Ground-fault detector for multifurnace installation

Abstract

An apparatus for detecting ground faults in a multifurnace installation with at least two induction furnaces and a multifurnace installation are described. A ground-fault sensor is associated with each induction furnace and is connected to the electrical supply line to the induction furnace coil. Furthermore, the apparatus has a ground-leak sensor. Moreover, the apparatus includes an additional ground-fault sensor that measures at the same location as the ground-leak sensor. In this manner an improvement of security during the operation of the system is obtained.

Claims

1. An installation comprising: two induction furnaces, two respective resonant load circuits each including a respective induction-furnace coil in a respective one of the furnaces, two respective parallel or series connected inverters that are connected to and each supply a respective one of the resonant load circuits, a respective electrical supply line connecting each of the parallel or series connected inverters to a common rectifier connected to an AC current source, a respective ground-fault sensor associated with each induction furnace and connected to the respective electrical supply line and to the respective induction-furnace coil, a ground-leak sensor connected to the inverters, and an additional ground-fault sensor measuring at the same location as the ground-leak sensor and injecting an AC current of low frequency into the furnace circuit and measuring the magnitude of this low-frequency circuit.

2. The installation according to claim 1, wherein the ground-leak sensor s and the additional ground-fault sensor are connected to the inverters.

3. The installation according to claim 1, wherein the two induction furnaces use current from a common power-sharing power supply.

4. An installation comprising: two induction furnaces, two respective resonant load circuits each including a respective induction-furnace coil in a respective one of the furnaces, two respective parallel or series connected inverters that are connected to and each supply a respective one of the resonant load circuits, a respective electrical supply line connecting each of the parallel or series connected inverters to a common rectifier connected to an AC current source, a respective ground-fault sensor associated with each induction furnace and connected to the respective electrical supply line and to the respective induction-furnace coil, a ground-leak sensor having R/C circuit and connected to the inverters, and an additional ground-fault sensor measuring at the same location as the ground-leak sensor.

5. The installation according to claim 4, wherein the additional ground-fault sensor as ground resistance monitor injects an AC current of low frequency into the furnace circuit and measures the magnitude of this low frequency current.

6. An installation comprising: two induction furnaces, two respective resonant load circuits each including a respective induction-furnace coil in a respective one of the furnaces, two respective parallel or series connected inverters that are each connected to a respective one of the resonant load circuits and that each supply a respective one of the resonant load circuits, a respective electrical supply line connecting each of the parallel or series connected inverters to a common rectifier connected to an AC current source, a respective ground-fault sensor associated with each induction furnace and connected to the respective electrical supply line and to the respective induction-furnace coil, a ground-leak sensor connected to the inverters, an additional ground-fault sensor measuring at the same location as the ground-leak sensor, and respective switches for isolating the load circuits of the respective induction furnaces from the corresponding inverters.

7. An installation comprising: two induction furnaces; two respective resonant load circuits each including a respective induction-furnace coil each in a respective one of the furnaces, two respective parallel or series connected inverters that are each connected to and that each supply a respective one of the resonant load circuits, a respective electrical supply line connecting each of the parallel or series connected inverters to a common rectifier connected to an AC current source, a respective ground-fault sensor associated with each induction furnace and connected to the respective electrical supply line and to the respective induction-furnace coil, a ground-leak sensor connected to the inverters, and an additional ground-fault sensor measuring at the same is location as the ground-leak sensor and having switches by means of which they are switchable on and off.

8. A multifurnace installation comprising two induction furnaces each having a respective induction-furnace coil that defines a part of a respective one of two series or parallel connected resonant load circuits, two parallel or series connected inverters each connected to and supplying a respective one of the resonant load circuits and operatively connected to a common rectifier that is connected to an AC current source, and an apparatus for detecting ground faults, the apparatus comprising a respective ground-fault sensor associated with each induction furnace and connected to the respective electrical supply line and to the respective induction-furnace coil, a ground-leak sensor connected to the two inverters, and an additional ground-fault sensor measuring at the same location as the ground-leak sensor and acting as ground resistance monitor by injecting an AC current of low frequency into the furnace circuit and measuring the magnitude of this low-frequency current.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) In the following the invention is described in detail by means of an example in connection with the drawing whose sole FIGURE shows schematically the construction of a multifurnace installation with ground-fault detection.

SPECIFIC DESCRIPTION OF THE INVENTION

(2) The FIGURE shows a multifurnace installation with two induction furnaces indicated schematically at 22 and 23 and using current from a common current supply with power sharing. According to this system a 3-phase AC line voltage from the utility network is applied to a rectifier 1. The rectifier 1 converts the 3-phase AC line voltage to a DC voltage. The DC voltage from the rectifier 1 flows through an inductor 2 that reduces ripple in the DC current. Then the DC current from the inductor 2 flows through two inverters 3 and 4. Each inverter 3 and 4 drives a respective resonant load circuit comprising a capacitor 5 and an induction-furnace coil 6 for the furnace 22 that is operatively associated with the inverter 3, and a capacitor 7 together with an induction-furnace coil 8 that is operatively associated with the inverter 4 and part of the furnace 23.

(3) The circuit configuration shown in the drawing is a power-sharing power supply according to which two induction furnaces 22 and 23 can share the available power from the common power supply 1, 2 in any desired proportion. It should be understood that the present invention is equally applicable to series or parallel connected load circuits and series or parallel connected inverters operatively associated with a common rectifier.

(4) The system includes ground-fault detecting means 13, 14 that are operatively connected to their respective induction-furnace coils 6 and 8. These ground-fault detecting means 13 and 14 are switched off by relays 15 and 16 when the respective furnaces 22 and 23 are switched on and are again switched on when the furnaces 22 and 23 are switched off so that they report an individual indication of the ground resistance to their respective induction furnaces 22 of 23. If one or both of the furnaces 22 and 23 are switched off, they are isolated from one another by their respective inverters 3 and 4 so that the ground resistance of each furnace can be individually determined.

(5) Furthermore, switches 18, 19 are provided in order to isolate the load circuits of the respective induction furnaces 22 and 23 from the respective inverters 3 and 4. That is, if a ground fault exists in one furnace it has to be isolated in order to be able to continue the operation of the other furnace. In addition to the switches 18, 19, bypass thyristors 20 and 21 can be provided so that the respective defective furnace 22 of 23 can be isolated by switching on the respective bypass thyristor 20 or 21 and switching off the corresponding inverter 3 or 4.

(6) Furthermore, the apparatus includes a ground-leak sensor (AC current ground-leak sensor) 10 that measures the leakage current through a R/C circuit 9. Its capacitor passes high frequency current typically associated with arcing and caused by insulation breakdown in the induction furnace refractory or the insulation that surrounds the coil while rejecting lower frequency voltage components. If the arcing current exceeds a preset threshold a trip signal is generated to shut down the furnaces 22 and 23 and an alarm is provided to signal the ground leakage fault to the furnace operator.

(7) Moreover, the apparatus has a ground-fault sensor (ground resistance monitor) 11 injecting an AC current of low frequency through a choke 12 into the furnace circuit. The magnitude of this low-frequency current that is proportional to the total resistance to ground of the furnace system is measured by ground-fault measuring. When the resistance to ground drops below a preset threshold, a trip signal is generated to shut down the furnaces 22 and 23 and provide an alarm to signal the ground resistance fault to the furnace operator.

(8) Accordingly, the ground-leak sensor 10 and the ground-fault sensor 11 are used in combination. Since the ground-fault sensor 11 functions independently of the furnace voltage while the ground-leak sensor 10 is characterized by a quick reaction, an improvement of security is obtained.