JPH0575981B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for measuring dielectric loss of a live cable.

(Prior Art) The dielectric loss of the insulator of a power cable is one of the important factors that determines the performance of the cable. Generally, the dielectric loss of a cable is evaluated using tanδ. The tan δ of a cable that has not yet been installed and is yet to be shipped is measured using a shearing bridge or the like. On the other hand, the measurement of tan δ for a live cable is performed as follows.

FIG. 6 shows an explanatory diagram of the measuring method.

The cable 1 shown in the figure has one end 2 of its conductor 2.
A is connected to the AC power supply 3, and the other end 2b is connected to the load 4. One end 2a of the cable 1 is connected to a detection resistor 5.On the other hand, one end of the sheath 7 of the cable 1 is directly grounded via a grounding wire 8.Normally, both ends of the cable sheath 7 If the wire is directly grounded, a large induced current will circulate in the cable sheath 7, so this grounding method is often adopted in actual lines.A current transformer 9 for current measurement is attached to this ground wire 8. ing.

The detection resistor 5 has a voltage measurement terminal 10.
and one end 9 of the output of the current transformer 9
A is grounded, and the other end 9b is a charging current measurement terminal 12.
will be established.

When connected in this manner, only the charging current based on the capacitance of the cable 1 flows through the ground wire 8 because the cable sheath 7 is grounded at one end. Therefore, the output voltage of the voltage measuring terminal 10 is proportional to the alternating voltage E applied to the conductor 2 of the cable 1, and the output voltage of the charging current measuring terminal 12 is proportional to the charging current I of the cable 1. tan δ can be determined from the phase angle between this alternating voltage E and charging current I. Generally, tan δ is determined by dividing the vector product of alternating voltage E and charging current I by the scalar product of both.

In addition, in reality, the detection unit 13 surrounded by a dashed line in the figure
A live line tan δ measuring device that integrates a measuring unit 14 for performing data processing such as calculations is already commercially available (Showa Cable TD: trademark of Showa Cable and Cable Co., Ltd.).

(Problem to be Solved by the Invention) By the way, during the measurement as described above, one end of the detection resistor 5 of the detection unit 13 shown in FIG. 6 is attached to one end 2a of the cable conductor 2 as follows. and extract the voltage.

FIG. 7 is an explanatory diagram of the voltage extraction method.

In the figure, the detection resistor 5 is embedded inside the insulator rod 15, and the tip 5 of the detection resistor 5
A hook 16 is attached to a. This hook 16 is connected to the lead wire 1 near the cable head 17.
It is installed on 8. The rear end 5b of the detection resistor 5 is
Voltage measurement terminal 10 of the detection unit 13 shown in FIG.
connected to.

However, when carrying out the above method, the operator uses rubber gloves or the like to grasp the rear end side of the insulator rod 15, and performs the above installation work on a scaffold (not shown) assembled near the cable head 17. I will do it.

Therefore, such work involves the danger of getting close to live cable conductors, and at the same time poses problems in that it requires assembling scaffolding and other preparatory work over a long period of time.

The present invention has been made focusing on the above points,
The object of the present invention is to provide a method for measuring dielectric loss of a cable, which allows measurement work to be carried out safely and in a short time.

(Means for Solving the Problems) A method for measuring dielectric loss of a cable according to the present invention detects the output of a voltage transformer installed in advance on a cable line, and detects the input voltage and output voltage of the voltage transformer. The alternating voltage applied to the conductor of the cable is measured by correcting the phase shift between the voltage and the charging current of the cable. It is characterized by seeking losses.

(Operation) In the above method, an existing instrument transformer for operating a cable line monitoring instrument is used to measure the alternating voltage applied to the cable conductor. However, potential transformers typically introduce a certain phase shift between the input voltage and the output voltage. Therefore, this deviation is corrected using a phase correction circuit, etc., and then, from the relationship with the cable charging current,
Measure the dielectric loss of the cable. In this way, since the output voltage of the potential transformer is relatively low, dangerous high-voltage work at high places can be avoided and the work can be simplified.

(Examples) Hereinafter, the present invention will be explained in detail using examples shown in the drawings.

FIG. 1 is an explanatory diagram of an embodiment of the method for measuring dielectric loss of a cable according to the present invention.

In the figure, the conductor 2 of the cable 1 is connected to one end 2a via a cable head 17 and a lead wire 18.
is connected to the AC power supply 3. Furthermore, an instrument transformer 20 is connected to one end 2a of the conductor 2. Furthermore, the sheath 7 of the cable 1 is connected to the ground wire 8
The current transformer 9 is attached to the ground.

The output of the instrument transformer 20 is wired to be input to a measuring section 22 via a phase correction circuit 21 . Further, the measuring section 22 is connected so that the output of the current transformer 9 is also input thereto.

The above-mentioned instrument transformer 20 is generally one that is already installed in a main part of a cable line at a substation or the like. That is, in order to monitor the operating status of power transmission and substation equipment, voltmeters and ammeters are usually installed at various locations. In order to operate these instruments, a voltage transformer and a current transformer are installed.
Take advantage of it.

Figure 2 shows an example of wiring for a very common potential transformer.

The illustrated instrument transformer 20 consists of a transformer whose primary side is connected to a cable conductor and whose secondary side extracts a voltage for operating instruments. For example, this input voltage 20a is about 3800V, and the output voltage 20a is about 3800V.
0b is installed at 110V.

Further, a wiring diagram of the current transformer 9 is shown in FIG. This current transformer 9 has the same structure as that already explained in FIG.

Next, the phase correction circuit 21 shown in FIG. 1 is a circuit for correcting the phase shift between the input voltage 20a and the output voltage 20b of the above-mentioned voltage transformer 20, and is a circuit that uses a known capacitor (not shown). It consists of a ladder-type circuit that combines resistors.

FIG. 4 shows an explanatory diagram of the circuit operation.

As shown in the figure, the voltage phase of the output voltage E' of the potential transformer deviates, for example, by α with respect to the alternating voltage E applied to the cable conductor. In the case of a transformer-type potential transformer as shown in Figure 2,
The phase of the output voltage is delayed compared to the phase of the input voltage,
In the case of capacitor voltage division type, there is a tendency to advance.

The phase correction circuit 21 in FIG. 1 corrects this,
An alternating voltage 21a having the same phase as the alternating voltage applied to the original cable conductor is output to the measuring section 22.

The measuring section 22 uses the known method already explained in FIG.
It consists of a tan δ measurement circuit, which determines the phase angle between the alternating voltage 21a output from the phase correction circuit 21 and the charging current 9c output from the current transformer 9, and measures tan δ.

That is, as shown in FIG. 5, the charging current I has a phase advance of (π/4)-δ with respect to the alternating voltage E, and due to the direct current resistance r of the cable insulator, tanδ in the figure
That is, a dielectric loss occurs by an amount corresponding to r/ωc.
Note that ω is the angular frequency of the power source, and c is the capacitance of the cable insulator. Based on this principle, tan δ may be calculated by a known method.

The present invention is not limited to the above embodiments.

The charging current of the cable is not necessarily measured using a current transformer, but may be measured using other methods.
Furthermore, the phase correction circuit may be integrated into the measuring section.

(Effects of the Invention) According to the method of the present invention explained above, the alternating voltage applied to the conductor of the cable is measured using the instrument transformer installed in advance on the cable line, so high-voltage work at high places is not required. can be avoided. Furthermore, preparation work for measurement, etc. can be performed safely and quickly, and work efficiency can be improved.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of an embodiment of the cable dielectric loss measurement method of the present invention, Fig. 2 is a wiring diagram of a potential transformer, Fig. 3 is a wiring diagram of a current transformer, and Fig. 4 is a phase correction circuit. 5 is an explanatory diagram of the operation of the measuring section, FIG. 6 is an explanatory diagram of a conventional cable dielectric loss measurement method, and FIG. 7 is an explanatory diagram of the voltage extraction method. DESCRIPTION OF SYMBOLS 1... Cable, 2... Conductor, 3... AC power supply, 7... Sheath, 8... Ground wire, 9... Current transformer, 17... Cable head, 20... Instrument transformer, 21... ...Phase correction circuit, 22...Measurement section.

Claims (1)

[Claims] 1. Detecting the output of a potential transformer installed in advance on a cable line, and correcting the phase shift between the input voltage and output voltage of the potential transformer,
A dielectric loss of a cable, characterized in that an alternating voltage applied to a conductor of the cable is measured, and a charging current of the cable is measured, and the dielectric loss is determined from the relationship between the alternating voltage and the charging current. Measuring method.