer I.

When the circuit is connected in this way, it can be made into an AC live line with no difference from the normal case in terms of AC, and it is equivalent to a bridge circuit as shown in FIG. 2 in terms of DC. Therefore, if you adjust the slide variable resistor r while observing the deflection of the galvanometer and balance the bridge circuit, Rx = (R1/ r) ・R2 = (2 ) can be used to calculate the insulation resistance Rx.

By the way, in this embodiment as well, if there is a non-negligible sheath resistance Re and a local battery Ec in the cable sheath, the sheath resistance Rc is reduced from the shielding conductor S as shown in FIG.
, it is also necessary to consider a circuit that is grounded via the local battery Ec.However, even in this case, in an equivalent bridge circuit using a DC component, the sheath resistance Re and the local battery Ec are connected to the DC current source P, as shown in Fig. 2. It is only added in parallel, ie.

The appearance is equivalent to the fluctuation of the DC current source P, and due to the nature of the bridge circuit, it does not have any effect on the balance. Therefore, (
As is clear from equation 2), there is no effect on the measured value of the insulation resistance R1, and accurate measurements can be made even if the sheath is degraded. Further, the only adjustment point is the slide variable resistor r, and it only needs to be adjusted once, and furthermore, even if constant monitoring is to be performed, operations such as opening and closing of switches are not necessary.

In this embodiment, if other power cables, devices, etc. are connected to the high voltage bus B, the influence of these can be removed by selecting a small reference resistance R1. Also, the connection positions of the slide variable resistor r and the reference resistor R2 may be exchanged, but - the value of the insulation resistance Rx that should be measured for boat fishing is extremely high, and the value of the reference resistor R1 is small as described above. Therefore, if the slide variable resistor r is directly connected to the insulation resistor RX in a direct current manner, it is necessary to select a variable one with a high resistance value. Further, it is desirable that the DC current source P be one that can extract a large amount of current, but if the slide variable resistor r and the reference resistor R2, especially the resistance value of the latter, are sufficiently large, a normal voltage source may be used.

Further, in the above embodiment, only the slide variable resistor r is made variable, but other reference resistances may be made variable using a variable resistor whose resistance value can be read.

[Effects of the Invention] As explained above, in the live wire insulation resistance measuring method according to the present invention, even if the cable sheath deteriorates or a local battery occurs, the balance of the bridge circuit is not affected. It can be completed in one operation, and it is easy to operate, and there is no risk of increasing measurement errors. Also, there is no need to open or close switches during constant monitoring, so there is no need to worry about erroneous operation and it is accurate. Live wire insulation diagnosis is possible.

[Brief explanation of drawings]

Drawings 1 and 2 show an embodiment of the live wire insulation resistance measuring method according to the present invention, FIG. 1 is a circuit diagram for carrying out this measuring method, and 2rgJ is its DC equivalent circuit diagram. , FIG. 3 is a circuit diagram of the conventional method, and FIG. 4 is its DC equivalent circuit diagram. Symbol B is the high voltage bus, GPT is the grounding transformer, L is the main conductor, S is the shield conductor, Rx is the insulation resistance to be measured, 1.2 is the AC grounder, R1 and R2 are the reference resistances, and r is the slide variable. Resistance, M is a galvanometer, P is a DC current source, Rc is a sheath resistance,
Ec is the local battery.

Claims (1)

[Claims] 1. Ground the neutral point of the grounding transformer using an AC grounder for AC, ground it via resistor R1 for DC, and connect it to the grounding transformer via the insulation resistance Rx to be measured. For AC, the shielded conductor is grounded by an AC grounder, and for DC, it is grounded through a series circuit of resistor R2 and resistor r, and the four elements of insulation resistance Rx, resistance r, resistance R1, and resistance R2 are A circuit of a bridge circuit is formed by connecting a DC power supply in parallel to the series circuit of the resistor R2 and the resistor r and the series circuit of the insulation resistor Rx and the resistor R1, and the resistor R1,
At least one of the resistor R2 and the resistor r is adjusted so that the node between the resistor R2 and the resistor r, and the resistor Rx and the resistor R1 are connected to each other.
A live wire insulation resistance measuring method, characterized in that the insulation resistance Rx is measured by balancing the bridge circuit so that the potential difference between the nodes is zero.