JPS5963925A - Protecting circuit for ac transformer - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection circuit for an AC transformer against surge voltage, and more particularly to a protection circuit for an AC transformer used for AC breakdown testing of power cables.

When developing acid caples, AC breakdown tests are conducted to confirm their insulation performance. This AC destructive test is generally performed using an AC transformer.

On the other hand, if a power cable to be tested is connected to such an AC transformer and power is applied, a surge voltage will be generated if the cable is damaged, and this high surge voltage will be applied to the AC transformer and cause a system failure. That damage is likely to occur. In order to prevent damage to the AC transformer due to surge voltage, it is necessary to provide a protection circuit between the AC transformer and the power cable to be tested. FIG. 1 shows an example. That is, a protection circuit 2 is provided between the tunnel cable 1 and the AC transformer 3, and this protection circuit 2 is generally made of a resistor. It has been found that a protection circuit such as this is effective when the cable 1 to be tested has a relatively small capacity. However, as the capacity of the cable increases, the cable charging current increases when power is applied, so when adopting such a protection circuit, it is necessary to make the current capacity of the resistor itself sufficiently large. , a contradiction arises in that the resistance value must be increased in order to suppress the voltage at the time of cable breakdown.

In order to simultaneously satisfy these contradictory demands, the resistance R
It is common practice to configure the protection circuit 2 with a series circuit of L and inductance L.

FIG. 2 shows a configuration for realizing such a reduced series circuit with equal constraints. In Fig. 2, a cylinder 4 made of an insulating material has a high resistance (
4) The metal wire 5 is wound to form an inductance L to form this series circuit. Note that a shield ring 6 is provided to alleviate the terminal electric field. Even in such a protection circuit, if the resistor R is increased to attenuate the surge voltage when the acid capeple breaks down, the Joule heat (R・I2) of the resistor will increase due to the passing current I when AC voltage is applied. growing. Therefore, in order to maintain the protective resistance within an allowable temperature range, it is necessary to increase the surface area of the resistor to enhance the heat dissipation effect, and as a result, the resistor must become extremely large.

SUMMARY OF THE INVENTION An object of the present invention is to provide a small and inexpensive protection circuit for an AC transformer against surge voltages, which solves the problems associated with the conventional protection circuits.

According to the present invention, the above object is achieved by configuring a protection circuit by connecting a resistor and a reactor in parallel. The present invention will be explained in detail below based on the drawings.

FIG. 3 shows an example of the configuration of the protection circuit of the present invention. In FIG. 3, a reactor 7 is formed by winding a low resistance metal wire around a cylinder 4 made of the same insulating material as in FIG.
is placed. This resistor 8 is connected in parallel with the reactor 7, and a shield ring 6 for mitigating the electric field at one end of the cylinder is provided as in FIG. 1. FIG. 4 shows an equivalent circuit of the circuit obtained by the configuration of FIG. 3. That is, the reactance L of the reactor 7 and the resistance R of the resistor 8 form a parallel circuit.

The protection circuit according to the invention provides a charging current xt during AC cardiac pressure control.
d, the surge current flows through the reactor (inductance is L), and the surge current at the time of cable breakdown flows through the resistor (resistance R). Therefore, in order to satisfy such conditions, the inductance L and the resistance 1t must each have selected values. In other words, in order to attenuate the voltage at the time of a cable breakdown, the resistance R needs to be 1 to 10 times the surge impedance of the lead wire between the cable and the resistor (usually 200 to 500 Ω), and therefore Q. 5000Ω
It is. In addition, the reactance L is smaller than the resistance value R within the duration range of the surge wave, that is, 1000 Ω, and if the surge frequency f = 100 KHz to 1 OMHz, then L
≧1.6 mH.

During power application at one side frequency, the voltage is selected to be sufficiently larger than 2πfL. For example, in order to make the ratio of both 10 or more, R='t o o oΩ, and at f:=soHz, L≦318 mH, so from these, the value of the inductance L is preferably set to 1.6 ml≦L≦318 mH.

In the present invention, the beam axle can be manufactured by winding a low-resistance insulated wire on the insulated cylinder 4 in a coil shape,
Further, the resistor can easily be constructed from an element with a small heat capacity. For example, a resistor with a small inductance component, such as a solid resistor, can be used.

When the protection circuit of the present invention is used, the charging current flows to the accelerator during alternating current charging, and the resistance produces a surge voltage suppressing effect on the charging transformer in the event of a cable ground fault. Since the resistance of the reactor is sufficiently small, it is possible to have a sufficiently large charging current capacity during AC 7 charging, and the surge voltage from entering the transformer 4 when the cable breaks can be suppressed by the resistor. Also, since the Joule heat caused by the resistor can be reduced,
A small one will suffice.

[Brief explanation of the drawing]

Figure 1 is a schematic connection diagram of a protection circuit, Figure 2 is a diagram showing a cross-sectional configuration of an example of a conventional protection circuit, Figure 3 is a diagram showing a cross-sectional configuration of a protection circuit of the present invention, and Figure 4 is a diagram showing a cross-sectional configuration of an example of a conventional protection circuit. This is an equivalent circuit of FIG. 3. 1...Test type cable, 2...Protection circuit, 3...
AC transformer, 4... Insulating cylinder, 5 Reactor, 6...
...Shield ring, 7...Reactor, 8...Resistor 7. Child 1 Accounting Z Show 3rd

Claims (2)

[Claims] (1) An AC transformer consisting of a parallel circuit of a resistor and a reactor, which has one end connected to the output of the AC transformer and the other end connected to the equipment used, and is designed to absorb surge voltages generated by abnormal operation of the equipment used above. protection circuit. (2) The AC transformer protection circuit according to claim 1, wherein the resistor is a solid P resistor, and the reactor is a solenoid P made of an insulated wire.