JPS6082067A - Protecting system of frequency converter - Google Patents

Abstract

PURPOSE:To enable to protect a frequency converter without increasing or decreasing or decreasing the number of CTs irrespective of the number of water- cooled cables by connecting the CTs with one of the cables at every phase to detect the disconnection of the cable. CONSTITUTION:When one of three water-cooled cables in any phase is disconnected, the coil current IF of a furnace must be shared by the remaining two cables for the phase of the disconnected cable, the current flowed to the CT of the disconnected cable becomes IF/2, while the current flowed to the CT of the normal phase becomes IF/3. Thus, a current difference occurs, this is detected by an operating circuit 7, and an induction heating frequency converter is stopped by a stop logic circuit 8.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a protection system for a frequency converter comprising a load commutated inverter.

[Technical Background of the Invention and Problems Therewith] Frequency converters constituted by load commutated inverters are generally used as power sources for induction furnaces, induction heating, and the like. Schematic structure of such a device
1 To the hill (-;, 7←A frequency converter 1 consisting of a load commutation inverter is connected to a coil 3' through a power factor correction capacitor 2, and supplies high-frequency alternating current to the coil 3 of the melting furnace 3a. ing.

On the other hand, in a melting furnace, it is necessary to release the molten material inside the furnace to the outside during the final stage of melting, and for this purpose, a method is generally adopted in which the furnace body is tilted to release the molten material inside the furnace. are doing. Therefore, the melting furnace 3a and the power factor correction capacitor 2
The wire connecting the two must be made of elastic cable, and the current flowing through coil 3 will be several thousand amperes or more.
A plurality of water cooling cables 4 are required.

Industrial water is used for the cooling water (droplet 112) used in the water cooling cable 4, and a device is installed to remove floating substances mixed in it. Over a long period of time, scale etc. may build up in the water cooling cable, causing the water cooling cable to heat up and break due to a lack of cooling water, or even though the cooling water is flowing normally, the cable may break due to deterioration of the electric wires used in the water cooling cable. As a detection method for these, a method has been adopted in which a disconnection of the water-cooled cables 4 is detected by connecting as many CT5s as there are water-cooled cables to a plurality of water-cooled cables 4 of each phase.

The detection circuit 6 is a circuit that does not output an output when current is flowing through the CT 5, but outputs a signal when the current no longer flows.The signal is input to the OR circuit 6a and sent to one or more of the plurality of detection circuits 6. When operated, a signal appears at the output of the OR circuit 6a, which is further input to the logic circuit 8, which performs protection interlocking and frequency conversion@
The station is designed to stop.

When a break in the water cooling cable 4 is detected using the method described above, it is necessary to increase or decrease the CT5 as the number of water cooling cables 4 increases or decreases.As the number of water cooling cables 4 increases, the installation space for the CT5 becomes a problem, and due to the increase in the number of parts. It had drawbacks such as an increased failure rate and an economic disadvantage.

[Objective of the Invention] The present invention solves the above-mentioned problems and allows installation space to be increased to 4 times without increasing or decreasing the number of CTs regardless of the number of water-cooled cables.
It is an object of the present invention to provide a protection method for a frequency converter that can safely protect the frequency converter by providing an economically advantageous detection device that requires less work, has a low failure rate, and has a low failure rate.

[Summary of the Invention] In order to achieve this object, the present invention connects a total of M12 CTs for each digit to any one of a plurality of water-cooled cables, and detects the current difference between the CTs. It is characterized by detecting disconnection of the water cooling cable and protecting the frequency converter.

[Embodiment of the Invention] FIG. 2 shows an embodiment of the present invention, and parts having the same functions as those in FIG. 1 showing a conventional example are denoted by the same reference numerals and explanations thereof will be omitted.

FIG. 2, which shows one embodiment of the present invention, shows the CT connected to each phase.
5, a converter 9 that converts current into voltage, an operating circuit 7 that outputs an output signal when there is a difference in the output of the converter 9, and a logic circuit 8 that stops the frequency converter based on the signal.

To explain the present invention in detail, taking as an example the case where there are three water cooling cables connected to each phase, if the current flowing through the furnace coil is IF, then a current of IF/3 flows through each CT5, and each There is no difference in the output of the converter 9, but if one of the three water-cooled cables of either phase is disconnected, the phase in which the disconnection occurred will receive the furnace coil current using the remaining two water-cooled cables. ■The current that flows through the CT that has to share 2 and has a disconnection becomes IF/2,
On the other hand, since the current flowing through the normal phase CT is IF/3, a difference of l Ip /2-1p, /31 appears, and this difference can be detected by the operating circuit 7, and the signal is sent to the stop logic circuit. It is possible to stop the frequency converter for induction heating by inputting 8. Although the above example is based on the case where there are three water-cooled cables, it is obvious that the same effect can be obtained regardless of the number of water-cooled cables.

[Effects of the Invention] As described above, the present invention makes it possible to detect a break in a water-cooled cable by using one CT for each phase and protect the frequency converter, no matter how many water-cooled cables there are. ,
It can provide a protection method that is advantageous in terms of installation space, failure rate, and economy.

[Brief explanation of drawings]

Figure 1 is a block diagram showing a conventional frequency conversion device, Figure 2 is a block diagram showing a conventional frequency conversion device.
The figure is a block diagram showing one embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Frequency converter, 2... Power factor improvement capacitor, 3... Coil, 3a... Melting furnace, 4... Water cooling cable, 5... CT, 6... Detection circuit , 6a...
-OR circuit, 7...operation circuit, 8...logic circuit, 9...converter. Applicant's agent Patent attorney Takehiko Suzue Figure 1 Figure 2

Claims (1)

[Claims] In a frequency converter configured with a load commutation type inverter, a current transformer is connected to any one of each phase of a plurality of water cooling cables connecting the load commutation type inverter and the load side. A protection method for a frequency converter characterized by detecting a differential current of a current transformer and stopping the frequency converter using that signal.